Synthetic and Mechanistic Studies of the Four-Electron Reduction of Dioxygen, NN, and NO Double Bonds by Tungsten(II) Aryloxide Compounds

Four-Electron Oxidative Addition of an N=N Double Bond at a Chromium Metallocyclopropene

This report describes the synthesis of a pseudo-tetrahedral chromium alkyne complex supported by a bidentate phosphinimide ligand and its reactivity with an azobenzene derivative. Characterization of the former by structural and computational methods reveals an unprecedented extent of alkyne activation by a formal chromium(II) center, suggesting that this complex is best described as a chromium(IV)-metallocyclopropene. Exposure of this compound to 4,4'-difluoroazobenzene results in the formation of a chromium(VI) diimido complex, which constitutes a rare 4-electron oxidative addition of an N=N double bond. The isolation of a chromium(IV)-hydrazido intermediate enabled mechanistic investigations of this challenging bond cleavage process. This work substantiates the notion that terminal phosphinimide ligands can engender first-row transition metal ions with exceptional reactivity.

Nonclassical oxygen atom transfer reactions of an eight-coordinate dioxomolybdenum(vi) complex

Eight-coordinate MoO₂(DOPO^Q)₂ can donate two oxygen atoms to substrates such as phosphines in a four-electron nonclassical oxygen atom transfer reaction.

A Dimeric Chromium(II) Pincer as an Electron Shuttle for N=N Bond Scission

Reduction of the bis-pyrazolyl pyridine complex [CrL]₂ with 4 KC₈ , followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K₃ Cr(PhNNPh)₃ ]. This has three κ² PhNNPh^2- ligands and K⁺ bound to nitrogen atoms of azobenzene. When the stoichiometry is modified to 1:4:3, the product is changed to [K₂ CrL(PhNNPh)₂ ], which has C₂ symmetry except for the intimate ion pairing of two K⁺ ions to reduced azobenzene nitrogen atoms, and to pyrazolate and phenyl rings. The origin of the observed delivery of reducing equivalents to several, not to a single N=N bond, is traced to the resistance of the one-electron-reduced substrate to receiving a second electron, and is thus a general phenomenon. [CrL]₂ alone is shown to be a two-electron reductant towards benzo[c]cinnoline (BCC) resulting in a product of formula [Cr₂ L₂ (BCC)], in which the reducing equivalents originate purely from Cr^II . An analogous study of the reaction of [CrL]₂ with azobenzene yields [Cr₂ L₂ (PhNNPh)(THF)], an adduct in which one THF has displaced one of four hydrazide nitrogen/Cr bonds. Together these illustrate different modes for the Cr₂ L₂ unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K⁰ , have the ability to reduce the N=N bond. Further KC₈ reduction of preformed Cr₂ L₂ (RNNR) inevitably gives products in which K⁺ stabilizes the charge in the increasingly electron-rich nitrogen atoms, in a phenomenon which mimics proton coupled electron transfer: K⁺ performs the role of H⁺ . A least-squares fit of the two singly reduced DFT structures shows that the only major change is a re-orientation of one of the two phenyl rings in order to avoid repulsion with potassium but to still allow interaction of that phenyl π system with K⁺ . This shows both the impact of K⁺ , being modest to nitrogen/chromium interactions, but nevertheless accommodating some π donation of phenyl to potassium. Finally, delivering increasing equivalents of KC₈ leads to complete cleavage of the N=N bond, and both N bind to three Cr^II . The varied impacts of the K⁺ electrophile on NN multiple bond reduction is discussed.

Reactivity of terminal imido complexes of group 4-6 metals: stoichiometric and catalytic reactions involving cycloaddition with unsaturated organic molecules

Imido complexes of early transition metals are key intermediates in the synthesis of many nitrogen-containing organic compounds. The metal-nitrogen double bond of the imido moiety undergoes [2+2] cycloaddition reactions with various unsaturated organic molecules to form new nitrogen-carbon and nitrogen-heteroatom bonds. This review article focuses on reactivity of the terminal imido complexes of Group 4-6 metals, summarizing their stoichiometric reactions and catalytic applications for a variety of reactions including alkyne hydroamination, alkyne carboamination, pyrrole formation, imine metathesis, and condensation reactions of carbonyl compounds with isocyanates.

Synthesis of Pyridylimido Complexes of Tantalum and Niobium by Reductive Cleavage of the N═N Bond of 2,2'-Azopyridine: Precursors for Early-Late Heterobimetallic Complexes

We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by N═N bond cleavage of 2,2'-azopyridine. Reaction of MCl5 (M = Ta and Nb) with 2,2'-azopyridine in the presence of 0.5 equiv of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated Si-Me-CHD) afforded a dark red solution (for Ta) and a dark blue solution (for Nb) with some insoluble precipitates. After removing the solids, another 0.5 equiv of Si-Me-CHD was added to each solution, giving [M(═Npy)Cl3]n (1a: M = Ta; 1b: M = Nb) through reductive cleavage of the N═N bond of 2,2'-azopyridine. The initial products of the above reactions were determined to be 2,2'-azopyridine-bridged dinuclear complexes, [(MCl4)2(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl5 with 2,2'-azopyridine and Si-Me-CHD in a 2:1:1 molar ratio. In 2a and 2b, the N═N bond was reduced to a single bond via two-electron reduction. Further reduction of complexes 2a and 2b with 1 equiv of Si-Me-CHD afforded complexes 1a and 1b. An anionic doubly μ-imido-bridged ditantalum complex, [nBu4N][Ta2(μ-Npy)2Cl7] (3a), was generated upon addition of nBu4NCl to complex 1a, while addition of nBu4NCl to niobium complex 1b gave a polymeric terminal imido complex, [nBu4N]n/2[{Nb(═Npy)Cl3}2(μ-Cl)]n/2 (3b). Complexations of 1a and 1b with 1 equiv of 2,2'-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M(═Npy)Cl3(bipy) (4a: M = Ta; 4b: M = Nb), whose main structural feature is intramolecular hydrogen bonding between the ortho hydrogen atom of 2,2'-bipyridine and the nitrogen atom of the pyridyl group on the imido ligand. Isolated 2-pyridylimido complexes 4a and 4b reacted with [RhCl(cod)]2 to produce the corresponding early-late heterobimetallic complexes, (bipy)MCl3(μ-Npy)RhCl(cod) (5a: M = Ta; 5b: M = Nb).

Photochemically induced intramolecular six-electron reductive elimination and oxidative addition of nitric oxide by the nitridoosmate(VIII) anion

UV photolysis of the nitridoosmate(VIII) anion, OsO3 N(-) , in low-temperature frozen matrices results in nitrogen-oxygen bond formation to give the Os(II) nitrosyl complex OsO2 (NO)(-) . Photolysis of the Os(II) nitrosyl product with visible wavelengths results in reversion to the parent Os(VIII) complex. Formally a six-electron reductive elimination and oxidative addition, respectively, this represents the first reported example of such an intramolecular transformation. DFT modelling of this reaction proceeds through a step-wise mechanism taking place through a side-on nitroxyl Os(VI) intermediate, OsO2 (η(2) -NO)(-) .

Direct cleavage of the NN bond of azobenzenes by MeOTf leading to N-arylbenzimidazoles

Direct cleavage of the NN bond of azo compounds by methyltriflate (MeOTf) leading to benzimidazoles has been described without the assistance of any metals or metalloids.

One- and two-electron reactivity of a tantalum(V) complex with a redox-active tris(amido) ligand

A new redox-active, tris(amido) ligand platform, bis(2-isopropylamino-4-methoxyphenylamine [NNN(cat)](3-), has been prepared and used in the preparation of tantalum(V) complexes. The ligand was prepared in its protonated form by a three-step procedure from commercially available 4-methoxy-2-nitroaniline and 1-iodo-4-methoxy-2-nitrobenzene. Direct reaction of [NNN(cat)]H(3) with TaCl(2)Me(3) afforded five-coordinate [NNN(cat)]TaCl(2) (1), which accepted the strong sigma-donor ligand (t)BuNC to form the six-coordinate adduct [NNN(cat)]TaCl(2)(CN(t)Bu) (2). Complex 1 is formally a d(0), Ta(V) complex; however, one- and two-electron reactivity is enabled at the metal center by the redox-activity of the ligand platform. Complex 1 was oxidized by one electron to afford the radical species [NNN(sq*)]TaCl(3) (3), which was characterized by solution EPR spectroscopy. Cyclic voltammetry studies of complex 3 showed clean one-electron oxidation and reduction processes at 0.148 and -0.324 V vs [Cp(2)Fe](+/0), indicating the accessibility of three oxidation states, [NNN(cat)](3-), [NNN(sq*)](2-), and [NNN(q)](-), for the metallated ligand. Complex 1 also can undergo two-electron reactions, as evidenced by the reaction with nitrene transfer reagents to form tantalum imido species. Thus 1 reacted with organic azides, RN(3) (R = Ph, p-C(6)H(4)Me, p-C(6)H(4)(t)Bu), to form [NNN(q)]TaCl(2)(NR) (4). Similarly, the tantalum diphenylmethylidenehydrazido complex, [NNN(q)]TaCl(2)(NNCPh(2)) (5), was formed by reaction of 1 with the diazoalkane, N(2)CPh(2).