THERMODYNAMIC AND KINETIC ASPECTS OF 2-ELECTRON AND 3-ELECTRON REDOX PROCESSES MEDIATED BY NITROGEN ATOM TRANSFER

Treatment of (meso-tetra-p-tolylporphyrinato)manganese(V) nitride, (TTP)Mn = N, with (octaethylporphyrinato)manganese(II), Mn(OEP), in toluene leads to the reversible transfer of the nitrido ligand between the two metal complexes to form (OEP)Mn = N and Mn(TTP). The net result is a formal three-electron reduction of (TTP)Mn(V)N to (TTP)Mn(II). This occurs with a second-order rate constant of (5.6 +/- 1.2) x 10(3) M-1 s-1 to form an equilibrium mixture with K(eq) = 1.2 +/- 0.5 at 20-degrees-C. The thermodynamic and activation parameters for this process are DELTA-H-degrees = 2.0 +/- 0.2 kcal/mol, DELTA-S-degrees = 7.1 +/- 0.6 cal/mol.K, DELTA-H-double-ended-dagger = 9.4 +/- 0.7 kcal/mol, and DELTA-S-double-ended-dagger = -10 +/- 2 cal/mol.K. In THF at 20-degrees-C, the equilibrium constant is 1.8 +/- 0.2 and the rate constant drops to 2.3 +/- 0.3 M-1 s-1. When a manganese(III) porphyrin complex is used as a reductant, reversible nitrogen atom transfer still occurs but mediates a formal two-electron process. At 22-degrees-C, the exchange process between (TTP)MnCl and (OEP)Mn = N produces (TTP)Mn = N and (OEP)MnCl with a second-order rate constant of 0.010 +/- 0.007 M-1 s-1 (DELTA-H-double-ended-dagger = 19 +/- 2 kcal/mol and DELTA-S-double-ended-dagger = -3 +/- 6 cal/mol.K) and forms an equilibrium mixture with K(eq) = 24.3 +/- 3.3 (DELTA-H-degrees = -7.0 +/- 0.6 kcal/mol and DELTA-S-degrees = -17 +/- 2 cal/mol.K). Evidence for the formation of a binuclear mu-nitrido intermediate is presented for both processes. For the two-electron redox reaction, kinetic studies and mechanistic probes support a pathway which involves an initial chloride dissociation from the Mn(III) complex. Nitrogen atom transfer subsequently occurs between the Mn = N complex and the four-coordinate Mn(III) cationic species.