DFT studies on the structure and stability of tetraaza macrocyclic nickel(II) complexes containing dicarbinolamine ligand moiety

Density functional theory calculations at M052X/6-311++G** level were performed to understand the structure and stability of Ni(II) tetraaza macrocyclic dicarbinolamine complex 1. The preferential stability of 1 over the hitherto unknown Ni(II) complex having fully conjugated macrocyclic ligand 2, is examined by analyzing geometric and electronic structures and energy considerations. The present calculations predict that in the trans (C-2) structure, 1 is 102 kcal/mol more stable than its components 2 and 2(OH) at M062X-D3/def2-QZVP//M052X/6-311++G** level. This significant stabilization explains the formation of 1 as experimentally observed. The calculations support a distorted square planar environment for Ni in 1, in agreement with the observed spectral and magnetic properties. In order to understand the stability of 1, we examined the second-order stabilizing interactions in natural bond orbital (NBO) basis, the role of the noncovalent dispersion energy, macrocyclic cavity size, Ni-ligand covalent bond strength, natural electronic population on the atomic centers and the nature of the frontier molecular orbitals in the complexes. The present study reveals that the higher stability of 1 over 2 is primarily due to the stronger covalent bonds between the Ni(II) centre, and two of the coordinating nitrogen atoms in 1 than in 2 and significant second-order stabilizing interactions originating from the NBOs involving the oxygen atoms.