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How Do Nitriles Compare with Isoelectronic Alkynyl Groups in the Electronic Communication between Iron Centers Bridged by Phenylenebis- and -tris(nitrile) Ligands? An Electronic and Crystal-Structure Study

2010; American Chemical Society; Volume: 50; Issue: 1 Linguagem: Inglês

10.1021/ic101415c

1520-510X

Lauréline Bonniard, Samia Kahlal, Abdou K. Diallo, Cátia Ornelas, Thierry Roisnel, Gabriele Manca, João Rodrigues, Jaimé Ruiz, Didier Astruc, Jean‐Yves Saillard,

Metal complexes synthesis and properties

Density functional theory (DFT) calculations on the model [{FeCp(dpe)}2{1,4-C6H4(CN)2}]2+ (32+; dpe = diphosphinoethane) of salts of the cations [{FeCp(dppe)}2{1,4-C6H4(CN)2}]2+ (12+; dppe = 1,2-bis[diphenyldiphosphino]ethane) and [{FeCp*(CO)2}2{1,4-C6H4(CN)2}]2+ (22+), for which the X-ray crystal structures have been determined, as well as on its isomer [{FeCp(dpe)}2{1,3-C6H4(CN)2}]2+ (42+) and on the related complex [{FeCp(dpe)}3{1,3,5-C6H3(CN)3}]3+ (52+), indicate that the highest occupied molecular orbitals (HOMOs) of these compounds are localized on the metal centers with negligible participation of the C6 ring. Thus, the poly(nitrile)phenylene ligand efficiently quenches the electronic communication between the metal centers. This is at variance with the related isoelectronic polyacetylene phenylene complexes, in which the iron centers have been shown to be electronically coupled. Consistently, apart from the case of 33+, which shows some degree of delocalization, all of the oxidized forms of 32+, 42+, and 52+ can be described as class II, localized mixed-valent species, in agreement with the electrochemical data showing two close oxidation potentials around 1 V vs FeCp*2. This is at variance with the p-phenylene-briged biethynyldiiron analogue, for which extended electronic delocalization was earlier shown to provide greater degree of delocalization of the mixed valency. Time-dependent DFT calculations on 32+, 42+, and 52+ indicate that the lowest-energy absorption band is associated with metal-to-ligand charge-transfer transitions involving the metallic HOMOs and the two lowest unoccupied molecular orbitals that derive from the lowest π*(phenylene) orbitals with some π*(CN) bonding admixture.