Synthesis, Characterization, and Activity of Yttrium(III) Nitrate Complexes Bearing Tripodal Phosphine Oxide and Mixed Phosphine–Phosphine Oxide Ligands

A series of four tripodal phosphine oxide ligands, (OPR2)2CHCH2POR2 (1a–1d), and four mixed phosphine–phosphine oxide ligands, (OPR2)2CHCH2PR2 (3a–3d), were synthesized and coordinated to yttrium to produce Y­(NO3)3[(OPR2)2CHCH2POR2] (2a–2d) and Y­(NO3)3[(OPR2)2CHCH2PR2]­(OPPh3) (4a–4d) complexes. The previously reported ligand 1a and unknown phosphine oxide ligands 1b–1d were generated in an unprecedented trisubstitution reaction of bromoacetaldehyde diethyl acetal, while the novel partially reduced ligands 3a–3d were synthesized from 1a–1d according to a known literature protocol for the selective monoreduction of bisphosphine oxides. The neutral yttrium complexes 2a–2d are nine-coordinate and display a tricapped trigonal-prismatic geometry. Complexes 4a–4d are also neutral, nine-coordinate species and have a pendant phosphine functionality, which provides the potential to form bimetallic early–late transition-metal complexes. Additionally, yttrium complexes 2a–2d were activated with base and tested for the ring-opening polymerization of ε-caprolactone, but the results showed that base by itself was significantly more effective than the yttrium species investigated.