η3-vinylborane complexes of platinum and nickel: Borataallyl- And alkyl/borataalkene-like coordination modes

Article abstract of DOI:10.1002/anie.200906931

(Figure Presented) Anyone for pi? The vinylborane PhHC= CH-B(C ₆F₅)₂ reacts with zero-valent Croup 10 transition-metal precursors to form η-vinylborane complexes. The platinum tri-tert-butylphosphine complex exhibits an η³-borataallyl-like coordination mode whereas the nickel bis(triphenylphosphine) complex tends towards alkyl/ borataalkene coordination.

Full text of DOI:10.1002/anie.200906931

Communications
DOI: 10.1002/anie.200906931
Vinylborane Complexes
h3-Vinylborane Complexes of Platinum and Nickel: Borataallyl- and
Alkyl/Borataalkene-Like Coordination Modes**
Kristopher B. Kolpin and David J. H. Emslie*
Complexes of carbon-based p ligands have occupied a central
position in d- and f-block element organometallic chemistry
since the remarkably early synthesis of Zeiseꢀs salt, K[PtCl3-
(C2H4)]·H2O, in 1827.[1] By contrast, p-ligand complexes
containing boron and carbon have only been developed in
the past 50 years. Formally monoanionic boratabenzene[2] and
dianionic borole ligands,[2] which are isoelectronic with arene
and cyclopentadienyl ligands (Figure 1), respectively, are now
prominent examples in this area.[3] However, boron analogues
of acyclic p ligands are rare. For example, mononuclear h2-
borataalkene complexes (Figure 1) are limited to the tanta-
lum complexes reported by Piers and co-workers, [Cp2TaLx-
{CH2B(C6F5)2}] (L = CNtBu and CO; x = 0 or 1; III in
Figure 2),[5] and h3-borataallyl complexes (Figure 1)[4] are
À
Figure 2. Selected complexes featuring M BR3 interactions. I and II
are h3-coordinated borane-bridged diylide complexes.[6] III is an h2-
borataalkene complex.[5] IV and V are h3-BCC-coordinated triarylborane
complexes.[8] VI is a metallocenylborane complex.[9] VII and VIII are
platinum–h1-borane complexes.[12–14]
limited to the palladium and zirconium complexes, [PdCl2{h3-
PhB(CHPPh3)2}] and [ZrCl4{h3-PhB(CHPPh3)2}] (I and II,
Figure 2), that were reported by the Shapiro group.[6] The
PhB(CHPPh3)2 ligand may be viewed as a borane-bridged
diylide or a zwitterionic 1,3-di-phosphonium-substituted 2-
borataallyl ligand.
Figure 1. Structural relationships between common hydrocarbon p li-
gands and their boron analogues. The inset shows four possible
bonding descriptions for an h3-coordinated vinylborane; the formal
charges on the ligand are shown in square brackets.[4]
The formation of complexes of h3-coordinated
R2CCRBR2 ligands or simple hydrocarbyl-substituted
R2CBRCR2 ligands is of particular interest given the gen-
erally enhanced reactivity of acyclic p ligands relative to
cyclic p ligands (e.g. allylÀ versus C5H5 ). Metal–allyl com-
À
[*] K. B. Kolpin, Prof. D. J. H. Emslie
plexes may be prepared from the reaction of a suitable metal
precursor with an allyl anion or cation source. Free vinyl-
boranes are conceptually analogous to an allyl cation; herein,
Department of Chemistry and Chemical Biology
McMaster University, 1280 Main Street West
Hamilton, Ontario, L8S 4M1 (Canada)
Fax: (+1)905-522-2509
E-mail: emslied@mcmaster.ca
Homepage: http://www.chemistry.mcmaster.ca/faculty/emslie/
=
À
we describe the reactions of (E)-PhHC CH B(C6F5)2
(VBPh)[7] with low-valent platinum and nickel precursors to
yield h3-BCC-coordinated vinylborane complexes. Several
possible bonding descriptions for an h3-vinylborane ligand are
[**] D.J.H.E. thanks the NSERC of Canada for a Discovery Grant, the
Ontario Ministry of Resarch and Innovation for an Early Researcher
Award, and the CFI and OIT for New Opportunities Grants. We are
also very grateful to I. Vargas-Baca for help with DFT calculations,
and J. F. Britten and H. A. Jenkins for help with X-ray crystallography.
shown in Figure 1.
Ph [7]
=
À
The reaction of (E)-PhHC CH B(C6F5)2 (VB ) with
[Pt(nb)3] (nb = norbornylene), followed by the addition of
PtBu3 afforded dark red [(tBu3P)Pt{h3-BCC-VBPh}] (1) in
68% yield (Scheme 1). Crystals of 1 suitable for X-ray
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906931.
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Chemie
is strongly pyramidalized in tightly bound h1-borane com-
À À
plexes, it is only modestly pyramidalized in 1 [S(C B C) =
353 and 3578].
The h3-coordination mode in 1 contrasts the h2-alkene
coordination mode found in the previously reported
[(CO)4Fe{H2C CH BR(NMe2)}] (R = Br, Me)[16] and
=
À
=
À
[Cp2Ti(ArH CH BCat)] (Ar= Ph and 4-MeOC6H4; Cat =
O2C6H4 or 4-tBuC6H3O2),[17] presumably owing to the much
higher borane Lewis acidity in 1. The related complexes
[(CO)3Fe{H2C CH B(R)NMe2}] (R = Br, Me, or tBu)[16] and
=
À
[18]
=
À
À
[(CO)4Cr{tBuHC CH BH N(SiMe3)2}]
are also known,
but in both cases the vinylborane is h4 coordinated, and acts as
a four-electron donor; the former is a boron- and nitrogen-
containing analogue of butadiene, whilst the latter binds
through alkene and s-borane interactions.
Scheme 1. Synthesis of VBPh,[7] [(tBu3P)Pt(h3-BCC-VBPh)] (1),
[(Ph3P)2Pt(h3-BCC-VBPh)] (2) and [(Ph3P)2Ni(h3-BCC-VBPh)] (3).
cod=1,5-cyclooctadiene.
For molecules A and B in the unit cell of 1, all exo
À À
substituents on the B C C core [C(3) and H(2)] are bent
away from the metal, whilst the endo [C(9) and C(15)] and
central [H(1)] substituents bend towards the metal.[23] Similar
distortions were observed in [PdCl2{h3-PhB(CHPPh3)2}],[6]
and are typical of late-transition-metal–allyl complexes.[19]
These results highlight striking similarities between the
VBPh ligand in 1 and p-allyl ligands.
The 11B NMR spectrum shows a single broad signal at d =
15.6 ppm; far upfield of the corresponding signal for the free
VBPh (d = 58.3 ppm),[7] and significantly upfield of I, IV (Pd
and Ni), and VIII (d = 35–18 ppm). Furthermore, 195Pt
satellites were observed for the BCHCHPh atoms
2
(1J 195Pt = 196 Hz; J1H;195Pt = 47 and 80 Hz) in the 1H and
13C;
13C NMR spectra of 1. These data are consistent with the tight
h3-BCC coordination of VBPh.
Figure 3. X-ray structure of 1 with thermal ellipsoids at 50% proba-
bility. Only molecule A (one of two independent molecules in the unit
cell) is shown. Most hydrogen atoms are removed for clarity.
To examine the generality of the h3-BCC bonding mode
À
À
for vinylborane ligands, and the extent to which M B and M
C bond distances are sensitive to changes in the central metal
and the phosphine coligands, [(Ph3P)2Pt{h3-BCC-VBPh}] (2)
and [(Ph3P)2Ni{h3-BCC-VBPh}] (3), were prepared as shown
analysis (Figure 3), containing two independent molecules (A
and B) in the unit cell, were obtained by recrystallization from
hexane at À308C.[10] In both A and B, the VBPh ligand is h3-
BCC coordinated, with PtBu3 positioned roughly trans to the
1
2
in Scheme 1.[24] Significant J
(171 Hz), J1H;
(40 and
195Pt
13C;195Pt
49 Hz), and 2J
(30 and 16 Hz) couplings for the
13C;13P
À
BCHCHPh atoms are indicative of strong metal carbon
interactions in 2 and 3. However, the 11B NMR signals for 2
and 3 (d = 25 and 34 ppm, respectively) were shifted to a
higher frequency than that for complex 1. 31P NMR spectros-
copy showed that the VBPh ligand in complexes 2 and 3
exhibited hindered rotation around the metal–ligand axis.
Substantial barriers to acyclic p-ligand rotation are also
encountered in 16-electron allyl complexes, such as
[(Ph3P)ClPt(h3-C3H5)],[20] and in a broad range of late-
transition-metal–alkene complexes.[21]
VBPh ligand centroid. The B C(1) distances of 1.519(7) and
À
1.517(6) ꢁ in 1 are shorter than those in free vinylboranes,
=
=
such as (Mes)2BCH CHPh (Mes = mesityl), [(F5C6)2B]2C
=
CHPh, and B[C(SiMe3) CHMe]3 [1.54–1.57 ꢁ], whilst the
À
C(1) (2) distances of 1.383(6) and 1.403(6) ꢁ are slightly
longer (cf. 1.33–1.365 ꢁ in the vinylboranes above).[11]
À
À
In molecule A, the Pt B distance is 2.273(5) ꢁ, Pt C(1) is
À
2.130(4) ꢁ, and Pt C(2) is 2.155(4) ꢁ; these distances are
indicative of strong h3 binding. In molecule B, the C6F5 rings
lie at different angles relative to the BCC core, leading to
Crystals of 3 suitable for X-ray analysis were obtained
from a benzene/hexane mixture at À308C (Figure 4).[10] Key
differences in the structure of 3 relative to 1 are: 1) a longer
À
À
À
slightly different Pt B, Pt C(1), and Pt C(2) distances of
À
2.319(5), 2.126(4), and 2.137(4) ꢁ, respectively. Similar M B
À
À
À
À
and M C distances (M = Pd and Ni) were observed for I and
M B distance of 2.660(3) ꢁ; 2) shorter M C(1) and M C(2)
distances of 2.025(2) and 2.032(2) ꢁ, respectively; 3) a shorter
À
IV (Figure 2), and the M C distances in 1 are comparable
with those in [Pd(h3-C3H4Me)2],[12] [(tBu3P)PtCl(h3-C3H5)][13]
B(1) C(1) bond of 1.483(4) ꢁ (see below); 4) a somewhat
À
and [(dmpe)Pt(h3-1,3-Ph2C3H3)]PF6 (2.13–2.23 ꢁ; dmpe =
longer C(1) C(2) distance of 1.431(3) ꢁ; 5) less pronounced
[14]
À
À
1,2-bis(dimethylphosphino)ethane). The M B distance in 1 is
distortions of the exo H, exo C6F5, endo C6F5, and the
central H substituents on the h3-coordinated core of 3; and
6) distortion of the endo Ph substituent away from the metal,
rather than towards it in 1.[22]
also comparable with those in h1-borane complexes, such as
VIII [2.224(4) ꢁ],[15] and is far shorter than those in metal-
locenylboranes (e.g. VI; Figure 2).[9] However, whereas boron
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2717

Communications
Figure 4. X-ray structure of 3 with thermal ellipsoids at 50% proba-
bility. Most hydrogen atoms are removed for clarity.
À
Returning to point (3) above, the B C distance in 3 is in
À
=
= =
fact more similar to that found in free R2B CR2 or R2B C
2À
BR2 anions (1.39–1.51 ꢁ)[23] than in vinylboranes (see
below), and is also shorter than those in the tantalum
borataalkene complexes reported by Piers and co-workers
(1.51–1.53 ꢁ).[5] These data, and the distortion away from the
metal of both substituents on the C(2) atom, are indicative of
a
more alkyl/borataalkene-like coordination (see inset,
Figure 1). A range of h3-coordination modes have also been
À
reported for azaallyl complexes, with more single C N and
[24]
=
double C C bond character in some cases.
DFT calculations (ADF 2008.01, TZ2P, VWN, PW91,
Figure 5. Key MOs involved in metal–VBPh bonding in 1 and 3. Insets
show SCF deformation density (SCFDD) isosurfaces (SCF electron
density for the molecule minus the sum of the SCF electron density for
the two fragments; blue is increased and pink is depleted electron
density).
ZORA) were carried out to further probe the nature of the
Ph
=
bonding in H2C CH-BH2, VB , 1, and 3. All of the geo-
metries were fully optimized, and those of 1 and 3 matched
À
the solid-state structures, including a shortening of the B
C(1) bond in 3 by 0.035 ꢁ relative to that in 1. However,
À
metal boron distances were over-estimated by 3% in 1 and
9% in 3, although this is not expected to significantly alter the
overall analysis. The bonding in 1 and 3 was then investigated
by using a fragment approach that considered the interaction
of an uncharged {(tBu3P)Pt} or {(Ph3P)2Ni} fragment with the
neutral VBPh ligand (all fragments were generated from the
geometry optimized structures of 1 and 3).
as shown by a value of À0.255 for the VBPh fragment obtained
from Hirshfeld charge analysis. Regions of depleted or
increased electron density, which arise from a combination
of the (tBu3P)Pt and VBPh fragments, are also illustrated in
the SCF deformation density isosurface in Figure 5.
The VBPh ligand in 3 also functions as both a donor and
acceptor (Figure 5). However, the frontier metal orbitals of
(Ph3P)2Ni are higher in energy than those of (tBu3P)Pt,[25]
leading to a value of À0.404 for the VBPh fragment from the
Hirshfeld charge analysis. Large contributions from the
LUMO of the VBPh ligand to both the HOMO-3 and
HOMO-4 of 3 are also observed, and more effective back-
donation into the VBPh LUMO is consistent with the shorter
=
In free H2C CH-BH2, the HOMO is fully bonding across
the BCC unit, and is analogous to the HOMO of an allyl
cation. The LUMO + 1 is also analogous to that of an allyl
cation, with nodes between B and C(1), and C(1) and C(2).
However, the vinylborane LUMO is bonding in character
between B and C(1), and antibonding in character between
C(1) and C(2) owing to the lower electronegativity of boron
relative to carbon. Analogous MOs were observed for VBPh
(free and in 1 and 3), although in this case several lower lying
orbitals also exhibit p-bonding character within the BCC
core.
À
B C(1) distance observed experimentally and by DFT for 3.
In summary, the first h3-coordinated vinylborane com-
plexes have been prepared, and adopt unique 1-borataallyl-
or alkyl/borataalkene-like coordination modes. Future studies
will explore the reactivity of these complexes with a focus on
the availability of boron to interact with external substrates.
In complex 1 (Figure 5), the LUMO receives major
contributions from the VBPh LUMO and both filled and
empty (tBu3P)Pt orbitals that have Pt 5d or 6p character.
Similarly, the HOMO-4 in 1 involves the VBPh HOMO and
both filled and empty (tBu3P)Pt orbitals that have Pt 5d or 6s/
6p character. The HOMO-7, HOMO-8, and HOMO-10 also
Received: December 9, 2009
Revised: January 20, 2010
Published online: March 15, 2010
play an important role in Pt VBPh bonding and are shown in
À
Figure 5. Overall, the VBPh ligand in 1 functions as both a
Keywords: boranes · isoelectronic analogues · Lewis acids ·
.
donor and an acceptor, with dominating acceptor interactions,
p interactions · transition metals
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Angewandte
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coordinated R2CCRBR2 ligand, regardless of hapticity. Given
the alkene and metallacyclopropane extremes for alkene bond-
ing, a charge ranging from 0 to À2 can also be assigned upon
coordination. However, the VBPh coordination modes in 1 and 3
are described as 1-borataallyl- and alkyl/borataalkene-like to
highlight their relationship to alkene, alkyl, and p-allyl bonding.
Specifically, the term “borataallyl” is used to refer to a C2BR5
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[22] For molecule A in the unit cell of 1, the exo H, exo C6F5,
endo Ph, endo C6F5 and central H substituents are distorted from
the BCC plane by 24.18, 43.78, À11.68, À16.18 and À20.88,
respectively. For molecule B, these same distortions are 25.68,
31.98, À10.58, À15.08 and À13.48, and for 3 they are 9.08, 17.28,
11.28, À5.28 and À1.08.
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0.15 mm3, triclinic, space group P1; a = 10.4467(7), b =
ꢀ
16.4391(11),
c = 19.1813(14) ꢁ,
a = 99.1070(10),
b =
=
=
97.1550(10), g = 101.7510(10), V= 3142.2(4) ꢁ3, Z = 4, 1calcd
1.787 MgmÀ3, m(MoKa) = 0.71073 mmÀ1, T= 100(2) K, 2qmax
578, 42232 reflections, 15473 indep. (Rint = 0.0530), R1 = 0.0350
and wR2 = 0.0641 [I > 2s(I)], max. res. density peaks: 1.13 to
À1.28 eꢁÀ3. Crystal data for 3: C56H37BF10P2Ni, Mr = 1031.34,
3
ꢀ
0.58 ꢂ 0.32 ꢂ 0.30 mm , triclinic, space group P1; a = 10.1954(9),
b = 13.3921(11),
c = 17.7377(15) ꢁ,
a = 96.319(2),
b =
103.476(2), g = 98.237(2), V= 2304.9(3) ꢁ3, Z = 2, 1calcd
=
=
1.486 MgmÀ3, m(MoKa) = 0.71073 mmÀ1, T= 100(2) K, 2qmax
538, 22713 reflections, 9340 indep. (Rint = 0.0301), R1 = 0.0351
and wR2 = 0.0786 [I > 2s(I)], max. res. density peaks: 0.560 to
À0.356 eꢁÀ3. Structures refined with Patterson methods. Hydro-
gen atoms in VBPh and PPh3 were located and refined isotropi-
cally with thermal parameters set to 1.2 ꢂ Cattached. Hydrogen
atoms in PtBu3 were placed at calculated positions on Cattached but
with methyl group rotation refined to match highest points of
electron density. CCDC 757289 (1) and CCDC 757290 (3)
contain the supplementary crystallographic data for this paper.
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