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 SiH Interactions

Abstract

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')(PR3)(H)(SiRnCl3-n)] (M=V, Nb; R'=2,6-diisopropylphenyl (Ar), 2,6-dimethylphenyl (Ar')), [Nb(Cp)(=NR')(PR''3)(H)(SiPhR2)] (R2=MeH, H2), [Nb(Cp)(==NR')(PR''3)(Cl)(SiHRnCl2-n)] and [Nb(Cp)(eta 3-N(R)SiR2--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(-->SiHClR2)R'}(PR''3)2] bearing a penta-coordinate silicon centre, which then rearranges into the final products through a Si--H or Si--Cl bond activation process. DFT calculations show that this imido-silane adduct is additionally stabilized by a Si--HM agostic interaction. Si--H activation is kinetically preferred even when Si--Cl activation affords thermodynamically more stable products. The niobium complexes [NbCp(=NAr)(PMe3)(H)(SiR2Cl)] (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)(PMe3)(H)(SiMe2Cl)]. The extent of Si--H activation in the beta-Si--HM agostic complexes [Cp{eta 3-N(R')SiR2--H}M(PR''3)(Cl)] (R''=PMe3, PMe2Ph) primarily depends on the identity of the ligand trans to the Si--H bond. A trans phosphine leads to a stronger Si--H bond, manifested by a larger J(Si--H) coupling constant. The Si--H activation diminishes slightly when a less basic phosphine is employed, consistent with decreased back-donation from the metal.

Heterometallic Aluminum–Chromium Phenazine and Thiophenazine Complexes. Formation of a Tetranuclear Chromium(I) Sandwich Complex

“…Organic molecules possessing large and delocalized π systems that are capable of embarking in a substantial charge transfer interaction with a transition metal − are known as “noninnocent” ligands. − , In fact, the most attractive feature of these molecules as ligands is their ability to act as electron reservoirs and to transfer spin density on demand to and from the metal center. − , This turns out to be particularly useful in the case of metals that cannot be readily reduced because of an intrinsic instability and reactivity of their low-valent states . Provided that the appropriate ligand system is present, it is possible to obtain species with the appearance of a formal low-valent complex. ,,,,,− ,− ,,,,, The low-valent appearance, however, is deceptive, since these species in reality consist of higher valent metals connected to radical anion forms of the ligand system. ,,,,,,,,,,,− ,,− ,,,, What is remarkable is that the chemical behavior truly remains that expected for a low-valent species.…”

A Fluorescent Polymeric Heparin Sensor

“…Although agostic silylamido complexes A have been known since 1992 (Scheme ),1, 2 their reactivity has been virtually unstudied, so they largely remain a laboratory curiosity 3. We recently discovered a silane–imide coupling reaction,4 allowing for the preparation of agostic silylamido complexes 1 and 2 a , b with a d 2 configuration (Scheme ) 2. Bonding in these species can be represented by two canonical forms ( B and C ; Scheme ), one of which has a silanimine character ( C ).…”

Vanadium, niobium and tantalum

“…56 The half-sandwich niobium imido complex, [NbCp(QNAr)(PMe 3 ) 2 ] (Ar = 2,6-diisopropylphenyl) was treated with silanes HSiXRR 0 (X = Cl, H; R = Me, Cl, Ph; R 0 = Cl, Me, Ph). 58 Different products, including agostic species [NbCpCl(PMe 3 )(QNAr)(H)(SiXRR 0 )] and imido hydride complex [NbCp(QNAr)(PMe 3 )(H)(SiXRR 0 )], were formed depending upon the silane used.…”