Elusive N₂, CO complexes made

Bis(cyclopentadienyl) titanium sandwich compounds (titanocenes) have intrigued chemists because their reactivity makes them potentially useful for dinitrogen activation. But the compounds also have perplexed chemists because the reactivity, which can lead to titanium dimer formation, has hindered the study of the monomeric complexes. Introducing bulky and electron-withdrawing silyl substituents onto the cyclopentadienyl rings has helped. This strategy has now led Tamara E. Hanna, Emil Lobkovsky, and Paul J. Chirik of Cornell University to prepare the first isolable monomeric titanocenes with one N₂ or CO ligand (one shown, R = Si[CH₃]₂C₆H₅), as well as disubstituted titanocenes containing N₂, CO, or both ligands (J. Am. Chem. Soc. 2006, 128, 6018). Chirik's group previously prepared disubstituted N₂ titanocenes in which the cyclopentadienyl R substituent was Si(CH₃)₃ or branched alkyl groups. It turns out that the slightly bulkier Si(CH₃)₂C₆H₅ groups are a perfect match to stabilize the monosubstituted complexes. The results provide new insight into how N₂ coordination to transition metals can be controlled by careful manipulation of cyclopentadienyl substituents, the team notes.