Fluorinated Anions Promoted “on Water” Activity of Di- and Tetranuclear Copper(I) Catalysts for Functional Triazole Synthesis

A set of di- and tetra-copper­(I) compounds [Cu₂(L¹H)₂]­[BAr^F]₂ (<b>1</b>) (L¹H = bis­(5,7-dimethyl-1,8-naphthyridin-2-yl)­amine; BAr^F = [B­{C₆H₃(CF₃)₂}₄]) and [Cu₄(L¹)₂(L²)₂]­[BNB^F]₂ (<b>2</b>) (L² = 5,7-dimethyl-1,8-naphthyridin-2-amine; BNB^F = [NH₂{B­(C₆F₅)₃}₂]), stabilized by naphthyridine-based ligands and containing fluorinated anions, is synthesized. Their catalytic utility for copper­(I)-catalyzed azide–alkyne coupling (CuAAC) reactions in organic solvents and “on water” is evaluated. The dimer analogue [Cu₂(L¹H)₂]­[BPh₄]₂ (<b>3</b>) with nonfluorinated anion is synthesized for the purpose of comparison. All three compounds show CuAAC activity in organic solvents, although the performance of <b>3</b> is considerably lower. Remarkable rate enhancement is displayed by compounds containing fluorinated anions (<b>1</b> and <b>2</b>) under “on water” conditions for the model reaction involving benzyl azide, affording 98% conversions in 15–20 min, where compound <b>3</b> gives 76% conversions in 50 min. Kinetic experiments reveal the involvement of two coppers in the cycloaddition process. Employing a host of substrates, the usefulness of fluorinated anions to dramatically improve the catalytic activity of Cu­(I) compounds under “on water” conditions is demonstrated

Oxidative Route to Abnormal NHC Compounds from Singly Bonded [M–M] (M = Ru, Rh, Pd) Precursors

A new base-free entry to metal–<i>a</i>NHC compounds from metal–metal bonded bimetallic precursors and imidazolium salts is reported. Regioselective metalation proceeds via C–I oxidative addition of an annulated imidazo­[1,2-<i>a</i>]­[1,8]­naphthyridine system to [Ru^I–Ru^I], [Rh^II–Rh^II], and [Pd^I–Pd^I] single bonds, affording C⁵-bound (abnormal) Ru^II–, Rh^III–, and Pd^II–NHC compounds, respectively, at room temperature and in high yields

Cyclometalations on the Imidazo[1,2‑<i>a</i>][1,8]naphthyridine Framework

Cyclometalation on the substituted imidazo­[1,2-<i>a</i>]­[1,8]­naphthyridine platform involves either the C₃-aryl or C₄′-aryl <i>ortho</i> carbon and the imidazo nitrogen N₃′. The higher donor strength of the imidazo nitrogen in comparison to that of the naphthyridine nitrogen aids regioselective orthometalation at the C₃/C₄′-aryl ring with Cp*Ir^III (Cp* = η⁵-pentamethylcyclopentadienyl). A longer reaction time led to double cyclometalations at C₃-aryl and imidazo C₅′-H, creating six- and five-membered metallacycles on a single skeleton. Mixed-metal Ir/Sn compounds are accessed by insertion of SnCl₂ into the Ir–Cl bond. Pd­(OAc)₂ afforded an acetate-bridged dinuclear ortho-metalated product involving the C₃-aryl unit. Metalation at the imidazo carbon (C₅′) was achieved via an oxidative route in the reaction of the bromo derivative with the Pd(0) precursor Pd₂(dba)₃ (dba = dibenzylideneacetone). Regioselective C–H/Br activation on a rigid and planar imidazonaphthyridine platform is described in this work

Cyclometalations on the Imidazo[1,2‑<i>a</i>][1,8]naphthyridine Framework

Cyclometalation on the substituted imidazo­[1,2-<i>a</i>]­[1,8]­naphthyridine platform involves either the C₃-aryl or C₄′-aryl <i>ortho</i> carbon and the imidazo nitrogen N₃′. The higher donor strength of the imidazo nitrogen in comparison to that of the naphthyridine nitrogen aids regioselective orthometalation at the C₃/C₄′-aryl ring with Cp*Ir^III (Cp* = η⁵-pentamethylcyclopentadienyl). A longer reaction time led to double cyclometalations at C₃-aryl and imidazo C₅′-H, creating six- and five-membered metallacycles on a single skeleton. Mixed-metal Ir/Sn compounds are accessed by insertion of SnCl₂ into the Ir–Cl bond. Pd­(OAc)₂ afforded an acetate-bridged dinuclear ortho-metalated product involving the C₃-aryl unit. Metalation at the imidazo carbon (C₅′) was achieved via an oxidative route in the reaction of the bromo derivative with the Pd(0) precursor Pd₂(dba)₃ (dba = dibenzylideneacetone). Regioselective C–H/Br activation on a rigid and planar imidazonaphthyridine platform is described in this work

Carbon Monoxide Induced Double Cyclometalation at the Iridium Center

Bubbling of CO into a dichloromethane solution of [Ir­(COD)­(CH₃CN)₂]­[BF₄] followed by the addition of 2-phenyl-1,8-naphthyridine (LH) at room temperature results in the bis-cyclometalated Ir^III complex [Ir­(C^∧N)₂(CO)­(LH)]­[BF₄] (C^∧N = L). The observed cyclometalation contradicts the classical role of CO, which is to hinder oxidative addition by lowering electron density on the metal. DFT calculations reveal that the first cyclometalation involves oxidative addition of the ligand. Subsequently, preferential electrophilic activation of the second ligand followed by elimination of dihydrogen affords the bis-cyclometalated Ir^III complex

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

Carbon Monoxide Induced Double Cyclometalation at the Iridium Center

Bubbling of CO into a dichloromethane solution of [Ir­(COD)­(CH₃CN)₂]­[BF₄] followed by the addition of 2-phenyl-1,8-naphthyridine (LH) at room temperature results in the bis-cyclometalated Ir^III complex [Ir­(C^∧N)₂(CO)­(LH)]­[BF₄] (C^∧N = L). The observed cyclometalation contradicts the classical role of CO, which is to hinder oxidative addition by lowering electron density on the metal. DFT calculations reveal that the first cyclometalation involves oxidative addition of the ligand. Subsequently, preferential electrophilic activation of the second ligand followed by elimination of dihydrogen affords the bis-cyclometalated Ir^III complex

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

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