KINETICALLY DESIGNED CONDITIONS FOR THE CATALYTIC FORMATION OF DISFAVORED PRODUCTS - THE REACTION OF (ETA(5)-C5H5)MO(CO)(3)(CENTER-DOT) WITH N,N,N',N'-TETRAMETHYL-1,4-PHENYLENEDIAMINE

In the absence of other reagents, the 17e molybdenum radical, (eta(5)-C5H5)Mo(CO)(3)(.), combines to form the stable dimer, [CpMo(CO)(3)](2). In the presence of TMPD, however, an electron transfer process ensues, in which the normally persistent radical TMPD(.+) is produced. Under these conditions, the absorbance of the TMPD(.+) radical disappears shortly thereafter. Various kinetic tests have been applied to show that this is the result of a sequence of two electron transfer steps. One is the reaction between CpMo(CO)(3.) (Mo-.) and TMPD, and the other is the reaction between Mo-. and TMPD(.+). The net result of the two reactions occurring in sequence is the disproportionation of the molybdenum radical, rather than the combination reaction that occurs in the absence of this redox-active amine. To the contrary, PhNMe(2) shows no such effect, confirming that these observations are correctly attributed to electron transfer and not to ligand-catalyzed disproportionation. That the TMPD-catalyzed sequence really is disproportionation was confirmed by the chemical identification of the products, CpMo(CO)(3)(-) and CpMo(CO)(3)NCCH3+ Since the disproportionation reaction, with K = 10(7), is less favored than radical combination, with K = 10(16) L mol(-1), this is a case where catalysis yields products that are less favored thermodynamically than those that would otherwise form spontaneously. In that sense, then, the light energy has been stored over long periods of time in intermediates that Lie at a higher Gibbs energy than the thermodynamic products.