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 MO 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 MO 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 MO 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 MnO bonds in 1 and the long CoO 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.

Graphical abstractAll the M(II) centres (M = Mn, Fe, Co, Ni, Cu or Zn) in the six complexes adopt coordination environments featuring eightfold oxygen coordination and all the MO bonds being abnormally long.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The six complexes have centres adopting unprecedented coordination environments. ► All the MO bond distances are abnormally long. ► M(II) centres having the high coordination number of 8 often form long MO bonds. ► Complexes with weak coordination environments would present different properties. ► The BDEs suggest that the long MO bonds in 1–6 are significantly weak.