TCNE and TCNQ ligands as efficient bridges in mixed-valence complexes containing iron-cyclopentadienyl and other organometallic systems

Reaction between the pi-accepting tetracyanoeethene (TCNE) or tetracyano-p-quinodimethane (TCNQ) ligands and the rr-electron-rich organometallic species Cp(dppe)Fe+ results in the formation of the complexes [{Cp(dppe)Fe}(n){eta(n)-TCNX}](PF6)(n) (n = 1, 2 and 4). Infrared spectroscopy, magnetic moment measurements, electron-spin resonance (ESR) and cyclic voltammetry data indicate a net transfer of one rr electron to the TCNX acceptor ligand. The polynuclear complexes have an intense intervalence electron transfer absorption band in the near-infrared region. The values for the intervalence parameters alpha = 0.02-0.06 and V-ab = 200-600 cm(-1) indicate that these complexes can be classified as class II according to Robin-Day. Estimation of the rate of electron transfer affords values in the order of 10(8) s(-1), which are compared and discussed with values estimated for other polynuclear systems containing TCNE and TCNQ ligands as bridges. The solvent effect on the intervalence transition follows Hush's prediction for highly polar solvents, thereby permitting evaluation of the reorganizational energy. For the related series [{Cp(dppe)Fe}(n)-(mu-X)] (PF6)(n), the reorganization energy appears to depend mainly on the inner reorganization energy through changes to vibrational modes of the bridge. Reorganizational energy is not as important in determining the electron transfer rate as the activation energy, E-a. A linear relationship is found between the rate constant ln(k(et)) and the activation energy. These results can be discussed within the Marcus theory of electron transfer. The fast and efficient electron transfer between the metal fragments is discussed using molecular orbital arguments. (C) 2000 Elsevier Science Ltd All rights reserved.