57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Reduction of CO₂ is of great interest because the reduced forms of CO₂ such as formic acid, methanol, or methane can be directly used as fuels, which establishes CO₂ as an abundant and inexpensive source for alternative energy. Although there have been continuing efforts in developing coordination complexes that can catalyze the reduction of CO₂,¹ none has shown the comparable efficiency and selectivity of the CO₂-reducing enzymes found in nature.

Formate dehydrogenase (FDHs) are enzymes that catalyze the two electron oxidation of formate (HCO₂^–) to CO₂ coupled to the reduction of NAD(P)⁺ to NAD(P)H.² In some prokaryotes, the FDHs contain molybdenum (Mo) or tungsten (W) cofactors, by which an efficient CO₂-reductase activity has been also observed. Recently, Hirst and coworkers have shown that a purified W-FDH can activate an electrode to reduce CO₂ to formate more efficiently than any of known synthetic catalysts.³

The X-ray structures of several Mo or W containing FDHs are known,⁴ in which the metal ion is in a square pyramid geometry with two equatorial pyranopterins and an –SH (or –OH) axial ligand in the reduced state, Figure 1.  In light of the known FDH structures as well as the efficient electrocatalytic activity of W-FDH, we have studied on the CO₂ reactivity of a synthetic model complex of W-FDH.⁵

Figure 1.  Active site structures in the reduced Mo-FDH (E. coli)^4a.

Holm has previously reported⁶ that a group of phenolate bound tungsten or molybdenum bis(dithiolene) complexes such as [W^IV(OPh)(S₂C₂Me₂)₂]^– undergo hydrolysis in the presence of excess water to yield W^IV=O complexes via formation of {W^IV-OH} species. These reports inspired us to explore CO₂ reactivity of [W^IV(OH)(S₂C₂Ph₂)₂]^– which can be prepared in situ through hydrolysis of [W^IV(OPh)(S₂C₂Ph₂)₂]^– (1) in the presence of water.

We first sought a hydrolysis condition that can be achieved with the minimum amount of water possible. We found that a relatively small amount of water was sufficient enough to induce hydrolysis of [W(OPh)(S₂C₂Ph₂)₂]^– (1) to yield [W^IV(O)(S₂C₂Ph₂)₂]^2– (2).  However, we noticed that 2 was not stable and it slowly converts into another well known compound, [W^V(O)(S₂C₂Ph₂)₂]^– (3) (Scheme 1), which is associated with the reduction of H⁺ to hydrogen.

Scheme 1

IR spectroscopy supports the generation of 3 as the final hydrolysis product as well as the released free phenol, Figure 2. The ¹⁸OH₂ isotope experiment indicates that the oxygen atoms in the reaction products originate from water through hydrolysis of [W^IV(OPh)(S₂C₂Ph₂)₂]^– (1), during which the intermediate [W^IV(OH)(S₂C₂Ph₂)₂]^– must have been generated.  The observed 937 cm^–1 peak from the crude reaction products is assigned to the stretching frequency of W^V=O in 3, while the additional peak at 915 cm^–1 is due to the hydrogen bonded W^V=O moiety to free phenol that is released from 1.

Figure 2.  IR spectra (Nujol) of (a) the crude reaction products of [W^IV(OPh)(S₂C₂Ph₂)₂] (1) and 10 equiv. of H₂O (black dotted) and those of 1 and 10 equiv. of H₂¹⁸O (blue solid), (b) the isolated reaction product [W^V(O)(S₂C₂Ph₂)₂]^– (3) (black dotted), 3 in the presence of 2 equiv of externally added phenol (green solid), and re-isolated 3 after removing phenol by precipitation (grey solid).

Next, we studied the CO₂ reactivity of [W^IV(OPh)(S₂C₂Ph₂)₂]^– (1) in the presence of water. The reaction of 1/CO₂/H₂O does not exhibit the anticipated FDH activity. Instead, [W^IV(OH)(S₂C₂Ph₂)₂]^– reacts with CO₂ to form a presumable (bi)carbonato intermediate which immediately undergoes decarboxylation to generate [W^IV(O)(S₂C₂Ph₂)₂]^– (2) and CO₂ (Scheme 2).  The formation of (bi)carbonato species from CO₂ with a metal-hydroxide is common for many transition metal complexes. In order to evaluate the prospect of the proposed (bi)carbonato intermediate during the reaction (Scheme 2), [W^IV(OPh)(S₂C₂Ph₂)₂]^– (1) was reacted with one equiv. of Et₄NHCO₃.  Upon addition of bicarbonate to 1, the oxo compound 2 formed immediately, as monitored by UV-Vis spectroscopy. The result supports a notion that the O atom of CO₂ is inserted into 2 through a (bi)carbonate-bound W intermediate.

Scheme 2

The FDHs utilize a bis(dithiolene) bound M^IV (where M = Mo or W) ion to reduce CO₂ to formate.  The use of higher-valent metal ions with non-innocent ligands by the enzyme is a very different strategy from most synthetic systems in which the metal ions in much lower valences, 0, +1, and +2, are utilized to activate CO₂. Our current study shows that the presence of a nucleophilic ligand such as OH^– is crucial to initiate CO₂ reactivity with a bis(dithiolene) tungsten(IV) species to likely form a tungsten (bi)carbonate intermediate. Indeed, a recent calculation study on the mechanism of Mo-FDH suggests that a Mo^IV-thiocarbonato intermediate forms during the oxidation cycle of formate to CO₂. With respect to the reverse reaction by W-FDH, one can expect the conversion of W^IV-thiocarbonate species to W^VI-formate (Scheme 3), similar to the typical O-atom abstraction chemistry known for this class of enzyme. In our current synthetic model system, however, we were not able to imitate the reduction step. This may be due to the lack of a proton delivery channel near the metal active site, i.e., selenocysteine and histidine, that is strictly conserved in the FDH proteins. Future studies will focus on the factors critical to induce an O-atom abstraction from a putative tungsten-(bi)carbonate intermediate over the oxide (O^2–) abstraction chemistry observed.

Scheme 3

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