Osmium- and Iridium-Promoted C–H Bond Activation of 2,2′-Bipyridines and Related Heterocycles: Kinetic and Thermodynamic Preferences

Photolysis-driven bond activation by thorium and uranium tetraosmate polyhydride complexes

Transition metal multimetallic complexes have seen intense study due to their unique bonding and potential for cooperative reactivity, but actinide-transition metal (An-TM) species are far less understood. We have synthesized uranium- and thorium-osmium heterometallic polyhydride complexes in order to study An-Os bonding and investigate the reactivity of An-Os interactions. Computational studies suggest the presence of a significant bonding interaction between the actinide center and the four coordinated osmium centers supported by bridging hydrides. Upon photolysis, these complexes undergo intramolecular C-H activation with the formation of an Os-Os bond, while the thorium complex may activate an additional C-H bond of the benzene solvent, resulting in a μ-η1,η1 phenyl ligand across one Th-Os interaction.

C-H, N-H, and O-H Bond Activations to Prepare Phosphorescent Hydride-Iridium(III)-Phosphine Emitters with Photocatalytic Achievement in C-C Coupling Reactions

Complex IrH5(PiPr3)2 (1) activates two different σ-bonds of 3-phenoxy-1-phenylisoquinoline, 2-(1H-benzimidazol-2-yl)-6-phenylpyridine, 2-(1H-indol-2-yl)-6-phenylpyridine, 2-(2-hydroxyphenyl)-6-phenylpyridine, N-(2-hydroxyphenyl)-N'-phenylimidazolylidene, and 1,3-di(2-pyridyl)-4,6-dimethylbenzene to give IrH{κ3-C,N,C-[C6H4-isoqui-O-C6H4]}(PiPr3)2 (2), IrH{κ3-N,N,C-[NBzim-py-C6H4]}(PiPr3)2 (3), IrH{κ3-N,N,C-[Ind-py-C6H4]}(PiPr3)2 (4), IrH{κ3-C,N,O-[C6H4-py-C6H4O]}(PiPr3)2 (5), IrH{κ3-C,C,O-[C6H4-Im-C6H4O]}(PiPr3)2 (6), and IrH{κ3-N,C,C-[py-C6HMe2-C5H3N]}(PiPr3)2 (7), respectively. The activations are sequential, with the second generally being the slowest. Accordingly, dihydride intermediates IrH2{κ2-C,N-[C6H4-isoqui-O-C6H5]}(PiPr3)2 (2d), IrH2{κ2-N,N-[NBzim-py-C6H5]}(PiPr3)2 (3d), IrH2{κ2-N,N-[Ind-py-C6H5]}(PiPr3)2 (4d), and IrH2{κ2-N,C-[py-C6HMe2-py]}(PiPr3)2 (7d) were characterized spectroscopically. Complexes 3 and 5 are green phosphorescent emitters upon photoexcitation, exhibiting good absorption over a wide range of wavelengths, emission quantum yields about 0.70 in solution, long enough lifetimes (10-17 μs), and reversible electrochemical behavior. In agreement with these features, complex 3 promotes the photocatalytic α-amino C(sp3)-H arylation of N,N-dimethylaniline and N-phenylpiperidine with 1,4-dicyanobenzene and 4-cyanopyridine under blue LED light irradiation. The C-C coupling products are isolated in high yields with only 2 mol % of photocatalyst after 24 h.

CH bond activation in aromatic ketones mediated by iridium-tris(pyrazolyl)borate complexes

Reaction of complex [TpMe2Ir(η4-CH2C(Me)C(Me)C2)] (1) with a series of aromatic ketones at 130 °C renders, by means of a selective ortho-CH activation, Ir(III)-metallacycles 2-5, which display an Ir-H bond. When [TpMe2Ir(C6H5)2N2] (6) is treated with 2-(trifluoromethyl)acetophenone and 2-fluoroacetophenone at 80 °C, the formation of dimeric (7) and trimeric architectures (8) is achieved through the meta- and para-CH activation of the aromatic ketone, respectively. The generation of complexes 2-5 is proposed to occur by the initial formation of Ir(III) η1-ketone adducts as key intermediates, followed by aromatic CH activations and the release of a butadiene ligand. The formation of complexes 7 and 8 involves an assisted process in which a metal center activation of the less sterically hindered C-H bond of the aromatic ketone takes place (releasing a benzene molecule), followed by the coordination of the carbonyl group, which generates the respective dimeric and trimeric structures. Complexes 7 and 8 are efficient catalysts for the transfer hydrogenation of ketones and aldehydes using isopropanol as the hydrogen source. All complexes have been fully characterized by NMR spectroscopy, FT-IR, elemental analysis and, in the cases of 7 and 8, X-ray crystallography. Details of the reaction conditions, isolation of the products, and proposals for the pathways of formation of complexes 2-5 and 7-8 are discussed.

Reactions of an Osmium-Hexahydride Complex with 2-Butyne and 3-Hexyne and Their Performance in the Migratory Hydroboration of Aliphatic Internal Alkynes

Reactions of the hexahydride OsH₆(PⁱPr₃)₂ (1) with 2-butyne and 3-hexyne and the behavior of the resulting species toward pinacolborane (pinBH) have been investigated in the search for new hydroboration processes. Complex 1 reacts with 2-butyne to give 1-butene and the osmacyclopropene OsH₂(η²-C₂Me₂)(PⁱPr₃)₂ (2). In toluene, at 80 °C, the coordinated hydrocarbon isomerizes into a η⁴-butenediyl form to afford OsH₂(η⁴-CH₂CHCHCH₂)(PⁱPr₃)₂ (3). Isotopic labeling experiments indicate that the isomerization involves Me-to-C_Os hydrogen 1,2-shifts, which take place through the metal. The reaction of 1 with 3-hexyne gives 1-hexene and OsH₂(η²-C₂Et₂)(PⁱPr₃)₂ (4). Similarly to 2, complex 4 evolves to η⁴-butenediyl derivatives OsH₂(η⁴-CH₂CHCHCHEt)(PⁱPr₃)₂ (5) and OsH₂(η⁴-MeCHCHCHCHMe)(PⁱPr₃)₂ (6). In the presence of pinBH, complex 2 generates 2-pinacolboryl-1-butene and OsH{κ²-H,H-(H₂Bpin)}(η²-HBpin)(PⁱPr₃)₂ (7). According to the formation of the borylated olefin, complex 2 is a catalyst precursor for the migratory hydroboration of 2-butyne and 3-hexyne to 2-pinacolboryl-1-butene and 4-pinacolboryl-1-hexene. During the hydroboration, complex 7 is the main osmium species. The hexahydride 1 also acts as a catalyst precursor, but it requires an induction period that causes the loss of 2 equiv of alkyne per equiv of osmium.

Metathesis between E-C(spn ) and H-C(sp3 ) σ-Bonds (E=Si, Ge; n=2, 3) on an Osmium-Polyhydride

The silylation of a phosphine of OsH₆(PⁱPr₃)₂ is performed via net‐metathesis between Si−C(spⁿ) and H−C(sp³) σ‐bonds (n=2, 3). Complex OsH₆(PⁱPr₃)₂ activates the Si−H bond of Et₃SiH and Ph₃SiH to give OsH₅(SiR₃)(PⁱPr₃)₂, which yield OsH₄{κ¹‐P,η²‐SiH‐[ⁱPr₂PCH(Me)CH₂SiR₂H]}(PⁱPr₃) and R−H (R=Et, Ph), by displacement of a silyl substituent with a methyl group of a phosphine. Such displacement is a first‐order process, with activation entropy consistent with a rate determining step occurring via a highly ordered transition state. It displays selectivity, releasing the hydrocarbon resulting from the rupture of the weakest Si‐substituent bond, when the silyl ligand bears different substituents. Accordingly, reactions of OsH₆(PⁱPr₃)₂ with dimethylphenylsilane, and 1,1,1,3,5,5,5‐heptamethyltrisiloxane afford OsH₅(SiR₂R′)(PⁱPr₃)₂, which evolve into OsH₄{κ¹‐P,η²‐GeH‐[ⁱPr₂PCH(Me)CH₂SiR₂H]}(PⁱPr₃) (R=Me, OSiMe₃) and R′−H (R′=Ph, Me). Exchange reaction is extended to Et₃GeH. The latter reacts with OsH₆(PⁱPr₃)₂ to give OsH₅(GeEt₃)(PⁱPr₃)₂, which loses ethane to form OsH₄{κ¹‐P,η²‐GeH‐[ⁱPr₂PCH(Me)CH₂GeEt₂H]}(PⁱPr₃).

Homogeneous catalysis with polyhydride complexes

This review analyzes the role of transition metal polyhydrides as homogeneous catalysts for organic reactions. Discussed reactions involve nearly every main organic functional group.

Azolium Control of the Osmium-Promoted Aromatic C-H Bond Activation in 1,3-Disubstituted Substrates

The hexahydride complex OsH₆(PⁱPr₃)₂ promotes the C-H bond activation of the 1,3-disubstituted phenyl group of the [BF₄]^- and [BPh₄]^- salts of the cations 1-(3-(isoquinolin-1-yl)phenyl)-3-methylimidazolium and 1-(3-(isoquinolin-1-yl)phenyl)-3-methylbenzimidazolium. The reactions selectively afford neutral and cationic trihydride-osmium(IV) derivatives bearing κ²-C,N- or κ²-C,C-chelating ligands, a cationic dihydride-osmium(IV) complex stabilized by a κ³-C,C,N-pincer group, and a bimetallic hexahydride formed by two trihydride-osmium(IV) fragments. The metal centers of the hexahydride are separated by a bridging ligand, composed of κ²-C,N- and κ²-C,C-chelating moieties, which allows electronic communication between the metal centers. The wide variety of obtained compounds and the high selectivity observed in their formation is a consequence of the main role of the azolium group during the activation and of the existence of significant differences in behavior between the azolium groups. The azolium role is governed by the anion of the salt, whereas the azolium behavior depends upon its imidazolium or benzimidazolium nature. While [BF₄]^- inhibits the azolium reactions, [BPh₄]^- favors the azolium participation in the activation process. In contrast to benzimidazolylidene, the imidazolylidene resulting from the deprotonation of the imidazolium substituent coordinates in an abnormal fashion to direct the phenyl C-H bond activation to the 2-position. The hydride ligands of the cationic dihydride-osmium(IV) pincer complex display intense quantum mechanical exchange coupling. Furthermore, this salt is a red phosphorescent emitter upon photoexcitation and displays a noticeable catalytic activity for the dehydrogenation of 1-phenylethanol to acetophenone and of 1,2-phenylenedimethanol to 1-isobenzofuranone. The bimetallic hexahydride shows catalytic synergism between the metals, in the dehydrogenation of 1,2,3,4-tetrahydroisoquinoline and alcohols.

Alternative Conceptual Approach to the Design of Bifunctional Catalysts: An Osmium Germylene System for the Dehydrogenation of Formic Acid

The reaction of the hexahydride OsH₆(PⁱPr₃)₂ with a P,Ge,P-germylene-diphosphine affords an osmium tetrahydride derivative bearing a Ge,P-chelate, which arises from the hydrogenolysis of a P–C(sp³) bond. This Os(IV)–Ge(II) compound is a pioneering example of a bifunctional catalyst based on the coordination of a σ-donor acid, which is active in the dehydrogenation of formic acid to H₂ and CO₂. The kinetics of the dehydrogenation, the characterization of the resting state of the catalysis, and DFT calculations point out that the hydrogen formation (the fast stage) exclusively occurs on the coordination sphere of the basic metal center, whereas both the metal center and the σ-donor Lewis acid cooperatively participate in the CO₂ release (the rate-determining step). During the process, the formate group pivots around the germanium to approach its hydrogen atom to the osmium center, which allows its transfer to the metal and the CO₂ release.

An alternative class of bifunctional catalysts can be assembled by coordination of σ-donor Lewis acids to platinum-group-metal basic fragments. In contrast to what happens with the previously reported bifunctional catalysts, this design allows enhancing the basicity of the base and the acidity of the acid. According to this, a bifunctional catalyst for the dehydrogenation of formic acid, based on an osmium(IV)-germylene cooperative system, has been prepared and the mechanism of the catalysis established.

Synthesis, structure and reactivity of iridium complexes containing a bis-cyclometalated tridentate C^N^C ligand

In an effort to synthesize cyclometalated iridium complexes containing a tridentate C^N^C ligand, transmetallation of [Hg(HC^N^C)Cl] (1) (H2C^N^C = 2,6-bis(4-tert-butylphenyl)pyridine) with various organoiridium starting materials has been studied. The treatment of 1 with [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene) in acetonitrile at room temperature afforded a hexanuclear Ir4Hg2 complex, [Cl(κ2C,N-HC^N^C)(cod)IrHgIr(cod)Cl2]2 (2), which features Ir-Hg-Ir and Ir-Cl-Ir bridges. Refluxing 2 with sodium acetate in tetrahydrofuran (thf) resulted in cyclometalation of the bidentate HC^N^C ligand and formation of trinuclear [(C^N^C)(cod)IrHgIr(cod)Cl2] (3). On the other hand, refluxing [Ir(cod)Cl]2 with 1 and sodium acetate in thf yielded [Ir(C^N^C)(cod)(HgCl)] (4). Chlorination of 4 with PhICl2 gave [Ir(C^N^C)(cod)Cl]·HgCl2 (5·HgCl2) that reacted with tricyclohexylphosphine to yield Hg-free [Ir(C^N^C)(cod)Cl] (5). Chloride abstraction of 5 with silver(i) triflate (AgOTf) gave [Ir(C^N^C)(cod)(H2O)](OTf) (6) that can catalyze the cyclopropanation of styrene with ethyl diazoacetate. Reaction of 1 and [Ir(CO)2Cl(py)] (py = pyridine) with sodium acetate in refluxing thf afforded [Ir(C^N^C)(HgCl)(py)(CO)] (7), in which the carbonyl ligand is coplanar with the C^N^C ligand. On the other hand, refluxing 1 with (PPh4)[Ir(CO)2Cl2] and sodium acetate in acetonitrile gave [Ir(C^N^C)(κ2C,N-HC^N^C)(CO)] (8), the carbonyl ligand of which is trans to the pyridyl ring of the bidentate HC^N^C ligand. Upon irradiation with UV light 8 in thf was isomerized to 8', in which the carbonyl is trans to a phenyl group of the bidentate HC^N^C ligand. The isomer pair 8 and 8' exhibited emission at 548 and 514 nm in EtOH/MeOH at 77 K with lifetime of 84.0 and 64.6 μs, respectively. Protonation of 8 with p-toluenesulfonic acid (TsOH) afforded the bis(bidentate) tosylate complex [Ir(κ2C,N-HC^N^C)2(CO)(OTs)] (9) that could be reconverted to 8 upon treatment with sodium acetate. The electrochemistry of the Ir(C^N^C) complexes has been studied using cyclic voltammetry. Reaction of [Ir(PPh3)3Cl] with 1 and sodium acetate in refluxing thf led to isolation of the previously reported compound [Ir(κ2P,C-C6H4PPh2)2(PPh3)Cl] (10). The crystal structures of 2-5, 8, 8', 9 and 10 have been determined.

Hydration of Aliphatic Nitriles Catalyzed by an Osmium Polyhydride: Evidence for an Alternative Mechanism

The hexahydride OsH₆(PⁱPr₃)₂ competently catalyzes the hydration of aliphatic nitriles to amides. The main metal species under the catalytic conditions are the trihydride osmium(IV) amidate derivatives OsH₃{κ²-N,O-[HNC(O)R]}(PⁱPr₃)₂, which have been isolated and fully characterized for R = ⁱPr and ^tBu. The rate of hydration is proportional to the concentrations of the catalyst precursor, nitrile, and water. When these experimental findings and density functional theory calculations are combined, the mechanism of catalysis has been established. Complexes OsH₃{κ²-N,O-[HNC(O)R]}(PⁱPr₃)₂ dissociate the carbonyl group of the chelate to afford κ¹-N-amidate derivatives, which coordinate the nitrile. The subsequent attack of an external water molecule to both the C(sp) atom of the nitrile and the N atom of the amidate affords the amide and regenerates the κ¹-N-amidate catalysts. The attack is concerted and takes place through a cyclic six-membered transition state, which involves C_nitrile···O–H···N_amidate interactions. Before the attack, the free carbonyl group of the κ¹-N-amidate ligand fixes the water molecule in the vicinity of the C(sp) atom of the nitrile.

The hexahydride complex OsH₆(PⁱPr₃)₂ competently catalyzes the hydration of aliphatic nitriles to amides. Isolation of the main metal species under the catalytic conditions, kinetics of hydration, and density functional theory calculations support an alternative mechanism to those previously reported.

Assembly of a Dihydrideborate and Two Aryl Nitriles to Form a C,N,N′-Pincer Ligand Coordinated to Osmium

The C,N,N′-donor aryl-diimineborate pincer ligand of the complexes OsH₂{κ³-C,N,N-[C₆H₃RCH=NB(cat)N=CHC₆H₄R]}(PⁱPr₃)₂ (R = H, Me) has been generated in a one-pot procedure, by the reaction of the hexahydride OsH₆(PⁱPr₃)₂ with catecholborane (catBH) and two molecules of the corresponding aryl nitrile. The osmium–pincer bonding situation has been analyzed by means of atoms in molecules (AIM), natural bond orbital (NBO), and energy decomposition analysis coupled with the natural orbitals for chemical valence (EDA-NOCV) methods. According to the results, the complexes exhibit a rather strong electron-sharing Os–C bond, two weaker donor–acceptor N–Os bonds, and two π-back-donations from the transition metal to vacant π* orbitals of the formed metallacycles. In addition, spectroscopic findings and DFT calculations reveal that the donor units of the pincer are incorporated in a sequential manner. First, the central Os–N bond is formed, by the reaction of the dihydrideborate ligand of the intermediate OsH₃{κ²-H,H-(H₂Bcat)}(PⁱPr₃)₂ with one of the aryl nitriles. The subsequent oxidative addition of the o-C–H bond of the aryl substituent of the resulting κ¹-N-(N-boryl-arylaldimine) affords the Os–C bond. Finally, the second Os–N bond is generated from a hydride, an ortho-metalated N-boryl-arylaldimine, and the second aryl nitrile.

Electronic Communication in Binuclear Osmium- and Iridium-Polyhydrides

Reactions of polyhydrides OsH₆(PⁱPr₃)₂ (1) and IrH₅(PⁱPr₃)₂ (2) with rollover cyclometalated hydride complexes have been investigated in order to explore the influence of a metal center on the MH_n unit of the other in mixed valence binuclear polyhydrides. Hexahydride 1 activates an ortho-CH bond of the heterocyclic moiety of the trihydride metal–ligand compounds OsH₃{κ²-C,N-[C₅RH₂N-py]}(PⁱPr₃)₂ (R = H (3), Me (4), Ph (5)). Reactions of 3 and 4 lead to the hexahydrides (PⁱPr₃)₂H₃Os{μ-[κ²-C,N-[C₅RH₂N-C₅H₃N]-N,C-κ²]}OsH₃(PⁱPr₃)₂ (R = H (6), Me (7)), whereas 5 gives the pentahydride (PⁱPr₃)₂H₃Os{μ-[κ²-C,N-[C₅H₃N-C₅(C₆H₄)H₂N]-C,N,C-κ³]}OsH₂(PⁱPr₃)₂ (8). Pentahydride 2 promotes C—H bond activation of 3 and the iridium-dihydride IrH₂{κ²-C,N-[C₅H₃N-py]}(PⁱPr₃)₂ (9) to afford the heterobinuclear pentahydride (PⁱPr₃)₂H₃Os{μ-[κ²-C,N-[C₅H₃N-C₅H₃N]-N,C-κ²]}IrH₂(PⁱPr₃)₂ (10) and the homobinuclear tetrahydride (PⁱPr₃)₂H₂Ir{μ-[κ²-C,N-[C₅H₃N-C₅H₃N]-N,C-κ²]}IrH₂(PⁱPr₃)₂ (11), respectively. Complexes 6–8 and 11 display HOMO delocalization throughout the metal–heterocycle-metal skeleton. Their sequential oxidation generates mono- and diradicals, which exhibit intervalence charge transfer transitions. This notable ability allows the tuning of the strength of the hydrogen–hydrogen and metal–hydrogen interactions within the MH_n units.

The influence of a metal center on the MH_n unit of the other in binuclear osmium- and iridium-polyhydride complexes bearing rollover cyclometalated bipyridines as a bridging ligand has been studied.

Rollover Cyclometalation as a Valuable Tool for Regioselective C-H Bond Activation and Functionalization

Rollover cyclometalation constitutes a particular case of cyclometallation reaction. This reaction occurs when a chelated heterocyclic ligand loses its bidentate coordination mode and undergoes an internal rotation, after which a remote C-H bond is regioselectively activated, affording an uncommon cyclometalated complex, called "rollover cyclometalated complex". The key of the process is the internal rotation of the ligand, which occurs before the C-H bond activation and releases from coordination a donor atom. The new "rollover" ligand has peculiar properties, being a ligand with multiple personalities, no more a spectator in the reactivity of the complex. The main reason of this peculiarity is the presence of an uncoordinated donor atom (the one initially involved in the chelation), able to promote a series of reactions not available for classic cyclometalated complexes. The rollover reaction is highly regioselective, because the activated C-H bond is usually in a symmetric position with respect to the donor atom which detaches from the metal stating the rollover process. Due to this novel behavior, a series of potential applications have appeared in the literature, in fields such as catalysis, organic synthesis, and advanced materials.