Reductive Trimerization of CO to the Deltate Dianion using Activated

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Reductive Trimerization of CO to the Deltate Dianion using Activated Magnesium(I) Compounds K. Yuvaraj, Iskander Douair, Albert Paparo, Laurent Maron, and Cameron Jones J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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Journal of the American Chemical Society

Reductive Trimerization of CO to the Deltate Dianion using Activated Magnesium(I) Compounds K. Yuvaraj,† Iskander Douair,‡ Albert Paparo,† Laurent Maron,*,‡ and Cameron Jones*,† † School

of Chemistry, Monash University, PO Box 23, Melbourne, VIC, 3800, Australia de Toulouse et CNRS, INSA, UPS, UMR 5215, LPCNO, 135 Avenue de Rangueil, F-31077 Toulouse, France

‡ Université

Supporting Information Placeholder ABSTRACT: This study details syntheses of unsymmetrical magnesium(I)-adduct complexes, [(ArNacnac)(D)Mg−Mg(ArNacnac)] (ArNacnac = [(ArNCMe)2CH]), Ar = xylyl (Xyl), mesityl (Mes), 2,6-diethylphenyl (Dep), or 2,6diisopropylphenyl (Dip); D = N-heterocyclic carbene or 4dimethylaminopyridine, DMAP), which X-ray crystallographic studies show to have markedly elongated Mg−Mg bonds. Two of these highly reactive species are shown to reductively trimerize CO to yield rare crystallographically characterized examples of the planar, aromatic deltate dianion, incorporated in the complexes [{(DipNacnac)(D)Mg(-C3O3)Mg(DipNacnac)}2] (D = DMAP or :C{N(Me)C(Me)}2). DFT calculations suggest these complexes form via stepwise two-electron reductions of three CO molecules, resulting in the formation of three C−C bonds within the cyclic deltate unit. This work highlights the utility activated magnesium(I) adduct complexes have as soluble organometallic models for the study of reductive C−C bond forming events in, for example, the heterogeneously catalyzed Fischer-Tropsch process.

Carbon monoxide is a cheap and abundant industrial feedstock, which is readily obtained from coal, natural gas or biomass on a huge scale. In combination with H2 (i.e. in synthesis gas: CO/H2) it is utilized as a C1 building block in, for example, the FischerTropsch (F-T) process. This typically employs heterogeneous transition metal catalysts to generate millions of tons of liquid hydrocarbons and oxygenates per annum.1 Despite the importance of Fischer-Tropsch, little is definitively known about the fundamental steps that lead to C−C bond formations in this, and related processes. To aid the understanding of reaction mechanisms associated with the F-T process, and to potentially enhance its selectivity, organometallic complexes are increasingly being used to model the chemistry that is central to F-T, but in a homogeneous setting.2 With regard to the study of mechanisms surrounding F-T induced C−C bond formations, particular recent interest has lain with the reductive homologation of CO by low-valent organometallic compounds, yielding cyclic and acyclic oxocarbon anions, e.g. ethynediolate [C2O2]2- and cyclic aromatic [CnOn]2- (n = 3-6), under mild conditions. Despite CO possessing one of the strongest bonds known (BDE = 257 kcal/mol),3 metal complexes from every block in the periodic table, and several low-valent "metal-free" compounds, have been shown capable of reductive oligomerization of the diatomic gas to oxocarbon anions.4-7 Although much of this work has been carried out in the last several decades, the first reports of oxocarbon anion formation date back

to the beginning of the 19th century, when it was proposed that the reduction of CO with molten alkali metals generated [CnOn]2- (n = 5 or 6).4a,b Related later reports described the formation of similar anions by treating CO with "solutions" of alkali metals in NH3 or THF/crown ether mixtures.4c,d Notwithstanding this, and to the best of our knowledge, there have been no reports of crystallographically characterized oxocarbon anions derived from s-block metal reductions of CO.8 Since their discovery in 2007,9 we have extensively developed the reduction chemistry of isolable magnesium(I) compounds, [(L)Mg−Mg(L)] (L = bulky guanidinate, -diketiminate, amide etc.),10,11 and believed they could prove useful as selective and soluble reductants for oxocarbon anion formations. However, we have previously shown that these compounds are unreactive towards CO, though when hydrogenated, the generated magnesium hydride complexes, [{(ArNacnac)Mg(-H)}2] (ArNacnac = [(ArNCMe)2CH]-), 2,6-diethylphenyl (Dep), or 2,6diisopropylphenyl (Dip)), do couple CO to selectively give an ethylenediolate, [{(DipNacnac)Mg}2(-O2C2H2)], or an unprecedented cyclopropanetriolate compound, [{(DepNacnac)Mg}3(-O3C3H3)], depending on the size of the diketiminate ligand employed.12 Here we show that magnesium(I) compounds can be activated by the addition of sub-stoichiometric amounts of Lewis bases, and that the resultant magnesium(I) adduct complexes do reductively trimerize CO to give the deltate dianion, [C3O3]2-. It is pertinent that the only prior crystallographic authentication of this elusive oxocarbon anion was by Cloke and co-workers, who in their landmark 2006 study, showed that it can be obtained by a related reductive trimerization of CO using an organo-uranium(III) complex.7a,b Reactions of -diketiminate coordinated magnesium(I) compounds, e.g. [{(ArNacnac)Mg}2] (Ar = xylyl (Xyl), mesityl (Mes), Dep, or Dip), with Lewis base donors (D) are known to give symmetric adduct complexes, [{(ArNacnac)(D)Mg}2], in which the Mg−Mg bond increases markedly from ca. 2.85 Å to > 3 Å.10,13 Although this lengthening might be expected to increase the reactivity of the Mg−Mg bond, the reverse is true due to the more coordinatively saturated nature of the Mg centers in the adducts. It seemed reasonable that related 1:1 adducts, [(ArNacnac)(D)Mg−Mg(ArNacnac)], might have similarly lengthened Mg−Mg bonds, but enhanced reactivity due to them possessing one 3-coordinate Mg center, which is able to coordinate reducible substrates. To explore this possibility, we reacted a series of magnesium(I) compounds with one equivalent of 4dimethylaminopyridine (DMAP) or N-heterocyclic carbenes (NHCs) to give the adduct complexes, 1-5, as orange to red crystalline solids in moderate isolated yields (Scheme 1).

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Scheme 1. Synthesis of compounds 1-5 (TMC = :C{N(Me)C(Me)}2, IPriMe = :C{N(Pri)C(Me)}2, DMAP = 4dimethylaminopyridine, Xyl = xylyl, Mes = mesityl, Dep = 2,6-diethylphenyl, Dip = 2,6-diisopropylphenyl).

The majority of the compounds are stable in the solid state, though the crystalline DMAP adduct 5 decomposes at room temperature over several days. All of the NHC adducts decompose slowly in benzene or toluene solutions at ambient temperature, but can be used in further reactions for several hours after dissolution. Variable temperature NMR spectroscopic studies of the adducts 15 revealed fluxional behavior, which is believed to arise from rapid "hopping" of the Lewis base donor between the two Mg centers. A full discussion of this phenomenon can be found in the SI. All of the adduct complexes were crystallographically characterized, which showed them to be structurally similar. As a result, only the molecular structure of 4 is depicted in Figure 1. The Mg−Mg bonds in 1-5 fall within a narrow range (2.938(1)-3.089(1) Å), and all are significantly longer than the Mg−Mg bonds in the magnesium(I) complexes from which they were derived.10 The bonds lengths are, however, similar to those in related 2:1 adduct complexes of magnesium(I) species, [{(ArNacnac)(D)Mg}2].10 Each complex possesses one distorted tetrahedral and one distorted trigonal planar Mg center, the former of which is coordinated by an NHC or DMAP ligand. Complexes 1-4 represent the first NHC adducts of magnesium(I) compounds, and their Mg−C distances (range: 2.296(3)-2.341(2) Å) are longer than those in all but one of the known NHC adducts of magnesium(II) fragments.14 This is not surprising, given the expected larger covalent radius of magnesium(I).

Figure 1. Molecular representation (20% probability surface) of [(DipNacnac)(TMC)Mg−Mg(DipNacnac)] in crystals of 4·(toluene)·0.5(benzene). Adducts 4 and 5 were chosen as exemplars to study the reaction of activated magnesium(I) dimers with CO. Toluene solutions of each, held at -78 °C, were treated with an excess of CO gas at 1 atm., then the reaction solutions were warmed to room temperature, before being left to stand overnight. During this time moderate

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yields of the colorless crystalline deltate complexes 6 (50% yield) and 7 (52% yield) deposited (Scheme 2). Monitoring the progress of the reactions by 1H NMR spectroscopy did not reveal any obvious intermediates in the formations of the deltate complexes, which are clean, and complete within ca. 3 hrs at ambient temperature. Repeating the reactions, but using a 1:1 mixture of CO and H2, again, led only to the isolation of deltate complexes, 6 and 7, with no evidence for the generation of hydrogenated products, cf. [{(DepNacnac)Mg}3(-O3C3H3)].12 Contrastingly, replacing CO with an excess of the isolobal isocyanide, :CNBut, in the reaction with 4, did not proceed to the clean formation of a C−C coupled product, cf. 6, but instead afforded a complex mixture of products. Although largely intractable, a low yield (7 %) of the crystalline magnesium cyanide complex, [(DipNacnac)(TMC)Mg(NC)], was obtained from this mixture, presumably after being formed by a reductive C−C cleavage reaction (see SI for further details).15

Scheme 2. Synthesis of deltate complexes 6 and 7.

The formations of 6 and 7 are closely related to the aforementioned uranium(III) induced reductive trimerization of CO to give the deltate dianion in [{U(COT†)(Cp*)}2(-C3O3)] (COT†= [C8H6(Pri)2-1,4]-; Cp* = [C5Me5]-).7a This was proposed to be the thermodynamic product of the CO reduction process. Cloke and co-workers subsequently synthesized the analogous CO dimerized ethynediolate complex, [{U(COT†)(Cp*)}2(-C2O2)], believed to be the kinetic product, and showed this to be unreactive towards excess CO.7f,j This confirmed that [{U(COT†)(Cp*)}2(C2O2)] is not an intermediate to [{U(COT†)(Cp*)}2(-C3O3)]. Indeed, a later computational study by Maron and co-workers indicated different (though related) mechanisms for the synthesis of the two compounds.16 So as to assess the potential intermediacy of magnesium ethynediolate complexes in the formation of 6 and 7, toluene solutions of magnesium(I)-adduct complexes 4 and 5 were treated with 2.2 equivalents of CO. This led to rapid reactions and the formation of product mixtures, which contained deltate complexes, 6 and 7, magnesium(I) starting materials, and the diketimine, DipNacnacH, amongst other lower yielding species. Although no ethynediolate, or other CO coupled products, could be isolated from the reaction mixtures, their presence as minor products cannot be definitively ruled out at this stage. Both deltate complexes 6 and 7 are thermally stable in solution and the solid state for weeks. Their solution state NMR spectra are indicative of them retaining their solid state structures in solution, though their low solubility in non-coordinating solvents hindered meaningful variable temperature NMR spectroscopic experiments. In the solid state, the compounds are essentially isostructural, so only the molecular structure of 7 is depicted in Figure 2. This shows it to be a centrosymmetric dimer with two distorted tetrahedral (DipNacnac)Mg fragments bridging two deltate dianions, generating an Mg2O4C4 ten-membered ring. In addition, the deltate moieties each coordinate a terminal (DipNacnac)(DMAP)Mg unit, which also contains a distorted tetrahedral metal center. The deltate dianions are essentially planar with close to equivalent C−C and C−O bond lengths (ranges for 7: 1.391(2)-1.402(2) Å and 1.269(2)-1.288(2) Å respectively), which lie between single and double bond distances,17 thus implying electronic delocalization over the aromatic cycles. This contrasts to the situation in


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[{U(COT†)(Cp*)}2(-C3O3)], in which the deltate dianion is only partially delocalized.7a

symmetrical magnesium(I) dimers. The substantially enhanced reactivity of the Mg−Mg bonds in the adduct compounds leads to them effecting facile reductive trimerizations of CO under ambient conditions, yielding deltate complexes. These represent the first structurally authenticated s-block metal complexes of any oxocarbon anion derived from reductive CO oligomerization, and only the second examples of deltate complexes. DFT calculations suggest that the deltate dianion assembles by a multi-step process, involving three C−C bond formations, similar to that previously proposed for UIII mediated CO trimerizations. This work highlights the utility of activated magnesium(I) adduct complexes as cheap, non-toxic, diamagnetic, non-radioactive alternatives to f-block organometallic models for the study of reductive C−C bond forming events in, for example, the Fischer-Tropsch process. We continue to explore the chemistry of activated magnesium(I) adducts, and their use in catalytically relevant small molecule activations, and further value adding functionalizations.

Figure 2. Molecular representation (20% probability surface) of [{(DipNacnac)(DMAP)Mg(-C3O3)Mg(DipNacnac)}2] in crystals of 7·4(toluene). Isopropyl groups omitted for sake of clarity.

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In order to gain insight into the reaction mechanism that yielded 6, DFT calculations (B3PW91) were carried out in the gas phase to determine a possible reaction pathway from 4 and CO (Figure 3). The reaction begins with insertion of CO into the Mg−Mg bond of 4, which is activated and lengthened (3.113 Å) by the presence of the coordinated TMC donor. The kinetic barrier to this insertion is low (9.7 kcal/mol), and in the transition state, the CO is η2-bonded to TMC free Mg1. For comparison, in the absence of TMC, the barrier to CO insertion is markedly higher (29.6 kcal/mol) and endothermic by 9.6 kcal/mol, which is in line with the lack of experimental reactivity between [{(DipNacnac)Mg}2] and the gas.15 Following the intrinsic reaction coordinate leads to the formation of a key intermediate with a doubly reduced CO sandwiched between two MgII centers in a κ1-C:η2-C,O fashion. The formation of this intermediate is exothermic by 7.0 kcal/mol. This intermediate then undergoes a facile CO insertion into the Mg1-C bond (barrier: 1.7 kcal/mol) yielding an intermediate containing a “zig-zag” [O=C−C=O]2- fragment, which is very similar to that reported for the mechanism of formation of [{U(COT†)(Cp*)}2(C2O2)].7f,16 Although very stable (-35.0 kcal/mol), the latter intermediate allows a barrierless insertion of CO to ultimately form the highly stable deltate product 6 (-103.3 kcal/mol). It is noteworthy that the low kinetic barriers from 4 to 6 are consistent with absence of a spectroscopically observable intermediate in the experimental reaction. In summary, a series of highly activated, unsymmetrical magnesium(I)-adduct complexes have been prepared by reaction of sub-stoichiometric amounts of Lewis bases with known,

Supporting Information Details of the synthesis and characterizing data for all new compounds. Full details and references for the crystallographic and computational data. Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Financial support from the Australian Research Council (CJ, LM), the U.S. Air Force Asian Office of Aerospace Research and Development (grant FA2386-18-1-0125 to CJ), the Indian Science and Engineering Research Board (fellowship for KY), and the Alexander von Humboldt Foundation (Feodor-Lynen fellowship for AP), is acknowledged. Part of this research was undertaken on the MX1 beamline at the Australian Synchrotron, Victoria, Australia.



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Figure 3. Computed enthalpy profile at 298 K for the formation of deltate complex 6 from magnesium(I)-adduct complex 4, and three molecules of CO (relevant bond lengths in Å).

REFERENCES (1) See for example: (a) Advances in Fischer–Tropsch Synthesis Catalysts and Catalysis, Davis B. H., Occelli, M. L., CRC, Boca Raton, FL, 2009. (b) Khodakov, A. Y.; Chu, W.; Fongarland, P. Advances in the development of novel cobalt Fischer−Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels. Chem. Rev. 2007, 107, 16921744. (c) Rofer-DePoorter, C. K. A comprehensive mechanism for the Fischer-Tropsch synthesis. Chem. Rev. 1981, 81, 447474. (d) Masters, C. The Fischer-Tropsch reaction. Adv. Organomet. Chem. 1979, 17, 61103. (2) See for example: (a) Durfee, L. D.; Rothwell, I. P. Chemistry of 2acyl, 2-iminoacyl and related functional groups. Chem. Rev. 1988, 88, 10591079. (b) Erker, G. Carbonylation of zirconium complexes. Acc. Chem. Res. 1984, 17, 103109. (c) Gladysz, J. A. Transition metal formyl complexes. Adv. Organomet. Chem. 1982, 20, 138. (d) Wolczanski, P. T.; Bercaw, J. E. Mechanisms of carbon monoxide reduction with zirconium hydrides. Acc. Chem. Res. 1980, 13, 121127. (3) Kalescky, R.; Kraka, E.; Cremer, D. Identification of the strongest bonds in chemistry. J. Phys. Chem. A 2013, 117, 8981−8995. (4) For examples of s-block reductive oligomerizations of CO see: (a) Gmelin, L. Ueber, einige merkwürdige, bei der darstellung des kaliums nach der Brunner'schen methode, erhaltene substanzen. Ann. Phys. 1825, 4, 31–62. (b) Liebig, J. Ueber das verhalten des kohlenoxyds zu kalium. Ann. Chem. Pharm. 1834, 11, 182189. (c) Büchner, W. Zur kenntnis der sogenannten «alkalicarbonyle» III. die «alkalicarbonyle» als substangemische einer metalloganischen verbindung und metallacetylendiolaten. Helv. Chim. Acta 1963, 46, 21112120. (d) Lednor, P. W.; Versloot, P. C. Radical-anion chemistry of carbon monoxide. J. Chem. Soc., Chem. Commun. 1983, 284285. (5) For examples of p-block element reductive oligomerizations of CO see: (a) Braunschweig, H.; Dellermann, T.; Dewhurst, R. D.; Ewing, W. C.; Hammond, K.; Jimenez-Halla, J. O. C.; Kramer, T.; Krummenacher, I.; Mies, J.; Phukan, A. K.; Vargas, A. Metal-free binding and coupling of carbon monoxide at a boron-boron triple bond. Nat. Chem. 2013, 5, 1025−1029. (b) Böhnke, J.; Braunschweig, H.; Dellermann, T.; Ewing, W. C.; Hammond, K.; Jimenez-Halla, J. O. C.; Kramer, T.; Mies, J. The synthesis of B2(SIDip)2 and its reactivity between the diboracumulenic and diborynic extremes. Angew. Chem. Int. Ed. 2015, 54, 13801−13805. (c) Wang, B.; Luo, G.; Nishiura, M.; Luo, Y.; Hou, Z. Cooperative trimerization of carbon monoxide by lithium and samarium boryls. J. Am. Chem. Soc. 2017, 139, 16967–16973. (d) Kong, R. Y.; Crimmin, M. R. Carbon chain growth by sequential reactions of CO and CO2 with [W(CO)6]

and an aluminum(I) reductant. J. Am. Chem. Soc. 2018, 140, 1361413617. (e) Protchenko, A. V.; Vasko, P.; Do, D. C. H.; Hicks, J.; Fuentes, M. A.; Jones, C.; Aldridge, S. Reduction of carbon oxides by an acyclic silylene: reductive coupling of CO. Angew. Chem. Int. Ed. 2019, 58, 18081812. (f) Wang, Y.; Kostenko, A.; Hadlington, T. J.; Luecke, M.-P.; Yao, S.; Driess, M. Silicon-mediated selective homo- and heterocoupling of carbon monoxide. J. Am. Chem. Soc. 2019, 141, 626634. (6) For examples of d-block reductive oligomerizations of CO see: (a) Bianconi, P. A.; Williams, I. D.; Engeler, M. P.; Lippard, S. J. Reductive coupling of two carbon monoxide ligands to form a coordinated alkyne. J. Am. Chem. Soc. 1986, 108, 311313. (b) Bianconi, P. A.; Vrtis, R. N.; Pulla Rao, Ch.; Williams, I. D.; Engeler, M. P.; Lippard, S. J. Reductive coupling of carbon monoxide ligands to form coordinated bis(trimethylsiloxy)ethyne in seven-coordinate niobium(I) and tantalum(I) [M(CO)2(dmpe)2Cl] complexes. Organometallics 1987, 6, 1968−1977. (c) Prostasiewicz, J. D.; Lippard, S. J. Vanadium-promoted reductive coupling of carbon monoxide and facile hydrogenation to form cis-disiloxyethylenes. J. Am. Chem. Soc. 1991, 113, 65646570. (d) Watanabe, T.; Ishida, Y.; Matsuo, T.; Kawaguchi, H. Reductive coupling of six carbon monoxides by a ditantalum hydride complex. J. Am. Chem. Soc. 2009, 131, 3474−3475. (e) Sharpe, H. R.; Geer, A. M.; Taylor, L. J.; Gridley, B. M.; Blundell, T. J.; Blake, A. J.; Davies, E. S.; Lewis, W.; McMaster, J.; Robinson, D.; Kays, D. L. Selective reduction and homologation of carbon monoxide by organometallic iron complexes. Nat. Commun. 2018, 9, 3757. (7) For examples of f-block reductive oligomerizations of CO see: (a) Summerscales, O. T.; Cloke, F. G. N.; Hitchcock, P. B.; Green, J. C.; Hazari, N. Reductive cyclotrimerization of carbon monoxide to the deltate dianion by an organometallic uranium complex. Science 2006, 311, 829−831. (b) Wayland, B.; Fu, X. Building molecules with carbon monoxide reductive coupling. Science 2006, 311, 790−791. (c) Evans, W. J.; Grate, J. W.; Hughes, L. A.; Zhang, H.; Atwood, J. L. Reductive homologation of CO to a ketenecarboxylate by a low-valent organolanthanide complex−synthesis and X-ray crystal-structure of [(C5Me5)4Sm2(O2CCCO)(THF)]2. J. Am. Chem. Soc. 1985, 107, 3728−3730. (d) Summerscales, O. T.; Cloke, F. G. N.; Hitchcock, P. B.; Green, J. C.; Hazari, N. Reductive cyclotetramerization of CO to squarate by a U(III) complex: the X-ray crystal structure of [(U(η-C8H6{(SiiPr3)1,4}2)(η-C5Me4H)]2(μ-η2:η2-C4O4). J. Am. Chem. Soc. 2006, 128, 9602−9603. (e) Evans, W. J.; Lee, D. S.; Ziller, J. W.; Kaltsoyannis, N. Trivalent [(C5Me5)2(THF)Ln]2(μ-η2:η2-N2) complexes as reducing agents including the reductive homologation of CO to a ketene carboxylate, (μ-η4O2C−C=C=O)2. J. Am. Chem. Soc. 2006, 128, 14176−14184. (f) Frey, A. S.; Cloke, F. G. N.; Hitchcock, P. B.; Day, I. J.; Green, J. C.; Aitkin, G. Mechanistic studies on the reductive cyclooligomerization of CO by U(III)


Page 5 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mixed sandwich complexes; the molecular structure of [(U(η-C8H6{SiiPr31,4}2)(η-Cp*)]2(μ-η1:η1-C2O2). J. Am. Chem. Soc. 2008, 130, 1381613817. (g) Arnold, P. L.; Turner, Z. R.; Bellabarba, R. M.; Tooze, R. P. Carbon monoxide coupling and functionalisation at a simple uranium coordination complex. Chem. Sci. 2011, 2, 77−79. (h) Mansell, S. M.; Kaltsoyannis, N.; Arnold, P. L. Small molecule activation by uranium tris(aryloxides): experimental and computational studies of binding of N2, coupling of CO, and deoxygenation insertion of CO2 under ambient conditions. J. Am. Chem. Soc. 2011, 133, 90369051. (i) Gardner, B. M.; Stewart, J. C.; Davis, A. L.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Homologation and functionalization of carbon monoxide by a recyclable uranium complex. Proc. Natl. Acad. Sci. 2012, 109, 92659270. (j) Tsoureas, N.; Summerscales, O. T.; Cloke, F. G. N.; Roe, S. M. Steric effects in the reductive coupling of CO by mixed-sandwich uranium(III) complexes. Organometallics 2013, 32, 13531362. (k) Castro-Rodriguez, I.; Meyer, K. Carbon dioxide reduction and carbon monoxide activation employing a reactive uranium(III) complex. J. Am. Chem. Soc. 2005, 127, 1124211243. (8) Alkali metal salts of deltate were first reported in 1976, though not from CO reductive trimerization, and not crystallographically characterized. Eggerding, D.; West, R. Synthesis and properties of deltic acid (dihydroxycyclopropenone) and the deltate ion. J. Am. Chem. Soc. 1976, 98, 36413644. (9) Green, S. P.; Jones, C.; Stasch, A. Stable magnesium(I) compounds with MgMg bonds. Science 2007, 318, 17541757. (10) (a) Jones, C. Dimeric magnesium(I) β-diketiminates: a new class of quasi-universal reducing agent. Nat. Rev. Chem. 2017, 0059. (b) Stasch, A.; Jones, C. Stable dimeric magnesium(I) compounds: from chemical landmarks to versatile reagents. Dalton Trans. 2011, 40, 56595672.

(11) Boutland, A. J.; Dange, D.; Stasch, A.; Maron, L.; Jones, C. Twocoordinate magnesium(I) dimers stabilized by superbulky amido ligands. Angew. Chem. Int. Ed. 2016, 55, 92399243. (12) Lalrempuia, R.; Kefalidis, C. E.; Bonyhady, S. J.; Schwarze, B.; Maron, L.; Stasch, A.; Jones, C. Activation of CO by hydrogenated magnesium(I) dimers: sterically controlled formation of ethenediolate and cyclopropanetriolate complexes. J. Am. Chem. Soc. 2015, 137, 8944−8947. (13) Significantly lengthened MgMg bonds can occur in very bulky three-coordinate MgI dimers. Gentner, T. X.; Rösch, B.; Ballmann, G.; Langer, J.; Elsen, H.; Harder, S. Low valent magnesium chemistry with a super bulky β‐diketiminate ligand. Angew. Chem. Int. Ed. 2019, 58, 607611. (14) As determined from a survey of the Cambridge Crystallographic Database, April, 2019. (15) Reductive CC cleavage of isocyanides by MgI dimers has previously been reported. Ma, M.; Stasch, A.; Jones, C. Magnesium(I) dimers as reagents for the reductive coupling of isonitriles and nitriles. Angew. Chem. Int. Ed. 2012, 18, 1066910676. (16) McKay, D.; Frey, A. S. P.; Green, J. C.; Cloke, F. G. N.; Maron, L. Computational insight into the reductive oligomerisation of CO at uranium(III) mixed-sandwich complexes. Chem. Commun. 2012, 48, 41184120. (17) These values are close to those calculated for the free deltate dianion. Franell, L.; Radom, L.; Vincent, M. A. The geometric and electronic structures of oxocarbons. An ab initio molecular orbital study. J. Mol. Struct. 1981, 76, 110.



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Fig 1



Fig 2


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Fig 3 348x171mm (300 x 300 DPI)



Scheme 1 95x48mm (300 x 300 DPI)


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Scheme 2 82x35mm (300 x 300 DPI)



Graphical abstract 77x43mm (300 x 300 DPI)


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Reductive Trimerization of CO to the Deltate Dianion using Activated

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