Francisco J. Fernández-Alvarez

Effective Fixation of CO2 by Iridium-Catalyzed Hydrosilylation†

Financial support from MINECO/FEDER (CTQ2010-15221, CSD2009-00050 CONSOLIDER-INGENIO 2010, CTQ2011-27593, “Ramon y Cajal” (P.J.S.M.) and “Juan de la Cierva” (M.I.) programs), and DGA/FSE (group E7), is acknowledged.

Homogeneous catalytic reduction of CO2 with hydrosilanes

Catalytic CO2 hydrosilylation is a thermodynamically favored chemical process that could be potentially applied to large-scale transformations of this greenhouse gas. During the last decade, there have been an increasing number of experimental studies regarding metal-catalyzed CO2 hydrosilylation processes. The first examples of catalytic systems used for CO2 hydrosilylation employed late transition metals such as ruthenium and iridium. Presently, there are several examples of other catalysts, including transition metal species acting alone or together with B(C6F5)3, as well as metal-free frustrated Lewis pairs (FLPs) and organocatalysts which are able to perform this reaction.

An Alternative Mechanistic Paradigm for the β-Z Hydrosilylation of Terminal Alkynes: The Role of Acetone as a Silane Shuttle

The β-Z selectivity in the hydrosilylation of terminal alkynes has been hitherto explained by introduction of isomerisation steps in classical mechanisms. DFT calculations and experimental observations on the system [M(I)2{κ-C,C,O,O-(bis-NHC)}]BF4 (M=Ir (3a), Rh (3b); bis-NHC=methylenebis(N-2-methoxyethyl)imidazole-2-ylidene) support a new mechanism, alternative to classical postulations, based on an outer-sphere model. Heterolytic splitting of the silane molecule by the metal centre and acetone (solvent) affords a metal hydride and the oxocarbenium ion [R3Si-O(CH3)2](+), which reacts with the corresponding alkyne in solution to give the silylation product [R3Si-CH=C-R](+). Thus, acetone acts as a silane shuttle by transferring the silyl moiety from the silane to the alkyne. Finally, nucleophilic attack of the hydrido ligand over [R3Si-CH=C-R](+) affords selectively the β-(Z)-vinylsilane. The β-Z selectivity is explained on the grounds of the steric interaction between the silyl moiety and the ligand system resulting from the geometry of the approach that leads to β-(E)-vinylsilanes.

CO2 activation and catalysis driven by iridium complexes

Recent reports on homogeneous catalytic transformation of carbon dioxide by iridium complexes have prompted us to review the area. Progress on new iridium catalysts for carbon dioxide transformations should take into account the interaction of carbon dioxide with the iridium center, which seems to be governed by the oxidation state of iridium and the nature of the carbon dioxide molecule. Most examples of iridium catalyzed carbon dioxide reductions are based on IrIII centers. These reactions take place through outer‐sphere mechanisms, by means of nucleophilic attack on the carbon atom. In all the reported systems, the nucleophile is always a hydrido ligand coordinated to a IrIII center. Future challenges on iridium catalyzed functionalization of carbon dioxide include the development of efficient electrophiles, compatible with the inclusion of appropriate nucleophiles, which would allow the preparation of value‐added organic molecules using CO2 as C1 feedstock.

Outer‐Sphere Ionic Hydrosilylation Catalysis

Hydrosilylation reactions play a key role in modern organic synthesis thanks to the low cost and non-toxic nature of silicon reagents, together with the mild reaction conditions required and the ease for further functionalization of their reaction products. The importance of this reaction has led to an extensive study of the mechanisms involved. The great wealth of mechanistic data present in the literature seems to prove that postulation of a general catalytic cycle that comprises all cases is a difficult task. The fact that the nature of the hydrosilylation mechanism is very much dependent on the substrate, catalyst or silane employed calls for individual analysis of each particular example. In the case of the hydrosilylation of carbonyl compounds with monohydrosilanes catalyzed by late transition metal complexes, the most commonly invoked mechanism is that proposed by Ojima and co-workers (Scheme 1).

Solvent-free iridium-catalyzed CO2 hydrosilylation: experiments and kinetic modeling

The iridium(III) complex [Ir(H)(CF3SO3)(NSiN)(coe)] (NSiN = bis(pyridine-2-yloxy)methylsilyl, coe = cyclooctene) has been demonstrated to be an active catalyst for the solvent-free hydrosilylation of CO2 with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) under mild reaction conditions (3 bar). The activity of this catalytic system depends on the reaction temperature. The best catalytic performance has been achieved at 75 °C. A kinetic study at variable temperature (from 25 °C to 75 °C) and constant pressure (3 bar) together with kinetic modeling has been carried out. The results from such a study show an activation energy of 73.8 kJ mol−1 for the process.

Solvent‐Free Iridium‐Catalyzed Reactivity of CO2 with Secondary Amines and Hydrosilanes

The complex [Ir(H)(CF3SO3)(NSiN)(coe)] (NSiN=bis(pyridine‐2‐yloxy)methylsilyl fac‐coordinated) (1) is an effective catalyst precursor for the solvent‐free synthesis of silyl carbamates from reaction of aliphatic secondary amines with CO2 and HSiMe(OSiMe3)2. The preferential formation of the silyl carbamate instead of the expected formamide or methylamine has proven to be consequence of an iridium‐catalyzed dehydrogenative Si−N coupling between the silane and the amine to afford the corresponding silyl amine, which under the reaction conditions reacts with CO2 to give the corresponding silyl carbamate.

Iridium-Catalyzed Hydrogen Production from Hydrosilanes and Water

The iridium(III) complex [Ir(H)(CF3SO3)(NSiN)(coe)] (NSiN=fac‐coordinated bis(pyridine‐2‐yloxy)methylsilyl, coe=cyclooctene) has been proven to be an effective catalyst precursor for hydrogen production from the hydrolysis of hydrosilanes at room temperature. The reaction performance depends both on the nature of the silane and the solvent. Interestingly, high turnover frequencies of around 105 h−1 were obtained by using Et2SiH2 or (Me2HSi)2O as hydrogen sources and THF as the solvent. Moreover a mechanistic insight into this Ir‐catalyzed hydrogen generation process, based on both theoretical calculations and NMR spectroscopy, is reported. The overall catalytic cycle can be viewed as a two‐stage process that involves water‐promoted SiH bond activation followed by water splitting by a proton transfer.

Synthesis of Poly(silyl ether)s by Rhodium(I)–NHC Catalyzed Hydrosilylation: Homogeneous versus Heterogeneous Catalysis

The preparation of 1‐(3‐triisopropoxysilylpropyl)‐3‐(2‐methoxyethyl)‐imidazolium bromide or chloride salts and their reaction with [Rh(COD)(μ‐OMe)]2 (COD=1,5‐cyclooctadiene) to afford the corresponding [Rh(COD)(NHC)X] (X=Br, Cl; NHC=1‐(3‐triisopropoxysilylpropyl)‐3‐(2‐methoxyethyl)‐2‐ilydene‐imidazol) species is described. These new compounds were used as catalyst precursors for acetophenone hydrosilylation. The higher activity of the rhodium‐chlorido complex evidences a clear halide effect in the activation of the catalyst. Immobilization of the catalytic precursor [Rh(COD)(NHC)Cl] on mobile crystalline material 41 (MCM‐41) allows for the preparation of the corresponding heterogeneous catalyst. Reduction of acetophenone to PhMeCH‐O‐SiMe(OSiMe3)2 by hydrosilylation with 1,1,1,3,5,5,5‐heptamethyltrisiloxane is effectively catalyzed by both the homogeneous and the heterogeneous catalysts, in which the homogeneous system is the more active. Interestingly, the heterogeneous catalyst is reusable. Both homo‐ and heterogeneous catalysts are also effective for the copolymerization of terephthalaldehyde and 1,1,3,3,5,5‐hexamethyltrisiloxane, which affords the corresponding poly(silyl ether). The catalyst yields the heterogeneous system polymers with higher molecular weights (Mw=94 000 g mol−1) and a narrow molecular weight distribution (PDI=1.5–1.7).

Heterogeneous catalysts based on supported Rh–NHC complexes: synthesis of high molecular weight poly(silyl ether)s by catalytic hydrosilylation

The new rhodium(I) complexes [Rh(Cl)(COD)(R-NHC-(CH2)3Si(OiPr3)3)] (R = 2,6-diisopropylphenyl (2a); n-butyl (2b)) have been synthesised and fully characterised. The study of their application as ketone hydrosilylation catalysts showed a clear N-substituent effect, 2a being the most active catalyst precursor. Complex 2a has been immobilised in the mesoporous materials MCM-41 and KIT-6. The new hybrid materials have been fully characterised and used as catalyst precursors for the preparation of poly(silyl ether)s by catalytic hydrosilylation. The heterogeneous catalytic systems based on the materials 2a–MCM-41 and 2a–KIT-6 afford polymers with high average molecular weight (Mw) Mw = 2.61 × 106 g mol−1 (2a–MCM-41) and Mw = 4.43 × 105 g mol−1 (2a–KIT-6).