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 Msingle bondO 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 O8 donor set. All the Msingle bondO bonds in the six complexes are abnormally long (2.444–2.528 Å). 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 N4 donor set. Theoretical calculations for the bond dissociation energies (BDEs) of the Msingle bondO 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 Mnsingle bondO bonds in 1 and the long Cosingle bondO 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.