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

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 [N activated = CHCH = CH(N..C..S)Li.hmpa]n 1. 2-Mercaptothiazoline (II), with a -SC(= S)NH-/-SC(-SH) = N- unit, gives [S activated CH2CH2(N..C..S)Li.tmen]n 2. 2-Mercaptobenzimidazole (III), having a -NHC(= S)NH-/-NHC(-SH) = N- unit, gives, when dilithiated, { activated C6H4[N..C(..S)..N]Li2.3hmpa}n 3 [hmpa = hexamethylphosphoramide, (Me2N)3P = O; tmen = Me2NCH2CH2NMe2]. The solid-state structures of complexes 1-3 were solved by X-ray crystallography. Complex 1 is polymeric (n = infinity): each unit contains a (N..C..S)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 N..C..S 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 mu-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 (N..C..S)Li chelating modes found in 1 and 3, and help to explain why direct S-Li bonding is found in 2.