COVALENTLY-LINKED OXOMOLYBDENUM(V) AND IRON(III) PORPHYRIN CENTERS - SYNTHETIC MODELS FOR THE MOLYBDENUM IRON INTERACTION IN SULFITE OXIDASE

The modified tetraarylporphyrins 5-(3,4-dihydroxyphenyl)-10,15,20-tri-p-tolylporphyrin (3,4-OH-TTP, 3) and 5-(2,3-dihydroxyphenyl)-10,15,20-tri-p-tolylporphyrin (2,3-OH-TTP, 4) that possess a single pendant catechol group at one of the meso positions have been synthesized and used to investigate the coupling between oxo-Mo(V) and Fe(III) centers as preliminary models for the inter-prosthetic group interaction in sulfite oxidase. Reaction of these novel ligands with LMoO(2+) (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) followed by insertion of Fe(III) into the porphyrin generates the bimetallic complexes FeCl(3,4-Mo-TTP) (11) and FeCl(2,3-Mo-TTP) (12) with controlled Mo...Fe distances of 9.4 and 7.3 Angstrom, respectively, as determined from computer modeling. Addition of excess N-methylimidazole (N-MeIm) produces the six-coordinate complexes Fe(N-MeIm)(2)(3,4-Mo-TTP)Cl (13) and Fe-(N-MeIm)(2)(2,3-Mo-TTP)Cl (14) with low-spin Fe(III) centers and two axial imidazole ligands. The formation constants for these bimetallic bis(N-methylimidazole) complexes are larger than those for related six-coordinated monometallic Fe(III) complexes of asymmetric porphyrin ligands. The interaction between the two S = 1/2 metal centers of Fe-(N-MeIm)(2)(3,4-Mo-TTP)Cl and Fe(N-MeIm)(2)(2,3-Mo-TTP)Cl has been probed by cyclic voltammetry and EPR spectroscopy. Simulation of the EPR spectra of these bimetallic Mo...Fe complexes shows that weak anisotropic dipolar coupling dominates the interaction between the two metals in Fe(N-MeIm)(2)(3,4-Mo-TTP)Cl (Mo...Fe similar to 9.4 Angstrom), whereas weak exchange interactions ((3-70) X 10(-3) cm(-1), 100-2000 MHz) predominate for Fe(N-MeIm)(2)(2,3-TTP)Cl (Mo...Fe similar to 7.9 Angstrom). The EPR simulations for both Fe(N-MeIm)(2)(3,4-Mo-TTP)Cl and Fe(N-MeIm)(2)(2,3-Mo-TTP)Cl require a distribution of coupling interaction energies, consistent with the presence of several rotamers of the N-methylimidazole ligand in solution that produce different orientations of the in-plane magnetic axes of the porphyrin. Cyclic voltammetry shows that two metal centers of Fe(N-MeIm)(2)(3,4-Mo-TTP)Cl and Fe(N-MeIm)(2)(2,3-Mo-TTP)Cl behave essentially as independent one-electron couples, but the potential of the Mo(V/IV) couple is sensitive to the electronic state and charge on the metalloporphyrin core.