Electron exchange by hexakis(tert-butyl isocyanide)- and hexakis(cyclohexyl isocyanide)manganese(I,II). Solvent effect on the rate constant and the volume of activation

Abstract

The rate of electron self-exchange of Mn(CNC(CH3)3)6+/2+ and Mn(CNC6Hn)6+/2+ as the BF4' salts has been measured by 55Mn NMR line broadening as a function of pressure, temperature, and concentration in acetonitrile, bromobenzene, benzonitrile, acetone, diethyl ketone, methanol, ethanol, methylene chloride, and trimethyl phosphate, and various binary mixtures of methylene chloride, bromobenzene, and acetonitrile. The values of AV* obtained are negative and cover a range of ca. 12 cm1 23/mol, which is limited by ion pairing in the solvents of lower dielectric constant. The variation of the ambient pressure rate constant with solvent is qualitatively different for the Mn(CNC(CH3)3)6+/2+ reaction than was observed for the Mn(CNC6Hn)6+/2+ reaction. This is taken as further evidence for a significant influence of rather subtle differences in solvation on the molecular level that are not approximated by dielectric continuum models.

Activation and Reaction Volumes in Solution. 3

“…Such ion pairs formed by cationic reactants in nonaqueous solvents are generally less reactive in electron-transfer reactions than the free ions according to Wherland. 272 The experimental ∆V q values are expected to be more negative than the calculated ones, since pressure in general breaks up ion pairs by favoring solvation of the separated ions. 404 Recently Swaddle et al 45,407 determined volumes of activation for heterogeneous self-exchange reactions of a series of complexes on the surface of an electrode.…”

“…Self-exchange reactions in polar organic solvents also exhibit distinctly negative volumes of activation (entries 930−960) but ion pairing with counterions is expected to influence the values. Such ion pairs formed by cationic reactants in nonaqueous solvents are generally less reactive in electron-transfer reactions than the free ions according to Wherland . The experimental Δ V ⧧ values are expected to be more negative than the calculated ones, since pressure in general breaks up ion pairs by favoring solvation of the separated ions …”

Homogeneous versus Heterogeneous Self-Exchange Electron Transfer Reactions of Metal Complexes: Insights from Pressure Effects

“…The remaining entries in Table are for nonaqueous systems studied by Wherland and co-workers. , For the Mn II/I isonitrile complexes, is strongly negative but still less so than ; this is unlikely to be due to ion pairing, which tends to retard electron transfer but is broken up by application of pressure, but rather reflects the failure of eq 46 for solvents of low ε because and (eqs 48 and 49) become numerically huge and the calculation becomes unrealistic. One might expect the cage (clathrochelate) couple Co(dmg‘ ‘(BF) 2 ) +/0 in the more polar solvent acetonitrile to be well-behaved, but its is similar to those for Co(en) 3 3+/2+ and Ru(en) 3 3+/2+ in water; it may not be coincidental, however, that the anomalous values for both Ru(en) 3 3+/2+ and Co(dmg 3 (BF) 2 ) +/0 were obtained from the cross-relation.…”

Application of High Pressure in the Elucidation of Inorganic and Bioinorganic Reaction Mechanisms

“…In the cases of significant deviations between experimental and calculated values of the volumes of activation for [Co(en) 3 ] 2 þ /3 þ and [Co (phen) 3 ] 2 þ /3 þ , exchange may follow an adiabatic pathway, with an explanation presented of attendant features relating to Co(II)/Co(III) spin states. [276][277][278][279][280][281] This section on redox reactions will now be devoted to an examination, in suitable detail, of case studies of reactions (not self-exchange) in which determination of kinetic parameters at different hydrostatic pressures has played some role in understanding the reaction mechanism. The reactions selected include some of background importance in bioinorganic chemistry.…”