Dehydrogenation, disproportionation and transfer hydrogenation reactions of formic acid catalyzed by molybdenum hydride compounds

Abstract

The cyclopentadienyl molybdenum hydride compounds, Cp^RMo(PMe₃)_3−x(CO)_xH (Cp^R = Cp, Cp*; x = 0, 1, 2 or 3), are catalysts for the dehydrogenation of formic acid, with the most active catalysts having the composition Cp^RMo(PMe₃)₂(CO)H. The mechanism of the catalytic cycle is proposed to involve (i) protonation of the molybdenum hydride complex, (ii) elimination of H₂ and coordination of formate, and (iii) decarboxylation of the formate ligand to regenerate the hydride species. NMR spectroscopy indicates that the nature of the resting state depends on the composition of the catalyst. For example, (i) the resting states for the CpMo(CO)₃H and CpMo(PMe₃)(CO)₂H systems are the hydride complexes themselves, (ii) the resting state for the CpMo(PMe₃)₃H system is the protonated species [CpMo(PMe₃)₃H₂]⁺, and (iii) the resting state for the CpMo(PMe₃)₂(CO)H system is the formate complex, CpMo(PMe₃)₂(CO)(κ¹-O₂CH), in the presence of a high concentration of formic acid, but CpMo(PMe₃)₂(CO)H when the concentration of acid is low. While CO₂ and H₂ are the principal products of the catalytic reaction induced by Cp^RMo(PMe₃)_3−x(CO)_xH, methanol and methyl formate are also observed. The generation of methanol is a consequence of disproportionation of formic acid, while methyl formate is a product of subsequent esterification. The disproportionation of formic acid is a manifestation of a transfer hydrogenation reaction, which may also be applied to the reduction of aldehydes and ketones. Thus, CpMo(CO)₃H also catalyzes the reduction of a variety of ketones and aldehydes to alcohols by formic acid, via a mechanism that involves ionic hydrogenation.

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Introduction

The growing demand for energy requires the development of alternative energy sources that are more sustainable than fossil fuels and have a reduced impact on the environment. As such, both the sun and the wind are considered to be major candidates for the production of renewable energy.¹ However, since energy production from these sources would vary with location and time, it is essential to couple the production of energy with efficient methods for storage and transportation.² In this regard, an attractive energy storage medium is provided by hydrogen,³ which can, for example, be consumed in a fuel cell with only water as a byproduct. Unfortunately, a problem with using hydrogen in this manner is that present storage and transportation techniques are inadequate.⁴ For example, not only does storing liquid hydrogen present a safety risk, considerable energy is also required to liquefy the hydrogen and maintain it in this form.^2c

In view of these issues, efforts have focused on the use of physisorption and chemical methods to provide hydrogen on demand.⁵ Of these approaches, the use of formic acid as a chemical medium for storing H₂ has garnered much attention.^2,6,7 Specifically, although formic acid contains only 4.4% hydrogen by mass, it is an attractive storage medium because (i) it is a liquid at room temperature and is therefore easy to handle and transport, (ii) it is commercially available on a large scale^8,9 and (iii) the byproduct of H₂ release is carbon dioxide which, in principle, can be trapped¹⁰ and either recycled^11,12 or used as a C₁ source for other chemicals.¹² With respect to achieving the release of H₂ from formic acid, much research has been directed towards the discovery of both heterogeneous¹³ and homogeneous^14–22 catalyst systems. However, since most of these catalyst systems feature precious metals (e.g. Pd, Pt, Au), an important current thrust is the discovery of catalysts that utilize earth abundant nonprecious metals.^19–22 Therefore, we report here a series of molybdenum compounds that are not only effective for the catalytic release of H₂ from formic acid, but are also capable of (i) forming methanol via disproportionation of formic acid and (ii) using formic acid as a reagent for the transfer hydrogenation of aldehydes and ketones.

Results and discussion

We have previously described the first example of a molybdenum compound, namely Cp*Mo(PMe₃)₂(CO)H,^20,23 to serve as a catalyst for the dehydrogenation (decarboxylation) of formic acid (Scheme 1).²⁴ In order to understand the factors that influence dehydrogenation, we sought to extend the investigation to the series of compounds, CpMo(PMe₃)_3−x(CO)_xH and Cp*Mo(PMe₃)_3−x(CO)_xH (x = 0, 1, 2 or 3), in which the substitution of (i) PMe₃ ligands by CO and (ii) Cp* ligands by Cp would be expected to exert significant electronic and structural effects, as illustrated by the fact that the oxidation potentials of the CpMo(PMe₃)_3−x(CO)_xH/[CpMo(PMe₃)_3−x(CO)_xH]⁺ couples span a range of ca. 2 V.²⁵

Scheme 1 Dehydrogenation and disproportionation of HCO₂H.

Indeed, comparison of CpMo(PMe₃)₃H,²⁶ CpMo(PMe₃)₂(CO)H,²⁷ CpMo(PMe₃)(CO)₂H^27,28 and CpMo(CO)₃H²⁹ indicates that substitution of a CO ligand by PMe₃ has a profound effect on the rate of decarboxylation and release of H₂ (Table 1).³⁰ For example, while the reaction catalyzed by the tricarbonyl derivative, CpMo(CO)₃H, requires temperatures of 100 °C in order to proceed efficiently in benzene, the bis(phosphine) derivative, CpMo(PMe₃)₂(CO)H, is an effective catalyst at room temperature.³¹ Most interestingly, the trend is not monotonic, such that the tris(phosphine) derivative CpMo(PMe₃)₃H exhibits no significant catalytic activity at room temperature. Thus, of the series of compounds CpMo(PMe₃)_3−x(CO)_xH, the bis(phosphine) derivative, CpMo(PMe₃)₂(CO)H, has the greatest activity.

Table 1 Turnover frequencies (h⁻¹) for dehydrogenation of formic acid by Cp^RMo(PMe₃)_3−x(CO)_xH at 100 °C^a

	Cp	Cp*
a [Cp^RMo(PMe3)3−x(CO)xH] = 0.016 M, [HCO2H]initial = 0.39 M, C6D6, values at 50% conversion.
Cp^RMo(PMe3)2(CO)H	31	54
Cp^RMo(PMe3)(CO)2H	0.64	1.2
Cp^RMo(CO)3H	0.67	0.33

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Although CpMo(PMe₃)₃H exhibits little activity at room temperature, it does, nevertheless, induce the decarboxylation of formic acid at elevated temperatures. However, it is evident that CpMo(PMe₃)₃H is not necessarily the catalytically active species because it is converted to the monocarbonyl compound CpMo(PMe₃)₂(CO)H under these conditions (Scheme 2).³²

Scheme 2 Conversion of CpMo(PMe₃)₃H to CpMo(PMe₃)₂(CO)H by formic acid.

The nature of the substituents on the cyclopentadienyl ring also influences the reactivity, with Cp*Mo(PMe₃)₂(CO)H being a more active catalyst than CpMo(PMe₃)₂(CO)H (Table 1).³¹ The difference is not, however, as pronounced as that achieved by variation of the number of PMe₃ and CO ligands.

The essential features of the mechanism proposed for the catalytic cycle are illustrated in Scheme 3 and involve two main sequences, namely (i) protonation, elimination of H₂ and coordination of formate, and (ii) decarboxylation of the formate ligand to regenerate the hydride species. In order to obtain evidence for such a mechanism, the nature of the molybdenum-containing compounds that form upon treatment of CpMo(PMe₃)_3−x(CO)_xH (x = 0, 1, 2, or 3) with formic acid has been probed by NMR spectroscopy.

Scheme 3 Essential features for the proposed mechanism of dehydrogenation of HCO₂H by Cp^RMo(PMe₃)_3−x(CO)_xH.

Evidence that CpMo(PMe₃)_3−x(CO)_xH may be protonated by formic acid is provided by the observation that treatment of CpMo(PMe₃)₃H with formic acid results in the formation of [CpMo(PMe₃)₃H₂][HCO₂], which is characterized by two hydride signals in a 1 : 1 ratio at −2.57 [m, ²J_P–H = 54, ²J_P–H = 49 and ²J_H–H = 8] and −5.74 [m, ²J_P–H = 46, ²J_P–H = 8 and ²J_H–H = 8] in the ¹H NMR spectrum (CD₃CN) at 239 K.³³ Support for the identification of [CpMo(PMe₃)₃H₂][HCO₂] is provided by previous studies in which [CpMo(PMe₃)₃H₂]⁺ has been generated by the reaction of CpMo(PMe₃)₃H with HBF₄.^25,26a Furthermore, we have also structurally characterized the chloride derivative [CpMo(PMe₃)₃H₂][Cl] by X-ray diffraction (Fig. 1), which reinforces its identification as a dihydride rather than a dihydrogen complex.³⁴

Fig. 1 Molecular structure of [CpMo(PMe₃)₃H₂][Cl] (only the cation is shown).

However, despite the fact that CpMo(PMe₃)₃H is readily protonated, it is not, as noted above, an effective catalyst for decarboxylation of formic acid at room temperature. Presumably, the catalytic inactivity of CpMo(PMe₃)₃H is a consequence of dissociation of H₂ from [CpMo(PMe₃)₃H₂][HCO₂] being non-facile at room temperature. Supporting this suggestion, the tetrafluoroborate derivative, [CpMo(PMe₃)₃H₂][BF₄], is stable with respect to dissociation of H₂ at 80 °C.²⁵

Evidence for the formation of a protonated species has also been obtained for the reaction of CpMo(PMe₃)₂(CO)H with formic acid. Specifically, treatment of CpMo(PMe₃)₂(CO)H in d₈-toluene with an excess of formic acid at low temperature (276 K) allows for observation of a protonated species, [CpMo(PMe₃)₂(CO)H₂]⁺, which is characterized by a triplet signal assignable to the hydride ligands at −3.33 ppm [t, ²J_P–H = 43 Hz] in the ¹H NMR spectrum.^35,36 In contrast to [CpMo(PMe₃)₃H₂]⁺, however, [CpMo(PMe₃)₂(CO)H₂]⁺ does not persist at room temperature, dissociating H₂ and coordinating formate to give CpMo(PMe₃)₂(CO)(κ¹-O₂CH). The latter compound is also unstable and only exists for a prolonged period in the presence of excess formic acid because it slowly dissociates CO₂ and regenerates CpMo(PMe₃)₂(CO)H.³⁷ It is, therefore, evident that both protonation and release of H₂ is facile in this system, such that decarboxylation is the turnover-limiting step of the catalytic cycle in benzene during the initial stages. However, as the formic acid is consumed, the pseudo first order rate constant for protonation of CpMo(PMe₃)₂(CO)H decreases relative to that for decarboxylation of CpMo(PMe₃)₂(CO)(κ¹-O₂CH), such that CpMo(PMe₃)₂(CO)H becomes the resting state during the latter stage of the catalytic transformation.

Unlike CpMo(PMe₃)₃H and CpMo(PMe₃)₂(CO)H, the dicarbonyl and tricarbonyl compounds, CpMo(PMe₃)(CO)₂H and CpMo(CO)₃H, do not react with formic acid in benzene at room temperature. As such, elevated temperatures are required for efficient catalysis. However, although it is unreactive towards formic acid at room temperature, previous studies have shown that CpMo(PMe₃)(CO)₂H reacts with a stronger acid, namely HBF₄, to release H₂, with no observation of the dihydride cation, [CpMo(PMe₃)(CO)₂H₂]⁺.^38,39 Likewise, CpMo(CO)₃H rapidly eliminates H₂ upon treatment with TfOH to afford CpMo(CO)₃OTf, with no observation of [CpMo(CO)₃H₂]⁺.^39a,40 On the basis of these observations, it is evident that the reduced basicity of the metal center resulting from the increased replacement of the PMe₃ ligands by CO^41,42 causes protonation to be turnover-limiting.

The above observations clearly indicate that, for the series of CpMo(PMe₃)_3−x(CO)_xH compounds, the turnover-limiting step depends on the distribution of PMe₃ and CO ligands. Specifically, (i) the catalytic activities of CpMo(CO)₃H and CpMo(PMe₃)(CO)₂H are limited by their susceptibility to protonation by formic acid, (ii) the activity of CpMo(PMe₃)₃H is limited by the inability of [CpMo(PMe₃)₃H₂]⁺ to dissociate H₂, and (iii) the activity of CpMo(PMe₃)₂(CO)H is limited by decarboxylation of the formate intermediate, CpMo(PMe₃)₂(CO)(κ¹-O₂CH), in the presence of a high concentration of formic acid, but by protonation of CpMo(PMe₃)₂(CO)H when the concentration of acid is low.

The different forms of the catalyst resting states provide some insight into the non-monotonic variation in activity upon substituting PMe₃ for carbonyl ligands. Thus, while CpMo(CO)₃H has a low activity because the metal center is not susceptible to protonation, CpMo(PMe₃)₃H has a low activity because the protonated derivative [CpMo(PMe₃)₃H₂]⁺ is relatively stable with respect to elimination of H₂. As such, the hybrid carbonyl/phosphine derivatives, Cp^RMo(PMe₃)₂(CO)H, have the greatest reactivity.

Interestingly, while CO₂ and H₂ are the principal products of the catalytic conversion of formic acid by Cp^RMo(PMe₃)_3−x(CO)_xH (R = H, Me), methanol and methyl formate are also observed. The formation of methanol is a consequence of disproportionation of formic acid (Scheme 1), while the methyl formate is a result of subsequent esterification. The observation of this additional pathway is of note in view of the fact that the generation of methanol is a key component of the potential methanol economy,⁴³ and yet there are only two reports for the formation of methanol by the homogeneous catalytic disproportionation of formic acid.^15g,17c,44 Furthermore, both of these reports describe catalysts that contain precious metals, namely iridium^17c and ruthenium.^15g As such, the ability to catalyze this transformation by compounds containing the nonprecious metal, molybdenum, is noteworthy. The selectivity for the catalytic formation of methanol and methyl formate relative to decarboxylation by Cp^RMo(PMe₃)_3−x(CO)_xH depends on the nature of the cyclopentadienyl substituents and the number of carbonyl ligands, and is greatest for CpMo(CO)₃H, which can achieve a selectivity⁴⁵ (21%) that is intermediate between the values for the iridium (12%)^17c and ruthenium (27%)^15g,46 systems.

A simplified mechanism for the formation of methanol is illustrated in Scheme 4 and is based on that previously proposed for the catalytic ionic hydrogenation of ketones.^{34,39,47–49} Thus, it is proposed that Cp^RMo(PMe₃)_3−x(CO)_xH delivers a hydride ligand to the protonated form of formic acid, [HC(OH)₂]⁺, which is present due to autoionization⁵⁰ or generated incipiently, thereby forming methylene diol. The formation of methanol from CH₂(OH)₂ can be achieved in principle by either (i) disproportionation to methanol and formic acid,⁵¹ or by (ii) dehydration to formaldehyde followed by catalytic ionic hydrogenation akin to that for the first reduction sequence of formic acid (Scheme 4).

Scheme 4 Essential features for the proposed mechanism of disproportionation of HCO₂H by Cp^RMo(PMe₃)_3−x(CO)_xH.

Support for a direct proton and hydride transfer, rather than reaction with the H₂ released via decarboxylation, is provided by the observation that performing the reaction under D₂ (1 atm) results in no deuterium incorporation into the methanol or methyl formate.

On the basis of the mechanisms illustrated in Scheme 3 and Scheme 4 for the dehydrogenation and disproportionation of formic acid, the relative selectivity of the two processes (dehydrogenation versus disproportionation) is influenced by the preference of the metal center to transfer a hydride ligand to [HC(OH)₂]⁺ (which is present in low concentration) relative to its tendency to undergo protonation and liberate H₂ (Scheme 5). As such, the pK_B and the hydricity of Cp^RMo(PMe₃)_3−x(CO)_xH are expected to play an important role in influencing the selectivity. In this regard, while such values have been reported for a variety of metal hydride derivatives,^41,52–54 data for all the compounds described here are not available. Nevertheless, the kinetic hydricity of CpMo(PMe₃)(CO)₂H is significantly greater than that of CpMo(CO)₃H, by a factor of ca. 10⁴,⁵² which is in accord with the notion that replacing a CO ligand by PMe₃ would be expected to promote dissociation of hydride.⁵⁵ However, it is also generally recognized that the basicity of a metal center increases upon replacing a CO ligand by PR₃ (vide supra),^41,42 and so it is nontrivial to predict a priori the relative influence of such substitution on the two pathways.

Scheme 5 Possible origin of the selectivity of disproportionation versus dehydrogenation of formic acid.

The ability of these molybdenum compounds to catalyze the disproportionation of formic acid prompted the possibility that a system could be developed to use formic acid as a reagent in the transfer hydrogenation of other substrates. In this regard, transfer hydrogenation,^56,57 often employing isopropanol as the reductant, offers considerable advantages over direct reduction by hydrogen because it obviates the need to use H₂.⁵⁶ However, disadvantages of using isopropanol are that (i) isopropanol is obtained commercially by the hydration of propene,⁵⁸ and (ii) the reactions are often reversible, such that the isopropanol needs to be used as a solvent to drive the equilibrium.⁵⁶ In contrast, formic acid is available from renewable sources, and the release of CO₂ as a byproduct renders the reaction effectively irreversible.⁵⁶ However, catalysts that are commonly used to effect transfer hydrogenation using formic acid are largely restricted to the platinum group metals, namely Ru, Rh, Pd and Ir,^59–61 with few reports employing other metals,⁶² and none employing Mo. It is, therefore, noteworthy that CpMo(CO)₃H is also capable of effecting transfer hydrogenation of a variety of carbonyl compounds, namely RCHO (R = Me, Prⁱ), RC(O)Me (R = Me, Prⁱ, Bu^t, Ph) and Ph₂CO, using formic acid as the reductant,⁶³ as illustrated in Scheme 6. For example, Ph₂CO is reduced by HCO₂H to Ph₂CH(OH) with a selectivity of 29% relative to decarboxylation and disproportionation, while PrⁱCHO is reduced to BuⁱOH and HCO₂Buⁱ (3 : 1) with a selectivity of 32%.^64–67

Scheme 6 Ionic hydrogenation of carbonyl compounds.

Conclusions

In summary, the cyclopentadienyl molybdenum hydride compounds, Cp^RMo(PMe₃)_3−x(CO)_xH (Cp^R = Cp, Cp*; x = 0, 1, 2 or 3), are catalysts for the dehydrogenation of formic acid. The activity of these catalysts is not a monotonic function of the number of CO or PMe₃ ligands and is greatest for Cp^RMo(PMe₃)₂(CO)H. Specifically, while CpMo(CO)₃H has a low activity because the metal center is not susceptible towards protonation, CpMo(PMe₃)₃H has a low activity because the protonated derivative [CpMo(PMe₃)₃H₂]⁺ is relatively stable with respect to elimination of H₂. As such, the hybrid carbonyl/phosphine derivatives, Cp^RMo(PMe₃)₂(CO)H, have the greatest reactivity. In addition to catalyzing the dehydrogenation of formic acid, Cp^RMo(PMe₃)_3−x(CO)_xH also catalyzes its disproportionation to methanol and CO₂via a transfer hydrogenation reaction. Similarly, CpMo(CO)₃H also catalyzes the reduction of aldehydes and ketones by formic acid via a mechanism that involves ionic hydrogenation. These investigations demonstrate that molybdenum hydride compounds have much potential with respect to transfer hydrogenation reactions involving formic acid.

Acknowledgements

We thank the US Department of Energy, Office of Basic Energy Sciences (DE-FG02-93ER14339) for support of this research. M. C. N. acknowledges the National Science Foundation for a Graduate Research Fellowship under Grant no. DGE 11-44155. Dr Deven P. Estes and Jonathan L. Kuo are thanked for technical assistance with high pressure experiments.

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32. Likewise, Cp*Mo(PMe₃)₃H also converts to Cp*Mo(PMe₃)₂(CO)H in the presence of formic acid. See ref. 20.
33. [CpMo(PMe₃)₃H₂][HCO₂] is best isolated by performing the reaction in THF or pentane, rather than benzene.
34. For the structure of [CpW(PMe₃)(CO)₂H₂][OTf], see: R. M. Bullock, J.-S. Song and D. J. Szalda, Organometallics, 1996, 15, 2504–2516.
35. It is worth noting that Poli has demonstrated that protonation of Cp*Mo(PMe₃)₂(CO)H by HBF₄ yields a complex that exists as the dihydrogen compound, [Cp*Mo(PMe₃)₂(CO)(η²-H₂)]⁺, in THF and as the dihydride, [Cp*Mo(PMe₃)₂(CO)H₂]⁺, in CH₂Cl₂, both of which dissociate H₂ at low temperature (230 K).^a The former is characterized by a broad singlet at δ –5.14 in the hydride region, while the latter is characterized by a triplet at δ –3.92 (J_P–H = 55 Hz), which is comparable to that observed for [CpMo(PMe₃)₂(CO)H₂]⁺. Bullock has also shown that protonation of CpMo(CO)(dppe)H by TfOH at low temperature generates a dihydride species [CpMo(CO)(dppe)H₂][OTf] which releases H₂ to form CpMo(CO)(dppe) at room temperature.^b See: (a) P. A. Dub, N. V. Belkova, O. A. Filippov, J. C. Daran, L. M. Epstein, A. Lledos, E. S. Shubina and R. Poli, Chem. Eur. J., 2010, 16, 189–201; (b) T.-Y. Cheng, D. J. Szalda, J. Zhang and R. M. Bullock, Inorg. Chem., 2006, 45, 4712–4720.
36. Weak Brønsted acids bind to the carbonyl ligands of Cp*Mo(PMe₃)₂(CO)H, while strong Brønsted acids interact preferentially with the hydride ligand to yield a dihydrogen adduct. See: P. A. Dub, O. A. Filippov, N. V. Belkova, J.-C. Daran, L. M. Epstein, R. Poli and E. S. Shubina, Dalton Trans., 2010, 39, 2008–2015.
37. However, although decarboxylation of CpMo(PMe₃)₂(CO)(κ¹-O₂CH) is thermodynamically downhill, treatment of CpMo(PMe₃)₂(CO)H with a high pressure of CO₂ (59.9 atm) demonstrates that CpMo(PMe₃)₂(CO)(κ¹-O₂CH) can be generated as a component of an equilibrium mixture (K = 0.01 atm⁻¹).
38. E. A. Quadrelli, H.-B. Kraatz and R. Poli, Inorg. Chem., 1996, 35, 5154–5162.
39. For protonation of CpMo(PPh₃)(CO)₂H with TfOH, see: (a) R. M. Bullock and J.-S. Song, J. Am. Chem. Soc., 1994, 116, 8602–8612; (b) K.-T. Smith, J. R. Norton and M. Tilset, Organometallics, 1996, 15, 4515–4520.
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63. It is pertinent to note that compounds of this class have been used as catalysts for ionic hydrogenation employing H₂. See, for example, ref. 48.
64. Selectivities are the average of two runs.
65. Selectivity for transfer hydrogenation is defined as {[ROH]_formed + [HCO₂R]_formed}/{[HCO₂H]_consumed − [HCO₂R]_formed − [HCO₂Me]_formed} × 100.
66. Formate esters have also been observed in other systems. See, for example, ref. 59l and 62a.
67. Note that it is more difficult to reduce esters by ionic hydrogenation than it is to reduce aldehydes and ketones by ionic hydrogenation. See, for example: J. S. Song, D. J. Szalda and R. M. Bullock, Organometallics, 2001, 20, 3337–3346.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1028432. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc03128h

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