Steric effects in metathesis and reduction reactions of phosphinimines with catechol- and pinacolboranes

Article abstract of DOI:10.1021/ic050230k

A series of catecholboryl-phosphinimide complexes with the general formula (μ(R₃PN)Bcat)_x (cat = O₂C₆H ₄) have been synthesized via associative metathetical reactions. For R = Et, n-Bu, Ph, and i-Pr a

Full text of DOI:10.1021/ic050230k

Inorg. Chem. 2005, 44, 4301−4308
Steric Effects in Metathesis and Reduction Reactions of Phosphinimines
with Catechol- and Pinacolboranes
Sarah Hawkeswood, Pingrong Wei, James W. Gauld, and Douglas W. Stephan*
Department of Chemistry & Biochemistry, UniVersity of Windsor, Windsor,
Ontario, Canada N9B 3P4
Received February 14, 2005
A series of catecholboryl
have been synthesized via associative metathetical reactions. For R
X-ray crystallography as well as solution NMR spectroscopy and reactivity studies reveal that these species are
dimeric. In the case of R t-Bu, the steric congestion results in the monomeric species, t-Bu3PNBcat. Similarly,
reactions of R(t-Bu)2PNH (R n-Bu, t-Bu) and i-Pr3PNH with pinacolborane (HBO2C2Me4 HBpin) led to the
formation of n-Bu(t-Bu2)PNBpin, t-Bu3PNBpin, and i-Pr3PNBpin. Analogous reactions of smaller phosphinimines
R3PNH (R Et or n-Bu) with pinacolborane (HBpin) generated free phosphine and the boron-containing product
HN(Bpin)2. In the related reactions of R3PNPh or R3PNAd (R Et and n-Bu) and HBpin, the white crystalline
−phosphinimide complexes with the general formula (
µ
-(R3PN)Bcat)x (cat
)
O2C6H4)
)
Et, n-Bu, Ph, and i-Pr and R3
) n-Bu t-Bu2
)
)
)
)
)
solids PhHN(Bpin) or AdHN(Bpin) were isolated. HN(Bpin)2 was also derived from the reaction of Et3PNSiMe3 and
4
1
HBpin. Kinetic studies showed this reaction is first order in both reagents with a rate constant of 1.3(7)
A mechanism involving a 1:1 donor
×
10- s-
.
−
acceptor interaction of the phosphinimine and borane affording reduction of
the phosphinimine to phosphine with concurrent formation of borylamine is proposed. Computational studies were
performed to probe the steric effects on these reactions of phosphinimine and borane. Model reactions involving
t-Bu3PNH showed a lower activation barrier for protonolysis in comparison to phosphinimine reduction. In contrast,
for the smaller phosphinimine H3PNH, the activation barriers for phosphinimine reduction are lower. The causes of
these steric effects are considered.
Introduction
and co-workers have demonstrated that phosphinimines
themselves undergo metathesis with imines and catalyze
imine/imine and imine/carbodiimine cross-metathesis.8,9 More-
over, phosphinimine-based ligands have been employed to
support novel transition metal complexes,10-13 while theoreti-
cal studies have examined the electronic nature of phos-
phinimines and phosphinimide complexes.14-16 Understand-
Phosphinimine and phosphinimide complexes of main
group elements and transition metals have been shown to
exhibit wide structural diversity.1,2 Recently, reactivity studies
of such compounds have drawn considerable attention. We
have shown that early transition metal derivatives can yield
highly effective catalysts for olefin polymerization.3-7 Meyer
(7) Yue, N.; Hollink, E.; Guerin, F.; Stephan, D. W. Organometallics 2001,
20, 4424-4433.
* Author to whom correspondence should be sent. E-mail: stephan@
uwindsor.ca.
(8) Bell, S. A.; Meyer, T. Y.; Geib, S. J. J. Am. Chem. Soc. 2002, 124,
10698-10705.
(1) Dehnicke, K.; Krieger, M.; Massa, W. Coord. Chem. ReV. 1999, 182,
19-65.
(9) Burland, M. C.; Meyer, T. Y. Inorg. Chem. 2003, 42, 3438-3444.
(10) Aparna, K.; Babu, R. P. K.; McDonald, R.; Cavell, R. G. Angew.
Chem., Int. Ed. 2001, 40, 4400-4402.
(2) Dehnicke, K.; Weller, F. Coord. Chem. ReV. 1997, 158, 103-169.
(3) Hollink, E.; Wei, P.; Stephan, D. W. Organometallics 2004, 23, 1562-
1569.
(11) Babu, R. P. K.; McDonald, R.; Cavell, R. G. Chem. Commun. 2000,
006, 481-482.
(4) Stephan, D. W.; Guerin, F.; Spence, R. E. v. H.; Koch, L.; Gao, X.;
Brown, S. J.; Swabey, J. W.; Wang, Q.; Xu, W.; Zoricak, P.; Harrison,
D. G. Organometallics 1999, 18, 2046-2048.
(12) Kasani, A.; Babu, R. P. K.; McDonald, R.; Cavell, R. G. Angew.
Chem., Int. Ed. 1999, 38, 1483-1484.
(5) Stephan, D. W.; Stewart, J. C.; Guerin, F.; Courtenay, S.; Kickham,
J.; Hollink, E.; Beddie, C.; Hoskin, A.; Graham, T.; Wei, P.; Spence,
R. E. v. H.; Xu, W.; Koch, L.; Gao, X.; Harrison, D. G. Organome-
tallics 2003, 22, 1937-1947.
(13) Kasani, A.; McDonald, R.; Cavell, R. G. Chem. Commun. 1999, 1993-
1994.
(14) Sundermann, A.; Uzan, O.; Milstein, D.; Martin, J. M. L. J. Am. Chem.
Soc. 2000, 122, 7095-7104.
(15) Lu, W. C.; Sun, C. C. THEOCHEM 2002, 593, 1-7.
(16) Xue, Y.; Kim, C. K. J. Phys. Chem. A 2003, 107, 7945-7951.
(6) Stephan, D. W.; Stewart, J. C.; Guerin, F.; Spence, R. E. v. H.; Xu,
W.; Harrison, D. G. Organometallics 1999, 18, 1116-1118.
10.1021/ic050230k CCC: $30.25
Published on Web 05/17/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 12, 2005 4301

Hawkeswood et al.
Synthesis of ((µ-R3PN)Bcat)2 (R3 ) Et3 (1), n-Bu3 (2), Ph3 (3),
i-Pr3 (4), n-But-Bu2 (5)), and t-Bu3PNBcat (6). These compounds
were prepared in a similar fashion, and thus, only one preparation
is detailed. A solution of ClBcat (0.270 g, 1.75 mmol) in 20 mL of
toluene was added to a solution of Et3PNSiMe3 (0.360 g, 1.75
mmol) in 2 mL of toluene and was stirred for 1 h at room
temperature. Toluene and SiMe3Cl were removed in vacuo. The
product was washed with pentanes (3 × 2 mL) and dried in vacuo.
ing the structure-reactivity relationship continues to stimulate
research endeavors. In our work to uncover strategies to
improved olefin polymerization catalysts, one recurring
theme has emerged: the resulting reactivity of phosphinimide
complexes is determined by the admixture of the polarity of
the P-N bond and the steric demands of the substituents on
phosphorus.17-19 In an effort to probe the generality of these
observations, we have begun to examine related main group-
phosphinimide chemistry. Initially we probed the role of the
steric demands and showed that while phosphinimines with
small substituents were known to afford species of formula
B(NPR3)3,20 sterically demanding phosphinimines gave the
linear borinium salt [(t-Bu3PN)2B]Cl.21 Similar steric effects
were observed in methyl-abstraction reactions of silyl and
tin phosphinimines.22 Herein we describe steric effects on
the reactions of phosphinimines with catechol and pina-
colboranes.
1
A white solid was isolated. Data for 1: yield 82%; H NMR 7.10
(m, 4H, C6H4 (o-H)), 6.89 (m, 4H, C6H4 (m-H)), 1.22 (dq, 12H,
2
3
3
CH2, JP-H ) 12 Hz, JH-H ) 8 Hz), 0.73 (dt, 18H, Me, JP-H
)
17 Hz, JH-H ) 8 Hz); 31P{1H} NMR 40.7; 13C{1H} NMR 153.4
(s, C6H4 (ipso-C)), 119.0 (s, C6H4 (o-C)), 109.2 (s, C6H4 (m-C)),
16.9, 5.7 (s, Me); 11B{1H} NMR 8.1. Anal. Calcd: H, 7.63; C,
57.41; N, 5.58. Found: H, 7.67; C, 57.59; N, 5.55. Data for 2:
3
1
yield 97%; H NMR (C6D6) 7.02 (m, 4H, C6H4 (o-H)), 6.84 (m,
4H, C6H4 (m-H)), 1.36 (m, 24H, CH2CH2), 1.07 (m, 12H, PCH2),
3
0.76 (t, 18H, Me, JH-H ) 7.2 Hz); 31P{1H} NMR 35.9; 13C{1H}
NMR 153.4 (s, C6H4 (ipso-C)), 118.9 (s, C6H4 (o-C)), 109.2 (s,
C6H4 (m-C)), 25.7 (s, PCH2), 24.8 (s, PCH2CH2), 24.0 (s,
PCH2CH2), 14.0 (s, Me); 11B{1H} NMR 6.0. Anal. Calcd: H, 9.32;
C, 64.49; N, 4.18. Found: H, 9.77; C, 64.25; N, 4.09. Data for 3:
Experimental Section
General Data. All preparations were performed under an
atmosphere of dry O2-free N2 by employing either Schlenk-line
techniques or a Vacuum Atmospheres glovebox. Solvents were
purified employing Grubbs-type column systems manufactured by
Innovative Technologies or were distilled from the appropriate
drying agents under N2. ClBcat, HBpin, n-Bu3P, Et3P, and N3SiMe3
were used as received from Sigma-Aldrich. t-Bu3P and i-Pr3P
were purchased from the Strem Chemical Co. Modified litera-
ture procedures were used to synthesize the phosphinimines.5 1H,
13C{1H}, 31P{1H}, 11B{1H}, and 19F{1H} NMR spectra were
recorded on Bruker Avance spectrometers operating at 300 and 500
MHz, respectively. Deuterated benzene and toluene were purchased
from Cambridge Isotopes Laboratories, vacuum distilled from the
appropriate drying agents, and freeze-pump-thaw-degassed (3×).
C6D6 was used to record the NMR spectra unless otherwise
indicated. Trace amounts of protonated solvents were used as
references, and 1H and 13C{1H} NMR chemical shifts (δ) are
reported relative to SiMe4. 31P{1H}, 11B{1H}, and 19F{1H}
NMR spectra are referenced to 85% H3PO4, BF3(OEt2) and
CFCl3, respectively. Line widths at half-height (∆ν1/2) are re-
ported in hertz. Combustion analyses were performed at the
University of Windsor Chemical Laboratories. Infrared spectra were
collected on a Bruker Vector 22 FT-IR spectrometer using Nujol
mulls. Modified literature procedures were used to synthesize
t-Bu3PNSiMe3, i-Pr3PNSiMe3, n-Bu3PNPh, Et3PNAd, Et3PNH,
n-Bu3PNH, t-Bu3PNH, PhN3, Et3PNH, and n-Bu3PNH.23-29
1
3
yield 69%; H NMR (C6D5CD3 ) 7.70 (dd, 12H, PPh, JP-H )13
Hz, 3JH-H ) 8 Hz), 6.99 (m, 22H, PPh, C6H4), 6.75 (m, 4H, C6H4);
31P{1H} NMR (C6D5CD3) 34.1; 13C{1H} NMR (C6D5CD3, partial)
132.8 (PPh), 131.8 (PPh), 128.8 (PPh), 121.3 (C6H4), 111.5 (C6H4);
11B{1H} NMR (C6D5CD3) 8.4. Anal. Calcd: H, 4.85; C, 72.94; N,
3.54. Found: H, 5.07; C, 72.61; N, 3.58. Data for 4: yield 83%;
1H NMR (C6D5CD3) 6.95 (m, 4H, C6H4 (o-H)), 6.76 (m, 4H, C6H4
2
3
(m-H)), 1.82 (dseptet, 6H, i-Pr, JP-H ) 12 Hz, JH-H ) 7 Hz),
0.96 (dd, 36H, i-Pr, 3JP-H ) 15 Hz, 3JH-H ) 7 Hz); 31P{1H} NMR
(C6D5CD3) 49.2; 13C{1H} NMR (C6D5CD3) 153.0 (C6H4 (o-C)),
121.6 (C6H4 (o-C)), 109.1 (C6H4 (m-C)), 25.2 (d, i-Pr), 17.2 (i-Pr);
11B{1H} NMR (C6D5CD3) 7.8. Anal. Calcd: H, 8.60; C, 61.46; N,
4.78. Found: H, 8.48; C, 61.23; N, 4.65. Data for 5: yield 60%;
1H NMR 7.09 (m, 4H, C6H4 (o-H)), 6.80 (m, 4H, C6H4 (m-H)),
1.67 (m, 4H, PCH2), 1.49 (m, 4H, CH2CH2), 1.26 (m, 4H,
3
CH2Me), 1.03 (d, 36H, t-Bu, JP-H ) 14 Hz), 0.83 (t, 6H, Me,
3JH-H ) 7 Hz); 31P{1H} NMR 38.8; 13C{1H} NMR 151.1 (s, C6H4
(ipso-C)), 121.4 (s, C6H4 (o-C)), 111.5 (s, C6H4 (m-C)), 36.6 (d,
t-Bu, 1JP-C ) 62 Hz), 27.4 (s, t-Bu, 26.5 (d, CH2, 3JP-C ) 13 Hz),
3
3
25.4 (d, CH2CH2, JP-C ) 13 Hz), 22.18 (d, CH2Me, JP-C ) 57
Hz), 14.7 (s, Me); 11B{1H} NMR 24.4. Anal. Calcd: H, 9.32; C,
64.49; N, 4.18. Found: H, 9.29; C, 64.71; N, 4.18. Data for 6:
1
yield 94%; H NMR (C6D5CD3) 7.09 (m, 2H, C6H4 (o-H)), 6.81
3
(m, 2H, C6H4 (m-H)), 1.19 (d, 27H, t-Bu, JP-H ) 13 Hz);
31P{1H} NMR (C6D5CD3) 42.7; 13C{1H} NMR 151.2 (s, C6H4
(ipso-C)), 121.2 (d, C6H4 , J ) 25 Hz), 111.4 (d, C6H4, J ) 28
1
Hz), 40.4 (d, t-Bu, JP-C ) 53 Hz), 29.7 (s, t-Bu); 11B{1H} NMR
(17) Kickham, J. E.; Guerin, F.; Stephan, D. W. J. Am. Chem. Soc. 2002,
124, 11486-11494.
(18) Kickham, J. E.; Guerin, F.; Stewart, J. C.; Stephan, D. W. Angew.
Chem., Int. Ed. 2000, 39, 3263-3266.
(C6D5CD3) 24.2. Anal. Calcd: H, 9.32; C, 64.49; N, 4.18. Found:
H, 9.44; C, 64.10; N, 4.29.
Synthesis of (µ-Et3PN)Bcat(B(C6F5)3) (7). X (0.034 g, 0.135
mmol) was dissolved in 3 mL of toluene. To this solution was added
solid B(C6F5)3 (0.069 g, 0.135 mmol). The resulting solution was
stirred at ambient temperature for 10 min. The solvent was removed
in vacuo, and the resulting white solid was isolated in 82% yield:
1H NMR 6.78 (m, 2H, C6H4 (o-H)), 6.64 (m, 2H, C6H4 (m-H)),
1.65 (dq, 6H, PCH2, 2JP-H ) 13 Hz, 3JH-H ) 8 Hz), 0.38 (dt, 9H,
Me, 3JP-H ) 19 Hz, 3JH-H ) 8 Hz); 31P{1H} NMR 68.0; 13C{1H}
NMR (C6D5CD3) 147.0 (s, BC6F5), 140.0 (s, C6H4 (ipso-C)), 139.1
(s, BC6F5), 135.3 (s, BC6F5), 122.7 (s, C6H4 (o-C)), 111.7 (s, C6H4
(m-C)), 17.6 (d, PCH2, 1JP-C ) 60 Hz), 5.9 (s, Me); 11B{1H} NMR
28.0 (BO), -6.4; 19F{1H} NMR -162.4, -156.4, -131.8. Anal.
(19) Kickham, J. E.; Guerin, F.; Stewart, J. C.; Urbanska, E.; Ong, C. M.;
Stephan, D. W. Organometallics 2001, 20, 1175-1182.
(20) Moehlen, M.; Neumueller, B.; Dehnicke, K. Z. Anorg. Allg. Chem.
1998, 624, 177-178.
(21) Courtenay, S.; Mutus, J. Y.; Schurko, R. W.; Stephan, D. W. Angew.
Chem., Int. Ed. 2002, 41, 498-501.
(22) Courtenay, S.; Ong, C. M.; Stephan, D. W. Organometallics 2003,
22, 818-825.
(23) Aksnes, G.; Froeyen, P. Acta Chem. Scand. 1969, 23, 2697-703.
(24) Birkofer, L.; Kim, S. M. Chem. Ber. 1964, 97, 2100-1.
(25) Froeyen, P. Acta Chem. Scand. 1972, 26, 1777-82.
(26) Lindsay, R. O.; Allen, C. F. H. Org. Synth. 1942, 22, 96-8.
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4302 Inorganic Chemistry, Vol. 44, No. 12, 2005

Metathesis/Reduction Reactions of Phosphinimines
Calcd: H, 2.58; C, 48.60; N, 1.89. Found: H, 2.37; C, 48.55; N,
1.79.
11B{1H} NMR 24.1. Anal. Calcd: H, 10.18; C, 69.33; N, 5.05.
Found: H, 9.59; C, 69.77; N, 4.66.
Kinetic Experiments. Using a 1.5 M stock solution of
Et3PNSiMe3 in toluene, aliquots (0.05, 0.1, 0.2, 0.4, or 0.6 mL)
were added to each of five 2 mL volumetric flasks containing
HBpin (1 mL) and the integration reference PPh3 (0.25 mL, 0.3 M
in toluene). Similarly, aliquots of a 0.69 M stock solution of
HBpin in toluene (0.57, 0.45, 0.34, or 0.227 mL) were added to
Et3PNSiMe3 (0.5 g) and the Ph3P reference solution. Aliquots of
each solution were placed in a resealable NMR tube. The NMR
tubes were heated in an oil bath maintained at 55 °C for the duration
of the experiment. All of the above reactions were monitored using
31P{1H} NMR spectroscopy. Each data set was collected at varying
time intervals over a period of 7 days. Each spectrum was recorded
with 32 consecutive 30° pulses and 1 s delay time. This delay time
was shown to be adequate on the basis of the experiments conducted
with varying delay times between pulses (1, 5, 10, 25, and 60 s)
and known concentrations of reactants and products.
Computational Studies. Density functional theory calculations
were carried out using the Gaussian 9830 suite of programs. The
B3 exchange functional,31,32 as implemented in GAUSSIAN 98,33
was used in combination with the correlation functional of Lee,
Yang, and Parr (LYP).34 Optimized geometries35 were obtained
using the B3-LYP functional in conjunction with the 6-31G(d) basis
set. Harmonic vibrational frequencies and zero-point vibrational
energy (ZPVE) corrections were also calculated at this level of
theory. Relative energies were calculated by performing single point
energy calculations at the B3-LYP/6-311+G(2df,p) level using the
above geometries, i.e., B3-LYP/6-311+G(2df)//B3-LYP/6-31G(d,p)+
ZPVE. All bond lengths are in angstroms (Å) unless otherwise
specified.
X-ray Data Collection and Reduction. Crystals were manipu-
lated and mounted in capillaries in a glovebox, thus maintaining a
dry, O2-free environment for each crystal. Diffraction experiments
were performed on a Siemens SMART System CCD diffractometer.
The data were collected in a hemisphere of data in 1329 frames
with 10 s exposure times. The observed extinctions were consistent
with the space groups in each case. The data sets were collected
(4.5° < 2θ < 45-50.0°). A measure of decay was obtained by
recollecting the first 50 frames of each data set. The intensities of
reflections within these frames showed no statistically significant
change over the duration of the data collections. The data were
processed using the SAINT and XPREP processing packages. An
empirical absorption correction based on redundant data was applied
to each data set. Subsequent solution and refinement was performed
Synthesis of n-Bu(t-Bu2)PNBpin (8), t-Bu3PNBpin (9), and
i-Pr3PNBpin (10). These compounds were prepared in a similar
fashion, and thus only one preparation is detailed. To a solution of
n-Bu(t-Bu)2PNH (0.050 g, 0.23 mmol) in 3 mL of pentane was
added HBpin (0.067 mL, 0.46 mmol). Gas evolution was observed
upon addition of HBpin. The resulting solution was stirred at room
temperature for 8 h. The solvent and excess HBpin were decanted,
and the solid was dried in vacuo and collected. Data for 8: yield
61%; 1H NMR 1.72 (m, 2H, PCH2), 1.50 (m, 2H, PCH2CH2), 1.30
3
(sextet, 2H, CH2CH2Me, JH-H ) 7 Hz), 1.19 (s, 12H, OCMe),
1.11 (d, 18H, t-Bu, 3JP-H ) 13 Hz), 0.89 (t, 3H, CH2Me, 3JH-H
)
7 Hz); 31P{1H} NMR 36.2; 13C{1H} NMR 80.1 (s, OCMe), 36.7
(d, t-Bu, 1JP-C ) 61 Hz), 27.7 (s, t-Bu), 26.6 (d, CH2CH2CH2, 3JP-C
) 26 Hz), 25.6 (d, PCH2CH2), 25.1 (s, OCMe), 22.5 (d, PCH2,
1JP-C ) 56 Hz), 14.4 (s, CH2Me); 11B{1H} NMR 22.6. Anal.
Calcd: H, 11.45; C, 62.98; N, 4.08. Found: H, 9.80; C, 61.97; N,
3.64. Data for 9: yield 94%; 1H NMR 1.27 (d, 27H, t-Bu, 3JP-H
)
13 Hz), 1.24 (s, 12H, Me); 31P{1H} NMR 40.2; 13C{1H} NMR
1
80.1 (s, BOC), 40.5 (d, t-Bu, JP-C ) 26 Hz), 30.0 (s, t-Bu), 25.6
(s, Me); 11B{1H} NMR 23.4 (∆ν1/2 ) 486 Hz). Anal. Calcd: C,
62.98; H, 11.45; N, 4.08. Found: C, 62.85; H, 11.43; N, 4.07. Data
1
3
for 10: yield 87%; H NMR 1.86 (d septet, 3H, CHMe2, JP-H
)
11 Hz, 3JH-H ) 7 Hz), 1.22 (s, 12H, CMe), 1.03 (dd, 18H, CHMe2,
3JP-H ) 15 Hz, 3JH-H ) 7 Hz); 31P{1H} NMR 36.0; 13C{1H} NMR
1
80.1 (CMe), 25.7 (d, i-Pr, JP-C ) 61 Hz), 25.5 (CMe), 17.5
(i-Pr); 11B{1H} NMR 23.9. Anal. Calcd: H, 11.04; C, 59.81; N,
4.65. Found: H, 11.52; C, 57.61; N, 5.50.
Synthesis of HN(Bpin)2 (11). Method i. HBpin (0.10 mL, 0.68
mmol) was slowly added to a pentane solution of n-Bu3PNH (0.074
g, 0.34 mmol). Immediate gas evolution was observed. The solution
was stirred at 25 °C for 2 h and concentrated to 1 mL from which
a white precipitate was collected. The solid was dried in vacuo.
This solid was redissolved in pentane from which X-ray-quality
crystals of HN(Bpin)2 were grown in 25% yield.
Method ii. HBpin (0.81 mL, 5.60 mmol) was slowly added to
a pentane solution of Et3PNH (0.355 g, 2.67 mmol). Immediate
gas evolution was observed. The solution was stirred at 25 °C for
2 h. The solvent, Et3P, and excess HBpin were removed in vacuo.
The resulting white solid was washed (3 × 1 mL) with cold pentane,
1
dried in vacuo, and isolated: yield 92%; H NMR (partial) 1.04
(s, 24H, Me); 13C{1H} NMR 82.8 (s, 4C, BOC), 25.1 (s, 8C, Me);
11B{1H} NMR 25.2; IR ν(N-H) 3334 cm-1. Anal. Calcd: C, 53.59;
H, 9.37; N, 5.21. Found: C, 54.09; H, 9.58; N, 5.22.
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Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
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Synthesis of RHN(Bpin) (R ) Ph (12), Ad (13)). These
compounds were prepared in a similar fashion using either the
Et3PNR or n-Bu3PNR derivatives, and thus, only a representative
preparation is detailed. HBpin (0.3 mL, 2.06 mmol) was added
slowly to a 5 mL pentane solution of PhNPn-Bu3 (0.576 g, 1.96
mmol) resulting in a bright yellow solution. After 6 h of stirring,
a colorless solution remained. At -33 °C the product precipitated
as a white crystalline solid in 48% yield. Preparation from
1
Et3PNPh gave 12. Data for 12: yield 81%; H NMR 7.09 (m, 4H,
m-Ph, o-Ph), 6.77 (t, 1H, p-Ph, 3JH-H ) 7 Hz), 1.07 (s, 12H, Me);
13C{1H} NMR 144.3 (s, ipso-Ph), 129.6 (s, m-Ph), 120.8 (s, o-Ph),
118.4 (s, p-Ph), 83.0 (s, BOC), 25.0 (s, Me); 11B{1H} NMR 24.1;
IR ν(N-H) 3357 cm-1. Anal. Calcd: C, 65.79; H, 8.28; N, 6.39.
(31) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-52.
(32) Becke, A. D. J. Chem. Phys. 1993, 98, 1372.
(33) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623.
(34) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(35) Optimized geometries and relative energies were also obtained at the
MP2/6-31G(d,p) level of theory. The structures and relative energetics
obtained are similar to those obtained at the B3-LYP/6-31G(d,p) level.
1
Found: C, 65.56; H, 8.21; N, 6.62. Data for 13: yield 83%; H
NMR 2.37 (s, 1H, NH), 1.93 (s, 3H, Ad-CH), 1.81 (s, 6H, Ad-
CH2), 1.52 (m, 6H, Ad-CH2), 1.14 (s, 12H, Me); 13C{1H} NMR
(partial) 46.0 (Ad-CH2), 37.0 (Ad-CH2), 30.7 (Ad-CH), 25.1 (CMe);
Inorganic Chemistry, Vol. 44, No. 12, 2005 4303

Hawkeswood et al.
Table 1. Crystallographic Dataa
1
2
3‚CH2Cl2
4
5
6
11
12
formula
fw
a (Å)
b (Å)
c (Å)
H19C12BNO2P H31C18BNO2P H19C24BNO2P·CH2Cl2 H25C15BNO2P H31C18BNO2P H31C18BNO2P H25B2C12NO4 H18BC12NO2
251.07
10.189(2)
11.550(3)
11. 881(3)
90
102.538(5)
90
monoclinic
P21/c
1364.9(5)
1.217
335.23
10.007(5)
10.283(5)
10.035(5)
65.115(8)
84.46(1)
61.646(7)
triclinic
P1h
494.16
13.7714(6)
9.5732(5)
17.6408(9)
90
96.568(1)
90
monoclinic
P21/n
2310.4(2)
1.380
293.14
12.298(2)
9.093(1)
15.410(2)
90
101.719(3)
90
monoclinic
P21/n
1687.1(3)
1.154
335.23
8.593(2)
16.421(3)
13.797(3)
90
103.341(5)
90
monoclinic
P21/n
1894.3(6)
1.175
335.23
14.167(7)
10.073(5)
15.201(8)
90
113.254(9)
90
monoclinic
P21/n
1993(2)
1.117
268.95
12.400(6)
10.446(5)
12.336(6)
90
90.24(1)
90
monoclinic
P21/c
1597(1)
1.118
219.09
9.928(6)
11.970(7)
12.500(7)
111.68(1)
92.85(1)
112.435(9)
triclinic
P1h
R (deg)
â (deg)
γ (deg)
cryst syst
space group
V (Å3)
980.3(8)
1.136
1244(1)
1.170
d(calc) (g cm-1
Z
)
4
2
6
4
4
4
2
2
abs coeff, µ (cm-1
)
0.191
0.148
0.374
0.163
0.154
0.146
0.079
0.077
data collcd
6502
4227
2810
11201
3335
8148
2423
9150
2722
8264
2830
6628
2287
5307
3508
data Fo2 > 3σ(Fo ) 1962
2
variables
154
208
289
181
208
208
175
290
Rb
Rw
0.0727
0.012 02
1 .052
0.0477
0.0662
1.000
0.0618
0.0943
1.033
0.0559
0.1123
1.012
0.0832
0.1601
1.024
0.0366
0.0534
0.884
0.0552
0.0863
1.026
0.0687
0.1695
1.003
c
GOF
2
2
a These data were collected at 20 °C with Mo KR radiation (λ ) 0.710 69 Å). b R ) Σ(Fo - Fc)/σ(Fo). c Rw ) {Σ[w(Fo - Fc )2]/Σ[σ(Fo)2]}1/2
.
Scheme 1
using the SHELXTL solution package. Crystallographic data are
listed in Table 1.
Structure Solution and Refinement. Non-hydrogen atomic
scattering factors were taken from literature tabulations.36 The heavy
atom positions were determined using direct methods employing
the SHELXTL direct methods routine. The remaining non-hydrogen
atoms were located from successive difference Fourier map
calculations. The refinements were carried out by using full-matrix
least- squares techniques on F. In the final cycles of each
refinement, all non-hydrogen atoms were assigned anisotropic
temperature factors in the absence of disorder or insufficient data.
In the latter cases atoms were treated isotropically. C-H atom
positions were calculated and allowed to ride on the carbon to which
they are bonded. H-atom temperature factors were fixed at 1.10
times the isotropic temperature factor of the C-atom to which they
are bonded. The H-atom contributions were calculated but not
refined. The locations of the largest peaks in the final difference
Fourier map calculation as well as the magnitude of the residual
electron densities in each case were of no chemical significance.
Additional details are provided in the Supporting Information.
Single-crystal X-ray diffraction studies revealed that
complexes 1-5 are dimeric in the solid state and thus
formulated as (µ-(R3PN)Bcat)2. These molecules have similar
structures, and representative ORTEP drawings of 1 and 3
are presented in Figures 1 and 2. In these compounds, the
phosphinimide nitrogen atoms bridge the two B centers
giving a planar four-membered B2N2 core with B-N-B′
and N-B-N′ angles that are all very close to 90°. The
resulting coordination sphere about B is thus composed of
two N-atoms and the two O-atoms of the catecholate ligand
yielding a pseudotetrahedral geometry. The N-B bond
distances in 1-5 range from 1.529(5) to 1.573(6) Å, which
are slightly longer than those observed for ((Br2B(µ-
Results and Discussion
The catecholboryl-phosphinimide complexes of the gen-
eral formula (µ-(R3PN)Bcat))x (R ) Et (1), n-Bu (2), Ph (3),
i-Pr 4, t-Bu (6), R3 ) n-Bu t-Bu2 (5)) were synthesized
via the straightforward metathetical reaction of the corre-
sponding R3PNSiMe3 and ClBcat in toluene at room tem-
perature (Scheme 1). The 31P{1H} chemical shifts of com-
pounds 1-6 are downfield shifted relative to the starting
material R3PNSiMe3, consistent with the deshielding of the
phosphorus center by the electron-withdrawing boron atom.
1
All of the resulting derivatives were characterized by H,
31P{1H}, 11B{1H}, and 13C{1H} NMR spectroscopy, elemen-
tal analysis, and X-ray crystallography. These data were
consistent with the empirical formulation although the nature
of the oligomerization could not be unambiguously deter-
mined.
Figure 1. ORTEP drawing of 1 with 30% thermal ellipsoids. Hydrogen
atoms are omitted.
(36) Cromer, D. T.; Waber, J. T. Int. Tables X-ray Crystallogr. 1974, 4,
71-147.
4304 Inorganic Chemistry, Vol. 44, No. 12, 2005

Metathesis/Reduction Reactions of Phosphinimines
Figure 4. 31P{1H} NMR spectra of 1, 4, and mixture of 1, 4, and (µ-Et3-
PN)(Bcat)2(µ-i-Pr3PN).
Figure 2. ORTEP drawing of 3 with 30% thermal ellipsoids. Hydrogen
atoms are omitted.
and 8.4 ppm, typical of four-coordinate B and thus consistent
with a dimeric structure in solution. In the case of 6 the
11B{1H} NMR shift is 24.2 ppm typical of three-coordinate
B, that is, a monomeric compound40-44 consistent with the
solid-state structure. Compound 5 which is dimeric in the
solid state exhibits a 11B{1H} NMR shift at 24.4 ppm. This
implies a monomer-dimer equilibrium in solution that favor
a monomer in solution. Such an equilibrium is also indicated
by 31P{1H} NMR spectroscopy on a mixture of 1 and 4.
While both compounds gives rise to a single resonance in
the 31P{1H} NMR spectrum, the mixture shows these signals
as well as two additional sets of doublets. The latter
resonances were was attributed to the dissymmetric dimer
(µ-Et3PN)(Bcat)2(µ-i-Pr3PN) (Figure 4). The P-P coupling
between the P centers was observed to be 21 Hz. This
exchange suggests that some small degree of dissociation
of the dimers 1 and 4 occurs in solution allowing recombina-
tion to give the observed statistical mixture of all possible
dimers. Similar exchange experiments were also conducted
using 2 and 6. This resulted only in the observation of the
resonances attributable to 2 and 6. This supports the view
that compound 6 retains its monomeric nature both in
solution and in the solid state. This is likely due to the steric
crowding about the N atom which precludes further associa-
tion.
Figure 3. ORTEP drawing of 6 with 30% thermal ellipsoids. Hydrogen
atoms are omitted.
NPMe3))2 (1.504(4) Å), (Cl2B(µ-NPPh3))2 (1.527(2) Å), and
(Cl2B(µ-NPEt3))2 (1.510(2) Å).37,38 The longer bond lengths
in 1-5 are attributed to the presence of the electron-donating
catecholate fragment. The P-N distances in 1-5 vary from
1.563(4) to 1.607(2) Å. The P-N-B angles range from
127.9(2) to 135.4(3)° and are generally correlated to the steric
bulk of substituents on P.
The X-ray crystallographic data for 6 revealed a mono-
meric structure (Figure 3). The P-N distance of 1.563(2) Å
is longer than those seen in 2-5 and the average P-N bond
lengths of the monomeric borylphosphinimide complexes:
B(NPPh3)3 (1.549(5) Å) and HB(NPt-Bu3)2 (1.531(5) Å).20,21
The B-N distance of 1.369(3) Å is much shorter than those
observed for 1-5 and those seen for B(NPPh3)3 (1.45(1) Å)
and HB(NPt-Bu3)2 (1.41(5) Å). The geometry about B is
approximately trigonal-planar with N-B-O angles of
123.6(2) and 128.8(2)° and a P-N-B angle of 145.6(2)°.
This latter, relatively large angle is consistent with N to B
π-donation. The monomeric nature of 6 is attributed to the
steric bulk of the tert-butyl groups. The steric demands of
the Pt-Bu3 fragment have been recognized since Tolman
developed the notion of cone angles.39
In a similar vein, 1 reacts with with 2 equiv of B(C6F5)3
to give the quantitative formation of (µ-Et3PN)Bcat(B(C6F5)3)
7 (Scheme 1). Spectroscopic data support this formulation.
In particular, the 31P{1H} NMR resonance shifts downfield
to 68.0 ppm, consistent with coordination of the electron-
deficient borane to the phosphinimide-N atom. In contrast
and consistent with the above exchange studies, addition of
(39) Tolman, C. A. Chem. ReV. 1977, 77, 313-348.
(40) Clegg, W.; Dai, C.; Lawlor, F. J.; Marder, T. B.; Nguyen, P.; Norman,
N. C.; Pickett, N. L.; Power, W. P.; Scott, A. J. Dalton Trans. 1997,
839-846.
(41) Coapes, R. B.; Souza, F. E. S.; Fox, M. A.; Batsanov, A. S.; Goeta,
A. E.; Yufit, D. S.; Leech, M. A.; Howard, J. A. K.; Scott, A. J.;
Clegg, W.; Marder, T. B. Dalton Trans. 2001, 1201-1209.
(42) Marder, T. B.; Smith, P. S.; Howard, J. A. K.; Fox, M. A.; Mason, S.
A. J. Organomet. Chem. 2003, 680, 165-172.
The dimeric nature of 1-5 in solution was probed. The
11B{1H} NMR shift for compounds 1-4 ranges between 6.0
(37) Moehlen, M.; Harms, K.; Dehnicke, K.; Magull, J.; Goesmann, H.;
Fenske, D. Z. Anorg. Allg. Chem. 1996, 622, 1692-1700.
(38) Moehlen, M.; Neumuller, B.; Faza, N.; Muller, C.; Massa, W.;
Dehnicke, K. Z. Anorg. Allg. Chem. 1997, 623, 1567-1576.
(43) Nguyen, P.; Dai, C.; Taylor, N. J.; Power, W. P.; Marder, T. B.; Pickett,
N. L.; Norman, N. C. Inorg. Chem. 1999, 34, 4290-4291.
(44) Wescott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T.; Calabrese,
J. C. Inorg. Chem. 1993, 32, 2175-2182.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4305

Hawkeswood et al.
Scheme 2
Figure 5. ORTEP drawing of 11 with 30% thermal ellipsoids. Hydrogen
atoms are omitted. Selected distances (Å) and angles (deg): B(1)-O(1)
1.370(4), B(1)-O(2) 1.390(4), B(1)-N(1) 1.418(4), B(2)-O(3) 1.348(4),
B(2)-O(4) 1.384(4), B(2)-N(1) 1.421(4); O(1)-B(1)-O(2) 112.5(3),
O(1)-B(1)-N(1) 126.7(3), O(2)-B(1)-N(1) 120.8(3), O(3)-B(2)-O(4)
112.7(2), O(3)-B(2)-N(1) 126.1(3), O(4)-B(2)-N(1) 121.3(3),
B(1)-N(1)-B(2) 132.9(3).
the Lewis acidic B(C6F5)3 to the monomeric complex 6
results in no change in the NMR spectra, again reflecting
the steric congestion about the nitrogen atom in 6.
Reactions of R(t-Bu)2PNH (R ) n-Bu, t-Bu) with HBpin
led to vigorous evolution of H2 and the formation of the
compounds of empirical formulas R(t-Bu)2PNBpin (R )
n-Bu (8), t-Bu (9)) (Scheme 2). These species were isolated
as colorless oils after a few minutes reaction time in toluene
or pentane solution, in yields of 61 and 94%, respectively.
Similarly, reaction of 1 equiv of i-Pr3PNH with 1 equiv of
HBpin in toluene at box temperature produced the bo-
rylphosphinimide of empirical formula i-Pr3PNBpin (10) as
the major product in 87% yield. The 1H, 31P{1H}, 13C{1H},
and 11B{1H} NMR spectra were consistent with the above
formulations. The 11B NMR shifts for 8-10 all range from
22.6 to 23.9 ppm typical of three-coordinate B centers
implying that all of these species are monomeric in
solution.40-44 This is not surprising given the lower Lewis
acidity of Bpin derivatives in comparison to Bcat analogues.
Interestingly, solutions of both 8 and 10 gave rise to
downfield 31P resonances upon addition of B(C6F5)3 presum-
ably as a result of the generation of (R3PNBpin)B(C6F5)3,
whereas the analogous addition to a solution of 9 results in
no apparent reaction. The absence of reaction in the latter
case is attributable to steric demands of the phosphinimide
ligand.
Figure 6. ORTEP drawing of 12 with 30% thermal ellipsoids. Hydrogen
atoms are omitted. Selected distances (Å) and angles (deg): N(1)-B(1)
1.406(4), N(1)-C(7) 1.411(3), O(1)-B(1) 1.378(3), O(2)-B(1) 1.376(3);
B(1)-N(1)-C(7) 126.8(2), O(2)-B(1)-O(1) 113.5(2), O(2)-B(1)-N(1)
124.3(2), O(1)-B(1)-N(1) 122.1(2).
Location and refinement of the proton on nitrogen revealed
that the geometry at N is planar, with a B-N-B angle of
132.9(3)°. This angle is similar to the corresponding angle
of 135.4(2)° observed for HN(BPh2)2.47
In the related reactions of R3PNPh or R3PNAd (R ) Et
and n-Bu) with HBpin, 31P{1H} NMR spectroscopy revealed
the liberation of free phosphine, consistent with reduction
of the phosphinimine precursor. Subsequent workup afforded
the white crystalline solid PhHN(Bpin) (12) and AdHN(Bpin)
(13) in yields of 81% and 83%, respectively. A crystal-
lographic study of 12 revealed a similar geometry with a
B-N bond length of 1.406(4) Å and an C-N-B angle of
126.8(2)° (Figure 6).
The formation of 11 from the reaction of phosphinimine
and HBpin was rapid; however, the species Et3PNSiMe3 was
shown to react with HBpin at a much slower rate, such that
the kinetics could be monitored. Reactions of excess HBpin
with varying concentrations of Et3PNSiMe3 were performed
and monitored as a function of time at 55 °C. Ph3P was used
as an internal integration standard. The formation of Et3P
and the consumption of Et3PNSiMe3 revealed that the rate-
determining step exhibits a first-order dependence on the
concentration of Et3PNSiMe3 (Figure 7a). Analogous experi-
ments using varying concentrations of HBpin and excess
Et3PNSiMe3 revealed that the reaction is first order in HBpin
(Figure 7b). The overall rate constant at 55 °C, k, was
determined to be 1.3(7) × 10-4 s-1. On the basis of these
data, a mechanism involving a 1:1 donor-acceptor interac-
tion of the phosphinimine and borane is proposed to prompt
Reaction of smaller phosphinimines R3PNH (R ) Et or
n-Bu) with HBpin also resulted in the liberation of H2.
However, monitoring these reactions by 31P{1H} NMR
spectroscopy revealed the generation of the corresponding
free phosphines. Subsequently, the boron-containing product
HN(Bpin)2 (11) was isolated as a white solid. 1H NMR data
revealed only the presence of the pinacolate protons, and IR
spectral data revealed an absorption at 3334 cm-1 consistent
with the presence of an N-H fragment. X-ray-quality crystals
of 11 were grown from pentane, and the X-ray data
confirmed the formulation as the diborylamine (Figure 5).
The B-N bond distances in 11 average 1.419(6) Å, which
is typical for B-N bond distances in diborylamines.45-48
(45) Al-Juaid, S. S.; Eaborn, C.; Hitchcock, P. B.; Kundu, K. K.; Molla,
M. E.; Smith, J. D. J. Organomet. Chem. 1990, 385, 13-21.
(46) Bartlett, R. A.; Chen, H.; Dias, H. V. R.; Olmstead, M. M.; Power, P.
P. J. Am. Chem. Soc. 1988, 110, 446-9.
(47) Maennig, D.; Noeth, H.; Prigge, H.; Rotsch, A. R.; Gopinathan, S.;
Wilson, J. W. J. Organomet. Chem. 1986, 310, 1-20.
(48) Paciorek, K. J. L.; Kratzer, R. H.; Kimble, P. F.; Nakahara, J. H.;
Wynne, K. J.; Day, C. S. Inorg. Chem. 1988, 27, 2432-6.
4306 Inorganic Chemistry, Vol. 44, No. 12, 2005

Metathesis/Reduction Reactions of Phosphinimines
Figure 8. Schematic gas-phase potential energy surface for the reactions
of H3PNH with HB(OCH2)2.
Figure 7. (a) Plot of log (rate)o versus log[Et3PNSiMe3]o. (b) Plot of
log(rate)o versus log[HBpin]o.
Scheme 3
Figure 9. Schematic gas-phase potential energy surface for the reaction
of t-Bu3PNH with HB(OCH2)2.
both pathways should be accessible for sterically less de-
manding phosphinimines at room temperature, this is not
observed experimentally. This observation suggests that the
differences in the activation energies are underestimated. This
could reflect a limitation of these gas-phase computations
and the need to consider solvation and temperature effects.
Nonetheless, the computational model data predict a cross-
over in preference from one pathway to the other with
changing the steric demands, an observation that is confirmed
experimentally.
The optimized geometries of the phosphinimine-borane
adducts (H3PNH)HBO2C2H4, (H3PNH)HBpin, (t-Bu3PNH)-
HBO2C2H4, and (t-Bu3PNH)HBpin shed some light on this
issue on the differing pathways. These computations pre-
dict that the adducts are pseudotetrahedral at B and trigonal
planar at N with B-N distances of 1.56, 1.60, 1.60, and 1.61
Å, respectively (Figure 10). Interestingly, the differences
among these models are the orientations of substituents about
the B-N bonds. In the case of (H3PNH)HBO2C2H4 and
(H3PNH)HBpin, the P-atom is oriented in close proximity
to one of the O-atoms of the diolate on boron (P···O: 2.13
and 2.25 Å) (Figure 10a). This proximity is evident in the
Newman projection (Figure 10b) down the B-N bond which
shows the almost eclipsed nature of the P on N and O on B.
This geometry, which provides a pseudo-OBNP four-mem-
bered ring, is reminiscent of the proposed phosphetidine
intermediate for phosphinimine metathesis.9 In the optimized
structures of (t-Bu3PNH)HBO2C2H4 and (t-Bu3PNH)HBpin,
these P‚‚‚O distances are increased to 3.47 and 3.53 Å, re-
the rate-determining reduction of the phosphinimine to
phosphine with the formation of the borylamine. This view
is supported by the isolation of 12 and 13. A rapid reaction
generating 11 follows (Scheme 3). Attempts to obtain
thermodynamic data for this reaction were thwarted by the
thermal decomposition of the HBpin above 55 °C and the
extremely slow reaction of Et3PNSiMe3 and HBpin at
temperatures below 55 °C.
The formation of 8-11 clearly demonstrates two reaction
pathways are accessible in the parent reaction of R3PNH and
HBpin. Computations at the B3LYP/6-311G(2df,p) level
on the two reaction pathways for t-Bu3PNH or H3PNH with
HB(OCH2)2 indicate that both H2 elimination and phosphin-
imine reduction are thermodynamically favorable. However,
for the model reactions involving t-Bu3PNH, the activation
barrier to H2 elimination is 11.8 kcal/mol (Figure 8) lower
than that of the reaction pathway involving phosphinimine
reduction. In contrast, for the reactions modeled with H3-
PNH the differences in the activation barriers are much
smaller with phosphinimine reduction being favored by 2.3
kcal/mol (Figure 9). While this small difference suggests that
Inorganic Chemistry, Vol. 44, No. 12, 2005 4307

Hawkeswood et al.
electrostatic P‚‚‚O attraction. The result is a staggered
position of the B- and N-bound H-atoms and an increased
separation. In this orientation, hydride transfer from B to N
is favored, resulting in the reduction of the phosphinimine.
Such hydride transfer for reactions of HNPR3 affords the
transient H2NBpin which reacts rapidly with a second 1 equiv
of HBpin to give 11 while the analogous reaction for RNPEt3
(R ) Ph, Ad) provides 12 and 13 directly.
Summary
Catecholborane-phosphinimide derivatives are easily
formed via reactions of chlorocatecholborane and silylphos-
phinimines. Steric factors determine whether the products
are monomers or dimers. Reactions of phosphinimines and
pinacolborane proceed via two pathways. In the case of
sterically demanding phosphinimines, pinacolborane-phos-
phinimide derivatives are obtained. In the case of phosphin-
imines with less demanding substituents on P, reduction to
free phosphine and the formation of borylamines occurs.
These differing pathways can be attributed to the role of steric
effects determining the orientation about the B-N bond in
the intermediate phosphinimine-borane adducts. The study
of the effects of steric factors on the reactivity of phosphin-
imines and their derivatives continues to be the subject of
investigation in our group.
Figure 10. Optimized structures for (a) (H3PNH)HBO2C2H4 and (b)
(t-Bu3PNH)HBO2C2H4. The corresponding Newman projections down the
B-N vector are shown in (c) and (d), respectively. Color scheme: O, red;
B, pink; N, blue; P, orange.
spectively (Figure 10c,d). Previous computational studies14-16
have noted the polarized nature of the P-N bond in
phosphinimines. Thus, the closer approach of P and O with
decreasing steric demands of the substituents on P in the
present systems suggests that the lowest energy orientation
about the B-N bond is determined by electrostatic attraction
between P(δ+) and the boron-bound oxygen atoms (δ-).
In the optimized geometry of (H3PNH)HBO2C2H4 and
(H3PNH)HBpin, the dihedral angles between the BH and
NH groups are 63.5 and 38.3°, respectively. This geometry
results in a BH‚‚‚HN separations of 2.90 and 2.70 Å. As the
steric bulk of the substituents is increased in the models,
(t-Bu3PNH)HBO2C2H4 and (t-Bu3PNH)HBpin, the corre-
sponding H-N-B-H dihedral angles decrease to 4.43 and
10.86°, respectively, while the H‚‚‚H separations decrease
to 2.20 and 2.13 Å, respectively. These data suggest that
sterically demanding substituents prompt closer proximity
of B- and N-bound H-atoms and thus favor H2 elimination
and formation of N-borylphosphinimines. In contrast, for
phosphinimines with small substituents, the preferred ori-
entation in the intermediate adducts is dictated by the
Acknowledgment. Financial support from the NSERC
of Canada (D.W.S., J.W.G.) and NOVA Chemicals Corp.
(D.W.S.) is gratefully acknowledged. S.H. is grateful for the
award of an Ontario Graduate Scholarship. We also thank
Professor Todd Marder of the University of Durham for his
insightful comments.
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org. CIF files are available on-line from the
Cambridge Crystallographic Data Centre (CCDC Nos. 235138-
235140 and 263395-263399).
IC050230K
4308 Inorganic Chemistry, Vol. 44, No. 12, 2005