A study of lanthanide complexes with di-2-pyridyl ligands

The ligands di(2-pyridyl) ketone (DPK) and cis-1,2 di(2-pyridyl) ethylene (DPE) are N,N,Odonor ligands that can undergo nucleophilic addition and become more flexible for coordination. The reaction between the lanthanide thiocyanate salt and DPK gave rise to seven novel complexes of the general formula [Ln(NCS)3(DPKOH)3], where Ln = La, Ce, Nd, Eu, Tb, Dy and Ho. 1H NMR spectroscopy verified that the ligand underwent nucleophilic addition upon coordination. This was further confirmed using UV-Vis spectroscopy which showed a shift in the absorption bands due to conjugation of electrons within the pyridyl ring upon coordination. UV-Vis spectroscopy was also utilised to test the covalent character of the neodymium and holmium complexes. X-ray diffraction and IR spectroscopy showed that three DPK ligands coordinated bidentately through a pyridinic nitrogen and a hydroxyl oxygen, while three isothiocyanato molecules completed the coordination environment around each metal. Furthermore, X-ray diffraction also revealed that these complexes are isostructural, ninecoordinate and the polyhedron which encloses each ion is of trigonal tricapped prismatic shape with D3h symmetry. Micro-analysis on all the complexes, except lanthanum and holmium confirmed the molecular formulae produced from the crystallographic data of each complex. The reaction between the lanthanide thiocyanate salt and DPE produced poor quality crystals which could not be detected by X-ray diffraction. The lanthanide salts used for this reaction were lanthanum, neodymium, europium, dysprosium and holmium. Upon coordination, conductivity measurements detected the presence of lanthanide ions in each solution. 1H NMR and IR spectroscopic studies showed that the ethylenic moiety of DPE underwent nucleophilic addition upon coordination. UV-Vis spectroscopy further confirmed nucleophilic addition upon coordination due to shifts in absorption bands. IR spectroscopy verified the possibility of a bidentate coordination to each metal through a pyridinic nitrogen and a hydroxyl oxygen as well as a monodentate coordination through isothiocyanato ligands. A micro-analysis on all the complexes provided the molecular formulae that can best fit each complex. The effect of the solvent molecules on the bonding parameters of the lanthanum complex was investigated. An analysis of the results produced from crystallographic data revealed the presence of intermolecular forces which interacted and stabilised the complex