A theoretical insight on the anion⋯anion interactions observed in the solid state structure of a hetero-trinuclear complex

Abstract

A heteronuclear cobalt(III)/potassium complex, [(L¹)₂Co₂K(L²)₂]I₃ {where H₂L¹ = N,N-bis(3-ethoxysalicylidene)2,2-dimethyl-1,3-propanediamine and HL² = 3-ethoxysalicylaldehyde}, has been synthesized and characterized by several analytical techniques including single crystal X-ray diffraction analysis. The I₃⁻ anions form discrete dimers in the solid state with an I⋯I distance of 3.55 Å. The molecular electrostatic potential (MEP) surface shows a depletion of the charge density at the extension of I–I covalent bond (σ-hole). Although this value is negative due to the overall negative charge of the system, the value is significantly smaller (in absolute value) than the value at the negative belt around the I atoms, indicating that the binding mode observed in the X-ray structure minimizes the electrostatic repulsion. The I₃⁻⋯I₃⁻ dimer in the gas phase is not stable. However, it is energetically favourable in water and acetonitrile, thus suggesting the possibility that an anti-electrostatic halogen⋯halogen interaction may exist in solution. This gives a plausible explanation for the formation of these counterintuitive dimers in the solid state, where the effect of the surrounding molecules is expected to be higher than water solvation. Agreeably, the NCI plot computational tool also suggests that the interaction is attractive in nature.