Suzanne J. Brudenell, Leone Spiccia, Alan M. Bond, Peter C. Mahon and David C. R. Hockless
Binuclear nickel(II) complexes of bis(pentadentate) ligands generated by functionalisation of the four secondary nitrogens in bis(tacn) macrocycles with 2-pyridylmethyl arms have been prepared, isolated and characterised. The pentadentate compartments are linked by (CH2)2 {tmpdtne, [Ni2(tmpdtne)(OH2)2][ClO4]4·5H2O 1}, (CH2)3 {tmpdtnp, [Ni2(tmpdtnp)(OH2)2][ClO4]4 2}, (CH2)4 {tmpdtnb, [Ni2(tmpdtnb)(OH2)2][ClO4]4 3}, CH2C6H4CH2-m {tmpdtnm-X, [Ni2(tmptdtm-X)(OH2)2][ClO4]4·3H2O 4} and CH2CHOHCH2 {tmpdtnp-OH, [Ni2(tmpdtnp-OH)(OH2)2][ClO4]4 5} bridging units. Single crystal X-ray diffraction studies of 3 have confirmed that each nickel(II) centre is in a distorted octahedral geometry defined by five N-donors from a pentadentate compartment of the ligand and an oxygen donor from a water ligand. Tetragonal elongation of the geometry is evidenced by the axial Ni–N distances, which are ca. 0.05 Å longer than the three equatorial Ni–N distances. The two pentadentate compartments are oriented in an anti configuration with the two aqua ligands pointing away from each other. Cyclic and square-wave voltammetric studies on 1–5 indicate that the complexes undergo oxidation to the nickel(III) state in two overlapping 1e– processes (i.e. NiIINIII → NiIINiIII → NiIIINiIII). Two partially resolved 1e– oxidation waves were observed for 1 and 5 while for 2–4 two 1e– processes were indicated by fact that the signals were broader than expected for a single 2e– process.