Novel multi-spin-state linear hexanickel complexes Ni611+ and their singly oxidized products Ni612+ with 1,8-naphthyridine-based ligands: Tuning the redox properties of the metal string

Abstract The new ligand, 2,7-bis(α-pyrimidylamino)-1,8-naphthyridine (H 2 bpmany), was prepared by the reaction of 2,7-dichloro-1,8-naphthyridine with 2-aminopyrimidine in the presence of sodium tert -butoxide under palladium-catalyzed conditions. The linear hexanickel Ni 6 11 + complexes [Ni 6 (μ 6 -bpmany) 4 X 2 ]Cl (X = Cl ( 1 ); X = NCS ( 2 )) and their singly oxidized products [Ni 6 (μ 6 -bpmany) 4 X 2 ](BF 4 ) 2 (X = Cl ( 3 ); X = NCS ( 4 )) have been synthesized, and compounds 1 , 2 and 4 have been crystallographically characterized. The crystal structures of the Ni 6 11 + complexes show remarkably short Ni–Ni distances (ca. 2.22 A), clearly indicative of partial metal–metal bonding in the mixed-valence Ni 2 3 + unit. This is also verified by the axial X-band EPR spectra of the complexes in solution. Magnetic susceptibility measurements reveal that the Ni 6 11 + complexes exhibit antiferromagnetic interactions ( J  = −47 cm −1 ) between the terminal Ni 2+ ion and the central Ni 2 3 + unit, and the Ni 6 12 + complexes exhibit weak antiferromagnetic interactions ( J  = −5 cm −1 ) between the two terminal Ni 2+ ions. The cyclic voltammograms display three reversible redox waves at E (1) 1/2  = +0.87, E (2) 1/2  = −0.02 and E (3) 1/2  = −0.46 V for 1 , and E (1) 1/2  = +0.96, E (2) 1/2  = −0.01, and E (3) 1/2  = −0.41 V for 2 . The relatively low potentials of E (2) 1/2 suggest that the Ni 6 11 + complexes can be easily converted to their Ni 6 12 + forms.