2D NMR INVESTIGATIONS OF THE ROTATION OF AXIAL LIGANDS IN 6-COORDINATE LOW-SPIN IRON(III) AND COBALT(III) TETRAPHENYLPORPHYRINATES HAVING 2,6-DISUBSTITUTED PHENYL RINGS - QUANTITATION OF RATE CONSTANTS FROM H-1 EXSY CROSS-PEAK INTENSITIES

Phase-sensitive NOESY/EXSY experiments have been utilized to measure t he rates of axial ligand rotation for (tetramesitylporphyrinato)iron(m ) and -cobalt(III) bis(2-methylimidazole), [(TMP)Fe(2-MeImH)(2)]+ClO4- and [(TMP)Co(2-MelmH)(2)]+BF4-, and several related complexes at vari ous low temperatures. The derivations of the expressions for EXSY cros s-peak volumes (Ernst, R. R.; Bodenhausen, G.; Wokaun, A, Principles o f Magnetic Resonance in One and Two Dimensions; Clarendon Press: Oxfor d, U.K., 1992; chapters 6 and 9) as a function of mixing time tau(m), longitudinal relaxation time T-1, and chemical exchange rate constant, k, have been extended to the case of cyclic four-site chemical exchan ge having a single rate constant. Cross-peak volumes were fit to the e xpressions, and the rate constants were calculated using a computer fi tting program developed in this laboratory. The dependence of the reli ability of the rate constant on T-1, tau(m), and other experimental fa ctors is discussed. The temperature dependence of the rate constants w as used to calculate the activation enthalpy and entropy for these com plexes and two others, [tetrakis(2,6- dichlorophenyl)porphyrinato]iron bis(2-methylimidazole) perchlorate, [(2,6-Cl-2)(4)(TPP)Fe(2-MeImH)(2) ]+ClO4- and its 2,6-dibromophenyl analog, [(2,6-Br-2)(4)(TPP)Fe(2-Melm H)(2)]+ClO4-, as well as the bis(1,2-dimethylimidazole) complexes of(T MP)Co-III. The values of Delta H-double dagger are very similar for al l Fe(III) complexes (46-51 kJ/mol), and Delta S-double dagger values a re close to zero. Nevertheless, the combined differences in these acti vation parameters led to rate constants for ligand rotation at 25 degr ees C ranging from 1.1 x 10(5) (2,6-Br-2) to 1 x 10(4) (TMP) s(-1). Fo r the [(TMP)CoL(2)]+BF4- complexes where L = 2-MeImH and 1,2-Me(2)Im, the values of Delta H-double dagger are very similar but slightly smal ler than those for the low-spin Fe(m) complexes, but the values of Del ta S-double dagger are rather negative (-63 and -84 J/(mol K), respect ively), which lead to rate constants at 25 degrees C of 14 and 5 s(-1) , respectively. The difference in Delta S-double dagger and thus the 1 0(3) difference in the rate constants for Fe(III) and Co(III) complexe s probably indicates either steric hindrance to rotation of the 2-meth yl group of the ''hindered'' ligand in the Co(III) complexes, where th e Co-N-ax bond lengths are expected to be somewhat shorter than the co rresponding Fe-N-ax bonds, or differences in solvation of the Co(III) complexes (BF4- anion) that lead to a more highly structured transitio n state than for those of the Fe(III) complexes (ClO4- anion). The met hods 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 co nstant.