Structural characterization of borate esters in which sodium acts as a support to the structural framework

The borate ester derivatives of phenol, trans-cyclohexane-1,2-diol and mandelic acid have been prepared from NaBH4 and structurally characterized by X-ray crystallography. The product from the reaction with phenol appears as a crystallographic disorder of [(THF)(2)Na{B(OPh)(3)H}](2) (1) and [(THF)(2)Na{B(OPh)(OH2)H}](2) (2). Both compounds are dimeric with bridging borate groups linking the Na cations. The reaction with trans-cyclohexane-1,2-diol in DMSO yields the infinite polymer, [(DMSO)Na{B(O2C6H10)(2)}](infinity) (3), in which the Na cations link [B(O2C6H10)(2)](-) anions. The unusual 5-coordinate geometry of the sodium is completed by the coordination of a disordered DMSO molecule. In a similar manner, mandelic acid reacts to form an infinite lattice [Na(py)(2)][B{O2CC(O)Ph}(2)] (4), in which each sodium is coordinated to three [B{O2CC(O)Ph}(2)](-) anions, one through two interactions involving the alkoxide and carboxylate groups of a chelate mandelic acid, and two interactions involving the carboxylate groups of adjacent anions. The role of the Group 1 cation in supporting the structural framework of the borate anions is discussed.