Facile rearrangement and electron transfer of 19-electron radicals from the reduction of the bischelated manganese carbonyl cation Mn(CO)2(PPh2(CH2)2PPh2)2+

The cathodic reduction of the bischelated manganese carbonly cation trans-Mn(CO)2(η2-DPPE)2+ (I+, where DPPE = PPh2(CH2)2PPh2) produces the metastable anion Mn(CO)2(eta2-DPPE)(η1-DPPE)- (II-) via an overall 2e process at a Pt electrode in either tetrahydrofuran or acetonitrile solution. The unusual observation of a cathodic curve crossing and two isopotential points in the cyclic voltammogram of I+ can be traced to an electron transfer with II- to afford a pair of neutral radicals, i.e., the 19e I* and the 17e II*. The former (which is also generated during the initial step in the reduction of I+) is a metastable species and spontaneously extrudes an end of one DPPE ligand to afford the second radical II* with the rate constant kr ∼ 4 x 106 s-1. The structures of the anionic and radical intermediates II-, II*, and I* are discussed in the context of their transient electrochemical behavior as well as their 31P NMR and ESR spectra. The rather slow rate of electron transfer between I+ and II- is ascribed to steric effects in carbonylmanganese ions encumbered with a pair of DPPE ligands. 59 refs., 7 figs., 1 tab