A pinboard by
Sungho Park

Graduate student, University of Wisconsin–Madison


Studies on a Ru(III) azide species as a precursor to a Ru(V) nitride for C–N bond formation

Nitrogen atoms are ubiquitous in biology. Molecules containing nitrogen atoms can participate not only in hydrogen bonding but also in biological acid/base chemistry in the form of either an ammonium (acidic) or an amine (basic). Metal nitride complexes (M≡N) have attracted attention for their ability to transfer a nitrogen atom to organic molecules. This type of transformation is important especially in the context of drug discovery and synthesis of pharmaceuticals containing C–N bonds. Photolysis of an azide precursor (M–N3) is widely utilized to expel N2 and generate these metal nitrides. With the aim of studying novel Ru≡N species, an air-stable Ru(II) azide complex was synthesized. This Ru(II) azide species immediately turns color from yellow to purple upon one-electron oxidation. EPR studies at 20 K reveal a single paramagnetic species that decomposes upon irradiation with visible light even at −196 °C, and thermal degradation at −25 °C in the dark leads to a diamagnetic Ru(II) solvent adduct. Direct interrogation of this metastable purple species revealed its identity as a rare example of a crystallographically characterized monometallic Ru(III) azide complex. Although photolysis of the Ru(III) azide complex leads to the Ru(II) solvent adduct instead of the desired Ru(V) nitride, a different Ru(III) azide complex has been reported in the literature to exhibit nitride-like reactivity towards triphenylphosphine (PPh3), giving a phosphinimide complex (Ru=N–PPh3). Hence, although isolation of the Ru(V) nitride is not directly achievable with photolysis, reactivity studies are ongoing to elucidate whether my Ru(III) azide can also demonstrate nitride-like reactivity and potentially perform C–N bond formation reactions. My Ru(III) azide complex is extremely Lewis acidic, capable of abstracting a chloride from dichloromethane and from the hexachloroantimonate anion. Its reactivity towards substrates such as triphenylphosphine, isocyanides, dihydroanthracene and 1,4-cyclohexadiene is currently being explored. Other routes that can lead to the Ru(V) nitride include using nitride transfer reagents such as N≡Cr(t-BuO)3 or oxidizing an ammonia complex (M–NH3). It should be noted that no monometallic Ru(V) nitride complex has been structurally characterized to date. This intriguing project is expected to contribute to the development of novel metal azide/nitride species capable of C–N bond formation and applicable in an industrial setting.


Generation, spectroscopic and chemical characterization of an octahedral iron (V) - nitrido species with a neutral ligand platform.

Abstract: Iron complex [FeIII(N3)(MePy2tacn)](PF6)2 (1), containing a neutral triazacyclononane-based pentadentate ligand, and a terminally bound azide ligand has been prepared and spectroscopically and structurally characterized. Structural details, magnetic susceptibility data and Mössbauer spectra demonstrate that 1 has a low-spin state (S = 1/2) ferric center. X-Ray diffraction analysis of 1 reveals remarkably short Fe - N (1.859 Å) and long FeN - N2 (1.246 Å) distances, respectively, and the FT-IR spectra showed an unusually low N - N stretching frequency (2019 cm-1), suggesting that the FeN - N2 bond is particularly weak. Photolysis of 1 at 470 nm or 530 nm in frozen solutions caused N2 elimination and generation of a nitride species that on the basis of Mössbauer, magnetic susceptibility, EPR, and X - ray absorption (XANES) data in conjunction with DFT computational analyses is formulated as [FeV(N)(MePy2tacn)]2+ (2). The results showed that 2 is a low-spin S =1/2 iron(V) species and exhibits a short Fe-N distance (1.64 Å), as deduced from EXAFS analysis. Compound 2 is only stable at cryogenic (liquid N2) temperatures, and frozen solutions as well as solid samples decompose rapidly upon warming, producing N2. The high-valent compound could also be generated in the gas phase and its reactivity against olefins, sulphides and substrates with weak C-H bonds has been studied. It proved to be a powerful two-electron oxidant that can add the nitride ligand to olefin and sulphide sites, and oxidizes cyclohexadiene substrates to benzene, in a formal H2 transfer process. In summary compound 2 constitutes the first case of an octahedral FeV(N) species prepared within a neutral ligand framework, and adds to the few examples of FeV species that could be spectroscopically and chemically characterized.

Pub.: 10 Jun '17, Pinned: 30 May '18

Reactivity of nitrido complexes of ruthenium(VI), osmium(VI), and manganese(V) bearing Schiff base and simple anionic ligands.

Abstract: Nitrido complexes (M≡N) may be key intermediates in chemical and biological nitrogen fixation and serve as useful reagents for nitrogenation of organic compounds. Osmium(VI) nitrido complexes bearing 2,2':6',2″-terpyridine (terpy), 2,2'-bipyridine (bpy), or hydrotris(1-pyrazolyl)borate anion (Tp) ligands are highly electrophilic: they can react with a variety of nucleophiles to generate novel osmium(IV)/(V) complexes. This Account describes our recent results studying the reactivity of nitridocomplexes of ruthenium(VI), osmium(VI), and manganese(V) that bear Schiff bases and other simple anionic ligands. We demonstrate that these nitrido complexes exhibit rich chemical reactivity. They react with various nucleophiles, activate C-H bonds, undergo N···N coupling, catalyze the oxidation of organic compounds, and show anticancer activities. Ruthenium(VI) nitrido complexes bearing Schiff base ligands, such as [Ru(VI)(N)(salchda)(CH3OH)](+) (salchda = N,N'-bis(salicylidene)o-cyclohexyldiamine dianion), are highly electrophilic. This complex reacts readily at ambient conditions with a variety of nucleophiles at rates that are much faster than similar reactions using Os(VI)≡N. This complex also carries out unique reactions, including the direct aziridination of alkenes, C-H bond activation of alkanes and C-N bond cleavage of anilines. The addition of ligands such as pyridine can enhance the reactivity of [Ru(VI)(N)(salchda)(CH3OH)](+). Therefore researchers can tune the reactivity of Ru≡N by adding a ligand L trans to nitride: L-Ru≡N. Moreover, the addition of various nucleophiles (Nu) to Ru(VI)≡N initially generate the ruthenium(IV) imido species Ru(IV)-N(Nu), a new class of hydrogen-atom transfer (HAT) reagents. Nucleophiles also readily add to coordinated Schiff base ligands in Os(VI)≡N and Ru(VI)≡N complexes. These additions are often stereospecific, suggesting that the nitrido ligand has a directing effect on the incoming nucleophile. M≡N is also a potential platform for the design of new oxidation catalysts. For example, [Os(VI)(N)Cl4](-) catalyzes the oxidation of alkanes by a variety of oxidants, and the addition of Lewis acids greatly accelerates these reactions. [Mn(V)(N)(CN)4]2(-) is another highly efficient oxidation catalyst, which facilitates the epoxidation of alkenes and the oxidation of alcohols to carbonyl compounds using H2O2. Finally, M≡N can potentially bind to and exert various effects on biomolecules. For example, a number of Os(VI)≡N complexes exhibit novel anticancer properties, which may be related to their ability to bind to DNA or other biomolecules.

Pub.: 21 Sep '13, Pinned: 30 May '18

Aryl C-H amination by diruthenium nitrides in the solid state and in solution at room temperature: experimental and computational study of the reaction mechanism.

Abstract: Diruthenium azido complexes Ru(2)(DPhF)(4)N(3) (1a, DPhF = N,N'-diphenylformamidinate) and Ru(2)(D(3,5-Cl(2))PhF)(4)N(3) (1b, D(3,5-Cl(2))PhF = N,N'-bis(3,5-dichlorophenyl)formamidinate) have been investigated by thermolytic and photolytic experiments to investigate the chemical reactivity of the corresponding diruthenium nitride species. Thermolysis of 1b at ~100 °C leads to the expulsion of N(2) and isolation of Ru(2)(D(3,5-Cl(2))PhF)(3)NH(C(13)H(6)N(2)Cl(4)) (3b), in which a nitrogen atom has been inserted into one of the proximal aryl C-H bonds of a D(3,5-Cl(2))PhF ligand. A similar C-H insertion product is obtained upon thawing a frozen CH(2)Cl(2) solution of the nitride complex Ru(2)(DPhF)(4)N (2a), formed via photolysis at -196 °C of 1a to yield Ru(2)(DPhF)(3)NH(C(13)H(10)N(2)) (3a). Evidence is provided here that both reactions proceed via direct intramolecular attack of an electrophilic terminal nitrido nitrogen atom on a proximal aryl ring. Thermodynamic and kinetic data for this reaction are obtained from differential scanning calorimetric measurements and thermal gravimetric analysis of the thermolysis of Ru(2)(D(3,5-Cl(2))PhF)(4)N(3), and by Arrhenius/Eyring analysis of the conversion of Ru(2)(DPhF)(4)N to its C-H insertion product, respectively. These data are used to develop a detailed, experimentally validated DFT reaction pathway for N(2) extrusion and C-H functionalization from Ru(2)(D(3,5-Cl(2))PhF)(4)N(3). The diruthenium nitrido complex is an intermediate in the calculated reaction pathway, and the C-H functionalization event shares a close resemblance to a classical electrophilic aromatic substitution mechanism.

Pub.: 13 Jul '11, Pinned: 29 May '18