A family of iron(II) carbonyl hydride complexes supported by either a bifunctional PNP ligand containing a secondary amine, or a PNP ligand with a tertiary amine that prevents metal–ligand cooperativity, were found to promote the catalytic hydrogenation of CO2 to formate in the presence of Brønsted base. In both cases a remarkable enhancement in catalytic activity was observed upon the addition of Lewis acid (LA) co-catalysts. For the secondary amine supported system, turnover numbers of approximately 9000 for formate production were achieved, while for catalysts supported by the tertiary amine ligand, nearly 60000 turnovers were observed; the highest activity reported for an earth abundant catalyst to date. The LA co-catalysts raise the turnover number by more than an order of magnitude in each case. In the secondary amine system, mechanistic investigations implicated the LA in disrupting an intramolecular hydrogen bond between the PNP ligand N–H moiety and the carbonyl oxygen of a formate ligand in the catalytic resting state. This destabilization of the iron-bound formate accelerates product extrusion, the rate-limiting step in catalysis. In systems supported by ligands with the tertiary amine, it was demonstrated that the LA enhancement originates from cation assisted substitution of formate for dihydrogen during the slow step in catalysis.

Reduction of the pincer nickel(II) complex [(PNP)NiBr] with sodium amalgam (Na/Hg) forms the mercury-bridged dimer [{(PNP)Ni}2{μ-Hg}], which homolytically cleaves dihydrogen to form [(PNP)NiH]. Reversible CO2 insertion into the Ni–H bond is observed for [(PNP)NiH], forming the monodentate κ1O-formate complex [(PNP)NiOC(O)H].

An iridium(III) trihydride complex supported by a pincer ligand with a hydrogen bond donor in the secondary coordination sphere promotes the electrocatalytic reduction of CO2 to formate in water/acetonitrile with excellent Faradaic efficiency and low overpotential. Preliminary mechanistic experiments indicate formate formation is facile while product release is a kinetically difficult step.

The reductive functionalization of carbon dioxide into high value organics was accomplished via the coupling with carbon monoxide and ethylene/propylene at a zerovalent nickel species bearing the 2-((di-t-butylphosphino)methyl)pyridine ligand (PN). An initial oxidative coupling between carbon dioxide, olefin, and (PN)Ni(1,5-cyclooctadiene) afforded five-membered nickelacycle lactone species, which were produced with regioselective 1,2-coupling in the case of propylene. The propylene derived nickelacycle lactone was isolated and characterized by X-ray diffraction. Addition of carbon monoxide, or a combination of carbon monoxide and diethyl zinc to the nickelacycle lactone complexes afforded cyclic anhydrides and 1,4-ketoacids, respectively, in moderate to high yields. The primary organometallic product of the transformation was zerovalent (PN)Ni(CO)2.

A series of zerovalent molybdenum complexes bearing triphosphine ligands, [Ar2PCH2CH2]2PPh, have been synthesized and evaluated for reductive functionalization of CO2 with ethylene. The ability to form dimeric triphosphine molybdenum(II) acrylate hydride species from CO2-ethylene coupling was found to be highly sensitive to steric encumbrance on the phosphine aryl substituents. Trapping of triphosphine molybdenum(II) acrylate hydride species using triphenylphosphine afforded isolable monomeric CO2 functionalization products with all ancillary ligands studied. Kinetic analysis of the acrylate formation reaction revealed a first-order dependence on molybdenum, but no influence from CO2 pressure or the triphenylphosphine trap. Systematic attenuation of steric and electronic features of the triphosphine ligands showed a strong CO2 functionalization rate influence for ligand size with [(3,5-tBu-C6H3)2PCH2CH2]2PPh coupling nearly four times slower than with [(3,5-Me-C6H3)2PCH2CH2]2PPh. A considerably milder electronic effect was observed with complexes bearing [(4-F-C6H4)2PCH2CH2]2PPh reducing CO2 at approximately half the rate as with [Ph2PCH2CH2]2PPh.

A family of new iridium phosphine–sulfonate complexes based on 2-dicyclohexylphosphino-4-benzenesulfonic acid was synthesized and characterized. An iridium(I) phosphine–sulfonate cyclooctadiene species was prepared from transmetalation of a silver salt of the phosphine–sulfonate ligand and iridium(I) cyclooctadiene chloride dimer. This diolefin iridium complex was found to be a functional precatalyst for cyclopentene hydrogenation. A corresponding iridium(I) phosphine–sulfonate dicarbonyl species was prepared by ligand substitution using carbon monoxide. Analysis by infrared spectroscopy established this metal–ligand platform as a relatively electron poor coordination environment. The iridium(I) phosphine–sulfonate cyclooctadiene species was further utilized as a precursor for the synthesis of two iridium(III) phosphine–sulfonate hydride compounds by treatment with strong acids.

The Lewis acid tris(pentafluorophenyl)borane was found to rapidly promote ring-opening β-hydride elimination in a 1,1′-bis(diphenylphosphino)ferrocene (dppf) nickelalactone complex under ambient conditions. The thermodynamic product of nickelalactone ring-opening was characterized as (dppf)Ni(CH(CH3)CO2BArf3), the result of β-hydride elimination and subsequent 2,1-insertion from a transient nickel(II) acrylate hydride intermediate. Treatment of (dppf)Ni(CH(CH3)CO2BArf3) with a nitrogen-containing base afforded a diphosphine nickel(0) η2-acryl borate adduct. Formation of the diphosphine nickel(0) η2-acryl borate adduct completes a net conversion of nickelalactone to acrylate species, a significant obstacle to catalytic acrylate production from CO2 and ethylene. Displacement of the η2-acrylate fragment from the nickel center was accomplished by addition of ethylene to yield a free acrylate salt and (dppf)Ni(CH2═CH2).

The molybdenum tetrahydride species (Triphos)MoH4PPh3 (Triphos = PhP(CH2CH2PPh2)2) generated from sodium triethylborohydride addition to (Triphos)MoCl3 was found to promote CO2 functionalization to afford acrylate, propionate, and formate species. The formation of (Triphos)MoH4PPh3 occurs via a (Triphos)Mo(H)Cl(PPh3) intermediate followed by dismutation of an unobserved six-coordinate molybdenum(II) dihydride complex. Addition of dihydrogen to the dismuation product mixture affords a nearly quantitative yield of (Triphos)MoH4PPh3. The molybdenum tetrahydride species facilitates CO2 insertion into a metal hydride to produce a formate complex, (Triphos)Mo(H)(κ2-CHO2)(PPh3), with an observed rate constant of [2.9(2)] × 10–4 s–1 (25 °C), which is independent of CO2 pressure. Selective formation of acrylate and propionate carbon dioxide–ethylene coupling products, (Triphos)Mo(H)(κ2-C3H3O2)(PPh3) and (Triphos)Mo(H)(κ2-C3H5O2)(PPh3), was achieved by sequential addition of olefin and heterocumulene to (Triphos)MoH4PPh3. A formally zerovalent TriphosMo(η2-C2H4)3 intermediate was characterized by NMR spectroscopy and computational analysis along the pathway for carbon dioxide–ethylene coupling.

The cobalt(III) dimethyl halide complexes cis,mer-(PMe3)3Co(CH3)2X (X = Cl, I) were found to undergo a degenerate cobalt-to-cobalt transfer of the methyl ligands during isotopic labeling experiments. Extensive mechanistic studies exclude radical, methyl iodide elimination, and disproportionation/comproportionation pathways for exchange of the methyl groups between metals. A related cobalt(III) dimethyl complex supported by the tridentate phosphine ligand MeP(CH2CH2PMe2)2 showed dramatically slower methyl ligand transfer, indicative of a mechanism for intermetallic exchange with a requisite phosphine dissociation. Crossover experiments between cobalt(III) dimethyl halide complexes supported by PMe3 and MeP(CH2CH2PMe2)2 are consistent with a dicobalt transition structure in which only one cobalt center requires phosphine dissociation prior to methyl transfer. An additional methyl group scrambling process between cis,mer-(PMe3)3Co(CH3)2I and free PMe3 was also identified during the investigation and originates from reversible P–CH3 bond cleavage.

Alkali metal reduction of tungsten tetrachloride in the presence of excess trimethylphosphite and ethylene affords moderate yields of trans-tetrakis(trimethylphosphite)tungsten bis(ethylene). This easily prepared species bearing inexpensive ancillary ligands promotes the oxidative coupling of carbon dioxide and ethylene at ambient temperature to produce two isomeric tetrakis(trimethylphosphite)tungsten acrylate hydride complexes. These isomers vary by the κ2-O,O and κ3-C,C,O coordination mode of the acrylate ligand, and swiftly interconvert in solution as detected by 2D NMR spectroscopy. The CO2-derived acrylate fragment may be released from the tungsten coordination sphere by treatment with methyl iodide to afford modest quantities of free methyl acrylate.

A cobalt(I) methyl species, (PMe3)4CoCH3, was found to promote C–CN bond oxidative addition of acetonitrile at ambient temperature. The isolated product of acetonitrile activation, cis,mer-(PMe3)3Co(CH3)2CN, was characterized by NMR, IR, and single-crystal X-ray diffraction studies and presents a higher valent metal in comparison to those previously observed for base-metal-mediated nitrile activations. A short-lived reaction intermediate was detected during nitrile cleavage and identified as fac-(PMe3)3Co(CH3)2CN, the kinetic product of C–CN oxidative addition. Conversion of the kinetic product to cis,mer-(PMe3)3Co(CH3)2CN proceeds with a rate constant of [1.0(1)] × 10–3 s–1 at 27 °C.

The diamagnetic cobalt(III) dimethyl complex, cis,mer-(PMe3)3Co(CH3)2I, was found to promote selective C–C bond formation, affording ethane and triplet (PMe3)3CoI. The mechanism of reductive elimination has been investigated by a series of kinetic and isotopic-labeling experiments. Ethane formation proceeds with a rate constant of 3.1(5) × 10–5 s–1 (50 °C) and activation parameters of ΔH⧧ = 31.4(8) kcal/mol and ΔS⧧ = 17(3) eu. Addition of free trimethylphosphine or coordinating solvent strongly inhibits reductive elimination, indicating reversible phosphine dissociation prior to C–C bond-coupling. EXSY NMR analysis established a rate constant of 9(2) s–1 for phosphine loss from cis,mer-(PMe3)3Co(CH3)2I. Radical trapping, crossover, and isotope effect experiments were consistent with a proposed mechanism for ethane extrusion where formation of an unobserved five-coordinate intermediate is followed by concerted C–C bond formation. An unusual intermolecular exchange of cobalt–methyl ligands was also observed by isotopic labeling.

Treatment of deprotonated N-(dimethylaminoethyl)-2-diphenylphosphinoaniline with bis(cyclooctene)iridium chloride dimer affords a thermally stable iridium(I) olefin complex. Infrared analysis of the corresponding monocarbonyl iridium(I) compound indicates a relatively electron rich metal center. Reaction of the iridium(I) cyclooctene complex with iodomethane effects oxidation of the metal yielding a five-coordinate iridium(III) methyl iodide complex which reversibly coordinates tetrahydrofuran. X-ray crystallography confirms coordination of ether to the iridium(III) methyl iodide complex and NMR spectroscopic experiments establish an equilibrium constant of 1.66(9) M for tetrahydrofuran binding. A five-coordinate iridium(III) dimethyl complex has also been prepared and characterized by X-ray diffraction. Hydrogenolysis of the dialkyl species permits identification of a short-lived classical iridium(III) dihydride complex.

A zerovalent molybdenum complex, [(Ph2PCH2CH2)2PPh]Mo(C2H4)(N2)2, was found to promote coupling of CO2 and ethylene to afford a molybdenum(II) acrylate hydride complex. The identity of the molybdenum(II) acrylate hydride complex was established by spectroscopy and reactivity studies. The mechanism of carbon dioxide functionalization with ethylene has been investigated by a series of kinetic and isotopic labeling studies, in addition to the observation of a formally molybdenum(0) carbon dioxide−ethylene intermediate along the reaction pathway. Acrylate formation from the molybdenum(0) intermediate proceeds with a rate constant of 3.8(3) × 10−5 s−1 and an isotope effect of 1.2(2) for C2H4 vs C2D4 at 23 °C. Measuring rate constants of the CO2 reduction over a 40 °C temperature range established activation parameters for acrylate formation of ΔS‡ = 1(7) eu and ΔH‡ = 24(3) kcal/mol. The mechanism of CO2−ethylene coupling is proposed to proceed from a molybdenum(0) carbon dioxide−ethylene adduct via rate-limiting oxidative C−C bond formation followed by rapid β-hydride elimination from a metallalactone complex.

Alkali metal reduction of tungsten tetrachloride in the presence of excess trimethylphosphite and ethylene affords moderate yields of trans-tetrakis(trimethylphosphite)tungsten bis(ethylene). This easily prepared species bearing inexpensive ancillary ligands promotes the oxidative coupling of carbon dioxide and ethylene at ambient temperature to produce two isomeric tetrakis(trimethylphosphite)tungsten acrylate hydride complexes. These isomers vary by the κ2-O,O and κ3-C,C,O coordination mode of the acrylate ligand, and swiftly interconvert in solution as detected by 2D NMR spectroscopy. The CO2-derived acrylate fragment may be released from the tungsten coordination sphere by treatment with methyl iodide to afford modest quantities of free methyl acrylate.

A family of iron(II) carbonyl hydride complexes supported by either a bifunctional PNP ligand containing a secondary amine, or a PNP ligand with a tertiary amine that prevents metal–ligand cooperativity, were found to promote the catalytic hydrogenation of CO2 to formate in the presence of Brønsted base. In both cases a remarkable enhancement in catalytic activity was observed upon the addition of Lewis acid (LA) co-catalysts. For the secondary amine supported system, turnover numbers of approximately 9000 for formate production were achieved, while for catalysts supported by the tertiary amine ligand, nearly 60000 turnovers were observed; the highest activity reported for an earth abundant catalyst to date. The LA co-catalysts raise the turnover number by more than an order of magnitude in each case. In the secondary amine system, mechanistic investigations implicated the LA in disrupting an intramolecular hydrogen bond between the PNP ligand N–H moiety and the carbonyl oxygen of a formate ligand in the catalytic resting state. This destabilization of the iron-bound formate accelerates product extrusion, the rate-limiting step in catalysis. In systems supported by ligands with the tertiary amine, it was demonstrated that the LA enhancement originates from cation assisted substitution of formate for dihydrogen during the slow step in catalysis.

An iridium(III) trihydride complex supported by a pincer ligand with a hydrogen bond donor in the secondary coordination sphere promotes the electrocatalytic reduction of CO2 to formate in water/acetonitrile with excellent Faradaic efficiency and low overpotential. Preliminary mechanistic experiments indicate formate formation is facile while product release is a kinetically difficult step.

The cobalt phenylthiolate complex, cis,mer-(PMe3)3Co(CH3)2SPh, was found to undergo competitive two-electron ethane reductive elimination and C–H bond cyclometallation. The thiophenolato bound cobaltacycle was generated via C–H bond oxidative addition to a five-coordinate intermediate followed by rapid methane elimination. A related cobalt isothiocyanate complex, cis,mer-(PMe3)3Co(CH3)2NCS, was also prepared and found to perform ethane elimination and S-atom transfer to yield trimethylphosphine sulfide. This rare example of S-atom donation from a isothiocyanate was characterized by NMR and GC-MS analysis, with cis,mer-(PMe3)3Co(CH3)2CN identified as one of the cobalt based products.

Reduction of the pincer nickel(II) complex [(PNP)NiBr] with sodium amalgam (Na/Hg) forms the mercury-bridged dimer [{(PNP)Ni}2{μ-Hg}], which homolytically cleaves dihydrogen to form [(PNP)NiH]. Reversible CO2 insertion into the Ni–H bond is observed for [(PNP)NiH], forming the monodentate κ1O-formate complex [(PNP)NiOC(O)H].

The reductive functionalization of carbon dioxide into high value organics was accomplished via the coupling with carbon monoxide and ethylene/propylene at a zerovalent nickel species bearing the 2-((di-t-butylphosphino)methyl)pyridine ligand (PN). An initial oxidative coupling between carbon dioxide, olefin, and (PN)Ni(1,5-cyclooctadiene) afforded five-membered nickelacycle lactone species, which were produced with regioselective 1,2-coupling in the case of propylene. The propylene derived nickelacycle lactone was isolated and characterized by X-ray diffraction. Addition of carbon monoxide, or a combination of carbon monoxide and diethyl zinc to the nickelacycle lactone complexes afforded cyclic anhydrides and 1,4-ketoacids, respectively, in moderate to high yields. The primary organometallic product of the transformation was zerovalent (PN)Ni(CO)2.

Treatment of deprotonated N-(dimethylaminoethyl)-2-diphenylphosphinoaniline with bis(cyclooctene)iridium chloride dimer affords a thermally stable iridium(I) olefin complex. Infrared analysis of the corresponding monocarbonyl iridium(I) compound indicates a relatively electron rich metal center. Reaction of the iridium(I) cyclooctene complex with iodomethane effects oxidation of the metal yielding a five-coordinate iridium(III) methyl iodide complex which reversibly coordinates tetrahydrofuran. X-ray crystallography confirms coordination of ether to the iridium(III) methyl iodide complex and NMR spectroscopic experiments establish an equilibrium constant of 1.66(9) M for tetrahydrofuran binding. A five-coordinate iridium(III) dimethyl complex has also been prepared and characterized by X-ray diffraction. Hydrogenolysis of the dialkyl species permits identification of a short-lived classical iridium(III) dihydride complex.