RATES OF AXIAL LIGAND ROTATION IN DIAMAGNETIC D(6) CO(III) AND FE(II)PORPHYRINATES

In order to investigate the rates of rotation of pyridine and imidazol e ligands in diamagnetic low-spin d(6) Co(III) and Fe(II) porphyrinate systems, we have synthesized tetramesitylporphyrinate (TMP) complexes of each of these metals with pyridine and imidazole ligands and inves tigated them as a function of temperature by LH NMR spectroscopy. We h ave already reported that for TMPFe(III) and -Co(III) complexes with h indered imidazoles the TMP o-CH3 resonances can be used to measure the rates of rotation (N.V Shokhirev, T.Kh. Shokhireva, J.R. Polam, C.T. Watson, K. Raffii, U. Simonis and F.A. Walker, J. Phys. Chern. A, 101 (1997) 2778). For the bis-1,2-dimethylimidazole complex, [TMPCo(1,2-Me (2)Im)(2)] BF4, at ambient temperatures ligand rotation is slow but me asureable on the NMR time scale, and four o-CH3 resonances are observe d, as we have already reported. In contrast, as shown in the present w ork, for the bis-4-dimethylaminopyridine complex, [TMPCo(4-NMe2Py)(2)] BF4, ligand rotation is extremely rapid at ambient temperatures. At t emperatures below -50 degrees C at 300 MHz the o-CH, resonance broaden s and the rates of rotation can be estimated using the modified Bloch equations simplified for the fast exchange regime. The activation para meters Delta H-not equal and Delta S-not equal have been determined, a nd the extrapolated rate constant at 25 degrees C, k(ex) greater than or equal to 1.1 x 10(6) s(-1). These results contradict previous repor ts (J. Huet and A. Gaudemer, Org. Magn. Reson., 15 (1981) 347; I. Cass idei, H. Bang, J.O. Edwards and R. G. Lawler, J. Phys. Chem., 95 (1991 ) 7186) that pyridine ligands bound to Co(III) porphyrinates do not ro tate at room temperature in homogeneous solution. For unhindered imida zole complexes, such as [TMPCo(NMeIm)(2)]+BF4-, no broadening of the o -CH3 resonance is observed, even at -90 degrees C, and thus the rate o f axial ligand rotation is too fast to measure, even at that low tempe rature (or the difference in chemical shift of the two resonances expe cted if ligand rotation is slow is very small). For the corresponding Fe(II) porphyrinate complexes, the rates of pyridine and unhindered im idazole rotation are too fast to measure, even at -90 degrees C. The 2 -methylimidazole complex undergoes chemical reactions that prevent det ailed study of this system by NMR spectroscopy, but the 1,2-dimethylim idazole complex is stable and of similar structure (ruffled porphyrina te ring, axial ligands in perpendicular planes) to the Co(III) and Fe( III) analogs, with the rate constant for ligand rotation, k(ex) simila r to 1 s(-1), at -90 degrees C. Assuming a similar activation enthalpy to those of the Co(III) and Fe(III) systems, the rate of rotation of axial ligands in [TMPFe(1,2-Me(2)Im)(2)] at 25 degrees C is estimated to be about 2 x 10(4) s(-1). (C) 1997 Elsevier Science S.A.