INVESTIGATIONS OF ROTATION OF AXIAL LIGANDS IN 6-COORDINATE LOW-SPIN IRON(III) TETRAPHENYLPORPHYRINATES - MEASUREMENT OF RATE CONSTANTS FROM SATURATION-TRANSFER EXPERIMENTS AND COMPARISON TO MOLECULAR MECHANICS CALCULATIONS

Saturation transfer experiments have been utilized to measure the rate of axial ligand rotation in (tetramesitylporphyrinato)iron(III) bis(2 -methylimidazole), [(TMP)Fe(2-MeImH)(2)](+). Saturation transfer peak intensities of four distinct pyrrole protons have been measured at a s eries of temperatures. Derivation of analytical expressions for steady -state peak intensities in the case of cyclic four-site exchange allow ed the determination of the exchange rate constant. Previously measure d longitudinal relaxation rate constants of the pyrrole protons of [(T MP)Fe(2-MeImH)(2)](+) have been used for rate constant determination. The temperature dependence of the rates has allowed estimation of the enthalpy barriers and entropy of this rotation. Modified MM2 potential s have also been used to study the rotation of axial ligands in [(TMP) Fe(1,2Me(2)Im)(2)](+) and (tetraphenylporphyrinato)iron(III) bis(1-met hylimidazole), [(TPP)Fe(1-MeIm)(2)](+). The ''adiabatic'' potential en ergy surfaces (PES) for rotation of axial ligands (minima achieved in all degrees of freedom except for constrained internal rotation coordi nates for the two axial ligands) have been constructed for both comple xes by combining a Ramachadran-type dihedral drive with geometry minim ization or Monte Carlo single minimum analysis with subsequent geometr y minimization. The PES of the TMP-hindered imidazole complex indicate s that the preferable mode of rotation is synchronous clockwise or cou nterclockwise rotation of the two axial ligands, with an enthalpy barr ier to such rotation of approximately 48 kJ/mol. For the TPP-nonhinder ed imidazole complex, enthalpy barriers to synchronous and asynchronou s rotation were found to be 3.3 and 5.4 kJ/mol, respectively, thus pro mpting the assumption that no particular mode of rotation is highly pr eferable in that complex. The rotational enthalpy barrier for the TMP- hindered imidazole complex was found to be consistent with experimenta l measurements of the current (59 kJ/mol) and previous work (50-54 kJ/ mol) (Shokhirev, N. V.; Shokhireva, T. Kh.; Polam, J. R.; Watson, C. T .; Raffii, K.; Simonis,.U.; Walker, F. A. J. Phys. Chem. A 1997, 101, 0000. Nakamura, M.; Groves, J. T. Tetrahedron 1988, 44, 3225). The rel ationship between the orientation of axial ligands, the distortion of the metalloporphyrin core from planarity, and the bulkiness of axial l igands and porphyrin substituents is discussed.