P.Basu, N.V.Shokhirev, J.H.Enemark, F.A.Walker.
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.
Journal of the American Chemical Society. 117: (35) 9042-9055, 1995.

Abstract:

A detailed H-1 NMR study has been carried out on the novel porphyrinatoiron(III)-Mo(V) complexes {tri-p-tolyl[2,3-[((hydrotris(3,5-dimethylpyrazolyl)borato) oxomolybdenum) dioxy]phenyl] porphyrinato}bis(L) iron-(III)chloride, [Fe(2,3-Mo-TTP)L₂]Cl-+(-), where L = N-methylimidazole (NMeIm), imidazole (ImH), or 4-(dimethylamino)pyridine (4DAP), and [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 axial ligands bound to the iron atom is prevented by the bulky (hydrotris(pyrazolyl)borato) oxomolybdenum substituent, as evidenced by the observation of eight unique pyrrole-H resonances that do not coalesce over most of the liquid range of the CD₂Cl₂ solvent (-90 to +30 degrees C). Moreover, the slow electron spin relaxation time of 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) center and each of the pyrrole protons of the low-spin iron(III) porphyrinate. Combination of results from measurement of the T(1)s of the eight pyrrole protons, the COSY coupling pattern, NOEs between protons not in the same pyrrole ring, and analysis of the effect of the orientation of the nodal plane of the nonrotating axial ligand on the rhombic dipolar contribution to the isotropic shift led to a complete and unambiguous assignment of these resonances. Theoretical analysis of the observed 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 orbital into which the unpaired electron is preferentially delocalized, and 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 the pyrrole-H resonances. Comparison to other systems, including the 3,4-MoO complex, demonstrates the large, dominating effect of a fixed axial ligand plane in determining the spread of the pyrrole-H resonances. The results demonstrate the relatively small effect of the orientation df the p(pi) orbital of the planar ligand on the in-plane magnetic anisotropy, and its much larger effect on spin d elocalization via the contact interaction. Thus, we conclude that it is likely that the spread of the methyl resonances in ferricytochromes b₂ and c and other low-spin ferriheme proteins is controlled largely by the effect of the orientation of the pp orbital of the strongest p donor ligand on the contact shift, rather than on the in-plane magnetic anisotropy created simultaneously by that same pp orbital and manifested in the dipolar term. Rates of axial ligand (L) exchange for [Fe(2,3-Mo-TTP)L₂]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 the bulky oxomolybdenum(V) group exchanges much more slowly than the one on the opposite side of the porphyrinate plane (anti).