A pendant proton shuttle on [Fe4N(CO)12]- alters product selectivity in formate vs. H2 production via the hydride [H-Fe4N(CO)12].

Proton relays are known to increase reaction rates for H2 evolution and lower overpotentials in electrocatalytic reactions. In this report we describe two electrocatalysts, [Fe4N(CO)11(PPh3)]- (1-) which has no proton relay, and hydroxyl-containing [Fe4N(CO)11(Ph2P(CH2)2OH)]- (2-). Solid state structures indicate that these phosphine-substituted clusters are direct analogs of [Fe4N(CO)12]- where one CO ligand has been replaced by a phosphine. We show that the proton relay changes the selectivity of reactions: CO2 is reduced selectively to formate by 1- in the absence of a relay, and protons are reduced to H2 under a CO2 atmosphere by 2-. These results implicate a hydride intermediate in the mechanism of the reactions and demonstrate the importance of controlling proton delivery to control product selectivity. Thermochemical measurements performed using infrared spectroelectrochemistry provided pKa and hydricity values for [HFe4N(CO)11(PPh3)]-, which are 23.7, and 45.5 kcal mol-1, respectively. The pKa of the hydroxyl group in 2- was determined to fall between 29 and 41, and this suggests that the proximity of the proton relay to the active catalytic site plays a significant role in the product selectivity observed, since the acidity alone does not account for the observed results. More generally, this work emphasizes the importance of substrate delivery kinetics in determining the selectivity of CO2 reduction reactions that proceed through metal-hydride intermediates

Renewable Formate from C–H Bond Formation with CO₂: Using Iron Carbonyl Clusters as Electrocatalysts

ConspectusAs a society, we are heavily dependent on nonrenewable petroleum-derived fuels and chemical feedstocks. Rapid depletion of these resources and the increasingly evident negative effects of excess atmospheric CO₂ drive our efforts to discover ways of converting excess CO₂ into energy dense chemical fuels through selective C–H bond formation and using renewable energy sources to supply electrons. In this way, a carbon-neutral fuel economy might be realized.To develop a molecular or heterogeneous catalyst for C–H bond formation with CO₂ requires a fundamental understanding of how to generate metal hydrides that selectively donate H^– to CO₂, rather than recombining with H⁺ to liberate H₂. Our work with a unique series of water-soluble and -stable, low-valent iron electrocatalysts offers mechanistic and thermochemical insights into formate production from CO₂. Of particular interest are the nitride- and carbide-containing clusters: [Fe₄N­(CO)₁₂]⁻ and its derivatives and [Fe₄C­(CO)₁₂]^2–. In both aqueous and mixed solvent conditions, [Fe₄N­(CO)₁₂]⁻ forms a reduced hydride intermediate, [H–Fe₄N­(CO)₁₂]⁻, through stepwise electron and proton transfers. This hydride selectively reacts with CO₂ and generates formate with >95% efficiency. The mechanism for this transformation is supported by crystallographic, cyclic voltammetry, and spectroelectrochemical (SEC) evidence. Furthermore, installation of a proton shuttle onto [Fe₄N­(CO)₁₂]⁻ facilitates proton transfer to the active site, successfully intercepting the hydride intermediate before it reacts with CO₂; only H₂ is observed in this case. In contrast, isoelectronic [Fe₄C­(CO)₁₂]^2– features a concerted proton–electron transfer mechanism to form [H–Fe₄C­(CO)₁₂]^2–, which is selective for H₂ production even in the presence of CO₂, in both aqueous and mixed solvent systems. Higher nuclearity clusters were also studied, and all are proton reduction electrocatalysts, but none promote C–H bond formation.Thermochemical insights into the disparate reactivities of these clusters were achieved through hydricity measurements using SEC. We found that only [H–Fe₄N­(CO)₁₂]⁻ and its derivative [H–Fe₄N­(CO)₁₁(PPh₃)]⁻ have hydricities modest enough to avoid H₂ production but strong enough to make formate. [H–Fe₄C­(CO)₁₂]^2– is a stronger hydride donor, theoretically capable of making formate, but due to an overwhelming thermodynamic driving force and the increased electrostatic attraction between the more negative cluster and H⁺, only H₂ is observed experimentally. This illustrates the fundamental importance of controlling thermochemistry when designing new catalysts selective for C–H bond formation and establishes a hydricity range of 15.5–24.1 or 44–49 kcal mol^–1 where C–H bond formation may be favored in water or MeCN, respectively

Formation of a Stable Complex, RuCl₂(S₂CPPh₃)(PPh₃)₂, Containing an Unstable Zwitterion from the Reaction of RuCl₂(PPh₃)₃ with Carbon Disulfide

New insight into the complexity of the reaction of the prominent catalyst RuCl₂(PPh₃)₃ with carbon disulfide has been obtained from a combination of X-ray diffraction and ³¹P NMR studies. The red-violet compound originally formulated as a cationic π-CS₂ complex, [RuCl­(π-CS₂)­(PPh₃)₃]­Cl, has been identified as a neutral molecule, RuCl₂(S₂CPPh₃)­(PPh₃)₂, which contains the unstable zwitterion S₂CPPh₃. In the absence of RuCl₂(PPh₃)₃, there is no sign of a reaction between triphenylphosphine and carbon disulfide, although more basic trialkylphosphines form red adducts, S₂CPR₃. Despite the presence of an unstable ligand, RuCl₂(S₂CPPh₃)­(PPh₃)₂ is remarkably stable. It survives melting at 173–174 °C intact, is stable to air, and undergoes reversible electrochemical oxidation to form a monocation. When the reaction of RuCl₂(PPh₃)₃ with carbon disulfide is conducted in the presence of methanol, crystals of orange [RuCl­(S₂CPPh₃)­(CS)­(PPh₃)₂]­Cl·2MeOH and yellow RuCl₂(CS)­(MeOH)­(PPh₃)₂ also form. ³¹P NMR studies indicate that the unsymmetrical dinuclear complex (SC)­(Ph₃P)₂Ru­(μ-Cl)₃Ru­(PPh₃)₂Cl is the initial product of the reaction of RuCl₂(PPh₃)₃ with carbon disulfide. A path connecting the isolated products is presented