Hydrogen bonds and other intermolecular interactions in organometallic crystals†
Organometallic compounds have been studied with X-ray crystallography from their very discovery. Yet structural organometallic chemists were almost exclusively concerned with the molecular structure and stereochemistry of organometallic compounds and clusters rather than with their crystal structures and packing characteristics. The growing importance of crystal engineering and supramolecular chemistry, however, led to interest in the nature of the interactions that bind organometallic molecules into crystals. In part, these interactions are similar to those found in purely organic crystals because the peripheries of these molecules often contain organic residues. Yet molecular features peculiar to organometallic compounds also do lead to distinctive supramolecular characteristics. Most notable among these intermolecular interactions are hydrogen bonds. Organometallic compounds contain a wealth and diversity of hydrogen bonds that are without counterpart in the organic world. These include C–H⋯O bonds to M–CO acceptors, and hydrogen bonds wherein the metal atom itself acts as a donor or as an acceptor. Even more exotic is the dihydrogen bond M1–H⋯H–M2. Despite this variety, all these weak interactions have properties that resemble those of the more familiar hydrogen bonds such as O–H⋯O, N–H⋯O, O–H⋯N and N–H⋯N. Other interactions that are distinctive to organometallic compounds are the agostic interaction to electron deficient metals (C–H)⋯M and the aurophilic interaction Au⋯Au. The Cambridge Structural Database (CSD) is an essential tool in the analysis of weak intermolecular interactions. Since the number of organometallic crystal structures in the CSD is very large, the weakest of intermolecular interactions may be studied with ever-increasing degrees of reliability. Through such analysis one is able to obtain a more complete idea of organometallic crystal architecture. Crystal engineering must pass through the stage of analysis before crystal synthesis can be attempted and organometallic crystal engineering is still in its infancy. However, the progress made so far in understanding the nature of intermolecular interactions in these crystals indicates that one may expect rapid progress in the engineering of organometallic crystals with desired structures and properties.