Tantalizing chemistry of the half-sandwich silylhydride complexes of niobium: Identification of likely intermediates on the way to agostic complexes

Article abstract of DOI:10.1021/ic026017v

Reactions of the compound [NbCp(ArN)(PMe₃)₂] with chlorosilanes HSiRR′Cl give a series of silyl complexes [NbCp(ArN)(PMe₃)(H)-(SiRR′Cl)] and [NbCp(ArN)(PMe₃)(Cl)(SiRR′H)] which are likely intermediates to the agostic complexes [NbCp(ArN(RR′₂Si-H...))-(PMe₃)(Cl)].

Full text of DOI:10.1021/ic026017v

Inorg. Chem. 2003, 42, 258−260
Tantalizing Chemistry of the Half-Sandwich Silylhydride Complexes of
Niobium: Identification of Likely Intermediates on the Way to Agostic
Complexes
,†
,‡
Georgii I. Nikonov,* Philip Mountford,* and Stuart R. Dubberley‡
Department of Chemistry, Moscow State UniVersity, Vorob’eVy Gory, 119899, Moscow, Russia,
and Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, U.K.
Received September 10, 2002
Scheme 1
Reactions of the compound [NbCp(ArN)(PMe3)2] with chlorosilanes
HSiRR′Cl give a series of silyl complexes [NbCp(ArN)(PMe3)(H)-
(SiRR′Cl)] and [NbCp(ArN)(PMe3)(Cl)(SiRR′H)] which are likely
intermediates to the agostic complexes [NbCp(ArN(RR′2Si−H‚‚‚))-
(PMe3)(Cl)].
Silane σ-complexes have been known for more than 30
years and are now well-studied.1 Agostic Si-H-M interac-
tions were first reported about 10 years ago.2-4 Recently,
new types of nonclassical complexes have been discovered,5
some of which were found to have interligand hyperValent
interactions (IHI).6,7 Examining the latter type of bonding
we observed that Cp(ArN)Ta(PMe3)2 (Ar ) 2,6-diisopro-
pylphenyl) reacts with HSiMe2Cl giving the compound
Cp(ArN)Ta(PMe3)(H)(SiMe2Cl) (1) with IHI, whereas its
niobium analogue, Cp(ArN)Nb(PMe3)2 (2), affords the first
stretched â-agostic Si-H‚‚‚M complex Cp(ArNMe2Si-H‚‚‚)-
Nb(PMe3)(Cl) (3) (Scheme 1).7 Whether this reaction
proceeds via silane addition across the NbdN bond or via
oxidative addition to the metal to give a tantalum-like
structure Cp(ArN)Nb(PMe3)(H)(SiMe2Cl) with subsequent
rearrangement was not clear at that time. Our further studies
in this field showed that, first, simple variation of the silane
dramatically changes the type of product to the tantalum-
like complex Cp(ArN)Nb(PMe3)(H)(SiR3) and, second,
unprecedented intramolecular exchange of hydrogen and
chlorine atoms between the silicon and niobium sites is
* Authors to whom correspondence should be addressed. E-mail:
nikonov@org.chem.msu.su (G.I.N.); Philip.Mountford@chem.ox.ac.uk (P.M.).
† Moscow State University.
‡ Inorganic Chemistry Laboratory.
(1) (a) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer
Academic/Plenum: New York, 2001. (b) Crabtree, R. H. Angew.
Chem., Int. Ed. Engl. 1993, 32, 789. (c) Schubert, U. AdV. Organomet.
Chem. 1990, 30, 151. (d) Corey, J. Y.; Braddock-Wilking, J. Chem.
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Soc. 1994, 116, 177. (c) Herrmann, W. A.; Eppinger, J.; Spiegler,
M.; Runte, O.; Anwander, R. Organometallics 1997, 16, 1813. (d)
Nagl, I.; Scherer, W.; Tafipolsky, M.; Anwander, R. Eur. J. Inorg.
Chem. 1999, 1405. (e) Eppinger, J.; Spiegler, M.; Hieringer, W.;
Herrmann, W. A.; Anwander, R. J. Am. Chem. Soc. 2000, 122, 3080.
(3) (a) Burckhardt, U.; Casty, G. L.; Gavenonis, J.; Tilley, T. D.
Organometallics 2002, 21, 3108. (b) Ohff, A.; Koesse, P.; Baumann,
W.; Tillack, A.; Kempe, R.; Go¨rls, H.; Burlakov, V. V.; U. Rosenthal,
J. Am. Chem. Soc. 1995, 117, 10399. (c) Schubert, U.; Giges, H.
Organometallics 1995, 152, 373. (d) Yin, J.; Klozin, J.; Abboud, K.;
Jones, W. M. J. Am. Chem. Soc. 1995, 117, 3298. (e) Sadow, A. D.;
Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 6814.
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V. J. Am. Chem. Soc. 1995, 117, 10133. (b) Nikonov, G. I.; Kuzmina,
L. G.; Lemenovskii, D. A.; Kotov, V. V. J. Am. Chem. Soc. 1996,
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A. G.; Mountford, P.; Nikonov, G. I. J. Am. Chem. Soc., accepted.
(7) Nikonov, G. I.; Mountford, P.; Ignatov, S. K.; Green, J. C.; Cooke, P.
A.; Leech, M. A.; Kuzmina, L. G.; Razuvaev, A. G.; Rees, N. H.;
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(8) Crystal data for 4: C29H33ClNSiPNb, orthorhombic, Pna21, a )
21.030(4) Å, b ) 8.730(2) Å, c ) 36.900(7) Å, V ) 6775(2) Å3, T )
150 K, Z ) 4, µ(Mo KR) ) 0.54 mm-1. A total of 12578 reflections
were measured (9549 observed). R1 ) 0.054, wR2 ) 0.1275, and
GOF ) 1.041.
258 Inorganic Chemistry, Vol. 42, No. 2, 2003
10.1021/ic026017v CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/14/2002

COMMUNICATION
Scheme 2
Figure 1. Molecular structure of 4. Selected bond lengths (Å) and angles
(deg) for molecule A: Nb1-P1 2.545(2), Nb1-Si1 2.587(2), Si1-Cl1
2.146(2), Nb1-N1 1.811(5), P1 -Nb1-Si1 117.13(6). For molecule B:
Nb2-P2 2.537(2), Nb2-Si2 2.577(2), Si2-Cl2 2.153(3), P2-Nb2-Si2
116.52(6).
possible. These findings underpin a likely mechanistic
pathway to â-agostic species.
interactions in 4 may still be present, although they are
significantly reduced in comparison with the related Nb and
Ta compounds. The reason for this may lie in the different
electronegativities of niobium and tantalum and in the
different electron releasing abilities of the Cp2 and Cp/ArN/
PMe3 ligand sets. Tantalum is known to stabilize higher
oxidation states better than niobium and thus, probably, is a
better electron donor, making the hydride in 1 more basic.
However, one should not also exclude the possibility of a
hyperconjugation between the π-system of the phenyl rings
and the (Si-Cl)* antibonding orbital, which can also reduce
interaction of this orbital with the Nb-H bonding orbital,
resulting in diminished IHI.
By reacting the precursor 2 with HSiPh2Cl instead of
HSiMe2Cl we unexpectedly obtained the silylhydrido com-
plex Cp(ArN)Nb(PMe3)(H)(SiPh2Cl) (4) similar to the
tantalum compound 1 (Scheme 2). Complex 4 has the
hydride signal as a doublet at 2.86 ppm (JP-H ) 63.5 Hz)
and two doublets for the nonequivalent Me groups of the
Ar ligand, which is typical for complexes of the types
Cp(RN)Nb(X)(Y) and Cp(RN)Nb(PR′3)(X)(Y). The Nb-H
bond stretching can be seen in the IR spectrum as a band at
1616 cm-1. The connectivity of 4 was further supported by
X-ray study.8 A similar product Cp(ArN)Nb(PMe3)(H)(SiCl3)
(5) is formed when 2 reacts with HSiCl3.
An orthorhombic crystal of 4 has two independent
molecules in the unit cell. The molecular structure of one of
them is shown in Figure 1. These two molecules have
somewhat different Nb-Si bond lengths of 2.587(2) and
2.577(2) Å, which can be attributed to crystal packing effects.
These values are comparable with the Ta-Si bond of
2.574(2) Å in the related complex Cp(ArN)Ta(PMe3)(H)-
(SiMe2Cl) (1) and with the Nb-Si bonds in the isolobal
niobocenes Cp2Nb(SiMe2Cl)H2 (2.579(2) Å) and Cp2Nb-
(SiMe2Cl)2H (2.597(1) Å). Short M-Si bonds and long Si-
Cl bonds (range 2.163-2.177(2) Å) in the latter three
compounds have been previously attributed to the presence
of interligand hypervalent interactions (IHIs) M-H‚‚‚Si-
Cl.6,7 Whereas the Nb-Si bond lengths in the two indepen-
dent molecules of 4 fall well within the range of distances
found for complexes with IHI, the Si-Cl bond lengths
observed for 4 are shorter (2.146(2) and 2.153(3) Å),
signifying diminished interligand interactions. Nevertheless,
these Si-Cl bonds are still at the upper end of values found
for classical complexes LnM(SiR2Cl) (R ) alkyl, aryl,
2.094-2.149 Å),9 among which the longest Si-Cl distance
of 2.149(2) Å is found in WCp2(SiMe3)(SiPri2Cl), which, in
its turn, may be affected by the increased bulkiness of the
isopropyl group.9d These data establish that some weak Si-H
Reaction of 2 with HSiMeCl2 during 30 min followed by
a quick workup affords a high yield of the complex NbCp-
(ArN)(PMe3)(H)(SiMeCl2) (6), which also has a “tantalum-
like” structure (Scheme 2). In addition to the Nb-H hydride
1
signal at 2.51 ppm (2JP-H ) 70.2 Hz) in the H NMR
spectrum, it exhibits the Nb-H band at 1620 cm-1 in the
IR spectrum. When isolated, this complex is stable at room
temperature at least for some weeks. When this reaction was
carried out in the NMR tube, complex 6 was observed as
the sole product in 15 min after mixing the reagents.
However, after about 30 min its transformation into another
product 7 was noticed, which was complete in few hours.
This reaction was performed on the Schlenk tube scale by
stirring the reagents overnight, and complex 7 was obtained
in 63% isolated yield. Its 1H NMR spectrum pattern is similar
to that of 6 apart from the absence of the Nb-H resonance
and appearance of a quartet at 6.24 ppm (3JH-H ) 3.3 Hz),
integrated as 1 and coupled to the Si-Me resonance (doublet
at 1.23 ppm). The IR spectrum of 7 no longer shows the
Nb-H band, but a new band at 2142 cm-1 in the typical
Si-H region appears. The 29Si NMR spectrum confirmed
the presence of a direct Si-H bond (J(Si-H) ) 207 Hz).
The elemental analysis of 6 and 7 shows that their composi-
tion is the same. All together these data establish that 7 has
the structure Cp(ArN)Nb(PMe3)(Cl)(SiMeClH), i.e., an
unexpected hydride for chloride exchange took place. This
reactivity contrasts to what is found for Rh and Ir complexes,
for which migration of chlorides from the metal to silyls
(9) (a) Lee, K. E.; Arif, A. M.; Gladysz, J. A. Chem. Ber. 1991, 124,
309. (b) Jagirdar, B. R.; Palmer, R.; Klabunde, K. J.; Radonovich, L.
Inorg. Chem. 1995, 34, 278. (c) Hays, M. K.; Eisenberg, R. Inorg.
Chem. 1991, 30, 2623. (d) Koloski, T. S.; Pestana, D. C.; Carroll, P.
J.; Berry, D. H. Organometallics 1994, 13, 489.
Inorganic Chemistry, Vol. 42, No. 2, 2003 259

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occurs.10 This difference can be attributed to the high
halophilicity of the early transition metals such as niobium.
We also found that 7 is stable in the presence of MeSiCl3
but decomposes to unknown products when HSiMeCl2 is
added and reacts with PMe3 to give Cp(PMe3)3NbCl2.
Although the nature of the rearrangement of 6 into 7 is not
clear at the moment, it is apparently assisted by the
ingredients of the reaction mixture. Interestingly, the related
compounds 4 and 5 do not show this type of transformation
in the mother liquor, most likely due to different electronic
and steric properties of the substituents at the silicon center.
Finally, when the silane HSiMePhCl, “intermediate” in
its properties to HSiMe2Cl and HSiPh2Cl, was reacted with
2, a mixture of all possible structural types (“tantalum-like”
4-6, agostic 3, rearranged 7), identified by their character-
istic Nb-H and Si-H signals, was formed. Therefore, it is
reasonable to assume that a possible pathway for the
formation of agostic species Cp(ArNMe2Si-H‚‚‚)Nb(PMe3)-
(Cl) (3) in the reaction of 2 with HSiMe2Cl includes, as the
first step, formation of the complex Cp(ArN)Nb(PMe3)(H)-
(SiMe2Cl) of the type 4-6 followed by its quick rearrange-
ment into Cp(ArN)Nb(PMe3)(Cl)(SiMe2H), an analogue of
7. Migration of the SiMe2H group onto the imido group
completes the reaction sequences. Such a migration of the
silyl group is precedented.11 Apparently, the particular choice
of substituents at the silicon in 4, 5, and 7 freezes each stage
of this unusual reaction.
Acknowledgment. G.I.N. and P.M. thank the Royal
Society (London) for support of this work through a joint
research grant.
Supporting Information Available: Preparation and charac-
terization details for new compounds and X-ray data (CIF) for 4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(10) (a) Tessier, C. A.; Youngs, W. J.; Gallucci, J. C.; Simons, R. S. J.
Organomet. Chem. 2002, 654, 224. (b) Osakada, K.; Sarai, S.;
Koizumi, T.; Yamamoto, T. Organometallics 1997, 163, 3973.
IC026017V
(11) Gountchev, T. I.; Tilley, T. D. J. Am. Chem. Soc. 1997, 119, 12831.
260 Inorganic Chemistry, Vol. 42, No. 2, 2003