Structural and Spectroscopic Features of Mono- and Binuclear Nickel(II) Complexes with Tetradentate N(amine)₂S(thiolate)₂ Ligation

Three complexes containing NiIIN(amine)2S(thiolate)2 units have been prepared and characterized. Both (R,R)-N,N‘-bis(1-carboxy-2-mercaptoethyl)-1,2-diaminoethane [(R,R)-1] and N,N‘-bis(2-methyl-2-mercaptoprop-1-yl)-1,3-diamino-2,2-dimethylpropane (4) act as tetradentate S−N−N−S ligands to form complexes Ni((R,R)-1) and Ni(4) with nearly planar cis-NiN2S2 units. The N−Ni−N and S−Ni−S angles differ significantly in the two complexes yet are very nearly supplementary. The 1,3-disubstituted cyclohexane species rac-N,N‘-bis(2-mercapto-2-methylprop-1-yl)-1,3-cyclohexanediamine (6) behaves as a bis(bidentate-N,S) ligand to form an unexpectedly intense-blue dinickel complex (1S*,3R*,1‘S*,3‘R*)-7, which contains two trans-NiN2S2 units bridged by 1,3-disubstituted cyclohexane groups. The coordination geometry in (1S*,3R*,1‘S*,3‘R*)-7 is distorted 15° toward tetrahedral, most likely as a result of steric crowding, suggested by several short contacts between the NiS2 units and both the cyclohexyl and gem-dimethyl groups of the N,S-chelate rings. The complexes exhibit rich UV−vis spectra, whose deconvoluted bands are now fully assigned, from low to high energy, as ligand field (LF), π(S) → Ni(II) ligand-to-metal charge transfer (LMCT), σ(S) → Ni(II) LMCT, σ(N) → Ni(II) LMCT, localized S, and S,N Rydberg transitions. The unusually intense LF absorptions shown by (1S*,3R*,1‘S*,3‘R*)-7 are thought to result from relaxation of the Laporte restriction arising from the 15° tetrahedral twist.