Structural, magnetic, high-frequency and high-field EPR investigation of double-stranded heterometallic [{Ni(en)2}2(µ-NCS)4Cd(NCS)2]n·nCH3CN polymer self-assembled from cadmium oxide, nickel thiocyanate and ethylenediamine

Abstract

A novel heterometallic 1D coordination polymer [{Ni(en)₂}₂(µ-NCS)₄Cd(NCS)₂]_n·nCH₃CN (en = ethylenediamine) has been prepared using the self-assembly process in a one-pot reaction of cadmium oxide, nickel and ammonium thiocyanates with an acetonitrile solution of ethylenediamine. The complex consists of an uncommon cis-Cd(SCN)₄(NCS)₂⁴⁻ fragment and a rare combination of cis-Ni(en)₂²⁺ and trans-Ni(en)₂²⁺ building blocks linked by µ_1,3-NCS bridges into a double-stranded zigzag chain structure. Each chain is comprised of [Ni₂Cd₂(µ-NCS–N,S)₄)] macrocycles with chair-like and rectangular-like shapes arrayed alternately. The shortest intrachain Cd⋯Cd separations are 9.535(1) and 10.868(2) Å, while the nearest Ni⋯Ni distances are 5.418(1) and 6.612(2) Å. A network of weak N–H⋯S hydrogen bonds, involving the terminal NCS ligands and NH₂-groups of en, links the infinite chains and results in the formation of an extended supramolecular three-dimensional framework. Variable-temperature (1.8–300 K) magnetic susceptibilities show a slight change of the µ_B value at low temperature, indicative of weak antiferromagnetic interactions (J = 1.55 cm⁻¹) between magnetic canters. High-field, high-frequency (100–400 GHz) EPR spectra were simulated using S = 1 ground state for separate Ni²⁺ ions with the spin Hamiltonian parameters g = 2.165, D = 0.45 cm⁻¹ and E = 0.03 cm⁻¹. According to DFT calculations, the D and E parameters are −0.35 cm⁻¹ and 0.049 cm⁻¹ for the cis arrangement of Ni²⁺ and 0.58 cm⁻¹ and 0.012 cm⁻¹ for trans.