NMR-STUDIES OF HINDERED LIGAND ROTATION, MAGNETIC-ANISOTROPY, CURIE BEHAVIOR, PROTON SPIN RELAXATION, AND LIGAND-EXCHANGE IN SOME NOVEL OXOMOLYBDENUM(V) IRON(III) PORPHYRINATE COMPLEXES

A detailed H-1 NMR study has been carried out on the novel porphyrinat oiron(III)-Mo(V) complexes ybdenum)dioxy]phenyl]porphyrinato}bis(L)iro n-(III) chloride, [Fe(2,3-Mo-TTP)L(2)]Cl-+(-), where L = N-methylimida zole (NMeIm), imidazole (ImH), or 4-(dimethylamino)pyridine (4DAP), an d [Fe(3,4-Mo-TTP)(NMeIm)(2)]Cl-+(-). Each of these compounds contains two S = 1/2 metal centers. In the 2,3-isomer, rotation of one of the a xial ligands bound to the iron atom is prevented by the bulky (hydrotr is(pyrazolyl)borato)oxomolybdenum substituent, as evidenced by the obs ervation of eight unique pyrrole-H resonances that do not coalesce ove r most of the liquid range of the CD2Cl2 solvent (-90 to +30 degrees C ). Moreover, the slow electron spin relaxation timeof oxomolybdenum(V) allows this center to function as a ''dipolar relaxation agent'' that provides a sensitive measure of the distance between the Mo(V)(V) cen ter and each of the pyrrole protons of the low-spin iron(III) porphyri nate. Combination of results from measurement of the T(1)s of the eigh t pyrrole protons, the COSY coupling pattern, NOEs between protons not in the same pyrrole ring, and analysis of the effect of the orientati on of the nodal plane of the nonrotating axial ligand on the rhombic d ipolar contribution to the isotropic shift led to a complete and unamb iguous assignment of these resonances. Theoretical analysis of the obs erved shifts and their temperature dependence made it possible to map the unpaired electron spin density at the beta-pyrrole positions, and thereby the unpaired electron spin density distribution in the pi orbi tal into which the unpaired electron is preferentially delocalized, an d to calculate the approximate energy separation, Delta E(pi), between it and its e(pi) counterpart. Thermal population of the higher-energy orbital accounts for the non-zero intercepts of the Curie plots of th e pyrrole-H resonances. Comparison to other systems, including the 3,4 -MoO complex, demonstrates the large, dominating effect of a fixed axi al ligand plane in determining the spread of the pyrrole-H resonances. The results demonstrate the relatively small effect of the orientatio n df the p(pi) orbital of the planar ligand on the in-plane magnetic a nisotropy, and its much larger effect on spin delocalization via the c ontact interaction. Thus, we conclude that it is likely that the sprea d of the methyl resonances in ferricytochromes b(5) and c and other lo w-spin ferriheme proteins is controlled largely by the effect of the o rientation of the p(pi) orbital of the strongest pi donor ligand on th e contact shift, rather than on the in-plane magnetic anisotropy creat ed simultaneously by that same p(pi) orbital and manifested in the dip olar term. Rates of axial ligand (L) exchange for [Fe(2,3-Mo-TTP)L(2)] Cl-+(-) (for L = NMeIm and 4DAP) have also been measured. It is found that the ligand on the same side of the porphyrinate plane (syn) as th e bulky oxomolybdenum(V) group exchanges much more slowly than the one on the opposite side of the porphyrinate plane (anti).