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

Dalton Transactions Pub Date: 2016-09-26 DOI: 10.1039/C6DT03223K

Abstract

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)4Cl2](CF3SO3) 1, [Ni4(phpyany)4Cl2](BF4)22, [Ni4(phpyany)4(NCS)2](ClO4) 3 and [Ni4(phpyany)4(NCS)2](CF3SO3)24 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 ?) for 1 and 3 indicate partial metal–metal bonding, which can be viewed as both complexes containing one mixed-valence Ni23+ unit. The measurements of the magnetic susceptibility reveal that Ni47+ 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 Ni23+ unit, while Ni48+ 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.

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