Crystal Structures of Spiroborates Derived from 2,2′-Dihydroxybiphenyl

Salts of bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] borate (4) were prepared by the reaction of boric acid with 2,2′-dihydroxybiphenyl in the presence of various cations. Ten salts were crystallized, and their structures were solved by X-ray diffraction. In all cases examined, spiroborate 4 favors enantiomeric (R,R) and (S,S) structures with a flattened D2 geometry. Moreover, the crystals analyzed all incorporate the two enantiomers in a 1:1 ratio, even when the cations themselves are chiral and enantiomerically pure. In crystals of certain salts (NH4+, NH2Me2+, NMe4+, PPh4+, and MBA+), the packing of spiroborate 4 can be considered to define homochiral layers; in other salts, however, the organization of the anions is helical (NMe4+), columnar (NPr4+), or can be described as either layered or columnar (NBu4+ and NPent4+). These variations presumably arise because neighboring anions are not positioned reliably by strong directional interactions between them. Derivatives of spiroborate 4 that can engage in such interactions are expected to have well-defined molecular geometries and to serve as effective subunits for engineering crystals and other ordered materials with predetermined structures and properties.