31P NMR Probes of Chemical Dynamics: Paramagnetic Relaxation Enhancement of the 1H and 31P NMR Resonances of Methyl Phosphite and Methylethyl Phosphate Anions by Selected Metal Complexes

enhancement is a measure of the rate at which the anionic probes come into physical contact with the paramagnetic center (i.e., enter the inner coordination shell); that is, !!"par ) kassn[M], where !"par is the difference between the line widths of the resonance in paramagnetic and diamagnetic solutions, and kassn is the second-order rate constant for association of the phosphorus ligand with the metal, M. Comparison of the 31 P T1 and T2 relaxation enhancements shows that rapid T2 relaxation by the metal ion is caused by scalar interaction with the electronic spin. Relaxation of the phosphorus-bound proton of MeOPH (1H-P) by MnIIITMPyP5+ displayed intermediate exchange kinetics over much of the observable temperature range. The field strength dependence of 1 H-P T2 enhancement and the independence of the 31 P T2 support these assertions. As in the case of the 31 P T2, the 1 H-P T2 relaxation enhancement results from scalar interaction with the electronic spin. The scalar coupling interpretation of the NMR data is supported by a pulsed EPR study of the interactions of Mn(H2O)6 2+ with the P-deuterated analogue of methyl phosphite, CH3OP(2H)(O)2 -. The electron to 31P and 2H nuclear scalar coupling constants were found to be 4.6 and 0.10 MHz, respectively. In contrast, the effects of paramagnetic ions on the methoxy and ethoxy 1 H resonances of MeOPH and MEP are weak, and the evidence suggests that relaxation of these nuclei occurs by a dipolar mechanism. The wide variation in the relaxation sensitivities of the 1 H and 31 P nuclei of MeOPH and MEP permits us to study how differences in the strengths of the interactions between an observed nucleus and a paramagnetic center affect NMR T2 relaxations. We propose that these anion ligand probes may be used to study ligand-exchange reactivities of manganese complexes without requiring variable temperature studies. The 31 P T2 is determined by chemical association kinetics when the following condition is met: (T2M,P/T2M,H)(!"P/!"HP - 1) < 0.2 where T2M,P and T2M,H are the transverse relaxation times of the 31 P and 1 H nuclei when the probe is bound to the metal, and !"P and !"HP are the paramagnetic line broadenings of the 31 P and 1 H-P nuclei, respectively. We assert that the ratio T2M,P/T2M,H can be estimated for a general metal complex using the results of EPR and NMR experiments.