Are J(Si-H) NMR coupling constants really a probe for the existence of nonclassical H-Si interactions?

Article abstract of DOI:10.1021/ja027935v

A series of hydridosilyl complexes of tantalum, Cp(ArN)Ta(PMe₃)(H)(SiCl_nR₃-_n) (n = 0-3), was prepared and studied by ²⁹Si NMR, X-ray diffraction, and DFT calculations. An unprecedented increase of the J(Si-H) coupling constant between the hydride and silyl ligands from 14 Hz for n = 0 to 50 Hz n = 3 was observed, which however, according to DFT calculations, does not correspond to stronger bonding interaction between silicon and hydride ligands, with the strongest interaction being for n = 1. Copyright

Full text of DOI:10.1021/ja027935v

Published on Web 12/20/2002
Are J(Si-H) NMR Coupling Constants Really a Probe for the Existence of
Nonclassical H-Si Interactions?
Stuart R. Dubberley,† Stanislav K. Ignatov,‡ Nicholas H. Rees,† Alexei G. Razuvaev,‡
Philip Mountford,*,† and Georgii I. Nikonov*,§
Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK, UniVersity of Nizhnii NoVgorod, 23,
Gagarin AVenue, Nizhnii NoVgorod 603600, Russia, and Department of Chemistry, Moscow State UniVersity,
Vorob’eVy Gory, 119899 Moscow, Russia
Received July 30, 2002 ; E-mail: nikonov@org.chem.msu.su
Recently, a great deal of progress has been made in studying
complexes with nonclassical interligand interactions.1 In the field
of silane Si-H bond σ-complexes LnM(η2-H-SiR3) the most
common means for probing the H-Si bonding is by measurement
1
of J(Si-H) coupling constants, with magnitudes higher than 20
Hz being attributed to the presence of a direct H-Si interaction.1c,d
Usually electron-donating groups at the silicon atom effect higher
1J(Si-H) values and, by implication, stronger Si-H interactions,
whereas electron-withdrawing groups render a deeper degree of
Si-H bond oxidative addition and hence a comparatively dimin-
1
ished J(Si-H).1a-d
Here we report 29Si NMR and DFT studies of a series of
silylhydrido complexes of Ta, having nonclassical M-H‚‚‚Si-X
interligand hypervalent interactions (IHIs).2,3 For the first time we
have observed a correlation between the magnitude of J(Si-H)
values and the identity of the substituent X which is opposite to
Figure 1. Molecular structure of 3. Displacement ellipsoids (non-H atoms)
are shown at 50% probability level. Hydrogens bonded to carbon atoms
that one found for silane σ-complexes. Moreover, DFT calculations
of model systems showed that an increase in J(Si-H) does not
necessarily correspond to the strengthening of the Si-H interaction.
Complexes of the type Cp(ArN)Ta(PMe3)(H)(SiR3) (1-4) (Ar
) 2,6-C6H3iPr2; SiR3 ) SiMePhH (1), SiMe2Cl (2), SiMeCl2 (3),
SiCl3 (4)) were prepared in 50-70% isolated yields by the oxidative
addition of silanes HSiR3 to Cp(ArN)Ta(PMe3)2 (eq 1) and
characterized by NMR and IR spectra and by X-ray diffraction
studies of 23 and 3.4
are omitted for clarity.
the bond to the “out-of-plane” chlorine (Si-Cl(1) ) 2.064(3) Å).
The Si-Cl bonds in transition metal silyls are normally elongated
relative to the parent silanes due to the operation of Bent’s rule.2a,5
Since in 3 this factor contributes equally to both the Si-Cl(1) and
Si-Cl(2) bond lengths, the relative lengthening of the latter by
0.053(4) Å manifests the presence of IHI. This is the first time
that a direct comparison of two types of different Si-Cl bonds,
one interacting with the hydride and the other noninteracting, is
possible at the same silicon center. It is also noteworthy that there
is no experimental difference in the Ta-Si bond lengths in 2 (2.574-
(1) Å) and 3 (2.569(2) Å; ∆ ) 0.005(2) Å) despite the presence of
two electron-withdrawing Cl substituents on Si in the latter. This
suggests that contribution of IHI to any expected shortening of the
Ta-Si bond2 in 3 is much less than that in 2. This conclusion is
further supported by the DFT study.
The 29Si NMR spectra of 1-4 show a remarkable trend in that
the magnitude of the J(H-Si) coupling constant sequentially
increases from 14 Hz in 1 to 33 Hz in 2 and 40 Hz in 3 to 50 Hz
in 4 as the number of chlorine groups on silicon increases. This
trend is opposite to that observed for normal Si-H bond
σ-complexes1c and could be at first glance considered as evidence
for strengthening of the Si-H interaction on going from 1 to 4.
However, how the strength of the Si-H IHI should change upon
increasing chlorination of the SiR3 groups is not a priori clear
because the electronic origin of this type of nonclassical bonding
is different from the more common σ-bond complexation.2a On one
hand, increasing n in SiR3-nCln would be expected to shorten the
Ta-Si bond due to an increase in Bent’s rule effects and thus bring
the silicon and hydride atoms into closer contact. On the other hand,
The molecular structure of 3 (Figure 1) strongly resembles the
previously reported structure of 23, with the chlorine Cl(2), and
hydride atoms lying trans to each other (mean deviation from the
TaSiClH plane ) 0.03 Å). In complexes with IHI such an
orientation allows for the overlap of the M-H bond orbital and
the (Si-Cl)* antibonding orbital which leads to the electron density
transfer from the former to the latter and elongation of the Si-Cl
bond.2 Importantly, the “in-plane” chlorine forms a Si-Cl(2) bond
of 2.117 (2) Å which is significantly longer (∆ ) 0.053(4) Å) than
† Oxford.
‡ University of Nizhnii Novgorod.
§ Moscow State University.
9
642
J. AM. CHEM. SOC. 2003, 125, 642-643
10.1021/ja027935v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Selected Results of DFT Calculations for
Cp(MeN)Ta(PMe3)(H)(SiH3-nCln) (n ) 0-3) (Bond Lengths in Å)
contraction of the Ta-Si bond due to IHI decreases, whereas
contraction due to Bent’s rule effect increases, and the two opposite
trends compensate each other.
SiXR
2
SiClH (6)a
SiClHCl(7)b
SiCl3 (8)
1.810
Therefore, the increase in the magnitude of J(H-Si) upon going
from 2 to 4 is not paralleled by a strengthening of the interligand
interaction. Other factors, such as possible increase of the through
two-bond Si-Ta-H magnetic interactions due to increasing silicon
3s character in the Ta-Si bond, should be considered to account
for this behavior.
In conclusion, we have observed for the first time9 a reverse
correlation between the number of electron-withdrawing substituents
at silicon and the silicon hydride coupling constants in transition
metal silyl hydride complexes and established that, contrarily to
expectation, an increase in the magnitude of J(H-Si) does not
necessarily correspond to stronger bonding interaction between
these two ligands.
Bond
SiH (5)
3
2
Ta-H
Ta-Si
Ta-P
1.810
2.606
2.531
2.288
1.828
1.819 [1.95(7)]
2.556 [2.574(1)] 2.545 [2.569(2)] 2.547
2.540 [2.550(1)] 2.538 [2.522(2)] 2.538
2.062
2.236 [2.177(2)] 2.208 [2.117(2)] 2.189
2.184 [2.064(3)] 2.174, 2.167
Si-Hc
Si-Cld
Si-Cle
2.115 [2.15(7)]
2.174
a Experimental values for 2 in brackets, the hydride atom was located
from the difference map but not refined. b Experimental values for 3 in
brackets. c Ta-bound hydride. d In-plane chlorine trans to hydride. e Out-
of-plane chlorine(s)
Table 2. Changes in Atom-Weighted Bond Orders (BO) and
Wiberg Indices (WI) in a Series Cp(MeN)Ta(PMe3)(H)(SiH3-nCln)
(n ) 0-3)
Acknowledgment. This work was supported by Royal Society
(London) through a joint research grant to G.I.N. and P.M.. Also,
G.I.N. is grateful to INTAS for a YS INTAS fellowship. S.K.I.
and A.G.R. thank RFBR for financial support (project 00-03-32094).
SiXR
2
bond
SiH (5)
SiClH (6)
SiClHCl(7)
SiCl3 (8)
3
2
Ta-Ha
BO
WI
BO
WI
BO
WI
BO
WI
BO
WI
BO
WI
0.4942
0.6376
0.6044
0.7581
0.1872
0.1350
0.7393
0.9135
0.7516c
0.9365 c
0.7512c
0.9410 c
0.4668
0.5881
0.6467
0.7684
0.2792
0.2095
0.5621
0.7090
0.7487c
0.9083 c
0.7480c
0.9119 c
0.4723
0.6028
0.6500
0.7547
0.2622
0.1831
0.6073
0.7495
0.6272d
0.7874d
0.7427c
0.8812 c
0.4798
0.6168
0.6690
0.7396
0.2458
0.1608
0.6500
0.7830
0.6668d
0.8115 d
0.6613d
0.8077 d
Supporting Information Available: Experimental details and
characterization data (PDF). X-ray crystallographic file in CIF format.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Ta-Si
Si-Ha
Si-Xb
Si-R1
Si-R2
References
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however, a larger n causes greater electron deficiency at silicon,
which would be accompanied by orbital contraction, and this could
result in a decrease of orbital oVerlap between the Si and H atoms.
In addition, the increase in the electron-withdrawing ability of the
SiR3-nCln group would make the Ta-H bond less basic.6
(6) Basicity of the hydride is a prerequisite of IHI.2
This matter has been clarified by means of DFT calculations7 of
a series of model complexes with the formula Cp(MeN)Ta(PMe3)(H)-
(SiH3-nCln) (n ) 0-3, compounds 5-8, respectively). Selected
results of the calculations are given in Table 1. The most important
result of the calculations is that instead of a monotonic change in
values of the parameters on going from n ) 0 to n ) 3 due to
Bent’s rule, the compound Cp(MeN)Ta(PMe3)(H)(SiH2Cl) (6)
exhibits extreme values. Thus, on going from Cp(MeN)Ta(PMe3)-
(H)(SiH3) (5) to 6 the Si-H distance first decreases (stronger Si-H
interaction), as predicted, from 2.288 to 2.064 Å and then increases
to 2.115 Å in 7 and 2.174 Å in 8, while the Ta-H bonds first
elongates (weaker Ta-H interaction) and then contracts again,
indicating the strongest IHI is found for 6. This is further supported
by the largest values of the bond orders and Wiberg indices8 for
the Si-H contact in 6, accompanied by the lowest values for the
Ta-H bonds (Table 2). The apparently anomalous changing of these
and other parameters, however, is totally consistent with the theory
of IHI and establishes that 6 has the strongest H-Si bonding. The
weaker IHI found in 7 (model for 3) compared to that in 6 (model
for 2) allows us to understand why the Ta-Si bond in 3 is only
marginally shortened in comparison with that in 2: in 3 the expected
(7) All calculations were carried out with the Gaussian 98 program package
(Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
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T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
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W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
98, revision A.3; Gaussian, Inc.: Pittsburgh, PA, 1998) using density
functional theory applying Becke’s 1988 nonlocal exchange functional
in conjunction with Perdew’s correlation functional, commonly aliased
as BP86. The compound basis set was used for the calculation consisted
of the 6-31G(d) basis set for the silicon, phosphorus and nitrogen atoms,
6-31G for the carbon atoms and the sylil hydrogens, and the 3-21G basis
set for the hydrogen atoms of Cp ring and methyl groups. The basis set
augmented by the p-polarization function (6-31G(d,p) basis set) was used
for the hydride H atom. The Hay-Wadt VDZ effective core potentials
(ECP) and the corresponding VDZ basis sets were used for the tantalum
atom, and the “Stuttgart” quasi-relativistic ECP were used for the atom
Cl in this model. For more details and references see ref 3.
(8) Wiberg K. B. Tetrahedron 1968, 24, 1024.
(9) F. R. Lemke has recently made
communication).
a similar observation (private
JA027935V
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