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

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 O-4 compartments, have been used to prepare four heterotrinuclear nickel(ii) complexes [Ni(L-1)Na(L-1)Ni(NCS)]center dot H2O (1), [Ni(L-1)Na(L-1)Ni(N-3)] (2), [Ni(L-2)Na(L-2)Ni]NCS center dot CH3OH (3), and [Ni(L-2)Na(L-2)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 O-4 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 Delta E = -26.1 kcal mol(-1).