Lithiations of mercaptoamines containing NC(S)NH–,–NHC(S)S– and –NHC(S)NH– units: syntheses, crystal structures and model molecular-orbital calculations

Abstract

Three mercaptoamines have been lithiated in the presence of various Lewis bases. 2-Mercaptopyrimidine (I), containing a NC(S)NH–(as an amine)/NC(–SH)N–(as a thiol) unit, affords [graphic omitted]S)Li·hmpa]_n1. 2-Mercaptothiazoline (II), with a –SC(S)NH–/–SC(–SH)N– unit, gives [graphic omitted]S)Li·tmen]_n2. 2-Mercaptobenzimidazole (III), having a –NHC(S)NH–/–NHC(–SH)N– unit, gives, when dilithiated, {[graphic omitted]Li₂·3hmpa}_n3[hmpa = hexamethylphosphoramide, (Me₂N)₃PO; tmen = Me₂NCH₂CH₂NMe₂]. The solid-state structures of complexes 1–3 were solved by X-ray crystallography. Complex 1 is polymeric (n=∞): each unit contains a (NCS)Li chelate feature with a terminal hmpa molecule on Li, and these units are then associated via intermolecular N→Li co-ordinations using the third (N) heteroatom of the organic anion. Complex 2 is a dimeric (n= 2) S–Li compound with each Li bearing a tmen molecule; dimerisation is achieved by N→Li intermonomer interactions, but the third (ring S) heteroatom is not involved with metal centres. The dilithiated species 3 is also a dimer (n= 2) and each Li is chelated by an (NCS) unit of its organic dianion; the two end-Li atoms of the dimer are each co-ordinated to two terminal hmpa molecules, while the two central Li atoms are linked by two µ-hmpa molecules, which effect dimerisation. The structural diversities displayed by 1–3 have been probed, and thereby in part rationalised, by ab initio(6-31G basis set) and MNDO molecular-orbital calculations on the amine/thiol isomers of I–III, and on their uncomplexed and complexed lithiated derivatives (as monomers). In particular, the optimised structures predict and reproduce the (NCS)Li chelating modes found in 1 and 3 and help to explain why direct S–Li bonding is found in 2.