The Trinuclear Gallium-Bridged Ferrocenophane [{Fe(η5-C5H4)2}3Ga2]: Synthesis, Bonding, Structure, and Coordination Chemistry

The trinuclear ferrocenophane [{Fe(η5‐C5H4)3}2Ga2] (3) featuring two sp2‐hybridized gallium atoms in bridging positions between three ferrocene‐1,1′‐diyl units represents a novel type of ferrocene derivative. Compound 3 is obtained by thermal treatment of 1,1′‐bis(dimethylgallyl)ferrocene (1) in nondonor solvents or in diethyl ether as solvent and subsequent thermal decomplexation. The [1.1]ferrocenophane [{Fe(η5‐C5H4)2}2{GaMe}2] (2) is an intermediate in the formation of 3. The reaction of 3 with an excess of trimethylgallium leads back to 1 and proves the reversibility of the multistep reaction sequence. Theoretical calculations reveal a carousel‐type D3h structure for 3. The compound can best be described as being composed of three only weakly interacting ferrocenediyl units covalently connected by gallium atoms without any π‐bond contribution in the GaC bonds. Owing to steric constraints 3 cannot be reduced to the dianion 32−, which would feature a GaGa bond. Compound 3 represents a stereochemically rigid difunctional Lewis acid allowing the formation of the adducts 3 a–3 d possessing linear donor‐aceptor‐aceptor‐donor arrangements. Crystal structure data for 3 a–3 d show a symmetry‐reduced chiral ferrocenophane core (D3h→D3). A polymeric rodlike structure is observed for 3 b and 3 d caused by π‐stacking effects (3 b) or by a difunctional donor–acceptor interaction (3 d). In solution, the chirality of the adducts is lost by rapid interconversion of the enantiomers. A cyclic voltammogram of 3 b in pyridine reveals three quasi‐reversible oxidation steps at −356, −154, and 8 mV, indicating only weak electron delocalization in the cationic species. The redox potentials of the pyridine adduct 3 b are compared with those of other pyridine‐stabilized gallyl‐sustituted ferrocene derivatives and with ferrocene itself. Chiral carousels: A carousel‐type ferrocenophane with bridging gallium atoms has been obtained by thermal treatment of 1,1′‐bis(dimethylgallyl)ferrocene in a reversible reaction sequence. Theoretical calculations reveal a D3h structure for the base‐free compound. Cyclic voltammetric data of the pyridine adduct 1 reveal three quasi‐reversible oxidation steps.