Titanium imido complexes of cyclooctatetraenyl ligands

Reaction of Ti(NR)Cl2(py)3] (R=tBu or 2,6-iPr2C6H3) with K(2)COT] (COT=C8H8) or Li2COT'] (COT'=1,4-C8H6(SiMe3)2) gave the monomeric complexes Ti(NR)(eta8-COT)] or Ti(NR)(eta8-COT')], respectively. The pseudo-two coordinate, "pogo stick" geometry for these complexes is unique in both early transition-metal and cyclooctatetraenyl ligand chemistry. In contrast, reaction of Ti(N-2,6-Me2C6H3)Cl2(py)3] with K2COT] gave the mu-imido-bridged dimer Ti2(mu-N-2,6-Me2C6H3)2(eta8-COT)2]. It appears that as the steric bulk of the imido and C8 ring substituents are decreased, dimerisation becomes more favourable. Aryl imido COT complexes were also prepared by imido ligand exchange reactions between anilines and Ti(NtBu)(eta(8)-COT)] or Ti(NtBu)(eta(8)-COT')]. The complexes Ti(NtBu)(eta(8)-COT)], Ti(N-2,6-iPr2C6H3)2(eta8-COT)] and Ti2(mu-N-2,6-Me2C6H3)2(eta8-COT)2] have been crystallographically characterised. The electronic structures of both the monomeric and dimeric complexes have been investigated by using density functional theory (DFT) calculations and gas-phase photoelectron spectroscopy. The most striking aspect of the bonding is that binding to the imido nitrogen atom is primarily through sigma and pi interactions, whereas that to the COT or COT' ring is almost exclusively through delta symmetry orbitals. A DFT-based comparison between the bonding in Ti(NtBu)(eta8-COT)] and the bonding in the previously reported late transition-metal "pogo stick"complexes Os(NtBu)(eta6-C6Me6)], Ir(NtBu)(eta5-C5Me5)] and Ni(NO)(eta5-C5H5)] has also been undertaken.