Tris(carbene)borate ligands featuring imidazole-2-ylidene, benzimidazol-2-ylidene, and 1,3,4-triazol-2-ylidene donors. Evaluation of donor properties in four-coordinate {NiNO}10 complexes

Article abstract of DOI:10.1021/ic301204b

The synthesis and characterization of new tris(carbene)borate ligand precursors containing substituted benzimidazol-2-ylidene and 1,3,4-triazol-2-ylidene donor groups, as well as a new tris(imidazol-2-ylidene) borate ligand precursor are reported. The relative donor strengths of the tris(carbene)borate ligands have been evaluated by the position of νNO) in four-coordinate {NiNO}¹⁰ complexes, and follow the order: imidazol-2-ylidene > benzimidazol-2-ylidene > 1,3,4-triazol-2-ylidene. There is a large variation in νNO), suggesting these ligands to have a wide range of donor strengths while maintaining a consistent ligand topology. All ligands are stronger donors than Tp* and Cp*.

Full text of DOI:10.1021/ic301204b

Article
pubs.acs.org/IC
Tris(carbene)borate Ligands Featuring Imidazole-2-ylidene,
Benzimidazol-2-ylidene, and 1,3,4-Triazol-2-ylidene Donors.
Evaluation of Donor Properties in Four-Coordinate {NiNO}10
Complexes
Salvador B. Munoz, III,† Wallace K. Foster,† Hsiu-Jung Lin,† Charles G. Margarit,† Diane A. Dickie,‡
̃
and Jeremy M. Smith*,†
†Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003, United States
‡Department of Chemistry and Chemical Biology, The University of New Mexico, Albuquerque, New Mexico 87131, United States
S
* Supporting Information
ABSTRACT: The synthesis and characterization of new tris(carbene)borate ligand precursors containing substituted
benzimidazol-2-ylidene and 1,3,4-triazol-2-ylidene donor groups, as well as a new tris(imidazol-2-ylidene)borate ligand precursor
are reported. The relative donor strengths of the tris(carbene)borate ligands have been evaluated by the position of ν(NO) in
four-coordinate {NiNO}10 complexes, and follow the order: imidazol-2-ylidene > benzimidazol-2-ylidene > 1,3,4-triazol-2-
ylidene. There is a large variation in ν(NO), suggesting these ligands to have a wide range of donor strengths while maintaining a
consistent ligand topology. All ligands are stronger donors than Tp* and Cp*.
iron, and cobalt complexes,10,11 and more recently a copper(I)
INTRODUCTION
■
cluster.12 Our group has developed synthetic routes for second
The development of multidentate ligands incorporating
multiple N-heterocyclic carbene (NHC) donors has been
driven in part by the desire to develop new catalysts with
generation tris(carbene)borate ligands in which the steric bulk
t
has been increased (R = Bu, Mes; R′ = H, Ph). As with the
topologically related tris(pyrazolyl)borate ligands, the intro-
increased stability. While the vast majority of multidentate
duction of steric bulk allows for the synthesis of low coordinate
NHC ligands are bidentate, higher denticity ligands, including
transition metal complexes.13 However, tris(carbene)borate
tridentate1 and even tetradentate2 donors have been reported.3
ligands are substantially stronger donors than tris(pyrazolyl)-
Tridentate ligands composed solely of NHC donors are mainly
borates,14 and have been shown to stabilize high valent
limited to a small number of C3-symmetric tripodal ligands in
complexes of first row transition metals,15 including an isolable
which three imidazol-2-ylidene donors are connected through a
iron(V) nitride.16
central linker atom or group.4−9
The donor strength of tris(pyrazolyl)borate ligands can be
modified by suitable changes to the pyrazolyl ring substitu-
tents.17 Similar flexibility in the donor strength of tris(carbene)-
borate ligands does not yet exist. In the case of monodentate
NHC ligands, changing the N-substituents in imidazol-2-
ylidenes does not typically have a major impact on the donor
strength of the NHC ligand,18 and thus similar changes to the
donors of tris(carbene)borate ligand are not expected to have a
major influence on the ligand donor strength. However, recent
work has shown that NHCs based on donors other than
imidazol-2-ylidene can lead to large changes in donor strength.
Thus, both experimental and computational studies of
Tripodal tris(carbene)borate ligands (Figure 1) connect
three imidazol-2-ylidene donors through a central borate
linker.4 First generation tris(carbene)borate ligands (R = Me,
Et; R′ = H) have been used to prepare octahedral manganese,
Figure 1. Tris(carbene)borate ligands based on imidazol-2-ylidene
donors.
Received: June 6, 2012
© XXXX American Chemical Society
A
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(10.3 mL, 7.38 mmol, 1.05 equiv) added and maintained at 0 °C for 4
h and left to stir overnight at room temperature. Hexanes (80 mL) and
diethyl ether (40 mL) were added, and the mixture was washed in a
separatory funnel with saline water (3 × 150 mL). The residual
dimethylsulfoxide was removed by rewashing the combined organic
fractions with three more portions of water (80 mL). The solvents
were removed in vacuo, and the product distilled with heating at
reduced pressure to yield a hygroscopic white solid. The solid was
dissolved in THF and dried over CaH2 (11.3 g, 97%). The compound
is spectroscopically identical to that reported in the literature.26
Preparation of 1-(Methylcyclohexyl)benzimidazole (2). A 1 L flask
was charged with benzimidazole (19.1 g, 0.162 mol, 1 equiv) and
stirred with freshly ground KOH (12.5 g, 0.223 mol, 1.37 equiv) in
220 mL of dimethylsulfoxide for 1 d. The yellow solution was cooled
in ice, bromomethylcyclohexane (23.3 mL, 0.166 mol, 1.02 equiv)
added, and the reaction allowed to warm to room temperature after 4
h. After stirring for 3 d, dichloromethane (250 mL) was added to the
resultant slurry. The mixture was washed with distilled water (3 × 250
mL). The solvents were removed in vacuo, and the product sublimed
with heating under reduced pressure to yield a white solid (32.1 g,
monodentate NHC ligands show that carbenes derived from
heterocycles other than imidazole can have substantially
different donor strengths.19
In this paper we report the synthesis of new tris(carbene)-
borate ligands, including ligands based on benzimidazol-2-
ylidene and 1,3,4-triazol-2-ylidene donors. Studies of mono-
dentate NHC ligands have shown that the donor strength of
these ligands decreases according to the order: imidazol-2-
ylidene > benzimidazol-2-ylidene > 1,3,4-triazol-2-ylidene
(Figure 2) The donor properties of the new tris(carbene)borate
Figure 2. Relative donor strength of selected NHC ligands.
1
94%). H NMR (CDCl3): δ 7.85 (s, 1H, Bz), 7.82 (d, 1H, JHH = 8.8,
ligands have been evaluated in four-coordinate {NiNO}10
complexes, allowing their donor properties to be compared
with other monoanionic facially coordinating ligands. A number
of proposals regarding the nature of bonding between Ni and
NO in complexes of this type have been put forth,20 with the
most comprehensive investigation describing the electronic
structure as having multireference character; the dominant
contributor being Ni(II) (SNi = 1) antiferromagnetically
coupled to NO− (SNO = 1).21
Bz), 7.40 (d, 1H, JHH = 7.2, Bz), 7.29 (m, 2H, Bz), 4.00 (d, 2H, JHH
=
7.2, CH2), 1.87 (m, 1H, Cy), 1.75−1.64 (m, 5H, Cy), 1.27−1.14 (m,
+
3H, Cy), 1.06−0.97 (m, 2H, Cy). ESI+-MS 215 (C14H19N2 ). Anal.
Cald. for C14H18N2: C 78.46, H 8.47, N 13.07. Found C 77.74, H 8.46,
N 12.71. Multiple microanalysis trials were consistently low in carbon.
Preparation of 4-Phenyl-1,2,4-triazole (3). 4-Phenyl-1,2,4-triazole
was prepared by modification of a literature procedure.23 A 500 mL
flask was charged with N,N-dimethylformamide azine dihydrochloride
(20.9 g, 9.73 mmol, 1 equiv), freshly distilled aniline (8.87 mL, 9.73
mmol, 1 equiv), and benzene (100 mL). The reaction was heated at
reflux overnight. After cooling to room temperature, the resultant
mixture was dried in vacuo. Dichloromethane (100 mL) was added to
the solid, and the mixture washed with alkaline water (3 × 200 mL).
The solvent was removed from the organic layer under reduced
pressure, and the white solid was purified by vacuum sublimation with
heating to yield a white solid (8.61 g, 61%). The spectral data are
identical to that reported in the literature.27
EXPERIMENTAL SECTION
■
General Considerations. Manipulations involving air-sensitive
materials were performed under a nitrogen atmosphere by standard
Schlenk techniques or in an M. Braun Labmaster glovebox. The quality
of the glovebox atmosphere was periodically checked with a toluene
solution of “titanocene”.22 Glassware was dried at 150 °C overnight.
Diethyl ether, pentane, tetrahydrofuran (THF), and toluene were
purified by the Glass Contour solvent purification system. Deuterated
benzene was dried first over CaH2, then over Na/benzophenone,
before vacuum transfer into a storage container. Before use, aliquots of
Et2O, THF, and toluene were tested with a drop of Na/benzophenone
in THF solution. N,N-Dimethylformamide azine dihydrochloride was
prepared according to a literature procedure.23 Florisil for column
chromatography was dried by heating at 100 °C under vacuum for 8 h
prior to use. The {NiNO}10 synthons Ni(PPh3)2(NO)Br24 and
Preparation of 4-(4-tert-Butylphenyl)-1,2,4-triazole (4). 4-(4-tert-
Butylphenyl)-1,2,4-triazole was prepared by modification of a literature
procedure.23 A 500 mL flask was charged with N,N-dimethylforma-
mide azine dihydrochloride (16.4 g, 7.61 mmol, 1 equiv), 4-tert-
butylaniline (12.3 mL, 7.61 mmol, 1 equiv), and benzene (80 mL).
The reaction was heated at reflux overnight. After cooling the resultant
mixture was dried in vacuo. Dichloromethane (100 mL) was added to
the solid, and the mixture washed with alkaline water (3 × 200 mL).
The solvent was removed from the organic layer under reduced
pressure, and the resulting white solid was purified by vacuum
25
Ni(NO)I(THF)2 were prepared according to literature procedures.
distillation while heating. A white solid was obtained upon cooling
All other reagents were purchased from commercial vendors and used
1
as received. 1H NMR data were recorded on a Varian Unity 400
(14.9 g, 97%). H NMR (CDCl3) δ 8.44 (s, 2H, Tz), 7.55 (d, JHH
=
t
t
1
8.4, 2H, o/m-C6H4 Bu), 7.32 (d, JHH = 8.8, 2H, o/m-C6H4 Bu), 1.36 (s,
spectrometer (400 MHz) at 22 °C. All resonances in the H NMR
t
+
9H, Bu). ESI+-MS 202 (C12H16N3 ). Anal. Cald. for C12H15N3: C
spectra are referenced to residual CHCl3 (δ 7.26 ppm), C6D5H (δ 7.16
ppm), C4D7HO (δ 3.57 and 1.72 ppm), and CD2HCN (δ 1.94 ppm).
Infrared spectra were recorded on Perkin−Elmer Spectrum One FTIR
and Thermo Scientific Nicolet iS10 SMART iTR spectrophotometers,
while UV−visible spectra were recorded on a Cary 100 spectrometer.
Atmospheric pressure positive and negative ionization mass spectral
data were collected using a Waters-Micromass ZQ2000 Mass
Spectrometer. Cyclic voltammograms were recorded under a N2
atmosphere using a CH Instruments CHI600D potentiostat in 0.1
M Bu4PF6 in THF with a glassy carbon working electrode, a Pt wire
counter electrode, and a Ag+/Ag reference electrode. All electro-
chemical data are referenced to the Cp2Fe+/Cp2Fe couple. Elemental
analyses were performed by Robertson Microlit Laboratories,
Madison, NJ.
71.61, H 7.51, N 20.88. Found C 71.57, H 7.34, N 20.78.
Preparation of 4-Mesityl-1,2,4-triazole (5). 4-Mesitylene-1,2,4-
triazole was prepared by modification of a literature procedure.23
A
500 mL flask was charged with N,N-dimethylformamide azine
dihydrochloride (20.6 g, 9.58 mmol, 1 equiv), 2,4,6-trimethylaniline
(13.9 mL, 9.58 mmol, 1 equiv), and heated to 180 °C in
chlorobenzene (100 mL) for 3 d. After cooling, hexanes (300 mL)
was added to the vessel, and the mixture cooled to −25 °C. The
supernatant was decanted from the solid that had precipitated.
Dichloromethane (100 mL) was added to the solid, and the mixture
washed with alkaline water (3 × 200 mL). The solvent was removed
from the organic layer under reduced pressure to yield a white to off-
white solid. Colored impurities were removed by recrystallization from
CH2Cl2/Et2O at −25 °C . The white solid obtained was purified by
vacuum sublimation with heating (10.4 g, 58%). The spectral data are
identical to that reported in the literature.28
Synthesis of Compounds. Preparation of 1-(Methylcyclohexyl)-
imidazole (1). A 250 mL flask was charged with imidazole (4.81 g,
0.700 mol, 1.0 equiv) and stirred with freshly ground KOH (5.94 g,
0.106 mol, 1.5 equiv) in dimethylsulfoxide (80 mL) for 3 h. The
golden solution was cooled in ice, and then bromomethylcyclohexane
Preparation of [PhB(CyCH2ImH)3](OTf)2 (6). N-Methylcyclohex-
ylimidazole 1 (3.50 g, 21.3 mmol, 3.04 equiv) was added to a solution
B
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Article
of PhBCl2 (1.11 g, 6.99 mmol, 1 equiv) in toluene (50 mL). After 30
min, TMSOTf (3.31 g, 14.9 mmol, 2.13 equiv) was added, and the
mixture was heated at 110 °C for 1 d. After cooling to room
temperature hexanes (100 mL) was added, leading to the formation of
a white precipitate. The mixture was cooled to −25 °C overnight, and
the resultant white solid collected by filtration and washed with
hexanes. The solid was recrystallized from CH2Cl2/Et2O at −25 °C.
The resulting white solid collected by vacuum filtration, washed with
hexanes, and dried in vacuo (5.14 g, 87%). 1H NMR (CDCl3): δ 8.49
(s, 3H, Im-H), 7.46 (s, 3H, Im-H), 7.18 (s, 3H, Im-H), 7.16 (m, 5H,
B(C6H5)), 4.10 (d, 6H, JHH = 7.6, CH2), 1.80−1.66 (m, 17H, Cy),
1.27−1.19 (m, 10H, Cy), 1.04−0.97 (m, 6H, Cy). ESI+-MS 879
Cald. for C24H25N6BBr2·H2O: C 49.18, H 4.64, N 14.34. Found C
49.18, H 4.64, N 14.35.
Preparation of [HB(CyCH2BzH)3](Br)2 (11). A solution of N-
methylcyclohexylbenzimidazole 2 (2.54 g, 11.8 mmol, 3.02 eqiuv) and
Me3N:BHBr2 (0.896 g, 3.90 mmol, 1 equiv) was heated in refluxing
chlorobenzene (30 mL) for 1 d. After cooling to room temperature,
hexanes (60 mL) were added, and the solution was heated at 50 °C for
15 min. On cooling, the white solid was separated by vacuum filtration,
washed with hexanes, and the residual solvent removed under reduced
pressure The solid was purified by multiple recrystallizations from
acetonitrile/Et2O at −25 °C. The white solid was then dried in vacuo
1
(2.8 g, 88%). H NMR (CDCl3): δ 10.47 (s, 3H, Bz), 8.05 (d, JHH
=
+
(C38H54N6BF6O6S2 ). Anal. Cald. for C38H53N6BF6O6S2: C 51.94, H
6.8, 3H, Bz), 7.66 (d, JHH = 8.0, 3H, Bz), 7.56 (m, 6H, Bz), 5.83 (br s,
1H, B−H), 4.48 (d, JHH = 7.2, 6H, CH2), 1.97 (m, 3H, Cy), 1.74−1.62
(m, 14H, Cy), 1.15−1.06 (m, 16H, Cy). IR (ATR, cm−1): 2431 (w,
B−H). ESI+-MS 815 (C42H56N6BBr2+). Anal. Cald. for
C42H55N6BBr2·H2O: C 60.59, H 6.90, N 10.09. Found C 60.21, H
6.76, N 9.98.
6.08, N 9.56. Found C 51.88, H 6.12, N 9.39.
Preparation of [PhB(PhTzH)3](OTf)2 (7). 4-Phenyl-1,2,4-triazole 3
(5.83 g, 40.0 mmol, 3.05 equiv) was added to a solution of PhBCl2
(2.15 g, 13.1 mmol, 1 equiv) in toluene (70 mL). After stirring for 30
min, TMSOTf (6.19 g, 27.6 mmol, 2.10 equiv) was added, and the
mixture was heated at 110 °C for 1 d. After cooling to room
temperature, a white solid was collected by vacuum filtration, washed
with diethyl ether, and dried in vacuo (9.64 g, 94%). 1H NMR
(CDCl3): δ 9.83 (s, 3H, Tz-H), 8.72 (s, 3H, Tz-H), 7.80 (m, 6H, o-
C6H5), 7.58 (m, 9H, m/p-C6H5), 7.50 (m, 5H, B(C6H5)). ESI+-MS
8 2 2 ( C 3 2 H 2 7 N 9 B F 6 O 6 S 2 + ) . A n a l . C a l d . f o r
C32H26N9BF6O6S2·0.25C7H8: C 48.00, H 3.34, N 14.93. Found C
47.73, H 3.42, N 14.60.
Preparation of [HB(p-tBuPhTzH)3](Br)2 (12). A solution of 4-(4-tert-
butylphenyl)-1,2,4-triazole 3 (2.16 g, 10.7 mmol, 3.05 equiv) and
Me3N:BHBr2 (0.809 g, 3.51 mmol, 1 equiv) in chlorobenzene (30
mL) was heated at 150 °C for 1 d. The solution was allowed to cool to
40 °C followed by dropwise addition of diethyl ether (30 mL) then
pentane (30 mL) with stirring. The mixture was then stored at −25 °C
overnight. The resulting solid was separated from supernatant and
dried under reduced pressure. Dichloromethane (50 mL) was added
to the solid, and the solution washed with alkaline/saline water (1 ×
10 mL). The organic layer was dried under reduced pressure to yield a
white to off-white solid. Impurities were removed by recrystallization
from acetonitrile/Et2O at −25 °C. The white solid obtained was dried
Preparation of [PhB(p-tBuPhTzH)3](OTf)2 (8). 4-(4-tert-Butylphen-
yl)-1,2,4-triazole 4 (1.98 g, 9.8 mmol, 3.03 equiv) was added to a
solution of PhBCl2 (0.513 g, 3.23 mmol, 1 equiv) in toluene (40 mL).
After 30 min, TMSOTf (1.49 g, 6.70 mmol, 2.07 equiv) was added to
the solidified reaction mixture, and the reaction heated at 110 °C for 1
d. After cooling to room temperature, the solid was collected by
vacuum filtration and washed with hexanes. The white solid was then
collected and dried in vacuo (2.49 g, 78%). 1H NMR (CDCl3): δ 9.76
(s, 3H, Tz-H), 8.67 (s, 3H, Tz-H), 7.71 (d, 6H, JHH = 8.8, o/m-
1
in vacuo (1.61 g, 59%). H NMR (CDCl3): δ 10.99 (s, 3H, Tz-H),
t
8.61 (s, 3H, Tz-H), 7.94 (d, JHH = 8.8, 6H o/m-C6H4 Bu), 7.61 (d, JHH
t
= 8.8, 6H o/m-C6H4 Bu), 5.28 (br s, 1H, B−H), 1.33 (s, 27H, tBu). IR
+
(ATR, cm−1): 2518 (w, B−H). ESI+-MS 776 (C36H47N9BBr2 ). Anal.
Cald. for C36H46N9BBr2: C 55.76, H 5.98, N 16.26. Found C 56.03, H
6.27, N 16.46.
t
t
C6H4 Bu), 7.59 (d, 6H, JHH = 8.8, o/m-C6H4 Bu), 7.49 (m, 5H,
t
+
B(C6H5)), 1.33 (s, 27H, Bu). ESI+-MS 990 (C32H27N9BF6O6S2 ).
Anal. Cald. for C32H26N9BF6O6S2: C 46.78, H 3.19, N 15.34. Found C
46.58, H 3.27, N 15.07.
Preparation of [HB(MesTzH)3](Br)2 (13). A toluene (100 mL)
solution of 4-mesitylene-1,2,4-triazole 5 (3.44 g, 18.4 mmol, 3.10
equiv) and Me3N:BHBr2 (1.37 g, 5.93 mmol, 1 eqiuv) in a round-
bottom flask equipped with reflux condenser was heated at vigorous
reflux for 3 d under nitrogen (heating in chlorobenzene at 150 °C
results in the formation of impurities). The solution was allowed to
cool, and the solvent removed under reduced pressure. The resulting
off-white solid was dissolved in dichloromethane (80 mL) and washed
with alkaline/saline water (1 × 15 mL). The organic layer was
collected, and to the aqueous wash solution was added dichloro-
methane (30 mL) a second time. The organic layers were combined
and dried under reduced pressure; the resulting solid was purified by
recrystallization from acetonitrile/Et2O at −25 °C. The white solid
Preparation of [PhB(MesTzH)3](OTf)2 (9). 4-Mesitylene-1,2,4-
triazole 5 (2.40 g, 12.8 mmol, 3.06 equiv) was partially dissolved by
stirring in toluene (80 mL) overnight. Phenylboranedichloride,
PhBCl2 (0.665 g, 4.18 mmol, 1 equiv) was added to the mixture,
and the reaction stirred for 8 h. Trimethylsilyl triflate, TMSOTf (1.91
g, 8.61 mmol, 2.06 eqiuv) was added to the cloudy solution, and the
reaction heated at 110 °C for 1 d. The resultant mixture was dried in
vacuo to yield a white solid. The solid was dissolved in dichloro-
methane (15 mL), and diethyl ether (130 mL) added to induce
precipitation. The solution was stored at −25 °C overnight, and the
resulting colorless crystals were dried in vacuo (3.70 g, 96%). 1H NMR
(CDCl3): δ 9.99 (s, 3H, Tz), 8.55 (s, 3H, Tz), 7.46 (m, 3H, m/p-
B(C6H5)), 7.32 (m, 2H, o-B(C6H5)), 7.03 (s, 6H, m-Mes), 2.34 (s,
1
was dried in vacuo (3.25 g, 74%). H NMR (CDCl3): δ 11.24 (s, 3H,
Tz-H), 8.44 (s, 3H, Tz-H), 7.01 (s, 6H, m-Mes), 5.28 (br s, 1H, B−
H), 2.33 (s, 9H, p-Me), 2.19 (s, 18H, o-Me). IR (ATR, cm−1): 2524
+
+
9H, p-Me), 2.16 (s, 18H, o-Me). ESI+-MS 948 (C41H45N9BF6O6S2 ).
(w, B−H). ESI+-MS 734 (C33H41N9BBr2 ). Anal. Cald. for
Anal. Cald. for C41H44N9BF6O6S2·H2O: C 50.99, H 4.80, N 13.05.
Found C 50.93, H 4.63, N 13.30.
C33H40N9BBr2: C 54.05, H 5.50, N 17.19. Found C 54.02, H 5.76,
N 17.12.
Preparation of [HB(MeBzH)3](Br)2 (10). A solution of N-
methylbenzimidazole (3.54 g, 26.8 mmol, 3.08 eqiuv) and
Me3N:BHBr2 (2.01 g, 8.69 mmol, 1 eqiuv) was heated in
chlorobenzene (50 mL) at 150 °C for 1 d. After cooling to room
temperature, the mixture was filtered to yield a white solid that was
washed with diethyl ether and hexanes and then dried under reduced
pressure. The white solid was dissolved in warm methanol (5 mL).
Diethyl ether (50 mL) and hexanes (20 mL) were added to initiate
precipitation, and the mixture was stored at −25 °C overnight. The
solid that formed was separated from supernatant, and the process
Preparation of PhB(CyCH2Im)3NiNO (14). A slurry of [PhB-
(CyCH2ImH)3](OTf)2 6 (304 mg, 0.345 mmol, 1 equiv) in toluene (5
mL) was cooled to −78 °C. To the mixture was added lithium
diisopropylamide (112 mg, 1.04 mmol, 3.01 equiv). The solution was
stirred for 1 h at −78 °C, allowed to warm to room temperature and
stirred for 3 h. To the resulting yellow solution was added
Ni(NO)I(THF)2 (124 mg, 0.345 mmol, 1 eqiuv) . The reaction was
left to stir for 3 h at room temperature. The solution was filtered
through Celite, and the filtrate dried under reduced pressure. The
residue was then extracted with hexanes (15 mL) and filtered. The
filtrate was dried under vacuum to give an orange-red solid (72 mg,
repeated twice more. The white solid obtained was dried in vacuo (4.6
1
1
g, 92%). H NMR (CDCl3): δ 10.82 (s, 3H, Bz), 7.75 (d, 3H, JHH
=
31%). H NMR (CDCl3): δ 7.86−7.85 (m, 2H, m-B(C6H5)), 7.70−
8.4, Bz), 7.70 (d, 3H, JHH = 8.0, Bz), 7.59 (t, 3H, JHH = 7.4, Bz), 7.52
(t, 3H, JHH = 7.4, Bz), 5.70 (br s, 1H, B−H), 4.27 (s, 9H, Me). IR
7.65 (m, 2H, o-B(C6H5)), 7.56−7.51 (m, 1H, p-B(C6H5)), 6.83 (d,
JHH = 1.6, 3H, Im-H), 6.72 (d, JHH = 2.0, 3H, Im-H), 4.07 (d, 6H, JHH
= 6.8, CH2), 1.73−1.71 (m, 17H, Cy), 1.20 (m, 10H, Cy), 1.09−1.03
+
(ATR, cm−1): 2482 (w, B−H). ESI+-MS 568 (C24H25N6BBr2 ). Anal.
C
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(m, 6H, Cy) . 13C NMR (CDCl3): δ 201.3 (carbene-C), 134.2, 127.5
127.4, 126.9, 122.8, 118.8, 57.0, 39.5, 30.7, 26.3, 25.8. IR (THF, cm−1):
1693 (s, N−O). ESI−-MS 668 (C36H53N7BNiO−). UV−vis (THF):
441 nm (ε = 790 M−1 cm−1). Eox = −0.22 V (irr). Despite multiple
attempts, we have been unable to obtain satisfactory microanalysis
results.
Ni). 1H NMR (C6D6): δ 7.55 (s, 3H, Tz-H), 7.39 (d, JHH = 8.8, 6H, o/
t
t
m-C6H4 Bu), 7.04 (d, JHH = 8.4, 6H, o/m-C6H4 Bu), 1.03 (s, 27H,
tBu). 13C NMR (CDCl3): δ 199.6 (carbene-C), 151.2, 141.3, 135.2,
126.4, 122.6, 34.7, 31.3. IR (THF, cm−1): 2544 (w, B−H), 1746 (s,
N−O). ESI−-MS 700 (C36H43N10BNiO−). UV−vis (THF): 467 nm (ε
= 500 M−1 cm−1). Eox = 0.38 V (irr); Ered = −1.78 V (irr).Despite
multiple attempts, we have been unable to obtain satisfactory
microanalysis data for this complex.
Preparation of HB(MeBz)3NiNO (15). To a solution of [HB-
(MeBzH)3](Br)2 10 (0.345 g, 0.607 mmol, 1 equiv) in dichloro-
methane (7 mL) was added Ag2O (0.227 g, 0.979 mmol, 1.6 eqiuv).
The reaction was heated at 45 °C overnight in air, dried under
Preparation of HB(MesTz)3NiNO (18). To a solution of [HB-
(MesTzH)3](Br)2 13 (184 mg, 0.250 mmol, 1 equiv) in acetonitrile (5
mL) was added Ag2O (93 mg, 0.40 mmol, 1.6 equiv). The reaction
was heated at reflux for 1 d, the reaction mixture was then dried under
vacuum, and KBr (45 mg, 0.38 mmol, 1.5 equiv) in THF (5 mL) was
added to the residue and stirred under nitrogen overnight. The solvent
was removed under reduced pressure, and the residue extracted with
toluene and filtered through Celite. The filtrate was dried under
reduced pressure to yield “HB(MesTz)3K” (89 mg). An aliquot of the
1
vacuum, and an aliquot was analyzed by H NMR spectroscopy. Two
sets of resonances associated with the deprotonated ligand are
observed. This is possibly due to the formation of different silver
complexes, but was not further investigated, (1H NMR (CD3Cl3)):
Species A: δ 7.92 (s, 3H, Bz), 7.79 (d, JHH = 8.0, 3H, Bz), 7.44 (m, 3H,
Bz), 7.35 (m, 3H, Bz), 3.86 (s, 9H, Me); Species B: δ 7.75 (d, 3H, Bz),
7.57 (d, JHH = 8.0, 3H, Bz), 7.43 (m, 3H, Bz), 7.34 (m, 3H, Bz), 3.69
(s, 9H, Me)). To the tris(carbene)borate ligand synthon was added
THF (7 mL), the vessel charged with Ni(NO)Br(PPh3)2 (0.463 g,
0.668 mmol, 1.1 equiv) under nitrogen at room temperature, and
stirred overnight. The solvent was removed under reduced pressure,
and the residue extracted with toluene and filtered through Celite. The
toluene filtrate was dried, and the residual solid was purified by column
chromatography under nitrogen (Florisil, THF eluent). The orange-
red colored fraction was collected and dried to give an orange-red solid
(101 mg, 33%). 1H NMR (C6D6): δ 7.86 (d, JHH = 8.0, 3H, Bz), 7.12−
7.05 (m, 6H, Bz), 6.77 (d, JHH = 8.0, 3H, Bz), 5.79 (br s, 1H, B−H),
3.69 (s, 9H, Me). 13C NMR (CDCl3): δ 210.7 (carbene-C), 138.34,
136.81, 121.48, 120.78, 111.79, 109.06, 34.64. IR (THF, cm−1): 2462
(w, B−H), 1714 (s, N−O). ESI−-MS 492 (C24H22N7BNiO−). Anal.
Calcd. for C24H22BN7NiO: C 58.35, H 4.49, N 19.85. Found C 58.04,
H 4.48, N 19.74. UV−vis (THF): 433 nm (ε = 1180 M−1 cm−1). Ered
= −1.43 V (irr).
Preparation of HB(CyCH2Bz)3NiNO (16). To a solution of
[HB(CyCH2BzH)3](Br)2 11 (341 mg, 0.418 mmol, 1 equiv) in
acetonitrile (6 mL) was added Ag2O (194 mg, 0.837 mmol, 2 equiv).
The reaction was heated at reflux for 1 d, dried under vacuum. The
poor solubility of the residue prevented any further characterization.
To the tris(carbene)borate ligand synthon was added THF (6 mL)
and Ni(NO)I(THF)2 (0.105 g, 0.292 mmol, 0.7 equiv).29 The
reaction was stirred overnight under nitrogen at room temperature.
The solvent was removed under reduced pressure, and the residue
extracted with toluene and filtered through Celite. The toluene filtrate
was dried, the resultant solid extracted with hexanes and filtered. The
hexanes solution was dried under vacuum, and the residue washed
with acetone followed by MeOH to give a red-orange colored solid
(87 mg, 40% based on Ni). 1H NMR (C6D6): δ 7.91 (d, JHH = 7.6, 3H,
Bz), 7.09−7.00 (m, 12H, Bz), 5.71 (br s, 1H, B−H), 4.33 (d, JHH = 7.2,
6H, CH2), 1.66 (br m, 7H, Cy), 1.57 (br m, 12H, Cy), 1.07 (br m,
14H, Cy). 13C NMR (CDCl3): δ 211.7 (carbene-C), 138.5, 136.6,
121.2, 120.4, 111.9, 109.5, 54.6, 38.8, 31.2, 26.3, 25.9. IR (THF, cm−1):
2463 (w, B−H), 1711 (s, N−O). ESI−-MS 739 (C42H52N7BNiO−).
UV−vis 435 nm (ε = 1490 M−1 cm−1). Ered = −1.93 V (irr).
1
residue analyzed by H NMR spectroscopy is consistent with 3-fold
deprotonation (1H NMR (C6D6)): δ 7.40 (s, 3H, Tz-H), 6.49 (s, 6H,
m-Mes), 1.98 (s, 9H, p-Me), 1.48 (s, 18H, o-Me). To the
tris(carbene)borate ligand synthon was added a toluene solution (3
mL) of Ni(NO)I(THF)2 (52 mg, 0.14 mmol, 0.6 equiv).29 The
reaction was stirred overnight at room temperature. The solution was
filtered through Celite and dried in vacuo to give a maroon colored
1
solid (51 mg, 53% based on Ni). H NMR (CDCl3): δ 7.84 (s, 3H,
Tz-H), 6.91 (s, 6H, m-Mes), 2.27 (s, 9H, p-Me), 1.97 (s, 18H, o-Me).
13C NMR (CDCl3): δ 202.1 (carbene-C), 142.5, 138.6, 134.6, 133.2,
129.0, 20.9, 18.0. IR (THF, cm−1): 2537 (w, B−H), 1742 (s, N−O).
ESI−-MS 658 (C33H37N10BNiO−). Anal. Calcd. for C33H37BN10NiO:
C 60.12, H 5.66, N 21.25. Found C 59.81, H 5.40, N 21.00. UV−vis
(THF): 451 nm (ε = 620 M−1 cm−1). Eox = 0.40 V (irr); Ered = −1.83
V (irr).
Solid Angle Calculations. Solid angle calculations were
performed using the program Solid-G.30,31 The ligand coordinates
used in the calculations were taken from X-ray crystal structure data of
the corresponding {NiNO}10 complexes. The metal ligand bond
lengths were set at a distance of 2.28 Å. In cases where there was more
than one molecule in the asymmetric unit, solid angles were calculated
for each molecule, and the average value is reported. For the truncated
calculations, ORTEP-3 (version 2.02)32 was used to define a sphere of
enclosure (4 Å, centered on Ni), and the resulting coordinates were
used for solid angle calculations.
RESULTS AND DISCUSSION
■
Synthesis of Heterocycles. N-Methylcyclohexylimidazole
1 was prepared by the reaction of deprotonated imidazole and
bromomethylcyclohexane (Scheme 1). Conducting the reaction
Scheme 1
Preparation of HB(p-tBuPhTz)3NiNO (17). To a solution of
[HB(p-tBuPhTzH)3](Br)2 12 (59 mg, 0.07 mmol, 1 equiv) in
acetonitrile (4 mL) was added Ag2O (35 mg, 0.15 mmol, 2.1
equiv). The reaction was heated at reflux for 1 d. The reaction mixture
was dried under vacuum, followed by addition of KBr (14 mg, 0.11
mmol, 1.5 equiv) in THF (4 mL) and the reaction stirred under
nitrogen overnight. The solvent was removed under reduced pressure,
and the residue extracted with toluene and filtered through Celite. The
filtrate was dried under reduced pressure to yield “HB(p-tBuPhTz)3K”
(49 mg). An aliquot of the residue analyzed by 1H NMR spectroscopy
is consistent with 3-fold deprotonation (1H NMR (C6D6)): δ 7.88 (s,
at low temperature and allowing for long reaction times
provides the product in almost quantitative yield following
distillation. This procedure is a substantial improvement over
that previously reported,26 leading to substantially greater yields
of product without the need for chromatography. The same
procedure yields N-methylcyclohexylbenzimidazole as a color-
less solid in almost quantitative yield. Both heterocycles have
been spectroscopically characterized.
t
3H, Tz-H), 7.00 (d, JHH = 8.4, 6H, o/m-C6H4 Bu), 6.50 (d, JHH = 8.4,
t
t
6H, o/m-C6H4 Bu), 1.11 (s, 27H, Bu). To the tris(carbene)borate
ligand synthon was added a toluene (3 mL) solution of Ni(NO)I-
(THF)2 (27 mg, 0.07 mmol, 1 equiv) at room temperature. The
reaction was stirred overnight, the solution filtered through Celite, and
dried in vacuo to give a maroon colored solid (29 mg, 52% based on
1-Aryl-1,3,4-triazoles were prepared by a similar procedure to
that reported for the synthesis of 1-phenyl-1,3,4-triazole 3.23
D
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This synthetic route was successfully extended to the synthesis
of p-tBuPhTz 4 and MesTz 5 by reacting the dihydrochloride of
N,N-dimethylformamide azine with para-tert-butylaniline and
2,4,6-trimethylaniline, respectively (Scheme 2).23 Increasing the
Scheme 4. Synthesis of HB(RTz)3NiNO Complexes 17 and
18
a
Scheme 2
a
Reagents and conditions: (i) 0.33 HBBr2:NMe3, C6H5CH3 or
C6H5Cl, Δ; (ii) 1. Ag2O, MeCN, Δ; 2. KBr, THF; 3. Ni(NO)I-
(THF)2, THF.
bulk of the aryl group requires longer reaction times and higher
temperatures to proceed to completion, for example, for Ar =
Ph, 1 d in refluxing benzene, while for Ar = Mes, 3 d heating in
refluxing chlorobenzene. The colorless solids were purified by
dications. Thus, for example, six resonances are observed in
the 1H NMR spectrum of HB(MesTz)3Br2 13. The most
notable feature of the spectrum is the downfield singlet at 11.24
ppm, which is assigned to the three acidic triazole protons. As
expected for the 2+ charge, all the other resonances arising
from the substitued triazole moeity are shifted downfield from
the corresponding triazole. Thus, the remaining triazole ring
protons resonate as a singlet at 8.55 ppm, while resonances
assigned to the mesityl substituent are observed at 7.01 (m-H),
2.33 (p-Me), and 2.19 ppm (o-Me). A broad resonance
attributed to the B−H group is observed at 5.28 ppm. Similar
features are observed in the 1H NMR spectrum of HB-
(p-tBuPhTzH)3Br2 12.
1
crystallization and characterized by H NMR spectroscopy.
Both PhTz 323 and MesTz 528 have been previously reported,
while p-tBuPhTz 4 is a new compound.
Synthesis of Ligand Precursors. Tris(carbene)borate
ligand precursors were prepared by the same synthetic routes
used for the synthesis of other tris(imidazol-2-ylidene)borate
ligands.6,13 Thus, for example, tris(benzimidazolium)-
hydroborane dications HB(RBzH)3Br2 (R = Me 10, CyCH2
11) were prepared by reaction of the appropriate benzimida-
zole with Me3N:BHBr2 in hot chlorobenzene, as with the
synthesis of tris(imidazolium)hydroborane dications (Scheme
3).13,33 As with the tris(triazolium)borane dications, the
We have also prepared a new tris(imidazolium)phenylborane
dication starting from 1-methylcyclohexylimidazole, namely,
PhB(CyCH2ImH)3OTf2 6. This salt was prepared by the same
method used for the synthesis of other tris(imidazolium)-
phenylborane triflate salts and has similar spectral proper-
ties.11,13 Colorless crystals of this salt were grown by diffusion
of pentane into a CH2Cl2 solution at −25 °C. The solid state
structure reveals the expected four-coordinate boron center that
is bound to a phenyl and three imidazolium groups (Figure 3).
Two triflate anions are also observed in the solid state structure.
NHC Formation. The new tris(carbene)borate ligands were
generated by 3-fold deprotonation of the borane dications to
yield the corresponding tris(carbene)borate ligands (Schemes 3
and 4). 3-fold deprotonation of the borane dications to
generate the corresponding tris(carbene)borate ligands was
found to be very sensitive to the nature of the base. While the
tris(methylcyclohexylimidazol-2-ylidene)phenylborate ligand
was readily formed by deprotonation of the corresponding
imidazole borane dication with LDA, as previously reported,13
similar attempts to generate the tris(benzimidazol-2-ylidene)-
hydroborate and tris(1,3,4-triazol-2-ylidene)hydroborate li-
gands were unsuccessful. A library of bases was canvassed,
including LDA, NaH, MeMgBr, all of which provided
Scheme 3. Synthesis of HB(RBz)3NiNO Complexes 15 and
16
a
a
Reagents and conditions: (i) 0.33 HBBr2:NMe3, C6H5Cl, reflux; (ii)
1. Ag2O, MeCN or CH2Cl2, Δ; 2. Ni(NO)I(THF)2 or Ni(NO)Br-
(PPh3)2, THF.
spectral data of these compounds is consistent with their
proposed formulation. In the case of HB(MeBzH)3Br2 10, the
resonance assigned to the acidic protons are observed at 10.82
ppm. Four resonances assigned to protons from the
benzimidazolium group at 7.75, 7.70, 7.59, and 7.52 ppm,
respectively, with the singlet at 4.27 ppm assigned to the methyl
substituent.
Tris(triazolium)hydroborane dications were prepared in high
yield by heating a toluene solution containing 3 equiv of the
substituted triazole with Me3N:BHBr2 (Scheme 4).33 We note
that a bis(1,3,4-triazole)hydroborane cation has been previously
reported, although a different synthetic route was used.34 The
resulting tris(triazolium)hydroborane dications 12 and 13 were
1
intractable mixtures of products, as determined by H NMR
spectroscopic analysis of the crude reaction mixtures.
The tris(benzimidazolium)hydroborane and tris(triazolium)-
hydroborane dications could be deprotonated by reaction with
Ag2O in hot MeCN or CH2Cl2. While the insolubility of most
of these materials prevented further characterization, subse-
quent reaction with {NiNO}10 synthons confirmed that
tris(benzimidazol-2-ylidene)hydroborate synthons were formed
(see below).
Unfortunately, the ligand synthons formed from the
tris(triazolium)hydroborane dications and Ag2O did not cleanly
transfer to nickel. These compounds were instead treated with
KBr to species tentatively formulated as potassium tris(1,3,4-
1
characterized by H NMR spectroscopy, ESI-MS and micro-
analysis. The triazolium salts are colorless air-stable compounds
that are less soluble in nonpolar solvents than the related
tris(imidazolium)hydroborane dications.13 Their solubility is
influenced by the nature of the triazole substituent, with the
relative solubility decreasing in the order p-tBuPh > Mes > Ph.
The spectral data for these compounds are consistent with
their proposed formulation as tris(triazolium)hydroborane
E
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24
synthons Ni(NO)Br(PPh3)2 or Ni(NO)I(THF)2,25 leading
to formation of new tris(carbene)borate {NiNO}10 complexes
(Schemes 3 and 4). The nickel nitrosyl complexes were found
to be diamagnetic, air stable complexes, characterized by a
1
number of techniques including H and 13C{1H} NMR, UV−
vis, and IR spectroscopy, single crystal X-ray diffraction, CV,
microanalysis, and ESI-MS. When prepared using Ni(NO)Br-
(PPh3)2, the complexes were often contaminated with PPh3,
PPh3O, and PPh3AgBr byproducts. For some complexes, these
byproducts could be removed by chromatography on a Florisil
column in an air-free environment. However, other complexes,
for example, PhB(CyCH2Im)3NiNO 14, were found to
decompose on the column, even when other solid supports
25
were used. The use of Ni(NO)I(THF)2 as the {NiNO}10
synthon was not universally successful, and in some cases the
use of a sub-stochiometric amount of the reagent was necessary
to avoid the formation of side products, for example, in the case
of HB(CyCH2Bz)3NiNO 16.
The complexes PhB(CyCH2Im)3NiNO 14, HB-
(MeBz)3NiNO 15, HB(CyCH2Bz)3NiNO 16, HB-
(p-tBuPhTz)NiNO 17, and HB(MesTz)NiNO 18 were also
crystallographically characterized.35 The molecular structures of
these complexes (Figure 4 and Supporting Information) reveal
the expected four-coordinate metal center bound to the
tripodal tris(carbene)borate ligand and terminal nitrosyl ligand.
The complexes all have similar metrical parameters around the
nickel center, regardless of the nature of the tripodal ligand.
Thus, for example, the Ni−NO bond lengths in these
complexes are in the narrow range 1.640(2)−1.646(2) Å,
which compares well with the Ni−N bond length of 1.620(5) Å
observed for HB(tBuIm)3NiNO.14 Similarly, the Ni−C bonds
show little variation between the different tripodal ligands.
There is greater variability in the Ni−N−O bond angle, which
varies from 169.3(2)°−176.3(3)°, compared with 178.5(4)° for
HB(tBuIm)3NiNO.14 However, it is likely these differences are
due to packing forces in the solid state, as evidenced by the
variation in the Ni−N−O bond angle (169.3(2)−174.8(2)°)
for the four independent molecules of HB(MeBz)3NiNO 15 in
the unit cell. There do not appear to be any trends in metrical
Figure 3. X-ray crystal structure of PhB(CyCH2ImH)3OTf2 6.
Thermal ellipsoids shown at 50% probability, hydrogen atoms omitted
for clarity. Selected bond lengths (Å) and angles (deg): C(11)−N(11)
1.337(3); C(11)−N(21) 1.322(3); C(12)−N(12) 1.332(3); C(12)−
N(22) 1.319(3); C(13)−N(13) 1.331(3); C(13)−N(23) 1.328(3;
N(11)−C(11)−N(21) 109.8(2); N(12)−C(12)−N(22) 110.1(2);
N(13)−C(13)−N(23) 110.2(2).
triazol-2-ylidene)hydroborates. Because of their greater sol-
ubility, deprotonation of the carbene precursors could be
1
confirmed by H NMR spectroscopy. Thus, for example, four
1
resonances are observed in the H NMR spectrum of product
prepared from [HB(p-tBuTz)3](Br)2: a singlet at δ 7.88 ppm,
assigned to the remaining triazolylidene proton, doublets at δ
7.00 and 6.50 ppm, assigned to the protons of the p-tert-
butylphenyl ring and a singlet at δ 1.11 ppm, assigned to the
tert-butyl group. The resonance for the B−H group is not
observed, presumably because of quadrupolar broadening.
Synthesis and Characterization of Nickel Nitrosyl
Complexes. The ligand synthons were not isolated, but
instead were used in a one-pot reaction with the {NiNO}10
Figure 4. X-ray crystal structures of representative nickel nitrosyl complexes. Thermal ellipsoids shown at 50% probability, most hydrogen atoms
omitted for clarity. (A) PhB(CyCH2Im)3NiNO 14. One of three independent molecules in the asymmetric unit. Selected bond lengths (Å) and
angles (deg): Ni(1)−N(7) 1.643(10); Ni(1)−C(1) 1.982(11); Ni(1)−C(11) 1.939(11); Ni(1)−C(21) 1.974(13); N(7)−Ni(1)−C(1) 122.8(5);
N(7)−Ni(1)−C(11) 123.2(5); N(7)−Ni(1)−C(21) 130.3(5); Ni(1)−N(7)−O(1) 173.7(11). (B) HB(MeBz)3NiNO 15. One of four independent
molecules in the asymmetric unit. Selected bond lengths (Å) and angles (deg): Ni(1)−N(7) 1.646(2); Ni(1)−C(15) 1.945(3); Ni(1)−C(1)
1.959(3); Ni(1)−C(8) 1.961(3); N(7)−Ni(1)−C(15) 118.85(11); N(7)−Ni(1)−C(1) 132.30(11); N(7)−Ni(1)−C(8) 123.90(11); O(1)−N(7)−
Ni(1) 169.3(2). (C) HB(p-tBuPhTz)3NiNO 17. Selected bond lengths (Å) and angles (deg): Ni(1)−N(10) 1.640(2); Ni(1)−C(1) 1.979(3);
Ni(1)−C(13) 1.989(3); Ni(1)−C(25) 1.985(3); N(10)−Ni(1)−C(1) 125.68(12); N(10)−Ni(1)−C(13) 122.51(12); N(10)−Ni(1)−C(25)
126.90(14); O(1)−N(10)−Ni(1) 176.3(3).
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parameters that are ligand-dependent, and thus X-ray
crystallography is unable to differentiate the donor abilities of
the different tris(carbene)borate ligands, for example, from N−
O or Ni−C bond lengths.
to make them similar to the alkyl-substituted tris(benzimidazol-
2-ylidene)borate. It is likely that the three NHC donors in
tris(carbene)borate ligands magnify these effects as compared
to the monodentate analogues, allowing for fine-tuning of the
tripodal ligand donor strength. Interestingly, for the imidazol-2-
ylidene based ligands, the tris(carbene)borate donor strength
does not follow the trend that would be predicted from the
inductive properties of the alkyl donors, with the tert-butyl
substituted ligand being the least donating. For this ligand it is
likely that steric crowding resulting from this very bulky ligand
reduces orbital overlap between nickel and the tris(carbene)-
borate. Evidence for this hypothesis comes from the average
Ni−C bond length, which is longer for PhB(tBuIm)3NiNO
(2.003(1) Å)14 than for PhB(CyCH2Im)3NiNO 14 (1.969(4)
Å).
Infrared spectroscopy was used to evaluate the relative donor
ability of the tris(carbene)borate ligands in solution, specifically
from νNO (Table 1). On the basis of the bonding descriptions
Table 1. Infrared Spectroscopic Data for Four-Coordinate
Tris(carbene)borate {NiNO}10 Complexes Measured in
THF
tris(carbene)borate
solution νNO (cm−1
)
reference
−
PhB(CyCH2Im)3
1693
1697
1701
1724
1711
1714
1742
1746
this work
14
−
PhB(MeIm)3
PhB(tBuIm)3
14
−
Comparing the donor properties of these tris(carbene)borate
ligands with related facially coordinating ligands is instructive
(Table 2). Thus, although the tris(1,3,4-triazol-2-ylidene)-
−
PhB(MesIm)3
14
−
HB(CyCH2Bz)3
this work
this work
this work
this work
−
HB(MeBz)3
−
HB(MesTz)3
Table 2. Solution IR Spectroscopic Data for Selected Four-
Coordinate {NiNO}10 Complexes
HB(p-tBuPhTz)3
−
ligand
νNO (cm−1
)
reference
for structurally related Tp*NiNO, the impact of the tripodal
ligand donor strength on νNO can be formulated in two ways. In
the traditional formulation,20a the magnitude of νNO is related
to the extent of π-backbonding from the Ni(0) center to the π*
orbitals of coordinated NO+.36 Thus, increasing the electron-
releasing ability of the tripodal ligand leads to a reduction in
νNO. In the revised formulation, the electronic structure is
described as having multireference character with NiI(S = 1/
2)−NO0(S = 1/2), NiII(S = 0)−NO−(S = 0), and NiII(S = 1)−
NO−(S = 1) configurations.21 In this description, the
magnitude of νNO can be related to the relative stabilization
of these different electronic configurations. Specifically,
increasing the donor strength of the tripodal ligand will
stabilize the Ni(II) configurations at the expense of the Ni(I)
configuration. Since the Ni(II) configurations have populated
N−O π* orbitals, this is expected to reduce the overall N−O
bond order. Thus, both electronic structure descriptions predict
that increasing the donor strength of the tripodal ligand will
Cp*
Tp*
1787
1786
1737
1741
37
38
39
36
−
PhB(CH2PPh2)3
TmtBu−
borates are the weakest of the tris(carbene)borate donors, they
are still more strongly donating than either Cp* and Tp*. In
fact, the tris(1,3,4-triazol-2-ylidene)borates bridge a gap in
donor strength between these classical ligands and the stronger
tris(phosphino)borate39 and tris(thioimidazolyl)borate
(TmR−) donors,36 while the tris(benzimidazol-2-ylidene)-
borates bridge a similar gap between these two ligands and
the alkyl-substituted tris(imidazol-2-ylidene)borates. An im-
portant difference with these other ligand classes is that the
ligand topology is consistent for all three types of tris(carbene)-
borate. Thus, the rigid tris(carbene)borates span a greater range
of donor strengths than either the tris(phosphino)borate or
tris(thioimidazolyl)borate ligands6 while maintaining a con-
sistent topology.
Attempts to use other physical measurements to delineate
the donor properties of the tris(carbene)borate ligands were
not successful, for example, 13C{1H} NMR, UV−vis, CV. Thus,
for example, while the resonances attributed to the carbene
carbon of the tris(carbene)borate ligand could be readily
identified in the 13C{1H} NMR spectra, the chemical shifts are
similar for all complexes, and there is no systematic order
related to the donor strength of the ligand.35 This is similar to
observations reported for a large series of monodentate carbene
ligands in Rh(CO)2(NHC)I complexes.19a
The range of substituents has also allowed us to explore the
steric properties of the tris(carbene)borate ligands using solid
angles. The solid angle can be visualized as follows: a light
source is placed at the metal center that is circumscribed by a
sphere of radius r. Since the ligand blocks the light source, it
will cast a shadow of area A on the inside of the sphere,
allowing the solid angle to be calculated in steradians as Ω = A/
r2. A convenient way of expressing the solid angle is as the
percentage of the sphere’s surface that is covered by the ligand
shadow, G = 100 Ω/4π (4π is the solid angle of a sphere in
steradians).30
result in a lowering of νNO
.
The IR data reveal that, in general, the donor strength of
these tris(carbene)borate ligands decreases in the order:
imidazol-2-ylidene > benzimidazol-2-ylidene > 1,3,4-triazol-2-
ylidene, which correlates well with the trend observed for
monodentate NHC ligands.19a,c,35 It has been shown in at least
one instance that the nature of the fourth group on the boron
atom (i.e., H− vs Ph−) has little impact on the donor strength of
the tris(carbene)borate ligand.14 It is notable that tris(carbene)-
borates provide an exceptional degree of electronic tunability,
with νNO varying over a range of more than 50 cm−1. This
should be compared with the smaller range of 25 cm−1
observed for Rh(CO)2(NHC)I complexes, where the mono-
dentate NHC varied from relatively weakly donating 1,3-
dimethylperimidin-2-ylidene to very strongly donating 1,2,3-
trimethyl-4-isopropylpyrazolin-5-ylidene.19
Interestingly, and in contrast to monodentate NHC ligands,
changing the ligand substituent influences the donor strength of
tris(carbene)borate ligands.19 Thus, for example, alkyl sub-
stituted ligands have a greater donor ability than aryl, as
reflected in the data for the imidazol-2-ylidene series. In fact,
replacing alkyl substituents in the tris(imidazol-2-ylidene)-
borates by an aryl group attenuates donor strength sufficiently
G
dx.doi.org/10.1021/ic301204b | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
The percent coverage of a sphere (G) for the alkyl-
substituted tris(carbene)borate ligands were calculated from
the coordinates determined by X-ray crystallography (Table
3).35 As expected, these calculations reveal that the methyl-
substituents appear to hinder effective orbital overlap, thereby
reducing the effective donor strength. Similarly to tris-
(pyrazolyl)borates, the consistent topology of the tris-
(carbene)borate ligand class allows for predictable changes to
their steric properties. An interesting finding in this regard is
that certain N-substituents, for example, methylcyclohexyl, may
provide an open environment close to the metal center, while
still providing a sufficiently bulky profile to stabilize low
coordination numbers. The combination of exceptional
electronic tunability and the consistent topology of the
tris(carbene)borate ligand class is likely to allow for predictable
variation in the properties of their transition metal complexes.
a
Table 3. Solid Angle Data (G) for Selected Alkyl-
Substituted Tris(carbene)borate Ligands in {NiNO}10
Complexes
b
ligand
full ligand
truncated ligand
c
−
HB(MeBz)3
HB(CyCH2Bz)3
36.9
56.3
52.8
53.2
37.0
55.4
37.0
36.9
39.0
53.2
37.0
54.1
−
c
−
PhB(CyCH2Im)3
HB(tBuIm)3
−
ASSOCIATED CONTENT
* Supporting Information
■
Tp*
S
−
PhB(CH2PPh2)3
Further details on the solid angle calculations, X-ray crystallo-
graphic data for all structurally characterized compounds,
including crystallographic information files (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
a
Solid angles expressed as percentage coverage of a sphere. The G
b
values were calculated using a metal ligand distance of 2.28 Å. Ligand
was truncated at a radius of 4 Å from the metal center. Average values
c
for the four independent molecules in the asymmetric unit.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jesmith@nmsu.edu.
■
subtituted tris(carbene)borate ligand is the least encompassing
ligand, but somewhat surprisingly, the tert-butyl and methyl-
cyclohexyl substituted ligands appear to exert a similar steric
effect. However, when G was determined with the ligand
artificially truncated at a radius 4 Å from the metal center, the
methylcyclohexyl and methyl substituents were found to be
similar in size. Similar observations have been made for some
monodentate NHC ligands.40 The truncated value of G, which
approximates the steric profile of the ligand as experienced in
the first coordination sphere of nickel, suggests that the
methylcyclohexyl substituent provides an open environment in
the immediate vicinity of the metal center, while the large value
of G for the full ligand suggests that this substituent may also be
sufficiently bulky to stabilize low coordination numbers.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding from the DOE-BES (DE-FG02-08ER15996) and the
Dreyfus Foundation is gratefully acknowledged. S.B.M. thanks
NSF-AMP (HRD 0929343) and NIH-RISE (R25 GM061222-
11) for financial support. The Bruker X8 X-ray diffractometer
was purchased via an NSF CRIF:MU award to The University
of New Mexico, CHE-0443580. We thank Eileen Duesler
(UNM) and Ranko P. Bontchev (Cabot Corporation) for
assistance with X-ray crystallography.
For comparative purposes, G was also calculated for two
other tripodal ligands using the X-ray crystal structure data of
Tp*NiNO20b and PhB(CH2PPh2)3NiNO.39 The data suggest,
not surprisingly, that the steric impact of tris(carbene)borate
and tris(pyrazolyl)borate ligands are similar, as shown by the
similar sizes of the methyl-substituted ligands HB(MeBz)3− and
Tp*. The solid angle data also reveal that the very bulky
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dx.doi.org/10.1021/ic301204b | Inorg. Chem. XXXX, XXX, XXX−XXX