Intramolecular activation of a pendant nitrile group in Ti and Zr metallocene complexes

Abstract

Several approaches to the intramolecular activation of a pendant nitrile group attached to the group 4 metallocene framework have been probed starting with the complexes [(η5-C5Me5)(η5-C5H4CMe2CH2CN)MCl2] (M = Zr, 1; M = Ti, 2). A reduction induced activation of the nitrile moiety was effected for the Zr derivative either by the treatment with Mg, or by the alkylation using t-BuMgCl followed by spontaneous reductive elimination. In both cases, the reactions finally yielded the zirconocene cyano complex with an intramolecularly tethered alkyl substituent, [(η5-C5Me5)(η5:κC1-C5H4CMe2CH2)Zr(CN-κC)] (4), which resulted from the C–C bond cleavage in the pendant arm. Furthermore, a by-product was isolated which contained two zirconocene fragments bridged through a MgCl2 moiety featuring a rare σ,π-bridging mode of the pendant nitrile group, [{(η5-C5Me5)(μ-η5:η2:κN-C5H4CMe2CH2CN)(μ-Cl)Zr}2Mg] (3). The solid-state structure of 3 was elucidated and this compound together with the product 4 and the tentative intermediate [{(η5-C5Me5)(η5:η2-C5H4CMe2CH2CN)Zr] (3′) were also investigated by DFT calculations. In another approach, 1 and 2 were treated with Li[B(C6F5)4]∙Et2O to generate cationic complexes. Accidental hydrolysis of these species afforded complexes bearing an intramolecularly tethered amide group [{(η5-C5Me5)(η5:κO-C5H4CMe2CH2CONH2)M], of which the Ti derivative (6) was isolated and structurally characterised. Finally, reaction of the zirconocene dimethyl complex [(η5-C5Me5)(η5-C5H4CMe2CH2CN)ZrMe2] (8) with (Ph3C)[B(C6F5)4] was studied.