Rhodium silyl boryl hydride complexes: Comparison of bonding and the rates of elimination of borane, silane, and dihydrogen

Article abstract of DOI:10.1002/anie.200460430

Spoilt for choice: a silane, dihydrogen, borane, or hydridoborate complex? The silyl boryl hydride complex [Cp*Rh(H₂) (Et ₃Si)(Bpin)] (Bpin = (pinacolato) boryl) could adopt any of these structures, but it appears to contain stronger B-H bonding than Si-H or H-H bonding and undergoes elimination of pinacolborane faster than of silane or hydrogen (see scheme). The bonding situation is studied by experimental and theoretical methods.

Full text of DOI:10.1002/anie.200460430

Communications
Borane Complexes
Rhodium Silyl Boryl Hydride Complexes:
Comparison of Bonding and the Rates of
Elimination of Borane, Silane, and Dihydrogen**
Kevin S. Cook, Christopher D. Incarvito,
Charles Edwin Webster, Yubo Fan, Michael B. Hall,*
and John F. Hartwig*
Transition-metal “s complexes”[1] of dihydrogen[2] and of
silanes[3] are common, and transition-metal s-borane com-
plexes are now known.[4–11] Dihydrogen complexes are
intermediates in the oxidative addition that occurs during
hydrogenation, and they are intermediates of the reductive
elimination that occurs during dehydrogenation.Similarly,
silane complexes are probably intermediates in the oxidative
additions of silanes that occur during hydrosilylation,[12] and
borane complexes have been shown to be intermediates in
catalytic hydroboration.[5] Borane complexes are also likely
intermediates[13] in the borylation of alkanes[14,15] and
arenes.[15–19]
A complex with hydride, silyl, and boryl groups bound to
the metal center of the same complex could adopt a structure
with a s-dihydrogen, -silane, or -borane ligand.No complexes
of this type have been isolated in pure form, but compounds
with this collection of ligands would allow direct assessment
of the similarities and differences between the three types of
s complexes.Herein, we report the synthesis, experimental
and calculated structures, as well as the reaction chemistry of
[Cp*RhH2(Bpin)(SiR3)] (Cp* = C5Me5, Bpin = (pinacolato)-
boryl) complexes, which contain two hydrides, one silyl and
one boryl group.Our data suggests that this complex contains
À
partial B H bonding and that borane eliminates from this
complex faster than dihydrogen or silane.
Reaction of the known bis(silyl) dihydride 1a[20] with a
large excess of HBpin in refluxing cyclohexane formed
[Cp*RhH2(SiEt3)(Bpin)] (2a) in 90% yield, as determined
by NMR spectroscopy with an internal standard [Eq.(1)].In
addition to a single pinacol methyl resonance at d = 1.09 ppm
[*] Dr. K. S. Cook, Dr. C. D. Incarvito, Prof. J. F. Hartwig
Department of Chemistry
Yale University
P.O. Box 208107, New Haven, Connecticut 06520-8107 (USA)
Fax: (+1)203-432-3917
E-mail: John.Hartiwg@yale.edu
Dr. C. E. Webster, Dr. Y. Fan, Prof. M. B. Hall
Department of Chemistry
Texas A&M University
P.O. Box 30012, College Station, TX 77842-3012 (USA)
Fax: (+1)979-845-2971
E-mail: mbhall@tamu.edu
[**] The NSF (Grant Nos. CHE 03-01907 and MRI 02-16275) and the
Welch Foundation (Grant No. A-648) are gratefullyacknowledged
for financial support. The NSERC is acknowledged for a post-
doctoral fellowship to K.S.C.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460430
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Angewandte
Chemie
and ethyl resonances from the silyl group, the 1H NMR
spectrum of 2a contained a doublet at d = À12.7 ppm
(1JRh-H = 42.0 Hz) for the hydride ligands. The 11B NMR
spectrum contained a resonance at d = 40 ppm, which con-
firmed the presence of a boryl ligand.Though 2a formed in
high yield by NMR spectroscopy, we were unable to isolate
crystalline material from these reactions.
However, irradiation of the bis(silyl) complex 1a and
HBpin in cyclohexane for 18 h allowed the isolation of
crystalline silyl boryl dihydride 2a in 61% yield.Reaction of
[Cp*RhH2(SiMe2Ph)2] (1b)[21] with excess HBpin formed
trans-[Cp*RhH2(SiMe2Ph)(Bpin)] (2b) under similar photo-
chemical conditions in 78% yield [Eq.(1)].Longer reaction
times did not cause replacement of the second silane of 2a or
2b with borane to form [Cp*Rh(H)2(Bpin)2] (3).
2.3681(15) Š is similar to the 2.379(2) Š Rh Si separation in
the bis(silyl) complex 1a.[20]
À
The hydride positions suggested a structure with more
À
À
À
residual B H bonding than H H or Si H bonding, despite
the caveats of defining hydride positions by X-ray diffraction.
An ideal four-legged piano-stool structure would contain a
mirror plane that is defined by the Cp* centroid, rhodium,
silicon, and boron atoms.In such a piano-stool geometry, the
two hydride units would lie in or near a vertical plane that
would be orthogonal to this mirror plane.Although one of the
hydrides in 2a lies near this plane, the second hydride lies out
of this plane on the side of boron atom.Thus, the H(1)-Rh(1)-
B(1) angle of 55.7(16)8 is measurably smaller than the H(2)-
Rh(1)-B(1) angle of 67.9(14)8 or the two H-Rh(1)-Si(1)
angles (66.58 and 70.38).The symmetrical starting complex 1a
contains H-Rh-Si angles of 658 and 698.[20] The boron–hydride
À
separation of the shorter B H bond of 1.74(4) Š is 0.3 Š
shorter than the longer boron–hydride bond.This distance is
À
much longer than the B H bonds of 1.30–1.36 Š in Mn, Re,
and Ti s-borane complexes.[7,10] However, it is shorter than
À
the B H bond in trans-[Rh(Cl)(H)(Bpin)(PiPr3)2].This
À
complex has a long B H separation of 2.013(5) Š, as
determined by neutron diffraction, but an acute H-Rh-B
angle that seems to result from B H bonding.
Considering the difficulty in defining the position of
hydrides and the series of different binding modes available to
[19,23]
À
Reaction of B2pin2 with 1 formed a single major product,
but the conversion of 1 was incomplete before this major
species underwent further reaction.We previously formu-
lated the major species as the bisboryl 3, based on the
presence in the NMR spectra of a broad resonance signal for
hydride groups and new resonances for pinacol methyl
groups.[15] With 2a now in hand, the spectral features of the
major product generated from 1 and B2pin2 clearly result from
2a.
À
2a and 2b, we sought to provide further evidence for the B H
interaction by computing the ground-state structure of 2a by
DFT-B3LYP methods.[24,25] An overlay of the calculated and
experimental structure of 2a is provided in Figure 2.The
The structure of 2a was confirmed by X-ray crystallog-
raphy.[22] All hydride units were located in the electron
difference map and were freely refined isotropically.Figure 1
À
shows a view of 2a along the Cp* Rh axis.The boryl and silyl
ligands are located in mutually trans positions of the four-
À
legged piano stool structure of 2a.The Rh Si bond length of
Figure 2. Overlayof the experimental and calculated (DFT-B3LYP)
structure of [Cp*RhH2(SiEt3)(Bpin)]. The experimental structure is
given in red and the computed structure is given in blue.
separations and angles in the calculated structure are
relatively close to those of the experimental structure.
Although the unsymmetrical property of the computed
structure is less pronounced than that of the experimental
structure, both structures display geometries with separations
À
appropriate for partial B H bonding, one hydride that is
closer to the boron atom than the other hydride, and one B-
Rh-H angle that is smaller than the other B-Rh-H angle or the
Si-Rh-H angles.As a result, even the computed structure in
the gas phase lacks a mirror plane containing the Cp*
centroid, rhodium, silicon, and boron.
The solution H NMR spectroscopic data of complexes
2a,b show a small, direct scalar coupling between the hydride
units and the boron center that further indicates the presence
Figure 1. Molecular structure of 2a (thermal ellipsoids set at 30%
probability). Selected bond lengths [Š] and angles [8]: Rh(1)-H(1)
1.57(5), Rh(1)-H(2) 1.59(4), Rh(1)-Si(1) 2.3681(15), Rh(1)-B(1)
2.038(5), Rh(1)-Cp*cent 1.919(4); Si(1)-Rh(1)-H(1) 70.3(15), H(1)-
Rh(1)-B(1) 55.7(16), B(1)-Rh(1)-H(2) 67.9(14), H(2)-Rh(1)-Si(1)
66.5(15).
1
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Communications
À
of B H bonding.The single resonance signal for the two
occurred in lower yields.Heating 2a and 2b in pentane at
1258C in a sealed vessel formed the pentylboronate ester in
30% and 19% yield, as determined by GC (Scheme 1).
Arylsilane or pentylsilane were not detected in these reaction
mixtures.Several rhodium products are formed from these
thermolysis reactions, and these materials have not yet been
fully characterized.Because borane dissociates from 2a and
2b more readily than H2 or silane, and the borylation of
arenes and alkanes typically occurs through metal boryl
complexes,[16,27,30,31] the mechanism of the reactions of 2a and
2b with hydrocarbons is likely to be complex.
hydride units in 2a and 2b is slightly broadened (2a, w1/2
=
11.7 Hz; 2b, w1/2 = 12.1 Hz) and sharpens upon 11B decoupling
(2a, w1/2 = 8.2 Hz; 2b, w1/2 = 8.2 Hz). 1H NMR spectra of
compounds containing discrete, cis-disposed boryl and hy-
dride ligands contain sharp hydride signals that remain
unchanged upon 11B decoupling.[17,26–28] Our NMR spectro-
À
scopic data imply that the B H interactions in 2a,b remain
intact in solution, though the two hydride positions are
averaged on the NMR time scale and the small magnitude of
the scalar coupling suggests a weak interaction.
The calculated potential energy surface (PES) for short-
In conclusion, two complexes that could possess a
dihydrogen, silane, or borane ligand have been prepared.
These complexes preferentially adopt geometries with s-
borane complexes and undergo dissociation of borane more
rapidly than dissociation of silane or dihydrogen.This
preference for dissociation of borane from 2a and 2b to
generate a silyl hydride intermediate instead of dissociation of
silane to generate a boryl hydride intermediate explains in
part why the silyl complexes are less effective than
[Cp*Rh(h6-C6Me6)] as catalyst precursors for the borylation
of hydrocarbons.
À1
À
ening of the B H bonding in 2a is nearly flat (< 0.5 kcalmol
À
for the decrease in B H separation from 1.970 Š to 1.670 Š),
À
but the PES is steeper for shortening the H Si separation.
These results suggest that dissociation of borane should occur
in preference to dissociation of silane.Further, the trans
orientation of the hydride units should make dissociation of
H2 unfavorable.
Consistent with this prediction, heating of 2a and 2b at
1008C with 1 equivalent of P(p-tol)3 gave as the major
products the silyl complexes [Cp*Rh(H)(SiR1 R2){P(p-
2
tol)3]}] (4a, R1 = R2 = Et; 4b, R1 = Me, R2 = Ph; Scheme 1)
Received: April 23, 2004
Revised: August 11, 2004
Keywords: boron · densityfunctional calculations · hydride
.
ligands · rhodium · Si ligands
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