Study of the Counter Cation Effects on the Supramolecular Structure and Electronic Properties of a Dianionic Oxamate-Based {Ni-2(II)} Helicate

Herein, we describe the synthesis, crystal structure, and electronic properties of {[K-2(dmso)(H2O)(5)][Ni-2(H(2)mpba)(3)]center dot dmso center dot 2H(2)O}(n) (1) and [Ni(H2O)(6)][Ni-2(H(2)mpba)(3)]center dot 3CH(3)OH center dot 4H(2)O (2) [dmso = dimethyl sulfoxide; CH3OH = methanol; and H(4)mpba = 1,3-phenylenebis(oxamic acid)] bearing the [Ni-2(H(2)mpba)(3)](2-) helicate, hereafter referred to as {Ni-2(II)}. SHAPE software calculations indicate that the coordination geometry of all the Ni-II atoms in 1 and 2 is a distorted octahedron (O-h) whereas the coordination environments for K1 and K2 atoms in 1 are Snub disphenoid J84 (D-2d) and distorted octahedron (O-h), respectively. The {Ni-2(II)} helicate in 1 is connected by K+ counter cations yielding a 2D coordination network with sql topology. In contrast to 1, the electroneutrality of the triple-stranded [Ni-2(H(2)mpba)(3)] (2-) dinuclear motif in 2 is achieved by a [Ni(H2O)(6)](2+) complex cation, where the three neighboring {Ni-2(II)} units interact in a supramolecular fashion through four R-2(2)(10) homosynthons yielding a 2D array. Voltammetric measurements reveal that both compounds are redox active (with the Ni-II/Ni-I pair being mediated by OH- ions) but with differences in formal potentials that reflect changes in the energy levels of molecular orbitals. The Ni-II ions from the helicate and the counter-ion (complex cation) in 2 can be reversibly reduced, resulting in the highest faradaic current intensities. The redox reactions in 1 also occur in an alkaline medium but at higher formal potentials. The connection of the helicate with the K+ counter cation has an impact on the energy levels of the molecular orbitals; this experimental behavior was further supported by X-ray absorption near-edge spectroscopy (XANES) experiments and computational calculations.