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

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 Å) 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 E_1/2⁽¹⁾ = 0.12 V, E_1/2⁽²⁾ = −0.65 V for 1, and at E_1/2⁽¹⁾ = 0.12 V, E_1/2⁽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.