A Chromium(II) Tetracarbene Complex Allows Unprecedented Oxidative Group Transfer

Article abstract of DOI:10.1021/acs.inorgchem.7b02253

Multiple distinct oxidative group transfer reactions to low valent chromium were examined. Six new chromium complexes were prepared from a highly electronically unsaturated Cr(II) square planar complex that was supported by a macrocyclic tetracarbene ligand. This complex's reactivity with Me₃NO and disparate azides was investigated. The reaction with Me₃NO generated a highly stable Cr(IV)-oxo complex. Less bulky organic azides such as p-tolyl and n-octyl azides gave rise to metallotetrazenes, while more sterically demanding mesityl and adamantyl azides generated Cr(IV)-imido complexes. The reaction of the square planar Cr(II) complex with TMS-azide yielded the first linearly bridging nitrido chromium species. Reductive group transfer was explored for a Cr(IV)-imido complex, and organic products, such as aziridines, were formed after addition. Cr(IV) imidos and oxos are quite rare, while tetrazenes and bridging nitridos are virtually unknown. This is the most detailed study on oxidative group transfer reactions using chromium based complexes on a single auxiliary ligand to date.

Full text of DOI:10.1021/acs.inorgchem.7b02253

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
A Chromium(II) Tetracarbene Complex Allows Unprecedented
Oxidative Group Transfer
Gaya R. Elpitiya,† Brian J. Malbrecht,‡ and David M. Jenkins*,†
†Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
‡Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States
S
* Supporting Information
ABSTRACT: Multiple distinct oxidative group transfer reactions to low valent
chromium were examined. Six new chromium complexes were prepared from a highly
electronically unsaturated Cr(II) square planar complex that was supported by a
macrocyclic tetracarbene ligand. This complex’s reactivity with Me3NO and disparate
azides was investigated. The reaction with Me3NO generated a highly stable Cr(IV)-
oxo complex. Less bulky organic azides such as p-tolyl and n-octyl azides gave rise to
metallotetrazenes, while more sterically demanding mesityl and adamantyl azides
generated Cr(IV)-imido complexes. The reaction of the square planar Cr(II) complex
with TMS-azide yielded the first linearly bridging nitrido chromium species. Reductive
group transfer was explored for a Cr(IV)-imido complex, and organic products, such as
aziridines, were formed after addition. Cr(IV) imidos and oxos are quite rare, while
tetrazenes and bridging nitridos are virtually unknown. This is the most detailed study on oxidative group transfer reactions using
chromium based complexes on a single auxiliary ligand to date.
INTRODUCTION
■
The exploration of new metal−ligand multiple bonds on earth
abundant transition metals continues to be an active area of
research in inorganic chemistry due to their importance as
intermediates in oxidative group transfer reactions, ranging
from alkane hydroxylation1−7 to aziridination.8−13 For example,
biomimetic iron-oxo complexes have been scrutinized exten-
sively due to their ability to mimic oxidative reactions that
occur in both cytochrome P450 and nonheme en-
zymes.3,6,7,14,15 Recent iron oxo and imido complexes have
demonstrated remarkable reactivity including novel routes for
C−H bond functionalization.12,16,17 In each of these cases, the
key iron−ligand multiple bond is formed through an oxidative
group transfer reaction. Despite widespread fresh investigations
of oxidative group transfer on iron complexes in the past
decade, few recent studies have been reported on early
transition metals such as chromium.17−20
While chromium oxo and imido complexes have been
synthesized via oxidative group transfer, they are almost all high
valent Cr(V) and Cr(VI) complexes supported with auxiliary
ligands such as porphyrins, phthalocyanins, salens, or β-
diketiminates.21−28 Furthermore, these high valent chromium
Figure 1. Selected examples of the limited midvalent Cr(IV) oxos (A),
Cr(IV) imidos (B), and Cr(III) tetrazenes (C) that were previously
reported.
complexes often showed limited reactivity for reductive transfer
of the oxo or imido.20,21,26,29−32 While lower valent chromium
complexes may have more facile group transfer of an
imido33−39 or oxo,40−44 only a very limited number of these
complexes have been synthesized (Figure 1).
One tactic to improve group transfer on chromium
complexes is to switch to stronger σ-donating auxiliary ligands,
which have proven very successful in developing novel oxidative
group transfer reactions on iron.45−51 Macrocyclic N-
heterocyclic carbenes (NHCs) are strong σ-donor ligands
that are structurally similar to, but electronically distinct from,
other macrocyclic ligands, such as porphyrins, that have been
Received: September 1, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02253
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Inorganic Chemistry
Article
studied extensively on chromium.40,42 Macrocyclic NHCs have
proven versatile for probing group transfer on iron as both
iron(IV) oxos and iron(IV) tetrazenes have been synthe-
sized.10,52−54 Furthermore, macrocyclic NHC iron complexes
are quite effective for supporting catalytic group transfer
reactions, particularly aziridination.9−11 Despite their potential
application for oxidative group transfer on chromium, only one
chromium tetra-NHC complex has been reported previously.55
In this paper, we assess whether midvalent chromium(IV)
imidos and oxos are viable for group transfer reactions in a
manner similar to that for iron(IV), by showcasing a
comprehensive survey of chromium(IV) complexes with
distinct metal−ligand multiple bonds supported by a single
auxiliary ligand. This begins with the synthesis of an
electronically unsaturated square planar Cr(II) complex with
our second generation NHC-borate macrocycle.56 This Cr(II)
complex is highly reactive toward group transfer oxidants such
as organic azides and Me3NO. Sterically bulky azides, such as
adamantyl azide, lead to the formation of rare Cr(IV) imido
complexes. Less encumbered organic azides, such as octyl azide,
lead to metallotetrazene complexes that are formed in a formal
[2 + 3] cycloaddition reaction from the metal-imido
intermediate. These metallotetrazenes are the first examples
of Cr(IV) tetrazenes. Notably, the reaction of the Cr(II)
complex with TMS-N3 gives the first isolable Cr(III/IV) μ-
nitrido species. In addition to the oxidative group transfer, the
reactivities of the Cr(IV) imido and oxo species were tested for
reductive group transfer. While the chromium oxo complex
showed no release of the oxo ligand, the chromium imido
complex showed excellent reactivity toward group transfer to
decene and triphenyl phosphine, yielding the aziridine and
phosphanimine, respectively. This broad selection of complexes
and reactions with azides makes this macrocyclic NHC system
the most versatile and widely characterized system for oxidative
group transfer reactions on chromium to date.
deuterated solvents. Despite multiple attempts, extreme sensitivity of
the complexes led to elemental analyses that were outside the range
viewed as establishing analytical purity. Elemental composition was
confirmed by high resolution mass spectrometry.
Synthesis and Characterization of Chromium Complexes.
[(BMe ,EtTCH)Cr], 2. (BMe ,EtTCH)(Br)2 (1) (0.300 g, 0.530 mmol) was
added to tetrahydrofuran (40 mL) in a 100 mL round-bottom flask
and stirred for 10 min. n-BuLi (0.586 g, 2.11 mmol) was pipetted into
2
2
the stirring (BMe ,EtTCH)(Br)2 mixture. After 20 min, chromium(II)
2
chloride (0.065 g, 0.53 mmol) was added to the reaction mixture. The
reaction mixture was allowed to stir for 18 h at room temperature. The
slurry was filtered over Celite to collect the filtrate. All volatiles were
removed under reduced pressure. Benzene (50 mL) was added to the
crude solid in 10 mL portions, and the product was extracted as yellow
solution. The yellow solution was then filtered, and the collected
filtrate was concentrated to 10 mL. The pure product was crystallized
via vapor diffusion of pentane into the benzene solution, and the bright
yellow crystals were collected in two batches (0.144 g, 60.0% yield).
1H NMR (benzene-d6, 499.74 MHz): δ −1.44, −30.59. UV−vis
(THF) λmax, nm (ε): 419 (320). DART HR MS (m/z): [M]+
454.20156 (found), 454.20282 (calcd).
[(BMe ,EtTCH)Cr(O)], 3. This was synthesized by two methods.
2
Method 1 follows: [(BMe ,EtTCH)Cr] (2) (0.100 g, 0.219 mmol) was
2
added to tetrahydrofuran (10 mL) in a 20 mL vial and stirred for 5
min. Me3NO (0.019 g, 0.25 mmol) was added in to the stirring
solution of 2. The reaction mixture was allowed to stir for 48 h at
room temperature. The slurry was filtered over Celite to collect the
filtrate. All volatiles were removed under reduced pressure. Benzene
(20 mL) was added to the crude solid in 10 mL portions, and the
product was extracted as green solution. The green solution was then
filtered, and the collected filtrate was concentrated to 5 mL. The pure
product was crystallized via vapor diffusion of pentane into the
benzene solution, and the bright green crystals were collected (0.0140
g, 14.0% yield). Method 2 follows: [(BMe ,EtTCH)Cr] (2) (0.100 g,
2
0.219 mmol) was added to tetrahydrofuran (10 mL) in a 50 mL
Schlenk flask inside the glovebox. The flask was thoroughly greased
and sealed and was brought out of the glovebox and connected to a
flow of N2 under Schlenk line. A 2.7 mL (0.011 mmol) portion of dry
O2 was syringed carefully into the mixture of 2 in tetrahydrofuran and
stirred for 5 min. The reaction mixture turned from bright yellow to
green immediately, and all volatiles were removed under reduced
pressure after 5 min. The reaction flask was brought into the glovebox,
and the green compound was extracted with benzene (20 mL) and was
filtered through Celite. The bright green crystals were obtained in the
same way as in Method 1. (0.052 g, 50% yield). 1H NMR (benzene-d6,
499.74 MHz): δ −1.39. IR: 2929, 1587, 1444, 1412, 1285, 1204, 1147,
EXPERIMENTAL SECTION
■
General Methods and Procedures. All reactions were
performed under a dry nitrogen atmosphere with the use of either a
drybox or standard Schlenk techniques. Solvents were dried on an
Innovative Technologies (Newburgport, MA) Pure Solv MD-7
Solvent Purification System and degassed by three freeze−pump−
thaw cycles on a Schlenk line to remove O2 prior to use. Benzene-d6
and chloroform-d were degassed by three freeze−pump−thaw cycles
prior to drying over activated molecular sieves. These NMR solvents
1113, 1031 (CrO), 942, 788, 734, 678 cm−1. UV−vis (THF) λmax
,
nm (ε): 483 (140), 627 (40). DART HR MS (m/z): [M + H]+
471.20696 (found), 471.20501 (calcd).
[(BMe ,EtTCH)Cr((p-tolyl)N4(p-tolyl))], 4. [(BMe ,EtTCH)Cr] (2) (0.100
g, 0.219 mmol) was added to tetrahydrofuran (10 mL) in a 20 mL vial
and stirred for 5 min. p-Tolylazide (0.087 g, 0.66 mmol) was added to
the stirring solution of 2. The reaction mixture was allowed to stir for
24 h at room temperature. The slurry was filtered to collect the filtrate.
All volatiles were removed under reduced pressure. Benzene (30 mL)
was added to the crude solid in 10 mL portions, and the product was
extracted as deep red solution. The deep red solution was then filtered,
and the collected filtrate was concentrated to 10 mL. The pure product
was crystallized via vapor diffusion of pentane into the benzene
solution, and the deep red crystals were collected (0.038 g, 40.0%
2
2
were then stored under N2 in a glovebox. The compounds n-octyl
56
azide, p-tolyl azide, mesityl azide, and (BMe ,EtTCH)(Br)2 (1) were
2
prepared from literature procedures. All other reagents were purchased
from commercial vendors and used without purification. 1H, 13C{1H},
and 31P NMR spectra were recorded at ambient temperature on a
Varian VNMRS 500 MHz narrow-bore broadband system. 1H and 13
C
NMR chemical shifts were referenced to the residual solvent. All mass
spectrometry analyses were conducted at the Mass Spectrometry
Center located in the Department of Chemistry at the University of
Tennessee. The DART analyses were performed using a JEOL
AccuTOF-D time-of-flight (TOF) mass spectrometer with a DART
(direct analysis in real time) ionization source from JEOL USA, Inc.
(Peabody, MA). Mass spectrometry sample solutions of metal
complexes were prepared in tetrahydrofuran. Mass spectrometry
sample solutions of organic compounds were prepared in ethyl acetate.
Infrared spectra were collected on a Thermo Scientific Nicolet iS10
with a Smart iTR accessory for attenuated total reflectance. UV−vis
measurements were taken inside a dry glovebox on an Ocean Optics
USB4000 UV−vis system with 1 cm path length quartz crystal cell.
Room temperature magnetic measurements using Evans NMR were
not successful due to poor solubility of the complexes in common
1
yield). H NMR (benzene-d6, 499.74 MHz): δ 91.79, 48.00, 26.67,
−1.20, −2.31, −10.30, −18.90, −82.75. UV−vis (THF) λmax, nm (ε):
544 (690). IR: 2925, 1500, 1422, 1284, 1196, 1148, 1111, 1034, 977,
957, 943, 821, 752, 736, 722, 703, 681 cm−1. DART HR MS (m/z):
[M + H]+ 693.33474 (found), 693.33194 (calcd).
[(BMe ,EtTCH)Cr(NMes)], 5. [(BMe ,EtTCH)Cr] (2) (0.100 g, 0.219
mmol) was added to tetrahydrofuran (10 mL) in a 20 mL vial and
stirred for 5 min. Mesityl azide (0.035 g, 0.22 mmol) was added in to
the stirring solution of 2. The reaction mixture was allowed to stir for
24 h at room temperature. The slurry was filtered over Celite to collect
2
2
B
DOI: 10.1021/acs.inorgchem.7b02253
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Inorganic Chemistry
Article
the filtrate. All volatiles were removed under reduced pressure.
Benzene (20 mL) was added to the crude solid in 10 mL portions, and
the product was extracted as orange solution. The orange solution was
then filtered, and the collected filtrate was concentrated to 5 mL. The
pure product was crystallized via vapor diffusion of pentane into the
benzene solution, and the orange crystals were collected (0.064 g,
26.73, 22.82, 20.56, 19.25, 14.25. DART HR MS (m/z): [M + H]+
274.25423 (found); [C19H32N]+ 274.25293 (calcd).
Synthesis of N-Mesityl-1,1,1-triphenylphosphanimine, 10.
[(BMe ,EtTCH)Cr(NMes)] (4) (0.021 g, 0.035 mmol) was added to a
2
solution of triphenylphosphine (0.009 g, 0.035 mmol) in deuterated
benzene and stirred at 75 °C for 48 h. Purification by column
chromatography through gradient elution (ethyl acetate/hexane)
1
50.0% yield). H NMR (benzene-d6, 499.74 MHz): δ 50.65, 35.73,
1
yielded the product as a white crystalline solid (0.014 g, 65%). H
9.96, −1.77, −16.78, −27.98, −46.35, −71.64. UV−vis (THF) λmax
,
NMR (C6D6, 499.74 MHz): δ 7.70−7.04 (m, 15H), 1.41−1.22 (m,
14H), 0.95 (s, 9H), 0.86 (t, J = 7.0 Hz, 3H). 31P NMR (CDCl3, 125.66
MHz): δ −13.86. DART HR MS (m/z): [M]+ 395.18120 (found);
[C27H26NP]+ 395.18029 (calcd).
nm (ε): 479 (800). IR: 2929, 1459, 1414, 1397, 1283, 1202, 1149,
1111, 1065, 1043, 955, 942, 852, 797, 722, 698, 680 cm−1. DART HR
MS (m/z): [M + H]+ 588.30150 (found), 588.29924 (calcd).
[(BMe ,EtTCH)Cr((n-octyl)N4(n-octyl))], 6. [(BMe ,EtTCH)Cr] (2) (0.098
g, 0.22 mmol) was added to tetrahydrofuran (10 mL) in a 20 mL vial
and stirred for 5 min. n-Octylazide (0.067 g, 0.43 mmol) was added to
the stirring solution of 2. The reaction mixture was allowed to stir for
24 h at room temperature. The slurry was filtered over Celite to collect
the filtrate. All volatiles were removed under reduced pressure.
Benzene (30 mL) was added to the crude solid in 10 mL portions, and
the product was extracted as brown solution. The brown solution was
then filtered, and the collected filtrate was concentrated to 10 mL. The
pure product was crystallized via vapor diffusion of pentane into the
benzene solution, and the brown-yellow crystals were collected (0.063
2
2
X-ray Structure Determinations. X-ray diffraction measurements
were performed on single crystals coated with Paratone oil and
mounted on glass fibers. Each crystal was frozen under a stream of N2
while data were collected on a Bruker APEX diffractometer. A matrix
scan using at least 12 centered reflections was used to determine initial
lattice parameters. Reflections were merged and corrected for Lorenz
and polarization effects, scan speed, and background using SAINT
4.05.57 Absorption corrections, including odd and even ordered
spherical harmonics, were performed using SADABS.58 Space group
assignments were based upon systematic absences, E statistics, and
successful refinement of the structure. The structures were solved by a
combination of direct and intrinsic phasing method and were refined
using the SHELXTL 5.059 software package and Olex260 software
package. All of the non-hydrogen atoms were refined anisotropically.
Hydrogen atoms attached to the carbons and nitrogens were fixed at
calculated positions and refined using a standard riding model.
Compounds 3, 5, and 8 had solvent molecule benzene cocrystallized in
the asymmetric unit cell, while compound 7 had solvent molecule
tetrahydrofuran cocrystallized in the asymmetric unit cell. Compounds
2 and 8 were symmetry grown, and compound 4 had two independent
molecules in the asymmetric unit cell. Crystal data, data collection
parameters, and results of the analyses are listed in the Supporting
Information.
1
g, 40% yield). H NMR (benzene-d6, 499.74 MHz): δ 16.22, 12.35,
−1.84, −17.89. UV−vis (THF) λmax, nm (ε): 467 (850). IR: 2951,
2923, 2853, 1464, 1422, 1398, 1287, 1199, 1109, 1076, 1061, 1030,
958, 943, 726, 702, 685, 672 cm−1. DART HR MS (m/z): [M + H]+
737.48628 (found), 737.48844 (calcd).
[(BMe ,EtTCH)Cr(NAd)], 7. [(BMe ,EtTCH)Cr] (2) (0.100 g, 0.219
mmol) was added to tetrahydrofuran (10 mL) in a 20 mL vial and
stirred for 5 min. Adamantyl azide (0.039 g, 0.22 mmol) was added to
the stirring solution of 2. The reaction mixture was allowed to stir for
24 h at room temperature. The slurry was filtered over Celite to collect
the filtrate. All volatiles were removed under reduced pressure.
Benzene (20 mL) was added to the crude solid in 10 mL portions, and
the product was extracted as orange solution. The orange solution was
then filtered, and the collected filtrate was concentrated to 5 mL. The
pure product was crystallized via vapor diffusion of pentane into a
benzene and tetrahydrofuran solution mix, and the bright orange
2
2
Collection of Magnetic Data. Magnetic data were collected using
a Quantum Design MPMS SQUID magnetometer. Measurements
were obtained for finely ground microcrystalline powders restrained in
a solid eicosane matrix with gelatin capsules. Samples were prepared
under a dry nitrogen atmosphere by loading the ground powder into a
capsule and then adding warm liquid eicosane. The resulting mixture
was agitated until homogeneous and then allowed to cool to form a
solid suspension. The capsule was then immobilized in a plastic straw
and mounted in the magnetometer. Direct current susceptibility
measurements were collected in the temperature range 5−300 K under
an applied dc field of 1 T. The susceptibility data were corrected for
contributions from the sample holder and eicosane, as well as the core
diamagnetism of the sample using Pascal’s constants.61
1
crystals were collected (0.026 g, 20% yield). H NMR (benzene-d6,
499.74 MHz): δ 37.68, 25.60, 10.68, −1.39, 1.86, −7.85, −18.55. UV−
vis (THF) λmax, nm (ε): 409 (370). IR: 2924, 2896, 1451, 1401, 1294,
1281, 1204, 1146, 1111, 1058, 1028, 1035, 945, 731, 721, 705, 677
cm−1. DART HR MS (m/z): [M + H]+ 604.32997 (found), 604.33054
(calcd).
[(BMe ,EtTCH)Cr]2(N), 8. [(BMe ,EtTCH)Cr] (2) (0.100 g, 0.219 mmol)
was added to tetrahydrofuran (10 mL) in a 20 mL vial and stirred for 5
min. Trimethylsilylazide (0.038 g, 0.3285 mmol) was added to the
stirring solution of 2. The reaction mixture was allowed to stir for 24 h
at room temperature. The slurry was filtered over Celite to collect the
filtrate. All volatiles were removed under reduced pressure. Benzene
(30 mL) was added to the crude solid in 10 mL portions, and the
product was extracted as brown solution. The brown solution was then
filtered, and the collected filtrate was concentrated to 10 mL. The pure
product was crystallized via vapor diffusion of pentane into the
benzene solution, and the red crystals were collected (0.100 g, 50.0%
yield). 1H NMR (benzene-d6, 499.74 MHz): δ −1.44, −22.26. UV−vis
(THF) λmax, nm (ε): 419 (570), 555 (320). IR: 2927, 1448, 1417,
1399, 1292, 1209, 1152, 1112, 1064, 1044, 942, 794, 727, 697, 672
cm−1. DART HR MS (m/z): [M + H]+ for the fragment
[(BMe2,EtTCH)Cr]N 469.21496 (found), 469.21207 (calcd).
2
2
Fitting of Magnetic Data. The data for 8 was fit using PHI.62 The
χMT versus T data were fit according to an isotropic exchange coupled
̂
̂
̂
̂
model with S1 = 1, S2 = 1/2, and Hex = −2JS1S2. The isotropic g values
for each chromium center were constrained to be equivalent.
RESULTS AND DISCUSSION
■
While a previous macrocyclic chromium tetracarbene
([(Me,EtTCPh)CrCl2]PF6) could be prepared by silver trans-
metalation,55 the second generation macrocycle,56
(
BMe ,EtTCH)(Br)2 (1), required a direct deprotonation
2
approach to synthesize a chromium complex (Scheme 1).
Compound 1 was successfully deprotonated with nBuLi at
room temperature in THF within 20 min. The addition of
anhydrous CrCl2 to the above reaction solution yielded
Group Transfer Reactions of [(BMe ,EtTCH)Cr(NMes)], 5. Syn-
2
thesis of 1-(Mesityl)-2-octylaziridine, 9. [(BMe ,EtTCH)Cr(NMes)] (4)
2
(0.050 g, 0.085 mmol) was added to a solution of 1-decene (0.012 g,
0.085 mmol) in deuterated benzene and stirred at 75 °C for 18 h.
Purification by column chromatography through gradient elution
(ethyl acetate/hexane) yielded the product as a yellow oil (0.020 g,
86.0%). 1H NMR (CDCl3, 499.74 MHz): δ 6.75 (s, 2H), 2.31 (s, 6H),
2.21(s, 3H), 2.10 (m, 1H), 1.49 (d, J = 7.4 Hz), 1.29 (m, 12H), 0.90
(t, J = 6.8 Hz, 3H). 13C NMR (CDCl3, 125.66 MHz): δ 149.01,
130.85, 129. 69, 129.08, 41.29, 37.04, 32.85, 32.02, 29.73, 29.71, 29.42,
[(BMe ,EtTCH)Cr] (2) in 60% yield after crystallization from a
2
mixture of benzene and pentane. Complex 2 is a highly
electronically unsaturated square planar complex, and it is
extremely sensitive to both oxygen and water. The 1H NMR of
this yellow complex confirmed its paramagnetism, as expected
for a Cr(II) complex. Collection of magnetic susceptibility data
C
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gave [(BMe ,EtTCH)Cr(O)] (3), as pale green crystals in 14%
and 50% yield, respectively (Scheme 1), showcasing the
chromium(II) complex’s ability to activate O2. Complex 3 is
2
Scheme 1. Synthesis of Cr(II) Complex from Macrocyclic
NHC Precursor and Its Reactions with a Variety of Group
Transfer Oxidants
1
paramagnetic on the basis of H NMR. Previously isolated
terminal Cr(IV) oxos have exhibited both diamagnetism (a
porphyrin complex by Groves42 and a cyclam complex by
Nam41) and paramagnetism (two siloxide chromium complexes
by Wolczanski38 and two Tp complexes by Theopold43,44).
Complex 3 has a strong stretch in the IR spectrum at 1031
cm−1 which can be attributed to the terminal oxo, and is
consistent with porphyrin and corrole based Cr oxo
complexes.22−24,40,42,78 Single crystal X-ray data reveals a square
pyramidal geometry for 3 with the Cr atom raised 0.525 Å
above the plane of the carbene carbons (Figure 3). The Cr−C
for 2 was complicated by its extreme air-sensitivity leading to
inconclusive results over multiple attempts. All of the
previously reported square planar Cr(II) complexes with d4
configuration had an S = 2 spin state.63−65 There is only one
example of a Cr(II) complex with an S = 1 spin state but with a
square pyramidal geometry.64
Single crystal X-ray diffraction demonstrated that 2 is a
square planar complex (Figure 2). The Cr−C bond distances in
Figure 3. X-ray crystal structure of [(BMe ,EtTCH)Cr(O)], 3. Purple,
2
red, blue, gray, and olive ellipsoids (30% probability) represent Cr, O,
N, C, and B atoms, respectively. Solvent molecules and H atoms are
omitted for clarity. Selected interatomic bond distances (Å) and angles
(deg) are as follows: Cr1−C1 = 2.094(5); Cr1−C2 = 2.031(5); Cr1−
C3 = 2.080(5); Cr1−C4 = 2.026(5); Cr1−O1 = 1.564(3); C4−Cr1−
C2 = 153.3(2); C3−Cr1−C1 = 147.7(2).
Figure 2. X-ray crystal structure of [(BMe ,EtTCH)Cr], 2. Purple, blue,
2
bond distances range from 2.026 to 2.094 Å which is
significantly shorter than the distances observed for 2, while
the trans C−Cr−C bond angles show only a mild distortion
from a bent square pyramidal geometry.
gray, and olive ellipsoids (50% probability) represent Cr, N, C, and B
atoms, respectively. Solvent molecules and H atoms are omitted for
clarity. Selected interatomic bond distances (Å) and angles (deg) are
as follows: Cr1−C1 = 2.076(3); Cr1−C2 = 2.132(3); C1−Cr1−C1′ =
170.54(14); C2−Cr1−C2′ = 177.39(12).
Since the few examples of terminal Cr(IV) oxos have shown
a wide range of reactivity, from highly stable to radical reactions
with C−H bonds, we tested 3 to elucidate its group transfer
potential.17,39,44 A test reaction with 3 and 1-decene showed no
reactivity toward epoxidation (Scheme 2). In a similar manner,
test reactions with 3 and triphenylphosphine showed no
reaction. These results are consistent with Che’s Cr(IV) oxo
supported by a porphyrin auxiliary ligand in a square pyramidal
geometry.79 In contrast, Theopold has shown that a tris-
(pyrazolyl)borate based Cr(IV) oxo complex in trigonal
bipyramidal geometry is capable of hydrogen atom abstraction
through activation of weak C−H bonds.44 The Cr−O distance
of 1.564 Å is the shortest bond for a chromium(IV) oxo and
combined with its no further reactivity toward group transfer
indicates a triple bond character between Cr and O.40−44
complex 2 range from 2.076(3) to 2.132(3) Å which is within
the expected range (2.08−2.18 Å) given the limited number of
Cr(II) NHC complexes.66−77 The trans C1−Cr−C1′ and C2−
Cr−C2′ angles are 170.54(14)° and 177.39(12)°, respectively,
and the Cr atom sits on the calculated centroid of the plane of
the NHC carbenes.
The significant electronic unsaturation of complex 2
combined with its low valent state makes 2 an excellent choice
for oxidative group transfer reactions. The oxidation of 2 with
nitrogen or oxygen transfer reagents should yield Cr(IV)
complexes with metal−ligand multiple bonds, which are quite
rare.33−44 The reactions of 2 with Me3NO and molecular O2
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ligand in 4 dianionic and not radical in nature which is
consistent with our previous studies on isostructural iron
tetrazenes.10 Previously reported Cr(III) tetrazenes have similar
double bond character on the central nitrogens.80,81 As a
consequence of attempting to relieve steric hindrance, the
crystal structure of 4 is slightly distorted from a trigonal
prismatic geometry (ϕav = 7.77°, perfect trigonal prism, ϕ =
0°). This distortion can also be observed in the distinction
between the two trans C−Cr−C angles which are quite
disparate (C4−Cr−C2 = 121.8(1)° and C1−Cr−C3 =
140.4(1)°).
Scheme 2. Group Transfer Reactions with 3 (Top) and 5
(Bottom)
Since sterically bulky azides can hinder the formation of a
metallotetrazene, we reacted 1 equiv of mesityl azide with 2
which resulted in an isolable Cr-imido complex 5 in 50% yield
(Scheme 1). The red-orange compound 5 is a paramagnetic
1
species as revealed by its H NMR, which showed all eight
expected peaks. While many chromium imidos have been
previously prepared, only eight examples of Cr(IV) imidos are
known.34−39,83,84 These Cr(IV) imido complexes are all
diamagnetic, except the three trigonal examples reported by
Groysman, the pseudotrigonal example by Wolczanski and co-
workers, and the only square pyramidal complex made by
West.84
The X-ray structure reveals that 5 adopts a bent square
pyramidal geometry with the central Cr atom sitting 0.542 Å
out of the tetracarbene plane (Figure 5). The Cr−C bond
In addition to chromium oxos, we were particularly
interested in the formation of midvalent chromium imidos.
Metal imidos are purported intermediates during catalytic
aziridination, and an isostructural iron complex to 2 was
recently reported to be the first general catalyst for the
formation of aziridines from alkyl azides.8 A variety of organic
azides were tested for potential conversion to imidos, which
yielded a diversity of structural motifs.
The reaction of 2 with tolyl azide yielded not the expected
chromium imido, but a metallotetrazene, 4 (Scheme 1). The
metallotetrazene is presumably formed in a [2 + 3] cyclo-
addition by addition of an additional equivalent of organic azide
with a transient metal imido.10 The Cr−C bond distances range
from 2.056 to 2.120 Å which is in the expected range (Figure
4). Tetrazenes are noninnocent ligands which potentially make
the oxidation state assignment of the Cr metal center
ambiguous.80−82 A comparison of the N−N bond distances
in the tetrazene shows that the N2−N3 bond distance (1.287
Å) is shorter than the N1−N2 (1.364 Å) and N3−N4 (1.359
Å) distances which suggests that there is a double bond
between the central nitrogens. This would make the tetrazene
Figure 5. X-ray crystal structure of [(BMe ,EtTCH)Cr(NMes)], 5.
2
Purple, blue, gray, and olive ellipsoids (50% probability) represent Cr,
N, C, and B atoms, respectively. Selected interatomic bond distances
(Å) and angles (deg) are as follows: Cr1−C1 = 2.104(6); Cr1−C2 =
2.097(6); Cr1−C3 = 2.065(6); Cr1−C4 = 2.082(6); Cr1−N1 =
1.703(4); N1−C21 = 1.386(6); C4−Cr1−C2 = 147.81(2); C3−Cr1−
C1 = 151.8(2); C21−N1−Cr1 = 166.1(4).
Figure 4. X-ray crystal structure of [(BMe ,EtTCH)Cr((p-tolyl)N4(p-
2
distances range from 2.067 to 2.102 Å which is between the
values for 3 and 4. The Cr−N1 bond distance of 1.704 Å in 5 is
the longest Cr(IV)-imido bond to date. In addition, the N1−
C21 distance is also extended to 1.386 Å which is longer than a
typical single bond. These bond distances combined with the
slightly bent Cr1−N1−C21 angle of 166.2° indicate a
delocalized double bond, which is in contrast to the eight
tolyl))], 4. Purple, blue, gray, and olive ellipsoids (50% probability)
represent Cr, N, C, and B atoms, respectively. Selected interatomic
bond distances (Å) and angles (deg) are as follows: Cr1−C1 =
2.056(3); Cr1−C2 = 2.120(3); Cr1−C3 = 2.063(3); Cr1−C4 =
2.118(3); Cr1−N1 = 1.977(3); Cr1−N4 = 1.964(3); N1−N2 =
1.359(3); N2−N3 = 1.287(3); N3−N4 = 1.358(3); C4−Cr1−C2 =
121.82(11); C1−Cr−C3 = 140.41(11).
E
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other examples of Cr(IV) imidos.34−36,38,39,83,84 The closest
example of a similar imido complex is a Ta species reported by
Bercaw in 1992 that showcased a double bond character with a
nearly linear geometry on the metal-imido bond due to the lone
pair on nitrogen residing on the empty p orbital instead of an
sp or an sp2 hybridized orbital.37 This distinct structural
difference between 3 and 5 suggested to us that 5 would be
more effective for group transfer since it does not have a
metal−ligand triple bond like 3.
substituents. A comparison of the N−N bond distances on the
tetrazene ligand shows that the central nitrogen bond distance
for N2−N3 (1.303 Å) is shorter compared to the other two
nitrogen bond distances, N1−N2 (1.347 Å) and N3−N4
(1.347 Å).
Shifting to a bulkier alkyl azide formed the expected
chromium imido complex. The reaction of adamantyl azide
and 2 gave [(BMe ,EtTCH)Cr(NAd)], 7 (Figure 7), albeit in only
2
While there are almost no examples of successful group
transfer reactions from Cr(IV) imido complexes,39 several high
valent chromium imidos showed reactivity toward hydrogen
abstraction and imido group transfer to phosphines.20 We
tested complex 5 for nitrene group transfer with 1-decene and
PPh3 to form the corresponding aziridine and triphenylphos-
phanimine, respectively. Stoichiometric addition of 1-decene to
5 at 75 °C in benzene yielded 1-(mesityl)-2-octylaziridine, 9, in
86% yield (Scheme 2). This formation of aziridine from an
imido and an alkene is the first successful group transfer of this
type with chromium. In a similar manner, addition of
triphenylphosphine to 5 yields N-mesityl-1,1,1-triphenylphos-
phanimine, 10, in 65% yield. Since both the oxidative group
transfer and reductive elimination of the imido are successful, 2
appeared to be a potential catalyst for aziridination, at least with
sterically bulky azides. Nonetheless, the catalytic reaction with
mesityl azide, decene, and 2 was not successful. Further
investigations revealed this was because 2 was susceptible to
thermal degradation at 75 °C.
Since alkyl azides are more challenging to activate compared
to aryl azides,85−87 we tested the effectiveness of oxidative
group transfer of these reagents with 2. The reaction of n-octyl
Figure 7. X-ray crystal structure of [(BMe ,EtTCH)Cr(NAd)], 7. Purple,
2
blue, gray, and olive ellipsoids (50% probability) represent Cr, N, C,
and B atoms, respectively. Selected interatomic bond distances (Å)
and angles (deg) are as follows: Cr1−C1 = 2.096(3); Cr1−C2 =
2.101(3); Cr1−C3 = 2.070(3); Cr1−C4 = 2.066(3); Cr1−N1 =
1.686(2); N1−C21 = 1.453(4); C4−Cr1−C2 = 143.9(11); C3−Cr1−
C1 = 142.0(11); Cr1−N1−C21 = 149.8(2).
azide with 2 resulted in a metallotetrazene, [(BMe ,EtTCH)Cr((n-
2
octyl)N4(n-octyl))], 6, that is analogous to 4 (Scheme 1). The
crystal structure showed that 6 has trigonal prismatic geometry
with a modest distortion (ϕav = 8.62°), similar to 4 (Figure 6).
The distortion for complex 6 is slightly higher than that of 4
due to increased steric repulsion imposed by bulkier alkyl
20% yield (Scheme 1). A comparison of the crystal structures of
5 and 7 shows that they are similar except for the bonding of
the imido ligand. The Cr−N1 bond distance on 7 is 1.686 Å
which is slightly shorter than the Cr−N1 distance on 5, but the
bond angle for Cr1−N1−C21 of 149.8° is considerably more
bent. Complex 7 has one of the most bent Cr−N−C angles for
a Cr(IV) imido complex while all of the other examples are
nearly linear.34−36,38,39,83 The combination of an elongated Cr−
N1 bond and a bent Cr1−N1−C21 angle is consistent with a
chromium imido double bond.
In addition to the reactions with organic azides, we tested a
reaction of 2 with TMS-azide. The TMS group is an easily
removable protecting group, and it has been employed as a
surrogate for hydrazoic acid.88 However, instead of isolating the
expected imido or tetrazene complex, a reaction of 2 and excess
TMS-azide yielded a bis-chromium bridging nitrido complex,
[(BMe ,EtTCH)Cr]2(N), 8, in 50% yield (Scheme 1). While
2
TMS-azide has not been demonstrated to form bridging
nitridos previously, there are examples where TMS-azide forms
terminal nitridos.89 Sodium azide has been demonstrated to
form bridging nitridos by delivering an azide to the metal which
is followed by attack of a second transition metal with the
extrusion of N2.90,91 A similar reaction pathway could explain
the synthesis of 8, where TMS-azide acts as a radical azide
transfer agent to the first equivalent of 2, followed by attack of a
second equivalent of 2 and loss of N2. These combined
oxidation processes would lead to Cr1 and Cr2 metal centers at
Figure 6. X-ray crystal structure of [(BMe ,EtTCH)Cr((n-octyl)N4(n-
2
octyl))], 6. Purple, blue, gray, and olive ellipsoids (50% probability)
represent Cr, N, C, and B atoms, respectively. Selected interatomic
bond distances (Å) and angles (deg) are as follows: Cr1−C1 =
2.0701(19); Cr1−C2 = 2.1288(19); Cr1−C3 = 2.0879(19); Cr1−C4
= 2.1591(19); Cr1−N1 = 1.9644(16); N1−N2 = 1.347(2); N2−N3 =
1.303(2); N3−N4 = 1.347(2); Cr1−N4 = 1.9531(16); C1−Cr1−C3
= 141.79 (7); C2−Cr1−C4 = 126.68(7).
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+4 and +3 oxidation states, respectively. The X-ray structural
data for 8 revealed an asymmetric bridging nitrido ligand
(Cr1−N1 = 1.667 Å versus Cr2−N1 = 1.899 Å) (Figure 8).
Figure 9. Variable temperature dc magnetic susceptibility for 8. χMT vs
T plot shown. Data was collected at 1.0 T. Fit parameters obtained
with PHI: gCr1 = gCr2 = 1.91, J = 38 cm−1.
mol−1). An alternative interpretation could be a set of S = 3/2
and S = 0 metal centers, but that should have a value of 1.87
cm3 K mol−1 and should yield a flat line for the magnetic
susceptibility independent of temperature. The increasing
magnetic susceptibility as temperature was lowered is
suggestive of a Cr(III)/Cr(IV) pair with weak ferromagnetic
coupling. A fit of the data with PHI62 according to this model
produced a satisfactory fit and yielded a coupling constant, J, of
38 cm−1.
Figure 8. X-ray crystal structure of [(BMe ,EtTCH)Cr]2(N), 8. Purple,
2
CONCLUSIONS
■
blue, gray, and olive ellipsoids (30% probability) represent Cr, N, C,
and B atoms, respectively. Selected interatomic bond distances (Å)
and angles (deg) are as follows: Cr1−C1 = 2.086(5); Cr1−C2 =
2.111(4); Cr2−C3 = 2.104(4); Cr2−C4 = 2.163(4); Cr1−N1 =
1.667(5); Cr2−N1 = 1.896(5); C1−Cr1−C1′ = 158.80 (2); C2−
Cr1−C2′ = 153.4(2); C3−Cr2−C3′ = 168.4(2); C4−Cr2−C4′ =
160.5(2); Cr1−N1−Cr2 = 180.
In conclusion, we have isolated and structurally characterized a
diverse family of chromium(IV) complexes that demonstrates
the distinctions in reactivity between different oxidative group
transfer reagents. The initial low valent square planar
tetracarbene complex, 2, was synthesized by an in situ
deprotonation strategy using nBuLi on a macrocyclic
tetraimidazolium and CrCl2. This highly electronically unsatu-
rated complex provides an extremely versatile synthon to
generate various multiply bonded metal−ligand chromium
complexes including a Cr(IV)-oxo, 3, Cr(IV)-imidos, 5 and 7,
and the first bridging mononitrido Cr(III)/Cr(IV), 8. In
addition to the chromium−ligand multiple bond species, less
sterically bulky organic azides lead to Cr(IV) metallotetrazenes,
4 and 7. A comparison of the bond distances obtained by X-ray
diffraction demonstrates that the oxo complex has a stronger π
donation compared to the imido ligands, resulting in a triple
bond for 3 versus double bonds for 5 and 7. This conclusion on
the difference in bond order is corroborated with reactivity
studies. The oxo complex, 3, shows no group transfer reactivity,
while the mesityl imido complex, 5, yields an aziridine in the
presence of 1-decene. This aziridination is the first reported
formation of an aziridine directly from a chromium imido
complex. Although numerous oxo and imido complexes of
chromium have been reported previously, the vast majority of
them were high valent (Cr(V) and Cr(VI)) and showed little to
no reactivity toward reductive group transfer. Finally, we
investigated the first linearly bridging nitrido of a bis-chromium
complex. This complex, 8, has two inequivalent chromium
centers based on X-ray crystallography and SQUID magneto-
metry and exhibits weak ferromagnetic coupling. This study is
an unprecedented comparison of oxidative group transfer
reactions on low valent chromium that is supported by a single
auxiliary ligand platform. Insights from this research can lead to
improved devolopment of catalytic redox reactions on
midvalent chromium.
The Cr1−N1−Cr2 angle is a perfect 180° showing a linear
arrangement between the bridging nitrido and the two metal
centers. The Cr1−C bond distances are shorter (Cr1−C1 =
2.082 Å, Cr1−C2 = 2.120 Å) compared to the Cr2−C bond
distances (Cr2−C3 = 2.104 Å, Cr2−C4 = 2.167 Å) which
supports the assertion that the metals are in different oxidation
states. To our knowledge, complex 8 is the first isolable,
bridging linear nitrido monomeric chromium complex. A
dimeric chromium(V) complex98 with bridging nitrido forming
a diamond core has been reported by Cummins and coworkers
while Wieghardt and co-workers in a previous report mention a
possible Cr(III)/Cr(V) mixed valent bridging nitrido species,
solely based on spectroscopic evidence, which was formed from
a photo-decomposition reaction.92 This proposed species was
never isolated. Complex 8 is particularly noteworthy since there
are numerous examples of linear bridging nitridos on vanadium
and iron on the first row, and molybdenum and tungsten in the
same column.91−97
Since 8 is the first isolated bridging mononitrido on
chromium and bridging nitridos and oxos are so critical in
magnetic materials, we decided to test the magnetic interactions
between the chromium centers. Variable temperature dc
susceptibility measurements for 8 were collected (5−300 K)
at 1 T (Figure 9). The resulting susceptibility curve achieves a
maximum value just below 1.7 cm3 K mol−1 at low temperature
before declining toward the spin-only value for two, non-
interacting spin centers with S = 1 and S = 0.5 (1.375 cm3 K
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02253.
■
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High resolution mass spectra and X-ray crystallographic
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AUTHOR INFORMATION
Corresponding Author
*E-mail: jenkins@ion.chem.utk.edu.
ORCID
David M. Jenkins: 0000-0003-2683-9157
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the National Science
Foundation (NSF-CAREER/CHE-1254536) for financial sup-
port of this work.
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