The trinuclear gallium-bridged ferrocenophane [{Fe(η5-C5H4)2}3Ga2]:: Synthesis, bonding, structure, and coordination chemistry

The trinuclear ferrocenophane [{Fe(eta(5)-C5H4)(3)}(2)Ga-2] (3) featuring two sp(2)-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(eta(5)-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 D-3h 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 pi-bond contribution in the Ga-C bonds. Owing to steric constraints 3 cannot be reduced to the dianion 3(2-), which would feature a Ga-Ga bond. Compound 3 represents a stereochemically rigid difunctional Lewis acid allowing the formation of the adducts 3a-3d possessing linear donor-aceptor-aceptor-donor arrangements. Crystal structure data for 3a-3d show a symmetry-reduced chiral ferrocenophane core (D-3h -> D-3). A polymeric rodlike structure is observed for 3b and 3d caused by it-stacking effects (3b) or by a difunctional donor-acceptor interaction (3d). In solution, the chirality of the adducts is lost by rapid interconversion of the enantiomers. A cyclic voltammogram of 3b 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 3b are compared with those of other pyridine-stabilized gallyl-sustituted ferrocene derivatives and with ferrocene itself.