Photophysical Studies of a Zr(IV) Complex with Two Pyrrolide-Based Tetradentate Schiff Base Ligands.

The reaction of two equivalents of ,-bis(2-pyrrolylmethylidene)-1,2-phenylenediamine (Hbppda) with tetrabenzylzirconium provided the air- and moisture-stable eight-coordinate complex Zr(bppda). Temperature-dependent steady-state and time-resolved emission spectroscopy established weak photoluminescence (Φ = 0.4% at 293 K) by a combination of prompt fluorescence and thermally activated delayed fluorescence (TADF) upon visible light excitation at and around room temperature.

Organometallic Intermediates in the Synthesis of Photoluminescent Zirconium and Hafnium Complexes with Pyridine Dipyrrolide Ligands.

The two commercially available zirconium complexes tetrakis(dimethylamido)zirconium, Zr(NMe), and tetrabenzylzirconium, ZrBn, were investigated for their utility as starting materials in the synthesis of bis(pyridine dipyrrolide)zirconium photosensitizers, Zr(PDP). Reaction with one equivalent of the ligand precursor 2,6-bis(5-methyl-3-phenyl-1-pyrrol-2-yl)pyridine, HPDP, resulted in the isolation and structural characterization of the complexes (PDP)Zr(NMe)thf and (PDP)ZrBn, which could be converted to the desired photosensitizer Zr(PDP) upon addition of a second equivalent of HPDP. Using the more sterically encumbered ligand precursor 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1-pyrrol-2-yl)pyridine, HPDP, only ZrBn yielded the desired bis-ligand complex Zr(PDP).

A Redox Transmetalation Step in Nickel-Catalyzed C-C Coupling Reactions.

Ni-catalyzed C-H functionalization reactions are becoming efficient routes to access a variety of functionalized arenes, yet the mechanisms of these catalytic C-C coupling reactions are not well understood. Here, we report the catalytic and stoichiometric arylation reactions of a nickel(II) metallacycle. Treatment of this species with silver(I)-aryl complexes results in facile arylation, consistent with a redox transmetalation step.

Long-lived photoluminescence from an eight-coordinate zirconium(IV) complex with four 2-(2'-pyridyl)pyrrolide ligands.

The photoluminescent eight-coordinate zirconium complex Zr(PMP) supported by four monoanionic 2-(2'-pyridine)pyrrolide ligands was synthesized. This molecule shows dual emission fluorescence and phosphorescence with an overall quantum efficiency of 4% at room temperature in solution. The phosphorescence lifetime is dependent on concentration, indicating excimer formation at higher concentrations, and reaches almost 800 μs at high dilution.

Long-Lived Photoluminescence of Molecular Group 14 Compounds through Thermally Activated Delayed Fluorescence.

Photoluminescent molecules exploiting the sizable spin-orbit coupling constants of main group metals and metalloids to access long-lived triplet excited states are relatively rare compared to phosphorescent transition metal complexes. Here we report the synthesis of three air- and moisture-stable group 14 compounds E(PDP), where E = Si, Ge, or Sn and [PDP] is the doubly deprotonated form of 2,6-bis(5-methyl-3-phenyl-1-pyrrol-2-yl)pyridine. In solution, all three molecules exhibit exceptionally long-lived triplet excited states with lifetimes in the millisecond range and show highly efficient photoluminescence (Φ ≤ 0.

High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex.

Among Earth-abundant catalyst systems, iron-carbene intermediates that perform C-C bond forming reactions such as cyclopropanation of olefins and C-H functionalization via carbene insertion are rare. Detailed descriptions of the possible electronic structures for iron-carbene bonds are imperative to obtain better mechanistic insights and enable rational catalyst design. Here, we report the first square-planar iron-carbene complex (PDP)Fe(CPh), where [PDP] is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1-pyrrol-2-yl)pyridine].

Effects of Ligand Substitution on the Optical and Electrochemical Properties of (Pyridinedipyrrolide)zirconium Photosensitizers.

A series of seven bis(pyridinedipyrrolide)zirconium complexes, Zr(PDP), where [PDP] is the doubly deprotonated form of [2,6-bis(5-R-3-R-1-pyrrol-2-yl)pyridine], were prepared and characterized in solution by NMR, UV/vis absorption, and emission spectroscopy and cyclic voltammetry. The molecular structures were determined by single-crystal X-ray crystallography. All complexes exhibit remarkably long emission lifetimes (τ = 190-576 μs) with high quantum efficiencies (Φ = 0.

Photochemical synthesis of a zirconium cyclobutadienyl complex.

Photolysis of (MePMPMe)2ZrBn2 (MePMPMe = 3,5-dimethyl-2-(2-pyridyl)pyrrolide) in the presence of diphenylacetylene yields the first η4-cyclobutadienyl zirconium complex, (MePMPMe)2Zr(η4-C4Ph4), through formal [2+2] cycloaddition of two alkynes at a putative low-valent zirconium intermediate. This unique reactivity expands the scope of alkyne coupling reactions at low-valent zirconium centers that traditionally produce zirconacyclopentadienes.

Delayed fluorescence from a zirconium(IV) photosensitizer with ligand-to-metal charge-transfer excited states.

Advances in chemical control of the photophysical properties of transition-metal complexes are revolutionizing a wide range of technologies, particularly photocatalysis and light-emitting diodes, but they rely heavily on molecules containing precious metals such as ruthenium and iridium. Although the application of earth-abundant 'early' transition metals in photosensitizers is clearly advantageous, a detailed understanding of excited states with ligand-to-metal charge transfer (LMCT) character is paramount to account for their distinct electron configurations. Here we report an air- and moisture-stable, visible light-absorbing Zr(IV) photosensitizer, Zr(PDP), where [PDP] is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine].

Reactivity of Pyridine Dipyrrolide Iron(II) Complexes with Organic Azides: C-H Amination and Iron Tetrazene Formation.

Reaction of (PDP)Fe(thf) (HPDP = 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine) with organic azides has been studied. The identity of the azide substituent had a profound impact on the transformation type and nature of the observed products. Reaction with aromatic -tolyl azide, NTol, resulted in exclusive formation of the corresponding iron tetrazene complex (PDP)Fe(NTol).

Photoluminescence of Seven-Coordinate Zirconium and Hafnium Complexes with 2,2'-Pyridylpyrrolide Ligands.

Luminescent seven-coordinated zirconium and hafnium complexes bearing three mono-anionic 2,2'-pyridylpyrrolide ligands and one chloride were synthesized. Solid-state structures and the dynamic behaviors in solution were probed by X-ray crystallography and variable temperature H NMR experiments, respectively. Absorption spectroscopy and time-dependent density functional theory (TD-DFT) calculations supported a hybrid of ligand-to-metal charge transfer (LMCT)/ligand-to-ligand charge transfer (LLCT) for the visible light absorption band.

Synthesis and Electronic Structure of Neutral Square-Planar High-Spin Iron(II) Complexes Supported by a Dianionic Pincer Ligand.

Two square-planar high-spin Fe complexes bearing a dianionic pyridine dipyrrolate pincer ligand and a diethyl ether or tetrahydrofuran ligand were synthesized and structurally characterized, and their electronic structures were elucidated by a combined spectroscopic and computational approach. In contrast to previous examples, the S = 2 ground states of these square-planar Fe complexes do not require an overall anionic charge of the compounds or incorporation of alkali metal cations. The tetrahydrofuran complex exhibits an equilibrium between four- and five-coordinate species in solution, which was supported by H NMR and Fe Mössbauer spectroscopy and comparison to a structurally characterized five-coordinate pyridine dipyrrolate iron bis-pyridine adduct.

Access to a pair of ambiphilic phosphine-borane regioisomers by rhodium-catalyzed hydroboration.

Lewis basic substrates, such as vinylphosphines and enamines, can be problematic for transition-metal catalysed hydrofunctionalization reactions due to their propensity to ligate and deactivate transition-metal catalysts as well as form direct Lewis adducts with reaction partners. While exploring rhodium-catalyzed hydroboration of diphenylvinylphosphine with pinacolborane, we found that a high degree of regiocontrol could be achieved without the need to diminish the Lewis basicity of the phosphine by oxidation or prior-protection. At slightly elevated temperature, a high yield of the previously unreported branched regioisomer, 1-pinacolatoborono-1-diphenylphosphinoethane, was achieved with regioselectivity greater than 10 : 1 using [Rh(COD)Cl]2 as the catalyst and AgOTf as a catalytic additive.

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(PDP) (PDP = 2,6-bis(5-methyl-3-phenyl-1 H-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.

Synthesis of a Cyclophane Bearing Two Benz[a]anthracene Units Connected at the 5 and 7 Positions with Two Naphth-1,4-diyl Groups.

A synthetic pathway to a cyclophane bearing two benz[a]anthracene units connected at the 5 and 7 positions through two naphth-1,4-diyl groups was developed, and its structure was confirmed by X-ray structure analysis. Because of structural constraints, the two naphthyl groups are distorted from planarity and the bonds connecting them to the benz[a]anthracene units are bent significantly. The UV-vis and fluorescence spectra of the cyclophane are red-shifted from those of 7-(1-naphthalenyl)benz[a]anthracene, which is the corresponding monomeric polycyclic aromatic hydrocarbon.

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(PDP)] (z = 1-, 2-, 3-) and [Mo(PDP)] (z = 0, 1-, 2-, 3-) were synthesized to study the redox properties of the pincer ligand PDP (HPDP = 2,6-bis(5-methyl-3-phenyl-1H-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 PDP ligands and +III oxidation states (d, S = 3/2) for the central metal ions.

Synthesis and Characterization of Functionalized [12]Cycloparaphenylenes Containing Four Alternating Biphenyl and Naphthyl Units.

Functionalized [12]cycloparaphenylenes ([12]CPPs) containing four alternating biphenyl and naphthyl units were synthesized. A macrocyclic furan-containing CPP precursor was used for the Diels-Alder reaction with the parent benzyne or 3,6-dimethoxybenzyne to form the corresponding macrocyclic carbon frameworks. The subsequent reductive deoxygenation of the Diels-Alder adducts with Fe(CO) followed by oxidative aromatization with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone produced the functionalized [12]CPPs.

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 CFS groups and results in a broad scope of 3-trifluoromethylthiolated coumarins, including analogues of natural products, in moderate to excellent yields.

Synthesis of Partially Hydrogenated Cycloparaphenylenes with Bent and Fused Structures Bearing Armchair Carbon Nanotube-like Connections.

The Diels-Alder reactions between 2 equiv of (E,E)-1,4-bis(4-bromophenyl)-1,3-butadiene and 1,4-benzoquinone led to the formation of a key intermediate with all four 4-bromophenyl substituents cis to one another. The subsequent nickel-mediated homocoupling reactions then produced partially hydrogenated cycloparaphenylenes, including a molecule bearing two units of tetrahydro[6]cycloparaphenylene (4H[6]CPP) fused together through two 1,4-dimethoxybenzene units in an armchair (6,6)carbon nanotube-like connection. Similarly, two 6H[9]CPPs were connected through three 1,4-dimethoxybenzene units in an armchair (9,9)carbon nanotube-like arrangement.

Regioselective Copper-Catalyzed Boracarboxylation of Vinyl Arenes.

Regioselective copper-catalyzed boracarboxylation of vinyl arenes with bis(pinacolato)diboron and carbon dioxide has been achieved. New boron-functionalized α-aryl carboxylic acids, including nonsteroidal anti-inflammatory drugs (NSAIDs), are obtained in moderate to excellent yields. The synthetic utility of the transformation was shown through subsequent derivatization of the carbon-boron bond yielding formal hydroxy- and fluorocarboxylation products as well as anionic difluoroboralactones.

Synthesis of Cycloparaphenylenes Bearing Furan-2,5-diyl or 2,2'-Bifuran-5,5'-diyl Units in the Macrocyclic Structures.

Synthetic pathways to furan-containing cycloparaphenylenes (CPPs) as molecular nanohoops bearing 10, 12, and 15 aromatic units including furan-2,5-diyl or 2,2'-bifuran-5,5'-diyl units have been developed. The X-ray structures of a partially hydrogenated bifuran-containing CPP precursor and the corresponding fully aromatized bifuran-containing CPP were obtained to allow the determination of their conformational arrangements in the crystal lattice. The optical and electrochemical properties of the furan-containing CPPs were investigated.

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(PDP) and Zr(PDP) (PDP = 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(PDP) 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.

Syntheses and Structures of Functionalized [9]Cycloparaphenylenes as Carbon Nanohoops Bearing Carbomethoxy and N-Phenylphthalimido Groups.

Functionalized [9]cycloparaphenylenes ([9]CPPs) bearing nine aromatic units in the macrocyclic structures were synthesized. The macrocyclic structures were substituted with carbomethoxy or N-phenylphthalimido groups. The Diels-Alder reaction of (E,E)-1,4-bis(4-bromophenyl)-1,3-butadiene or a related diene with dimethyl acetylenedicarboxylate followed by the nickel-mediated homocoupling reactions and oxidative aromatization produced the functionalized [9]CPPs.

Syntheses and Structures of Thiophene-Containing Cycloparaphenylenes and Related Carbon Nanohoops.

Thiophene-containing cycloparaphenylenes (CPPs) bearing 8, 10, and 16 aromatic and heteroaromatic units in the macrocyclic ring structures were synthesized. Specifically, two and four thiophene-2,5-diyl units were incorporated into functionalized [6]- and [12]CPP macrocyclic carbon frameworks, respectively. In addition, two 2,2'-bithiophene-5,5'-diyl units were inserted into a functionalized [6]CPP carbon framework.

Ligand-Assisted Gold-Catalyzed Cross-Coupling with Aryldiazonium Salts: Redox Gold Catalysis without an External Oxidant.

Gold-catalyzed C(sp)-C(sp(2)) and C(sp(2))-C(sp(2)) cross-coupling reactions are accomplished with aryldiazonium salts as the coupling partner. With the assistance of bpy ligand, gold(I) species were oxidized to gold(III) by diazonium without any external oxidants. Monitoring the reaction with NMR and ESI-MS provided strong evidence for the nitrogen extrusion followed by Au(III) reductive elimination as the key step.