Reductive Coupling Of CO Research Articles

Highly Activated Terminal Carbon Monoxide Ligand in an Iron-Sulfur Cluster Model of FeMco with Intermediate Local Spin State at Fe.

Nitrogenases, the enzymes that convert N2 to NH3, also catalyze the reductive coupling of CO to yield hydrocarbons. CO-coordinated species of nitrogenase clusters have been isolated and used to infer mechanistic information. However, synthetic FeS clusters displaying CO ligands remain rare, which limits benchmarking. Starting from a synthetic cluster that models a cubane portion of the FeMo cofactor (FeMoco), including a bridging carbyne ligand, we report a heterometallic tungsten-iron-sulfur cluster with a single terminal CO coordination in two oxidation states with a high level of CO activation (νCO = 1851 and 1751 cm-1). The local Fe coordination environment (2S, 1C, 1CO) is identical to that in the protein making this system a suitable benchmark. Computational studies find an unusual intermediate spin electronic configuration at the Fe sites promoted by the presence the carbyne ligand. This electronic feature is partly responsible for the high degree of CO activation in the reduced cluster.

Reductive coupling of CO with magnesium anthracene complexes: formation of magnesium enediolates.

Two β-diketiminato-magnesium anthracene complexes, [{(ArNacnac)Mg}2(μ-C14H10)] (ArNacnac = [HC(MeCNAr)2]-, Ar = mesityl (Mes) or 2,6-diethylphenyl (Dep)) react with 1 atm. CO at room temperature to give tetra-magnesium enediolate complexes [{(ArNacnac)Mg}4(O2C16H10)2] via coupling of two molecules of CO with the anthracenediyl fragment. Similarly, reaction of magnesium anthracene, [Mg(THF)3(C14H10)], with CO afforded a low isolated yield of a tetra-magnesium complex bearing both dianthraceneylenediolate and dibenzocyclohepteneolateyl ligands. The presented reactions represent very rare examples of magnesium alkyl carbonylations that occur under mild conditions, and in the absence of catalysts.

Carbon Monoxide Coupling Reactions: A New Concept for the Formation of Hexahydroxybenzene.

For linear and cyclic coupling reactions of CO, among other products, the formation of the hexapotassium salt of hexahydroxybenzene is of particular interesting. The interaction of metallic potassium and CO offers, via the assumed K[OC≡CO]K as the result of several carbon monoxide coupling reactions, the formation of C6(OK)6 among other products. To date, only speculations exist about the reaction pathway for these products, which were first described by Liebig in 1834. A novel concept is suggested here, which consists of the single steps (i) reductive coupling of CO, (ii) formation of dihetero‐metallacyclopentynes (cis‐2,5‐diheterobutatriene as formal ethylenedione O=C=C=O complexes), (iii) formation of its dinuclear 1‐metalla‐2,5‐dioxo‐cyclopentyne complexes by external coordination of the triple bond, (iv) insertion of CO into the M−C bond of the formed metallacyclopropene, and (v) the reductive elimination of C6(OK)6. The novel aspect of this concept is the formation of dihetero‐metallacyclopentynes (in analogy to the well characterized all‐C‐metallacyclopentynes), which have not been considered in the mechanism of reductive CO coupling reactions. It is expected that the presence of transition‐metal impurities would promote the reaction.

Reduction of Carbon Oxides by an Acyclic Silylene: Reductive Coupling of CO.

Reactions of a boryl-substituted acyclic silylene with carbon dioxide and monoxide are reported. The former proceeds through oxygen atom abstraction, generating CO (with rearrangement of the putative silanone product through silyl-group transfer). The latter is characterized by reductive coupling of CO to give an ethynediolate fragment, which undergoes formal insertion into the Si-B bond. The net conversion of carbon dioxide with two equivalents of silylene offers a route for the three-electron reduction of CO2 to [C2 O2 ]2- .

Reduction of Carbon Oxides by an Acyclic Silylene: Reductive Coupling of CO

AbstractReactions of a boryl‐substituted acyclic silylene with carbon dioxide and monoxide are reported. The former proceeds through oxygen atom abstraction, generating CO (with rearrangement of the putative silanone product through silyl‐group transfer). The latter is characterized by reductive coupling of CO to give an ethynediolate fragment, which undergoes formal insertion into the Si−B bond. The net conversion of carbon dioxide with two equivalents of silylene offers a route for the three‐electron reduction of CO2 to [C2O2]2−.

Silicon-Mediated Selective Homo- and Heterocoupling of Carbon Monoxide.

While the transformation of carbon monoxide to multicarbon compounds (fuels and organic bulk chemicals) via reductive scission of the enormously strong CO bond is dominated by transition metals, splitting and deoxygenative reductive coupling of CO under nonmatrix conditions using silicon, the second most abundant nonmetal of the earth's crust, is extremely scarce and mechanistically not well understood. Herein, we report the selective deoxygenative homocoupling of carbon monoxide by divalent silicon utilizing the (LSi:)2Xant 1a [Xant = 9,9-dimethyl-xanthene-4,5-diyl; PhC(N tBu)2] and (LSi:)2Fc 1b (Fc = 1,1'-ferrocenyl) as four-electron reduction reagents under mild reaction conditions (RT, 1 atm), affording the corresponding disilylketenes, Xant(LSi)2(μ-O)(μ-CCO) 2a and Fc(LSi)2(μ-O)(μ-CCO) 2b, respectively. However, the dibenzofuran analogue of 1b, compound 1c, was unreactive toward CO due to the longer distance between the two SiII atoms, which demonstrated the crucial role of the Si···Si distance on cooperative CO binding and activation. This is confirmed by density functional theory (DFT) calculations, and further theoretical investigations on CO homocoupling with 1a and 1b revealed that the initial step of CO binding and scission involved CO acting as a Lewis acid (four-electron acceptor), in sharp contrast to CO activation mediated by transition metals where CO serves as a Lewis base (two-electron donor). This mechanism was strongly reinforced by the reaction of 1a with isocyanide Xyl-NC (Xyl = 2,6-Me2C6H3), isoelectronic with CO. Treatment of 1a with one or two molecules of Xyl-NC furnished the unique (silyl)(imido)silene 3a and the C═C coupled bis(Xyl-NC) product 5, respectively, via the isolable doubly bridged Xant(LSi)2(μ-XylNC)2 intermediate 4. Moreover, compound 3a reacts with 1 molar equivalent of CO to give the disilylketenimine Xant(LSi)2(μ-O)(μ-CCNR) 6, representing, for the first time, a selective heterocoupling product of CO with isoelectronic isocyanide.

Reductive Coupling of CO2 , Primary Amine, and Aldehyde at Room Temperature: A Versatile Approach to Unsymmetrically N,N-Disubstituted Formamides.

We present a simple, metal-free, and versatile route to synthesize unsymmetrically N,N-disubstituted formamides (NNFAs) from CO2 , primary amine, and aldehyde promoted by an ionic liquid (1-butyl-3-methylimidazolium chloride) at room temperature. This approach features wide scopes of amines and aldehydes, and various unsymmetrical NNFAs could be obtained in good to excellent yields. The ionic liquid can be reused for at least five runs without obvious activity loss.

Mechanism of Molybdenum-Mediated Carbon Monoxide Deoxygenation and Coupling: Mono- and Dicarbyne Complexes Precede C-O Bond Cleavage and C-C Bond Formation.

Deoxygenative coupling of CO to value-added C≥2 products is challenging and mechanistically poorly understood. Herein, we report a mechanistic investigation into the reductive coupling of CO, which provides new fundamental insights into a multielectron bond-breaking and bond-making transformation. In our studies, the formation of a bis(siloxycarbyne) complex precedes C-O bond cleavage. At -78 °C, over days, C-C coupling occurs without C-O cleavage. However, upon warming to 0 °C, C-O cleavage is observed from this bis(siloxycarbyne) complex. A siloxycarbyne/CO species undergoes C-O bond cleavage at lower temperatures, indicating that monosilylation, and a more electron-rich Mo center, favors deoxygenative pathways. From the bis(siloxycarbyne), isotopic labeling experiments and kinetics are consistent with a mechanism involving unimolecular silyl loss or C-O cleavage as rate-determining steps toward carbide formation. Reduction of Mo(IV) CO adducts of carbide and silylcarbyne species allowed for the spectroscopic detection of reduced silylcarbyne/CO and mixed silylcarbyne/siloxycarbyne complexes, respectively. Upon warming, both of these silylcarbynes undergo C-C bond formation, releasing silylated C2O1 fragments and demonstrating that the multiple bonded terminal Mo≡C moiety is an intermediate on the path to deoxygenated, C-C coupled products. The electronic structures of Mo carbide and carbyne species were investigated quantum mechanically. Overall, the present studies establish the elementary reactions steps by which CO is cleaved and coupled at a single metal site.

Metal-metal multiple bonds with “half-bond” components in paramagnetic organometallics of f-block metals: Cyclopentadienyluranium carbonyls as molecular relatives of diuranium

The experimentally observed reductive CO coupling in organouranium systems observed by Cloke and co-workers makes of interest the exploration of the structures and thermochemistry of simple Cp2U2(CO)n (n = 5, 4, 3, 2) derivatives. In this connection, the low-energy Cp2U2(CO)n (n = 2, 3, 4) structures contain two to four bridging η2-μ-CO groups supplemented, in some cases, by one or two terminal CO groups. No examples of CO coupling to form C2O2 ligands are found in any of these Cp2U2(CO)n structures in contrast to the previously studied Cp2Th2(CO)4 system in which CO coupling to form C2O2 ligands is observed. The retention of uranium valence electrons in low-energy Cp2U2(CO)n (n = 3, 4) structures, even after partial donation of uranium electrons to reduce Cp to Cp– and CO to CO2-, leads to a rich and complicated variety of UU bonding modes as well as spin states from singlet to quintet. Of particular interest are uranium-uranium multiple bonds containing multiple single electron half-bond components in triplet and quintet spin state Cp2U2(CO)n structures. The formal uranium-uranium multiple bonds in such Cp2U2(CO)n structures exhibit some of the features of the quintuple bond of diuranium, U2, in which a typical σ + 2π triple bond similar to the CC triple bond of acetylene is supplemented by no less than four separate one-electron half-bonds. Thus the UU bonds in the lowest energy Cp2U2(CO)n (n = 2, 3) structures can be interpreted as formal triple bonds of various types with unusually short bond distances of ∼2.5 Å and Wiberg bond indices of ∼3.

A Thermodynamic Analysis of Rhenium(I)–Formyl C–H Bond Formation via Base-Assisted Heterolytic H2Cleavage in the Secondary Coordination Sphere

Conversion of synthesis gas, a mixture of carbon monoxide and hydrogen, into value-added Cn≥2 products requires both C–H and C–C bond-forming events. Our group has developed a series of molecular complexes, based on group 7 (manganese and rhenium) carbonyl complexes, to interrogate the elementary steps involved in the homogeneous hydrogenative reductive coupling of CO. Here, we explore a new mode of H2 activation, in which strong bases in the secondary coordination sphere are positioned to assist in the heterolytic cleavage of H2 to form a formyl C–H bond at a rhenium-bound carbonyl. A series of cationic rhenium(I) complexes of the type [ReI(P∼B:-κ1-P)(CO)5]n (n = 0, +1), where P∼B: is a phosphine ligand with a tethered strong base, are prepared and characterized; measurement of their protonation equilibria demonstrates a pronounced attenuation of the basicity upon coordination. Formyl complexes supported by these ligands can be prepared in good yield by hydride delivery to the parent pentacarbonyl comple...

A CO-Derived Iron Dicarbyne That Releases Olefin upon Hydrogenation

An iron diphosphineborane platform that was previously reported to facilitate a high degree of N2 functionalization is herein shown to effect reductive CO coupling. Disilylation of an iron dicarbonyl precursor furnishes a structurally unprecedented iron dicarbyne complex. Several complexes related to this process are also characterized which allows for a comparative analysis of their respective Fe-B and Fe-C bonding. Facile hydrogenation of the iron dicarbyne at ambient temperature and 1 atm H2 results in release of a CO-derived olefin.

Steric Effects in the Reductive Coupling of CO by Mixed-Sandwich Uranium(III) Complexes

The selectivity of the mixed-sandwich U(III) complexes of the type [U(η-C8H6{SiiR3-1,4}2)(η-CpR′)] (R = Me, iPr; R′ = Me4H, Me5, Me4iPr, Me4SiMe3, Me4Et) toward the reductive coupling of CO to form uranium-bound oxocarbons has been explored. In this context, the new U(III) mixed-sandwich complexes [U(η-C8H6{SiiPr3-1,4}2)(η-CpMe4TMS)], [U(η-C8H6{SiiPr3-1,4}2)(η-CpMe4iPr)], [U(η-C8H6{SiiPr3-1,4}2)(η-CpMe4Et)], [U(η-C8H6{SiMe3-1,4}2)(η-Cp*)], and [U(η-C8H6{SiMe3-1,4}2)(η-CpMe4TMS)] have been prepared and structurally characterized. The reactivity toward CO is dominated by the “global” sterics around the uranium center, while selectivity for oxocarbon formation is largely regulated by the steric bulk of the CpR′ ligand. Accordingly, with excess CO [U(η-C8H6{SiiPr3-1,4}2)(η-CpMe4TMS)] and [U(η-C8H6{SiiPr3-1,4}2)(η-CpMe4iPr)] show no reactivity, [U(η-C8H6{SiMe3-1,4}2)(η-CpMe4TMS)] is completely selective for the formation of the ynediolate complex [U(η-C8H6{SiMe3-1,4}2)(η-CpMe4TMS)]2(μ-η1:η1-13C2O2), [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)] affords only the deltate complex [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)]2(μ-η2:η2-C3O3), and [U(η-C8H6{SiiPr3-1,4}2)(η-CpMe4H)] gives solely the squarate complex [U(η-C8H6{SiiPr3-1,4}2)(η-CpMe4H)]2(μ-η2:η2-C4O4). Additionally, the squarate moiety has been removed from the uranium center in the last complex by reaction with Me3SiCl to afford the TMS ester of squaric acid, C4O2(OTMS)2.

Reductive Coupling of CO Templated by Iron Bound to the Tris(pyrrolide)ethane Scaffold

The reactivity of the high-spin (S = 2) [(Mestpe)Fe(THF)][Li(THF)4] (1) complex (Mestpe = tris(mesitylpyrrolide)ethane) with isocyanide and CO substrates is explored. Reaction of 1 with excess tBuNC forms a low-spin (S = 0), six-coordinate iron(II) species with three tBuNC ligands bound to iron, producing a notable tautomerization of one of the pyrrolide units from N- to C-ligation to iron. Reaction of 1 with an atmosphere of CO also produces a new diamagnetic complex, wherein two molecules of CO are consecutively reductively coupled, driven by the two-electron oxidation and fragmentation of the tris(pyrrolide)ethane ligand. The product features a six-coordinate Fe(II) species bound to a dipyrromethene ligand (resulting from oxidative fragmentation of the Mestpe ligand), an oxalyl-imino pyrrole fragment from pyrrole coupling to two molecules of CO. The reactions of 1 with tBuNC and CO provide insight into how tautomerization of the tris(pyrrolide) ligand upon substrate binding initiates the contiguous reductive coupling of CO.

Homogeneous CO Hydrogenation: Ligand Effects on the Lewis Acid-Assisted Reductive Coupling of Carbon Monoxide

Structure−function studies on the role of pendent Lewis acids in the reductive coupling of CO are reported. Cationic rhenium carbonyl complexes containing zero, one, or two phosphinoborane ligands (Ph2P(CH2)nB(C8H14), n = 1−3) react with the nucleophilic hydride [HPt(dmpe)2]+ to reduce [M−CO]+ to M−CHO; this step is relatively insensitive to the Lewis acid, as both pendent (internal) and external boranes of appropriate acid strength can be used. In contrast, whether a second hydride transfer and C−C bond forming steps occur depends strongly on the number of carbon atoms between P and B in the phosphinoborane ligands, as well as the number of pendent acids in the complex: shorter linker chain lengths favor such reductive coupling, whereas longer chains and external boranes are ineffective. A number of different species containing partially reduced CO groups, whose exact structures vary considerably with the nature and number of phosphinoborane ligands, have been crystallographically characterized. The reac...

An unprecedented two-dimensional Eu(III) coordination polymer Eu(OOC–C 5H 4N–CH 2–CH 2–COO)(OOC–COO)·2H 2O formed by in situ reaction of fumaric acid and isonicotinic acid: Crystal structure and luminescent properties

Hydrothermal reaction of fumaric acid and isonicotinic acid in presence of trace quantity of nitric acid produced an unprecedented two-dimensional coordination polymer Eu(OOC–C 5H 4N–CH 2–CH 2–COO)(OOC–COO)·2H 2O containing in situ formed new ligands, 3-(4-pyridinecarboxylate)propionic acid (inpro) and oxalate (ox) moieties. The formation of inpro ligand from isonicotinic acid and fumaric acid is presumably through dehydration between 3-hydroxylpropionic acid and 4-carboxypyridinium moieties present under the above conditions, and the oxalate formation due to the reductive coupling of CO 2 radicals generated from 2-hydroxysuccinic acid through C–C bond cleavage. The compound crystallizes in monoclinic P2 1/ C space group and contains nine-coordinated Eu 3+ ion having a distorted mono-capped square-antiprismatic geometry. While two chelatively bridging oxalate moieties coordinating through four of their O atoms lead to the formation of zigzag Eu(III)–oxalate chains, four uniquely bridging anionic inpro ligands which coordinate through five O centers altogether make the system take an overall two-dimensional network arrangement. Upon excitation at 359 nm the compound exhibits interesting luminescent properties in solid state with several intense bands in the visible region; the most intense and sharp emission being in the red region at 615 nm due to 5D 0 → 7F 2 transition, facilitated by the low-symmetry coordination environment around Eu 3+ ion by the ligands.

In-situ carboxylation and synthesis of two novel Sm(III) coordination polymers assembled from 5-hydroxyisophthalate and nitrate, chloride in hydrothermal reaction

By reactions of 5-hydroxyisophthalic acid (H 2hisp) with Sm(NO 3) 3·6H 2O or SmCl 3·6H 2O in the presence of NaOH, two kinds of samarium coordination polymers, [Sm(H 2hbtc)(ox) 0.5(H 2O) 3] n · nH 2O ( 1) (H 2hbtc 2−=6-hydroxy-1,2,4-benzenetricarboxylate) and [Sm(hisp)(Hhisp)(H 2O) 2] n ·2 nH 2O ( 2), have been hydrothermal synthesized and characterized. Single-crystal X-ray analyses reveal that compound 1 features a novel two-dimensional (2D) stair-like structure with oxalate ligands and the new organic ligand (H 2hbtc 2−) but without 5-hydroxyisophthalate ligands, while compound 2 gives the expected product and displays a novel layer structure. The oxalate ligands have been formed via the in-situ reductive coupling of CO 2 molecules released from the decomposition of carboxylate ligands with the reduction of NO 3 − and the new organic ligands have been formed via the in-situ carboxylation under the presence of NO 3 −. Reported herein are the syntheses of compounds 1 and 2, crystal structures and possible mechanism information regarding the in-situ carboxylation.

Terbium-oxalate-pyridinedicarboxylate coordination polymers suggesting the reductive coupling of carbon dioxide (CO 2) to oxalate (C 2O 42−):[Tb 2 (3,5-PDC) 2(H 2O) 4(C 2O 4)]·2H 2O and [Tb(2,4-PDC)(H 2O)(C 2O 4) 0.5] (PDC = pyridinedicarboxylate)

Under hydrothermal conditions, Tb(NO 3) 3·5H 2O reacted with 3,5-pyridinedicarboxylic acid (3,5-PDCH 2) to give a 3-D coordination polymer with the empirical formula of [Tb 2 (3,5-PDC) 2(H 2O) 4(C 2O 4)]·2H 2O ( 1). Tb(NO 3) 3·5H 2O also reacted with 2,4-pyridinedicarboxylic acid (2,4-PDCH 2) to give another 3-D coordination polymer [Tb 2 (2,4-PDC) 2(H 2O) 2(C 2O 4)] ( 2). The structures of both polymers have been determined by X-ray diffraction. X-ray structure analyses show that both polymers contain bridging oxalate (C 2O 4 2−) ligands, which might have been formed by the reductive coupling of CO 2 molecules released from the PDC 2− ligands through the C–C bond cleavage.

Catalytic hydrosilylation of carbon monoxide with cobalt carbonyls

The hydrosilylation of carbon monoxide with cobalt carbonyls under relatively mild conditions resulted in a Fischer-Tropsch-analogous reductive coupling of CO. Alkyl silyl ethers, 1,2-bis(silyloxy)-1-alkenes and higher molecular weight compounds with at least three silyloxy groups were also formed, each of these containing up to seven carbon atoms derived from CO and for the most part having a linear structure. A (silyloxycarbyne)cobalt complex was postulated as the key intermediate.

Mechanisms of reductive addition of CO 2 to quinones in acetonitrile

In this paper we report the mechanisms of reductive coupling of CO 2 to quinone anions in acetonitrile (AN). Organic compounds such as unsaturated hydrocarbons, organic halides, phenols and ketones add CO 2 under reducing conditions, and the addition involves either one or two molecules of CO 2 [1-8]. Compounds with anionic oxygens add CO 2 in aprotic solvents to yield carbonates as primary products

ChemInform Abstract: Vanadium‐Promoted Reductive Coupling of CO and Facile Hydrogenation to Form cis‐Disiloxyethylenes.

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