First investigation at elevated pressures to confirm the exact nature of the gated electron-transfer systems: volume profiles of the gated reduction reaction and nongated reverse oxidation reaction involving a [Cu(dmp)2(solvent)]2+/[Cu(dmp)2]+ couple (dmp = 2,9-dimethyl-1,10-phenanthroline)

Redox reactions involving the Cu(dmp)2]2+/+ couple (dmp = 2,9-dimethyl-1,10-phenanthroline) in acetonitrile were examined at elevated pressures up to 200 MPa. Activation volumes were determined as -8.8 and -6.3 cm3 mol-1 for the reduction cross-reaction by Co(bipy)3]2+ (bipy = 2,2'-bipyridine) and for the oxidation cross-reaction by Ni(tacn)2]3+ (tacn = 1,4,7-triazacyclononane), respectively. The activation volume for the hypothetical gated mode of the self-exchange reaction estimated from the reduction cross-reaction was -13.9 cm3 mol-1, indicating extensive electrostrictive rearrangement of solvent molecules around the CuII complex during the change in the coordination geometry before the electron-transfer step. On the other hand, the activation volume for the self-exchange reaction estimated from the oxidation cross-reaction was -2.7 +/- 1.5 cm3 mol-1. Although this value was within the range that can be interpreted by the concept of the ordinary concerted process, from theoretical considerations it was concluded that the reverse (oxidation) cross-reaction of the gated reduction reaction of the Cu(dmp)2(CH3CN)]2+/Cu(dmp)2]+ couple proceeds through the product excited state while the direct self-exchange reaction between Cu(dmp)2(CH3CN)]2+ and Cu(dmp)2]+ proceeds through an ordinary concerted process.