Stacking interactions of organometallic sandwich and half-sandwich compounds: crystallographic evidence and quantum chemical support

This review provides crystallographic insight into stacking interactions between aromatic ligands in organometallic sandwich and half-sandwich compounds obtained by analyzing all crystal structures deposited in the Cambridge Structural Database. The influence of various structural features of aromatic ligands onto their stacking interactions were considered, including ring size (cyclopentadienyl, benzene, tropylium, cyclooctatetraenide), substituents (toluene, p-cymene, methylcyclopentadienyl, pentamethylcyclopentadienyl) and fused rings (indenyl, naphthalene). The crystallographic data were supported by quantum chemical calculations of interaction energies, which show that coordination to transition metals can significantly strengthen stacking interactions of aromatic compounds. Particular attention was given to stacking interactions with large horizontal displacements, which are very important in molecular recognition process and enable formation of additional simultaneous interactions. Even though stacking interactions are shown to be the weakest type of interactions between metallocene compounds, they are shown to be very important supramolecular forces in crystal structures.