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 perip hery. In the solid state, the nickel(II) complex Ni(TCHP) has a ruffled por phyrin conformation while Zn(TCHP) exhibits a stepped distortion of the mac rocycle. In chloroform solution, fast rotation of the cyclohexyl groups on the NMR time scale is observed at room temperature for both complexes. Temp erature-dependent H-1 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 (Delta G(c)d ouble dagger = 10-12 kcal mol(-1)). In both complexes, the porphyrin ring i s 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 HOM O-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 othe r odor active proteins. Crystal data: Ni(TCHP). CHCl3. CH3CN, monoelinic, C 2/c, a = 27.405(12), b = 10.004 (21), c = 32.877(24) Angstrom, beta = 107.7 1(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) Angstrom, beta = 102.85(16)degrees at 127 K, Z = 2.