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.

One-dimensional complexes with metal-containing backbones have been attractive to physicists and chemists for several decades, [1] and their interest has been further increased in the last ten years by potential applications in nanoelectronics.[2] Apart from the family of platinum and iridium blues, most chains of metal atoms were constrained to be aligned by means of purposely designed ligands. [3,4] The polypyridylamido ligands have produced to date the widest variety of metal chains with controlled lengths, as far as the nature and the number of metal atoms are concerned. [3] Strings with three to nine metal atoms were obtained with Cr and Ni. Chains containing three or five metal atoms, characterized for Co, Cu, Ru, and Rh, have unprecedented structural versatility and magnetic behavior that is hard to predict.[3] Even though heterometallic chain complexes are not uncommon [5] and often exhibit similarity to platinum blues [6] or involve weak noncovalent interactions between heavy atoms, [7] all linear complexes with polypyridylamido or closely related ligands characterized to date incorporate a single type of metal atoms. We report here the synthesis, Xray structure, and magnetic properties of [Co 2 PdCl 2 (dpa) 4 ] (1; dpa = bis(2-pyridyl)amide), the first trinuclear complex of dpa with a heteronuclear backbone. The magnetic behavior of 1 was investigated and found to be consistent with a complex interaction involving zero-field splitting superimposed on strong exchange coupling between two high-spin Co II ions separated by a diamagnetic Pd center. The ground-state electronic structure of 1 was interpreted by means of DFT calculations.Complex 1 was synthesized by a variant of the protocol currently used to obtain trinuclear dipyridylamide complexes.[8] Single-crystal X-ray analysis of 1·(C 2 H 5 ) 2 O resulted in the structure displayed in Figure 1. The heterometallic chain is helically wrapped by four syn-syn-type ligands, and the complex exhibits approximate D 4 symmetry. The average CoÀN (2.16 ) and CoÀCl (2.34 ) distances are quite similar to those observed at room temperature for the isolated, highspin Co II ion in the unsymmetrical form of [Co 3 Cl 2 (dpa) 4 ].[9]The average Pd À N (2.01 ) is much shorter than the average Co À N distance and suggests that the Pd atom is essentially low-spin (S = 0) in square-planar coordination. The relatively short PdÀCo distance (2.52 ) is consistent with a weak metal-metal interaction, as confirmed by DFT calculations.

The synthesis and crystal structure of two heteronuclear compounds stabilized by four dipyridylamide (dpa) ligands is reported. Cu2Pd(dpa)4Cl2 (1) and Cu2Pt(dpa)4Cl2 (2) exhibit an approximate D4 symmetry and a linear metal framework. They are structurally similar to the homotrinuclear complexes M3(dpa)4L2 already characterized with various transition metals (M=Cr, Co, Ni, Cu, Rh, Ru). With 26 metal valence electrons, they are also isoelectronic to the oxidized form of the tricopper complex [Cu3(dpa)4Cl2]+ (3), previously characterized and investigated by Berry et al.10 The magnetic properties and the EPR spectra of 1 and 2 are reported. The results for 1 are interpreted in terms of a weak antiferromagnetic interaction (2J=-7.45 cm(-1) within the framework of the Heisenberg Hamiltonian H=-2JAB ŝAŝB) between the Cu(II) magnetic centers. For 2, the antiferromagnetic interaction sharply decreases to <1 cm(-1). These properties are at variance with those of (3), for which a relatively strong antiferromagnetic interaction (2J=-34 cm(-1)) had been reported. DFT/UB3LYP calculations reproduce the decrease of the magnetic interaction from 3 to 1 and assign it to the role of the nonmagnetic metal in the transference of the superexchange coupling. However, the vanishing of the magnetic interaction in 2 could not be reproduced at this level of theory and is tentatively assigned to spin-orbit coupling.