Topochemical control of covalent bond formation by dihydrogen bonding

This report explores the potential of the unconventional hydrogen bonds between the hydridic hydrogens in X-BH₃/^- (X = H, CN) and traditional OH or NH proton donors (dihydrogen bonds) to serve as preorganizing interactions for the topochemical assembly of covalent materials. Evidence for such topochemical control in the reaction B-H · · · H-X → B-X + H₂ was obtained in studies of the solid-state structures and reactivities of N-[2- (6-aminopyridyl)]acetamidine (NAPA) cyanoborohydride and triethanolamine (TEA) complexes of NaBH₄ and NaCNBH₃. The X-ray crystal structures of all three new compounds studied exhibit multiple dihydrogen bonds which are significant in defining the packing of the molecules in the solid state. Moreover, this new type of interaction is a powerful tool for crystal engineering; as planned, NAPA H₃BCN crystallized in the desired (NAPA H₃BCN)₂ closed loop coordination. Solid-state decomposition of NaBH₄ · TEA is topochemical, leading to a trialkoxyborohydride, which is not achievable in solution or melt. In addition to close H-H contacts, the relative acidity/basicity of the proton-hydride pairs make a significant contribution to the solid-state reactivity of dihydrogen-bonded systems, as demonstrated by the contrasting reactivities of the NaBH₄ · TEA and NaCNBH₃ · TEA complexes.