Diboron complexes of binucleating bis(amidinate) ligands

Article abstract of DOI:10.1016/j.ica.2006.02.008

The synthesis and X-ray crystal structures of the following bis(amidinate)-substituted boron halides are reported: 1,3-C₆H₄[C{N(SiMe₃)}₂BCl₂]₂ (3), 1,4-C₆H₄[C{N(SiMe

Full text of DOI:10.1016/j.ica.2006.02.008

Inorganica Chimica Acta 360 (2007) 1316–1322
www.elsevier.com/locate/ica
Diboron complexes of binucleating bis(amidinate) ligands
*
Zheng Lu, Nicholas J. Hill, Michael Findlater, Alan H. Cowley
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX 78712, USA
Received 20 January 2006; accepted 2 February 2006
Available online 15 March 2006
Dedicated to Professor Michael F. Lappert in honor of his outstanding contributions to chemistry.
Abstract
The synthesis and X-ray crystal structures of the following bis(amidinate)-substituted boron halides are reported: 1,3-C6H4[C{N-
(SiMe3)}2BCl2]2 (3), 1,4-C6H4[C{N(SiMe3)}2BCl2]2 (4), 1,4-C6H4[C{N(SiMe3)}2B(Ph)Cl]2 (5), 1,4-C6H4[C{NCy}2BCl2]2 (6), and 1,4-
C6H4[C{NCy}2B(Ph)Cl]2 (7). Compounds 3–5 were prepared by trimethylsilyl chloride elimination, while 6 and 7 were prepared via salt
metathesis reactions of the appropriate dilithium bis(amidinates) with BCl3 or PhBCl2. The molecular structures of complexes 3, 5, and 6
were determined by single-crystal X-ray diffraction, along with that of the free bis(amidine) 1a.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Boron; Bis(amidinate); X-ray crystal structure
1. Introduction
with the above systems, the chemistry of the related bisben-
zene(amidinate) systems 1,3- and 1,4-C6H4[C(NR)2]ꢀ
(N = SiMe3 (1), Cy (2), Cy = cyclohexyl, Chart 1) is less
developed. These ligands are readily synthesized and, on
account of their rigidity, have the ability to bridge two
coordinated metal moieties. However, reports of their
coordination chemistry are restricted to a bis-trichlorotin
species [11], a bis(phosphenium) dication [12], a zwitter-
ionic titanium compound [13], and a pair of neutral bime-
tallic aluminum complexes, 1,4-C6H4[C(NiPr)2AlMeX]2
(X = Me, Cl) [14]. Compounds 1a and 1b have also been
employed for the preparation of bifunctional 1,2,3,5-
dithia- and diselena-diazolyl radicals, a class of compounds
which has potential as single component molecular semi-
conductors [15–17].
Amidinate complexes featuring group 13 metal–alkyl
and metal–halide fragments have received sustained inter-
est during the last decade due to the discovery of useful
applications for these compounds in key technological
areas. For example, amidinate-supported alkylaluminum
cations have proved to be active catalysts for the polymer-
ization of olefins, while gallium amidinate complexes have
been employed as single-source precursors for nitride mate-
Amidinate anions are four-electron, N-donor bidentate
chelates of general formula [RC(NR0)2]ꢀ. On account of
their readily tunable steric and electronic properties, these
anions have been widely used as ligands in main group,
transition metal and f-block coordination chemistry [1,2].
Early work in this field was focused on the benzamidinate
derivative [PhC{NSiMe3}2]ꢀ, which is readily synthesized
via the reaction of LiN(SiMe3)2 with PhCN [3]. More
recently, a range of binucleating amidinate ligands have
been developed. For example, bimetallic complexes of
bis(amidinates) based upon dibenzofuran (A) [4–7], 9,9-
dimethylxanthene (B) [4–7], and 1,2-disubstituted cyclo-
hexyl (C) [8–10] frameworks have been reported. In these
compounds, the proximity of the two metal fragments is
governed by the nature of the structural unit linking the
amidinate functional groups, hence such complexes may
have applications as bifunctional catalysts. In comparison
*
Corresponding author. Tel.: +1 512 471 7484; fax: +1 512 471 6822.
E-mail address: cowley@mail.utexas.edu (A.H. Cowley).
0020-1693/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.02.008

Z. Lu et al. / Inorganica Chimica Acta 360 (2007) 1316–1322
1317
Ph
R
O
R
N
N
R
R
R
R
N
N
R
R
O
N
N
Ph
B
A
C
SiMe
SiMe
N
(SiMe )
3 2
SiMe
N
Cy
N
Cy
3
3
3
N
NH
N
N
N
N
N
HN
Cy
N
(SiMe )
(SiMe )
(SiMe )
SiMe
3
Cy
3 2
3 2
3 2
1b
1a
2
Chart 1.
rials [18–31]. We have recently extended the range of group
13 amidinates to include boron halide derivatives, a rela-
tively unexplored area prior to our work [32–34]. In the
present contribution, we describe the synthetic and struc-
tural chemistry of a series of diboron amidinates derived
from 1 and 2.
SiMe4 (d 0.00), using residual solvent resonances as inter-
nal standards. 11B NMR data are referenced to BF3ÆOEt2
(d 0.00).
2.3. Single crystal X-ray structure determinations
For compounds 1a, 3, 5, and 6 a crystal of suitable qual-
ity was removed from a Schlenk flask under positive argon
pressure, placed on a glass slide, covered immediately with
degassed hydrocarbon oil and mounted on a thin glass
fiber. The X-ray diffraction data were collected at 153 K
on a Nonius Kappa CCD diffractometer equipped with
an Oxford Cryostream low-temperature device and a
graphite-monochromated Mo Ka radiation source
2. Experimental
2.1. General procedures
All manipulations and reactions were performed under a
dry, oxygen-free, catalyst-scrubbed argon atmosphere
using a combination of standard Schlenk techniques or in
an M-Braun or Vacuum Atmospheres drybox. All glass-
ware was oven-dried and vacuum- and argon flow-
degassed before use. All solvents were distilled over sodium
benzophenone ketyl, except dichloromethane which was
distilled over CaH2, and degassed prior to use. Compounds
1a, 1b and 2 were prepared according to the literature pro-
cedures [14,35]. The reagents 1,3- and 1,4-dicyanobenzene,
1,4-dibromobenzene, lithium bis(trimethylsilyl)amide, 1,3-
dicyclohexylcarbodiimide, boron halides and n-butyl lith-
ium solutions were obtained commercially and used as
received.
˚
(k = 0.71073 A). Corrections were applied for Lorentz
and polarization effects. All structures were solved by direct
methods [36] and refined by full-matrix least-squares cycles
on F2. All non-hydrogen atoms were allowed anisotropic
thermal motion, and hydrogen atoms were placed in fixed,
˚
calculated positions using a riding model (C–H 0.96 A).
Selected crystal data, and data collection and refinement
parameters are listed in Table 1.
2.4. Synthesis of 1,3-C6H4 [C{N(SiMe3)}2BCl2]2 (3)
BCl3 (10 mL, 1.0 M in hexane, 10 mmol) was added to a
stirred solution of 1a (3.0 g, 5 mmol) in 20 mL of toluene at
room temperature. After being stirred overnight, the reac-
tion mixture was filtered through Celiteꢁ and the solvent
was stripped from the filtrate to afford a white powder.
Recrystallization of this powder from dichloromethane
solution afforded a crop of colorless crystals of 3 (91%
yield).
2.2. Spectroscopic measurements
Low-resolution CI mass spectra were obtained on a
Finnigan MAT TSQ-700 mass spectrometer and high-reso-
lution CI mass spectra were recorded on a VG Analytical
ZAB-VE sector instrument. All MS analyses were per-
formed on samples that had been sealed in glass capillaries
under an argon atmosphere. H, 13C{1H} and 11B NMR
1H NMR (CDCl3): d 7.69 (m, Ph, 3H), 7.50 (s, Ph, 1H),
0.170 (s, SiMe3, 36H); 13C{1H} NMR (C6D6): d 169.37
(NCN), 132.96, 129.72, 129.13, 124.12 (Ph), 0.35 (SiMe3);
11B NMR (CDCl3): d 5.73. MS (CI+, CH3): m/z
[M + H]+ 612, [M ꢀ Cl]+ 576. HRMS (CI, CH4) Calc.
for C20H40N4B2Si4Cl4, 610.1270. Found: 610.1278.
1
spectra were obtained at 295 K in C6D6 or CDCl3 solutions
on a GE QE-300 instrument (1H, 300 MHz; 13C, 75 MHz,
11B, 96 MHz) immediately following removal of the sam-
ples from the drybox. 1H and 13C{1H} NMR chemical shift
values are reported in parts per million (ppm) relative to

1318
Z. Lu et al. / Inorganica Chimica Acta 360 (2007) 1316–1322
Table 1
Selected crystal data, data collection and refinement parameters for 1a, 3, 5, and 6
1a
3
5
6
Formula
C26H58N4Si6
595.30
monoclinic
C2/c
C20H40N4B2Cl4Si4
612.34
orthorhombic
Pca21
C32H50N4B2Cl4Si4
695.64
monoclinic
P21/c
C32H48N4B4Cl4
652.16
orthorhombic
Pbcn
Formula weight
Crystal system
Space group
˚
a/A
27.732(5)
9.515(5)
17.665(5)
90
124.245(5)
90
3853(2)
4
1.026
21.111(5)
12.901(5)
12.380(5)
90
90
90
3372(2)
4
1.206
10.293(5)
12.988(5)
16.781(5)
90
115.973(5)
90
2016.8(14)
2
1.146
13.870(3)
14.466(3)
19.406(4)
90
90
90
3893(5)
4
1.113
˚
b/A
˚
c/A
a/ꢂ
b/ꢂ
c/ꢂ
3
˚
V/A
Z
q
Calc/g cmꢀ3
F(000)
1304
1288
740
1384
Crystal size/mm
h range/ꢂ
Number of reflections collected
Number of independent reflections
R1 [I > 2r(I)]
0.30 · 0.30 · 0.30
1.78–27.51
6793
4331
0.0633
0.20 · 0.15 · 0.10
1.93–27.41
12923
7169
0.0544
0.20 · 0.20 · 0.20
2.63–27.52
14344
4593
0.0972
0.20 · 0.20 · 0.15
2.29–27.49
8409
4452
0.0655
wR2 (all data)
Peak and hole/e A
0.1900
0.365 and ꢀ0.322
0.1347
0.492 and ꢀ0.432
0.3215
0.580 and ꢀ0.724
0.1864
0.443 and ꢀ0.488
ꢀ2
˚
2.5. Synthesis of 1,4-C6H4 [C{N(SiMe3)}2BCl2]2 (4)
dichloromethane solution afforded a crop of colorless crys-
tals of 6 (92% yield).
Colorless crystalline 4 was prepared in 90% yield from
BCl3 (10 mL, 1.0 M in hexane, 10 mmol) and 1b (3.0 g,
5 mmol) using the procedure described for 3.
1H NMR (CDCl3): d 7.71 (m, Ph, 4H), 3.44 (m, NCH,
4H), 1.76–1.12 (m, Cy, 40H); 13C{1H} NMR (C6D6): d
173.25 (NCN), 138.60, 120.19 (Ph), 54.87 (Cy–C1), 33.45
(Cy), 25.49, 25.25 (Cy); 11B NMR (CDCl3): d 5.97. MS
(CI+, CH3): m/z [M + H]+ 652, [M ꢀ Cl]+ 617. HRMS
(CI, CH4) Calc. for C32H48N4B2Cl4, 650.2819. Found:
650.2822.
1H NMR (CDCl3): d 7.47 (m, Ph, 4H), 0.15 (s, SiMe3,
36H); 13C{1H} NMR (C6D6): d 168.95 (NCN), 136.40,
122.20 (Ph), 0.32 (SiMe3); 11B NMR (CDCl3): d 5.62. MS
(CI+, CH3): m/z [M + H]+ 612, [M ꢀ Cl]+ 576. HRMS
(CI, CH4) Calc. for C20H40N4B2Si4Cl4, 610.1270. Found:
610.1280.
2.8. Synthesis of 1,4-C6H4 [C{NCy}2B(Ph)Cl]2 (7)
2.6. Synthesis of 1,4-C6H4 [C{N(SiMe3)}2B(Ph)Cl]2 (5)
Colorless crystalline 7 was prepared in 93% yield from
nBuLi (4 mL of 2.5 M in hexanes, 10 mmol), PhBCl2
(1.6 g, 10 mmol) and 2 (1.67 g, 5 mmol) in 30 mL of toluene
using the procedure described for 6.
Colorless crystalline 5 was prepared in 91% yield from
PhBCl2 (1.6 g, 10 mmol) and 1b (3.0 g, 5 mmol) in 20 mL
of toluene using the procedure described for 3.
1H NMR (CDCl3): d 8.17 (d, Ph, 4H), 7.49–7.25 (m, Ph,
10H), 3.29 (m, NCH, 4H), 1.90–0.74 (m, Cy, 40H); 13C{1H}
NMR (CDCl3): d 169.54 (NCN), 146.74, 138.62, 135.49,
133.20, 130.19 (Ph), 54.83 (Cy–C1), 34.51, 33.95, 25.49,
25.23 (Cy); 11B NMR (CDCl3): d 9.69. MS (CI+, CH3):
m/z [M + H]+ 736, [M ꢀ Cl]+ 698. HRMS (CI, CH4) Calc.
for C44H58N4B2Cl2, 734.4225. Found: 734.4222.
1H NMR (CDCl3): d 7.45 (m, Ph, 14H), ꢀ0.05 (s, SiMe3,
36H); 13C{1H} NMR (CDCl3): d 178.85 (NCN), 140.78,
136.11, 133.66, 132.14 (Ph), 0.42 (SiMe3); 11B NMR
(CDCl3): d 9.05. MS (CI+, CH3): m/z [M + H]+ 695,
[M ꢀ Cl]+ 660, [M ꢀ Ph]+ 617. HRMS (CI, CH4) Calc.
for C32H50N4B2Si4Cl2, 694.2676. Found: 694.2678.
2.7. Synthesis of 1,4-C6H4 [C{NCy}2BCl2]2 (6)
3. Results and discussion
nBuLi (4 mL of 2.5 M in hexanes, 10 mmol) was added
dropwise to a solution of 2 (1.67 g, 5 mmol) in 30 mL of
toluene at room temperature. The resulting slurry was stir-
red for 2 h, after which BCl3 (10 mL, 1.0 M in hexane,
10 mmol) was added dropwise. After being stirred over-
night, the reaction mixture was filtered through Celiteꢁ
and the solvent was stripped from the filtrate to afford a
white powder. Recrystallization of this powder from
Several methods have been reported for the synthesis of
metal amidinate complexes, including: (i) insertion of car-
bodiimides into metal–alkyl or metal–amide bonds; (ii)
protonolysis reactions employing neutral amidines; (iii) salt
metathesis reactions of lithium amidinates (prepared by
treatment of carbodiimides with alkyl lithium reagents)
with metal halides; (iv) reaction of metal halides with
N,N,N0-tris(trimethylsilyl)amidines.

Z. Lu et al. / Inorganica Chimica Acta 360 (2007) 1316–1322
1319
The reaction of 1a or 1b with two equivalents of BXCl2
(X = Cl, Ph) at room temperature proceeded via trimethyl-
silyl chloride elimination and afforded compounds 3–5 as
colorless crystalline solids in high yields (Scheme 1). The
1H and 13C{1H} NMR spectra of these compounds are
similar, and show signals due to equivalent trimethylsilyl
groups and the bridging phenyl group, along with a weak
13C{1H} resonance at d 169 attributable to electron delo-
calization within the N–C–N fragment. The 11B NMR
spectra exhibit strong singlet resonances at d 5.73 for 3, d
5.62 for 4, and d 9.05 for 5, values that fall in the typical
range for four-coordinate boron atoms [37]. The mass spec-
tra of 3–5 indicated the binding of two BXCl fragments to
the binucleating bis(amidinate) ligand. In order to gain fur-
ther insight into the molecular structures of these com-
pounds, we undertook single crystal X-ray diffraction
experiments on 3 and 5.
Compound 3 is monomeric in the solid state and features
two planar, four-membered B–N–C–N chelate rings linked
by a phenyl group. The chelate rings are arranged in a per-
pendicular fashion with respect to the bridging phenyl
group. The boron atoms are bound to two chlorine and
two nitrogen atoms, giving rise to an overall distorted tet-
rahedral coordination environment. The B–N, B–Cl, and
N–C bond distances and internal bond angles of the
four-membered ring are very similar to those observed
for the parent boron amidinate [PhC{NSiMe3}2BCl2],
and are indicative of electron delocalization within the
N–C–N junction [32]. The molecular structure of 5 is that
predicted on the basis of spectroscopic data. However, dis-
order within the phenyl and trimethylsilyl groups prevented
a satisfactory refinement of the data set, thus precluding a
detailed comparison of the metrical parameters with those
of 3. The molecular structure of the free amidine 1a was
also determined (Fig. 2). The molecule contains two crys-
tallographically identical amidine groups, with N(1)–C(5)
Crystalline samples of 3 and 5 suitable for X-ray diffrac-
tion studies were obtained by recrystallization from dichlo-
romethane solutions. The molecular structure of 3 is shown
in Fig. 1, experimental data are detailed in Table 1 and
selected metrical parameters are summarized in Table 2.
˚
and N(2)–C(5) bond lengths of 1.402(4) and 1.279(4) A,
respectively, which are indicative of localized C–N and
C@N bonds. For comparison, the corresponding bond
SiMe
N
SiMe
N
SiMe
N
(SiMe )
3 2
3
3
3
3
3
Cl
X
N
X
BXCl
2
B
B
Cl
- Me SiCl
N
N
3
N
N
SiMe
SiMe
(SiMe )
SiMe
3 2
3
4: X = Cl
5: X = Ph
1b
Scheme 1. Preparation of 4 and 5 by the Me3SiCl elimination method.
Fig. 1. ORTEP diagram of 3 with thermal ellipsoids at 40% probability. All hydrogen atoms have been omitted for clarity.

1320
Z. Lu et al. / Inorganica Chimica Acta 360 (2007) 1316–1322
Table 2
˚
Selected bond distances (A) and bond angles (ꢂ) for 1a, 3, 5 and 6
1a
3
5
6
N(1)–C(5)
N(2)–C(5)
N(1)–Si(1)
N(1)–Si(2)
N(2)–Si(3)
C(2)–C(5)
N(1)–C(5)–N(2)
Si(1)–N(1)–Si(2)
Si(1)–N(1)–C(5)
Si(2)–N(1)–C(5)
Si(3)–N(2)–C(5)
C(5)–C(2)–C(1)
C(5)–C(2)–C(3)
1.402(4)
1.279(4)
1.774(3)
1.769(3)
1.729(3)
1.503(5)
B(1)–Cl(1)
B(1)–Cl(2)
B(1)–N(1)
B(1)–N(2)
N(1)–C(1)
N(1)–C(2)
N(1)–Si(1)
N(2)–Si(2)
1.835(4)
1.839(5)
1.576(6)
1.568(5)
1.343(5)
1.335(5)
1.766(3)
1.767(3)
1.472(5)
B(1)–Cl(1)
B(1)–N(1)
B(1)–N(2)
B(1)–C(2)
N(1)–C(1)
N(1)–C(2)
N(1)–C(3)
N(1)–Si(1)
1.869(8)
1.613(8)
1.573(8)
1.580(10)
1.326(6)
1.331(6)
1.463(3)
1.760(5)
1.731(5)
B(1)–Cl(1)
B(1)–Cl(2)
B(1)–N(1)
B(1)–N(2)
N(1)–C(1)
N(1)–C(2)
N(1)–C(5)
N(2)–C(11)
1.843(4)
1.828(4)
1.552(5)
1.567(5)
1.331(4)
1.332(4)
1.473(4)
1.459(4)
1.466(4)
119.8(3)
123.1(5)
122.1(2)
113.1(2)
136.6(3)
120.2(4)
121.2(3)
C(1)–C(2)
N(2)–Si(2)
C(1)–C(2)
Cl(1)–B(1)–Cl(2)
N(1)–B(1)–N(2)
N(1)–C(1)–N(2)
N(1)–B(1)–Cl(1)
N(1)–B(1)–Cl(2)
N(2)–B(1)–Cl(1)
N(2)–B(1)–Cl(2)
112.5(2)
Cl(1)–B(1)–C(2)
N(1)–B(1)–N(2)
N(1)–C(1)–N(2)
N(1)–B(1)–Cl(1)
N(1)–B(1)–C(2)
N(2)–B(1)–Cl(1)
N(2)–B(1)–C(2)
114.5(5)
Cl(1)–B(1)–Cl(2)
N(1)–B(1)–N(2)
N(1)–C(1)–N(2)
N(1)–B(1)–Cl(1)
N(1)–B(1)–Cl(2)
N(2)–B(1)–Cl(1)
N(2)–B(1)–Cl(2)
110.5(2)
84.4(3)
104.2(3)
114.1(3)
113.9(3)
114.3(3)
114.8(3)
82.7(4)
104.8(4)
112.5(5)
113.4(5)
112.25(5)
117.6(7)
83.1(2)
101.9(3)
115.4(2)
115.4(2)
114.5(2)
114.5(2)
Fig. 2. ORTEP diagram of 1a with thermal ellipsoids at 40% probability. All hydrogen atoms have been omitted for clarity.
distances in the 5-tert-butyl analog of 1a are 1.410(5) and
solids in high yields (Scheme 2). Multinuclear NMR and
mass spectral data again indicated the formation of binu-
clear complexes, and this indication was confirmed by an
X-ray diffraction study on a single crystal of 6 (Fig. 3).
The molecular structure of 6 is very similar to those of 3
and 5, in the sense that a BCl2 fragment is bound by each
˚
1.264(5) A, respectively [17], while those in 1b are
˚
1.413(3) and 1.271(3) A, respectively [38].
Treatment of bis(amidine) 2 with two equivalents of
nBuL, followed by addition of the requisite boron halide
furnished, after workup, 6 and 7 as colorless crystalline
Cy
N
Cy
Cy
N
Cy
N
n
NH
X
Cl
X
1) BuLi
2) BCl
B
B
3
NH
Cy
N
Cl
N
N
- LiCl
Cy
Cy
Cy
2
6: X = Cl
7: X = Ph
Scheme 2. Preparation of 6 and 7 by salt metathesis reactions.

Z. Lu et al. / Inorganica Chimica Acta 360 (2007) 1316–1322
1321
Fig. 3. ORTEP diagram of 6 with thermal ellipsoids at 40% probability. All hydrogen atoms have been omitted for clarity.
of the two N-donors of the amidinate in a symmetrical
bidentate fashion thereby resulting in a planar B–N–C–N
ring. The B–Cl, B–N and C–N bond distances and angles
in 6 are similar to those in 3 and the bulky boron amidinate
[Mes*C{NCy}2BCl2] (Mes* = 2,4,6-tri(tert-butyl)phenyl)
[32].
www.ccdc.cam.ac.uk), by quoting the publication citation
and the deposition number. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.ica.2006.02.008.
References
In summary, we have extended the range of boron ami-
dinates to include binuclear complexes. On the basis of
spectroscopic and X-ray structural data, the bonding in
the four-membered B–N–C–N chelate rings in these com-
pounds can be described in terms of equal contributions
from two diaza-allyl resonance forms which results in elec-
tron delocalization at the N–C–N junctions. This structural
feature is characteristic of the majority of group 13 amidi-
nate compounds [18–34]. Bridged binuclear compounds
such as 3–7 may find use as synthons for the construction
of coordination networks and advanced materials.
[1] F.T. Edelmann, Coord. Chem. Rev. 137 (1994) 403.
[2] J. Barker, M. Kilner, Coord. Chem. Rev. 133 (1994) 219.
[3] A.R. Sanger, Inorg. Nucl. Chem. Lett. 9 (1973) 351.
[4] J.R. Hagadorn, Chem. Commun. (2001) 2144.
[5] J.R. Hagadorn, M.J. McNevin, G. Wiedenfeld, R. Shoemaker,
Organometallics 22 (2003) 4818.
[6] M.J. McNevin, J.R. Hagadorn, Inorg. Chem. 43 (2004) 8547.
[7] B. Clare, N. Sarker, R. Shoemaker, J.R. Hagadorn, Inorg. Chem. 43
(2004) 1159.
[8] J.R. Hagadorn, J. Arnold, Angew. Chem., Int. Ed. 37 (1998) 1729.
[9] J.F. Li, L.H. Weng, X.H. Wei, D.S. Liu, Dalton Trans. (2002) 1401.
[10] H. Kawaguchi, T. Matsuo, Chem. Commun. (2002) 958.
[11] S. Appel, F. Weller, K. Dehnicke, Z. Anorg. Allg. Chem. 583 (1990)
7.
Acknowledgments
`
[12] C. Rouques, M.-R. Mazieres, J.-P. Majoral, M. Sanchez, Inorg.
Chem. 28 (1989) 3932.
We gratefully acknowledge the National Science Foun-
dation (Grant CHE-0240008) and the Robert A. Welch
Foundation (Grant F-135) for support of this work. We
also thank Jennifer A. Moore for collection of the X-ray
diffraction data for compound 4.
[13] J. Grundy, M.P. Coles, P.B. Hitchcock, New J. Chem. 28 (2004)
1195.
[14] J. Grundy, M.P. Coles, P.B. Hitchcock, J. Organomet. Chem. 662
(2002) 178.
[15] A.W. Cordes, R.C. Haddon, R.T. Oakley, L.F. Scheemeyer, J.V.
Waszczak, K.M. Young, N.M. Zimmerman, J. Am. Chem. Soc. 113
(1991) 582.
[16] M.P. Andrews, A.W. Cordes, D.C. Douglas, R.M. Fleming, S.H.
Glarum, R.C. Haddon, P. Marsh, R.T. Oakley, T.T.M. Palstra, L.F.
Scheemeyer, G.W. Trucks, R. Tycko, J.V. Waszczak, K.M. Young,
N.M. Zimmerman, J. Am. Chem. Soc. 113 (1991) 3559.
[17] R.A. Beekman, R.T. Boere´, K.M. Moock, M. Parvez, Can. J. Chem.
76 (1998) 85.
[18] M.L. Cole, C. Jones, P.C. Junk, M. Kloth, A. Stasch, Chem. Eur. J.
11 (2005) 4482.
[19] V. Amo, R. Andres, E. de Jesus, F.J. de la Mata, J.C. Flores, R.
Gomez, M.P. Gomez- Sal, J.F.C. Turner, Organometallics 29 (2005)
2331.
Appendix A. Supplementary material
Full crystallographic data for the structural analyses
have been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 294839 (3), 294840 (5), 294841
(6), and 294842 (1a). Copies of this information can be
obtained free of charge upon request from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax
+44 1223 336 033 or e-mail: deposit@ccdc.cam.ac.uk or

1322
Z. Lu et al. / Inorganica Chimica Acta 360 (2007) 1316–1322
´
[20] R.T. Boere, M.L. Cole, P.C. Junk, New J. Chem. 29 (2005) 128.
[21] H.A. Jenkins, D. Abeysekara, D.A. Dickie, J.A.C. Clyburne, Dalton
Trans. (2002) 3919.
[29] J. Barker, N.C. Blacker, P.R. Phillips, N.W. Alcock, W.W.
Errington, M.G.H. Wallbridge, J. Chem. Soc., Dalton Trans.
(1996) 431.
[22] D. Abeysekara, K.N. Robertson, T.S. Cameron, J.A.C. Clyburne,
Organometallics 20 (2001) 5532.
[30] Y. Zhou, D.S. Richeson, Inorg. Chem. 35 (1996) 2448.
[31] Y. Zhou, D.S. Richeson, Inorg. Chem. 35 (1996) 1423.
[32] N.J. Hill, M. Findlater, A.H. Cowley, Dalton Trans. (2005) 3229.
[33] N.J. Hill, J.A. Moore, M. Findlater, A.H. Cowley, Chem. Commun.
(2005) 5462.
[23] K. Kincaid, C.P. Gerlach, G.R. Giesbrecht, J.R. Hagadorn, G.D.
Wheeler, A. Shafir, J. Arnold, Organometallics 18 (1999) 5360.
[24] C.C. Chang, C.S. Hsiung, H.L. Su, B. Srinivas, M.Y. Chiang, G.H.
Lee, Y. Wang, Organometallics 17 (1998) 1595.
[25] M.P. Coles, D.C. Swenson, R.F. Jordan, V.G. Young, Organomet-
allics 17 (1998) 4042.
[26] M.P. Coles, D.C. Swenson, R.F. Jordan, V.G. Young, Organomet-
allics 16 (1997) 5183.
[27] J.T. Patton, M.M. Bokota, K.A. Abboud, Organometallics 21 (2002)
2145.
[34] M. Findlater, N.J. Hill, A.H. Cowley, Polyhedron 25 (2006) 983.
´
[35] R.T. Boere, R.G. Hicks, R.T. Oakley, Inorg. Synth. 31 (1997) 94.
[36] G.M. Sheldrick, SHELL-PC Version 5; 03, Siemens Analytical X-ray
Instruments, Inc., Madison, WI, USA, 1994.
[37] H. No¨th, B. Wrackmeyer, in: Nuclear Magnetic Resonance Spec-
troscopy of Boron Compounds, Springer, Berlin, 1978, pp. 74–101
(Chapter 7).
[28] S. Dagorne, R.F. Jordan, V.G. Young, Organometallics 18 (1999)
4619.
[38] F. Weller, F. Schmick, K. Dehnicke, Z. Natrurforsch. B 44 (1999)
548.