Is bridging water in metal complexes the strongest hydrogen bond donor? Crystallographic and DFT insights

Arguably the most famous type of noncovalent interactions, hydrogen bonds are very important in numerous chemical and biological systems, including the ones with aqua complexes. Metal coordination enhances the strength of hydrogen bonds of water by amplifying the positive charge on interacting hydrogen atoms. In this work, the hydrogen bonding ability of water molecules acting as bridging ligand in metal complexes was studied by the means of crystallographic analysis, as well as density functional calculations.

A survey of crystal structures of high quality from the Cambridge Structural Database (CSD) was performed using the ConQuest program to find hydrogen bonds between bridging water in metal complexes as a hydrogen bond donor and the water molecule as a hydrogen bond acceptor. The energies of hydrogen bonds found in the CSD were calculated in the Gaussian 09 suite of programs using the dispersion-corrected B97D density functional and the def2-TZVP basis set.

A total of 88 hydrogen bonds of bridging water in metal complexes was found. The majority of these hydrogen bonds have short H∙∙∙O distances, mostly between 1.7 Å and 1.9 Å, with several examples even below 1.6 Å. Additionally, these hydrogen bonds possess a high degree of linearity, with the majority of O-H∙∙∙O angles between 165° and 175°. These geometrical parameters indicate short and linear, and therefore strong, hydrogen bonds. DFT calculations show that these interactions are indeed very strong. Specifically, neutral complexes containing bridging water form hydrogen bonds with free water that can reach an energy of -15.4 kcal/mol. This is significantly stronger than hydrogen bonds of neutral metal complexes of non-bridging water, which do not surpass -10.0 kcal/mol.

The joined crystallographic and computational study demonstrates the enhanced ability of bridging water in metal complexes to act as hydrogen bond donor.