Syntheses and structures of eight-semi-coordinate M(II) (M = Mn, Fe, Co, Ni, Cu, Zn) complexes and density functional theory study of bond dissociation energies for the M-O semi coordinate bonds

Six new complexes 1-6 with the common formula [M(NiL)(4)][M(NCS)(4)] (M = Mn, Fe, Co, Ni, Cu and Zn for 1,2,3, 4, 5 and 6, respectively; the same below) were synthesised and structurally characterised by X-ray single crystal analysis. NiL acts as a complex ligand. L denotes the dianion of dimethyl 5,6,7,8,15,16-hexahydro-6,7-dioxodibenzo-[1,4,8,11]tetraazacyclotetradecine-13,18-dicarboxylate. Each M(II) centre of the [M(NiL)(4)](2+) complex cations in 1-6 adopts a distorted square-antiprism coordination geometry with a O-8 donor set. All the M - O bonds in the six complexes are abnormally long (2.444-2.528 angstrom). M(II) complexes having such weak coordination environments have not been reported, and eight-coordinate M(II) complexes with all the eight oxygen donor atoms coming from metalloligands have also not been documented. Each M(II) centre of the [M(NCS)(4)](2-) anions in 1-6 has a distorted tetrahedral coordination environment with a N-4 donor set. Theoretical calculations for the bond dissociation energies (BDEs) of the M - O semi coordinate bonds were performed using density functional theory at B3LYP level. The calculated BDE values are 23.8, 25.5, 20.0, 22.3, 19.8 and 18.2 kcal/mol for 1, 2, 3, 4, 5 and 6, respectively. The BDE values suggest that the long Mn - 0 bonds in 1 and the long Co - O bonds in 3 are significantly weaker than their significantly shorter counterparts in the formerly reported [Mn(NiL)(2)(NCS)(2)] and [Co(NiL)(2)(NCS)(2)], respectively. (C) 2012 Elsevier By. All rights reserved.