Effective fixation of CO2 by iridium-catalyzed hydrosilylation.

Development of new chemical processes using CO2 as raw material has become a high priority for scientists. Utilisation of CO2 as feedstock would have the advantages of being naturally occurring, abundant, and inexpensive. However, due to the thermodynamic and kinetic stability of CO2 its activation represents a challenge for chemists. In this context, the low reactivity of CO2 can be overcome by catalytic activation and functionalization. For example, a number of transition-metal catalysts are known to be effective for the hydrogenation of CO2 to formic acid. However, these catalytic systems have not yet been applied industrially. The main reason is that direct catalysed hydrogenation of CO2 to formic acid is thermodynamically unfavourable (ΔG = +33 kJ/mol) requiring high H2 and CO2 pressures, exhibiting low activities even at these conditions. 7] The catalytic hydrosilylation of CO2 shows great promise for large scale transformation of this greenhouse gas into value-added chemicals. For instance silyl formates, which are easily hydrolysable to afford formic acid, have been applied for the production of silicon-based polymeric materials in industry and as reactive intermediates in organic syntheses. Thus, metal-catalysed hydrosilylation of CO2 to silyl formates is emerging as an alternative methodology for catalytic CO2 fixation. Ruthenium-catalysed hydrosilylation of CO2 allows formation of silyl formates, being disiloxanes the only by-products observed in these reactions. Despite, the good selectivities exhibited by these ruthenium catalytic systems high CO2 pressure is needed. On the other hand, iridium-catalysed hydrosilylation of CO2 to the methoxide level, first reported by Eisenberg and Eisenschmid in 1989, takes place under mild conditions (r.t. and 1 atm). The authors monitored the reaction of CO2 with Me3SiH at 40 oC in the presence of [Ir(CN)(CO)(dppe)] as catalyst by H and C NMR spectroscopy. This study revealed that reduction of CO2 produced, in a first stage, the silyl formate Me3Si-O-CHO. This species was further reduced to (Me3SiO)2(μ-CH2), which finally reacts with an additional equivalent of Me3SiH to afford Me3SiOCH3 and (Me3SiO)2(μ-O). In situ generated zirconium cationic species, known to be air and moisture sensitive, have also been used as effective catalyst for the hydrosilylation of CO2 to the methane level under mild conditions. H and C NMR studies of these reactions evidenced formation of (R3SiO)2(μ-CH2) and/or (R3SiO)2(μ-O) together with CH4. Some transition metal free catalytic systems are also effective for CO2 hydrosilylation. The large-scale applicability of the catalytic reduction of CO2 to silyl formates, which would represent a real breakthrough, lies on the improvement of the selectivity and activity under mild reaction conditions. For this purpose, hydrosiloxanes, as for instance 1,1,1,3,5,5,5-heptamethyltrisiloxane (HTMS), are attractive reducing agents since they are commercially available, non-toxic, soluble in most organic solvents and stable to air and moisture. Herein, we report the first example of a solvent free gramscale synthesis and isolation of a silyl formate by iridium-catalysed reduction of CO2 with HMTS. This reaction is selective, proceeds effectively under mild conditions and generates no waste. Additionally, a mechanistic insight into the catalytic hydrosilylation of CO2 is also provided. We have synthetized a new tridentate bis-(pyridine-2yloxy)methylsilyl (NSiN) ligand precursor with a geometry that favours facial coordination modes, in which the trans-labilising properties of the silicon atom are reduced by the electronic effect of the two Si–O bonds (Eq. 1).