2D NMR Investigations of the Rotation of Axial Ligands in Six-Coordinate Low-Spin Iron(III) and Cobalt(III) Tetraphenylporphyrinates Having 2,6-Disubstituted Phenyl Rings: Quantitation of Rate Constants from 1H EXSY Cross-Peak Intensities

Phase-sensitive NOESY/EXSY experiments have been utilized to measure the rates of axial ligand rotation for (tetramesitylporphyrinato)iron(III) and -cobalt(III) bis(2-methylimidazole), [(TMP)Fe(2-MeImH) 2 ] + ClO 4 - and [(TMP)Co(2-MeImH) 2 ] + BF 4 -, and several related complexes at various low temperatures. The derivations of the expressions for EXSY cross-peak volumes (Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principles of Magnetic Resonance in One and Two Dimensions; Clarendon Press: Oxford, U.K., 1992; chapters 6 and 9) as a function of mixing time τ m, longitudinal relaxation time T 1, and chemical exchange rate constant, k, have been extended to the case of cyclic four-site chemical exchange having a single rate constant. Cross-peak volumes were fit to the expressions, and the rate constants were calculated using a computer fitting program developed in this laboratory. The dependence of the reliability of the rate constant on T 1, τ m, and other experimental factors is discussed. The temperature dependence of the rate constants was used to calculate the activation enthalpy and entropy for these complexes and two others, [tetrakis(2,6-dichlorophenyl)porphyrinato]iron bis(2-methylimidazole) perchlorate, [(2,6-Cl 2 ) 4 (TPP)Fe(2-MeImH) 2 ] + ClO 4 -, and its 2,6-dibromophenyl analog, [(2,6-Br 2 ) 4 (TPP)Fe(2-MeImH) 2 ] + ClO 4 -, as well as the bis(1,2-dimethylimidazole) complexes of (TMP)Co III . The values of Δ H ⧧ are very similar for all Fe(III) complexes (46−51 kJ/mol), and Δ S ⧧ values are close to zero. Nevertheless, the combined differences in these activation parameters led to rate constants for ligand rotation at 25 °C ranging from 1.1 × 10 5 (2,6-Br 2 ) to 1 × 10 4 (TMP) s -1 . For the [(TMP)CoL 2 ] + BF 4 - complexes where L = 2-MeImH and 1,2-Me 2 Im, the values of Δ H ⧧ are very similar but slightly smaller than those for the low-spin Fe(III) complexes, but the values of Δ S ⧧ are rather negative (−63 and −84 J/(mol K), respectively), which lead to rate constants at 25 °C of 14 and 5 s -1, respectively. The difference in Δ S ⧧ and thus the 10 3 difference in the rate constants for Fe(III) and Co(III) complexes probably indicates either steric hindrance to rotation of the 2-methyl group of the “hindered” ligand in the Co(III) complexes, where the Co−N ax bond lengths are expected to be somewhat shorter than the corresponding Fe−N ax bonds, or differences in solvation of the Co(III) complexes (BF 4 - anion) that lead to a more highly structured transition state than for those of the Fe(III) complexes (ClO 4 - anion). The methods developed for analysis of the EXSY data are general and could be used for any case of four-site chemical exchange with a single rate constant.