Persistent Covalency and Planarity in the B_<i>n</i>Al_6–<i>n</i>^2– and LiB_<i>n</i>Al_6–<i>n</i>^– (<i>n</i> = 0–6) Cluster Ions

The chemical bonding being covalent, metallic, or mixed reflects in the structure and properties of solids. How does this play out on the small cluster scale? We report on the interplay between covalent and strongly delocalized bonding in the series of mixed boron–aluminum cluster ions, B_<i>n</i>Al_6–<i>n</i>^2– (<i>n</i> = 0–6), and their lithium salts and show that covalent bonding is an extraordinarily resilient effect that governs the cluster shape more than the delocalized bonding does. The covalent bonding achieved only through the direct B–B interactions is persistent in the considered clusters down to the smallest concentrations of B atoms. As a result, clusters remain planar, and the quality of the delocalized bonding is unavoidably compromised. We explain this trend on the basis of the s–p hybridization of atomic orbitals affordable in the B versus Al atoms. The found effect may be general and not specific to the considered systems

Protonation of Nickel–Iron Hydrogenase Models Proceeds after Isomerization at Nickel

Theory and experiment indicate that the protonation of reduced NiFe dithiolates proceeds via a previously undetected isomer with enhanced basicity. In particular, it is proposed that protonation of (OC)₃Fe­(pdt)­Ni­(dppe) (<b>1</b>; pdt^2– = ^–S­(CH₂)₃S^–; dppe = Ph₂P­(CH₂)₂PPh₂) occurs at the Fe site of the two-electron mixed-valence Fe(0)­Ni­(II) species, not the Fe­(I)-Ni­(I) bond for the homovalence isomer of <b>1</b>. The new pathway, which may have implications for protonation of other complexes and clusters, was uncovered through studies on the homologous series L­(OC)₂Fe­(pdt)­M­(dppe), where M = Ni, Pd (<b>2</b>), and Pt (<b>3</b>) and L = CO, PCy₃. Similar to <b>1</b>, complexes <b>2</b> and <b>3</b> undergo both protonation and 1e^– oxidation to afford well-characterized hydrides ([<b>2</b>H]⁺ and [<b>3</b>H]⁺) and mixed-valence derivatives ([<b>2</b>]⁺ and [<b>3</b>]⁺), respectively. Whereas the Pd site is tetrahedral in <b>2</b>, the Pt site is square-planar in <b>3</b>, indicating that this complex is best described as Fe(0)­Pt­(II). In view of the results on <b>2</b> and <b>3</b>, the potential energy surface of <b>1</b> was reinvestigated with density functional theory. These calculations revealed the existence of an energetically accessible and more basic Fe(0)­Ni­(II) isomer with a square-planar Ni site

Computational Investigation of [FeFe]-Hydrogenase Models: Characterization of Singly and Doubly Protonated Intermediates and Mechanistic Insights

The [FeFe]-hydrogenase enzymes catalyze hydrogen oxidation and production efficiently with binuclear Fe metal centers. Recently the bioinspired H₂-producing model system Fe₂(adt)­(CO)₂(dppv)₂ (adt=azadithiolate and dppv=diphosphine) was synthesized and studied experimentally. In this system, the azadithiolate bridge facilitates the formation of a doubly protonated ammonium-hydride species through a proton relay. Herein computational methods are utilized to examine this system in the various oxidation states and protonation states along proposed mechanistic pathways for H₂ production. The calculated results agree well with the experimental data for the geometries, CO vibrational stretching frequencies, and reduction potentials. The calculations illustrate that the NH···HFe dihydrogen bonding distance in the doubly protonated species is highly sensitive to the effects of ion-pairing between the ammonium and BF₄^– counterions, which are present in the crystal structure, in that the inclusion of BF₄^– counterions leads to a significantly longer dihydrogen bond. The non-hydride Fe center was found to be the site of reduction for terminal hydride species and unsymmetric bridging hydride species, whereas the reduced symmetric bridging hydride species exhibited spin delocalization between the Fe centers. According to both experimental measurements and theoretical calculations of the relative p<i>K</i>_a values, the Fe_d center of the neutral species is more basic than the amine, and the bridging hydride species is more thermodynamically stable than the terminal hydride species. The calculations implicate a possible pathway for H₂ evolution that involves an intermediate with H₂ weakly bonded to one Fe, a short H₂ distance similar to the molecular bond length, the spin density delocalized over the two Fe centers, and a nearly symmetrically bridged CO ligand. Overall, this study illustrates the mechanistic roles of the ammonium-hydride interaction, flexibility of the bridging CO ligand, and intramolecular electron transfer between the Fe centers in the catalytic cycle. Such insights will assist in the design of more effective bioinspired catalysts for H₂ production

Quinone 1 e^– and 2 e^–/2 H⁺ Reduction Potentials: Identification and Analysis of Deviations from Systematic Scaling Relationships

Quinones participate in diverse electron transfer and proton-coupled electron transfer processes in chemistry and biology. To understand the relationship between these redox processes, an experimental study was carried out to probe the 1 e^– and 2 e^–/​2 H⁺ reduction potentials of a number of common quinones. The results reveal a non-linear correlation between the 1 e^– and 2 e^–/​2 H⁺ reduction potentials. This unexpected observation prompted a computational study of 134 different quinones, probing their 1 e^– reduction potentials, p<i>K</i>_a values, and 2 e^–/​2 H⁺ reduction potentials. The density functional theory calculations reveal an approximately linear correlation between these three properties and an effective Hammett constant associated with the quinone substituent(s). However, deviations from this linear scaling relationship are evident for quinones that feature intramolecular hydrogen bonding, halogen substituents, charged substituents, and/or sterically bulky substituents. These results, particularly the different substituent effects on the 1 e^– versus 2 e^–/​2 H⁺ reduction potentials, have important implications for designing quinones with tailored redox properties

Models of the Ni-L and Ni-SI_a States of the [NiFe]-Hydrogenase Active Site

A new class of synthetic models for the active site of [NiFe]-hydrogenases are described. The Ni^I/II(SCys)₂ and Fe^II(CN)₂CO sites are represented with (RC₅H₄)­Ni^I/II and Fe^II(diphos)­(CO) modules, where diphos = 1,2-C₂H₄(PPh₂)₂(dppe) or <i>cis</i>-1,2-C₂H₂(PPh₂)₂(dppv). The two bridging thiolate ligands are represented by CH₂(CH₂S)₂^2– (pdt^2–), Me₂C­(CH₂S)₂^2– (Me₂pdt^2–), and (C₆H₅S)₂^2–. The reaction of Fe­(pdt)­(CO)₂(dppe) and [(C₅H₅)₃Ni₂]­BF₄ affords [(C₅H₅)­Ni­(pdt)­Fe­(dppe)­(CO)]­BF₄ ([<b>1a</b>]­BF₄). Monocarbonyl [<b>1a</b>]­BF₄ features an <i>S</i> = 0 Ni^IIFe^II center with five-coordinated iron, as proposed for the Ni-SI_a state of the enzyme. One-electron reduction of [<b>1a</b>]⁺ affords the <i>S</i> = ¹/₂ derivative [<b>1a</b>]⁰, which, according to density functional theory (DFT) calculations and electron paramagnetic resonance and Mössbauer spectroscopies, is best described as a Ni^IFe^II compound. The Ni^IFe^II assignment matches that for the Ni-L state in [NiFe]-hydrogenase, unlike recently reported Ni^IIFe^I-based models. Compound [<b>1a</b>]⁰ reacts with strong acids to liberate 0.5 equiv of H₂ and regenerate [<b>1a</b>]⁺, indicating that H₂ evolution is catalyzed by [<b>1a</b>]⁰. DFT calculations were used to investigate the pathway for H₂ evolution and revealed that the mechanism can proceed through two isomers of [<b>1a</b>]⁰ that differ in the stereochemistry of the Fe­(dppe)­CO center. Calculations suggest that protonation of [<b>1a</b>]⁰ (both isomers) affords Ni^III–H–Fe^II intermediates, which represent mimics of the Ni-C state of the enzyme

Models of the Ni-L and Ni-SI_a States of the [NiFe]-Hydrogenase Active Site

A new class of synthetic models for the active site of [NiFe]-hydrogenases are described. The Ni^I/II(SCys)₂ and Fe^II(CN)₂CO sites are represented with (RC₅H₄)­Ni^I/II and Fe^II(diphos)­(CO) modules, where diphos = 1,2-C₂H₄(PPh₂)₂(dppe) or <i>cis</i>-1,2-C₂H₂(PPh₂)₂(dppv). The two bridging thiolate ligands are represented by CH₂(CH₂S)₂^2– (pdt^2–), Me₂C­(CH₂S)₂^2– (Me₂pdt^2–), and (C₆H₅S)₂^2–. The reaction of Fe­(pdt)­(CO)₂(dppe) and [(C₅H₅)₃Ni₂]­BF₄ affords [(C₅H₅)­Ni­(pdt)­Fe­(dppe)­(CO)]­BF₄ ([<b>1a</b>]­BF₄). Monocarbonyl [<b>1a</b>]­BF₄ features an <i>S</i> = 0 Ni^IIFe^II center with five-coordinated iron, as proposed for the Ni-SI_a state of the enzyme. One-electron reduction of [<b>1a</b>]⁺ affords the <i>S</i> = ¹/₂ derivative [<b>1a</b>]⁰, which, according to density functional theory (DFT) calculations and electron paramagnetic resonance and Mössbauer spectroscopies, is best described as a Ni^IFe^II compound. The Ni^IFe^II assignment matches that for the Ni-L state in [NiFe]-hydrogenase, unlike recently reported Ni^IIFe^I-based models. Compound [<b>1a</b>]⁰ reacts with strong acids to liberate 0.5 equiv of H₂ and regenerate [<b>1a</b>]⁺, indicating that H₂ evolution is catalyzed by [<b>1a</b>]⁰. DFT calculations were used to investigate the pathway for H₂ evolution and revealed that the mechanism can proceed through two isomers of [<b>1a</b>]⁰ that differ in the stereochemistry of the Fe­(dppe)­CO center. Calculations suggest that protonation of [<b>1a</b>]⁰ (both isomers) affords Ni^III–H–Fe^II intermediates, which represent mimics of the Ni-C state of the enzyme

Interplay between Terminal and Bridging Diiron Hydrides in Neutral and Oxidized States

This study describes the structural, spectroscopic, and electrochemical properties of electronically unsymmetrical diiron hydrides. The terminal hydride Cp*Fe­(pdt)­Fe­(dppe)­(CO)H ([<b>1</b>(<i>t</i>-H)]⁰, Cp*^– = Me₅C₅^–, pdt^2– = CH₂(CH₂S^–)₂, dppe = Ph₂PC₂H₄PPh₂) was prepared by hydride reduction of [Cp*Fe­(pdt)­Fe­(dppe)­(CO)­(NCMe)]⁺. As established by X-ray crystallography, [<b>1</b>(<i>t</i>-H)]⁰ features a terminal hydride ligand. Unlike previous examples of terminal diiron hydrides, [<b>1</b>(<i>t</i>-H)]⁰ does not isomerize to the bridging hydride [<b>1</b>(μ-H)]⁰. Oxidation of [<b>1</b>(<i>t</i>-H)]⁰ gives [<b>1</b>(<i>t</i>-H)]⁺, which was also characterized crystallographically as its BF₄^– salt. Density functional theory (DFT) calculations indicate that [<b>1</b>(<i>t</i>-H)]⁺ is best described as containing an Cp*Fe^III center. In solution, [<b>1</b>(<i>t</i>-H)]⁺ isomerizes to [<b>1</b>(μ-H)]⁺, as anticipated by DFT. Reduction of [<b>1</b>(μ-H)]⁺ by Cp₂Co afforded the diferrous bridging hydride [<b>1</b>(μ-H)]⁰. Electrochemical measurements and DFT calculations indicate that the couples [<b>1</b>(<i>t</i>-H)]^+/0 and [<b>1</b>(μ-H)]^+/0 differ by 210 mV. Qualitative measurements indicate that [<b>1</b>(<i>t</i>-H)]⁰ and [<b>1</b>(μ-H)]⁰ are close in free energy. Protonation of [<b>1</b>(<i>t</i>-H)]⁰ in MeCN solution affords H₂ even with weak acids via hydride transfer. In contrast, protonation of [<b>1</b>(μ-H)]⁰ yields 0.5 equiv of H₂ by a proposed protonation-induced electron transfer process. Isotopic labeling indicates that μ-H/D ligands are inert

Mechanism of H₂ Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides

The intermediacy of a reduced nickel–iron hydride in hydrogen evolution catalyzed by Ni–Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)­Ni­(μ-pdt)­Fe­(CO)­(dppv) ([<b>1</b>]⁰; dppv = <i>cis</i>-C₂H₂­(PPh₂)₂) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [<b>1</b>]⁰ initially produces <i>unsym</i>-[H<b>1</b>]⁺, which converts by a first-order pathway to <i>sym</i>-[H<b>1</b>]⁺. These species have <i>C</i>₁ (unsym) and <i>C</i>_<i>s</i> (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H<b>1</b>]⁺ protonates at sulfur. The <i>S</i> = 1/2 hydride [H<b>1</b>]⁰ was generated by reduction of [H<b>1</b>]⁺ with Cp*₂Co. Density functional theory (DFT) calculations indicate that [H<b>1</b>]⁰ is best described as a Ni­(I)–Fe­(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H<b>1</b>]⁰. Whereas [H<b>1</b>]⁺ does not evolve H₂ upon protonation, treatment of [H<b>1</b>]⁰ with acids gives H₂. The redox state of the “remote” metal (Ni) modulates the hydridic character of the Fe­(II)–H center. As supported by DFT calculations, H₂ evolution proceeds either directly from [H<b>1</b>]⁰ and external acid or from protonation of the Fe–H bond in [H<b>1</b>]⁰ to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H<b>1</b>]⁰ initially produces [<b>1</b>]⁺, which is reduced by [H<b>1</b>]⁰. Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit

Concerted One-Electron Two-Proton Transfer Processes in Models Inspired by the Tyr-His Couple of Photosystem II

Nature employs a Tyr_Z-His pair as a redox relay that couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II. Artificial redox relays composed of different benzimidazole–phenol dyads (benzimidazole models His and phenol models Tyr) with substituents designed to simulate the hydrogen bond network surrounding the Tyr_Z-His pair have been prepared. When the benzimidazole substituents are strong proton acceptors such as primary or tertiary amines, theory predicts that a concerted two proton transfer process associated with the electrochemical oxidation of the phenol will take place. Also, theory predicts a decrease in the redox potential of the phenol by ∼300 mV and a small kinetic isotope effect (KIE). Indeed, electrochemical, spectroelectrochemical, and KIE experimental data are consistent with these predictions. Notably, these results were obtained by using theory to guide the rational design of artificial systems and have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction