Donor-acceptor interactions and electron transfer in cyano-bridged trinuclear compounds

LUIS. M. BARALDO; P. ALBORES; ROSSI, M. B.; L. D. SLEP

INORGANIC CHEMISTRY

AMER CHEMICAL SOC

Lugar: Washington, USA; Año: 2006 vol. 45 p. 10595 - 10604

0020-1669

The NIR donor - acceptor charge transfer (DACT) bands of the series of trinuclear complexes trans- [(NC)(5)Fe-II/III- (mu-CN)(RuL4)-L-II(mu-NC)Fe-III(CN)(5)](5/4-) (L) pyridine, 4-tert-butylpyridine, and 4-methoxypyridine) are analyzed in terms of a simplified molecular orbital picture that reflects the interaction between the donor and acceptor fragments. The degree of electronic coupling between the fragments is estimated by a full fit of the DACT band profiles according to a three-state model inspired in the Mulliken - Hush formalism. The information is complemented with determinations performed on the asymmetric heterotrinuclear species trans-[(NC)(5)Co-III(mu-CN)Ru-II(py)(4)(mu-NC)Fe-III(CN)(5)](4-), whose preparation is reported here for the first time. The analysis of the NIR spectra of the symmetric trans-[(NC)(5)Fe-III-(mu-CN)(RuL4)-L-II(mu-NC)Fe-III(CN)(5)](4-) species reveals a low degree of mixing between the terminal acceptor fragments and the bridging moiety containing Ru-II, with H-12 values between 1.0 x 10(3) and 1.5 x 10(3) cm(-1). The reorganization energy contributions seem to be the same for the three species, even when the spectra were recorded in different media. This observation also applies for the Co-III-substituted compound. The computed potential energy surfaces (PES) of the ground state for these complexes show only one stationary point, suggesting that the Fe-II - Ru-III - Fe-III ( or Fe-II - Ru-III - Co-III) electronic isomers are not thermally accessible. One-electron reduction leads to asymmetric trans-[(NC)(5)Fe-II(mu-CN)(RuL4)-L-II(mu-NC)Fe-III(CN)(5)](5-) compounds with potentially two DACT bands involving the Ru-II and the Fe-II donor fragments. These species reveal a similar degree of electronic mixing but the PES shows three minima. We explore the role of the bridging fragment in the long-range thermally induced electron transfer between the distant iron centers. The results suggest that superexchange and hopping might become competitive paths, depending on the substituents in the bridging fragment.