A Step Too Far? Assessment of the Boroxide Ligand in Ring-Opening Polymerization

Dimesityl borinic acid, (mes)2BOH [mes = 2,4,6-Me3C6H2], has been employed as a source
of boroxide ligand in lithium, zinc, and magnesium chemistry; for structural comparisons,
the alkoxide ligand [OCH(mes)2]- has also been used. The lithium compounds [Li{OB(mes)2}(L)n]2 [1a, L = Et2O, n = 1; 1b, L = Py, n = 1; 1c, L = MeCN, n = 2] have been structurally
characterized, showing a common bimetallic core with bridging boroxide ligands that differ
in the relative rotation of the “BC2” unit about the Li2O2 metallacycle. The zinc compounds
[Zn{OR}Me]2 [2, R = B(mes)2; 3, R = CH(mes)2] have been synthesized from the reaction of
ZnMe2 with the corresponding ROH species. As for the Li salts, 2 and 3 display a dimeric
solid-state structure with μ-OR ligands. Reaction of the in situ generated lithium boroxide
1a with MgBr2·Et2O afforded an unprecedented tetrametallic compound, [Mg{OB(mes)2}Br·LiBr(OEt2)2]2 (4), in which the lithium bromide side product is retained in a tricyclic
structure containing a “Li(μ-Br)2Mg{μ-OB(mes)2}Mg(μ-Br)2Li” core. The magnesium alkyl
compounds [Mg{OB(mes)2}R(THF)]2 [5, R = Me; 6, R = Bu] were obtained from the reaction
of the Li salt with the Grignard reagent, MgMeBr, and protonolysis of MgBu2 with (mes)2BOH, respectively. X-ray crystallography showed formation of a dimer in each case, where
in 6 only the n-butyl group was observed in the solid state. Attempts at combining the
boroxide ligand with the widely used β-diketiminate ligand, [HC{C(Me)NAr}2]- (BDI), at a
zinc center are discussed, and a brief study into the potential of selected zinc complexes to
initiate the ring-opening polymerization of ε-caprolactone and rac-lactide is described.