Evaluation of Organic Hydride Donors as Reagents for the Reduction

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

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Evaluation of Organic Hydride Donors as Reagents for the Reduction of Carbon Dioxide and Metal-Bound Formates Timothy E. Elton,† Graham E. Ball,*,† Mohan Bhadbhade,‡ Leslie D. Field,† and Stephen B. Colbran*,† †

School of Chemistry and ‡Mark Wainwright Analytical Centre, The University of New South Wales, Sydney, New South Wales 2052, Australia

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S Supporting Information *

ABSTRACT: A variety of organic hydride donors (OHDs) have been tested as reagents for the transfer of hydride to iron formato complexes in the activation and reduction of carbon dioxide. Theoretical calculations show that the selection of OHD and solvent is crucial when planning systems involving OHD cooperativity. Strong consideration is given to the likelihood that metal centers may deactivate formate to hydride attack, since, in general, the formate group has more resonance stabilization energy when complexed to a metal center compared to an organoformate or formic acid. It is experimentally demonstrated that 1,2dihydropyridine is not a competent reducing agent for carbon dioxide.

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INTRODUCTION Carbon dioxide has the potential to become an important C1 chemical feedstock, and a formidable body of research is currently dedicated to investigating this possibility.1−10 Owing to the reckless use of fossil fuel sources to provide energy and materials, carbon dioxide from industry is readily available as a concentrated emission for capture and transformation.11,12 At the forefront of efforts to utilize this waste product are numerous studies of organotransition metal complexes as catalysts for the chemical and electrochemical reduction of carbon dioxide. The literature is vast, but well-surveyed,1−12 given the currency and importance of the area. Of importance to the work to be described in this paper is the coordination of carbon dioxide to transition metals: Binding of carbon dioxide to a metal center is generally accepted to activate it and to lower the activation energy for subsequent reactions.1−13 The coordination of carbon dioxide to a metal center is thus a process that has received constant attention, with η1-C, η2-CO, and less common η1-O binding modes of carbon dioxide to a metal center being wellunderstood, as are the compounds produced when coupling these species with reactive substrates.13,14 Moreover, insertion of carbon dioxide into an existing metal−ligand bond has also been extensively studied, and ligands that are most amenable to insertion of carbon dioxide include those with silicon, oxygen, phosphorus, nitrogen, and carbon donors, along with other metals and hydrides.13 The insertion of carbon dioxide into a metal hydride bond is often facile and affords a metal−formato species (eq 1). M − H + CO2 ⇆ M−OCHO

the metal, its ligands, and the environment (solvent) all affecting the energetics of the insertion.17,18 Waldie and Kubiak et al. have recently considered the thermodynamics of this reaction in detail. In short, the hydricity of formate, ΔG°H−(HCO2−), in acetonitrile is ∼185 kJ mol−1; therefore, the hydricity of a hydride donor, be it transition metal or organic,19 must be less than this for an exergonic transfer of hydride to CO2.20 Their work showed a most useful and robust correlation between the one-electron reduction potential of a metal hydride complex and its hydricity, thus allowing easy measurement and prediction of the reactivity of a metal hydride species with CO2. A thoroughly characterized iron−hydride complex that displays a well-understood insertion reaction chemistry with carbon dioxide is cis-[Fe(dmpe)2H2] (1: dmpe = 1,2bis(dimethylphosphino)ethane).21 When placed under an atmosphere of carbon dioxide, solutions of 1 selectively form trans-[Fe(dmpe)2H(OCHO)] (2) in quantitative yield, Scheme 1.21 Increasing the pressure of CO2 within the reaction vessel causes the double-insertion product, transScheme 1. Reactions of cis-[Fe(dmpe)2H2] (1) with CO2

(1)

The reaction can occur for a variety of metal−hydride centers bound by many different ancillary ligands,8,15,16 with © XXXX American Chemical Society

Received: August 19, 2018

A

DOI: 10.1021/acs.organomet.8b00600 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

subsequent addition of a proton (pKa 4.1) and 1e−-reduction being spontaneous and producing dihydropyridine 6.36−38 From crucial dihydropyridine 6, the overall multistep reduction of carbon dioxide was proposed to proceed as a sequence of hydride transfer (HT) then proton transfer (PT) steps as is depicted in Scheme 2.36−38 That pyridine is a catalyst for the electrochemical reduction CO2 is controversial: First, most homogeneous catalysts do not catalyze electrochemical reduction of CO2 beyond the twoelectron level (thus carbon monoxide or formate are the normal reduction product(s)); second, pyridine is such a simple and readily accessible heterocycle. Third, several authors have found the experimental evidence suffers from a lack of reproducibility.49−57 For noble metal electrodes, the consensus viewpoint has recently trended toward weak initial catalysis, followed by poisoning by produced carbon monoxide, which in sum leads to very poor turnovers. At semiconductor electrodes, photoelectrocatalysis of CO2 reduction by pyridine is understood to be a heterogeneous process.46,48,58 Other studies, however, do provide evidence that pyridine may be an important homogeneous catalyst, such as in the [Ru(bpy)3]2+photosensitized production of methanol from CO2 in the presence of pyridine catalyst without the use of electrodes, albeit with low turnover numbers.59−61 The ongoing debate certainly indicates that simple heterocycles may possess OHD functionality useful in the electrochemical, and/or chemical, reduction of carbon dioxide. Therefore, we surmised that OHDs could perhaps add hydride ion directly to a metal−formato intermediate available from reaction of a metal hydride and CO2. Such a reaction would lead to a metal-bound hydroxymethanolate species, which is illustrated for a hypothetical reaction of iron−formato complex 2 to afford the corresponding iron−hydroxymethanolato complex 9 in Scheme 3. The reaction takes reduction of

[Fe(dmpe)2(OCHO)2] (3) to form.21 The first insertion is irreversible, and the produced formato−iron complex is stable even to mild heating.21 The second insertion reaction is readily reversed: At lower than ∼2 atm of carbon dioxide, loss of the second CO2 molecule leads to reformation of 2.21 Darensbourg et al. have calculated insertion of carbon dioxide into the iron-hydride bonds of 1 and 2 to be exothermic by −75.2 and −61.0 kJ mol−1, respectively.17 These heats of reaction are significantly greater than for the corresponding ruthenium analogues for which the estimated reaction enthalpies were −54.8 and −5.9 kJ mol−1 for CO2 insertion into the first and second hydride, respectively.17 Also central to the research work described in this paper are organic hydride donors (OHDs), which are organic molecules that are able to donate hydride ion and are typically heterocycles such as those illustrated in Chart 1.22−25 Chart 1. Organic Hydride Donors Considered in This Worka

a

From left to right: 1-benzyl-1,4-dihydronicotinamide (4), Hantzsch’s ester (5), 1,2-dihydropyridine (6), 2-methyl-1,2-dihydrobipyridine (7), and 2-methyl-1,2-dihydropyridine (8).

Bioinspired examples of OHDs have recently found extensive utility in asymmetric reduction.22,23 Conveniently, OHDs are available with thermodynamic hydricities (bond enthalpies for C−H heterolysis) and kinetic hydricities (nucleophilicities) tailored over a wide range, and many (most) are stable in the presence of Brønsted acids.24,25 Moreover, a body of experimental results, backed by independent computational studies, have suggested that pyridine may act as an electrochemically recyclable OHD (via dihydropyridine) capable of reduction of carbon dioxide to methanol. Bocarsly et al. and others reported that pyridine catalyzes the electrochemical reduction of carbon dioxide to methanol. 26−35 Theoretical studies of Musgrave, 36−38 Keith,39−45 and Carter39−43,45−48 and their co-workers in particular implicate 1,2-dihydropyridine 6 as the active reducing agent in homogeneous solution. Foremost, Musgrave’s mechanism for reduction relies upon the four step formation of dihydropyridine species 6: Protonation of pyridine (pKa 5.3) is followed by 1e−-reduction at −1.31 V to give the pyridinyl radical (•pyH), with

Scheme 3. Hypothetical Scheme for the Reaction of Iron− Formato Complex 2 with an OHDa

a

2-Methyl-1,2-dihydropyridine as the OHD is illustrated (with the hydridic H in blue and the acidic H in red) in a hypothetical reaction with the iron-formato complex 2 to afford the corresponding ironhydroxymethanolato complex 9.

Scheme 2. Multistep Reduction of Carbon Dioxide by 1,2-Dihydropyridine (6)a

a

As proposed by Musgrave et al.36−38 The DFT calculations of Musgrave et al. revealed the hydride transfer (HT) steps were assisted by polarization from hydrogen-bonded water molecules that were explicitly included in their modeling. The HT reaction to CO2 afforded formate, which proton transfer (PT) converted to formic acid. Repetition of HTPT steps then produced methanediol, which gave formaldehyde on dehydration. A final HTPT sequence led to the formation of methanol. B


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Organometallics

Figure 1. 150.9 MHz 13C{1H} DEPT-135 NMR spectrum of Lansbury’s reagent after hydrolysis in d8-THF before (top) and after (bottom) addition of 13CO2. The peak assignments follow those of Tanner et al.65 Methylene-C peaks appear with negative phase and CH-signals with positive phase; the signal for 13CO2 (*), which is suppressed in the DEPT experiment, still appears as an out of phase peak due to its high intensity.

spectroscopy. 1,2-Dihydropyridine 6 was generated in situ using Lansbury’s reagent, [Li{Al(NC5H6)4}], a 4-coordinate aluminum dihydropyridine adduct that gives a mixture of the 1,2-, 1,4-, and 2,5-isomers of dihydropyridine when hydrolyzed with water.65 Though the reactivity of the dihydropyridine isomers is slightly different,65 we assessed that the difference would be unlikely to affect the overall reaction outcome (and some 1,2-dihydropyridine would be present in any case). Accordingly, a mixture of 1,2-, 1,4-, and 2,5-isomers of dihydropyridine was formed in situ by the hydrolysis of Lansbury’s reagent, Li[Al(NC5H6)4], in d8-THF held within an NMR tube fitted with a concentric Teflon valve under an atmosphere of nitrogen. 1H NMR, 13C{1H}, and 13C DEPTedited NMR spectra confirmed the clean hydrolysis to afford the 1,2-, 1,4-, and 2,5-isomers of dihydropyridine, e.g., Figure 1 (top trace). Under the reaction conditions, 1,4-dihydropyridine was the major product, typically 60−70%, with 1,2dihydropyridine always present in 40−15% and a trace amount to 15% of the 2,5-isomer. The 13C{1H} NMR spectrum was entirely consistent with the literature65 for these dihydropyridines, noting particularly the 13C resonances at ca. δ 22.6 and 41.6 ppm that are characteristic of the methylene carbon atom of 1,4-dihydropyridine and 1,2-dihydropyridine, respectively. The solution was degassed, and 13CO2 was admitted.66 13C NMR spectroscopy of the sealed reaction mixture showed that no 13CO2 was incorporated into any reaction products, as illustrated by the essentially unperturbed spectrum in Figure 1 (bottom trace). The 13C{1H} DEPT-135 NMR spectrum of the sample after adding 13CO2 showed only the resonances of unreacted 13CO2 and the dihydropyridine reactants as well as signals for the d8THF solvent and for pyridine. Under these reaction conditions, none of the isomers of dihydropyridine react with CO2, i.e., 1,2-dihydropyridine is inert to the presence of CO2. Additional degassed water (0.5 equiv) was added (since Musgrave et al. have predicted water should assist HT from 6 to CO2, see above), and the sealed reaction mixture heated at 60 °C for a further 3 h, then let sit for 3 days. No reaction of

CO2 beyond the two-electron level, i.e., beyond carbon monoxide or formate as product. At the outset, it seemed the hypothetical hydride transfer to a metal-coordinated formate would be more favored if coupled to a proton transfer to the coordinated hydroxymethanolate ligand to avoid buildup of charge in the product species (as illustrated in Scheme 3). The study reported in this article was designed to shed light upon two fundamentally important questions: (1) Can a synthetic OHD such as dihydropyridine 6, in the absence of catalysts or other forcing reaction conditions, reduce carbon dioxide? Given the vast amount of literature that reports this to be so, we sought convincing evidence of a simple and unambiguous chemical reaction to reinforce or deny these claims. (2) Can a metal center assist in the OHD-catalyzed reduction of carbon dioxide? Metal-catalyzed reduction of carbon dioxide is highly topical in the literature, so further information to combine this chemistry with that of OHDs would be extremely valuable to both fields. To investigate these key questions, we assessed the reactivity of OHDs 4−8 with carbon dioxide and with the metal− formato complexes, [(dmpe)2Fe(H)(OCHO)] (2) and [(dmpe)2Fe(OCHO)2] (3). OHDs 4−8 (Chart 1) were chosen for their synthetic accessibility, with the series also spanning a range of thermodynamic and kinetic hydricity values.37,62,63 Additionally, OHDs 5−8 are all neutral dihydropyridines with an acidic N-bound proton and therefore capable of proton-coupled transfer of hydride to a substrate.

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RESULTS AND DISCUSSION Experimental Studies. Reaction of 1,2-/1,4-/2,5-Dihydropyridines with CO2. Unsubstituted dihydropyridine species are highly reactive, a fact that in itself lends some upfront credence to the notion that these species may reduce CO2.64 We proposed that a reasonable way to investigate the reaction of 1,2-dihydropyridine (6) with CO2 and to observe any products formed would be to generate 6 in situ and then to add isotopically labeled 13CO2 and monitor the reaction by NMR C


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Organometallics the dihydropyridines mixture with 13CO2 was observed. Addition of excess water (1:4 H2O-THF) and 13CO2 resulted in the 1,4-isomer being the only dihydropyridine observed in the spectra, but as noted above, the 1,4-isomer is only slightly less effective as a HTPT reagent than is the 1,2-isomer from a thermodynamic perspective (∼13 kJ mol−1 difference calculated, see below). Given that 1,2-dihydropyridine 6 and its 1,4isomer are significantly better hydride donors than OHDs 4 and 5, the implication is that these reagents would also be insufficient to reduce CO2 under these reaction conditions. Most importantly, the results clearly imply that 6 and its 1,4isomer are not active species in the reduction of CO2. Formation of Formato Complexes. We found the solution behavior of complex 2 was consistent with that previously reported,21 with the pale yellow solution of iron dihydride 1 undergoing insertion of CO2 at 1 atm pressure to afford an orange solution of 2 and a double insertion of CO2 providing dark green 3 at ca. 2 atm pressure. Decreasing the pressure of CO2 above the reaction mixture caused green 3 to revert to orange 2 (Scheme 1). Green crystals of trans-[Fe(dmpe)2(OCHO)2] 3 were obtained from tetrahydrofuran under ca. 2 atm CO2, and an X-ray crystal structure was determined at ambient pressure at 150 K. Although the crystals partially degraded upon release of the external CO2 pressure as the crystals were mounted, the current refinement represents a considerable improvement over the previous partially determined structure for which no crystallographic data was reported.21 The crystal structure confirmed the constitution of complex 3 with a Fe(dmpe)2 core and two mutually transoid η1-formate ligands. The rhombohedral unit cell contains two crystallographically independent molecules of 3. Molecule B sits with the Fe ion on the 3-fold axis and is 3-fold disordered (see the Supporting Information). It is imprudent to take metric data from this highly disordered molecule. The other independent molecule, molecule A, was 2-fold disordered at the ethylene linkers of the dmpe ligands and the formate ligands but well-defined and is depicted below in Figure 2. The refinement reveals a mean Fe−P bond distance at 2.25 Å for molecule A that is comparable to that for the analogous iron thioformate complex [Fe(dmpe)2(SCHO)H].21 The Fe−O(formate) bond distance

is 2.032(12) Å, while the O−C−O bond angle for the bound formate ligands at 129(2)° is consistent with sp2 hybridization of the central carbon atoms. Important metric parameters are summarized in Table 1. Table 1. Selected Bond Distances (Å) and Angles (deg) for the Better-Defined Molecule in the Crystal Structure of trans-[Fe(dmpe)2(OCHO)2] (3) Fe(1)−P(1) Fe(1)−P(2)

P(1)−Fe(1)−P(2)′ P(1)−Fe(1)−P(2) P(1)−Fe(1)−P(1)′ P(2)−Fe(1)−P(2)′

bond distances 2.245(2) Fe(1)−O(1) 2.251(2) O(1)−C(1) C(1)−O(2) bond angles 94.52(9) O(1)−Fe(1)−O(1)′ 85.48(9) Fe(1)−O(1)−C(1) 180.0 O(1)−C(1)−O(2) 180.0

2.032(12) 1.25(2) 1.220(18) 180.0 132(2) 129(2)

Reaction of trans-[Fe(dmpe)2H(O13CHO)] (2-13C) with OHDs. ODHs 4−765,67−69 were prepared using literature procedures. Complex 2-13C, i.e., with the formato ligand 13Clabeled, was generated using 13CO2 and treated with four different OHDs, namely, Hantzsch’s ester (4), 1-benzyl-1,4dihydronicotinamide (6), 1,2-dihydropyridine (7), and 2methyl-1,2-dihydrobipyridine (7). To perform these reactions, [Fe(dmpe)2H2] (1) was dissolved in d8-THF with stable OHD 4 or 5 under vacuum in an NMR tube fitted with a concentric Teflon valve, resulting in a clear yellow solution. 13CO2 was then admitted, and a 13C{1H} NMR spectrum was recorded to confirm formation of 2-13C. In the case of OHD 6 or 8, the dihydropyridines were separately generated in the d8-THF and then combined with 2 in the NMR tube under the atmosphere of 13CO2. Table 2 summarizes the specific reaction conditions that were employed for each experiment and provides a summary of the outcomes observed. Figure 3 presents a typical 13C{1H} NMR spectrum of a reaction mixture. In each of the reactions, no reduction products of 13CO2 (other than 2-13C) were observed by 13C NMR spectroscopy. Addition of phenol or lutidinium ion as a Brønsted acid in the case of OHD 5 did not alter the outcome of the reaction. Pyridine was observed as a product in the case of reaction of OHD 6 and 2-13C. However, the doublet at δ 168 for the C atom of the formato ligand in 2-13C remained, and no new peaks for incorporation of 13CO2 were observed. The origin of the pyridine is uncertain. Addition of OHD 7 to 2-13C produced an unknown species in solution with single peak at δ 61.9 in the 31P{1H} NMR spectrum suggestive for a metal−containing species with symmetrically disposed phosphine ligand(s). The 13C NMR spectrum, showed peaks associated with bipyridine, methyl bipyridine, and menthol, plus a small number of unidentified weak peaks that are not associated with anticipated reduction products of carbon dioxide, such as formate, hydroxymethanolate, and methanol, given their chemical shifts. In summary, in all of the reactions, no 13CO2 incorporation into any products other than 2-13C was observed. DFT Studies. DFT calculations71 were undertaken to further understand the thermodynamic feasibility of the intermolecular reaction of OHDs 4−8 with iron(II) formato species such as complex 2, depicted below in Scheme 4. Hydride transfer reactions from OHD to iron(II)-formato species afford iron(II)-hydroxymethanolato species (Scheme

Figure 2. ORTEP diagram of the better defined of the two independent molecules in the crystal structure of trans-[Fe(dmpe)2(OCHO)2] 3. Only 50% ellipsoids at 150 K are displayed; for clarity, hydrogen atoms and the alternate positions for the atoms displaying 2-fold disorder are omitted. See the Supporting Information. D


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Organometallics

Table 2. Summary of Observations for Reactions of Organic Hydride Donors with [Fe(dmpe)2H(OCOH)] (2) under CO2 OHD 4 5 6a 7b

Brønsted acid phenol, lutidinium

H2O present?

other species present

yes yes

observation(s) 13

C NMR: peaks for unreacted starting materials remain; no incorporation of 13CO2 into reduction products 13 C NMR: peaks for unreacted starting materials remain; no incorporation of 13CO2 into reduction products

no no Li+, Al3+ 2,2′-bipyridine, menthol, Li+

13 31

C NMR: 2-13C formato ligand peak unchanged; pyridine present P{1H} NMR: δ 61.9 (s). 13C NMR: see the Supporting Information. 13C peaks do not align with any anticipated CO2 reduction products; the Fe−OC(O)H peak disappears; no appreciable 13C-containing product

a Generation method for 6: Li[Al(NC5H6)4] + 4.2 H2O; see ref 65. bGeneration method for 7: consecutive titration of 2,2′-bipyridine with (i) MeLi and then (ii) menthol; see refs 69 and 70.

calculations were dihydropyridines 6 and 8. A full table summarizing the vacuum phase calculations is available in the Supporting Information. To gain an improved approximation of the Gibbs reaction energy, solvation energies using the SMD model of Cramer and Truhlar73 were undertaken for four different solvents (acetonitrile, pyridine, tetrahydrofuran, and water). For all of the reactions between a complex and an OHD as depicted above in Scheme 4 with solvation modeled, none was exergonic (Table 3). Nevertheless, the computational results have value. First, and most importantly, the results corroborate the experimental findings of nil reaction between 2-13C and OHDs 4−8. Second, it is clear that the nature of the OHD has the most significant impact on the energetics of the reaction as evidenced by the change in the order of magnitude for the free energy change of the reaction when moving from the top to the bottom of Table 3 across all iron complexes. The most reactive OHD was dihydropyridine 8 and close derivatives, due to the stability conferred by aromatization of the dihydropyridine to afford the delocalized, planar pyridine. Third, the metal complex also impacts on the reaction energetics but to a smaller, yet significant, extent. For example, the decrease in the magnitudes of the Gibbs reaction energies when 8 is used as the OHD is noteworthy across the series of iron complexes; see the bottom row of Table 3. These results suggest OHDs, specifically 1,2-dihydropyridine 6, will not undergo HTPT with a metal−formate complex to form the desired metal−hydroxymethanolate complex. In an attempt to understand why this may be the case, we calculated using our methodology the overall change in Gibbs free energy from formic acid to methanediol and found it to be exergonic (at −46 kJ mol−1 in water). This value is similar to that obtained by Musgrave et al. (−38 kJ mol−1) when water is not explicitly modeled.37 Calculations for the reaction of 6 with several other organic formates also gave exergonic energetics (−91, −63, and −46 kJ mol−1 for trifluoromethyl-, phenyl-, and t-butyl-formate respectively in water). The infrared stretching frequency of a carbonyl reflects the electron density of the CO bond, and the frequency of the CO stretch of a formate group is an indicator of its reactivity.74 Estimated carbonyl stretching frequencies for the formate groups were available from our DFT calculations and were corrected with a vibrational scaling factor of 1.0337.75 The resulting carbonyl stretching frequencies matched well with those experimental frequencies that were directly available.21,76,77 Figure 4 presents a plot of the calculated ν̃(CO) stretching frequencies against the Gibbs free energy change for the reaction of the formato species with the representative OHD, 1,2-dihydropyridine (6), in water. There is a good linear relationship between ν̃(CO) and the ΔGxrn (R2 = 0.97). The

Figure 3. 100.6 MHz 13C NMR spectrum of [Fe(dmpe)2(H)(O13C(O)H)] (2-13C) and Hantszch’s ester (5) under 13CO2 (1 atm). The doublet at δ 168 ppm is due to the −O13C(O)H ligand. The minor peaks visible in the spectrum are from the dmpe ligands of 2-13C and from Hantszch’s ester (5). There are no new peaks that result from further reactions of 13CO2. The peaks for the d8-THF solvent are marked.

Scheme 4. Hydride Transfer Reactions from a Generic OHD to Iron(II)-formato Species (Left) To Afford an Iron(II)-hydroxymethanolato Species (Right) Studied by DFT Calculationsa

a

Hydridic and acidic H atoms explicitly drawn blue and red, respectively. (a) OHD capable of HTPT (OHDs 6−8 and similar species); (b) OHD capable of HT only, with PT from an appropriate Brønsted acid. Lutidinium ion and phenol were modeled as the acids for consistency with the experimental work. The aromatized forms of the heterocyclic OHDs are stoichiometrically generated after the theorized HTPT steps.

4). Vacuum phase geometry optimizations were conducted using BP86-D3/def2TZVP methodology and energies were calculated at the PBE0-D3(BJ)/def2QZVP//BP86-D3/ def2TZVP level as this combination has been reported to more accurately model small molecules at an iron center.72 Calculations were undertaken for a variety of OHD molecules and on variations of the iron complex 2 in order to observe how changes to the ligand field or OHD would affect the overall free energy of the reaction. The free energy change for all combinations OHD and metal complexes studied were tabulated. The most reactive OHD species from this series of E


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Table 3. Summary of the Calculated Gibbs Free Energy (ΔGrxn/kJ mol−1) for the Intermolecular Reaction of a Protic OHD or an Aprotic OHD Plus a Brønsted Acid and a Bis(chelate)iron(II)formato Species Shown in Scheme 4a X=

OCHO

H

OCHO

CO

Cl

OCHO

L=

tmeda

dmpe

dmpe

dmpe

dmpe

bpy

4 + phenol 4 + lutidinium 5 dihydrophenanthroline 7 6 1,4-dihydropyridine 8

139.8 101.5 89.1 73.9 28.8 32.8 41.6 21.6

144.0 105.7 93.3 78.1 33.0 37.0 45.8 25.8

143.3 105.0 92.5 77.3 32.2 36.3 45.1 25.0

130.9 92.6 80.1 64.9 19.9 23.9 32.7 12.6

137.5 99.2 86.7 71.5 26.5 30.5 39.3 19.2

127.5 89.2 76.7 61.5 16.5 20.5 29.3 9.2

a

Values calculated in H2O solution using PBE0-D3(BJ)/def2QZVP method and the application of the universal solvation model (SMD) using the parameters of water. Values for the free energies of the hydride transfer reactions in pyridine, acetonitrile and THF are comparable and are available in the Supporting Information.

difference in ΔGrxn between dihydropyridine 6 and OHD A with any particular formate species, e.g., those from Table 3. Some values of m (cm−1/kJ mol−1) and C (cm−1) are as follows: water m = −1.57, C = 1712; pyridine m = −1.63, C = 1733; acetonitrile m = −1.60, C = 1732; THF m = −1.64, C = 1733; and vacuum m = −1.738, C = 1729. A plot of each data set used to derive these values can be found in the Supporting Information. Generally, metal−formate species with a ν̃(CO) stretch above approximately 1760 cm−1 would be required to enable the reaction for weaker OHD species such as 5, while for the strongest OHDs such as dihydropyridine 8, a metal−formate species with a ν̃(CO) stretch of approximately 1690 cm−1 would be sufficient for HTPT to take place. Using this relationship, a simple free energy calculation for any OHD before and after hydride donation can be used to assess the likelihood of a reaction with a metal−formate species. The underlying rationale for the variation in reactivity for the different formato species presented in Figure 4 is that an increase in resonance stabilization/delocalization of the formate group contributes to the decrease in reactivity.70 Possible resonance structures for esters/formic acid/metal− formate species are shown in Scheme 5.

Figure 4. Plot of estimated vibrational stretching frequency of the formate carbonyl against the free energy change for HTPT from 1,2dihydropyridine (6) to the illustrated formate species in water, i.e., for the HTPT reactions of 6 shown in Scheme 4; both organometallic and organic formate species fall on the same trendline.

plot suggests that below a carbonyl stretching frequency of approximately 1710 cm−1 a formate moiety is too deactivated to undergo HTPT with dihydropyridine 6 in water. A brief literature survey shows most metal−formato complexes have ν̃(CO) frequencies below 1650 cm−1,78 revealing that metal complexation may not be appropriate to facilitate HTPT reactions from OHD molecules to formate derived from CO2. This result suggests that in most cases reaction of CO2 with a metal hydride to form a metal-bound formate actually deactivates the CO2 toward further reduction with an OHD. The line of best fit depicted in the plot shown in Figure 4 can be expressed by a straight-line formula (eq 2) and a rearrangement of eq 2 gives eq 3 that expresses the minimum ν̃(CO)required for a formate species to undergo a spontaneous reaction with any OHD, A, from this study (corresponding to ΔGrxn = 0; for the derivation, see the Supporting Information). ν̃(CO) = m(ΔGrxn) + C

(2)

ν̃(CO)required = m × −Δ(ΔGrxn)6 → A + 1712 cm−1

(3)

Scheme 5. Resonance Forms of Formate Esters (X = Alkyl/ Aryl), Formic Acid (X = H), and Metal−Formate Complexes (X = Metal Ion/Center)a

a

Species with electron-withdrawing groups attached to the formate, e.g., HCOOCF3, lower the overall resonance stabilization by reducing the contributions of forms (ii) and (iii) and an increase in resonance form (i). Resonance form (iv) contributes significantly in the metal− formates where the bonding is more ionic in nature (HCO2− M+), and so the formate is more stabilized due to the larger amount of delocalization in the metal complexes, making it thermodynamically more difficult to hydrogenate.

An alternative, easily calculated, simple indicator of the extent of delocalization/resonance stabilization is the carbon− oxygen bond length of the C−O−X unit in the formate moiety. Resonance form (i) has discrete short CO double and long C−O single bonds with maximum difference in length; increasing the contribution of resonance form (ii) leads to a

In eq 2, m is the slope and C is the intercept at ΔGrxn = 0 of the plot shown in Figure 4. In eq 3, the term Δ(ΔGrxn)6→A is the F


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Organometallics shortening of the C−O(−X) bond length. In a free HCO2− anion, the two C−O bond lengths would be equal, with formal bond order of 1.5, and the delocalization is maximized. A plot of the calculated C−O(−X) bond length versus calculated free energy change of the reaction of the formate species with 1,2dihydropyridine (6) in water is shown in Figure 5.

than in formic acid or formate esters making it less prone to reduction. The results suggest that the iron complex studied in this work or indeed any metal complex may not activate the coordinated formate ligand sufficiently for reduction by an OHD. Clearly, the stronger the pyridine-based OHD, the more favorable the overall thermodynamic potential for successful reduction of formate. For example, dihydrobipyridine 8 has significantly higher hydricity than those of the other OHDs investigated in this study and may transfer hydride to a particularly activated metal-bound formate. It is crucial to consider the hydricity of an OHD, the activation of the formate by the metal complex, and the mode of carbon dioxide coordination to develop successful tandem metal−OHD catalysts.

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EXPERIMENTAL METHODS

General Information. All manipulations involving air-sensitive compounds were carried out under an atmosphere of dry nitrogen or argon unless otherwise specified. THF, diethyl ether, and d8-THF were dried and distilled from sodium benzophenone ketyl and stored over activated 3 Å molecular sieves prior to use. Pyridine was dried over calcium hydride, distilled, and stored over 3 Å molecular sieves prior to use. Lithium aluminum hydride was purified by Soxhlet extraction with anhydrous diethyl ether under argon prior to use. All other deuterated solvents were dried using activated 3 Å molecular sieves and degassed prior to use. OHDs 5−8 were synthesized according to literature procedures.67,68,79,80 Iron complexes 1−3 were synthesized according to literature procedures.21,81,82 For convenience, full synthetic details are provided in the Supporting Information. NMR Spectroscopy. All NMR spectra were obtained from either a Bruker Avance III 400 MHz spectrometer equipped with a Prodigy nitrogen cooled Cryoprobe, a Bruker Avance III 600 MHz spectrometer equipped with a BBFO Plus probe, or a Bruker Avance III 600 MHz spectrometer equipped with a 5 mm TCI cryoprobe. All air-sensitive experiments requiring NMR spectroscopy were conducted using borosilicate NMR tubes fitted with a J. Young highvacuum concentric PTFE valve. When required, deuterated NMR solvents were transferred under high vacuum to a Young’s tube from a storage ampule fitted with a Young’s tap. Although 1H and 13C NMR chemical shifts are reported relative to tetramethylsilane, 1H and 13C NMR spectra were referenced relative to the residual solvent signal. 31 P NMR spectra chemical shifts were referenced to an external trimethylphosphite standard. X-ray Crystal Structure Analysis. The X-ray diffraction measurements were performed on a Bruker Kappa-II CCD diffractometer at 150 K using a l μS Incoatec Microfocus Source with Mo Kα radiation (λ = 0.710723 Å). The structure was solved using charge flipping, and the full matrix least-squares refinement was performed using ShelXL83 in Olex2.84 Complex 3 crystallized in rhombohedral space group R3̅ with the asymmetric unit consisting of 1 /2 molecule at the center of inversion (Fe at the center) and 1/3 molecule on the 3-fold axis (Fe and the bound formate O-donor on the symmetry axis). The former molecule at the center of inversion (molecule A) is well-defined compared the latter orientationally disordered one (molecule B). In molecule A, the ethylene bridge atoms C1A, C2A (along with associated P-methyl carbons), and axially coordinated formate ligand are doubly disordered with equal occupancies. In molecule B, the molecule is rotationally disordered over three orientations with equal occupancy of each, and it was difficult to assign the ethylene bridge C atoms of the phosphine ligand as these positions in one orientation overlapped with the P atoms in the orientations (full details of the structure refinement are provided in the Supporting Information). The least-squares refinement was carried out with appropriate restraints (SADI and RIGU) on molecules A and B and resulted in the final R value of ∼0.094. DFT Calculations. DFT calculations were performed with the Gaussian09 software package.71 Geometry optimizations in vacuum

Figure 5. Plot of the calculated C−O bond length of the C−O−X moiety of the formate against the free energy change for HTPT from 1,2-dihydropyridine (6) to the illustrated formate species in water, i.e., for the HTPT reactions of 6 shown in Scheme 4; both organometallic and organic formate species fall on the same trendline.

The plot includes a data point for free formate ion with a calculated C−O bond length of 1.26 Å. Using a similar analysis to that applied in the case of the IR stretching frequencies, the plot suggests that in order for an exergonic reaction to occur between 1,2-dihydropyridne and a generalized HC(O)−O−X species the BP86-D3/def2TZVP calculated C−O bond length must be at least 1.33 Å (or at least 1.31 Å to be reduced by 1,2 dihydro-2-methylpyridine).

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CONCLUSIONS Previous theoretical studies conclusively predict the reduction of carbon dioxide by 1,2-dihydropyridine (6). However, in our experimental work, we definitively observe no reduction of carbon dioxide by any OHD, including 6. Carbon dioxide is reduced to formate by initial insertion into the Fe−H bond(s) of 1 in our experimental work, but beyond this initial insertion of CO2, there is no further reaction with OHDs. Neither 6 nor any of the other dihydropyridines studied are capable of reducing CO2 under our reaction conditions. The results demonstrate that at least some of the mechanisms of reduction of carbon dioxide by OHDs suggested in literature need to be called into question. The experimental demonstration that 1,2dihydropyridine 6 or its more stable 1,4-isomer does not reduce carbon dioxide, including with excess water present, evidences that theoretical predictions and experimental reality do not always match. Our calculations show a strong correlation between the carbonyl stretching frequency and length of the C−O bond in a formate species HC(O)−O−X and the overall Gibbs energy change of the reaction between an OHD and that species. The formate unit in the metal−η1-formato species studied herein is likely to be stabilized by delocalization more G


Article

Organometallics

165 (BNAH), 146.4 (BNAH), 144.2 (BNAH), 134.0 (BNAH), 132.1 (BNAH), 130.0 (BNAH), 129.5 (BNAH), 129.1 (BNAH), 128.4 (BNAH), 125.72 (13CO2) 65.07 (BNAH), 53.34 (BNAH), 30.2 (BNAH), 17.36 (Fe), 14.40 (Fe), 14.03 (Fe). Reaction of Lansbury’s Reagent, Water, [Fe(dmpe)2H2], and 13 CO2. To a sample of Lansbury’s reagent (10 mg, 0.087 mmol) in 0.3 mL of d8-THF that had been hydrolyzed with a substoichiometric amount of degassed water (6 μL), contained within an NMR tube fitted with a J. Young’s tap, was added [Fe(dmpe)2H(O13CHO)] in 0.3 mL of d8-THF (generated as above) under an atmosphere of nitrogen. The resulting solution showed no color change, but formation of some white sediment was observed. No reduction products of CO2 were observed in the 13C{1H}-NMR spectrum. No change to the formate signal of [Fe(dmpe)2H(OCHO)] was observed. 13 C{ 1 H}-NMR (150.9 MHz, d 8 -THF): δ 168.63 (OCHO), 150.59 (Py), 136.06 (Py), 128.50 (dhp), 128.41 (dhp), 125.68 (dhp), 124.14 (Py), 95.78 (dhp), 67.21 (THF), 32.08 (dmpe), 25.31 (THF), 23.42 (dhp), 22.96 (dmpe), 15.19 (dmpe). Reaction of Dihydrobipyridine, Water, [Fe(dmpe)2H2] and 12CO2. Using standard Schlenk techniques with manipulation carried out on a Schlenk line fitted with a CO2 cylinder and N2 outlet, a colorless solution of bipyridine (10 mg, 0.064 mmol) in anhydrous tetrahydrofuran (10 mL) was titrated with MeLi (1.2 M) under an atmosphere of nitrogen until a blood red color persisted. The atmosphere within the apparatus was then changed to 12CO2, and the persistent red solution allowed to equilibrate with the 12CO2 atmosphere before it was quenched by addition of a solution of menthol in anhydrous tetrahydrofuran (0.42 M) to give a permanently yellow reaction mixture, indicating the presence of the desired 2-methyl-1,2-dihydrobipyiridine.69 The dihydrobipyridine solution was then added to [Fe(dmpe)2(H)(OCHO)] (10 mg, 0.028 mmol) under an atmosphere of 12CO2, and the resulting solution frozen before the addition of further 12CO2 to increase the internal pressure of the flask to approximately 2 atm. The reaction mixture was then stirred for 2 h before an aliquot was taken for NMR spectroscopy. The resulting NMR spectra did not show peaks for any reduction products of CO2. 31P NMR (161.9 MHz, THF): δ 61.88. 13 C{1H} NMR (100.6 MHz, THF): δ 158.95 (bpy), 149.91 (bpy), 135.62 (bpy), 124.96 (CO2), 123.83 (bpy), 121.94 (bpy), 118.90 (bpy).

for OHDs 4−8, formic acid, lutidinium, phenol, and the iron complexes were performed using the gradient-corrected BP86 exchange correlation functional along with the implemented valence triple-ζ def2-TZVP basis set for all atoms. Dispersion corrections were included by adding the D3 version of Grimme’s dispersion. The geometry optimizations were performed initially without imposing any symmetry constraints, and if symmetrical species were produced, then these were reoptimized in the correct point group to generate appropriate entropic contributions. The optimizations in vacuo were followed by the calculation of vibrational frequencies to ensure the located coordinates were global energy minima with no imaginary frequencies. In cases where several conformational possibilities existed, these were surveyed, and the conformation with the lowest free energy at the BP86-D3/def2TZVP level was selected. Singlepoint energies on all of the optimized geometries where calculated at the PBE0-D3(BJ)/def2QZVP level of theory85 as this hybrid method gives reasonably accurate energies in a range of inorganic complexes.83 To determine bulk solvent effects, the SMD universal solvation method developed by Truhlar et al.73 was employed for acetonitrile, tetrahydrofuran, pyridine, and water by calculating the free energy of solvation from the difference in B3LYP/6-31G(d) energies in SMD solvent and vacuum employing the BP86-D3/ def2TZVP vacuum geometries. Attempted Reactions. Reaction of 1,2-/1,4-/2,5-Dihydropyridine with 13CO2 in THF. To a solution of Lansbury’s reagent62 (10 mg, 0.087 mmol) in dry d8-THF (0.4 mL) under an atmosphere of nitrogen was added a substoichiometric amount of degassed water (6 μL). At this point, 1H and 13C DEPT-135 NMR spectra showed the presence of a 1,2-/1,4-/2,5-dihydropyridine mixture. Nitrogen was removed from the sample by freeze−pump−thaw cycles (×3) before the sample was placed under approximately 2 atm of 13CO2. The resulting 13C{1H} and 13C DEPT-135 NMR spectra showed the presence of pyridine, THF, and the dihydropyridines. No products due to the reduction of 13CO2 were observed. 13C DEPT NMR (150.9 MHz, d8-THF): 159.3 (2,5-DHP, CH), 149.8 (py, CH), 135.4 (py, CH), 133.5 (1,2-DHP, CH), 125.9 (1,4-DHP, CH), 125.2 (1,2DHP, CH) 125 (13CO2), 124.7 (2,5-DHP, CH), 123.5 (py, CH), 120.4 (2,5-DHP, CH), 109.6 (1,2-DHP, CH), 94.9 (1,4-DHP, CH), 94.6 (1,2-DHP, CH), 48.6 (2,5-DHP, CH2), 41.5 (1,2-DHP, CH2), 27.8 (2,5-DHP, CH2), 22.6 (1,4-DHP, CH2). The sample was heated at 60 °C for 3 h and allowed stand for 3 days; the 13C{1H} and 13C DEPT-135 NMR spectra remained unchanged. Water (100 μL) was then added, and 13C{1H} and 13C DEPT-135 NMR spectra acquired. Conversion of the 1,2-dihydropyridine to the 1,4-isomer was observed without any incorporation of 13CO2 into product(s). Reaction of Hantszch’s Ester and [Fe(dmpe)2H2] with 13CO2. To a sample of Hantszch’s ester 5 (10 mg, 0.040 mmol) and [Fe(dmpe)2H2] (10 mg, 0.028 mmol) contained in an NMR tube fitted with a J. Young’s tap was added anhydrous d8-THF (0.4 mL) by transfer under vacuum. To the resulting pale-yellow solution, 13CO2 was added, resulting in an orange solution with a pressure of ∼2 atm contained within the NMR tube. The reaction mixture was allowed to stand for 1 h. The resulting 13C{1H}-NMR spectrum showed no evidence for reduction products of 13CO2. The formate signal of [Fe(dmpe)2H(OCHO)] remained unchanged, as did the 13C resonances associated with Hantszch’s ester. 13C{1H}-NMR (100.6 MHz, d8-THF): δ 168.72 (d, J = 187 Hz, OCHO), 165.9 (HE), 146 (HE), 125.72 (13CO2), 98.81 (HE), 60.58 (HE), 59.10 (Fe), 28.01 (HE), 18.66 (HE), 17.36 (Fe), 15.20 (HE), 14.40 (Fe), 14.03 (Fe). Reaction of BNAH, [Fe(dmpe)2H2], and 13CO2. To a sample of BNAH ester 5 (10 mg, 0.047 mmol) and [Fe(dmpe)2H2] (10 mg, 0.028 mmol) contained in an NMR tube fitted with a J. Young’s tap was added dry d8-THF (0.4 mL) via transfer under vacuum. To the resulting medium yellow solution was added 13CO2, resulting in an orange solution with a pressure of ∼2 atm contained within the NMR tube. The reaction mixture was allowed to stand for 1 h. The resulting 13 C{1H} NMR spectrum showed no evidence for reduction products of 13CO2. The formate signal of [Fe(dmpe)2H(OCHO)] remained unchanged, as did the 13C resonances associated with BNAH. 13 C{1H}-NMR (100.6 MHz, d8-THF): δ 168.72 (d, J = 187, OCHO),

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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.8b00600. Experimental details, representative NMR spectra, tabulated energies of reactions, coordinates obtained from optimized geometries (PDF) Cartesian coordinates (XYZ) Accession Codes

CCDC 1862505 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Timothy E. Elton: 0000-0003-4269-2765 Graham E. Ball: 0000-0002-0716-2286 H


Article

Organometallics

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Mohan Bhadbhade: 0000-0003-3693-9063 Leslie D. Field: 0000-0001-5519-5472 Stephen B. Colbran: 0000-0002-1119-4950 Funding

This research was funded by the Australian Research Council (Grant No. DP160104383). Notes

The authors declare no competing financial interest.

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ABBREVIATIONS BNAH 1-benzyl-1,4-dihydronicotinamide; bpy 2,2′-bipyridin; DFT density functional theory; DHP dihydropyridine; dmpe 1,2-bis(dimethylphosphino)ethane; HT hydride transfer; OHD organic hydride donor; PT proton transfer; py pyridine; THF tetrahydrofuran; tmeda tetramethylenediamine

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