A new generation of metal string complexes: structure, magnetism, spectroscopy, theoretical analysis, and single molecular conductance of an unusual mixed-valence linear [Ni5]8+ complex

Two new linear pentanickel complexes Ni5(bna)4(Cl)2]PF6]2 (1) and Ni5(bna)4(Cl)2]PF6]4 (2; bna=binaphthyridylamide), were synthesized and structurally characterized. A derivative of 1, Ni5(bna)4(NCS)2]NCS]2 (3), was also isolated for the purpose of the conductance experiments carried out in comparison with Ni5(tpda)4(NCS)2] (4; tpda=tripyridyldiamide). The metal framework of complex 2 is a standard Ni5]10+ core, isoelectronic with that of Ni5(tpda)4Cl2] (5). Also as in 5, complex 2 has an antiferromagnetic ground state (J=-15.86 cm(-1)) resulting from a coupling between the terminal nickel atoms, both in high-spin sate (S=1). Complex 1 displays the first characterized linear nickel framework in which the usual sequence of NiII atoms has been reduced by two electrons. Each dinickel unit attached to the naphthyridyl moieties is assumed to undergo a one-electron reduction, whereas the central nickel formally remains NiII. DFT calculations suggest that the metal framework of the mixed-valence complex 1 should be described as intermediate between a localized picture corresponding to NiII-NiI-NiII-NiI-NiII and a fully delocalized model represented as (Ni2)3+-NiII-(Ni2)3+. Assuming the latter model, the ground state of 1 results from an antiferromagnetic coupling (J=-34.03 cm(-1)) between the two (Ni2)3+ fragments, considered each as a single magnetic centre (S=3/2). An intervalence charge-transfer band is observed in the NIR spectrum of 1 at 1186 nm, suggesting, in accordance with DFT calculations, that 1 should be assigned to Robin-Day class II of mixed-valent complexes. Scanning tunnelling microscopy (STM) methodology was used to assess the conductance of single molecules of 3 and 4. Compound 3 was found approximately 40% more conductive than 4, a result that could be assigned to the electron mobility induced by mixed-valency in the naphthyridyl fragments.