Synthetic and structural chemistry of amidinate-substituted boron halides

Article abstract of DOI:10.1039/b509392a

The following new amidinate-substituted boron halides are reported: [PhC{N(SiMe₃)}₂]BCl₂ (6), [MeC{NCy} ₂]BCl₂ (10), [Mes*C{NCy}₂]BCl₂ (11), [MeC{NⁱPr}₂]BCl

Full text of DOI:10.1039/b509392a

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F U L L P A P E R
Synthetic and structural chemistry of amidinate-substituted
boron halides
Nicholas J. Hill, Michael Findlater and Alan H. Cowley*
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas,
78712, USA. E-mail: cowley@mail.utexas.edu; Fax: +1 512 471-6822; Tel: +1 512 471 7484
Received 5th July 2005, Accepted 29th July 2005
First published as an Advance Article on the web 25th August 2005
The following new amidinate-substituted boron halides are reported: [PhC{N(SiMe3)}2]BCl2 (6), [MeC{NCy}2]BCl2
(10), [Mes*C{NCy}2]BCl2 (11), [MeC{NiPr}2]BCl2 (12), and [FcC{NCy}2]BBr2 (13). Compound 6 was prepared via
the trimethylsilyl chloride elimination reaction of BCl3 with N,N,Nꢀ-tris(trimethylsilyl)benzamidine, and compounds
10–12 were prepared by salt metathesis between the lithium amidinates [RC(NRꢀ)2]Li and BX3. Compound 13 was
prepared via the insertion of 1,3-dicyclohexylcarbodiimide into the B-C bond of ferrocenyldibromoborane FcBBr2.
The molecular structures of 6, 10, 11, 13 and the known compound [PhC{N(SiMe3)}2]BBr2 (1) were established by
single-crystal X-ray diffraction.
of Cl− from BCl4−, during reaction of the propylidyne complex
Introduction
t
t
8
= =
[Cp(CO)2Mn≡CEt][BCl4] with BuN C NBu and NEt3. The
The coordination chemistry of amidinate anions [RC(NRꢀ)2]−
with main group, transition metal, lanthanide and actinide
elements is well established (R, Rꢀ = alkyl, aryl, silyl).1 These
anions, which function typically as four-electron, N-donor
bidentate chelating ligands, have found widespread use due to
the ease with which the stereoelectronic properties of the N-
and C-atom substituents can be tailored. Moreover, the ligand
architecture can be readily modified, for instance to include
two amidinate groups, pendant donor atoms, or a chiral center.2
Several synthetic routes to metal amidinate complexes have been
developed, the most prevalent being (i) reaction of a metal halide
with an N,N,Nꢀ-tris(trimethylsilyl)amidine; (ii) protonolysis of
an amidine using a metal alkyl; (iii) carbodiimide insertion into
a metal–alkyl bond; and (iv) salt metathesis between a metal
halide substrate and a lithium amidinate (generated in situ by
route (ii) or (iii)).
Within the realm of p-block chemistry, amidinate complexes
featuring group 13 metal–alkyl and metal–halide fragments have
been the subject of sustained interest during the last decade3,4
due to the discovery of useful applications in a number of
key technological areas. For example, group 13 amidinate com-
plexes represent promising single-source precursors for nitride
materials4c while amidinate-supported alkylaluminium cations
have proved to be active catalysts for olefin polymerization.5
Considering the rich chemistry displayed by these group 13
species and their relative ease of synthesis, it is remarkable
that the coordination chemistry of amidinate ligands with
boron substrates remains undeveloped. In fact, examples of
such compounds obtained by rational synthetic routes are
confined to the dibromo compound 1,6 and the chloro–phenyl
derivative 2.7 A related example is the vinylidene–amidinate
compound 3, which was obtained serendipitously via the in situ
generation of an amidinate fragment, followed by displacement
bis(trifluoromethyl)-substituted amidinate 4 was formed by
rearrangement of an unstable adduct resulting from the [2 +
9
=
= =
2] cycloaddition of (CF3)2B NMe2 to PhN C NPh. In the
present contribution we report the syntheses and molecular
structures of a range of new boron-amidinate complexes.
Experimental
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 be-
fore use. All solvents were distilled over sodium benzophe-
none ketyl, except dichloromethane, which was distilled over
CaH2, and degassed prior to use. N,N,Nꢀ-Tris(trimethylsilyl)-
benzamidine10 and ferrocenyldibromoborane (FcBBr2)11 were
prepared according to the literature procedures. The compounds
1,3-dicyclohexylcarbodiimide, 1,3-diisopropylcarbodiimide, 1-
bromo-2,4,6-tri(tert-butyl)benzene (Mes*Br), boron halides,
and alkyl lithium solutions were obtained commercially and
used without further purification.
Physical measurements
Low-resolution CI mass spectra were obtained on a Finnigan
MAT TSQ-700 mass spectrometer and high-resolution CI mass
spectrarecorded onaVG AnalyticalZAB-VE sector instrument.
All MS analyses were performed on samples that had been sealed
1
in glass capillaries under an argon atmosphere. 1H, 13C{ H}, and
11B NMR spectra were recorded at 295 K in C6D6 solutions on
a GE QE-300 instrument (1H, 300 MHz; 13C, 75 MHz, 11B,
96 MHz) immediately following removal of the sample from the
1
1
drybox. H and 13C{ H} chemical shift values are reported in
parts per million (ppm) relative to SiMe4 (d 0.00), using residual
solvent resonances as internal standards. 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.
X-Ray crystallography
For compounds 1, 6, 10, 11 and 13, a crystal of suitable
quality was removed from a Schlenk flask under positive argon
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 3 2 2 9 – 3 2 3 4
3 2 2 9
©

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Table 1 Selected crystal data, data collection and refinement parameters for 1, 6, 10, 11 and 13
1
6
10
11
13
Formula
C13H23N2BBr2Si2
434.14
Monoclinic
C2/c
15.329(5)
10.887(5)
13.019(5)
116.223(5)
1949.1(13)
4
1.479
872
0.30 × 0.20 × 0.20
2.39–26.99
3207
C13H23N2BCl2Si2
345.22
Monoclinic
C2/c
15.026(5)
10.799(5)
12.711(5)
114.477(5)
1877.2(13)
4
1.222
728
0.20 × 0.20 × 0.20
2.59–26.99
3282
C14H25N2BCl2
303.07
Monoclinic
P21/n
13.590(5)
7.324(5)
16.451(5)
94.622(5)
1632.1(14)
4
1.233
648
0.30 × 0.20 × 0.20
2.48–24.99
5482
C37H57N2BCl2
611.56
Monoclinic
P21/n
13.338(5)
18.972(5)
15.336(5)
92.677(5)
3867(15)
4
1.050
1328
0.20 × 0.20 × 0.20
1.71–25.00
13301
C23H31N2BBr2Fe
561.98
Monoclinic
P21/n
13.953(5)
10.716(5)
15.723(5)
103.837
2282.7(15)
4
1.635
1136
0.30 × 0.30 × 0.20
2.42–27.00
8684
Formula weight
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
Dc/g cm−3
F(000)
Crystal size/mm
h Range/◦
No. of reflns collected
No. of indep. reflns
R1 [I > 2r(I)]
wR2 (all data)
1718
0.0259
0.0636
0.406 and 0.371
2045
0.0543
0.1569
0.485 and −0.325
2883
0.0435
0.1191
0.273 and −0.287
6811
0.0653
0.1990
0.446 and −0.300
4966
0.0431
0.0849
0.699 and −0.643
−3
˚
Peak and hole/e A
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 diffractometer
equipped with an Oxford Cryostream low-temperature device
and a graphite-monochromated Mo-Ka radiation source (k =
then cooled to −78 ◦C following which BCl3 (4.85 mL 1.0 M
solution in hexane, 4.85 mmol) was added dropwise. After being
stirred at room temperature for 2 h, the reaction mixture was
R
filtered through Celiteꢀ and the solvent was stripped from the
filtrate to afford a white powder. Recrystallization of this powder
from toluene solution afforded a crop of pale yellow crystals of
10 (1.24 g, 85% yield, mp 82–84 ◦C).
˚
0.71073 A). Corrections were applied for Lorentz and polar-
ization effects. All structures were solved by direct methods12
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
1H NMR (C6D6): d 3.41 (m, 2H); 2.14 (s, 3H); 2.07 (m, 4H);
1.59–1.24 (br m, 10H); 1.18–1.03 (m, 6H); 13C NMR (C6D6):
d 173.41 (NCN); 52.12 (Cy-C1); 33.55 (Cy); 26.95 (Cy); 25.97
(Cy); 10.82 (MeC). 11B NMR (C6D6): d 6.01 (s). MS (CI+, CH4):
m/z 302 (M + H), 267 (M − Cl). HRMS (CI, CH4): calc. for
C14H25N2BCl2: 302.1488; found: 302.1488.
˚
a riding model (C–H 0.96 A). Selected crystal data, and data
collection and refinement parameters are listed in Table 1.
CCDC reference numbers 275321–275325.
See http://dx.doi.org/10.1039/b509392a for crystallographic
data in CIF or other electronic format.
Preparation of [Mes*C{NCy}2]BCl2 (11). A solution of
nBuLi (10.2 mmol, 6.4 mL of 1.6 M solution in hexanes) was
added to a solution of Mes*Br (10 mmol) in 25 mL of diethyl
ether at −78 ◦C. The stirred reaction mixture was allowed to
warm to ambient temperature over a 2 h period. The resulting
Mes*Li solution (10 mmol) was chilled to −78 ◦C and an
equimolar solution of 1,3-dicyclohexylcarbodiimide in 10 mL
diethyl ether was added dropwise. The reaction mixture was
warmed to room temperature and stirred for 2 h, following
which it was cooled to −78 ◦C. Boron trichloride (10 mL of
1.0 M solution in hexanes, 10 mmol) was added dropwise,
and the stirred reaction mixture was allowed to warm to
room temperature overnight. The resulting white slurry was
Preparation of [PhC{N(SiMe3)}2]BBr2 (1). A solution of
BBr3 (10 mmol) in CH2Br2 (10 mL) was added to a stirred,
equimolar solution of N,N,Nꢀ-tris(trimethylsilyl)benzamidine
(3.4 g, 10 mmol) in CH2Br2 (30 mL) at room temperature. The
reaction mixture was warmed to 45 ◦C for several minutes, then
cooled to room temperature and stirred for 6 h. Concentration
1
2
of the resulting solution (approx.
volume) under reduced
pressure followed by storage at −30 ◦C resulted in a crop of
colourless crystals which was isolated by filtration. A seco◦nd
crop of crystals formed upon cooling of the filtrate to −30 C
(3.9 g, 90% yield, mp 127–128 ◦C).
R
filtered through Celiteꢀ and the solvent was removed from the
1H NMR (C6D6): d 6.86 (m, 5H, Ph), 0.16 (s, 18H, SiMe3);
13C NMR (C6D6): d 174.36 (NCN); 131.63; 128.88; 126.99 (Ph);
0.63 (SiMe3); 11B NMR (C6D6): d − 3.84 (s). MS (CI+, CH4):
m/z 435 (M + H), 355 (M − Br). HRMS (CI, CH4): calc. for
C13H24N2BBr2: 434.9917; found: 434.9927.
filtrate under reduced pressure to give a pale yellow residue.
Recrystallization of this residue from toluene solution afforded
a crop of pale yellow crystals of 11 (3.2 g, 83% yield, mp 178–
180 ◦C).
1H NMR (C6D6): d 6.87 (s, 2H); 3.38 (m, 2H); 1.74 (m, 4H);
1.36–1.26 (m, 10H); 1.19 (s, 27 H); 1.17–1.03 (m, 6H); 13C NMR
(C6D6): d 176.43 (NCN), 152.13 (Ph); 148.86 (Ph); 126.73 (Ph);
120.03 (Ph); 55.84 (Cy-C1); 39.44 (C(CH3)3); 35.47 (C(CH3)3);
33.14 (Cy); 26.76 (Cy); 25.53 (Cy); 11B NMR (C6D6): d 6.40 (s).
MS (CI+, CH4): m/z 534 (M + H), 497 (M − Cl). HRMS (CI,
CH4): calc. for C31H51N2BCl2: 532.3522; found: 532.3516.
Preparation of [PhC{N(SiMe3)}2]BCl2 (6). Colourless crys-
talline 6 (mp 129–131 ◦C) was prepared in 84% yield from
BCl3 (10 mmol, 10 mL 1.0 M solution in CH2Cl2) and
N,N,Nꢀ-tris(trimethylsilyl)benzamidine (3.4 g, 10 mmol) using
the procedure described for 1. 1H NMR (C6D6): d 6.82 (m, 5H,
Ph), 0.14 (s, 18H, SiMe3); 13C NMR (C6D6): d 174.68 (NCN);
131.35; 129.02; 125.77 (Ph); 0.66 (SiMe3); 11B NMR (C6D6): d
6.04 (s). MS (CI+, CH4): m/z 344 (M + H), 309 (M − Cl). HRMS
(CI, CH4): calc. for C13H24N2BCl2: 344.0870; found: 344.0871.
Preparation of [MeC{NiPr}2]BCl2 (12). Methyllithium
(3.8 mL of 1.6 M solution in diethyl ether, 6.0 mmol) was added
to a cold (−78 ◦C) solution of diisopropylcarbodiimide (0.75 g,
5.94 mmol) in 15 mL of diethyl ether. The stirred colourless
reaction mixture was allowed to warm to room temperature,
following which it was cooled to −78 ◦C and BCl3 (6 mL of 1.0 M
solution in hexane, 6 mmol) was added dropwise. After being
stirred at room temperature for 2 h, the reaction mixture was
Preparation of [MeC{NCy}2]BCl2 (10). Methyllithium
(3.1 mL of 1.6 M solution in diethyl ether, 4.85 mmol) was added
to a cold (−78 ◦C) solution of 1,3-dicyclohexylcarbodiimide
(1.00 g, 4.85 mmol) in 15 mL of diethyl ether. The stirred
colourless reaction mixture was warmed to room temperature,
3 2 3 0
D a l t o n T r a n s . , 2 0 0 5 , 3 2 2 9 – 3 2 3 4

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R
filtered through Celiteꢀ and the solvent was stripped from the
filtrate to afford a white powder, 12 (1.24 g, 94% yield, mp 38–
40 ◦C). 1H NMR (C6D6): 3.43 (sept, 2H); 2.17 (s, 3H); 1.19 (d,
12H); 13C NMR (C6D6): 173.80 (NCN); 46.17 (CHMe2); 23.08
(CHMe2); 10.66 (MeC); 11B NMR (C6D6): 5.78 (s). MS (CI+,
CH4): m/z 222 (M + H), 187 (M − Cl). HRMS (CI, CH4): calc.
for C8H17N2BCl2: 222.0862; found: 222.0863.
Preparation of [FcC{NCy}2]BBr2 (13). A solution of FcBBr2
(0.89 g, 2.5 mmol) in 20 mL of hexane was added to a cold
(−78 ◦C) solution of 1,3-dicyclohexylcarbodiimide (0.52 g,
2.5 mmol) in 15 mL of hexane. The resulting yellow reaction
mixture was stirred at room temperature for 2 h, following which
the solvent and volatiles were removed under reduced pressure
to give an orange powder, 13. Recrystallization of the crude
product from toluene afforded o◦range–red block crystals of 13
1
(0.98 g, 71% yield, mp 177–179 C). H NMR (C6D6): d 4.83–
1
4.03 (br, 9H); 3.64 (m, 2H); 2.20–1.11 (m, 20H); 13C{ H} NMR
(C6D6): d 75.13; 72.90; 71.17; 70.54; 69.55; 68.63; 56.03; 33.89;
26.46; 25.95 (NCN resonance not observed due to low intensity);
11B NMR (C6D6): d −4.05 (s). MS (CI+, CH4): m/z 562 (M +
H), 483 (M − Br). HRMS (CI, CH4): calc. for C23H31N2BBr2Fe:
560.0296; found: 560.0295.
Fig. 1 ORTEP diagram of 1 with thermal ellipsoids at 40% probability
and H-atoms omitted for clarity.
Results and discussion
the B–N–C–N torsion angles for both compounds are zero,
indicating delocalization about the N–C–N junction. The B–N
The coordination chemistry of boron amidinates is ripe for de-
velopment since, apart from intrinsic interest in their molecular
and electronic structures, complexes such as [RC(NRꢀ)2]BX2
(X = Cl, Br) represent potentially valuable starting materials for
the synthesis of new classes of boron-containing compounds.
Our initial approach to the synthesis of amidinate-substituted
boron halides focused on the elimination of trimethylsilyl
halide from the readily prepared ligand precursor N,N,Nꢀ-
tris(trimethylsilyl)benzamidine (5).10 Thus, treatment of a
methylene dihalide solution of 5 with an equimolar amount of
BX3 (X = Cl, Br) at room temperature resulted, after work-up
of the reaction mixtures, in high yields of pale yellow, thermally
stable crystalline solids 1 and 6.
˚
bond distances of 1.559(4) (1) and 1.580(5) A (6) fall within the
˚
typical range of 1.55–1.61 A for a B–N bond derived from a four-
coordinate boron atom bound to a three-coordinate nitrogen
atom.15 In comparison, the B–N bond distances in 9, an amido
boron compound containing a base-stabilized, three-coordinate
16
˚
boron atom, are 1.535(8) and 1.635(8) A. The bite angles of
the amidinate fragment (N(1)–B(1)–N(1A)) are 85.2(3) (1) and
86.1(◦3)◦ (6), and thus ca. 4◦ wider than the equivalent angle
(81.6 ) in 2,7 but closer to the mean bite angles of 84.0◦ in
3 and 83.8◦ in 4 (angles averaged for two crystallographically
independent molecules in the asymmetric units of 3 and 4).8,9
By contrast, the N–Al–N bond angle in the congeneric complex
[PhC{N(TMS)}2]AlCl2 is 72.9(2)◦ and the Al–N bond distance
◦
6
˚
is 1.882(3) A. The average N–B–X bond angle is 114.6 in 1
and 114.8◦ in 6, hence the geometry about the boron atom is
appreciably distorted from that of a regular tetrahedron.
Although 1 has been reported previously,6 the characteriza-
tion of this compound was based solely on an infrared spectrum
and microanalytical data. Curiously, in the same report, it was
mentioned that attempts to prepare 6 resulted in the isolation
of an uncharacterized yellow oil.6 However, we found that
crystalline 6 is indefinitely stable under an inert atmosphere and
shows no sign of reverting to an oil, even upon gentle heating.
In order to assess the structural and bonding features, single-
crystal X-ray diffraction experiments were performed on 1 and
6. Both compounds crystallize as monomers in the monoclinic
space group C2/c, and have very similar unit cell dimensions.
The molecular structure of 1 is illustrated in Fig. 1 along
with the numbering scheme. An identical numbering scheme
was employed for 6. Individual molecules of 1 and 6, which
reside on a two-fold axis passing through atoms B(1)–C(1)–
C(2)–C(5), feature a four-membered B–N–C–N chelate ring and
a phenyl group which is orthogonal to the B(1)–N(1)–C(1)–
N(1A) plane. The B–X, B–N, and C–N bond distances in 1
and 6 are similar to those reported for 2–4, and the related
amido-pyridyl compounds 7 and 8.13,14 The C(1)–N(1) bond
1
1
The H, 13C{ H} and 11B NMR spectra of 1 and 6 confirm
that the C2v-symmetric structures observed in the solid-state
are retained in solution. The 11B NMR spectra exhibit intense
singlet resonances at d −3.8 (1) and 6.0 (6), values which are
typical for a four-coordinate boron atom.17 The 1H and 13C{ H}
1
spectra exhibit peaks due to the phenyl group and two equivalent
trimethylsilyl groups. A low-intensity 13C{ H} peak attributable
1
to the carbon atom of the NCN fragment was detected at d 174.
Although the trimethylsilyl halide elimination method proved
to be effective for the preparation of the [RC(NRꢀ)2]BX2 com-
plexes described above, it was necessary to employ a different
synthetic strategy to extend the range of these compounds. In
this context, the salt metathesis reaction between BX3 and a
lithium amidinate [RC(NRꢀ)2]Li seemed like a more versatile
approach for the introduction of a wide variety of R and
Rꢀ groups, thereby offering the possibility of tuning the steric
environment of the BX2 fragment. Indeed, the validity of this
approach has already been demonstrated by the successful
synthesis of mono- and bis(amidinate) complexes of aluminium
and gallium.3,4
˚
distances for 1 and 6 are 1.339(3) and 1.332(4) A, respectively.
These values are approximately intermediate between typical
C–N double bond and C–N single bond distances. Moreover,
D a l t o n T r a n s . , 2 0 0 5 , 3 2 2 9 – 3 2 3 4
3 2 3 1

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The requisite lithium amidinates were prepared by addition of
diethyl ether solutions of LiMe or LiMes* (Mes* = 2,4,6-tri(tert-
butyl)phenyl) to cold (−78 ◦C) diethyl ether solutions of either
1,3-dicyclohexylcarbodiimide or 1,3-diisopropylcarbodiimide.
Subsequent treatment with one equivalent of BX3 and work-
up of the reaction mixtures afforded good yields of the desired
boron amidinate complexes 10–12 as colourless solids.
oil which did not exhibit a 11B resonance. However, the reaction
of equimolar quantities of 1,3-dicyclohexylcarbodiimide and
ferrocenyldibromoborane FcBBr2 in hexane solution at room
temperature afforded, upon work-up, orange crystalline 13 in
good yield. A similar carbodiimide insertion reaction takes place
with PhBCl2.20 The question of whether or not such insertions
occur therefore appears to depend upon the Lewis acidity of the
borane (i.e. on the ability to form a Lewis acid–base complex
with the carbodiimide). The 11B NMR spectrum of 13 revealed
an intense resonance at d −4.05, which falls in a similar region
to that observed for 1, thus suggesting that the desired insertion
1
1
reaction had occurred. H and 13C{ H} NMR data were also
consistent with such a suggestion (the NCN bridgehead carbon
atom was not observed due to weak intensity).
Single crystals of 10 and 11 suitable for X-ray diffraction
experiments were obtained by recrystallization from toluene
solution. The molecular structures of 10 and 11 (Fig. 2) are
very similar to those of the boron amidinates discussed above,
in the sense that the ligand is chelated to the BCl2 fragment
in a symmetrical bidentate fashion, resulting in a planar, four-
membered B–N–C–N heterocycle with a delocalized N–C–N
moiety. As expected, the bond distances and angles for 10 and
11 are also similar to those for 1 and 6. Likewise, the N(1)–B(1)–
N(2) bite angles and average N–B–Cl bond angles of 82.17(2)
and 115.23◦ for 10, and 82.1(4) and 115.4◦ for 11, indicate
substantial distortion from the ideal tetrahedral value. However,
the N(1)–C(1)–N(2) bond angles of 101.24(3) and 100.7(3)◦ in
10 and 11, respectively, are slightly more acute that those in 1
and 6 (av. 104◦).
Recrystallization of 13 from toluene solution produced a
crop of orange block crystals suitable for X-ray diffraction
experiments. The X-ray crystal structure (Fig. 3) confirmed
the structure proposed for 13. Although carbodiimide insertion
into an M-alkyl bond is well known, to our knowledge this
is the first example of such a process occurring at a p-block-
aryl bond (contrary to the assertion of Barry et al.,19b the
[PhC{N(TMS)}2]AlCl2 complex6 was prepared via trimethylsilyl
chloride elimination). As in the case of the boron amidinates
discussed earlier, the B–N–C–N heterocycle is planar and the
boron atom possesses a distorted tetrahedral geometry (bite
angle 83.5(2)◦, and a mean N–B–Br bond angle of 114.9◦. The
other metrical parameters are very similar to those of 1, 6, 10,
and 11 (Table 2).
Fig. 2 ORTEP diagram of 11 with thermal ellipsoids at 40% probability
and H-atoms omitted for clarity.
The insertion of a carbodiimide into a metal–hydrogen or
metal–alkyl bond has proved to be a facile route to amidinate
complexes of the heavier group 13 elements Al, Ga, and
In. For example, 1,3-bis(trimethylsilyl)carbodiimide reacts with
MMe3 (M = Al, Ga, In) to afford the insertion products
[MeC{N(SiMe3)}2]MMe2.18 A similar reaction occurs when 1,3-
diisopropylcarbodiimide is treated with AlMe3, and a recent
elegant theoretical study has provided insight into the mecha-
nism of carbodiimide insertion into Al–C and Al–N bonds.19
However, to the best of our knowledge this type of reaction
has not been employed previously for the synthesis of boron
amidinates.
Fig. 3 ORTEP diagram of 13 with thermal ellipsoids at 40% probability
and H-atoms omitted for clarity.
The incorporation of a ferrocene unit into the framework of an
amidinate ligand has precedent in the work of Arnold et al.,21,22
who prepared a ferrocene-substituted amidine via the reaction
of FcLi with 1,3-dicyclohexylcarbodiimide and subsequently
explored its coordination chemistry with Fe(II), Co(II) and Rh(I)
halides. To our knowledge, 13 represents the first example of a p-
block complex of the ferrocenylamidinate, and also constitutes
the first report of the insertion of a carbodiimide ligand into a
Cp–group 13 element bond.
Interestingly, the reaction of 1,3-dicyclohexylcarbodiimide
with BEt3 was unsuccessful and resulted in a thick, colourless
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The C5H5 plane of the Fc moiety is almost coplanar with the
NCN plane of the amidinate ligand (torsion angles N(1)–C(1)–
C(14)–C15) 10.0(5)◦, N(2)–C(1)–C(14)–C(18) 15.1(6)◦), which
may indicate some degree of p–p interaction between the two
˚
fragments (C(1)–C(14) 1.450(4) A) This arrangement contrasts
with that of the uncomplexed ferrocene-substitued amidine, on
which the Fc group is approximately perpendicular to the NCN
plane (corresponding torsion angles ∼60◦), and those of the
Fe(II), Co(II) and Rh(I) complexes in which the torsion angle is
∼45◦.21,22
Conclusions
We have demonstrated that a range of complementary synthetic
routes are applicable for the preparation of boron amidinate
compounds. On the basis of NMR and X-ray structural data,
the bonding in the four-membered B–N–C–N heterocycles may
be described in terms of equal contributions from two diaza-allyl
resonance forms, giving rise to delocalization about the N–C–N
junction.
Although the presence of bulky Mes* and Fc groups in 11
and 13 does not exert any marked effect upon the key metrical
parameters of the B–N–C–N chelate ring, we anticipate that
the use of sterically demanding N- and C-substituents will have
an influence on the subsequent chemistry of these complexes.
Moreover, the presence of the electrochemically active ferrocene
group in 13 may impart some intriguing redox properties upon
this compound. Studies of the reactivity patterns of these and
other boron amidinate complexes are currently in progress.
Acknowledgements
We gratefully acknowledge the National Science Foundation
(Grant CHE-0240008) and the Robert A. Welch Foundation
(Grant F-135) for support of this work.
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