Structural, spectroscopic and electrochemical studies of binuclear nickel(II) complexes of bis(pentadentate) ligands derived from bis(1,4,7-triazacyclononane) macrocycles
Abstract
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.