Alkali Metal Complexes of a Phosphine-Functionalized Cyclooctatetraene

The distinctive electronic properties and wide range of applications of cyclooctatetraene (COT) sandwich complexes make exploring this class of compounds essential. The new phosphine-functionalized proligand 1,4-bis(dimethylsilylmethyl)diphenylphosphinecycloocta-2,5,7-triene, {C₈H₈-1,4-(Me₂SiCH₂PPh₂)₂}, was synthesized by reaction of K₂COT with Me₂Si(Cl)CH₂PPh₂ first. Subsequent deprotonation with various alkali metal bases led to a series of alkali metal complexes [M₂{C₈H₆-1,4-(Me₂SiCH₂PPh₂)₂}] (M = Na, K, Rb, Cs). Notably, the crystallization solvent and the alkali metals significantly influenced the polymeric architecture and led to structural diversity of all complexes. For instance, the two phosphine arms either coordinate both to one metal atom, while the other metal ion is uncoordinated, or, in other structures, each metal atom is equally coordinated by just one phosphine function. Additionally, the coordination behavior of the pendant phosphine groups determines the emissive nature of these complexes. In all cases, polymeric structures are formed, in which intermolecular interactions between the COT ring and a metal atom are observed. Additionally, Rb forms a benzene-bridged eight-membered polymeric structure [{Rb₂(C₈H₆-1,4-(Me₂SiCH₂PPh₂)₂) (C₆H₆)₂}₈]_∞. Additionally, the lighter alkali metals in [M₂{C₈H₆-1,4-(Me₂SiCH₂PPh₂)₂}] can be exchanged by the heavier congeners by using the corresponding tert-butoxides.