Isolated Neutral Organic Radical Unveiled Solvent-Radical Interaction in Highly Reducing Photocatalysis.

Diffusion-limited kinetics is a key mechanistic debate when consecutive photoelectron transfer (conPET) is discussed in photoredox catalysis. In-situ generated organic photoactive radicals can access catalytic systems as reducing as alkaline metals that can activate remarkably stable bonds. However, in many cases, the extremely short-lived transient nature of these doublet state open-shell species has led to debatable mechanistic studies, hindering adoption and development.

Red Light-Blue Light Chromoselective C(sp)-X Bond Activation by Organic Helicenium-Based Photocatalysis.

Chromoselective bond activation has been achieved in organic helicenium (Pr-DMQA)-based photoredox catalysis. Consequently, control over chromoselective C(sp)-X bond activation in multihalogenated aromatics has been demonstrated. Pr-DMQA can only initiate the halogen atom transfer (XAT) pathway under red light irradiation to activate low-energy-accessible C(sp)-I bonds.

Synthesis of CF-Containing Spirocyclic Indolines via a Red-Light-Mediated Trifluoromethylation/Dearomatization Cascade.

A red-light-mediated Pr-DMQA-catalyzed cascade intramolecular trifluoromethylation and dearomatization of indole derivatives with Umemoto's reagent has been developed. This protocol provides a facile and efficient approach for the construction of functionalized and potentially biologically important CF-containing 3,3-spirocyclic indolines with moderate to high yields and excellent diastereoselectivities under mild conditions. The success of multiple gram-scale (1 and 10 g) experiments further highlights the robustness and practicality of this protocol and the merit of the employment of red light.

Trioxatriangulenium (TOTA) as a robust carbon-based Lewis acid in frustrated Lewis pair chemistry.

We report the reactivity between the water stable Lewis acidic trioxatriangulenium ion (TOTA) and a series of Lewis bases such as phosphines and N-heterocyclic carbene (NHC). The nature of the Lewis acid-base interaction was analyzed variable temperature (VT) NMR spectroscopy, single-crystal X-ray diffraction, UV-visible spectroscopy, and DFT calculations. While small and strongly nucleophilic phosphines, such as PMe, led to the formation of a Lewis acid-base adduct, frustrated Lewis pairs (FLPs) were observed for sterically hindered bases such as P( Bu).

Persistent, highly localized, and tunable [4]helicene radicals.

Persistent organic radicals have gained considerable attention in the fields of catalysis and materials science. In particular, helical molecules are of great interest for the development and application of novel organic radicals in optoelectronic and spintronic materials. Here we report the syntheses of easily tunable and stable neutral quinolinoacridine radicals under anaerobic conditions by chemical reduction of their quinolinoacridinium cation analogs.

Helical Carbenium Ion: A Versatile Organic Photoredox Catalyst for Red-Light-Mediated Reactions.

Red light has the advantages of low energy, less health risks, and high penetration depth through various media. Herein, a helical carbenium ion (-di--propyl-1,13-dimethoxyquinacridinium (Pr-DMQA) tetrafluoroborate) has been used as an organic photoredox catalyst for photoreductions and photooxidations in the presence of red light (λ = 640 nm). It has catalyzed red-light-mediated dual transition-metal/photo-redox-catalyzed C-H arylation and intermolecular atom-transfer radical addition through oxidative quenching.

Tunable carbocation-based redox active ambiphilic ligands: synthesis, coordination and characterization.

The synthesis of novel redox active ambiphilic ligands L1-L3 and their coordination chemistry to first-row late transition metal halides (M = Co and Ni) is reported. The heterocyclic carbocation scaffolds act as Lewis acid moieties while the pyridine anchor acts as the coordinating Lewis base. The high synthetic tunability of this ligand scaffold allows for control of its rigidity and electronic properties.

Low-valent homobimetallic Rh complexes: influence of ligands on the structure and the intramolecular reactivity of Rh-H intermediates.

Supporting two metal binding sites by a tailored polydentate trop-based (trop = 5-dibenzo[,]cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(i) complexes. Their reaction with hydrogen rapidly forms Rh hydrides that undergo an intramolecular semihydrogenation of two C[triple bond, length as m-dash]C bonds of the trop ligand. This reaction is chemoselective and converts C[triple bond, length as m-dash]C bonds to a bridging carbene and an olefinic ligand in the first and the second semihydrogenation steps, respectively.

Molecular Orbital Insights of Transition Metal-Stabilized Carbocations.

Transition metal-stabilized carbocations are characterized by synthetically valuable interactions, yet, to date there are no comprehensive reports of the many bonding modes that can exist between a metal and carbocation. This review summarizes developments in these complexes to provide a clear picture of their properties and reactivities. In order to strategically exploit them, we propose this summary of the different bonding modes for transition metal-carbocation complexes.

A low-valent dinuclear ruthenium diazadiene complex catalyzes the oxidation of dihydrogen and reversible hydrogenation of quinones.

The dinuclear ruthenium complex [RuH(μ-H)(Medad)(dbcot)] contains a 1,4-dimethyl-diazabuta-1,3-diene (Medad) as a non-innocent bridging ligand between the metal centers to give a [Ru(Medad)] core. In addition, each ruthenium is bound to one dibenzo[,]cyclooctatetraene (dbcot) ligand. This Ru dimer converts H to protons and electrons.

Zero-Valent Amino-Olefin Cobalt Complexes as Catalysts for Oxygen Atom Transfer Reactions from Nitrous Oxide.

The synthesis and characterization of several zero-valent cobalt complexes with a bis(olefin)-amino ligand is presented. Some of these complexes proved to be efficient catalysts for the selective oxidation of secondary and allylic phosphanes, as well as diphosphanes, even with a direct P-P bond. With 5 mol % catalyst loadings the oxidations proceed under mild conditions (25-70 °C, 7-22 h, 2 bar N O) and afford good to excellent yields (65-98 %).

From 0 to II in One-Electron Steps: A Series of Ruthenium Complexes Supported by TropPPh2.

We report the synthesis of a series of ruthenium complexes supported by the phosphine olefin ligand tropPPh2 (trop=5-H-dibenzo-[a,d]cyclohepten-5-yl) in the oxidation states 0, +I, and +II, formed via successive one-electron oxidization steps from Ru(0) (tropPPh2 )2 . The bidentate character of the tropPPh2 ligand and its steric hindrance force the complexes to adopt uncommon geometries, which were investigated by X-ray diffraction analysis. EPR data of the mononuclear Ru(I) complex reveal couplings of the unpaired spin with the ruthenium and two phosphorus nuclei, as well as the olefinic protons which show that the spin is mainly localized on the Ru(I) center.

Activation of heteroallenes by coordinatively unsaturated nickel(ii) alkyl complexes supported by the hydrotris(3-phenyl-5-methyl)pyrazolyl borate (Tp(Ph,Me)) ligand.

Activation of sulfur containing heteroallenes by nickel(ii) alkyl complexes supported by the bulky hydrotris(3-phenyl-5-methylpyrazolyl)borate (Tp(Ph,Me)) ligand is described. Exposure of Tp(Ph,Me)NiCH2Ph (1a) and Tp(Ph,Me)NiCH2Si(CH3)3 (1b) to CS2 resulted in formation of the insertion products Tp(Ph,Me)Ni(η(2)-CS2)CH2Ph (2a) and Tp(Ph,Me)Ni(η(2)-CS2)CH2Si(CH3)3 (2b) in moderate yields. Reaction of 1a and MeNCS produced two species in a 1 : 1 ratio, identified as Tp(Ph,Me)Ni(η(2)-MeNC)CH2Ph (3) and Tp(Ph,Me)Ni(η(2)-MeNCS)SCH2Ph (4).

Nitrous Oxide as a Hydrogen Acceptor for the Dehydrogenative Coupling of Alcohols.

The oxidation of alcohols with N2O as the hydrogen acceptor was achieved with low catalyst loadings of a rhodium complex that features a cooperative bis(olefin)amido ligand under mild conditions. Two different methods enable the formation of either the corresponding carboxylic acid or the ester. N2 and water are the only by-products.

Diolefins with an ether/thioether functionality as ligands in the coordination sphere of Ni and Rh.

A diolefin ether, trop2O (2), and a diolefin thioether, trop2S (3), have been investigated as ligand analogues of the well-established diolefin amine, trop2NH (1). Compounds 2 and 3 form different conformers in solution and in the solid state. Whereas 2 could be coordinated to Ni(0), 3 was found to be more suited for coordination to Rh(I).

C-F sp(2) bond functionalization mediated by niobium complexes.

Insertion chemistry of isocyanide molecules was used to functionalize C-F sp(2) bonds after their oxidative addition across the metal center in a β-diketiminate niobium(iii) imido complex (BDI)Nb(N(t)Bu)(C6H6). The complexes formed, 3a-b ([BDI]Nb(PhC[double bond, length as m-dash]N)(N(t)Bu)(F) (R = 1,6-diisopropylphenyl, tert-butyl), were characterized by NMR spectroscopy and X-ray analysis. Further treatment with phenylsilane induced H/F exchange under mild conditions, which was followed by hydride transfer to the inserted isocyanide.

Electron localization in a mixed-valence diniobium benzene complex.

Reaction of the neutral diniobium benzene complex {[Nb(BDI)N Bu](μ-CH)} (BDI = ,'-diisopropylbenzene-β-diketiminate) with Ag[B(CF)] results in a single electron oxidation to produce a cationic diniobium arene complex, {[Nb(BDI)N Bu](μ-CH)}{B(CF)}. Investigation of the solid state and solution phase structure using single-crystal X-ray diffraction, cyclic voltammetry, magnetic susceptibility, and multinuclear NMR spectroscopy indicates that the oxidation results in an asymmetric molecule with two chemically inequivalent Nb atoms. Further characterization using density functional theory (DFT) calculations, UV-visible, Nb L-edge X-ray absorption near-edge structure (XANES), and EPR spectroscopies supports assignment of a diniobium complex, in which one Nb atom carries a single unpaired electron that is not largely delocalized on the second Nb atom.

Regioselective [2+2] and [4+2] cycloaddition reactivity in an asymmetric niobium(bisimido) moiety towards unsaturated organic molecules.

The asymmetric bis-imido structure and the lability of the diethyl ether linkage in complex 1 provide a niobium complex that undergoes regioselective [4+2] cycloaddition reactions with an α,β-unsaturated ketone and cycloaddition reactions that involve bond formation to the MAD ligand (MonoAzabutaDiene). DFT calculations have been used to support an initial azametallacyclobutene intermediate in the alkyne reaction.

Reaction of (bisimido)niobium(V) complexes with organic azides: [3 + 2] cycloaddition and reversible cleavage of β-diketiminato ligands involving nitrene transfer.

We describe the unusual reactivity of a highly labile diethyl ether adduct of an asymmetric niobium(V) bis(imide) 2.OEt2 containing the monoazabutadiene (MAD) ligand. This species undergoes clean nitrene transfer on treatment with tert-butyl- or di-isopropylphenyl azide resulting in the unprecedented reformation of nacnac ligands bound to the metal center.

Synthesis and characterization of coordinatively unsaturated nickel(II) and manganese(II) alkyl complexes supported by the hydrotris(3-phenyl-5-methylpyrazolyl)borate (Tp(Ph,Me)) ligand.

The synthesis of several nickel(II) and manganese(II) alkyl complexes supported by hydrotris(3-phenyl-5-methylpyrazolyl)borate (Tp(Ph,Me)) ligand is reported. The metal halide complexes Tp(Ph,Me)MnCl(CH3CN) (1) and Tp(Ph,Me)NiCl (4) were used as precursors for synthesis of Tp(Ph,Me)MnCH2Si(Me)3 (2), Tp(Ph,Me)MnCH2Ph (3), Tp(Ph,Me)NiCH2Si(Me)3 (5) and Tp(Ph,Me)NiCH2Ph (6). The resulting Mn(II) and Ni(II) alkyl complexes, 2-3 and 5-6, were characterized by X-ray crystallography, NMR spectroscopy, and FT-IR spectroscopy.

Dis-assembly of a benzylic CF3 group mediated by a niobium(III) imido complex.

All three C-F bonds in CF3-substituted arenes are activated by a niobium imido complex, driven by the formation of strong Nb-F bonds. The mechanism of this transformation was studied by NMR spectroscopy, which revealed the involvement of Nb(III). Attempts to extend this chemistry to nonaromatic CF3 groups led to intramolecular reactivity.

Diniobium inverted sandwich complexes with μ-η6:η6-arene ligands: synthesis, kinetics of formation, and electronic structure.

Monometallic niobium arene complexes [Nb(BDI)(N(t)Bu)(R-C(6)H(5))] (2a: R = H and 2b: R = Me, BDI = N,N'-diisopropylbenzene-β-diketiminate) were synthesized and found to undergo slow conversion into the diniobium inverted arene sandwich complexes [[(BDI)Nb(N(t)Bu)](2)(μ-RC(6)H(5))] (7a: R = H and 7b: R = Me) in solution. The kinetics of this reaction were followed by (1)H NMR spectroscopy and are in agreement with a dissociative mechanism. Compounds 7a-b showed a lack of reactivity toward small molecules, even at elevated temperatures, which is unusual in the chemistry of inverted sandwich complexes.