Iron Carbonyl Clusters Research Articles

Oxidative Amidation of Ferrocenyl aldehydes with Amines Catalyzed by Chalcogenised Fe3Se2(CO)9 Cluster: Direct Transformation of Aldehyde to Amides.

With the interest and knowing the importance of ferrocenyl conjugates, a direct amidation of ferrocenyl aldehyde has been developed under feasible conditions. Varieties of amines has been oxidatively coupled with ferrocene aldehyde in the presence of highly economical and robustly stable iron chalcogenide ironcarbonyl clusters [Fe3E2(CO)9, E= S, Se, and Te] and TBHP. The reaction worked in greener solvent water at a moderate temperature of 70 °C and synthesized the ferrocenyl-amides in just 30 minutes. All the varieties of amines were found to be well tolerated for the present method, and due to the high medicinal importance of ferrocenyl-amides, sole focus of this report was concentrate on the synthesis of various new ferrocenyl-amides in good to excellent amounts.

Facile Photochemical/Thermal Assisted Hydration of Alkynes Catalysed under Aqueous Media by a Chalcogen Stabilized, Robust, Economical, and Reusable Fe3Se2(CO)9 Cluster

In this report, the catalytic potential of chalcogen-stabilized iron carbonyl clusters [Fe3E2(CO)9 (E = S, Se, Te)] for the photolytic hydration of alkynes has been explored. The iron chalcogenide clusters bring excellent transformations of terminal and internal alkynes to their respective keto products in just 25 min photolysis at −5 °C in inert free and aqueous conditions. After the completion of the reaction, the product can be extracted from organic solvent, and due to the lower solubility of the catalyst in water, it can also be isolated and further reused several times prior to any activation. The catalyst was also found to be active in thermal conditions and bring about the desired transformations with average to good catalytic efficiency. Moreover, during the thermal reaction, the catalyst decomposed and formed the nanoparticles of iron selenides, which worked as a single-source precursor for FeSe nanomaterials. The presented photolysis methodology was found to be most feasible, economical, instantly produce the desired product, and work for a wide range of internal and terminal alkynes; hence, all these features made this method superior to the other reported ones. This report also serves as the first catalytic report of chalcogen-stabilized iron carbonyl clusters for alkyne hydrations.

Iron- and copper-based bifunctional catalysts for the base- and solvent-free C–N coupling of amines and aryl/benzyl chlorides under aerobic conditions

The chalcogenised ironcarbonyl clusters Fe3Se2(CO)9 and Cu(OAc)2 were found to be outstanding bimetallic catalyst for the Buchwald type C–N cross coupling reaction of amine and aryl chloride.

Visible-light photoredox-catalyzed selective carboxylation of C(sp3)−F bonds with CO2

Visible-light photoredox-catalyzed selective carboxylation of C(sp3)−F bonds with CO2

Graphitic Carbon Nitride/CdSe Quantum Dot/Iron Carbonyl Cluster Composite for Enhanced Photocatalytic Hydrogen Evolution

A g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite has been successfully constructed. The structure and chemical composition of the composite were investigated via, inter alia, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The ability of the assembly to act as a photocatalyst for proton reduction to form hydrogen gas was studied. With visible light irradiation for 4 h, the total H2 production catalyzed by the g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite was found to be 9 times as high as a corresponding CdSe/[Fe2S2(CO)6] assembly and significantly higher than either the CdSe quantum dots or g-C3N4 alone. The g-C3N4 support/matrix was found to enhance the stability and efficiency of the CdSe quantum dot/iron carbonyl cluster assembly in the photocatalytic hydrogen evolution process. Results from recycling tests showed that the g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite is a sustainable and robust photocatalyst, maintaining the same activity after three cycles. The photoinduced charge carrier transfer dynamics in the g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite system has been investigated by transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopies. The spectroscopic results indicate efficient hole transfer from the valence band of the excited CdSe quantum dots to the molecular iron carbonyl clusters and from the defect state of the quantum dots to g-C3N4 in the g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite, which significantly inhibits the recombination of photogenerated charge carriers in CdSe quantum dots and boosts the photocatalytic activity and stability for hydrogen evolution. Energy transfer from g-C3N4 to the CdSe quantum dot/[Fe2S2(CO)6] assembly with a time constant of 0.7 ns also contributed to the charge transfer process.

Quantification of the Electrostatic Effect on Redox Potential by Positive Charges in a Catalyst Microenvironment

Charged functional groups in the secondary coordination sphere (SCS) of a heterogeneous nanoparticle or homogeneous electrocatalyst are of growing interest due to enhancements in reactivity that derive from specific interactions that stabilize substrate binding or charged intermediates. At the same time, accurate benchmarking of electrocatalyst systems most often depends on the development of linear free-energy scaling relationships. However, the thermodynamic axis in those kinetic-thermodynamic correlations is most often obtained by a direct electrochemical measurement of the catalyst redox potential and might be influenced by electrostatic effects of a charged SCS. In this report, we systematically probe positive charges in a SCS and their electrostatic contributions to the electrocatalyst redox potential. A series of 11 iron carbonyl clusters modified with charged and uncharged ligands was probed, and a linear correlation between the νCO absorption band energy and electrochemical redox potentials is observed except where the SCS is positively charged.

Ternary antimony–chalcogen iron carbonyl complexes and their derivatives: Syntheses, structures, reactivities, and low-energy-gap characteristics

A novel series of ternary antimony–chalcogen iron carbonyl clusters, [{SbTeFe3(CO)9}{Te2Fe3(CO)9}]– (1), [{SbSeFe3(CO)9}{Se2Fe2(CO)6}]– (2), and [{SbSFe3(CO)9}{SFe3(CO)9}]– (3), were synthesized in moderate yields from reactions of [EFe3(CO)9]2– (E = Te, Se, S) with SbCl3. X-ray analyses revealed that complexes 1–3 each can be viewed as a square pyramidal geometry [SbEFe3(CO)9] (E = Te, Se, S), where the Sb atom was further coordinated with pendant cluster fragments [Te2Fe3(CO)9], [Se2Fe2(CO)6], and [SFe3(CO)9], respectively. Interestingly, the oxidation state of the Sb atom in complexes 1–3 was 0, which was evidenced by XPS and XANES. Complexes 1–3 showed high electrophilicity toward a series of metal carbonylates, which produced transmetallated products, the “spiked” square pyramidal complexes [{SbEFe3(CO)9}{M(CO)x}] [M(CO)x = Fe(CO)4, E = Te, 1-Fe; Se, 2-Fe; S, 3-Fe; M(CO)x = Cr(CO)5, Se, 2-Cr; S, 3-Cr] and the Mn(CO)4-bridged di-ESbFe3(CO)9-based clusters [{SbEFe3(CO)9}2Mn(CO)4]– (E = Se, 2-Mn; S, 3-Mn). Furthermore, the diffuse reflectance spectra showed that these ternary and quaternary antimony-chalcogenide metal carbonyl clusters possessed low energy gaps of 0.84–1.48 eV, suggesting possible electron transports within the frameworks, which was supported by crystal packings and DFT calculations.

Electron Transfer Mediated by Iron Carbonyl Clusters Enhance Light-Driven Hydrogen Evolution in Water by Quantum Dots.

Photocatalytic water splitting has become a promising strategy for converting solar energy into clean and carbon‐neutral solar fuels in a low‐cost and environmentally benign way. Hydrogen gas is such a potential solar fuel/energy carrier. In a classical artificial photosynthetic system, a photosensitizer is generally associated with a co‐catalyst to convert photogenerated charge into (a) chemical bond(s). In the present study, assemblies consisting of CdSe quantum dots that are coupled with one of two molecular complexes/catalysts, that is, [Fe2S2(CO)6] or [Fe3Te2(CO)9], using an interface‐directed approach, have been tested as catalytic systems for hydrogen production in aqueous solution/organic solution. In the presence of ascorbic acid as a sacrificial electron donor and proton source, these assemblies exhibit enhanced activities for the rate of hydrogen production under visible light irradiation for 8 h in aqueous solution at pH 4.0 with up to 110 μmol of H2 per mg of assembly, almost 8.5 times that of pure CdSe quantum dots under the same conditions. Transient absorption and time‐resolved photoluminescence spectroscopies have been used to investigate the charge carrier transfer dynamics in the quantum dot/iron carbonyl cluster assemblies. The spectroscopic results indicate that effective electron transfer from the molecular iron complex to the valence band of the excited CdSe quantum dots significantly inhibits the recombination of photogenerated charge carriers, boosting the photocatalytic activity for hydrogen generation; that is, the iron clusters function as effective intermediaries for electron transfer from the sacrificial electron donor to the valence band of the quantum dots.

(Electrochemical) Properties and Computational Investigations of Ferrocenyl-substituted Fe3(μ3-PFc)2(CO)9 and Co4(μ4-PFc)2(CO)9 Clusters and Their Reduced Species.

The formation of ferrocenyl-functionalized iron and cobalt carbonyl clusters is reported, based on a reaction of FcPCl2 (3) (Fc = Fe(η5-C5H5)(η5-C5H4)) with Fe2(CO)9 and Co2(CO)8, respectively. Therein, nido-Fe3(CO)9(μ3-PFc)2 (4) and nido-Co4(CO)10(μ3-PFc)2 (5) clusters were obtained as the first diferrocenyl-substituted carbonyl clusters with a symmetrical cluster core. Cluster 4 shows two reversible one-electron processes within the anodic region, based on Fc/Fc+ redox events, as well as two processes in the cathodic region. In situ IR and electron paramagnetic resonance (EPR) measurements of all electronic states confirmed an Fc-based oxidation and a core-based reduction. On the basis of the results of a single-crystal X-ray analysis of structures of 4 and 5, computational studies of the highest occupied molecular orbital-lowest unoccupied molecular orbital energies, the spin density, quantum theory of atom-in-molecule delocalization indices, and the atomic charges were performed to explain the experimental results. The latter revealed a reorganization of the cluster core upon reduction and the existence of weak P···P interactions in 4 and 5. Ferrocenyl-related redox processes, occurring reversibly in case of 4, were absent for 5, due to a different distribution of the HOMO energies. EPR measurements furthermore confirmed the core-based radical anion and the formation of a decomposition product at potentials lower than [M]2- (M = Fe, Co).

Theoretical study of Fe–Fe bonding in a Series of Iron Carbonyl clusters [(μ-H)2Fe3(CO)9(µ3-As)Mn(CO)5], [Et4N][(µ-H)2Fe3(CO)9(µ3-As)Fe(CO)4] and [Et4N][HAs{Fe2(CO)6(µ-CO)(µ-H)}{Fe(CO)4}] by QTAIM Perspective

Abstract The bonding interactions such as Fe-Fe, Fe-H and Fe-CO, existing in the iron carbonyl clusters; [(μ-H)2Fe3(CO)9(µ3-As)Mn(CO)5] (1), [Et4N][(µ-H)2Fe3(CO)9(µ3-As)Fe(CO)4] (2) and [Et4N][HAs{Fe2(CO)6(µ-CO)(µ-H)}{Fe(CO)4}] (3), have been studied using atoms in molecules (AIM) approach. Many topological parameters of the electron density for these clusters have been computed at bond critical points (bcp’s). The conceptual framework of AIM theory indicates the absence of Fe-Fe direct bonding, since no bond critical point is found between Fe metals and therefore, no bond path connecting them in 1, 2 and 3. Also, from these results, a comparison was done for related but different interactions involving; different Fe…Fe interactions and bridged Fe-H bond versus other bridged Fe-ligand such as Fe-CO. An interaction of (Fe1-H1-Fe2-H2-Fe3) plan existing in each of cluster 1 and 2 is 5c-4e type. While in cluster 3, an interaction core (Fe1-H1-Fe2) is 3c-2e. Finally, the existence of hydride bridging ligands has an efficient role to reduce the electron density delocalized between hydride bridged iron pairs oppositely with hydride unbridged ones.

Bond-Order Time Series Analysis for Detecting Reaction Events in Ab Initio Molecular Dynamics Simulations.

Ab initio molecular dynamics is able to predict novel reaction mechanisms by directly observing the individual reaction events that occur in simulation trajectories. In this article, we describe an approach for detecting reaction events from simulation trajectories using a physically motivated model based on time series analysis of ab initio bond orders. We found that applying a threshold to the bond order was insufficient for accurate detection, whereas peak finding on the first time derivative resulted in significantly improved accuracy. The model is trained on a reference set of reaction events representing the ideal result given unlimited computing resources. Our study includes two model systems: a heptanylium carbocation that undergoes hydride shifts and an unsaturated iron carbonyl cluster that features CO ligand migration and bridging behavior. The results indicate a high level of promise for this analysis approach to be used in mechanistic analysis of reactive AIMD simulations more generally.

In Search of Multi-Stage Mutation Resistant HIV Drugs: A DFT Study

Aims: Searching of highly active (activity at the pico molar level) multi-stage, mutation registrant HIV-1 drug through theoretical and computational methodologies. Background: Recent advances of computational methodology to predict the important properties of a drug, like IC50, LogP, CC50, etc., assist in model and in-silico characterization of compounds as drug for variant deadly diseases. The drug potency of thio-propionate derivatives of iron, ruthenium and osmium carbonyl clusters are reported as less toxic,water-soluble, easy metabolic and effective as cancer drugs. Objective: The study of the drug-like activity of organo-metallic clusters of group-8 metals as HIV drug using the latest computational aids is the objective of the present research. Method: Quantum computational methodologies are used to predict the IC50, and LogP, CC50 of the modeled compounds. Density Functional Theory (DFT) calculations are used where it is required. Molecular docking methods are used for the study of the metabolic probability of the compound which has the best activity as an HIV drug. Result: It is observed that IC50 of Ru5(CO)14(µ−H)(µ−S(CH2)2COO−)Na+ is only 220 pico-mole as HIV-1 capsid -A inhibitor. Its inhibition activities in other enzymatic processes involved in the HIV life cycle in the human body are also studied and compared with the most used market available drugs. This modeled compound has supreme activity and less toxicity compared to the FDA approved drugs chosen for the comparison. Conclusion: From this study, it could be concluded that this modeled compound would be a multi-stage, mutation resistant HIV drug. Other: These findings would enrich the HIV drug search. Funding Statement: None to declare Declaration of Interests: None to declare

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

As 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.

Metal Hydrides as Catalytic Intermediates: The Curious Case of Iron Carbonyl Clusters

Metal Hydrides as Catalytic Intermediates: The Curious Case of Iron Carbonyl Clusters

Structures, Interconversions, and Spectroscopy of Iron Carbonyl Clusters with an Interstitial Carbide: Localized Metal Center Reduction by Overall Cluster Oxidation.

Structures, Interconversions, and Spectroscopy of Iron Carbonyl Clusters with an Interstitial Carbide: Localized Metal Center Reduction by Overall Cluster Oxidation.

Enhancement of biocompatibility and photoacoustic contrast activity of metal clusters

Organometallic carbonyl clusters (OMCC) of group VIII elements are water soluble, bio-compatible and stable high-contrast photoacoustic agents for live cell imaging. But, they have limited application due to weak absorption within 700–1000nm wavelength which is known as the biological window of absorption. In this article, we report that hexa-nuclear iron (Fe6) carbonyl cluster derivatized with sodium thio-propanoate has very good absorption within 700–1600nm wave length. This modeled compound is water soluble and bio-compatible. The bio-compatibility of this compound is tested through cytotoxicity, LogP and metabolic probability at CYP450-2D6 enzyme.

Structures, Interconversions, and Spectroscopy of Iron Carbonyl Clusters with an Interstitial Carbide: Localized Metal Center Reduction by Overall Cluster Oxidation.

The syntheses, interconversions, and spectroscopic properties of a set of iron carbonyl clusters containing an interstitial carbide are reported. This includes the low temperature X-ray structures of the six-iron clusters (Y)2[Fe6(μ6-C)(μ2-CO)4(CO)12] (1a-c; where Y = NMe4, NEt4, PPh4); the five-iron cluster [Fe5(μ5-C)(CO)15] (3); and the novel formulation of the five-iron cluster (NMe4)2[Fe5(μ5-C)(μ2-CO)(CO)13] (4). Also included in this set is the novel charge-neutral cluster, [Fe6(μ6-C)(CO)18] (2), for which we were unable to obtain a crystallographic structure. As synthetic proof for the identity of 2, we performed a closed loop of interconversions within a family of crystallographically defined species (1, 3, and 4): [Fe6]2- → [Fe6]0 → [Fe5]0 → [Fe5]2- → [Fe6]2-. The structural, spectroscopic, and electronic properties of this "missing link" cluster 2 were investigated by IR, Raman, XPS, and Mössbauer spectroscopies-as well as by DFT calculations. A single νCO feature (1965 cm-1) in the IR spectrum of 2, as well as a prominent Raman feature (νsymm = 1550 cm-1), are consistent with the presence of terminal carbonyls and a {(μ6-C)Fe6} arrangement of iron centers around the central carbide. The XPS of 2 exhibits a higher energy Fe 2p3/2 feature (707.4 eV) as compared to that of 1 (705.5 eV), consistent with the two-electron oxidation induced by treatment of 1 with two equivalents of [Fc](PF6) under CO atmosphere (for the two added CO ligands). DFT calculations indicate two axial and four equatorial Fe sites in 1, all of which have the same or similar oxidation states, for example, two Fe(0) and four Fe(+0.5). These assignments are supported by Mössbauer spectra for 1, which exhibit two closely spaced quadrupole doublets with δ = 0.076 and 0.064 mm s-1. The high-field Mössbauer spectrum of 2 (4.2 K) exhibits three prominent quadrupole doublets with δ = -0.18, -0.11, and +0.41 mm s-1. This indicates three pairs of chemically equivalent Fe sites. The first two pairs arise from irons of a similar oxidation state, while the last pair arises from irons in a different oxidation state, indicating a mixed-valent cluster. Variable field Mössbauer spectra for 2 were simulated assuming these two groups and a diamagnetic ground state. Taken together, the Mössbauer results and DFT calculations for 2 indicate two axial Fe(II) sites and four equatorial sites of lower valence, probably Fe(0). In the DFT optimized pentagonal bipyramidal structure for 2, the Fe(II)-Ccarbide distances are compressed (∼1.84 Å), while the Fe(0)-Ccarbide distances are elongated (∼2.05 Å). Analysis of the formulations for 1 (closo-square bipyramid) and 2 (nido-pentagonal bipyramid) is considered in the context of the textbook electron-counting rules of 14n+2 and 14n+4 for closo and nido clusters, respectively. This redox-dependent intracluster disproportionation of Fe oxidation states is concluded to arise from changes in bonding to the central carbide. A similar phenomenon may be promoted by the central carbide of the FeMoco cluster of nitrogenase, which may in turn stimulate N2 reduction.

Iron carbonyl clusters with ECl2 units (E = P, As)

Reaction of [PPN][HFe(CO)4] (PPN = bis(triphenylphosphine)iminium) with PCl3 in a 1:1 ratio produced small amounts of [PPN][P{Fe(CO)4}2Cl2] ([PPN][Ia]). Reaction of [Et4N][HFe(CO)4] with AsCl3 in a 1:0.75 ratio in THF at −78 °C produced [Et4N][As{Fe(CO)4}2Cl2] ([PPN][Ib]) as the majority product. The compound [PPN][(CO)4FePCl2O] ([PPN][II]) was obtained from the reaction of Fe2(CO)9 with PCl3 in THF. In contrast, [Et4N][HFe(CO)4] reacted with PCl3 at −78 °C in a 2:1 ratio to yield [Et4N][Fe2(CO)6{(μ4-PFe(CO)4)2(μ-CO)}{μ-PCl2}] ([Et4N][III]) as the majority product. The compounds were characterized spectroscopically and by single-crystal X-ray diffraction analyses.

Small iron-carbonyl clusters bearing imidazolium-2-trithioperoxycarboxylate ligands.

The reaction of [Fe2(CO)9] with two representative imidazolium-2-dithiocarboxylate zwitterions derived from common N-heterocyclic carbenes (NHCs) bearing mesityl (IMes) or 2,6-diisopropylphenyl substituents on their nitrogen atoms (IDip) unexpectedly afforded two small bimetallic iron-carbonyl clusters with the generic formula [Fe2(CO)6(μ-κ2-S,S'-κ2-S,S'-S3C·NHC)]. After a brief optimization of the reaction conditions, these two "sulfur-enriched" products were isolated in low yields. They were fully characterized by IR, NMR, UV/Visible, and ESI-MS techniques, and their molecular structures were determined by single crystal X-ray diffraction analysis. The two compounds adopted a butterfly-type disposition in the solid state, with an [Fe2(CO)6] core bridged by the trithioperoxycarboxylate moiety of the in situ generated NHC·CS3 ligands. Bond lengths recorded for the CS3- unit revealed that its negative charge was mostly located on the remote sulfur atom.

Tailoring Electrocatalysts for Selective CO2 or H(+) Reduction: Iron Carbonyl Clusters as a Case Study.

The design of electrocatalysts that will selectively transfer hydride equivalents to either H(+) or CO2 to afford H2 or formate is a long-standing goal in molecular electrocatalysis. In this Forum Article, we use experimentally determined thermochemical parameters, hydricity and pKa values, to rationalize our observations that the carbide-containing iron carbonyl cluster [Fe4C(CO)12](2-) reduces H(+) to H2 in the presence of CO2 in either acetonitrile (MeCN), MeCN with 5% water, or buffered water (pH 5-13), with no traces of formate or other carbon-containing products observed. Our previous work has shown that the closely related nitride-containing clusters [Fe4N(CO)12](-) and [Fe4N(CO)11(PPh3)](-) will also reduce H(+) to H2 in either MeCN with 5% water or buffered water (pH 5-13), but upon the addition of CO2, they selectively generate formate. The thermochemical measurements on [Fe4C(CO)12](2-) predict that the free energy for transfer of hydride, in MeCN, from the intermediate [HFe4C(CO)12](2-) to CO2 is thermoneutral and to H(+) is -32 kcal mol(-1). In water, these values are less than -19.2 and -8.6 kcal mol(-1), respectively. These results suggest that generation of both H2 and formate should be favorable in aqueous solution and that kinetic effects, such as a fast rate of H2 evolution, must influence the observed selectivity for generation of H2. The hydride-donating ability of [HFe4N(CO)12](-) is lower than that of [HFe4C(CO)12](2-) by 5 kcal mol(-1) in MeCN and by at least 3 kcal mol(-1) in water, and we speculate that this more modest reactivity provides the needed selectivity to obtain formate. We also discuss predictions that may guide future catalyst design.