Structural, spectroscopic and electrochemical studies of nickel(II) "sandwich" complexes with ligands featuring tethered 1,4,7-triazacyclononane macrocycles

A structural and electrochemical study of the nickel(ii) complexes, [Ni(tacn)(2)](ClO4)(2) (1), [NiLeth](ClO4)(2) (2), [NiLox](ClO4)(2).H2O (3), [Ni2Ldur](ClO4)(4). 2H(2)O (4), [Ni3Ldur(H2O)(3)](ClO4)(6). 9H(2)O (5) and [Ni2Lisomes(H2O)(3)](ClO4)(4). 7H(2)O (6) [tacn=1,4,7-triazacyclononane, L-eth=1,2-bis(1,4,7-triazacyclonon-1-yl)ethane, L-ox=1,2-bis(1,4,7-triazacyclonon-1-ylmethyl)benzene, L-dur=1,2,4,5-tetrakis(1,4,7-triazacyclonon-1-ylmethyl)benzene, L-isomes=1,2,3-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene] is reported. Single-crystal X-ray crystallography has established that 2 and 6 feature nickel(ii) centres in a distorted octahedral geometry, which are sandwiched between pairs of tacn rings. In 6, a second nickel(ii) centre coordinates the third tacn ring of L-isomes and three water molecules complete the distorted octahedral coordination sphere. For the sandwiched Ni(ii) centres, steric contraints introduced by the ethane and ortho-oriented tacn rings in L-isomes cause significant distortions to the Ni(ii) coordination spheres. Cyclic, square-wave and near steady-state voltammetric studies on 1-6 indicate that the sandwiched nickel(ii) centres may be reversibly oxidised to the nickel(iii) state, with the potential for the nickel(ii)/(iii) couple showing a dependence on the nature of the tether linking the sandwiching tacn rings. Complex 4 undergoes oxidation in two overlapping one-electron processes [i.e., Ni(ii)Ni(ii)--> Ni(iii)Ni(ii)--> Ni(iii)Ni(iii)], indicating that the two nickel centres within the complex behave in an essentially independent fashion.