Cooperative H2 Activation Across a Metal-Metal Multiple Bond and

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Cooperative H Activation Across a Metal-Metal Multiple Bond and Hydrogenation Reactions Catalyzed by a Zr/Co Heterobimetallic Complex Kathryn M Gramigna, Diane A. Dickie, Bruce M Foxman, and Christine M Thomas ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04390 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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Cooperative H2 Activation Across a Metal-Metal Multiple Bond and Hydrogenation Reactions Catalyzed by a Zr/Co Heterobimetallic Complex Kathryn M. Gramigna,† Diane A. Dickie,†,§ Bruce M. Foxman,† Christine M. Thomas*,†,‡ †Department

of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02453, United States

‡Department

of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States KEYWORDS: heterobimetallic, dihydrogen activation, alkyne semi-hydrogenation, alkene hydrogenation, metal-metal bonds, cooperative reactivity ABSTRACT: In the quest for active and selective catalysts featuring non-precious metals, bimetallic cooperativity poses a unique opportunity to promote catalytic reactions and influence selectivity. While examples of stoichiometric H2 activation across metal-metal bonds have been reported, there have been limited advances towards the incorporation of a well-defined cooperative bimetallic H2 activation process into a catalytic cycle for the hydrogenation of unsaturated hydrocarbons. Herein, we demonstrate that facile activation of H2 by two non-precious metals is facilitated by metalmetal cooperativity in coordinatively unsaturated Zr/Co bis(phosphinoamide) complexes, (THF)(I)Zr(XylNPiPr2)2CoPR3 (3-PMe3 and 3-PMePh2), that feature highly polar Zr-Co triple bonds. Owing to the stabilizing nature of the metal-metal bond, the H2 activation products (THF)(I)Zr(-H)(XylNPiPr2)2Co(H)(PR3) (4-PMe3 and 4-PMePh2), which feature one terminally bound Co-hydride and one hydride bridging the two metals, have been isolated and crystallographically characterized. The Zr/Co bimetallic complex 3-PMePh2 is found to be an active catalyst for the hydrogenation of alkenes and semi-hydrogenation of alkynes, and relevant intermediates including 4-PMePh2 and alkene (5) and alkyne (6) adducts have been identified spectroscopically in situ and isolated and characterized through independent synthesis. The alkyne semi-hydrogenation reaction catalyzed by 3-PMePh2 exhibits high selectivity for alkenes over alkane products, and generates an unselective distribution of E and Z alkenes via direct formation of both stereoisomers. These findings lend insight into the roles that metal-metal bonding and cooperativity play in the activation of small molecules and the promotion and selectivity of subsequent catalytic transformations.

INTRODUCTION Bimetallic complexes have attracted interest in the literature owing to their ability to exhibit reactivity or selectivity divergent from that of analogous monometallic compounds.1 Cooperativity between two base metal centers can facilitate multielectron redox events inaccessible to related monometallic compounds, providing advantages in applications such as small molecule activation and catalysis. In particular, tethering a Lewis acidic early metal to an electron-rich late metal can (1) enable more facile access to stabilized reduced species poised for redox processes at the late metal site and (2) induce a highly polarized metal-metal interaction, which is advantageous for synergistic substrate activation across the metal-metal bond.1a, 1d, 1f, 1h, 2 Although the early-late heterobimetallic strategy seems best suited to

polar substrates, the concept has been applied herein to the activation of the smallest nonpolar molecule, H2.1c, 1g Dihydrogen activation is classically depicted to transpire via homolytic cleavage of H2 by a precious metal catalyst, which is oxidized by two electrons to generate a metal-dihydride complex (Scheme 1, top). H2 activation by a bimetallic complex, however, can proceed through one of three different pathways1f, 3 (Scheme 1, bottom): (1) oxidative addition at one metal center to generate a classical metal dihydride complex in which in the metal center involved is oxidized by two electrons; (2) cooperative homolytic H2 cleavage, in which both metal centers are oxidized by one electron; or (3) cooperative heterolytic cleavage of H2 across the metal-metal bond, in which one metal center is oxidized by two electrons. The third pathway is likely to occur across a polarized metalmetal bond in an early-late heterobimetallic complex,

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inducing the formation of one hydridic M-H bond with the early metal and one acidic M-H bond with the late metal.1e The effective polarization of a traditionally nonpolar substrate offers plausibility for atypical reactivity of the resulting dihydride bimetallic complex. The Lu group reported a transition metal-main group metal bimetallic complex capable of binding H2 to afford the nonclassical H2 adduct (2-H2)Ni(N(oi (NCH2P( Pr)2)C6H4)3)Ga, which is catalytically active towards alkene hydrogenation.4 This reaction is proposed to proceed through a HNi(-H)Ga intermediate, but no spectroscopic or experimental evidence for this hypothesis could be obtained.3 The employment of a more coordinatively unsaturated bimetallic complex could serve to stabilize such an intermediate and influence reactivity. Scheme 1. Top: Classical H2 activation by a monometallic complex. Bottom: Three different pathways for activating H2 by a heterobimetallic complex.

Several examples of isolable heterobimetallic H2 activation products in which hydrogen oxidatively adds to one metal center5 and in which H2 is cooperatively activated6 have been reported in the literature. Of these, only the Cp2Ta(-CH2)2Ir(CO)(PPh3) complex (A, Scheme 2) reported by Bergman and coworkers shows activity towards hydrogenation.5a A more recent report by Bergman and Ess outlines a mechanism by which A catalyzes the hydrogenation of alkenes, and it is shown that the bimetallic complex goes through a different mechanism than does the related monometallic complex Ph2P(-CH2)2Ir(CO)(PPh3), emphasizing the importance of the second metal site for the high catalytic activity.7 In this case, H2 activation occurs solely at the Ir center to generate an Ir dihydride complex (pathway (1), vide supra), and the early metal facilitates the reductive elimination and oxidative addition of substrates through the methylene bridge, which comparatively cannot be accomplished by the phosphorus ylide analogue. A related Zr/Ir complex Cp2Zr(-NtBu)IrCp* (B) was shown

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to activate H2 across the metal-metal bond (pathway (3)) to generate a complex with a bridging hydride and terminal hydride bound to Zr (Scheme 2).6a Exposure of B to H2 in the presence of alkenes generates the expected hydrogenation products; however, it was determined that the active catalyst is likely to be a small impurity rather than the characterized heterobimetallic complex. Scheme 2. H2 activation by early-late heterobimetallics reported by Bergman.5a

A previous example of H2 activation by a Zr/Co heterobimetallic complex was reported by our group in 2010.6d Exposure of (THF)Zr(MesNPiPr2)3CoN2 (C, Mes = 2,4,6-trimethylphenyl) to 1 atm H2 for 12 hours results in the addition of two equivalents of H2 to generate the dihydride complex D (Scheme 3), in which one equivalent of H2 cleaves and hydrogenates the P-N bond of one phosphinoamide ligand (possibly the cause of the long reaction time). The second equivalent of H2 adds across the metal-metal bond to generate one hydride bridging the two metals and a terminal hydride bound to the Co center. Further reactivity with this dihydride complex was not pursued owing to the requisite ligand degradation. In addition to H2 activation, the C=O bonds of CO2 and diaryl ketones were shown to oxidatively add across the Zr-Co bond of C, in both cases requiring dissociation of one of the phosphinoamide ligands from cobalt.8 Scheme 3. P-N bond cleavage previously observed upon treatment of a tris(phosphinoamide) Zr/Co complex with H2.6d

The P-N bond cleavage and requisite ligand dissociation processes observed with the tris(phosphinoamide) Zr/Co system prompted us to construct more reactive compound bis(phosphinoamide) complexes. Indeed, a Ti/Co bis(phosphinoamide) complex ClTi(XylNPiPr2)2CoPMe3 (Xyl = 3,5dimethylphenyl) was found to be much more reactive

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towards ketones than its tris(phosphinoamide) analogue, promoting stoichiometric reductive coupling of ketones to alkenes (McMurry coupling).8c, 9 Given than Zr/Co complexes have, in our hands, generally proven to be significantly more reactive than their Ti/Co analogues,8c we now turn our attention to bis(phosphinoamide) Zr/Co complexes and highlight their enhanced reactivity with dihydrogen, including applications to catalytic hydrogenation of unsaturated C-C bonds.

oxidation state, halide identity, and coordination geometry. Scheme 4. Synthesis complexes 1, 2 and 3.

of

Zr/Co

heterobimetallic

RESULTS AND DISCUSSION Synthesis of Zr-Co bis(phosphinoamide) complexes. To generate a complex analogous to ClTi(XylNPiPr2)2CoPMe3,9 a stepwise synthetic procedure was developed. The monometallic precursor (Me2N)2Zr(XylNPiPr2)2 was prepared using a reported procedure.10 CoI2 was then added under reducing conditions using one equivalent of KC8 to afford the bimetallic ZrIV/CoI complex (Me2N)Zr(i Me2N)(XylNP Pr2)2CoI (1, Scheme 4). The 1H NMR spectrum of 1 features five paramagnetically shifted resonances consistent with a Cs-symmetric structure. Owing to their proximity to the unpaired electrons, the 1H NMR signals for both sets of dimethylamide and isopropyl methine resonances are too broad to resolve. Treatment of 1 with two equivalents of TMSI generates the iodide-bridged dimer [IZr(-I)(XylNPiPr2)2CoI]2 (2). The symmetry of 2 is akin to that of 1 such that a similar pattern of five paramagnetically shifted peaks are apparent in the 1H NMR spectrum. The effective magnetic moments of 1 and 2 were found using Evans’ method to be 3.52 and 4.87 B, consistent with S = 1 (SO = 2.83 B) and S = 2 (SO = 4.90 B) ground states, respectively. Comparisons between metal-metal distances are simplified by the use of Cotton’s formal shortness ratio (FSR),11 defined by the ratio of the metal-metal distance to the sum of the single-bond atomic radii (Pauling’s R1)12 of each metal. X-ray crystallography of single crystals of 1 and 2 (Figures 1A and S31) confirms their connectivity and reveals that the metal-metal distance in 2 (2.6579(4) Å; FSR = 1.02) is slightly elongated from that in the precursor 1 (2.5832(4) Å; FSR = 0.99) owing to either the modest increase in electron density at Zr resulting from the coordination of an additional  donating ligand or the difference in geometric constraints imposed by the different bridging ligands. Both distances suggest a single dative CoZr bond and are comparable to the metalmetal interaction in the related complex (XylNPiPr2)Zr(Br)(XylNPiPr2)2CoBr, in which the Zr-Co distance is 2.7602(3) Å (FSR = 1.01).13 All three metal-metal distances are longer than that in one-electron reduced [(Cl)Ti(XylNPiPr2)2CoI]2 (2.2051(4) Å; FSR = 0.89).9 Further comparison between the structures of the Ti/Co complex and 2 is not straightforward owing to the differences in

Reduction of 2 with four equivalents of Na/Hg in the presence of either PMe3 or PMePh2 affords the monomeric complexes (THF)(I)Zr(XylNPiPr2)2CoPR3 (3PMe3 and 3-PMePh2, Scheme 4). The 31P{1H} NMR spectra of 3-PMe3 and 3-PMePh2 display two peaks in a 2:1 ratio, namely a doublet corresponding to the equivalent phosphines of the phosphinoamide ligand centered at 48.9 (3-PMe3) and 46.2 (3-PMePh2) ppm and a triplet corresponding to the terminally-bound phosphine donor centered at -24.8 (3-PMe3) and 2.8 ppm (3-PMePh2). By analogy to Zr/Co complex C, whose oxidation states were assigned based on Co K-edge X-ray absorption spectroscopy,14 the oxidation states of the metal centers in complexes 3-PMe3 and 3-PMePh2 are assigned as ZrIV/CoI. This assignment was further corroborated by structural changes that occur upon reduction of 2. Single-crystal X-ray diffraction of 3-PMePh2 (Figure 1B) reveals a decrease in the average Co-PN distance (2.20 Å) of about 0.1 Å from that in the precursor 2 (2.30 Å), while the Zr-NP distance remains virtually unchanged (2.13 Å). The Co-PN contraction is consistent with the two-electron reduction occurring at the Co center14-15 owing to increased metal to ligand -


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backbonding. Notably, a marked decrease in the Zr-Co distance to 2.2733(6) Å (FSR = 0.84) indicates a significant increase in metal-metal bond order. Corrected for differences in radii, the short intermetallic distance in 3PMePh2 is comparable to the Ti-Co triple bond in the analogous Ti/Co complex ClTi(XylNPiPr2)2CoPMe3, in which the Ti-Co distance is 2.0236(9) Å (FSR = 0.81).9 The modest increase in FSR in 3-PMePh2 can be attributed to poorer 4d-3d orbital overlap. Furthermore, the slightly shortened Zr-Co distance in 3-PMePh2 as compared with that in the tris(phosphinoamide) analogue, (THF)Zr(XylNPiPr2)3CoN2 (2.3816 Å; FSR = 0.91),13 is likely a consequence of the poorer π-accepting and stronger σ-donating properties of PMePh2 compared to N2, amplifying donation from Co to Zr.

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therefore, the bonding is best classified as a compilation of three dative bonds. The results of a natural bond orbital (NBO) calculation support this description by revealing three Zr-Co NBOs that correspond to one  and two  bonds (Figure 3), which are all comprised of significantly more Co character (average of 88% Co) than Zr character (average of 12% Zr). The computed Wiberg bond index (WBI, 1.23) and Mayer bond order (MBO, 1.73) are also indicative of metal-metal multiple bonding. While both computationally determined values tend to underestimate the extent of metal-metal bonding in earlylate heterobimetallic systems owing to the highly dative nature of the interaction, comparisons between related compounds remain informative. These values fall in between those previously reported for doubly-bonded (THF)Zr(MesNPiPr2)3CoN2 (FSR = 0.90;15b WBI = 0.95;16 MBO = 1.4914) and for ClTi(XylNPiPr2)2CoPMe3 (FSR = 0.81; WBI = 1.59; MBO = 1.76),9 which features a more covalent Ti-Co triple bond.

Figure 2. Depictions of the calculated frontier molecular orbitals of 3-PMePh2. Figure 1. Displacement ellipsoid (50%) representations of 1 (A) and 3-PMePh2 (B). Hydrogen atoms have been omitted for clarity. In the case of 1, the phosphorus-bound isopropyl groups are disordered over two positions, and only one position is shown for clarity.

The Zr-Co bonding in 3-PMePh2 was probed computationally using density functional theory (DFT) to examine the frontier molecular orbitals (MOs, Figure 2). Appreciable overlap between the dz2 orbitals of both metal centers gives rise to a Zr-Co  bond, and two  bonds are evident as a result of overlap between the two dxz and the two dyz orbitals. It is apparent that the electron density is localized predominantly on the Co center;

Figure 3. Calculated NBO surfaces depicting the Zr-Co bonding interactions in 3-PMePh2.

H2 activation. Both 3-PMe3 and 3-PMePh2 were found to readily activate H2 across the Zr-Co bond at room temperature in less than five minutes to generate the ZrIV/CoI dihydride complexes (THF)(I)Zr(H)(XylNPiPr2)2Co(H)(PR3) (4-PMe3 and 4-PMePh2,


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Scheme 5). Two hydride signals are evident in the 1H NMR spectra of both complexes (Figures S9 and S12, respectively). For 4-PMe3, one doublet of triplets centered at -7.3 ppm corresponds to the bridging hydride and another centered at -18.0 ppm corresponds to the terminal hydride. The 1H NMR spectrum of 4-PMePh2 reveals the same pattern in the hydride region shifted slightly downfield to -7.1 and -17.4 ppm. The 31P{1H} NMR spectra of both complexes show broad signals in a 2:1 ratio for the phosphinoamide ligands at 69.1 and 66.5 ppm and for the terminal phosphine ligands at -1.9 and 31.1 ppm of 4PMe3 and 4-PMePh2, respectively. Scheme 5. H2 activation by 3-PMe3 and 3-PMePh2.

The solid state structures of both 4-PMe3 and 4PMePh2 were determined via X-ray diffraction of single crystals (Figure 4, left, and Figure S43), and the Zr-Co distances (2.4543(3) and 2.4695(3) Å, respectively; FSR = 0.94) reveal an increase from that in 3-PMePh2 by ~0.2 Å. The hydrides were located on the Fourier difference map and refined. In 4-PMePh2, the bridging hydride is 2.03(3) Å and 1.45(3) Å from the Zr and Co centers, respectively, with a Zr-H-Co angle of 89.0°. The terminal hydride is located 1.32(4) Å from Co, in a position trans to the bridging hydride (H-Co-H = 175(2)°). The acute Zr-H-Co angle suggests a “closed” M-H-M interaction (Figure 4, right) in which there is overlap between the d orbitals of the two metal centers and thus a distinct metal-metal interaction.17 Although the metal centers are in the same ZrIV/CoI oxidation states as in both 1 and 2, the Zr-Co distance is shorter in 4-PMe3 and 4-PMePh2 owing to the bridging hydride and the more electron-releasing nature of the phosphine ligand vs. iodide or amide ligands, allowing for increased CoZr donation. Thus, in this case, the shortened intermetallic distance is indicative of a stronger

been omitted for clarity. The Zr-bound THF ligand and one of the Ph groups of the PMePh2 ligand were disordered over two positions, but only one position is shown for clarity. Right: Qualitative representations of “open” and “closed” 3center-2-electron bonding depicting varying degrees of M-M interaction.

metal-metal bond. This is further corroborated by the calculated WBI (0.89) and MBO (1.13) for 4-PMe3, which suggest a formal Zr-Co single bond but a stronger interaction than that in singly-bonded i 16 ClZr(MesNP Pr2)3CoI (WBI = 0.52; MBO = 0.7315a). The frontier molecular orbital diagram (Figure 5), computed using DFT, offers insight into the bonding interactions in 4-PMe3. Upon examination of the highest occupied MOs, one Zr-Co  bond and one 3-center-2electron Zr-H-Co bond are evident owing to overlap between d orbitals of appropriate symmetry. The two metal dxz orbitals interact with the hydride s orbital to form three molecular orbitals of M-H bonding, nonbonding, and antibonding character (Figure 5). The LUMO, which lies over 2 eV higher in energy than the HOMO, is M-H nonbonding in character. The HOMO is mostly nonbonding Co dxy in character, and the electron density in the highest occupied orbitals is predominantly Co-based. In addition to the interaction through the bridging hydride, the Zr and Co centers are also involved in a 2-center-2-electron  bond. Both bonding motifs are additionally corroborated by NBO calculations (Figure 6). The NBO depicting the Zr-Co  bond is 89.6% Co and 10.4% Zr in character, supporting a dative interaction in which electron density is donated from the electron-rich Co center to the electron-deficient Zr center. The 3-center2-electron Zr-H-Co bond

Figure 4. Left: Displacement ellipsoid (50%) representation of 4-PMePh2. Hydrogen atoms (except for hydrides) have


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Catalytic hydrogenation of unsaturated C-C bonds. In light of hydride lability demonstrated by H/D exchange studies, the activity of 3 towards the hydrogenation of alkenes was investigated. Reported base metal catalysts that promote alkene hydrogenation from H2 employ different types of cooperativity to circumvent their propensity towards one-electron redox processes.1c, 18 Related examples include the installation of highly donating, strong-field ligands to Co to stabilize two-electron redox cycles,19 Co complexes that utilize metal-ligand cooperativity via redox non-innocence of the ligand or direct participation of the ligand in substrate coordination,20 a Zr+/amine FLP complex,21 and a Ni/Ga bimetallic complex.4 Thus, we reasoned that cooperativity between a redox-active Co center and a Lewis acidic Zr center could allow for novel reactivity.

Figure 5. Depictions of the calculated frontier molecular orbitals of 4-PMe3 and relevant orbitals depicting M-H-M 3center-2-electron bonding.

is represented by an NBO comprised of 18.1% Zr, 52.5% H, and 29.4% Co character.

As an initial probe of the catalytic alkene hydrogenation activity of the Zr/Co system, a mixture of 3-PMe3 and one equivalent of styrene was subjected to 1 atm H2. After 20 hours at room temperature, only 22% of the styrene had been converted to ethylbenzene. However, by simply substituting PMe3 for the less donating PMePh2, the reaction proceeded to 93% conversion in the same amount of time (20 h) at room temperature. The origin of the increased reaction rate is evident while monitoring

Figure 6. Calculated NBO surfaces depicting the Zr-H-Co (left) and Zr-Co (right) bonding interactions in 4-PMe3.

H/D exchange studies were carried out using both dihydride 4-PMePh2 and dideuteride 4D-PMePh2. When 4-PMePh2 is subjected to 1 atm D2, both hydrides are substituted for deuterides after four hours at 40°C. This is evident in the 1H NMR spectra (Figure S36), as the hydride signals disappear upon formation of 4D-PMePh2. Similarly, exchange of the deuterides in 4D-PMePh2 for hydrides under complementary conditions was observed by 2H NMR spectroscopy (Figure S37), which depicts the loss of the two deuteride signals as 4-PMePh2 is generated. The relative integration of the µ-H/D and CoH/D signals with respect to each other remains identical during the course of these reactions and no HD is observed in either the 1H or 2H NMR spectra, suggesting that H/D exchange occurs via concerted loss/addition of H2/D2.

Figure 7. Progression of the hydrogenation of one equivalent of styrene monitored by 31P{1H} NMR spectroscopy. (A) Immediately after addition of 1 equiv styrene to 3-PMePh2 under 1 atm H2. (B) After 2 h. (C) After 4 h. (D) After 8 h. Signal labeled as * denotes the small amount of free HXylNPiPr2 that is carried through synthetically.


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the reaction by 31P NMR spectroscopy (Figure 7). After addition of H2 to the mixture of 3-PMePh2 and styrene, 4PMePh2 is immediately generated (Figure 7A). As the reaction progresses, two peaks centered at 49.9 and 39.4 ppm become evident, along with a signal at -26.6 ppm corresponding to free PMePh2 (Figure 7B-D). The two new downfield peaks correspond to asymmetric complex 5, in which PMePh2 has dissociated and styrene is bound in its place. 5 can also be independently synthesized by adding one equivalent of styrene to 3-PMePh2 and stirring for two hours at room temperature or for 30 minutes at 40°C. As the hydrogenation of styrene to ethylbenzene proceeds, continued monitoring of the reaction by NMR spectroscopy reveals accumulation of 4PMePh2 in the reaction mixture along with requisite consumption of 5, until only 4-PMePh2 is evident in the 31P{1H} NMR spectrum upon full conversion of styrene. Addition of further equivalents of styrene to this reaction mixture regenerates 5 and exposure to 1 atm H2 resumes hydrogenation. Catalytic turnover can be achieved using 3-PMePh2: at 10 mol % catalyst loading, 93% of styrene is converted to ethylbenzene in 3 hours at 60°C under 1 atm H2. In contrast, neither free PMe3 nor 5 are observed in the 31P{1H} NMR spectrum at any point during styrene hydrogenation using 3-PMe3, even after addition of excess styrene or exposure to elevated temperatures. Therefore, under the same conditions, 3-PMe3 catalyzes the hydrogenation of styrene in only 71% conversion in 3 hours and requires twice as long as 3-PMePh2 (6 h) to reach 93% conversion. The lability of the terminal Cobound ligand is an essential consideration in the development of more active hydrogenation catalysts. Having established 3-PMePh2 as a competent catalyst for the hydrogenation of styrene, the reactivity towards the hydrogenation of several additional olefinic substrates was probed (Table S1). More hindered terminal alkenes such as 3,3-dimethylbutene were hydrogenated more slowly (Table S1, entry 2), likely owing to the diminished ability to form the alkene adduct analogous to 5 as a result of increased steric hindrance between the tert-butyl group of the substrate and the isopropyl groups of the phosphinoamide ligand. This is corroborated by the absence of a new metal species (analogous to 5) by 31P{1H} and 1H NMR spectroscopy when 3-PMePh2 is treated with 3,3-dimethylbutene. Similarly, internal alkenes such as cyclohexene are hydrogenated much more slowly than terminal alkenes (Table S1, entry 3). It was also found that 3-PMePh2 catalyzes the isomerization of terminal to internal olefins (Table S1, entries 3, 4 and 5). When chain walking occurs more rapidly than hydrogenation, longer reaction times are required to hydrogenate the resulting internal olefin (Table S1, entries 5a and 5b).

Given that 3-PMePh2 demonstrates very slow activity towards the hydrogenation of internal alkenes, we were interested in probing its propensity towards the semi-hydrogenation of internal alkynes. Traditional hydrogenation catalysts for the semi-hydrogenation of alkynes, such as the Schrock-Osborn catalyst22 or Lindlar’s catalyst,23 undergo syn-addition of H2 to the alkyne to generate the cis-alkene, and rely on a twoelectron oxidation at a single precious metal center, typically Ru, Rh, or Pd. Owing to the nature of H2 addition, elimination of the trans-alkene is less common. Thus, the most pervasive route for achieving this selectivity does not use H2 as the terminal reductant and instead involves dissolving stoichiometric amounts of Li or Na in ammonia. Several recently reported semihydrogenation catalysts that employ H2 as the reductant achieve E-selectivity via initial generation of the Z isomer followed by catalytic Z- to E-isomerization.24 In 2015, the Mankad group reported the heterobimetallic complex (NHC)Au-RuCp(CO)2 (NHC = N,N’-bis(2,4,6trimethylphenyl)imidazole-2-ylidene), which catalyzes alkyne semihydrogenation under 1 atm of H2 in 24 hours at 150°C and is selective for the E alkene after isomerization from the Z to the E isomer under the reaction conditions.24b Although no H2 activation product is observed upon exposure of (NHC)Au-RuCp(CO)2 to H2, the addition of alkynes at elevated temperatures generates the semi-hydrogenation products. It was proposed that H2 is activated across the metal-metal bond of (NHC)Au-RuCp(CO)2 to generate short-lived monometallic metal hydride complexes, followed by alkyne 1,2-insertion and protonolysis to eliminate the alkene, and it was suggested that H2 activation is the turnover-limiting step in the catalytic cycle.24b, 25 Among the rare examples of alkyne semi-hydrogenation processes that have been shown to directly form E alkenes, Fürstner’s [Cp*Rh(COD)Cl] catalyst, which has been shown to directly perform trans-hydrogenation via a carbene intermediate, is the most practical and widely applicable.26 Muetterties and coworkers reported the selective formation of trans olefins by a homobimetallic Rh complex, and Bargon and coworkers reported an NMR spectroscopy study confirming direct transhydrogenation of alkynes by a homobimetallic Ru complex.27 In both of the latter cases, it is proposed that this unique selectivity is a result of the hydrogenation of the same alkyne by two different metal centers upon insertion into a bridging metal hydride. We reasoned that 3 could promote similar bifunctional catalysis but might allow for divergent reactivity owing to the facile H2 activation that occurs at an intact metal-metal bond.


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orbital is poised to interact with both a terminal and a bridging hydride.

Figure 9. (A) Displacement ellipsoid (50%) representation of 6. Hydrogen atoms have been omitted for clarity. The THF and iodide ligands were disordered over two positions, but only one position is shown for clarity. (B) Calculated representation of the highest occupied molecular orbital in 6.

Figure 8. Progression of the hydrogenation of three equivalents of diphenylacetylene monitored by 31P{1H} NMR spectroscopy. (A) After addition of 3 equivalents of diphenylacetylene to 3-PMePh2. (B) Following the addition of 1 atm H2. (C) After 6 h. (D) After 16 h. Signal labeled as * denotes the small amount of free HXylNPiPr2 that is carried through synthetically.

As in the hydrogenation of styrene, 3-PMePh2 is significantly more active than 3-PMe3 towards the catalytic hydrogenation of diphenylacetylene. Upon addition of three equivalents of diphenylacetylene to 3PMePh2, a new complex is generated after two hours at room temperature. The 31P{1H} NMR spectrum of the reaction mixture (Figure 8A) shows two distinct peaks: one at -26.6 ppm, confirming PMePh2 dissociation, and another at 55.4 ppm, which corresponds to the phosphinoamide ligands of a new Cs-symmetric complex. The structure of this compound was determined crystallographically to be the diphenylacetylene adduct 6 (Figure 9A). The Zr-Co distance in 6 is elongated from that in 3-PMePh2 to 2.3716(4) Å (FSR = 0.91), in line with the stronger  accepting nature of the diphenylacetylene ligand as compared with PMePh2. -backbonding from the Co dyz orbital to a diphenylacetylene * orbital significantly disrupts the second Zr-Co  bond, supporting the presence of a formal Zr-Co double bond in 6. Similar attenuation of the metal-metal bond order has been observed in other Zr/Co heterobimetallics when -acids such as N2 bind to Co.15b Activation of the alkyne C-C bond is substantiated by both the elongated C1-C2 distance (1.292(3) Å) and the bent Ph-C1-C2 angle (135.8(2)°). DFT calculations reveal that the HOMO is predominantly Co dz2 in character (Figure 9B), and the

Upon exposure of a 3:1 mixture of diphenylacetylene and 3-PMePh2 to an atmosphere of H2, the peaks corresponding to 4-PMePh2 begin to appear in the 31P{1H} NMR spectrum as the hydrogenation reaction proceeds (Figure 8B-D). The signals assigned to 6 and free PMePh2 simultaneously decrease until all of the diphenylacetylene is consumed, at which point only 4PMePh2 remains in the 31P{1H} NMR spectrum. 92% conversion is achieved in 18 hours at room temperature to predominantly produce the two possible stilbene products (Z/E = 72:28) over 1,2-diphenylethane in a ratio of 92:8 (Table S2, entry 1b). The semi-hydrogenation reaction proceeds catalytically at room temperature at 10 mol % catalyst loading, but requires 30 hours to achieve 85% conversion (Table 1, entry 1). Both Z- and E-stilbene products are observed in a ratio of 79:21 under these conditions, along with a small amount of the over-hydrogenated alkane (stilbene/1,2-diphenylacetylene = 93:7). Reaction times can be reduced by elevating the reaction temperature. When the hydrogenation reaction is run for three hours at 60°C and 10 mol % catalyst loading under 1 atm H2, 98% of the diphenylacetylene is converted to stilbene (Z/E = 57:43) and 1,2-diphenylethane in a ratio of 98:2 (entry 2). For comparison, after three hours at room temperature only 31% conversion is achieved but with marginally increased selectivity for the Z-alkene (Table S2, entry 2a). Further elevation of the reaction temperature to 75°C leads to nearly complete conversion in just one hour without generation of additional overhydrogenation products (Table 1, entry 3). On the other hand, reducing the catalyst loading to 5 mol % and running the reaction at 60°C still allows for full conversion with similar selectivity, but predictably the reaction takes twice as long (Table 1, entry 4 vs entry 2). Catalytic turnover can be achieved using 3-PMe3; however, at identical catalyst loadings the reaction is much slower and requires double


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the reaction time to reach full conversion (Table 1, entry 5 vs entry 2), pursuant to the trend in reactivity towards the hydrogenation of one equivalent of styrene (vide supra). Catalytic semi-hydrogenation facilitated by isolated 6 proceeds more rapidly (Table 1, entry 6), reaching 93% conversion in 2 h, confirming that 6 is an on-cycle intermediate. However, an increase in selectivity for the Z-isomer was observed in this case (Z/E = 69:31).

hypothesized that a Z- to E-isomerization process was occurring on the same timescale as the semihydrogenation reaction, and that 1,2-insertion of the cisalkene into the M-H bond is competitive with alkyne insertion. The limited amount of 1,2-diphenylethane produced would then suggest that -hydride elimination to generate the trans-alkene is initially more facile than reductive elimination to generate the alkane. To investigate this hypothesis, the reaction mixture from entry 2 (10 mol % catalyst loading, 60°C) that had already reached 98% conversion was allowed to continue heating at 60°C for a total reaction time of 72 hours (Table S2, entry 3c). The ratio of Z/E-stilbene decreases significantly to 22:78 after 72 h, but the majority of this results from the depletion of the Z-isomer as it is overhydrogenated to the alkane (stilbene/1,2-diphenylethane = 72:28).

Both E- and Z-stilbene are formed under all conditions investigated, even at early time points in the reaction with low conversion (Table S2), and minimal changes in the ratios of stilbene/1,2-diphenylethane and Z/E-stilbene are observed over the course of the semihydrogenation reaction (Table S2, entries 1a vs 1b, 2a vs 2b, 3a vs 3b, 5a vs 5b, 6a vs 6b). Based on other semihydrogenation catalysts in the literature,24 we initially Table 1. Comparison of activity and selectivity for the catalytic semi-hydrogenation of diphenylacetylene promoted by 3-PMePh2, 3-PMe3, and several relevant previously reported catalysts.a

cat. loading

stilbene/1,2-

T (°C)

t (h)

solvent

conv (%)b

diphenylethaneb

10

23

30

C6D6

85

93:7

79:21

3-PMePh2

10

60

3

C6D6

98

98:2

57:43

3

3-PMePh2

10

75

1

C6D6

98

97:3

51:49

4

3-PMePh2

5

60

6

C6D6

99

98:2

56:44

5

3-PMe3

10

60

6

C6D6

94

98:2

61:39

6

6

10

60

2

C6D6

93

97:3

69:31

7d

(MesCCC)Co(N2)(PPh3)

1

30

17

THF

100

82:18

0:100

8e

(IMes)Ag-RuCp(CO)2

20

150

24

xylenes

96

99:1

4:96

9e

(IMes)Cu-FeCp(CO)2

20

150

24

xylenes

64

97:3

81:19

10f

[Cp*2Zr(OCH2CH2NiPr2)]+

2

rt

3

C6D5Br

>99

44:56

100:0

entry

catalyst

1

3-PMePh2

2c

(mol %)

Z/Eb

aCatalytic

conditions for entries 1-5: 0.1 M diphenylacetylene in ca. 700 L C6D6, 1 atm H2. bDetermined by 1H NMR integration. of three runs. dRef. 24c; MesCCC = bis(mesityl-benzimidazol-2-ylidenephenyl). eRef. 24b; IMes = N,N’bis(2,4,6-trimethylphenyl)imidazole-2-ylidene). fRef. 21; 1.5 bar H2. cAverage

To probe Z/E isomerization directly, Z-stilbene and 1 atm H2 were added to 10 mol % 3-PMePh2. After one hour at 75°C, the Z/E ratio was determined to be 81:19 by 1H NMR spectroscopy, confirming the modest ability of 3-PMePh2 to catalyze Z/E isomerization. However, under the same conditions diphenylacetylene is hydrogenated nearly quantitatively to stilbene with a Z/E ratio of 51:49 after one hour (Table 1, entry 3). The generation of significantly more E-stilbene in the alkyne semi-hydrogenation reaction suggests that a second hydrogenation pathway, in which diphenylacetylene is directly hydrogenated to the trans-alkene, may be operative. This is also consistent

with the observation that the Z/E ratio increases very little throughout the course of the semi-hydrogenation reactions under all conditions (Table S2), indicating that Z to E isomerization is not a dominant reaction pathway during the semi-hydrogenation of diphenylacetylene. To further explore the selectivity of the semihydrogenation reaction and its origins, the substrate and product distribution was monitored every 30 min throughout the 3 h reaction run at 60°C and 10 mol % loading of 3-PMePh2 (Figure 10A). After 30 minutes (Table S2, entry 3a), the reaction proceeds to 35% conversion and an appreciable quantity of E-stilbene is


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already generated (Z/E = 52:48). Throughout the reaction, hydrogenation to Z-stilbene proceeds more rapidly; however, the E isomer is present in substantial concentrations at early reaction times and continues to accumulate throughout the progression of the reaction.

Figure 10. (A) Progression of the hydrogenation of diphenylacetylene at 60°C. (B) Progression of the hydrogenation reaction at 75°C after full diphenylacetylene conversion.

In an attempt to fully convert the diphenylacetylene to a single product, the reaction was run at 75°C (for convenience) and 10 mol % loading of 3PMePh2, and was monitored past the point of full conversion for a total time of 84 h. As shown in Figure 10B, there is minimal change in the amount of E-stilbene over time following consumption of the alkyne. However, as the mole fraction of Z-stilbene decreases over time, the amount of 1,2-diphenylethane increases in a similar proportion. This observation suggests that Z-stilbene hydrogenation occurs at a much faster rate than either isomerization to E-stilbene or E-stilbene hydrogenation, accounting for the lower Z/E ratio observed at later reaction times following the consumption of diphenylacetylene (e.g. Table S2, entries 4b and 4c). Under the optimized catalytic conditions (60 °C, 1 atm H2, 10 mol% 3-PMePh2), hydrogenation of Z-stilbene was confirmed to proceed moderately faster than hydrogenation of E-stilbene (Table S3). The observations (1) that both E- and Z-stilbene are generated at early reaction times and (2) that the mole fraction of E-stilbene changes very little at longer reaction times support the

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possibility of a secondary mechanism in which much of the trans-alkene is generated directly by alkyne hydrogenation, rather than being formed exclusively via a Z- to E- isomerization process. The possibility of a bimolecular reaction pathway for direct trans-hydrogenation was initially probed by varying the concentration of the catalytic reactions. Running the reactions under the optimized conditions (10 mol% 3-PMePh2, 60 °C, 1 atm H2, 3 h) but in a Schlenk flask equipped with a stir bar rather than a J. Young tube led to greater selectivity for the Z-isomer (Z/E = 80:20). However, changing the concentrations of the reactions carried out under these conditions resulted in identical conversions and selectivities (Table S4), suggesting that either a bimolecular pathway mediated by two Co sites is not responsible for the generation of the E isomers. To gain further insight into the mechanism, the diphenylacetylene semi-hydrogenation reaction was carried out in a J. Young tube under the optimized reaction conditions using a 50:50 mixture of H2:D2. Generation of the mixed H/D alkene for both stilbene isomers was confirmed by GC-MS (see Figures S38-S39), along with di-protonated and di-deuterated stilbene. H/D scrambling into only the E isomer would have suggested that it was forming via Z/E isomerization; however the observation of comparable incorporation of a single deuterium atom into both alkene isomers does not confirm or refute the proposed direct formation of transstilbene and more detailed mechanistic studies are required to evaluate the mechanism further. The scope of the alkyne semi-hydrogenation catalysis facilitated by 3-PMePh2 was probed using several different internal alkynes (Table S5). Asymmetric alkynes were hydrogenated to predominantly afford the corresponding alkene over the alkane (Table S5, entries 2 and 3). Selectivity for the alkene over the alkane in these cases is slightly diminished compared with that observed during the hydrogenation of diphenylacetylene; however, a concomitant increase in Z/E selectivity is apparent. The effect of alkyne coordination on increased hydrogenation activity is emphasized by the prolonged reaction times necessary for hydrogenation of alkynes with more sterically encumbered substituents (Table S5, entries 3b and 4b). Hydrogenation of 1,2bis(trimethylsilyl)acetylene is very slow and results in predominant generation of overhydrogenated 1,2bis(trimethylsilyl)ethane (Table S5, entry 4). Dialkylsubstituted 4-octyne is selectively hydrogenated to cis-4octene: minimal overhydrogenation is observed and no formation of trans-4-octene is detected during any point in the reaction (Table S5, entry 5). However, after 3 h significant catalyst decomposition was discerned by both 1H and 31P{1H} NMR spectroscopies; thus the observed


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activity and selectivity in this case should be considered with caution as it cannot be definitively attributed to 3PMePh2 or 4-PMePh2. While it is not the case that all monometallic Co or monometallic Zr complexes are generally ineffective alkyne semi-hydrogenation catalysts (Table 1, entries 7 and 10), the lack of reactivity of monometallic complexes analogous to bimetallic 3 and 4, I2Zr(XylNPiPr2)2 and ICo(PPh2HNiPr)3, towards either H2 activation or diphenylacetylene hydrogenation highlights the requisite nature of the bimetallic system (Table S6, entries 2 and 3). Furthermore, the absence of reactivity by 2 (Table S6, entry 4) emphasizes the significance of the electron-rich Co center and open coordination sites in 3-PMePh2. The presence of the Lewis acidic Zr site serves to stabilize a highly reduced Co center, allows for two-electron redox processes at Co, and supports the binding of substrates across the two metal centers.2 The catalytic activity of 3PMePh2 is relatively similar to the best reported examples of monometallic Co and Zr semi-hydrogenation catalysts (MesCCC)Co(N2)(PPh3) and [Cp*2Zr(OCH2CH2NiPr2)]+,21, 24c but the observed differences in selectivity (alkene/alkane and E/Z) suggest a different mechanistic pathway that may provide further opportunities for optimization once fully understood. The catalytic activity of 3-PMePh2 towards alkyne semihydrogenation is comparable to that of other recently reported base metal catalysts21, 24 (Table 1, entries 7, 9, and 10), and the mild reaction conditions compare favorably. However, the poor selectivity for one stilbene isomer over the other could likely be improved by tuning the steric bulk of the phosphinoamide ligand and by employing insights gleaned from a mechanistic investigation. Both of these research directions will be the subject of future work. While the aforementioned (NHC)Ag-RuCp(CO)2 catalyst demonstrates high Eselectivity (entry 8), substitution of the composite precious metals for base metals Cu and Fe results in a catalyst that exhibits diminished activity and selectivity, and is more selective for the Z-isomer (entry 9). In this system, the turnover-limiting step was proposed to be H2 activation, and the authors suggest that in order to improve catalytic activity (particularly with Earthabundant metals) and lower the reaction temperature, it would be necessary to lower the barrier for heterobimetallic H2 activation and to stabilize the (NHC)M’-H intermediates.25 Thus the enhancement in activity (including more favorable catalyst loading, reaction time, and temperature) of the base metalmediated bimetallic catalysis reported herein is likely a direct consequence of the remarkably facile H2 activation by 3 and the stabilized dihydride complex 4. Both features are facilitated by the simultaneous flexibility and support

imparted by the metal-metal multiple bond. While the order of the Zr-Co bond fluctuates throughout catalysis to accommodate changes in coordination number and delectron count, the foundational bonding interaction remains intact and therefore imparts advantageous stability. CONCLUSIONS A highly reactive reduced Zr/Co complex was synthesized by designing a coordinatively unsaturated bis(phosphinoamide)-ligated complex with a formal triple bond between the metal centers. The metal-metal multiple bonding stabilizes the complex while maintaining open coordination sites to facilitate substrate binding. The polarized nature of the interaction lends itself to extremely facile heterolytic cleavage of H2 at room temperature. The reactivity of 3 and 4 are highly dependent on the nature of the Co-bound terminal ligand, as it needs to be donating enough to stabilize 3, but labile enough to dissociate during catalysis. 3-PMePh2 exhibits good catalytic activity towards the hydrogenation of styrene and semihydrogenation of diphenylacetylene, and the significant improvement in bimetallic-mediated activity can be largely attributed to the dual nature of the metal-metal bond in helping to lower the barrier to H2 activation and to accommodate structural and electronic changes to the catalyst. The tractable, well-defined hydrogenation catalysis can be easily monitored by 1H and 31P NMR spectroscopies, and relevant reactive species have been isolated and characterized structurally and computationally, lending valuable insight into the metalmetal cooperativity that is often postulated for heterobimetallic species. To the best of our knowledge, 4PMe3 and 4-PMePh2 are the first isolated and structurally characterized H2 activation products of heterobimetallic complexes that are active for further stoichiometric or catalytic reactivity. Furthermore, 3 and 4 employ two base metals to work in tandem to catalyze the two-electron transformations necessary for H2 activation and hydrogenation that are typically regulated by precious metal catalysts. Although the semi-hydrogenation of internal alkynes is not selective for one alkene isomer over the other, experimental evidence suggests the possibility of two simultaneous mechanistic pathways, one of which results in direct trans-hydrogenation. At present, we speculate that insertion of the alkyne into the terminal CoH or the bridging Zr-H-Co hydride could lead to direct formation of either Z or E alkene products. This hypothesis would be consistent with literature examples in which dirhodium and diruthenium complexes facilitate the direct formation of E-alkenes in semihydrogenation reactions following alkyne insertion into a


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bridging hydride, further cementing the importance of bimetallic cooperativity in tuning both catalyst activity and selectivity. A mechanistic pathway similar to Fürstner’s that involves a carbene intermediate is also possible. Nonetheless, mechanistic studies will be required to probe the role of bimetallic cooperativity in this transformation and determine exactly how the trans alkene products are formed. Additional catalytic applications of these complexes along with further catalyst optimization and mechanistic studies are currently underway.

AUTHOR INFORMATION Corresponding Author *[email protected]

Present Addresses §Department

of Chemistry, University of Virginia, Charlottesville, VA 22904, United States

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental procedures, spectroscopic data for complexes 1-6 and representative catalytic runs, crystallographic data collection and refinement details for 1, 2, 3-PMePh2, 4-PMe3, 4-PMePh2, and 6, additional computational details, and XYZ coordinates of DFT-optimized geometries for 3PMePh2, 4-PMe3, and 6 (PDF) Crystallographic data for 1, 2, 3-PMePh2, 4-PMe3, 4PMePh2, and 6 (CIF)

ACKNOWLEDGMENTS This material is based upon work support by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under Award No. DE-SC0014151. The Ohio State University Department of Chemistry and Biochemistry and Sustainable and Resilient Economy program are also gratefully acknowledged for financial support. The authors thank Dr. Benjamin R. Reiner for insightful discussions. The authors are also grateful for access to the Brandeis University high-performance computing cluster and the Ohio Supercomputer Center.28

REFERENCES 1. (a) Powers, I. G.; Uyeda, C. Metal–Metal Bonds in Catalysis. ACS Catalysis 2017, 7, 936-958; (b) Mankad, N. P. Selectivity Effects in Bimetallic Catalysis. Chem. Eur. J. 2016, 22, 5822-5829; (c) Karunananda, M. K.; Mankad, N. P. Cooperative Strategies for Catalytic Hydrogenation of Unsaturated Hydrocarbons. ACS Catalysis 2017, 7, 6110-6119; (d) Gade,

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L. H. Highly Polar Metal-Metal Bonds in "Early-Late" Heterodimetallic Complexes. Angew. Chem. Int. Ed. 2000, 39, 2658-2678; (e) Bullock, R. M.; Casey, C. P. Heterobimetallic Compounds Linked by Heterodifunctional Ligands. Acc. Chem. Res. 1987, 20, 167-173; (f) Cooper, B. G.; Napoline, J. W.; Thomas, C. M. Catalytic Applications of Early/Late Heterobimetallic Complexes. Cat. Rev. - Sci. Eng. 2012, 54, 140; (g) Wheatley, N.; Kalck, P. Structure and Reactivity of Early-Late Heterobimetallic Complexes. Chem. Rev. 1999, 99, 3379-3420; (h) Thomas, C. M. Metal-Metal Multiple Bonds In Early/Late Heterobimetallic Complexes: Applications Toward Small Molecule Activation and Catalysis. Comments Inorg. Chem. 2011, 32, 14-38. 2. Coombs, J.; Perry, D.; Kwon, D.-H.; Thomas, C. M.; Ess, D. H. Why Two Metals Are Better Than One for Cobalt Phosphinoamide Catalyzed Kumada Coupling. Organometallics 2018, 37, 4195-4203. 3. Cammarota, R. C.; Clouston, L. J.; Lu, C. C. Leveraging Molecular Metal–Support Interactions for H2 and N2 Activation. Coord. Chem. Rev. 2017, 334, 100-111. 4. Cammarota, R. C.; Lu, C. C. Tuning Nickel with Lewis Acidic Group 13 Metalloligands for Catalytic Olefin Hydrogenation. J. Am. Chem. Soc. 2015, 137, 12486-12489. 5. (a) Hostetler, M. J.; Butts, M. D.; Bergman, R. G. Scope and Mechanism of Alkene Hydrogenation/Isomerization Catalyzed by Complexes of the Type R2E(CH2)2M(CO)(L) (R = Cp, Me, Ph; E = Phosphorus, Tantalum; M = Rhodium, Iridium; L = CO, PPh3). J. Am. Chem. Soc. 1993, 115, 2743-2752; (b) Ferguson, G. S.; Wolczanski, P. T.; Parkanyi, L.; Zonnevylle, M. C. Synthesis and Reactivity of Heterobimetallic "A-frames" and Rh-Zr Bonded Complexes: Structure of Cp*Zr(-OCH2Ph2P)2RhMe2. Organometallics 1988, 7, 1967-1979; (c) Jones, C. M.; Doherty, N. M. The Nature of the Bridging Nitrido Ligand. Synthesis and Reactivity of Heterobimetallic Nitrido-Bridged Compounds. Polyhedron 1995, 14, 81-91. 6. (a) Baranger, A. M.; Bergman, R. G. Cooperative Reactivity in the Interactions of X-H Bonds with a Zirconium-Iridium Bridging Imido Complex. J. Am. Chem. Soc. 1994, 116, 3822-3835; (b) Riddlestone, I. M.; Rajabi, N. A.; Lowe, J. P.; Mahon, M. F.; Macgregor, S. A.; Whittlesey, M. K. Activation of H2 over the Ru−Zn Bond in the Transition Metal−Lewis Acid Heterobimetallic Species [Ru(IPr)2(CO)ZnEt]+. J. Am. Chem. Soc. 2016, 138, 11081-11084; (c) Campos, J. Dihydrogen and Acetylene Activation by a Gold(I)/Platinum(0) Transition Metal Only Frustrated Lewis Pair. J. Am. Chem. Soc. 2017, 139, 2944-2947; (d) Thomas, C. M.; Napoline, J. W.; Rowe, G. T.; Foxman, B. M. Oxidative Addition Across Co/Zr Multiple Bonds in Early/Late Heterobimetallic Complexes. Chem. Commun. 2010, 46, 5790-5792. 7. Zhang, Y.; Roberts, S. P.; Bergman, R. G.; Ess, D. H. Mechanism and Catalytic Impact of Ir–Ta Heterobimetallic and Ir–P Transition Metal/Main Group Interactions on Alkene Hydrogenation. ACS Catalysis 2015, 5, 1840-1849. 8. (a) Krogman, J. P.; Foxman, B. M.; Thomas, C. M. Activation of CO2 by a Heterobimetallic Zr/Co Complex. J. Am. Chem. Soc. 2011, 133, 14582-14585; (b) Marquard, S. L.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Stoichiometric C-O Bond Oxidative Addition of Benzophenone by a Discrete Radical Intermediate To Form a Cobalt(I) Carbene. J. Am. Chem. Soc. 2013, 135, 6018-6021; (c) Zhang, H.; Wu, B.; Marquard, S. L.; Litle, E. D.; Dickie, D. A.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Investigation of Ketone C═O Bond Activation Processes by Heterobimetallic Zr/Co and Ti/Co Tris(phosphinoamide) Complexes. Organometallics 2017, 36, 3498-3507. 9. Wu, B.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. A Heterobimetallic Complex Featuring a Ti-Co Multiple Bond and its Application to the Reductive Coupling of Ketones to Alkenes. Chem. Sci. 2015, 6, 2044-2049. 10. Saper, N. I.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Synthesis of Chiral Heterobimetallic Tris(phosphinoamide) Zr/Co Complexes. Polyhedron 2016, 114, 88-95. 11. Cotton, F. A.; Murillo, C. A.; Walton, R. A., Multiple Bonds Between Metal Atoms. 3rd ed.; Springer Science and Business Media, Inc.: New York, 2005. 12. Pauling, L., The Nature of the Chemical Bond. 3rd ed.; Cornell University Press: Ithaca, NY, 1960. 13. Zhou, W.; Saper, N. I.; Krogman, J. P.; Foxman, B. M.; Thomas, C. M. Effect of Ligand Modification on the Reactivity of PhosphinoamideBridged Heterobimetallic Zr/Co Complexes. Dalton Trans. 2014, 43, 19841989. 14. Krogman, J. P.; Gallagher, J. R.; Zhang, G.; Hock, A. S.; Miller, J. T.; Thomas, C. M. Assignment of the Oxidation States of Zr and Co in a


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ACS Catalysis

Highly Reactive Heterobimetallic Zr/Co Complex Using X-ray Absorption Spectroscopy (XANES). Dalton Trans. 2014, 43, 13852-13857. 15. (a) Greenwood, B. P.; Forman, S. I.; Rowe, G. T.; Chen, C.-H.; Foxman, B. M.; Thomas, C. M. Multielectron Redox Activity Facilitated by Metal−Metal Interactions in Early/Late Heterobimetallics: Co/Zr Complexes Supported by Phosphinoamide Ligands. Inorg. Chem. 2009, 48, 6251-6260; (b) Greenwood, B. P.; Rowe, G. T.; Chen, C.-H.; Foxman, B. M.; Thomas, C. M. Metal-Metal Multiple Bonds in Early/Late Heterobimetallics Support Unusual Trigonal Monopyramidal Geometries at both Zr and Co. J. Am. Chem. Soc. 2010, 132, 44-45. 16. Napoline, J. W.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. N-H Activation of Hydrazines by a Heterobimetallic Zr-Co Complex: Promotion of One-Electron Chemistry at Zr. Chem. Commun. 2013, 49, 4388-4390. 17. (a) Baik, M.-H.; Friesner, R. A.; Parkin, G. Theoretical Investigation of the Metal–Metal interaction in Dimolybdenum Complexes with Bridging Hydride and Methyl Ligands. Polyhedron 2004, 23, 28792900; (b) Parkin, G., Metal–Metal Bonding in Bridging Hydride and Alkyl Compounds. In Metal-Metal Bonding, Parkin, G., Ed. Springer Berlin Heidelberg: Berlin, Heidelberg, 2010; pp 113-145. 18. (a) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. Preparation and Molecular and Electronic Structures of Iron(0) Dinitrogen and Silane Complexes and Their Application to Catalytic Hydrogenation and Hydrosilation. J. Am. Chem. Soc. 2004, 126, 13794-13807; (b) Friedfeld, M. R.; Shevlin, M.; Hoyt, J. M.; Krska, S. W.; Tudge, M. T.; Chirik, P. J. Cobalt Precursors for High-Throughput Discovery of Base Metal Asymmetric Alkene Hydrogenation Catalysts. Science 2013, 342, 10761080; (c) Ma, X.; Lei, M. Mechanistic Insights into the Directed Hydrogenation of Hydroxylated Alkene Catalyzed by Bis(phosphine)cobalt Dialkyl Complexes. J. Org. Chem. 2017, 82, 2703-2712; (d) Harman, W. H.; Peters, J. C. Reversible H2 Addition Across a Nickel-Borane Unit as a Promising Strategy for Catalysis. J. Am. Chem. Soc. 2012, 134, 5080-5082; (e) Fong, H.; Moret, M.-E.; Lee, Y.; Peters, J. C. Heterolytic H2 Cleavage and Catalytic Hydrogenation by an Iron Metallaboratrane. Organometallics 2013, 32, 3053-3062; (f) Vasudevan, K. V.; Scott, B. L.; Hanson, S. K. Alkene Hydrogenation Catalyzed by Nickel Hydride Complexes of an Aliphatic PNP Pincer Ligand. Eur. J. Inorg. Chem. 2012, 2012, 4898-4906; (g) Xu, R.; Chakraborty, S.; Bellows, S. M.; Yuan, H.; Cundari, T. R.; Jones, W. D. Iron-Catalyzed Homogeneous Hydrogenation of Alkenes under Mild Conditions by a Stepwise, Bifunctional Mechanism. ACS Catalysis 2016, 6, 2127-2135. 19. (a) Friedfeld, M. R.; Margulieux, G. W.; Schaefer, B. A.; Chirik, P. J. Bis(phosphine)cobalt Dialkyl Complexes for Directed Catalytic Alkene Hydrogenation. J. Am. Chem. Soc. 2014, 136, 13178-13181; (b) Ingleson, M.; Fan, H.; Pink, M.; Tomaszewski, J.; Caulton, K. G. ThreeCoordinate Co(I) Provides Access to Unsaturated Dihydrido-Co(III) and Seven-Coordinate Co(V). J. Am. Chem. Soc. 2006, 128, 1804-1805; (c) Tokmic, K.; Markus, C. R.; Zhu, L.; Fout, A. R. Well-Defined Cobalt(I) Dihydrogen Catalyst: Experimental Evidence for a Co(I)/Co(III) Redox Process in Olefin Hydrogenation. J. Am. Chem. Soc. 2016, 138, 1190711913. 20. (a) Knijnenburg, Q.; Horton, A. D.; van der Heijden, H.; Kooistra, T. M.; Hetterscheid, D. G. H.; Smits, J. M. M.; de Bruin, B.; Budzelaar, P. H. M.; Gal, A. W. Olefin Hydrogenation Using Diimine Pyridine Complexes of Co and Rh. J. Mol. Catal. A: Chem. 2005, 232, 151159; (b) Friedfeld, M. R.; Shevlin, M.; Margulieux, G. W.; Campeau, L.C.; Chirik, P. J. Cobalt-Catalyzed Enantioselective Hydrogenation of Minimally Functionalized Alkenes: Isotopic Labeling Provides Insight into the Origin of Stereoselectivity and Alkene Insertion Preferences. J. Am. Chem. Soc. 2016, 138, 3314-3324; (c) Monfette, S.; Turner, Z. R.; Semproni, S. P.; Chirik, P. J. Enantiopure C1-Symmetric Bis(imino)pyridine Cobalt Complexes for Asymmetric Alkene Hydrogenation. J. Am. Chem. Soc. 2012, 134, 4561-4564; (d) Yu, R. P.;

Darmon, J. M.; Milsmann, C.; Margulieux, G. W.; Stieber, S. C. E.; DeBeer, S.; Chirik, P. J. Catalytic Hydrogenation Activity and Electronic Structure Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes. J. Am. Chem. Soc. 2013, 135, 13168-13184; (e) Chirik, P. J. Iron- and Cobalt-Catalyzed Alkene Hydrogenation: Catalysis with Both Redox-Active and Strong Field Ligands. Acc. Chem. Res. 2015, 48, 1687-1695; (f) Lin, T.-P.; Peters, J. C. Boryl–Metal Bonds Facilitate Cobalt/Nickel-Catalyzed Olefin Hydrogenation. J. Am. Chem. Soc. 2014, 136, 13672-13683; (g) Lin, T.-P.; Peters, J. C. Boryl-Mediated Reversible H2 Activation at Cobalt: Catalytic Hydrogenation, Dehydrogenation, and Transfer Hydrogenation. J. Am. Chem. Soc. 2013, 135, 15310-15313; (h) Zhang, G.; Scott, B. L.; Hanson, S. K. Mild and Homogeneous CobaltCatalyzed Hydrogenation of C=C, C=O, and C=N Bonds. Angew. Chem. Int. Ed. 2012, 51, 12102-12106; (i) Zhang, G.; Vasudevan, K. V.; Scott, B. L.; Hanson, S. K. Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions. J. Am. Chem. Soc. 2013, 135, 8668-8681; (j) Jing, Y.; Chen, X.; Yang, X. Computational Mechanistic Study of the Hydrogenation and Dehydrogenation Reactions Catalyzed by Cobalt Pincer Complexes. Organometallics 2015, 34, 57165722. 21. Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. Stoichiometric Reactions and Catalytic Hydrogenation with a Reactive Intramolecular Zr+/Amine Frustrated Lewis Pair. J. Am. Chem. Soc. 2015, 137, 4550-4557. 22. Schrock, R. R.; Osborn, J. A. Catalytic Hydrogenation Using Cationic Rhodium Complexes. II. The Selective Hydrogenation of Alkynes to cis Olefins. J. Am. Chem. Soc. 1976, 98, 2143-2147. 23. Lindlar, H.; Dubuis, R. Palladium Catalyst for the Partial Reduction of Acetylenes. Organic Syntheses 1966, 46. 24. (a) Srimani, D.; Diskin-Posner, Y.; Ben-David, Y.; Milstein, D. Iron Pincer Complex Catalyzed, Environmentally Benign, E-Selective Semi-Hydrogenation of Alkynes. Angew. Chem. Int. Ed. 2013, 52, 1413114134; (b) Karunananda, M. K.; Mankad, N. P. E-Selective SemiHydrogenation of Alkynes by Heterobimetallic Catalysis. J. Am. Chem. Soc. 2015, 137, 14598-14601; (c) Tokmic, K.; Fout, A. R. Alkyne Semihydrogenation with a Well-Defined Nonclassical Co–H2 Catalyst: A H2 Spin on Isomerization and E-Selectivity. J. Am. Chem. Soc. 2016, 138, 13700-13705. 25. Karunananda, M. K.; Mankad, N. P. Heterobimetallic H2 Addition and Alkene/Alkane Elimination Reactions Related to the Mechanism of E-Selective Alkyne Semihydrogenation. Organometallics 2017, 36, 220-227. 26. (a) Guthertz, A.; Leutzsch, M.; Wolf, L. M.; Gupta, P.; Rummelt, S. M.; Goddard, R.; Farès, C.; Thiel, W.; Fürstner, A. HalfSandwich Ruthenium Carbene Complexes Link trans-Hydrogenation and gem-Hydrogenation of Internal Alkynes. J. Am. Chem. Soc. 2018, 140, 3156-3169; (b) Radkowski, K.; Sundararaju, B.; Fürstner, A. A FunctionalGroup-Tolerant Catalytic trans Hydrogenation of Alkynes. Angew. Chem. Int. Ed. 2013, 52, 355-360; (c) Fürstner, A. trans-Hydrogenation, gemHydrogenation, and trans-Hydrometalation of Alkynes: An Interim Report on an Unorthodox Reactivity Paradigm. J. Am. Chem. Soc. 2019, 141, 1124. 27. (a) Burch, R. R.; Muetterties, E. L.; Teller, R. G.; Williams, J. M. Selective Formation of trans Olefins by a Catalytic Hydrogenation of Alkynes Mediated at Two Adjacent Metal Centers. J. Am. Chem. Soc. 1982, 104, 4257-4258; (b) Burch, R. R.; Shusterman, A. J.; Muetterties, E. L.; Teller, R. G.; Williams, J. M. Coordinately Unsaturated Clusters. A Novel Catalytic Reaction. J. Am. Chem. Soc. 1983, 105, 3546-3556; (c) Schleyer, D.; Niessen, H. G.; Bargon, J. In situ 1H-PHIP-NMR Studies of the Stereoselective Hydrogenation of Alkynes to (E)-Alkenes Catalyzed by a Homogeneous [Cp*Ru]+ Catalyst. New J. Chem. 2001, 25, 423-426. 28. Ohio Supercomputer Center. Columbus, OH, 1987.


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