Structural and spectroscopic features of mono- and binuclear Nickel(II) complexes with tetradentate N(amine)₂S(thiolate)₂ ligation

Abstract

Three complexes containing Ni^IIN(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-NiN₂S₂units. 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.