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

Density functional theory (DFT) calculations on the model [{FeCp(dpe)}(2){1,4-C6H4(CN)(2)}](2+) (3(2+); dpe = diphosphinoethane) of salts of the cations [{FeCp(dppe)}(2){1,4-C6H4(CN)(2)}}(2+) (1(2+); dppe = 1,2-bis[diphenyldiphosphino]ethane) and [{FeCp star(CO)(2)}(2){1,4-C6H4(CN)(2)}](2+) (2(2+)), 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+) (4(2+)) and on the related complex [{FeCp(dpe)}(3){1,3,5-C6H3(CN)(3)}](3+) (5(2+)), indicate that the highest occupied molecular orbitals (HOMOs) of these compounds are localized on the metal centers with negligible participation of the C-6 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 3(3+), which shows some degree of delocalization, all of the oxidized forms of 3(2+), 4(2+), and 5(2+) 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 FeCp2 star. 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 3(2+), 4(2+), and 5(2+) 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 pi(star)(phenylene) orbitals with some pi(star)(CN) bonding admixture.