Nickel(II) and Zinc(II) meso-Tetracyclohexylporphyrins. Structural and Electronic Effects Induced by meso-Cyclohexyl Substitution in Metalloporphyrins.

The synthesis and X-ray structures of the zinc(II) and nickel(II) complexes of meso-tetracyclohexylporphyrin H(2)(TCHP) are described. The nonplanarity of the meso substituents results in steric crowding at the porphyrin periphery. In the solid state, the nickel(II) complex Ni(TCHP) has a ruffled porphyrin conformation while Zn(TCHP) exhibits a stepped distortion of the macrocycle. In chloroform solution, fast rotation of the cyclohexyl groups on the NMR time scale is observed at room temperature for both complexes. Temperature-dependent (1)H NMR spectra showed that the (-g,g,-g,g) conformer of Zn(TCHP) and Ni(TCHP) is prevalent in solution at low temperatures and gave an estimate for the rotation barrier of the cyclohexyl groups (DeltaG(c)() = 10-12 kcal mol(-)(1)). In both complexes, the porphyrin ring is easier to oxidize and harder to reduce than in their tetraphenylporphyrin M(TPP) congeners, in agreement with the stronger electron-donating effect of the cyclohexyl group. The magnitude of the potential shift is larger for the first oxidation than for the first reduction, reflecting a smaller HOMO-LUMO energy gap and a greater degree of macrocycle distortion than in the M(TPP) derivatives. This information is of importance to understanding the protein regulation of electron-transfer processes by cytochrome c and other redox active proteins. Crystal data: Ni(TCHP).CHCl(3).CH(3)CN, monoclinic, C2/c, a = 27.405(12), b = 10.004(21), c = 32.877(24) Å, beta = 107.71(3) degrees at 127 K, Z = 8. Zn(TCHP), monoclinic, P2(1)/a, a = 11.159(15), b = 11.992(7), c = 13.465(20) Å, beta = 102.85(16) degrees at 127 K, Z = 2.