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 Ti(2)Pn(2)(dagger), (1) (Pn(dagger) = 1,4{(SiPr3)-Pr-i}(2),C8H4) with CO2, is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO2, reaction is performed at -78 degrees 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 mu:eta(2),eta(2) 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,mu-O dimer in which the Ti-Ti bond has been cleaved and one pentalene ligand binds in an eta(8) 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 Ti(2)Pn(2) (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) 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 Ti-2, 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(mu-S) mono(CO) species 10. Ph3PS is an effective sulfur transfer agent for 1, enabling the synthesis of mono(mu-S) complex 11 with a double-sandwich structure and bis(mu-S) dimer 12 in which the Ti-Ti bond has been cleaved.