title: The Reductive Activation of CO2Across a Ti=Ti Double Bond: Synthetic, Structural, and Mechanistic Studies.
abstract: The reactivity of the bis(pentalene)dititanium double-sandwich compound Ti2Pn†2(1) (Pn†= 1,4-{SiiPr3}2C8H4) with CO2is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO2reaction is performed at -78 °C, the 1:1 adduct4is formed, and low-temperature spectroscopic measurements are consistent with a CO2molecule bound symmetrically to the two Ti centers in a μ:η2,η2binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO2is quantitatively reduced over a period of minutes to give the bis(oxo)-bridged dimer2and the dicarbonyl complex3. In situ NMR studies indicated that this decomposition proceeds in a stepwise process via monooxo (5) and monocarbonyl (7) double-sandwich complexes, which have been independently synthesized and structurally characterized.5is thermally unstable with respect to a μ-O dimer in which the Ti-Ti bond has been cleaved and one pentalene ligand binds in an η8fashion to each of the formally TiIIIcenters. The molecular structure of7shows a "side-on" bound carbonyl ligand. Bonding of the double-sandwich species Ti2Pn2(Pn = C8H6) to other fragments has been investigated by density functional theory calculations and fragment analysis, providing insight into the CO2reaction pathway consistent with the experimentally observed intermediates. A key step in the proposed mechanism is disproportionation of a mono(oxo) di-TiIIIspecies to yield di-TiIIand di-TiIVproducts.1forms a structurally characterized, thermally stable CS2adduct8that shows symmetrical binding to the Ti2unit and supports the formulation of4. The reaction of1with COS forms a thermally unstable complex9that undergoes scission to give mono(μ-S) mono(CO) species10. Ph3PS is an effective sulfur transfer agent for1, enabling the synthesis of mono(μ-S) complex11with a double-sandwich structure and bis(μ-S) dimer12in which the Ti-Ti bond has been cleaved.
authors.affiliation: Department of Chemistry, School of Life Sciences, University of Sussex , Brighton BN1 9QJ, U.K.
authors.forename: Alexander F R
authors.initials: AF
authors.lastname: Kilpatrick
authors.affiliation: Department of Chemistry, University of Oxford , Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, U.K.
authors.forename: Jennifer C
authors.initials: JC
authors.lastname: Green
authors.affiliation: Department of Chemistry, School of Life Sciences, University of Sussex , Brighton BN1 9QJ, U.K.
authors.forename: F Geoffrey N
authors.initials: FG
authors.lastname: Cloke