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

2001, Inorganic Chemistry

Methyl phosphite ((CH 3 O)P(H)(O) 2 -; MeOPH) and methylethyl phosphate ((CH 3 O)P(OCH 2 CH 3 )(O) 2 -; MEP) are two members of a class of anionic ligands whose 31 P T 2 relaxation rates are remarkably sensitive to paramagnetic metal ions. The temperature dependence of the 31 P NMR line broadenings caused by the Mn(H 2 O) 6 2+ ion and a water-soluble manganese(III) porphyrin (Mn III TMPyP 5+ ) indicates that the extent of paramagnetic relaxation 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, π∆ν par ) k assn [M], where ∆ν par is the difference between the line widths of the resonance in paramagnetic and diamagnetic solutions, and k assn is the second-order rate constant for association of the phosphorus ligand with the metal, M. Comparison of the 31 P T 1 and T 2 relaxation enhancements shows that rapid T 2 relaxation by the metal ion is caused by scalar interaction with the electronic spin. Relaxation of the phosphorus-bound proton of MeOPH ( 1 H-P) by Mn III TMPyP 5+ displayed intermediate exchange kinetics over much of the observable temperature range. The field strength dependence of 1 H-P T 2 enhancement and the independence of the 31 P T 2 support these assertions. As in the case of the 31 P T 2 , the 1 H-P T 2 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(H 2 O) 6 2+ with the P-deuterated analogue of methyl phosphite, CH 3 OP( 2 H)(O) 2 -. The electron to 31 P and 2 H 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 T 2 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 T 2 is determined by chemical association kinetics when the following condition is met: (T 2M,P /T 2M,H )-(∆ν P /∆ν HP -1) < 0.2 where T 2M,P and T 2M,H are the transverse relaxation times of the 31 P and 1 H nuclei when the probe is bound to the metal, and ∆ν P and ∆ν HP are the paramagnetic line broadenings of the 31 P and 1 H-P nuclei, respectively. We assert that the ratio T 2M,P /T 2M,H can be estimated for a general metal complex using the results of EPR and NMR experiments.