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.