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Cooperative Iron-Oxygen-Copper Catalysis in the Reduction of Benzaldehyde Under Water-Gas Shift Reaction Conditions Arundhoti Chakraborty, Richard Garrison Kinney, Jeanette A. Krause, and Hairong Guan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01994 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016

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Cooperative Iron-Oxygen-Copper Catalysis in the Reduction of Benzaldehyde Under Water-Gas Shift Reaction Conditions Arundhoti Chakraborty, R. Garrison Kinney, Jeanette A. Krause, and Hairong Guan* Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 452210172, United States

Fe

2O

Ph CH

SiMe3 O

CO

Ph

CH

O

TABLE OF CONTENTS GRAPH

H

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OC OC

Cu SiMe 3

H

N N

2O CO H 2

[Fe-O-Cu] catalyst

ABSTRACT Two Fe-Cu heterobimetallic complexes have been synthesized from the reactions of (cyclopentadienone)iron complexes {2,5-(EMe3)2-3,4-(CH2)4(η4-C4C=O)}Fe(CO)3 (E = Si, 1a; E = C, 1b) with (IPr)CuOH (IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene).

X-ray

crystallographic studies show that these complexes adopt a structure featuring a bridging hydride and described by the formula {2,5-(EMe3)2-3,4-(CH2)4(η4-C4C=O)}(CO)2Fe(µ-H)Cu(IPr) (E = Si, 4a'; E = C, 4b'). When dissolved in toluene, THF, or cyclohexane, these complexes form a rapidly equilibrated isomeric mixture of 4a' or 4b' and a terminal iron hydride {2,5-(EMe3)2-3,4(CH2)4(η5-C4COCuIPr)}Fe(CO)2H (E = Si, 4a; E = C, 4b). The solution structure for the Me3Siderivative is dominated by 4a. Both 4a and 4b/4b' react with HCO2H to form a monometallic iron hydride and (IPr)CuOCHO. They also undergo displacement of (IPr)CuH by CO. The heterobimetallic complexes are effective catalysts for the reduction of PhCHO under water-gas1 ACS Paragon Plus Environment

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shift conditions; 4a exhibits higher activity than 4b/4b'. Control experiments with monometallic species establish the cooperativity between a bifunctional iron fragment and a copper fragment during the catalytic reaction.

Mechanistic investigation including stoichiometric reactions,

various control experiments, and labeling studies have led to the proposal of two different catalytic cycles. KEYWORDS: cooperativity, heterobimetallic complexes, Knölker complex, iron, copper, water-gas shift reaction, reduction

INTRODUCTION Enzymes perform catalytic reactions in a remarkably efficient and selective manner by virtue of their ability to bind and activate substrates with synergistic action of amino acid side chains and cofactors. 1 Inspired by biological systems, synthetic chemists have successfully designed metal complexes, non-metal compounds, or the combination thereof that can also activate molecules with multiple points of interaction. 2 Recent advances in homogeneous catalysis have largely benefited from new strategies developed for small molecule activation, particularly utilizing the synergy between the two metals of a heterobimetallic complex,3 metal and a heteroatom (or a carbon atom) in a metal-ligand bifunctional catalyst, 4 or a frustrated Lewis pair.5 More complex systems featuring the interaction of substrates with more than two functional sites are being developed and showing great promise for homogeneous catalysis.6 Understanding the intricate role of each functional site of these well-defined systems is crucial to the rational design of new catalysts with improved efficiency and selectivity. In recent years, a number of iron-based metal-ligand bifunctional catalysts have been developed for the reduction of carbonyl groups,7 a research area primarily driven by the interest in catalysis using earth-abundant metals such as iron.8 The first such catalyst is the Knölker 2 ACS Paragon Plus Environment

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complex (Figure 1),9 which was reported in 1999 but not known as a hydrogenation catalyst until Casey’s work published in 2007.10 Mechanistic studies have suggested that the acidic OH and hydridic FeH hydrogens are simultaneously transferred to a C=O bond,11 resulting in selective hydrogenation of aldehydes and ketones. Structural modifications to the Knölker complex have been made, including changing substituents on the cyclopentadienyl ring,12 substituting a chiral phosphoramidite for one of the CO ligands, 13 and replacing the OH group with a silyl or germyl group (Figure 1). 14 These complexes and the related (cyclopentadienone)iron tricarbonyl complexes 15 have been demonstrated as effective catalysts for a broad range of chemical transformations.16 SiMe3 O

Ph

SiMe3

Fe

OC H OC Knölker complex

Fe

OC OC

Fe

R

H

Ph

Fe Ph

OC OC

H

H

Guan

X Fe

O Fe

H X = O, CH2 Beller, Berkessel, Casey SiPh3 O

H SiMe3 H

L* =

H SiMe3

OC OC

SiMe3 O

OC OC H L* OC i R = SiEt3, Si( Pr)Me2, SiCyMe2 Beller

H Tol

H

R O

O

O

H

SiMe3 O

Ph

Tol

Fe

O O

PNMe2

Berkessel

E SiPh3

H OC H OC E = SiEt2, GeEt2, SiMe2OSiMe2 Nakazawa

Figure 1. The Knölker complex and its derivatives (structural modifications highlighted in red). We present here a new class of compounds bearing three potential functional sites as highlighted in Figure 2. Incorporating copper into the metal-ligand bifunctional system could create additional reaction pathways that are unavailable or inefficient with Knölker-type complexes alone. Mankad and coworkers have reported a variety of Fe-Cu heterobimetallic complexes supported by cyclopentadienyl (or pentamethylcyclopentadienyl) and N-heterocyclic

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carbene (NHC) ligands.17 Our compounds differ from Mankad’s complexes in the sense that the reactivity is governed not only by the polarization of the Fe–Cu bond but also by the involvement of the oxygen atom. Our preliminary study shows that these “metal-ligand-metal trifunctional” catalysts, as one might call them, can promote the reduction of benzaldehyde under water-gas shift reaction (WGSR) conditions. Although homogeneous WGSR systems are well known in the literature,18 combining the reduction of C=O bonds with the WGSR in one catalytic system is rare.19,20 Beller and coworkers have shown that Knölker’s (cyclopentadienone)iron tricarbonyl complex catalyzes the reduction of PhCHO in DMSO/H2O under CO.19

This

reaction requires at least 1 equiv of K2CO3 (with respect to PhCHO) as an additive; without a base, no alcohol product was observed. In a related study, Wu et al. have employed the same iron tricarbonyl complex as the catalyst and 1 equiv of Na2CO3 as the additive for the reduction of PhCHO in DMSO/H2O, in which case paraformaldehyde is used as the surrogate for CO.21 In this work, we demonstrate that with the heterobimetallic system, the base additive is not needed. Reactivity exhibited by these new complexes versus the component complexes establishes the synergy of iron, oxygen and copper during the catalytic process. R O Fe OC OC

R

CuL

H

Figure 2. Heterobimetallic complexes with three potentially functional sites (L = ligand).

RESULTS AND DISCUSSION Synthesis and Characterization of Fe-Cu Heterobimetallic Complexes. According to Knölker’s original report,9 mixing a THF solution of iron tricarbonyl complex 1a with a 1 M aqueous solution of NaOH (NaOH/1a = 8.4) produces the Knölker complex 2a and its 4 ACS Paragon Plus Environment

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deprotonated form 3a in a 13 : 1 ratio (eq 1). This result implies that 3a reacts with water reversibly to yield 2a and NaOH, and the equilibrium lies to the 2a/NaOH side. We therefore surmised that, to synthesize the targeted Fe-Cu heterobimetallic complexes as illustrated in Figure 2, we could simply replace NaOH with a copper hydroxide complex (LCuOH) and carry out the reaction in a non-aqueous medium to avoid hydrolysis of the product. SiMe3 O

SiMe3 O Fe

OC OC

SiMe3+ NaOH (aq) CO

1a

SiMe3 O Na

THF H Fe SiMe3 Fe SiMe3 + RT, 2.5 h OC OC H - CO 2 H OC OC 3a 2a

(1)

In searching for copper hydroxide complexes,22 the one that caught our attention the most was Nolan’s (IPr)CuOH (IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene), 23 which was reported to react with a wide variety of molecules containing acidic hydrogens. Its hydroxide group is also nucleophilic, showing similar reactivity as NaOH when mixed with iron tricarbonyl complexes 1a-c in C6D6 (eq 2). At room temperature, 1a is completely consumed after 48 h, yielding a hydride species 4a with a characteristic 1H resonance at –12.58 ppm. A similar reaction with 1b is much more sluggish; after 72 h, roughly 50% of 1b is converted to 4b (δH = – 13.37 ppm). It is well known from other studies that the tendency for metal carbonyl complexes to undergo attack by nucleophiles correlates with the stretching frequencies of the CO ligands.24 For example, (arene)manganese tricarbonyl complexes with CO stretching bands below 2000 cm-1 do not react with amines.25 Consistent with these studies, the CO stretching frequencies of 1b (2057, 1997 and 1980 cm-1, in CH2Cl2) were found to be lower than those of 1a (2064, 2008 and 1985 cm-1, in CCl4).26 In fact, the reaction of 1b with NaOH in THF/H2O is sluggish too. Under reflux conditions for 16 h, only 14% of 1b is converted to the corresponding iron hydride species 2b. In contrast, the same reaction of 1a at room temperature reaches completion within 5 ACS Paragon Plus Environment

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2.5 h (eq 1). Interestingly, the phenyl-substituted complex 1c reacts with (IPr)CuOH more rapidly, which is attributed to a less sterically demanding cyclopentadienone ring.27 The reaction is typically complete within 6 h, resulting in a new iron hydride (δH = –10.94 ppm) as confirmed by 1H NMR. R

R O Fe

R +

OC CO OC 1a (R = SiMe3) 1b (R = tBu) 1c (R = Ph)

N

N Cu OH

O

C 6D 6 RT - CO2

Fe OC OC

R H

Cu

N

(2)

N

4a (R = SiMe3) 4b (R = tBu) 4c (R = Ph)

Complexes 4a and 4b can be isolated in an analytically pure form when the reaction of the iron tricarbonyl complexes with (IPr)CuOH is carried out overnight at 60 °C (Scheme 1). Unfortunately, isolating pure 4c is not possible due to its facile decomposition during work-up. This is not surprising considering that the related Knölker-type complex (i.e., replacing (IPr)Cu in 4c with H) is also short-lived.12a Compound 4a can be independently synthesized by reacting the Knölker complex 2a with (IPr)CuOH (or alternatively (IPr)CuOtBu 28 ) for only 30 min. Because one equivalent of water is generated as the by-product, the success of this method suggests that these Fe-Cu heterobimetallic complexes are not as moisture sensitive as originally thought. On the contrary, they are very stable in the presence of water. Monitoring the mixture of 4a and 10 equiv of water (in C6D6) at 60 °C for 48 h shows no sign of any reaction, although at 100 °C, 4a starts to decompose to some black precipitate. Unlike the Knölker complex, which changes color under ambient fluorescent lighting,11a 4a and 4b are not light sensitive.


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R

R O Fe

N

N

toluene 60 oC Fe 16-18 h OC OC - CO2

R + Cu

OC CO OC 1a (R = SiMe3) 1b (R = tBu)

OH

O R H

Cu

N N

4a (R = SiMe3) 4b (R = tBu) SiMe3 O

N

N

H

Fe SiMe3 + OC H OC 2a

Cu

toluene RT, 30 min - H2O

OH

Scheme 1. Synthetic routes to the Fe-Cu heterobimetallic complexes. Solid-State and Solution Structures. Solid-state structures of the two isolated Fe-Cu heterobimetallic complexes were studied by X-ray crystallography. To our surprise, in both cases (IPr)Cu+ was found to bind to the Fe–H moiety rather than to the oxygen (Figure 3). The short C29–O1 distance [1.250(5) Å for 4a', 1.244(5) Å for 4b'] is best described as a C=O unit of a cyclopentadienone ligand, which adopts an envelope-shape conformation with C29 representing the flap.

Like other cyclopentadienone iron complexes reported in the

literature,13,14,15e-h,26,29 the Fe…C29 distance is 0.24-0.30 Å longer than the Fe–C bonds formed between iron and the cyclopentadienone ligand (Table 1). Compounds 4a' and 4b' can thus be viewed as (IPr)Cu–H being σ bonded to a formally 16-electron Fe(0) species.30 The Fe–Cu distance of 4a' [2.4959(9) Å] is comparable to those of many other Fe-Cu heterobimetallic complexes; 31 however, it is noticeably longer than the Fe–Cu distance of 4b' [2.4059(8) Å], (NHC)Cu–Fe(C5R5)(CO)2 (R = H or Me) [2.3215(3)-2.3514(7) Å]17a,c and (Ph2EtP)3Fe(µH)3Cu(PEtPh2) [2.319(2) Å], 32 likely due to greater steric hindrance exerted by the SiMe3 groups. In the case of 4a', the hydride was located from the difference map and shown to bridge both iron and copper. One of the CO ligands has close contact with copper [d(Cu…C) =


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2.486(5) Å], which is similar to what was observed for (NHC)Cu–FeCp(CO)217a and categorized as a semibridging binding CO.33 Compared to 4a', the CO ligands in 4b' approach copper more closely. Consistent with the X-ray structure analysis, the FT-IR spectrum of each solid sample reveals two C≡O bands (1967 and 1903 cm-1 for 4a', 1960 and 1896 cm-1 for 4b') and one C=O band (1577 cm-1 for 4a', 1582 cm-1 for 4b'). The bridging hydride band could be very weak or overlapped with ligand-based vibrational bands. An attempt to locate its position by inspecting the IR spectrum of 4a'-D (made from 1a and (IPr)CuOD) was unsuccessful.

tBu

SiMe3 O Fe

OC OC

O tBu

SiMe3 H Cu N

Fe OC OC

H Cu N

N

N 4b'

4a'

Figure 3. ORTEP drawings of 4a' (top left) and 4b' (top right) at the 50% probability level. Several carbon atoms as well as the semibridging CO (C44–O3) of 4b' are disordered; only one component of each disordered group is shown. The bridging hydride of 4b' could not be located from the difference map.


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Table 1. Selected Bond or Interatomic Distances (Å) and Angles (deg)

Fe–Cu Fe–C43 Fe–C44 Cu…C43 Cu…C44 Fe–H Cu–H C29–O1 C43–O2 C44–O3 Cu–C1 Fe–C28 Fe…C29 Fe–C30 Fe–C31 Fe–C32 Fe–Cu–C1 Fe–C43–O2 Fe–C44–O3 Cu–C44–O3 Cu–Fe–C43 Cu–Fe–C44

4a' 2.4959(9) 1.766(6) 1.767(6) 2.901(5) 2.486(5) 1.78(4) 1.73(4) 1.250(5) 1.153(6) 1.154(6) 1.913(4) 2.131(4) 2.374(5) 2.126(4) 2.073(4) 2.072(4) 160.14(14) 175.7(5) 172.5(4) 117.9(4) 83.93(16) 68.91(16)

4b' 2.4059(8) 1.761(4) 1.843(6), 2.049(19) 2.856(4) 2.279(5), 2.283(19) N/A N/A 1.244(5) 1.148(5) 1.177(7), 1.22(3) 1.903(3) 2.130(4) 2.376(4) 2.101(3) 2.091(4) 2.076(5) 166.49(11) 177.9(4) 173.3(4), 176.3(17) 116.2(4), 111.1(14) 85.04(13) 63.25(14), 61.0(5)

Given the solid-state structures (4a' and 4b') revealed by X-ray crystallography, it is worth questioning whether the Fe(µ-H)Cu core is intact in solution or actually breaks apart to form the terminal-hydride species (4a and 4b) as proposed in eq 2. In this particular case, because of the involvement of a d10 metal (i.e., CuI), a chemical shift value for the hydride will not provide definitive information concerning its bonding mode. 34 Nevertheless, the hydride resonance for 4a appears upfield of that for the Knölker complex 2a (Table 2). It is noted that deprotonation of the OH group of 2a shows a similar effect on the hydride resonance. The 13

C{1H} NMR spectra of these complexes are much more informative, especially the resonance 9 ACS Paragon Plus Environment

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for the cyclopentadienyl ring C29 carbon (referring to the numbering scheme in Figure 3) which reports the strength of the C–O bond. As expected, C–O bond order increases going from 2a to 3a to 1a, so does the chemical shift value for the C29 carbon (Figure 4). A solid sample of 4a', when dissolved in THF-d8,35 displays a resonance at 168.10 ppm for the C29 carbon. This value is in between those of 2a and 3a, suggesting that in solution the Fe-Cu heterobimetallic complex adopts an analogous structure, hence a terminal hydride (4a). The close proximity to the value of 3a also indicates that the O–Cu bond has significant ionic character. Titrating a solution of 4a in C6D6 with DMSO (up to 20 equiv) results in a noticeable shift of the hydride resonance (from – 12.58 to –12.82 ppm) as well as the imidazole resonance (from 6.28 to 6.61 ppm). It is possible that DMSO promotes the formation of a solvent-separated ion pair. Table 2. 1H NMR Hydride Resonances of the Iron Complexes in C6D6 (δ) in THF-d8 (δ) –11.63 –12.00 2a N.D.a –13.03 3a –12.58 –13.26 4a a N.D. = not determined. 3a has poor solubility in C6D6. increased C-O bond strength 146.82 ppm SiMe3

173.38 ppm SiMe3 O Na

O

H Fe SiMe3

OC OC

Fe SiMe3 OC H OC 3a

H 2a 168.10 ppm SiMe3

Fe OC OC

O Cu SiMe3 H

181.74 ppm SiMe3 O SiMe3 Fe OC CO OC 1a

N N

4a

Figure 4. Trend of the C29 resonance (in THF-d8). 10 ACS Paragon Plus Environment

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Additional evidence supporting 4a being the dominant solution structure comes from IR analysis of the solution samples. The strong C=O stretching band at 1577 cm-1 is absent when 4a' is dissolved in toluene, THF, or cyclohexane. The two C≡O bands, however, remain and are shifted to higher wavenumbers (by 8-14 cm-1) from those observed for the solid sample. All in all, both isomers can be described as a tight ion pair of [3a minus Na]– and [(IPr)Cu]+ except that copper resides near oxygen in 4a while with the Fe–H bond in 4a' (eq 3). The energy difference between 4a and 4a' is probably small, so are the kinetic barriers for their interconversion. Crystal packing might provide sufficient driving force to shift the equilibrium to the more compact structure 4a' when the solvent is removed. Variable-temperature 1H NMR spectra of 4a' dissolved in toluene-d8 show that the hydride resonance broadens significantly below 0 °C, coalesces around –30 °C, and disappears between –30 and –73 °C, but becomes sharper when the temperature rises above 30 °C (see Supporting Information for details). These observations support a dynamic process in solution, which may involve a rapid interconversion between 4a and 4a'. SiMe3 O

SiMe3 Fe OC OC

O SiMe3 H

N

Cu

N

SiMe3 H

Fe

OC OC

Cu N

(3)

N 4a

dominant structure in solution

4a' solid-state structure

When 4b' is dissolved in toluene, its IR spectrum shows three C≡O bands at 1973, 1960, and 1902 cm-1 as well as one C=O band at 1611 cm-1. The 1902 cm-1 band is relatively broad, likely due to the presence of two overlapping bands. The C≡O bands at 1960 and 1902 cm-1 match reasonably well with those for the solid sample of 4b'. This result suggests that structures


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4b' and 4b coexist in solution. Assuming similar molar absorptivities for the two isomers, the ratio between the two is estimated to be 2 : 1 favoring 4b'. The IR data for the sample dissolved in THF and cyclohexane are similar, except that in cyclohexane, the four C≡O bands are better resolved. The estimated isomeric ratio of 4b' and 4b is 1 : 1 in both THF and cyclohexane. Variable-temperature 1H NMR spectra of 4b' dissolved in toluene-d8 also indicate some dynamic behavior, displaying a broad hydride resonance at low temperatures (below 0 °C) with a coalescence temperature of around –40 °C (see Supporting Information). At 30 °C or higher, the hydride resonance becomes sharp, similar to what has been observed with the trimethylsilyl derivative.

The lack of synthetic routes to pure iron hydride 2b precludes a systematic

comparison of the carbon resonances as made for the trimethylsilyl analog (Figure 4). However, the C29 resonance of 4b/4b' in C6D6 (170.17 ppm)36 is quite close to the cyclopentadienone carbonyl resonance of 1b (173.73 ppm), consistent with the IR data showing that 4b' exists to a greater extent in solution. Replacing the SiMe3 groups in 4a' with less bulky tBu groups may reduce the steric repulsion with the IPr ligand and narrow the energy gap between the two isomers. Given the structural elucidation described above, for the remainder of the paper, we will use 4a and 4b/4b' to denote the species in solution, and 4a' and 4b' for the solid samples. Stoichiometric Reactions of the Heterobimetallic Complexes. Having synthesized the Fe-Cu heterobimetallic complexes, we shifted our attention to the reactivity of these compounds towards various substrates. At room temperature, complex 4a reacts with HCO2H readily to afford 2a9 and (IPr)CuOCHO37 (eq 4). This process is analogous to the protonation of 3a with H3PO4 to yield 2a,9 except that the conjugate base generated here (i.e., HCO2–) is trapped by


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[(IPr)Cu]+. The reaction of 4b/4b' with HCO2H is similar, producing 2b and (IPr)CuOCHO within 15 min. SiMe3 O Cu Fe SiMe3 OC OC

H

SiMe3 O N N

HCO2H C6D6, RT 15 min

Fe

OC OC

H SiMe3 + H

N

N (4) Cu O

O

2a

4a

H

In contrast to facile insertion of alkynes28,38 and CO237 into (IPr)CuH, 4a is inert to PhC≡CPh and CO2 (1 atm) at room temperature or 60 °C over a period of 2 days. The lack of reactivity with CO2 was somewhat expected because CO2 is a by-product formed during the synthesis of the heterobimetallic complexes (eq 2). On the other hand, this result suggests that under these conditions, dissociation of 4a to a 16-electron iron dicarbonyl species and (IPr)CuH is not favorable. Complex 4a does not react with PhCHO at room temperature; however, at 60 °C, several benzyloxide species (δ 5.07-4.30) including free PhCH2OH appear after 4 h. The reaction is slow with ~50% of 4a remaining after a week. Although the insertion of aldehydes or ketones into (IPr)CuH has been previously proposed 39 and studied computationally, 40 the reaction of 4a with PhCHO is more likely to proceed without the involvement of (IPr)CuH. If it were otherwise, 4a would react with PhC≡CPh and CO2 to generate insertion products. Perhaps 4a resembles the Knölker complex 2a, transferring hydride and copper to the aldehyde in a concerted manner11a (Figure 5) to yield a 16-electron iron dicarbonyl species and (IPr)CuOCH2Ph.

The latter compound is available from a room temperature reaction of

(IPr)CuOH with PhCH2OH, or alternatively from a metathesis reaction between (IPr)CuCl and NaOCH2Ph. However, it is unstable at 60 °C, especially in the presence of water, reverting back to (IPr)CuOH and PhCH2OH. 13 ACS Paragon Plus Environment

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TMS O Fe TMS OC OC

H Ph

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N

Cu

N

O

H

Figure 5. Proposed reduction of PhCHO by 4a. Complex 4a reacts with 1 atm of CO at 60 °C, albeit at a slow rate (eq 5). The identifiable product is the iron tricarbonyl complex 1a, although the fate of the "(IPr)Cu" part is not clear to us. The reaction of 4a with CO (6.4 atm) in the presence of 3-hexyne gives (IPr)Cu(CEt=CHEt),28 suggesting that the initial copper product is (IPr)CuH.

A control

experiment shows that [(IPr)CuH]228 is unstable under a CO atmosphere, producing an intractable mixture.

The reaction of 4a with CO was investigated further under different

conditions and the results are summarized in Table 3. Increasing the CO pressure from 1 atm to 6.4 atm results in a higher conversion of 4a to 1a (entry 3). Replacing C6D6 with a more polar solvent THF-d8 has no effect on the rate of the reaction over 24 h (entry 4 vs. entry 1), but gives a slightly higher conversion over 72 h (entry 5 vs. entry 2). The tert-butyl derivative 4b/4b' is completely unreactive with CO under similar conditions. SiMe3 Fe OC OC

H 4a

SiMe3 O

O Cu SiMe3

N N

CO 60 oC

Fe

SiMe3 + "(IPr)CuH" (5)

OC CO OC 1a


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Table 3. Reactivity of the Fe-Cu heterobimetallic complexes towards CO Entry Complex Solvent Temp. pCO Time Conversiona 1 C6D6 60 °C 1 atm 24 h 17% 4a 2 C6D6 60 °C 1 atm 72 h 21% 4a 3 C6D6 60 °C 6.4 atm 48 h 71% 4a o 1 atm 24 h 17% 4 THF-d8 60 C 4a 60 °C 1 atm 72 h 30% 5 THF-d8 °C 4a 6 4b/4b' THF-d8 60 °C 1 atm 72 h 0% a 1 Determined by H NMR (the remaining iron species was 4a or 4b/4b').

Catalytic Reduction of PhCHO under WGSR conditions.

The reactivity of 4a

towards PhCHO (Figure 5) and CO (eq 5), the hydrolysis of (IPr)CuOCH2Ph, and how 4a is synthesized (eq 2) suggest that we can construct a catalytic cycle for the reduction of PhCHO using CO and H2O, or under WGSR conditions. Compared to the monometallic system based on 1a,19,21 employing Fe-Cu heterobimetallic complexes 4a and 4b/4b' as the catalysts does not require an external base. The in-situ generated (IPr)CuOH should play the role of a base to regenerate the hydride species. Moreover, a solvent less polar than DMSO can be used. In Beller’s work, switching DMSO/H2O (1 : 1) to toluene/H2O (1 : 1) shuts down the catalysis completely.19 In contrast, in toluene, 4a alone is an effective catalyst for the reduction of PhCHO to PhCH2OH by water (20 equiv) and CO (Table 4). At 100 oC and under 6.4 atm of CO pressure, a nearly quantitative conversion can be achieved in 24 h with a catalyst loading of 1.5 mol% (entry 2). Lowering the loading to 1 mol % (entry 3) leads to a partial conversion of PhCHO, which is further compromised by reducing the CO pressure to 5 atm (entry 5). Compared to 4a, the tert-butyl derivative 4b/4b' is a less active catalyst (entry 2 vs. entry 6). The reaction is also catalyzed by a 1 : 1 mixture of 1a and (IPr)CuOH with efficiency similar to that of 4a (entry 7). Interestingly, the mixture of 1b and (IPr)CuOH is more active than the


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heterobimetallic complex 4b/4b' (entry 8 vs. entry 6), suggesting that in this case it is more advantageous to keep the iron and copper fragments apart. Table 4. Reduction of PhCHO Under WGSR Conditions catalyst PhCHO + H 2O + CO PhCH 2OH + CO 2 toluene (1 equiv) (20 equiv) 100 oC, 24 h

entry catalyst cat. loading (mol%) pCO (atm) conversion (%)b 1 5 6.4 >99c 4a 2 1.5 6.4 >99 4a 3 1 6.4 87 4a 4 5 5.0 >99 4a 5 1 5.0 72 4a 6 4b/4b' 1.5 6.4 40 a 7 1a + (IPr)CuOH 1.5 6.4 >99 a 8 1b + (IPr)CuOH 1.5 6.4 62 a b The iron and copper species were mixed in a 1 : 1 ratio. NMR spectra showed no other products originated from PhCHO. c GC yield for this reaction was >99%.

To gain a better understanding of the synergy between the iron and copper components, several control experiments were performed using monometallic iron or copper complexes (Scheme 2) under the catalytic conditions optimized for 4a (Table 4, entry 2). Iron complex 1a proves to be an inactive catalyst for the reduction of PhCHO. Iron hydride 2a shows very low catalytic activity, converting only 5% of PhCHO to PhCH2OH. The small amount of alcohol was expected because 2a reacts with PhCHO stoichiometrically to generate a thermally unstable iron alcohol complex.11a While the majority of this alcohol complex should be converted to 1a under CO, some may decompose to species that exhibit additional but limited catalytic activity. In any case, 4a is a significantly more active catalyst, confirming that the copper component is indispensable.

Catalytic performance of copper-only complexes including (IPr)CuOH,

[(IPr)CuH]2, and (IPr)CuOCH2Ph was investigated similarly.

In each case, the 1H NMR

spectrum shows a mixture of unreacted PhCHO, PhCH2OH, the Tischenko product 16 ACS Paragon Plus Environment

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PhCO2CH2Ph, and some unidentified products. In contrast, using 4a as the catalyst leads to a clean conversion of PhCHO to PhCH2OH. Taken together, these results suggest that the iron component enhances both the efficiency and the selectivity of the reaction. 1.5 mol% [Fe] or [Cu]

PhCHO + H2O + CO (1 equiv) (20 equiv) (6.4 atm)

SiMe3 O

SiMe3 O [Fe]

Fe

SiMe3

OC CO OC 1a 0% conversion [Cu]

(IPr)CuOH

PhCH2OH + CO2

toluene 100 oC, 24 h

Fe

H SiMe3

OC H OC 2a 5% conversion

[(IPr)CuH]2 (IPr)CuOCH2Ph

gave unreacted PhCHO, PhCH2OH, PhCO2CH2Ph and other products

Scheme 2. Control experiments with monometallic complexes.

Without PhCHO, 4a catalyzes the WGSR, although its catalytic activity has yet to be quantified. The formation of H2 is confirmed by 1H NMR spectroscopy, when the reaction is carried out at 60 °C. Higher temperatures make it more difficult to detect H2 because of lower solubility of the gas. A catalytic reaction performed under 1 atm of evidence for the formation of CO2 (Scheme 3). resonances of both

13

The

13

13

CO provides strong

C{1H} NMR spectrum displays

CO2 (δ 124.88) and (IPr)CuO13CHO (δ 167.44). The copper formate

complex is likely generated from

13

CO2 insertion into (IPr)CuH. A control experiment of

(IPr)CuOH mixed with CO rules out an alternative mechanism that involves CO insertion into (IPr)CuOH followed by isomerization of the resulting metallo-carboxylic acid. In addition to 13

CO2, (IPr)CuO13CHO, and the unreacted

13

CO (δ 184.52), 4a-13CO and 1a-13CO (δ 209.84)


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are present in the reaction mixture. It should be mentioned that the Knӧlker complex 2a also exchanges with 13CO and at the same time is converted to 1a-13CO via the loss of H2. 13CO

(1 atm)

+ H 2O

13C-labelled

O13C

SiMe3 O Cu SiMe 3 Fe

OC

H

5 mol% 4a toluene-d8 100 oC, 1-4 d

13CO

2

+ H2

organometallic products

N N

4a-13CO

SiMe3 O SiMe3

Fe OC 13CO OC

O 13 C

(IPr)Cu

H

O

1a-13CO

Scheme 3. Catalytic WGSR using 13CO. Mechanisms for the Reduction of PhCHO. On the basis of the stoichiometric reactions and labeling study, we propose two different catalytic cycles (Scheme 4, color coded in red and blue) that converge at the step to regenerate the heterobimetallic species (in magenta). Reduction of PhCHO by 4a via the transition state depicted in Figure 5 produces 5a and (IPr)CuOCH2Ph. The separate iron and copper fragments independently react with CO and H2O to give 1a and (IPr)CuOH, respectively. The catalytic cycle in red is completed by the reaction of 1a with (IPr)CuOH to release CO2. Direct dissociation of 4a to 5a and (IPr)CuH is possible at 100 °C; however, this process is more favorable in the presence of CO, resulting in 1a and (IPr)CuH (the blue cycle). The copper hydride then reacts with PhCHO and H2O sequentially to transform into (IPr)CuOH for the awaiting 1a.


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SiMe3 O SiMe3 H

Fe

OC OC

SiMe3 O

Cu N N

4a'

Fe OC OC 5a

PhCHO

N

SiMe3 +

N Cu OCH2Ph H2 O

CO SiMe3 O Fe OC OC

SiMe3 O

Cu SiMe3

H

PhCH2OH

N

-CO2

N

4a

SiMe3 + Fe OC CO OC 1a

CO

N

OC OC

SiMe3 + CO

1a

-PhCH2OH

Cu N

OH H2 H2O

SiMe3 O Fe

H2O

N

N

N Cu OCH2Ph + 1a

N Cu H

PhCHO

Scheme 4. Catalytic cycles for the reduction of PhCHO with 4a under WGSR conditions.

We cannot rule out an alternative mechanism involving 4a-catalyzed WGSR followed by hydrogenation of PhCHO with H2 generated from CO and H2O. As a matter of fact, at 100 °C, 4a catalyzes the hydrogenation of PhCHO (in toluene) to PhCH2OH under 6.4 atm of H2 pressure. With a catalyst loading of 1.5 mol%, a quantitative conversion is achieved within 24 h. If the hydrolysis of (IPr)CuH (Scheme 4, highlighted in green) is more competitive than PhCHO insertion, the WGSR would occur prior to the reduction of PhCHO. In an attempt to test the competition between water and PhCHO for the reaction with (IPr)CuH, [(IPr)CuH]2 (synthesized from (IPr)CuOtBu and (EtO)3SiH)28 was treated with H2O/PhCHO (20 : 1) in C6D6. H2 was detected; however, due to the excess of water, it was not possible to quantify how much PhCHO was reduced. 19 ACS Paragon Plus Environment

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The side reactions of (IPr)CuH with CO2 and CO negatively impact the catalytic reaction. CO2 inserts into (IPr)CuH to form (IPr)CuOCHO, which may re-enter into the catalytic cycle via deinsertion of CO2 but reduces the steady-state concentration of the active (IPr)CuH. CO causes decomposition of (IPr)CuH, as confirmed by the control experiment. It is noted that as the catalytic reaction progresses, the color of the reaction mixture darkens. To test the possibility of heterogeneous catalysis, a mercury poisoning experiment was conducted.

Adding 100-fold

excess of elemental mercury shows no retardation to the 4a-catalyzed reduction of PhCHO under WGSR conditions, suggesting that the catalytic reaction is homogenous.41

CONCLUSIONS We have successfully synthesized a unique class of Fe-Cu heterobimetallic complexes that preserves the core structure of the Knölker complex but incorporates an (NHC)Cu fragment. In the solid state these complexes adopt a structure featuring a hydride bridging iron and copper; however, in solution they are present as both the bridging hydride and a terminal iron hydride that are in a rapid equilibrium.

The isomeric ratio depends on the substituents on the

cyclopentadienone/cyclopentadienyl ring; the complex bearing bulky Me3Si groups favors the terminal hydride species. These complexes show stoichiometric reactivity towards HCO2H, PhCHO and CO, and catalytic activity for the reduction of PhCHO under WGSR conditions. Control experiments with monometallic iron or copper complexes have highlighted the importance of having both iron and copper components for the catalytic reaction to be efficient and selective.

Compared to the catalytic system using only iron complexes,19 the

heterobimetallic system does not require a large excess of base additive and avoids the use of a high-boiling polar solvent (i.e., DMSO). Though effective, the heterobimetallic complexes and


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the copper component in particular decompose over the time. Our future efforts will be focused on building similar molecules with more robust ligand scaffolds.

EXPERIMENTAL SECTION Materials and General Considerations. Unless otherwise mentioned, all air-sensitive compounds were handled under an inert atmosphere using standard Schlenk line and inertatmosphere box techniques. Toluene, pentane, and THF were deoxygenated and dried in a solvent purification system by passing through an activated alumina column and an oxygenscavenging column under argon. Cyclohexane, glyme, and water were degassed by bubbling argon or nitrogen through them for 30 min prior to use. C6D6 was distilled from Na and benzophenone under an argon atmosphere. Other deuterated solvents such as CDCl3 and THF-d8 (packed in an ampule) were used as received from commercial sources. Benzaldehyde was purchased from Sigma-Aldrich and purified by vacuum distillation prior to use. Infrared spectra were recorded on a Thermo Scientiﬁc Nicolet 6700 FT-IR spectrometer equipped with smart orbit diamond attenuated total reﬂectance (ATR) accessory.

Complexes 1a, 42 2a,9, 43 and

(IPr)CuOH23 were synthesized according to literature procedures. Synthesis of 4a/4a'. Method A (from 1a): In a glove box, an oven-dried Schlenk flask equipped with a stir bar was charged with (IPr)CuOH (150 mg, 0.32 mmol), 1a (134 mg, 0.32 mmol), and 25 mL of toluene. The mixture was heated at 60 oC for 16 h, resulting in a yellow solution. After cooling to room temperature, the solution was filtered through a short plug of Celite and dried under vacuum. The remaining sticky material was washed with a small amount of pentane (~ 5 mL) and dried to afford a yellow powder. The desired pure product was isolated by recrystallization from a saturated toluene solution layered with pentane and kept at –30 oC


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(124 mg, 46%). Method B (from 2a): In a glove box, an oven-dried Schlenk flask equipped with a stir bar was charged with (IPr)CuOH (186 mg, 0.40 mmol), the Knӧlker complex 2a (156 mg, 0.40 mmol), and 25 ml of toluene. The resulting yellow solution was stirred under argon for 30 min and then filtered through a short plug of Celite. The filtrate was concentrated under vacuum to give a yellow solid, which was washed with pentane (5 mL) and dried. The desired product was isolated as a yellow powder (201 mg, 60%). 1H NMR (400 MHz, C6D6, δ): 7.23 (t, 2H, 3JHH

= 7.6 Hz, ArH), 7.08 (d, 4H, 3JH-H = 7.6 Hz, ArH), 6.28 (s, 2H, NCH=CHN), 2.63-2.51 (m, 6H,

CH(CH3)2 + CH2), 2.25-2.21 (m, 2H, CH2), 1.61-1.58 (m, 2H, CH2), 1.37-1.35 (m, 14H, CH(CH3)2 + CH2), 1.03 (d, 12H, 3JH-H = 7.2 Hz, CH(CH3)2), 0.31 (s, 18H, Si(CH3)3), –12.58 (s, 1H, FeH).

13

C{1H} NMR (101 MHz, C6D6, δ): 218.46 (Fe(CO)2), 181.24 (CCu), 168.92

(CpCO), 145.60 (ortho-ArC), 135.15 (ipso-ArC), 130.85 (para-ArC), 124.61 (meta-ArC), 123.06 (NCH=CHN), 103.12 (CpC), 68.37 (CpC), 29.00 (CH(CH3)2), 26.49 (CH2), 24.60 (CH(CH3)2), 24.26 (CH(CH3)2), 23.58 (CH2), 1.65 (Si(CH3)3).

13

C{1H} NMR (101 MHz, THF-

d8, δ): 218.63 (Fe(CO)2), 180.65 (CCu), 168.10 (CpCO), 146.25 (ortho-ArC), 136.00 (ipso-ArC), 130.96 (para-ArC), 124.89 (meta-ArC), 124.78 (NCH=CHN), 102.48 (CpC), 68.12 (CpC), 29.45 (CH(CH3)2), 26.77 (CH2), 24.64 (CH(CH3)2), 24.26 (CH(CH3)2), 23.93 (CH2), 1.39 (Si(CH3)3). ATR-IR (solid, cm-1): 1967 (νC≡O), 1903 (νC≡O), 1577 (νC=O). The νC≡O values obtained from transmission-IR of solution samples (cm-1): 1977 and 1912 (in toluene), 1976 and 1911 (in THF), 1980 and 1917 (in cyclohexane). Anal. Calcd for C44H63N2O3Si2FeCu: C, 62.65; H, 7.53; N, 3.32. Found: C, 62.91; H, 7.40; N, 3.10. Synthesis of {2,5-(tBu)2-3,4-[(CH2)4](η4-C4CO)}Fe(CO)3 (1b).

Under a nitrogen

atmosphere, tBuC≡C(CH2)4C≡CtBu (4.37 g, 20 mmol), Fe(CO)5 (5.26 mL, 40 mmol), and glyme (5 mL) were mixed in a Fisher-Porter glass tube. The reaction vessel was flushed several times


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ACS Catalysis

with 3 atm of CO before disconnecting from the CO source. The closed system was heated by a 120 ºC oil bath for 22 h. After cooling, the reaction mixture was concentrated under vacuum and the residue was purified by column chromatography (eluted with 1 : 1 hexanes/CH2Cl2 first, and then with 1 : 1 hexanes/EtOAc). The desired product was isolated as an air-stable yellow solid (3.55 g, 46% yield). 1H NMR (300 MHz, CDCl3, δ): 2.88-2.75 (m, 2H, CH2), 2.68-2.52 (m, 2H, CH2), 1.82-1.73 (m, 4H, CH2), 1.33 (s, 18H, CH3). 1H NMR (400 MHz, C6D6, δ): 2.45-2.38 (m, 2H, CH2), 2.11-2.05 (m, 2H, CH2), 1.36 (s, 18H, CH3), 1.26-1.14 (m, 4H, CH2).

13

C{1H} NMR

(90 MHz, CDCl3, δ): 210.53 (Fe(CO)3), 173.25 (CpCO), 100.63 (CpC), 92.52 (CpC), 33.50 (C(CH3)3), 30.22 (C(CH3)3), 25.15 (CH2), 22.32 (CH2).

13

C{1H} NMR (101 MHz, C6D6, δ):

211.08 (Fe(CO)3), 173.73 (CpCO), 100.57 (CpC), 92.66 (CpC), 33.62 (C(CH3)3), 30.13 (C(CH3)3), 25.00 (CH2), 22.07 (CH2). ATR-IR (solid, cm-1): 2048 (νC≡O), 1979 (νC≡O), 1950 (νC≡O), 1620 (νC=O). IR (toluene, cm-1): 2055 (νC≡O), 1996 (νC≡O), 1975 (νC≡O), 1644 (νC=O). IR (CH2Cl2, cm-1): 2057 (νC≡O), 1997 (νC≡O), 1980 (νC≡O), 1630 (νC=O). HRMS-ESI (m/z): [M+H]+ calcd for C20H27O4Fe, 387.1258; found, 387.1245. Synthesis of 4b/4b'. In a glove box, an oven-dried Schlenk flask equipped with a stir bar was charged with (IPr)CuOH (121 mg, 0.26 mmol), 1b (100 mg, 0.26 mmol), and 25 mL of toluene.

The resulting mixture was heated at 60 oC for 18 h, and upon cooling to room

temperature filtered through a short plug of Celite. Removal of the volatiles under vacuum afforded a yellow residue, which was washed with a small amount of pentane (~ 5 mL) and then dried. An analytically pure product was obtained from recrystallization in toluene-pentane at – 30 oC (102 mg, 49%).

1

H NMR (400 MHz, C6D6, δ): 7.26-7.07 (m, 6H, ArH), 6.33 (s, 2H,

NCH=CHN), 2.67-2.62 (m, 6H, CH(CH3)2 + CH2), 2.37-2.29 (m, 2H, CH2), 1.45 (s, 18H, C(CH3)3), 1.41 (d, 12H, 3JH-H = 8.0 Hz, CH(CH3)2), 1.36-1.32 (m, 4H, CH2), 1.04 (d, 12H, 3JH-H


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= 10.4 Hz, CH(CH3)2), –13.37 (s, 1H, FeH).

Page 24 of 33

13

C{1H} NMR (101 MHz, C6D6, δ): 216.97

(Fe(CO)2), 182.39 (CCu), 170.17 (CpCO), 145.66 (ortho-ArC), 135.14 (ipso-ArC), 130.79 (para-ArC), 124.46 (meta-ArC), 122.67 (NCH=CHN), 95.37 (CpC), 88.67 (CpC), 33.03 (C(CH3)3), 31.14 (C(CH3)3), 29.05 (CH(CH3)2), 26.68 (CH2), 24.51 (CH(CH3)2), 24.23 (CH(CH3)2), 23.19 (CH2).

ATR-IR (solid, cm-1): 1960 (νC≡O), 1896 (νC≡O), 1582 (νC=O).

Transmission-IR (toluene, cm-1): 1973 (νC≡O), 1960 (νC≡O), 1902 (νC≡O), 1611 (νC=O). Transmission-IR (THF, cm-1): 1972 (νC≡O), 1962 (νC≡O), 1903 (νC≡O), 1881 (νC≡O), 1606 (νC=O). Transmission-IR (cyclohexane, cm-1): 1978 (νC≡O), 1962 (νC≡O), 1905 (νC≡O), 1894 (νC≡O), 1618 (νC=O). Anal. Calcd for C46H63N2O3FeCu: C, 68.09; H, 7.83; N, 3.45. Found: C, 67.86; H, 7.62; N, 3.23. General Procedure for the Reduction of PhCHO under WGSR Conditions. In a glove box, an oven-dried pressure tube equipped with a stir bar was charged with a 4.73 mM stock solution of 4a or 4b/4b' in toluene (3 mL, 14.2 µmol) and PhCHO (96.6 µL, 0.95 mmol, for a catalyst loading of 1.5 mol%). After mixing, the pressure tube was connected to a FisherPorter apparatus, which was subsequently attached to a Schlenk line filled with argon. Under a slightly positive argon pressure, degassed water (0.34 mL, 18.9 mmol, 20 equiv with respect to PhCHO) was added to the reaction vessel, and the system was flushed at least three times with 5 atm of CO. The final CO pressure was set to 6.4 atm, and the Fisher-Porter apparatus was disconnected from the CO gas cylinder. The closed reaction system was heated at 100 oC with an oil bath and stirred at 600 rpm. After 24 h, H2 was slowly vented and the reaction mixture was cooled to room temperature. The resulting yellow solution was filtered through a Pasteur pipette packed with a short plug of silica gel, which was washed thoroughly with THF until the solution coming out of the pipette became almost colorless. Evaporation of the filtrate to dryness


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ACS Catalysis

afforded an oily residue, which was dissolved in CDCl3 and its 1H NMR spectrum recorded. The conversion from PhCHO to PhCH2OH was calculated based on the 1H NMR integrations of these two species. For reactions with a different catalyst loading, the volume of the catalyst stock solution (4.73 mM) was fixed at 3 mL. The amount of PhCHO and water was varied but the PhCHO to water ratio was kept at 1 : 20. Control experiments using a complex bearing only iron or copper were conducted similarly. The concentration of the complex (1.5 mol% loading) was kept at 4.73 mM and the ratio between PhCHO and water was maintained at 1 : 20.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.xxxxxxx. Experimental procedures, complete details of the crystallographic study (PDF and CIF), and characterization data of 4a/4a' and 4b/4b'. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-1156449 and CHE-1464734) and the University of Cincinnati (Doctoral Enhancement Research Fellowship to A.C.) for support of this research. Crystallographic data were collected on a Bruker SMART6000 diffractometer, which was funded by an NSF-MRI grant (CHE-0215950).


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REFERENCES 1

(a) Enzyme Catalysis in Organic Synthesis, 3rd ed.; Drauz, K., Gröger, H., May, O., Eds.;

Wiley-VCH: Weinheim, Germany, 2012. (b) Rauwerdink, A.; Kazlauskas, R. J. ACS Catal. 2015, 5, 6153-6176. (c) Palermo, G.; Cavalli, A.; Klein, M. L.; Alfonso-Prieto, M.; Dal Peraro, M.; De Vivo, M. Acc. Chem. Res. 2015, 48, 220-228. (d) Hanoian, P.; Liu, C. T.; HammesSchiffer, S.; Benkovic, S. Acc. Chem. Res. 2015, 48, 482-489. 2

(a) Bifunctional Molecular Catalysis; Ikariya, T., Shibasaki, M., Eds.; Topics in

Organometallic Chemistry, Vol. 37, Springer-Verlag, Berlin, Heidelberg, Germany, 2011. (b) Askevold, B.; Roesky, H. W.; Schneider, S. ChemCatChem 2012, 4, 307-320. (c) van der Vlugt, J. I. Eur. J. Inorg. Chem. 2012, 363-375. (d) Cooperative Catalysis: Designing Efficient Catalysts for Synthesis; Peters, R., Ed.; Wiley-VCH: Weinheim, Germany, 2015. 3

(a) Bullock, R. M.; Casey, C. P. Acc. Chem. Res. 1987, 20, 167-173. (b) Stephan, D. W. Coord.

Chem. Rev. 1989, 95, 41-107. (c) Wheatley, N.; Kalck, P. Chem. Rev. 1999, 99, 3379-3419. (d) Gade, L. H. Angew. Chem. Int. Ed. 2000, 39, 2658-2678. (e) Thomas, C. M. Comment Inorg. Chem. 2011, 32, 14-38. (f) Cooper, B. G.; Napoline, J. W.; Thomas, C. M. Catal. Rev. Sci. Eng. 2012, 54, 1-40. (g) Mata, J. A.; Hahn, F. E.; Peris, E. Chem. Sci. 2014, 5, 1723-1732. (h) Mankad, N. P. Chem. Eur. J. 2016, 22, 5822-5829. 4

(a) Noyori, R.; Ohkuma, T. Angew. Chem. Int. Ed. 2001, 40, 40-73. (b) Noyori, R.; Kitamura,

M.; Ohkuma, T. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5356-5362. (c) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201-2237. (d) Kanai, M.; Kato, N.; Ichikawa, E.; Shibasaki, M. Synlett 2005, 1491-1508. (e) Muñiz, K. Angew. Chem. Int. Ed. 2005, 44, 6622-6627. (f) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4, 393-406. (g)


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THF-d8 was chosen as the solvent as it can dissolve all the iron complexes in Figure 4.

36

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Although iron is not one of these metals that form an amalgam readily with Hg(0), copper

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