Catalytic Tetrazole Synthesis via [3+2] Cycloaddition of NaN₃ to Organonitriles Promoted by Co(II)-complex: Isolation and Characterization of a Co(II)-diazido Intermediate

The [3+2] cycloaddition of sodium azide to nitriles to give 5-substituted 1H-tetrazoles is efficiently catalyzed by a Cobalt(II) complex (1) with a tetradentate ligand N,N-bis(pyridin-2-ylmethyl)quinolin-8-amine. Detailed mechanistic investigation shows the intermediacy of the cobalt(II) diazido complex (2), which has been isolated and structurally characterized. Complex 2 also shows good catalytic activity for the synthesis of 5-substituted 1H-tetrazoles. These are the first examples of cobalt complexes used for the [3+2] cycloaddition reaction for the synthesis of 1H-tetrazoles under homogeneous conditions

N‑Heterocyclic Silyl Pincer Ligands

The reaction of 1,2-C₆H₄(NHCH₂PPh₂)₂ with chlorosilanes Cl₂SiHR (R = Ph, Cl) affords the benzosiladiazoles RHSi­(NCH₂PPh₂)₂C₆H₄ (R = Ph, <b>1</b>; Cl, <b>2</b>). The phenyl derivative <b>1</b> undergoes chelate-assisted Si–H activation with [RuPhCl­(CO)­(PPh₃)₂] and [RhCl­(PPh₃)₃] to afford the structurally characterized silyl pincer complexes [RuCl­(CO)­(PPh₃)­{κ³-<i>P,Si,P</i>′-SiPh­(NCH₂PPh₂)₂C₆H₄}] (<b>3</b>) and [RhHCl­(PPh₃)­{κ³-<i>P,Si,P</i>′-SiPh­(NCH₂PPh₂)₂C₆H₄}] (<b>4</b>). The reaction of <b>4</b> with [Et₂NH₂]­[S₂CNEt₂] affords the complex [RhH­(S₂CNEt₂)­{κ³-<i>P,Si,P</i>′-SiPh­(NCH₂PPh₂)₂C₆H₄}] (<b>5</b>), structural data for which demonstrate a pronounced <i>trans</i> influence for the σ-silyl donor

Understanding C–H Bond Activation on a Diruthenium(I) Platform

Activation of the C–H bond at the axial site of a [Ru^I–Ru^I] platform has been achieved. Room-temperature treatment of 2-(R-phenyl)-1,8-naphthyridine (R = H, F, OMe) with [Ru₂(CO)₄(CH₃CN)₆]­[BF₄]₂ in CH₂Cl₂ affords the corresponding diruthenium­(I) complexes, which carry two ligands, one of which is orthometalated and the second ligand engages an axial site via a Ru···C–H interaction. Reaction with 2-(2-<i>N</i>-methylpyrrolyl)-1,8-naphthyridine under identical conditions affords another orthometalated/nonmetalated (<i>om</i>/<i>nm</i>) complex. At low temperature (4 °C), however, a nonmetalated complex is isolated that reveals axial Ru···C–H interactions involving both ligands at sites <i>trans</i> to the Ru–Ru bond. A nonmetalated (<i>nm</i>/<i>nm</i>) complex was characterized for 2-pyrrolyl-1,8-naphthyridine at room temperature. Orthometalation of both ligands on a single [Ru–Ru] platform could not be accomplished even at elevated temperature. X-ray metrical parameters clearly distinguish between the orthometalated and nonmetalated ligands. NMR investigation reveals the identity of each proton and sheds light on the nature of [Ru–Ru]···C–H interactions (preagostic/agostic). An electrophilic mechanism is proposed for C–H bond cleavage that involves a C­(p_π)–H → σ* [Ru–Ru] interaction, resulting in a Wheland-type intermediate. The heteroatom stabilization is credited to the isolation of nonmetalated complexes for pyrrolyl C–H, whereas lack of such stabilization for phenyl C–H causes rapid proton elimination, giving rise to orthometalation. NPA charge analysis suggests that the first orthometalation makes the [Ru–Ru] core sufficiently electron rich, which does not allow significant interaction with the other axial C–H bond, making the second metalation very difficult

Understanding C–H Bond Activation on a Diruthenium(I) Platform

Activation of the C–H bond at the axial site of a [Ru^I–Ru^I] platform has been achieved. Room-temperature treatment of 2-(R-phenyl)-1,8-naphthyridine (R = H, F, OMe) with [Ru₂(CO)₄(CH₃CN)₆]­[BF₄]₂ in CH₂Cl₂ affords the corresponding diruthenium­(I) complexes, which carry two ligands, one of which is orthometalated and the second ligand engages an axial site via a Ru···C–H interaction. Reaction with 2-(2-<i>N</i>-methylpyrrolyl)-1,8-naphthyridine under identical conditions affords another orthometalated/nonmetalated (<i>om</i>/<i>nm</i>) complex. At low temperature (4 °C), however, a nonmetalated complex is isolated that reveals axial Ru···C–H interactions involving both ligands at sites <i>trans</i> to the Ru–Ru bond. A nonmetalated (<i>nm</i>/<i>nm</i>) complex was characterized for 2-pyrrolyl-1,8-naphthyridine at room temperature. Orthometalation of both ligands on a single [Ru–Ru] platform could not be accomplished even at elevated temperature. X-ray metrical parameters clearly distinguish between the orthometalated and nonmetalated ligands. NMR investigation reveals the identity of each proton and sheds light on the nature of [Ru–Ru]···C–H interactions (preagostic/agostic). An electrophilic mechanism is proposed for C–H bond cleavage that involves a C­(p_π)–H → σ* [Ru–Ru] interaction, resulting in a Wheland-type intermediate. The heteroatom stabilization is credited to the isolation of nonmetalated complexes for pyrrolyl C–H, whereas lack of such stabilization for phenyl C–H causes rapid proton elimination, giving rise to orthometalation. NPA charge analysis suggests that the first orthometalation makes the [Ru–Ru] core sufficiently electron rich, which does not allow significant interaction with the other axial C–H bond, making the second metalation very difficult

Utricularia crenata

The reactions between [Ir­(COD)­(μ-OAc)]₂ and the functionalized imidazolium salt 1-mesityl-3-(pyrid-2-yl)­imidazolium bromide (MesIPy·HBr) or 1-benzyl-3-(5,7-dimethylnaphthyrid-2-yl)­imidazolium bromide (BnIN·HBr) at room temperature afford the COD-activated Ir^III–N-heterocyclic carbene (NHC) complexes [Ir­(1-κ-4,5,6-η-C₈H₁₂)­(κ²<i>C</i>,<i>N</i>-MesIPy)­Br] (<b>1</b>) and [Ir­(1-κ-4,5,6-η-C₈H₁₂)­(κ²<i>C</i>,<i>N</i>-BnIN)­Br] (<b>2</b>), respectively. In contrast, the methoxy analogue [Ir­(COD)­(μ-OMe)]₂ on reaction with MesIPy·HBr under identical conditions affords the Ir^I–NHC complex [Ir­(COD)­(κ²<i>C</i>,<i>N</i>-MesIPy)­Br]. Treatment of [Ir­(COD)­(κ²<i>C</i>,<i>N</i>-MesIPy)­Br] with sodium acetate does not lead to COD activation. Further, use of 2,2′-bipyridine (bpy) with [Ir­(COD)­(μ-X)]₂ (X = MeO or AcO) in the presence of [ⁿBu₄N]­[BF₄] affords exclusively [Ir­(bpy)­(COD)]­[BF₄] (<b>3</b>). Metal-bound acetate is shown to be an essential promoter for activation of the COD allylic C–H bond. An examination of products reveals the following transformations of the precursor components: cleavage of the imidazolium C₂–H and subsequent NHC metalation, metal oxidation from Ir^I to Ir^III, and 2e reduction of COD, effectively resulting in 1-κ-4,5,6-η-C₈H₁₂ coordination to the metal. Mechanistic investigation at the DFT/B3LYP level of theory strongly suggests that NHC metalation precedes COD allylic C–H activation. Two distinct pathways have been examined which involve initial C₂–H oxidative addition to the metal followed by acetate-assisted allylic C–H activation (path A) and the reverse sequence, i.e., deprotonation of C₂–H by the acetate and subsequent allylic C–H oxidative addition to the metal (path B). The result is an Ir^III–NHC–hydride−κ¹,η²-C₈H₁₁ complex. Subsequent intramolecular insertion of the COD double bond into the metal–hydride bond followed by isomerization gives the final product. An acetate-assisted facile COD allylic C–H bond activation, in comparison to oxidative addition of the same to Ir, makes path A the favored pathway. This work thus raises questions about the innocence of COD, especially when metal acetates are used for the synthesis of NHC complexes from the corresponding imidazolium salts

N‑Heterocyclic Silyl Pincer Ligands

The reaction of 1,2-C₆H₄(NHCH₂PPh₂)₂ with chlorosilanes Cl₂SiHR (R = Ph, Cl) affords the benzosiladiazoles RHSi­(NCH₂PPh₂)₂C₆H₄ (R = Ph, <b>1</b>; Cl, <b>2</b>). The phenyl derivative <b>1</b> undergoes chelate-assisted Si–H activation with [RuPhCl­(CO)­(PPh₃)₂] and [RhCl­(PPh₃)₃] to afford the structurally characterized silyl pincer complexes [RuCl­(CO)­(PPh₃)­{κ³-<i>P,Si,P</i>′-SiPh­(NCH₂PPh₂)₂C₆H₄}] (<b>3</b>) and [RhHCl­(PPh₃)­{κ³-<i>P,Si,P</i>′-SiPh­(NCH₂PPh₂)₂C₆H₄}] (<b>4</b>). The reaction of <b>4</b> with [Et₂NH₂]­[S₂CNEt₂] affords the complex [RhH­(S₂CNEt₂)­{κ³-<i>P,Si,P</i>′-SiPh­(NCH₂PPh₂)₂C₆H₄}] (<b>5</b>), structural data for which demonstrate a pronounced <i>trans</i> influence for the σ-silyl donor

Bifunctional Water Activation for Catalytic Hydration of Organonitriles

Treatment of [Rh­(COD)­(μ-Cl)]₂ with excess ^<i>t</i>BuOK and subsequent addition of 2 equiv of PIN·HBr in THF afforded [Rh­(COD)­(κC₂-PIN)­Br] (<b>1</b>) (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)­imidazol-2-ylidene, COD = 1,5-cyclooctadiene). The X-ray structure of <b>1</b> confirms ligand coordination to “Rh­(COD)­Br” through the carbene carbon featuring an unbound naphthyridine. Compound <b>1</b> is shown to be an excellent catalyst for the hydration of a wide variety of organonitriles at ambient temperature, providing the corresponding organoamides. In general, smaller substrates gave higher yields compared with sterically bulky nitriles. A turnover frequency of 20 000 h^–1 was achieved for the acrylonitrile. A similar Rh­(I) catalyst without the naphthyridine appendage turned out to be inactive. DFT studies are undertaken to gain insight on the hydration mechanism. A 1:1 catalyst–water adduct was identified, which indicates that the naphthyridine group steers the catalytically relevant water molecule to the active metal site via double hydrogen-bonding interactions, providing significant entropic advantage to the hydration process. The calculated transition state (TS) reveals multicomponent cooperativity involving proton movement from the water to the naphthyridine nitrogen and a complementary interaction between the hydroxide and the nitrile carbon. Bifunctional water activation and cooperative proton migration are recognized as the key steps in the catalytic cycle

Bifunctional Water Activation for Catalytic Hydration of Organonitriles

Treatment of [Rh­(COD)­(μ-Cl)]₂ with excess ^<i>t</i>BuOK and subsequent addition of 2 equiv of PIN·HBr in THF afforded [Rh­(COD)­(κC₂-PIN)­Br] (<b>1</b>) (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)­imidazol-2-ylidene, COD = 1,5-cyclooctadiene). The X-ray structure of <b>1</b> confirms ligand coordination to “Rh­(COD)­Br” through the carbene carbon featuring an unbound naphthyridine. Compound <b>1</b> is shown to be an excellent catalyst for the hydration of a wide variety of organonitriles at ambient temperature, providing the corresponding organoamides. In general, smaller substrates gave higher yields compared with sterically bulky nitriles. A turnover frequency of 20 000 h^–1 was achieved for the acrylonitrile. A similar Rh­(I) catalyst without the naphthyridine appendage turned out to be inactive. DFT studies are undertaken to gain insight on the hydration mechanism. A 1:1 catalyst–water adduct was identified, which indicates that the naphthyridine group steers the catalytically relevant water molecule to the active metal site via double hydrogen-bonding interactions, providing significant entropic advantage to the hydration process. The calculated transition state (TS) reveals multicomponent cooperativity involving proton movement from the water to the naphthyridine nitrogen and a complementary interaction between the hydroxide and the nitrile carbon. Bifunctional water activation and cooperative proton migration are recognized as the key steps in the catalytic cycle

Relationship between Hydrogen-Atom Transfer Driving Force and Reaction Rates for an Oxomanganese(IV) Adduct

Hydrogen atom transfer (HAT) reactions by high-valent metal-oxo intermediates are important in both biological and synthetic systems. While the HAT reactivity of Fe^IV-oxo adducts has been extensively investigated, studies of analogous Mn^IV-oxo systems are less common. There are several recent reports of Mn^IV-oxo complexes, supported by neutral pentadentate ligands, capable of cleaving strong C–H bonds at rates approaching those of analogous Fe^IV-oxo species. In this study, we provide a thorough analysis of the HAT reactivity of one of these Mn^IV-oxo complexes, [Mn^IV(O)­(2pyN2Q)]²⁺, which is supported by an N5 ligand with equatorial pyridine and quinoline donors. This complex is able to oxidize the strong C–H bonds of cyclohexane with rates exceeding those of Fe^IV-oxo complexes with similar ligands. In the presence of excess oxidant (iodosobenzene), cyclohexane oxidation by [Mn^IV(O)­(2pyN2Q)]²⁺ is catalytic, albeit with modest turnover numbers. Because the rate of cyclohexane oxidation by [Mn^IV(O)­(2pyN2Q)]²⁺ was faster than that predicted by a previously published Bells–Evans–Polanyi correlation, we expanded the scope of this relationship by determining HAT reaction rates for substrates with bond dissociation energies spanning 20 kcal/mol. This extensive analysis showed the expected correlation between reaction rate and the strength of the substrate C–H bond, albeit with a shallow slope. The implications of this result with regard to Mn^IV-oxo and Fe^IV-oxo reactivity are discussed

Volatility and Chain Length Interplay of Primary Amines: Mechanistic Investigation on the Stability and Reversibility of Ammonia-Responsive Hybrid Perovskites

Hybrid organic–inorganic perovskites possess promising signal transduction properties, which can be exploited in a variety of sensing applications. Interestingly, the highly polar nature of these materials, while being a bane in terms of stability, can be a boon for sensitivity when they are exposed to polar gases in a controlled atmosphere. However, signal transduction during sensing induces irreversible changes in the chemical and physical structure, which is one of the major lacuna preventing its utility in commercial applications. In the context of developing alkylammonium lead­(II) iodide perovskite materials for sensing, here we address major issues such as reversibility of structure and properties, correlation between instability and properties of alkylamines, and relation between packing of alkyl chains inside the crystal lattice and the response time toward NH₃ gas. The current investigation highlights that the vapor pressure of alkylamine formed in the presence of NH₃ determines the reversibility and stability of the original perovskite lattice. In addition, close packing of alkyl chains inside the perovskite crystal lattice reduces the response toward NH₃ gas. The mechanistic study addresses three important factors such as quick response, reversibility, and stability of perovskite materials in the presence of NH₃ gas, which could lead to the design of stable and sensitive two-dimensional hybrid perovskite materials for developing sensors