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Oct 2, 2017 - Synthesis and Catalytic Activity of PNP-Supported Iron Complexes with Ancillary Isonitrile Ligands. Nicholas E. Smith,. †. Wesley H...
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Synthesis and Catalytic Activity of PNP-Supported Iron Complexes with Ancillary Isonitrile Ligands Nicholas E. Smith,† Wesley H. Bernskoetter,*,‡ Nilay Hazari,*,† and Brandon Q. Mercado† †

The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States The Department of Chemistry, The University of Missouri, Columbia, Missouri 65211, United States

‡

S Supporting Information *

ABSTRACT: Pincer-supported iron complexes of the form [(iPrPNHP)Fe(H)(HBH3)(CO)] (iPrPNHP = HN(CH 2 CH 2 P i Pr 2 ) 2 ), [( i P r PN H P)Fe(H){OC(O)H}(CO)], [(iPrPNP)Fe(H)(CO)]; iPrPNP = N(CH2CH2PiPr2)2−) have proven to be a privileged class of catalysts for hydrogenation and dehydrogenation reactions. Here, the synthesis, characterization, and reactivity of a family of complexes related to these species, in which the ancillary carbonyl ligand has been replaced with an aryl isonitrile ligand, is described. The complexes [(iPrPNHP)Fe(Cl)2(CNR)] (R = 2,6-dimethylphenyl (1a), 4methoxyphenyl (1b)) were prepared in good yield through the dropwise addition of the aryl isonitrile to an in situ generated solution of [(iPrPNHP)Fe(Cl)2]. If the aryl isonitrile was added too quickly, significant amounts of the cationic bis(isonitrile) complexes [(iPrPNHP)Fe(Cl)(CNR)2][Cl] (R = 2,6-dimethylphenyl (2a); 4-methoxyphenyl (2b)) were also formed. Treatment of 1a with excess NaBH4 generated [(iPrPNHP)Fe(H)(HBH3)(C NR)] (3a), but the corresponding reaction with 1b was unsuccessful. In contrast, the reaction of 1a or 1b with 1 equiv of n Bu4NBH4 generated the hydrido chloride complexes [(iPrPNHP)Fe(H)(Cl)(CNR)] (R = 2,6-dimethylphenyl (4a), 4methoxyphenyl (4b)). The reaction of 4a or 4b with KOtBu resulted in deprotonation of the pincer ligand and the formation of the five-coordinate amido complexes [(iPrPNP)Fe(H)(CNR)] (R = 2,6-dimethylphenyl (5a), 4-methoxyphenyl (5b)). The ability of the pincer ligand to participate in bifunctional reactivity was demonstrated through the reactions of 5a,b with H2 and a H2/CO2 mixture, which generated the unstable complexes [(iPrPNHP)Fe(H)2(CNR)] (R = 2,6-dimethylphenyl (6a), 4methoxyphenyl (6b)) and [(iPrPNHP)Fe(H){OC(O)H}(CNR)] (R = 2,6-dimethylphenyl (7a), 4-methoxyphenyl (7b)), respectively. The five-coordinate amido complexes 5a,b were shown to be catalysts for the hydrogenation of CO2 to formate. Complexes 1a,b, 2a, 4b, 5a, and 7a were characterized by X-ray crystallography.

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INTRODUCTION Traditionally, homogeneous catalysts for both hydrogenation and dehydrogenation reactions have been based on precious metals such as iridium and ruthenium.1 As a result of the high cost, limited supply, and in some cases toxicity of precious metals, there is currently significant interest in the development of catalysts based on earth-abundant first-row transition metals, such as iron.2 In recent years, iron complexes supported by the bifunctional HN(CH2CH2PR2)2 (RPNHP; R = Et, iPr, Cy) pincer ligand, as well as an ancillary CO ligand, have been demonstrated to be remarkably active catalysts for the hydrogenation or dehydrogenation of both organic molecules and substrates related to energy applications (Figure 1).3 For example, the related precatalysts [(RPNHP)Fe(H)(HBH3)(CO)] and [(RPNHP)Fe(H){OC(O)H}(CO)] have been shown to be active for the hydrogenation of esters,4 ketones,4c,5 nitriles,6 amides,7 olefins,8 nitrogen-containing heterocycles,9 and CO2.10 The same complexes have also shown high activity for the dehydrogenation of formic acid,11 methanol,12 primary and secondary alcohols,4c,5 polyols such as glycerol,13 nitrogencontaining heterocycles,9 and ammonia−borane.14 Additionally, © XXXX American Chemical Society

they have been used as catalysts for the dehydrogenative synthesis of lactones, 15 lactams,15 and amides. 16 Both [(RPNHP)Fe(H)(HBH3)(CO)] and [(RPNHP)Fe(H){OC(O)H}(CO)] are proposed to access the coordinatively unsaturated five-coordinate iron(II) complex [(RPNP)Fe(H)(CO)] (RPNP = N(CH2CH2PR2)2−) in situ, and this species can be isolated and used directly as a catalyst. In many cases iron complexes with RPNHP ligands perform better than most if not all other first-row transition-metal systems for the same transformation, but their productivity is often inferior in comparison to systems based on precious metals, particularly in areas relevant to energy storage applications.3b,c Consequently, improvement in the performance of catalysts based on the R PNHP/RPNP ligand is still required. Modification of the ancillary ligands is a common strategy for improving the performance of homogeneous transition-metal catalysts. To date there has been some investigation into the effect of changing the substituents on the phosphine donors of Received: August 4, 2017

A

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Figure 1. Examples of previously prepared RPNHP and RPNP supported iron pincer complexes and those prepared in this work.

Scheme 1. Synthesis of 1a,b from

iPr

PNHP, FeCl2, and CNR

[(RPNHP)Fe(H)(HBH3)(CO)] and related complexes.4e,7b This is synthetically difficult and has not resulted in catalysts which are significantly more productive. Additionally, in the case of [(RPNHP)Fe(H)(HBH3)(CO)] and related catalysts, the variation of the substituents on the nitrogen atom of the R PNHP/RPNP ligand is likely to be infeasible for many reactions due to the apparent need for a bifunctional N−H moiety for good activity. Therefore, in order to design new catalysts, we decided to replace the carbonyl ligand with another strong π acceptor. The replacement of carbonyl ligands with isoelectronic isonitrile ligands is well-precedented in organometallic chemistry17 and provides benefits due to the commercial availability of a wide variety of alkyl and aryl isonitriles, which allows for tuning of both the steric and the electronic properties of the π-accepting ligand. Furthermore, in a fashion analogous to carbonyl ligands, isonitrile ligands have a distinctive IR signature that assists with both characterization and the monitoring of reactivity. Here, we describe the synthesis of complexes related to [(RPNHP)Fe(H)(HBH3)(CO)] and [(RPNP)Fe(H)(CO)], in which isonitrile ancillary ligands are used in place of the ubiquitous CO ligand. The catalytic activity of these complexes for CO2 hydrogenation to formate is reported.

that was formed was batch dependent, 31P NMR spectroscopy indicated that most batches contained more than 50% of the bis(isonitrile) product after the crude solid had been washed with pentane to remove excess free ligand. Multiple recrystallizations were required to separate the mono(isonitrile) complexes 1a,b from the bis(isonitrile) species 2a,b, and as a result the isolated yield of 1a,b was low. To minimize the formation of the bis(isonitrile) species 2, a new synthetic procedure was developed. The five-coordinate iron(II) dichloride complex [(iPrPNHP)Fe(Cl)2] (B) was generated in situ on the basis of the procedure previously reported (Scheme 1).20 Subsequent dropwise addition of a dilute (∼30 mM) solution of aryl isonitrile over a period of 1 h to the solution containing B yielded the desired product 1 with minimal amounts of the bis(isonitrile) byproduct, for both 2,6dimethylphenyl isonitrile and 4-methoxyphenyl isonitrile. Starting from free ligand and FeCl2, a 75% yield was obtained for the synthesis of 1a and a 58% yield for 1b. Even with our optimized reaction conditions, we were unable to form mono(isonitrile) complexes with alkyl isonitriles. Treatment of in situ generated B with isopropyl, tert-butyl, or adamantyl isonitrile resulted in the formation of a mixture of species, with cationic bis(isonitrile) species similar to 2 postulated to be the major products (see the Supporting Information). Additionally, the preparation of complexes similar to 1a,b with the tert-butylsubstituted ligand tBuPNHP was also unsuccessful. This is analogous with previous work demonstrating that it is not possible to form [(tBuPNHP)Fe(Cl)2(CO)] from tBuPNHP, FeCl2, and CO,18 presumably due to steric factors. NMR spectroscopy showed that 1a,b are both diamagnetic, indicative of a low-spin d6 electronic configuration. A single resonance was observed in the 31P{1H} NMR spectra at 64.7 and 65.5 ppm for 1a,b, respectively. The resonance for 1a is broadened slightly, presumably due to restricted rotation of the 2,6-dimethylphenyl substituent on the isonitrile (see the Supporting Information), while that of 1b is sharp. IR spectroscopy showed broad C−N absorptions at 2063 and 2037 cm−1 for 1a,b, respectively. Consistent with our difficulties in synthesizing mono(isonitrile) complexes in the presence of significant quantities of free isonitrile, treatment of 1a,b with additional aryl isonitrile resulted in rapid conversion to the

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RESULTS AND DISCUSSION Synthesis of Iron Dichloride Complexes. The synthesis of the carbonyl-containing precatalysts [(iPrPNHP)Fe(H)(HBH3)(CO)] and [(iPrPNP)Fe(H)(CO)] was achieved through the initial formation of the dichloride complex [(iPrPNHP)Fe(Cl)2(CO)] (A).18 We attempted to prepare species of the form [(iPrPNHP)Fe(Cl)2(CNR)] via the same synthetic route we had utilized to prepare A. However, mixing FeCl2, iPrPNHP, and 1 equiv of either 2,6-dimethylphenyl isonitrile or 4-methoxyphenyl isonitrile in one pot in THF resulted in the formation of two products. Although one of these products was the desired complex [(iPrPNHP)Fe(Cl)2(CNR)] (R = 2,6-dimethylphenyl (1a), 4-methoxyphenyl (1b)), a significant amount of the cationic bis(isonitrile) complex [(iPrPNHP)Fe(Cl)(CNR)2][Cl] (R = 2,6-dimethylphenyl (2a), 4-methoxyphenyl (2b)) was also formed (see the Supporting Information).19 Although the exact amount of 2a,b B

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planes is 85.3°). In contrast, in 1b the angle between these two planes is 28.7 or 51.7° (there are two values because of positional disorder in the phenyl group of the isonitrile ligand). Presumably, in 1a the increased bulk associated with the omethyl groups on the isonitrile ligand causes the twisting of the ligand and also leads to restricted rotation, which results in the broadening of the NMR spectrum (vide supra). Synthesis of Complexes Containing Iron−Hydride Bonds. The hydrido−borohydride complex [(iPrPNHP)Fe(H)(HBH3)(CO)] (C) is an effective precatalyst for a number of different reactions.7a,c,11−13 One synthetic route to prepare C is treatment of [(iPrPNHP)Fe(Cl)2(CO)] (A) with excess NaBH4.18 Similarly, treatment of 1a with 10 equiv of NaBH4 in a 1/1 mixture of benzene and ethanol generates a bright yellow solution (Scheme 3). A yellow powder, characterized as [(iPrPNHP)Fe(H)(HBH3)(CNR)] (R = 2,6-dimethylphenyl (3a)), was isolated in 35% yield. Complex 3a decomposes upon exposure to vacuum for extended periods, presumably due to release of BH3. At room temperature, 3a exhibits a resonance at −21.10 ppm in the 1H NMR spectrum that we assign to the iron hydride, as well as a broad resonance centered at −2.89 ppm which integrates to four hydrogen atoms and is assigned to the κ1-HBH3 ligand. At low temperature (−60 °C), the broad resonance at −2.89 ppm is no longer present and a new signal at −15.47 ppm, which integrates to one proton, appears.22 This indicates that the κ1-HBH3 ligand is fluxional and undergoes rapid exchange between the BH4 hydrogen atoms at room temperature (see the Supporting Information), which is common for these types of complexes.18,23 The IR spectrum of 3a exhibits a strong absorption at 1984 cm−1, which is assigned as νCN, as well as absorptions at 1780 cm−1 (νFeH) and 2334 cm−1 (νBH4). This assignment is consistent with that proposed for related complexes.18 Treatment of the 4methoxyphenyl isonitrile ligated complex 1b with 10 equiv of NaBH4 produced an intractable mixture of compounds. The complexes [(iPrPNHP)Fe(H)(Cl)(CNR)] (R = 2,6dimethylphenyl (4a), 4-methoxyphenyl (4b)), which are related to 3a, were prepared by treatment of the appropriate dichloride species 1a,b with nBu4NBH4 (instead of NaBH4) in THF at room temperature (Scheme 4). Although the corresponding reaction between [(iPrPNHP)Fe(Cl)2(CO)] (A) and nBu4NBH4 to form [(iPrPNHP)Fe(H)(Cl)(CO)] (D) can be performed in acetonitrile,18 this was not possible in the

cationic species 2a,b (see Scheme 2 and the Supporting Information). In contrast, there is no observed reaction when Scheme 2. Synthesis of 2a from 1a

the carbonyl-containing complex [(iPrPNHP)Fe(Cl)2(CO)] (A) is placed under 1 atm of CO. These results suggest that the aryl isonitrile ligands studied in this work result in a more electron rich iron center in comparison with a carbonyl ligand, as they are able to stabilize cationic complexes.17f The solid-state structures of 1a,b were determined using single-crystal X-ray diffraction (Figure 2 and Table 1). The structures of 1a,b are similar to that of the previously reported carbonyl species [(iPrPNHP)Fe(Cl)2(CO)] (A).18 The coordination geometry around the iron center in 1a,b is distorted octahedral, with P−Fe−P bond angles of 169.6(3) and 168.62(2)°, respectively, associated with the pincer ligand. The largest difference between the structures of 1a,b and A is the Fe−C distance. In A, the Fe−C bond distance for the carbonyl ligand is 1.7221(19) Å, whereas in 1a,b the Fe−C bond distances to the isonitrile ligands are 1.789(3) and 1.793(2) Å, respectively. This presumably reflects that the carbonyl ligand is a better π acceptor than the isonitrile ligands studied in this work.17f The CN bond distance in the isonitrile ligand in 1a is 1.167(3) Å, while the corresponding distance in 1b is 1.167(3) Å. The CN−C bond angle of the isonitrile ligand is further away from linearity in 1a (169.6(3)°) in comparison to 1b (175.8(2)°). All of the geometrical parameters associated with the isonitrile ligands in 1a,b are typical of those observed in other iron complexes containing terminal isonitrile ligands.21 In 1a the plane containing the phenyl ring of the isonitrile ligand is essentially perpendicular to the plane consisting of the iron center and the three donor atoms from the iPrPNHP ligand (the angle between the two

Figure 2. Solid-state structures of 1a (left) and 1b (right) with ellipsoids drawn at 30% probability. Hydrogen atoms and solvent of recrystallization have been omitted for clarity. Where disorder is present, only one conformation is shown. C

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Table 1. Selected Bond Distances (Å) and Angles (deg) for the (iPrPNHP)Fe and (iPrPNP)Fe Complexes 1a,b, 4b, 5a, 7a, A, D, and E*

*

See the Supporting Information for details about the structure of 2a. aDue to disorder in the position of one of the Cl atoms, three distances are reported. bThree independent molecules are present in the asymmetric unit. cTwo independent molecules are present in the asymmetric unit.18

at −20.08 and −23.85 ppm, respectively. When it was left in solution (C6D6) for 2 days at room temperature, 4b converted exclusively to the initially predominant isomer. The signals observed in the 1H and 31P NMR spectra are consistent with meridional coordination of the iPrPNHP ligand and the hydride in a position trans to a Cl ligand.18 On the basis of similar observations made for the carbonyl complex D,18 we propose that the two structures have either syn- or anti-coplanar arrangements of the N−H and Fe−H moieties, with the anti isomer being the predominant complex (Scheme 4).24 In contrast, although 4a also forms as a mixture of isomers in an approximately 7:1 ratio with 31P{1H} resonances at 93.2 and 99.1 ppm and 1H hydride resonances at −25.12 and −21.87 ppm, respectively, these two isomers do not interconvert even upon heating at 50 °C for 96 h. We suggest that this is related to the increased steric bulk of the 2,6-dimethylphenyl isonitrile ligand. The overall yields for the formation of 4a,b are low, 40 and 37%, respectively, but they are similar to the reported yield

Scheme 3. Synthesis of 3a from 1a

case of the isonitrile complexes. Dissolution of 1a,b in acetonitrile led to rapid decomposition of the complexes, presumably due to the formation of cationic complexes related to 2a,b containing acetonitrile ligands. Initially, two isomers of 4 were formed. For example, 4b was formed as a mixture of isomers in a roughly 1:2 ratio with 31P{1H} NMR resonances present at 97.5 and 92.4 ppm and 1H NMR hydride resonances

Scheme 4. Synthesis of the Hydride Complexes 4a,b and 5a,b from 1a,b

D

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Organometallics for D (38%).18 The low yields are likely due to the formation of borane−phosphine adducts.7b The IR spectra of 4a,b both show multiple strong stretches in the region where νCN is expected. For example, a sample containing only anti-4b showed two strong, broad absorptions at 1812 and 1847 cm−1. We propose that these bands arise from a combination of νCN and νFeH. When an isotopologue of 4b containing an Fe−D bond (instead of an Fe−H) was prepared using NaBD4, two new signals were observed at 1837 and 1319 cm−1 (see the Supporting Information). The appearance of two new stretches upon deuteration suggests that there is coupling between νCN and νFeH, and at this stage definitive assignment of the bands is not possible. Similar behavior has previously been observed for complexes containing carbonyl ligands.18 The solid-state structure of the anti isomer of 4b is shown in Figure 3, with selected structural

It is likely that one of the major reasons that catalysts supported by the RPNHP ligand are so active is the bifunctional nature of the ligand, which allows for facile interconversion between the neutral RPNHP ligand and the anionic RPNP ligand.3 The reaction of 4a,b with KOtBu results in rapid deprotonation of the iPrPNHP ligand and the formation of the five-coordinate iron(II) hydride species 5a,b in good yield (Scheme 4).25 In the 1H NMR spectra resonances consistent with the presence of iron hydrides that integrate to one hydrogen atom are observed at −25.86 and −22.74 ppm for 5a,b, respectively. As with the dichloride species 1a, the sole resonance in the 31P{1H} NMR spectrum of 5a is slightly broadened, but variable-temperature NMR spectroscopy suggests that only one species is present (see the Supporting Information). In the IR spectra two strong absorptions that are assigned to νCN are observed at 1871 and 1849 cm−1 for 5a,b, respectively, which suggests that there is significant backdonation from the iron center to the π* orbitals of the isonitrile ligands. The solid-state structure of 5a is shown in Figure 4,

Figure 3. Solid-state structure of 4b with ellipsoids drawn at 30% probability. There are three independent molecules in the asymmetric unit (only one is shown here). Selected hydrogen atoms and solvent of recrystallization have been omitted for clarity.

Figure 4. Solid-state structure of 5a with ellipsoids drawn at 30% probability. Selected hydrogen atoms have been omitted for clarity.

parameters provided in Table 1. The data were of insufficient quality to freely refine the hydrogen atoms involved in the Fe− H bonds, and their positions were restrained. There are three unique molecules of 4b in the asymmetric unit, which differ primarily in the angle between the plane formed by the atoms of the phenyl ring of the isonitrile ligand and that formed by the iron center and the three donor atoms from the iPrPNHP ligand. This angle ranges from 6.3 to 45.7°. One potential reason for the large difference in this angle is the presence of an intramolecular interaction between the N−H moiety of the iPr PNHP ligand of one iron center with the chloride atom on a neighboring iron center (see the Supporting Information). The overall coordination geometry around the iron center in 4b is similar to that of 1b. The major difference relates to the Fe−Cl bond length, which increases from 2.3334(11) and 2.3409(10) Å in 1b to 2.4233(15), 2.4290(15), and 2.4233(15) Å in 4b. This is in agreement with the increased trans influence of the hydride ligand. There is also a contraction in the Fe−C bond distance to the isonitrile ligand from 1.804(3) Å in 1b to 1.764(5), 1.757(5), and 1.763(5) Å in 4b, suggesting that the iron center in 4b is more electron rich and donates more electron density into the π* orbital of the isonitrile ligand. Consistent with this proposal the CN bond distance is 1.165(5) Å in 1b in comparison with 1.183(6) and 1.191(6) Å in 4b.

with selected geometrical parameters provided in Table 1. The data were of sufficient quality that the hydrogen atom involved in the Fe−H bond was located in the Fourier map and its position refined without restraint. The geometry around the iron center is best described as distorted trigonal bipyramidal with the phosphorus atoms of the iPrPNP ligand in the axial positions and the hydride, nitrogen donor of the iPrPNP ligand, and carbon donor of the isonitrile ligand in the equatorial positions. The angles between the atoms in the equatorial positions, with one being significantly larger than 120° and another significantly smaller (H(1)−Fe(1)−N(1) 123.6(7)°, N(1)−Fe(1)−C(1) 150.39(7)°, H(1)−Fe(1)−C(1) 85.9(7)°), indicate a Y-shaped distortion from trigonal bipyramidal, which has previously been proposed by Schneider and co-workers to be indicative of π donation from the amide to the iron center.26 Consistent with this hypothesis, the Fe−N bond distance is shorter than those in the structures containing an iPrPNHP ligand reported in this work (Table 1). Additionally, in agreement with the decrease in the νCN stretching frequency, the Fe−C bond distance (1.7435(17) Å) is shorter and the CN bond distance (1.215(2) Å) is longer than in other complexes described here. There is also a dramatic decrease in the CN−C bond angle of the isonitrile ligand (148.48(16)°), indicative of strong back-bonding from the iron center to the E

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Organometallics Scheme 5. In Situ Preparation of 6a,b and 7a,b from 5a,b

isonitrile ligand.27 Overall, the structural differences between iPr PNP-supported 5a and related isonitrile-containing complexes with a iPrPNHP ligand are similar to those observed upon pincer ligand deprotonation in carbonyl-containing systems,11,18 and the geometrical parameters around the iron center in 5a are comparable to those for [(iPrPNP)Fe(H)(CO)] (E). Stoichiometric Reactions with Five-Coordinate Amido Species 5a,b. The reactivity of 5a,b with H2 was probed to evaluate the ability of the iPrPNP ligand in 5a,b to act in a bifunctional manner. At room temperature, under 1 bar of H2, a rapid reaction was observed. The 1H and 31P{1H} NMR spectra indicated the formation of a new product. However, in both reactions a significant amount of starting material persisted. On the basis of the NMR spectra and comparison to a related reaction of the five-coordinate amido species E with H2,11 we propose that the new complexes are the trans-dihydride species [(iPrPNHP)Fe(H)2(CNR)] (R = 2,6-dimethylphenyl (6a), 4methoxyphenyl (6b)) (Scheme 5). For 6a two new overlapping hydride resonances were observed at −9.28 and −9.54 ppm in the 1H NMR spectra, while for 6b the corresponding resonances were observed between −7.50 and −7.95 ppm. At room temperature under 1 bar of H2 the ratio of 6a to 5a was 11:8, while the ratio of 6b to 5b was 5:2. When 1H NMR spectra were recorded at low temperature (−80 °C), the starting five-coordinate species 5a,b were only observed in trace amounts, suggesting a temperature-dependent equilibrium between 5a/6a and 5b/6b and H2 (see the Supporting Information). This is similar to the reactivity of the carbonylsupported complex E with H2.11 However, in the case of E only the corresponding dihydride is observed at room temperature, and at 50 °C both the five-coordinate and dihydride species are observed. Although the trans-dihydrides 6a,b were the predominant products (>95%), 1H NMR spectroscopy indicated that small amounts of cis-dihydride species were also formed but were observed in significant quantities only at lowered temperature (see the Supporting Information).18 When the H2 was removed from samples of 6a,b (via freeze−pump−thaw cycles), 1H and 31P{1H} NMR spectroscopy indicated that the starting five-coordinate amido complexes 5a,b were recovered in essentially quantitative yields. As a result of this instability to vacuum and the incomplete reaction under 1 bar of H2, no attempts were made to isolate 6a,b. Nevertheless, these results demonstrate that the isonitrile-supported complexes 5a,b can participate in the type of bifunctional reactivity that is proposed to be crucial to the high catalytic activity of E.3c In several catalytic reactions involving E, formate species such as [(iPrPNHP)Fe(H){OC(O)H}(CO)] (F) are proposed to be a crucial intermediate.10−12 One synthetic route to prepare formate complexes involves CO2 insertion into a metal hydride. In situ generation of the dihydride complexes 6a,b,

followed by treatment with 1 bar of a 1:1 mixture of H2:CO2 results in the appearance of new signals at −27.47 and −26.20 ppm in the 1H NMR spectra from reactions starting with 6a,b, respectively. These signals integrate to one hydrogen atom, and new peaks also integrating to one hydrogen atom are observed at 9.13 and 8.68 ppm in the reaction of 6a and at 9.26 and 8.60 ppm in the reaction of 6b. The chemical shifts of these peaks are consistent with a formate {OC(O)H} proton and the N−H proton of the iPrPNHP ligand when it is it involved in a hydrogen bond.11,13,28 On this basis we propose that the formate complexes [(iPrPNHP)Fe(H){OC(O)H}(CNR)] (R = 2,6-dimethylphenyl (7a), 4-methoxyphenyl (7b)) are formed. However, other products are also formed in these reactions, and the peaks associated with the postulated formate species slowly decompose in hours upon standing at room temperature. Additionally, removal of the atmosphere of H2/CO2 resulted in an increased rate of decomposition to a mixture of products. Despite the apparent instability of 7a, single crystals suitable for X-ray crystallography were obtained from the reaction mixture of 6a with CO2 at −35 °C. The solid-state structure is shown in Figure 5. The geometric parameters

Figure 5. Solid-state structure of 7a with ellipsoids drawn at 30% probability. Selected hydrogen atoms and solvent of recrystallization have been omitted for clarity.

around the iron center in 7a are similar to those in the other six-coordinate iron systems described in this work (Table 1). The only significant difference is the CN−C bond angle of the isonitrile ligand (162.0(3)°), which is lower than those for other six-coordinate species. The orientation of the formate ligand is consistent with a hydrogen bond with the N−H proton of the ligand, supporting the NMR assignment. Catalytic Hydrogenation of CO2. Two proposed elementary steps in the catalytic hydrogenation of CO2 using E are 1,2-addition of H2 to E to generate a dihydride complex, F

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Organometallics followed by CO2 insertion to form a metal formate.10 Given that complexes 5a,b can undergo these transformations (vide supra), we tested their ability to perform CO2 hydrogenation (Table 2). We utilized the same conditions that were

glovebox or using a stainless steel cannula on a Schlenk line. Solvents were dried by passage through a column of activated alumina and stored under dinitrogen unless otherwise noted. Ethanol (200 proof) was purchased from Decon Laboratories, Inc., and degassed and stored under dinitrogen prior to use. All commercial chemicals were used as received except where noted. iPrPNHP and tBuPNHP were purchased from Strem. Anhydrous FeCl2 was purchased from Alfa Aesar. Sodium borohydride was purchased from Acros Organics. Tetrabutylammonium borohydride purchased from Acros was dried in-melt before use. 2,6-Dimethylphenyl isonitrile and 4-methoxyphenyl isonitrile were used as received from Santa Cruz Biotech or Sigma-Aldrich. Potassium tert-butoxide was purchased from Sigma-Aldrich. Formic acid was purchased from EMD Millipore Inc. and dried over phthalic anhydride prior to use. Anhydrous dihydrogen and a 1/1 H2/CO2 mixture were obtained from Airgas, Inc. Sodium borodeuteride and deuterated solvents were obtained from Cambridge Isotope Laboratories. C6D6 and C6D5CD3 were dried over sodium metal or by passage through a plug of activated alumina. NMR spectra were recorded on Bruker AMX-400, AMX-500, and AMX-600 spectrometers at ambient probe temperatures, unless otherwise noted. Chemical shifts are reported in ppm with respect to residual internal protio solvent for 1H and 13 C{1H} NMR spectra. 31P{1H} NMR spectra are referenced via the 1 H resonances on the basis of the relative gyromagnetic ratios.29 All J coupling constant values are given in Hertz. Elemental analyses were performed by Robertson Microlit Laboratories, Inc. Infrared data were obtained on a Bruker ALPHA FTIR spectrometer with a platinum ATR attachment inside a N2-filled glovebox. All samples were taken of the neat solid. UV−vis data were collected on a Hewlett-Packard 8453 diode array spectrophotometer. X-ray Crystallography. Crystal samples were mounted on polyimide loops with immersion oil. Low-temperature diffraction data (ω scans) were collected on a Rigaku R-AXIS RAPID diffractometer coupled to an R-AXIS RAPID imaging plate detector with Mo Kα radiation (λ = 0.71073 Å) for 1a,b and 5a or a Rigaku SCX Mini diffractometer coupled to a Rigaku Mercury275R CCD with Mo Kα radiation (λ = 0.71073 Å) for 2a,b, 4b, and 7a. The diffraction images were processed and scaled using the Rigaku CrystalClear software.30 The structure was solved with SHELXT and was refined against F2 on all data by full-matrix least squares with SHELXL.31 Details of the crystal and refinement data for 1a,b, 2a, 4b, 5a, and 7a are described in the Supporting Information. Synthetic Procedures and Characterization Data for New Compounds. [(iPrPNHP)Fe(Cl)2(2,6-dimethylphenyl isonitrile)] (1a). A three-necked round-bottom flask was charged with iPrPNHP (203 mg, 0.665 mmol), FeCl2 (83 mg, 0.66 mmol), and 100 mL of THF. The mixture was refluxed for 2 h to give a homogeneous solution, to which a solution of 2,6-dimethylphenyl isonitrile (84 mg, 0.64 mmol) in 25 mL of THF was added dropwise over the course of 1 h. Upon addition of the isonitrile solution, the reaction mixture turned dark green. The mixture was stirred for an additional 1 h. The volatiles were removed in vacuo. The crude product was washed with 3 × 5 mL of pentane and extracted with 3 × 5 mL of toluene, which was subsequently removed in vacuo to yield 1a (270 mg, 75%) as a dark green solid. Crystals suitable for X-ray diffraction were grown from slow diffusion of pentane into a concentrated solution in benzene. Anal. Found (calcd) for C25H46P2N2Cl2Fe: C, 53.30 (53.30); H, 8.16 (8.23); N, 4.89 (4.97). 1H NMR (500 MHz, C6D6): 6.88 (d, 2H, CHAr J = 7.5 Hz), 6.80 (m, 1H, CHAr), 4.74 (t, 1H, NH, J = 12.0 Hz), 3.18 (m, 2H, CH2), 2.88 (m, 4H, CH2), 2.70 (br, 2H, CH2), 2.66 (s, 6H, CArCH3), 2.08 (m, 2H, CH), 1.74 (td, 2H, CH, J = 4.9 Hz, J = 14.4 Hz), 1.48 (m, CHCH3, 18H), 1.27 (br, CHCH3, 6H). 13C{1H} NMR (151 MHz, C6D6): 191.2, 134.6, 131.5, 128.6, 125.4, 49.8, 26.9, 23.4, 22.1, 20.5, 20.3, 19.4, 19.2, 18.9. 31P{1H} NMR (202 MHz, C6D6): 64.7. IR (cm−1): 3188 (ννNH), 2063 (νCN). UV−vis (toluene (λmax, ε)): (291 nm, 9698 M−1 cm−1); (334 nm, 9236 M−1 cm−1). [(iPrPNHP)Fe(Cl)2(4-methoxyphenyl isonitrile)] (1b). A three-necked round-bottom flask was charged with iPrPNHP (150 mg, 0.491 mmol), FeCl2 (62.3 mg, 0.491 mmol), and 75 mL of THF. The mixture was refluxed for 2 h to give a homogeneous solution, to which a solution of 4-methoxyphenyl isonitrile (65.4 mg, 491 mmol) in 25 mL of THF

Table 2. Comparison of Iron Catalysts for CO2 Hydrogenation to Formatea

entry

catalyst

DBU/LiOTf

TONb,c

1 2 310

5a 5b E

7.5/1 7.5/1 7.5/1

613 333 6030

Reaction conditions: 69 atm of CO2:H2 (1:1), 0.3 μmol of catalyst in 10 mL THF (ca. 0.01 M), 3.600 g DBU at 80 °C. bFormate production quantified by 1H NMR spectroscopy. cReported values are the average of at least two trials. a

successfully used for CO2 hydrogenation catalyzed by E.10 5a,b are both active catalysts for CO2 hydrogenation to formate, giving 613 and 333 turnovers, respectively. Although these values are roughly comparable to those for other iron catalysts in the literature, they are significantly lower than the TON obtained using E.3c In CO2 hydrogenation using E the formate complex F is proposed to be the catalyst resting state. The observed instability of the isonitrile-supported formate complexes 7a,b provides an explanation for their relatively poor catalytic performance.

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CONCLUSIONS In this work we have prepared a number of iron aryl isonitrile complexes supported by iPrPNHP and iPrPNP ligands. These complexes are direct analogues of related carbonyl-ligated species that are highly active for a range of hydrogenation and dehydrogenation reactions. Our results show that in most cases the synthetic chemistry for preparing the aryl isonitrile complexes is similar to that described for the corresponding carbonyl species. However, there are differences in the stability and the stoichiometric and catalytic reactivity of the aryl isonitrile complexes. At this stage replacing the carbonyl ligand with aryl isonitrile ligands has only resulted in the development of systems which perform relatively poorly for CO2 hydrogenation to formate. Nevertheless, the synthetic work developed here provides a framework for further modification of the aryl isonitrile ancillary ligand, which may result in more active catalytic systems or provide a framework for supporting the complexes to generate systems suitable for heterogeneous catalysis. Additional synthetic work is still required to prepare complexes containing alkyl isonitrile ligands, and this will be a focus of future research in our laboratories.

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EXPERIMENTAL SECTION

General Methods. Experiments were performed under a dinitrogen atmosphere in an MBraun glovebox or using standard Schlenk techniques unless otherwise noted. Under typical operating conditions, the glovebox was not purged between uses of benzene, diethyl ether, pentane, THF, or toluene. As a consequence, each solvent should be assumed to contain trace amounts of the others. All moisture- and air-sensitive liquids were either transferred inside the G

DOI: 10.1021/acs.organomet.7b00602 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

19.5, 19.13, 19.08, 17.8, 17.7, 15.2, 13.7. Isonitrile carbon not observed. 31P{1H} NMR (161 MHz, C6D6): major isomer, 93.2; minor isomer, 99.1. IR (cm−1): 3187 (νNH), 1964 (νCN, br). UV−vis (toluene (λmax, ε)): (313 nm, 15900 M−1 cm−1); (363 nm, 9233 M−1 cm−1). [(iPrPNHP)Fe(H)(Cl)(4-methoxyphenyl isonitrile)] (4b). A Schlenk flask was charged with [(iPrPNHP)Fe(Cl)2(4-methoxyphenyl isonitrile)] (1b; 150.8 mg, 0.2668 mmol) and nBu4NBH4 (68.4 mg, 0.266 mmol), a magnetic stir bar, and 15 mL of THF. Upon addition of THF the solution developed an orange color. The reaction mixture was stirred for 48 h, after which the volatiles were removed in vacuo to yield an orange residue. The residue was extracted with 3 × 2 mL of 5/ 1 Et2O/benzene and cooled to −30 °C. Compound 4b was isolated as an orange solid. Yield: 52.1 mg, 37%. Additional yield was obtained through removal of volatiles in vacuo from the supernatant and subsequent recrystallization of the solid from 5/1 Et2O/benzene. The reported yield is from multiple crops of different recrystallizations. Crystals suitable for X-ray diffraction were grown from a solution in 5/ 1 Et2O/benzene at −35 °C. Anal. Found (calcd) for C25H46P2N2Cl2Fe: C, 54.49 (54.30); H, 8.71 (8.54); N, 5.14 (5.28). 1H NMR (500 MHz, C6D6): 7.19 (m, 2H, CHAr, apparent J = 8.8 Hz), 6.59 (m, 2H, CHAr, apparent J = 8.5 Hz), 3.35 (t, 1H, NH, J = 11.8 Hz), 3.18 (s, 3H, OCH3,), 3.04−3.12 (m, 2H, CH2), 2.54 (m, 2H, CH2), 2.04−2.15 (m, 2H, CH2), 1.66−1.80 (m, 9H, CH(CH3)2), 1.52 (m, 2H, CH2), 1.18−1.34 (m, 12H, CH(CH3)2), 0.98−1.09 (m, 7H, CH(CH3)2), −23.85 (t, 1H, Fe-H, 2 JH−P = 53.5 Hz). 13C{1H} NMR (126 MHz, C6D6): 128.6, 128.4, 125.7, 114.8, 54.9, 53.4 (t, J = 5.7 Hz), 29.6 (t, J = 6.6 Hz), 26.8 (t, J = 7.9 Hz), 25.1 (t, JP = 9.9 Hz), 20.9 (m), 19.4, 18.2. Isonitrile carbon not observed. 31P{1H} NMR (202 MHz, C6D6): 92.4. IR (cm−1): 3191 (νNH), 1847 (νCN, νFeH), 1812 (νCN, νFeH). UV−vis (toluene (λmax, ε)): (307 nm shoulder, 16910 M−1 cm−1); (365 nm shoulder, 7794 M−1 cm−1). Selected peaks for the minor isomer, which is initially present upon formation, are as follows. 1H NMR (500 MHz, C6D6): −20.08 (t, 1H, Fe-H, 2JH−P= 52.2 Hz). 31P{1H} (202 MHz, C6D6): 97.5. [(iPrPNP)Fe(H)(2,6-dimethylphenyl isonitrile)] (5a). A Schlenk flask was charged with [(iPrPNHP)Fe(H)(Cl)(2,6-dimethylphenyl isonitrile)] (4a; 52 mg, 0.098 mmol), KOtBu (13 mg, 0.12 mmol), a magnetic stir bar, and 10 mL mTHF. The dark red-purple reaction mixture was stirred for 30 min, after which the volatiles were removed in vacuo. The resultant maroon residue was extracted with pentane at −30 °C, and the solvent was removed in vacuo. The extraction was then repeated to yield 5a as a maroon solid. Yield: 39 mg (0.0795 mmol, 81%). Crystals suitable for X-ray diffraction were grown from a solution of 2/1 pentane/hexamethyldisiloxane at −35 °C. Anal. Found (calcd) for C25H46P2N2Fe: C, 60.86 (60.98); H, 9.23 (9.42); N, 5.50 (5.69). 1H NMR (500 MHz, C6D6): 6.91 (d, 2H, CHAr, J = 7.4 Hz), 6.79 (t, 1H, CHAr,J = 7.4 Hz), 3.26 (br, 2H, CH2), 3.09 (br, 2H, CH2), 2.51 (s, 6H, CH3), 2.41 (br, 2H, CH2), 2.07 (m, 2H, CH2), 1.81 (br, 4H, CH), 1.38 (m, 6H, CH3), 1.18 (m, 12H, CH3), 1.09 (m, 6H, CH3), −25.86 (t, 1H, Fe-H, 2JH−P = 54.3 Hz). 13 C{1H} NMR (151 MHz, C6D6): 208.1, 135.5, 133.7, 130.4, 122.7, 64.1 (t, J = 10.6 Hz), 34.4, 27.4 (t, J = 11.2 Hz), 25.2 (t, J = 9.5 Hz), 24.3 (t, J = 5.6 Hz), 22.7, 20.1, 19.6 (t, J = 2.2 Hz), 18.1 (t, J = 2.2 Hz), 18.1 (br), 17.6. 31P{1H} NMR (202 MHz, C6D6): 114.8. IR (cm−1) 1871 (νCN). UV−vis (toluene (λmax, ε)): (293 nm, 40820 M−1 cm−1); (330 nm, 36540 M−1 cm−1). [(iPrPNP)Fe(H)(4-methoxyphenyl isonitrile)] (5b). A Schlenk flask was charged with [(iPrPNHP)Fe(H)(Cl)(4-methoxyphenyl isonitrile)] (4b; 29.8 mg, 0.0561 mmol), KOtBu (8.9 mg, 0.079 mmol), a magnetic stir bar, and 10 mL of THF. The dark red-purple reaction mixture was stirred for 30 min, after which the volatiles were removed in vacuo. The resultant dark red residue was extracted with pentane at −30 °C, and the solvent was removed in vacuo. The extraction was then repeated to yield 5b as a dark red solid. Yield: 16.7 mg (0.0338 mmol, 60%). Anal. Found (calcd) for C25H46P2N2Cl2Fe: C, 58.36 (58.30); H, 9.15 (8.97); N, 5.60 (5.67). 1H NMR (500 MHz, C6D5CD3): 7.15 (m, 2H, CHAr, apparent J = 8.8 Hz), 6.65 (m, 2H, CHAr, apparent J = 8.9

was added dropwise over the course of 1 h. Upon addition of the isonitrile solution, the reaction mixture turned green. The mixture was stirred for an additional 1 h. The volatiles were removed in vacuo. The crude product was washed with 3 × 5 mL of pentane and extracted with 3 × 5 mL of toluene. Pentane (10 mL) was added, causing an unknown yellow impurity to precipitate, and the supernatant was collected. The volatiles were removed in vacuo to yield 1b (160.0 mg, 58%) as a dark green solid. Crystals suitable for X-ray diffraction were grown from slow-diffusion of pentane into a concentrated solution of benzene. Anal. Found (calcd) for C25H46P2N2Cl2Fe: C, 51.04 (50.99); H, 7.63 (7.85); N, 4.88 (4.96). 1H NMR (600 MHz, C6D6): 7.35 (m, 2H, CHAr, apparent J = 8.7 Hz), 6.55 (m, 2H, CHAr, apparent J = 8.7 Hz), 4.83 (t, 1H, NH, J = 11.9 Hz), 3.21−3.15 (overlapping multiplet, 5H, CH2, OCH3), 2.89 (br, 4H, CH2), 2.73 (br, 2H, CH2), 2.10 (m, 2H, CH), 1.78 (td, 2H, CH, J = 13.3 Hz, 3.2 Hz), 1.65 (m, CHCH3, 6H), 1.58 (m, CHCH3, 6H), 1.48 (m, CHCH3, 6H), 1.35 (br, CHCH3, 6H). 13C{1H} NMR (151 MHz, C6D6): 157.9, 128.6, 126.9, 115.0 54.9, 49.8, 26.9 (br), 23.4 (t, J = 9.6 Hz), 21.8 (t, J = 7.5 Hz), 20.7, 20.4, 19.4, 18.9. Isonitrile carbon not observed. 31P{1H} NMR (161 MHz, C6D6): 65.5. IR (cm−1): 3207 (νNH), 2037 (νCN). UV−vis (toluene (λmax, ε)): (316 nm, 9425 M−1 cm−1); (363 nm, 5541 M−1 cm−1). [(iPrPNHP)Fe(H)(HBH3)(2,6-dimethylphenyl isonitrile)] (3a). A Schlenk flask was charged with [(iPrPNHP)Fe(Cl)2(2,6-dimethylphenyl isonitrile)] (1a; 202.0 mg, 0.36 mmol) and NaBH4 (136 mg, 3.6 mmol). This mixture was suspended in 15 mL of benzene, and then 15 mL of ethanol was added. Upon addition of ethanol, gas evolution was observed with a concomitant color change of the solution to bright yellow. The mixture was stirred for 12 h. The volatiles were removed in vacuo until a minimum amount of solvent was present, after which dry N2 was passed over the mixture to remove the remaining solvent. Subsequent extraction of the product with diethyl ether and drying under a constant flow of dry N2 provided 3a as a yellow powder (103.6 mg, 35%). Due to the instability of this compound to vacuum, elemental analysis was not performed. 1 H NMR (500 MHz, C6D6): 6.83 (d, 2H, CHAr J = 7.6 Hz), 6.72 (t, 1H, CHAr, J = 7.5 Hz), 3.90 (t, 1H, NH, J = 11.7 Hz), 2.80−2.90 (m, 2H, CH2), 2.52 (s, 6H, CArCH3), 2.40−2.49 (m, 2H, CH2), 2.06 (m, 2H, CH), 1.80 (m, 2H, CH), 1.59 (m, 2H, CH), 1.51 (m, 2H, CH), 1.44 (m, 7H, CHCH3), 1.20 (m, 14H, CHCH3), 1.02 (m, 7H, CHCH3), −2.79 (br, 4H, HBH3), −21.07 (t, 1H, Fe-H, 2JH−P = 53.9). 13 C{1H} NMR (151 MHz, C6D6): 133.5, 124.1, 53.6 (t, J = 5.8 Hz), 29.2 (t, J = 8.2 Hz), 28.5(t, J = 6.6 Hz), 25.8 (t, J = 12.0 Hz), 20.9 (t, J = 2.4 Hz), 20.5, 19.6, 19.2, 18.8. Isonitrile carbon not observed. 31 1 P{ H} NMR (202 MHz, C6D6): 96.8. IR (cm−1): 3199 (νNH), 2334 (νFe‑HBH3), 1984 (νCN). UV−vis (toluene (λmax, ε)): (281 nm, 7350 M−1 cm−1); (337 nm, 8775 M−1 cm−1). [(iPrPNHP)Fe(H)(Cl)(2,6-dimethylphenyl isonitrile)] (4a). A Schlenk flask was charged with [(iPrPNHP)Fe(Cl)2(2,6-dimethylphenyl isonitrile)] (1a; 141.0 mg, 0.2503 mmol) and nBu4NBH4 (64.6 mg, 0.251 mmol), a magnetic stir bar, and 15 mL of THF. Upon addition of THF the solution developed an orange color. The reaction mixture was stirred for 4 h, after which the volatiles were removed in vacuo to yield an orange residue. The residue was extracted with 3 × 3 mL of 5/1 Et2O/benzene and cooled to −30 °C. Compound 4a was isolated as orange crystals of two isomers in a 7/1 ratio. Yield: 52.2 mg, 40%. Additional yield was obtained through removal of volatiles in vacuo from the supernatant and subsequent recrystallization of the residue from 5/1 Et2O/benzene. The reported yield is from multiple crops of different recrystallizations. Selected 1H NMR data are provided for each isomer, while 13C NMR data is for both isomers. Anal. Found (calcd) for C25H47P2N2ClFe: C, 56.95 (56.77); H, 9.02 (8.96); N, 5.28 (5.30). 1H NMR (600 MHz, C6D6): major isomer, 6.87 (d, 2H, CHAr, J = 7.5 Hz), 6.75 (t, 1H, CHAr, J = 7.5 Hz), 2.57 (s, 6H, CArCH3), −25.12 (t, 1H, Fe-H, 2JH−P = 55.4 Hz); minor isomer, 6.83 (d, 2H, CHAr, J = 7.5 Hz), 6.72 (t, 1H, CHAr, J = 7.5 Hz), 2.53 (s, 6H, CArCH3), −21.87 (t, 1H, Fe-H, 2JH−P = 54.2 Hz). 13C{1H} NMR (126 MHz, C6D6): 132.9, 123.0, 65.5, 58.6, 47.4 (t, J = 5.6 Hz), 28.2, 25.9 (t, J = 5.6 Hz), 25.5 (m), 24.2, 20.2 (m), 20.0 (m), 19.9 (br), H

DOI: 10.1021/acs.organomet.7b00602 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Hz), 3.27 (s, 3H, OCH3,), 3.22 (m, 2H, CH2), 3.12 (m, 2H, CH2), 2.43 (m, 2H, CH2), 1.97 (m, 2H, CH2), 1.76 (m, 4H, CH), 1.36 (m, 6H, CH3), 1.17 (m, 6H, CH3), 1.08 (m, 12H, CH3), −22.92 (t, 1H, Fe-H, 2JH−P = 53.2 Hz). 13C{1H} NMR (151 MHz, C6D6): 156.0, 134.9, 124.1, 114.8, 64.4 (t, JP = 10.3 Hz), 54.9, 26.7 (t, JP = 11.6 Hz), 25.2 (t, JP = 9.9 Hz), 24.2 (t, JP = 5.9 Hz), 22.8, 19.6 (br), 18.4 (br), 18.2 (br), 17.5. Isonitrile carbon not observed. 31P{1H} NMR (202 MHz, C6D5CD3): 114.9. IR (neat, cm−1): 1847 (νCN), 1776 (νFeH) UV−vis (toluene (λmax, ε)): (296 nm, 14536 M−1 cm−1); (334 nm, 9300 M−1 cm−1); (531 nm, 730 M−1 cm−1). In Situ Synthesis of [(iPrPNHP)Fe(H)2(2,6-dimethylphenyl isonitrile)] (6a). A J. Young NMR tube was charged with [(iPrPNP)Fe(H)(2,6-dimethylphenyl isonitrile)] (5a; 10 mg, 0.02 mmol) and 600 μL of d8-toluene in a N2 glovebox. On a Schlenk line, the N2 atmosphere was replaced with an atmosphere of H2 (1 bar) using three consecutive freeze−pump−thaw cycles. The reaction mixture was allowed to stand for 10 min, during which time there was no observable change in the color of the solution. 1H and 31P{1H} NMR spectra were recorded. These indicated the formation of predominantly the trans isomer of [(iPrPNHP)Fe(H)2(2,6-dimethylphenyl isonitrile)] (6a) with a significant amount of 5a present (the ratio of 6a to 5a was approximately 11/8). Selected peaks for 6a are reported here. See the Supporting Information for complete spectra. There was evidence for the formation of a small amount of the cis isomer at lower temperatures (see the Supporting Information). 1 H NMR (500 MHz, C6D5CD3): 6.84 (br, 2H, CHAr), 6.72 (m, 1H, CHAr), 2.47 (br, 6H, CArCH3), −9.28 (br, H−Fe-H), −9.54 (t, H−FeH, JH−P = 44.6 Hz). 31P{1H} NMR (202 MHz, C6D5CD3): 114.7. In Situ Synthesis of [(iPrPNHP)Fe(H)2(4-methoxyphenyl isonitrile)] (6b). A J. Young NMR tube was charged with [(iPrPNP)Fe(H)(4methoxyphenyl isonitrile)] (5b; 10 mg, 0.02 mmol) and 600 μL of d6benzene in a N2 glovebox. On a Schlenk line, the N2 atmosphere was replaced with an atmosphere of H2 (1 bar) using three consecutive freeze−pump−thaw cycles. The reaction mixture was allowed to stand for 10 min, during which time there was no observable change in the color of the solution. 1H and 31P{1H} NMR spectra were recorded, showing predominantly the trans isomer of [(iPrPNHP)Fe(H)2(4methoxyphenyl isonitrile)] (6b), with some unreacted 5b present. The ratio of 6b to 5b was approximately 5/2. Selected peaks for 6b are reported here. See the Supporting Information for complete spectra. 1 H NMR (500 MHz, C6D6): 7.31 (br, 2H, CHAr), 6.73 (br 2H, CHAr), 3.58 (br, 1H, NH), 3.24 (s, 3H, OCH3), −7.50 to −7.95 (m, 2H, H-Fe-H). 31P{1H} NMR (202 MHz, C6D6): 112.7. In Situ Synthesis of [(iPrPNHP)Fe(H){OC(O)H}(2,6-dimethylphenyl isonitrile)] (7a). A J. Young NMR tube was charged with [(iPrPNP)Fe(H)(2,6-dimethylphenyl isonitrile)] (5a; 10 mg, 0.02 mmol) and 600 μL of d6-benzene in a N2 glovebox. On a Schlenk line, the N2 atmosphere was replaced with an atmosphere of H2 (1 bar) using three consecutive freeze−pump−thaw cycles. The reaction mixture was allowed to stand for 5 min, and then the atmosphere was replaced with a 1/1 H2/CO2 mixture (1 bar) using three consecutive freeze−pump−thaw cycles. The reaction mixture was allowed to stand for 10 min, during which the solution changed from a deep, dark red to a persistent bright yellow color. 1H and 31P{1H} NMR spectra were recorded and are consistent with the formation of 7a. Selected peaks are reported. See the Supporting Information for complete spectra. Crystals suitable for X-ray diffraction were grown by removing volatiles in vacuo and then dissolving in 1 mL of pentane followed by slow diffusion of an equivalent amount of hexamethyldisiloxane at −35 °C. See the Supporting Information for NMR data of single crystals. Selected NMR Peaks. 1H NMR (500 MHz, C6D6): 9.13 (s, 1H, OC(O)H), 8.68 (br, 1H, NH), 2.51 (s, 6H, CArCH3), −27.47 (t, 1H, 53.5 Hz). 31P{1H} NMR (202 MHz, C6D6): 92.7. In Situ Synthesis of [(iPrPNHP)Fe(H){OC(O)H}(4-methoxyphenyl isonitrile)] (7b). A J. Young NMR tube was charged with [(iPrPNP)Fe(H)(4-methoxyphenyl isonitrile)] (5b; 10 mg, 0.02 mmol) and 600 μL of d6-benzene in a N2 glovebox. On a Schlenk line, the N2 atmosphere was replaced with an atmosphere of H2 (1 bar) using three consecutive freeze−pump−thaw cycles. After the tube was allowed to stand for 5 min, the atmosphere was replaced with a 1/

1 H2/CO2 mixture (1 bar) using three consecutive freeze−pump− thaw cycles. The reaction mixture was allowed to stand for 10 min, during which time the solution changed from a deep, dark red to a persistent bright yellow color. 1H and 31P{1H} NMR spectra were recorded and are consistent with the formation of 7b. Selected peaks are reported. See the Supporting Information for complete spectra. 1 H NMR (500 MHz, C6D6): 9.26 (s, 1H, OC(O)H), 8.60 (br, 1H, NH), 3.17 (s, 3H, CArOCH3), −26.20 (t, 1H, Fe-H, JH−P = 52.1 Hz). 31 1 P{ H} NMR (202 MHz, C6D6): 93.3. General Methods for Catalytic CO2 Hydrogenation. In a glovebox, a 50 mL glass reactor liner was charged with catalyst as a stock solution in THF (ca. 0.02 M, 0.3 μmol catalyst), LiOTf (492 mg, 3.15 mmol), DBU (3.6 g, 24 mmol), and 10 mL of THF. The cylinder liner was placed into the Parr reactor and the vessel sealed. The reactor was removed from the glovebox and pressurized with 500 psi of CO2 and 500 psi of H2 at ambient temperature. The reactor was then heated, and the contents were stirred at 80 °C for 24 h. The reaction was stopped by removal of the heat source and venting of the vessel’s atmosphere. The contents of the reactor were then transferred to a 100 mL round-bottomed flask, using D2O to dissolve the solid products. All volatiles were removed in vacuo. The residue was then dissolved in D2O, and DMF was added as an internal standard for quantification of the formate product by 1H NMR spectroscopy.

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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.organomet.7b00602. Additional information about selected experiments, NMR spectra, and details about the X-ray structures of 1a,b, 2a, 4b, 5a, and 7a (PDF) Accession Codes

CCDC 1566441−1566444 and 1566691−1566693 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]. 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 Authors

*E-mail for W.H.B.: [email protected]. *E-mail for N.H.: [email protected]. ORCID

Nilay Hazari: 0000-0001-8337-198X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.H. acknowledges support from the National Science Foundation through Grant CHE-1150826. N.H. and W.H.B. are fellows of the Alfred P. Sloan Foundation. We thank Julia Curley for valuable discussions and Matthew Mills for assistance in evaluating the activity of isonitrile-containing species for CO2 hydrogenation.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00602 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00602 Organometallics XXXX, XXX, XXX−XXX