Preparation and structural characterization of group 1 metal complexes containing a chelating biphenolato phosphine ligand

The coordination chemistry of a potentially tridentate, dianionic biphenolato phosphine ligand with respect to group 1 metals is described. Deprotonation of bis-(3,5-di-tert-butyl-2-hydroxyphenyl)phenylphosphine (H2[OPO]) with two equivalents of n-BuLi, NaH, or KH in dimethoxyethane (DME) solutions produces the corresponding dinuclear alkali metal complexes [OPO]M2(DME)2 (M = Li, Na, K). The X-ray structure of [OPO]Li2(DME)2 reveals that the two lithium atoms are bridged by both phenolato oxygen donors with only one lithium being coordinated to the phosphorus donor. Consistently, variable-temperature 31P{1H} and 7Li{1H} NMR spectroscopic studies elucidate the coordination of the phosphorus donor in [OPO]Li2(DME)2 to one of the lithium atoms in solution. Interestingly, an X-ray diffraction study of the potassium complex indicates a dimeric structure with S2 symmetry for this species in which the four potassium atoms are bridged by both phosphorus and oxygen donors of the biphenolato phosphine ligands. These alkali metal complexes are active initiators for catalytic ring-opening polymerization of ε-caprolactone.