A THERMODYNAMIC STUDY OF THE ELECTRON-TRANSFER CHAIN CATALYZED SUBSTITUTION OF TRIPHENYLPHOSPHINE FOR IODIDE ON CPFE(CO)(2)I

The thermodynamic parameters governing the electron transfer chain cat alyzed substitution of triphenylphosphine for iodide on CpFe(CO)(2)I h ave been studied. The reaction is driven by the much higher stability of the triphenylphosphine complex relative to the iodide complex, and proceeds to completion even though the electron transfer which propaga tes the catalytic chain is endergonic. The standard reduction potentia l of CpFe(CO)(2)I is -1.64 V vs. Fc(+)/Fc, while that of CpFe(CO)(2)(P Ph3)(+) is -1.59 V. Nevertheless, the association constant for triphen ylphosphine with the 17-electron CpFe(CO)(2) fragment is 4 X 10(5) tim es that for iodide(log K-p = 1.9 +/- 1.9, log K-I = -3.7 +/- 1.9). The rate of the reaction is accelerated enormously by reduction of the io dide complex, which allows the substitution to proceed through the mor e labile 17/19-electron complexes. The contrast between the pi-basicit y of iodide and the pi-acidity of triphenylphosphine is proposed to be responsible for the favored complexation of triphenylphosphine by the relatively electron-rich CpFe(CO)(2) fragment. The application of red ox catalysis and redox equilibration to the study of such 17/19e equil ibria shows great promise for obtaining these difficult-to-measure for mation constants. (C) 1998 Elsevier Science S.A.