Tin Complexes Derived from the Acids Ph2C(X)CO2H (X = OH, NH2): Structure and ROP Capability

Interaction of [Sn(OtBu)4] with the acid 2,2′-diphenylgylcine, Ph2C(X)CO2H (X = NH2), affords the complex {Sn[Ph2C(NH2)(CO2)]4}·2MeCN (1·2MeCN) after work-up, whereas when X = OH (benzilic acid), the complex {Sn[Ph2C(O)(CO2)]2(CH3CO2H)2} (2) is isolated. In 1·2MeCN, the four 2,2′-diphenylglycinate ligands adopt three different coordination modes (two N,O-chelates, an O,O-chelate, and a monodentate carboxylate ligand), whilst in 2, two cis-O,O-chelate ligands are present along with two acetic acid ligands, the latter being derived from hydrolysis of acetonitrile. Both 1 and 2 have been screened as catalysts for the ring opening polymerization of ε-caprolactone and δ-valerolactone; for comparison, the commercial catalyst [Sn(Oct)2], where Oct = 2-ethylhexanoate, and the precursor [Sn(OtBu)4] have been screened under similar conditions. The products were of low to high molecular weight for PCL and low to moderate molecular weight for PVL, with wide Ð values, and they comprised several types of polymer families, including OH-terminated, OH/OMe-terminated, and cyclic polymers. For both monomers, kinetic profiles indicated that [Sn(Oct)2] outperformed 1, 2, and [Sn(OtBu)4], though under certain conditions, 1 and 2 afforded high-molecular weight products with better control.