Synthesis and characterization of dinickel(II) and dipalladium(II) complexes of the macrocyclic binucleating ligand 3,13-dimethyl-3,13-dinitro-1,5,11,15-tetra-azacycloeicosane-8,18-dithiol (L5). Crystal structure of the complex [Ni2(L5– 2H)][NO2]2·3.5H2O

Abstract

Condensation of the bis(1,5-diaminopentane-3-thiolato)dinickel(II) ion or the dipalladium(II) analogue with formaldehyde and nitroethane in basic methanol yields the dianion of the macromonocyclic ligand 3,13-dimethyl-3,13-dinitro-1,5,11,15-tetra-azacycloeicosane-8,18-dithiol (L⁵) as the dimetal(II) complex. Spectroscopic properties of the diamagnetic complexes are reported, together with the crystal-structure analysis of the dinickel(II) complex. The nitrite salt of the latter crystallizes in the space group Pbnm, a= 7.762(3), b= 14.921 (3), c= 25.459(3)Å, Z= 4, R= 0.057 for 1 739 ‘observed’ reflections. The complex is a syn isomer, the first time this geometry has been observed from this type of condensation reaction, with the two N₂NiS₂ planes folded slightly towards each other about the mirror plane through S ⋯ S in a ‘butterfly’ arrangement (interplanar angle 162.8°). Oxygen atoms of the pendant nitro groups form weak axial interactions, the six-membered rings at the ‘ends’ having skew-boat conformations.