Structure of non-symmetric lithium amidinate complexes prepared by addition of lithium amides to various nitriles

Abstract

Lithium amidinate complexes were prepared by direct reactions from various nitriles - R1-CN (R = Ph, n-Bu, t-Bu) and trimethylsilyl-substituted lithium amides - R2(Me3Si)NLi (R2 = Ph, 2,6-(CH3)2C6H3 (Dmp) and 2,6-[(CH3)2CH]2C6H3 (Dipp)). All the reactions proceed only in coordinating solvents as diethylether or THF where the migration of the trimethylsilyl group to the nitrogen atom of the parent nitrile is favoured. In the case of t-BuCN, virtually no conversion to appropriate lithium amidinate was observed under the same reaction protocol. Only the adduct of the composition {(t-BuCN){2,6-[(CH3)2CH]2C6H3}(Me3Si)NLi]}2 was isolated from the reaction of t-BuCN with {2,6-[(CH3)2CH]2C6H3}(Me3Si)NLi]} carried out in hexane or toluene solution. Lithium atoms in this adduct appear in trigonal planar to pyramidal coordination neighborhoods. The structure of lithium amidinates in the solid state is strongly dependent on the substitution pattern, when the Dipp substituent is present, complexes always crystallize as mononuclear species with isobidentate bonding fashion of the amidinate and two molecules of solvent being extra-coordinated to the lithium atom. On contrary, smaller peripheral groups do not protect the lithium atom from further association and those complexes are described as di- or trinuclear adducts with less than two coordinated molecules of solvent per lithium atom. In all lithium amidinates prepared, the central lithium atom has distorted tetrahedral coordination geometry with ligands going from anisobidenate or bidentate-bridging to tridentate bonding fashion except the trinuclear complex bearing the smallest combination of ligands where one of the lithium atoms is found in the trigonal planar vicinity.