The Reductive Activation of CO₂ Across a Ti=Ti Double Bond: Synthetic, Structural, and Mechanistic Studies

The reactivity of the bis(pentalene)dititanium double-sandwich compound Ti₂Pn^†₂ (1) (Pn^† = 1,4-{SiⁱPr₃}₂C₈H₄) with CO₂ is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO₂ reaction is performed at -78 °C, the 1:1 adduct 4 is formed, and low-temperature spectroscopic measurements are consistent with a CO₂ molecule bound symmetrically to the two Ti centers in a μ:η²,η² binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO₂ is quantitatively reduced over a period of minutes to give the bis(oxo)-bridged dimer 2 and the dicarbonyl complex 3. 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. 5 is 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 η⁸ fashion to each of the formally Ti^III centers. The molecular structure of 7 shows 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 CO₂ reaction pathway consistent with the experimentally observed intermediates. A key step in the proposed mechanism is disproportionation of a mono(oxo) di-Ti^III species to yield di-Ti^II and di-Ti^IV products. 1 forms a structurally characterized, thermally stable CS₂ adduct 8 that shows symmetrical binding to the Ti₂ unit and supports the formulation of 4. The reaction of 1 with COS forms a thermally unstable complex 9 that undergoes scission to give mono(μ-S) mono(CO) species 10. Ph₃PS is an effective sulfur transfer agent for 1, enabling the synthesis of mono(μ-S) complex 11 with a double-sandwich structure and bis(μ-S) dimer 12 in which the Ti-Ti bond has been cleaved.