Mechanistic Studies on the Selective Reduction of CO2 to the Aldehyde Level by a Bis(phosphino)boryl (PBP)-Supported Nickel Complex

Four-electron reduction of CO2: from formaldehyde and acetal synthesis to complex transformations

The expansive and dynamic field of the CO2 Reduction Reaction (CO2RR) seeks to harness CO2 as a sustainable carbon source or energy carrier. While significant progress has been made in two, six, and eight-electron reductions of CO2, the four-electron reduction remains understudied. This review fills this gap, comprehensively exploring CO2 reduction into formaldehyde (HCHO) or acetal-type compounds (EOCH2OE, with E = [Si], [B], [Zr], [U], [Y], [Nb], [Ta] or -R) using various CO2RR systems. These encompass (photo)electro-, bio-, and thermal reduction processes with diverse reductants. Formaldehyde, a versatile C1 product, is challenging to synthesize and isolate from the CO2RR. The review also discusses acetal compounds, emphasizing their significance as pathways to formaldehyde with distinct reactivity. Providing an overview of the state of four-electron CO2 reduction, this review highlights achievements, challenges, and the potential of the produced compounds - formaldehyde and acetals - as sustainable sources for valuable product synthesis, including chiral compounds.

A-Frame-Templated High-Coordinate Platinum(IV) cis-Bis(boryl) Complexes

The addition of Et2O·BF3 or Me2S·BCl3 to the BNBN-cumulene-bridged Pt(II) A-frame complexes [(μ-1,1-BNBN(TMS)2)(μ-dmpm)2Pt2X2] (TMS = SiMe3, dmpm = CH2(PMe2)2, X = Br 1Br, I 1I) resulted in the oxidative addition of one B-F or B-Cl bond, respectively, to the internal BN bond of the bridging, iminoborane-like B-N≡B-N moiety, and coordination of one Pt(II) center to the resulting adjacent BF2 (complex 2Br-F) or BCl2 (complexes 2Br-Cl and 2I-Cl) moiety, respectively. X-ray crystallographic and multinuclear NMR-spectroscopic data show that the Pt→BF2 interaction in 2Br-F is very weak and merely electrostatic, while the Pt→BCl2 interaction in 2Br-Cl and 2I-Cl is a stronger donor-acceptor bond. In contrast, the reaction of Me2S·BBr3 with 1Br yielded a ca. 3:2 mixture of the analogous B-Br addition product to the iminoborane, 2Br-Br, and the product of a subsequent oxidative addition of one B-Br bond of the chelating BBr2 moiety to the adjacent platinum center, the mixed-valence boranediyl-bridged, Pt(II)-Pt(IV)-bromoboryl complex 3-Br5. The analogous reactions of Me2S·BI3 with 1Br and Me2S·BBr3 with 1I yielded complex product mixtures of Pt(II)-Pt(II)-borane (2Br-I and 2I-Br, respectively) and Pt(II)-Pt(IV)-boryl complexes (3-BrnI5-n, n = 1-3) analogous to 2X-Y and 3-Br5, respectively, the proportion of the latter increasing with the proportion of iodide in the precursor mixture. Both multinuclear NMR-spectroscopic and X-ray crystallographic data show evidence of complex and extensive inter- and intramolecular bromide-iodide exchanges between the soft, iodide-affine platinum centers and the harder, more bromide-affine boron centers. A clue to the mechanism of these halide exchanges is provided by the reactions of BBr2Ar (Ar = 2,4,6-Me3C6H2 (Mes), 2,3,5,6-Me4C6H (Dur)) with 1Br, which yielded the cationic Pt(II)-Pt(II)-borenium analogues of 2Br-Br, the complexes 4Br-Ar, generated by the sterics-induced displacement of the bromide substituent from the chelating Pt→BBrAr moiety, and displaying a rare metal→borenium donor-acceptor bond.

Nitrous oxide activation by picoline-derived Ni-CNP hydrides

Oxygen atom transfer (OAT) from N2O to the Ni-H bond of proton-responsive picoline-derived CNP nickel complexes has been investigated both experimentally and theoretically. These Ni-CNP complexes efficiently catalyse the reduction of N2O with pinacolborane (HBpin) under mild conditions.

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Cleavage of Carbon Dioxide C=O Bond Promoted by Nickel-Boron Cooperativity in a PBP-Ni Complex

The synthesis and characterization of (tBu PBP)Ni(OAc) (5) by insertion of carbon dioxide into the Ni-C bond of (tBu PBP)NiMe (1) is presented. An unexpected CO2 cleavage process involving the formation of new B-O and Ni-CO bonds leads to the generation of a butterfly-structured tetra-nickel cluster (tBu PBOP)2 Ni4 (μ-CO)2 (6). Mechanistic investigation of this reaction indicates a reductive scission of CO2 by O-atom transfer to the boron atom via a cooperative nickel-boron mechanism. The CO2 activation reaction produces a three-coordinate (tBu P2 BO)Ni-acyl intermediate (A) that leads to a (tBu P2 BO)-NiI complex (B) via a likely radical pathway. The NiI species is trapped by treatment with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to give (tBu P2 BO)NiII (η2 -TEMPO) (7). Additionally, 13 C and 1 H NMR spectroscopy analysis using 13 C-enriched CO2 provides information about the species involved in the CO2 activation process.

PNP-type ligands enabled copper-catalyzed N-formylation of amines with CO2 in the presence of silanes

The sustainable catalytic transformation of carbon dioxide into valuable fine chemicals with high efficiency is a global challenge as although CO₂ is an abundant, nontoxic, and sustainable carbon feedstock it is also the most important factor behind the Greenhouse Effect. We describe herein a PNP-type ligand-enabled copper-catalyzed N-formylation of amines utilizing CO₂ as the building block in the presence of hydrosilane as the reductant. Our current protocol featured newly synthesized PNP-type ligands with broad substrate scope under mild reaction conditions.

Ethylene Dimerization and Oligomerization Using Bis(phosphino)boryl Supported Ni Complexes

We report the dimerization and oligomerization of ethylene using bis(phosphino)boryl supported Ni(II) complexes as catalyst precursors. By using alkylaluminum(III) compounds or other Lewis acid additives, Ni(II) complexes of the type (^RPBP)NiBr (R = tBu or Ph) show activity for the production of butenes and higher olefins. Optimized turnover frequencies of 640 mol_ethylene·mol_Ni^-1·s^-1 for the formation of butenes with 41(1)% selectivity for 1-butene using (^PhPBP)NiBr, and 68 mol_ethylene·mol_Ni^-1·s^-1 for butenes production with 87.2(3)% selectivity for 1-butene using (^tBuPBP)NiBr, have been demonstrated. With methylaluminoxane as a co-catalyst and (^tBuPBP)NiBr as the precatalyst, ethylene oligomerization to form C₄ through C₂₀ products was achieved, while the use of (^PhPBP)NiBr as the pre-catalyst retained selectivity for C₄ products. Our studies suggest that the ethylene dimerization is not initiated by Ni hydride or alkyl intermediates. Rather, our studies point to a mechanism that involves a cooperative B/Ni activation of ethylene to form a key 6-membered borametallacycle intermediate. Thus, a cooperative activation of ethylene by the Ni-B unit of the (^RPBP)Ni catalysts is proposed as a key element of the Ni catalysis.

Mechanism Insights into the Iridium(III)- and B(C6F5)3-Catalyzed Reduction of CO2 to the Formaldehyde Level with Tertiary Silanes

The catalytic system [Ir(CF3CO2)(κ2-NSiMe)2] [1; NSiMe = (4-methylpyridin-2-yloxy)dimethylsilyl]/B(C6F5)3 promotes the selective reduction of CO2 with tertiary silanes to the corresponding bis(silyl)acetal. Stoichiometric and catalytic studies evidenced that species [Ir(CF3COO-B(C6F5)3)(κ2-NSiMe)2] (3), [Ir(κ2-NSiMe)2][HB(C6F5)3] (4), and [Ir(HCOO-B(C6F5)3)(κ2-NSiMe)2] (5) are intermediates of the catalytic process. The structure of 3 has been determined by X-ray diffraction methods. Theoretical calculations show that the rate-limiting step for the 1/B(C6F5)3-catalyzed hydrosilylation of CO2 to bis(silyl)acetal is a boron-promoted Si-H bond cleavage via an iridium silylacetal borane adduct.

Challenges and Prospects in the Catalytic Conversion of Carbon Dioxide to Formaldehyde

Formaldehyde (HCHO) is a crucial C1 building block for daily-life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy- and cost-intensive gas-phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO2 ) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO2 -to-HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO2 to HCHO are discussed.

Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides

The kinetics of hydride transfer from Re(^Rbpy)(CO)₃H (bpy = 4,4'-R-2,2'-bipyridine; R = OMe, ^tBu, Me, H, Br, COOMe, CF₃) to CO₂ and seven different cationic N-heterocycles were determined. Additionally, the thermodynamic hydricities of complexes of the type Re(^Rbpy)(CO)₃H were established primarily using computational methods. Linear free-energy relationships (LFERs) derived by correlating thermodynamic and kinetic hydricities indicate that, in general, the rate of hydride transfer increases as the thermodynamic driving force for the reaction increases. Kinetic isotope effects range from inverse for hydride transfer reactions with a small driving force to normal for reactions with a large driving force. Hammett analysis indicates that hydride transfer reactions with greater thermodynamic driving force are less sensitive to changes in the electronic properties of the metal hydride, presumably because there is less buildup of charge in the increasingly early transition state. Bronsted α values were obtained for a range of hydride transfer reactions and along with DFT calculations suggest the reactions are concerted, which enables the use of Marcus theory to analyze hydride transfer reactions involving transition metal hydrides. It is notable, however, that even slight perturbations in the steric properties of the Re hydride or the hydride acceptor result in large deviations in the predicted rate of hydride transfer based on thermodynamic driving forces. This indicates that thermodynamic considerations alone cannot be used to predict the rate of hydride transfer, which has implications for catalyst design.

Activation of Si-H and B-H bonds by Lewis acidic transition metals and p-block elements: same, but different

In this Perspective we discuss the ability of transition metal complexes to activate and cleave the Si-H and B-H bonds of hydrosilanes and hydroboranes (tri- and tetra-coordinated) in an electrophilic manner, avoiding the need for the metal centre to undergo two-electron processes (oxidative addition/reductive elimination). A formal polarization of E-H bonds (E = Si, B) upon their coordination to the metal centre to form σ-EH complexes (with coordination modes η1 or η2) favors this type of bond activation that can lead to reactivities involving the formation of transient silylium and borenium/boronium cations similar to those proposed in silylation and borylation processes catalysed by boron and aluminium Lewis acids. We compare the reactivity of transition metal complexes and boron/aluminium Lewis acids through a series of catalytic reactions in which pieces of evidence suggest mechanisms involving electrophilic reaction pathways.

Catalytic reduction of carbon dioxide by a zinc hydride compound, [Tptm]ZnH, and conversion to the methanol level

The zinc hydride compound, [Tptm]ZnH, may achieve the reduction of CO₂ by (RO)₃SiH (R = Me, Et) to the methanol oxidation level, (MeO)_xSi(OR)_4-x, via the formate species, HCO₂Si(OR)₃. However, because insertion of CO₂ into the Zn-H bond is more facile than insertion of HCO₂Si(OR)₃, conversion of HCO₂Si(OR)₃ to the methanol level only occurs to a significant extent in the absence of CO₂.

Ligand and solvent effects on CO2 insertion into group 10 metal alkyl bonds

The insertion of carbon dioxide into metal element σ-bonds is an important elementary step in many catalytic reactions for carbon dioxide valorization. Here, the insertion of carbon dioxide into a family of group 10 alkyl complexes of the type (^RPBP)M(CH₃) (^RPBP = B(NCH₂PR₂)₂C₆H₄⁻; R = Cy or ^tBu; M = Ni or Pd) to generate κ¹-acetate complexes of the form (^RPBP)M{OC(O)CH₃} is investigated. This involved the preparation and characterization of a number of new complexes supported by the unusual ^RPBP ligand, which features a central boryl donor that exerts a strong trans-influence, and the identification of a new decomposition pathway that results in C–B bond formation. In contrast to other group 10 methyl complexes supported by pincer ligands, carbon dioxide insertion into (^RPBP)M(CH₃) is facile and occurs at room temperature because of the high trans-influence of the boryl donor. Given the mild conditions for carbon dioxide insertion, we perform a rare kinetic study on carbon dioxide insertion into a late-transition metal alkyl species using (^tBuPBP)Pd(CH₃). These studies demonstrate that the Dimroth–Reichardt parameter for a solvent correlates with the rate of carbon dioxide insertion and that Lewis acids do not promote insertion. DFT calculations indicate that insertion into (^tBuPBP)M(CH₃) (M = Ni or Pd) proceeds via an S_E2 mechanism and we compare the reaction pathway for carbon dioxide insertion into group 10 methyl complexes with insertion into group 10 hydrides. Overall, this work provides fundamental insight that will be valuable for the development of improved and new catalysts for carbon dioxide utilization.

The kinetics of carbon dioxide insertion into a pincer-supported palladium methyl complex are studied. The complex inserts carbon dioxide at room temperature, and we explore both solvent and Lewis acid effects on carbon dioxide insertion.

Cobalt-Catalyzed Hydrosilylation of Carbon Dioxide to the Formic Acid, Formaldehyde, and Methanol Level-How to Control the Catalytic Network?

The selective hydrosilylation of carbon dioxide (CO₂) to either the formic acid, formaldehyde, or methanol level using a molecular cobalt(II) triazine complex can be controlled based on reaction parameters such as temperature, CO₂ pressure, and concentration. Here, we rationalize the catalytic mechanism that enables the selective arrival at each product platform. Key reactive intermediates were prepared and spectroscopically characterized, while the catalytic mechanism and the energy profile were analyzed with density functional theory (DFT) methods and microkinetic modeling. It transpired that the stepwise reduction of CO₂ involves three consecutive catalytic cycles, including the same cobalt(I) triazine hydride complex as the active species. The increasing kinetic barriers associated with each reduction step and the competing hydride transfer steps in the three cycles corroborate the strong influence of the catalyst environment on the product selectivity. The fundamental mechanistic insights provide a consistent description of the catalytic system and rationalize, in particular, the experimentally verified opportunity to steer the reaction toward the formaldehyde product as the chemically most challenging reduction level.

Partnering a Three-Coordinate Gallium Cation with a Hydroborate Counter-Ion for the Catalytic Hydrosilylation of CO2

A novel β-diketiminate stabilized gallium hydride, (^Dipp L)Ga(Ad)H (where (^Dipp L)={HC(MeCDippN)₂ }, Dipp=2,6-diisopropylphenyl and Ad=1-adamantyl), has been synthesized and shown to undergo insertion of carbon dioxide into the Ga-H bond under mild conditions. In this case, treatment of the resulting κ¹ -formate complex with triethylsilane does not lead to regeneration of the hydride precursor. However, when combined with B(C₆ F₅ )₃ , (^Dipp L)Ga(Ad)H catalyses the reductive hydrosilylation of CO₂ . Under stoichiometric conditions, the addition of one equivalent of B(C₆ F₅ )₃ to (^Dipp L)Ga(Ad)H leads to the formation of a 3-coordinate cationic gallane complex, partnered with a hydroborate anion, [(^Dipp L)Ga(Ad)][HB(C₆ F₅ )₃ ]. This complex rapidly hydrometallates carbon dioxide and catalyses the selective reduction of CO₂ to the formaldehyde oxidation level at 60 °C in the presence of Et₃ SiH (yielding H₂ C(OSiEt₃ )₂ ). When catalysis is undertaken in the presence of excess B(C₆ F₅ )₃ , appreciable enhancement of activity is observed, with a corresponding reduction in selectivity: the product distribution includes H₂ C(OSiEt₃ )₂ , CH₄ and O(SiEt₃ )₂ . While this system represents proof-of-concept in CO₂ hydrosilylation by a gallium hydride system, the TOF values obtained are relatively modest (max. 10 h^-1 ). This is attributed to the strength of binding of the formatoborate anion to the gallium centre in the catalytic intermediate (^Dipp L)Ga(Ad){OC(H)OB(C₆ F₅ )₃ }, and the correspondingly slow rate of the turnover-limiting hydrosilylation step. In turn, this strength of binding can be related to the relatively high Lewis acidity measured for the [(^Dipp L)Ga(Ad)]⁺ cation (AN=69.8).

Recent advances in pincer-nickel catalyzed reactions

Organometallic catalysts have played a key role in accomplishing numerous synthetically valuable organic transformations that are either otherwise not possible or inefficient. The use of precious, sparse and toxic 4d and 5d metals are an apparent downside of several such catalytic systems despite their immense success over the last several decades. The use of complexes containing Earth-abundant, inexpensive and less hazardous 3d metals, such as nickel, as catalysts for organic transformations has been an emerging field in recent times. In particular, the versatile nature of the corresponding pincer-metal complexes, which offers great control of their reactivity via countless variations, has garnered great interest among organometallic chemists who are looking for greener and cheaper alternatives. In this context, the current review attempts to provide a glimpse of recent developments in the chemistry of pincer-nickel catalyzed reactions. Notably, there have been examples of pincer-nickel catalyzed reactions involving two electron changes via purely organometallic mechanisms that are strikingly similar to those observed with heavier Pd and Pt analogues. On the other hand, there have been distinct differences where the pincer-nickel complexes catalyze single-electron radical reactions. The applicability of pincer-nickel complexes in catalyzing cross-coupling reactions, oxidation reactions, (de)hydrogenation reactions, dehydrogenative coupling, hydrosilylation, hydroboration, C-H activation and carbon dioxide functionalization has been reviewed here from synthesis and mechanistic points of view. The flurry of global pincer-nickel related activities offer promising avenues in catalyzing synthetically valuable organic transformations.

Visible-Light Photocatalytic Conversion of Carbon Dioxide by Ni(II) Complexes with N4S2 Coordination: Highly Efficient and Selective Production of Formate

The efficient and selective light-driven conversion of carbon dioxide to formate is a scientific challenge for green chemistry and energy science, especially utilizing visible-light energy and earth-abundant catalytic materials. In this report, two mononuclear Ni(II) complexes of pyridylbenzimidazole (pbi) and pyridylbenzothiazole (pbt), such as Ni(pbt)(pyS)₂ (1) and Ni(pbi)(pyS)₂ (2) (pyS = pyridine-2-thiolate), were prepared and their reactivities studied. The two Ni complexes were examined for CO₂ conversion using eosin Y as a photosensitizer upon visible-light irradiation in a H₂O/ethanol solvent. The photoreaction of CO₂ catalyzed by complexes 1 and 2 selectively affords formate with a high efficiency (14 000 turnover number) and a high catalytic selectivity of ∼99%. Undesirable proton reduction pathways were completely suppressed in the photocatalytic reactions with these sulfur-rich Ni catalysts under CO₂. Hydrogen photoproduction was also studied under argon. Their kinetic isotope effects and influence of solution pH for formate and H₂ production in the photocatalytic reactions are described in relation to the reaction mechanisms. These bioinspired Ni(II) catalysts with N/S ligation in relation to [NiFe]-hydrogenases are the first examples of early transition metal complexes affording such high selectivity and efficiencies, providing a future path to design solar-to-fuel processes for artificial photosynthesis.

Kinetic and mechanistic analysis of a synthetic reversible CO2/HCO2- electrocatalyst

[Pt(depe)2](PF6)2 electrocatalyzes the reversible conversion between CO2 and HCO2- with high selectivity and low overpotential but low rates. A comprehensive kinetic analysis indicates the rate determining step for CO2 reduction is the reactivity of a Pt hydride intermediate to produce HCO2-. To accelerate catalysis, the use of cationic and hydrogen-bond donor additives are explored.

The DFT Quest for Possible Reaction Pathways, Catalytic Species, and Regioselectivity in the InCl3-Catalyzed Cycloaddition of N-Tosyl Formaldimine with Olefins or Allenes

The present work focuses on a theoretical investigation of the plausible mechanism, determination of catalytically active species, and understanding of the regioselectivity in the InCl₃-catalyzed cycloaddition of N-tosyl formaldimine with alkenes or allenes. InCl₃ and InCl₂⁺ coordinated by dichloroethane (InCl₂⁺-DCE) were investigated as model catalytic systems. DFT data supported that InCl₂⁺-DCE represent the plausible in situ generated catalytic species. The catalytic cycle starts from the coordination of N-tosyl formaldimine to InCl₂⁺-DCE, generating an In-complexed iminium intermediate. This then undergoes intermolecular reaction (aza-Prins) with alkene substrate to form a carbocation intermediate, which is chemoselectively attacked by the second N-tosyl formaldimine molecule to form a formaldiminium intermediate. In a final step, this intermediate undergoes the ring closure, leading to hexahydropyrimidine along with the regeneration of catalyst. In addition, our DFT results indicate that N-tosyl formaldimine not only acts as a reactant but also accelerates the 1,3-H-shift as a proton acceptor, giving an experimentally observed allylamide product. Also, the "iminium/alkene/imine" path was supported by calculation results for diastereoselective [2 + 2 + 2] reaction using an internal alkene. Finally, the regioselectivity of the InCl₃-catalyzed cycloaddition using allenes along with N-tosyl formaldimine was also analyzed.

Unravelling the mechanism of the organocatalyzed aminolytic kinetic resolution of α-nitroepoxides: a theoretical study

The mechanism of the aminolytic kinetic resolution of 2-methyl-2-nitro-3-phenyl oxirane with aniline in the presence of Takemoto's catalyst is disclosed by way of DFT calculations.

Tandem deoxygenative hydrosilation of carbon dioxide with a cationic scandium hydridoborate and B(C6F5)3

A scandium hydridoborate complex supported by the dianionic pentadentate ligand B₂Pz₄Py is prepared via hydride abstraction from the previously reported scandium hydride complex with tris-pentafluorophenyl borane.

Bis(alkyl) scandium and yttrium complexes coordinated by an amidopyridinate ligand: synthesis, characterization and catalytic performance in isoprene polymerization, hydroelementation and carbon dioxide hydrosilylation

Highly versatile and robust organolanthanides as catalysts or catalyst precursors for a variety of challenging transformations.

Diverse Catalytic Systems and Mechanistic Pathways for Hydrosilylative Reduction of CO2

Catalytic hydrosilylation of carbon dioxide has emerged as a promising approach for carbon dioxide utilization. It allows the reductive transformation of carbon dioxide into value-added products at the levels of formate, formaldehyde, methanol, and methane. Tremendous progress has been made in the area of carbon dioxide hydrosilylation since the first reports in 1981. This focus review describes recent advances in the design and catalytic performance of leading catalyst systems, including transition-metal, main-group, and transition-metal/main-group and main-group/main-group tandem catalysts. Emphasis is placed on discussions of key mechanistic features of these systems and efforts towards the development of more selective, efficient, and sustainable carbon dioxide hydrosilylation processes.

Nanofibrous rhodium with a new morphology for the hydrogenation of CO2 to formate

Herein, fibrous rhodium (Rh) was engineered using a microemulsion system.

Ir-catalyzed selective reduction of CO2 to the methoxy or formate level with HSiMe(OSiMe3)2

Ir-NSi-based catalysts allow controlling the selective reduction of CO₂ with HSiMe(OSiMe₃)₂ to afford methoxysilane or silyl formate.

Acceleration of CO2 insertion into metal hydrides: ligand, Lewis acid, and solvent effects on reaction kinetics

The insertion of CO2 into metal hydrides and the microscopic reverse decarboxylation of metal formates are important elementary steps in catalytic cycles for both CO2 hydrogenation to formic acid and methanol as well as formic acid and methanol dehydrogenation. Here, we use rapid mixing stopped-flow techniques to study the kinetics and mechanism of CO2 insertion into transition metal hydrides. The investigation finds that the most effective method to accelerate the rate of CO2 insertion into a metal hydride can be dependent on the nature of the rate-determining transition state (TS). We demonstrate that for an innersphere CO2 insertion reaction, which is proposed to have a direct interaction between CO2 and the metal in the rate-determining TS, the rate of insertion increases as the ancillary ligand becomes more electron rich or less sterically bulky. There is, however, no rate enhancement from Lewis acids (LA). In comparison, we establish that for an outersphere CO2 insertion, proposed to proceed with no interaction between CO2 and the metal in the rate-determining TS, there is a dramatic LA effect. Furthermore, for both inner- and outersphere reactions, we show that there is a small solvent effect on the rate of CO2 insertion. Solvents that have higher acceptor numbers generally lead to faster CO2 insertion. Our results provide an experimental method to determine the pathway for CO2 insertion and offer guidance for rate enhancement in CO2 reduction catalysis.

Computational mechanistic study of Ru-catalyzed CO2 reduction by pinacolborane revealing the σ-π coupling mechanism for CO2 decarbonylation

It has been reported that RuH2(η2-H2)2(PCy3)2 (1) could mediate CO2 reduction by pinacolborane (HBpin), affording pinBOBpin (7), pinBOCH3 (8), pinBOCHO (9), pinBOCH2OBpin (10), and an unprecedented C2 species pinBOCH2OCHO (11), which meanwhile is converted to the Ru complexes, including the transient 3 (RuH(κ2-O2CH)(CO)(PCy3)2) and 5 (RuH{(μ-H)2Bpin}(CO)(PCy3)2), and the persistent 4 (RuH(κ2-O2CH)(CO)2(PCy3)2) and 6 (RuH2(CO)2(PCy3)2). To gain an insight into the catalysis, a DFT study was carried out. The study identified the key active catalyst to be the hydride 13 (RuH2(CO)(PCy3)2) and characterized the mechanisms leading to the experimentally observed species (3-11). By investigating the experimental system, we learned a new mechanism called σ-π coupling for CO2 decarbonylation. Under this mechanism, CO2 and HBpin first co-coordinate to the Ru center of 13, then σ-π coupling takes place, forming a B-O bond between CO2 and HBpin, Ru-H and Ru-C bonds, and simultaneously breaking the H-Bpin bond, followed by -OBpin group migration to the Ru center, completing the CO2 decarbonylation. An interesting feature regarding the Ru catalysis was the involvement of η1-Hη1-H → η2-H2 and η1-Hη1-Bpin → η2-HBpin reductions, which facilitated the oxidative H-Bpin addition or the coordination mode change of CO2 from η1-O to η2-CO for CO2 activation or σ-π coupling. The facilitation effects could be attributed to the reductions enhancing the electron donations from the Ru center to the antibonding orbitals of the activating bonds.

Insight into catalytic reduction of CO2 to methane with silanes using Brookhart's cationic Ir(iii) pincer complex

Using density functional theory computations, we investigated in detail the underlying reaction mechanism and crucial intermediates present during the reduction of carbon dioxide to methane with silanes, catalyzed by the cationic Ir-pincer complex ((POCOP)Ir(H)(acetone)+, POCOP = 2,6-bis(dibutylphosphinito)phenyl). Our study postulates a plausible catalytic cycle, which involves four stages, by sequentially transferring silane hydrogen to the CO2 molecule to give silylformate, bis(silyl)acetal, methoxysilane and the final product, methane. The first stage of reducing carbon dioxide to silylformate is the rate-determining step in the overall conversion, which occurs via the direct dissociation of the silane Si-H bond to the C[double bond, length as m-dash]O bond of a weakly coordinated Ir-CO2 moiety, with a free energy barrier of 29.5 kcal mol-1. The ionic SN2 outer-sphere pathway in which the CO2 molecule nucleophilically attacks at the η1-silane iridium complex to cleave the η1-Si-H bond, followed by the hydride transferring from iridium dihydride [(POCOP)IrH2] to the cation [O[double bond, length as m-dash]C-OSiMe3]+, is a slightly less favorable pathway, with a free energy barrier of 33.0 kcal mol-1 in solvent. The subsequent three reducing steps follow similar pathways: the ionic SN2 outer-sphere process with silylformate, bis(silyl)acetal and methoxysilane substrates nucleophilically attacking the η1-silane iridium complex to give the ion pairs [(POCOP)IrH2] [HC(OSiMe3)2]+, [(POCOP)IrH2] [CH2(OSiMe3)2(SiMe3)]+, and [(POCOP)IrH2] [CH3O(SiMe3)2]+, respectively, followed by the hydride transfer process. The rate-limiting steps of the three reducing stages are calculated to possess free energy barriers of 12.2, 16.4 and 22.9 kcal mol-1, respectively. Furthermore, our study indicates that the natural iridium dihydride [(POCOP)IrH2] generated along the ionic SN2 outer-sphere pathway could greatly facilitate the silylation of CO2, with a potential energy barrier calculated at a low value of 16.7 kcal mol-1.

Computational Analysis of Transition Metal-Terminal Boride Complexes

A computational analysis of model transition-metal terminal boride [MB(PNP^R)] complexes is reported. A combination of density functional theory methods, natural bond orbital analysis, and multiconfiguration self-consistent field calculations were employed to investigate the structure and bonding of terminal boride complexes, in particular, the extent of metal dπ-boron pπ bonding. Comparison of metal-boride, -borylene, and-boryl bond lengths confirms the presence of metal-boron π bonds, albeit the modest shortening (∼3%) of the metal-boron bond suggests that the π-bonding is very weak in terminal borides. Calculated free energies of H₂ addition to the boride complexes to yield the corresponding boryl complexes indicate that metal-boride π-bond strengths are 22 kcal/mol or less as compared to 44 kcal/mol for an analogous nitride complex. It is concluded that, for the boride complexes studied, covering a range of different 4d and 5d metals, that the metal-boride bond consists of a reasonably covalent σ but two very polarized metal-boron π bonds. The high polarization of the boron-to-metal π bonds indicates that the terminal boride is an acceptor or Z-type ligand.