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

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

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