Diminishing accessibility of electrophilic nickel(ii) centres due to incorporation of a methylene spacer in the pendant side arm of a series of heterotrinuclear nickel(ii)/sodium complexes: a DFT study using a homodesmotic equation

Abstract

Two compartmental N₂O₄ donor Schiff bases, H₂L¹ [N,N-bis(3-methoxysalicylidene)-2,2-dimethylpropane-1,3-diamine] and H₂L² [N,N-bis(3-ethoxysalicylidene)-2,2-dimethylpropane-1,3-diamine], each having inner N₂O₂ and outer O₄ compartments, have been used to prepare four heterotrinuclear nickel(II) complexes [Ni(L¹)Na(L¹)Ni(NCS)]·H₂O (1), [Ni(L¹)Na(L¹)Ni(N₃)] (2), [Ni(L²)Na(L²)Ni]NCS·CH₃OH (3), and [Ni(L²)Na(L²)Ni]ClO₄ (4). All the complexes have been characterized by elemental and spectral analyses. Single crystal X-ray diffraction analyses have confirmed their structures. In each complex, nickel(II) is placed in the inner N₂O₂ compartment and sodium is placed in the outer O₄ compartment of the respective Schiff base ligand. In complexes 1 and 2, one nickel(II) centre is tetra-coordinated (square planar) and the other is penta coordinated (square pyramidal), whereas in complexes 3 and 4, both nickel(II) centres are tetra-coordinated. The geometries of complexes 1 and 3 have been optimized without a counterion (denoted as 1⁺ and 3⁺). The electrophilic nickel(II) centre is found to be accessible in complex 1⁺ and, conversely, it is unreachable in complex 3⁺, in agreement with the experimental result. Starting from the hypothetical 1⁺ complex where both nickel(II) centres are tetra-coordinated, the energy change for replacing four methyl groups with four ethyl groups has been computed using ethane and methane to complete the homodesmotic equation. The result is that the intra-molecular interaction of the four ethyl groups favours complex 3⁺ by ΔE = −26.1 kcal mol⁻¹.