Diminishing accessibility of electrophilic nickel(II) centres on incorporating methyl spacer in the pendant side arm of a series of hetero-trinuclear nickel(II)/sodium complexes: DFT study using 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 O4 compartments, have been used to prepare four hetero-trinuclear 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), [Ni(L2)Na(L2)Ni]ClO4 (4) All 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 inner N2O2 compartment and sodium is placed in outer O4 compartment of the respective Schiff base ligand. In complexes 1 and 2, one nickel(II) centre is tetra-coordinated (square planar) and 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 the counter ion (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, energy change of replacing four methyl groups by four ethyl groups have 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.