CO Coupling Chemistry of a Terminal Mo Carbide: Sequential Addition of Proton, Hydride, and CO Releases Ethenone Joshua A

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Subscriber access provided by Caltech Library Article CO Coupling Chemistry of a Terminal Mo Carbide: Sequential Addition of Proton, Hydride, and CO Releases Ethenone Joshua A. Buss, Gwendolyn A. Bailey, Julius Oppenheim, David G. VanderVelde, William A. Goddard, and Theodor Agapie J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07743 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019

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1 2 3 4 5 6 7 CO Coupling Chemistry of a Terminal Mo Carbide: Sequential Addition 8 of Proton, Hydride, and CO Releases Ethenone 9 10 Joshua A. Buss, Gwendolyn A. Bailey, Julius Oppenheim, David G. VanderVelde, William A. Goddard 11 12 III, and Theodor Agapie 13 Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E. California Blvd. MC 127- 14 72, Pasadena, CA, USA 15 16 17 ABSTRACT: The mechanism originally proposed by Fischer and Tropsch for carbon monoxide (CO) hydrogenative catenation 18 involves C–C coupling from a carbide-derived surface methylidene. A single molecular system capable of capturing these 19 complex chemical steps is hitherto unknown. Herein, we demonstrate the sequential addition of proton and hydride to a 20 terminal Mo carbide derived from CO. The resulting anionic methylidene couples with CO (1 atm.) at low-temperature (-78 21 °C) to release ethenone. Importantly, the synchronized delivery of two reducing equivalents and an electrophile, in the form 22 of a hydride (H- = 2e- + H+), promotes alkylidene formation from the carbyne precursor and enables coupling chemistry, under 23 conditions milder than those previously described with strong one-electron reductants and electrophiles. Thermodynamic 24 measurements bracket the hydricity and acidity requirements for promoting methylidene formation from carbide as ener- 25 getically viable upon formal heterolysis of H2. Methylidene formation prior to C–C coupling proves critical for organic product 26 release, as evidenced by direct carbide carbonylation experiments. Spectroscopic studies, a monosilylated model system, and 27 Quantum Mechanics computations provide insight into the mechanistic details of this reaction sequence, which serves as a 28 rare model of the initial stages of the Fischer Tropsch synthesis. 29 30 31 INTRODUCTION Previous work in our group has focused on reductive C–O 32 catenation chemistry employing one electron reductants The Fischer-Tropsch (FT) synthesis is a well-established 33 (KC8) and silyl electrophiles, demonstrating the deoxygena- 34 industrial-scale process that uses synthesis gas, a mixture of tion of bound CO to a terminal carbide (1, Scheme 1).25 This 35 CO and H2, as a feedstock for the generation of a broad range reactivity models the early steps proposed for FT, achieving 1-5 36 of catenated products. While the variation in outputs can the six-electron cleavage of CO using reducing equivalents be versatile, it is often cost prohibitive in regards to petro- 37 stored in a redox non-innocent supporting ligand. C–C cou- leum substitution.1-2 As efforts increase to synthesize bio- 38 pling was achieved via an isolated silyl alkylidyne complex, diesels from renewably sourced synthesis gas,6-7 shifting FT which, upon addition of two equiv. of strong reductant (KC8 39 catalysis toward specific desirable products becomes ever or naphthalenide) followed by strong electrophiles (silyl 40 more important.7 Understanding the operative mecha- chlorides), afforded disilylketene (Scheme 1, top).25-26 41 nism(s) of FT provides potential for rational catalyst design 42 and, in turn, improved selectivity.1-2, 8 In contrast to these relatively harsh reducing conditions, several molecular FT models have employed proton and hy- 43 While there is little consensus as to the operative mecha- dride equivalents as the products of heterolytic cleavage of 44 nism(s) of FT,9-10 all proposals share two fundamental com- 45 H2.27-29 Herein, we revisit CO coupling chemistry from ter- monalities—CO reduction (via C–H bond formation and/or minal carbide 1, the first reported example of a terminal 46 C–O cleavage) and C–C coupling. The elementary step of C– 47 group VI carbide bearing d-electrons. While C–C bond for- C coupling between CO and surface hydrocarbons has been mation from this species is facile, it ultimately results in del- 48 1,11-12,13-14 studied extensively in model systems. In contrast, eterious ligand functionalization. Sequential proton and hy- 49 the investigation of carbide/CO coupling remains largely dride delivery convert complex 1 to a terminal methylidene 50 underexplored. In seminal studies of carbonyl-bridged via an isolated terminal methylidyne (Scheme 1, bottom). metal carbide ensembles,15 more exposed carbides demon- 51 Importantly, thermochemical experiments with NaBHPh3 strated enhanced reactivity, including C/CO coupling in the 52 and [Ar2PhPMe]Cl (Ar = 2,4,6-trimethoxyphenyl) provide a Fe3 cluster.16 Additional examples of C≡O cleavage and cou- 53 formal demonstration that thermodynamically H2 can pro- pling have been reported, likely via carbide intermediacy, 54 mote reduction and protonation of complex 1, illustrating with oxophilic metals.17-19 To our knowledge, CO coupling 55 the feasibility of using proton and hydride as H2 surrogates has yet to be demonstrated from a terminal carbide precur- 56 in carbide conversion chemistry. Addition of exogenous CO sor, likely due to the paucity of reactive yet well-character- 57 to the formed methylidene releases ketene, demonstrating ized examples of such motifs.20-24 58 the sequential formation of two C–H bonds and C/CO cou- 59 pling, to yield a silicon-free, metal-free C2O1 organic. 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 12

Scheme 1. Mechanistically Distinct Modes of C/CO Cou- a Carbide 1 was synthesized cleanly in situ via deprotonation 1 pling to Free Ketene Fragments 30 of P2Mo(CH)(CO)(Cl) 4 with benzyl potassium. For more de- 2 tails, see the Supporting Information. 3 A single-crystal X-ray diffraction (XRD) study of 2 con- 4 firmed the proposed assignment (Figure 1). The short C31– 5 C32 bond length and wide C31–C32-O1 angle (1.2885(11) 6 Å and 179.01°, respectively) support assignment of the η1- 7 C(P)=C=O bonding motif.33 Mo engages the central arene in 8 an η6 fashion, to stabilize the low-valent metal center result- 9 ing from C–C coupling. Transition metal-bound phospho- 10 ranylidene ketenes, relatively rare motifs, are generally pre- 11 pared via reaction of a metal precursor with preformed ke- 34 12 tene, R3PCCO. In one example, the P–C linkage was formed 35 13 at the metal center, from cleavage of carbon suboxide. Complex 2 therefore represents the first example of such a 14 motif in which both the P–C and C–C bonds were formed at 15 the metal center. 16 RESULTS AND DISCUSSION 17 18 Carbide/CO Coupling Reactivity 19 Direct carbide carbonylation represents an early bifurca- 20 tion in the mechanistically complicated FT process; surface 21 C2 formation via carbide/CO coupling has been proposed for 22 syngas conversion on iron foils.31 To explore carbide/CO 23 coupling in a well-defined system, complex 1 was treated 30,32 24 with CO (Scheme 2). Admission of one atmosphere (1 atm) of 13CO to a THF solution of 1 at -80 °C showed no ef- 25 fect; however, upon warming to 25 °C, new spectral features 26 were observed. A peculiar high-field doublet of doublets (- 27 32.14 ppm, 1J(P,C) = 118 and 1J(C,C) = 103 Hz) was present 28 in the 13C{1H} NMR spectrum, in addition to two downfield 29 resonances. Concomitantly, two new features were ob- 30 served in the 31P{ 1H} NMR spectrum, including a doublet of Figure 1. Solid-state structure of 2. Thermal anisotropic dis- 31 doublet of doublets (37.64 ppm, 1J(P,C) = 118.1 Hz, 2J(P,C) = placement ellipsoids are shown at the 50% probability level. H- 32 33.2 Hz, & 3J(P,P) = 4.2 Hz) displaying strong scalar coupling atoms are omitted. Selected bond distances [Å] and angles [°]: P1–C31 1.7144(11), C31–C32 1.2885(11), C32–O1 1.1865(10), 33 to the high-field carbon. The large P–C and C–C spin-spin coupling constants, in addition to the broken symmetry of Mo1–C31 2.2899(16), Mo1–C33 1.9563(12), C33–O2 34 1.1693(10), C31–C32–O1 179.01(8), P1–C31–Mo1 130.73(5). 35 the terphenyl diphosphine (P2) ligand, are most consistent 36 with formation of a Mo-bound phosphoranylideneketene, 2. We hypothesized that CO binding could induce C–C bond 37 Scheme 2. CO Coupling Chemistry of Terminal Molyb- formation36-37 in 1 via a putative dicarbonyl carbide inter- 38 denum Carbidesa mediate (Scheme 2). Labeling experiments were conducted to explore this process. Addition of 12CO to 1 results in a 39 50:50 mixture of isotopes at the phosphoranylideneketene 40 α-carbon (Figure 2). Likewise, the 31P{1H} NMR spectrum 41 displays a broad resonance for the Mo-bound phosphine 42 arm comprised of an overlapping doublet of doublets and 43 doublet, the former split by partial (ca. 50%) 13C enrichment 44 at the carbonyl carbon. Taken together, these data are con- 45 sistent with an intramolecular coupling pathway from a 46 proposed dicarbonyl carbide complex. This process con- 47 trasts the ostensibly similar cleavage and coupling of CO by 48 Ta(silox)3,38 which, through a series of detailed mechanistic 49 studies, was shown to form the C–C bond prior to the first 50 deoxygenation event, en route to a ditantalum dicarbide 51 ((silox)3Ta≡CC≡Ta(silox)3).39 Related transformations start- 52 ing from alkylidyne carbonyl complexes, with addition of 53 phosphines leading to the assembly of neutral PC(R)=C=O ligands, have also been reported.40-43 Combined, these mo- 54 lecular systems represent complementary precedents for 55 potential elementary steps in the early stages of the FT syn- 56 thesis. 57 58 59 60 ACS Paragon Plus Environment Page 3 of 12 Journal of the American Chemical Society

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Figure 2. High-field partial 13C{1H} NMR spectra of 2 generated 18 under 13CO (top) and 12CO (bottom) showing splittings for the 13 19 phosphoranylidene C nucleus circled in grey. 20 Recognizing that carbide 1 need not bind an additional 21 molecule of CO to form a ketenylidene, it was slowly a Carbide 1 was synthesized cleanly in situ via deprotonation 22 warmed to RT. While this results in a composite mixture of of P2Mo(CH)(CO)(Cl) 4 with benzyl potassium. For more de- 23 largely intractable species,25 one of these complexes proved tails, see the Supporting Information. highly soluble. Extraction of the crude residue with hexame- 24 Gratifyingly, 4 is thermally stable, facilitating both its use 25 thyldisiloxane afforded a deep purple solution, which upon standing at -35 °C, precipitated 3 as a purple powder. While for reaction chemistry and permitting growth of single crys- 26 single crystals of 3 have proven elusive, the structure can be tals. A single-crystal XRD study corroborates the structure 27 confidently inferred from multinuclear NMR spectroscopy. inferred from spectroscopy (Figure 3). The Mo center sits above the central arene ring at a distance of 2.749 Å, sug- 28 A diagnostic carbidic 13C{1H} resonance is observed at gesting negligible metal arene interaction. The Mo≡CH dis- 29 569.86 ppm, further downfield shifted from that of 1. Two tance is appropriately short (1.764(2) Å) but slightly longer 30 distinct features in the 31P{1H} NMR spectrum at 91.44 and than that in a structurally characterized Mo(VI) methyli- 31 -4.08 ppm support an arm-on arm-off binding mode for the dyne (1.702(5) Å),44 likely a manifestation of the more-re- 32 P2 ligand, and four upfield aromatic 1H shifts corroborate duced metal center. 33 an η6 metal-arene interaction. Addition of 1 atm of CO to so- 34 lutions of 3 likewise did not afford the coupled product 2, 35 however. Instead, an intractable mixture is formed, to- 36 gether with free P2, reflecting additional, deleterious path- 37 ways that are enabled by dissociation of the second phos- 38 phine arm. 39 Carbide Protonation, Hydride Transfer, and Methylidene/CO 40 Coupling 41 Toward modeling the proposed role of surface hydrides in 42 FT, and ultimately forming C–H bonds, carbide protonation 43 was targeted.21, 44 Addition of Et3NHCl to a -78 °C THF solu- 44 tion of 1 results in formation of a new compound upon 45 warming to room temperature. A single resonance between 46 3.5 and 6.5 ppm is observed in the 1H NMR spectrum, a dou- 47 blet of triplets centered at 5.06 ppm (1J(C,H) = 147.8 Hz, 48 3J(P,H) = 3.1 Hz), with a relative integration of one. This sug- gests both a terminal methylidyne20, 24, 45-51 and a five-coor- Figure 3. Solid-state structure of 4. Thermal anisotropic dis- 49 placement ellipsoids are shown at the 50% probability level. H- 50 dinate pseudo-square pyramidal structure, as in 4, without 31 1 atoms are omitted, except for the methylidyne proton, which 51 metal-arene interactions (vide infra; Scheme 3). The P{ H} and 13C{1H} NMR data further corroborate the proposed as- was refined freely. Selected bond distances [Å] and angles [°]: 52 Mo1–C31 1.764(2), Mo1–C32 1.930(4), O1–C32 1.197(6). signment of 4, with a doublet of doublets at dP 40.21 ppm 53 (2J(C,P) = 18.6 and 10.2 Hz) and a characteristic methyli- Hydride addition to both terminal21 and bridging52 me- 54 dyne resonance at dC 281.19 ppm, respectively.45, 51 thylidyne ligands has precedent, and we sought to target 55 this reactivity, considering proton and hydride as a crude 56 Scheme 3. Sequential Proton and Hydride Addition en route to Ketene Formation a surrogate for H2 (vide infra). NaBEt3H was added to a C7D8 57 58 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 4 of 12

solution of 4 at -78 °C. Mixing the reagents immediately re- methylidene dicarbonyl complex. We favor the latter, given 1 sulted in a color change to dark brown, and VT NMR spec- that coordination to Mo facilitates C–C coupling chemistry 2 troscopy showed signals in the 31P{1H} (s, 52.07 ppm), in the related carbide/CO chemistry described above. 3 13C{1H} (br s, 296.91 and br s, 237.52 ppm), and 1H (15.03, Reactivity of a Silylcarbyne Model System 4 d, 1J(C,H) = 114.90 Hz and 13.27, d, 1J(C,H) = 142.7 Hz) spec- 5 tra, in line with formation of methylidene 5 (Scheme 3).21 Expecting that similar C/CO coupling chemistry would be 6 Warming the sample to 0 °C, showed quantitative conver- achieved from silyl carbyne 9, affording a more stable or- 7 sion to a new asymmetric complex with a triplet of doublets ganic product, a related reactivity sequence was explored 2 2 with this silyl model system (Scheme 4). Low-temperature 8 (84.68 ppm, J(P,CCO) = 15.5 & J(P,CCH2) = 6.0 Hz) and doublet of doublets (47.41 ppm, 1J(P,CCH2) = 21.9 ppm & addition of NaBEt3H to a C7D8 solution of alkylidyne 9 and 9 2J(P,CCO) = 17.0 Hz) in the 31P{1H} NMR spectrum. The subsequent warming to -20 °C resulted in the gradual for- 10 13C{1H} NMR spectrum likewise showed two signals, a mation of a new species demarcated by a 31P{1H} resonance 11 1 broad doublet of triplets centered at 251.88 ppm (2J(CCO,P) at 48.53 ppm and low-field doublet in the H NMR spectrum 1 12 = 15.3 & 2J(CCO,CCH2) = 2.2 Hz) and an extremely high-field (δ = 15.79 ppm, J(C,H) = 121.7 Hz). Such a signal could be 74-77 13 resonance at -34.87 ppm (ddd, 1J(CCH2,P) = 21.8, 2J(CCH2,P) = consistent with either formyl generation, via CO inser- 14 6.2 Hz & 2J(CCH2,CCO) = 2.2 Hz). The high-field chemical shift tion into an intermediate Mo hydride (a process that is fa- 15 in the 13C{1H} NMR spectrum49 and the C1 symmetry are cilitated by borane Lewis acids),29, 78-80 or hydride attack at 16 consistent with P–C bond formation, giving 6,53-54 unexpect- the carbyne to give a silyl alkylidene. The 13C NMR spectrum 17 edly suggesting some electrophilic carbene character in was more informative, displaying a broad doublet at 315.09 1 18 electron-rich, anion 5.53-55 The small 1J(P,CCH2) spin-spin ppm ( J(C,H) = 121.7 Hz) and a broad singlet at 251.83 ppm 19 coupling56 observed for 6 is consistent with phosphonium (Figure S28). C–H coupling to the more downfield reso- 20 ylide complexation to transition metal centers; ligation re- nance is suggestive of a carbene/carbonyl isomer rather 2 3 than the alternative carbyne/formyl complex. 2D 1H/13C 21 sults in rehybridization—C(sp ) to C(sp )—that is reflected in the smaller one-bond scalar coupling constant.57 The correlation experiments further disambiguated the struc- 22 structure of 6 was confirmed in a single-crystal XRD study, ture (Figure S29), with strong HMBC cross peaks between 23 which corroborated the assignment of the h1-C-bound ylide the H-bound carbon atom and the SiMe3 methyl protons. 24 moiety (see the SI). These data point to hydrogen transfer to the alkylidyne car- 25 21, 81-83 We hypothesized that similarly to the effect observed for bon, affording silyl carbene 10, in analogy to the reac- 26 carbide/CO coupling, methylidene/CO coupling may be fa- tivity observed for parent methylidyne 4 (vide supra). 27 vored by addition of exogenous CO, either via direct CO at- Scheme 4 Hydride-Initiated C–C Coupling. 28 tack58 or coordination to Mo. Placing a -78 °C solution of in 29 situ generated 5 under a 13CO atmosphere resulted in a 30 gradual color change to bright orange, as the gas diffused 31 down the sample (Scheme 3). The 13C{1H} NMR spectrum at 32 -78 °C, showed both free 13CO and three resonances in the 33 metal-bound region—the CO signals of previously charac- 34 terized P2Mo(13CO)3.59 35 Anticipating that zero-valent Mo was indicative of a C–C 36 coupling process, identification of ethenone by 13C{1H} NMR 37 spectroscopy was attempted. The diagnostic signals for free 38 ketene (13C δ = 193.7 and 2.7 ppm)60 were absent, but cou- 39 pling doublets at 154.69 and 83.90 ppm (1J(C,C) = 76.1 Hz) 40 were observed, attributed to a putative ethenone-triethyl 41 borane adduct.61 While further efforts to detect free 42 ethenone (7) were unsuccessful, ketene trapping with eth- 43 anol has precedent, giving stable and easily detectable ethyl acetate (EtOAc).60, 62 Forming 5 at low temperature, adding 44 13CO, and condensing EtOH into the reaction tube afforded 45 While 10 was the major species observed at -20 °C, it EtOAc-13C2, 8, unequivocally demonstrating the generation proved transient, slowly reacting further to give a mixture 46 and ejection of parent ketene (Scheme 3). Alkylidene car- of two Mo complexes with a doublet and singlet in the 31P 47 bonylation has been demonstrated on a variety of molecular NMR spectrum at 58.38 and 76.24 ppm, respectively. The 48 scaffolds,13 but often requires CO overpressures,58, 63-65 pro- latter corresponds to previously characterized Mo0-N2 ad- 49 ceeds from activated Fischer carbene complexes,13, 66-68 or duct 12,59, 84 demonstrating loss of the carbon-based lig- 50 occurs at bridging alkylidene ligands.69-70 Rarely is this cou- ands, presumably via C–C coupling. The isotopically en- 51 pling achieved at monometallic complexes with carbene riched 13C signals at 84.76 and 191.35 ppm showed strong fragments derived from CO.67, 71-73 52 scalar coupling (1J(C,C) = 69.7 Hz), the latter split into a dou- 12 53 Isotopic labeling studies were conducted in which CO blet of triplets (2J(C,P) = 26.3 Hz) by the trans-spanning 54 was added to 5. Mixed isotopolog P2Mo(12/13CO)3 was phosphines of the P2 ancillary ligand. This spectral signa- 13 55 formed, consistent with either i) rapid exchange of the CO ture is consistent with C–C bond formation affording the tri- 56 ligand in 5 with free 12CO and on-metal C–C bond formation methylsilyl ketene complex 11. Interestingly, in this case, 57 or ii) 12CO binding and C–C coupling preceding from a CO addition is not requisite for C–C coupling. 58 59 60 ACS Paragon Plus Environment Page 5 of 12 Journal of the American Chemical Society

The assignment of 11 was further supported via inde- level. H-atoms are omitted for clarity. Selected bond dis- 1 pendent synthesis.85 Anticipating that the observed mixture tances [Å] and angles [°]: Mo1–O1 2.1858(7), Mo1–C31 2 of 11 and 12 resulted from a ligand substitution equilib- 2.1629(10), Mo1–Carene (ave.) 2.2599(9), O1–C31 3 rium between the isoelectronic Mo0 adducts, a C6D6 solution 1.2799(12), C31–C32 1.3536(14), ∠O1-C31-C32 132.32(9). 4 of 12 was treated with trimethylsilyl ketene.86 While only Quantum Mechanics Computations 5 starting material was observed under an N2 atmosphere, 6 degassing the reaction mixture resulted in significant con- Akin to the sequential addition of potassium graphite and 31 1 25-26 7 version to 11, as evidenced by P{ H} NMR spectroscopy. silyl electrophile to alkylidyne 9, NaBEt3H proved profi- cient for enacting the reductive coupling of silyl carbyne 8 The N2/ketene exchange was borne out in the hydride-initi- with CO. The latter reactivity manifold, preceding through a 9 ated C–C coupling chemistry; in addition to the coupling doublets of the bound ketene, a pair of doublets at 192.48 carbene intermediate (10) rather than a dicarbyne com- 10 and 60.28 ppm (1J(C,C) = 73.5 Hz) were observed in the 13C plex, is accessible under much milder conditions and shows 11 NMR spectrum, corresponding to the metal-free C2 frag- equal efficacy in C–C coupling. Given the facility with which 12 ment.61 the catenation proceeds, Quantum Mechanics computation 13 93 An XRD study of single crystals of 11 grown from slow (using the M06-2X flavor of Density Functional Theory) 14 evaporation of liquid butane confirmed the inferred η2-CO was employed to interrogate the electronic structure re- 15 ketene assignment (Figure 4), with comparable O1-C31 quirements in the critical bond-forming step. The optimized 16 (1.2799(12) Å) and C31-C32 (1.3536(14) Å) distances.87-89 structure following hydride addition to 9 resembles silyl al- 94 17 η2-CC ketene adducts are known,90-92 but the O-bound form kylidene 10, in agreement with experiment; however, a + 18 is often thermodynamically favored.90 Contrasting the close contact between Na and the Cl ligand is observed in 95 19 structure of 9,25 As in the structure of 2 above, the central addition to borane binding to the carbonyl (Figure S42). 20 arene is engaged in η6 coordination to Mo, consistent with The HOMO of this complex, with dxz parentage, is dominated 21 the more reduced Mo center formed on C–C coupling. by a π-bonding interaction with CO (10•BEt3, Figure 5, in- 22 set). As the C–C distance contracts, the alkylidene ligand ro- 1 23 tates, directing the Mo–CSi π-system towards CO ( X, Figure 5). This geometry has a dyz HOMO stabilized by M-arene 24 backbonding. With further C–C shortening, the ground state 25 crosses to a triplet, resulting from electron promotion from 26 the dyz (Mo– arene π) to the dxz (Mo–CSi π*) and representing 27 cleavage of the Mo–C π-bond (1X to 3X, Figure 5, right in- 28 sets). This alkylidene activation process, with significant 29 electron density localization on the carbon, is facilitated by 30 the proximal Na+ ion, without which a low-energy pathway 31 could not be identified. Bond formation is additionally as- 32 sisted by borane coordination, stabilizing the oxygen lone 33 pair as electron density is transferred to CO.95 The direct im- 34 pact of the Lewis acids Na+ and BH3 on this C–C coupling 35 process is particularly notable. While BEt3 has no direct rel- 36 evance to FT, there is precedent for both Lewis acidic pro- 10 37 moters in FT catalysis and selectivity perturbations resulting from alkali metal cation addition in electrochemical 38 CO96-97 reduction. Our computations highlight in a well-de- 39 Figure 4. Solid-state structure of 11. Thermal anisotropic fined molecular system the necessity of these interactions 40 displacement ellipsoids are shown at the 50% probability for coupling chemistry resulting in ketene. 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 6 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Figure 5. Potential Energy Landscape for C–C Bond Formation. Truncated transition state structures, computed C–C bond distances, 19 and energies (relative to 10•BEt3) are provided. The insets depict key molecular orbitals (0.05 eÅ-3 isosurfaces) involved in bond- 20 breaking and making steps. 21 22 Thermochemistry of Proton and Hydride Transfer to Carbide The conjugate ylide Ar2PhP=CH2 was not detected, how- 31 23 1 ever (dP 7.00 ppm). Instead, a new peak in the P NMR at 98 7.4 ppm was observed, together with free P2 (dP –3.45 24 Thus far, a moderate acid (Et3NHCl, pKa = 14.9 in THF) and ppm), suggesting displacement of P2 by ylide in the formed 25 a strong hydride donor (NaBHEt3, ΔG°H- = 26 kcal/mol in methylidyne complex. Competing decomposition of the car- 26 MeCN99) have been described in the reaction chemistry for the formation of ethenone from 1. These reagents are only bide complex by other pathways, however, cannot be 27 strictly ruled out. Taken together, these protonation studies 28 a crude surrogate for H2, as the free energy of hydrogen formation is favorable by ca. 25 kcal/mol, considering the re- suggest a lower limit of 33 for the pKa (Cl) of methylidyne 4, 29 roughly three orders of magnitude higher than the closest action free energy of H2 heterolysis in MeCN.100 Given the 30 20 basicity of related Mo(VI) and W(VI) carbide complexes previously reported comparator, Mo(≡CH)(NRAr)3. The 31 higher effective basicity of carbide 1 may reflect diminished (Mo(≡CH)(NRAr)3, pKa ~ 30 kcal/mol in THF;20 32 π-donation from the carbide ligand in this more electron- W(≡CH)(CO)2(Tp*), pKa = 28.7 kcal/mol in THF21), we an- 33 ticipated that weaker acids could be employed to access me- rich, formally Mo(IV), complex, and/or the favored energet- 34 thylidyne 4. ics of chloride binding to Mo. 35 Further protonation studies assessed the basicity of car- Scheme 5. Protonation studies of carbide 1.a, b 36 bide 1, and supported a pKa (Cl) for methylidyne 4 of ≥ 33.101

37 Treatment of carbide 1 with the phosphazene base 38 P2Et•HCl (Scheme 5; pKa 26.6 in THF102) resulted in quanti- 39 tative conversion to methylidyne 4, with observation of the 40 expected conjugate base P2Et at 15.1 ppm in the 31P{1H} 41 NMR spectrum. Reaction with the weaker acid 42 [Ar2PhPMe]Cl (Ar = 2,4,6-trimethoxyphenyl; pKa 33.5 in 43 THF102) likewise furnished methylidyne 4, along with a new 44 species, 4’, whose NMR signature mirrored that of 4, sug- 31 45 gesting a closely related isomer. Most diagnostic was a P chemical shift at 41.81 ppm, shifted only ca. 1.5 ppm down- 46 field from 4, and a diagnostic 13C resonance at 267.8 ppm, 47 which correlated by HMQC analysis to a triplet in the 1H 48 NMR spectrum at 2.64 ppm for the methylidyne proton

49 (3J(H,P)= 3.8 Hz). Independent synthesis, together with a 50 single-crystal XRD study, confirmed 4’ to be an isomer of 4 apKa values of HCl salts in THF: P2Et•HCl, 26.6;98 102 b 51 in which the methylidyne is oriented cis with respect to the [PhAr2PMe][Cl], 33.5. Carbide 1 was synthesized cleanly in 52 central arene (Figure 6). As in the previously reported cis situ via deprotonation of P2Mo(CH)(CO)(Cl) 4 with benzyl potassium. For more details, see the Supporting Information. 53 silylcarbyne isomer,26 the cis methylidyne isomer adopts a 54 pseudo-octahedral geometry with a clear h2-arene interac- 55 tion trans to CO. The Mo–C31 distances are similar between 56 the two isomers (cis/trans 1.781 Å vs 1.764(2), respec- 57 tively), and both are consistent with a Mo-C triple bond. 58 59 60 ACS Paragon Plus Environment Page 7 of 12 Journal of the American Chemical Society

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14 Figure 6. Solid-state structure of 4’. Thermal anisotropic 15 displacement ellipsoids are shown at the 50% probability 16 level. H-atoms are omitted for clarity, except the methyli- CONCLUSION 17 dyne proton. Selected bond distances [Å] and angles [°]: In summary, a terminal Mo carbide complex has been 18 Mo1–C31 1.781(2), Mo1–C32 1.972(2), O1–C32 1.155(2). demonstrated to undergo direct carbonylation, in conjunc- 19 20 tion with ligand functionalization, affording a phospho- To gain insight into the hydride affinity of methylidyne 4, ranylideneketene complex. Carbide protonation provides a 21 weaker hydride sources were employed. Hydride transfer thermally stable methylidyne carbonyl complex, which, 22 was complete using NaBHPh3 (DG°H– = 36 in MeCN99), as upon sequential treatment with hydride and CO, induces C– 23 judged by the quantitative formation of 6 upon warming the C coupling. Experimental and Quantum Mechanics compu- 24 reaction to RT. However, in a variable temperature NMR tational interrogation of reactions between the silyl alkyli- 25 study, methylidene 5 was not observed on maintaining the dyne congener and hydride suggest that this C–C bond for- 26 reaction at –78 °C. Instead, observation of 5 necessitated mation differs from the mechanism established for stepwise 27 warming to ca. –30 °C, consistent with a higher activation reduction/electrophile addition, proceeding through car- 28 energy of hydride transfer from the bulkier borohydride. Up bene carbonyl intermediates rather than dicarbynes. 29 to equal proportions of 5 and 4 were observed at –20 °C, Notably, this reaction sequence models, with high fidelity, 30 suggesting that the hydride affinity of methylidyne 4 is at the proposed mechanism of FT: surrogates of H2 react with 31 least comparable to that of NaBHPh3. Competing conversion a terminal carbide (1) to afford a reactive methylidene (5) 32 of 5 to 6 occurred at these temperatures; however, prevent- (cf. B – F, Figure 7). This methylidene is the precursor to C– 33 ing extraction of the exact thermodynamic parameters. C bond formation, coupling with CO to give ketene. These 34 Addition of NaBHPh3 at -78 °C, followed by reaction with steps provide valuable precedent for the reactivity pro- 35 CO (1 atm), likewise furnished P2Mo(CO)3 on warming to posed for the heterogenous FT process, mimicking the 36 RT, as expected based on the thermodynamic favorability of multi-electron C–C coupling reactivity postulated following 37 conversion from 5 described above. A small peak assigned C–O scission at a well-defined monometallic Mo carbide to 5 was apparent only at intermediate temperatures (0 °C), 38 complex. Critically, the use of hydride as a FT relevant two- again consistent with the higher activation barrier using electron reductant provides an efficient strategy to accom- 39 NaBHPh3. plish C–C coupling and C–H bond formation. The reaction of 40 the 1 with weak acid and hydride sources (the products of 41 Reconsidering the relevance to H2 splitting, the pKa of [Ar2PhPMe]Cl and hydricity of NaBHPh3 suggest that a reac- formal H2 heterolytic cleavage) to access a methylidene con- 42 tion between carbide 1 and H2 is thermodynamically down- trasts the lack of reactivity observed when treated with H2 43 hill by at least 5 kcal / mol (Scheme 6).103 These experi- directly, highlighting the need for pre-activation to achieve 44 ments therefore constitute a thermochemical demonstra- productive carbide hydrogenation in either the present mo- 45 tion that H2 is a suitable source of protons and hydrides in lecular model system or via cleavage on the catalyst surface 46 the studied system. However, warming a THF solution of 1 in FT. 47 under 1 atm H2 showed only slight conversion to 3. Direct 48 reaction of carbides with H2 therefore appears to be kinet- 49 ically disfavored.104 Cleavage of H2―either heterolytically or 50 on a surface―may be important to reaction with carbide 51 and subsequent enabling of C–C coupling reactivity. This is 52 in contrast to the reactivity of transition metal imides and 53 carbenes, for which direct activation of R–H bonds (R = C, 105-107 54 H) is kinetically allowed. 55 Scheme 6. Thermochemical Cycle Evaluating Relevance 56 to Syngas Chemistry 57 58 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 8 of 12

Catalysts and Catalytic Reactions. ACS Catal. 2019, 3026- 1 3053. 2 6. Ail, S. S.; Dasappa, S. Biomass to liquid 3 transportation fuel via Fischer Tropsch synthesis – 4 Technology review and current scenario. Renewable and 5 Sustainable Energy Rev. 2016, 58, 267-286. 6 7. Zhang, X. Essential scientific mapping of the value 7 chain of thermochemically converted second-generation 8 bio-fuels. Green Chem. 2016, 18 (19), 5086-5117. 8. Jiao, F.; Li, J.; Pan, X.; Xiao, J.; Li, H.; Ma, H.; Wei, M.; 9 Pan, Y.; Zhou, Z.; Li, M.; Miao, S.; Li, J.; Zhu, Y.; Xiao, D.; He, T.; 10 Yang, J.; Qi, F.; Fu, Q.; Bao, X. Selective conversion of syngas 11 to light olefins. Science 2016, 351 (6277), 1065. 12 9. Muetterties, E. L.; Stein, J. Mechanistic features of 13 catalytic carbon monoxide hydrogenation reactions. Chem. 14 Rev. 1979, 79 (6), 479-490. 15 Figure 7. i) Schematic representation of the early steps of 10. Maitlis, P. M.; Zanotti, V. The role of electrophilic 16 Fischer-Tropsch synthesis. ii) Molecular Mo Complexes species in the Fischer–Tropsch reaction. Chem. Commun. 17 Modeling C/CO Coupling to Metal-Free Organics. 2009, (13), 1619-1634. 18 11. Mayr, A.; Hoffmeister, H., Recent Advances in the 19 Chemistry of Metal-Carbon Triple Bonds. In Adv. Organomet. 20 ASSOCIATED CONTENT Chem., Stone, F. G. A.; West, R., Eds. Academic Press: San 21 Diego, 1991; Vol. 32, pp 227-324. Supporting Information 12. Geoffroy, G. L.; Bassner, S. L., Interaction of Ketenes 22 Detailed experimental procedures, full characterization, crys- with Organometaliic Compounds: Ketene, Ketenyl, and 23 tallographic details (CIF), and spectroscopic data. This material Ketenylidene Complexes. In Adv. Organomet. Chem., Stone, F. 24 is available free of charge via the Internet at G. A.; West, R., Eds. Academic Press: San Diego, 1988; Vol. 28, 25 http://pubs.acs.org. pp 1-83. 26 13. Zhang, Z.; Zhang, Y.; Wang, J. Carbonylation of Metal 27 AUTHOR INFORMATION Carbene with Carbon Monoxide: Generation of Ketene. ACS 28 Catal. 2011, 1 (11), 1621-1630. Corresponding Author 29 14. In several recent noteworthy examples, the 30 *E-mail: [email protected] deoxygenation and reductive cleavage of CO has been extended beyond transition metal systems to the main 31 Notes 32 group. See, for example: (a) Wang, Y.; Kostenko, A.; 33 The authors declare no competing financial interests. Hadlington, T. J.; Luecke, M.-P.; Yao, S.; Driess, M. Silicon- 34 Mediated Selective Homo- and Heterocoupling of Carbon ACKNOWLEDGMENT Monoxide. J. Am. Chem. Soc. 2019, 141, 626-634. (b) 35 Majumdar, M.; Omlor, I.; Yildiz, C. B.; Azizoglu, A.; Huch, V.; 36 We thank Larry Henling and Mike Takase for invaluable crys- tallographic assistance. J.A.B. is grateful for an NSF graduate re- Scheschkewitz, D. Reductive Cleavage of Carbon Monoxide 37 by a Disilenide. Angew. Chem. Int. Ed. 2015, 54, 8746-8750. 38 search fellowship, G.A.B. for NSERC and Resnick Sustainability Institute fellowships, and J.O. for an Ernest H. Swift Summer (c) Arrowsmith, M.; Böhnke, J.; Braunschweig, H.; Celik, M. 39 Undergraduate Research Fellowship. We thank the NSF (CHE- A. Reactivity of a Dihydrodiborene with CO: Coordination, 40 1800501), the Dow Next Generation Education Fund (instru- Insertion, Cleavage, and Spontaneous Formation of a Cyclic 41 mentation), and Caltech for funding. The computational studies Alkyne. Angew. Chem. Int. Ed. 2017, 56, 14287-14292. 42 were supported by the NSF (CBET-1805022). 15. Shriver, D. F.; Sailor, M. J. Transformations of carbon 43 monoxide and related ligands on metal ensembles. Acc. 44 REFERENCES Chem. Res. 1988, 21 (10), 374-379. 45 1. West, N. M.; Miller, A. J. M.; Labinger, J. A.; Bercaw, J. 16. Kolis, J. W.; Holt, E. M.; Drezdzon, M.; Whitmire, K. 46 E. Homogeneous syngas conversion. Coord. Chem. Rev. H.; Shriver, D. F. 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Some degree of O–borane interaction is apparently 53 kinetic study of Lewis acid induced alkyl migration. Inorg. also present in methylidene 5, as judged by parallel 54 Chem. 1982, 21 (3), 1272-1273. experiments with BPh3 (described below), in which the 31P 55 81. Fischer, E. O.; Frank, A. Übergangsmetall-Carbin- chemical shift for 5 is detected slightly upfield at 49.4 ppm 56 Komplexe, XLIII: Reduktion einer Metall-Kohlenstoff- (cf. 52.1 ppm in the reaction with BEt3). No such change in 57 Dreifachbindung. Chem. Ber. 1978, 111 (11), 3740-3744. chemical shift was detected for 6 in these two reactions, 58 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 12 of 12

however; nor was it detected for bound ketene complex 11 103. Because of the limited availability of hydricity 1 generated in the presence or absence of BEt3. We conclude values reported in THF, and the reactivity of Mo-P2 2 that O–borane interactions are likely important for complexes in MeCN, we rely on the hydricity values in MeCN, 3 stabilizing reaction intermediates during C–C coupling, but but use the pKa values determined in THF, to get an 4 that they are not ubiquitously present in the ground-state approximation of the Gibbs free energy of H2 addition to 1. 5 structures. To correct for these solvent effects, we used Morris’ method 6 96. Pérez-Gallent, E.; Marcandalli, G.; Figueiredo, M. C.; to estimate a lower bound for the pKa of methylidyne 4 in 7 Calle-Vallejo, F.; Koper, M. T. M. Structure- and Potential- MeCN from the experimentally determined pKa in THF. Even 8 Dependent Cation Effects on CO Reduction at Copper Single- with this correction factor applied, the addition of H2 to 1 Crystal Electrodes. J. Am. Chem. Soc. 2017, 139 (45), 16412- remains thermoneutral (pKaMeCN > 28; ΔG < 1.5 kcal / mol). 9 16419. See: Morris, R. H. Estimating the Acidity of Transition Metal 10 97. Paik, W.; Andersen, T. N.; Eyring, H. Kinetic studies Hydride and Dihydrogen Complexes by Adding Ligand 11 of the electrolytic reduction of carbon dioxide on the Acidity Constants. J. Am. Chem. Soc. 2014, 136 (5), 1948- 12 mercury electrode. Electrochim. Acta 1969, 14 (12), 1217- 1959. 13 1232. 104. Soluble KCl is carried forward from the in situ 14 98. Garrido, G.; Koort, E.; Ràfols, C.; Bosch, E.; Rodima, synthesis of 1, as evidenced by experiments in which 1 was 15 T.; Leito, I.; Rosés, M. Acid−Base Equilibria in Nonpolar treated with acids bearing BArF4 counterions, and chloride- 16 Media. Absolute pKa Scale of Bases in Tetrahydrofuran. J. bound methylidynes 4/4’ were generated as the primary 17 Org. Chem. 2006, 71 (24), 9062-9067. products (see the SI). Even so, no reaction was observed 18 99. Heiden, Z. M.; Lathem, A. P. Establishing the between carbide 1 and H2. 19 Hydride Donor Abilities of Main Group Hydrides. 105. Davies, H. M. L.; Manning, J. R. Catalytic C–H 20 Organometallics 2015, 34 (10), 1818-1827. functionalization by metal carbenoid and nitrenoid 21 100. Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. insertion. Nature 2008, 451, 417. Measurement of the Hydride Donor Abilities of 22 106. Park, Y.; Kim, Y.; Chang, S. Transition Metal- [HM(diphosphine)2]+Complexes (M = Ni, Pt) by Heterolytic Catalyzed C–H Amination: Scope, Mechanism, and 23 Activation of Hydrogen. J. Am. Chem. Soc. 2002, 124 (9), Applications. Chem. Rev. 2017, 117 (13), 9247-9301. 24 1918-1925. 107. Chu, J. C. K.; Rovis, T. Complementary Strategies for 25 101. The effective pKa of methylidyne 4 is designated as Directed C(sp3)−H Functionalization: A Comparison of 26 pKa(Cl) to indicate that the observed reactivity reflects both Transition-Metal-Catalyzed Activation, Hydrogen Atom 27 the free energy of proton transfer (pKa) and the free energy Transfer, and Carbene/Nitrene Transfer. Angew. Chem. Int. 28 of chloride binding to Mo. Ed. 2018, 57 (1), 62-101. 29 102. Saame, J.; Rodima, T.; Tshepelevitsh, S.; Kütt, A.; 30 Kaljurand, I.; Haljasorg, T.; Koppel, I. A.; Leito, I. 31 Experimental Basicities of Superbasic Phosphonium Ylides 32 and Phosphazenes. J. Org. Chem. 2016, 81 (17), 7349-7361. 33 34 35 Insert Table of Contents artwork here 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Insert Table of Contents artwork here 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment