Ni(II) tetraphosphine complexes as catalysts/initiators in the ring opening polymerization of ε-caprolactone

Reactions of the tetraphosphine ligand N,N,N',N'-tetra(diphenylphosphanylmethyl)ethylene diamine) (dppeda) or 1,4-N,N,N',N'-tetra(diphenylphosphanylmethyl)benzene diamine (dpppda) with NiCl2, ancillary ligand (C2H5)2NCS2Na (Na(dtc)) and NH4PF6 afforded two binuclear complexes [(dtc)Ni(dppeda)Ni(dtc)](PF6)2 (1) and [(dtc)Ni(dpppda)Ni(dtc)](PF6)2 (2). Compounds 1 and 2 were characterized by elemental analyses, IR spectra, (1)H, (13)C{(1)H} NMR and (31)P{(1)H} NMR, electrospray ionization (ESI) mass spectra, thermogravimetric (TGA) experiments and single crystal X-ray diffraction. The Ni(ii) atoms in both compounds form a square planar coordination geometry while the ligands hold a similar end-to-end coordination mode. Both products can catalyze the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) while 1 exhibited a higher catalytic activity than 2. Influencing aspects such as solvent, temperature, the ratio of ε-CL to catalyst, the presence of benzyl alcohol, concentration of ε-CL and reaction intervals were investigated.