Dioxo-, Oxo-imido-, and Bis-imido-Molybdenum(VI) Complexes with a Bis-phenolate-NHC Ligand

Article abstract of DOI:10.1021/acs.organomet.9b00472

We report the facile synthesis of five rare high-valent Mo(VI)-NHC complexes with a dianionic OCO-pincer benzimidazolylidene ligand (L¹) with the general formula L¹MoO₂ (1), L¹Mo(O)(N^tBu)(THF) (2), L<

Full text of DOI:10.1021/acs.organomet.9b00472

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Dioxo‑, Oxo-imido‑, and Bis-imido-Molybdenum(VI) Complexes with
a Bis-phenolate-NHC Ligand
Marc Baltrun,† Fabian A. Watt,† Roland Schoch,† and Stephan Hohloch*,†
†Paderborn University, Faculty of Science, Department of Chemistry, Warburger Straße 100, 33098 Paderborn, Germany
S
* Supporting Information
ABSTRACT: We report the facile synthesis of five rare high-valent
Mo(VI)-NHC complexes with a dianionic OCO-pincer benzimidazolylidene
ligand (L1) with the general formula L1MoO2 (1), L1Mo(O)(NtBu)(THF)
(2), L1Mo(NtBu)(NHtBu)Cl (3), L1Mo(NtBu)2 (4), and L1 Mo(NMes)2
(5). These complexes represent a complete series of high-valent
molybdenum(VI) complexes, obtained by consecutive exchange of the oxo
for imido ligands from the dioxo complex (1) to the bis-imido complexes (4
and 5). The complexes can be synthesized in high yields following a simple
deprotonation protocol, using either triethylamine or lithium diisopropyla-
mide (LDA) as a base. All complexes have been characterized by various
techniques, including 1H, 13C{1H}, and 15N NMR spectroscopy, IR
spectroscopy, and X-ray diffraction analysis. Except for the dioxo complex
1, all complexes adopt monomeric structures in the solid state. Compounds
1, 4, and 5 are rare examples of five-coordinated molybdenum(VI)
complexes in the solid state, while 2 is six-coordinated, having a weakly bound THF ligand in the axial position. The dioxo
complex 1 was found to be stable under air and moisture in the solid state as well as in solution for several days. Furthermore,
preliminary reactivity studies and an approximate determination of the pKa value of the imido ligands in 4 are presented.
dim. However, in 2010, Hor and co-workers showed that in
situ generated (NHC)CpMoO2 complexes are indeed potent
catalysts for the epoxidation of cyclooctene.24 Two years later,
the same work group also structurally characterized the first
example of a cationic Mo(VI)Cp(NHC) complex.25 However,
the structure was found to have the Lindqvist-type26 anion
[Mo6O24]2−, which most likely resulted from the decom-
position of the synthesized tetrafluoroborate salt (Figure 1).
INTRODUCTION
■
High-valent molybdenum-oxo complexes as catalysts are of
considerable interest for a large variety of reactions, including
oxidative or reductive transformations1−4 via oxygen atom
transfer reactions (OATR), arene coupling reactions,5 oxygen
activation,6−8 and polymerizations.9−13 One big advantage of
molybdenum compared with many other 4d and 5d metals
(e.g., rhenium, tungsten, tantalum, or niobium) is its low
toxicity, biocompatibility, and low price.14 In this context, a
large variety of ligands have been explored in recent years for
the synthesis of high-valent molybdenum complexes.3,15−21
However, searching through the library of ligands explored, it
becomes obvious that examples of one of the most prominent
ligands in catalysis are rare and that, despite the ongoing
interest in N-heterocyclic carbenes (NHCs), high-valent dioxo-
molybdenum complexes with NHC ligands are scarce.
Pioneering work performed by Herrmann et al. in 1996
showed that NHC ligands are in principle well suited for the
use in high-valent molybdenum chemistry; however, the
resulting complexes were extremely sensitive toward air,
moisture, and “acidic” solvents (e.g., chloroform), resulting
in fast hydrolysis of the complexes.22 The sensitivity of these
complexes was further probed by Royo et al., showing that the
substituents on the nitrogen atoms as well as on the NHC
backbone have a major influence on the stability of the
complexes toward hydrolysis.23 According to the authors, the
high sensitivity of molybdenum NHC complexes toward
hydrolysis renders their application in chemical catalysis rather
Figure 1. Previously reported attempts to isolate stable molybdenum-
(VI)-NHC complexes.
Inspired by the work of Buchmeiser et al.27 who showed that
NHC complexes of high-valent molybdenum alkylidene and
alkylidyne complexes are valuable catalysts in metathesis
reactions and polymerizations,10,28−40 we were interested in
exploring the coordination chemistry of other high-valent
molybdenum(VI) units, especially the above-mentioned dioxo
Received: July 11, 2019
© XXXX American Chemical Society
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Scheme 1. Synthesis of the Four Molybdenum Complexes 1−4 Featuring a Dianionic Bis-phenolate-NHC Ligand
Figure 2. ORTEP plot of 1-mono (left) and 1-dimer (right). tBu groups have been truncated and hydrogen atoms have been omitted for clarity.
Thermal ellipsoids are shown at a probability level of 50%.
and the bis-imido frameworks. However, since previous studies
showed that these complexes can be quite challenging in their
synthesis, we sought for NHC ligands with anionic linker
atoms, which facilitate the metal binding of the NHC ligand
and enforce a stronger bonding of the NHC to the metal
center. Employing this strategy has led to the isolation of
various unusual main group,41−45 early transition metal,45−52
late transition metal43,53−56 and f-element NHC com-
plexes57−73 with unique reactivity patterns and spectroscopic
as well as electrochemical properties. In our search for a NHC
ligand that is capable of forcing the metal center to bind to the
NHC moiety by using anionic linker groups, we came across
the bis-phenol benzimidazolium chloride [H3L1]Cl introduced
by Bercaw et al.74 and by Bellemin-Lapponnaz.75 Besides
forcing the NHC moiety in a close proximity to the
molybdenum center, this ligand also holds two phenolates,
which already have independently shown to be valuable
coligands in molybdenum chemistry.76−85 Additionally, L1 as
well as other structurally related bis-phenolate imidazolylidene
ligands have been shown to be excellent supporting ligands for
the synthesis and design of main group complexes such as
aluminum,86 and high-valent early transition metal complexes
including titanium,87,88 zirconium,88−91 hafnium,88,91 vanadi-
um,92 and manganese.92 Only recently Buchmeiser et al. have
been able to stabilize a molybdenum alkylidine complex using
this ligand.93
complex 1 can be isolated. After the removal of triethylammo-
nium chloride by filtration and subsequent precipitation with
n-hexane, 1 is isolated in good yields of 75% as a faint yellow
solid (Scheme 1). The 1H NMR spectrum showed the
presence of a single, C2v symmetric species, and the absence of
the OH as well as of the benzimidazolium-2H proton indicated
the formation of a NHC complex. The assignment of the new
compound as a carbene complex was further supported by the
presence of a NHC carbene resonance at δ 183.1 ppm in the
13C{1H} NMR spectrum. Additionally, the presence of
stretching frequencies at ṽ
914 and 939 cm−1 in the IR
spectrum, corresponding to the asymmetric and symmetric
Mo−O vibrations, confirmed the presence of the MoO2
unit.15−18,20 Ultimate proof for the successful formation of
the desired complex was given by X-ray structure analysis.
Single crystals of 1 were obtained by diffusion of pentane into a
concentrated solution of 1 in THF. The complex crystallizes in
the triclinic space group P-1 and was found to crystallize with
two different molecules in the asymmetric unit, a
molybdenum(VI)-dimer (1-dimer) with two six-coordinate
molybdenum atoms in a strongly distorted octahedral
environment and a molybdenum(VI) monomer (1-mono)
with the molybdenum atom being five-coordinate lying in
between a square pyramidal and a trigonal bipyramidal
coordination environment displaying a τ5-value of 0.462
(Figure 2).95−98 The main differences between the monomeric
and the dimeric form can be found in the conformation of the
phenolate linkers of the NHC ligand. While in the dimeric
structure, the phenolate rings are both twisted into the same
direction in reference to the benzimidazolylidene plane by
39.6(1) and 29.7(1) °, the phenolate rings are twisted into
opposite directions by 30.7(1) and −37.4(2) ° in the
RESULTS AND DISCUSSION
■
Treatment of a THF solution of the ligand precursor [H3L1]Cl
with triethylamine at room temperature, followed by the
addition of (DME)MoO2Cl2,94 resulted in the formation of an
orange solution, from which the desired dioxo molybdenum
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monomeric structure. Because of the strong distortion of the
ligand in the dimer, the benzimidazolylidene plane is also
strongly bent out of the Mo−C bond line by 29.1(1) °.
However, in solution, we only observe a single set of
resonances for the ligand and one resonance for the carbene
atom C1, indicating a high flexibility of the phenolate linkers in
solution. The Mo−O distances for the terminally bound oxo
ligands are 1.706(3) Å (for Mo1−O10) and 1.702(3) Å (for
Mo1−O11) in 1-mono and compare well to other previously
reported MoO2 complexes.15,16,18 For 1-dimer, the Mo2−O20
distance with nonbridging oxo ligand was found to be 1.695(3)
Å. The bridging oxygen atom O21 is asymmetrically bound to
the two molybdenum centers displaying a distance of 1.761(2)
Å and 2.439(3) Å to the two molybdenum atoms, respectively.
The Mo−C1 distance was found to be 2.260(4) Å in 1-dimer
and 2.193(4) Å in 1-mono. To further probe the structural
identity of the complex in solution and to see if there is a
temperature-dependent equilibrium between the complexes 1-
mono and 1-dimer in solution, we investigated the complexes
molybdenum(VI) oxo-imido complexes.2,8 We have been able
to grow crystals of the complex from a concentrated THF
solution at −40 °C over 2 days. Unfortunately, this method
and all other attempts yielded only low quality crystals of the
desired complex 2. Nevertheless, the connectivity could be
confirmed and the molecular structure of 2 showed the
formation of the desired NHC-oxo-imido complex (Figure 3,
left). The molybdenum center in 2 can be best described as six
coordinate with a weakly coordinating THF ligand occupying
the sixth coordination site. It is noteworthy that the THF
ligand is only present because of the crystallization from pure
THF. The material isolated after the workup as described
above does not contain any ligated THF, which was also
verified by 1H NMR spectroscopy (compare Figure S15).
Because of the low quality of the crystals, bond lengths and
angles cannot be discussed at this point.
Switching to the bis-imido precursor (DME)Mo-
100,101
(NtBu)2Cl2
and applying triethylamine as a base again,
we were not able to isolate the desired bis-imido complex 4
from the reaction mixture, but instead the imido-amido
complex 3 was obtained in high purity and yield (85%) as
orange crystals after crystallization from THF. The formation
of the imido-amido complex was indicated by several
spectroscopic features: (i) the presence of more than three
tert-butyl resonances in both the 1H and 13C {1H}NMR
spectrum; (ii) the presence of a very low-field shifted
resonance at δ 12.20 ppm corresponding to the proton of
1
by variable-temperature H NMR spectroscopy in toluene-d8
as well as in THF-d8 (Figures S13 and S14). However, we did
1
not observe any changes in the H NMR spectra over a
temperature range from −30 °C to +60 °C, indicating that the
observation of the dimerization of 1 in the solid state is most
likely related to packing effects, influencing the ligand
geometry and thus leading to the formation of 1-mono and
1-dimer. Notably, opening a NMR sample of 1 in C6D6 to air
and moisture, we have not observed any signs of decom-
position of 1 in solution over 5 days, rendering the dioxo
complex 1 as fairly air stable (compare Figure S12).
1
the amido ligand in the H NMR spectrum; (iii) the presence
of two different tBuN resonances in the 1H−15N HMBC NMR
spectrum at δ 82.5 and 337.6 ppm corresponding to the imido
and the amido group (Table 1); (iv) the presence of a doublet
Having established such a simple route to molybdenum
dioxo complexes, we were curious whether this route would
also be applicable for the formation of oxo-imido or bis-tert-
butyl-imido complexes 2 and 4. Following the same reaction
protocol and workup conditions used for 1 and employing the
1
in the H−15N HMBC spectrum at δ 12.15/337.6 ppm and δ
12.24/337.6 ppm (J = 63 Hz) characteristic for NH protons
(Table 1 and Figure S25); and (v) the presence of an IR
stretch at v
̃
3234 cm−1, which is characteristic for NH protons.
99
oxo-imido-Mo(VI) precursor (DME)Mo(O)(NtBu)Cl2 re-
The carbene resonance for this complex was found to be at δ
191.1 ppm in the 13C{1H} NMR spectrum. It is noteworthy
that only one of three possible isomers was obtained in which
the amido function is trans to the carbene carbon (3-trans,
Figure S45, middle), while we have not seen the formation of
any cis-coordinated product with the amido ligand being cis to
the carbene ligand (3-cis, Figure S45, left). The third isomer,
3-Cl (Figure S45, right) where the imido and the amido ligand
are trans to each other is most likely unfavored due to the
competitive π-bonding between the imido and the amido
ligand in this arrangement. The assignment of complex 3 as the
trans isomer is also obvious by the presence of a cross peak in
sulted in the clean formation of a new yellow compound.
Again, the absence of the OH resonances as well as the
1
characteristic benzimidazolium-2H resonance in its H NMR
spectrum indicated successful formation of the desired oxo-
imido complex 2. The presence of a carbene complex was
furthermore supported by the presence of a low-field
resonance at δ 191.6 ppm in the 13C{1H} NMR spectrum.
The presence of the imido-ligand was verified by a cross peak
1
in the H−15N HMBC spectrum at δ 1.38/102.9 ppm (Table
1/Figure S19). Additionally, the presence of the oxo-ligand
was indicated by the presence of a single IR stretch at ṽ 910
1
the H−13C HMBC spectrum at δ 12.20/191.1 ppm. Such a
cm−1, which lies in the range for other, previously reported
coupling is favorably seen in a trans orientation of the carbene
carbon and the NH function of the amido ligand (compare
Figure S24). The trans conformation was also proven by X-ray
crystallography (vide infra). Single crystals of 3 were obtained
from a concentrated THF solution at room temperature over
the course of several days. The complex crystallizes in the
monoclinic space group P21/c with one molecule in the
asymmetric unit. The molybdenum atom in 3 was found to be
six-coordinate in a distorted octahedral fashion (Figure 3,
middle), coordinated by the three atoms of the carbene ligand,
the imido, the amido, and additionally by one chlorido ligand.
The Mo−N distances were found to be 1.711(3) Å for Mo1−
N40 (imido ligand) and 1.954(3) Å for Mo1−N50 (amido
ligand). In addition, the Mo1−N40−C40 bond angle was
found to be nearly linear with 168.6(2) °, while the
1
Table 1. List of the H−15N HMBC Data of 2, 3, and 4 (in
ppm), Showing the Chemical Shifts of the Benzimidazole
Nitrogen, as well as the Amido and the Imido Nitrogen
Atoms of the Corresponding Complexes
a
complex
NNHC
NHtBu
NtBu
2
3
7.66/177.9
7.88/176.1
-
1.38/102.9
0.75/82.3
12.15/337.6
12.24/337.6
1.52/337.6
-
4
7.84/179.5
7.67/179.5
1.48/83.8
a
See also Figures S19 (for 2), S25 (for 3), and S30 (for 4).
C
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Figure 3. Ball and stick representation of 2 (left). ORTEP plot of 3 (middle) and 4 (right). tBu groups have been partly truncated and hydrogen
atoms have been omitted for clarity. Thermal ellipsoids are shown at a probability level of 50%.
corresponding Mo1−N50−C50 bond angle was found to be
strongly bent with 142.4(2) °. These pronounced differences
in bond distances and angles, in combination with the
spectroscopic data, clearly support the assignment of N40 as
the imido nitrogen and N50 as the amido nitrogen and
therefore unambiguously prove the isolation of the (NHC)-
Mo(VI)-imido-amido complex 3. The structure also unambig-
uously confirms the amido group being trans to the carbene as
already inferred from NMR spectroscopy.
compared to that of the two-step procedure via the amido-
imido complex 3, resulting in much lower yields. It is also
noteworthy at this point that lithium bases need to be used.
Applying KHMDS under the same conditions, we have not
observed the formation of the desired complex 4 at all, but
instead, complicated mixtures have been obtained. Addition-
ally, the oxo-imido complex 2 can also be synthesized via a
ligand exchange reaction between the dioxo complex 1 and the
bis-imido complex 4 at elevated temperatures (Scheme 1).
Because it is well-known that aryl-imido ligands are less
reactive compared with their alkyl congeners,104−107 we
assumed that the use of electron-deficient imido ligands
(e.g., mesityl-imido) might result in the direct formation of the
Probing the acidity of the NH proton in 3, we found that the
complex can be easily deprotonated using either lithium
diisopropylamide (LDA) or potassium hexamethyldisilazide
(KHMDS), resulting in the clean formation of the originally
1
bis-imido complex 5 using weaker bases than LDA. Following
targeted bis-imido complex 4. The H NMR spectrum shows
108,109
our general protocol using (DME)Mo(NMes)2Cl2
as a
the clean formation of a single product exhibiting C2v
symmetry and disappearance of the NH resonance at δ
12.20 ppm in 3 as well as the merging of the two tBu signals at
δ 0.75 ppm and δ 1.52 ppm to a single resonance at δ 1.46
ppm in 4. Additionally, only a single cross peak was observed
molybdenum source, we indeed isolated the desired bis-imido
complex 5 in good yields of 79% as a dark orange
microcrystalline powder (compare Scheme 2). Formation of
1
in the H−15N HMBC spectrum of 4 at δ 1.48/83.8 ppm
Scheme 2. Synthesis of the Mesityl-Substituted Bis-imido
Complex 5
(Table 1/Figure S30). The assignment of 4 still being a NHC
complex was furthermore supported by the observation of a
low-field shifted resonance in its 13C{1H} NMR spectrum
shifting from δ 191.1 ppm in 3 to δ 199.3 ppm in 4. Single
crystals of 4 were grown by slow evaporation of a concentrated
n-hexane solution of 4 at room temperature. The complex
crystallizes in the triclinic space group P-1 as n-hexane solvate
with one equivalent of n-hexane in the asymmetric unit. The
molybdenum atom was found to be five-coordinate in a slightly
distorted square pyramidal coordination environment (Figure
3, right; τ5 = 0.167). The Mo−N distances were found to be
1.745(2) Å and 1.754(2) Å for Mo1−N40 and Mo1−N50
respectively, lying in the range of previously reported examples
of molybdenum bis-imido complexes.17,102 While the two
Mo−N distances are fairly similar, there is a big difference in
the bond angle between the two imido ligands; that is, the
Mo1−N50−C50 angle is close to linear with 173.1(2) °
compared to the Mo1−N40−C40 angle, which is strongly bent
with 154.9(2) °.
the desired bis-imido NHC complex was indicated by the
absence of the benzimidazolium-2H resonance in the 1H NMR
spectrum as well as by the observation of a low-field resonance
at δ 193.2 ppm in the 13C{1H} NMR spectrum of the complex.
Unfortunately, in the case of the mesityl-imido ligands, it is not
possible to have an efficient polarization transfer from the six
protons on the ortho-methyl groups on the mesityl ligand to
the nitrogen atom, and thus, we have not been able to prove
1
the assignment as a bis-imido complex by H−15N HMBC
spectroscopy. However, we have been able to grow X-ray
quality single crystals by the slow evaporation of a
concentrated THF solution at −40 °C. The complex
crystallized in the triclinic space group P-1 with one molecule
of 5 and three formula units of THF in the asymmetric unit of
which one needed to be removed using the SQUEEZE
operation.113 The molybdenum atom in 5 is five-coordinate in
a strongly distorted trigonal bipyramidal fashion (Figure 4, left;
Notably, the bis imido complex 4 can also be synthesized
directly from the corresponding bis-imido molybdenum
precursor (DME)Mo(NtBu)2Cl2 and the ligand precursor
[H3L1]Cl if a stronger base than triethylamine was used (e.g.,
LDA or LiHMDS) and the reaction is initiated at −40 °C.103
However, the reaction does not proceed as cleanly as
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t
Figure 4. ORTEP plot of 5 (left), 6 (middle), and 7 (right). Bu groups have been partly truncated, and hydrogen atoms have been omitted for
clarity. Thermal ellipsoids are shown at a probability level of 50%.
Scheme 3. Reactivity of Complex 4 toward Various Substrates of Decreasing Acidity
τ5 = 0.53), with the two phenolate ligands occupying the axial
coordination sites, while the imido ligands as well as the
carbene unit are in the equatorial plane. The bond distances of
Mo1−N40 and Mo1−N50 are 1.762(4) Å and 1.745(4) Å,
thus unambiguously confirming the assignment of 5 as a bis-
imido complex of high-valent Mo(VI). The molybdenum
carbene distance of 2.171(5) Å for Mo1−C1 lies within the
range of the distances of complexes 1−4. Further structural
details regarding the molecular structure of 5 can be found in
the SI. To the best of our knowledge, the bis-imido complexes
4 and 5 are only the second examples of molybdenum bis-
imido complexes ligated by a NHC ligand. We have recently
reported a similar complex coordinated by a structurally
related MIC ligand.103
Having these new complexes in hand, we next turned our
interest toward their chemical properties. Especially the bis-
imido complex 4 caught our attention. We believe that the
isolation of the imido-amido complex 3 results from the initial
formation of 4 in the presence of triethylammonium chloride,
which is then protonated by triethylammonium chloride to
give the observed complex 3. To probe this hypothesis, we
have conducted a NMR experiment adding a suspension of
triethylammonium chloride to complex 4 in C6D6, which
indeed resulted in the clean protonation of complex 4 to give
complex 3 within an hour (compare Figure S35). Thus, we
have been interested in the reactivity of 4 towards dihydrogen
and if it is a potential catalyst for the hydrogenation of
unactivated alkynes. Arnold and Toste et al. have recently
presented a cationic bis-imido vanadium complex, which is an
effective catalyst for the hydrogenation of alkynes.110 However,
we found that complex 4 does not react with dihydrogen at all
and is also not catalytically active for the hydrogenation of
alkynes at mild conditions comparable to Arnold’s work. To
probe the basicity of the bis-imido complex 4, we subsequently
reacted the complex with mesitole (pKa = 9), benzimidazole
(pKa = 16), and carbazole (pKa = 20). While the reactions for
mesitole and for benzimidazole proceeded very messily, along
with the formation of a characteristic singlet above δ 11 ppm
corresponding to the NHtBu moiety, for carbazole no reaction
was observed (compare Scheme 3 and Figures S36−S38). This
states that the pKa value of complex 4 is somewhere between
16 and 20, which proves that the complex is not basic enough
to activate dihydrogen having a pKa value of 36. Even though
the reaction between 4 and mesitole was very messy leading to
a myriad of products, we were able to crystallize dark red and
dark purple blocks from a concentrated pentane solution.
Unfortunately, it was not possible to further segregate these
two products to obtain NMR spectra of the pure compounds.
Nevertheless, we have been able to investigate both of these
crystals by X-ray crystallography. X-ray diffraction analysis of
the red crystals revealed the successful formation of the desired
imido-amido complex 7 (Figure 4, right), while the purple
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yellow suspension was stirred for 30 min at room temperature before
it was added dropwise to a stirred solution of (DME)MoO2Cl2 (1
equiv, 0.5 mmol, 145 mg) in THF (5 mL), resulting in the formation
of a brownish yellow suspension. The reaction mixture was stirred for
16 h, before it was filtered to remove triethylammonium chloride. The
solution was concentrated to about 1 mL, and n-hexane was slowly
added while swirling the vial to induce precipitation of a yellow solid.
The yellow solids were isolated by decanting of the solvent and
washed multiple times with n-hexane (4 × 5 mL) to give pure
complex 1 in moderate yields of 75% (238 mg, 0.375 mmol). X-ray
quality crystals were grown by slow diffusion of pentane into a
concentrated THF solution of 1 at room temperature. 1H NMR
(C6D6, 700 MHz, 25 °C): δ (ppm) = 7.87 (m, 2H, Aryl-H), 7.64 (m,
4H, Aryl-H), 7.00 (m, 2H, Aryl-H), 1.70 (s, 18H, C(CH3)3), 1.37 (s,
18H, C(CH3)3); 13C{1H} NMR (C6D6, 175 MHz, 25 °C): δ (ppm) =
183.1 (Carbene-C), 150.9 (Aryl-C-O), 141.5, 141.3, 132.5, 126.1
(Aryl-C), 125.0, 123.2, 115.5, 114.2 (Aryl-CH), 35.9, 34.5
crystals were found to be the twofold activated product 6
(Figure 4, middle), in which one imido ligand deprotonated
two equivalents of mesitole, resulting in a bis-mesitole-imido
complex. In both complexes 6 and 7, the coordination
environment of the molybdenum atom is distorted octahedral.
Interestingly, in 6, the bis-phenolate benzimidazolylidene
ligand does not coordinate in a mer-fashion, as it would be
expected for a tridentate pincer type ligand, but in a fac-
conformation. This underlines the high flexibility of this ligand
and its ability to stabilize various metal cations of different sizes
and steric demands. The bond lengths between the
molybdenum atom to the carbene carbon was found to be
2.159(6) Å in 6 and 2.185(4) Å in 7 being in the normal range
as in the complexes 1−5 (vide supra). The bond lengths
between the molybdenum atom and the imido ligand in 6 and
7 were found to be 1.704(4) Å and 1.715(4) Å and are
comparable to those found in the imido complexes 3, 4, and 5
(vide supra). Strikingly, none of the above-mentioned reagents
were able to protonate the mesityl-imido ligands in 5,
rendering this complex to be much less reactive.
(C(CH3)3), 31.4, 30.0 (C(CH3)3); IR: v
̃
(cm−1) = 913 (O = 
MoO, asymmetric), 939 (OMoO, symmetric); elemental
analysis (%) calcd for C35H44N2O4Mo: C, 64.41; H, 6.80; N, 4.29;
found: C, 64.62; H, 7.11; N, 4.20.
L1 Mo(O)(NtBu) (2). Method A. In an argon-filled glovebox,
[H3L1]Cl (1 equiv, 0.5 mmol, 281 mg) was dissolved in 8 mL of THF
and stirred for 5 min at room temperature. To the colorless solution,
triethylamine (3.5 equiv, 1.75 mmol, 186 mg) dissolved in 3 mL of
THF was added, resulting in a fast color change to bright yellow. The
yellow suspension was stirred for 30 min at room temperature before
it was added dropwise to a stirred solution of (DME)MoO(NtBu)Cl2
(1 equiv, 0.5 mmol, 172 mg) in THF (5 mL), resulting in the
formation of a brownish yellow suspension. The reaction mixture was
stirred for 16 h, before it was filtered to remove triethylammonium
chloride. The solution was concentrated to about 1 mL, and n-hexane
was slowly added while swirling the vial to induce precipitation of a
yellow solid. The yellow solids were isolated by decanting of the
solvent and washed multiple times with n-hexane (4 × 5 mL) to give
pure complex 2 in moderate yields of 67% (237 mg, 0.335 mmol).
Method B. Complex 1 (1 equiv, 0.025 mmol, 16 mg) and complex
4 (1 equiv, 0.025 mmol, 19 mg) were dissolved in C6D6 (0.65 mL)
and heated to 80 °C overnight. The resulting NMR spectra was
identical to the NMR obtained from the isolated complexes 2 (vide
infra). Crystals suitable for X-ray diffraction analysis were grown from
a concentrated THF solution (0.15g in 1 mL) of 2 at −40 °C over 2
CONCLUSIONS
■
In conclusion, we have presented a simple yet very efficient
route to prepare NHC complexes of high-valent molybdenum-
(VI), featuring oxo and imido ligands. We have been able to
show that the outcome of the reactions leading to bis-imido
complexes featuring dianionic bis-phenolate NHC ligands
strongly depends on the nature (and the basicity) of the
corresponding imido ligand. Electron-rich (tBu) imido
complexes have additionally been shown to react with protic
substances up to a pKa value of 16, while electron-deficient
(mesityl) imido ligands are barely reactive. Additionally, we
have been able to show that the rare NHC dioxo complex 1 is
(in contrast to previously reported (NHC)MoO2 complexes)
stable toward air and moisture even in solution, making these
complexes ideal candidates for chemical catalysis. We are
currently exploring the catalytic potential of the new complexes
in oxygen-transfer reactions as well as trying to find strategies
to make them more reactive towards molecular hydrogen.
1
days. H NMR (C6D6, 700 MHz, 25 °C): δ (ppm) = 7.83 (m, 2H,
Aryl-H), 7.65 (d, J = 2.3 Hz, 2H, Aryl-H), 7.61 (d, J = 2.3 Hz, 2H,
Aryl-H), 6.95 (m, 2H, Aryl-H), 1.78 (s, 18H, C(CH3)3), 1.38 (s, 27H,
C(CH3)3 and Mo = NC(CH3)3); 13C{1H} NMR (C6D6, 175 MHz,
25 °C): δ (ppm) = 191.6 (Carbene-C), 154.6 (Aryl-C-O), 141.3,
140.6, 133.7, 127.4 (Aryl-C), 125.0, 123.0 116.4, 114.4 (Aryl-CH),
72.2 (MoNC(CH3)3), 36.7, 35.0 (C(CH3)3), 32.1 (C(CH3)3), 30.9
(MoNC(CH3)3), 30.8 (C(CH3)3); 15N NMR (C6D6, 71 MHz, 25
EXPERIMENTAL SECTION
■
General Remarks. If not otherwise mentioned, all trans-
formations were carried out in an argon-filled glovebox under inert
conditions. Solvents were dried by a MBraun SPS system and stored
over activated molecular sieves (3 Å) for at least 1 day. Prior to use,
deuterated C6D6, THF-d8 and toluene-d8 were dried over sodium/
benzophenone and CDCl3 and CD2Cl2 over calcium hydride,
followed by vacuum transfer and three freeze−pump−thaw cycles.
Starting materials [H3L1]Cl,74,75 (DME)MoO2Cl2,94 (DME)Mo(O)-
°C): δ (ppm) = 102.9 (MoNC(CH3)3). IR: v
̃
(cm−1) = 911 (Mo
O); elemental analysis (%) calcd for C39H53N3O3Mo: C, 66.18; H,
7.55; N, 5.94; found: C, 66.48; H, 7.41; N, 5.74.
L1 Mo(NtBu)(NHtBu)Cl (3). In an argon-filled glovebox, NEt3 (3.5
equiv, 1.75 mmol, 186 mg) diluted in THF (2 mL) was slowly added
dropwise to a stirring solution of the benzimidazolium salt [H3L1]Cl
(1 equiv, 0.5 mmol, 281 mg) in THF (4 mL). The color changed to
pale yellow and a colorless precipitate formed. The suspension was
stirred for 30 min and added at once to a solution of (DME)Mo-
(NtBu)2Cl2 (1 equiv, 0.5 mmol, 199 mg) in THF (2 mL). The color
changed to dark red, and the reaction was stirred for another 16 h.
The mixture was subsequently filtered and the solvent was
concentrated to about 2 mL. Over the course of 1 day, a large mass
of crystals formed which was isolated by decanting of the solvent and
washing the crystals with n-hexane (5 mL) to give pure 3 in good
yields of 85% (0.425 mmol, 340 mg) as an orange solid. X-ray quality
crystals were grown form a concentrated solution of 3 in THF (0.1 g
(NtBu)Cl2,99 (DME)Mo(NtBu)2Cl2,100,101 and (DME)Mo-
108
(NMes)2Cl2
have been synthesized following known procedures
found in the literature. If not otherwise stated, triethylamine (NEt3)
and all other chemicals have been used as received. IR spectra were
recorded using a Bruker Vertex 70 with ATR equipment. NMR
spectra were collected at ambient temperature on a Bruker AV-500 or
1
an Ascent 700 spectrometer using a J-Young NMR tube. H and 13C
NMR chemical shifts (δ) are reported in ppm and were calibrated to
residual solvent peaks. 15N spectra were referenced vs liquid ammonia
(NH3). Elemental analysis was performed using an Elementar vario
microcube instrument.
Synthetic Procedures. L1 MoO2 (1). In an argon-filled glovebox,
[H3L1]Cl (1 equiv, 0.5 mmol, 281 mg) was dissolved in 8 mL of THF
and stirred for 5 min at room temperature. To the colorless solution,
triethylamine (3.5 equiv, 1.75 mmol, 186 mg) dissolved in 3 mL of
THF was added, resulting in a fast color change to bright yellow. The
1
in 1 mL) over the course of 4 days at room temperature. H NMR
(C6D6, 700 MHz, 25 °C): δ (ppm) = 12.20 (s, 1H, Mo−
NH(C(CH3)3)), 7.91 (m, 2H, Aryl-H), 7.88 (d, J = 2.3 Hz, 2H,
F
DOI: 10.1021/acs.organomet.9b00472
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Aryl-H), 7.60 (d, J = 2.3 Hz, 2H, Aryl-H), 6.96 (m, 2H, Aryl-H), 1.71
(s, 18H, C(CH3)3), 1.52 (s, 9H, Mo-NH(C(CH3)3)), 1.38 (s, 18H,
C(CH3)3), 0.75 (s, 9H, MoNC(CH3)3); 13C{1H} NMR (C6D6,
175 MHz, 25 °C): δ (ppm) 191.1 (Carbene-C), 156.8 (Aryl-C-O),
140.0, 139.8, 134.0, 127.4 (Aryl-C), 124.7, 122.2, 116.6, 114.9 (Aryl-
CH), 73.8 (MoNC(CH3)3), 61,9 (Mo-NH(C(CH3)3)), 36.6, 35.0
(C(CH3)3), 33.6 (Mo-NH(C(CH3)3)), 32.2, 31.1 (C(CH3)3), 30.5
(MoNC(CH3)3); 15N NMR (C6D6, 71 MHz, 25 °C): δ (ppm) =
337.6 (Mo-NH(C(CH3)3)), 176.1 (Benzimidazole-N), 82.5 (Mo
4H, m−CH), 2.42 (s, 12H, o-CH3), 2.09 (s, 6H, p-CH3), 1.64 (s,
18H, C(CH3)3), 1.43 (s, 18H, C(CH3)3); 13C{1H} NMR (C6D6, 175
MHz, 25 °C): δ (ppm) = 193.2 (Carbene-C), 156.2 (Aryl-C-O),
155.1 (N-Cipso), 140.7, 139,7, 135.3, 133.7, 133.6, 127.4 (Aryl-C),
124.5, 122.7, 116.5, 114.0 (Aryl−CH), 36.3, 34.7 (C(CH3)3), 31.8
(C(CH3)3), 30.3 (C(CH3)3), 21.1 (p-CH3), 19.3 (o-CH3); elemental
analysis (%) calcd for C53H66N4O2Mo: C, 71.76; H, 7.50; N, 6.32;
found: C, 71.55; H, 7.63; N, 6.42.
L1 Mo(NtBu)(OMes)2 (6) and L1 Mo(NtBu)(NHtBu)(OMes) (7). In
an argon-filled glovebox, complex 4 (1 equiv, 0.05 mmol, 38 mg) was
dissolved in C6D6 (0.55 mL) in a J-Young NMR tube. To this was
added mesitole (1.05 equiv, 0.0525 mmol, 7.5 mg) dissolved in C6D6
(0.1 mL). The color changed initially from bright yellow to deep
orange. The resulting mixture was left to stand for 24 h at room
temperature during which the color changed to dark purple. 1H NMR
spectroscopy indicated a wild mixture of compounds being formed
(Figure S36). The C6D6 was evaporated, and the resulting purple
solid was dissolved in pentane, filtered, and concentrated to 1 mL.
After 5 days, a mixture of purple and dark red blocks formed at room
temperature. Despite extensive efforts, we found no conditions to
selectively crystallize only one species. The purple blocks have been
found to be complex 6, while the dark red blocks were found to be
complex 7 by X-ray crystallography.
X-ray Crystallography. Single crystals for X-ray diffraction
experiments were measured at the analytical facility of the Paderborn
University, the Free University of Berlin and the University of
Duisburg-Essen using a Bruker Smart AXS (Paderborn). All crystals
were kept at 130(2) K throughout data collection. Data collection was
performed using either the APEXIII or the Smart software package.
Data refinement and reduction were performed with Bruker Saint
(V8.34A). All structures were solved with SHELXT and refined using
the OLEX 2 software package.111,112 A strongly disordered THF
solvent molecule in 5 was removed using the SQUEEZE operation.113
All non-hydrogen atoms were refined anisotropically, and hydrogen
atoms were included at the geometrically calculated positions and
refined using a riding model. The quality of the data obtained for 2
was very poor. Problems during the integration led to high Rint values
and an incomplete data set. Many atomic displacement parameters
refine to negative values, and R1 will not improve to less than about
25%. The model as such cannot be considered reliable; however, it
matches well with other analytical means, suggesting that at least the
connectivity and overall conformation of the molecule are correctly
described. CCDC 1936967−1936972 contain the CIF files for
complexes 1 and 3−7, respectively. For further crystallographic
details, please check Tables S1 and S2 in the Supporting Information.
NC(CH3)3). IR: ṽ
(cm−1) = 3254 (Mo-NH−C(CH3)3); elemental
analysis (%) calcd for C43H63N4O2ClMo: C, 64.61; H, 7.94; N, 7.01;
found: C, 64.25; H, 7.71; N, 7.13.
L1 Mo(NtBu)2 (4). Method A. In an argon-filled glovebox, to a
solution of 3 (1 equiv, 0.5 mmol, 400 mg) in toluene (10 mL) was
added dropwise a solution of KHMDS (1.1 equiv, 0.55 mmol, 110
mg) diluted in toluene (2 mL). The color changed immediately from
orange to bright yellow, and the reaction mixture was stirred for
another 2 h at room temperature to ensure complete deprotonation of
3. The reaction mixture was filtered and evaporated to dryness. The
yellow residue was then redissolved in dichloromethane (5 mL),
filtered again, and concentrated to 0.5 mL. Addition of n-hexane while
swirling around the vial induced precipitation of the clean complex 4
which was isolated by decanting of the solution and drying under high
vacuum in very good yields of 92% (0.460 mmol, 351 mg).
Depending on the amount of dichloromethane left in the solution,
it has been observed that precipitation of 4 starts with a short delay. If
the precipitation is not starting right away, we recommend stirring the
mixture of n-hexane and dichloromethane overnight.
Method B. The proligand [H3L]Cl (1 equiv, 0.3 mmol, 168 mg)
was dissolved in a 1:1 mixture of Et2O/THF (5 mL) and cooled to
−40 °C. To the mixture was added a cold solution of LDA (3 equiv,
0.9 mmol, 90 mg) in Et2O (3 mL), and the mixture was stirred for 2
min and added dropwise to a −40 °C cold solution of (DME)Mo-
(NtBu)2Cl2(1 equiv, 0.3 mmol, 120 mg) in THF/Et2O (1:1). The
resulting bright yellow mixture was stirred for 18 h, and the cloudy
yellow suspension was filtered and evaporated to dryness. The yellow
residue was redissolved in toluene (10 mL), filtered again, and
concentrated to about 1 mL. Pentane was added dropwise to this
solution to induce precipitation of the desired complex. The complex
was isolated as a yellow powder in moderate yields of 42% (96 mg,
0.126 mmol). X-ray quality crystals were grown by slow evaporation
1
of a saturated n-hexane solution of 4 at room temperature. H NMR
(C6D6, 700 MHz, 25 °C): δ (ppm) = 7.77 (m, 2H, Aryl-H), 7.65 (d, J
= 2.3 Hz, 2H, Aryl-H), 7.59 (d, J = 2.3 Hz, 2H, Aryl-H), 6.89 (m, 2H,
Aryl-H), 1.80 (s, 18H, C(CH3)3), 1.46 (s, 18H, MoNC(CH3)3),
1.37 (s, 18H, C(CH3)3); 13C{1H} NMR (C6D6, 175 MHz, 25 °C): δ
(ppm) = 199.3 (Carbene-C), 155.8 (Aryl-C-O), 141.0, 139.3, 134.4,
127.9 (Aryl-C), 124.5, 122.5, 117.2, 114.4 (Aryl−CH), 70.1 (Mo
NC(CH3)3), 36.8, 34.9 (C(CH3)3), 32.2 (C(CH3)3), 32.2 (Mo
NC(CH3)3), 30.8 (C(CH3)3); 15N NMR (C6D6, 71 MHz, 25 °C): δ
(ppm) = 179.5 (Benzimidazole-N), 83.4 (MoNC(CH3)3);
elemental analysis (%) calcd for C43H62N4O2Mo·0.3 C6H14: C,
68.22; H, 8.46; N, 7.10; found: C, 68.33; H, 8.15; N, 6.82.
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.9b00472.
L1 Mo(NMes)2 (5). In an argon-filled glovebox, triethylamine (3.5
equiv, 1.75 mmol, 186 mg) diluted in THF (2 mL) was slowly added
dropwise to a stirring solution of the benzimidazolium salt [H3L1]Cl
(1 equiv, 0.5 mmol, 281 mg) in THF (4 mL). The color changed to
pale yellow, and a colorless precipitate formed. The suspension was
stirred for 30 minutes and added dropwise to a solution of
(DME)Mo(NMes)2Cl2 (1 equiv, 0.5 mmol, 262 mg) in THF (2
mL) at room temperature. The color changed to dark red, and the
reaction was stirred for another 16 h. The mixture was subsequently
filtered, and the filtrate was evaporated to dryness. The orange red
residue was extracted with pentane, filtered again, and dried under
high vacuum to give the desired bis-imido complex 5 as a red orange
powder in good yields of 79% (0.395 mmol, 350 mg). X-ray quality
crystals of 5 could be grown from a concentrated THF solution (100
Synthetic details, NMR spectra, IR spectra, UV−vis
spectra, and crystallographic details for all compounds
(PDF)
Accession Codes
CCDC 1936967−1936972 contain the supplementary crys-
tallographic 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
1
■
mg in 1 mL) at −40 °C overnight. H NMR (C6D6, 700 MHz, 25
°C): δ (ppm) = 7.89 (m, 2H, Aryl-H), 7.74 (d, J = 2.3 Hz, 2H, Aryl-
H), 7.61 (d, J = 2.3 Hz, 2H, Aryl-H), 7.00 (m, 2H, Aryl-H), 6.72 (s,
Corresponding Author
*E-mail: Stephan.Hohloch@upb.de.
G
DOI: 10.1021/acs.organomet.9b00472
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(13) Varjas, C. J.; Powell, D. R.; Thomson, R. K. Rapid Access to an
Oxido-Alkylidene Complex of Mo(VI). Organometallics 2015, 34,
4806−4809.
ORCID
Stephan Hohloch: 0000-0002-5353-0801
Author Contributions
(14) Heinze, K. Bioinspired functional analogs of the active site of
molybdenum enzymes: Intermediates and mechanisms. Coord. Chem.
Rev. 2015, 300, 121−141.
The project was designed by S.H. Experimental work has been
carried out by M.B., F.A.W., and S.H. IR spectra have been
recorded by F.A.W. UV−vis data have been recorded by S.H.
X-ray structure analyses have been performed by S.H. and R.S.
The manuscript was written by S.H., F.A.W., and M.B.
(15) Hossain, M. K.; Schachner, J. A.; Haukka, M.; Lehtonen, A.;
̈
Mosch-Zanetti, N. C.; Nordlander, E. Dioxidomolybdenum(VI) and
-tungsten(VI) complexes with tripodal amino bisphenolate ligands as
epoxidation and oxo-transfer catalysts. Polyhedron 2017, 134, 275−
281.
Notes
́
(16) Dupe, A.; Hossain, M. K.; Schachner, J. A.; Belaj, F.; Lehtonen,
The authors declare no competing financial interest.
̈
A.; Nordlander, E.; Mosch-Zanetti, N. C. Dioxomolybdenum(VI) and
-tungsten(VI) Complexes with Multidentate Aminobisphenol Ligands
as Catalysts for Olefin Epoxidation. Eur. J. Inorg. Chem. 2015, 2015,
3572−3579.
ACKNOWLEDGMENTS
■
We are grateful to the Daimler and Benz Foundation, to the
Fonds der Chemischen Industrie, to the Young Academy of
the North Rhine-Westphalian Academy of Sciences, Human-
ities and the Arts, and to the University of Paderborn for
funding of this work. Prof. Dr. Matthias Bauer, Prof. Dr. Jan
̈
Paradies, and Prof. Dr. Thomas Kuhne are acknowledged for
helpful discussions. We also thank Linus Kuckling and Philipp
Boos for experimental help.
̈
(17) Lyashenko, G.; Herbst-Irmer, R.; Jancik, V.; Pal, A.; Mosch-
Zanetti, N. C. Molybdenum oxo and imido complexes of beta-
diketiminate ligands: synthesis and structural aspects. Inorg. Chem.
2008, 47, 113−120.
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(18) Leppin, J.; Forster, C.; Heinze, K. Molybdenum complex with
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Chem. 2014, 53, 12416−12427.
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(19) Leppin, J.; Forster, C.; Heinze, K. Ligand dynamics of tert-butyl
isocyanide oxido complexes of molybdenum(IV). Inorg. Chem. 2014,
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