DFT Mechanistic Insights into the Alkyne Insertion Reaction Affording Diiron μ-Vinyliminium Complexes and New Functionalization Pathways

Article abstract of DOI:10.1021/acs.organomet.8b00448

Insertion of propyne or 2-butyne into the Fe-carbyne bond belonging to the fragment [Fe₂Cp₂(CO)(μ-CO){μ-CNMe(R)}]⁺ (R = Me or Xyl = 2,6-C₆H₃Me₂) was investigated via density functional theo

Full text of DOI:10.1021/acs.organomet.8b00448

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
DFT Mechanistic Insights into the Alkyne Insertion Reaction
Affording Diiron μ‑Vinyliminium Complexes and New
Functionalization Pathways
Gianluca Ciancaleoni,*,†,‡ Stefano Zacchini,‡,§ Valerio Zanotti,‡,§ and Fabio Marchetti*,†,‡
†Dipartimento di Chimica e Chimica Industriale, University of Pisa, Via G. Moruzzi 13, I-56124 Pisa, Italy
‡CIRCC, Via Celso Ulpiani 27, I-70126 Bari, Italy
§Dipartimento di Chimica Industriale “Toso Montanari”, University of Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy
S
* Supporting Information
ABSTRACT: Insertion of propyne or 2-butyne into the Fe−carbyne
bond belonging to the fragment [Fe2Cp2(CO)(μ-CO){μ-CNMe(R)}]+
(R = Me or Xyl = 2,6-C6H3Me2) was investigated via density functional
theory (DFT), and plausible intermediates were identified along the
formation of the vinyliminium complexes [Fe2Cp2(CO)(μ-CO){μ-
η1:η3-C(Me)C(R″)CN(Me)(R)}]SO3CF3, [2a−d]+, thus allowing us to
explain regio- and stereochemical features. The X-ray structure of
[2a]SO3CF3 (R = Me, R″ = H) was determined by single crystal X-ray
diffraction. Novel C−C and C−S bond forming pathways involving the
vinyliminium ligand were then explored. Thus, [2b]SO3CF3 (R = Xyl,
R″ = H) reacted with cyclopentadiene (or cyclopentene), triphenyl-
phosphonium methylide, and benzyl bromide, in tetrahydrofuran in the
presence of sodium hydride, respectively, to give [Fe2Cp2(CO)(μ-
CO){μ-η1:η2-C(Me)C{C(CH)4}CN(Me)(Xyl)}], 3, [Fe2Cp2(CO)(μ-
CO){μ-η1:η2-C(Me)C(CH2)CN(Me)(Xyl)}], 4, and [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(Me)C(CH2Ph)CN(Me)(Xyl)}]Br,
[5]Br, in good yields. Unstable complex 4 (detected by IR spectroscopy) readily converted into [2c]SO3CF3 (R = Xyl, R″
= Me) upon HSO3CF3 protonation of the methylide function. [5]Br was obtained as E/Z isomeric mixture, which was then
quantitatively converted into the most stable Z form, by heating in methanol solution at 50 °C. The reactions of [2c,d]SO3CF3
(R = Me, Xyl, R″ = Me) with PhSSPh/NaH selectively yielded the aminoalkylidyne species [Fe2Cp2(SPh)(CO)(μ-CO){μ-
CN(Me)(Xyl)}], 6, and the bis-alkylidene [Fe2Cp2(CO)(μ-CO){μ-η1:η2-C(Me)C(Me)(SPh)CN(Me)2}], 7, respectively,
probably via the intermediacy of radical compounds 2c,d. The structures of 3−7 and 2c,d were elucidated by DFT calculations,
and the isolated products were characterized by analytical and spectroscopic methods.
and the awareness that nature uses diiron organometallic units
to perform amazingly efficient enzymatic processes.8
INTRODUCTION
■
Alkynes are useful and versatile reagents in organometallic
chemistry. They can behave as η2 terminal ligands or occupy
bridging sites in polynuclear complexes,1 and this preliminary
coordination may prelude to an impressive variety of reactions
pathways.2 In particular, the intramolecular alkyne insertion
into a metal−carbon bond, in di- and trinuclear metal
complexes, represents an intriguing strategy to grow various
hydrocarbyl fragments stabilized by bridging multisite
coordination.3
However, [Fe2Cp2(CO)4] is a long time known, easily
available and robust compound,4 previously used as a starting
material for pioneering studies to explore new C−C bond
coupling reactions and to model the Fischer−Tropsch
process.5 The construction of organometallic motifs supported
on the [Fe2Cp2(CO)x] skeleton has witnessed a renewed
interest,6 in view of the current huge effort to develop
sustainable synthetic routes based on earth abundant metals,7
Diiron compounds [Fe2Cp2(CO)3(L)] are known to
undergo alkyne insertion between one Fe atom and the
bridging L ligand, the latter being CO9 or a fragment isolobal
to CO (i.e., thiocarbonyl10 or isocyanide);11 see Scheme 1.
This general reaction requires the preliminary photolytic
displacement of one terminal carbonyl ligand, in order to
generate a vacant site available to the entrance of the alkyne.
The analogous interaction of alkynes with aminoalkylidyne
complexes, [Fe2Cp2(CO)2(μ-CO){μ-CNMe(R)}]+ (R = Me,
[1a]+; R = Xyl = 2,6-C6H3Me2, [1b]+), benefits from the net
cationic charge of the complexes, weakening the terminal CO
binding. Therefore, CO elimination is viable with a chemical
approach, rather than with the much less selective photo-
chemical method. More precisely, the required vacant site is
Received: June 30, 2018
© XXXX American Chemical Society
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a
Scheme 1. Insertion Reactions of Alkynes
a
Into iron−carbonyl (E = O);9 iron-thiocarbonyl (E = S);10 iron-isocyanide (E = NR).11.
conveniently generated removing one carbonyl with trimethyl-
amine-N-oxide and can be temporarily occupied by a labile
acetonitrile ligand (or chloride, to be abstracted with a silver
salt). The subsequent alkyne insertion into iron−carbyne
bond, affording vinyliminium complexes, is usually regiospe-
cific, i.e., placing the most hindered alkyne substituent (R′) far
from the newly generated iminium moiety.12 The representa-
tive synthesis of [2a−d]+ from [1a−b]+ and propyne or 2-
butyne, respectively, is shown in Scheme 2.
can be quantitatively converted into the more stable cis
counterpart upon thermal treatment. Moreover, E/Z isomery
can be found in [2]+, as a consequence of the two possible
spatial orientations adopted by two different N-substituents
(Scheme 2, inset).
Herein, we present the results of a density functional theory
(DFT) study to give insight into the mechanism of formation
of [2a−c]+ and to provide explanation for the stereochemistry
exhibited by the products. Furthermore, [2b−d]SO3CF3 have
been selected as starting materials for exploring new synthetic
pathways to derivatize the vinyliminium ligand, resulting in
unprecedented hydrocarbyl fragments.
a
Scheme 2. Synthesis of Vinyliminium Complexes
RESULTS AND DISCUSSION
■
DFT and Crystallographic Studies. First, the insertion
reaction of propyne into the Fe−carbyne bond of [1a]+
12a
affording [2a]+ was investigated (Scheme 3). The alkyne
Scheme 3. Proposed DFT Intermediates along the
Formation of [2a]+ via Propyne Insertion into Fe−μ-
Carbyne Bond (L = Cl or NCMe)
a
(i) generation of a coordination vacancy and (ii) alkyne insertion
into Fe−carbyne bond (Xyl = 2,6-C6H3Me2). Inset: E/Z geometry
around the iminium bond in [2b−c]+.
The μ-vinyliminium ligand in [2]+ displays a rather rare
coordination fashion13 and a rich reactivity, essentially related
to the cationic charge of the complex and the cooperative
effects provided by the two adjacent iron atoms.14 Such
reactivity ranges from nucleophilic additions,15 incorporation
of small molecular units,16 and synthesis of unusually
functionalized sandwich compounds,17 or other monoiron
species.18
The mechanism of formation of diiron vinyliminium
compounds [2]+ is not obvious, for at least two reasons.
First, iron−alkyne intermediates are likely but elusive19 and
have not been detected experimentally so far. The second
point is that the diiron frame seems flexible, from a
stereochemical point of view, during the alkyne insertion.
Thus, the final products may display both cis and trans
configuration of the Cp ligands (see the specific cases in
Scheme 2), while starting complexes [1a,b]+ are exclusively
cis.20,21 When observed, the trans product is a kinetic one and
is expected to initially coordinate one iron center, although this
modality has not been recognized experimentally. In the DFT-
optimized geometry of 2a-I1 (Figure 1a), the bridging
aminoalkylidyne is migrated to a terminal position, binding
one single iron atom (Mayer bond orders22 for Fe1−C1 and
C1−N1 are 1.64 and 1.38, respectively). This rearrangement
allows propyne to coordinate the other iron according to a
slightly asymmetric η2-mode (Fe2−C3 = 2.075 Å, Fe2−C4 =
2.101 Å). The two carbonyl ligands occupy terminal sites.
Possible intermediate 2a-I1 is 12.0 kcal mol−1 less stable
than 2a-I2, wherein the alkyne bridges the two iron centers,
and the conversion of the former into the latter requires a small
activation energy (ΔG⧧ = 1.6 kcal mol−1). In 2a-I2 (Figure
B
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Figure 1. DFT-optimized geometries of 2a-I1 (a) and 2a-I2 (b). Hydrogen atoms (except H1) omitted for clarity. Selected distances (Å) and
angles (deg): 2a-I1: Fe1−Fe2 = 2.745, Fe1−C2 = 3.125, Fe2−C2 = 2.086, Fe2−C3 = 2.110, Fe2−C4 = 3.251, Fe1−C1 = 1.674, C1−N1 = 1.303,
C1−C2 = 3.169, C2−C3 = 1.274, H1−C2−C3 = 151.8, C2−C3−C4 = 159.8, Fe1−C1−N1 = 172.9, C1−Fe1−Fe2−C2 = 72.6; 2a-I2: Fe1−Fe2 =
2.685, Fe1−C2 = 2.010, Fe2−C2 = 2.678, Fe2−C3 = 2.010, Fe1−C1 = 1.655, C1−N1 = 1.304, C1−C2 = 2.458, C2−C3 = 1.326, H1−C2−C3 =
128.6, C2−C3−C4 = 131.2, Fe1−C1−N1 = 173.6.
1b), the orientation of the alkyne Me-substituent minimizes
steric repulsions and seems responsible for the observed
regiospecificity of the process (see the Introduction and
Scheme 2). The distance between the alkyne carbon C2 and
the carbyne C1 is 2.458 Å; this configuration is ideal for C1−
C2 bond formation (insertion reaction), and 2a-TS2 has been
identified as the related transition state. In 2a-TS2, the C1−C2
distance is shortened to 2.230 Å and the corresponding energy
is only 0.4 kcal mol−1 higher than that of 2a-I2.
Table 1. Experimental and Calculated Values for Selected
Bond Distances (Å) and Angles (deg) for [2a]+
X-ray
DFT
Fe(1)−Fe(2)
Fe(1)−C(11)
Fe(1)−C(12)
Fe(2)−C(12)
Fe(1)−C(3)
Fe(2)−C(3)
Fe(2)−C(1)
Fe(2)−C(2)
C(11)−O(1)
C(12)−O(2)
C(1)−C(2)
C(2)−C(3)
C(1)−N(1)
N(1)−C(4)
N(1)−C(5)
2.5535(3)
1.7689(17)
1.9116(17)
1.9305(17)
1.9565(16)
2.0414(16)
1.8550(16)
2.0628(16)
1.139(2)
1.176(2)
1.422(2)
1.415(2)
1.281(2)
2.538
1.728
1.894
1.928
2.062
1.954
1.844
2.103
1.194
1.217
1.448
1.448
1.317
1.490
1.493
After insertion, the system collapses to final product [2a]+,
being 47.8 kcal mol−1 more stable than 2a-I2 (a view of the
calculated structure of [2a]+ is shown in Figure S1). The
structure of [2a]SO3CF3 was experimentally determined by X-
ray diffraction: A view of the cation is given in Figure 2, while
1.470(2)
1.468(2)
Fe(1)−C(11)−O(1)
Fe(1)−C(12)−Fe(2)
Fe(1)−C(3)−Fe(2)
Fe(1)−C(3)−C(2)
C(3)−C(2)−C(1)
C(2)−C(1)−N(1)
C(2)−C(1)−Fe(2)
C(1)−N(1)−C(4)
C(1)−N(1)−C(5)
C(4)−N(1)−C(5)
177.18(15)
83.30(7)
79.36(6)
119.51(12)
116.06(14)
133.68(15)
76.76(9)
121.56(14)
122.42(14)
116.02(13)
179.01
83.24
78.37
120.38
113.27
133.78
78.40
121.56
122.88
115.48
well described as a vinyliminium, with the alkenyl moiety C2−
C3 being elongated due to coordination to Fe1. The Mayer
bond orders for C1−Fe1, C2−Fe1, C3−Fe1, and C3−Fe2 are
1.01, 0.38, 0.54, and 0.91, respectively.
Figure 2. Molecular structure of [2a]+ with key atoms labeled.
Displacement ellipsoids are at the 30% probability level.
the salient bonding parameters are reported in Table 1,
showing substantial agreement with the corresponding DFT
data and with analogous structures previously reported but
comprising a xylyl N-substituent.12
The trans isomer of [2a]+ (experimentally not observed)12a
could be originated from 2a-I1, by means of rotation around
the Fe−Fe axis affording 2a-I1T (2a-I1T is 30.1 kcal mol−1
more stable than 2a-I1). The transition state for this rotation
lies 11.9 kcal mol−1 higher than 2a-I1; therefore, it is expected
to be accessible at ambient temperature (2a-TScis−trans).
Indeed, intramolecular ligand interchanges between cis/trans
and bridging/terminal sites in related diiron systems were
previously explained in a similar way (Adams−Cotton
mechanism).23 DFT calculations point that 2a-I1T has a
It is interesting to note that in [2a]+ (Figure S1) the bonds
C1−C2 and C2−C3 exhibit very similar lengths [calculated
values: 1.448 (C1−C2) and 1.448 Å (C2−C3); X-ray values:
1.422(2) (C1−C2) and 1.415(2) Å (C2−C3)]. The Mayer
bond orders for N−C1, C1−C2, and C2−C3 are 1.38, 1.12,
and 1.05, respectively, confirming that the C3 ligand can be
C
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Figure 3. DFT-optimized geometries of 2a-I1T (a), 2a-I2T (b), and trans-[2a]+ (c). Hydrogen atoms (except H1) omitted for clarity. Selected
distances (Å) and angles (deg): 2a-I1T: Fe1−Fe2 = 2.516, Fe1−C1 = 1.852, Fe2−C1 = 1.862, Fe2−C2 = 2.104, Fe2−C3 = 2.148, C1−N1 =
1.328, C2−C3 = 1.270, H1−C2−C3 = 153.5, C2−C3−C4 = 160.1, Fe1−C1−N1 = 136.7; 2a-I2T: Fe1−Fe2 = 2.685, Fe1−C1 = 1.649, C1−N1 =
1.309, Fe1−C2 = 2.008, Fe2−C3 = 2.014, Fe2−C2 = 2.663, C2−C3 = 1.321, H1−C2−C3 = 129.3, C2−C3−C4 = 131.9, Fe1−C1−N1 = 176.8;
trans-[2a]+: Fe1−Fe2 = 2.529, Fe1−C1 = 1.837, C1−N1 = 1.319, Fe1−C2 = 2.090, Fe1−C2 = 2.059, Fe2−C3 = 1.940, C1−C2 = 1.449, C2−C3 =
1.438, H1−C2−C3 = 121.5, C2−C3−C4 = 119.3, Fe1−C1−N1 = 147.5.
different geometry than 2a-I1, in that the aminocarbyne
bridging coordination is maintained after alkyne binding to one
Fe atom (Figure 3a). Nevertheless, further rearrangement to
2a-I2T, bearing a bridging alkyne (Figure 3b), appears
necessary to allow the C1−C2 bond formation. However,
the activation energy for the trans to cis isomerization is much
higher than that requested for the formation of 2a-I1, which
makes the whole trans path unlikely. The trans isomer (Figure
3c) is not favored even on thermodynamic basis, since it is 2.3
kcal mol−1 less stable than cis-[2a]+. A comparative overview of
the energy profiles of the calculated pathways leading to cis-
[2a]+ and trans-[2a]+ is given in Figure 4.
Trying to elucidate the synthesis of [2b]+,12a the picture is
qualitatively similar to what discussed for [2a]+, but E and Z
isomerism, beyond cis/trans, is also possible. Indeed the energy
difference between (cis-E)-[2b]+ and (cis-Z)-[2b]+ is only 0.8
kcal mol−1, thus justifying the formation of both forms
(experimental E to Z ratio = 2). The obtained product
exclusively displays cis geometry, which is more stable than the
trans one by 1.8 kcal mol−1. Views of the calculated structures
of (cis-E)-[2b]+ and (cis-Z)-[2b]+ are provided as Figure S2.
Regarding [2c]+, the experimental outcome points out that
the trans-Z isomer (Figure 5) is the kinetic product, converting
into the cis-Z isomer by thermal treatment.12b DFT
calculations show that the coordination of 2-butyne forces
the aminoalkylidyne to shift to one single iron atom (2c-I1).
This configuration is stabilized by a CH···π weak interaction
between the aromatic ring of the xylyl moiety and the methyl
belonging to the alkyne. Due to this interaction, which is lost in
the TS, the activation barrier for the migration of the alkyne to
bridging position (2c-I2, ΔG = −9.1 kcal mol−1) is 12.6 kcal
mol−1 (2c-TS1), i.e., higher respect to the case of [2a]+. The
subsequent insertion step leads to the cis-Z isomer (ΔG(2c-
TS2) = −7.5 kcal mol−1; ΔG(cis-[2c]+) = −47.2 kcal mol−1).
The route to the trans isomer is more probable: Starting from
2c-I1, the activation barrier for the rotation around the Fe−Fe
bond is 9.9 kcal mol−1 (2c-TScis−trans), i.e., lower than the
activation energy required by 2c-TS1. The rotation leads to the
trans isomer, 2c-I1T (Figure 5), being 29.2 kcal mol−1 more
stable than 2c-I1. Then, the alkyne can move to bridging
position (ΔG(2c-TS1) = 4.7 kcal mol−1, ΔG([2c-I2T]+) =
Figure 4. Energy profiles of the pathways leading to cis-[2a]+ (black
line) and trans-[2a]+ (red line).
D
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Figure 5. DFT-optimized geometries of 2c-I1T (a), 2c-I2T (b) and (trans-Z)-[2c]+ (c). Hydrogen atoms omitted for clarity. Selected distances
(Å) and angles (deg): 2c-I1T: Fe1−Fe2 = 2.513, Fe1−C1 = 1.870, Fe2−C1 = 1.848, Fe2−C2 = 2.124, Fe2−C3 = 2.140, C1−N1 = 1.337, C1−C2
= 2.742, C2−C3 = 1.271, C2−C4 = 1.483, C4−C2−C3 = 153.0, C2−C3−C5 = 157.5, Fe1−C1−N1 = 134.6; 2c-I2T: Fe1−Fe2 = 2.651, Fe1−C1
= 1.644, Fe1−C2 = 2.018, Fe2−C2 = 2.703, Fe2−C3 = 2.606, C1−N1 = 1.310, C2−C3 = 1.324, C2−C4 = 1.511, C4−C2−C3 = 131.5, C2−C3−
C5 = 130.6, Fe1−C1−N1 = 175.1; (trans-Z)-[2c]+: Fe1−Fe2 = 2.531, Fe1−C1 = 1.337, Fe1−C2 = 2.123, Fe1−C3 = 2.034, Fe2−C3 = 1.938,
C1−N1 = 1.326, C1−C2 = 1.452, C2−C3 = 1.444, C2−C4 = 1.526, C1−C2−C3 = 113.3, N1−C1−C2 = 132.2, C2−C3−C5 = 120.2.
−4.8 kcal mol−1; see Figure 5), and finally, the insertion step
takes place almost barrierless (ΔG(2c-TS2) = −7.7 kcal
mol−1), leading to the product (ΔG((cis-Z)-[2c]+) = −53.6
kcal mol−1, Figure 6). It is possible that the thermal
isomerization of (trans-Z)-[2c]+ to (cis-Z)-[2c]+.12b proceeds
through fast de-insertion of the alkyne, thus allowing the
system to reach the thermodynamic equilibrium. The
calculated structure of (cis-Z)-[2c]+ is shown in Figure S4,
while a comparison of calculated and experimental bonding
parameters is given in Table S1.
C−C Coupling Reactions. The vinyliminium ligand is a
useful substrate to generate C−C bonds, and previous reports
regarded the addition of cyanide ion,24 acetylides,25 and
isocyanides.26 We came interested in the possibility of
tethering alkenes to the vinyliminium skeleton upon C2−H
removal. The investigation of the reactivity of [2a,b]SO3CF3
with a series of alkenes did not afford stable products, except in
one case. Thus, the reaction of [2b]SO3CF3 with cyclopentene,
in tetrahydrofuran in the presence of sodium hydride, afforded
a modest amount of fulvene−bis-alkylidene complex 3, in
admixture with other nonidentified species (Scheme 4).
Scheme 4. Modification of a Vinyliminium Ligand with a
Fulvene Moiety via Unusual Activation of Cyclopentadiene/
Cyclopentene
Figure 6. Energy profiles of the pathways leading to (cis-Z)-[2c]+
(black line) and (trans-Z)-[2c]+ (red line).
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The formation of 3 is notable, since it requires a cycloalkene
activation (from cyclopentene to fulvene) almost unknown in
the literature. Compound 3 was purified by alumina
chromatography and fully characterized by elemental analysis,
mass spectrometry, and IR and NMR spectroscopy. Once the
structure of 3 had been elucidated, we evaluated that
cyclopentadiene could be a more suitable reactant to the
synthesis. As a matter of fact, 3 was obtained with an optimized
yield of 61% from [2b]SO3CF3, freshly distilled cyclo-
pentadiene, and sodium hydride (Scheme 4). The formation
of 3 appears the result of coupling of dehydrogenated
cyclopentadiene with the deprotonated form of [2b]+,
possessing some carbene character (Figure 7).26 It should be
Figure 8. DFT-optimized geometry of 3. Hydrogen atoms omitted for
clarity. Selected bond lengths (Å) and angles (deg): Fe1−Fe2 =
2.503, Fe1−C1 = 1.865, Fe1−C3 = 2.014, Fe1−C2 = 2.403, Fe2−C3
= 1.981, C1−C2 = 1.477, C2−C3 = 1.486, C1−N1 = 1.339, C2−C4
= 1.414, C4−C5 = 1.472, C5−C7 = 1.389, C7−C8 = 1.470, C6−C8
= 1.390, C6−C4= 1.472, C1−C2−C3= 102.5, C2−C1−N1= 127.7,
C1−C2−C6 = 125.0, N1−C1−C2−C4 = −48.1.
Figure 7. Resonance forms representing the fragment derived from
vinyliminium C2−H deprotonation.
mentioned here that the uncommon cyclopentadiene to
fulvene conversion was previously reported upon base-assisted
condensation with ketones,27 but it is unprecedented for an
organometallic structure.
The IR spectrum of 3 (in CH2Cl2 solution) displays two
bands due to the carbonyl ligands (at 1949 and 1783 cm−1)
and an additional absorption at 1551 cm−1, assigned to the
coupled vibrations of the C1N and the C2C3 bonds by
comparison with the calculated IR spectrum (vide infra). The
1H NMR spectrum (in CDCl3) contains one set of resonances,
and the signals related to the fulvene moiety consist of two
multiplets at 6.49 and 6.36 ppm, thus indicating free rotation at
ambient temperature around the C2−C bond (vide infra). A
NOE experiment by irradiating the Cp resonance at 4.35 ppm
resulted in a significant enhancement of the remaining Cp
signal at 4.97 ppm, pointing that the two rings are arranged in
relative cis geometry. However, irradiation of the resonance at
3.27 ppm (NMe) revealed NOE effect with the CH fulvene
resonance at 6.49 ppm, but not with any Cp resonance. This
outcome clearly indicates that the N-bound methyl group
points far from the Fe−Fe axis, the same Z-configuration being
usually adopted by diiron complexes comprising C2-substituted
vinyliminium and bis-alkylidene ligands.16a,12b,28
Figure 9. Resonance formulas describing the structure of 3 (compare
to Figure 7).
of an oxygen atom in the place of the fulvene moiety.28 It has
to be remarked that the latter two complexes exhibit very close
IR spectra and comparable values of 13C NMR chemical shifts
for C1 and C3.29 Structure II in Figure 9 accounts for the
experimentally observed free rotation of the C5 ring around the
C2−C4 bond at ambient temperature (see above).
The reaction of [2b]SO3CF3 with PPh3CH2/NaH, in
tetrahydrofuran, readily proceeded with the clean formation of
a neutral product, as suggested by the IR spectrum recorded on
the reaction solution after 30 min [νCO = 1952 and 1773
cm−1]. It is reasonable to assume that the detected compound
is bis-alkylidene 4, analogous to 3 (Figure 9, structure I). The
formation of 4 might be viewed as the coupling between the
carbene moiety belonging to deprotonated [2b]+ (Figure 7,
structure I) and the [CH2] fragment originating from PPh3
CH2. Examples of [CH2] transfer from Ph3PCH2 to carbene
groups were indeed previously documented.30 Attempts to
isolate 4 were unsuccessful and led to recovery of a small
amount of [2b]SO3CF3. Instead, the treatment of the reaction
mixture with HSO3CF3 resulted in the fast and clean formation
of [2c]SO3CF3 (Scheme 5), which was isolated in 75% yield
after work up and unambiguously identified by elemental
The 13C resonances related to the C1 and C3 carbons fall at
typical low fields (247.4 and 195.1 ppm), in agreement with
their amino-alkylidene and alkylidene character, respective-
ly.15b Several attempts were done to collect X-ray quality
crystals of 3, but unsuccessfully. The structure of 3 (in the cis-Z
form, as indicated by NMR) was therefore optimized by DFT
calculations, and a view of the structure is shown in Figure 8
with calculated bonding parameters reported in the caption.
The Mayer bond orders for N1−C1, C1−C2, C2−C3, and
C2−C4 are 1.36, 0.97, 0.83, and 1.24, respectively (the
calculated C2−C4 bond length is 1.414 Å). The DFT data
suggest some zwitterionic character in the structure of 3, whose
bridging C3 ligand should be alternatively described as a
vinyliminium with the negative charge delocalized on the five-
membered substituent (Figure 9). Analogous vinyliminium/
bis-alkylidene hybrid structure was revealed by the crystallo-
graphic characterization of [Fe2Cp2(CO)(μ-CO){μ-η1:η2-C-
(Me)C(O)CN(Me)(Xyl)}], differing from 3 in the presence
1
analysis, IR, and H NMR spectroscopy.
The structure of 4 was optimized by DFT calculations; the
calculated bonding parameters are in well agreement with a
bis-alkylidene nature of the bridging C3 ligand15b (Figure 10).
On theoretical grounds, the E isomer is slightly more stable
than the opposite Z one (ΔE = 2.0 kcal mol−1), presumably
due to attractive forces between the [CH2] group and the
F
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Scheme 5. Two-Step H+−CH3 Substitution at a Vinyliminium Ligand
Article
+
the two carbonyl bands are seen at 1986 and 1814 cm−1, while
the absorption due to the NC1C2 moiety falls at 1603 cm−1. As
a comparison, the corresponding values for (cis-Z)-[2c]-
SO3CF3 are 1986, 1818, and 1613 cm−1.12b
The NMR spectra of [5]Br (in CDCl3) evidenced the
presence of an isomeric mixture. In general, E/Z and cis/trans
isomers of diiron vinyliminium complexes can be easily
distinguished by 1H and 13C NMR data (especially the
resonances due to Cp and methyl groups).12 Therefore, in this
case the isomers were identified as cis-E and cis-Z (E/Z ratio =
2, based on 1H NMR). The 19F spectrum ruled out the
presence of residual triflate anion.
In order to give insight into thermodynamic and structural
aspects, we calculated the structures of E-[5]+ and Z-[5]+
(Figure 11). The Z isomer resulted 2.0 kcal mol−1 more stable
than the E one. In accordance with the DFT outcome, the
isomeric mixture was cleanly converted into the most stable Z
form by heating in methanol solution.
Figure 10. DFT-optimized geometry of E-4. Hydrogen atoms (except
CH2) omitted for clarity. Selected bond lengths (Å) and angles (deg):
Fe1−Fe2 = 2.487, Fe1−C1 = 1.857, Fe1−C2 = 2.514, Fe1−C3 =
1.980, C1−N1 = 1.346, C1−C2 = 1.486, C2−C3 = 1.504, C2−C4 =
1.370, Fe2−C3 = 1.990, C1−C2−C3 = 98.3, C2−C1−N1 = 127.4,
C3−C2−C6 = 128.5, N1−C1−C2−C4 = −17.9.
The synthesis of [5]+ is formally the result of deprotonation
of the parent vinyliminium followed by nucleophilic attack of
the resulting zwitterionic species (Figure 7, structure II) to the
alkyl halide. Despite the tendency of benzyl bromide to be
engaged in radical reactions,31 the possibility of an alternative,
radical pathway starting with the NaH-reduction of [2b]+
should be ruled out: As a matter of fact, the reaction of
[2b]SO3CF3 with benzyl bromide in the presence of
cobaltocene, i.e., a typical mono electron reductant employed
in organometallic chemistry,32 resulted in the formation of a
mixture of products not including [5]+.
aromatic xylyl ring (CH−π interaction). However, final
product [2c]+ was recognized in the Z form (Scheme 2).
This fact indicates that E to Z conversion must take place on
going from [2b]+ to [2c]+ via 4, possibly through rotation
around the C1−N axis in 4 (calculated C1−N distance = 1.346
Å).
Synthesis of Thiophenolate Compounds. The chem-
istry of the vinyliminium compounds [2c,d]SO3CF3 is limited
by the absence of C2-bound hydrogen, not allowing the
structural modifications otherwise described for [2a,b]+
(Schemes 4−6). Notwithstanding, it has been documented
that C2-substituted vinyliminium compounds may convert into
monoiron species leaving the C1−C2−C3 chain intact, upon
reaction with strong reducing agents (NaH or LitBu).33 Sulfur
or selenium can be incorporated during the fragmentation
process, leading to functionalized metallacycles.18
With the aim of further exploring the possibility of C−C
bond formation, we studied the reaction of [2b]SO3CF3 with
benzyl bromide, in the presence of sodium hydride. This
reaction afforded vinyliminium compound [5]Br in 55% yield,
Scheme 6.
Compound [5]Br was purified by quick chromatography
through alumina and then characterized by elemental analysis,
IR, and NMR spectroscopy. The IR spectrum (in CH2Cl2)
displays a pattern matching that of analogous complexes
bearing alkyl substituents at both C2 and C3.12b More precisely,
Scheme 6. Derivatization of Vinyliminium Ligand via C−H Cleavage and C−C Coupling
G
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Figure 11. DFT-optimized geometries of (cis-E)-[5]+ (a) and (cis-Z)-[5]+ (b). Hydrogen atoms omitted for clarity. Selected distances (Å) and
angles (deg): (cis-E)-[5]+: Fe1−Fe2 = 2.529, Fe1−C1 = 1.845, Fe1−C2 = 2.098, Fe1−C3 = 2.048, Fe2−C3 = 1.952, C1−N1 = 1.322, C1−C2 =
1.447, C2−C3 = 1.452, C1−C2−C3 = 144.5, Fe1−C1−N1 = 144.9; (cis-Z)-[5]+: Fe1−Fe2 = 2.543, Fe1−C1 = 1.849, Fe1−C2 = 2.104, Fe1−C3 =
2.063, Fe2−C3 = 1.945, C1−N1 = 1.320, C1−C2 = 1.445, C2−C3 = 1.446, C1−C2−C3 = 114.6, Fe1−C1−N1 = 148.3.
We studied the reactivity of [2c,d]SO3CF3 with NaH in the
presence of PhSSPh. The reaction involving [2c]+ proceeded
with alkyne de-insertion and binding of the [SPh] group to one
metal center, affording the μ-aminocarbyne complex
[Fe2Cp2(SPh)(CO)(μ-CO){μ-CN(Me)(Xyl)}], 6, in 68%
yield (Scheme 7a and Figure 12a). The aminocarbyne moiety
manifests itself with a diagnostic 13C NMR resonance at
characteristic low field (338.5 ppm, CDCl3 solution).20,34 The
IR and NMR features of 6 match those of similar diiron μ-
aminocarbyne complexes with cis-oriented Cp ligands.34f,g,35
Interestingly, the reaction of [2d]SO3CF3 with PhSSPh/
NaH revealed a completely different outcome and afforded the
bis-alkylidene product [Fe2Cp2(CO)(μ-CO){μ-η1:η2-C(Me)-
C(Me)(SPh)CN(Me)2}], 7, in 55% yield (Scheme 7b and
Figure 12b).
The IR spectrum of 7 displays carbonyl bands at typically
low wavenumbers (1929 and 1752 cm−1), as already found for
other diiron complexes containing the bis-alkylidene skel-
eton.15b In the H NMR spectrum, the Cp ligands resonate at
1
4.61 and 4.39 ppm; these values indicating a cis arrangement
by comparison with a library of data referring to analogous
compounds.15b The major NMR features are given by the low
field 13C resonances at 251.6 and 177.0 ppm, attributed to the
alkylidene carbons C1 and C3, respectively. In accordance with
the aminocarbene character of C1, the two N-bound methyl
groups are not equivalent due to inhibited rotation around the
N−C1 bond.
Scheme 7. Reactions of Vinyliminium Complexes with
a
Diphenyldisulfide in the Presence of Sodium Hydride
The distinct outcomes of the reactions of homologous
vinyliminium salts [2c,d]SO3CF3 with PhSSPh/NaH were
investigated by DFT calculations. It is reasonable to assume
that both reactions proceed with initial electron transfer from
sodium hydride to cationic complexes [2c,d]+ to give 2c,d and
subsequent capture of the thiophenolato radical by 2c,d.
However, the nucleophilic attack to [2c,d]+ of the
thiophenolate anion, with this anion possibly generated from
the interaction of diphenyldisulfide with sodium hydride, was
ruled out on the basis of the experimental evidence that the
reactions of [2c,d]SO3CF3 with 1−2 equiv of NaSPh in
tetrahydrofuran did not produce 6 or 7. More precisely,
[2d]SO3CF3 revealed to be substantially unreactive, while the
addition of NaSPh to [2c]SO3CF3 led to a mixture of
decomposition products. The structures of radical species 2c,d
were optimized by DFT (doublet state), resulting rather
similar to those of the corresponding parent complexes
[2c,d]+. However, a slight lengthening of the C1−N bond is
observable on going from [2c,d]+ to 2c,d (from 1.317 to 1.337
Å; Mayer bond order from 1.38 to 1.21). The spin density
maps related to 2c,d are shown in Figure 13: In both
compounds, the spin density has a significant component on
the bridging ligand (see Table S2 for details). This favors the
SPh attack to the C2 carbon of 2d to give 7. However, the
major steric hindrance exerted by the iminium group in 2c is
a
(a) De-insertion reaction; (b) formation of [SPh]-functionalized
bridging ligand.
H
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Figure 12. DFT-optimized structures of 6 (a) and 7 (b). Hydrogen atoms omitted for clarity. Selected distances (Å) and angles (deg): 6: Fe1−Fe2
= 2.473, Fe1−C1 = 1.861, Fe2−C1 = 1.807, C1−N1 = 1.338, Fe2−S1 = 2.371, Fe1−C1−N1 = 137.0, C1−Fe2−S = 85.4, Fe2−S1−C3 = 104.1; 7:
Fe1−Fe2 = 2.484, Fe1−C1 = 1.868, Fe1−C2 = 2.559, Fe1−C3 = 1.957, Fe2−C1 = 3.072, Fe2−C3 = 2.022, C1−C2 = 1.526, C2−C3 = 1.528,
C2−S1 = 2.108, C1−N1 = 1.337, Fe1−C1−N1 = 136.8, C1−C2−C3 = 94.4, C1−C2−S1 = 97.8.
incorporating the [SPh] fragment in different ways according
to the iminium substituents.
EXPERIMENTAL SECTION
■
Materials and Methods. All the reactions were routinely carried
out under nitrogen atmosphere, using standard Schlenk techniques.
Organic reactants (Sigma-Aldrich or TCI Europe) were commercial
products of the highest purity available. Compounds [1a,b]SO3CF3,20
triphenylphosphonium methylide (from methyltriphenylphospho-
12b
nium bromide and LiBu),36 and [2c,d]SO3CF3
were prepared
according to published procedures. Solvents were distilled before use
under nitrogen over appropriate drying agents. Once isolated, the
metal products were conserved under nitrogen. Chromatography
separations were carried out on columns of deactivated alumina
(Sigma-Aldrich, 4% w/w water). C, H, and N analyses were
performed on a ThermoQuest Flash 1112 Series EA Instrument.
The bromide content of [5]Br was determined by the Mohr method37
on a solution prepared by dissolution of the solid in aqueous KOH at
boiling temperature, followed by cooling to room temperature and
addition of HNO3 up to neutralization. ESI-MS spectrum was
recorded on Waters Micromass ZQ 4000 with the sample dissolved in
CH3CN. Infrared spectra were recorded on liquid samples with a
PerkinElmer Spectrum 2000 FT-IR spectrophotometer. NMR spectra
were recorded at 298 K on a Mercury Plus 400 instrument. The
Figure 13. Spin density maps for radical complexes 2c and 2d.
presumably responsible for addressing the SPh addition to iron
to give 6.
CONCLUSIONS
■
Vinyliminium ligands in diiron complexes exhibit a rare
bridging coordination fashion and a unique reactivity
associated with the cooperative effects provided by the two
iron centers and the net cationic charge of the compounds.
Herein, we have proposed, on the basis of DFT calculations, a
mechanistic pathway for the alkyne insertion implicated in the
synthesis of the vinyliminium complexes. The described
mechanism is in accordance with the regiospecificity of the
reaction and the stereochemistry exhibited by the products
(cis/trans and E/Z), depending on the iminium substituents
and the alkyne. These findings may have some general validity
and might be extended to analogous alkyne insertion reactions
occurring on dimetal frames.3,9−11 Furthermore, we have
described the synthesis of unprecedented organometallic
motifs via vinyliminium C−H bond cleavage and subsequent
C−C coupling with a variety of organic reactants, including the
unusual activation of a cycloalkene. Vinyliminium complexes
lacking of appropriate C−H function may be derivatized
through one-electron reduction, the resulting radical species
1
chemical shifts for H and 13C were referenced to the nondeuterated
1
aliquot of the solvent. The H and 13C NMR spectra were assigned
with the assistance of 1H,13C correlation measured through gs-HSQC
and gs-HMBC experiments.38 NMR signals due to a second isomeric
form (where it has been possible to detect them) are italicized. NOE
measurements were recorded using the DPFGSE-NOE sequence.21
Synthesis of [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C3(Me)C2HC1N(Me)-
(R)}]SO3CF3 (R = Me, [2a]SO3CF3, Chart 1; R = Xyl, [2b]SO3CF3,
Chart 2). The title compounds were prepared by using a modified
literature procedure.12b
A solution of [1a]SO3CF3 (460 mg, 0.866 mmol) in acetonitrile
(ca. 10 mL) was treated with Me3NO (85 mg, 1.13 mmol). The
solution turned darker and was stirred at room temperature for 40
min, allowing continuous flux of the produced gas (CO2) away from
the reaction system. Thus, IR spectroscopy indicated the complete
conversion of the starting material into [Fe2Cp2(CO)(μ-CO)-
(NCMe){μ-CNMe2}]SO3CF3.20 The volatiles were removed under
vacuum. The residue was dissolved in CH2Cl2 (20 mL), and a
solution of propyne in THF (3.5 mL, ca. 1 mol/L) was added. The
resulting mixture was allowed to stir at room temperature for 72 h,
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Chart 1. Structure of [2a]+
and then the volatiles were removed under vacuum. The residue was
dissolved in CH2Cl2 and charged on an alumina column; after
washing with dichloromethane, a dark gray fraction corresponding to
3 was collected using neat THF as eluent. Yield 105 mg, 61%. Anal.
Calcd for C30H29Fe2NO2: C, 65.84; H, 5.34; N, 2.56. Found: C,
65.61; H, 5.40; N, 2.49. IR (CH2Cl2) ν/cm−1 = 1949 vs (CO), 1783 s
1
(μ-CO), 1551 m (C1N + C2C3). H NMR (CDCl3) δ/ppm =
7.37−7.22 (3 H, C6H3Me2); 6.49, 6.36 (dd, 4 H, CH); 4.97, 4.35 (s,
10 H, Cp); 4.23 (s, 3 H, C3 Me); 3.27 (s, 3 H, NMe); 2.71, 2.09 (s, 6
H, C6H3Me2). 13C{1H} NMR (CDCl3) δ/ppm = 267.2 (μ-CO);
247.4 (C1); 213.7 (CO); 195.1 (C3); 142.0 (ipso-C6H3Me2); 134.9,
134.2, 129.6, 128.8, 128.4 (C6H3Me2); 118.5, 118.2 (CH); 95.7
(C2CCH); 89.3, 88.1 (Cp); 81.2 (C2); 46.5 (NMe); 45.7 (C3Me);
20.0, 19.3 (C6H3Me2). ESI-MS (ES+) 547 m/z [M+].
Reaction of [2b]SO3CF3 with Ph3PCH2/NaH/CF3SO3H: For-
mation of [Fe2Cp2(CO)(μ-CO){μ-η1:η2-C3(Me)C2(CH2)C1N(Me)-
(Xyl)}], 4, and [2c]SO3CF3 (Chart 4).12. The reaction of
then charged on an alumina column. After washing with CH2Cl2/
THF (up to 2:1 v/v) mixtures, the product was collected as a brown
fraction using neat MeOH as eluent. A brown solid was obtained
upon removal of the solvent under reduced pressure. Yield 451 mg,
96%. Anal. Calcd for C19H20F3Fe2NO5S: C, 42.02; H, 3.71; N, 2.58.
Found: C, 41.89; H, 3.64; N, 2.68. IR (CH2Cl2) ν
̃
/cm−1 = 1990 vs
(CO), 1806 s (μ-CO), 1684 m (C2C1N). 1H NMR (dmso-d6) δ/ppm
= 5.48, 5.14 (s, 10 H, Cp); 4.51 (s, 1 H, C2H); 3.82, 3.77 (s, 6 H,
NMe + C3 Me); 3.18 (s, 6 H, NMe). 13C{1H} NMR (dmso-d6) δ/
ppm = 258.4 (μ-CO); 225.6 (C1); 211.4 (CO); 208.0 (C3); 91.2,
88.0 (Cp); 52.1 (C2); 51.0, 44.8 (NMe2); 41.7 (C3 Me). Crystals
suitable for X-ray analysis were obtained by slow diffusion of diethyl
ether into a dichloromethane solution of [2a]SO3CF3 at ambient
temperature.
Chart 4. Structures of E-4 and Z-[2c]+
Compound [2b]SO3CF3 was obtained using the same procedure
employed for the synthesis of [2a]SO3CF3, from [1b]SO3CF3 (450
Chart 2. Structure of [2b]+
[2b]SO3CF3 (152 mg, 0.240 mmol) with Ph3PCH2 (265 mg, 0.959
mmol) and NaH (17 mg, 0.71 mmol) was carried out by a procedure
similar to that described for the synthesis of 3. After 30 min, the IR
spectrum of the mixture cleanly exhibited two bands (1952 vs, 1773
s), assigned to 4. Every attempt at purification through stationary
phases resulted in prevalent decomposition. Thus, the liquid phase
was separated and treated with CF3SO3H (0.022 mL, 0.249 mmol).
Clean formation of cis-[2c]SO3CF3 was recognized after 5 min by IR
spectroscopy. The product was purified by alumina chromatography
and finally obtained as a dark-red solid. Yield: 117 mg, 75%. Anal.
Calcd for C27H28F3Fe2NO5S: C, 50.10; H, 4.36; N, 2.16. Found: C,
49.86; H, 4.28; N, 2.18. IR (CH2Cl2) ν/cm−1 = 1987 vs (CO), 1819 s
mg, 0.724 mmol) and an excess of propyne. Yield 390 mg, 85%. Anal.
Calcd for C26H26F3Fe2NO5S: C, 49.31; H, 4.14; N, 2.21. Found: C,
49.20; H, 4.22; N, 2.16. IR (CH2Cl2) ν/cm−1 = 2000 vs (CO), 1815 s
1
(μ-CO), 1610 m (NC1C2). H NMR (CDCl3) δ/ppm = 7.48−7.00
1
(μ-CO), 1632 m (NC1C2). H NMR (CDCl3) δ/ppm = 7.42−7.09,
(3 H, C6H3Me2); 5.29, 4.68 (s, 10 H, Cp); 3.88, (s, 3 H, C3 Me); 3.37
(s, 3 H, NMe); 2.49, 1.99 (s, 6 H, C6H3Me2); 2.10 (s, 3 H, C2 Me).
Synthesis of [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C3(Me)C2(CH2Ph)-
C1N(Me)(Xyl)}]Br, [5]Br, and Isomerization Reaction (Chart
5). The synthesis of [5]Br was carried out by a procedure analogous
6.92 (m, 3 H, C6H3Me2); 5.39, 5.28, 5.16, 4.72 (s, 10 H, Cp); 5.23,
3.99 (s, 1 H, C2H); 4.16, 3.48 (s, 3 H, NMe); 3.99, 3.82 (s, 3 H, C3
Me); 2.53, 2.28, 1.96, 1.75 (s, 6 H, C6H3Me2). E/Z ratio ≅ 2. 13C{1H}
NMR (CDCl3) δ/ppm = 254.6 (μ-CO); 233.4, 230.9 (C1); 211.6,
211.4 (C3); 210.6, 209.7 (CO); 144.8, 141.1 (ipso-C6H3Me2); 134.0,
133.7, 131.6, 129.6, 129.5, 129.3, 129.0 (C6H3Me2); 91.0, 90.7, 87.8,
87.6 (Cp); 53.5, 52.7 (C2); 52.1, 46.0 (NMe); 42.7, 42.3 (C3Me);
17.9, 17.8, 17.7, 17.2 (C6H3Me2).
Chart 5. Structures of E-[5]+ and Z-[5]+
Synthesis of [Fe2Cp2(CO)(μ-CO){μ-η1:η2-C3(Me)C2{C(CH)4}-
C1N(Me)(Xyl)}], 3 (Chart 3). A solution of [2b]SO3CF3 (200 mg,
0.316 mmol) in THF (20 mL) was treated with freshly distilled
cyclopentadiene (CpH; 0.18 mL, 2.1 mmol) and then with NaH (23
mg, 0.96 mmol). The resulting mixture was left stirring for 2 h. The
final mixture was filtered on a short alumina pad using THF as eluent,
Chart 3. Structure of 3
to that described for the synthesis of 3 (reaction time = 24 h). The
final mixture was dried under reduced pressure, and the residue was
dissolved in dichloromethane and quickly chromatographed through a
short alumina pad. After washing with THF, a fraction was collected
using neat MeOH as eluent. The title compound was isolated as
isomeric mixture (E/Z) upon removal of the solvent under reduced
pressure. Full conversion into the Z isomer was achieved by heating a
solution of the mixture in MeOH for 40 min at ca. 50 °C. The final
product was purified by filtration on a short alumina column and then
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obtained as a crystalline solid upon slow evaporation of the solvent
from the methanol solution.
mmol) in THF (15 mL) were added PhSSPh (320 mg, 1.47 mmol)
and then NaH (23 mg, 0.96 mmol). The resulting mixture was stirred
for 1 h, then filtered on a short alumina pad. Addition of diethyl ether
(50 mL) gave a green precipitate which was isolated and dried under
vacuum. Yield 66 mg, 55%. Anal. Calcd for C25H27Fe2NO2S: C, 58.05;
H, 5.26; N, 2.71. Found: C, 58.13; H, 5.39; N, 2.66. IR (CH2Cl2) ν/
E-[5]Br + Z-[5]Br. Brown solid, from [2b]SO3CF3 (180 mg, 0.284
mmol), PhCH2Br (0.31 mL, 2.6 mmol), and NaH (21 mg, 0.88
mmol). Yield 113 mg, 55%. Anal. Calcd for C32H32BrFe2NO2: C,
58.75; H, 4.93; N, 2.14; Br, 12.21. Found: C, 58.38; H, 4.98; N, 2.26;
Br, 11.89. IR (CH2Cl2) ν/cm−1 = 1986 vs (CO), 1814 s (μ-CO),
1
cm−1 = 1929 vs (CO), 1752 s (μ-CO), 1624 w (C1N). H NMR
1
1603 m (NC1C2), 1586 w (CC). H NMR (CDCl3) δ/ppm =
(CDCl3) δ/ppm = 7.58−6.84 (5 H, Ph); 4.61, 4.39 (s, 10 H, Cp);
3.71 (s, 3 H, C3 Me); 3.51, 3.00 (s, 6 H, NMe2); 1.37 (s, 3 H, C2 Me).
13C{1H} NMR (CDCl3) δ/ppm = 278.4 (μ-CO); 251.6 (C1); 215.7
(CO); 177.0 (C3); 152.6 (ipso-Ph); 144.7−122.8 (Ph); 87.2, 86.5
(Cp); 84.3 (C2); 46.3, 45.8 (NMe2); 43.6 (C3Me); 29.5 (C2Me).
Computational Studies. All the geometries were optimized with
ORCA 4.0.1.2,39 using the B97 functional40 in conjunction with a
triple-ζ quality basis set (def2-TZVP). The dispersion corrections
were taken into account using the Grimme D3-parametrized
correction and the Becke−Jonhson damping to the DFT energy.41
All the structures were confirmed to be local energy minima (no
imaginary frequencies) for intermediate species and saddle points
(one imaginary frequency) for transition states. In some cases, an
unavoidable very low negative frequency is present, correlated to the
rotation of the Cp moiety around the Cp−Fe bond axis.
X-ray Crystallography. Crystal data and collection details for
[2a]SO3CF3 are reported in Table S3. Data were recorded on a
Bruker APEX II diffractometer equipped with a PHOTON100
detector using Mo Kα radiation. Data were corrected for Lorentz
polarization and absorption effects (empirical absorption correction
SADABS).42 The structure was solved by direct methods and refined
by full-matrix least-squares based on all data using F2.43
7.42−7.05 (8 H, Ph + C6H3Me2); 5.49, 5.44, 5.24, 4.99 (s, 10 H, Cp);
2
4.57, 4.47 (d, 2 H, JHH = 19.9 Hz, CH2); 4.10, 3.15 (s, 3 H, NMe);
3.99, 3.87 (s, 3 H, C3 Me); 2.57, 2.32, 2.01, 1.74 (s, 6 H, C6H3Me2).
E/Z ratio = 2. 13C{1H} NMR (CDCl3) δ/ppm = 254.7, 254.4 (μ-
CO); 233.2, 232.5 (C1); 211.5, 211.3, 210.5, 209.6 (CO + C3); 144.6,
140.6 (ipso-C6H3Me2); 131.5−126.9 (Ph + C6H3Me2); 90.9, 90.6,
87.7, 87.4 (Cp); 64.4, 63.8 (C2); 59.0 (CH2); 52.1, 46.1 (NMe); 43.0,
42.5 (C3Me); 18.2, 18.1, 17.5, 17.0 (C6H3Me2).
Z-[5]Br. Light brown solid, yield 70%. Anal. Calcd for
C32H32BrFe2NO2: C, 58.75; H, 4.93; N, 2.14; Br, 12.21. Found: C,
58.41; H, 5.03; N, 2.20; Br, 12.04. IR (CH2Cl2) ν/cm−1 = 1981 vs
(CO), 1818 s (μ-CO), 1613 m (NC1C2), 1587 w (CC). 1H NMR
(CDCl3) δ/ppm = 7.43−7.34 (8 H, Ph + C6H3Me2); 5.49, 4.99 (s, 10
H, Cp); 4.62, 4.58 (m, 2 H, CH2); 3.99 (s, 3 H, C3 Me); 3.15 (s, 3 H,
NMe); 2.57, 2.01 (s, 6 H, C6H3Me2). 13C{1H} NMR (dmso-d6) δ/
ppm = 254.8 (μ-CO); 229.5 (C1); 212.6 (CO); 206.8 (C3); 162.6
(ipso-Ph); 141.1 (ipso-C6H3Me2); 138.3, 134.3, 130.0, 129.8, 129.4,
128.0, 127.2 (Ph + C6H3Me2); 92.4, 88.8 (Cp); 66.6 (C2); 58.4
(CH2); 50.9 (NMe); 38.2 (C3Me); 18.2 (C6H3Me2).
Reaction of [2c]SO3CF3 with PhSSPh/NaH: Synthesis of
[Fe2Cp2(SPh)(CO)(μ-CO){μ-C1N(Me)(Xyl)}], 6 (Chart 6). To a
Chart 6. Structure of 6
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00448.
DFT-optimized geometries of compounds; IR and NMR
spectra (PDF)
Computed Cartesian coordinates of all of the com-
pounds reported in this study (XYZ)
stirred solution of cis-[2c]SO3CF3 (120 mg, 0.185 mmol) in
tetrahydrofuran (15 mL) were added PhSSPh (295 mg, 1.35
mmol) and then NaH (23 mg, 0.96 mmol). The resulting mixture
was stirred overnight. Hence, the volatile materials were removed
under vacuum. The residue was dissolved in CH2Cl2 and filtered
through alumina. Compound 6 was recovered as a red microcrystal-
line powder upon removal of the solvent. Yield 70 mg, 68%. Anal.
Calcd for C28H27Fe2NO2S: C, 60.78; H, 4.92; N, 2.53. Found: C,
60.63; H, 5.01; N, 2.49. IR (CH2Cl2) ν/cm−1 = 1969 vs (CO), 1791 s
Accession Codes
CCDC 1849098 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
(μ-CO), 1512 m (C1N). H NMR (CDCl3) δ/ppm = 7.49−7.00 (5
AUTHOR INFORMATION
H, Ph + C6H3Me2); 4.68, 4.26 (s, 10 H, Cp); 4.64 (s, 3 H, NMe);
2.71, 2.25 (s, 6 H, C6H3Me2). 13C{1H} NMR (CDCl3) δ/ppm =
338.5 (C1); 265.0 (μ-CO); 213.8 (CO); 148.5 (ipso-C6H3Me2);
137.0−123.2 (Ph + C6H3Me2); 87.5, 86.1 (Cp); 51.2 (NMe); 18.5,
17.7 (C6H3Me2).
Reaction of [2d]SO3CF3 with PhSSPh/NaH: Synthesis of
[Fe2Cp2(CO)(μ-CO){μ-η1:η2-C3(Me)C2(Me)(SPh)C1N(Me)2}], 7
(Chart 7). To a stirred solution of [2d]SO3CF3 (130 mg, 0.233
■
Corresponding Authors
*E-mail: gianluca.ciancaleoni@unipi.it.
*E-mail: fabio.marchetti1974@unipi.it.
ORCID
Gianluca Ciancaleoni: 0000-0001-5113-2351
Stefano Zacchini: 0000-0003-0739-0518
Valerio Zanotti: 0000-0003-4190-7218
Fabio Marchetti: 0000-0002-3683-8708
Chart 7. Structure of 7
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Universities of Bologna and Pisa for financial
support.
K
DOI: 10.1021/acs.organomet.8b00448
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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