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

摘要

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, [Ni-2(tmpdtne) (OH2)(2)][ClO4](4). 5H(2)O 1}, (CH2)(3) {tmpdtnp, [Ni-2(tmpdtnp) (OH2)(2)][ClO4](4) 2}, (CH2)(4) {tmpdtnb, [Ni-2(tmpdtnb)(OH2)(2)][ClO4](4) 3}, CH2C6H4CH2-m {tmpdtnm-X, [Ni-2(tmptdtm-X) (OH2)(2)][ClO4](4). 3 H2O 4} and CH2CHOHCH2 {tmpdtnp-OH, [Ni-2(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 A 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 le(-) processes (ie. (NiNIII)-N-II --> (NiNiIII)-Ni-II --> (NiNiIII)-Ni-III). Two partially resolved le- oxidation waves were observed for 1 and 5 while for 2-4 two le(-) processes were indicated by fact that the signals were broader than expected for a single 2e(-) process. [References: 41]