A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis

Titanium and zirconium complexes carrying two 2,6-bis­(pyrrolyl)­pyridine ligands have been synthesized and characterized. The neutral complexes Ti­(^MePDP)₂ and Zr­(^MePDP)₂ (^MePDP = 2,6-bis­(5-methyl-3-phenyl-1H-pyrrol-2-yl)­pyridine) show intense ligand-to-metal charge-transfer bands in the visible region and undergo multiple reversible redox events under highly reducing conditions. Zr­(^MePDP)₂ exhibits photoluminescent behavior and its excited state can be quenched by mild reductants to generate a powerful electron transfer reagent with a ground state potential of −2.16 V vs Fc^+/0. This reactivity was utilized to facilitate dehalogenation reactions, the reduction of electron-poor olefins, and the reductive coupling of benzyl bromide via photoredox catalysis. In these reactions, the earth-abundant metal complex Zr­(^MePDP)₂ acts as a substitute for the precious metal photosensitizer [Ru­(bpy)₃]²⁺

A Luminescent Zirconium(IV) Complex as a Molecular Photosensitizer for Visible Light Photoredox Catalysis

Titanium and zirconium complexes carrying two 2,6-bis­(pyrrolyl)­pyridine ligands have been synthesized and characterized. The neutral complexes Ti­(^MePDP)₂ and Zr­(^MePDP)₂ (^MePDP = 2,6-bis­(5-methyl-3-phenyl-1H-pyrrol-2-yl)­pyridine) show intense ligand-to-metal charge-transfer bands in the visible region and undergo multiple reversible redox events under highly reducing conditions. Zr­(^MePDP)₂ exhibits photoluminescent behavior and its excited state can be quenched by mild reductants to generate a powerful electron transfer reagent with a ground state potential of −2.16 V vs Fc^+/0. This reactivity was utilized to facilitate dehalogenation reactions, the reduction of electron-poor olefins, and the reductive coupling of benzyl bromide via photoredox catalysis. In these reactions, the earth-abundant metal complex Zr­(^MePDP)₂ acts as a substitute for the precious metal photosensitizer [Ru­(bpy)₃]²⁺

Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional Theoretical Study

The three- and four-membered redox series [Cr­(^MePDP)₂]^<i>z</i> (<i>z</i> = 1–, 2–, 3−) and [Mo­(^MePDP)₂]^<i>z</i> (<i>z</i> = 0, 1–, 2–, 3−) were synthesized to study the redox properties of the pincer ligand ^MePDP^2– (H₂^MePDP = 2,6-bis­(5-methyl-3-phenyl-1<i>H</i>-pyrrol-2-yl)­pyridine). The monoanionic complexes were characterized by X-ray crystallography, UV/vis/NIR spectroscopy, and magnetic susceptibility measurements. Experimental and density functional theory (DFT) studies are consistent with closed-shell ^MePDP^2– ligands and +III oxidation states (d³, <i>S</i> = 3/2) for the central metal ions. Cyclic voltammetry established multiple reversible redox processes for [M­(^MePDP)₂]^1– (M = Cr, Mo), which were further investigated via chemical oxidation and reduction. For molybdenum, one-electron oxidation yielded Mo­(^MePDP)₂ which was characterized by X-ray crystallography, UV/vis/NIR, and magnetic susceptibility measurements. The experimental and computational data indicate metal-centered oxidation to a Mo^IV complex (d², <i>S</i> = 1) with two ^MePDP^2– ligands. In contrast, one- and two-electron reductions were found to be ligand centered resulting in the formation of ^MePDP^•3– radicals, in which the unpaired electron is predominantly located on the central pyridine ring of the ligand. The presence of ligand radicals was established experimentally by observation of ligand-to-ligand intervalence charge transfer (LLIVCT) bands in the UV/vis/NIR spectra of the dianionic and trianionic complexes and further supported by broken-symmetry DFT calculations. X-ray crystallographic analyses of the one-electron-reduced species [M­(^MePDP)₂]^2– (<i>S</i> = 1, M = Cr, Mo) established structural indicators for pincer reduction and showed localization of the radical on one of the two pincer ligands. The two-electron-reduced, trianionic complexes (<i>S</i> = 1/2) were characterized by UV/vis/NIR spectroscopy, magnetic susceptibility measurements, and EPR spectroscopy. The electronic structures of the reduced complexes are best described as containing +III metal ions (d³) antiferromagnetically coupled to one and two radical ligands for the dianionic and trianionic species, respectively

Silver-Mediated Oxidative Decarboxylative Trifluoromethylthiolation of Coumarin-3-carboxylic Acids

The introduction of trifluoromethylthio groups into organic compounds, in particular heterocycles, is important because of the prevalence of these structures in medicinally and agriculturally relevant molecules. Herein, the silver-mediated oxidative decarboxylative trifluoromethylthiolation of coumarin-3-carboxylic acids is reported. This methodology utilizes existing carboxylic acid functionalities for the direct conversion to CF₃S groups and results in a broad scope of 3-trifluoromethylthiolated coumarins, including analogues of natural products, in moderate to excellent yields

Ambient Benzotriazole Ring Opening through Intermolecular Radical Addition to Vinyltriazole

Radical addition to vinyltriazole was developed as a new approach to achieve 1,2,3-triazole ring opening under mild conditions. Through reagent control, excellent chemoselectivity was achieved, giving either nitrile under basic conditions or quinoxaline under neutral conditions. Reactivities made this method an attractive new reaction mode

Unsupervised feature selection based on the Morisita estimator of intrinsic dimension

Time-resolved emission spectroscopy for the luminescent zirconium complex Zr­(^MePDP)₂ (^MePDP = 2,6-bis­(5-methyl-3-phenyl-1<i>H</i>-pyrrol-2-yl)­pyridine) revealed a long-lived excited state with a lifetime τ = 325 ± 10 μs. Computational studies using time-dependent density functional theory were conducted to identify the nature of the luminescent excited state as a mixed triplet intraligand/ligand-to-metal charge-transfer state. Stern–Volmer experiments showed a strong dependence of the quenching rate on the redox potential of the quencher indicating photoinduced single-electron transfer (SET) as the quenching pathway. Mechanistic investigations of the photocatalytic homocoupling of benzyl bromide allowed the detection of organic radical intermediates during turnover and provided further evidence for SET mediated by Zr­(^MePDP)₂. Isolation of the one-electron-reduced form of the photosensitizer, [Zr­(^MePDP)₂]⁻, enabled studies of its electronic structure by a combination of experimental and computational techniques and confirmed its role as a strong reductant. Additionally, the role of the benz­imid­azolium hydride derivatives as two-electron sacrificial reductants during photoredox catalysis was investigated. In combination, the results presented in this report establish a detailed mechanistic picture of a photoredox catalytic reaction promoted by an earth-abundant early transition metal photosensitizer

A Zirconium Photosensitizer with a Long-Lived Excited State: Mechanistic Insight into Photoinduced Single-Electron Transfer

Time-resolved emission spectroscopy for the luminescent zirconium complex Zr­(^MePDP)₂ (^MePDP = 2,6-bis­(5-methyl-3-phenyl-1<i>H</i>-pyrrol-2-yl)­pyridine) revealed a long-lived excited state with a lifetime τ = 325 ± 10 μs. Computational studies using time-dependent density functional theory were conducted to identify the nature of the luminescent excited state as a mixed triplet intraligand/ligand-to-metal charge-transfer state. Stern–Volmer experiments showed a strong dependence of the quenching rate on the redox potential of the quencher indicating photoinduced single-electron transfer (SET) as the quenching pathway. Mechanistic investigations of the photocatalytic homocoupling of benzyl bromide allowed the detection of organic radical intermediates during turnover and provided further evidence for SET mediated by Zr­(^MePDP)₂. Isolation of the one-electron-reduced form of the photosensitizer, [Zr­(^MePDP)₂]⁻, enabled studies of its electronic structure by a combination of experimental and computational techniques and confirmed its role as a strong reductant. Additionally, the role of the benz­imid­azolium hydride derivatives as two-electron sacrificial reductants during photoredox catalysis was investigated. In combination, the results presented in this report establish a detailed mechanistic picture of a photoredox catalytic reaction promoted by an earth-abundant early transition metal photosensitizer

1,2,3-Triazole: Unique Ligand in Promoting Iron-Catalyzed Propargyl Alcohol Dehydration

A 1,2,3-traizole-promoted iron(III)-catalyzed propargyl alcohol dehydration was developed for the synthesis of conjugated enynes. The desired conjugated enynes were prepared in good to excellent yields (up to 95%) with a large substrate scope and excellent stereoselectivity (only <i>Z</i>-isomers)

Ambient Schmittel Cyclization Promoted by Chemoselective Triazole-Gold Catalyst

The Schmittel cyclization was achieved at room temperature through triazole–gold (TA–Au) catalyzed propargyl vinyl ether rearrangement. Other tested [L–Au]⁺ catalysts gave complex reaction mixtures under identical conditions with no desired products observed. Importantly, because of the employment of mild conditions, sterically hindered groups (such as <i>t</i>-Bu) on allene termini were no longer required, which allowed successful synthesis of previously challenging substrates

1,2,3-Triazole: Unique Ligand in Promoting Iron-Catalyzed Propargyl Alcohol Dehydration

A 1,2,3-traizole-promoted iron(III)-catalyzed propargyl alcohol dehydration was developed for the synthesis of conjugated enynes. The desired conjugated enynes were prepared in good to excellent yields (up to 95%) with a large substrate scope and excellent stereoselectivity (only <i>Z</i>-isomers)