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

Abstract

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 (180°). 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_⊥ = 2.143 ± 0.005 and g_‖ = 2.002 ± 0.005. No half field (ΔM = 2) signals were observed. RT EPR spectrum of polycrystalline [Cu^II(Me₆Cy)](ClO₄)₂ yields an EPR signal with g_‖ = 2.394 ± 0.005 and g_⊥ = 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 = ½). 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.