Precursor catenand complexes: synthesis, structure, and electrochemistry of bis(2,6-di-iminopyridyl) complexes of nickel(II). The single-crystal X-ray structureof [NiL42][BF4]2

Abstract

Reaction of Ni(BF₄)₂·6H₂O with 2 equivalents of the tridentate 2,6-di-iminopyridyl ligands (L)[L = NC₅H₃(CMeNR)-2,6 (R = Et, Ph, CH₂Ph, or CH₂CH₂CH₂Ph)] in EtOH-CH₂Cl₂, affords the octahedral complexes [NiL₂][BF₄]₂. The complex [NiL⁴₂][BF₄]₂(R = CH₂CH₂CH₂Ph) crystallises in the triclinic space group P, with a= 11.640(4), b= 12.884 6(27), c= l8.906(6)Å, a= 90.270(22), β= 95.226(28), γ= 112.098(24)°, and Z = 2. The single-crystal X-ray structure shows a distorted octahedral geometry around nickel(II) with four imine N-donors occupying equatorial sites [Ni–N(imine) 2.135(7), 2.121(7), 2.142(7), and 2.160(7)Å] and two shorter axial bonds to pyridyl N-donors [Ni–N (py) 1.989(7) and 1.981(7)Å]. The four propylphenyl groups of L extend past the inner co-ordination sphere of the metal ion into positions which could potentially be linked together to form octahedral catenand species. Cyclic voltammetry of [NiL₂][BF₄]₂(R = CH₂Ph or CH₂CH₂CH₂Ph) in MeCN at platinum electrodes shows for each complex two reversible one-electron reductions at ¹E_½=–1.56 and –1.53 V and at ²E_½=–1.89 and –1.87 V respectively, and one one-electron oxidation process at E_½=+1.22 and +1.12 V vs. ferrocene–ferrocenium respectively. The quantitative electrogeneration of the corresponding mono-reduced and -oxidised species has been accomplished and assigned by e.s.r. spectroscopy to the formation of nickel-(I) and -(III) complexes respectively.