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. Ni^IINI^II → Ni^IINi^III → Ni^IIINi^III). 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.