Asymmetric tetranuclear nickel chains with unidirectionally ordered 2-(α-(5-phenyl)pyridylamino)-1,8-naphthyridine ligands.

The new ligand, 2-(α-(5-phenyl)pyridylamino)-1,8-naphthyridine (Hphpyany), was synthesised by a palladium(0)-catalysed reaction of 2-chloro-1,8-naphthyridine with 2-amino-5-phenylpyridine in the presence of potassium tert-butoxide. Linear tetranickel metal complexes, [Ni4 (phpyany)4 Cl2 ](CF3 SO3 ) 1, [Ni4 (phpyany)4 Cl2 ](BF4 )2 2, [Ni4 (phpyany)4 (NCS)2 ](ClO4 ) 3 and [Ni4 (phpyany)4 (NCS)2 ](CF3 SO3 )2 4 were prepared and crystallographically characterised. Complexes 1-4 demonstrate that, for the first time, four asymmetric ligands align unidirectionally and thus configure (4,0)-form tetranickel strings, specifically, with the phenyl groups of the four phpyany- pointing to one side of the Ni4 chain and naphthyridyl to the other. The remarkably short Ni-Ni distances (ca. 2.33 Å) for 1 and 3 indicate partial metal-metal bonding, which can be viewed as both complexes containing one mixed-valence Ni2 3+ unit. The measurements of the magnetic susceptibility reveal that Ni4 7+ complexes 1 and 3 exhibit antiferromagnetic interactions (J = -42 cm-1 for 1 and -46 cm-1 for 3) between the terminal Ni2+ ion and the Ni2 3+ unit, while Ni4 8+ complexes 2 and 4 exhibit antiferromagnetic interactions (J = -33 cm-1 for 2 and -35 cm-1 for 4) between the two terminal Ni2+ ions. The results of the cyclic voltammetry indicate the presence of two reversible redox couples at E1/2 (1) = 0.12 V, E1/2 (2) = -0.65 V for 1, and at E1/2 (1) = 0.12 V, E1/2 (2) = -0.72 V for 3. The products of the first oxidation for 1 and 3 are the oxidised species 2 and 4, respectively. The values of single-molecule resistance (15.4 (±3.46) MΩ for 3 and 16.2 (±5.04) MΩ for 4) were determined by STM-based break-junction methods. The results represent the first conductance measurements of linear tetranickel chains.