Influences of Bifunctional PNP-Pincer Ligands on Low Valent Cobalt

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Influences of Bifunctional PNP-Pincer Ligands on Low Valent Cobalt Complexes Relevant to CO2 Hydrogenation Matthew R. Mills, Charles L. Barnes, and Wesley H. Bernskoetter* Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States S Supporting Information *

ABSTRACT: Pincer ligated coordination complexes bearing bifunctional sites have been at the center of recent developments in reversible hydrogenation catalysis, especially in cases utilizing base metals. The influence of bifunctional ligands on low valent cobalt complexes is detailed here using comparisons between the PNP-pincer ligands MeN[CH2CH2(PR2)]2 and HN[CH2CH2(PR2)]2 (R = iPr, Cy). Comparative catalytic studies of CO2 hydrogenation show that cobalt(I) precatalysts bearing the tertiary amine ligand dramatically outperform those bearing the secondary amine pincer ligand. Despite strong similarities between the precatalyst ground state structure and the redox potentials of the two systems, ligand bifunctionality was found to be detrimental to catalyst productivity. The enhanced stability imparted by the MeN[CH2CH2(PR2)]2 ligand also enabled isolation and characterization of a zero-valent cobalt dicarbonyl species, which was used to study the catalytically active oxidation state of cobalt in CO2 hydrogenation.

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INTRODUCTION Coordination compounds with bifunctional ligands have found wide application in homogeneous catalysis.1,2 Bifunctional ligands are typically chelators containing one or more sites that can be readily protonated or deprotonated, often in concert with other transformations at the proximal metal.3 The site of protonation/deprotonation is typically a heteroatom4−7 or the benzylic position of a ligand containing an arene or pyridine ring (Figure 1).8 Over the past decade or more,

application of bifunctional ligands in the asymmetric hydrogenation of aromatic ketones using ruthenium, which inspired a wealth of catalyst development in subsequent years.16−21 Bifunctional sites contained within the pincer ligand framework have been found to be particularly effective for reversible hydrogenation reactions.22 Milstein, Beller and other contributors have repeatedly demonstrated the utility of bifunctional pincer ruthenium complexes for the catalytic hydrogenation of esters23 and amides,24 the acceptorless dehydrogenation of secondary alcohols to form ketones,25 and dehydrogenative coupling of alcohols to form esters26 or alcohols and amines to form amides.27 These significant advances in reversible hydrogenation catalysis coupled with the high cost and scarcity of precious metals have shifted attention to the study of first-row metal congeners.28−36 Among base metals, PNP-iron and cobalt species have been particularly successful in hydrogenation reactions. For example, Jones and co-workers have described the bifunctional RPNP-Fe catalysts (RPNP = N[CH2CH2(PR2)]2, R = iPr, Cy), iPrPNPFeH(CO), and iPrPNHPFeH(CO)(HBH3) for hydrogenating ketones to secondary alcohols and alkenes to saturated hydrocarbons.37,38 Additionally, the hydrogenation of amides to yield amines and alcohols has been reported by several groups using related catalysts.33,39 Beller and co-workers have emphasized the importance of the proton-labile nitrogen on the pincer ligand by showing that the hydrogenation of esters5 and nitriles6 proceeds smoothly with iPrPNHPFeH(CO)(HBH3) but not with the N-methylated analogue iPrPNMePFeH(CO)(HBH3). Reactions that cobalt pincer complexes have been shown to

Figure 1. Common routes of H2 activation in metal complexes with bifunctional ligands.

transition metal catalysts supported by bifunctional ligands have seen a notable increase in research activity.9,10 The vanguard of this catalyst development approach primarily focused on the use of precious metals, with several significant advancements made in C−C and C−N bond-forming reactions utilizing ruthenium and iridium catalysts.11−15 Moreover, bifunctional ligands have been especially impactful in the area of catalytic hydrogenation and dehydrogenation reactions. The ability of the bifunctional ligand site and the metal to work cooperatively in splitting and forming H2 has been crucial in these reactions. Noyori and co-workers provided a seminal example of this © XXXX American Chemical Society

Received: November 17, 2017

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DOI: 10.1021/acs.inorgchem.7b02931 Inorg. Chem. XXXX, XXX, XXX−XXX

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catalyze include direct40 and transfer41 hydrogenation of nitriles to amines; hydrogenation of alkenes, imines, ketones, and aldehydes;7 and the dehydrogenation of alcohols to form aldehydes and ketones.42 In contrast to much of the recent pincer base-metal catalysis described above, work in our laboratory has found that the PNP-iron catalyzed hydrogenation of carbon dioxide (CO2) is greatly enhanced by “blocking” the bifunctionality of the ligand with the addition of a methyl group to the central nitrogen atom on the ligand.43 The reversible hydrogenation of CO2 is a particularly interesting target as it could be used to produce either formic acid or methanol to serve as a chemical hydrogen storage pathway, enabling energy storage and transportation with isolated or intermittent producers.44−47 In this context, it is significant that both turnover number (TON) and turnover frequency (TOF) for CO2 to formate conversion were more than 20-fold greater for the N-methyl iron catalyst iPr PNMePFeH(CO)(HBH3) complex compared to the analogous iPrPNHPFeH(CO)(HBH3) complex. The effect of bifunctional ligands on cobalt-catalyzed (de)hydrogenation reactions is also a focus of several research efforts. Several reported cobalt systems emphasize why the study of bifunctionality remains an important field of study. Hanson and co-workers have reported cobalt catalysts of the type [CyPNRPCo(CH2SiMe3)]BArF4 (R = H, CH3). The secondary amine ligated catalyst is effective for the hydrogenation of alkenes and ketones and the dehydrogenation of secondary alcohols.48 However, the tertiary amine form of the catalyst is only effective for alkene hydrogenation and alcohol dehydrogenation; no conversion is achieved for hydrogenation of ketones. Jones and co-workers have reported that both forms of Hanson’s catalyst are effective for ester hydrogenation. Elsevier, de Bruin, and co-workers reported the hydrogenation of esters and carboxylic acids using Co(triphos) (triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane).49 Although the triphos ligand itself does not contain a bifunctional site, the proposed mechanism indicates the carboxyl moieties act as transient bifunctional ligands in the concerted 1,2-addition of H2. Our group has recently reported a cobalt pincer complex, [iPrPNMePCo(CO)2]Cl (Figure 2), that catalyzes the hydro-

RESULTS AND DISCUSSION Bifunctional PNP Ligand Influences on Catalytic CO2 Hydrogenation. Prior investigations of iron PNP catalysts have found that the presence of a methyl group on the central nitrogen donor of the PNP ligand enhances the TON for carbon dioxide hydrogenation approximately 20-fold.43 This contrasts other reactions performed with similar pincer iron catalysts in which metal−ligand cooperativity plays a major role in catalysis.51,52 In order to gain insight into the influence of bifunctional ligands in the related cobalt chemistry, the secondary amine substituted complex [iPrPNHPCo(CO)2]Cl (1a, Figure 3) complex was prepared and its catalytic performance tested.

Figure 3. Cobalt pincer catalysts described in this work.

Complex 1a was prepared in the same manner as the Nmethyl congener, by mixing the neutral ligand with (PPh3)3CoCl in the presence of carbon monoxide. Crystals suitable for X-ray diffraction were obtained by slow diffusion of diethyl ether into an acetonitrile solution of 1a. The molecular structure of 1a is depicted in Figure 4, and selected metrical parameters are listed in Table 1. In comparison to 2a and 2b, 1a recrystallizes readily, likely due to the N−H···Cl interaction, which has a hydrogen bonding interaction of roughly 2.26 Å (there are two molecules in the asymmetric unit). Complex 1a is a distorted square pyramid (τ = 0.27),53 with a P(1)− Co(2)−P(2) angle of 159.27(2)°, and a N(1)−Co(1)−C(2) of 143.05(6)°. The Co(1)−N(1) distance of 2.061(1) Å shows a slight contraction compared to 2a (Table 1), possibly due to the sterically smaller nitrogen substituent. The performance of 1a as a precatalyst for the hydrogenation of carbon dioxide was also compared to 2a, using conditions previously optimized for the tertiary amine congener. In this reaction, the substitution of the N-Me substituent by N−H decreased the TON for formate production by a factor of approximately 50 (Table 2), indicating that metal−ligand cooperativity does not hold a particular advantage for CO2 hydrogenation in these cobalt systems, though it is not clear if this originates from a lower native activity or a more rapid catalyst decomposition. The PCy2 substituted variant of 2a, [CyPNMePCo(CO)2]Cl (2b), was also prepared and its catalytic performance was analyzed for any ancillary ligand influence. In contrast to 2a and 1a, 2b proved significantly more difficult to recrystallize as the chloride salt, instead precipitating from solution as a fluffy tan solid. Exchanging the chloride anion for BArF4− (BArF4 = B(3,5(CF3)2C6H3)) enabled the growth of crystals of sufficient quality for the determination of structure by X-ray crystallography (Figure 4). Metrical parameters about the cobalt center of 2b are nearly isostructural with those observed in 2a (Table 1). Additionally, the catalytic hydrogenation of carbon dioxide using the chloride salt of 2b performed similarly to 2a, showing only a slightly reduced TON relative to the isopropyl analogue (Table 1). This observation suggests that the steric effect from the phosphine donors of the PNP ligand has only a limited influence. Electronic differences between 2a and 2b were

Figure 2. Lewis acid-assisted, cobalt-catalyzed hydrogenation of carbon dioxide.

genation of carbon dioxide to formate in the presence of lithium triflate and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), achieving a TON of 29 000, which is the current benchmark productivity for cobalt catalysts in this field.50 Given the interest in the role of bifunctional ligands in enhancing hydrogenation catalysis, our attention was immediately drawn to studying its influence in this system. Our investigations into the impact of bifunctionality in these catalytic cobalt systems as well as other mechanistic considerations of CO2 hydrogenation are described herein. B

DOI: 10.1021/acs.inorgchem.7b02931 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Molecular structures of 1a (left) and 2b (right) at 30% ellipsoids. All hydrogen atoms (except that attached to nitrogen) as well as a noncoordinating [BArF4] anion and disordered solvent molecule for 2b have been removed for clarity. Two nearly identical molecules were found in the asymmetric unit of 1a; only one is shown here.

cobalt(II) followed by rapid disproportionation of the transient cobalt(II) species. This motivated an alternative synthetic approach beginning with the cobalt(II) complex, [CyPNMePCo(CH2SiMe3)]BArF4, previously reported by Hanson and coworkers. 7 However, addition of carbon monoxide or dihydrogen to [CyPNMePCo(CH2SiMe3)]BArF4 again resulted in an intractable mixture of diamagnetic species, hinting that cobalt(II) species related to 2a/2b may be inherently unstable. This instability is reinforced by Jones and co-workers who have recently reported that [CyPNHPCo(CH2SiMe3)]BArF4 decomposes to [CyPNHPCo(CO)2]BArF4 in the presence of 1 bar of carbon monoxide.54 In order to better determine the feasibility of preparing the desired complex, electrochemical studies were performed. Cyclic voltammetry experiments revealed that oxidation of 2a at 0.62 V (v. Fc) was essentially irreversible even at 500 mv s−1 (Figure 5; Table 3). After oxidation, the return wave shows three smaller reduction peaks, further indicating instability of the resulting cobalt(II) species (Figure 5). Interestingly, examination of the voltammogram does indicate a reduction of 2a at −1.67 V, which appears reversible given the separation and similar magnitude of the anodic and cathodic peaks as well as a linear dependence of the anodic and

Table 1. Selected Bond Distances (Å) for 1a, 2a, 2b, and 3b 1a Co−N(1) Co−P(1) Co−P(2) Co−C(1) Co−C(2) C(1)−O(1) C(2)−O(2)

2.061(1) 2.2206(6) 2.2225(6) 1.800(1) 1.721(1) 1.144(2) 1.160(2)

2a

50

2.117(2) 2.2350(4) 2.2350(4) 1.764(2) 1.744(2) 1.152(3) 1.155(3)

2b

3b

2.113(7) 2.2246(6) 2.2275(6) 1.731(2) 1.774(2) 1.155(2) 1.148(3)

2.2341(6) 2.2298(6) 1.776(2) 1.787(2) 1.153(2) 1.155(2)

Table 2. Comparison of PNP Supported Cobalt Precatalysts for CO2 Hydrogenationa

catalyst

TONb

2a 2b 1a

29,000c 24,000 450

a Conditions: 0.3 μmol catalyst, 3.6 g (23.6 mmol) of DBU, 500 mg (3.2 mmol) of LiOTf, 5 mL of MeCN, 500 psi CO2, 500 psi H2, 45 °C, 16 h. bAverage value of three trials. cFrom ref 50.

viewed as very minor given electrochemical data collected for both species (vida inf ra). Redox Behavior of [PNPCo] Pincer Complexes. Despite the state-of-the-art productivity of 2a/2b for cobalt catalysts in CO2 hydrogenation, very limited information is available regarding the mechanism of action.50 Indeed, even the metal oxidation states of the precatalysts that facilitate CO2 reduction has not been firmly established (though extensive prior work does suggest that the active species is homogeneous). In order to investigate the influence of the cobalt precatalyst oxidation state, attempts were made to prepare analogues of 2a/2b in the more common cobalt(II) oxidation state. The direct oxidation of 2a and 2b by ferrocenium tetrafluoroborate proved unsuccessful in isolating cobalt(II) species, instead producing a mixture containing mostly diamagnetic compounds. This magnetism is inconsistent with the presence of cobalt(II) species and may result from an initial oxidation of cobalt(I) to

Figure 5. Cyclic voltammogram of 2a in acetonitrile. Conditions: mM 2a, 0.1 M tert-butyl ammonium hexafluorophosphate (TBAPF6), 500 mV s−1. C

DOI: 10.1021/acs.inorgchem.7b02931 Inorg. Chem. XXXX, XXX, XXX−XXX

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heated or placed under a vacuum. Like 3b, this brown material was 1H NMR silent in the region between 100 and −50 ppm. However, 3b and its thermal conversion product could be distinguished by IR spectroscopy and UV−vis spectrophotometry. The solution IR spectrum of 3b displays bands at 1962 and 1863 cm−1, indicative of a dicarbonyl complex. When heated at 45 °C for 6 h these bands decrease in intensity, and a new band with a shoulder appears at 1822 cm−1 (Figure 7, left). These spectral changes, along with the chemical behavior under a vacuum, are consistent with a return to κ3-CyPNMeP coordination concomitant with loss of one CO ligand to form CyPNMePCo(CO) (4b) (Figure 8). The shoulder on the carbonyl stretch of 4b may be the result of two isomeric forms as the N-Me fragment renders the two faces of the PNP ligand inequivalent. A similar study monitoring the UV−vis spectrum also showed a smooth conversion with clean isosbestic points (411 and 660 nm) and the appearance of two new absorbances at 490 and 592 assigned to 4b (Figure 7; right). Analysis of the IR and UV−vis data both yielded similar observed rate constants of (1.66 ± 0.03) × 10−4 s−1 (UV−vis) and (1.5 ± 0.1) × 10−4 s−1 (IR) at 45 °C for the conversion of 3b to 4b. Despite considerable effort, we have failed to obtain crystals of 4b suitable for X-ray diffraction. Likewise, synthetic experiments starting from 2a afforded visually analogous chemistry, but no structural information. As mentioned earlier, the oxidation state of the active cobalt species during CO2 hydrogenation has not been conclusively determined; thus the availability of 3b provided an excellent opportunity to probe the competency of a cobalt(0) species in catalysis. Complex 3b was employed as a precatalyst for formate production under the previously optimized conditions described above (Table 2). After 16 h 3b afforded a TON of 3200 for CO2 hydrogenation, which is moderately productive, but far below the 24 000 TON achieved by the cobalt(I) congener 2b. These data suggest that cobalt(0) is not part of the active catalytic cycle for 2b. The modest activity observed from 3b could originate from partial conversion to the active form, or a separate but less productive cycle including a cobalt(0) intermediate. The possible mediation of CO2 reduction by the cobalt(0) monocarbonyl species 4b was also assayed. Aging a stock solution of 3b for 24 h in order to generate 4b and then applying the catalytic conditions yielded a TON of only 240. This dramatic decline from even 3b indicates that 4b is also an unlike intermediate in 2b catalyzed CO2 hydrogenation. While these experiments along with the observed instability of cobalt(II) congeners by cyclic voltammetry cannot be used to conclusively establish the active oxidation states reached by 2a/2b catalysis, this information is consistent with a cobalt(I) and/or cobalt(III) mediated pathway of formate production.

Table 3. Reversible Reduction Potential for Co(I/0) Redox Couple (v Fc) complex

CoI/Co0 (v Fc)

2a 2b 1a

−1.67 V −1.71 V −1.79 V

cathodic peak currents relative to the square root of the scan rate (Figure S3). Although isolation of the initially targeted cobalt(II) species appeared unlikely, the reversibility of the cobalt(I/0) couple presented an opportunity to prepare the cobalt(0) congener of 2a/2b. Such a species would be of interest as metal hydrides generated from a cobalt(0) precursor should be more hydridic than those generated from cobalt(I), a feature that has been linked to the performance of other cobalt CO2 hydrogenation catalysts.55 The synthesis CyPNMePCo(CO)2 (3b) was achieved by treatment of a suspension of 2b with KC8 in either toluene or THF which produced a green solution. After filtration and removal of solvent the green solution became a brown oil. Extraction of the oil with pentane afforded a green solution and left behind a brown residue. Crystals of 3b were obtained by cooling the pentane solution, and the molecular structure is shown in Figure 6 with select bond distances listed in Table 1.

Figure 6. Molecular structure of 3b at 30% ellipsoids. All hydrogen atoms have been removed for clarity.

The crystal structure reveals a dramatic lengthening of the Co− N distance from 2.113(7) Å in the Co(I) complex to over 2.7 Å in the Co(0) analogue, significantly longer than other Co−N bonds found in pincer complexes.56−60 Thus, 3b is best described as a 17-electron distorted tetrahedral complex with a κ2-PNP ligand. The κ2 structure has precedent in the reaction of 2a with trialkyl borohydrides to yield an equilibrium mixture of κ2-iPrPNMePCo(CO)2H and κ3-iPrPNMePCo(CO)(H).50 Similar κ3 to κ2 hapticity changes have also been reported by Gray and Wong in cobalt(II) dihalide species supported by a (PhCH2)N{CH2CH2(PPh2)}2 ligand.61 Most other metrical parameters for 3b are similar to 2b (Table 2), notably including the carbonyl C−O bond distances. Complex 3b is fairly stable in the solid state at ambient temperature; however, in solution it decays into a brown oil similar to the residue left behind during its initial isolation. This conversion appears to be accelerated when the solution is

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CONCLUDING REMARKS The work presented here offers insight into the differences in reactivity and coordination chemistry of PNP-cobalt complexes brought about by the presence of a bifunctional amine moiety. In terms of coordination chemistry, very little impact is observed from the N−H versus N-Me substituents on the cobalt(I) compounds 1a, 2a, and 2b. The cobalt(I) dicarbonyl species exhibit very similar metrical parameters, with only a slight elongation of the Co−N distance in the N-Me congeners. Alternatively, the presence of a bifunctional ligand in 1a has a significant deleterious impact on the catalytic CO2 hydrogenation formate. The TONs of 29 000 and 24 000 obtained D

DOI: 10.1021/acs.inorgchem.7b02931 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Infrared and UV−vis spectroscopic traces of the reaction of κ2-CyPNMePCo(CO)2 at 45 °C in toluene. The insert displays the kinetic trace for the growth of 4b.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out using standard vacuum, Schlenk, cannula, or glovebox techniques. Hydrogen, carbon dioxide, and high purity carbon monoxide were purchased from Airgas and used as received. (Ph3P)3CoCl, MeN{CH2CH2(PR2)}2 (R = iPr, Cy), and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate were prepared as previously described.5,65,66 All other chemicals were purchased from Aldrich, Fisher, VWR, Strem, or Cambridge Isotope Laboratories. Nonvolatile solids were dried under a vacuum at 60 °C for 2 days. 1,8-Diazabicycloundec-7-ene (DBU) was dried over CaH2 and twice distilled prior to use. Solvents were dried and deoxygenated using literature procedures.67 1H, 13C, and 31P NMR spectra were recorded on Bruker 300 MHz DRX, 500 MHz DRX, or 600 MHz spectrometers at ambient temperature, unless otherwise noted. 1H and 13C chemical shifts are referenced to residual solvent signals; 31P chemical shifts are referenced to an external standard of H3PO4. Probe temperatures were calibrated using ethylene glycol and methanol as previously described.68 IR spectra were recorded on a Thermo-Nicolet FTIR. X-ray crystallographic data were collected on a Bruker CCD or CMOS diffractometer with Mo−Kα radiation or Cu−Kα radiation. Samples were collected in inert oil and quickly transferred to a cold gas stream. The structures were solved from direct methods and Fourier syntheses and refined by full-matrix least-squares procedures with anisotropic thermal parameters for all non-hydrogen atoms. Crystallographic calculations were carried out using the SHELX programs. Cyclic voltammetry measurements were conducted using CH Instruments model 700D potentiostat. Samples were prepared to 3 mM in a 0.1 M solution of TBAPF6 in acetonitrile solution under inert atmosphere. A three-electrode cell, consisting of a glassy carbon working electrode, platinum counter electrode, and silver wire pseudoreference electrode was used. Upon completion of the analysis, ferrocene (5 mM) was added to the samples as an internal standard. High-pressure catalytic CO2 hydrogenation reactions were performed using a Parr 5500 series compact reactor with glass insert. Elemental analyses were performed at Robertson Microlit Laboratory in Ledgewood, NJ. Preparation of [CyPNMePCo(CO)2]Cl (2b). This compound was prepared in a manner analogous to that reported for [iPrPNMePCo(CO)2].50 In a nitrogen-filled glovebox, 440 mg (0.5 mmol) of (PPh3)3CoCl was added to a 100 mL high-pressure flask, suspended in 5 mL of diethyl ether, and then frozen in a cold well. Once frozen, a solution of 264 mg (0.55 mmol) of CyPNMeP ligand in 5 mL of diethyl ether was carefully layered on top and also frozen. The flask was sealed, quickly removed from the glovebox, and kept in liquid nitrogen. The flask was evacuated and pressurized with 400 Torr of carbon monoxide on a high vacuum line. The flask was allowed to stir

Figure 8. Proposed conversion of 3b to 4b via loss of one carbon monoxide ligand.

using 2a and 2b, respectively, dramatically outperform 1a which affords a TON of only 450. These results mirror the relative activity reported for bifunctional/nonbifunctional iron catalysts applied to the same CO2 hydrogenation reaction.43 This clearly demonstrates that despite the recent and impressive successes in using bifunctional ligands to develop base-metal catalysis,6,8,29,35,62−64 such frameworks are not universally advantageous. The electrochemically measured redox potentials of 1a, 2a, and 2b indicates only modest influence from the bifunctional ligand on the redox chemistry. Each complex exhibits similar cyclic voltammograms with irreversible oxidation waves near 0.62 V, suggesting the cobalt(II) congeners are relatively unstable. Additional reversible reduction waves near −1.7 V were also observed for 1a, 2a, and 2b; however chemical reduction experiments indicate substantial differences in the stability of the Co(0) congeners. Reduction of 2b afforded an isolable κ2-PNMePCo(CO)2 complex which later isomerized with loss of CO to CyPNMePCo(CO). On the other hand, chemical reduction of 1a initially formed a red solution which rapidly decayed to a mixture containing both diamagnetic and paramagnetic species. It is quite possible that these differences in redox behavior are linked to the differences in catalytic performance, though it should be noted that the isolated cobalt(0) complexes supported by CyPNMeP do not perform as well in CO2 reduction as the cobalt(I) congeners. It is our hope that elucidating these subtle differences in the coordination chemistry between bifunctional and more traditional pincer ligands will enable the field to more rationally approach the development of base metal catalysts, to understand when each ligand class may be advantageous, and to gain more insight into recent catalytic breakthroughs. E

DOI: 10.1021/acs.inorgchem.7b02931 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry at ambient temperature for 4 h. The flask was then degassed and returned to the nitrogen-filled glovebox. The resulting tan suspension was filtered, and the precipitate was washed repeatedly with diethyl ether followed by small amount of tetrahydrofuran. The remaining solid was then taken up in acetonitrile, filtered, and precipitated using diethyl ether. The tan solid was dried under a vacuum to give 100 mg (33% yield) of (2b). 1 H NMR (600 MHz, CD3CN): δ 2.86−2.70 (4H, m, NCH2CH2), 2.47 (2H, m, NCH2CH2), 2.40 (3H, s, NCH3), 2.38 (2H, m, NCH2CH2), 2.25 (4H, d, Cy), 2.18 (2H, d, Cy), 1.98 (6H, d, Cy), 1.87 (6H, q, Cy), 1.75 (4H, b, Cy), 1.61 (4H, m, Cy), 1.51−1.24 (16H, m, Cy). 31P NMR (121 MHz, CD3CN): δ 80.42 (s). 13C NMR (75 MHz, CD3CN): 65.49 (NCH2CH2), 53.47 (NCH3), 37.56 (t, J = 12 Hz, PCH(CH3)2), 37.18 (t, J = 13.1 Hz, PCH(CH3)2), 28.99, 28.62, 27.84−27.19 (m) 26.47, 26.37 (PCy), 24.53 (t, J = 9.8, NCH2CH2) IR (KBr): νCO = 1961, 1908 cm−1. Anal. Calcd for C31H55ClCoNO2P2: C, 59.09; H, 8.80; N, 2.22. Found: C, 58.86; H, 8.75; N, 2.19. Conversion of 2b to [CyPNMePCo(CO)2]BArF4. A solution of 51 mg of NaBArF4 (0.05 mmol) in diethyl ether was added to a suspension of 31 mg (0.05 mmol) 2b in diethyl ether. The suspension quickly became an orange solution, which was filtered to remove precipitated sodium chloride and the filtrate was concentrated under a vacuum. The concentrated solution was held at −35 °C for several days in order to obtain crystals of [CyPNMePCo(CO)2]BArF4 as orange blocks. The spectral features of this complex were identical to 2b except for peaks associated with the noncoordinating anion. Preparation of [iPrPNHPCo(CO)2]Cl (1a). In a nitrogen-filled glovebox, 837 mg (0.95 mmol) of (PPh3)3CoCl was added to a 100 mL high-pressure flask, suspended in 5 mL of diethyl ether, and the suspension was frozen in a cold well. Once frozen, a solution of 3.05 g (10% solution w/w in THF, 1.0 mmol) iPrPNHP ligand was carefully layered on top and also frozen. The flask was sealed, quickly removed from the glovebox, and kept in liquid nitrogen. The flask was evacuated and pressurized with 400 Torr of carbon monoxide on a high vacuum line. The flask was allowed to stir at ambient temperature for 4 h. The flask was then degassed and returned to the nitrogen-filled glovebox. The resulting orange suspension was filtered, and the precipitate was washed repeatedly with diethyl ether followed by a small amount of tetrahydrofuran to give 396 mg (99%) of 1a. Crystals of sufficient quality for determination of structure by X-ray crystallography were grown by diffusion of diethyl ether into a solution of 1a in acetonitrile at ambient temperature. 1H NMR (300 MHz, CD3CN): δ 6.58 (s, 1H, NH), 3.25−3.12 (m, 2H, NCH2CH2P), 2.56−2.35 (m, 4H, PCH(CH3)2), 2.31−2.02 (m, 6H, NCH2CH2P (2H) & NCH2CH2P (4H)), 1.43−1.22 (m, 24H, PCH(CH3)2). 31P NMR (121 MHz, CD3CN): δ 99.71 (s). 13C NMR (75 MHz, CD3CN): 55.01 (NCH2CH2), 27.84 (t, J = 13.2 Hz, PCH(CH3)), 26.83 (t, J = 13.7 Hz, PCH(CH3)), 25.40 (t, J = 10.5 Hz, NCH2CH2P), 19.41, 18.14, 18.02, 17.54. IR (KBr) νCO = 1970, 1906 cm−1. Anal. Calcd for C18H37ClCoNO2P2: C, 47.43; H, 8.18; N, 3.07. Found: C, 47.47; H, 8.16; N, 2.98. Preparation of κ2-CyPNMePCo(CO)2 (3b). Inside a nitrogen-filled glovebox, 100 mg (0.16 mmol) of 2b was suspended in 2 mL of THF and chilled to −35 °C. Potassium graphite (22 mg, 0.16 mmol) was suspended in 2 mL THF at −35 °C and added to the suspension of 2b. The reaction mixture turned from a tan suspension to a dark green solution. This was filtered to remove graphite and KCl, and the solvent was removed under a vacuum. The green-brown residue was taken up in pentane and recrystallized at −35 °C to give 41 mg (45% yield) of 3b as bright green crystals. IR (toluene) νCO = 1962, 1863. General Procedure for the Hydrogenation of Carbon Dioxide. Inside the glovebox, 3.2 mmol of LiOTf, 24 mmol of DBU, 0.3 μmol of catalyst, and 5 mL of acetonitrile were combined in a glass insert which was then placed into a Parr reactor. The Parr reactor was assembled and removed from the glovebox, pressurized with 500 psi each of carbon dioxide and hydrogen, then heated to 45 °C for 16 h. The reactor was then placed on ice to cool, vented, and opened. The reaction mixture was transferred to a round-bottom flask, and the reactor was rinsed several times with D2O to dissolve any

remaining residue. The washes were also transferred to the roundbottom flask, and the bulk of the solvent was removed by rotary evaporation. The resulting formate was quantified by 1H NMR spectroscopy using dimethylformamide as an internal standard.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02931. Additional experimental data (PDF) Accession Codes

CCDC 1585644−1585646 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 [email protected], 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

Matthew R. Mills: 0000-0001-8975-2855 Wesley H. Bernskoetter: 0000-0003-0738-5946 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This article is based upon work supported by the National Science Foundation (CHE-1350047) and the Curators of the Univ. of Missouri. W.H.B. is a fellow of the Alfred P. Sloan Foundation.

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REFERENCES

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