5 research outputs found
Recommended from our members
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indicate that these phosphine-substituted clusters are direct analogs of [Fe4N(CO)12]- where one CO ligand has been replaced by a phosphine.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
A pendant proton shuttle on [Fe 4 N(CO) 12 ] − alters product selectivity in formate vs. H 2 production via the hydride [H–Fe 4 N(CO) 12 ] −
Renewable Formate from C–H Bond Formation with CO<sub>2</sub>: 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 CO<sub>2</sub> drive our efforts to discover ways of converting
excess CO<sub>2</sub> 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 CO<sub>2</sub> requires a fundamental understanding
of how to generate metal hydrides that selectively donate H<sup>–</sup> to CO<sub>2</sub>, rather than recombining with H<sup>+</sup> to
liberate H<sub>2</sub>. Our work with a unique series of water-soluble
and -stable, low-valent iron electrocatalysts offers mechanistic and
thermochemical insights into formate production from CO<sub>2</sub>. Of particular interest are the nitride- and carbide-containing
clusters: [Fe<sub>4</sub>NÂ(CO)<sub>12</sub>]<sup>−</sup> and
its derivatives and [Fe<sub>4</sub>CÂ(CO)<sub>12</sub>]<sup>2–</sup>. In both aqueous and mixed solvent conditions, [Fe<sub>4</sub>NÂ(CO)<sub>12</sub>]<sup>−</sup> forms a reduced hydride intermediate,
[H–Fe<sub>4</sub>NÂ(CO)<sub>12</sub>]<sup>−</sup>, through
stepwise electron and proton transfers. This hydride selectively reacts
with CO<sub>2</sub> 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 [Fe<sub>4</sub>NÂ(CO)<sub>12</sub>]<sup>−</sup> facilitates proton transfer to the active site,
successfully intercepting the hydride intermediate before it reacts
with CO<sub>2</sub>; only H<sub>2</sub> is observed in this case.
In contrast, isoelectronic [Fe<sub>4</sub>CÂ(CO)<sub>12</sub>]<sup>2–</sup> features a concerted proton–electron transfer
mechanism to form [H–Fe<sub>4</sub>CÂ(CO)<sub>12</sub>]<sup>2–</sup>, which is selective for H<sub>2</sub> production
even in the presence of CO<sub>2</sub>, 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–Fe<sub>4</sub>NÂ(CO)<sub>12</sub>]<sup>−</sup> and its derivative [H–Fe<sub>4</sub>NÂ(CO)<sub>11</sub>(PPh<sub>3</sub>)]<sup>−</sup> have hydricities modest
enough to avoid H<sub>2</sub> production but strong enough to make
formate. [H–Fe<sub>4</sub>CÂ(CO)<sub>12</sub>]<sup>2–</sup> 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<sup>+</sup>, only H<sub>2</sub> 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<sup>–1</sup> where C–H bond formation may be favored in
water or MeCN, respectively
Formation of a Stable Complex, RuCl<sub>2</sub>(S<sub>2</sub>CPPh<sub>3</sub>)(PPh<sub>3</sub>)<sub>2</sub>, Containing an Unstable Zwitterion from the Reaction of RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub> with Carbon Disulfide
New insight into the complexity of
the reaction of the prominent catalyst RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub> with carbon disulfide has been obtained from a combination
of X-ray diffraction and <sup>31</sup>P NMR studies. The red-violet
compound originally formulated as a cationic Ï€-CS<sub>2</sub> complex, [RuClÂ(Ï€-CS<sub>2</sub>)Â(PPh<sub>3</sub>)<sub>3</sub>]ÂCl, has been identified as a neutral molecule, RuCl<sub>2</sub>(S<sub>2</sub>CPPh<sub>3</sub>)Â(PPh<sub>3</sub>)<sub>2</sub>, which contains
the unstable zwitterion S<sub>2</sub>CPPh<sub>3</sub>. In the absence
of RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub>, there is no sign
of a reaction between triphenylphosphine and carbon disulfide, although
more basic trialkylphosphines form red adducts, S<sub>2</sub>CPR<sub>3</sub>. Despite the presence of an unstable ligand, RuCl<sub>2</sub>(S<sub>2</sub>CPPh<sub>3</sub>)Â(PPh<sub>3</sub>)<sub>2</sub> 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 RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub> with carbon disulfide is conducted in the presence
of methanol, crystals of orange [RuClÂ(S<sub>2</sub>CPPh<sub>3</sub>)Â(CS)Â(PPh<sub>3</sub>)<sub>2</sub>]ÂCl·2MeOH and yellow RuCl<sub>2</sub>(CS)Â(MeOH)Â(PPh<sub>3</sub>)<sub>2</sub> also form. <sup>31</sup>P NMR studies indicate that the unsymmetrical dinuclear complex (SC)Â(Ph<sub>3</sub>P)<sub>2</sub>RuÂ(μ-Cl)<sub>3</sub>RuÂ(PPh<sub>3</sub>)<sub>2</sub>Cl is the initial product of the reaction of RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub> with carbon disulfide. A path
connecting the isolated products is presented