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Non‐Innocent Behaviour of Imido Ligands in the Reactions of Silanes with Half‐Sandwich Imido Complexes of Nb and V: A Silane/Imido Coupling Route to Compounds with Nonclassical SiH Interactions
Author(s) -
Ignatov Stanislav K.,
Rees Nicholas H.,
Merkoulov Alexei A.,
Dubberley Stuart R.,
Razuvaev Alexei G.,
Mountford Philip,
Nikonov Georgii I.
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700271
Subject(s) - chemistry , agostic interaction , silanes , silane , adduct , substituent , steric effects , associative substitution , phosphine , dichlorosilane , stereochemistry , hypervalent molecule , medicinal chemistry , crystallography , silicon , metal , catalysis , organic chemistry , reagent
Reactions of imido complexes [M(Cp)(NR′)(PR′′ 3 ) 2 ] (M=V, Nb) with silanes afford a plethora of products, depending on the nature of the metal, substitution at silicon and nitrogen and the steric properties of the phosphine. The main products are [M(Cp)(NR′)(PR 3 )(H)(SiR n Cl 3− n )] (M=V, Nb; R′=2,6‐diisopropylphenyl (Ar), 2,6‐dimethylphenyl (Ar′)), [Nb(Cp)(NR′)(PR′′ 3 )(H)(SiPhR 2 )] (R 2 =MeH, H 2 ), [Nb(Cp)(NR′)(PR′′ 3 )(Cl)(SiHR n Cl 2− n )] and [Nb(Cp)(η 3 ‐N(R)SiR 2 H⋅⋅⋅)(PR′′ 3 )(Cl)]. Complexes with the smaller Ar′ substituent at nitrogen react faster, as do more acidic silanes. Bulkier groups at silicon and phosphorus slow down the reaction substantially. Kinetic NMR experiments supported by DFT calculations reveal an associative mechanism going via an intermediate N‐silane adduct [Nb(Cp){N(→SiHClR 2 )R′}(PR′′ 3 ) 2 ] bearing a penta‐coordinate silicon centre, which then rearranges into the final products through a SiH or SiCl bond activation process. DFT calculations show that this imido‐silane adduct is additionally stabilized by a SiH⋅⋅⋅M agostic interaction. SiH activation is kinetically preferred even when SiCl activation affords thermodynamically more stable products. The niobium complexes [NbCp(NAr)(PMe 3 )(H)(SiR 2 Cl)] (R=Ph, Cl) are classical according to X‐ray studies, but DFT calculations suggest the presence of interligand hypervalent interactions (IHI) in the model complex [Nb(Cp) (NMe)(PMe 3 )(H)(SiMe 2 Cl)]. The extent of SiH activation in the β‐SiH⋅⋅⋅M agostic complexes [Cp{η 3 ‐N(R′)SiR 2 H⋅⋅⋅}M(PR′′ 3 )(Cl)] (R′′=PMe 3 , PMe 2 Ph) primarily depends on the identity of the ligand trans to the SiH bond. A trans phosphine leads to a stronger SiH bond, manifested by a larger J (SiH) coupling constant. The SiH activation diminishes slightly when a less basic phosphine is employed, consistent with decreased back‐donation from the metal.