Can Coordinated Water be a Good Hydrogen Bond Acceptor? Crystallographic and Quantum Chemical Study

In an attempt to reveal the hydrogen bond-accepting abilities of coordinated water, a survey of Cambridge Structural Database crystal structures yielded 1229 hydrogen bonds between free water as a hydrogen bond donor and coordinated water as a hydrogen bond acceptor. These hydrogen bonds can be divided into two major groups: short linear and long nonlinear hydrogen bonds, the former being more frequent. It was revealed that the short linear hydrogen bonds of acceptor-coordinated water are longer than the hydrogen bonds of donor-coordinated water, which suggests that they are weaker. DFT calculations at the B97D/def2-TZVP level demonstrated that these interactions usually do not surpass the energy of the hydrogen bond between free water molecules (−5.02 kcal/mol) since electrostatic potentials on coordinated water oxygen are less negative than the one on free water oxygen. However, if hydrogen bonds of acceptor-coordinated water are accompanied by substantial secondary interactions, then the interaction can be stronger. The strongest calculated interaction involving a neutral transition metal complex has the energy of −9.31 kcal/mol; these interactions become stronger if complexes are negatively charged, reaching the energy of −13.19 kcal/mol. Long nonlinear hydrogen bonds of acceptor-coordinated water appear only as additional interactions to other hydrogen bonds (short and linear). This study shows that hydrogen bonds of acceptor-coordinated water are abundant in crystal structures and can provide significant stabilization to supramolecular systems with metal complexes, despite them being weaker than hydrogen bonds of donor-coordinated water.