Oxomolybdenum(V)/Iron(III) Porphyrinate Complexes: Effect of Axial Ligand Plane Orientation on Complex Stability, Reduction Potential, and NMR and EPR Spectra.

Abstract

The compounds {5,10,15-tri-p-tolyl-20-[[2,3-[((hydrotris(3,5-dimethylpyrazolyl)borato)oxomolybdenio)dioxy]phenyl]porphyrinato}bis(2-methylimidazole)iron(III) chloride, Fe(2,3-Mo-TTP)(2MeImH)(2)Cl (1), and {5,10,15-tri-p-tolyl-20-[3,4-[((hydrotris(3,5-dimethylpyrazolyl)borato)oxomolybdenio)dioxy]phenyl]porphyrinato}bis(2-methylimidazole)iron(III) chloride, Fe(3,4-Mo-TTP)(2MeImH)(2)Cl) (2), have been prepared in order to assess the effect of axial ligand plane orientation upon the stability, reduction potential, and NMR and EPR spectra of these novel (porphryinato)iron(III)-Mo(V) systems that possess two S = (1)/(2) metal centers. The proton NMR spectra of 1 and 2 are characteristic of perpendicular orientation of the planes of the axial 2MeImH ligands. These results contrast with those previously reported (Basu, P.; Shokirev, N. V.; Enemark, J. H.; Walker, F. A. J. Am. Chem. Soc. 1995, 117, 9042-9055) for the analogous compounds with NMeIm as the axial base (3, 4) whose (1)H NMR spectra are characteristic of one or both axial ligands in parallel planes. The equilibrium constants (beta(2)) for binding the bulky 2MeImH ligands of 1 and 2 are more than an order of magnitude smaller than those for NMeIm binding to 3 and 4. Three distinct pseudo-Nernstian one-electron couples are observed for 1 and 2 in DMF that can be assigned to the Fe(III/II), Mo(V/IV), and Fe(II/I) reductions, with the Fe(III/II) couple being most positive. The Fe(III/II) and Mo(V/IV) potentials are similar to those for 3 and 4 and only slightly perturbed from those of the individual isolated components. The EPR spectrum of 1 shows features due to Mo(V) and low-spin Fe(III) that are perturbed by weak exchange coupling (2.6 GHz, 0.078 cm(-)(1)) between the two metal centers which are separated by approximately 7.9 Å. The "large g(max)" feature characteristic of the 2MeImH adducts of Fe(III) tetraphenylporphyrinates is shifted toward the Mo(V) signal to 2.85; the anisotropy of the Mo(V) signal is lost, and no molybdenum hyperfine can be detected. The EPR spectrum of 2, which has a metal-metal separation of approximately 9.4 Å, shows an unperturbed "large g(max)" value of 3.41 for the Fe(III) center. The Mo(V) part of the spectrum is slightly perturbed from that of the precursor catecholate complex but is essentially identical to that of 4, which exhibits a rhombic Fe(III) signal.