Spin coupling in metalloporphyrin .pi.-cation radicals

Abstract

The magnetic interactions between metal and ligand spins in metalloporphyrin ..pi..-cation radicals have been investigated as a function of d orbital type for synthetically accessible S = 1/2 metals from the first transition series. Most instructive is the case of copper(II) where the presumably planar (Cu(TMP)/sup ./))/sup +/ (TMP/sup ./ = ..pi..-cation radical of tetramesitylporphyrinate) has an overall S = 1 state (..mu../sub eff/ = 2.99 ..mu../sub B/). The ferromagnetic coupling of spins is rationalized as arising from the exchange interaction of unpaired electrons in orthogonal magnetic orbitals. By contrast, (Cu(TPP/sup ./))/sup +/ has a ruffled porphyrin core in the solid state and is diamagnetic (S = 0). The antiferromagnetic coupling is rationalized in terms of overlap of the magnetic orbitals, i.e., bond formation with spin pairing in the bonding molecular orbital. The crystal and molecular structure of (Cu(TPP/sup ./))(SbCl/sub 6/) has been determined. The structure consists of (Cu(TPP/sup ./))/sup +/ ions that interact in pairs in the crystalline lattice. Dimer formation is believed to be the cause of the exceptionally small dihedral angles between the peripheral phenyl rings and the porphinato core (average value 41.5/sup 0/). This dimerization is also responsible for the unusual saddle-shaped core conformation. This confirmationmore » has also been observed in other ..pi..-cation radical species. The observed Cu-N bond distance of 1.988 (4) A is typical of that for copper(II) and confirms the ring-oxidized nature of the compound. Crystal data: a = 13.484 (2) A, b = 13.924 (3) A, c = 12.886 (2) A, ..cap alpha.. = 98.90 (2)/sup 0/, ..beta.. = 111.08 (1)/sup 0/, ..gamma.. = 106.97 (2)/sup 0/, triclinic, P1, Z = 2, CuN/sub 4/C/sub 44/H/sub 28/SbCl/sub 6/.« less