Amidinate-substituted boron halides: Synthesis and structure

Article abstract of DOI:10.1016/j.poly.2005.11.012

The synthesis and X-ray crystal structures of the following amidinate-substituted boron halides are reported: [^tBuC{N(Cy)} ₂]BCl(Ph) (1), [ⁿBuC{NCy}₂]BCl(Ph) (2), [(Me₃Si)₂NC{NCy}₂]BCl₂ (3) and [MeC{NⁱPr}₂]BCl₂ (4). Compounds 1, 2, and 3 were prepared via salt metathesis reactions of the appropriate lithium amidinates with PhBCl₂ or BCl₃. Compound 3 was also prepared by carbodiimide insertion into the B-N bond of (Me₃Si) ₂N-BCl₂. The latter method represents the first example of the insertion of a carbodiimide into a B-N moiety. The molecular structures of 1, 2, and 4 were determined by single-crystal X-ray diffraction.

Full text of DOI:10.1016/j.poly.2005.11.012

Polyhedron 25 (2006) 983–988
www.elsevier.com/locate/poly
Amidinate-substituted boron halides: Synthesis and structure
*
Michael Findlater, Nicholas J. Hill, Alan H. Cowley
Department of Chemistry and Biochemistry, University of Texas at Austin, 1 University Station A5300, Austin, TX 78712, USA
Received 25 August 2005; accepted 10 November 2005
Available online 27 December 2005
Dedicated to Professor Michael B. Hursthouse on the occasion of his 65th birthday.
Abstract
The synthesis and X-ray crystal structures of the following amidinate-substituted boron halides are reported: [tBuC{N(Cy)}2]BCl(Ph)
(1), [nBuC{NCy}2]BCl(Ph) (2), [(Me3Si)2NC{NCy}2]BCl2 (3) and [MeC{NiPr}2]BCl2 (4). Compounds 1, 2, and 3 were prepared via salt
metathesis reactions of the appropriate lithium amidinates with PhBCl2 or BCl3. Compound 3 was also prepared by carbodiimide inser-
tion into the B–N bond of (Me3Si)2N–BCl2. The latter method represents the first example of the insertion of a carbodiimide into a B–N
moiety. The molecular structures of 1, 2, and 4 were determined by single-crystal X-ray diffraction.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Boron; Amidinates; Guanidine; Insertion; Carbodiimide; Crystal structure
1. Introduction
(i) protonolysis reactions employing neutral amidines; (ii)
insertion of carbodiimides into metal-alkyl bonds; (iii)
reaction of metal halides with N,N,N0-tris(trimethylsi-
lyl)amidines; and (iv) salt metathesis reactions of lithium
amidinates (prepared by treatment of alkyl-lithium
reagents with carbodiimides) with metal halides.
A major advantage of the use of the amidinate ligand is
the ability to tune, with minimum synthetic effort, the steric
and (to a lesser extent) the electronic environment of the
resulting complex. This flexibility affords a degree of con-
trol over the chemistry at the metal center and is responsi-
ble for the widespread use of amidinates as supporting
ligands in main group, transition metal, lanthanide and
actinide chemistry [1,2].
Amidinate anions function typically as four-electron, N-
donor bidentate chelating ligands and have the general for-
mula [RNC(R0)NR00]ꢀ. Early work in this field was focused
on the benzamidinate derivative [PhC{NSiMe3}2]ꢀ [1,2],
which is readily synthesized via the reaction of LiN(TMS)2
with benzonitrile [3]. More recently, amidinate chemistry
has witnessed a significant expansion in the number and
types of ligands by elaboration of both the N- and C-sub-
stituents. Moreover, several new methods have emerged for
the synthesis of metal amidinate complexes, including:
Amidinate complexes featuring group 13 metal–alkyl
and metal–halide fragments have been the subject of sus-
tained interest during the last decade due to the discovery
of a number of useful applications in key technological
areas. For example, gallium amidinate complexes represent
promising single-source precursors for nitride materials
while amidinate-supported alkylaluminum cations have
proved to be active catalysts for olefin polymerization
[4–18]. However, despite this increased attention to and
development of the field in general, the chemistry of
boron-substituted amidinates remains relatively unex-
plored. Our recent work in the field [19,20] has prompted
us to undertake the synthesis and structural assay of a
wider range of these amidinates and in the present contri-
bution we report the synthesis and structural characteriza-
tion of three novel boron-substituted amidinates (1, 2, and
3). One of the synthetic approaches to 3 represents the first
*
Corresponding author. Tel.: +1 512 471 7484; fax: +1 512 471 6822.
E-mail address: cowley@mail.utexas.edu (A.H. Cowley).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.11.012

984
M. Findlater et al. / Polyhedron 25 (2006) 983–988
example of the insertion of a carbodiimide into a boron–
nitrogen bond. We also report a single-crystal X-ray dif-
fraction study of previously synthesized 4 [19].
96 MHz) immediately following removal of the samples
1
from the drybox. H and 13C{1H} chemical shift values
are reported in parts per million (ppm) relative to SiMe4
(d 0.00), using residual solvent resonances as internal stan-
dards. 11B NMR data are referenced to BF3:OEt2 (d 0.00).
Melting points (uncorrected) were obtained on a Fisher–
Johns apparatus after flame-sealing the samples in glass
capillaries under argon.
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 glassware
was oven-dried and vacuum- and argon flow-degassed
before use. All solvents were distilled over sodium benzophe-
none ketyl, except dichloromethane, which was distilled over
CaH2, and degassed prior to use. N,N,N0-Tris(trimethylsi-
lyl)benzamidine [21], bis(trimethylsilyl)amido boron dichlo-
ride [22], and [MeC{NiPr}2]BCl2 (4) [19] were prepared
according to the literature procedures. The compounds
1,3-dicyclohexylcarbodiimide, 1,3-diisopropylcarbodii-
mide, boron halides, bis(trimethylsilyl)amidolithium, and
alkyl-lithium solutions were obtained commercially and
used without further purification.
2.3. Single-crystal X-ray structure determinations
For compounds 1, 2, and 4, a crystal of suitable quality
was removed from a Schlenk flask under positive argon
pressure, covered immediately with degassed hydrocarbon
oil and mounted on a glass fiber. The X-ray diffraction data
were collected at 153 K on a Nonius Kappa CCD diffrac-
tometer equipped with an Oxford Cryostream low-temper-
ature device and a graphite-monochromated Mo Ka
˚
radiation source (k = 0.71073 A). Corrections were applied
for Lorentz and polarization effects. All structures were
solved by direct methods [23] 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
˚
2.2. Spectroscopic measurements
model (C–H 0.96 A). Selected crystal data, and data collec-
tion and refinement parameters are listed in Table 1.
Low-resolution CI mass spectra were obtained on a
Finnigan MAT TSQ-700 mass spectrometer and high-reso-
lution CI mass spectra recorded on a VG Analytical ZAB-
VE sector instrument. All MS analyses were performed on
samples that had been sealed in glass capillaries under an
argon atmosphere. 1H, 13C{1H}, and 11B NMR spectra
were recorded at 295 K in C6D6 solutions on a GE EQ-
300 instrument (1H, 300 MHz; 13C, 75 MHz; 11B,
2.4. Synthesis of [tBuC{N(Cy)}2]BCl(Ph) (1)
tert-Butyllithium (2 mL of a 2.5 M solution in diethyl
ether, 5.0 mmol) was added to a cold (ꢀ78 ꢁC) solution
of 1,3-dicyclohexylcarbodiimide (1.03 g, 5.0 mmol) in
15 mL of diethyl ether. The stirred colorless reaction mix-
ture was warmed to room temperature, then cooled to
Table 1
Selected crystal data, data collection, and refinement parameters for 1, 2, and 4
Compound
1
2
4
Formula
C
386.80
23H36N2BCl
C23H36N2BCl
386.80
monoclinic
P21/c
9.939(5)
20.598(5)
11.052(5)
90
93.204(5)
90
C8H17N2BCl2
222.95
triclinic
Formula weight
Crystal system
Space group
monoclinic
P21/n
8.893(5)
17.194(5)
14.511(5)
90
92.754(5)
90
2216.3(16)
4
ꢀ
P1
˚
a (A)
7.409(5)
8.127(5)
10.436(5)
94.529(5)
92.529(5)
104.188(5)
606.0(6)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
2259.1(16)
4
1.137
Z
qcalc (g cmꢀ3
F(000)
)
1.159
840
1.222
236
840
Crystal size (mm)
h Range (ꢁ)
Number of reflections collected
Independent reflections
R1 [I > 2r(I)]
0.30 · 0.20 · 0.20
2.75–27.49
9399
5028
0.0432
0.10 · 0.10 · 0.10
2.05–27.48
8308
5147
0.0590
0.10 · 0.10 · 0.05
3.11–27.48
2750
1588
0.0486
wR2 (all data)
Peak and hole (e A
0.1019
0.311 and ꢀ0.256
0.1366
0.436 and ꢀ0.487
0.1095
0.380 and ꢀ0.269
ꢀ3
˚
)

M. Findlater et al. / Polyhedron 25 (2006) 983–988
985
ꢀ78 ꢁC following which PhBCl2 (0.794 g, 5.0 mmol) was
added dropwise. After being stirred at room temperature
for 2 h, the reaction mixture was filtered through Celiteꢂ
and the solvent was stripped from the filtrate to afford a
white powder. Recrystallization of this powder from hex-
ane solution afforded a crop of colorless crystals of 1
(1.12 g, 58% yield, mp 124–126 ꢁC).
(CH2CH2CH2CH3). 11B NMR (C6D6): d 6.47 (s). MS
(CI+, CH4): m/z 387 (M+H), 352 (MꢀCl). HRMS (CI,
CH4) calcd. for C23H36N2BCl, 386.2660; found: 386.2660.
2.6. Synthesis of [(Me3Si)2NC{NCy}2]BCl2 (3) via
metathesis
1H NMR (C6D6): d 8.02 (d, 2H, Ph); 7.35 (m, 2H, Ph);
7.26 (m, 1H, Ph); 3.53 (m, 2H, Cy); 2.07 (m, 4H, Cy); 1.62–
1.53 (br m, 10H, Cy); 1.28–1.15 (m, 6H, Cy); 1.08 (s, 9H,
tBu). 13C NMR (C6D6): d 178.25 (NCN); 135.84; 135.62;
133.68; 127.83 (Ph); 56.39 (Cy–C1); 35.08 (Cy); 34.60
(Cy); 28.49 (C–Me3); 26.45; 26.43 (Cy); 25.80 (C–Me3).
11B NMR (C6D6): d 6.50 (s). MS (CI+, CH4): m/z 386
(M+H), 351 (MꢀCl). HRMS (CI, CH4) calcd. for
C23H36N2BCl, 386.2660; found: 386.2655.
Bis(trimethylsilyl)amidolithium (0.81 g, 4.8 mmol) was
added to a cold (ꢀ78 ꢁC) solution of dicyclohexylcarbodii-
mide (1.00 g, 4.8 mmol) in 15 mL of hexane. The stirred
colorless reaction mixture was allowed to warm to room
temperature, following which it was cooled to ꢀ78 ꢁC
and BCl3 (4.8 mL of 1.0 M solution in hexane, 4.8 mmol)
was added dropwise. After being stirred at room tempera-
ture for 2 h, the reaction mixture was filtered through Cel-
iteꢂ and the solvent was stripped from the filtrate to afford
a white powder. Recrystallization from toluene at ꢀ40 ꢁC
afforded a crop of pale yellow crystals of 3 (1.63 g, 77%,
2.5. Synthesis of [nBuC{N(Cy)}2]BCl(Ph) (2)
1
mp 97–99 ꢁC). H NMR (C6D6): 3.31 (m, 2H); 2.01 (m,
Colorless crystalline 2 (mp 99–101 ꢁC) was prepared in
4H); 1.64 (m, 8H); 1.05 (m, 8H); 0.143 (s, 18H); 13C
NMR (C6D6): 166.57 (NCN); 53.78 (Cy–C1); 33.82 (Cy);
26.41 (Cy); 25.89 (Cy); 2.15 (SiMe3); 11B NMR (C6D6):
4.16 (s). MS (CI+, CH4): m/z 449 (M+H), 412 (MꢀCl).
HRMS (CI, CH4) calcd. for C19H40N3BCl2Si2, 447.2231;
found: 447.2227.
n
63% yield from butyllithium (2 mL of a 2.5 M solution
in diethyl ether, 5.0 mmol), 1,3-dicyclohexylcarbodiimide
(1.03 g, 5.0 mmol) in 15 mL of diethyl ether and PhBCl2
(0.794 g, 5.0 mmol) using the procedure described above
for 1.
1H NMR (C6D6): d 8.08 (d, 2H, Ph); 7.39 (m, 2H, Ph);
7.26 (m, 1H, Ph); 3.19 (m, 2H); 1.98 (s, 4H); 1.83 (m, 2H);
1.57–1.28 (br m, 10H); 1.16–0.96 (br m, 10H); 0.74 (t, 3H,
CH3). 13C NMR (C6D6): d 166.93 (NCN); 133.54; 128.07;
127.99; 125.53 (Ph); 54.73 (Cy–C1); 46.71 (CH2CH2CH2-
CH3); 34.50 (Cy); 29.10 (Cy); 26.55; 26.45 (Cy); 25.77
(CH2CH2CH2CH3); 24.95 (CH2CH2CH2CH3); 14.03
2.7. Synthesis of [(Me3Si)2NC{NCy}2]BCl2 (3) via
carbodiimide insertion
Bis(trimethylsilylamido)boron
dichloride
(0.589 g,
2.43 mmol) in 5 mL of hexane was added to a solution
of dicyclohexylcarbodiimide (0.50 g, 2.43 mmol) in
Cy
Cy
N
N
X
XBCl2
-LiCl
LiR'
CyN=C=NCy
Li
R'
R'
B
Cl
N
N
Cy
Cy
t
1: R' = Bu; X = Ph
2: R' = nBu; X = Ph
3: R' = N(SiMe3)2; X = Cl
Scheme 1. Preparation of 1–3 by metathesis.
Cy
N
Cl
CyN=C=NCy + (Me3Si)2NBCl2
(Me3Si)2N
B
Cl
N
Cy
3
Scheme 2. Preparation of 3 by carbodiimide insertion.

986
M. Findlater et al. / Polyhedron 25 (2006) 983–988
3. Results and discussion
3.1. Synthesis of boron amidinates
Alkyl-lithium insertion into carbodiimides represents a
well precedented method for the synthesis of lithium amidi-
nates (Scheme 1). In the present case, the requisite lithium
reagents were prepared by treatment of 1,3-dicyclohexyl-
t
n
carbodiimide with BuLi, BuLi, or (Me3Si)2NLi at low
temperature (ꢀ78 ꢁC). In turn, reaction of these lithium
amidinates with PhBCl2 or BCl3 afforded the boron amidi-
nates 1, 2, and 3 as stable crystalline solids in yields of 58%,
63% and 77%, respectively. Compound 3 can also be pre-
pared by insertion of CyN@C@NCy into the B–N bond
of (Me3Si)2NBCl2 as summarized in Scheme 2. The yield
obtained by this method is slightly higher than that
obtained by the metathesis route.
In previous work, we have demonstrated that the carbo-
diimides will insert into the B–C bonds of both ferrocenyl-
dibromoborane [19] and dichlorophenylborane [20].
Collectively, these represent the first examples of the synthe-
sis of boron amidinates by means of carbodiimide insertion
[24]. Intriguingly, we have found that BEt3 and B(NMe2)3
fail to undergo carbodiimide insertion reactions. Recent the-
oretical studies indicate that the initial step in this insertion
reaction is coordination of the carbodiimide nitrogen lone-
pairs to the vacant pz orbital of the borane [20,25,26].
Clearly, the Lewis acidity of the borane depends on the ste-
reoelectronic characteristics of the substituents. In the case
of tris(dimethylamido)borane the Lewis acidity is dimin-
ished by N ! B p-bonding involving the nitrogen lone pairs
and the vacant boron 2p orbital thus precluding formation
of the initial donor–acceptor complex with the carbodii-
mide. In the case of the attempted reaction of CyN@C@NCy
with BEt3, initial formation is disfavored due to both elec-
tronic and steric effects of the C2H5 substituents.
Fig. 1. ORTEP diagram of 1 with thermal ellipsoids at 40% probability.
All hydrogen atoms have been omitted for clarity.
15 mL of diethyl ether at ambient temperature. A color-
less reaction mixture resulted, which was stirred overnight
at room temperature, following which the solvent was
removed under reduced pressure to afford a white powder
(0.94 g, 89% yield, mp 97–99 ꢁC). All NMR data were
identical to those obtained for 3 via the metathesis route.
MS (CI+, CH4): m/z 449 (M+H), 412 (MꢀCl). HRMS
(CI, CH4) calcd. for C19H40N3BCl2Si2, 447.2231; found:
447.2233.
Fig. 2. ORTEP diagram of 4 with thermal ellipsoids at 40% probability. All hydrogen atoms have been omitted for clarity.

M. Findlater et al. / Polyhedron 25 (2006) 983–988
987
Table 2
Selected bond distances (A) and angles (ꢁ) for 1, 2, and 4
˚
Compound 1
Compound 2
Compound 4
B(1)–Cl(1)
B(1)–C(18)
B(1)–N(1)
B(1)–N(2)
N(1)–C(1)
N(2)–C(1)
N(1)–C(6)
N(2)–C(12)
C(1)–C(2)
1.866(2)
1.595(2)
1.583(2)
1.572(2)
1.345(2)
1.336(2)
1.461(2)
1.463(2)
1.518(2)
B(1)–Cl(1)
B(1)–C(18)
B(1)–N(1)
B(1)–N(2)
N(1)–C(1)
N(2)–C(1)
N(1)–C(6)
N(2)–C(12)
C(1)–C(2)
1.862(4)
1.591(5)
1.590(4)
1.580(4)
1.333(4)
1.332(4)
1.447(4)
1.456(4)
1.485(4)
B(1)–Cl(1)
B(1)–Cl(2)
B(1)–N(1)
B(1)–N(2)
N(1)–C(1)
N(2)–C(1)
N(1)–C(3)
N(2)–C(6)
C(1)–C(2)
1.835(3)
1.842(3)
1.561(4)
1.567(4)
1.325(3)
1.334(3)
1.462(3)
1.457(3)
1.475(4)
N(1)–B(1)–N(2)
N(1)–C(1)–N(2)
Cl(1)–B(1)–C(18)
N(1)–B(1)–Cl(1)
N(1)–B(1)–C(18)
N(2)–B(1)–Cl(1)
N(2)–B(1)–C(18)
81.36(10)
100.20(12)
113.77(11)
113.41(11)
115.89(13)
112.81(11)
115.75(13)
N(1)–B(1)–N(2)
N(1)–C(1)–N(2)
Cl(1)–B(1)–C(18)
N(1)–B(1)–Cl(1)
N(1)–B(1)–C(18)
N(2)–B(1)–Cl(1)
N(2)–B(1)–C(18)
80.90(19)
101.1(2)
113.7(2)
112.6(2)
116.5(3)
112.5(2)
116.6(2)
N(1)–B(1)–N(2)
N(1)–C(1)–N(2)
Cl(1)–B(1)–Cl(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)
82.15(18)
101.2(2)
110.52(16)
115.25(19)
115.62(18)
115.36(18)
115.5(2)
3.2. Crystal structures
our knowledge, this represents the first example of the
insertion of a carbodiimide into a boron–nitrogen bond.
Finally, since all four compounds feature boron–chlorine
bonds, it is anticipated that they will prove to be useful
synthons.
Single-crystal X-ray diffraction studies were carried out
on both 1 and 2 to assess their structural and bonding char-
acteristics. Compounds 1 and 2 crystallize as monomers and
there are no unusually short intermolecular contacts. The
molecular structure of 1 is illustrated in Fig. 1. Both mole-
cules feature an essentially planar B–N–C–N ring and an
approximately tetrahedral geometry around the boron
atom. The B–N–C–N four-membered ring torsion angles
are ꢀ3.35(10)ꢁ and ꢀ1.1(2)ꢁ for 1 and 2, respectively. The
B–N, B–Cl, and B–CPh bond distances are similar to those
reported previously for [nBuC{N(tBu)}2]BCl(Ph) [27]. The
Acknowledgments
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.
Appendix A. Supplementary data
˚
C–N bond distances (averages 1.34035(19) A for 1 and
˚
1.3325(4) A for 2) are intermediate between those antici-
Full crystallographic data for the structural analyses
have been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 281291 (1), 281292 (2) and
281290 (4). 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 www:
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.poly.2005.11.012.
pated for single and double bonds and are indicative of
delocalization at the NCN moiety. The B–N bonds in 1
and 2 are similar in length and fall in the range observed
for B–N bonds in four-coordinate boron centers bound to
three-coordinate nitrogen atoms [28].
The molecular structure of 4 was of interest to determine
the effect, if any, of changing the steric bulk of the nitrogen
and boron substituents (see Fig. 2). Overall, the metrical
parameters for 4 are very similar to those for 1 and 2 (Table
2). However, the B–N–C–N ring is less distorted from pla-
narity (0.69(18)ꢁ) than those of the cyclohexyl-substituted
amidinates, presumably due to the smaller size of the diiso-
propyl substituent.
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