Hydrogenation and electrocatalytic reduction of carbon dioxide to formate with a single Co catalyst.

A cobalt(i) complex is shown to be capable of both electrocatalytic reduction and hydrogenation of CO2 to formate. Several proposed intermediates are characterized and thus form the basis for a proposed mechanism that allows for the dual reactivity: reduction of CO2via H2 addition, and H+/e- equivalents. The work makes use of a novel tris(phosphino) ligand. When a pendent amine is attached to the ligand, no change in catalytic reactivity is observed.

Toward Combined Carbon Capture and Recycling: Addition of an Amine Alters Product Selectivity from CO to Formic Acid in Manganese Catalyzed Reduction of CO2.

Owing to the energetic cost associated with CO2 release in carbon capture (CC), the combination of carbon capture and recycling (CCR) is an emerging area of research. In this approach, "captured CO2," typically generated by addition of amines, serves as a substrate for subsequent reduction. Herein, we report that the reduction of CO2 in the presence of morpholine (generating mixtures of the corresponding carbamate and carbamic acid) with a well-established Mn electrocatalyst changes the product selectivity from CO to H2 and formate. The change in selectivity is attributed to in situ generation of the morpholinium carbamic acid, which is sufficiently acidic to protonate the reduced Mn species and generate an intermediate Mn hydride. Thermodynamic studies indicate that the hydride is not sufficiently hydritic to reduce CO2 to formate, unless the apparent hydricity, which encompasses formate binding to the Mn, is considered. Increasing steric bulk around the Mn shuts down rapid homolytic H2 evolution rendering the intermediate Mn hydride more stable; subsequent CO2 insertion appears to be faster than heterolytic H2 production. A comprehensive mechanistic scheme is proposed that illustrates how thermodynamic analysis can provide further insight. Relevant to a range of hydrogenations and reductions is the modulation of the hydricity with substrate binding that makes the reaction favorable. Significantly, this work illustrates a new role for amines in CO2 reduction: changing the product selectivity; this is pertinent more broadly to advancing CCR.

The role of leached Zr in the photocatalytic reduction of CO2 to formate by derivatives of UiO-66 metal organic frameworks.

This work reports the photocatalytic reduction of CO2 to formate that is mediated by UiO-66 Zr MOF derivatives. Amino-substituted UiO-66 is a known photocatalyst for this transformation, and herein we identify that this catalysis is due to leached Zr, not the MOF itself. No correlation between catalytic activity and crystal size is observed for UiO-66-NH2. Recycling studies along with SEM images of the crystals prior to and after catalysis support our conclusion that the catalytic performance can be described by the amount of leached Zr. Moreover, when the effect of the linker on the catalytic reaction is probed, all MOFs that facilitate the reduction of CO2 are found to leach Zr into solution. Correlation of the MOF (or linker) band gap energies to formate production indicates that this is an important parameter to the leached species. Combined with a linker exchange study, this indicates that the leached Zr still coordinates the linker. These results indicate that the UiO-66 Zr MOFs are not stable under typical photochemical conditions, and emphasize the importance of considering the role that leached metals play in catalysis.

Low Reorganization Energy for Electron Self-Exchange by a Formally Copper(III,II) Redox Couple.

The rate constant for electron self-exchange (k11) between LCuOH and [LCuOH]- (L = bis-2,6-(2,6-diisopropylphenyl)carboximidopyridine) was determined using the Marcus cross relation. This work involved measurement of the rate of the cross-reaction between [Bu4N][LCuOH] and [Fc][BAr4F] (Fc+ = ferrocenium; BAr4F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)) by stopped-flow methods at -88 °C in CH2Cl2 and measurement of the equilibrium constant for the redox process by UV-vis titrations under the same conditions. A value of k11 = 3 × 104 M-1 s-1 (-88 °C) led to estimation of a value 9 × 106 M-1 s-1 at 25 °C, which is among the highest values known for copper redox couples. Further Marcus analysis enabled determination of a low reorganization energy, λ = 0.95 ± 0.17 eV, attributed to minimal structural variation between the redox partners. In addition, the reaction entropy (ΔS°) associated with the LCuOH/[LCuOH]- self-exchange was determined from the temperature dependence of the redox potentials, and found to be dependent upon ionic strength. Comparisons to other Cu redox systems and potential new applications for the formally CuIII,II system are discussed.

Thermodynamic Analysis of Metal-Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition.

The hydrogenation of CO2 in the presence of amines to formate, formamides, and methanol (MeOH) is a promising approach to streamlining carbon capture and recycling. To achieve this, understanding how catalyst design impacts selectivity and performance is critical. Herein we describe a thorough thermochemical analysis of the (de)hydrogenation catalyst, (PNP)Ru-Cl (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; Ru = Ru(CO)(H)) and correlate our findings to catalyst performance. Although this catalyst is known to hydrogenate CO2 to formate with a mild base, we show that MeOH is produced when using a strong base. Consistent with pKa measurements, the requirement for a strong base suggests that the deprotonation of a six-coordinate Ru species is integral to the catalytic cycle that produces MeOH. Our studies also indicate that the concentration of MeOH produced is independent of catalyst concentration, consistent with a deactivation pathway that is dependent on methanol concentration, not equivalency. Our temperature-dependent equilibrium studies of the dearomatized congener, (*PNP)Ru, with various H-X species (to give (PNP)Ru-X; X = H, OH, OMe, OCHO, OC(O)NMe2) reveal that formic acid equilibrium is approximately temperature-independent; relative to H2, it is more favored at elevated temperatures. We also measure the hydricity of (PNP)Ru-H in THF and show how subsequent coordination of the substrate can impact the apparent hydricity. The implications of this work are broadly applicable to hydrogenation and dehydrogenation catalysis and, in particular, to those that can undergo metal-ligand cooperativity (MLC) at the catalyst. These results serve to benchmark future studies by allowing comparisons to be made among catalysts and will positively impact rational catalyst design.

Sodium-coupled electron transfer reactivity of metal-organic frameworks containing titanium clusters: the importance of cations in redox chemistry.

Stoichiometric reduction reactions of two metal-organic frameworks (MOFs) by the solution reagents (M = Cr, Co) are described. The two MOFs contain clusters with Ti8O8 rings: Ti8O8(OH)4(bdc)6; bdc = terephthalate (MIL-125) and Ti8O8(OH)4(bdc-NH2)6; bdc-NH2 = 2-aminoterephthalate (NH2-MIL-125). The stoichiometry of the redox reactions was probed using solution NMR methods. The extent of reduction is greatly enhanced by the presence of Na+, which is incorporated into the bulk of the material. The roughly 1 : 1 stoichiometry of electrons and cations indicates that the storage of e- in the MOF is tightly coupled to a cation within the architecture, for charge balance.

Bulk-to-Surface Proton-Coupled Electron Transfer Reactivity of the Metal-Organic Framework MIL-125.

Stoichiometric proton-coupled electron transfer (PCET) reactions of the metal-organic framework (MOF) MIL-125, Ti8O8(OH)4(bdc)6 (bdc = terephthalate), are described. In the presence of UV light and 2-propanol, MIL-125 was photoreduced to a maximum of 2( e-/H+) per Ti8 node. This stoichiometry was shown by subsequent titration of the photoreduced material with the 2,4,6-tri- tert-butylphenoxyl radical. This reaction occurred by PCET to give the corresponding phenol and the original, oxidized MOF. The high level of charging, and the independence of charging amount with particle size of the MOF samples, shows that the MOF was photocharged throughout the bulk and not only at the surface. NMR studies showed that the product phenol is too large to fit in the pores, so the phenoxyl reaction must have occurred at the surface. Attempts to oxidize photoreduced MIL-125 with pure electron acceptors resulted in multiple products, underscoring the importance of removing e- and H+ together. Our results require that the e- and H+ stored within the MOF architecture must both be mobile to transfer to the surface for reaction. Analogous studies on the soluble cluster Ti8O8(OOC tBu)16 support the notion that reduction occurs at the Ti8 MOF nodes and furthermore that this reduction occurs via e-/H+ (H-atom) equivalents. The soluble cluster also suggests degradation pathways for the MOFs under extended irradiation. The methods described are a facile characterization technique to study redox-active materials and should be broadly applicable to, for example, porous materials like MOFs.

Protonation and Proton-Coupled Electron Transfer at S-Ligated [4Fe-4S] Clusters.

Biological [Fe-S] clusters are increasingly recognized to undergo proton-coupled electron transfer (PCET), but the site of protonation, mechanism, and role for PCET remains largely unknown. Here we explore this reactivity with synthetic model clusters. Protonation of the arylthiolate-ligated [4Fe-4S] cluster [Fe4 S4 (SAr)4 ](2-) (1, SAr=S-2,4-6-(iPr)3 C6 H2 ) leads to thiol dissociation, reversibly forming [Fe4 S4 (SAr)3 L](1-) (2) and ArSH (L=solvent, and/or conjugate base). Solutions of 2+ArSH react with the nitroxyl radical TEMPO to give [Fe4 S4 (SAr)4 ](1-) (1ox ) and TEMPOH. This reaction involves PCET coupled to thiolate association and may proceed via the unobserved protonated cluster [Fe4 S4 (SAr)3 (HSAr)](1-) (1-H). Similar reactions with this and related clusters proceed comparably. An understanding of the PCET thermochemistry of this cluster system has been developed, encompassing three different redox levels and two protonation states.

Electron transfer and proton-coupled electron transfer reactivity and self-exchange of synthetic [2Fe-2S] complexes: models for Rieske and mitoNEET clusters.

This report describes the thermochemistry, proton-coupled electron transfer (PCET) reactions and self-exchange rate constants for a set of bis-benzimidazolate-ligated [2Fe-2S] clusters. These clusters serve as a model for the chemistry of biological Rieske and mitoNEET clusters. PCET from [Fe2S2((Pr)bbim)((Pr)bbimH)](2-) (4) and [Fe2S2((Pr)bbim)((Pr)bbimH2)](1-) (5) to TEMPO occurs via concerted proton-electron transfer (CPET) mechanisms ((Pr)bbimH2 = 4,4-bis-(benzimidazol-2-yl)heptane). Intermolecular electron transfer (ET) self-exchange between [Fe2S2((Pr)bbim)2](2-) (1) and [Fe2S2((Pr)bbim)2](3-) (2) occurs with a rate constant of (1.20 ± 0.06) × 10(5) M(-1) s(-1) at 26 °C. A similar self-exchange rate constant is found for the related [2Fe-2S] cluster [Fe2S2(SArO)2](2-/3-), SArO(2-) = thiosalicylate. These are roughly an order of magnitude slower than that reported for larger [4Fe-4S] clusters and 1 order of magnitude faster than that reported for N-ligated high-spin iron complexes. These results suggest that the rate of intermolecular ET to/from [Fe-S] clusters is modulated by cluster size. The measured PCET self-exchange rate constant for 1 and 4 at -30 °C is (3.8 ± 0.7) × 10(4) M(-1) s(-1). Analysis of rate constants using the Marcus cross-relation suggests that this process likely occurs via a concerted proton-electron transfer (CPET) mechanism. The implications of these findings to biological systems are also discussed, including the conclusion that histidine-ligated [2Fe-2S] clusters should not have a strong bias to undergo concerted e(-)/H(+) transfers.

Fast proton-coupled electron transfer observed for a high-fidelity structural and functional [2Fe-2S] Rieske model.

Rieske cofactors have a [2Fe-2S] cluster with unique {His2Cys2} ligation and distinct Fe subsites. The histidine ligands are functionally relevant, since they allow for coupling of electron and proton transfer (PCET) during quinol oxidation in respiratory and photosynthetic ET chains. Here we present the highest fidelity synthetic analogue for the Rieske [2Fe-2S] cluster reported so far. This synthetic analogue 5(x-) emulates the heteroleptic {His2Cys2} ligation of the [2Fe-2S] core, and it also serves as a functional model that undergoes fast concerted proton and electron transfer (CPET) upon reaction of the mixed-valent (ferrous/ferric) protonated 5H(2-) with TEMPO. The thermodynamics of the PCET square scheme for 5(x-) have been determined, and three species (diferric 5(2-), protonated diferric 5H(-), and mixed-valent 5(3-)) have been characterized by X-ray diffraction. pKa values for 5H(-) and 5H(2-) differ by about 4 units, and the reduction potential of 5H(-) is shifted anodically by about +230 mV compared to that of 5(2-). While the N-H bond dissociation free energy of 5H(2-) (60.2 ± 0.5 kcal mol(-1)) and the free energy, ΔG°CPET, of its reaction with TEMPO (-6.3 kcal mol(-1)) are similar to values recently reported for a homoleptic {N2/N2}-coordinated [2Fe-2S] cluster, CPET is significantly faster for 5H(2-) with biomimetic {N2/S2} ligation (k = (9.5 ± 1.2) × 10(4) M(-1) s(-1), ΔH(‡) = 8.7 ± 1.0 kJ mol(-1), ΔS(‡) = -120 ± 40 J mol(-1) K(-1), and ΔG(‡) = 43.8 ± 0.3 kJ mol(-1) at 293 K). These parameters, and the comparison with homoleptic analogues, provide important information and new perspectives for the mechanistic understanding of the biological Rieske cofactor.

Do Spin State and Spin Density Affect Hydrogen Atom Transfer Reactivity?

The prevalence of hydrogen atom transfer (HAT) reactions in chemical and biological systems has prompted much interest in establishing and understanding the underlying factors that enable this reactivity. Arguments have been advanced that the electronic spin state of the abstractor and/or the spin-density at the abstracting atom are critical for HAT reactivity. This is consistent with the intuition derived from introductory organic chemistry courses. Herein we present an alternative view on the role of spin state and spin-density in HAT reactions. After a brief introduction, the second section introduces a new and simple fundamental kinetic analysis, which shows that unpaired spin cannot be the dominant effect. The third section examines published computational studies of HAT reactions, which indicates that the spin state affects these reactions indirectly, primarily via changes in driving force. The essay concludes with a broader view of HAT reactivity, including indirect effects of spin and other properties on reactivity. It is suggested that some of the controversy in this area may arise from the diversity of HAT reactions and their overlap with proton-coupled electron transfer (PCET) reactions.

Decomposition of a Mixed-Valence [2Fe-2S] Cluster to Linear Tetra-Ferric and Ferrous Clusters.

Despite the ease of preparing di-ferric [2Fe-2S] clusters, preparing stable mixed-valence analogues remains a challenge, as these clusters have limited thermal stability. Herein we identify two decomposition products of the mixed-valence thiosalicylate-ligated [2Fe-2S] cluster, [Fe2S2(SArCOO)2]3- ((SArCOO)2- = thiosalicylate).

Mononuclear five- and six-coordinate iron hydrazido and hydrazine species.

This article describes the synthesis and characterization of several low-spin iron(II) complexes that coordinate hydrazine (N(2)H(4)), hydrazido (N(2)H(3)(-)), and ammonia. The sterically encumbered tris(di-meta-terphenylphosphino)borate ligand, [PhBP(mter)(3)](-), is introduced to provide access to species that cannot be stabilized with the [PhBP(Ph)(3)](-) ligand ([PhBP(R)(3)](-) = PhB(CH(2)PR(2))(3)(-)). Treatment of [PhBP(mter)(3)]FeMe with hydrazine generates the unusual 5-coordinate hydrazido complex [PhBP(mter)(3)]Fe(η(2)-N(2)H(3)) (1), in which the hydrazido serves as an L(2)X-type ligand. Upon coordination of an L-type ligand, the hydrazido shifts to an LX-type ligand, generating [PhBP(mter)(3)]Fe(L)(η(2)-N(2)H(3)) (L = N(2)H(4) (2) or NH(3) (3)). In contrast, treatment of [PhBP(Ph)(3)]FeMe with hydrazine forms the adduct [PhBP(Ph)(3)]Fe(Me)(η(2)-N(2)H(4)) (5). Complex 5 is thermally unstable to methane loss, generating intermediate [PhBP(Ph)(3)]Fe(η(2)-N(2)H(3)), which undergoes bimolecular coupling to produce {[PhBP(Ph)(3)]Fe}(2)(µ-η(1):η(1)-N(2)H(4))(µ-η(2):η(2)-N(2)H(2)). The oxidation of these and related hydrazine and hydrazido species is also presented. For example, oxidation of 1 or 5 with Pb(OAc)(4) results in disproportionation of the N(2)H(x) ligand (x = 3, 4), and formation of [PhBP(R)(3)]Fe(NH(3))(OAc) (R = Ph (9) and mter (11)).

Modeling the signatures of hydrides in metalloenzymes: ENDOR analysis of a Di-iron Fe(µ-NH)(µ-H)Fe core.

The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L(3)Fe(µ-NH)(µ-H)FeL(3) (L(3) = [PhB(CH(2)PPh(2))(3)](-)) complex, 3, is a novel S = (1)/(2) type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. (1,2)H and (14,15)N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (µ-H) and the proton of the (µ-NH) of the crystallographically characterized 3 show the proposed signature of a 'bridging' hydride that is essentially equidistant between two 'anchor' metal ions: a rhombic dipolar interaction tensor, T ≈ [T, -T, 0]. The point-dipole model for describing the anisotropic interaction of a bridging H as the sum of the point-dipole couplings to the 'anchor' metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T ≈ [-T, -T, 2T], thus validating both the model and the signatures. This validation in turn lends strong support to the assignment, based on such a point-dipole analysis, that the molybdenum-iron cofactor of nitrogenase contains two [Fe-H(-)-Fe] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E(4)). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E(4). This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.

Protonation and concerted proton-electron transfer reactivity of a bis-benzimidazolate ligated [2Fe-2S] model for Rieske clusters.

A model system for biological Rieske clusters that incorporates bis-benzimidazolate ligands ((Pr)bbim)(2-) has been developed ((Pr)bbimH(2) = 4,4-bis(benzimidazol-2-yl)heptane). The diferric and mixed-valence clusters have been prepared and characterized in both their protonated and deprotonated states. The thermochemistry of interconversions of these species has been measured, and the effect of protonation on the reduction potential is in good agreement to that observed in the biological systems. The mixed-valence and protonated congener [Fe(2)S(2)((Pr)bbim)((Pr)bbimH)](Et(4)N)(2) (4) reacts rapidly with TEMPO or p-benzoquinones to generate diferric and deprotonated [Fe(2)S(2)((Pr)bbim)(2)](Et(4)N)(2) (1) and 1 equiv of TEMPOH or 0.5 equiv of p-benzohydroquinones, respectively. The reaction with TEMPO is the first well-defined example of concerted proton-electron transfer (CPET) at a synthetic ferric/ferrous [Fe-S] cluster.

A five-coordinate phosphino/acetate iron(II) scaffold that binds N2, N2H2, N2H4, and NH3 in the sixth site.

A family of iron(II) complexes that coordinate dinitrogen, diazene, hydrazine, and ammonia are presented. This series of complexes is unusual in that the complexes within it feature a common auxiliary ligand set and differ only by virtue of the nitrogenous N(x)H(y) ligand that occupies the sixth binding site. The ability of an iron center to bind N(2), N(2)H(2), N(2)H(4), and NH(3) is important to establish in the context of evaluating catalytic N(2) reduction schemes that invoke these nitrogenous species. Such a scenario has been proposed as an iron-mediated, alternating reduction scheme within the cofactor of nitrogenase enzymes.

M≡E and M=E Complexes of Iron and Cobalt that Emphasize Three-fold Symmetry (E = O, N, NR).

Mid-to-late transition metal complexes that feature terminal, multiply bonded ligands such as oxos, imides, and nitrides have been invoked as intermediates in several catalytic transformations of synthetic and biological significance. Until about ten years ago, isolable examples of such species were virtually unknown. Over the past decade or so, numerous chemically well-defined examples of such species have been discovered. In this context, the presentreview summarizes the development of 4- and 5-coordinate Fe(E) and Co(E) species under local three-fold symmetry.

Characterization of structurally unusual diiron N(x)H(y) complexes.

A series of fascinating diiron complexes featuring bridging N(x)H(y) ligands stabilized by tris(phosphine)borate ([PhB(CH(2)PR(2))(3)] = [PhBP(R)(3)]) ligands have been characterized. Hydrazine activation by [PhBP(R)(3)]Fe-Me (R = Ph or cyclohexylmethyl) leads to diiron Fe(2)(mu-eta(1):eta(1)-N(2)H(4))(mu-eta(2):eta(2)-N(2)H(2)) complexes featuring both bridging hydrazine and hydrazido ligands. Thermolysis of {[PhBP(CH(2))(Cy)(3)]Fe}(2)(mu-eta(1):eta(1)-N(2)H(4))(mu-eta(2):eta(2)-N(2)H(2)) at 22 degrees C leads to a structurally unusual {[PhBP(CH(2))(Cy)]Fe}(2)(mu-eta(1):eta(1)-N(2)H(2))(mu-NH(2))(2) complex featuring bridging HN horizontal lineNH and NH(2)(-) ligands. This contrasts with {[PhBP(Ph)(3)]Fe}(2)(mu-eta(1):eta(1)-N(2)H(4))(mu-eta(2):eta(2)-N(2)H(2)), which can be chemically oxidized to produce either {[PhBP(Ph)(3)]Fe}(2)(mu-eta(1):eta(1)-N(2)H(2))(mu-eta(2):eta(2)-N(2)H(2)) or {[PhBP(Ph)(3)]Fe}(2)(mu-NH)(2), depending on the conditions. The former product is the only known complex to contain bridging N(2)H(2) ligands in each of their limiting states of oxidation (HN horizontal lineNH vs HN-NH(2-)). The latter product constitutes the first example of a diiron Fe(2)(mu-NH)(2) diamond-shaped core.