A step too far? Assessment of the boroxide ligand in ring-opening polymerization

Dimesityl borinic acid, (meS)(2)BOH [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)(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 p-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( mu-Br)(2)Mg{mu-OB(mes)(2)}Mg(mu-Br)(2)Li" core. The magnesium alkyl compounds [Mg{OB(mes)(2)}R(THF)](2) [5, R = Me; 6, R = Bu] were obtained from the reaction 1)for 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 beta-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 E-caprolactone and rac-lactide is described.