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Inorg Chem ; 59(12): 8146-8160, 2020 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-32459480


A solar-driven conversion of CO2 into fuels by artificial photosynthesis would not only mitigate the greenhouse effect but also provide an alternative to obtain fuels in a renewable fashion. To this end, the new iron polypyridine catalyst [Fe(bpyNHEtPY2Me)L2]2+ (L = H2O, CH3CN) was recently developed for the electrochemical reduction of CO2 to CO. In this study, we performed density functional theory (DFT) electronic structure calculations to shed light on a possible pathway for CO2 reduction and the origin of the selectivity between CO2 reduction versus the hydrogen evolution reaction. The metal center remains Lewis acidic throughout the reduction process due to ligand loss and mainly ligand-based reduction stabilized by antiferromagnetic coupling to a high-spin Fe(II) center. This results in a high barrier for hydride formation but a facile addition and activation of CO2 via an η2 coordination and stabilizing hydrogen bonding by the amine group. The second unoccupied equatorial coordination site opens up the possibility for an intramolecular protonation with a coordinated water ligand. This facilitates protonation because not only CO2 but also the proton source H2O is activated and properly aligned for a proton transfer due to the Fe-OH2 bond; consequently, both protonation steps are facile. The moderate ligand field allows a rapid ligand exchange for a second intramolecular protonation step and facilitates an exergonic CO release. The lower selectivity of the related [Fe(bpyOHPY2Me)L2]2+ complex can be related to its more acidic second coordination sphere, which enables an intramolecular proton transfer that is kinetically competitive with CO2 addition.

Inorg Chem ; 2020 Mar 26.
Artigo em Inglês | MEDLINE | ID: mdl-32212626


The development of noble-metal-free catalysts capable of electrochemically converting carbon dioxide (CO2) selectively into value-added compounds remains one of the central challenges in catalysis research. Here, we present a systematic study of Fe(II) complexes of the functionalized ligands bpyRPY2Me (bpyPY2Me = 6-(1,1-bis(pyridin-2-yl)ethyl)-2,2'-bipyridine) in the pursuit of water-stable molecular Fe complexes that are selective for the catalytic formation of CO from CO2. Taking advantage of the inherently high degree of tunability of this ligand manifold, we followed a bioinspired approach by installing protic functional groups of varying acidities (-H, -OH, -OMe, -NHEt, and -NEt2) into the ligand framework to systematically modify the second coordination sphere of the Fe center. This family of [(bpyRPY2Me)FeII] complexes was characterized using single-crystal X-ray analysis, 1H NMR spectroscopy, and mass spectrometry. Comparative catalytic evaluation of this set of compounds via voltammetry and electrolysis experiments identified [(bpyNHEtPY2Me)Fe]2+ in particular as an efficient, iron-based, non-heme CO2 electroreduction catalyst that displays significant selectivity for the conversion of CO2 to CO in acetonitrile solution with 11 M H2O. We propose that the NH group acts as a local proton source for cleaving the C-O bond in CO2 to form CO. Interestingly, the complex with the most acidic functional group in the second coordination sphere, [(bpyOHPY2Me)Fe]2+, favors formation of H2 over CO. Our results correlate the selectivity of water versus carbon dioxide reduction to the acidity of the second coordination sphere functional group and emphasize the continued untapped potential that synthetic molecular chemistry offers in the pursuit of next-generation CO2 reduction electrocatalysts.

Chem Sci ; 10(17): 4652-4661, 2019 May 07.
Artigo em Inglês | MEDLINE | ID: mdl-31123575


Materials with switchable magnetic and electrical properties may enable future spintronic technologies, and thus hold the potential to revolutionize how information is processed and stored. While reversible switching of magnetic order or electrical conductivity has been independently realized in materials, the ability to simultaneously switch both properties in a single material presents a formidable challenge. Here, we report the 2D manganese benzoquinoid framework (Me4N)2[MnII2(L2-)3] (H2L = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone), as synthesized via post-synthetic counterion exchange. This material is paramagnetic above 1.8 K and exhibits an ambient-temperature electrical conductivity of σ 295 K = 1.14(3) × 10-13 S cm-1 (E a = 0.74(3) eV). Upon soaking in a solution of sodium naphthalenide and 1,2-dihydroacenaphthylene, this compound undergoes a single-crystal-to-single-crystal (SC-SC) reduction to give Na3(Me4N)2[Mn2L3]. Structural and spectroscopic analyses confirm this reduction to be ligand-based, and as such the anionic framework is formulated as [MnII2(L3-˙)3]5-. Magnetic measurements confirm that this reduced material is a permanent magnet below T c = 41 K and exhibits a conductivity value of σ 295 K = 2.27(1) × 10-8 S cm-1 (E a = 0.489(8) eV), representing a remarkable 200 000-fold increase over the parent material. Finally, soaking the reduced compound in a solution of [Cp2Fe]+ affords Na(Me4N)[MnII2(L2-)3] via a SC-SC process, with magnetic and electrical properties similar to those observed for the original oxidized material. Taken together, these results highlight the ability of metal benzoquinoid frameworks to undergo reversible, simultaneous redox switching of magnetic order and electrical conductivity.

J Am Chem Soc ; 141(9): 3940-3951, 2019 03 06.
Artigo em Inglês | MEDLINE | ID: mdl-30715871


Here we describe the combination of an archetypal redox-active metal sulfide cluster, Fe4S4, with an organic linker, 1,4-benzenedithiolate, to prepare coordination polymers containing infinite chains of Fe4S4 clusters. The crystal structures of two solid materials have been solved from synchrotron X-ray powder diffraction data using simulated annealing and refined by a least-squares Rietveld refinement procedure. The electronic properties of these chains have also been characterized by UV-visible and Mössbauer spectroscopies. Additional experiments demonstrated that these chains can be solubilized by variation of the countercation and that the chain structure is maintained in solution. The redox-activity of the Fe4S4 clusters can be accessed with chemical reagents. Introduction of charge carriers by reduction of the Fe4S4 clusters is found to increase the electrical conductivity of the materials by up to 4 orders of magnitude. These results highlight the utility of Fe4S4 clusters as redox-active building blocks in preparing new classes of coordination polymers.

Acc Chem Res ; 48(7): 2027-36, 2015 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-26101803


Climate change, rising global energy demand, and energy security concerns motivate research into alternative, sustainable energy sources. In principle, solar energy can meet the world's energy needs, but the intermittent nature of solar illumination means that it is temporally and spatially separated from its consumption. Developing systems that promote solar-to-fuel conversion, such as via reduction of protons to hydrogen, could bridge this production-consumption gap, but this effort requires invention of catalysts that are cheap, robust, and efficient and that use earth-abundant elements. In this context, catalysts that utilize water as both an earth-abundant, environmentally benign substrate and a solvent for proton reduction are highly desirable. This Account summarizes our studies of molecular metal-polypyridyl catalysts for electrochemical and photochemical reduction of protons to hydrogen. Inspired by concept transfer from biological and materials catalysts, these scaffolds are remarkably resistant to decomposition in water, with fast and selective electrocatalytic and photocatalytic conversions that are sustainable for several days. Their modular nature offers a broad range of opportunities for tuning reactivity by molecular design, including altering ancillary ligand electronics, denticity, and/or incorporating redox-active elements. Our first-generation complex, [(PY4)Co(CH3CN)2](2+), catalyzes the reduction of protons from a strong organic acid to hydrogen in 50% water. Subsequent investigations with the pentapyridyl ligand PY5Me2 furnished molybdenum and cobalt complexes capable of catalyzing the reduction of water in fully aqueous electrolyte with 100% Faradaic efficiency. Of particular note, the complex [(PY5Me2)MoO](2+) possesses extremely high activity and durability in neutral water, with turnover frequencies at least 8500 mol of H2 per mole of catalyst per hour and turnover numbers over 600 000 mol of H2 per mole of catalyst over 3 days at an overpotential of 1.0 V, without apparent loss in activity. Replacing the oxo moiety with a disulfide affords [(PY5Me2)MoS2](2+), which bears a molecular MoS2 triangle that structurally and functionally mimics bulk molybdenum disulfide, improving the catalytic activity for water reduction. In water buffered to pH 3, catalysis by [(PY5Me2)MoS2](2+) onsets at 400 mV of overpotential, whereas [(PY5Me2)MoO](2+) requires an additional 300 mV of driving force to operate at the same current density. Metalation of the PY5Me2 ligand with an appropriate Co(ii) source also furnishes electrocatalysts that are active in water. Importantly, the onset of catalysis by the [(PY5Me2)Co(H2O)](2+) series is anodically shifted by introducing electron-withdrawing functional groups on the ligand. With the [(bpy2PYMe)Co(CF3SO3)](1+) system, we showed that introducing a redox-active moiety can facilitate the electro- and photochemical reduction of protons from weak acids such as acetic acid or water. Using a high-throughput photochemical reactor, we examined the structure-reactivity relationship of a series of cobalt(ii) complexes. Taken together, these findings set the stage for the broader application of polypyridyl systems to catalysis under environmentally benign aqueous conditions.