Control of Catalyst Isomers Using an N-Phenyl-Substituted RN(CH2CH2PiPr2)2 Pincer Ligand in CO2 Hydrogenation and Formic Acid Dehydrogenation.

A novel pincer ligand, iPrPNPhP [PhN(CH2CH2PiPr2)2], which is an analogue of the versatile MACHO ligand, iPrPNHP [HN(CH2CH2PiPr2)2], was synthesized and characterized. The ligand was coordinated to ruthenium, and a series of hydride-containing complexes were isolated and characterized by NMR and IR spectroscopies, as well as X-ray diffraction. Comparisons to previously published analogues ligated by iPrPNHP and iPrPNMeP [CH3N(CH2CH2PiPr2)2] illustrate that there are large changes in the coordination chemistry that occur when the nitrogen substituent of the pincer ligand is altered. For example, ruthenium hydrides supported by the iPrPNPhP ligand always form the syn isomer (where syn/anti refer to the relative orientation of the group on nitrogen and the hydride ligand on ruthenium), whereas complexes supported by iPrPNHP form the anti isomer and complexes supported by iPrPNMeP form a mixture of syn and anti isomers. We evaluated the impact of the nitrogen substituent of the pincer ligand in catalysis by comparing a series of iPrPNRP (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO2) hydrogenation to formate. The iPrPNPhP-ligated species is the most active for formic acid dehydrogenation, and mechanistic studies suggest that this is likely because there are kinetic advantages for catalysts that operate via the syn isomer. In CO2 hydrogenation, the iPrPNPhP-ligated species is again the most active under our optimal conditions, and we report some of the highest turnover frequencies for homogeneous catalysts. Experimental and theoretical insights into the turnover-limiting step of catalysis provide a basis for the observed trends in catalytic activity. Additionally, the stability of our complexes enabled us to detect a previously unobserved autocatalytic effect involving the base that is added to drive the reaction. Overall, by modifying the nitrogen substituent on the MACHO ligand, we have developed highly active catalysts for formic acid dehydrogenation and CO2 hydrogenation and also provided a framework for future catalyst development.

Rational selection of co-catalysts for the deaminative hydrogenation of amides.

The catalytic hydrogenation of amides is an atom economical method to synthesize amines. Previously, it was serendipitously discovered that the combination of a secondary amide co-catalyst with (iPrPNP)Fe(H)(CO) (iPrPNP = N[CH2CH2(PiPr2)]2 -), results in a highly active base metal system for deaminative amide hydrogenation. Here, we use DFT to develop an improved co-catalyst for amide hydrogenation. Initially, we computationally evaluated the ability of a series of co-catalysts to accelerate the turnover-limiting proton transfer during C-N bond cleavage and poison the (iPrPNP)Fe(H)(CO) catalyst through a side reaction. TBD (triazabicyclodecene) was identified as the leading co-catalyst. It was experimentally confirmed that when TBD is combined with (iPrPNP)Fe(H)(CO) a remarkably active system for amide hydrogenation is generated. TBD also enhances the activity of other catalysts for amide hydrogenation and our results provide guidelines for the rational design of future co-catalysts.

The Role of Proton Shuttles in the Reversible Activation of Hydrogen via Metal-Ligand Cooperation.

The reversible activation of H2 via a pathway involving metal-ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Nevertheless, there is a paucity of experimental information probing the mechanism of this transformation. Here, we present an in-depth kinetic study of the 1,2-addition of H2 via an MLC pathway to the widely used iron catalyst [(iPrPNP)FeH(CO)] (1) (iPrPNP = N(CH2CH2PiPr2)2-). We report one of the first experimental demonstrations of an enhancement in rate for the activation of H2 using protic additives, which operate as "proton shuttles". Our results indicate that proton shuttles need to be able to both simultaneously donate and accept a proton, and the best shuttles are molecules that are strong hydrogen bond donors but sufficiently weak acids to avoid deleterious protonation of the transition metal complex. Additionally, comparison of the rate of H2 activation via an MLC pathway between 1 and two widely used ruthenium catalysts enables more general conclusions about the role of the metal, ancillary ligand, and proton shuttles in H2 activation. The results of this study provide guidance about the design of catalysts and additives to promote H2 activation via an MLC pathway.

Understanding the Individual and Combined Effects of Solvent and Lewis Acid on CO2 Insertion into a Metal Hydride.

The insertion of CO2 into a metal hydride bond to form a metal formate is a key elementary step in many catalytic cycles for CO2 conversion. Similarly, the microscopic reverse reaction, the decarboxylation of a metal formate to form a metal hydride and CO2, is important in both organic synthesis and strategies for hydrogen storage using organic liquids. There are however few experimental studies probing the mechanism of these reactions and identifying the effects of specific variables such as Lewis acid (LA) additives or solvent, which have been shown to significantly impact catalytic performance. In this study, we use a rapid mixing stopped-flow instrument to study the kinetics of CO2 insertion into the cationic ruthenium hydride [Ru(tpy)bpy)H]PF6 (tpy = 2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine) in various solvents, both in the presence and in the absence of a LA. We show that LAs can increase the observed rate of this reaction and determine the first quantitative trends for the rate enhancement observed for CO2 insertion in the presence of cationic LAs, Li+ ≫ Na+ > K+ > Rb+. Furthermore, we show that the rate enhancement observed with LAs is solvent dependent. Specifically, as the acceptor number (AN) of the solvent increases, the effect of the LA becomes smaller. Last, we demonstrate that there is a significant solvent effect on CO2 insertion in the absence of a LA. Although the AN of the solvent has been previously used to predict the rate of CO2 insertion, this work shows that the best model for the rate of insertion is based on the Dimroth-Reichardt ET(30) value of the solvent, a parameter that better accounts for specific solute/solvent interactions.

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.

Iron-catalyzed urea synthesis: dehydrogenative coupling of methanol and amines.

Substituted ureas have numerous applications but their synthesis typically requires the use of highly toxic starting materials. Herein we describe the first base-metal catalyst for the selective synthesis of symmetric ureas via the dehydrogenative coupling of methanol with primary amines. Using a pincer supported iron catalyst, a range of ureas was generated with isolated yields of up to 80% (corresponding to a catalytic turnover of up to 160) and with H2 as the sole byproduct. Mechanistic studies indicate a stepwise pathway beginning with methanol dehydrogenation to give formaldehyde, which is trapped by amine to afford a formamide. The formamide is then dehydrogenated to produce a transient isocyanate, which reacts with another equivalent of amine to form a urea. These mechanistic insights enabled the development of an iron-catalyzed method for the synthesis of unsymmetric ureas from amides and amines.

Influences of Bifunctional PNP-Pincer Ligands on Low Valent Cobalt Complexes Relevant to CO2 Hydrogenation.

Pincer ligated coordination complexes bearing bifunctional sites have been at the center of recent developments in reversible hydrogenation catalysis, especially in cases utilizing base metals. The influence of bifunctional ligands on low valent cobalt complexes is detailed here using comparisons between the PNP-pincer ligands MeN[CH2CH2(PR2)]2 and HN[CH2CH2(PR2)]2 (R = iPr, Cy). Comparative catalytic studies of CO2 hydrogenation show that cobalt(I) precatalysts bearing the tertiary amine ligand dramatically outperform those bearing the secondary amine pincer ligand. Despite strong similarities between the precatalyst ground state structure and the redox potentials of the two systems, ligand bifunctionality was found to be detrimental to catalyst productivity. The enhanced stability imparted by the MeN[CH2CH2(PR2)]2 ligand also enabled isolation and characterization of a zero-valent cobalt dicarbonyl species, which was used to study the catalytically active oxidation state of cobalt in CO2 hydrogenation.

Reversible Hydrogenation of Carbon Dioxide to Formic Acid and Methanol: Lewis Acid Enhancement of Base Metal Catalysts.

New and sustainable energy vectors are required as a consequence of the environmental issues associated with the continued use of fossil fuels. H2 is a potential clean energy source, but as a result of problems associated with its storage and transport as a gas, chemical H2 storage (CHS), which involves the dehydrogenation of small molecules, is an attractive alternative. In principle, formic acid (FA, 4.4 wt % H2) and methanol (MeOH, 12.6 wt % H2) can be obtained renewably and are excellent prospective liquid CHS materials. In addition, MeOH has considerable potential both as a direct replacement for gasoline and as a fuel cell input. The current commercial syntheses of FA and MeOH, however, use nonrenewable feedstocks and will not facilitate the use of these molecules for CHS. An appealing option for the sustainable synthesis of both FA and MeOH, which could be implemented on a large scale, is the direct metal catalyzed hydrogenation of CO2. Furthermore, given that CO2 is a readily available, nontoxic and inexpensive source of carbon, it is expected that there will be economic and environmental benefits from using CO2 as a feedstock. One strategy to facilitate both the dehydrogenation of FA and MeOH and the hydrogenation of CO2 and H2 to FA and MeOH is to utilize a homogeneous transition metal catalyst. In particular, the development of catalysts based on first row transition metals, which are cheaper, and more abundant than their precious metal counterparts, is desirable. In this Account, we describe recent advances in the development of iron and cobalt systems for the hydrogenation of CO2 to FA and MeOH and the dehydrogenation of FA and MeOH and provide a brief comparison between precious metal and base metal systems. We highlight the different ligands that have been used to stabilize first row transition metal catalysts and discuss the use of additives to promote catalytic activity. In particular, the Account focuses on the crucial role that alkali metal Lewis acid cocatalysts can play in promoting increased activity and catalyst stability for first row transition metal systems. We relate these effects to the nature of the elementary steps in the catalytic cycle and describe how the Lewis acids stabilize the crucial transition states. For all four transformations, we discuss in detail the currently proposed catalytic pathways, and throughout the Account we identify mechanistic similarities among catalysts for the four processes. The limitations of current catalytic systems are detailed, and suggestions are provided on the improvements that are likely required to develop catalysts that are more stable, active, and practical.

Effective Pincer Cobalt Precatalysts for Lewis Acid Assisted CO2 Hydrogenation.

The pincer ligand MeN[CH2CH2(P(i)Pr2)]2 ((iPr)PNP) was employed to support a series of cobalt(I) complexes, which were crystallographically characterized. A cobalt monochloride species, ((iPr)PNP)CoCl, served as a precursor for the preparation of several cobalt precatalysts for CO2 hydrogenation, including a cationic dicarbonyl cobalt complex, [((iPr)PNP)Co(CO)2](+). When paired with the Lewis acid lithium triflate, [((iPr)PNP)Co(CO)2](+) affords turnover numbers near 30 000 (at 1000 psi, 45 °C) for CO2-to-formate hydrogenation, which is a notable increase in activity from previously reported homogeneous cobalt catalysts. Though mechanistic information regarding the function of the precatalysts remains limited, multiple experiments suggest the active species is a molecular, homogeneous [((iPr)PNP)Co] complex.

Enhanced CO2 electroreduction efficiency through secondary coordination effects on a pincer iridium catalyst.

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.

Homolytic H2 cleavage by a mercury-bridged Ni(I) pincer complex [{(PNP)Ni}2{µ-Hg}].

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 κ(1)O-formate complex [(PNP)NiOC(O)H].

Iron catalyzed CO2 hydrogenation to formate enhanced by Lewis acid co-catalysts.

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 60 000 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.

Nickel promoted functionalization of CO2 to anhydrides and ketoacids.

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.

C-H bond activation and S-atom transfer from cobalt(III) thiolate and isothiocyanate complexes.

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.

Lewis acid-assisted formic acid dehydrogenation using a pincer-supported iron catalyst.

Formic acid (FA) is an attractive compound for H2 storage. Currently, the most active catalysts for FA dehydrogenation use precious metals. Here, we report a homogeneous iron catalyst that, when used with a Lewis acid (LA) co-catalyst, gives approximately 1,000,000 turnovers for FA dehydrogenation. To date, this is the highest turnover number reported for a first-row transition metal catalyst. Preliminary studies suggest that the LA assists in the decarboxylation of a key iron formate intermediate and can also be used to enhance the reverse process of CO2 hydrogenation.

Effect of sodium cation on metallacycle ß-hydride elimination in CO2-ethylene coupling to acrylates.

The catalytic conversion of carbon dioxide and olefins into acrylates has been a long standing target, because society attempts to synthesize commodity chemicals in a more economical and sustainable fashion. Although nickel complexes have been known to successfully couple CO2 and ethylene for decades, a key ß-hydride elimination step has proven a major obstacle to the development of a catalytic process. Recent studies have shown that Lewis acid additives can be used to create a lower-energy pathway for ß-hydride elimination and facilitate a low number of catalytic turnovers. However, the exact manner, in which the Lewis acid promotes ß-hydride elimination remains to be elucidated. Herein, we describe the kinetic and thermodynamic role that commercially relevant and weakly Lewis acidic sodium salts play in promoting ß-hydride elimination from nickelalactones synthesized from CO2 and ethylene. This process is compared to a non-Lewis acid promoted pathway, and DFT calculations were used to identify differences between the two systems. The sodium-free isomerization reaction gave a rare CO2 -derived ß-nickelalactone complex, which was structurally characterized.

Synthesis and structure of six-coordinate iron borohydride complexes supported by PNP ligands.

The preparation of a number of iron complexes supported by ligands of the type HN{CH2CH2(PR2)}2 [R = isopropyl (((i)Pr)PNP) or cyclohexyl ((Cy)PNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron(II) complexes (((i)Pr)PNP)FeCl2(CO) (1a) and ((Cy)PNP)FeCl2(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl2 in the presence of CO. The iron(0) complex (((i)Pr)PNP)Fe(CO)2 (2a) was prepared through the reaction of Fe(CO)5 with ((i)Pr)PNP, while irradiating with UV light. Compound 2a is unstable in CH2Cl2 and is oxidized to 1a via the intermediate iron(II) complex [(((i)Pr)PNP)FeCl(CO)2]Cl (3a). The reaction of 2a with HCl generated the related complex [(((i)Pr)PNP)FeH(CO)2]Cl (4a), while the neutral iron hydrides (((i)Pr)PNP)FeHCl(CO) (5a) and ((Cy)PNP)FeHCl(CO) (5b) were synthesized through the reaction of 1a or 1b with 1 equiv of (n)Bu4NBH4. The related reaction between 1a and excess NaBH4 generated the unusual η(1)-HBH3 complex (((i)Pr)PNP)FeH(η(1)-HBH3)(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η(1)-HBH3 ligand and the N-H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of 6a with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(((i)Pr)PNP)FeH(CO)}2(µ2,η(1):η(1)-H2BH2)][BPh4] (7a), which features a rare example of a µ2,η(1):η(1)-H2BH2 ligand. Unlike all previous examples of complexes with a µ2,η(1):η(1)-H2BH2 ligand, there is no metal-metal bond and additional bridging ligand supporting the borohydride ligand in 7a; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes 1a, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.

Reductive functionalization of carbon dioxide to methyl acrylate at zerovalent tungsten.

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 κ(2)-O,O and κ(3)-C,C,O coordination mode of the acrylate ligand, and swiftly interconvert in solution as detected by 2D NMR spectroscopy. The CO(2)-derived acrylate fragment may be released from the tungsten coordination sphere by treatment with methyl iodide to afford modest quantities of free methyl acrylate.

Dihydrogen complexes of iridium and rhodium.

A series of iridium and rhodium pincer complexes have been synthesized and characterized: [(POCOP)Ir(H)(H(2))] [BAr(f)(4)] (1-H(3)), (POCOP)Rh(H(2)) (2-H(2)), [(PONOP)Ir(C(2)H(4))] [BAr(f)(4)] (3-C(2)H(4)), [(PONOP)Ir(H)(2))] [BAr(f)(4)] (3-H(2)), [(PONOP)Rh(C(2)H(4))] [BAr(f)(4)] (4-C(2)H(4)) and [(PONOP)Rh(H(2))] [BAr(f)(4)] (4-H(2)) (POCOP = κ(3)-C(6)H(3)-2,6-[OP(tBu)(2)](2); PONOP = 2,6-(tBu(2)PO)(2)C(5)H(3)N; BAr(f)(4) = tetrakis(3,5-trifluoromethylphenyl)borate). The nature of the dihydrogen-metal interaction was probed using NMR spectroscopic studies. Complexes 1-H(3), 2-H(2), and 4-H(2) retain the H-H bond and are classified as η(2)-dihydrogen adducts. In contrast, complex 3-H(2) is best described as a classical dihydride system. The presence of bound dihydrogen was determined using both T(1) and (1)J(HD) coupling values: T(1) = 14 ms, (1)J(HD) = 33 Hz for the dihydrogen ligand in 1-H(3), T(1)(min) = 23 ms, (1)J(HD) = 32 Hz for 2-H(2), T(1)(min) = 873 ms for 3-H(2), T(1)(min) = 33 ms, (1)J(HD) = 30.1 Hz for 4-H(2).

Mechanistic considerations for C-C bond reductive coupling at a cobalt(III) center.

The diamagnetic cobalt(III) dimethyl complex, cis,mer-(PMe(3))(3)Co(CH(3))(2)I, was found to promote selective C-C bond formation, affording ethane and triplet (PMe(3))(3)CoI. 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(double dagger) = 31.4(8) kcal/mol and ΔS(double dagger) = 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-(PMe(3))(3)Co(CH(3))(2)I. 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.