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.