Cobalt Complexes Supported by cis-Macrocyclic Diphosphines

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Cobalt Complexes Supported by cis-Macrocyclic Diphosphines: Synthesis, Reactivity, and Activity toward Coupling Carbon Dioxide and Ethylene Ioana Knopf, Marc-André Courtemanche, and Christopher C. Cummins* Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, United States S Supporting Information *

ABSTRACT: The coordination chemistry of cis-macrocyclic diphosphines readily accessed from white phosphorus was explored, with a focus on preparing and studying cobalt complexes. cis-1,6-Dicyclohexyl-3,4,8,9-tetramethyl-2,5,7,10-tetrahydro-1,6-DiPhospheCine, or Cy2−DPC (1), was primarily used as a model diphosphine. Cobalt(II) dihalide diphosphine complexes such as (Cy2−DPC)CoX2, X = Cl (2) and I (3), were prepared, and their reactivity toward a variety of reducing agents was studied. We were successful in preparing and structurally characterizing an unusual iodide-bridged cobalt(I) dimer, [(Cy2−DPC)CoI]2 (8). These cobalt complexes were also investigated as potential catalysts for the coupling of carbon dioxide and ethylene to produce acrylate, a valuable polymer precursor. Although not yet catalytic, the first examples of cobalt complexes capable of mediating this transformation are reported. Notably, the well-known commercial complex ClCo(PPh3)3 was also found to be active in mediating acrylate production. As part of our mechanistic investigation, a pseudotetrahedral cobalt methyl acrylate complex, (Cy2− DPC)CoI(CH2CHCOOMe) (10), was prepared and structurally characterized.

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INTRODUCTION Although diphosphines are ubiquitous ligands in transition metal coordination chemistry, macrocyclic diphosphines2 are an underexplored class of ligands.3 A medium-sized (7−12membered ring) macrocycle is expected to impart rigidity to the ligand backbone, thus rendering macrocyclic diphosphines an attractive class of ligands to study. Since the trans isomer of a medium-sized macrocyclic diphosphine cannot act as a bidentate ligand,4 selectively generating the cis isomer is a prerequisite to studying its coordination chemistry. This synthetic challenge was first overcome by Alder and coworkers, who developed a selective route to cis-macrocyclic diphosphines.5 Perhaps because this ligand synthesis was lengthy and cumbersome, Alder and co-workers did not report any coordination complexes for these phosphines.6 Recently, we developed a synthetic protocol to access a family of cismacrocyclic diphosphines in a streamlined and atom-economical fashion directly from white phosphorus (Figure 1).1 With these molecules in hand, we set out to explore their coordination chemistry and learn more about their properties. We chose to focus our initial efforts on preparing cobalt complexes, given that cobalt has a variety of accessible oxidation states and coordination geometries. In addition to exploring the fundamental coordination chemistry of these ligands, we were curious to expand their use in the synthesis of acrylate from carbon dioxide and ethylene, an important endeavor in the field of carbon dioxide utilization.7 Significant efforts have been devoted in recent years to improve this transformation, mostly using nickel complexes,8−11 but turn© XXXX American Chemical Society

Figure 1. General synthetic route for preparing cis-macrocyclic diphosphines via P4 photolysis.1 (a) R = Me, iBu, Bn; X = Br, I. (b) R′ = Me, Cy, Ph, Mes; [M] = Li, MgCl, MgBr. (ii) R = R′ = Cy, [M] = MgCl.

over numbers remain low. We have recently shown that our family of cis-macrocyclic diphosphines performed as well or better than benchmark diphosphine ligands in facilitating the nickel-catalyzed synthesis of acrylate from CO2 and ethylene.1 However, this process still needs to see a dramatic improvement in activity in order to be implemented on an industrial scale. Looking for a paradigm shift, we were inspired by the work of Linehan and co-workers, who developed an efficient cobaltcatalyzed CO2 hydrogenation protocol.12,13 We wondered if we could use an analogous Co(I)/Co(III) catalytic cycle for generating acrylate from CO2 and ethylene. This would fundamentally change the current system, and potentially allow for improvements beyond those achieved with the Ni(0)/ Ni(II) catalytic cycle. One of the main challenges in the nickel Received: September 29, 2017

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DOI: 10.1021/acs.organomet.7b00734 Organometallics XXXX, XXX, XXX−XXX

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Organometallics system is promoting β−H elimination from the stable, squareplanar nickel lactones. If a Co(I)/Co(III) cycle would be operational, the putative Co(III) lactone species would adopt either a five-coordinate or an octahedral geometry, thus changing the coordination chemistry of the problematic β−H elimination step. Since the chemistry of nickel lactones has been so critical in developing the nickel-based catalytic systems, we were curious to learn more about the properties of cobalt lactones. We were surprised to learn that there are no known cobalt lactones, although rhodium14 and iridium15 lactones have been reported. The lack of an isolated cobalt lactone example might be an opportunity for CO2/ethylene catalysis, assuming this type of complex has not yet been isolated because it is not as stable as its nickel analogue. While nickel lactones have provided valuable mechanistic insight, their stability has made achieving turnover a challenge. We were curious to investigate whether a cobalt-based catalytic cycle might exhibit better kinetics than the known nickel-based cycle.

conversion to a neutral, tetrahedral Co(II) complex in our system. Complex 2 is silent by 31P{1H} NMR spectroscopy and features 1H NMR resonances from 264.9 to −18.5 ppm. The paramagnetic shifting of all the 1H NMR resonances provides great resolution for all the individual peaks, allowing us to identify 16 distinct resonances. While we expected 17 different resonances from this complex, two inequivalent proton peaks accidentally overlap. Accounting for the Cl1−Co1−Cl2 mirror plane that bisects the ligand, all the protons within each half of the ligand are inequivalent. Therefore, this 1H NMR spectrum shows that complex 2 is fully locked in the endo-exo conformation at room temperature on the NMR time scale. The analogous cobalt(II) complex (Cy2−DPC)CoI2 (3) was prepared from Cy2−DPC (1) and CoI2 in THF. Brick-colored 3 has very similar spectroscopic signatures to those of 2 in both 31 1 P{ H} NMR (silent) and 1H NMR spectroscopies. As with 2, the complex is locked in the endo-exo conformation and displays 16 distinct proton resonances in CDCl3. One notable difference is that complex 3 is more soluble than 2 in organic solvents. In the 1H NMR spectrum acquired in C6D6, all 17 inequivalent proton resonances of (Cy2−DPC)CoI2 were fully resolved. With these cobalt(II) complexes in hand, we were curious to explore their reduction chemistry and see if we could generate some interesting cobalt(I) complexes. Most cobalt(I) species supported by phosphines have three phosphorus donors in their coordination sphere (e.g., the well-known ClCo(PPh3)3),18 so we were interested to study the reactivity of cobalt(I) species supported by only two phosphines. In order to easily identify the reaction products from reduction reactions, we wanted to use a derivative endowed with a distinctive 1 H NMR handle. Therefore, we prepared [NpP2dmb2]I (4) from P2dmb2 and neopentyl iodide and subsequently treated it with neopentyllithium to yield Np2− DPC (5). With this new ligand in hand, we prepared (Np2− DPC)CoCl2 (6) starting from CoCl2 in THF. The resulting blue-green complex has a distinctive resonance at −8.23 ppm in its 1H NMR spectrum for the nine equivalent hydrogens of the neopentyl group. Furthermore, complex 6 is much more soluble in organic solvents such as THF than its cyclohexyl analogue 2. Efforts to Prepare Cobalt(I) Complexes via Reduction. Treatment of (Np2−DPC)CoCl2 (6) with cobaltocene, lithium triethylborohydride, or Ti[N(tBu)(Ar)]3 (Ar = 3,5-Me2C6H3)19 resulted in a mixture of unreacted 6 and free Np2−DPC, as well as decomposition to a black insoluble material. Reduction of 6 with decamethylcobaltocene in various solvents (toluene, THF, CH3CN) resulted in full consumption of the starting material; however, decomposition to free Np2−DPC and brown-black unidentified species was always observed, even in the presence of other ligands such as diphenylacetylene. Addition of organometallic reagents such as methyllithium, mesitylmagnesium bromide, allylmagnesium bromide, or trimethylsilylmethylmagnesium bromide to 6 resulted in brown or black reaction mixtures, with free Np2−DPC detected by 31P{1H} NMR spectroscopy. Only when 6 was treated with tertbutylmagnesium chloride at low temperature was a new diamagnetic species with two inequivalent phosphorus signals at 40.0 and −15.9 ppm observed by 31P{1H} NMR spectroscopy. However, the isolated solids were brown-black and contaminated with free Np2−DPC. Because of the very soluble and greasy nature of the ligand, we were unable to purify or

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RESULTS AND DISCUSSION Synthesis of Cobalt(II) Complexes. We decided to begin investigating the coordination chemistry of our macrocyclic diphosphines by preparing a few cobalt(II) complexes. As a representative cis-macrocyclic diphosphine, we chose the previously reported cis-1,6-dicyclohexyl-3,4,8,9-tetramethyl2,5,7,10-tetrahydro-1,6-DiPhospheCine, or Cy2−DPC (1).1 Treatment of 1 with CoCl2 in THF at room temperature overnight resulted in the formation of a turquoise complex, (Cy2−DPC)CoCl2 (2). The solid-state structure of complex 2 (Figure 2) shows the cobalt center in a pseudotetrahedral

Figure 2. Solid-state structure of (Cy2−DPC)CoCl2 (2) with thermal ellipsoids at the 50% probability level, and hydrogen atoms omitted for clarity. Representative interatomic distances [Å] and angles [deg]: Co1−Cl1 2.2295(3), Co1−Cl2 2.2340(3), Co1−P1 2.3479(4), Co1− P2 2.3618(4); P1−Co1−P2 93.087(12), Cl1−Co1−Cl2 112.957(14).

geometry, with a ligand bite angle of 93.087(12)°. While there are many examples of similar tetrahedral cobalt diphosphine complexes that have been structurally characterized,16 this coordination chemistry is not always straightforward. For example, diphenylphosphinoethane (dppe) has been shown to have surprisingly complex coordination chemistry with cobalt halides, forming ionic complexes of the form [CoX(dppe)2]2[Co2X6(dppe)].17 We were pleased to see full B


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Organometallics

and allowed us to determine its structure by X-ray diffraction methods. As seen in Figure 4, the new product [(Cy2−DPC)CoI]2 (8) is a dimer with two bridging iodide ligands. Given the halogen

crystallize this species. Therefore, we moved back to the more crystalline cyclohexyl ligand system. Treatment of (Cy2−DPC)CoCl2 (2) with tert-butylmagnesium chloride (2.5 equiv) in thawing Et2O led to the formation of an analogous diamagnetic species with broad 31P{1H} NMR resonances at 37.1 and 20.8 ppm. Although the crude reaction mixture was brown-black, we were able to obtain orange crystals of the product by cooling a pentane solution to −35 °C. With pure material in hand, we acquired low temperature 31 1 P{ H} NMR and 31P NMR spectra. At −30 °C, the resonances became sharp enough to resolve the “hidden” P− H coupling in the upfield 31P NMR signal, which appeared as a doublet at 23.6 ppm with a 1JPH of 308 Hz. The 1H NMR spectrum corroborated the presence of a P−H bond, with the corresponding proton resonance appearing as a doublet of doublets centered at 4.13 ppm. While these data clearly indicated that the ligand framework had been altered upon reduction, it was not until a single crystal X-ray diffraction study was performed that we could fully elucidate the structure of the product. Figure 3 shows the product, (Cy2−mDPC)Co(Me-

Figure 4. Solid-state structure of [(Cy2−DPC)CoI]2 (8) with thermal ellipsoids at the 50% probability level, and hydrogen atoms omitted for clarity. Representative interatomic distances [Å] and angles [deg]: Co1−I1 2.6771(6), Co1−I2 2.6090(6), Co2−I1 2.6139(6), Co2−I2 2.6964(7), Co1−P1 2.2246(12), Co1−P2 2.2332(12), Co2−P3 2.2358(11), Co2−P4 2.2375(11), Co1−Co2 2.8884(8); Co1−I1− Co2 66.160(17), Co1−I2−Co2 65.947(18), P1−Co1−P2 92.26(4), P3−Co2−P4 92.20(4).

exchange, an inner sphere mechanism is probably operating in the reduction process. The geometry around each cobalt center is pseudotetrahedral, with the Cy2−DPC ligands’ bite angles of 92.26(4)° and 92.20(4)°. The diamond core is not planar and shows acute Co1−I1−Co2 and Co1−I2−Co2 angles of 66.160(17)° and 65.947(18)°, respectively. The two cobalt centers are not within bonding distance, as the Co1−Co2 distance is 2.8884(8) Å. While similar diamond core structures are common for rhodium and iridium,21 this appears to be the first structurally characterized halide-bridged cobalt diphosphine dimer. We must note that the aforementioned rhodium and iridium analogues of 8 are not exactly isostructural since they have square-planar geometries around the metal centers. Although reduction of 2 with SmI2 afforded 8, the isolated dimer was contaminated with ca. 10% samarium salts, as determined through energy-dispersive X-ray spectroscopy (EDX). We developed an alternative preparation for 8 by reducing (Cy2−DPC)CoI2 (3) with excess zinc dust (Figure 5). While this reduction proceeded slowly at room temperature, we were able to purify and isolate dimer 8 in 72% yield. Reactivity of the Cobalt(I) Dimer. With pure 8 in hand, we started exploring its reactivity with various ligands. Treatment of [(Cy2−DPC)CoI]2 with tert-butyl isocyanide resulted in the formation of the diamagnetic species [(Cy2− DPC)Co(CNtBu)3]I (9) (Figure 5). This yellow complex precipitated from toluene, a first indication that the iodide had dissociated from the cobalt center and the resulting product was a salt. The X-ray structure of 9 (Figure 6) shows a trigonal bipyramidal cobalt(I) center with three isocyanide ligands in

Figure 3. Right: Simplified line drawing of (Cy2−mDPC)Co(Me-allyl) (7). Left: Solid-state structure of 7 with thermal ellipsoids at the 50% probability level, and the hydrogen atoms of the cyclohexyl groups omitted for clarity. Representative interatomic distances [Å] and angles [deg]: Co1−C1 2.0920(10), Co1−C2 1.9957(10), Co1−C3 2.0708(10), Co1−C11 2.0248(10), Co1−C12 2.0891(10), Co1−P1 2.1968(3), Co1−P2 2.1761(3), C11−C12 1.4253(15), C12−C13 1.4771(15), C13−C16 1.3444(14); P1−Co1−P2 98.803(11).

allyl) (7), which features a methylallyl group coordinated to the Co(I) center, likely originating from the tert-butylmagnesium chloride reductant. One of the 2,3-dimethylbutene-1,4-diyl linkers of the macrocycle has formally transferred a hydrogen atom to one of the phosphorus atoms and isomerized to a feature a terminal olefin that was now bound to cobalt. We were unable to find a literature precedent for such a conversion of a tert-butyl organometallic reagent to a methylallyl group. Synthesis of a Halide-Bridged Cobalt(I) Dimer. Still searching for a way to access a Co(I) species, we treated 2 with a solution of samarium diiodide in THF.20 The mixture remained heterogeneous, but the initial turquoise complex turned into an olive-green species overnight. After isolating the precipitate from the THF reaction mixture, we noted that the new species obtained was still paramagnetic, but had a different 1 H NMR signature from the starting Co(II) complex. Single crystals of this product were grown from a benzene solution C


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Organometallics

The 1H NMR spectrum of the degassed sample verified the regeneration of Co(I) dimer 8 (Figure 5). After confirming ethylene binding to our cobalt diphosphine complex, we turned toward investigating the coupling of ethylene with carbon dioxide at a cobalt center. Activity of Cobalt Complexes toward CO2/Ethylene Coupling. Having accessed cobalt(II) and cobalt(I) species supported by our cis-macrocyclic ligands, we were curious to test their reactivity in the synthesis of acrylate from carbon dioxide and ethylene. We began our studies by testing a model cobalt complex, (Cy2−DPC)CoCl2 (2), under conditions previously employed for nickel complexes. We envisioned that the Co(II) complex would get reduced in situ to a Co(I) species by the zinc dust typically employed in these protocols. No acrylate could be observed when using conditions similar to those used by Limbach and co-workers,10 namely, sodium 2fluorophenoxide (50 equiv), Zn (50 equiv), CO2 (10 bar), C2H4 (10 bar), THF, 100 °C, 20 h. However, we were able to see acrylate formation when using conditions similar to those developed by Vogt and co-workers,9 namely, LiI (25 equiv), NEt3 (70 equiv), Zn (50 equiv), CO2 (10 bar), C2H4 (20 bar), chlorobenzene, 60 °C, 22 h. A control experiment replacing (Cy2−DPC)CoCl2 with just CoCl2 did not yield any acrylate. Another control experiment showed that no product is generated in the absence of a cobalt source.23 Furthermore, attempting to generate the cobalt diphosphine species in situ from the Cy2−DPC (1) diphosphine and cobalt halides was not successful in producing acrylate (Table 1).

Figure 5. Preparation of [(Cy2−DPC)CoI]2 (8) from either (Cy2− DPC)CoCl2 (2) or (Cy2−DPC)CoI2 (3) and subsequent reactivity of the cobalt(I) dimer with ethylene and tert-butyl isocyanide.

Table 1. Testing Various Cobalt Sources for Acrylate Production from CO2 and Ethylene

Figure 6. Solid-state structure of [(Cy2−DPC)Co(CNtBu)3]I (9) with thermal ellipsoids at the 50% probability level, with the iodide anion, hydrogen atoms, the disordered crystallization solvent, and the minor part of the disordered cyclohexyl group omitted for clarity. Representative interatomic distances [Å] and angles [deg]: Co1−C1 1.818(3), Co1−C2 1.818(3), Co1−C3 1.850(3), Co1−P1 2.2024(7), Co1−P2 2.2066(8), C1−N1 1.157(4), C2−N2 1.170(4), C3−N3 1.157(4); P1−Co1−P2 88.86(3), C1−Co1−P1 173.42(9), C1−Co1− P2 95.92(9), C2−Co1−P2 110.50(9), C3−Co1−P2 113.76(9).

cobalt species

% acrylatea

no cobalt CoX2 + Cy2−DPC (1) (Cy2−DPC)CoCl2 (2) (Cy2−DPC)CoI2 (3) [(Cy2−DPC)CoI]2 (8) (PPh3)3CoCl

0 0b 3 0c 0 16

a

Cobalt complex (0.03 mmol), LiI (0.75 mmol), Zn (1.5 mmol), NEt3 (2.1 mmol), chlorobenzene (2 mL), pressurized with C2H4 (20 bar) and CO2 (10 bar) and heated to 60 °C for 22 h. % acrylate = (mmol acrylate)/(mmol Co) × 100. bX = Cl, Br, or I. Cy2−DPC ligand (0.03 mmol) used in addition to the respective cobalt halide. cNo LiI was added.

addition to diphosphine 1, which displays a bite angle of 88.86(3)°. In solution, this complex is fluxional and all the tertbutyl isocyanide ligands appear as a singlet at 1.49 ppm by 1H NMR spectroscopy. The 31P{1H} NMR spectrum of 9 also displays a single resonance at 23.5 ppm. In the solid-state ATRIR spectrum of 9, the CN stretches appear at 2133 and 2027 cm−1. Similar cobalt complexes bearing three isocyanides and two phosphines have been reported previously.22 Next, we found that a weaker ligand such as ethylene reacted reversibly with dimer 8. Exposing a yellow-green solution of the dimer in C6D6 to 1 atm of ethylene led to the formation of a green solution. The 1H NMR spectrum of this reaction mixture showed a broad ethylene signal, indicating fast chemical exchange between the bound and free ethylene. Degassing this solution via four consecutive freeze−pump−thaw cycles resulted in a color change back to the original yellow-green.

Having determined that using the isolated complex (Cy2− DPC)CoCl2 (2) led to the generation of small amounts of acrylate, we were curious to learn more about the reaction and see if we could improve the yield of acrylate. At this point, we were intrigued by the role of LiI in this reaction. We noted a color change from turquoise to brown when mixing the reagents inside the glovebox, indicating that the (Cy2− DPC)CoCl2 was undergoing some initial transformation. Adding excess LiI (ca. 20 equiv) to a sample of 2 in CDCl3 led to a gradual color change from blue-turquoise to green, then orange, and eventually brown. After 3 h at room temperature, a 1 H NMR spectrum of the reaction mixture confirmed full conversion to the previously characterized (Cy2−DPC)CoI2 D


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Organometallics (3) (Figure 7). Knowing that LiI is effecting this halogen exchange, we wondered if the lithium cation is indeed necessary

Table 2. Testing Bases, Solvents, and Temperatures for Acrylate Production from CO2 and Ethylene

Figure 7. Stacked 1H NMR spectra taken at 20 °C in CDCl3 of (Cy2− DPC)CoCl2 (2) (top), (Cy2−DPC)CoCl2 (2) + excess LiI (middle), and (Cy2−DPC)CoI2 (3) (bottom).

for the CO2/C2H4 coupling reaction to take place. We therefore ran the coupling reaction using complex (Cy2− DPC)CoI2 (3), which was previously being generated in situ, and excluding the lithium iodide. No acrylate was observed, signifying that the lithium cation is a necessary ingredient for this transformation (Table 1). This finding is consistent with the critical role of Lewis acids in acrylate synthesis from CO2 and ethylene already established in the literature.8−11,24,25 Knowing that 3 gets reduced by zinc dust to the cobalt(I) dimer [(Cy2−DPC)CoI]2 (8) (vide supra), we suspected this reduction was operating under the reaction conditions of the CO2/C2H4 coupling. However, we were not able to detect acrylate formation when using the isolated 8 as the cobalt source (Table 1). This dimer has very low solubility in organic solvents such as chlorobenzene, so this might be the reason for its apparent lack of activity. When 3 gets reduced in situ, the resulting cobalt(I) species is likely to bind ethylene or another ligand before it gets the chance to dimerize. Given the assumption of a cobalt(I)/cobalt(III) cycle being operational for this conversion of CO2 and ethylene to acrylate, we decided also to test a simple, commercial cobalt(I) complex: (PPh3)3CoCl. We were pleasantly surprised to find that this cobalt complex could generate acrylate under the same conditions as our complex. Therefore, we decided to use (PPh3)3CoCl as a model complex for performing optimization experiments on other parameters of the system such as base, solvent, and temperature. Optimization of Reaction Conditions Using ClCo(PPh3)3. The first parameter we changed systematically was the identity of the base. Replacing triethylamine with other bases such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4diazabicyclo[2.2.2]octane (DABCO), pyridine, 4-dimethylaminopyridine (DMAP), or 2,4,6-collidine did not lead to the production of any acrylate (Table 2). We then turned to testing the influence of the solvent on the CO2/C2H4 coupling reaction. Running the reaction in tetrahydrofuran or neat triethylamine did not yield any product (Table 2). In the case of triethylamine, the cobalt complex was visibly insoluble, so this might have been a factor in the lack of activity observed. Toluene proved to be a good solvent, so we decided to move forward with similar weakly coordinating solvents, testing

base

solvent

temp. (°C)

% acrylatea

NEt3 DBU DABCO pyridine DMAP 2,4,6-collidine NEt3 n/a NEt3 NEt3 NEt3 NEt3 NEt3 NEt3 NEt3 NEt3 NEt3 NEt3 NEt3

PhCl PhCl PhCl PhCl PhCl PhCl THF NEt3 PhCH3 PhCH3 PhCH3 PhCl PhCl PhOCH3 PhOCH3 PhOCH3 PhOCH3 PhOCH3 PhOCH3

60 60 60 60 60 60 60 60 60 60 100 60 100 60 100 60 60 60 25

16 0 0 0 0 0 0b 0b 30b 4 trace 10 2 22 3 0c 9d d,e 0 13d

a

(PPh3)3CoCl (0.03 mmol), LiI (0.75 mmol), Zn (1.5 mmol), base (2.1 mmol), solvent (2 mL), pressurized with C2H4 (20 bar) and CO2 (10 bar) and stirred at indicated temperature for 22 h. % acrylate = (mmol acrylate)/(mmol Co) × 100. bDifferent workup and internal standard used because of overlapping 1H NMR signals with typical standard. cNo LiI was added. dNo Zn was added. eCy2−DPC (0.03 mmol) was added.

toluene, chlorobenzene, and anisole. At 60 °C, all three solvents were suitable for acrylate formation. Increasing the temperature to 100 °C was detrimental to the reaction, and only traces of product were observed for all three solvents (Table 2). We decided to perform further experiments using anisole, as it is more polar than toluene, more effective at dissolving lithium iodide, and does not contain potentially reactive bonds as in the case of chlorobenzene.26 As previously observed when testing 3, excluding LiI resulted in no acrylate formation. However, excluding zinc was tolerated since the starting cobalt source, (PPh3)3CoCl, is already in the +1 oxidation state (Table 2). Notably, the reaction is poisoned by the addition of the diphosphine ligand 1. The last entry in Table 2 shows that acrylate was still generated when conducting the reaction at room temperature. At this point, the system had been simplified significantly by eliminating the elevated temperature and zinc additive. However, the amounts of product generated remained low. With the optimized solvent and temperature in hand, we went back to testing cobalt sources and found that the cobalt(I) dimer [(Cy2−DPC)CoI]2 (8) was inactive under these new conditions as well. As discussed above, the poor solubility properties of this species are likely playing a role in the lack of activity observed. Continuing our optimization efforts, we turned back to using cobalt(II) precursors in conjunction with reducing agents. While (Cy2−DPC)CoI2 (3) generated some acrylate, (Np2−DPC)CoCl2 (6) was unsuccessful. The necessity of a cobalt(I) species for effecting CO2/C2H4 coupling was corroborated by the lack of any observed acrylate when the cobalt(II) complex 3 was used without the zinc reductant (Table 3). It was not clear whether the process of generating acrylate was simply very slow or if the active cobalt species became E


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Organometallics

NEt3 (4.2 mmol) in 4 mL of anisole was stirred at room temperature under 20 bar C2H4 and 10 bar CO2 for 22 h. After removing volatiles from the reaction mixture, the residue was dissolved in C6D6 and filtered to remove excess LiI. This solution showed no signals by 31P{1H} NMR spectroscopy. The 1H NMR spectrum showed a myriad of signals up to 290 ppm, indicating that all the cobalt-containing products were paramagnetic. While we could not identify many of the resonances, we were able to determine that some of them corresponded to the previously synthesized cobalt(I) dimer, [(Cy2−DPC)CoI]2 (8). Synthesis of a Cobalt Acrylate Complex. Aiming to identify some of the species from the CO2/C2H4 reaction mixture, we sought to synthesize independently a cobalt acrylate complex. Treatment of [(Cy2−DPC)CoI]2 (8) with excess methyl acrylate resulted in formation of an emerald green complex. Unfortunately, we could not isolate readily this species in pure form because acrylate binding is reversible; we obtained a mixture of dimer 8 and the acrylate complex after removing volatiles from the reaction mixture. Adding excess methyl acrylate to a toluene-d8 sample of 8 allowed us to obtain a 1H NMR spectrum of the new complex. However, the 1H NMR resonances of this acrylate complex do not match the resonances we observe when analyzing the CO2/C2H4 coupling reaction mixture by 1H NMR spectroscopy. We confirmed our formulation of t his species as (Cy 2 −DPC)CoI(CH2CHCOOMe) (10) through an X-ray diffraction study carried out on a single crystal grown from the toluene-d8 solution containing excess methyl acrylate (Figure 8). The

Table 3. Testing Various Cobalt Complex Sources for Acrylate Production from CO2 and Ethylene cobalt complex

zinc additive

% acrylatea

(PPh3)3CoCl [(Cy2−DPC)CoI]2 (8) (Cy2−DPC)CoI2 (3) (Cy2−DPC)CoI2 (3) (Np2−DPC)CoCl2 (6) (PPh3)3CoCl (PPh3)3CoCl (Cy2−DPC)CoI2 (3) (Cy2−DPC)CoI2 (3)

− − − + + − − + +

13 0 0 6 0 15b 3c 9b 2c

a

Cobalt complex (0.03 mmol), LiI (0.75 mmol), Zn (1.5 mmol when used), NEt3 (2.1 mmol), anisole (2 mL), pressurized with C2H4 (20 bar) and CO2 (10 bar) and stirred at room temperature for 22 h. % acrylate = (mmol acrylate)/(mmol Co) × 100. bReaction was run for 3 days instead of 22 h. cThe excess reagents were reduced 10-fold, so LiI (0.075 mmol), Zn (0.15 mmol when used), and NEt3 (0.21 mmol) were used.

deactivated after making a certain amount of product. Running the reaction for 3 days instead of 22 h did not lead to significantly larger amounts of acrylate, suggesting that the key cobalt mediator(s) had been deactivated. Since the amounts of acrylate generated are substoichiometric with respect to the cobalt complex used, we also tested whether the large excess of reagents (LiI, Zn, NEt3) was necessary. Reducing the excess 10fold still yielded acrylate, albeit in apparently reduced amounts (Table 3). Analysis of the Reaction Mixtures Produced after CO2/C2H4 Coupling. While working on optimizing reaction conditions, we were curious to know what species might be involved in the conversion of CO2 and ethylene to acrylate. To this end, we conducted an experiment using conditions in which we saw acrylate production and brought the reaction mixture back inside the glovebox instead of submitting it to the typical aqueous workup. In this experiment, a mixture of (PPh3)3CoCl (0.06 mmol), LiI (1.5 mmol), Zn (3 mmol), and NEt3 (4.2 mmol) in 4 mL of chlorobenzene was heated to 60 °C under 20 bar C2H4 and 10 bar CO2 for 22 h. Analyzing the crude mixture by 31P{1H} NMR spectroscopy revealed PPh3, OPPh3, as well as a new species displaying a resonance at 55 ppm. After removing volatiles and dissolving the residue in C6D6, a 1H NMR spectrum was acquired. In addition to the resonances for PPh3, OPPh3, and (PPh3)3CoCl, new signals, potentially corresponding to paramagnetic species, were detected. One notable resonance, a triplet with a coupling constant of 41 Hz, appeared at −10.3 ppm. While present in a very small quantity, this resonance clearly invokes the presence of a cobalt hydride. Complexes with similar P−H coupling patterns have been reported.27−30 Among them, complex HCo(CO)2(PPh3)2 was reported to have the same 1H NMR chemical shift and coupling constant27 as the species in our reaction mixture, as well as the same 31P{1H} NMR shift at 55 ppm.27,28 On the basis of these data, we have assigned the species present in our reaction mixture as HCo(CO)2(PPh3)2. Linehan and co-workers have also observed the formation of a cobalt carbonyl complex while investigating decomposition pathways for their cobalt-catalyzed CO2 hydrogenation.13 A similar experiment was conducted using conditions that showed acrylate production from (Cy2−DPC)CoI2 (3). A mixture of 3 (0.06 mmol), LiI (1.5 mmol), Zn (3 mmol), and

Figure 8. Solid-state structure of (Cy2−DPC)CoI(CH2CHCOOMe) (10) with ellipsoids at the 50% probability level, with hydrogen atoms and minor disorder omitted for clarity. Representative interatomic distances [Å] and angles [deg]: Co1−C1 2.079(4), Co1−C2 2.082(5), Co1−P1 2.2689(10), Co1−P2 2.2900(10), Co1−I1 2.5851(6), C1− C2 1.375(7), C2−C3 1.504(8); P1−Co1−P2 89.14(3), P1−Co1−I1 106.07(3).

complex adopts a pseudotetrahedral geometry around the cobalt center, with a ligand bite angle (P1−Co1−P2) of 89.14(3)°. Complex 10 is one of few structurally characterized cobalt diphosphine complexes bearing both a halide and an olefin ligand.31 Furthermore, to the best of our knowledge, there is only one other reported example of a structurally characterized cobalt methyl acrylate complex.32 While we cannot implicate complex 10 in the CO2/C2H4 coupling, this F


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Organometallics

brick-colored powder was obtained (303 mg, 0.43 mmol, 90% yield). H NMR (500 MHz, CDCl3, 20 °C) δ: 288.90 (s, 1H), 165.86 (s, 1H), 152.58 (s, 1H), 111.94 (s, 1H), 28.82 (s, 1H), 21.39 (s, 3H), 7.95 (s, 1H), 7.48 (s, 1H), 6.87 (s, 3H), −3.12 (s, 2H), −3.78 (s, 1H), −6.12 (s, 1H), −16.30 (s, 1H), −18.81 (s, 1H), −19.69 (s, 1H), −20.91 (s, 1H) ppm. 1H NMR (500 MHz, C6D6, 20 °C) δ: 278.86 (s, 1H), 166.12 (s, 1H), 153.61 (s, 1H), 126.25 (s, 1H), 23.94 (s, 3H), 22.19 (s, 1H), 6.12 (s, 1H), 5.29 (s, 1H), 4.74 (s, 3H), −3.46 (s, 1H), −3.62 (s, 1H), −4.46 (s, 1H), −7.30 (s, 1H), −16.55 (s, 1H), −19.61 (s, 1H), −20.77 (s, 1H), −22.84 (s, 1H) ppm. 31P{1H} NMR (162 MHz, CDCl3, 25 °C) δ: silent. Evans Method (1% (Me3Si)2O in CDCl3, 20 °C): μeff = 4.34 μB. ATR-IR: 2924, 2850, 1441, 1207, 1117, 1070, 1043, 852, 832, 802, 427 cm−1. Elemental analysis [%] found (calculated for C24H42I2CoP2): C 40.92 (40.87), H 6.21 (6.00), N < 0.02 (0). Synthesis of [NpP2dmb2]I (4). P2dmb21,34 (1.01 g, 4.48 mmol, 1 equiv) was loaded along with a stir bar into a thick walled vessel. Neopentyl iodide (2.0 mL, 2.99 g, 15.1 mmol, 3.3 equiv) was added by syringe. The vessel was capped with its Teflon cap, then taped and taken outside the glovebox. The neat mixture was heated at 130 °C overnight. After 14 h, the vessel was cooled to room temperature and brought back into the glovebox. The residue was dissolved in CH2Cl2 (12 mL) and filtered through a pipet microfiber filter paper. Additional CH2Cl2 (10 mL) was used to rinse the vessel and wash the pipet filter. A very small amount of yellow product was collected on the filter. The CH2Cl2 filtrate was set under vacuum and all volatiles were removed. The residue was slurried in pentane (10 mL) and filtered. The precipitate was washed with pentane (2 × 8 mL). After drying under vacuum, the solid obtained was analyzed by 1H NMR spectroscopy, showing that some neopentyl iodide (ca. 10%) was contaminating the final product. The solids were stirred with pentane (10 mL) for 3 h, then filtered and washed with pentane (3 × 8 mL), and then dried under vacuum to yield [NpP2dmb2]I (1.65 g, 3.88 mmol, 87% yield). 1 H NMR (400 MHz, CDCl3, 25 °C) δ: 3.47 (d, J = 13.9 Hz, 4H), 2.85 (dd, J = 12.4, 5.1 Hz, 2H), 2.80−2.50 (m, 4H), 1.92 (s, 6H), 1.82 (d, J = 5.9 Hz, 6H), 1.20 (s, 9H) ppm. 13C{1H} NMR (101 MHz, CDCl3, 25 °C) δ: 130.75 (dd, J = 12.7, 2.0 Hz), 122.61 (dd, J = 10.4, 4.2 Hz), 36.83 (dd, J = 33.4, 13.5 Hz), 32.61 (d, J = 5.1 Hz), 32.07 (dd, J = 7.1, 4.6 Hz), 27.30 (dd, J = 27.9, 5.6 Hz), 26.83 (dd, J = 27.4, 2.5 Hz), 21.54 (d, J = 3.6 Hz), 21.39 (dd, J = 4.5, 1.2 Hz) ppm. 31P{1H} NMR (162 MHz, CDCl3, 25 °C) δ: 44.12 (d, J = 297.5 Hz), −67.94 (d, J = 297.5 Hz) ppm. ATR-IR: 2958, 2929, 2863, 1469, 1441, 1424, 1385, 1372, 1191, 1173, 1060, 1029, 851, 843, 833, 805, 428 cm−1. Elemental analysis [%] found (calculated for C17H31IP2): C 47.27 (48.12), H 6.84 (7.36), N < 0.02 (0). Synthesis of Np2−DPC (5). Neopentyllithium was generated in situ and used without further purification.35 A solution of neopentyl iodide (246 mg, 1.244 mmol, 1.05 equiv) in Et2O (6 mL) was frozen in the cold well. To this thawing solution, a solution of 1.7 M tertbutyllithium in pentane (1.5 mL, 2.55 mmol, 2.1 equiv) was added slowly using a syringe. The thawed solution was allowed to stir at room temperature for 30 min, then added to [NpP2dmb2]I (see below). [NpP2dmb2]I (503 mg, 1.185 mmol, 1 equiv) was slurried in Et2O (20 mL). To this slurry, the previously generated neopentyllithium solution was added dropwise at room temperature. Most solids dissolved, but some still remained. After 1.5 h, the mixture was filtered through an alumina pad, washed with Et2O (2 × 20 mL), and then set under vacuum to remove all volatiles from the filtrate. The resulting white solids were dissolved in pentane (6 mL) and filtered through glass microfiber filter paper using a pipet. The flask was rinsed with pentane (2 × 2 mL) and filtered through the same pipet. The pentane filtrate was set under vacuum and all volatiles were removed. A white solid was obtained (393 mg, 1.06 mmol, 90% yield). 1H NMR (400 MHz, C6D6, 25 °C) δ: 3.04 (dd, J = 12.9, 5.6 Hz, 4H), 1.92 (d, J = 13.8 Hz, 4H), 1.88 (s, 12H), 1.31 (d, J = 2.7 Hz, 4H), 1.07 (s, 18H) ppm. 13C{1H} NMR (101 MHz, C6D6, 25 °C) δ: 125.74 (t, J = 5.3 Hz), 45.82 (m), 39.69 (m), 31.54 (m), 31.17 (m), 19.95 (m) ppm. 31 1 P{ H} NMR (162 MHz, C6D6, 25 °C) δ: −59.37 ppm. ATR-IR: 2945, 2930, 2905, 2861, 1473, 1463, 1380, 1361, 1241, 1218, 853, 813

structure demonstrates that our cobalt diphosphine platform is capable of supporting an acrylate ligand.

1

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CONCLUSION The cobalt complexes described herein represent our initial exploration of the fundamental coordination chemistry of our recently reported cis-macrocyclic diphosphines.33 The unique steric profile of these ligands1 has led to the isolation of an unusual halide-bridged cobalt(I) diphosphine dimer. Future work aims at further understanding and developing the coordination chemistry of these cis-macrocyclic diphosphines. The first examples of cobalt complexes capable of mediating the coupling of carbon dioxide and ethylene have also been described. Further work should focus on combining experimental and computational studies in order to identify the optimal reaction conditions and ligand framework to promote this transformation.

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EXPERIMENTAL SECTION

All manipulations were performed using standard Schlenk techniques or in a nitrogen atmosphere glovebox, unless otherwise stated. All reagents were purchased from Sigma-Aldrich, Alfa Aesar, or Strem Chemicals. Basic alumina was dried by heating at 250 °C under dynamic vacuum for at least 4 days prior to use. Solvents (EMD Chemicals) were either used as received or purified on a Glass Contour Solvent Purification System built by SG Water USA, LLC. High pressure reactions were conducted in a model no. 4792 50 mL high pressure/high temperature general purpose vessel from Parr Instrument Company. IR spectra were recorded on a Bruker Tensor 37 Fourier transform IR (FTIR) spectrometer. Elemental analyses were performed by Robertson Microlit Laboratories, Inc. NMR solvents were obtained from Cambridge Isotope Laboratories, degassed, and dried over 4 Å molecular sieves for at least 2 days prior to use. NMR spectra were obtained on a Varian 300 MHz, a Bruker 400 MHz, a Varian 500 MHz, or a JEOL 500 MHz spectrometer. 1H NMR and 13C NMR chemical shifts are referenced to residual protio-solvent signals, and 31P NMR spectra are referenced to a 85% H3PO4 external standard (δ = 0 ppm). Many of the 1H NMR and 13C NMR spectra display non-first-order multiplets; in those cases, please be aware that the J values reported are only the apparent J values. Energy-dispersive X-ray spectroscopy (EDX) data were collected on a JEOL 6610LV scanning electron microscope. Synthesis of (Cy2−DPC)CoCl2 (2). To a blue suspension of CoCl2 (104 mg, 0.8 mmol, 1 equiv) in THF (10 mL), a solution of Cy2− DPC1 (321 mg, 0.82 mmol, 1.02 equiv) in THF (15 mL) was added. The resulting mixture was stirred at room temperature overnight. After 21 h, the solution was turquoise with a lot of precipitate. The mixture was placed under vacuum and all volatiles were removed. The resulting solids were slurried in pentane (8 mL), filtered, and then washed with pentane (2 × 8 mL). After drying under vacuum, a turquoise powder was obtained (372 mg, 0.71 mmol, 89% yield). 1H NMR (500 MHz, CDCl3, 20 °C) δ: 264.90 (s, 1H), 176.13 (s, 1H), 136.91 (s, 1H), 105.32 (s, 1H), 20.61 (s, 1H), 18.59 (s, 3H), 8.55 (s, 1H), 5.75 (s, 1H), 4.89 (s, 3H), −2.37 (s, 2H), −3.05 (s, 1H), −4.87 (s, 1H), −14.06 (s, 1H), −15.06 (s, 1H), −17.57 (s, 1H), −18.53 (s, 1H) ppm. 31 1 P{ H} NMR (162 MHz, CDCl3, 25 °C) δ: silent. Evans Method (1% (Me3Si)2O in CDCl3, 20 °C): μeff = 4.44 μB. ATR-IR: 2920, 2853, 1439, 1380, 1198, 1045, 851, 834, 798, 448, 421 cm−1. Elemental analysis [%] found (calculated for C24H42Cl2CoP2): C 54.53 (55.18), H 7.77 (8.10), N < 0.02 (0). Synthesis of (Cy2−DPC)CoI2 (3). To a dark blue solution of CoI2 (150 mg, 0.48 mmol, 1 equiv) in THF (2 mL), a solution of Cy2− DPC (197 mg, 0.5 mmol, 1.05 equiv) in THF (7 mL) was added, yielding an orange-brown solution, which developed some brown precipitate. The mixture was stirred for 2 h; then volatiles were removed under vacuum. The solids were slurried in pentane (6 mL), filtered, and then washed with pentane (2 × 6 mL). After drying, a G


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Organometallics cm−1. Elemental analysis [%] found (calculated for C22H42P2): C 71.22 (71.70), H 10.75 (11.49), N < 0.02 (0). Synthesis of (Np2−DPC)CoCl2 (6). To a blue suspension of CoCl2 (145 mg, 1.12 mmol, 1 equiv) in THF (6 mL), a solution of Np2−DPC (413 mg, 1.12 mmol, 1 equiv) in THF (16 mL) was added. The resulting mixture was stirred at room temperature for 5 h, at which point the solution was dark green and almost homogeneous. The solution was filtered through a pipet microfiber glass filter and placed under vacuum to remove all volatiles. Et2O was added to the resulting solid; then all volatiles were removed under vacuum. This process was repeated twice, yielding a blue-green powder (532 mg, 1.07 mmol, 95% yield). 1H NMR (500 MHz, CDCl3, 20 °C) δ: 278.40 (s, 1H), 234.05 (s, 1H), 166.73 (s, 1H), 124.21 (s, 1H), 19.93 (s, 3H), 15.58 (s, 1H), 4.32 (s, 3H), −7.91 (s, 1H, overlapping with −8.23 ppm resonance), −8.23 (s, 9H) ppm. 31P{1H} NMR (162 MHz, CDCl3, 25 °C) δ: silent. ATR-IR: 2952, 2911, 2869, 1467, 1439, 1389, 1367, 1023, 855, 828, 817, 430 cm−1. Elemental analysis [%] found (calculated for C22H42Cl2CoP2): C 53.29 (53.02), H 8.31 (8.50), N < 0.02 (0). Synthesis of (Cy2−mDPC)Co(Me-allyl) (7). A slurry of (Cy2− DPC)CoCl2 (84 mg, 0.16 mmol, 1 equiv) in Et2O (12 mL) was placed in the cold well and frozen. To the thawing slurry, a solution of 1 M tert-butylmagnesium chloride in THF (0.4 mL, 0.4 mmol, 2.5 equiv) was added dropwise. The resulting slurry remained green for a few minutes, then slowly darkened as it warmed up and finally turned very dark brown. The reaction was allowed to stir at room temperature for 1 h; then the solvent was removed under vacuum. The residue was extracted with pentane (10 mL), filtered through a pipet microfiber glass filter, and washed with pentane (2 × 2 mL). This dark filtrate was placed under vacuum and all volatiles were removed. The residue was slurried in pentane (2 mL) and filtered through a pipet filter. The filtrate was black, and the precipitate yellow. The filtrate was discarded. The yellow precipitate was extracted using pentane (3 × 1.5 mL). The resulting yellow solution was placed in the freezer (−35 °C) overnight. The next day, orange crystals had formed. The mixture was filtered, and the crystals were washed with cold pentane (2 mL). The crystals were transferred to a tared vial and further dried under vacuum to yield orange crystalline material (21 mg, 0.04 mmol, 25% yield). 1H NMR (500 MHz, C6D6, 20 °C) δ: 4.93 (d, J = 1.9 Hz, 1H), 4.55 (s, 1H), 4.13 (dd, J = 305.5, 9.5 Hz, 1H, PH), 3.09−2.92 (m, 2H), 2.82 (dt, J = 11.5, 3.6 Hz, 1H), 2.62 (d, J = 5.8 Hz, 3H), 2.39 (t, J = 11.4 Hz, 1H), 2.13 (d, J = 12.7 Hz, 1H), 1.98−1.87 (m, 2H), 1.83 (d, J = 12.6 Hz, 1H), 1.76−1.29 (m, 24H), 1.16−0.91 (m, 10H), 0.67 (dd, J = 22.4, 4.3 Hz, 1H) ppm. 13C{1H} NMR (126 MHz, C6D6, 20 °C) δ: 153.24 (d, J = 6.7 Hz), 128.85 (m), 123.03, 97.42 (d, J = 10.9 Hz), 93.82 (d, J = 3.3 Hz), 65.42, 44.53 (dd, J = 12.5, 4.2 Hz), 44.36, 40.65 (m), 35.98 (m), 33.01 (d, J = 5.3 Hz), 32.56 (d, J = 8.5 Hz), 32.09 (d, J = 11.5 Hz), 29.52, 29.24 (d, J = 8.8 Hz), 29.14, 28.05 (d, J = 4.9 Hz), 27.60 (d, J = 6.1 Hz), 27.47 (d, J = 10.0 Hz), 27.25 (d, J = 6.6 Hz), 26.97 (d, J = 8.3 Hz), 26.65, 26.45, 25.80 (d, J = 16.3 Hz), 25.38, 23.65 (d, J = 10.3 Hz), 23.19, 20.25 ppm. 31P{1H} NMR (203 MHz, C6D6, 20 °C) δ: 37.12 (br s), 20.78 (br s) ppm. 31P{1H} NMR (202 MHz, toluene-d8, −50 °C) δ: 37.20, 23.63 (d, J = 308.0 Hz) ppm. ATR-IR: 2922, 2847, 2285 (P−H), 1590, 1448, 1175, 1114, 1027, 914, 866, 833, 740, 673, 569, 509, 469, 448, 424 cm−1. Elemental analysis [%] found (calculated for C28H49CoP2): C 66.68 (66.39), H 9.52 (9.75), N < 0.02 (0). Synthesis of [(Cy2−DPC)CoI]2 (8). Route 1: The turquoise (Cy2− DPC)CoCl2 complex (104 mg, 0.2 mmol, 1 equiv) was slurried in THF (3 mL). To this slurry, a dark blue solution of 0.1 M SmI2 in THF (2 mL, 0.2 mmol, 1 equiv) was added dropwise at room temperature. The color of the resulting mixture was dark brown. After stirring at room temperature overnight (16 h), the resulting heterogeneous mixture was filtered. The precipitate was washed with THF (3 × 4 mL) and dried under vacuum. An olive-green powder was obtained (76 mg, 0.13 mmol, 66% yield). The material obtained through this route is contaminated with ca. 10 mol % of the byproduct samarium(III) salt. The presence of this samarium-containing impurity was corroborated by energy-dispersive X-ray spectroscopy (atomic ratio Co:P:I:Sm:Cl = 1.0:2.1:1.4:0.1:0.1), as well as by elemental

analysis (calculated for C48H84Co2I2P4: C 49.84, H 7.32, N 0; found: C 47.72, H 6.83, N < 0.02). Route 2: Zinc dust (75 mg, 1.14 mmol, 4 equiv, 8-fold excess) was added to a solution of (Cy2−DPC)CoI2 (200 mg, 0.284 mmol, 1 equiv) in THF (8 mL). This reaction mixture was stirred at room temperature for 44 h, then filtered. In order to catch the very fine unreacted zinc, two layers of microfiber filter paper were set on top of a typical medium porosity fritted funnel. The solids collected (a mixture of [(Cy2−DPC)CoI]2 and zinc) were washed with THF (2 × 3 mL). The filter flask was then changed to a new one. The solids were extracted with toluene (7 × 2 mL), leaving behind only gray powder on the filter paper (excess zinc). The toluene filtrate was set under vacuum and all volatiles were removed. The solids were slurried in pentane (10 mL) and transferred to a tared vial; then all volatiles were removed and the solid dried. An olive-green powder was obtained (119 mg, 0.206 mmol, 72% yield). 1H NMR (500 MHz, C6D6, 20 °C) δ: 41.46 (s, 2H), 32.80 (s, 2H), 16.32 (s, 2H), 9.63 (s, 2H), 4.87−1.02 (m, 28H), −2.42 (s, 6H) ppm. 31P{1H} NMR (162 MHz, C6D6, 25 °C) δ: silent. Evans Method: the solubility of this complex was too poor for an accurate measurement. ATR-IR: 2918, 2848, 1441, 1066, 1022, 887, 851, 826, 792, 643, 448, 413 cm−1. Elemental analysis [%] found (calculated for C48H84Co2I2P4): C 50.33 (49.84), H 7.27 (7.32), N < 0.02 (0). Synthesis of [(Cy2−DPC)Co(CNtBu)3]I (9). Inside the glovebox, [(Cy2−DPC)CoI]2 (73 mg, 0.063 mmol, 1 equiv) was dissolved in toluene (4 mL). To this solution, a solution of tert-butyl isocyanide (97 mg, 1.17 mmol, 18 equiv) in toluene (2 mL) was added dropwise at room temperature. The resulting mixture changed color from the starting green-brown to orange during the addition, then shortly after a precipitate formed. The mixture was stirred at room temperature for 1 h, then filtered. The yellow precipitate was washed with pentane (3 × 3 mL), then dried under vacuum. A yellow powder was obtained (82 mg, 0.099 mmol, 78% yield). 1H NMR (500 MHz, CDCl3, 20 °C) δ: 2.60 (br s, 4H), 2.49 (d, J = 11.0 Hz, 4H), 2.08 (d, J = 10.7 Hz, 4H), 1.96 (d, J = 10.0 Hz, 4H), 1.82 (d, J = 9.4 Hz, 2H), 1.72 (s, 12H), 1.49 (s, 27H), 1.39−1.22 (m, 12H) ppm. 13C{1H} NMR (126 MHz, CDCl3, 20 °C) δ: 158.69, 127.73, 57.30, 42.79, 32.81, 31.19, 27.49, 27.22, 26.22, 22.59 ppm. 31P{1H} NMR (203 MHz, CDCl3, 20 °C) δ: 23.48 ppm. ATR-IR: 2974, 2917, 2850, 2133 (CN), 2027 (CN), 1445, 1368, 1198, 820, 650, 545, 522, 449 cm−1. Elemental analysis [%] found (calc. for C39H69CoN3P2I): C 56.23 (56.59), H 8.20 (8.40), N 4.94 (5.08).

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org/. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00734. Full experimental, crystallographic (CCDC 1541575, 1541579, 1541581, 1541582, 1548011), and spectroscopic data (PDF) Accession Codes

CCDC 1541575, 1541579, 1541581−1541582, and 1548011 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Christopher C. Cummins: 0000-0003-2568-3269 H


Article

Organometallics Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank SABIC (Saudi Arabia Basic Industries Corporation) for funding this work. M.-A.C. acknowledges the NSERC for a postdoctoral fellowship. Dr. Peter Müller and Dr. Jonathan Becker are gratefully acknowledged for their helpful advice on crystallographic refinement. Xray diffraction data were collected on an instrument purchased with the aid of the National Science Foundation (NSF) under CHE-0946721. We would also like to thank Dr. Bruce Adams for his assistance and guidance with advanced NMR spectroscopy experiments.

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Cobalt Complexes Supported by cis-Macrocyclic Diphosphines

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