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

Dimesityl borinic acid, (mes) 2 BOH [mes = 2,4,6-Me 3 C 6 H 2 ], 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 = Et 2 O, 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 “BC 2 ” unit about the Li 2 O 2 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 ZnMe 2 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 MgBr 2 ·Et 2 O afforded an unprecedented tetrametallic compound, [Mg{OB(mes) 2 }Br·LiBr(OEt 2 ) 2 ] 2 ( 4 ), in which the lithium bromide side product is retained in a tricyclic structure containing a “Li(μ-Br) 2 Mg{μ-OB(mes) 2 }Mg(μ-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 of the Li salt with the Grignard reagent, MgMeBr, and protonolysis of MgBu 2 with (mes) 2 BOH, 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.