Ambarish Ray

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 CuII–NiII 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 [CuII(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.

Geometrical isomers of [TEAH][Co(LSe)2].xH2O: synthesis, structural, spectroscopic and computational studies.

Trans-1, [HNEt3][CoIII(LSe)2].H2O and cis-1, [HNEt3][CoIII(LSe)2].3H2O have been synthesized and characterized by single-crystal X-ray studies. The counter ion Et3NH+ plays a crucial role in the crystal packing leading to the formation of two distinctly different supramolecular assemblies in the two complexes. In trans-1, Co-bisphenolate units and triethylamine molecules are arranged in a linear fashion leading to a supramolecular columnar assembly along the crystallographic a-axis. In this assembly, triethylammonium ions are sandwiched between successive Co-bisphenolate units and act as gluing agents joining Co-bisphenolate units on either side through C-Hpi interactions. In sharp contrast to trans-1, Co-bisphenolate units and triethylammonium ions in cis-1 are arranged in a helical supramolecular assembly through similar C-Hpi interactions along the crystallographic b-axis. The SeSe van der Waals interactions may be responsible for the predominant occurrence of the cis- isomer. The cyclic voltammetric studies showed quasi-reversible waves for the cobalt(III)-->cobalt(II) reductions with E1/2=0.635 and 0.628 V vs. Ag/AgCl for cis-1 (at approximately 5 degrees C) and trans-1 (at approximately 25 degrees C), respectively. DFT calculations show that the trans-form is the thermodynamic product with higher stability than the cis- one, which is consistent with the variable temperature 1H NMR studies.