Synthesis, crystal structure, EPR and magnetic properties of a cyano-bridged CuII-NiII heterobimetallic complex:: an unusual structure with long-range ferromagnetic exchange through hydrogen bonding

A cyano-bridged Cu-II-Ni-II dimeric complex [Cu(Me6Cy)][Ni(CN)(4)] has been synthesized and characterized by its single crystal X-ray structure determination. The square planar [Cu(Me6Cy)](2+) cation is converted to a distorted trigonal bipyramidal (TBP) geometry through ligation to one of the cyano groups of [Ni(CN)(4)](2-) in the equatorial plane, keeping the other three as terminal. Inspection of the X-ray data reveals that the Cu - N(6) - C(6) angle of the bridging cyano group is perfectly linear (180degrees). The room temperature (RT) EPR spectrum of polycrystalline [Cu(Me6Cy)][Ni(CN)(4)] is typical for mononuclear copper(II) complexes of axial symmetry, where the flattened out symmetry axis is characterized by g(perpendicular to) = 2.143 +/- 0.005 and g(parallel to) = 2.002 +/- 0.005. No half field (DeltaM = 2) signals were observed. RT EPR spectrum of polycrystalline [Cu-II(Me6Cy)](ClO4)(2) yields an EPR signal with g(parallel to) = 2.394 +/- 0.005 and g(perpendicular to) = 2.045 +/- 0.005, indicating that this parent compound has a different geometry than [Cu(Me6Cy)][Ni(CN)(4)]. Variable temperature magnetic moment studies on [Cu(Me6Cy)][Ni(CN)(4)] indicate a very weak long-range ferromagnetic exchange through hydrogen bonding between copper(II) ions (S = 1/2). The present investigation provides a unique example where the original square planar [Cu(Me6Cy)](2+) moiety rearranges itself to a trigonal bipyramidal geometry under the influence of [Ni(CN)(4)](2-). This is probably only the second, rare example, and perhaps the first, for a macrocyclic precursor using only one cyano group of [Ni(CN)(4)](2-) for bridging, leading to a discrete dinuclear complex.