Copper(ii) and nickel(ii) hexafluorophosphate complexes derived from a monoanionic porphyrin analogue: Solvato- and thermochromism of the Ni complexes by spin-interconversion

Abstract

Phenanporphodimethene (1) is a porphyrin analogue which has a dipyrromethene unit replaced by a 1,10-phenanthroline moiety. This modification effectively coverts a dianionic porphyrin to a monoanionic porphyrin analogue with a porphyrin-like N4 coordination sphere. The ligand 1 forms copper and nickel hexafluorophosphate complexes, Cu-1 and Ni-1, respectively. X-ray crystallographic analysis of Cu-1 indicates that the Cu(II) complex is tetracoordinated by two pyrrolic nitrogen atoms and two phenanthrolic nitrogen atoms and includes a non-bonding PF₆ counter-anion. The Ni-1 complex has similar geometry with a tetracoordinate square-planar structure in non-coordinating solvents such as CHCl₃. In coordinating solvents such as MeOH, the coordination structure adopts an octahedral geometry. These results indicate that Ni-1 can be converted from a low-spin to high-spin configuration by the coordination of two solvent molecules to the nickel center. This solvatochromic conversion of Ni-1 is accompanied by thermochromic behavior resulting from the transformation between square planar and octahedral configurations in THF solution. The redox peak responsible for the nickel-centered redox reaction of Ni-1 is observed at −0.66 V (vs. Fc/Fc⁺) in CH₂Cl₂ solution, which indicates generation of low valent Ni(I) species. Thus, Ni-1 may be useful for future investigations as a novel structural model of the active site of cofactor F₄₃₀ in methyl-coenzyme M reductase.