Can Coordinated\nWater be a Good Hydrogen Bond Acceptor?\nCrystallographic and Quantum Chemical Study

In\nan attempt to reveal the hydrogen bond-accepting abilities of\ncoordinated water, a survey of Cambridge Structural Database crystal\nstructures yielded 1229 hydrogen bonds between free water as a hydrogen\nbond donor and coordinated water as a hydrogen bond acceptor. These\nhydrogen bonds can be divided into two major groups: short linear\nand long nonlinear hydrogen bonds, the former being more frequent.\nIt was revealed that the short linear hydrogen bonds of acceptor-coordinated\nwater are longer than the hydrogen bonds of donor-coordinated water,\nwhich suggests that they are weaker. DFT calculations at the B97D/def2-TZVP\nlevel demonstrated that these interactions usually do not surpass\nthe energy of the hydrogen bond between free water molecules (−5.02\nkcal/mol) since electrostatic potentials on coordinated water oxygen\nare less negative than the one on free water oxygen. However, if hydrogen\nbonds of acceptor-coordinated water are accompanied by substantial\nsecondary interactions, then the interaction can be stronger. The\nstrongest calculated interaction involving a neutral transition metal\ncomplex has the energy of −9.31 kcal/mol; these interactions\nbecome stronger if complexes are negatively charged, reaching the\nenergy of −13.19 kcal/mol. Long nonlinear hydrogen bonds of\nacceptor-coordinated water appear only as additional interactions\nto other hydrogen bonds (short and linear). This study shows that\nhydrogen bonds of acceptor-coordinated water are abundant in crystal\nstructures and can provide significant stabilization to supramolecular\nsystems with metal complexes, despite them being weaker than hydrogen\nbonds of donor-coordinated water.