Bis(alkylamido)phenylborane complexes of zirconium, hafnium, and vanadium

Article abstract of DOI:10.1021/ic0340478

The coordination chemistry of the bis(tert-butylamido)phenylborane ligand, [^tBuN-B^Ph-N^tBu]^2-, is developed. The ligand can be delivered to metals of groups 4 and 5 from its dilithio salt. The reactions of PhB(

Full text of DOI:10.1021/ic0340478

Inorg. Chem. 2003, 42, 4431−4436
Bis(alkylamido)phenylborane Complexes of Zirconium, Hafnium, and
Vanadium
David R. Manke and Daniel G. Nocera*
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139-4307
Received January 16, 2003
t
The coordination chemistry of the bis(tert-butylamido)phenylborane ligand, [tBuN−BPh−NBu]2-, is developed. The
t
ligand can be delivered to metals of groups 4 and 5 from its dilithio salt. The reactions of PhB(BuNLi)2, 1, with
t
metal halides of zirconium, hafnium, and vanadium generate complexes of the general formulas (BuN−BPh−
t
t
t
t
t
NBu)2M(THF) (M ) Zr (2), Hf (3)), Li2[M(BuN−BPh−NBu)3] (M ) Zr (4), Hf (5)), and M(BuN−BPh−NBu)2 (M )
V(6)). 1H and 11B{1H} NMR and single-crystal X-ray analysis show that these amido metal complexes are structurally
analogous to amidinates.
Introduction
tion metal chemistry could be elaborated for ligands of the
antithetical π-donor-acceptor-donor motif. One strategy to
developing such an electronic structure is to catenate the
π-accepting orbital of a bridgehead boron to the π-donor
orbitals of amides, as observed for bis(alkylamido)phenyl-
boranes, PhB(RN)22-. The ligand framework is structurally
similar to the widely used monoanionic amidinate ligands
with the additional caveat that an electron-accepting group
is situated at the bridgehead. Like amidinates, the electronic
and steric properties of the ligand can be tuned with
substituents on the amide nitrogen. Additionally, the donating
ability of the coordinating amides can be further modulated
by R substitution at the electron-withdrawing B(R) bridge-
head.
Simple ligand frameworks that juxtapose π-accepting
groups directly adjacent to π-donating groups are unusual
in transition metal chemistry. One such example that
embodies this electronic construct is the bis(difluorophos-
phino)methylamine (dfpma) ligand, CH3N(PF2)2.1,2 In the
dfpma architecture, π-accepting fluorophosphine groups are
adjacent to the lone pair of an amine bridgehead, giving rise
to an acceptor-donor-acceptor ligand motif. Because the
phosphine groups may π-accept electrons from the metal or
from the lone pair of nitrogen, the ligand is able to
concomitantly accommodate metals both in low and moder-
ate oxidation states. We have exploited this property of
dfpma and the related bis(phosphito)amine ligands to develop
a rich ground and excited state multielectron chemistry of
transition metal complexes.3-10 Against this backdrop of
reactivity, we became interested in exploring whether transi-
Diamines of bis(alkylamido)phenylboranes have been
known since 1957,11 and they can be prepared readily from
the reaction of dichlorophenylborane with 4 equiv of a
primary amine.12 The lithiated amide derivatives of these
amines were first reported in 1990;13 a subsequent chemistry
* To whom correspondence should be addressed. E-mail: nocera@
mit.edu.
2-
has evolved for PhB(RN)2 complexed to main group
(1) Nixon, J. F. Chem. Commun. 1967, 669.
(2) King, R. B. Acc. Chem. Res. 1980, 13, 243-248.
(3) Dulebohn, J. I.; Ward, D. L.; Nocera, D. G. J. Am. Chem. Soc. 1988,
110, 4054-4056.
elements.14-22 The transition metal chemistry of these ligands,
(4) Dulebohn, J. I.; Ward, D. L.; Nocera, D. G. J. Am. Chem. Soc. 1990,
112, 2969-2677.
(11) Mikhailov, B. M.; Kostroma, T. V. Bull. Acad. Sci. USSR, DiV. Chem.
Sci. 1957, 1149-1152.
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Inorg. Chem. 1996, 35, 811-817.
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1999, 121, 5023-5032.
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Pauer, F. Chem. Ber. 1990, 123, 703-706.
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(8) Odom, A. L.; Heyduk, A. F.; Nocera, D. G. Inorg. Chim. Acta 2000,
297, 330-337.
(14) Heine, A.; Fest, D.; Stalke, D.; Habben, C. D.; Meller, A.; Sheldrick,
G. M. J. Chem. Soc., Chem. Commun. 1990, 742-743.
(15) Habben, C. D.; Heine, A.; Sheldrick, G. M.; Stalke, D.; Bu¨hl, M.;
Schleyer, P. v. R. Chem. Ber. 1991, 124, 47-50.
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46B, 625-628.
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10.1021/ic0340478 CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/18/2003
Inorganic Chemistry, Vol. 42, No. 14, 2003 4431

Manke and Nocera
vacuum evaporation, and 10 mL of hexanes was added. The solution
was filtered through Celite to remove LiX. Concentration of the
solution followed by cooling to -35 °C and filtration afforded 108
mg of colorless crystals (56% yield) of 4 and 173 mg of colorless
crystals (64% yield) of 5. Anal. data for 4: 1H NMR (300 MHz,
C6D6, 25 °C) δ 1.400 (s, 54H), 7.1-7.8 (m, 15H); 11B{1H} NMR
(96.205 MHz, C6D6, 25 °C) δ 36.4. Anal. Calcd for C42H69B3-
Li2N6Zr: C, 63.41; H, 8.74; N, 10.56. Found: C, 63.35; H, 8.80;
N, 10.47. Anal. data for 5: 1H NMR (300 MHz, C6D6, 25 °C) δ
1.398 (s, 54H), 7.1-7.8 (m, 15H); 11B{1H} NMR (96.205 MHz,
C6D6, 25 °C) δ 36.3. Anal. Calcd for C42H69B3Li2N6Hf: C, 57.14;
H, 7.88; N, 9.52. Found: C, 56.80; H, 7.74; N, 9.20.
however, has been little explored, with only three such
complexes reported in the literature.13,23 Herein we expand
the coordination chemistry of this novel ligand architecture
with the synthesis of a series of zirconium, hafnium, and
vanadium complexes.
Experimental Section
General Procedures. All synthetic manipulations were carried
out using modified Schlenk techniques under an atmosphere of N2
or within the confines of a Vacuum Atmosphere HE-553-2
glovebox. Solutions were frozen in the cold well of the glovebox.
Solvents for synthesis were of reagent grade or better and were
dried according to standard methods.24 Bis(tert-butylamino)-
phenylborane,12 tetrachlorobis(tetrahydrofuran)hafnium(IV),25 and
N,N’-dilithiobis(tert-butylamino)phenylborane13 were prepared by
literature methods. All other materials were used as received.
(tBuN-BPh-NtBu)2M(THF), M ) Zr (2) and M ) Hf (3).
MCl4(THF)2 (M ) Zr, 100 mg; M ) Hf, 120 mg) and PhB-
(tBuNLi)2 (129 mg for the Zr reaction and 126 mg for the Hf
reaction) were each combined with 7 mL of diethyl ether to give
a suspension and a solution, respectively. Both mixtures were
frozen, and upon thawing, the PhB(tBuNLi)2 solution was added
dropwise over 7 min to the partially thawed MCl4(THF)2 suspen-
sion. The resulting mixture was allowed to slowly warm to room
temperature. After stirring overnight, solvent was removed by
vacuum evaporation, and 10 mL of hexanes was added. The solution
was filtered through Celite to remove LiCl. Concentration of the
solution followed by cooling to -35 °C and finally filtration
afforded 84 mg of colorless crystals (52% yield) of 2 and 124 mg
of colorless crystals (67% yield) of 3. Anal. data for 2: 1H NMR
(300 MHz, C6D6, 25 °C) δ 1.293 (s, 36H), 1.373 (m, 4H), 3.656
(br, 4H), 7.1-7.7 (m, 10H); 11B{1H} NMR (96.205 MHz, C6D6,
25 °C) δ 34.3. Anal. Calcd for C35H61B2N4OZr: C, 63.05; H, 9.22;
N, 8.40. Found: C, 62.98; H, 9.08; N, 8.38. Anal. data for 3: 1H
NMR (300 MHz, C6D6, 25 °C) δ 1.301 (s, 36H), 1.374 (m, 4H),
3.627 (br, 4H), 7.1-7.7 (m, 10H); 11B{1H} NMR (96.205 MHz,
C6D6, 25 °C) δ 33.7. Anal. Calcd for C32H54B2N4OHf: C, 54.06;
H, 7.66; N, 7.88. Found: C, 54.14; H, 7.68; N, 7.67.
V(tBuN-BPh-NtBu)2 (6). Diethyl ether solutions (25 mL)
containing 0.136 mL of VCl4 and 633 mg of PhB(tBuNLi)2 were
cooled to -78 °C in a dry ice/acetone bath. The PhB(tBuNLi)2
solution was added dropwise via cannula to the VCl4 solution. The
mixture was allowed to stir at -78 °C for 15 min and then allowed
to warm slowly to room temperature. Solvent was removed by
vacuum evaporation, and 20 mL of hexanes was added. The solution
was filtered through Celite to remove LiCl. Concentration of the
solution followed by cooling to -35 °C and filtration afforded 382
mg of red crystals (59% yield). Anal. Calcd for C28H46B2N4V: C,
65.78; H, 9.07; N, 10.96. Found: C, 65.67; H, 9.15; N, 11.06. UV-
vis (pentane) λmax,abs/nm (ꢀ/M-1 cm-1): 482 (4893), 351 nm (2924),
and 260 (sh) (13934). IR (pentane) ν(V-N) 605 cm-1
.
Physical Methods. 1H NMR spectra were recorded on solutions
at 25 °C within the magnetic fields of Varian Unity 300 or Mercury
300 spectrometers, which were located in the Department of
Chemistry Instrumentation Facility (DCIF) at MIT. Chemical shifts
are reported using the standard δ notation in ppm. 1H spectra were
referenced to the residual solvent peak. 11B{1H} NMR spectra were
collected at the DCIF on a Varian Unity 300 spectrometer and
referenced to an external BF3‚OEt2 standard at 0 ppm. Elemental
analyses were performed at H. Kolbe Mikroanalytisches Labora-
torium. EPR spectra were recorded on a modified Bruker EMX
X-band spectrometer (9.303 GHz) with a field modulation amplitude
of 2 G. Single scans of 4096 points were acquired on samples
maintained at 77 K. UV-vis absorption spectra were collected on
an Aviv 14DS spectrophotometer. IR spectra were collected on a
Perkin-Elmer model 2000 spectrophotometer (DCIF) in pentane
solutions. Only absorptions not observed in the IR spectrum of the
free ligand are reported.
Li2[M(tBuN-BPh-NtBu)3], M ) Zr (4) and M ) Hf (5). The
procedure for preparing these compounds was similar, only differing
in the metal tetrahalide starting material. MX4 (100 mg) (M ) Zr,
X ) Br; M ) Hf, X ) Cl) and PhB(tBuNLi)2 (178 mg for the Zr
reaction and 228 mg for the Hf reaction) were each combined with
7 mL of diethyl ether to give a suspension and a solution,
respectively. Both were frozen, and upon thawing, the PhB(tBuNLi)2
solution was added dropwise over 7 min to the partially thawed
MX4 suspension. The mixture was allowed to slowly warm to room
temperature. After stirring overnight, solvent was removed by
X-ray diffraction experiments were performed on single crystals
grown from concentrated pentane or hexanes solutions at -35 °C.
Crystals were removed from the supernatant liquid and transferred
onto a microscope slide coated with Paratone N oil. Selected crystals
were affixed to a glass fiber in wax and Paratone N oil and cooled
to -90 °C. Data collection was performed by shining Mo KR (λ
) 0.71073 Å) radiation onto crystals mounted on a three-circle
goniometer Siemens Platform equipped with a CCD detector. The
data were processed and refined by using the program SAINT
supplied by Siemens Industrial Automation, Inc. The structures were
solved by direct methods (SHELXTL v6.10, Sheldrick, G. M., and
Siemens Industrial Automation, Inc., 2000) in conjunction with
standard difference Fourier techniques. All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were placed in
calculated positions. Some details regarding the refined data and
cell parameters are provided in Table 1.
(17) Habben, C. D.; Heine, A.; Sheldrick, G. M.; Stalke, D. Z. Naturforsch.
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Allg. Chem. 1994, 620, 1403-1408.
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34, 2549-2551.
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Chem. 1998, 624, 1514-1518.
(22) Chivers, T.; Fedorchuk, C.; Schatte, G.; Parvez, M. Inorg. Chem. 2003,
42, 2084-2093.
(23) Koch, H.-J.; Roesky, H. W.; Bohra, R.; Noltemeyer, M.; Schmidt,
H.-G. Angew. Chem., Int. Ed. Engl. 1992, 31, 598-599.
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Results and Discussion
The dilithio salt of bis(tert-butylamino)phenylborane pro-
vides a convenient platform for delivery of the bis amide
4432 Inorganic Chemistry, Vol. 42, No. 14, 2003

Zr, Hf, V Bis(alkylamido)phenylborane Complexes
Table 1. Crystallographic Data for (tBuN-BPh-NtBu)2Zr(THF) (2), (tBuN-BPh-NtBu)2Hf(THF) (3), Li2[Zr(tBuN-BPh-NtBu)3] (4),
Li2[Hf(tBuN-BPh-NtBu)3] (5), and V(tBuN-BPh-NtBu)2 (6)
2‚1/2n-hexane
3
4
5
6
empirical formula
fw
T (K)
λ (Å)
space group
a (Å)
C35H61B2N4OZr
666.72
183(2)
0.71073
P21/n
10.8574(13)
13.3389(16)
26.086(3)
90
91.119(2)
90
3777.3(8)
4
1.172
0.321
C32H54B2HfN4O
710.90
183(2)
0.71073
P21/n
10.6370(11)
13.3956(13)
24.502(2)
90
94.930(2)
90
3478.3(6)
4
1.358
3.027
C42H69B3Li2N6Zr
795.56
183(2)
0.71073
P212121
10.1211(15)
20.060(3)
22.383(3)
90
C42H69B3HfLi2N6
882.83
183(2)
0.71073
P212121
10.1198(6)
19.8226(12)
22.2641(13)
90
90
90
4466.2(5)
4
1.313
C28H46B2N4V
511.26
183(2)
0.71073
Pbcn
17.297(7)
8.976(4)
20.238(9)
90
90
90
3142(2)
4
1.081
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
4544.5(12)
4
1.163
0.276
0.0430
0.0924
d
calcd (Mg/m3)
abs coeff (mm-1
)
2.371
0.0488
0.0682
0.336
0.0435
0.1064
R1a
0.0428
0.0993
0.0346
0.0915
wR2a
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
2
2
2
Chart 1
ligand [tBuN-BPh-NtBu]2- to metal centers. The action of
2 equiv of n-BuLi on the parent amine affords salt 1,
PhB(tBuNLi)2. Single-crystal X-ray diffraction analysis of
1 reveals the heterocubane Li and N core that is familiar to
other difunctional lithium amides.26 During our studies, the
crystal structure of 1 was reported by Chivers and co-
workers;27 our observations are entirely consistent with their
results. The Li-N distances are unremarkable, ranging from
2.029 to 2.077 Å, and N-B distances of 1.442(5) and
1.452(5) Å as well as an N-B-N angle of 109.5(3)° are
similar to the metric parameters for the ligand when it is
complexed to a transition metal (vide infra).
Synthesis. Complexes of bis(tert-butylamido)phenylborane
ligands are readily prepared via metathesis reaction between
PhB(tBuNLi)2, 1, and the appropriate metal halide (Chart 1)
in yields ranging from 52% to 67%. The reaction of 2 equiv
of 1 with the tetrahydrofuran adducts of zirconium and
hafnium tetrachloride (MCl4(THF)2) yields colorless com-
plexes possessing two bis(tert-butylamido)phenylborane
ligands and a single tetrahydrofuran, (tBuN-BPh-NtBu)2M-
(THF) (M ) Zr (2), Hf (3)). The addition of 3 equiv of 1 to
(26) (a) Hellmann, K. W.; Bergner, A.; Gade, L. H.; Scowen, I. J.;
McPartlin, M. J. Organomet. Chem. 1999, 573, 156-164. (b) Brauer,
D. J.; Bu¨rger, H.; Liewald, G. R. J. Organomet. Chem. 1986, 308,
119-130. (c) Gardiner, M. G.; Raston, C. L. Inorg. Chem. 1995, 34,
4206-4212.
(27) Chivers, T.; Fedorchuk, C.; Schatte, G.; Brask, J. K. Can. J. Chem.
2002, 80, 821-831.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4433

Manke and Nocera
These distances are comparable to the M-N bond distances
of Zr and Hf complexes (d(M-N) ) 2.087(2)-2.109(3)
Å29-31) of the alkyl substituted dianionic guanidinato ligand,
[R′NC(RN)2]2- (R ) Pr, Bu), but are significantly shorter
(d(M-N) ) 2.161(2)-2.329(2) Å) than the M-N bonds of
coordinated monoanionic amidinato and guanidinato ligands
possessing alkyl groups on the amido nitrogens.32-40 The
ligand binds to the metal to form an in plane M-N-B-N
spirocycle, with a bite angle ranging from 70.47(15)° to
71.0(2)°, and a N-B-N angle ranging from 111.2(4)° to
111.9(4)°. For comparison, amidinato and guanidinato
ligands form similar M-N-C-N spirocycles (bite angles
of 57.15(8)-61.06(7)° and N-C-N angles of 110.5(2)-
113.9(5)°).32-40 The M-N-C angles provide a measure of
the steric protection conferred by the ligand on the metal
center; M-N-C angles of 137.3(4)-146.2(4)° for 2 and 3
show the alkyl groups to be pushed away from the metal
center. Finally, substantial π-bonding character between the
boron bridgehead and nitrogen amides is signified by d(N-
B) ) 1.443(9)-1.468(7) Å.41 These distances concur with
those of group 4 amido complexes containing nonbridging
boryl groups.42,43
i
t
Figure 1. Solid-state structure of (tBuN-BPh-NtBu)2Zr(THF) (2) with
thermal ellipsoids shown at the 50% probability level.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
(tBuN-BPh-NtBu)2Zr(THF) (2) and (tBuN-BPh-NtBu)2Hf(THF) (3)
Bond Lengths
2
3
Zr(1)-O(1)
Zr(1)-N(3)
Zr(1)-N(4)
N(1)-B(1)
2.299(3)
2.122(4)
2.044(4)
1.467(7)
Hf(1)-O(1)
Hf(1)-N(3)
Hf(1)-N(4)
N(1)-B(1)
2.261(5)
2.103(5)
2.030(5)
1.464(9)
Complexes 4 and 5, possessing three chelating diamido
ligands, are nearly superimposable (Figure 2). These “ate”
complexes exhibit pseudo-D3 symmetry, if the two lithium
counterions are ignored; their inclusion lowers the symmetry
of the molecules to pseudo-C2 symmetry. The standard
polyhedron that best describes these compounds is a trigonal
prism,28 though significant deviations from ideal geometry
are observed. Table 3 lists selected bond lengths and angles
for compounds 4 and 5. One coordinated bis(alkylamido)-
phenylborane demonstrates metrics that are consistent with
those of compounds 2 and 3, while the other two diamido
ligands are significantly elongated due to their association
with the lithium cations. On the two affected ligands, one
nitrogen is in close contact with a single lithium cation
Bond Angles
2
3
N(1)-B(1)-N(2)
111.2(4)
N(1)-B(1)-N(2)
111.2(4)
N(1)-Zr(1)-N(3) 111.93(15)
N(2)-Zr(1)-N(3) 174.14(15)
N(1)-Hf(1)-N(3) 114.32(19)
N(2)-Hf(1)-N(3) 173.50(19)
N(1)-Zr(1)-N(2)
70.47(15)
N(1)-Hf(1)-N(2)
71.0(2)
Zr(1)-N(1)-C(1) 140.3(3)
Zr(1)-N(2)-C(5) 146.0(3)
Hf(1)-N(1)-C(1) 137.3(4)
Hf(1)-N(2)-C(5) 144.6(4)
zirconium tetrabromide or hafnium tetrachloride produces
colorless dianionic complexes containing three bis(tert-
butylamido)phenylborane ligands, Li2[M(tBuN-BPh-NtBu)3]
(M ) Zr (4), Hf (5)). Finally, the reaction of vanadium(IV)
chloride with 2 equiv of 1 generates a paramagnetic blood
red complex containing two bis(tert-butylamido)phenyl-
borane ligands, V(tBuN-BPh-NtBu)2 (6). The H NMR
1
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spectra for complexes 2-5 exhibit large singlets correspond-
ing to equivalent tert-butyl protons, as well as multiplets in
the aromatic region. Compounds 2 and 3 also show peaks
corresponding to a coordinated tetrahydrofuran. The 11B{1H}
NMR spectra of these compounds demonstrate only a single
resonance. All complexes gave satisfactory elemental analy-
sis.
Structural Analysis. X-ray diffraction studies were per-
formed on crystals of compounds 2-5. Complexes 2 and 3
possess five coordinate metal centers, containing two chelat-
ing diamido ligands and a single ligating tetrahydrofuran.
The solid-state structures of 2 and 3 are nearly superimpos-
able and demonstrate pseudo-C2 symmetry (Figure 1). The
compounds are best described as trigonal bipyramidal on the
basis of deviation from standard polyhedra.28 Table 2 lists
selected bond lengths and angles for compounds 2 and 3.
The observed M-N distances of 2 and 3 range from 2.044(4)
to 2.122(4) Å, and from 2.019(5) to 2.105(5) Å, respectively.
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4434 Inorganic Chemistry, Vol. 42, No. 14, 2003

Zr, Hf, V Bis(alkylamido)phenylborane Complexes
Figure 3. Solid-state structure of V(tBuN-BPh-NtBu)2 (6), with thermal
ellipsoids shown at the 50% probability level.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
V(tBuN-BPh-NtBu)2 (6)
Bond Lengths
Bond Angles
N(1)-B(1)-N(2A)
N(1)-V(1)-N(2A)
N(1)-V(1)-N(2)
V(1)-N(1)-C(7)
V(1)-N(2)-C(11)
V(1)-N(1)
1.853(3)
1.851(3)
1.460(5)
1.459(6)
106.9(4)
78.52(14)
126.57(14)
140.8(3)
141.2(3)
V(1)-N(2)
N(1)-B(1)
N(2)-B(1A)
Figure 2. Solid-state structure of Li2[Hf(tBuN-BPh-NtBu)3] (5) with
thermal ellipsoids shown at the 50% probability level.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
Li2[Zr(tBuN-BPh-NtBu)3] (4) and Li2[Hf(tBuN-BPh-NtBu)3] (5)
Spectroscopy of 6. Room temperature EPR spectra of 6
show a strong signal with no hyperfine coupling. Conversely,
a frozen pentane solution (77 K) of 6 gives a well-resolved
octet in the EPR spectrum arising from the interaction of
Bond Lengths
4
5
Zr(1)-N(1)
Zr(1)-N(2)
Zr(1)-N(3)
N(1)-B(1)
N(2)-B(1)
N(1)-Li(1)
N(2)-Li(2)
2.802(3)
2.211(3)
2.068(3)
1.446(5)
1.474(5)
1.992(7)
2.028(7)
Hf(1)-N(1)
Hf(1)-N(2)
Hf(1)-N(3)
N(1)-B(1)
N(2)-B(1)
N(1)-Li(2)
N(1)-Li(1)
2.752(6)
2.177(6)
2.052(6)
1.419(11)
1.460(10)
1.966(14)
2.024(14)
7
the unpaired electron with 51V(I ) /2) (g ) 1.979; A(51V)
) 49.0 G). There is significant line broadening and an
increased amplitude in the first peak and the final dip, due
to superhyperfine coupling to the quadrapolar nitrogens. A
simulated spectrum49 is in accordance with the observed
spectrum, yielding A(14N) ) 4.7 G (Figure 4). The IR
spectrum of 6 exhibits a strong band at 605 cm-1, which is
absent in the free ligand; on this basis, the absorption is
assigned to the V-N stretching mode. An empty p-orbital
at the B(R) bridgehead might be expected to attenuate the
π-back-bonding interaction between the amido nitrogens and
metal center and consequently give rise to a reduced V-N
frequency. However, such simple stereoelectronic correla-
tions may be obscured by mass and steric effects of the
ligand. Accordingly, it is not unexpected that the V-N
stretching frequency of 6 falls between that observed for
V(NMe2)4 and V(NEt2)4.50 Finally, the absorption spectrum
of 6 is consistent with a d1 metal center in a tetrahedrally
distorted crystal field. The 2E and 2T2 terms of Td symmetry
are split in D2d symmetry to a 2B1 ground state and 2A1, 2B2,
Bond Angles
4
5
N(1)-B(1)-N(2)
C(5)-N(2)-Zr(1)
C(9)-N(3)-Zr(1)
N(4)-Zr(1)-N(3)
110.4(3)
128.5(2)
145.3(2)
N(1)-B(1)-N(2)
C(5)-N(2)-Hf(1)
C(9)-N(3)-Hf(1)
N(4)-Hf(1)-N(3)
110.9(8)
129.0(5)
145.5(5)
71.8(2)
71.92(11)
(d(M-N) ) 2.177(6)-2.211(3) Å) while the other contacts
both lithium cations (d(M-N) ) 2.536(6)-2.802(3) Å). The
N-B-N and N-M-N planes differ from each other by
35.6-38.9° for the ligands perturbed by lithium interactions.
The homoleptic vanadium complex of [tBuN-BPh-
NtBu]2-, 6, exhibits a tetragonally compressed coordination
geometry (D2d) about the vanadium(IV) center (Figure 3).
The distortion is almost identical to that observed by Fest
and co-workers in Ti[tBuN-BPh-NtBu]2.13 The V-N bond
distances of 1.851(3) and 1.853(3) Å are consistent with the
V-N distances observed in the bis(tert-butylamido)silane
congener, V[Me2Si(tBuN)2]2 (Table 4).44 The 78.52(14)° bite
angle of 6 is considerably larger than those observed for 2-4.
The metric parameters of 6 contrast those of alkyl amidinate
vanadium complexes, which exhibit longer V-N distances
(d ) 1.944(2)-2.232(3) Å) and more acute bite angles
(63.46(19)-64.84(1)°).45-48
2
and E excited states; for these three excited states, the
2E r 2B1 transition is Laporte allowed. An LMCT transition
at 260 nm dominates the UV absorption region.
In summary, bis(tert-butylamido)phenylborane is a com-
petent ligand for group 4 and 5 transition metals. The ligand
demonstrates significantly shorter metal-nitrogen bonds than
amidinate ligands owing to its dianionic nature, and a larger
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Inorganic Chemistry, Vol. 42, No. 14, 2003 4435

Manke and Nocera
Chart 2
of these ligand classes has not been forthcoming. By far,
the most extensively studied [RN-X-NR]2- system has
been the bis(amido)silanes (D).44,68-71 The bis(alkylamido)-
phenylborane ligands offer a versatile counterpart to this
system inasmuch as the steric electronic properties of the
three-atom dianionic chelate can be tuned with substitution
at nitrogen and the boron bridgehead, which is strongly
coupled to the amide termini via π-bonding. Future directions
include expanding the coordination chemistry of bis(alky-
lamido)phenylborane ligands as chelates and also as a
bridging ligand of bimetallic metal centers.
Acknowledgment. This work was supported by a grant
from the National Science Foundation (CHE-0132680). We
thank Dr. Alan F. Heyduk for helpful discussions, and for
help with X-ray diffraction studies.
Supporting Information Available: Table of X-ray crystal-
lographic data for complexes 2 through 6 including a fully labeled
thermal ellipsoid plot, final atomic coordinates, equivalent isotropic
displacement parameters, anisotropic displacement parameters, and
bond distances and angles. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 4. EPR spectrum (X-band) of V(tBuN-BPh-NtBu)2 (6) at 77 K
in frozen pentane: (a) experimental; (b) simulated.
IC0340478
bite angle, which accompanies the shorter M-N distances.
The dianionic charge of bis(tert-butylamido)phenylborane
ligands also relaxes the need for additional ancillary anionic
ligands on metal centers of high oxidation state. More
generally, the class of diamido ligands possessing a single-
atom bridgehead, [RN-X-NR]2-, is limited. A compilation
of these ligands is shown in Chart 2. From this roster of
ligands, the transition metal coordination chemistry of the
bis(alkylamido)phosphino51 and bis(alkylamido)arsino52 ligands
(A) has yet to be developed. The early transition metal
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