Electron-transfer sensitization of H2 oxidation and CO2 reduction catalysts using a single chromophore Nathan T. La Porte, Davis B. Moravec, and Michael D. Hopkins1 Department of Chemistry, The University of Chicago, Chicago, IL 60637 Edited by Harry B. Gray, California Institute of Technology, Pasadena, CA, and approved May 27, 2014 (received for review November 15, 2013) Energy-storing artificial-photosynthetic systems for CO2 reduction Neumann and coworkers recently reported a photochemical must derive the reducing equivalents from a renewable source system for the reduction of CO2 to CO in which the reducing rather than from sacrificial donors. To this end, a homogeneous, equivalents are derived from the oxidation of H2 by colloidal I + integrated chromophore/two-catalyst system is described that platinum (15). This system, which contains a [Re (phen)(CO)3L] is thermodynamically capable of photochemically driving the (phen is 1,10-phenanthroline) CO2 reduction catalyst linked with energy-storing reverse water–gas shift reaction (CO2 + H2 → CO + a polyoxometalate cluster, drives the reverse water–gas-shift H2O), where the reducing equivalents are provided by renewable reaction (RWGS) (CO2 + H2 → CO + H2O), using light as the H2. The system consists of the chromophore zinc tetraphenylpor- energy source, via the reactions shown in Eqs. 1–3. Unlike R – phyrin (ZnTPP), H2 oxidation catalysts of the form [Cp Cr(CO)3] , photochemical reactions that consume sacrificial donors, this ′ –1 and CO2 reduction catalysts of the type Re(bpy-4,4 -R2)(CO)3Cl. Us- energy-storing system [ΔHf = 41.2 kJ·mol (17)] catalytically ing time-resolved spectroscopic methods, a comprehensive mech- extracts renewable reducing equivalents that can be sourced anistic and kinetic picture of the photoinitiated reactions of to water. mixtures of these compounds has been developed. It has been − + found that absorption of a single photon by broadly absorbing H2 → 2e + 2H [1] ZnTPP sensitizes intercatalyst electron transfer to produce the sub- strate-active forms of each. The initial photochemical step is the − + CO2 + 2e + 2H → CO + H2O [2] heretofore unobserved reductive quenching of the low-energy T1 state of ZnTPP. Under the experimental conditions, the catalyti- + → + [3] cally competent state decays with a second-order half-life of CO2 H2 CO H2O ∼15 μs, which is of the right magnitude for substrate trapping The mechanistic, thermodynamic, and kinetic integration of of sensitized catalyst intermediates. two catalytic cycles with a chromophore is a general challenge that cuts across homogeneous molecular approaches to forming artificial photosynthesis | catalysis | dual catalysis | photoredox | solar fuels from CO and H O. A photochemical system for the photocatalysis 2 2 RWGS reaction that used a homogeneous H2 oxidation catalyst CHEMISTRY would both provide insights into the fundamental factors that he inexorable growth of global energy consumption and govern this integration and opportunities to exert greater control Tconcern over its attendant environmental consequences has over them than is possible with heterogeneous catalysts. Moti- spurred considerable research into developing means to store vated by these possibilities, we report here the photochemistry solar energy in the form of renewable chemical fuels (1, 2). of a homogeneous system composed of a photosensitizer, CO2- Among the potential feedstocks for these solar fuels, CO2 is reduction catalyst, and H2-oxidation catalyst that, upon excita- a desirable target because it is the end product of the combustion tion with long-wavelength light (λ > 590 nm), yields a product of fossil fuels. Thus, developing solar-driven mechanisms for chemically reducing CO2 to energy-rich products holds the po- Significance tential to recycle conventional fuels and mitigate their carbon – impact (3 7). In natural photosynthesis, the reducing equivalents for the Numerous studies over the past 30 y have investigated ho- reduction of CO2 are derived from photochemical water split- mogeneous artificial-photosynthetic systems for CO2 reduction, ting. In homogeneous artificial-photosynthetic systems for CO2 in which a photoexcited chromophore accomplishes the transfer reduction, the source of the reducing equivalents is generally of electrons from a source of reducing equivalents to a CO2 a nonrenewable sacrificial electron donor. These sacrificial reduction catalyst (8–14). With very rare exceptions (15), the reagents aid proof-of-concept experiments by suppressing reducing equivalents consumed in these photochemical CO2 unproductive electron-transfer reactions but negate the solar- reduction reactions have been supplied by sacrificial electron energy-storing potential for the system. Here, we describe an donors. These reagents are used because their prompt decom- integrated homogeneous system in which a zinc porphyrin position following photoinitiated oxidation suppresses un- photosensitizes CO2 reduction and H2 oxidation catalysts, productive back-electron-transfer pathways, which are generally allowing the reducing equivalents for CO2 photoreduction to fast compared with substrate transformation, and because their be derived from renewable H2. This system is thermodynami- decomposition products can provide additional reducing equiv- cally competent to photochemically drive the energy-storing reverse water–gas shift reaction. alents needed for some CO2 reduction reactions, thus circum- venting the one-photon/one-electron limit of molecular photo- Author contributions: N.T.L., D.B.M., and M.D.H. designed research; N.T.L. and D.B.M. sensitizers. Offsetting these practical advantages, however, is the performed research; N.T.L., D.B.M., and M.D.H. analyzed data; and N.T.L., D.B.M., and fact that the stoichiometric consumption of conventional sacri- M.D.H. wrote the paper. ficial donors in these reactions negates their energy-storing The authors declare no conflict of interest. potential. In order for homogeneous systems to drive CO2 re- This article is a PNAS Direct Submission. duction reactions that store energy, these sacrificial reagents 1To whom correspondence should be addressed. E-mail: [email protected]. must be replaced by a second catalytic cycle that extracts the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. reducing equivalents from a renewable source (16). 1073/pnas.1321375111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1321375111 PNAS | July 8, 2014 | vol. 111 | no. 27 | 9745–9750 Downloaded by guest on September 24, 2021 state thermodynamically capable of accomplishing the RWGS Molecular catalysts for H2 oxidation may be divided into two reaction. The system operates via reductive quenching of the mechanistic classes: those with metalloradical active states that low-energy T1 excited state of the common chromophore zinc homolytically cleave H2 via a termolecular process (27), and tetraphenylporphyrin (ZnTPP) by a compound of the form hydrogenase-inspired catalysts that heterolytically activate H2 at a R – – R 5 Cp Cr(CO)3 (1 ;Cp = η -cyclopentadienyl), followed by closed-shell metal center with assistance of a second-coordination- – thermal electron transfer from the product ZnTPP radical to sphere Brønsted base (28). Although the hydrogenase-inspired ′ 2 ′ a complex of the type Re(bpy-4,4 -R2)(CO)3Cl ( ; bpy is 2,2 - catalysts exhibit faster rates for H2 activation, their catalytic cycles bipyridyl). The electron-transfer-sensitized radical products of involve three metal oxidation states. In contrast, catalysts with R – these reactions—Cp Cr(CO)3 (1•)and[Re(bpy-4,4′-R2)(CO)3Cl] metalloradical active states cycle through two oxidation states and, – (2 )—can initiate the oxidation of H2 and reduction of CO2, re- thus, can potentially be activated by a single photoinduced elec- spectively. The ligand substituents within each of these classes of tron-transfer reaction. The most extensively studied of these cata- R – compounds allow control over the driving forces and, thus, the lysts are of the form [Cp Cr(CO)3] , which participate in the cycle rates of the productive (and unproductive) electron-transfer of reactions shown in Eqs. 4–6: reactions available to the components. A comprehensive picture  Ã−  à R ð Þ ð1−Þ → R ð Þ ð1•Þ + − [4] of the mechanism and kinetics of this system has been elucidated Cp Cr CO 3 Cp Cr CO 3 e using time-resolved spectroscopic methods.  à R ð Þ + 1 = → R ð Þ ð1HÞ [5] Results and Discussion Cp Cr CO 3 2H2 Cp Cr CO 3H Design Criteria. Shown in Fig. 1 is a general scheme for accom-  Ã− R ð Þ + ð :Þ → R ð Þ + +; [6] plishing the RWGS reaction via photochemical electron-transfer Cp Cr CO 3H base B Cp Cr CO 3 BH sensitization of H2-oxidation (CatH2)andCO2-reduction (CatCO2) catalysts by a chromophore (Chr). This scheme imposes where Eq. 5 is a composite of two steps (1• + H2 → 1(H2); 1(H2) + – – a number of fundamental and practical constraints on the com- 1• → 2 1H) (27). The derivatives [Cp*Cr(CO)3] (1a ) and [CpCr – – ponents of the system, which are described here to explain the (CO)3] (1b ) were selected for this study because they possess particular chromophore and catalysts chosen for the present suitable oxidation potentials and pKas for the relevant intermedi- study. These constraints include the obvious thermodynamic ates (29). Further, the deprotonation step that completes the H2 requirements for the pairs of chromophore–catalyst electron- activation cycle (Eq. 6) can be accomplished with alkoxide bases t –