Homo- and Heterodinuclear Ir and Rh Imine-functionalized Protic NHC Complexes: Synthetic, Structural Studies, and Tautomerization/Metallotropism Insights

Article abstract of DOI:10.1002/chem.201504030

The influence of the potentially chelating imino group of imine-functionalized Ir and Rh imidazole complexes on the formation of functionalized protic N-heterocyclic carbene (pNHC) complexes by tautomerization/metallotropism sequences was investigated. Chloride abstraction in [Ir(cod)Cl{C₃H₃N₂(DippN=CMe)-κN3}] (1 a) (cod=1,5-cyclooctadiene, Dipp=2,6-diisopropylphenyl) with TlPF₆ gave [Ir(cod){C₃H₃N₂(DippN=CMe)-κ²(C2,N_imine)}]⁺[PF₆]^- (3 a⁺[PF₆]^-). Plausible mechanisms for the tautomerization of complex 1 a to 3 a⁺[PF₆]^- involving C2-H bond activation either in 1 a or in [Ir(cod){C₃H₃N₂(DippN=CMe)-κN3}₂]⁺[PF₆]^- (6 a⁺[PF₆]^-) were postulated. Addition of PR₃ to complex 3 a⁺[PF₆]^- afforded the eighteen-valence-electron complexes [Ir(cod)(PR₃){C₃H₃N₂(DippN=CMe)-κ²(C2,N_imine)}]⁺[PF₆]^- (7 a⁺[PF₆]^- (R=Ph) and 7 b⁺[PF₆]^- (R=Me)). In contrast to Ir, chloride abstraction from [Rh(cod)Cl{C₃H₃N₂(DippN=CMe)-κN3}] (1 b) at room temperature afforded [Rh(cod){C₃H₃N₂(DippN=CMe)-κN3}₂]⁺[PF₆]^- (6 b⁺[PF₆]^-) and [Rh(cod){C₃H₃N₂(DippN=CMe)-κ²(C2,N_imine)}]⁺[PF₆]^- (3 b⁺[PF₆]^-) (minor); the reaction yielded exclusively the latter product in toluene at 110 C. Double metallation of the azole ring (at both the C2 and the N3 atom) was also achieved: [Ir₂(cod)₂Cl{μ-C₃H₂N₂(DippN=CMe)-κ²(C2,N_imine),κN3}] (10) and the heterodinuclear complex [IrRh(cod)₂Cl{μ-C₃H₂N₂(DippN=CMe)-κ²(C2,N_imine),κN3}] (12) were fully characterized. The structures of complexes 1 b, 3 b⁺[PF₆]^-, 6 a⁺[PF₆]^-, 7 a⁺[PF₆]^-, [Ir(cod){C₃HN₂(DippN=CMe)(DippN=CH)(Me)-κ²(N3,N_imine)}]⁺[PF₆]^- (9⁺[PF₆]^-), 10 Et₂O'toluene, [Ir₂(CO)₄Cl{μ-C₃H₂N₂(DippN=CMe)-κ²(C2,N_imine),κN3}] (11), and 12-2 THF were determined by X-ray diffraction.

Full text of DOI:10.1002/chem.201504030

DOI: 10.1002/chem.201504030
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&
N-Heterocyclic Carbenes |Hot Paper|
Homo- and Heterodinuclear Ir and Rh Imine-functionalized Protic
NHC Complexes: Synthetic, Structural Studies, and
Tautomerization/Metallotropism Insights**
Fan He,[a] Marcel Wesolek,[a] Andreas A. Danopoulos,*[a, b] and Pierre Braunstein*[a]
Dedicated to Prof. Dr. Herbert W. Roesky on the occasion of his 80th birthday
Abstract: The influence of the potentially chelating imino
group of imine-functionalized Ir and Rh imidazole complexes
on the formation of functionalized protic N-heterocyclic car-
bene (pNHC) complexes by tautomerization/metallotropism
sequences was investigated. Chloride abstraction in
[Ir(cod)Cl{C3H3N2(DippN=CMe)-kN3}] (1a) (cod=1,5-cyclooc-
tadiene, Dipp=2,6-diisopropylphenyl) with TlPF6 gave
[Ir(cod){C3H3N2(DippN=CMe)-k2(C2,Nimine)}]+[PF6]À (3a+[PF6]À).
Plausible mechanisms for the tautomerization of complex
1a to 3a+[PF6]À involving C2ÀH bond activation either in
1a or in [Ir(cod){C3H3N2(DippN=CMe)-kN3}2]+[PF6]À (6a+
[PF6]À) were postulated. Addition of PR3 to complex 3a+
[PF6]À afforded the eighteen-valence-electron complexes
[Ir(cod)(PR3){C3H3N2(DippN=CMe)-k2(C2,Nimine)}]+[PF6]À (7a+
[PF6]À (R=Ph) and 7b+[PF6]À (R=Me)). In contrast to Ir, chlo-
ride abstraction from [Rh(cod)Cl{C3H3N2(DippN=CMe)-kN3}]
(1b) at room temperature afforded [Rh(cod){C3H3N2(DippN=
CMe)-kN3}2]+[PF6]À (6b+[PF6]À) and [Rh(cod){C3H3N2(DippN=
CMe)-k2(C2,Nimine)}]+[PF6]À (3b+[PF6]À) (minor); the reaction
yielded exclusively the latter product in toluene at 110 8C.
Double metallation of the azole ring (at both the C2 and the
N3 atom) was also achieved: [Ir2(cod)2Cl{m-C3H2N2(DippN=
CMe)-k2(C2,Nimine),kN3}] (10) and the heterodinuclear com-
plex
[IrRh(cod)2Cl{m-C3H2N2(DippN=CMe)-k2(C2,Nimine),kN3}]
(12) were fully characterized. The structures of complexes
1b, 3b+[PF6]À, 6a+[PF6]À, 7a+[PF6]À, [Ir(cod){C3HN2(DippN=
CMe)(DippN=CH)(Me)-k2(N3,Nimine)}]+[PF6]À (9+[PF6]À), 10·
Et2O·toluene, [Ir2(CO)4Cl{m-C3H2N2(DippN=CMe)-k2(C2,Nimine),-
kN3}] (11), and 12·2THF were determined by X-ray diffrac-
tion.
Introduction
Since the isolation of N-heterocyclic carbenes (NHCs),[1] their
study and complexation chemistry have generated a rapidly in-
creasing interest.[2] In most of their metal complexes, both N
atoms carry substituents R (R=R’ or R¼ R’ in Scheme 1A) that
allow fine-tuning of the steric and electronic properties of the
NHC ligands. Protic NHC (pNHC) metal complexes (Scheme 1B)
are less common. The NÀH moiety can further be a reactive
site, for example, with bases or hydrogen-bond acceptors, the
latter being relevant to substrate recognition in homogeneous
catalysis.[3]
Scheme 1. Different structures of substituted NHC metal complexes
pNHCs cannot be generally obtained by simple deprotona-
tion of the corresponding imidazolium salts owing to the pres-
ence of sites with acidity (i.e., NÀH group) comparable to the
C2ÀH group on the heterocycle; furthermore, the free pNHCs
are not stable and tend to isomerize to the corresponding imi-
dazoles.[4] This has motivated the development of various
methods to access pNHC metal complexes, which have been
recently summarized and reviewed.[3b,c,5] The first transition-
metal complex bearing 1H-imidazol-2-ylidene (R=H in
Scheme 1B) was obtained by the acid-catalyzed rearrangement
of a RuII–imidazole to a RuII–pNHC system (Scheme 2).[6] Re-
cently, the preparation of the parent (benz)imidazole-type
pNHC complexes through oxidative addition of 2-halogeno-
azoles to a zerovalent transition-metal center was reported by
the group of Hahn (Scheme 2).[7]
[a] F. He, Dr. M. Wesolek, Dr. A. A. Danopoulos, Dr. P. Braunstein
Laboratoire de Chimie de Coordination
Institut de Chimie (UMR 7177 CNRS)
UniversitØ de Strasbourg, 4 rue Blaise Pascal
67081 Strasbourg Cedex (France)
E-mail: braunstein@unistra.fr
[b] Dr. A. A. Danopoulos
UniversitØ de Strasbourg Institute for Advanced Study (USIAS)
4 rue Blaise Pascal, 67081 Strasbourg Cedex (France)
E-mail: danopoulos@unistra.fr
[**] NHC=N-heterocyclic carbene.
The N-substituent (R in Scheme 1B) is expected to be crucial
for the tuning of the stereoelectronic properties and the coor-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504030.
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ported by Hahn et al., the former revealed intermolecular hy-
drogen bonding between the NÀH group and the 1,3-dimeth-
yltetrahydropyrimidin-2(1H)-one acting as hydrogen-bond ac-
ceptor. This property appears to be common and could be uti-
lized for substrate recognition and regioselective catalysis with
pNHC complexes.[3b,c]
In view of the remarkable catalytic properties of a-diimine
and pyridine diimine complexes,[15] it appeared attractive to
design metal complexes bearing imine-functionalized NHC li-
gands.[16] The features of the imino–NHCs as hybrid ligands are
based on the association of a s-donor/p-acceptor imine and
a strong s-donor/poor p-acceptor NHC functionality. It is note-
worthy that an imine-functionalized NHC RhI complex shows
high activity and cis-selectivity in the catalytic cyclopropana-
tion of alkenes,[17] and that imine-functionalized pNHC com-
plexes are potentially bifunctional catalysts.[18]
We have recently demonstrated[19] that the reaction of 1-
(2,6-diisopropylphenylimino)ethylimidazole (HC2L)[16f] (where
the superscript associated with “H” indicates its position in the
ring) with 0.5 equivalent of [Ir(cod)(m-Cl)]2 (cod=1,5-cycloocta-
diene) afforded [Ir(cod)Cl{C3H3N2(DippN=CMe)-kN3}] (1a)
(Dipp=2,6-diisopropylphenyl), abbreviated as [Ir(cod)Cl(HC2LN3)]
where “LN3” represents an N3-metalated, functionalized imida-
zole (the superscript associated with “L” indicates the site(s) of
metalation) (Scheme 4).
Scheme 2. Examples of transition-metal complexes containing 1H-(benz)imi-
dazol-2-ylidenes.[6,7]
dination behavior of the pNHCs as well as for the stability and
catalytic properties of the resulting metal complexes.[8] In most
pNHC complexes, the R substituent is an alkyl or an aryl group,
but there are relatively few examples where a donor functional
group is attached to the N substituent that could potentially
be involved in chelate formation. The number of complexes
with functionalized pNHCs is still limited, and some have
found catalytic applications (Scheme 3).
Scheme 3. Transition-metal complexes with bidentate functionalized pNHCs.
Cy=cyclohexyl, Cp*=1,2,3,4,5-pentamethylcyclopentadiene, Cp=cyclopen-
tadiene.
Scheme 4. Tautomerism/metallotropism leading to the pNHC complex
3a+[PF6]À.[19]
Thus, Bergman et al. reported an intramolecular coupling re-
action sequence of alkenes to 2-functionalized azole deriva-
tives, in which an alkene-functionalized pNHC RhI complex was
isolated as an intermediate (Scheme 3).[9] This intramolecular
reaction was extended to an intermolecular coupling reac-
tion.[10] The pyridyl-functionalized pNHC RuII complex reported
by Kuwata et al. (Scheme 3) was used as a catalyst for the de-
hydrative condensation of N-(2-pyridyl)benzimidazole and allyl
alcohol.[11] The phosphorus-functionalized pNHC IrIII and RuII
complexes reported by Grotjahn et al. (Scheme 3) were devel-
oped as catalysts for the activation of dihydrogen and in vari-
ous transfer hydrogenation reactions.[12] Related phosphorus-
functionalized pNHC RuII[13] and RhI[14] complexes have been re-
Reaction of complex 1a with an additional half equivalent of
[Ir(cod)(m-Cl)]2, or the direct reaction of HC2L with one equiva-
lent of [Ir(cod)(m-Cl)]2, afforded [Ir2(cod)2HCl2{m-C3H2N2(DippN=
CMe)-k2(C2,Nimine),kN3}] (2), abbreviated as [Ir2(cod)2-
HCl2(LC2,Nimine,N3)], which represents a mixed-valence homodinu-
clear complex comprising one N-bound IrI center and one
C2,Nimine-chelated IrIII center. The imine-functionalized pNHC IrI
complex [Ir(cod){C3H3N2(DippN=CMe)-k2(C2,Nimine)}]+[PF6]À (3a+
[PF6]À), abbreviated as [Ir(cod)(HN3LC2,Nimine)]+[PF6]À, was ob-
tained from the reaction of either complex 1a or complex 2
with TlPF6 (Scheme 4).[19]
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Here, we further investigate the synthesis and reactivity of
imine-functionalized pNHC iridium complexes by the metala-
tion of the C2ÀH bond in N-bonded imidazole iridium com-
plexes, and extend these studies to rhodium and to homo-
and heterodinuclear complexes.
Results and Discussion
Chelate assistance by the imine functionality in the C2-met-
allation of Ir imidazole complexes
Scheme 6. Synthesis of complex 6a+[PF6]À.
In order to examine the influence of the potentially chelating
imino group on the reactivity of complex 1a, in particular in
the course of the chloride abstraction which led to products
arising from tautomerization/metallotropism, we prepared the
complexes [Ir(cod)Cl{C3H3N2(nBu)-kN3}] (4a) and [Ir(cod)-
Cl{C3H3N2(Mes)-kN3}] (4b)[20] (Mes=mesityl) (Scheme 5), with
the N-butyl- and N-mesityl-substituents, respectively.
1
In the H NMR spectrum, the 2:1 ratio between the imida-
zole and the 1,5-cod protons in complex 6a+[PF6]À is consis-
tent with the molecular structure of complex 6a+[PF6]À in the
solid state (Figure 1). The sixteen valence-electron iridium
center in complex 6a+[PF6]À adopts an approximate square-
planar coordination geometry, defined by the two olefinic
bonds of the 1,5-cod ligand and two nitrogen atoms from the
imidazole ligands.
Complexes with functionalized pNHC ligands, related to
complex 3a+[PF6]À, and bearing bidentate (benz)imidazolin-2-
ylidene/donor ligands, have been obtained by formal tautome-
rization/metallotropism,[11–13] Hahn et al. proposed the occur-
rence of a “redox tautomerization” involving an initial C2ÀH
oxidative addition of azoles followed by reductive elimination
of the proton on the metal center.[3b,14,21]
In the present case of the tautomerization of complex 1a to
1
complex 3a+[PF6]À, we suggest on the basis of H NMR moni-
Scheme 5. Synthesis of complexes 4a and 4b as well as complexes
5a+[PF6]À and 5b+[PF6]À.
toring that complex 2 (Scheme 4) is a reaction intermediate. It
was isolated in an separate experiment and can be viewed as
the product of a C2ÀH oxidative addition of the N-bound iridi-
um complex 1a (Scheme 4), requiring the presence of a catalyt-
Under conditions similar to those used for the chloride ab-
straction reaction of complex 1a, complexes 4a and 4b only
afforded the complexes [Ir(cod){C3H3N2(R)-kN3}2]+[PF6]À [i.e.,
5a+[PF6]À (R=nBu), 5b+[PF6]À (R=Mes)]; this was evidenced
by the ratio of the coordinated imidazoles and the 1,5-cyclooc-
1
ic amount of [Ir(cod)(m-Cl)]2. Then, H NMR monitoring of the
reaction between complex 2 and TlPF6 in CD2Cl2 (Scheme 4) re-
1
tadiene (2:1) in the H NMR spectra. The reaction was accom-
panied by the formation of [Ir(cod)(m-Cl)]2 (NMR evidence).
These results point to the crucial role of the N-arylimino func-
tional group for the tautomerization/metallotropism manifest-
ed by the conversion of the N-bound IrI to the IrIÀpNHC com-
plex (Scheme 4).
In comparison, although the formation of [Ir(cod)-
{C3H3N2(DippN=CMe)-kN3}2]+[PF6]À (6a+[PF6]À), abbreviated as
[Ir(cod)(HC2LN3)2]+[PF6]À, was observed in the initial stages of
the halide abstraction reaction from complex 1a, as evidenced
by monitoring the reaction of the latter with TlPF6 in CD2Cl2 at
room temperature by 1H NMR spectroscopy, it could not be
isolated. Informatively, we show below that complex 6a+[PF6]À
can further react with [Ir(cod)(m-Cl)]2 to give complex 3a+
[PF6]À (see Scheme 8). In contrast to complexes 5a+[PF6]À and
5b+[PF6]À (Scheme 5), complex 1a was fully converted to com-
plex 3a+[PF6]À at the end of the reaction (Scheme 4). Addition
of one equivalent of HC2L was required in order to isolate com-
plex 6a+[PF6]À (Scheme 6).
Figure 1. Molecular structure of the cation in complex 6a+[PF6]À. Hydrogen
atoms are omitted for clarity. Thermal ellipsoids are at the 30% level. The
crystallographic labels C1 and N1 correspond to the conventional C2 and N3
numbering used in the text. Selected bond lengths [Š] and angles [8]: C1À
N1 1.322(5), C1ÀN2 1.363(5), C2ÀN1 1.393(5), C2ÀC3 1.352(6), C3ÀN2
1.379(5), C4ÀN2 1.435(4), C4ÀN3 1.255(5), C4ÀC5 1.491(5), C6ÀN3 1.422(4),
Ir1ÀN1 2.078(3), Ir1ÀC19 2.125(5), Ir1ÀC20 2.120(5), C19ÀC20 1.391(9); N1-
C1-N2 110.2(3), N1-Ir1-C19 91.8(2), N1-Ir1-C20 92.3(2), N1-Ir1-N1’ 88.3(2).
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vealed a hydride resonance at d=À14.41 ppm, which could
be tentatively assigned to the intermediate [2ÀClÀ]+[PF6]À in
1
cycle I (compare d=À15.12 ppm in complex 2). In a H NMR
experiment for monitoring the chloride abstraction of complex
1a with TlPF6 in CD2Cl2 at room temperature, the resonances
of complex 6a+[PF6]À and of three hydride species were ob-
served in the initial stages of the reaction, in a ratio of approxi-
mately 1:20:4 at d=À14.41 ([2ÀClÀ]+[PF6]À), À14.99,
À15.12 ppm (2) (Figure S1 in the Supporting Information), re-
spectively, which progressively disappeared. The hydride spe-
cies observed at d=À14.41 ([2ÀClÀ]+[PF6]À) and À15.12 ppm
(2) are consistent with the suggested steps shown in cycle I of
Scheme 7.
Scheme 8. The reaction between complexes 6a+[PF6]À and [Ir(cod)(m-Cl)]2
that was monitored by H NMR spectroscopy.
1
plex 6a+[PF6]À is an intermediate in the tautomerization of
complex 1a to complex 3a+[PF6]À and therefore, cannot be
isolated by this sequence of steps.
The reaction between complex 1a and TlPF6 (cycle II in
Scheme 7) would give a reactive Ir species, that is, [1aÀClÀ]+
[PF6]À, which could react with complex 1a still present to yield
complex 6a+[PF6]À by ligand transfer, and [Ir(cod)(m-Cl)]2. The
latter could activate the C2ÀH bond either of complex 1a to
give complex 2 (cycle I in Scheme 7) or of complex 6a+[PF6]À
to give complex 3a+[PF6]À (Scheme 8 and cycle II in
Scheme 7), through a postulated IrIII/IrI intermediate, which is
supposed to be the hydride species resonating at d=
À14.99 ppm. Proton transfer from the IrIII center to the N3
atom of the metalated heterocycle with concomitant breaking
of the N3ÀIrI bond would lead to complexes 3a+[PF6]À and
1a, the latter remaining in the cycle until complete conversion
to complex 3a+[PF6]À. In summary, the species postulated in
cycles I and II in Scheme 7 point to the different elementary
steps that lead to tautomerization (H-shift) and “apparent”
metallotropism (Ir-to-N3 shift), the latter formally involving dif-
ferent iridium species.
With the aim to explore the scope of the H-shift from N to Ir
in pNHCs as a way to access valence tautomers, we reacted
complex 3a+[PF6]À with phosphine ligands to render the
metal center more electron-rich, hoping to favor a transforma-
tion from IrI to IrIII. Addition of triphenylphosphine/trimethyl-
phosphine afforded the eighteen-valence-electron addition
products [Ir(cod)(PR3){C3H3N2(DippN=CMe)-k2(C2,Nimine)}]+[PF6]À
(7a+[PF6]À (R=Ph) and 7b+[PF6]À (R=Me)), abbreviated as
[Ir(cod)(PR3)(HN3LC2,Nimine)]+[PF6]À (Scheme 9).
Scheme 7. Proposed mechanism for the tautomerization of complex 1a to
complex 3a+[PF6]À through a neutral and a cationic IrIII/IrI intermediate.
In order to prove that C2ÀH bond activation can occur on
the N3-coordinated imidazole in complexes of the type
[Ir(cod)(HC2LN3)2]+ (compare behavior of complexes 4a and 4b
giving complexes 5a+ and 5b+, respectively, Scheme 5), a reac-
tion between complexes 6a+[PF6]À and [Ir(cod)(m-Cl)]2 in
In the 31P{1H} NMR spectrum of complex 7a+[PF6]À, the PPh3
ligand gives rise to a singlet at d=À1.9 ppm. In complex 7b+
[PF6]À,
a
characteristic 31P{1H} NMR singlet peak at d=
1
CD2Cl2 was monitored by H NMR spectroscopy (Scheme 8). In-
À44.8 ppm and the CNHC resonance at d=171.1 ppm (d,
2J(P,C)=10.9 Hz) in the 13C{1H} NMR spectrum suggest that co-
ordination of the PMe3 ligand occurs in cis position to the CNHC
atom. In the structure of complex 7a+[PF6]À (Figure 2), the iri-
dium center adopts a distorted trigonal-bipyramidal coordina-
tion geometry, with the C1 atom and the C22=C23 double
bond in the apical positions.[22] The Ir1ÀC1 bond length in
terestingly, we observed the same intermediate hydride spe-
cies as in the chloride abstraction reaction of complex 1a (Fig-
ure S2 in the Supporting Information). These postulated steps
for the tautomerization of complex 1a to complex 3a+[PF6]À
are summarized in cycle II of Scheme 7. Furthermore, the ob-
servation of complexes 1a and 3a+[PF6]À indicates that com-
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Scheme 9. Synthesis of complexes 7a+[PF6]À and 7b+[PF6]À.
Scheme 10. Synthesis of the ligand HC2L’ and complexes 8 and 9+[PF6]À.
PTSA=p-toluenesulfonic acid.
Abstraction of the chloride ligand from complex 8 in CH2Cl2
did not lead to “Ir migration” from N to C2, but to rapid chela-
tion of the proximal imino group to give [Ir(cod)-
{C3HN2(DippN=CMe)(DippN=CH)(Me)-k2(N3,Nimine)}]+[PF6]À (9+
[PF6]À), abbreviated as [Ir(cod)(HC2L’N3,Nimine)]+[PF6]À, the struc-
ture of which was confirmed crystallographically (Figure 3). In
contrast to complex 1a, which can lead to the dinuclear IrIII/IrI
complex 2 (Scheme 4), complex 9+[PF6]À does not react further
with [Ir(cod)(m-Cl)]2, possibly for steric reasons, the orientation
of the Ir(cod) moiety being locked owing to N,N chelation.
Figure 2. Molecular structure of the cation in complex 7a+[PF6]À. Hydrogen
atoms are omitted for clarity, except H1N. Thermal ellipsoids are at the 30%
level. Selected bond lengths [Š] and angles [8]: C1ÀN1 1.345(2), C1ÀN2
1.371(2), C2ÀN1 1.388(3), C2ÀC3 1.337(3), C3ÀN2 1.402(2), C4ÀN2 1.402(2),
C4ÀN3 1.284(2), C4ÀC5 1.495(3), C6ÀN3 1.453(2), Ir1ÀP1 2.3388(5), Ir1ÀC1
1.982(2), Ir1ÀN3 2.347(2), Ir1ÀC18 2.118(2), Ir1ÀC19 2.142(2), Ir1ÀC22
2.220(2), Ir1ÀC23 2.269(2), C18ÀC19 1.445(3), C22ÀC23 1.390(3), N1ÀH1N
0.82(3); N1-C1-N2 104.0(2), C1-Ir1-N3 73.61(6), C1-Ir1-P1 89.59(5), C1-Ir1-C18
89.36(7), C1-Ir1-C19 90.85(8), N3-Ir1-P1 108.63(4).
complex 7a+[PF6]À (1.982(2) Š) is similar to that in complex
3a+[PF6]À (1.984(3) Š), but the Ir1ÀN3 bond in complex 7a+
[PF6]À is considerably longer than the corresponding bond in
complex 3a+[PF6]À (2.347(2) vs. 2.115(3) Š), which is consistent
with the respective electron counts of the metals and the
steric hindrance in complex 7a+. The closest NÀH···F(PF5) dis-
tance of 2.21(3) Š is consistent with a hydrogen-bonding inter-
action. Although as expected, phosphine coordination has
taken place in the reactions given in Scheme 9, a transforma-
tion from IrI to IrIII species was not observed in these reactions,
in contrast to findings involving a 1,9-phenanthroline-derived
pNHC system.[22c] The stability of complexes 7a+[PF6]À and
7b+[PF6]À is consistent with their eighteen-valence-electron
count.
Figure 3. Molecular structure of the cation in 9+[PF6]À. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are at the 30% level. Selected bond
lengths [Š] and angles [8]: C1ÀN1 1.31(1), C1ÀN2 1.38(1), C2ÀN1 1.394(9),
C2ÀC3 1.37(1), C3ÀN2 1.41(1), C4ÀN3 1.31(1), Ir1ÀN1 2.083(7), Ir1ÀN3
2.094(6), Ir1ÀC32 2.154(9), Ir1ÀC33 2.137(8), Ir1ÀC36 2.110(9), Ir1ÀC37
2.123(9), C32ÀC33 1.43(2), C36ÀC37 1.38(1); N1-C1-N2 111.1(7), N1-Ir1-N3
78.9(3), N1-Ir1-C32 98.6(3), N1-Ir1-C33 94.2(3), N3-Ir1-C36 97.5(3), N3-Ir1-C37
93.6(3).
To further study the possible contribution of the imino
group on the tautomerization/metallotropism, the ligand HC2L’,
which is a derivative of HC2L, was designed in which the
carbon atoms C5 and C4 of the heterocycle are substituted by
methyl and imino groups in order to prevent the formation of
abnormal NHC complexes and to provide chelate assistance by
imine coordination, respectively. Reaction of HC2L’ with
Synthesis of imine-functionalized pNHC RhI complexes
Similarly to complex 1a, the reaction of the ligand HC2L
[Ir(cod)(m-Cl)]2 led to the formation of [Ir(cod)Cl{C3HN2(DippN= with 0.5 equivalent of [Rh(cod)(m-Cl)]2 in THF gave
CMe)(DippN=CH)(Me)-kN3}] (8), abbreviated as [Ir(cod)-
Cl(HC2L’N3)] (Scheme 10).
[Rh(cod)Cl{C3H3N2(DippN=CMe)-kN3}] (1b), abbreviated as
[Rh(cod)Cl(HC2LN3)], in nearly quantitative yield (Scheme 11).
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Scheme 11. Synthesis of the RhI complexes 1b, 3b+[PF6]À, and 6b+[PF6]À.
chloromethane at room temperature, the color of the solution
slightly turned to red and the pNHC RhI complex
[Rh(cod){C3H3N2(DippN=CMe)-k2(C2,Nimine)}]+[PF6]À (3b+[PF6]À),
abbreviated as [Rh(cod)(HN3LC2,Nimine)]+[PF6]À, was isolated in
4% yield as dark red crystals by fractional crystallization
(Scheme 11 b) (see the Experimental Section). The low yield
partly explains why complex 3b+[PF6]À had not been observed
in the 1H NMR experiment. From the supernatant solution,
complex 6b+[PF6]À was isolated as a pale yellow powder in
Figure 4. Molecular structure of complex 1b. Hydrogen atoms are omitted
for clarity. Thermal ellipsoids are at the 30% level. Selected bond lengths [Š]
and angles [8]: C1ÀN1 1.315(2), C1ÀN2 1.359(2), C2ÀN1 1.383(2), C2ÀC3
1.355(2), C3ÀN2 1.381(2), C4ÀN2 1.428(2), C4ÀN3 1.260(2), C4ÀC5 1.496(2),
C6ÀN3 1.428(2), Rh1ÀN1 2.104(1), Rh1ÀCl1 2.3748(5), Rh1ÀC18 2.120(2),
Rh1ÀC19 2.101(2), Rh1ÀC22 2.143(2), Rh1ÀC23 2.127(2), C18ÀC19 1.399(3),
C22ÀC23 1.396(3); N1-C1-N2 111.1(1), N1-Rh1-Cl1 88.36(4), N1-Rh1-C18
93.69(6), N1-Rh1-C19 90.02(6), Cl1-Rh1-C22 92.20(6), Cl1-Rh1-C23 91.16(6).
1
good yield (90% based on HC2L). The H NMR data of complex
6b+[PF6]À in CD2Cl2 established the ratio of the imidazole and
the 1,5-cod ligands to be 2:1 and the upfield shift by 0.31 ppm
(compared to complex 1b) of the resonance of the C2ÀH
proton at d=8.42 ppm. In the 13C NMR spectrum, the reso-
nance at d=137.0 ppm was assigned to the C2ÀH carbon
atom. The IR band for the n(C=N) vibration of a dangling imine
group in complex 6b+[PF6]À was observed at n˜ =1687 cmÀ1.
1
1
In the H NMR spectrum of complex 1b in CD2Cl2, the C2ÀH
In the H NMR spectrum of complex 3b+[PF6]À in CD2Cl2, the
exhibits
a
broad singlet at d=8.73 ppm (compare d=
NÀH proton appears as a broad singlet at d=10.31 ppm and
8.11 ppm in the ligand HC2L). The molecular structure of com-
plex 1b is shown in Figure 4.
the 13C{1H} NMR spectrum contains resonances for the C2NHC
1
and Cimine carbon atoms at d=176.9 (d, J(Rh,C)=55.1 Hz) and
Defined by the two double bonds of the 1,5-cod ligand, one
nitrogen atom of the imidazole, and one terminal chloride
ligand, the rhodium center adopts a distorted square-planar
coordination geometry, as evidenced by the bond angles at
Rh, namely, N1-Rh1-Cl1 88.36(4), N1-Rh1-C18/C19 93.69(6)/
90.02(6), and Cl1-Rh1-C22/C23 92.20(6)/91.16(6)8. This is consis-
tent with the IR absorption band at n˜ =1685 cmÀ1 assigned to
the n(C=N) vibration of the dangling imino functionality and
the 13C{1H} NMR signal at d=149.0 ppm assigned to the C=N
moiety.
163.7 ppm, respectively (compare d=138.6 and 149.0 ppm, re-
spectively, in complex 1b). The IR absorptions at n˜ =3405 and
1627 cmÀ1 can be assigned to NÀH[19,22c,23] and coordinated C=
N stretching vibrations, respectively. All these data indicate
that complex 3b+[PF6]À is a k2(C2,Nimine)-chelated imino–NHC
rhodium complex, which was further confirmed crystallograph-
ically (Figure 5).
In the structure of complex 3b+[PF6]À, the square-planar co-
ordination geometry around the metal center, which is defined
by the k2(C2,Nimine) five-membered ring chelate and the two
olefinic bonds of the 1,5-cod ligand, is strongly distorted, with
an acute C1-Rh1-N3 angle of 77.7(1)8. In agreement with
a lower trans influence of the imino group compared to the
NHC donor, the Rh1ÀC18/C25 (2.137(3)/2.122(3) Š) bond
lengths are shorter than the Rh1ÀC21/C22 (2.239(3)/2.256(3) Š)
bond length and, consistently, the C18=C25 double bond
(1.390(5) Š) is slightly longer than the C21=C22 (1.345(5) Š)
double bond, owing to increased back-bonding from the
metal. The RhÀCNHC bond length of 1.972(3) Š is similar to that
found in the k2(C2,Nimine)-chelated imino–NHC rhodium com-
plex with a N-methyl substituent (1.99(1) Š).[17b] Owing to the
coordination of the Nimine atom to the Rh atom, the C4ÀN3
Inspired by the conversion of complex 1a to complex 3a+
[PF6]À, we investigated analogous reactivity with the rhodium
1
complex 1b. In a H NMR monitoring experiment, the reaction
of complex 1b with 1.0 equivalent of TlPF6 in CD2Cl2 at room
temperature gave a white precipitate of TlCl and a yellow solu-
1
tion, which (as proven by H NMR spectroscopy) contained half
one equivalent of
a
new rhodium species, that is,
[Rh(cod){C3H3N2(DippN=CMe)-kN3}2]+[PF6]À (6b+[PF6]À), abbre-
viated as [Rh(cod)(HC2LN3)2]+[PF6]À, and 0.25 equivalent of
[Rh(cod)(m-Cl)]2 resulting from a ligand transfer reaction, but
no pNHC Rh complex was detected (Scheme 11 a). However,
when this experiment was repeated on a larger scale in di-
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Figure 5. Molecular structure of the cation in complex 3b+[PF6]À. Hydrogen
atoms are omitted for clarity, except H1N. Thermal ellipsoids are at the 30%
level. Selected bond lengths [Š] and angles [8]: C1ÀN1 1.342(4), C1ÀN2
1.369(4), C2ÀN1 1.380(4), C2ÀC3 1.340(5), C3ÀN2 1.392(3), C4ÀN2 1.399(3),
C4ÀN3 1.287(3), C4ÀC5 1.484(4), C6ÀN3 1.444(3), Rh1ÀC1 1.972(3), Rh1ÀN3
2.125(2), Rh1ÀC18 2.137(3), Rh1ÀC21 2.239(3), Rh1ÀC22 2.256(3), Rh1ÀC25
2.122(3), C18ÀC25 1.390(5), C21ÀC22 1.345(5), N1ÀH1N 0.82(4); N1-C1-N2
103.0(2), C1-Rh1-N3 77.7(1), C1-Rh1-C18 95.9(1), C1-Rh1-C25 94.8(1), N3-Rh1-
C21 100.6(1), N3-Rh1-C22 97.9(1).
Scheme 12. Improved synthesis of complex 3b+[PF6]À.
1a to complex 3a+[PF6]À, but a different mechanism may be
operative because RhÀH species were not detected in situ by
1H NMR spectroscopy, although this may be attributable to the
poor solubility of the cationic hydride species in [D8]toluene.
Homo- and heterodinuclear Ir and Rh complexes
bond in complex 3b+[PF6]À (1.287(3) Š) is longer than that in
complex 1b (1.260(2) Š), whereas the C4ÀN2 bond in complex
3b+[PF6]À (1.399(3) Š) is shorter than that in complex 1b
(1.428(2) Š). As expected from the decreased p delocalization
over the azole ring, the bond lengths of the C1ÀN1/N2 bonds
in complex 3b+[PF6]À (1.342(4)/1.369(4) Š) are longer than
those in complex 1b (1.321(2)/1.359(2) Š). As a result, the N1-
C1-N2 bond angle in complex 3b+[PF6]À (103.0(2)8) is also sig-
nificantly less obtuse than that in complex 1b (111.1(1)8). The
shortest NÀH···F(PF5) distances of 2.43(4) and 2.49(4) Š are con-
sistent with hydrogen-bonding interactions.
To expand the scope and the reactivity of complexes 1a and
3a+[PF6]À, we considered to use them as precursors for dinu-
clear complexes. The in situ deprotonation of complex 1a by
potassium bis(trimethylsilyl)amide (KHMDS) in THF at À308C
initially gave a red solution; after addition of 0.5 equivalent of
[Ir(cod)(m-Cl)]2 the dark green [Ir2(cod)2Cl{m-C3H2N2(DippN=
CMe)-k2(C2,Nimine),kN3}] (10), abbreviated as [Ir2(cod)2Cl-
(LC2,Nimine,N3)], was isolated (Scheme 13).
In an attempt to improve the yield of complex 3b+[PF6]À,
1
we used toluene as the reaction solvent. In a H NMR experi-
ment in [D8]toluene at room temperature, the reaction of com-
plex 1b with one equivalent of TlPF6 was found to give the
same products as in CD2Cl2. However, after heating the reac-
tion mixture to 110 8C for twelve hours, the yellow solution
became colorless and a red precipitate was formed. The
1H NMR spectroscopic data of the solution indicated complete
consumption of complexes 6b+[PF6]À and [Rh(cod)(m-Cl)]2. The
red precipitate was isolated by filtration and extracted in
CD2Cl2 giving pure 3b+[PF6]À in solution and a white precipi-
tate of TlCl. Repeating the reaction on a larger scale led to the
isolation of complex 3b+[PF6]À in 90% yield (Scheme 12).
When a solution of complex 6b+[PF6]À, in the presence or
not of 0.5 equivalent of [Rh(cod)(m-Cl)]2, was heated at 110 8C
for more than 24 hours, no 3b+[PF6]À was observed but only
Scheme 13. Synthesis of the dinuclear IrI complexes 10 and 11.
An IR absorption band at n˜ =1577 cmÀ1 for the C=N stretch-
ing vibration and the 13C{1H} NMR resonance due to the C=N
carbon atom (d=166.0 ppm) support the coordination of the
Nimine atom to the cationic IrI center in complex 10. In addition,
a
slight decomposition of complex 6b+[PF6]À occurred
(Scheme 12). These results suggest that complex 6b+[PF6]À is
an intermediate in the synthesis of complex 3b+[PF6]À only in
the presence of both [Rh(cod)(m-Cl)]2 and TlPF6 (compare the
corresponding Ir complexes described above).
1
the disappearance of a C2ÀH resonance in the H NMR spec-
trum of complex 10 (in C6D6), and the appearance of a signal
at d=180.6 ppm in the 13C{1H} NMR spectrum, confirmed the
formation of an IrÀCNHC bond.
The tautomerization of complex 1b to complex 3b+[PF6]À
may apparently proceed similarly to the conversion of complex
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The structure of complex 10·Et2O·toluene (Figure 6) revealed
a dinuclear complex comprising one N-bound IrI center and
one k2(C2,Nimine)-chelated IrI center (compare the k2(C2,Nimine
)
chelation of the IrIII center in complex 2). Both iridium centers
adopt distorted square-planar coordination geometries. In
1
a H NMR experiment performed in CD2Cl2, complex 10 was al-
ternatively synthesized from the reaction of complex 3a+[PF6]À
with one equivalent of NaOtBu (which led to an equilibrium
between a mononuclear neutral complex containing a C-
bound “anionic” imidazolide and its dimer in which the imida-
zolide binds in a m-C,N bridging mode)[19] followed by the addi-
Figure 7. Molecular structure of complex 11. Thermal ellipsoids are at the
30% level. Selected bond lengths [Š] and angles [8]: C1ÀN1 1.336(9), C1ÀN2
1.391(9), C2ÀN1 1.386(9), C2ÀC3 1.35(1), C3ÀN2 1.392(8), C4ÀN2 1.378(9),
C4ÀN3 1.302(9), C4ÀC5 1.47(1), C6ÀN3 1.443(8), C1ÀIr2 2.052(7), N3ÀIr2
2.097(6), C20ÀIr2 1.838(8), C21ÀIr2 1.907(8), N1ÀIr1 2.074(6), Cl1ÀIr1 2.339(2),
C18ÀIr1 1.848(9), C19ÀIr1 1.85(1), C18ÀO1 1.15(1), C19ÀO2 1.14(1), C20ÀO3
1.148(9), C21ÀO4 1.131(9); N1-C1-N2 106.6(6), C1-Ir2-N3 77.5(2), C1-Ir2-C20
97.6(3), C20-Ir2-C21 89.9(3), C21-Ir2-N3 95.0(3), N1-Ir1-Cl1 86.6(2), N1-Ir1-C19
91.8(3), C18-Ir1-C19 88.9(4), C18-Ir1-Cl1 92.7(3).
1
tion of 0.5 equivalent of [Ir(cod)(m-Cl)]2. The yield (by H NMR
spectroscopy) is higher than 80%.
Figure 6. Molecular structure of complex 10 in 10·Et2O·toluene. Hydrogen
atoms and the solvent molecules are omitted for clarity. Thermal ellipsoids
are at the 30% level. Selected bond lengths [Š] and angles [8]: C1ÀN1
1.335(6), C1ÀN2 1.397(7), C2ÀN1 1.391(7), C2ÀC3 1.342(8), C3ÀN2 1.387(7),
C4ÀN2 1.382(6), C4ÀN3 1.289(7), C4ÀC5 1.486(8), C6ÀN3 1.454(6), Ir1ÀC1
2.041(5), Ir1ÀN3 2.112(4), Ir1ÀC18 2.137(6), Ir1ÀC19 2.118(6), Ir1ÀC22 2.185(5),
Ir1ÀC23 2.184(5), C18ÀC19 1.400(9), C22ÀC23 1.380(9), Ir2ÀCl1 2.367(1), Ir2À
C26 2.149(5), Ir2ÀC27 2.111(6), Ir2ÀC30 2.113(5), Ir2ÀC31 2.099(5), C26ÀC27
1.396(8), C30ÀC31 1.418(8); N1-C1-N2 105.9(4), C1-Ir1-N3 78.3(2), C1-Ir1-C18
100.2(2), C1-Ir1-C19 95.8(2), N3-Ir1-C22 98.4(2), N3-Ir1-C23 94.3(2), N1-Ir2-Cl1
88.1(1), N1-Ir2-C30 93.4(2), N1-Ir2-C31 91.0(2), Cl1-Ir2-C26 92.3(2), Cl1-Ir2-C27
91.6(2).
Scheme 14. Syntheses of complex 12.
The two cod ligands in complex 10 were readily replaced by
CO at room temperature to afford the red [Ir2(CO)4Cl{m-
C3H2N2(DippN=CMe)-k2(C2,Nimine),kN3}] (11), abbreviated as
[Ir2(CO)4Cl(LC2,Nimine,N3)]. Its IR spectrum showed two strong
v(CO) bands at n˜ =2054 (s) and 1971 cmÀ1 (vbr), which is con-
sistent with a mutually cis disposition of the CO ligands, fur-
ther confirmed crystallographically (Figure 7).
Figure 8. Molecular structure complex 12 in 12·2THF. Hydrogen atoms and
the solvent molecules are omitted for clarity. Thermal ellipsoids are at the
30% level. Selected bond lengths [Š] and angles [8]: C1ÀN1 1.331(7), C1ÀN2
1.418(7), C2ÀN1 1.392(8), C2ÀC3 1.349(9), C3ÀN2 1.387(7), C4ÀN2 1.381(7),
C4ÀN3 1.303(7), C4ÀC5 1.480(8), C6ÀN3 1.434(7), Ir1ÀC1 2.004(6), Ir1ÀN3
2.111(4), Ir1ÀC26 2.202(6), Ir1ÀC27 2.189(6), Ir1ÀC30 2.140(5), Ir1ÀC31
2.113(6), C26ÀC27 1.38(1), C30ÀC31 1.39(1), Rh1ÀN1 2.079(5), Rh1ÀCl1
2.371(2), Rh1ÀC18 2.091(6), Rh1ÀC19 2.113(6), Rh1ÀC22 2.134(6), Rh1ÀC23
2.127(6), C18ÀC19 1.407(9), C22ÀC23 1.38(1); N1-C1-N2 105.0(5), C1-Ir1-N3
78.4(2), C1-Ir1-C30 98.4(2), C1-Ir1-C31 95.0(2), N3-Ir1-C26 95.7(2), N3-Ir1-C27
99.7(2), N1-Rh1-Cl1 88.4 (2), N1-Rh1-C18 90.3(2), N1-Rh1-C19 91.2(2), Cl1-
Rh1-C22 94.1(2), Cl1-Rh1-C23 90.5(3).
Unexpectedly, both the in situ deprotonation of complex 1b
by KHMDS followed by the addition of 0.5 equivalent of
[Ir(cod)(m-Cl)]2 and the in situ deprotonation of complex 1a by
KHMDS followed by the addition of 0.5 equivalent of
[Rh(cod)(m-Cl)]2 afforded the same heterodinuclear complex
[IrRh(cod)2Cl{m-C3H2N2(DippN=CMe) -k2(C2,Nimine),kN3}] (12), ab-
breviated as [IrRh(cod)2Cl(LC2,Nimine,N3)], in 60 and 51% yield, re-
spectively (Scheme 14).
In the 13C{1H} NMR spectrum of complex 12, a characteristic
singlet at d=179.8 ppm was assigned to the CNHC carbon
atom. The structure of complex 12·2THF (Figure 8) revealed
a dinuclear complex comprising one N-bound RhI center and
one k2(C2,Nimine)-chelated IrI center. Both metals adopt distort-
ed square-planar coordination geometries. The reactions
shown in Scheme 14 clearly indicate that the formation of the
IrÀC bond represent a thermodynamic driving force.
A comparative 1H NMR experiment was performed with
either complex 1a and 0.5 equivalent of [Rh(cod)(m-Cl)]2 or
complex 1b with 0.5 equivalent of [Ir(cod)(m-Cl)]2 in CD2Cl2 at
room temperature, in the absence of KHMDS. A mixture of
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Experimental Section
General considerations: All manipulations involving organometal-
lics were performed under argon in a Braun glovebox or by using
standard Schlenk techniques. Solvents were dried by using stan-
dard methods and distilled over sodium/benzophenone under
argon prior to use or passed through columns of activated alumina
and subsequently purged with argon. [Ir(cod)(m-Cl)]2 is commercial-
ly available from Johnson Matthey PLC. [Rh(cod)(m-Cl)]2,[24] 1-(2,6-
diisopropylphenylimino)ethylimidazole (HC2L),[16f] 4(5)-carbethoxy-
5(4)-methylimidazole,[25] N-(2,6-diisopropylphenyl)acetimidoyl chlor-
ide,[16f] complexes 1a,[19] 2,[19] 3a+[PF6]À,[19] and 4b[20] were pre-
pared according to the literature. NMR spectra of organic com-
pounds and complexes were recorded on a Bruker 300, 400, or
500 MHz instrument at ambient temperature and referenced by
using the proton (1H) or carbon (13C) resonance of the residual sol-
vent. Assignments are based on 1H,1H-COSY, 1H-NOESY, 1H/13C-
Scheme 15. The reactions between 1a and [Rh(cod)(m-Cl)]2, 1b and
[Ir(cod)(m-Cl)]2 in the absence of KHMDS that were monitored by H NMR
spectroscopy.
1
1
HSQC, and H/13C-HMBC experiments. 31P{1H} NMR spectra were re-
corded on a Bruker Avance 300 instrument at 121.49 MHz by using
H3PO4 (85% in D2O) as external standard. IR spectra were recorded
in the region n˜ =4000–100 cmÀ1 on a Nicolet 6700 FT-IR spectrom-
eter (ATR mode, diamond crystal). Elemental analyses were per-
formed by the “Service de microanalyses” of the UniversitØ de
Strasbourg.
complexes 1a, 1b, and 2 in a 3:1:1 ratio was obtained, indicat-
ing the occurrence of competing reactions (Scheme 15).
The reaction between complex 1a and 0.5 equivalent of
[Rh(cod)(m-Cl)]2 could be viewed as
a two-step process
(Scheme 15), the first step would be a partial N-bound metal
exchange, liberating some [Ir(cod)(m-Cl)]2 that could react in
a second step with complex 1a to give complex 2 as a result
of the C2ÀH bond activation. Starting from complex 1b and
[Ir(cod)(m-Cl)]2 a metal exchange is thermodynamically favored
and the resulting complex 1a reacts then with [Ir(cod)(m-Cl)]2
as described above. Somewhat surprisingly, a C2ÀH bond acti-
vation of complex 1b by [Ir(cod)(m-Cl)]2 to give
a k2(C2,Nimine)IrIII,k(N3)RhI complex was never observed.
Synthesis of 4(5)-methyl-5(4)-hydroxymethylimidazole: To
a
stirred suspension of 4(5)-carbethoxy-5(4)-methylimidazole
(1.54 g, 10.00 mmol) in THF (100 mL) at 08C was added slowly lithi-
um aluminium hydride (1.00 g, 26.35 mmol). The reaction mixture
was heated to reflux for 4 h and then quenched by sequential ad-
dition of H2O (1 mL), 15% aqueous NaOH solution (1 mL), and H2O
(3 mL) at 08C. After an appropriate amount of sodium sulfate was
added to the mixture, it was further stirred for 30 min. The resul-
tant solution was filtered through Celite and the filtrate was evapo-
rated under reduced pressure to yield a white solid (1.00 g,
8.92 mmol, 89%).
Conclusion
Synthesis
of
4(5)-methyl-5(4)-(2,6-diisopropylphenylimino)
methylimidazole: To a stirred solution of 4(5)-methyl-5(4)-hydroxy-
methylimidazole (0.56 g, 5.00 mmol) in EtOH (50 mL) was added
MnO2 (0.87 g, 10.00 mmol). The reaction mixture was heated to
reflux for 12 h. After filtration through Celite, 2,6-diisopropylaniline
(0.89 g, 5.00 mmol) and a catalytic amount of 4-methylbenzenesul-
fonic acid were added to the filtrate. The reaction mixture was fur-
ther heated to reflux for 6 h. After the solvent was evaporated
under reduced pressure, the residue was washed with petroleum
ether (3”3 mL) and dried under vacuum to give a white powder
A comparative study between IrI and RhI by using imine-func-
tionalized imidazoles as potential precursors to functionalized
pNHC complexes has revealed details of the mechanistic steps
prior and during the C2ÀH metallation that leads to the pNHC
complexes. In the case of iridium, the observation of IrÀH in-
1
termediates by H NMR spectroscopy is consistent with the in-
volvement of the (isolable) NimidazoleÀIr complex precursors un-
dergoing tautomerization/metallotropism, followed by a che-
late assisted C2ÀH bond activation in a neutral and a cationic
intermediate. In the case of rhodium, C2-metallation proceed-
ed only under forcing conditions at higher temperatures and
intermediates analogous to those observed for Ir remained elu-
sive. A new RhI–IrI heterodinuclear complex was obtained in
a stepwise manner, which allowed the chemoselectivity of the
synthetic approach to be investigated and understood. The
k2(C2,Nimine)IrI,k(N3)RhI complex was selectively isolated, either
starting from the N-bound Ir or from the N-bound Rh precur-
sor. These results are in agreement with the preferred forma-
tion of strong, inert IrÀC bonds and can rationalize the carbo-
philic migration of the iridium (from complex 1a to complex
12, Scheme 14). It is anticipated that the results described
above could be useful for the further development of synthetic
methodologies to pNHC, C-bound “anionic” imidazolide, and
relevant homo- and heterodinuclear complexes.
1
(0.68 g, 2.52 mmol, 50%). H NMR (500 MHz, CDCl3): d=11.90 (brs,
1H; NH), 8.13 (s, 1H; N=CH), 7.21–7.17 (m, 3H; CH(Dipp)), 6.75 (brs,
1H; NCHNH), 3.00 (sept, 3J=6.9 Hz, 2H; CH(CH3)2), 2.36 (s, 3H;
CH3(imidazole)), 1.12 ppm (d, 3J=6.9 Hz, 12H; CH(CH3)2); 13C{1H} NMR
(125 MHz, CDCl3): d=151.3 (N=CH), 147.9 (C(Dipp)), 144.2 (brs,
C(imidazole)), 138.8 (C(Dipp)), 138.3 (NCHNH), 125.5 (brs, C(imidazole)), 125.3
(CH(Dipp)), 123.6 (CH(Dipp)), 28.1 (CH(CH3)2), 23.7 (CH(CH3)2), 12.7 ppm
(CH3(imidazole)); IR (pure, orbit diamond): n˜max =1637 cmÀ1 (v(C=N)); el-
emental analysis calcd for C17H23N3 (269.39): C 75.80, H 8.61, N
15.60; found: C 75.41, H 8.64, N 15.93.
Synthesis of 1-(2,6-diisopropylphenylimino)ethyl-4-(2,6-diisopro-
pylphenylimino)methyl-5-methylimidazole (HC2L’): To a stirred so-
lution of 4(5)-methyl-5(4)-(2,6-diisopropylphenylimino) methylimi-
dazole (1.00 g, 3.71 mmol) and triethylamine (1.0 mL, 7.17 mmol) in
CH2Cl2 (10 mL) was added N-(2,6-diisopropylphenyl) acetimidoyl
chloride (0.88 g, 3.71 mmol). The reaction mixture was stirred for
12 h at room temperature. After removal of the volatiles under re-
duced pressure, the residue was extracted with Et2O and the solu-
Chem. Eur. J. 2016, 22, 2658 – 2671
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Full Paper
tion was filtered through Celite. The filtrate was evaporated under
reduced pressure. Then the residue was washed with pentane (3”
1 mL) and dried under vacuum to give a yellow powder (1.57 g,
(pure, orbit diamond): n˜max =3405 (v(NÀH)), 1627 (v(C=N)),
827 cmÀ1 (v(PÀF)); elemental analysis calcd for C25H35F6RhN3P
(625.45): C 48.01, H 5.64, N 6.72; found: C 47.92, H 5.65, N 6.71.
1
3.34 mmol, 90%). H NMR (500 MHz, CDCl3): d=8.68 (s, 1H; N=CH),
The above-described filtrate was further concentrated under re-
duced pressure to approximately 2 mL. A yellow precipitate was
formed after pentane (5 mL) was added to the solution. The pre-
cipitate was filtered, washed with pentane (2”3 mL) and dried
under vacuum to give complex 6b+[PF6]À as a pale yellow powder
(0.081 g, 0.090 mmol, 90% (based on L)). 1H NMR (500 MHz,
CD2Cl2): d=8.42 (brs, 2H; NCHN(near imine)), 7.80 (s, 2H; NCHCHN(near
imine)), 7.23–7.09 (m, 6H; CH(Dipp)), 7.04 (s, 2H; NCHCHN(near imine)), 4.24
(brs, 4H; CH(cod)), 2.67 (sept, 3J=6.7 Hz, 4H; CH(CH3)2), 2.62–2.52
(m, 4H; CH2(cod)), 2.19 (s, 6H; CH3(imine)), 2.02–1.86 (m, 4H; CH2(cod)),
1.14 (d, 3J=6.7 Hz, 12H; CH(CH3)2), 1.09ppm (d, 3J=6.7 Hz, 12H;
CH(CH3)2); 13C{1H} NMR (125 MHz, CD2Cl2): d=149.2 (C=N), 142.2
(ipso-C(Dipp)), 137.2 (o-C(Dipp)), 137.0 (NCHN(near imine)), 129.0
(NCHCHN(near imine)), 125.1 (p-CH(Dipp)), 123.7 (m-CH(Dipp)), 117.6
(NCHCHN(near imine)), 83.8 (br, CH(cod)), 31.1 (CH2(cod)), 28.7 (CH(CH3)2),
23.3 (CH(CH3)2), 22.9 (CH(CH3)2), 16.3 ppm (CH3(imine)); 31P{1H} NMR
(121.5 MHz, CD2Cl2): d=À144.3 ppm (sept, 1J(P,F)=712 Hz, PF6À);
8.00 (s, 1H; NCHN(near imine)), 7.23–7.00 (m, 6H; CH(Dipp)), 3.00 (sept,
3
3J=6.9 Hz, 2H; CH(CH3)2), 2.79 (sept, J=6.9 Hz, 2H; CH(CH3)2), 2.61
3
(s, 3H; CH3(imidazole)), 2.28 (s, 3H; CH3(imine)), 1.12 (d, J=6.9 Hz, 18H;
CH(CH3)2), 1.05 ppm (d, 3J=6.9 Hz, 6H; CH(CH3)2); 13C{1H} NMR
(125 MHz, CDCl3): d=153.2 (N=CH), 152.3 (N=CCH3), 149.9 (C(Dipp)),
146.9 (C(imidazole)), 142.5 (C(Dipp)), 137.9 (NCHN(near imine)), 137.7 (C(Dipp)),
136.8 (C(Dipp)), 125.2 (C(imidazole)), 124.7 (CH(Dipp)), 124.1 (CH(Dipp)), 123.4
(CH(Dipp)), 123.0 (CH(Dipp)), 28.4 (CH(CH3)2), 27.9 (CH(CH3)2), 23.7
(CH(CH3)2), 23.5 (CH(CH3)2), 23.2 (CH(CH3)2), 19.1 (CH3(imine)), 15.0
(CH3(imidazole)); IR (pure, orbit diamond): n˜max =1670 cmÀ1 (v(C=N)); el-
emental analysis calcd for C31H42N4 (470.71): C 79.10, H 8.99, N
11.90; found: C 78.67, H 8.94, N 12.33.
Synthesis of [Rh(cod)Cl(HC2LN3)] (1b); A solution of the ligand
HC2L (0.054 g, 0.20 mmol) in THF (2 mL) was added to a stirred so-
lution of [Rh(cod)(m-Cl)]2 (0.050 g, 0.10 mmol) in THF (3 mL). The re-
action mixture was stirred for 1 h at room temperature and then
the solvent was removed under reduced pressure. The residue was
washed with pentane (3”1 mL) and dried under vacuum to give
a yellow solid (0.093 g, 0.18 mmol, 90%). Single crystals of complex
1b suitable for X-ray diffraction were obtained by slow evapora-
tion of a saturated solution of complex 1b in Et2O at ambient tem-
1
19F{1H} NMR (282.4 MHz, CD2Cl2): d=À73.4 ppm (d, J(P,F)=712 Hz,
PF6À); IR (pure, orbit diamond): n˜max =1685 (v(C=N)), 840 cmÀ1
(v(PÀF)); elemental analysis calcd for C42H58F6N6PRh (894.84): C
56.37, H 6.53, N 9.39; found: C 56.90, H 6.75, N 9.74.
Improved synthesis of [Rh(cod)(HN3LC2,Nimine)]+[PF6]À (3b+[PF6]À):
To a stirred solution of complex 1b (0.044 g, 0.086 mmol) in tolu-
ene (2 mL) was added TlPF6 (0.030 g, 0.086 mmol). The reaction
mixture was stirred for 12 h at 1108C. The resulting red precipitate
was collected by filtration and dissolved in CH2Cl2 (3 mL). After fil-
tration through Celite, the filtrate was evaporated under reduced
pressure to yield a dark red crystalline solid (0.048 g, 0.077 mmol,
90%).
1
perature. H NMR (500 MHz, CD2Cl2): d=8.72 (s, 1H; NCHN(near imine)),
7.80 (apparent t, 3,4J=1.5 Hz, 1H; NCHCHN(near imine)), 7.20–7.09 (m,
3H; CH(Dipp)), 6.95 (apparent t, 3,4J=1.5 Hz, 1H; NCHCHN(near imine)),
4.80–3.70 (an overlap of two brs, 4H; CH(cod)), 2.68 (sept, 3J=
6.9 Hz, 2H; CH(CH3)2), 2.56–2.40 (m, 4H; CH2(cod)), 2.17 (s, 3H;
CH3(imine)), 1.93–1.77 (m, 4H; CH2(cod)), 1.14 (d, 3J=6.9 Hz, 6H;
CH(CH3)2), 1.10 ppm (d, 3J=6.9 Hz, 6H; CH(CH3)2); 13C{1H} NMR
(125 MHz, CD2Cl2): d=149.0 (C=N), 142.2 (ipso-C(Dipp)), 138.6
(NCHN(near imine)), 137.2 (o-C(Dipp)), 127.9 (NCHCHN(near imine)), 125.0 (p-
CH(Dipp)), 123.7 (m-CH(Dipp)), 117.0 (NCHCHN(near imine)), 83.0 (br, CH(cod)),
76.2 (br, CH(cod)), 31.2 (br, CH2(cod)), 28.7 (CH(CH3)2), 23.3 (CH(CH3)2),
23.0 (CH(CH3)2), 16.4 ppm (CH3(imine)). IR (pure, orbit diamond):
n˜max =1687 (v(C=N)), 280 cmÀ1 (v(RhÀCl)); elemental analysis calcd
for C25H35ClN3Rh (515.92): C 58.20, H 6.84, N 8.14; found: C 58.18, H
6.77, N 8.27.
Synthesis of [Ir(cod)Cl{C3H3N2(nBu)-kN3}] (4a): A solution of 1-n-
butylimidazole (0.019 g, 0.15 mmol) in THF (2 mL) was added to
a stirred solution of [Ir(cod)(m-Cl)]2 (0.050 g, 0.074 mmol) in THF
(3 mL). The reaction mixture was stirred for 1 h at room tempera-
ture and then the solvent was removed in vacuo. The residue was
washed with pentane (3”1 mL) and dried under vacuum to give
a
yellow solid (0.062 g, 0.13 mmol, 90%). 1H NMR (500 MHz,
CD2Cl2): d=8.08 (brs, 1H; NCHN(nBu)), 7.01 (s, 1H; NCHCHN(nBu)),
3
Synthesis of [Rh(cod)(HN3LC2,Nimine)]+[PF6]À (3b+[PF6]À) and
[Rh(cod)(HC2LN3)2]+[PF6]À (6b+[PF6]À): To a stirred solution of com-
plex 1b (0.103 g, 0.20 mmol) in CH2Cl2 (5 mL) was added TlPF6
(0.070 g, 0.20 mmol). The reaction mixture was stirred for 12 h at
room temperature. After filtration through Celite, the filtrate was
concentrated under reduced pressure to approximately 2 mL and
then the residue was stratified with Et2O to yield 3b+[PF6]À as dark
red crystals, which were collected by filtration and dried in vacuo
(0.005 g, 0.008 mmol, 4%). 1H NMR (500 MHz, CD2Cl2): d=10.31
(brs, 1H; NH), 7.36–7.25 (m, 4H; NHCHCHN(near imine), CH(Dipp)), 7.23
6.96 (s, 1H; NCHCHN(nBu)), 3.94 (t, J=7.4 Hz, 2H; NCH2CH2CH2CH3),
3
3.89 (brs, 4H; CH(cod)), 2.30–2.14 (m, 4H; CH2(cod)), 1.75 (quint, J=
7.4 Hz, 2H; NCH2CH2CH2CH3), 1.62–1.46 (m, 4H; CH2(cod)), 1.31 (sext,
3J=7.4 Hz, 2H; NCH2CH2CH2CH3), 0.92 ppm (t, 3J=7.4 Hz, 3H;
NCH2CH2CH2CH3); 13C{1H} NMR (125 MHz, CD2Cl2): d=139.1
(NCHN(nBu)), 127.2 (NCHCHN(nBu)), 119.8 (NCHCHN(nBu)), 62.5 (br,
CH(cod)), 48.1 (NCH2CH2CH2CH3), 33.0 (NCH2CH2CH2CH3), 31.8
(CH2(cod)), 20.0 (NCH2CH2CH2CH3), 13.6 ppm (NCH2CH2CH2CH3); ele-
mental analysis calcd for C15H24ClIrN2 (460.04): C 39.16, H 5.26, N
6.09; found: C 38.64, H 5.43, N 6.55.
3
(apparent t, J=2.3 Hz, 1H; NHCHCHN(near imine)), 4.68–4.60 (m, 2H;
Synthesis of [Ir(cod){C3H3N2(nBu)-kN3}2]+[PF6]À (5a+[PF6]À): To
a stirred solution of complex 4a (0.046 g, 0.10 mmol) in CH2Cl2
(5 mL) was added TlPF6 (0.035 g, 0.10 mmol). The reaction mixture
was stirred for 12 h at room temperature. After filtration through
Celite, the filtrate was evaporated to dryness under reduced pres-
sure. The residue was washed with Et2O (3”1 mL) and dried under
CH(cod)), 4.27–4.19 (m, 2H; CH(cod)), 3.10 (sept, 3J=6.8 Hz, 2H;
CH(CH3)2), 2.44–2.32 (m, 5H; CH2(cod), CH3(imine)), 2.29–2.17 (m, 4H;
CH2(cod)), 2.12–2.00 (m, 2H; CH2(cod)), 1.37 (d, 3J=6.8 Hz, 6H;
CH(CH3)2), 1.13 ppm (d, 3J=6.8 Hz, 6H; CH(CH3)2); 13C{1H} NMR
(125 MHz, CD2Cl2): d=176.9 (d, 1J(Rh,C)=55.1 Hz, NHCN(near imine)),
163.7 (C=N), 140.6 (o-C(Dipp)), 138.7 (ipso-C(Dipp)), 128.5 (p-CH(Dipp)),
124.9 (m-CH(Dipp)), 121.4 (NHCHCHN(near imine)), 116.1 (NHCHCHN(near
1
vacuum to give a yellow solid (0.028 g, 0.041 mmol, 82%). H NMR
(500 MHz, CD2Cl2): d=7.42 (s, 2H; NCHN(nBu)), 7.07 (s, 2H;
NCHCHN(nBu)), 6.81 (s, 2H; NCHCHN(nBu)), 3.99 (t, 3J=7.4 Hz, 4H;
NCH2CH2CH2CH3), 3.83 (brs, 4H; CH(cod)), 2.38–2.28 (m, 4H; CH2(cod)),
1
1
imine)), 105.6 (d, J(Rh,C)=6.8 Hz, CH(cod)), 78.9 (d, J(Rh,C)=12.6 Hz,
CH(cod)), 32.0 (CH2(cod)), 29.0 (CH2(cod)), 28.9 (CH(CH3)2), 25.0 (CH(CH3)2),
23.3 (CH(CH3)2), 16.3 ppm (CH3(imine)); 31P{1H} NMR (121.5 MHz,
CD2Cl2): d=À144.1 ppm (sept, 1J(P,F)=712 Hz, PF6À); 19F{1H} NMR
(282.4 MHz, CD2Cl2): d=À73.5 ppm (d, 1J(P,F)=712 Hz, PF6À); IR
3
1.82–1.66 (m, 8H; NCH2CH2CH2CH3, CH2(cod)), 1.27 (sext, J=7.4 Hz,
4H;
NCH2CH2CH2CH3),
0.91 ppm
(t,
3J=7.4 Hz,
6H;
NCH2CH2CH2CH3); 13C{1H} NMR (125 MHz, CD2Cl2): d=138.0
Chem. Eur. J. 2016, 22, 2658 – 2671
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imine)), 5.10–4.00 (brs, 2H; CH(cod)), 4.00–3.20 (brs, 2H; CH(cod)), 2.75
(brs„ 2H; CH(CH3)2), 2.37 (s, 3H; CH3(imine)), 1.89 (brs, 4H; CH2(cod)),
1.71 (brs, 4H; CH2(cod)), 1.48–0.56 ppm (m, 12H; CH(CH3)2);
13C{1H} NMR (125 MHz, CD2Cl2): d=173.8 (NHCN(near imine)), 158.7 (br,
(NCHN(nBu)), 127.9 (NCHCHN(nBu)), 121.2 (NCHCHN(nBu)), 67.2 (CH(cod)),
48.6 (NCH2CH2CH2CH3), 32.7 (NCH2CH2CH2CH3), 31.6 (CH2(cod)), 19.9
(NCH2CH2CH2CH3), 13.6 ppm (NCH2CH2CH2CH3); 31P{1H} NMR
(121.5 MHz, CD2Cl2): d=À144.4 ppm (sept, 1J(P,F)=712 Hz, PF6À);
19F{1H} NMR (282.4 MHz, CD2Cl2): d=À73.4 (d, 1J(P,F)=712 Hz,
PF6À); elemental analysis calcd for C22H36F6IrN4P (693.74): C 38.09, H
5.23, N 8.08; found: C 37.61, H 5.70, N 8.55.
1
C=N), 141.7 (C(Dipp)), 141.2 (br, C(Dipp)), 134.7 (d, J(P,C)=39.4 Hz, ipso-
C
), 133.1 (d, J(P,C)=5.3 Hz, CH
), 130.5 (CH
), 129.2 (d,
ðPPh3
Þ
ðPPh3
Þ
ðPPh3Þ
J(P,C)=9.1 Hz, CH
), 127.9 (CH(Dipp)), 125.0 (br, CH(Dipp)), 122.0
ðPPh3
Þ
(NHCHCHN(near imine)), 117.0 (NHCHCHN(near imine)), 88.7 (br, CH(cod)), 30.1
(CH2(cod)), 28.4 (br, CH(CH3)2), 25.3 (CH(CH3)2), 24.2 (br, CH(CH3)2),
19.3 ppm (CH3(imine)); 31P{1H} NMR (121.5 MHz, CD2Cl2): d=À1.9 (s,
PPh3), À144.1 ppm (sept, 1J(P,F)=712 Hz, PF6À); 19F{1H} NMR
(282.4 MHz, CD2Cl2): d=À73.5 ppm (d, 1J(P,F)=712 Hz, PF6À); IR
(pure, orbit diamond): n˜max =3390 (v(NÀH)), 1617 (v(C=N)),
839 cmÀ1 (v(PÀF)); elemental analysis calcd for C43H50F6IrN3P2
(977.05): C 52.86, H 5.16, N 4.30; found: C 52.39, H 5.33, N 4.14.
Synthesis of [Ir(cod){C3H3N2(Mes)-kN3}2]+[PF6]À (5b+[PF6]À): To
a stirred solution of complex 4b (0.052 g, 0.10 mmol) in CH2Cl2
(5 mL) was added TlPF6 (0.035 g, 0.10 mmol). The reaction mixture
was stirred for 12 h at room temperature. After filtration through
Celite, the filtrate was evaporated to dryness under reduced pres-
sure. The residue was washed with Et2O (3”1 mL) and dried under
1
vacuum to give a yellow solid (0.038 g, 0.046 mmol, 92%). H NMR
4
(500 MHz, CD2Cl2): d=7.52 (apparent t, J=1.3 Hz, 2H; NCHN(Mes)),
7.11 (apparent t, 3,4J=1.3 Hz, 1H; NCHCHN(Mes)), 7.07 (apparent t,
3,4J=1.3 Hz, 2H; NCHCHN(Mes)), 7.02 (s, 4H; m-CH(Mes)), 3.97–3.89 (m,
4H; CH(cod)), 2.43–2.35 (m, 4H; CH2(cod)), 2.34 (s, 6H; p-CH3(Mes)), 1.92
(s, 12H; o-CH3(Mes)), 1.85–1.77 ppm (m, 4H; CH2(cod)); 13C{1H} NMR
(125 MHz, CD2Cl2): d=141.0 (p-C(Mes)), 138.4 (NCHN(Mes)), 134.9 (o-
C(Mes)), 131.9 (ipso-C(Mes)), 129.8 (m-CH(Mes)), 128.5 (NCHCHN(Mes)),
123.1 (NCHCHN(Mes)), 68.2 (CH(cod)), 31.7 (CH2(cod)), 21.2 (p-CH3(Mes)),
17.4 ppm (o-CH3(Mes)); 31P{1H} NMR (121.5 MHz, CD2Cl2): d=
À144.5 ppm (sept, 1J(P,F)=712 Hz, PF6À); 19F{1H} NMR (282.4 MHz,
Synthesis of [Ir(cod)(PMe3)(HN3LC2,Nimine)]+[PF6]À (7b+[PF6]À): To
a stirred solution of complex 3a+[PF6]À (0.040 g, 0.056 mmol) in
CH2Cl2 (3 mL) was added a solution of PMe3 (0.1m in Et2O, 0.60 mL,
0.060 mmol). The reaction mixture was stirred for 1 h at room tem-
perature. After removal of the volatiles under reduced pressure,
the residue was washed with Et2O (3”1 mL) to yield a yellow
powder, which was collected by filtration and dried in vacuo
(0.038 g, 0.048 mmol, 86%). 1H NMR (500 MHz, CD2Cl2): d=10.08
(brs, 1H; NH), 7.47 (apparent t, J=2.4 Hz, 1H; NHCHCHN(near imine)),
1
CD2Cl2): d=À73.4 ppm (d, J(P,F)=712 Hz, PF6À); elemental analysis
3
7.36 (apparent t, J=2.4 Hz, 1H; NHCHCHN(near imine)), 7.35–7.21 (m,
calcd for C32H40F6IrN4P (817.88): C 46.99, H 4.93, N 6.85; found: C
47.07, H 4.60, N 7.24.
3H; CH(Dipp)), 4.00–3.86 (m, 1H; CH(cod)), 3.64–3.48 (m, 2H; CH(cod)),
3
2.86–2.78 (m, 1H; CH(cod)), 2.72 (sept, J=6.7 Hz, 1H; CH(CH3)2), 2.58
3
Synthesis of [Ir(cod)(HC2LN3)2]+[PF6]À (6a+[PF6]À): To a stirred solu-
tion of complex 1a (0.030 g, 0.050 mmol) and HC2L (0.014 g,
0.052 mmol) in CH2Cl2 (2 mL) was added TlPF6 (0.018 g,
0.052 mmol). The reaction mixture was stirred for 2 h at room tem-
perature. After filtration through Celite, the filtrate was evaporated
to dryness under reduced pressure. The residue was washed with
Et2O (3”1 mL) and dried under vacuum to give a yellow powder
(0.045 g, 0.046 mmol, 92%). Single crystals of complex 6a+[PF6]À
suitable for X-ray diffraction were obtained by slow evaporation of
a saturated solution in Et2O at ambient temperature. 1H NMR
(500 MHz, CD2Cl2): d=8.41 (s, 2H; NCHN(near imine)), 7.91 (s, 2H;
NCHCHN(near imine)), 7.20–7.07 (m, 8H; CH(Dipp), NCHCHN(near imine)), 3.98
(brs, 4H; CH(cod)), 2.67 (sept, 3J=6.9 Hz, 4H; CH(CH3)2), 2.51–2.35
(m, 4H; CH2(cod)), 2.22 (s, 6H; CH3(imine)), 1.91–1.75 (m, 4H; CH2(cod)),
1.14 (d, 3J=6.9 Hz, 12H; CH(CH3)2), 1.08 ppm (d, 3J=6.9 Hz, 12H;
CH(CH3)2); 13C{1H} NMR (125 MHz, CD2Cl2): d=149.2 (C=N), 141.9
(ipso-C(Dipp)), 137.1 (o-C(Dipp)), 136.7 (NCHN(near imine)), 129.0
(NCHCHN(near imine)), 125.3 (p-CH(Dipp)), 123.7 (m-CH(Dipp)), 118.4
(NCHCHN(near imine)), 68.9 (CH(cod)), 31.7 (CH2(cod)), 28.7 (CH(CH3)2), 23.4
(CH(CH3)2), 22.9 (CH(CH3)2), 16.2 ppm (CH3(imine)); 31P{1H} NMR
(121.5 MHz, CD2Cl2): d0À144.4 ppm (sept, 1J(P,F)=712 Hz, PF6À);
(sept, J=6.7 Hz, 1H; CH(CH3)2), 2.52–2.38 (m, 2H; CH2(cod)), 2.26 (d,
5J(H,P)=4.3 Hz, 3H; CH3(imine)), 2.20–1.94 (m, 3H; CH2(cod)), 1.46–1.32
(m, 3H; CH2(cod)), 1.30 (d, 3J=6.7 Hz, 3H; CH(CH3)2), 1.26–1.20 (m,
12H; P(CH3)3, CH(CH3)2), 1.09 (d, 3J=6.7 Hz, 3H; CH(CH3)2),
1.02 ppm (d, 3J=6.7 Hz, 3H; CH(CH3)2); 13C{1H} NMR (125 MHz,
CD2Cl2): d=171.1 (d, 2J(P,C)=10.9 Hz, NHCN(near imine)), 155.3 (d,
3J(P,C)=6.2 Hz, C=N), 142.6 (C(Dipp)), 140.9 (C(Dipp)), 140.2 (C(Dipp)),
128.1 (CH(Dipp)), 125.5 (CH(Dipp)), 124.5 (CH(Dipp)), 121.8 (NHCHCHN(near
2
imine)), 115.3 (NHCHCHN(near imine)), 94.4 (d, J(P,C)=4.6 Hz, CH(cod)), 73.4
(CH(cod)), 58.2 (d, 2J(P,C)=16.6 Hz, CH(cod)), 42.0 (CH2(cod)), 41.1 (d,
2J(P,C)=2.3 Hz, CH(cod)), 33.3 (CH2(cod)), 30.0 (CH2(cod)), 29.5 (CH2(cod)),
27.8 (d, 5J(P,C)=3.9 Hz, CH(CH3)2), 27.5 (CH(CH3)2), 24.8 (CH(CH3)2),
24.7 (CH(CH3)2), 24.4 (CH(CH3)2), 24.2 (CH(CH3)2), 17.5 (CH3(imine)),
16.1 ppm (d, 1J(P,C)=28.1 Hz, PCH3); 31P{1H} NMR (121.5 MHz,
CD2Cl2): d=À44.8 (s, P(CH3)3), À144.3 ppm (sept, 1J(P,F)=712 Hz,
PF6À); 19F{1H} NMR (282.4 MHz, CD2Cl2): d=À73.5 ppm (d, 1J(P,F)=
712 Hz, PF6À). IR (pure, orbit diamond): n˜max =3381 (v(NÀH)), 1608
(v(C=N)), 835 cmÀ1 (v(PÀF)); elemental analysis calcd for
C28H44F6IrN3P2 (790.84): C 42.53, H 5.61, N 5.31; found: C 42.32, H
5.71, N 5.11.
Synthesis of [Ir(cod)Cl(HC2L’N3)] (8): A solution of HC2L’ (0.071 g,
0.15 mmol) in THF (2 mL) was added to a stirred solution of
[Ir(cod)(m-Cl)]2 (0.050 g, 0.074 mmol) in THF (3 mL). The reaction
mixture was stirred for 1 h at room temperature and then the sol-
vent was removed under reduced pressure. The residue was
washed with pentane (3”1 mL) and dried under vacuum to give
1
19F{1H} NMR (282.4 MHz, CD2Cl2): d=À73.4 ppm (d, J(P,F)=712 Hz,
PF6À); IR (pure, orbit diamond): n˜max =1687 (v(C=N)), 833 cmÀ1
(v(PÀF)); elemental analysis calcd for C42H58F6N6PIr (984.15): C 51.26,
H 5.94, N 8.54; found: C 50.81, H 5.93, N 8.44.
Synthesis of [Ir(cod)(PPh3)(HN3LC2,Nimine)]+[PF6]À (7a+[PF6]À): To
a stirred solution of complex 3a+[PF6]À (0.040 g, 0.056 mmol) in
CH2Cl2 (3 mL) was added a solution of PPh3 (0.015 g, 0.057 mmol)
in CH2Cl2 (2 mL). The reaction mixture was stirred for 1 h at room
temperature. After the solution was concentrated under reduced
pressure to approximately 2 mL, the solution was stratified with
Et2O to yield after diffusion green crystals, which were collected by
filtration and dried in vacuo (0.046 g, 0.047 mmol, 84%). 1H NMR
(500 MHz, CD2Cl2): d=9.69 (brs, 1H; NH), 7.53–7.26 (m, 12H;
CH(Ar)), 7.22 (apparent t, 3J=2.3 Hz, 1H; NHCHCHN(near imine)), 7.06
(brs, 6H; CH(Ar)), 6.99 (apparent t, 3J=2.3 Hz, 1H; NHCHCHN(near
a
yellow solid (0.103 g, 0.13 mmol, 85%). 1H NMR (500 MHz,
CD2Cl2): d=8.70 (s, 1H; N=CH), 8.19 (s, 1H; NCHN(near imine)), 7.20–
7.00 (m, 6H; CH(Dipp)), 4.42–4.34 (m, 2H; CH(cod)), 3.38–3.30 (m, 2H;
CH(cod)), 3.00–2.92 (m, 5H; CH(CH3)2, CH3(imidazole)), 2.69 (sept, 3J=
6.9 Hz, 2H; CH(CH3)2), 2.37–2.29 (m, 4H; CH2(cod)), 2.28 (s, 3H;
CH3(imine)), 1.72–1.48 (m, 4H; CH2(cod)), 1.11 (d, 3J=6.9 Hz, 6H;
CH(CH3)2), 1.09 (d, 3J=6.9 Hz, 12H; CH(CH3)2), 0.98 ppm (d, 3J=
6.9 Hz, 6H; CH(CH3)2); 13C{1H} NMR (125 MHz, CD2Cl2): d=153.5 (N=
CH), 151.6 (N=CCH3), 149.6 (C(Dipp)), 144.5 (C(imidazole)), 142.0 (C(Dipp)),
137.8 (C(Dipp)), 136.9 (C(Dipp)), 136.7 (NCHN(near imine)), 126.4 (C(imidazole)),
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125.3 (CH(Dipp)), 124.7 (CH(Dipp)), 123.8 (CH(Dipp)), 123.2 (CH(Dipp)), 70.1
(CH(cod)), 58.5 (CH(cod)), 32.8 (CH2(cod)), 31.5 (CH2(cod)), 28.6 (CH(CH3)2),
28.1 (CH(CH3)2), 23.7 (CH(CH3)2), 23.2 (CH(CH3)2), 23.2 (CH(CH3)2),
18.8 (CH3(imine)), 16.0 ppm (CH3(imidazole)); IR (pure, orbit diamond):
n˜max =1684 cmÀ1 (v(C=N)); elemental analysis calcd for C39H54ClIrN4
(806.56): C 58.08, H 6.75, N 6.95; found: C 58.29, H 6.39, N 6.58.
(NCN(near imine)), 166.0 (C=N), 142.0 (o-C(Dipp)), 141.8 (o-C(Dipp)), 138.8
(ipso-C(Dipp)), 131.8 (NCHCHN(near imine)), 128.5 (p-CH(Dipp)), 124.3 (m-
CH(Dipp)), 123.9 (m-CH(Dipp)), 114.9 (NCHCHN(near imine)), 83.1, 82.1, 70.8,
69.3, 68.1, 66.0, 59.0, 56.3 (CH(cod)), 35.0, 33.8, 33.1, 32.6, 32.3, 31.6,
30.8, 29.4 (CH2(cod)), 28.4, 28.0 (CH(CH3)2), 25.0, 24.7, 23.6, 23.0
(CH(CH3)2), 15.2 ppm (CH3(imine)); IR (pure, orbit diamond): n˜max
=
1577 (v(C=N)), 292cmÀ1 (v(IrÀCl)); elemental analysis calcd for
C33H46ClIr2N3 (904.64): C 43.81, H 5.13, N 4.65; found: C 43.38, H
4.85, N 4.21.
Synthesis of [Ir(cod)(HC2L’N3,Nimine)]+[PF6]À (9+[PF6]À): TlPF6
(0.035 g, 0.10 mmol) was added to a stirred solution of complex 8
(0.081 g, 0.10 mmol) in CH2Cl2 (5 mL). The reaction mixture was fur-
ther stirred for 2 h at room temperature. After filtration through
Celite, the filtrate was evaporated under reduced pressure. Then
the residue was washed with pentane (3”1 mL) and dried under
vacuum to give a dark red crystalline solid (0.080 g, 0.087 mmol,
87%). Single crystals of complex 9+[PF6]À suitable for X-ray diffrac-
tion were obtained by slow diffusion of a layer of pentane into
a solution of complex 9+[PF6]À in THF at ambient temperature
under argon. 1H NMR (500 MHz, CD2Cl2): d=8.30 (s, 2H; N=CH,
NCHN(near imine)), 7.36–7.12 (m, 6H; CH(Dipp)), 4.88–4.80 (m, 2H;
CH(cod)), 3.35–3.27 (m, 2H; CH(cod)), 3.22 (sept, 3J=6.9 Hz, 2H;
CH(CH3)2), 2.87 (s, 3H; CH3(imidazole)), 2.69 (sept, 3J=6.9 Hz, 2H;
CH(CH3)2), 2.36 (s, 3H; CH3(imine)), 2.33–2.25 (m, 2H; CH2(cod)), 2.25–
2.17 (m, 2H; CH2(cod)), 1.95–1.87 (m, 2H; CH2(cod)), 1.78–1.70 (m, 2H;
From complex 3a+[PF6]À: To solution of complex 3a+[PF6]À
(0.007 g, 0.010 mmol) in CD2Cl2 (0.5 mL) in a Young NMR tube, was
added NaOtBu (0.001 g, 0.010 mmol). The deprotonation was con-
1
firmed by H NMR spectroscopic data after 30 min at room temper-
ature.[19] Then [Ir(cod)(m-Cl)]2 (0.004 g, 0.006 mmol) was added to
the reaction mixture. The formation of complex 10 (>80%) was
confirmed by 1H NMR spectroscopic data after 30 min at room
temperature.
Synthesis of [Ir2(CO)4Cl(LC2,Nimine,N3)] (11): A solution of complex 10
(0.090 g, 0.10 mmol) in THF (2 mL) was stirred under CO (1 bar) at
room temperature for 30 min and then filtered through a cannula.
After removal of the volatiles under reduced pressure, the residue
was washed with cold Et2O (2”1 mL) to yield a red crystalline
solid, which was collected by filtration and dried in vacuo (0.066 g,
0.082 mmol, 82%). Single crystals of complex 11 suitable for X-ray
diffraction were obtained by slow diffusion of a layer of Et2O into
a solution of complex 11 in dichloromethane at ambient tempera-
3
3
CH2(cod)), 1.40 (d, J=6.9 Hz, 6H; CH(CH3)2), 1.21 (d, J=6.9 Hz, 6H;
CH(CH3)2), 1.14 (d, 3J=6.9 Hz, 6H; CH(CH3)2), 1.13 ppm (d, 3J=
6.9 Hz, 6H; CH(CH3)2); 13C{1H} NMR (125 MHz, CD2Cl2): d=165.1 (N=
CH), 151.8 (N=CCH3), 142.3 (C(Ar)), 141.7 (C(Ar)), 141.5 (C(Ar)), 141.1
(NCHN(near imine)), 140.8 (C(Ar)), 139.0 (C(Ar)), 136.6 (C(Ar)), 129.1 (CH(Ar)),
125.7 (CH(Ar)), 124.1 (CH(Ar)), 123.9 (CH(Ar)), 71.1 (CH(cod)), 70.3 (CH(cod)),
31.6 (CH2(cod)), 31.4 (CH2(cod)), 28.8 (CH(CH3)2), 28.6 (CH(CH3)2), 25.9
(CH(CH3)2), 23.8 (CH(CH3)2), 22.8 (CH(CH3)2), 22.4 (CH(CH3)2), 18.1
(CH3(imine)), 14.6 ppm (CH3(imidazole)); 31P{1H} NMR (121.5 MHz, CD2Cl2):
d=À144.2 ppm (sept, 1J(P,F)=712 Hz, PF6À); 19F{1H} NMR
(282.4 MHz, CD2Cl2): d=À73.3 (d, 1J(P,F)=712 Hz, PF6À); IR (pure,
orbit diamond): n˜max =1689 (v(C=N)), 1623 cmÀ1 (v(C=N)); elemental
analysis calcd for C39H54F6IrN4P (916.07): C 51.13, H 5.94, N 6.12;
found: C 50.56, H 5.96, N 5.88.
1
ture under argon. H NMR (500 MHz, CD2Cl2): d=7.40–7.30 (m, 3H;
CH(Dipp)), 7.22 (d, 3J=2.1 Hz, 1H; NCHCHN(near imine)), 7.19 (d, 3J=
3
2.1 Hz, 1H; NCHCHN(near imine)), 3.09 (sept, J=6.8 Hz, 2H; CH(CH3)2),
3
2.34 (s, 1H; CH3(imine)), 1.34 (d, J=6.8 Hz, 6H; CH(CH3)2), 1.18 ppm
(d, 3J=6.8 Hz, 6H; CH(CH3)2); 13C{1H} NMR (125 MHz, CD2Cl2): d=
186.5 (NCN(near imine)), 183.0 (CO), 174.5 (CO), 172.3 (CO), 168.8 (C=N),
168.5 (CO), 142.7 (ipso-C(Dipp)), 140.8 (o-C(Dipp)), 133.0 (NCHCHN(near
imine)), 129.5 (p-CH(Dipp)), 125.1 (m-CH(Dipp)), 117.5 (NCHCHN(near imine)),
28.9 (CH(CH3)2), 24.2 (CH(CH3)2), 23.7 (CH(CH3)2), 15.0 ppm
(CH3(imine)); IR (pure, orbit diamond): n˜max =2054, 1971 (v(CO)), 1606
(v(C=N)), 320 cmÀ1 (v(IrÀCl)); elemental analysis calcd for
C21H22ClIr2N3O4 (800.30): C 31.52, H 2.77, N 5.25; found: C 32.06, H
3.23, N 5.32.
Synthesis of [Ir2(cod)2Cl(LC2,Nimine,N3)] (10)
From complex 1a: To a stirred yellow solution of complex 1a
(0.151 g, 0.25 mmol) in THF (10 mL) was added slowly a solution of
KHMDS (0.55 g, 0.28 mmol) in THF (5 mL) at À788C. The reaction
mixture was stirred for 1 h at À308C and the color of the solution
Synthesis of [IrRh(cod)2Cl(LC2,Nimine,N3)] (12)
From complex 1b: To a stirred, yellow solution of complex 1b
(0.103 g, 0.20 mmol) in THF (10 mL) was added a solution of
KHMDS (0.044 g, 0.22 mmol) in THF (5 mL) slowly at À788C. The
mixture was stirred for 1 h at À308C and the color of the solution
turned to orange. Then a solution of [Ir(cod)(m-Cl)]2 (0.067 g,
0.10 mmol) in THF (5 mL) was added to the mixture at À788C. The
mixture was further stirred for 12 h at room temperature during
which time the color of the solution turned to maroon. After re-
moval of the volatiles under reduced pressure, the residue was ex-
tracted with toluene and the solution was filtered through Celite.
The filtrate was evaporated to dryness under reduced pressure.
The residue was dissolved in THF (2 mL) and Et2O (2 mL) was
added. Then the solution was cooled to À308C to yield a maroon
crystalline solid, which was collected by filtration and dried in
vacuo (0.098 g, 0.12 mmol, 60%). Single crystals of complex
12·2THF suitable for X-ray diffraction were obtained by crystalliza-
tion in a THF/Et2O (1:1) solution at À308C. 1H NMR (500 MHz,
turned to red. Then
a solution of [Ir(cod)(m-Cl)]2 (0.084 g,
0.13 mmol) in THF (5 mL) was added to the reaction mixture at
À788C. The reaction mixture was further stirred for 12 h at room
temperature during which time the color of the solution turned
dark green. After removal of the solvent under reduced pressure,
the residue was extracted with toluene and the solution was fil-
tered through Celite. The filtrate was concentrated to approximate-
ly 3 mL. After Et2O (3 mL) was added, the solution was cooled to
À308C to yield dark green crystals, which were collected by filtra-
tion and dried in vacuo (0.145 g, 0.16 mmol, 64%). Single crystals
of complex 10·Et2O·toluene suitable for X-ray diffraction were ob-
tained by crystallization from a toluene/Et2O (1:1) solution at
À308C. 1H NMR (400 MHz, C6D6): d=7.29 (d, 3J=2.0 Hz, 1H;
NCHCHN(near imine)), 7.07–6.86 (m, 3H; CH(Dipp)), 6.30 (d, 3J=2.0 Hz,
1H; NCHCHN(near imine)), 6.28–6.18 (m, 2H; CH(cod)), 4.97–4.89 (m, 1H;
CH(cod)), 4.88–4.80 (m, 1H; CH(cod)), 3.65–3.57 (m, 1H; CH(cod)), 3.44–
3.36 (m, 1H; CH(cod)), 3.36–3.28 (m, 1H; CH(cod)), 3.27–3.19 (m, 1H;
CH(cod)), 2.92 (sept, 3J=6.7 Hz, 2H; CH(CH3)2), 2.60–2.08 (m, 7H;
3
CD2Cl2): d=7.30 (d, J=1.8 Hz, 1H; NCHCHN(near imine)), 7.29–7.21 (m,
3H; CH(Dipp)), 6.92 (d, 3J=1.8 Hz, 1H; NCHCHN(near imine)), 6.26–6.18
(m, 1H; CH(cod)), 5.57–5.49 (m, 1H; CH(cod)), 4.57–4.49 (m, 1H;
CH(cod)), 4.46–4.38 (m, 1H; CH(cod)), 3.66–3.54 (m, 2H; CH(cod)), 3.52–
3.40 (m, 1H; CH(cod)), 3.32–3.22 (m, 1H; CH(cod)), 3.21 (sept, 3J=
6.7 Hz, 1H; CH(CH3)2), 3.07 (sept, 3J=6.7 Hz, 1H; CH(CH3)2), 2.70–
CH2(cod)), 2.06–1.76 (m, 3H; CH2(cod)), 1.72–1.30 (m, 9H; CH2(cod)
,
3
3
CH3(imine)), 1.18 (d, J=6.7 Hz, 3H; CH(CH3)2), 1.11 (d, J=6.7 Hz, 3H;
CH(CH3)2), 0.86 (d, 3J=6.7 Hz, 3H; CH(CH3)2), 0.54 ppm (d, 3J=
6.7 Hz, 3H; CH(CH3)2). 13C{1H} NMR (75 MHz, C6D6): d=180.6
Chem. Eur. J. 2016, 22, 2658 – 2671
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2.56 (m, 2H; CH2(cod)), 2.44–2.28 (m, 4H; CH2(cod)), 2.27–2.07 (m, 5H;
CH2(cod), CH3(imine)), 2.06–1.96 (m, 2H; CH2(cod)), 1.95–1.85 (m, 2H;
CH2(cod)), 1.78–1.56 (m, 4H; CH2(cod)), 1.35 (d, 3J=6.7 Hz, 3H;
CH(CH3)2), 1.34 (d, 3J=6.7 Hz, 3H; CH(CH3)2), 1.12 (d, 3J=6.7 Hz,
Scholarship Council for a PhD grant to F.H., and the Service de
Radiocristallographie (Institut de Chimie, Strasbourg) for the
determination of the crystal structures.
3
3H; CH(CH3)2), 1.03 ppm (d, J=6.7 Hz, 3H; CH(CH3)2); 13C{1H} NMR
Keywords: CÀH activation
· imidazoles · iridium · N-
heterocyclic carbenes · rhodium
(125 MHz, CD2Cl2): d=179.8 (NCN(near imine)), 166.3 (C=N), 142.2 (o-
C(Dipp)), 142.0 (o-C(Dipp)), 138.8 (ipso-C(Dipp)), 131.7 (NCHCHN(near imine)),
128.2 (p-CH(Dipp)), 124.5 (m-CH(Dipp)), 124.2 (m-CH(Dipp)), 115.2
1
(NCHCHN(near imine)), 84.8 (d, J(Rh,C)=11.2 Hz, CH(cod)), 84.2 (CH(cod)),
83.4 (d, 1J(Rh,C)=11.4 Hz, CH(cod)), 83.0 (CH(cod)), 77.9 (d, 1J(Rh,C)=
14.6 Hz, CH(cod)), 73.8 (d, 1J(Rh,C)=13.8 Hz, CH(cod)), 69.2 (CH(cod)),
65.4 (CH(cod)), 34.5, 33.1, 32.8, 31.9, 31.3, 30.7, 29.5, 29.4 (CH2(cod)),
28.6, 28.3 (CH(CH3)2), 25.3, 23.7, 23.4 (CH(CH3)2), 15.9 ppm
(CH3(imine)); IR (pure, orbit diamond): n˜max =1600 (v(C=N)), 290 cmÀ1
(v(RhÀCl)); elemental analysis calcd for C33H46ClIrRhN3 (815.32): C
48.61, H 5.69, N 5.15; found: C 48.40, H 6.19, N 4.63.
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From complex 1a: To a stirred solution of complex 1a (0.121 g,
0.20 mmol) in THF (10 mL) was added slowly a solution of KHMDS
(0044 g, 0.22 mmol) in THF (5 mL) at À788C. The reaction mixture
was stirred for 1 h at À308C and the color of the solution turned
to red. Then a solution of [Rh(cod)(m-Cl)]2 (0.050 g, 0.10 mmol) in
THF (5 mL) was added to the mixture at À788C. The reaction mix-
ture was further stirred for 12 h at room temperature during which
time the color of the solution turned maroon. After removal of the
volatiles under reduced pressure, the residue was extracted with
toluene and the solution was filtered through Celite. The filtrate
was evaporated to dryness under reduced pressure. The residue
was dissolved in THF (2 mL) and Et2O (2 mL) was added. The solu-
tion was cooled to À308C to yield a maroon crystalline solid,
which was collected by filtration and dried in vacuo (0.083 g,
0.10 mmol, 51%).
X-ray data collection, structure solution, and refinement for all
compounds: Suitable crystals for the X-ray analysis of all com-
pounds were obtained as described above. Data for complexes 1b,
3b+[PF6]À, 7a+[PF6]À, 10·Et2O·toluene, and 12·2THF were collect-
ed on an APEX-II CCD (graphite-monochromated MoKa radiation,
l=0.71073 Š) at 173(2) K, data for complexes 6a+[PF6]À, 9+[PF6]À,
and 11 were collected on a Kappa CCD diffractometer (graphite-
monochromated MoKa radiation, l=0.71073 Š) at 173(2) K. Crystal-
lographic and experimental details for these structures are sum-
marized in Tables S1 and S2 in the Supporting Information. The
structures were solved by direct methods (SHELXS-97[26]) and re-
fined by full-matrix least-squares procedures (based on F2, SHELXL-
97[26]) with anisotropic thermal parameters for all the non-hydro-
gen atoms. The hydrogen atoms were introduced into the geomet-
rically calculated positions (SHELXS-97 procedures). Details of the
specific comments applied for the structures are provided in the
Supporting Information. CCDC 1426156 (1b), 1426157 (3b+[PF6]À),
1426158 (6a+[PF6]À), 1426159 (7a+[PF6]À), 1426160 (9+[PF6]À),
1426161 (10·Et2O·toluene), 1426162 (11), and 1426163 (12·2THF)
contain the supplementary crystallographic data. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre.
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Received: October 8, 2015
Published online on January 15, 2016
Chem. Eur. J. 2016, 22, 2658 – 2671
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim