Insight in cyclopentadienyl metal complexes with superbulky ligands: The crystal structure of [CpBIGK]∞

Reaction of (4-nBu-C6H4)5CpH (=CpBIGH) with KH in THF and subsequent recrystallization from hexane/Et2O gave the solvent-free product CpBIGK in good crystalline yield (55%). Crystal structure determination revealed the one-dimensional coordination polymer [CpBIGK]∞. Despite the steric bulk of the CpBIG ligands, the chains are not perfectly linear. They can be pictured as an array of highly symmetric (CpBIG)2K-(CpBIG)2K- sandwiches bridged by K+ ions at an angle of 167.7(2)°. The C–K bond distances are much shorter than those in other cyclopentadienyl potassium complexes. This is explained by networks of attractive C–H⋯C(π) interactions between neighbouring CpBIG ligands. In all cases the C–H donors and C acceptors are represented by the ortho  -CH units in the aryl substituents. The C–H···C(π) networks within the supersandwich are less distinct than those observed in the metallocene sandwich complexes (CpBIG)2(CpBIG)2M (M = Ca, Sr, Ba, Yb, Sm). Significant out-of-plane bending of the aryl substituents indicates that, within the supersandwich chain, pairs of tighter C–H⋯C(π) networks exist. Large displacement factors for the K+ ions are explained by assuming induced polarization of these soft metals. This results in enforcement of the CpBIG–K interactions.

Graphical abstractThe solvent-free supersandwich [CpBIGK]∞ (CpBIG = (4-nBu-C6H4)5Cp) contains centrosymmetric as well as slightly bent (CpBIG)2K-(CpBIG)2K- units. Remarkably short Cp–K contacts are explained by pairs of attractive interactions between the CpBIG ligands (C–H⋯C(π) hydrogen bonding).Figure optionsDownload full-size imageDownload as PowerPoint slide