RESUMEN
Two extended nickel string complexes, [Ni(7)(bnapy)(4)Cl(2)](Cl)(2) (2) and [Ni(9)(bnapya)(4)Cl(2)](PF(6))(2) (3) (bnapy(2-) = 2,6-bis(1,8-naphthyridylamido)pyridine and bnapya(3-) = bis(6-(1,8-naphthyridylamido)pyridyl)amido), which possess two redox-active [Ni(2)(napy)(4)](3+) units, were synthesized and characterized. The electronic communication between the two redox-active units in both complexes can be investigated not only by magnetic measurements but also by analyzing the difference between two consecutive one-electron oxidation peaks (DeltaE(1/2)) of 2 and 3. The antiferromagnetic coupling between the two [Ni(2)(napy)(4)](3+) fragments become weaker as the metal frameworks are elongated (J = -13.21 and -1.48 cm(-1) for 2 and 3, respectively). Moreover, the DeltaE(1/2) values of 2 and 3 are 110 and 84 mV, respectively, which are smaller than that (300 mV) of their pentanickel analogue [Ni(5)(bna)(4)(Cl)(2)](PF(6))(2) (bna(-) = bisnaphthyridylamido) (1). These DeltaE(1/2) values indicate that the electronic communication decreases with increasing number of inner diamagnetic nickel ions in nickel string complexes.
RESUMEN
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.
