Catalytic Formation of Asymmetric Carbodiimides at Mononuclear Chromium(II/IV) Bis(alkoxide) Complexes

Article abstract of DOI:10.1021/acs.organomet.5b00703

Herein we report the synthesis of Cr imido complexes in bis(alkoxide) ligand environments and their nitrene transfer reactivity with isocyanides. The reaction of Cr₂(OR)₄ (OR = OC^tBu₂Ph) with bulky aryl or alkyl azide results in the formation of the trigonal-planar Cr(IV) mono(imido) complexes Cr(OR)₂(NR¹), whereas less bulky aryl azides form the Cr(VI) bis(imido) complexes Cr(OR)₂(NR¹)₂. Cr(IV) mono(imido) complexes undergo facile reaction with 1 equiv of 2,6-dimethylphenyl isocyanide (CNR²) to form the corresponding carbodiimides R¹NCNR². In contrast, no reaction of Cr(OR)₂(NR¹)₂ complexes with CNR² is observed. The reaction of Cr(OR)₂(NR¹) with excess isocyanide leads to the isolation of the Cr(II) complex Cr(OR)₂(CNR²)₄, along with the observation of the anticipated carbodimide product. Cr(OR)₂(CNR²)₄, which can also be obtained by treating Cr₂(OR)₄ with 4 equiv of isocyanide, reacts with azides N₃R¹ (R¹ = adamantyl, mesityl) to produce the respective carbodiimides. Catalytic formation of carbodiimides R¹NCNR² is observed from the mixtures of azides R¹N₃ (R¹ = mesityl, 2,6-diethylphenyl, 2-isopropylphenyl, adamantyl) and several different aryl isocyanides CNR² using 2.5 mol % of Cr₂(OR)₄.

Full text of DOI:10.1021/acs.organomet.5b00703

Article
pubs.acs.org/Organometallics
Catalytic Formation of Asymmetric Carbodiimides at Mononuclear
Chromium(II/IV) Bis(alkoxide) Complexes
Maryam Yousif,† Daniel J. Tjapkes,‡ Richard L. Lord,‡ and Stanislav Groysman*,†
†Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, Michigan 48202, United States
‡Department of Chemistry, Grand Valley State University, Allendale, Michigan 49401, United States
S
* Supporting Information
ABSTRACT: Herein we report the synthesis of Cr imido complexes in
bis(alkoxide) ligand environments and their nitrene transfer reactivity with
isocyanides. The reaction of Cr2(OR)4 (OR = OCtBu2Ph) with bulky aryl
or alkyl azide results in the formation of the trigonal-planar Cr(IV)
mono(imido) complexes Cr(OR)2(NR1), whereas less bulky aryl azides
form the Cr(VI) bis(imido) complexes Cr(OR)2(NR1)2. Cr(IV) mono-
(imido) complexes undergo facile reaction with 1 equiv of 2,6-
dimethylphenyl isocyanide (CNR2) to form the corresponding carbodii-
mides R1NCNR2. In contrast, no reaction of Cr(OR)2(NR1)2 complexes
with CNR2 is observed. The reaction of Cr(OR)2(NR1) with excess
isocyanide leads to the isolation of the Cr(II) complex Cr(OR)2(CNR2)4, along with the observation of the anticipated
carbodimide product. Cr(OR)2(CNR2)4, which can also be obtained by treating Cr2(OR)4 with 4 equiv of isocyanide, reacts with
azides N3R1 (R1 = adamantyl, mesityl) to produce the respective carbodiimides. Catalytic formation of carbodiimides R1NCNR2
is observed from the mixtures of azides R1N3 (R1 = mesityl, 2,6-diethylphenyl, 2-isopropylphenyl, adamantyl) and several
different aryl isocyanides CNR2 using 2.5 mol % of Cr2(OR)4.
earlier transition metals are significantly more stable, because
the higher oxidation state of the early transition metals and the
availability of empty d orbitals allow them to form strong
multiple M−N bonds with a powerful σ and π donor, the imido
group (NR). Thus, whereas the stoichiometric and catalytic
reactivity of the oxophilic early-transition-metal imido com-
plexes with isocyanates to form symmetric carbodiimides is
well-established (Figure 1A),22 their reactivity in the imido-
INTRODUCTION
■
Carbodiimides are widely used organic compounds that
hydrolyze readily and thus serve as excellent dehydration
agents. Large-scale applications of carbodiimides include as
stabilizers and antihydrolysis agents in polymers production, as
coupling agents in peptide synthesis, and in various organic
transformations.1−6 Carbodiimides are synthesized by a
plethora of transition-metal-catalyzed routes,7 including oxida-
tive coupling of amines with isocyanides8 and coupling of two
isocyanates R′NCO with loss of 1 equiv of CO2,9 as well as by
transition-metal-free stoichiometric routes such as the reaction
of isocyanates with phosphinimines (aza-Wittig)10 and by tin-
mediated addition of silylamines to isocyanates.11 Coupling of
an [NR] (nitrene) group (originating in organoazides) with
isocyanide is among the most attractive methods to synthesize
carbodiimides, as (1) a large variety of azides are easily
accessible using simple synthetic procedures, (2) both sym-
metrical and asymmetrical carbodiimides can be synthesized,
and (3) the only side product in this reaction is the
environmentally friendly dinitrogen. Therefore, numerous
efforts have been made to make carbodiimides via coupling
of isocyanides and organoazides. Most of these coupling
reactions take place at low-coordinate late-transition-metal
catalysts stabilized by nitrogen, carbon, or phosphorus ligands,
where the MNR functionality is suggested to serve as the
reactive intermediate.12−21 The combination of the soft,
electron-rich late transition metals and the hard π-donating
imido functionality make the imido functionality reactive and
thus susceptible to transfer. In contrast, imido compounds of
Figure 1. Two common catalytic methods to synthesize carbodii-
mides: (A) synthesis of symmetric carbodiimides via early-transition-
metal catalysis; (B) synthesis of asymmetric carbodiimides via late-
transition-metal catalysis.
group transfer to isocyanides to form asymmetric carbodii-
midies is not very common (Figure 1B).23−26 Furthermore,
these imido−isocyanide couplings tend to be stoichiometric at
the early-transition-metal centers, whereas catalysis is common
with later metals. Heyduk and co-workers have recently
Received: August 14, 2015
© XXXX American Chemical Society
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reported an early-transition-metal-based (Zr(IV)) catalyst for
catalytic nitrene−isocyanide coupling where catalysis is
facilitated by the use of a redox-active ligand at Zr.27 Herein
we report that a low-coordinate early-transition-metal (Cr(IV))
imido complex in the bis(alkoxide) ligand environment serves
as an active catalyst in the formation of a variety of asymmetric
aryl isocyanides.
We have recently reported the synthesis of a series of 3d
complexes in bis(alkoxide) ligand environments (M-
(OR)2(THF)2, M = Mn−Co, OR = OCtBu2Ph).28 The
mononuclear iron complex Fe(OR)2(THF)2 was found to
undergo strikingly different reactivity with alkyl versus aryl
azides.29 In the case of an alkyl (adamantyl, Ad) azide, reductive
coupling was observed to yield the Fe(III)-bound hexazene
complex (RO)2Fe(μ-κ2:κ2-AdNNNNNNAd)Fe(OR)2.29a In
the case of aryl azides, nitrene formation occurs followed by
dimerization to form the respective diazene or complex
rearrangement to form bridging imido species (Scheme 1).29b
stable bis(isocyanide) complex Fe(OR)2(CNR2)2, which
proved unreactive with excess azide.29b Thus, since the
isocyanide coordination at the FeII(OR)2 center precludes
formation of the metal nitrene functionality, we decided to
investigate the interaction of preformed stable metal nitrene
(imido) species in bis(alkoxide) ligand environments with
various isocyanides. To obtain a stable metal imido complex, we
turned toward chromium. While Cr-imido complexes are
generally stable, we hoped that in the case of the coordinatively
unsaturated Cr(IV) imido complex we would be able to
observe ligation of an isocyanide next to the imido functionality
and perhaps to force C−N bond formation and carbodiimide
liberation via heating or photolysis. In this paper, we
demonstrate that trigonal planar CrIV(OR)2(NR1) species
undergo surprisingly facile reaction with several different
isocyanides to give Cr(II) isocyanide complexes and the
respective carbodiimides. Furthermore, we also report catalytic
conversion of mixtures of bulky aryl azides and isocyanides to
asymmetric carbodiimides.
Scheme 1. Consecutive Treatment of Fe(OR)2-Type
Complexes with Azides/Isocyanides Does Not Lead to the
Formation of Carbodiimides
RESULTS AND DISCUSSION
■
Synthesis of the Cr(II) Precursor Cr2(OR)4 (1). The
synthesis and reactivity of chromium complexes reported in this
paper are summarized in Scheme 2. Unlike the previously
synthesized M(OR)2(THF)2 (M = Mn−Co),28b we were not
able to obtain “Cr(OR)2(THF)2” by treating the seesaw cluster
[Cr2Li2X2(OR)4]28a with TlPF6. Seeking an alternative route to
Cr(OR)2(THF)2, we synthesized the previously reported
Cr((N(SiMe3)2)2(THF)2.30 Purple Cr(N(SiMe3)2)2(THF)2
undergoes protonolysis with HOR31 to form green homo-
dinuclear Cr2(OR)4 (1; 60% yield). It is worth noting that the
protonolysis reaction was conducted in THF, and no
Cr(OR)2(THF)2 formation was observed. The structure of 1,
featuring two trigonal-planar Cr(II) centers each bridged by
two alkoxides, is given in Figure 2. Compound 1 is related to
32
the previously synthesized Cr2(OSitBu3)4
and
Cr2(OCHtBu2)2(OCtBu3)2.33 The Cr−Cr separation is
2.651(1) Å in complex 1, in comparison with 2.637(2)/
2.6800(2) Å in Cr2(OSitBu3)4 and 3.075(1) Å in
Cr2(OCHtBu2)2(OCtBu3)2 (Table 1). The solution magnetic
moment of compound 1 was found to be μobs = 2.5 0.3 μB
(2.8 μB in Cr2(OSitBu3)4), indicative of some antiferromagnetic
We have attempted to intercept the transient nitrene complex
by the addition of isocyanide (2,6-dimethylphenyl isocyanide,
CNR2), in order to form a new C−N bond in the carbodiimide.
However, the addition of a mixture of isocyanide and azide to
the Fe(II) precursor has led instead to the formation of the
Scheme 2. Synthetic Transformations at Chromium Bis(alkoxide) Platforms Described in This Paper
B
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found to be extremely air sensitive. Complexes 2 and 3 were
characterized by X-ray crystallography, IR spectroscopy, UV−
vis spectroscopy, solution magnetic moment measurements,
and elemental analysis. Compound 4 was isolated in low yield,
due to its high solubility in pentane, and therefore was
characterized only by X-ray diffraction and IR spectroscopy. X-
ray-quality crystals of the imido complexes 2−4 were obtained
from pentane (2 and 4) and hexane (3) at −35 °C. The
structures (Figure 3) display trigonal-planar Cr centers (sums
of angles around Cr are 359.9° (2), 360.0° (3), and 359.6° (4))
with relatively short Cr−imido distances of 1.651(2) Å (2),
1.632(1) Å (3), and 1.656(2) (4). We note that Wolczanski
and co-workers reported several related trigonal Cr imido
species bearing a silox ligand, Cr(OSitBu3)2(NR). The structure
of Cr(OSitBu3)2(N-2,6-diphenylphenyl) similarly demonstrated
trigonal geometry at the Cr center and a short Cr−N bond
distance of 1.649(2) Å.32 Solution magnetic moments of
compounds 2 and 3, obtained using the Evans method,34 were
Figure 2. Molecular structure of Cr2(OCtBu2Ph)4 (1).
Table 1. Selected Structural Data for the Compounds
Reported in This Paper
found to be μobs = 2.5
0.3 μB and μobs = 3.3
0.4 μB,
respectively, in comparison with 2.7−2.8 μB values observed for
Cr(OSitBu3)2(NR).32 These values are indicative of triplet
ground states, which are consistent with DFT calculations (see
below).
complex
Cr−ORterminal (Å)
Cr−NR′ (Å)
RO−Cr−OR (deg)
1
2
3
4
5
6
7
1.833(2)/1.835(2)
1.766(2)/1.777(2)
1.780(1)/1.783(1)
1.760(1)/1.789(1)
1.776(4)/1.795(4)
1.651(2)
114.41(8)
117.57(5)
116.38(7)
111.41(18)
113.4(3)
1.632(1)
The bis(alkoxide) monoimido species 2 and 3 were
optimized as a singlet, triplet, and quintet at the B3LYP/6-
31G(d) level of theory. No simplifications were made to the
bulky alkoxide ligands. For 2, the triplet was calculated to be
lowest in free energy, followed by the singlet (+11.42 kcal/mol)
and then the quintet (+20.74 kcal/mol). A similar pattern was
observed for 3 with triplet < singlet (+13.31 kcal/mol) <
quintet (+33.97 kcal/mol). This finding is consistent with the
experimental and calculated spin states in the related CrNR′
species supported by siloxide ligands.32 On the basis of the
large thermodynamic preference for the triplet state that
matches the experimental magnetic data, we focused on the
triplet for the structural and electronic analysis. A trigonal-
planar arrangement about Cr was observed in the optimized
structures of 2 and 3 (Figure 4). Excellent agreement with the
X-ray data is observed for the Cr−N bond lengths in 2 (1.655
vs 1.651 Å) and 3 (1.635 vs 1.632 Å). It is worth noting that
one of the alkoxide arms is oriented with the phenyl group anti
to the imido group in 2, in contrast to the crystallographic
1.656(2)
1.641(5)/1.664(5)
1.648(7)/1.667(7)
a
1.770(5)/1.780(5)
1.945(4)/1.944(4)
173.17(16)
a
Of the two independent molecules present in the asymmetric unit,
data for only one are presented. The second molecule features similar
structural data.
interaction between the Cr(II) centers. As the electronic
structure of the related compound Cr2(OSitBu3)4 has been
studied in great detail,32 it was not pursued in this work.
Synthesis and Characterization of Cr(IV) Imido
Complexes. To study the group-transfer chemistry of
Cr2(OR)4 (1), we treated complex 1 with 2 equiv of mesityl
azide (N3Mes), adamantyl azide (N3Ad), or 2,6-diethylphenyl
azide (N3DEP) (Scheme 2). This reaction, followed by
recrystallization, affords the deep red Cr mono(imido)
complexes Cr(OR)2(NMes) (2, 81%), Cr(OR)2(NAd) (3,
64%), and Cr(OR)2(NDEP) (4, 23%). Treatment of 1 with
excess azide N3R1 (R1 = Mes, Ad) does not form the
corresponding bis(imido) complexes. We also note that the
treatment of 1 with bulkier azides (2,6-diisopropylphenyl azide
and 2,4,6-triphenylphenyl azide) failed to form the correspond-
ing imido complexes. All Cr mono(imido) compounds were
t
orientation, where one of the Bu groups is anti to the imido
group in both alkoxide arms.
The electronic structure of 2 was investigated by calculating
the corresponding orbitals35 that are shown in Figure 5. A
similar analysis for 3 may be found in the Supporting
Information. There are two singly occupied molecular orbitals
Figure 3. Molecular structures of compounds 2 (left), 3 (middle), and 4 (right).
C
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bulky Ar groups (Ar = 4-methoxyphenyl, 4-trifluorophenyl).
The reaction of compound 1 with 2 equiv of 4-methoxyphenyl
azide or 4-trifluorophenyl azide results in the formation of deep
red-brown Cr(VI) diimido complexes Cr(OR)2(NAr)2 that are
isolated as brown crystals in 46% and 30% yields, respectively.
The reaction of 1 with 1 equiv of 4-methoxyphenyl azide also
leads to the formation of Cr(VI) bis(imido) species, as
1
demonstrated by H NMR and IR spectroscopy. We were
not able to isolate or observe mono(imido) species from these
reactions. For [Cr(OSitBu3)2], it was also reported that bulky
azides form mono(imido) species while nonbulky azides form
bis(imido) complexes at room temperature.32
Figure 4. Optimized structures of triplet 2 (left) and 3 (right).
Important bond lengths are given in Å. Note that parts of the alkoxide
ligands are excluded from the image for clarity.
Cr(VI) bis(imido) complexes 5 and 6 are diamagnetic
1
species that were characterized by H and 13C NMR (and 19F
for 6) spectroscopy, IR spectroscopy, X-ray crystallography,
and elemental analysis. NMR spectroscopy is consistent with an
effective C2v symmetry in solution, displaying a single type of
tBu group and a phenyl group for both [OR] ligands. X-ray
crystallography (Figure 6) demonstrates pseudotetrahedral
complexes featuring RO−Cr−OR angles of 111.4(2)° (5)
and 113.4(3)° (6) and ArN−Cr−NAr angles of 104.9(3)° (5)
and 106.6(4)° (6). Cr−imido bond distances are 1.641(5)/
1.664(5) Å for 5 and 1.648(7)/1.667(7) Å for 6.
Reactions of Cr Imido Complexes with Isocyanides.
The focus of the current work is on the investigation of the
reactivity of Cr imido species with isocyanides. To test the
stoichiometric reactivity of the Cr mono(imido) complexes,
complexes 2 and 3 were treated with 1 equiv of 2,6-
dimethylphenyl isocyanide (2,6-Me2PhNC) in C6D6 in the
presence of an internal standard. Monitoring of the reaction by
1H NMR spectroscopy (see Figure 7) revealed the disappear-
ance of the isocyanide peaks and the appearance of new peaks
that were attributed to formation of the respective
carbodiimides N-(2,6-dimethylphenyl)-N-mesitylmethanedii-
mine19 and N-adamantyl-N-(2,6-dimethylphenyl)-
methanediimine (eq 1). The observed NMR yields of the
carbodiimides were 100% and 68%, respectively. The formation
of carbodiimides was also confirmed by mass spectrometry.
Figure 5. Corresponding orbitals for triplet 2. Orbital isosurfaces are
plotted at an isodensity value of 0.05 au.
based on the Cr ion and two doubly occupied π orbitals
between the imido fragment and Cr that are mainly based on
NR′. On the basis of this orbital occupation pattern, 2 and 3 are
best characterized as having a Cr(IV) ion that is triply bonded
to the NR′ fragment.36
1
Cr(OR)2(NR ) + 2,6‐Me2PhNC
1
→ (2,6‐Me2Ph)NCNR + [Cr]
(1)
Synthesis of Cr(VI) Bis(imido) Complexes. Azides
featuring relatively bulky R groups (R = adamantyl, mesityl,
2,6-diethylphenyl) led selectively to the formation of mono-
(imido) complexes. We have also investigated the reactivity of
the Cr(II) precursor with selected aryl azides featuring less
To gain insight into this reaction, we calculated the binding
of 2,6-dimethylphenyl isocyanide to the triplet Cr(IV) imido
intermediates. We denote these isocyanide-bound intermedi-
ates as 2−CNR″ and 3−CNR″ for mesityl and adamantyl,
Figure 6. Molecular structures of compounds 5 (left) and 6 (right).
D
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and 1.924 Å is observed. The new C−N bond is still relatively
long at 1.899 Å; however, visualization of the normal mode
corresponding to the imaginary frequency shows significant
motion along the C−N bond. The triple-bond character of
isocyanide is slightly reduced, with elongation from 1.172 Å in
3−CNR″ to 1.197 Å in this transition state. An intrinsic
reaction coordinate37 calculation demonstrated that this
transition state connects 3−CNR″ to a species with
carbodiimide bound to Cr(OR)2. Optimization of the
carbodiimide adducts for both mesityl and adamantyl (see
the Supporting Information for structural details) resulted in
structures that are uphill relative to free isocyanide and 2/3 by
9.2 and 0.6 kcal/mol, respectively. The endergonic reaction
energy for 2 may indicate why we have been unable to identify
the transition state for carbodiimide formation with this species.
Alternative pathways involving multiple isocyanides bound to
Cr (vide infra) and the possible role of the quintet surface in
this mechanism are currently being explored.
Using 1 equiv of isocyanide, we were able to observe the
formation of carbodiimide; however, we were not able to isolate
the Cr-containing product. Thus, we decided to treat Cr
mono(imido) species with excess isocyanide. The reaction of
complex 2 with 5 equiv of 2,6-methylphenyl isocyanide resulted
in complete conversion to the respective carbodiimide, as
1
indicated by H NMR spectroscopy (eq 2). Solvent removal
and crystallization at −35 °C from hexanes produced deep
brown crystals of CrII(OR)2(CNR2)4 (7; 56%). Chromium-
ligated isocyanides can be further converted to carbodiimides:
the reaction of complex 7 with 5 equiv of MesN3 produced the
corresponding carbodiimide in 96% yield (based on iso-
cyanide), as indicated by H NMR spectroscopy (eq 3). We
were not able to isolate a chromium-containing product from
this reaction.
Figure 7. (A) C6D6 spectrum of isocyanide 2,6-Me2NC (δ 6.71, 6.57,
and 2.05 ppm) in the presence of an internal standard (trimethox-
ybenzene, δ 6.25 and 3.30 ppm) in the 1−8 ppm region. (B) Spectrum
of the mixture from (A) with complex 2, demonstrating the formation
of (2,6-Me2Ph)NCNMes (δ 6.85, 6.65, 2.30, 2.28, and 2.07
ppm).
1
respectively. Formation of 2−CNR″ and 3−CNR″ is slightly
uphill by 11.7 and 5.3 kcal/mol. Isocyanide binding disrupts π
bonding to the imido group (see Figure 8, left), as evidenced by
elongation of the Cr−N bond from 1.655 to 1.672 Å for 2−
CNR″ and from 1.635 to 1.653 Å for 3−CNR″. Concurrently,
the Cr−N−R′ angle decreases from 177.1 to 168.5° in 2−
CNR″ and from 178.6 to 160.9° in 3−CNR″. Next, we
attempted to find a transition state for nucleophilic attack by
this coordinated isocyanide on the imido group. Attempts with
mesityl have been unsuccessful thus far, but the optimized
transition state for adamantyl is shown in Figure 8 (right).
This transition state is calculated to be uphill in free energy
by 23.3 kcal/mol relative to free isocyanide and 3, which should
be accessible under the experimental conditions. Elongation of
the Cr−N and shortening of the Cr−C bond lengths to 1.721
Cr(OR)2(NMes) + 5ArNC
→ MesNCNAr (100%)
+ Cr(OR)2(CNAr)4 (56%)
(2)
Cr(OR)2(CNAr)4 + 5MesN3
→ MesNCNAr (96%) + [Cr]
(3)
Compound 7 was characterized by X-ray crystallography, IR
spectroscopy, and solution magnetic moment measurements.
The X-ray structure of compound 7 (Figure 9) reveals a
pseudooctahedral Cr(II) complex containing two alkoxide
ligands and four isocyanides. This compound is a rare example
Figure 8. Optimized structure of 3−CNR″ (left) and the transition state for carbodiimide formation for 3 (right). Important bond lengths are
labeled in Å and bond angles in deg. Note that parts of the alkoxide ligands and isocyanide/imido substituents are excluded from the image for
clarity.
E
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Figure 9. Molecular structure of complex 7.
of a Cr complex combining isocyanide and alkoxide ligands.38
Wolczanski and co-workers synthesized [Cr-
(OSitBu3)2(CNtBu)2]. On the basis of the two CN absorptions
(major peak at 2192 cm−1, minor peak at 2099 cm−1), the
compound was proposed to exist as a mixture of cis and trans
(major) isomers.32 In 7, the bulky alkoxides occupy mutually
trans (axial) positions, whereas isocyanides form an equatorial
plane. The IR spectrum of 7 reveals the CN group absorption
at 2063 cm−1 (singlet), versus 2116 cm−1 in free 2,6-
dimethylphenyl isocyanide.39,40 Other Cr(II) isocyanide
complexes generally demonstrate lower CN stretching
frequencies,41 thus indicating relatively electron-deficient
isocyanides in the bis(alkoxide) complex 7 or in [Cr-
(OSitBu3)2(CNtBu)2].
1
Figure 10. (A) H NMR spectrum of complex 6 in the 1−9 ppm
range. (B) Spectrum of 2,6-dimethylphenyl isocyanide in the presence
of an internal standard (TMB). (C) Spectrum of the mixture of the
isocyanide and complex 6, demonstrating the lack of formation of the
corresponding carbodiimide.
Cr(OR)2(N(4‐CFPh))2 + ArNC → no reaction
(4)
3
Catalytic Formation of Carbodiimides. Having estab-
lished the stoichiometric reactivity modes of bis(alkoxide) Cr
imido complexes with 2,6-dimethylphenyl isocyanide, we
turned to investigate the catalytic reactivity of the Cr(II)
precursor with various mixtures of aryl/alkyl azides and
isocyanides. The experiments were conducted by treating 2.5
mol % of complex 1 with R2NC (R2 = 2,6-Me2Ph, 4-OMePh,
Ad, 2-Cl-6-MePh) and various organoazides at room temper-
ature for 24 h in C6D6. In selected cases (Table 2, entries 4, 8,
and 9), the mixture was heated to 60 °C. The results of the
catalytic experiments are given in Table 2. The combination of
organoazides R1N3 (entries 1−3 and 5−7) featuring relatively
bulky aryl substituents (R1 = Mes, 2,6-Et2Ph, 2-iPrPh) with aryl
isocyanides R2NC (R2 = 2,6-Me2Ph, 4-OMePh, 2-Cl-6-MePh)
successfully forms carbodiimides in high yields. In contrast,
combination of an adamantyl group at either the azide or
isocyanide with a mesityl/2,6-dimethylphenyl counterpart
produced only low yields of the resulting carbodiimide product
(see entries 4 and 8) at room temperature. The yield can be
somewhat increased for R1 = Mes and R2 = Ad by heating to 60
°C for several hours; further heating leads to the decomposition
of starting materials. As expected from the stoichiometric
experiments (see above), no carbodiimides form for aryl azides
with aryl groups lacking ortho substituents (R1 = 4-CF3Ph, 4-
OMePh; entries 10 and 11). Similarly, no carbodiimides form
for the very bulky aryl azides 2,6-diisopropylphenyl azide and
2,4,6-triphenylphenyl azide (for an example of 2,6-diisopropyl-
phenyl azide, see entry 12), for which no Cr(IV) imido
formation was observed.
Reactions of Cr(VI) Bis(imido) Complexes with
Isocyanides. We have also investigated the reactivity of
Cr(VI) bis(imido) complexes 5 and 6 with 2,6-methylphenyl
isocyanide. The experiments were conducted in a manner
similar to the experiments involving [NR] transfer from Cr(IV)
mono(imido) complexes: C6D6 solutions of the relevant
Cr(VI) complex were treated with C6D6 solutions containing
6 equiv of 2,6-methylphenyl isocyanide and the internal
standard trimethoxybenzene. In both cases, no reaction was
observed (eq 4). Figure 10 demonstrates the 1H NMR
spectrum of complex 6 in the range 1−9 ppm (A), the
spectrum of the isocyanide in the presence of the internal
1
standard in the same range (B), and the H NMR spectrum
resulting upon mixing a solution of the isocyanide/standard
from A with the solution of the complex from B (C). Spectrum
C clearly demonstrates that no change takes place. We
therefore conclude that the formation of Cr(VI) bis(imido)
complexes encumbers the imido group transfer to isocyanides.
It is possible that, due to the steric hindrance, Cr(VI)
bis(imido) cannot bind an additional ligand (isocyanide),
thus preventing formation of carbodiimide. We note that
Wilkinson, Hursthouse, and co-workers reported imido group
transfer to an isocyanide at the Cr(VI) bis(imido) complex
Cr(NtBu)2(Mes)2 at room temperature to form an isolable
metal-bound carbodiimide.42 We have also investigated the
possibility of carbodiimide formation by treating complex 7
with the nonbulky azides N3R1 (R1 = 4-methoxyphenyl, 4-
trifluorophenyl). Monitoring the reaction by 1H NMR
spectroscopy indicated no formation of the respective
carbodiimides (see the Supporting Information).
The carbodiimides N-(2,6-dimethylphenyl)-N′-mesitylme-
thanediimine (entry 1), N-(2,6-diethylphenyl)-N′-(2,6-
F
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f
Table 2. Catalytic Formation of Asymmetric Carbodiimides from the Corresponding Azides and Isocyanides
a
b
Heated to 60 °C for 15 h. The reaction at room temperature produced only a small amount of the product; we were not able to quantify the
c
d
amount of carbodiimide formed due to its small amount, and peaks overlap with those of the starting materials. Heated to 60 °C for 8 h. Heated to
60 °C for 14 h. We were not able to obtain his carbodiimide in pure form due to the presence of a small unknown impurity. All of the experiments
e
f
were conducted at room temperature for 24 h unless otherwise indicated.
dimethylphenyl)methanediimine (entry 2), N-(2,6-dimethyl-
phenyl)-N-(2-isopropylphenyl)methanediimine (entry 3), N-
mesityl-N-(4-methoxyphenyl)methanediimine (entry 5), and
N-mesityl-N-(2-chloro-6-methylphenyl)methanediimine (entry
co-workers,19c other carbodiimides are new compounds that
have not been reported before. All newly synthesized
carbodiimides appear to be mildly air sensitive. Their NMR
spectra are unremarkable and are given in the Supporting
Information. All carbodiimides give rise to a molecular ion in
ESI-MS (see the Experimental Section). The most notable IR
feature of carbodiimides involves strong signals around 2140
cm−1 (see the Supporting Information for details) attributed to
the NCN stretch.43
1
7) were isolated in moderate yields and characterized by H
and 13C NMR spectroscopy, IR spectroscopy, mass spectrom-
etry, and elemental analysis (see the Supporting Information
for details). We also obtained catalytic formation of N-(2,6-
diethylphenyl)-N-(4-methoxyphenyl)methanediimine (entry 6)
in good yield; however, we were not able to obtain this
carbodiimide in pure form due to a persistent unknown
impurity. The product was detected by ESI-MS. Whereas
catalytic formation and isolation of N-(2,6-dimethylphenyl)-N′-
mesitylmethanediimine was previously reported by Warren and
SUMMARY AND CONCLUSIONS
■
We have demonstrated that bulky aryl and alkyl azides form
selectively Cr(IV) imido complexes supported by two bulky
G
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Article
(m), 1202 (s), 1094 (s), 1053 (m), 1012 (m), 893 (w), 851 (m), 777
(s), 702 (s), 762 (s). μeff = 2.5 0.3 μB (calcd 2.8 μB). Anal. Calcd for
C39H57CrNO2: C, 75.1; H, 9.2. Found: C, 74.9; H, 9.4.
alkoxides, whereas nonbulky aryl azides lead to the formation of
Cr(VI) bis(imido) complexes. Trigonal-planar Cr(IV) imido
bis(alkoxide) complexes are capable of nitrene transfer to
isocyanides to form carbodiimides, whereas Cr(VI) bis(imido)
complexes are inert to this transformation. DFT calculations
indicate a possible mechanism for carbodiimide formation;
however, the calculated product with carbodiimide coordinated
to Cr(OR)2 was unable to be isolated experimentally. The
reaction is catalytic for selected azides and aryl isocyanides, with
two major limitations: (1) relatively small azides do not form
carbodiimide product due to the formation of unreactive
Cr(VI) bis(imido) complexes and (2) excessively bulky azides
do not form any carbodiimide product, as they are incapable of
imido formation. Our future endeavors in this project will focus
on bulkier alkoxide ligands which may restrict even small azides
to the formation of mono(imido) complexes, thus increasing
the scope of possible substrates for carbodiimide formation.
Cr(OR)2(NAd) (3). A solution of 5 mL of AdN3 (4.7 mg, 0.0265
mmol) in toluene was added to a solution of compound 1 (12.3 mg,
0.0125 mmol) in toluene in one portion. The solution immediately
changed color from green to deep red. The reaction mixture was
stirred for 4 h, upon which the solvent was removed. Crystallization
from hexanes at −35 °C overnight gave crystals of deep red
Cr(OR)2(NAd) (10.1 mg, 64% yield). IR (cm−1): 2900 (m), 2891
(m), 1487 (m), 1440 (m), 1387 (m), 1360 (m), 1206 (m), 1053 (s),
1009 (s), 901 (m), 771 (s), 704 (s). μeff = 3.3 0.4 μB (calcd 2.8 μB).
Anal. Calcd for C40H61CrNO2: C, 75.1; H, 9.6, N, 2.2. Found: C, 75.2;
H, 9.8, N, 2.2.
Cr(OR)2(NDEP) (4). A solution of 5 mL of N3DEP (2.4 mg, 0.014
mmol) in toluene was added to a solution of compound 1 (28.0 mg,
0.029 mmol) in toluene in one portion. The solution immediately
changed color from green to deep red. The reaction mixture was
stirred for 4 h, upon which the solvent was removed. Crystallization
from pentane at −35 °C over 2 weeks gave crystals of deep red
Cr(OR)2(NDEP) (8.5 mg, 23% yield). The low isolated yield of
compound 4 is likely due to its high solubility. The crystallized
compound was characterized by X-ray crystallography and IR
spectroscopy; the low isolated yield of the compound and prolonged
crystallization times prevented us from further characterization of this
compound. IR (cm−1): 2967 (w), 2878 (w), 2832 (w), 1597 (w), 1474
(w), 1432 (w), 1204 (m), 1092 (w), 1067 (s), 744 (s), 706 (s).
Cr(OR)2(N(4-CH3OPh))2 (5). A solution of 4-azidoanisole (0.5 M in
tert-butyl methyl ether, 0.110 mL) was added to a 5 mL solution of
complex 1 (26.0 mg, 0.0265 mmol) in toluene. The solution color
changed immediately from green to deep brown. The reaction mixture
was stirred for 4 h, upon which the volatiles were removed.
Crystallization from hexanes at −35 °C overnight gave deep brown
crystals of Cr(OR)2(N(4-CH3OPh))2 (18.2 mg, 46%). IR (cm−1):
2963 (w), 1585 (s), 1485 (m), 1250 (s), 1157 (s), 991 (m), 829 (s).
EXPERIMENTAL SECTION
■
General Methods and Procedures. All reactions involving air-
sensitive materials were executed in a nitrogen-filled glovebox. LiOR,28
mesityl azide,19c 2-isopropylphenyl azide,44a 2,6-diisopropylphenyl
azide,44b 2,6-diethylphenyl azide,44c and Cr[N(SiMe3)2]2(THF)2
30
were synthesized according to previously reported procedures.
Chromium(II) chloride was purchased from Strem. Lithium bis-
(trimethylsilyl)amide, adamantyl azide, 4-(trifluoromethyl)phenyl
azide solution, 4-azidoanisole solution, 2,6-dimethylphenyl isocyanide,
4-methoxyphenyl isocyanide, 1-adamantyl isocyanide, and 2-chloro-6-
methylphenyl isocyanide were purchased from Aldrich and used as
received. All solvents were purchased from Fisher Scientific and were
of HPLC grade. The solvents were purified using an MBraun solvent
purification system and stored over 3 Å molecular sieves. Compounds
1−7 were characterized by 1H and 13C NMR, IR, and mass
spectroscopy, solution state magnetic susceptibility, X-ray crystallog-
raphy, and elemental analysis. Isolated carbodiimides were charac-
terized by 1H and 13C NMR, IR, and mass spectrometry and elemental
analysis. NMR spectra were recorded at the Lumigen Instrument
Center (Wayne State University) on a Varian Mercury 400 MHz
NMR spectrometer in C6D6 at room temperature. Chemical shifts and
coupling constants (J) are reported in parts per million (δ) and hertz,
respectively. IR spectra of powdered samples were recorded on a
Shimadzu IR Affinity-1 FT-IR spectrometer outfitted with a
MIRacle10 attenuated total reflectance accessory with a monolithic
diamond crystal stage and pressure clamp. Solution state effective
magnetic moments were determined using the Evans method. Low-
resolution mass spectra were obtained at the Lumigen Instrument
Center utilizing a Waters Micromass ZQ mass spectrometer (direct
injection, with capillary at 3.573 kV and cone voltage of 20.000 V).
Only selected peaks in the mass spectra and in the IR spectra are
reported below. Analyses were performed by Midwest Microlab LLC
and Galbraith Laboratories, Inc.
3
3
1H NMR (C6D6): δ 8.66 (d, JHH = 7.6 Hz, 2H), 7.75 (d, JHH = 8.8
3
Hz, 2H), 7.39 (m, 2H), 7.17 (m, 8H), 6.37 (d, JHH = 8.8 Hz, 4H),
3.05 (s, 6H), 1.49 (s, 36H). 13C NMR (C6D6): δ 147.76, 130.17,
129.00, 127.11, 127.05, 125.88, 125.69, 113.61, 54.81, 44.59, 31.05
ppm. Anal. Calcd for complex 5: C, 72.1; H, 8.3, N, 3.8. Found: C,
72.3; H, 8.4; N, 3.7.
Cr(OR)2(N(4-CF3Ph))2 (6). To a solution of 5.0 mL of complex 1
(37.2 mg, 3.79 mmol) in toluene was added 4-(trifluoromethyl)phenyl
azide solution (0.5 M in tert-butyl methyl ether, 0.30 mL) dropwise.
Immediately gas evolution was observed, along with a change in
solution color to deep brown. The reaction mixture was stirred for 4 h,
upon which the solvents were removed in vacuo to yield a brown
residue. Crystallization from hexanes at −35 °C overnight gave deep
brown crystals of Cr(OR)2(N(4-CF3Ph))2 (15.6 mg, 30%). IR (cm−1):
2966 (w), 1597 (w), 1319 (s), 1165(m), 1103 (m), 975 (s), 833 (m).
3
3
1H NMR (C6D6): δ 8.38 (d, JHH = 9.2 Hz, 2H), 7.68 (d, JHH = 6.8
3
Hz, 2H), 7.01 (m, 4H), 7.22 (m, 2H), 6.93 (d, JHH = 8.0 Hz, 4H),
6.76 (d, JHH = 8.4 Hz, 4H), 1.38 (s, 36H) . 13C NMR (C6D6): δ
3
Synthesis and Characterization of Chromium Complexes.
[Cr2(OR)4] (1). A solution of 5 mL of HOR31 (57.6 mg, 0.261 mmol) in
THF was added at once to Cr[N(SiMe3)2]2(THF)2 (67.5 mg, 0.131
mmol) in THF. The solution gradually turned from light violet to
green. The reaction mixture was stirred for 6 h, upon which the solvent
was removed, and crystallization from hexanes at −35 °C overnight
produced green crystals of Cr2(OR)4 (38.2 mg, 60% yield). IR (cm−1):
2996 (m), 2916 (m), 2349 (m), 1059 (m), 949 (s), 745 (s), 706 (s).
Anal. Calcd for C60H92Cr2O4: C, 73.4; H, 9.5. Found: C, 72.8; H, 9.2.
μeff = 2.5 0.3 μB (calcd 2.8 μB).
Cr(OR)2(NMes) (2). A solution of 74 μL of MesN3 in ether (0.764
M) was added to a solution of complex 1 (28.1 mg, 0.0286 mmol) in
toluene in one portion. The solution immediately changed color from
green to deep red. The reaction mixture was stirred for 4 h, upon
which the solvent was removed. Crystallization from pentane at −35
°C gave crystals of deep red Cr(OR)2(NMes) (29.1, 81% yield). IR
(cm−1): 2943 (m), 2877 (w), 2122 (m), 1601 (m), 1479 (m), 1389
163.76, 146.44, 129.73, 129.00, 127.03, 126.40, 126.18, 126.00, 124.21,
100.70, 44.86, 30.91 ppm. Anal. Calcd for complex 6: C, 65.3; H, 6.7,
N, 3.5. Found: C, 65.4; H, 6.6; N, 3.4.
Reaction of Cr(OR)2(NMes) (2) with 1 Equiv of 2,6-Dimethyl
Isocyanide. To a solution of 5.0 mL of complex 2 (35.2 mg, 0.056
mmol) in C6D6 were added 1 equiv of 2,6-dimethylphenyl isocyanide
(7.3 mg, 0.056 mmol) and 1 equiv of 1,3,5-trimethoxybenzene (TMB,
9.3 mg, 0.055 mmol) in C6D6. The solution changed color from green
to deep brown and was stirred for 2 h, upon which a 1H NMR
spectrum was taken to show complete conversion to the
corresponding carbodiimide.
Reaction of Cr(OR)2(NMes) (2) with 5 Equiv of 2,6-
Dimethylphenyl Isocyanide: Observation of Carbodiimides
and Isolation of Compound 7. To a solution of 5.0 mL of complex
2 (19.8 mg, 0.032 mmol) in C6D6 were added 5 equiv of 2,6-
dimethylphenyl isocyanide (19.5 mg, 0.149 mmol) and 1 equiv of
TMB (5.6 mg, 0.0333 mmol) in C6D6. The solution changed color
H
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1
from green to deep brown and was stirred for 2 h, upon which a H
NMR spectrum was taken to show complete conversion to the
corresponding carbodiimide. Once the complete conversion was
established, the volatiles were removed in vacuo to yield a brown
residue. The residue was dissolved in hexanes and placed in the freezer
at −35 °C to produce deep orange crystals of complex 7 (16.9 mg,
56%).
Independent Synthesis of Cr(OR)2(CN(2,6-Me2Ph))4 (7) from
the Cr(II) Precursor Cr2(OR)4 (1). A solution of 5 mL of CN(2,6-
Me2Ph) (34.0 mg, 0.259 mmol) in toluene was added to a solution of
compound 1 (30.9 mg, 0.0315 mmol) in toluene in one portion. The
solution immediately changed color from green to deep brown. The
reaction mixture was stirred for 4 h, upon which the solvent was
removed. Crystallization from hexanes at −35 °C overnight produced
crystals of deep orange Cr(OR)2(CN(2,6-Me2Ph))4 (40.1 mg, 65%
yield). μeff = 2.3 0.2 μB (calcd 2.8 μB). Anal. Calcd for complex 7: C,
78.1; H, 8.1, N, 5.5. Found: C, 77.2; H, 8.6; N, 5.4.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00703.
Crystallographic data (CIF)
Cartesian coordinates for the calculated structures (XYZ)
Characterization data for carbodiimides, NMR, IR, UV−
vis, and mass spectra, Evans data, X-ray crystallographic
details, and DFT calculation details (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.G.: groysman@chem.wayne.edu.
■
Notes
The authors declare no competing financial interest.
Reaction of Cr(OR)2(N(4-CH3OPh))2 (5) with 2,6-Dimethyl-
phenyl Isocyanide. To a solution of 5.0 mL of complex 5 (12.7 mg,
0.0173 mmol) in C6D6 were added 6 equiv of 2,6-dimethylphenyl
isocyanide (14.4 mg, 0.110 mmol) and 1 equiv of TMB (5.30 mg,
0.0315 mmol) in C6D6. The solution was stirred for 2 days, upon
ACKNOWLEDGMENTS
■
Acknowledgment is made to the donors of the American
Chemical Society Petroleum Research Fund for support of this
research (under Grant No. 54178-DNI3). M.Y. thanks the
WSU Graduate School for the Rumble Fellowship. R.L.L. and
D.J.T. acknowledge financial support from the Research
Corporation through a Cottrell College Science Award to
R.L.L. Computational resources were provided by NSF-MRI
award No. CHE-1039925 through the Midwest Undergraduate
Computational Chemistry Consortium.
1
which an H NMR spectrum was taken to show no formation of the
corresponding carbodiimide and no consumption of the starting
isocyanide and complex 5.
Reaction of Cr(OR)2(N(4-CF3Ph))2 (6) with 2,6-Dimethylphen-
yl Isocyanide. To a solution of 5.0 mL of complex 6 (26.6 mg, 0.033
mmol) in C6D6 were added 6 equiv of 2,6-dimethylphenyl isocyanide
(2.5 mg, 0.192 mmol) and 1 equiv of TMB (6.0 mg, 0.036 mmol) in
1
C6D6. The solution was stirred for 2 days, upon which a H NMR
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■
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DOI: 10.1021/acs.organomet.5b00703
Organometallics XXXX, XXX, XXX−XXX