Use of Phosphorus Ligand NMR Probes To Investigate Electronic and Second-Sphere Solvent Effects in Ligand Substitution Reactions at Manganese(II) and Manganese(III)

Abstract

Manganese/ligand association dynamics were studied using a series of structurally related anionic phosphorus ester ligand probes [CH(3)OP(O)(X)(Y)(-), where X = CH(3)O, CH(3)CH(2), or H and Y = O, S, or BH(3)]. Reactions of the probe ions with Mn(H(2)O)(6)(2+) and a manganese(III) porphyrin (Mn(III)TMPyP(5+)) were studied in aqueous solution by paramagnetic (31)P NMR line-broadening techniques. A satisfactory linear free energy relationship for reactions of the probe ions with Mn(H(2)O)(6)(2+) and Mn(III)TMPyP(5+) required consideration of both the basicity and solvent affinity of the probe ligands: log(k(app)) = log(k(0)) + alpha pK(a) + beta log(K(ext)), where k(0), alpha, and beta are metal complex dependent parameters and pK(a) and K(ext) represent the measured Bronsted acidity and water/n-butanol extraction constant for the probe anions, respectively. Reactions of Mn(H(2)O)(6)(2+) were relatively insensitive to changes in ligand basicity (alpha = -0.04) and favored the more hydrophilic anions (beta = -0.54). These observations are consistent with a dissociative ligand exchange mechanism wherein the outer-sphere complex is stabilized by hydrogen bonding between Mn(H(2)O)(6)(2+) and the incoming ligand. In contrast, reactions with Mn(III)TMPyP(5+) are accelerated by decreases in both the basicity (alpha = -0.43) and the hydrophilicity (beta = +0.97) of the probe. We conclude that reactions of Mn(III)TMPyP(5+) are also dissociative but that the aromatic groups of the porphyrin provide a hydrophobic environment surrounding the ligand binding site in Mn(III)TMPyP(5+). Thus, the probe/water solvent interactions must be significantly weakened in order to form the outer-sphere complex that leads to ligand substitution. This work demonstrates the utility of phosphorus relaxation enhancement (PhoRE) techniques for characterizing the second coordination sphere environment of metal complexes leading to ligation and will allow comparison of the second coordination spheres of Mn(H(2)O)(6)(2+) and Mn(III)TMPyP(5+) to those of other metal complexes.

A “Push−Pull” Mechanism for Heterolytic O−O Bond Cleavage in Hydroperoxo Manganese Porphyrins

“…This is corroborated by the fact that axial aqua ligands for Mn III porphyrins in aqueous solution are very labile. For example, the rate for displacement of an aqua ligand in Mn III TM-4-PyP by various phosphorus ligands was measured by Summers et al using 31 P NMR line-broadening techniques and ligand exchange rates ranging from 5 × 10 5 to 9 × 10 6 M −1 s −1 …”

Water exchange rates of water-soluble manganese(iii) porphyrins of therapeutical potential

“…That effect alone is sufficient for bringing the rate constants to the same order of magnitude. Furthermore, it was shown that the aromatic groups of the porphyrins provide a hydrophobic environment that may also change the accessibility of central metal ions to the negatively charged species involved in the above mentioned dismutation and reduction processes, 41 as well that the charges of the complexes are either above or below the plane, which channels the negativelycharged reactants toward the axial positions of Mn porphyrin more efficiently. 6 Both of the latter two effects are entirely ignored in the application of the Fuoss equation.…”

Mechanisms of reactions in solution

“…A comprehensive study of reactions between [CH 3 OP(O)(X)(Y)] À , X = CH 3 O, CH 3 CH 2 or H and Y = O, S or BH 3 and [Mn(H 2 O) 6 ] 3+ or [Mn III (TMPyP)] 5+ using paramagnetic 31 P NMR line-broadening shows that the reactions with the aqua complex involve an I d mechanism, crucially involving H-bonding with the aqua ion in the formation of an outer-sphere species, so that the rate was not sensitive to ligand basicity. 89 Although the reaction with the [Mn III (TMPyP)] 5+ is also dissociative, it is modified by the porphyrin's hydrophobic environment, decreasing basicity and hydrophilicity and increasing reaction rates. Rates of reaction of [Mn III (TEPyP)] and [Mn III (TPPS)] with HNO donors are rapid, k = B10 5 M À1 mol À1 , but they react slowly, if at all, with NO donors, allowing discrimination between them.…”