Unexpected features of stretched Si-H...Mo β-agostic interactions

Article abstract of DOI:10.1039/b315517j

Coupling of silanes with the imido group of (Ar′N) ₂Mo(PMe₃)₃ gives either the silanimine dimer (ArN-SiHCl)₂ or Si-H agostic silylamido complexes which do not exhibit the commonly expected correlation between th

Full text of DOI:10.1039/b315517j

Unexpected features of stretched Si–H…Mo b-agostic interactions†‡
Stanislav K. Ignatov,a Nicholas H. Rees,b Stuart R. Dubberley,b Alexei G. Razuvaev,a Philip
Mountford*b and Georgii I. Nikonov*c
a University of Nizhnii Novgorod, 23, Gagarin Avenue, Nizhnii Novgorod 603600, Russia
b Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford,
UK OX1 3TA. E-mail: philip.mountford@chem.ox.ac.uk; Fax: + 44 01865 285141; Tel: +44 01865
285140
c
Moscow State University, Vorob’evy Gory, 119992, Moscow, Russia. E-mail: nikonov@org.chem.msu.su;
Fax: +7 095 9328846; Tel: +7 095 9391976
Received (in Cambridge, UK) 1st December 2003, Accepted 12th February 2004
First published as an Advance Article on the web 10th March 2004
Coupling of silanes with the imido group of (ArAN)2Mo(PMe3)3
gives either the silanimine dimer (ArN–SiHCl)2 or Si–H agostic
silylamido complexes which do not exhibit the commonly
expected correlation between the nature of the substituents on
silicon, the degree of Si–H addition and the value of the Si–H
coupling constant.
The question of how electronegative substituents at silicon affect
the strength of Si–H…M interactions, and the associated Si–H
coupling constants, in the more widespread agostic complexes of
the type 1 has so far remained unprobed. Since this issue cannot be
resolved for the complexes 5 (because the reaction of 3 with more
chlorinated silanes gives only silyl hydrides 4) we turned to the
related bis(imido) complexes (RN)2Mo(PMe3)3 (Cp2 ligand is
isolobal to the (RN)22 ligand). Here we present preliminary results
of a comprehensive NMR, X-ray and DFT study of stretched b-Si–
H agostic complexes of molybdenum, which offer for the first time
a unique insight into new and unexpected patterns of Si–H…M
agostic interactions.
Si–H…M Agostic bonding (schematically represented in 1) is a
relatively recent phenomenon compared to the long-established Si–
H bond s-complexes 2a.1–6 Whereas high-order (g-, d-) Si–H…M
agostic interactions are virtually indistinguishable from s-bond
complexation and can be considered as an intramolecular version of
2a, much less is known about the bonding in a-6a,b and b-Si–H…M
agostic species.2 It is widely accepted1a,c that sequential substitu-
tion at silicon by electron-withdrawing groups leads to advanced
Si–H bond oxidative addition. For s-bond complexes a decrease in
Si–H interaction is accompanied by a decrease in the magnitude of
1J(Si–H) from ca. 70 Hz in 2a to below 20 Hz for authentic silyl
hydrides 2b.1a–d Considerable importance has been attached to the
observed value of 1J(Si–H) as a means of assessing the extent of Si–
H…M interactions.1c,1d However, it has very recently been pointed
out7 that Si–H coupling constants measured for species on the
2a?2b reaction coordinate are sensitive to a number of contribu-
tions and do not necessarily reflect absolutely the extent of
oxidative addition.
Reactions of (ArAN)2Mo(PMe3)3 (6, ArA = 2,6-dimethylphenyl)
with the chlorosilanes HSiClMe2 and HSiMeCl2 afforded ex-
clusively the structurally characterised‡ b-agostic Si–H…Mo d2
complexes 7 (Fig. 1) and 8, respectively. The analogous reaction of
6 with HSiCl3 resulted in the unprecedented formation of the
9
silanimine dimer (ArAN–SiHCl)2 and (ArAN)MoCl2(PMe3)3. In
none of the reactions are simple d0 silyl hydride products analogous
to 4 formed.
Complex 7 (R = Me) is an analog of the agostic species 5 and
exhibits the same 1J(Si–H) of 97 Hz. Contrary to expectations
based on the trends observed in the previously studied systems,1
introduction of an electron-withdrawing group in 8 (R = Cl) does
1
not decrease the value of the coupling constant J(Si–H), rather it
increases significantly to 129 Hz. Such a trend is normally
indicative of a strengthening of the Si–H interaction (shorter Si–H
bond) and a corresponding lengthening of the M–Si and M–H
bonds.1 However, examination of the X-ray structures‡ of iso-
morphous 7 and 8 (selected bond lengths listed in Table 1) does not
support this view since they possess virtually identical Mo–H, Si–H
and Mo–Si bond distances. Indeed, if anything, there is in fact a
marginal shortening of the Mo–Si bond on going from 7 to 8. In the
case of complexes of the type 1 and 2a/b such a shortening would
We have recently reported that reactions of the d2 Group 5
bis(phosphine) imido complexes 3 (Ar = 2,6-diisopropylphenyl)
with silanes HSiClRRA (R, RA = alkyl and/or Cl) give two types of
nonclassical complexes depending on the nature of the metal (Nb,
Ta) and R, RA.8 Complexes 4 (d0) feature a M–H…Si interligand
hypervalent interaction and 5 (d2) possesses a stretched b-Si–
H…Nb agostic interaction. Remarkably for 4 we found that
increased chlorine substitution at silicon led to an increase in 1J(Si–
H) (range 30–50 Hz) but to a decrease in Si–H bonding, i.e. the
opposite to that expected for common s-bond complexes
2a?2b.
† Dedicated to Professor Malcolm Green, colleague and mentor, in
recognition of his contributions to the chemistry of agostic compounds.
‡ Electronic supplementary information (ESI) available: details of prepara-
tions, X-rays studies, DFT calculations and ORTEP Figure for 8. See http:
//www.rsc.org/suppdata/cc/b3/b315517j/
Fig. 1 Molecular structure of the complex 7.§
952
C h e m . C o m m u n . , 2 0 0 4 , 9 5 2 – 9 5 3
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 4

Table 1 DFT calculated bond lengths (in Å) for (MeN)(MeNSiMe22nCln–
H)Mo(Cl)(PMe3)2 (n = 0–2)
SiCl2
SiCl2 (11) (11a)
Bond\SiR2
SiMe2 (9)a
SiMeCl (10)a
Mo–Si
Mo–N(Si)
Mo–Pb
Mo–H
Mo–Cl
Si–H
2.673 [2.668(1)]
2.092 [2.122(3)]
2.453 [2.482(2)]
1.953 [1.92(4)]
2.620 [2.551(1)]
1.618 [1.54(4)]
1.716 [1.676(4)]
2.662 [2.657(1)]
2.112 [2.157(4)]
2.443 [2.474(2)]
1.992 [1.93 (4)]
2.613 [2.538(1)]
1.589 [1.51(3)]
1.684 [1.643(4)]
2.661
2.137
2.431
2.085
2.562
1.558
1.666
3.093
2.142
2.496
—
2.534
1.493
1.678
Chart 1 DCD model for the Si–H…M bonding
increasing Mo?(H–Si) s* back-donation.1a,1c These changes
affect the Mo–Si and Mo–H interactions unevenly, since the Si–H
bonding orbital is more localized on the H atom, whereas the (Si–
H) s* orbital has a bigger contribution from Si.
Si–N
a Experimental values for 7 and 8 in brackets; the Si–H atom was located
from difference maps and refined. b PMe3 trans to Mo…H–Si.
GIN is grateful to the Royal Society of Chemistry for an
International author Award and INTAS for a YS INTAS fellow-
ship. SKI and AGR thank RFBR for financial support (project
03-03-33120). PM and GIN thank the Royal Society (London) for
generous support.
normally be rationalized in terms of a more advanced oxidative
addition of the H–Si bond to the metal which clearly contradicts the
increase in 1J(Si–H) from 7 to 8.
To shed more light on these apparently conflicting results we
carried out DFT calculations on the model complexes (MeN)(MeN-
SiMe22nCln–H)MoCl(PMe3)2 (n = 0 (9), 1 (10), 2 (11)).‡ The
optimized structures for 9 (model for 7) and 10 (model for 8) are in
good accord with the experimental ones (Table 1). As the number
of Cl groups on the silicon atom increases, the Mo–Si bond lengths
decrease only slightly and, unexpectedly, become slightly weaker,
as evidenced by the decrease in the Wiberg bond indices (WI =
0.1471 for 9, 0.1445 for 10 and 0.1426 for 11).10‡ Moreover,
increased chlorine substitution does not tend towards cleavage of
the Si–H bond. In fact, the Si–H bond contracts and strengthens (WI
= 0.5830 for 9 versus 0.6171 for 10 and 0.6649 for 11), whereas the
Mo–H bond length increases from 1.953 Å to 2.085 Å and weakens
(WI = 0.2190, 0.1888, 0.1453 from 9 to 11). This is also reflected
in the Mo–P bond length to the PMe3 trans to Mo…H–Si, which
becomes shorter and stronger as the Si–H binds less strongly.
An AIM (atoms in molecules) analysis11 of 9–11 revealed a
bifurcated topological structure with a Mo–Si bond critical point
(rc) coalescing with the ring critical point (3,+1), leading to a
degenerate critical point structure. As is typical for agostic
systems12 the M–H bond critical point has a large ellipticity, which
increases from 9 to 11 (1.547 to 6.880), thus confirming the
weakening of the Mo–H bond. In contrast, the Si–H bond
strengthens as the number of Cl groups increases, as shown by a
significant decrease of the energy density values11b,c from 20.3682
to 20.4540 hartree Å23.‡
Notes and references
§ CCDC 226162 and 226263. See http://www.rsc.org/suppdata/cc/b3/
b315517j/ for crystallographic data in .cif or other electronic format.
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The formation of (ArAN–SiHCl)2 and (ArAN)MoCl2(PMe3)3
(rather than a product analogous to compounds 7 and 8 and model
11) in the reaction with HSiCl3 provides further insight into this
system. Optimization of the Si–Cl…Mo bonded structure 11a
(model for a likely intermediate) gave an energy only ca. 1 kcal
above that of agostic 11. This is accounted for by the expected
increase in Si–H bond strengths on increased Cl substitution at
silicon due to increased Si 3s contribution in accordance with the
Bent’s rule.13 b-Cl elimination from the real intermediate corre-
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(ArAN–SiHCl)2 and (ArAN)MoCl2(PMe3)3.
These surprising results can be rationalized by a Dewar–Chatt–
Duncanson (DCD) model (Chart 1) adjusted by Bent’s rule.7b
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C h e m . C o m m u n . , 2 0 0 4 , 9 5 2 – 9 5 3
953