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

Abstract

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-tert-butylthiobenzaldehyde 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(^tBuL¹)₂], 1, which crystallises in the monoclinic space group P2₁/n with cell dimensions, a = 14.665(4), b = 14.800(7), c = 14.923(6) Å, β = 94.45(3)°. 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 Å). These C–H· · ·Ni interactions are retained in solution as reflected in the ¹H NMR spectra of 1. With other nickel salts, the same benzothiazolidine ligand reacts in ethanol to form dinuclear species [Ni(L¹)]₂ 2, after loss of the protecting tertiary butyl group. Complex 2 crystallises in the monoclinic space group P2₁/c with cell dimensions, a = 11.753(3), b = 11.977(3), c = 20.275(4) Å, β = 123.67(1)°. 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(L²)]₂ 3. Complex 3 crystallises in the monoclinic space group P2₁/c with cell dimensions a = 15.049(4), b = 10.554(2), c = 12.921(4) Å, β = 108.68(2)°. 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.