Intermetal Oxygen, Sulfur, Selenium, and Nitrogen Atom Transfer Reactions

The importance of electron-transfer reactions is clearly indicated by the extensive literature describing this field and the present research emphasis devoted toward understanding these fundamental processes.1 The intricacies of electron-transfer reactions have been vigorously examined experimentally and theoretically for over 40 years.2·3 Because of continuing, intensive effort, fundamental aspects of these vital processes are still being elaborated

Multielectron Redox Reactions between Manganese Porphyrins Mediated by Nitrogen Atom Transfer

From the pioneering work of Taube, 1 electron-transfer reactions can be mechanistically categorized into either inner sphere or outer sphere processes. The most well-studied systems in either case typically involve one-electron changes. Redox processes involving transfers of a multiple number of electrons, especially between two metals, are much less prevalent and consequently less well understood. The most extensive studies on multiple electron changes have involved atom transfer processes.2 These are typically two electron-transfer reactions mediated by either a bridging halogen3•4 or a bridging oxo5 ligand. The consideration of a nitrido ligand as a bridging species in redox reactions has received little attention.6 In this regard, we initiated studies on the bridging capabilities of the nitrido complexes of metalloporphyrins. We report herein the first example of a reversible net three electron redox process mediated by nitrogen atom transfer

Rational Design of Linear Trinuclear Metal Complexes

A great deal of interest in multi metallic complexes derives from the special chemical and physical properties resulting from the mutual interaction of two or more metal centers.1 Of particu.lar interest are one-dimensional chains of metal complexes.2 Materials of this type have exhibited unusual magnetic, optical, and conduction properties.3 Despite the great interest in this area, the development of widely applicable, rational methods for synthesis of desired metal chain complexes remains elusive

Iridium Porphyrin Catalyzed N−H Insertion Reactions: Scope and Mechanism

Ir(TTP)CH3 catalyzed N–H insertion reactions between ethyl diazoacetate (EDA) or methyl phenyldiazoacetate (MPDA) and a variety of aryl, aliphatic, primary, and secondary amines to generate substituted glycine esters with modest to high yields. Aniline substrates generally gave yields above 80%, with up to 105 catalyst turnovers, and without slow addition of the diazo reagent. Good yields were also achieved with aliphatic amines, though higher catalyst loadings and slow addition of the amine were necessary in some cases. Primary amines reacted with EDA to generate both single- and double-insertion products, either of which could be produced selectively in high yield with the proper choice of stoichiometric ratios and reaction temperature. Notably, mixed trisubstituted amines, RN(CH2CO2Et)(CHPhCO2Me), were generated from the insertion of 1 equiv of EDA and 1 equiv of MPDA into primary amines. The N–H insertion mechanism was examined using substrate competition studies, trapping experiments, and multiple spectroscopic techniques. Substrate competition studies using pairs of amines with EDA or MPDA revealed Hammett correlations with respective slopes of ρ = 0.15 and ρ+ = −0.56 as well as kinetic isotope ratios of kH/kD = 1.0 ± 0.2 and 2.7 ± 0.2. Competitive amine binding to the iridium center was demonstrated by kinetics and equilibrium binding studies. Equilibrium binding constants ranged from 102 to 105. Monitoring the reaction by absorption spectroscopy revealed a transient metalloporphyrin complex. The lifetime of this species was dependent on the nature of the amine substrate, which suggests that the catalytic cycle proceeds through a metal–ylide intermediate

Synthesis, Characterization, and Reactivity of Group 4 Metalloporphyrin Diolate Complexes

A number of group 4 metalloporphyrin diolate complexes were synthesized via various approaches. For example, treatment of imido complex (TTP)HfNAriPr with diols resulted in formation of the corresponding diolato complexes (TTP)Hf[OCR1R2CR1R2O] (R1 = R2 = Me, 1; R1 = Me, R2 = Ph, 2; R1 = R2 = Ph, 3). Treatment of (TTP)TiNiPr with diols generated (TTP)Ti[OCR1R2CR1R2O] (R1 = R2 = Me, 5; R1 = Me, R2 = Ph, 6; R1 = H, R2 = Ph, 7; R1 = H, R2= p-tolyl, 8). Alternatively hafnium and titanium pinacolates 1 and 5 were prepared through metathetical reactions of (TTP)MCl2 (M = Hf, Ti) with disodium pinacolate. The substitution chemistry of hafnium complexes correlated well with the basicity of the diolato ligands. Complexes 1−6 underwent oxidative cleavage reaction, producing carbonyl compounds and oxometalloporphyrin species. For less substituted diolates 7 and 8, an array of products including the enediolate complexes (TTP)Ti[OC(Ar)C(Ar)O] (Ar = Ph, 9; Ar = p-tolyl, 10) was observed. The possible cleavage reaction pathways are discussed

Titanium(II) Porphyrin Complexes: Versatile One- and Two-Electron Reducing Agents. Reduction of Organic Chlorides, Epoxides, and Sulfoxides

Treatment of the well-defined complexes (TTP)Ti(η2-EtC⋮CEt) or trans-(TTP)Ti(THF)2 with vicinal dichloroalkanes or dichloroalkenes results in the production of alkenes or alkynes and 2 equiv of (TTP)TiCl. This net two-electron redox reaction arises from two formal one-electron reduction processes mediated by chlorine atom transfer. Oxygen atom transfer occurs when the Ti(II) porphyrins are treated with several different sulfoxides or epoxides, resulting in two-electron redox products, (TTP)TiO, the sulfide or alkene, and EtC⋮CEt or THF. The electronic properties of the substituents on the sulfoxides or epoxides correlate with the yield and rate of the deoxygenation reactions