Weak antiferromagnetic coupling for novel linear hexanuclear nickel(ii) string complexes (Ni612+) and partial metal–metal bonds in their one-electron reduction products (Ni611+)

Abstract

The preparation, crystal structures, magnetic properties and electrochemistry of novel linear hexanuclear nickel string complexes (Ni₆¹²⁺) and their corresponding 1-e⁻ reduction products (Ni₆¹¹⁺) are reported. In these complexes, the hexanickel chain is in a symmetrical arrangement (approximately D₄ symmetry) and is helically supported by four bpyany(2−) ligands [bpyany(2−) = the dianion of 2,7-bis(α-pyridylamino)-1,8-naphthyridine]. The Ni₆¹²⁺ complexes show that the two terminal nickel ions have high-spin states (S = 1) and the four inner ones have low-spin states (S = 0). The two terminal nickel ions exhibit weak antiferromagnetic coupling of ca. −5 cm⁻¹. All of Ni₆¹²⁺ complexes display three reversible redox couples at about −0.70, −0.20 and +1.10 V (vs. Ag/AgCl). The first reduction wave at about −0.20 V suggests facility of 1-e⁻ reduction for the Ni₆¹²⁺ compounds. The reaction of Ni₆¹²⁺ complexes with hydrazine afforded the 1-e⁻ reduction products (Ni₆¹¹⁺). As far as we are aware, the shortest bond distance of 2.202 Å with a partial metal–metal bond was observed in Ni₆¹¹⁺ compounds. The magnetic results of these Ni₆¹¹⁺ compounds are in agreement with a localized model, in which the two terminal nickel ions are in a spin state of S = 1 whereas the central Ni(3)–Ni(4) pair in a spin state of S = 1/2. The N₆¹¹⁺ compounds show relatively strong antiferromagnetic coupling of about 60 cm⁻¹ between the terminal and the central dinickel ions.