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 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 TiIII 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-TiIII species to yield di-TiII and di-TiIV 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.

Bonding in Complexes of Bis(pentalene)dititanium, Ti2(C8H6)2

“…17 Optimizing the geometry of Ti 2 Pn 2 CO 2 from various starting geometries led to a minimum-energy structure with C 2 v symmetry ( 3 ). Selected geometric parameters are given in Table 1.…”

“…17 Geometry optimization of Ti 2 Pn 2 CS 2 led to structure 4 , analogous to 3 . Key structural parameters are given in Table 1, and selected MOs are shown in Figure 9.…”

“…The structure of Ti 2 Pn 2 CO ( 6 ) has C s symmetry with the CO bound sideways-on to the Ti 2 core, in agreement with the experimentally determined structure of the monocarbonyl complex Ti 2 (μ:η 5 ,η 5 -Pn † ) 2 CO. 17 …”

“…16,17 The mechanism of the reaction of its CO 2 complex is described in the companion paper; 17 here we examine the bonding in a range of derivatives in more detail.…”

Reactivity of a Dititanium Bis(pentalene) Complex toward Heteroallenes and Main-Group Element–Element Bonds

“…The molecular structure ( Figure 2) shows a bent p-TCD ligand in a μ:η 2 ,η 2 -binding mode between two Ti centers forming a dimetallocyclopropane-type motif with a TiCTi angle of 68.2(2)°. In a similar fashion to the related mono(oxo) complex, (μ:η 5 ,η 5 -Pn † )2Ti2(μ-O), 14 3 posses a S2 axis passing through the carbodiimide carbon (C53) and the midpoint of the TiTi bond, which results in one half of the dimer being generated by symmetry. 27 Indeed, the TiC distance in 3 is comparable with the shorter TiCcarbene distances of titanium complexes with NHC ligands reported to date, which range from 2.160 (3) 28 to 2.212 Å, 29 however, it is significantly longer than those for Schrock-type titanium carbene complexes (ca.…”