Actinide Complexes Possessing Six-Membered N‑Heterocyclic Iminato Moieties: Synthesis and Reactivity

A novel class of ligand systems possessing a six-membered N-heterocyclic iminato [perimidin-2-iminato (Pr^RN, where R = isopropyl, cycloheptyl)] moiety is introduced. The complexation of these ligands with early actinides (An = Th and U) results in powerful catalysts [(Pr^RN)­An­(N­{SiMe₃)₂}₃] (<b>3</b>–<b>6</b>) for exigent insertion of alcohols into carbodiimides to produce the corresponding isoureas in short reaction times with excellent yields. Experimental, thermodynamic, and kinetic data as well as the results of stoichiometric reactions provide cumulative evidence that supports a plausible mechanism for the reaction

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

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

Olefin Oxygenation by Water on an Iridium Center

Oxygenation of 1,5-cyclooctadiene (COD) is achieved on an iridium center using water as a reagent. A hydrogen-bonding interaction with an unbound nitrogen atom of the naphthyridine-based ligand architecture promotes nucleophilic attack of water to the metal-bound COD. Irida-oxetane and oxo-irida-allyl compounds are isolated, products which are normally accessed from reactions with H₂O₂ or O₂. DFT studies support a ligand-assisted water activation mechanism

Olefin Oxygenation by Water on an Iridium Center

Oxygenation of 1,5-cyclooctadiene (COD) is achieved on an iridium center using water as a reagent. A hydrogen-bonding interaction with an unbound nitrogen atom of the naphthyridine-based ligand architecture promotes nucleophilic attack of water to the metal-bound COD. Irida-oxetane and oxo-irida-allyl compounds are isolated, products which are normally accessed from reactions with H₂O₂ or O₂. DFT studies support a ligand-assisted water activation mechanism

Reactions of Acids with Naphthyridine-Functionalized Ferrocenes: Protonation and Metal Extrusion

Reaction of 1,8-naphthyrid-2-yl-ferrocene (FcNP) with a variety of acids affords protonated salts at first, whereas longer reaction time leads to partial demetalation of FcNP resulting in a series of Fe complexes. The corresponding salts [FcNP·H]­[X] (X = BF₄ or CF₃SO₃ (<b>1</b>)) are isolated for HBF₄ and CF₃SO₃H. Reaction of FcNP with equimolar amount of CF₃CO₂H for 12 h affords a neutral complex [Fe­(FcNP)₂(O₂CCF₃)₂(OH₂)₂] (<b>2</b>). Use of excess acid gave a trinuclear Fe^II complex [Fe₃(H₂O)₂(O₂CCF₃)₈(FcNP·H)₂] (<b>3</b>). Three linear iron atoms are held together by four bridging trifluoroacetates and two aqua ligands in a symmetric fashion. Reaction with ethereal solution of HCl afforded [(FcNP·H)₃(Cl)]­[FeCl₄]₂ (<b>4</b>) irrespective of the amount of the acid used. Even the picric acid (HPic) led to metal extrusion giving rise to [Fe₂(Cl)₂(FcNP)₂(Pic)₂] (<b>5</b>) when crystallized from dichloromethane. Metal extrusion was also observed for CF₃SO₃H, but an analytically pure compound could not be isolated. The demetalation reaction proceeds with an initial proton attack to the distal nitrogen of the NP unit. Subsequently, coordination of the conjugate base to the electrophilic Fe facilitates the release of Cp rings from metal. The conjugate base plays an important role in the demetalation process and favors the isolation of the Fe complex as well. The 1,1′-bis­(1,8-naphthyrid-2-yl)­ferrocene (FcNP₂) does not undergo demetalation under identical conditions. Two NP units share one positive charge causing the Fe-Cp bonds weakened to an extent that is not sufficient for demetalation. X-ray structure of the monoprotonated FcNP₂ reveals a discrete dimer [(FcNP₂·H)]₂[OTf]₂ (<b>6</b>) supported by two N–H<b>···</b>N hydrogen bonds. Crystal packing and dispersive forces associated with intra- and intermolecular π–π stacking interactions (NP···NP and Cp···NP) allow the formation of the dimer in the solid-state. The protonation and demetalation reactions of FcNP and FcNP₂ with a variety of acids are reported