Rates of axial ligand rotation in diamagnetic d6 Co(III) and Fe(II) porphyrinates

Article abstract of DOI:10.1016/s0020-1693(97)05597-7

In order to investigate the rates of rotation of pyridine and imidazole ligands in diamagnetic low-spin d⁶ Co(III) and Fe(II) porphyrinate systems, we have synthesized tetramesitylporphyrinate (TMP) complexes of each of these metals with pyridine and imidazole ligands and investigated them as a function of temperature by ¹H NMR spectroscopy. We have already reported that for TMPFe(III) and -Co(III) complexes with hindered 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. Chem. A, 101 (1997) 2778). For the bis-1,2-dimethylimidazole complex, [TMPCo(1,2-Me₂Im)₂]BF₄, at ambient temperatures ligand rotation is slow but measureable on the NMR time scale, and four o-CH₃ resonances are observed, as we have already reported. In contrast, as shown in the present work, for the bis-4-dimethylaminopyridine complex, [TMPCo(4-NMe₂Py)₂] BF₄, ligand rotation is extremely rapid at ambient temperatures. At temperatures below - 50°C at 300 MHz the o-CH₃ resonance broadens and the rates of rotation can be estimated using the modified Bloch equations simplified for the fast exchange regime. The activation parameters ΔH^≠ and ΔS^≠ have been determined, and the extrapolated rate constant at 25°C, k_ex ≥ 1.1 × 10⁶ s^-1. These results contradict previous reports (J. Huet and A. Gaudemer, Org. Magn. Reson., 15 (1981) 347; I. Cassidei, 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 rotate at room temperature in homogeneous solution. For unhindered imidazole complexes, such as [TMPCo(NMeIm)₂]+BF₄^-, no broadening of the o-CH₃ resonance is observed, even at -90°C, and thus the rate of axial ligand rotation is too fast to measure, even at that low temperature (or the difference in chemical shift of the two resonances expected if ligand rotation is slow is very small). For the corresponding Fe(II) porphyrinate complexes, the rates of pyridine and unhindered imidazole rotation are too fast to measure, even at -90°C. The 2-methylimidazole complex undergoes chemical reactions that prevent detailed study of this system by NMR spectroscopy, but the 1,2-dimethylimidazole complex is stable and of similar structure (ruffled porphyrinate ring, axial ligands in perpendicular planes) to the Co(III) and Fe(III) analogs, with the rate constant for ligand rotation, k_ex ～ 1 s^-1, at -90°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-Me₂Im)₂] at 25°C is estimated to be about 2 × 10⁴ s^-1.