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

Herein, we describe the synthesis, crystal structure, and electronic properties of {[K 2 (dmso)(H 2 O) 5 ][Ni 2 (H 2 mpba) 3 ]·dmso·2H 2 O} n ( 1 ) and [Ni(H 2 O) 6 ][Ni 2 (H 2 mpba) 3 ]·3CH 3 OH·4H 2 O ( 2 ) [dmso = dimethyl sulfoxide; CH 3 OH = 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 II 2 }. 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 II 2 } 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(H 2 O) 6 ] 2+ complex cation, where the three neighboring {Ni II 2 } 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.