Anion dependent deprotection of a thioether group in Schiff base NS2 ligands results in new mononuclear and dinuclear thiolato nickel complexes

The synthesis and characterisation of three Schiff base compounds of nickel with NS2 donor groups is described as part of our research in structural modelling of nickel hydrogenase enzymes. 2-Aminothiophenol and 2-tertbutylthiobenzaldehyde reacted in ethanol to form a benzothiazolidine derivative, which is isolated as a yellow solid. The benzothiazolidine ring opens upon reaction with nickel acetate in ethanol to form a mononuclear complex, [Ni((BuL1)-Bu-t)(2)], 1, which crystallises in the monoclinic space group P2(1)/n with cell dimensions, a = 14.665(4), b = 14.800(7), c = 14.923(6) Angstrom, beta = 94.45(3)degrees. Compound 1 is mononuclear with a cis N2S2 chromophore, which is square planar, but slightly distorted towards tetrahedral, and which shows weak interactions with two hydrogens of the ligands (Ni-H distances of 2.52 and 2.58 Angstrom). These C-H ... Ni interactions are retained in solution as reflected in the H-1 NMR spectra of 1. With other nickel salts, the same benzothiazolidine ligand reacts in ethanol to form dinuclear species [Ni(L-1)](2) 2, after loss of the protecting tertiary butyl group. Complex 2 crystallises in the monoclinic space group P2(1)/c with cell dimensions, a = 11.753(3), b = 11.977(3), c = 20.275(4) Angstrom, beta = 123.67(1)degrees. An analogous ligand, synthesised from 2-aminoethanethiol and 2-tert-butylthiobenzaldehyde, was not isolated, but was used in a template reaction with nickel salts in ethanol to form the dinuclear compound [Ni(L-2)](2) 3. Complex 3 crystallises in the monoclinic space group P2(1)/c with cell dimensions a = 15.049(4), b = 10.554(2), c = 12.921(4) Angstrom, beta = 108.68(2)degrees. Compounds 2 and 3 are dinuclear, in a 'butterfly' shape, with bridging thiolates. The nickel ions in these two dinuclear complexes are in a NS3 chromophore with a square planar geometry.