A Novel Bulky Heteroaromatic-Substituted Methanide Mimicking NacNac: Bis(4,6-tert-butylbenzoxazol-2-yl)methanide in s-Block Metal Coordination

Article abstract of DOI:10.1002/chem.201702378

A novel bulky bis(4,6-tBu-benzoxazol-2-yl)methane ligand was synthesized in a straightforward three-step synthesis. The corresponding complexes [Li{(4,6-tBu-NCOC₆H₂)₂CH}THF], [K{η⁵-(4,6-tBu-NCOC₆H

Full text of DOI:10.1002/chem.201702378

DOI: 10.1002/chem.201702378
Full Paper
&
Metal–Ligand Complexes
A Novel Bulky Heteroaromatic-Substituted Methanide Mimicking
NacNac: Bis(4,6-tert-butylbenzoxazol-2-yl)methanide in s-Block
Metal Coordination**
Ingo Koehne, Sebastian Bachmann, Thomas Niklas, Regine Herbst-Irmer, and
Dietmar Stalke*[a]
Dedicated to Professor Dieter Fenske on the occasion of his 75th birthday
Abstract: A novel bulky bis(4,6-tBu-benzoxazol-2-yl)methane
ligand was synthesized in a straightforward three-step syn-
um metal or KC8 led to the formation of the homoleptic
compound [Mg{(4,6-tBu-NCOC6H2)2CH}2]. All compounds
were fully characterized. Their solid-state structures as well
as their behavior in solution, which was analyzed with the
help of advanced NMR spectroscopic techniques, are dis-
cussed in detail.
thesis.
The
corresponding
complexes
[Li{(4,6-tBu-
NCOC6H2)2CH}THF],
[K{h5-(4,6-tBu-NCOC6H2)2CH}] ,
and
1
[MgCl{(4,6-tBu-NCOC6H2)2CH}(THF)2] were obtained upon
metalation with alkaline or alkaline-earth-metal reagents. Re-
duction of [MgCl{(4,6-tBu-NCOC6H2)2CH}(THF)2] with potassi-
Introduction
a coordinated metal ion and thus avoids oligomerization or
electrophilic attack.[9]
Since the introduction of the first transition-metal complexes
of the versatile b-diketiminate (NacNac) ligand in 1968 this
ligand platform has gained more-and-more popularity.[1–3] Due
to the easy modification of the electronic and steric properties
of NacNac by simply varying the substituents at the imine moi-
eties, a variety of differently substituted derivatives can be syn-
To further increase the bulkiness at the ligand periphery of
the NacNac system, tBu groups were successfully introduced
by Budzelaar et al.[10] Furthermore, supersized imine residues
containing N-terphenyl substituents could be generated.[11]
With regards to modification at the bridging moiety, it was
shown that a coordinating NacNac ligand cleanly reacts with
diphenylketene to yield a tripodal ligated diimine-enolate com-
plex.[12]
Consequently, in the last two decades a tremendous variety
of b-diketiminate-derived main-group compounds have been
synthesized and proven to be as catalytically active as transi-
tion-metal catalysts.[13–15] For example, with the alkaline-earth
metals (Mg, Ca, Sr, and Ba) these complexes range from alkyl-
to mono- and dimeric hydrido- and halido-substituted com-
pounds. Remarkably, starting from this ligand platform it was
also possible to access dimeric low-oxidation-state magnesi-
um(I) complexes, which turned out to be versatile two-center
two-electron reducing agents.[16–23]
Scheme 1. Synthesis of aryl-substituted NacNacH ligands.
thesized (Scheme 1).[4,5] In this context, the Dipp (diisopropyl-
phenyl) substituted NacNac derivative in particular has been
one of the most-studied ligand systems since its first synthesis
in 1997.[6–8] The phenyl groups within this system display an
almost perpendicular orientation with respect to the residual
imine ligand backbone, which offers unique shielding towards
This rising interest also initiated research into other promis-
ing ligand platforms that mimic the chelating ability of the
ubiquitous NacNac ligand, in particular the feature that two
imine nitrogen atoms function as Lewis donors to a coordinat-
ing metal ion to form a six-membered metallaheterocycle. Ex-
amples are the bis(2-pyridyl)methane ligand system and its N,
P, and As bridged derivatives with benzo-fused imine moieties
(Scheme 2, left).[24–33] Furthermore, a new ligand class arose
when the 2-pyridyl residues were exchanged for benzannulat-
ed oxazoline sidearms. Despite the fact that bis(heterocyclo)-
methane ligands (L) can act as both neutral (LH) and mono-
anionic (Lꢀ) chelates suitable for transition-metal chemistry,
[a] I. Koehne, Dr. S. Bachmann, Dr. T. Niklas, Dr. R. Herbst-Irmer,
Prof. Dr. D. Stalke
Universitꢀt Gçttingen
Institut fꢁr Anorganische Chemie
Tammannstraße 4, 37077 Gçttingen (Germany)
E-mail: dstalke@chemie.uni-goettingen.de
Homepage: www.uni-goettingen.de/de/53448.html
[**] NacNac=b-diketiminate.
Supporting information and the ORCID number(s) for the author(s) of this
article can be found under https://doi.org/10.1002/chem.201702378.
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quent hydration of pure 1 with H2 (1.5 bar) and a heterogene-
ous hydration catalyst (Pd/C) gave 3,5-di-tert-butyl-2-amino-
phenol (2) in an excellent yield (90%) within 24 h. Again, the
IR spectrum of 2 showed characteristic vibrational bands for
the symmetric (n˜ =3316 cmꢀ1) and asymmetric (n˜ =3415 cmꢀ1)
NH2 stretch. Compound 2 (2 equiv) was reacted with the C3
linker ethyl-bisimidate dihydrochloride (1 equiv), whereupon a
double cyclocondensation afforded the desired ligand bis(4,6-
tBu-benzoxazol-2-yl)methane (3) in sufficient yield (18–21%,
Scheme 3, bottom).
Scheme 2. NacNac related bis(heterocyclo)methane, -amine, -phosphane,
and -arsine ligand systems.
they have attracted little attention in coordination chemistry
so far.[34,35]
Nevertheless, our group showed that bis(benzoxazol-2-yl)-
and bis(benzothiazol-2-yl)methanides are versatile platforms
for Group 1 and 13 metal complexes.[36,37] This chemistry was
extended to the corresponding N-bridged derivatives[38] and,
most recently, to benzimidazol- and asymmetrically substituted
bis(heterocyclo)methane species (Scheme 2, right).[39] All the
abovementioned methanide or amide ligand systems with H,
Me, or OMe substituents at the C4 position only display limited
steric demand in close proximity to the coordination pocket.
Herein, we present our successful effort to introduce bulky tBu
Compound 3 crystallizes in the monoclinic space group
P21/n and contains one molecule in the asymmetric unit
(Figure 1). The whole molecule shows positional disorder
substituents to
a bis(benzoxazol-2-yl)methane system to
supply enhanced steric shielding and enable s-block metal-ion
coordination.
Figure 1. Molecular structure of 3. Anisotropic displacement parameters are
depicted at the 50% probability level. Hydrogen atoms are omitted for clari-
ty, except for those at the bridging methylene position. See Tables 1 and 4
for structural data.
Results and Discussion
To obtain a bis(benzoxazol-2-yl)methane derivative that pro-
vides analogous steric demand to the NacNac five-membered
coordination ring, a synthetic route to the corresponding 2-
aminophenol derivative had to be established; with the 2-ami-
nophenol derivative in hand, the desired ligand species should
be easily accessible through a double cyclocondensation reac-
tion with a suitable C3-linker unit according to literature proce-
dures.[37,40] With this in mind, and to cope with the unselective
nature of the common nitration reaction of aromatic com-
pounds with nitric acid, the synthetic protocol commenced
from symmetrically substituted 3,5-di-tert-butylphenol, which
features two equivalent ortho positions and a sterically hin-
dered para position. Addition of nitric acid (1 equiv) to the
starting material gave the corresponding mono-ortho-substi-
tuted derivative 3,5-di-tert-butyl-2-nitrophenol (1) in an appre-
ciable yield of 45% (Scheme 3, top).[1] The IR spectrum of 1
shows characteristic vibrational bands for the symmetric (n˜ =
1366 cmꢀ1) and asymmetric (n˜ =1518 cmꢀ1) NO2 stretch. Subse-
Table 1. Selected angles [8] and average bond lengths [ꢁ] for I, II, and 3.
Compound
CipsoꢀC(1)
CipsoꢀN
Cipso-C(1)-Cipso
I
II
3
1.489(2)
1.487(2)
1.495(2)
1.286(2)
1.291(2)
1.289(2)
120.2(3)
110.8(1)
111.2(1)
about two positions. Some general trends among the related
bis(benzoxazol-2-yl)methane derivatives (NCOC6H4)2CH2 (I)[36]
and (4-MeNCOC6H3)2CH2 (II)[37] are discussed below (Table 1).
In all three compounds the corresponding averaged Cipso
ꢀ
C(1) and CipsoꢀN bond lengths are within the same range and
show no significant deviation as the steric demand of the sub-
stituents increases. However, the Cipso-C(1)-Cipso angle is signifi-
cantly narrowed from 120.2(3)8 in unsubstituted I to 110.8(1)8
in 4-methyl-substituted compound II. This strong deviation
might also be caused by the formation of a 3D network of Cꢀ
H···N hydrogen bonds in the solid state of II. In 4,6-tert-butyl-
substituted compound 3 the parent Cipso-C(1)-Cipso angle is
slightly increased to 111.2(1)8. The two benzoxazole moieties in
I, II, and 3 display a twisted orientation due to the distorted
tetrahedral coordination around the bridging C(1) atom. In I
and II one heterocycle stays almost in plane with the C3-linker
unit, whereas the other shows almost perpendicular torsion
with respect to the linker. In contrast, 3 shows a notably stron-
ger twisted orientation due to its greater steric requirements.
Scheme 3. Synthesis of the aminophenol derivative 2 (top) and bis(4,6-tBu-
benzoxazol-2-yl)methane (3) (bottom).
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In 3 both N-Cipso-C(1)-Cipso torsion angles display similar torsion
out of the C3-linker unit plane (average=131.68). Notably, the
coordination pocket in I and II in the solid state comprises the
ed by the 4-tBu substituents of the ligand periphery, which
prevents it from being tetrahedrally coordinated by inclusion
of a second THF donor base. Furthermore, the steric repulsion
of the tBu substituents forces the lithium cation 0.696(7) ꢁ out
of the plan of the chelating C3N2 ring. To the best of our
knowledge 4 is the first NacNac-related bis(benzoxazol-2-yl)-
methanide–lithium complex that displays trigonal pyramidal
coordination and is reminiscent of the related three-coordinate
trigonal planar lithium complexes [Li(L)(Dipp2NacNac][8] (L=
Et2O or THF). The reported averaged LiꢀN bond lengths of
these complexes [1.915(4) (L=Et2O) and 1.958(5) ꢁ (L=THF)]
are in good agreement with the average distance in 4
(1.951(6) ꢁ), which most closely matches the THF-solvated
complex. The same is true for the corresponding N-Li-N bite
angles, though in this case the value of the Et2O-solvated com-
plex (99.9(2)8) is most similar to 4 (99.0(3)8). The values for the
Cipso-C(1)-Cipso bridging moiety and the ring nitrogen atoms,
whereas in 3 the oxygen atoms are twisted inwards and
occupy the position of the donor atoms, which minimizes
steric repulsion between the neighboring tBu groups.
When 3 was treated overnight with tBuLi at ꢀ508C the pre-
cursor complex 4 was obtained in good yield (74%, Scheme 4).
Cipso-C(1)-Cipso angle in the ligand periphery of the NacNac com-
plexes are very similar (129.5(2)8 and 128.6(3)8), whereas in 4
this angle experiences a considerable reduction to 121.7(3)8.
Just like in 4, the lithium cation in the related complex [Li-
(Dipp2PhNacNac)(Et2O)],[41] which bears phenyl rather than
methyl groups on the ligand backbone, is also reported to
show a slight dislocation of the metal ion from the plane of
the ligand framework. Nevertheless, the metal ion was still
considered to be in a trigonal planar geometry. Furthermore,
even the related four-coordinate bis(pyridyl)methanide com-
plex [Li{(2-NC5H4)2CH}(THF)2][26] and its higher homologue the
bis(pyridyl)phosphide complex [Li{(2-NC5H4)2P}(THF)2][24] show
comparable structural features to 4.
Scheme 4. Synthesis of [Li{(4,6-tBu-NCOC6H2)2CH}THF] (4).
Lithium species 4 crystallizes in the monoclinic space group
C2/c and contains one molecule in the asymmetric unit.
(Figure 2 and Table 2). The central lithium cation adopts trigo-
nal pyramidal geometry from coordination by two ring nitro-
gen atoms of the parent monoanionic methanide ligand and
one oxygen atom of a THF molecule. The hard metal coordi-
nates with the nitrogen atoms despite the steric strain provid-
ed by the cross-ligand tBu substituents. Additionally, lithium is
lifted 0.629(6) ꢁ from the plane defined by N(1)-O(3)-N(2). In-
terestingly, the fourth coordination site at the Li+ ion is shield-
Reaction of the parent ligand system 3 with potassium hy-
dride in THF gives the corresponding polymeric precursor 5 in
excellent yield (80%, Scheme 5). Complex 5 crystallizes in the
Scheme 5. Synthesis of [K{h5-(4,6-tBu-NCOC6H2)2CH}] (5).
1
Figure 2. Molecular structure of 4. Anisotropic displacement parameters are
depicted at the 50% probability level. Hydrogen atoms are omitted for clari-
ty, except for that at the bridging methylene position. See Tables 2 and 4 for
structural data.
ꢀ
triclinic space group P1. The asymmetric unit contains one
ligand molecule and two potassium cations, which both lie on
an inversion center (see the Supporting Information). The
bond lengths and angles determined for this structure suffer
from a certain unreliability due to the formation of small acicu-
lar crystals with decreased scattering abilities, which cause low
data resolution. Hence, no detailed discussion of these param-
eters is attempted. Only the unambiguous data for the N-M-N
bite angle and the dislocation of the potassium cation from
the chelating C3N2 plane are discussed below.
Table 2. Selected average bond lengths [ꢁ] and angles [8] for 4–6.
Compound NꢀM
N-M-N
C
ipso-C(1)-Cipso MꢀC3N2 plane distance
4
5
6
1.951(6)
–
2.1786(2), 99.83(6) 128.9(2)
2.1602(2)
99.0(3) 121.7(3)
65.99(2) 125.5(6)
0.696(7)
2.311(7)
0.010(2)
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Figure 3. Excerpt from the molecular structure of 5, which forms infinite
linear coordination-polymer strands. Anisotropic displacement parameters
are depicted at the 50% probability level. Hydrogen atoms are omitted for
clarity, except for those at the bridging methylene positions. See Tables 2
and 4 for structural data.
Figure 4. Molecular structure of 6. Anisotropic displacement parameters are
depicted at the 50% probability level. The co-crystalized pentane molecule
and hydrogen atoms, except for that at the bridging methylene position,
are omitted for clarity. See Tables 2 and 4 for structural data.
The central potassium cation is sandwiched by h5 coordina-
tion to two C3N2 planes, which forms infinite linear strands
(Figure 3). Again, the steric demand of the tBu substituents in-
hibits coordination of THF donor molecules to the metal ion.
Relative to the structures of 4 and 6 the N-M-N bite angle in 5
is strongly reduced to realize the favored h5 coordination of
the soft potassium ion to the soft, undirected p density of the
deprotonated ligand molecules.[42,43] Nevertheless, the average
dislocation of the metal ions from the C3N2 plane (2.311(7) ꢁ)
and the average N-M-N bite angle (65.99(2)8) are in good
agreement with the values found for a related dimeric NacNac-
derived complex with an average C3N2 plane distance of
2.29(1) ꢁ and mean bite angle of 65.7(2)8.[44] In contrast, the
dislocation found in 5 falls at the short end of the range for
other non-NacNac related compounds that display potassium–
p-system interactions. For example, the h6- and h5-bound spe-
magnesium ion by two chelating ring nitrogen atoms and a
chloride ligand forming the equatorial plane and two THF
donor molecules at the apical positions (Figure 4). The Mg2+
cation shows an in-planar arrangement to the chelating C3N2
moiety (a dislocation of only 0.010(2) ꢁ) and fits perfectly into
the provided coordination pocket. The N-M-N bite angle is not
significantly changed relative to 4, whereas the corresponding
NꢀM bond lengths and Cipso-C(1)-Cipso angle are increased rela-
tive to 4 (2.1786(2) and 2.1602(2) ꢁ vs 1.951(6) ꢁ; 128.9(2)8 vs
121.7(3)8), which is in good agreement with the elevated ionic
radius of the Mg2+ cation. Complex 6 is the first example of a
NacNac-related bis(benzoxazol-2-yl)methanide magnesium
complex with a trigonal bipyramidal geometry. Comparable
trigonal bipyramidal NacNac-based magnesium complexes,
such as the dinuclear [Mg2({N(SiMe3)C(tBu)C(H)}2{2,3-
C4H2N2})Br2(THF)4][44] and the tripodal species [({k3-N,N’,O-
(DippNCMe)2(OCCPh2)CH}Mg{m-I})2] and [({k3-N,N’,O-(DepNC-
[45]
[46]
cies [K(PhCH2)(PMDETA)]
and [K(C5Me5)·2Py]
(PMDETA=
1
1
(Me2NCH2CH2)2NMe), Py=pyridyl) also adopt a polymeric struc-
ture in the solid state but have significantly longer potassium–
p-plane distances (3.150(2) and 2.79(1) ꢁ, respectively).
Me)2(OCCPh2)CH}Mg)2(m-k2:k2-S2O4)]
(Dep=2,6-diethylphen-
yl),[22] exhibit values from 2.155–2.132 ꢁ, thus have slightly
shorter MgꢀN bonds than in 6. Interestingly, in the six-coordi-
nate complex [Mg{(pz*)3C}2] (pz*=3,5-dimethylpyrazolyl), the
MgꢀN bond length is only slightly longer (2.197 ꢁ) than in 6.[48]
Reduction of 6 was attempted to synthesize a potentially di-
meric, low-oxidation-state MgI–MgI complex. Unfortunately, re-
actions with potassium metal or potassium graphite as the re-
ducing agent at room temperature or 08C gave the homolep-
tic Mg2+ complex 7 in good yield (76–80%, Scheme 7). The
When 3 (1 equiv) was suspended with the Hauser-type base
(iPr)2NMgCl (2 equiv) the corresponding heteroleptic magnesi-
um chloride complex 6 was obtained in a moderate yield
(48%) within 18 h (Scheme 6). Complex 6 crystallizes in the
Scheme 6. Synthesis of [MgCl{(4,6-tBu-NCOC6H2)2CH}(THF)2] (6).
monoclinic space group P21/n. It contains one molecule and a
co-crystallized pentane molecule in the asymmetric unit. An
Addison parameter[47] of 0.74 undoubtedly indicates a five-co-
ordinate trigonal bipyramidal coordination around the central
Scheme 7. Synthesis of [Mg{(4,6-tBu-NCOC6H2)2CH}2] (7).
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formation of complex 7 could be the result of a Schlenk equi-
librium redistribution of a heteroleptic complex towards its ho-
moleptic equivalent or a transient Mg+ complex that is prone
to disproportionation to Mg0 and Mg2+ and the resulting
excess of negatively charged ligand molecules. Similar observa-
tions were made by Bonyhady et al. during their attempts to
reduce
the
related
NacNac
precursor
complex
[MgI(PhNacNac)(Et2O)] with sodium or potassium metal.[21] Be-
cause of the high solubility of 7 no single crystals suitable for
X-ray diffraction experiments have been obtained as yet. Nev-
ertheless, formation of the homoleptic compound 7 was con-
firmed by LIFDI MS (liquid-injection field-desorption ionization
mass spectrometry), which undoubtedly displayed the isotopic
pattern and the mass of an Mg2+ ion chelated by two ligand
molecules.
The presence of N- and O-donor sites within the deprotonat-
ed bis(4,6-tBu-benzoxazol-2-yl)methanide ligand 3 means that
two symmetrical coordination modes are feasible: an exclusive-
ly N-bound (7) and a solely O-bound species (7a) (Figure 5).
1
Figure 6. Excerpt from the H-NOESY spectrum of 7 in C6D6 (*). For clarity,
only the relevant cross-peaks are shown.
DOSY investigations
1
The results of the H-DOSY external calibration curve (ECC) mo-
lecular weight (MW) estimation[49–51] used to clarify the struc-
tures of 4–7 in solution are discussed in detail below. Previous
studies showed that for most organometallic compounds the
dissipated spheres and ellipsoids (DSE) calibration curve is the
most suitable for an accurate estimation.[52] Therefore, only
values from the DSE and, for comparison, from the merge cali-
bration curve are considered (Table 3). The MW of 4 in solution
was estimated to be 575 (DSE) and 643 gmolꢀ1 (merge).
Within the error range these values either fit a three-
(553 gmolꢀ1) or four-coordinate (625 gmolꢀ1) species with one
or two attached THF molecules, respectively, which indicates
the presence of a dynamic exchange process. Compared with
Figure 5. Comparison of N-bound (7) and O-bound (7a) coordination
modes.
1
Due to the presence of only one set of signals in the H NMR
spectrum of 7 a third feasible coordination motif comprising
one N- and one O-bound ligand can be directly ruled out.
From a coordination chemistry point of view a solely N-bound
complex should be less favored due to the steric crowding
caused by the C(4)ꢀtBu groups occupying space around the
Mg2+ ion. Hence, advanced NMR techniques were applied to
determine the correct coordination mode.
1
the H NMR spectrum of 4, which shows one THF molecule at-
tached to the complex, it is most likely that the three-coordi-
nate structure observed in the solid state is retained in solu-
1
7
tion. Furthermore, superimposition of the H- and Li-DOSY ex-
periments for 4 shows that all complex-related signals have
First, the N-binding motif of deprotonated 3 to the Mg2+
ion in solution was evaluated by using the dN value deter-
mined from a 15N-1H HMBC experiment. The resonance at dN =
ꢀ213 ppm is close to the shift that 6 exhibits (dN =ꢀ217 ppm)
in the N-binding mode, which was confirmed by single-crystal
X-ray diffraction. This motif is further corroborated by NOESY
experiments. The cross-peak between the C(4)ꢀtBu substitu-
ents and the methylene bridging moiety of the second ligand
molecule is much stronger than between the C(6)ꢀtBu groups
and that bridging motif (Figure 6). The correlated spatial prox-
imity can only be rationalized by the N-binding mode of 7 (see
the structural models in the Supporting Information). Addition-
ally, we conducted a preliminary study which showed that
one-bond residual dipolar couplings (RDCs) between carbon
and hydrogen atoms can also be employed to solve this ste-
reochemical problem (see the Supporting Information).
Table 3. 1H-DOSY ECC MW estimation of 4–7 in C6D6 at r.t.
[a]
Aggregate
x
MWtheo
MWDSE
MWmerge
[gmolꢀ1]
(MWdif [%])
[gmolꢀ1]
[gmolꢀ1]
[b]
(MWdif [%])
[Li(tBuBox)(THF)x][c] (4)
[K(tBuBox)(C6H6)x] (5)
[MgCl(tBuBox)(THF)x] (6)
[Mg(tBuBox)2] (7)
0
1
2
0
1
2
0
1
2
–
481
553
625
513
591
669
533
606
678
972
575 (ꢀ16)
575 (ꢀ4)
575 (9)
490 (ꢀ8)
490 (6)
490 (20)
609 (ꢀ12)
609 (0)
609 (11)
857 (14)
643 (25)
643 (ꢀ14)
643 (ꢀ3)
620 (ꢀ17)
620 (ꢀ5)
620 (8)
685 (ꢀ22)
685 (ꢀ12)
685 (ꢀ1)
993 (ꢀ2)
[a] MWtheo =Theoretical MW. [b] Percentage difference between the theo-
retical and calculated MW value. [c] tBuBox=bis(4,6-tBu-benzoxazol-2-yl)-
methanide.
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proton signal of the deuterated solvent.[56] Deuterated solvents
were dried over activated molecular sieves (3 ꢁ) then stored in an
argon drybox. Elemental analyses (CHN) were carried out with a
Vario EL3 instrument at the Mikroanalytisches Labor, Institut fꢂr
Anorganische Chemie, University of Gçttingen. All LIFDI MS spectra
were performed with a Jeol AccuTOF spectrometer. NOESY spectra
were recorded with a Bruker Avance III 400 MHz instrument with a
relaxation delay of 1.5 s and 0.5 s mixing time. 2048ꢃ512 data
points were sampled over a spectral width of d=12 ppm (number
of scans=4). HMBC spectra were recorded with a Bruker Avance III
400 MHz instrument by using a relaxation delay of 2 s. 2048ꢃ256
data points were sampled over a spectral width of d=12 ppm in
F2 and d=200 ppm in F1 (number of scans=2).
one diffusion coefficient, which indicates the presence of a
strong contact ion pair (see the Supporting Information).
The MW of potassium complex 5 was estimated to be 490
(DSE) and 620 gmolꢀ1 (merge), which excludes retention of the
polymeric structure in solution. More likely, monomeric struc-
tures are formed with one additional benzene molecule to sat-
isfy the coordination sphere of the cation; a theoretical MW of
591 gmolꢀ1 calculated for this scenario provided the best fit to
the data.
The MW of magnesium complex 6 in solution was estimated
to be 609 (DSE) and 685 gmolꢀ1 (merge), which fits to a four-
or five-coordinate species with one or two attached THF mole-
cules, respectively. Again, these findings show that the THF li-
gands are subject to rapid dynamic exchange processes, which
exacerbates a reliable conclusion about the molecular shape of
6 in solution. The MW of the homoleptic magnesium complex
7 in solution was estimated to be 857 (DSE) and 993 gmolꢀ1
(merge). The latter estimation, in particular, is in good agree-
ment with the theoretical MW of 972 gmolꢀ1 for 7, and also
supports the LIFDI MS results.
Synthesis
3,5-Di-tert-butyl-2-nitrophenol (1): 3,5-Di-tert-butyl-phenol (10.0 g,
48.5 mmol, 1.00 equiv) was dissolved in ethyl acetate (800 mL).
Under vigorous stirring, nitric acid (1:2 HNO3/H2SO4, 9.32 mL,
48.5 mmol, 1.00 equiv) was slowly added portionwise
(0.20 mLminꢀ1). After complete addition, the mixture was stirred
for an additional 15 minutes then transferred to a separation
funnel. The organic phase was washed with H2O and brine until an
almost neutral pH of the aqueous phase was achieved. The organic
phase was separated, dried over anhydrous MgSO4, and the sol-
vent was removed under reduced pressure. The reddish-orange
residue was recrystallized several times from pentane to give 1
(5.44 g, 45%) as a yellow solid. 1H NMR (300 MHz, [D6]acetone,
Conclusion
The family of bis(benzoxazol-2-yl)methane ligands was extend-
ed by introducing the new bulky bis(4,6-tBu-benzoxazol-2-yl)-
methane ligand 3. Concerted deprotonation–metalation reac-
tions were carried out on 3 with typical alkali-metal reagents
such as tBuLi or KH, and the corresponding precursor com-
plexes 4 and 5 were formed. Furthermore, reaction of 3 with
the Hauser-type base (iPr)2NMgCl yielded trigonal bipyramidal
magnesium chloride species 6. All of the compounds show in-
teresting structural features in the solid state and in solution,
studied by single-crystal X-ray diffraction and DOSY NMR spec-
troscopy, respectively. Attempts to obtain a low-oxidation-
state, potentially dimeric MgI–MgI species by reduction of 6
with potassium metal or KC8 afforded the homoleptic Mg2+
complex 7, which was assumed to form by disproportionation
or a Schlenk equilibrium type redistribution. Despite the lack
of X-ray data for 7, its structure was successfully determined
by advanced NMR spectroscopic techniques (NOESY, clean-in-
phase HSQC, and DOSY) and LIFDI MS. Our efforts to expand
the class of sterically demanding, heteroaromatic-substituted
methanides that mimic the ubiquitous NacNac ligand system
and their application as platforms for the metal-mediated acti-
vation of small molecules are ongoing.
4
258C): d=9.12 (s, 1H; OH), 7.12 (d, J(H,H)=1.9 Hz, 1H; H4), 7.01
(d, 4J(H,H)=1.9 Hz, 1H; H6), 1.37 (s, 9H; 3-C(CH3)3), 1.30 ppm (s,
9H; 5-C(CH3)3); 13C NMR (75 MHz, [D6]acetone, 258C): d=154.3 (1C;
C5), 149.6 (1C; C1), 141.7 (1C; C3), 138.9 (1C; C2), 116.6 (1C; C4),
112.8 (1C, C6), 36.6 (1C; 3-C(CH3)3), 35.8 (1C; 5-C(CH3)3), 31.5 (1C;
3-C(CH3)3), 31.2 ppm (1C; 5-C(CH3)3); IR (ATR): n˜ =3423, 2956, 2908,
2872, 1592, 1518 (NO2 asymmetric stretch (as.)), 1366 (NO2 sym-
metric stretch (s.)), 1291, 978, 860, 666 cmꢀ1; MS (EI, 70 eV): m/z
(%): 251.3 (30) [M+], 236.2 (100) [MꢀOH+H+]; elemental analysis
calcd (%) for C14H21NO3: C 66.91, H 8.42, N 5.57; found: C 65.67, H
8.37, N 5.39.
3,5-Di-tert-butyl-2-aminophenol (2): In a 100 mL pressure flask
with screw cap and Young valve, 1 (4.00 g, 15.9 mmol, 1.00 equiv)
was dissolved in MeOH (20 mL) and Pd/C (10%, 180 mg,
1.59 mmol, 0.10 equiv) was added under cooling with a liquid N2
bath. The resulting suspension was degassed via three freeze–
pump–thaw cycles, and then a H2 atmosphere (1.5 bar) was estab-
lished above the frozen suspension and the mixture was stirred at
RT for 24 h. All solid material was removed by filtration via cannula
under an argon atmosphere and then the filtrate was evaporated
under reduced pressure. The crude product was purified by recrys-
tallization from CHCl3 to give 2 (3.17 g, 90%) as a white fluffy solid.
1H NMR (300 MHz, [D6]acetone, 258C): d=7.86 (s, 1H; OH), 6.81 (d,
4
4J(H,H)=2.2 Hz, 1H; H4), 6.74 (d, J(H,H)=2.2 Hz, 1H; H6), 4.02 (s,
Experimental Section
2H; NH2), 1.41 (s, 9H; 3-C(CH3)3), 1.22 ppm (s, 9H; 5-C(CH3)3);
13C NMR (75 MHz, [D6]acetone, 258C): d=152.9 (1C; C5), 145.2 (1C;
C1), 135.4 (1C; C3), 133.4 (1C; C2), 114.8 (1C; C4), 104.5 (1C; C6),
35.3 (1C; 3-C(CH3)3), 35.2 (1C; 5-C(CH3)3), 32.2 (1C; 3-C(CH3)3),
30.0 ppm (1C; 5-C(CH3)3); IR (ATR): n˜ =3415 (NH2 as.), 3316 (NH2 s.),
2954, 1580, 1418, 1301, 1171, 957, 860, 789 cmꢀ1; MS (EI, 70 eV):
m/z (%): 221.3 (25) [M+], 206.3 (100) [MꢀNH2+]; elemental analysis
calcd (%) for C14H23NO: C 75.97, H 10.47, N 6.33; found: C 74.08, H
10.12, N 6.14.
Procedures
All manipulations were carried out under an inert argon atmos-
phere by using Schlenk techniques[53] or in an argon drybox. All
solvents used for the metalation reactions were distilled from Na
or K before use. Starting materials were purchased commercially
and were used as received, unless stated otherwise. (iPr)2NMgCl
and KC8 were synthesized according to literature procedures.[54,55]
1H- and 13C NMR spectra were recorded with a Bruker Avance 300
or 400 MHz spectrometer and are referenced to the residual
(4,6-tBu-NCOC6H2)2CH2 (3): Compound
2 (8.00 g, 36.1 mmol,
2.00 equiv) and ethylbisimidate dihydrochloride (4.17 g, 18.1 mmol,
&
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6
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Full Paper
1.00 equiv) were dissolved in MeOH (60 mL) and heated at 858C
for 48 h. The mixture was cooled to RT and the solvent was re-
moved under reduced pressure. The residue was extracted with
pentane (3ꢃ30 mL) with the help of a supersonic bath, and then
filtered. The filtrate was evaporated under reduced pressure and
the crude product was recrystallized from EtOH to give 3 as a
white crystalline solid (1.55–1.80 g, 18–21%). 1H NMR (300 MHz,
(2C; C7), 55.3 (1C; Cbridge), 35.9 (2C; 6-C(CH3)3), 35.5 (2C; 4-C(CH3)3),
32.6 (6C; 6-C(CH3)3), 30.9 ppm (6C; 4-C(CH3)3); MS (LIFDI): m/z (%):
551.1 (33) [MꢀTHF+K+], 512.2 (100) [MꢀTHF], 474.2 (42) [4,6-tBu-
NCOC6H2)2CH2]; elemental analysis calcd (%) for C31H41KN2O2: C
72.61, H 8.06, N 5.46; found: C 70.34, H 8.12, N 5.25.
[MgCl{(4,6-tBu-NCOC6H2)2CH}(THF)2] (6): Compound 3 (300 mg,
633 mmol, 1.00 equiv) and (iPr2N)MgCl·(THF)0.65 (283 mg, 1.39 mmol,
2.20 equiv) were suspended in THF (15 mL) and stirred at RT for
18 h. The solvent was removed under reduced pressure and the
residue was extracted with pentane (3ꢃ10 mL) then filtered via
cannula. The filtrate was concentrated to two thirds of its original
volume then stored at ꢀ288C to afford 6 (228 mg, 48%) as color-
less plate-shaped crystals suitable for X-ray diffraction. 1H NMR
4
4
CDCl3, 258C): d=7.35 (d, J(H,H)=1.7 Hz, 2H; H5), 7.28 (d, J(H,H)=
1.7 Hz, 2H; H7), 4.61 (s, 2H; CH2), 1.55 (s, 18H; 4-C(CH3)3), 1.36 ppm
(s, 18H; 6-C(CH3)3); 1H NMR (300 MHz, C6D6, 258C): d=7.40 (d,
4
4J(H,H)=1.7 Hz, 2H; H5), 7.21 (d, J(H,H)=1.7 Hz, 2H; H7), 4.20 (s,
2H; CH2), 1.68 (s, 18H; 4-C(CH3)3), 1.23 ppm (s, 18H; 6-C(CH3)3);
4
1H NMR (300 MHz, [D8]THF, 258C): d=7.42 (d, J(H,H)=1.7 Hz, 2H;
H5), 7.30 (d, 4J(H,H)=1.7 Hz, 2H; H7), 4.66 (s, 2H; CH2), 1.54 (s,
18H; 4-C(CH3)3), 1.35 ppm (s, 18H; 6-C(CH3)3); 13C NMR (75 MHz,
CDCl3, 258C): d=157.7 (2C; C1), 152.0 (2C; C8), 148.5 (2C; C6),
142.6 (2C; C4), 137.2 (2C; C3), 118.2 (2C; C5), 105.2 (2C; C7), 35.6
(2C; 4-C(CH3)3), 35.4 (2C; 6-C(CH3)3), 31.9 (2C; 6-C(CH3)3), 30.5 (2C;
4-C(CH3)3), 29.8 ppm (1C, Cbridge); 13C NMR (75 MHz, C6D6, 258C): d=
158.3 (2C; C1), 152.6 (2C; C8), 148.6 (2C; C6), 142.9 (2C; C4), 137.8
(2C; C3), 118.2 (2C; C5), 105.6 (2C; C7), 35.9 (2C; 4-C(CH3)3), 35.3
(2C; 6-C(CH3)3), 31.8 (2C; 6-C(CH3)3), 30.6 (2C; 4-C(CH3)3), 29.5 ppm
(1C; Cbridge); IR (ATR): n˜ =2956, 1601 (C=N), 1396, 1243, 1153, 993,
853, 777 cmꢀ1; MS (EI, 70 eV): m/z (%): 474.3 (40) [M+], 459.3 (100)
[MꢀCH3+]; elemental analysis calcd (%) for C31H42N2O2: C 78.44, H
8.92, N 5.90; found: C 77.45, H 8.55, N 5.84.
4
(300 MHz, C6D6, 258C): d=7.42 (d, J(H,H)=2.0 Hz, 2H; H7), 7.28 (d,
4J(H,H)=2.0 Hz, 2H; H5), 5.62 (s, 1H; Hbridge), 3.57 (m, 8H;
OCH2CH2), 1.84 (s, 18H; 4-C(CH3)3), 1.27 (s, 18H; 6-C(CH3)3),
1.07 ppm (m, 8H; OCH2CH2); 13C NMR (75 MHz, C6D6, 258C): d=
168.3 (2C; C1), 149.7 (2C; C8), 145.3 (2C; C6), 138.6 (2C; C4), 136.5
(2C; C3), 118.1 (2C; C5), 104.2 (2C; C7), 69.1 (4C; OCH2CH2), 58.6
(1C; Cbridge), 35.5 (2C; 6-C(CH3)3), 34.7 (2C; 4-C(CH3)3), 32.0 (6C; 6-
C(CH3)3), 31.6 (6C; 4-C(CH3)3), 25.1 ppm (4C; OCH2CH2); 15N NMR
(30 MHz, C6D6, 258C): d=ꢀ217 ppm (s); MS (LIFDI): m/z (%): 532.2
(100) [Mꢀ2(THF)], 474.3 (30) [4,6-tBuNCOC6H2)2CH2]; elemental
analysis calcd (%) for C44H69ClMgN2O4: C 70.48, H 9.28, N 3.74;
found: C 69.30, H 8.72, N 3.81.
[Mg{(4,6-tBu-NCOC6H2)2CH}2] (7), Method a: Complex 6 (228 mg,
304 mmol, 1.00 equiv) and KC8 (41.0 mg, 304 mmol, 1.00 equiv) were
suspended in toluene (15 mL) and stirred at RT for 72 h. The sus-
pension was filtered via cannula and the solvent was removed
under reduced pressure to give 7 (118 mg, 80%) as a reddish-
orange solid.
[Li{(4,6-tBu-NCOC6H2)2CH}THF] (4): Compound
3
(300 mg,
632 mmol, 1.00 equiv) was dissolved in THF (15 mL) and cooled to
ꢀ508C. Under stirring, tBuLi (2.05m in hexane, 0.32 mL, 656 mmol,
1.04 equiv) was slowly added dropwise. After complete addition
the mixture was stirred at ꢀ508C for an additional 15 min. The
cooling bath was removed and the solution was stirred at RT for
5 h. The solvent was removed under reduced pressure then the
residue was washed with ice cold pentane (3ꢃ10 mL), filtered via
cannula, and dried under reduced pressure to give 4 (259 mg,
74%) as a pale-yellow solid. Crystals suitable for X-ray diffraction
Method b: Complex 6 and KC8 were suspended in toluene (15 mL)
at 08C then stirred for 72 h, during which time the cooling bath
was allowed to slowly warm to rt. The suspension was filtered via
cannula and the solvent was removed under reduced pressure to
give 7 as a reddish-orange solid.
1
were obtained from a saturated solution in hexane at rt. H NMR
Method c: Complex 6 (300 mg, 412 mmol, 1.00 equiv) was dis-
solved in toluene then stirred over a potassium mirror (56.0 mg,
412 mmol, 1.00 equiv) at RT for 72 h. The suspension was filtered
via cannula and the solvent was removed under reduced pressure
to give 7 (152 mg, 76%) as a reddish-orange solid. 1H NMR
4
(300 MHz, C6D6, 258C): d=7.34 (d, J(H,H)=1.8 Hz, 2H; H7), 7.30 (d,
4J(H,H)=1.8 Hz, 2H; H5), 5.60 (s, 1H; Hbridge), 3.15 (m, 4H;
OCH2CH2), 1.58 (s, 18H; 4-C(CH3)3), 1.34 (s, 18H; 6-C(CH3)3),
0.93 ppm (m, 4H; OCH2CH2); 13C NMR (75 MHz, C6D6, 258C): d=
169.9 (2C; C1), 150.8 (2C; C8), 143.1 (2C; C6), 139.0 (2C; C4), 134.1
(2C; C3), 117.5 (2C; C5), 104.4 (2C; C7), 68.6 (2C; OCH2CH2), 57.3
(1C; Cbridge), 35.1 (2C; 6-C(CH3)3), 35.0 (2C; 4-C(CH3)3), 32.1 (6C; 6-
C(CH3)3), 31.1 (6C; 4-C(CH3)3), 25.1 ppm (2C; OCH2CH2); 7Li NMR
(116 MHz, C6D6, 258C): d=2.76 ppm (s); MS (LIFDI): m/z (%): 480.2
(20) [MꢀTHF], 474.3 (100) [4,6-tBu-NCOC6H2)2CH2]; elemental analy-
sis calcd (%) for C35H49LiN2O3: C 76.06, H 8.94, N 5.07; found: C
75.18, H 8.86, N 5.36.
4
(400 MHz, [D12]cyclohexane, 258C): d=7.05 (d, J(H,H)=2.0 Hz, 4H;
4
H3), 7.01 (d, J(H,H)=2.0 Hz, 4H; H5), 5.38 (s, 2H; Hbridge), 1.25 (s,
36H; 4-C(CH3)3), 1.15 ppm (s, 36H; 6-C(CH3)3); 13C NMR (100 MHz,
[D12]cyclohexane, 258C): d=168.5 (4C; C1), 150.1 (4C; C3), 145.3
(4C; C6), 138.7 (4C; C4), 136.2 (4C; C8), 118.6 (4C; C5), 104.3 (4C;
C7), 60.9 (2C; Cbridge), 35.2 (4C; 6-C(CH3)3), 35.2 (4C; 4-C(CH3)3), 31.9
(12C; 6-C(CH3)3), 30.9 ppm (12C; 4-C(CH3)3); 15N NMR (30 MHz, C6D6,
258C): d=ꢀ213 ppm (s); MS (LIFDI): m/z (%): 970.7 (100) [M]; a
suitable elemental analysis could not be obtained, presumably to
magnesium nitride formation.
[K{h5-(4,6-tBu-NCOC6H2)2CH}]
(5): KH (33.0 mg, 696 mmol,
1.10 equiv) was suspended in THF (10 mL) and (300 mg,
1
3
633 mmol, 1.00 equiv) in THF (5 mL) was added dropwise via sy-
ringe. The mixture was stirred at RT for 18 h. The suspension was
filtered via cannula, and the filtrate was evaporated under reduced
pressure. The residue was washed with pentane (3ꢃ10 mL), fil-
tered, and dried under reduced pressure to give 5 as a white solid
(296 mg, 80%). Recrystallization by evaporation of pentane into a
saturated solution of 5 in THF at RT gave colorless needle-shaped
crystals. 1H NMR (300 MHz, [D8]THF, 258C): d=7.01 (d, 4J(H,H)=
1.7 Hz, 2H; H7), 6.97 (d, 4J(H,H)=1.8 Hz, 2H; H5), 4.56 (s, 1H;
Hbridge), 1.54 (s, 18H; 4-C(CH3)3), 1.32 ppm (s, 18H; 6-C(CH3)3);
13C NMR (75 MHz, [D8]THF, 258C): d=169.0 (2C; C1), 151.2 (2C; C8),
142.7 (2C; C6), 141.3 (2C; C4), 135.4 (2C; C3), 116.1 (2C; C5), 103.2
Crystallographic details
Shock-cooled crystals were selected from a Schlenk flask under an
argon atmosphere by using an X-TEMP2 device.[57–59] The data were
collected with an ImS microfocus source.[60] All data were integrated
with the SAINT program.[61] A multiscan absorption correction
(SADABS),[62] and for 3 and 6 a 3l correction, were applied.[63] The
structures were solved by direct methods (SHELXT)[64] and refined
on F2 by using the full-matrix least-squares method of SHELXL[65]
within the SHELXLE GUI.[66] CCDC 1551829 (3), 1551830 (4),
Chem. Eur. J. 2017, 23, 1 – 10
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7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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[10] P. H. M. Budzelaar, A. B. van Oort, A. G. Orpen, Eur. J. Inorg. Chem. 1998,
1485–1494.
Table 4. Crystallographic data for compounds 3–6.
[11] A. L. Kenward, J. A. Ross, W. E. Piers, M. Parvez, Organometallics 2009,
28, 3625–3628.
3
4
5
6·pentane
[12] F. Basuli, J. C. Huffman, D. J. Mindiola, Inorg. Chem. 2003, 42, 8003–
8010.
[13] P. P. Power, Nature 2010, 463, 171–177.
[14] Z. Yang, M. Zhong, X. Ma, S. De, C. Anusha, P. Parameswaran, H. W.
Roesky, Angew. Chem. Int. Ed. 2015, 54, 10225–10229; Angew. Chem.
2015, 127, 10363–10367.
[15] Z. Yang, M. Zhong, X. Ma, K. Nijesh, S. De, P. Parameswaran, H. W.
Roesky, J. Am. Chem. Soc. 2016, 138, 2548–2551.
[16] S. Harder, J. Brettar, Angew. Chem. Int. Ed. 2006, 45, 3474–3478; Angew.
Chem. 2006, 118, 3554–3558.
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[18] S. P. Green, C. Jones, A. Stasch, Science 2007, 318, 1754–1757.
[19] S. P. Green, C. Jones, A. Stasch, Angew. Chem. Int. Ed. 2008, 47, 9079–
9083; Angew. Chem. 2008, 120, 9219–9223.
[20] S. J. Bonyhady, S. P. Green, C. Jones, S. Nembenna, A. Stasch, Angew.
Chem. Int. Ed. 2009, 48, 2973–2977; Angew. Chem. 2009, 121, 3017–
3021.
formula
MW [gmolꢀ1
T [K]
C31H42N2O2
474.66
100(2)
C35H49LiN2O3 C31H41KN2O2 C44H69ClMgN2O4
]
552.70
180(2)
C2/c
512.76
100(2)
¯
P1
749.77
100(2)
P21/n
space group P21/n
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
18.068(2)
34.402(6)
12.474(2)
15.983(3)
90
103.89(2)
90
9.242(2)
9.733(2)
17.430(3)
86.98(2)
88.01(2)
69.48(2)
1466.1(5)
2
10.775(3)
27.033(7)
14.765(4)
90
90.92(2)
90
7.992(2)
19.848(2)
90
100.75(2)
90
2815.7(8)
4
0.069
b [8]
g [8]
V [ꢁ]3
Z
6658(2)
8
4300(2)
4
m [mmꢀ1
qmax [8]
]
0.069
0.210
0.145
26.062
50265
25.510
49428
23.301
37614
26.103
64634
reflns mea-
sured
reflns unique 5570
6154
4010
8491
[21] S. J. Bonyhady, C. Jones, S. Nembenna, A. Stasch, A. J. Edwards, G. J.
McIntyre, Chem. Eur. J. 2010, 16, 938–955.
[22] A. J. Boutland, I. Pernik, A. Stasch, C. Jones, Chem. Eur. J. 2015, 21,
15749–15758.
[23] M. S. Hill, D. J. Liptrot, C. Weetman, Chem. Soc. Rev. 2016, 45, 972–988.
[24] A. Steiner, D. Stalke, J. Chem. Soc. Chem. Commun. 1993, 444–446.
[25] H. Gornitzka, D. Stalke, Angew. Chem. Int. Ed. Engl. 1994, 33, 693–695;
Angew. Chem. 1994, 106, 695–698.
[26] H. Gornitzka, D. Stalke, Organometallics 1994, 13, 4398–4405.
[27] A. Steiner, D. Stalke, Organometallics 1995, 14, 2422–2429.
[28] A. Steiner, D. Stalke, Angew. Chem. Int. Ed. Engl. 1995, 34, 1752–1755;
Angew. Chem. 1995, 107, 1908–1910.
Rint
0.0564
0.0905
0.1275
0.0410
data/restr./
para.
5570/1781/ 6154/1712/ 4010/712/ 8491/771/546
606
494
393
R1 [I>2s(I)][a] 0.0436
0.0805
0.2347
–
0.328/
ꢀ0.351
0.0859
0.2061
–
0.0438
0.1024
–
0.345/ꢀ0.293
wR2 (all)[b]
0.1055
0.0029(4)
0.230/
ꢀ0.168
e
D1fin [eꢁꢀ3
]
0.378/
ꢀ0.384
[a] R1 =(SjFo jꢀjFc j)/(SjFo j). [b] wR2 =[{Sw(Fo ꢀFc2)2}/{Sw(Fo2)2}]1/2
.
2
1551831 (5), and 1551832 (6·pentane) contain the supplementary
crystallographic data for this paper. These data are provided free
of charge by The Cambridge Crystallographic Data Centre. See
Table 4 for details; additional details about the crystallographic
data can be found in the Supporting Information.
[29] T. Kottke, D. Stalke, Chem. Ber. 1997, 130, 1365–1374.
[30] H. Gornitzka, D. Stalke, Eur. J. Inorg. Chem. 1998, 311–317.
[31] H. Gornitzka, C. Hemmert, G. Bertrand, M. Pfeiffer, D. Stalke, Organome-
tallics 2000, 19, 112–114.
[32] M. Pfeiffer, F. Baier, T. Stey, D. Leusser, D. Stalke, B. Engels, D. Moigno, W.
Kiefer, J. Mol. Model. 2000, 6, 299–311.
[33] M. Pfeiffer, T. Stey, H. Jehle, B. Klꢂpfel, W. Malisch, V. Chandrasekhar, D.
Stalke, Chem. Commun. 2001, 337–338.
Acknowledgements
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2002, 67, 5753–5772.
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Manuscript received: May 24, 2017
Version of record online: && &&, 0000
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FULL PAPER
NacNac mimic: Metalation of the meth-
ylene bridging moiety in a novel bulky
bis(4,6-tBu-benzoxazol-2-yl)methanide
ligand with s-block organometallic re-
agents gave the corresponding N,N-che-
lated metal salts. Structural features in
the solid state (see figure) and in solu-
tion were compared to related com-
plexes derived from the ubiquitous
NacNac ligand.
&
Metal–Ligand Complexes
I. Koehne, S. Bachmann, T. Niklas,
R. Herbst-Irmer, D. Stalke*
&& – &&
A Novel Bulky Heteroaromatic-
Substituted Methanide Mimicking
NacNac: Bis(4,6-tert-butylbenzoxazol-
2-yl)methanide in s-Block Metal
Coordination
&
&
Chem. Eur. J. 2017, 23, 1 – 10
www.chemeurj.org
10
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!