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

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Abstract

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]+ClO4- and [(TMP)Co(2-MeImH)2]+BF4-, 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 τm, longitudinal relaxation time T1, 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 T1, τm, 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-Cl2)4(TPP)Fe(2-MeImH)2]+ClO4-, and its 2,6-dibromophenyl analog, [(2,6-Br2)4(TPP)Fe(2-MeImH)2]+ClO4-, as well as the bis(1,2-dimethylimidazole) complexes of (TMP)CoIII. The values of ΔH⧧ are very similar for all Fe(III) complexes (46−51 kJ/mol), and ΔS⧧ 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 × 105 (2,6-Br2) to 1 × 104 (TMP) s-1. For the [(TMP)CoL2]+BF4- complexes where L = 2-MeImH and 1,2-Me2Im, the values of ΔH⧧ are very similar but slightly smaller than those for the low-spin Fe(III) complexes, but the values of ΔS⧧ 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 ΔS⧧ and thus the 103 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−Nax bond lengths are expected to be somewhat shorter than the corresponding Fe−Nax bonds, or differences in solvation of the Co(III) complexes (BF4- anion) that lead to a more highly structured transition state than for those of the Fe(III) complexes (ClO4- 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.

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