Two Synthetic Tools to Deepen the Understanding of the Influence of Stereochemistry on the Properties of Iridium(III) Heteroleptic Emitters.

Two complementary procedures are presented to prepare cis-pyridyl-iridium(III) emitters of the class [3b+3b+3b'] with two orthometalated ligands of the 2-phenylpyridine type (3b) and a third ligand (3b'). They allowed to obtain four emitters of this class and to compare their properties with those of the trans-pyridyl isomers. The finding starts from IrH5(PiPr3)2, which reacts with 2-(p-tolyl)pyridine to give fac-[Ir{κ2-C,N-[C6MeH3-py]}3] with an almost quantitative yield. Stirring the latter in the appropriate amount of a saturated solution of HCl in toluene results in the cis-pyridyl adduct IrCl{κ2-C,N-[C6MeH3-py]}2{κ1-Cl-[Cl-H-py-C6MeH4]} stabilized with p-tolylpyridinium chloride, which can also be transformed into dimer cis-[Ir(µ-OH){κ2-C,N-[C6MeH3-py]}2]2. Adduct IrCl{κ2-C,N-[C6MeH3-py]}2{κ1-Cl-[Cl-H-py-C6MeH4]} directly generates cis-[Ir{κ2-C,N-[C6MeH3-py]}2{κ2-C,N-[C6H4-Isoqui]}] and cis-[Ir{κ2-C,N-[C6MeH3-py]}2{κ2-C,N-[C6H4-py]}] by transmetalation from Li[2-(isoquinolin-1-yl)-C6H4] and Li[py-2-C6H4]. Dimer cis-[Ir(µ-OH){κ2-C,N-[C6MeH3-py]}2]2 is also a useful starting complex when the precursor molecule of 3b' has a fairly acidic hydrogen atom, suitable for removal by hydroxide groups. Thus, its reactions with 2-picolinic acid and acetylacetone (Hacac) lead to cis-Ir{κ2-C,N-[C6MeH3-py]}2{κ2-O,N-[OC(O)-py]} and cis-Ir{κ2-C,N-[C6MeH3-py]}2{κ2-O,O-[acac]}. The stereochemistry of the emitter does not significantly influence the emission wavelengths. On the contrary, its efficiency is highly dependent on and associated with the stability of the isomer. The more stable isomer shows a higher quantum yield and color purity.

Homogeneous catalysis with polyhydride complexes.

Roles of the hydrogen atoms attached to the metal center of transition metal polyhydride complexes, LnMHx (x ≥ 3), are analyzed for about forty types of organic reactions catalyzed by such class of species. Reactions involve nearly every main organic functional group and represent friendly environmental procedures of synthesis of relevant and necessary molecules in several areas ranging from energy and environment to medicine or pharmacology. Catalysts are mainly complexes of group 8 metals, along with rhenium and iridium, and manganese and cobalt to a lesser extent. Their MHx units can be formed by Kubas-type dihydrogen, elongated dihydrogen, or hydride ligands, which facilitate both the homolytic and heterolytic σ-bond activation reactions and hydrogen transfer processes from the metal center to unsaturated organic molecules. As a consequence of the ability of polyhydride complexes to activate σ-bonds, the vast majority of the reactions catalyzed by derivatives of this class involve at least one σ-bond activation elemental step, whereas two sequential ruptures of σ-bonds and the cross-coupling of the resulting fragments take place in a variety of reactions of C-H functionalization and hydrodefluorination. The hydrogen transfer processes usually generate highly unsaturated metal fragments, which are very reactive and extremely active in interesting C-C coupling reactions. Polyhydride complexes bearing Kubas-type dihydrogen ligands are the last intermediates in dehydrogenation processes, while they can be the first ones in hydrogenation reactions. Polyhydrides coordinating elongated dihydrogen ligands are acidic, while classical hydride complexes behave as Brønsted bases. The combination of the properties of both types of species in a catalytic cycle gives rise to interesting outer-sphere processes. The basic character of the classical hydride ligands also confers them the ability of cooperating in the coordination of acidic molecules such as boranes, which is of great relevance for reactions involving the activation of a B-H bond. Multiple bonds of unsaturated organic molecules also undergo insertion into the M-H bond of the catalysts. Such insertions are a key step in many processes.

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-COs 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.

Silyl-Osmium(IV)-Trihydride Complexes Stabilized by a Pincer Ether-Diphosphine: Formation and Reactions with Alkynes.

Complex OsH4{κ3-P,O,P-[xant(PiPr2)2]} (1) activates the Si-H bond of triethylsilane, triphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane to give the silyl-osmium(IV)-trihydride derivatives OsH3(SiR3){κ3-P,O,P-[xant(PiPr2)2]} [SiR3 = SiEt3 (2), SiPh3 (3), SiMe(OSiMe3)2 (4)] and H2. The activation takes place via an unsaturated tetrahydride intermediate, resulting from the dissociation of the oxygen atom of the pincer ligand 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(PiPr2)2). This intermediate, which has been trapped to form OsH4{κ2-P,P-[xant(PiPr2)2]}(PiPr3) (5), coordinates the Si-H bond of the silanes to subsequently undergo a homolytic cleavage. Kinetics of the reaction along with the observed primary isotope effect demonstrates that the Si-H rupture is the rate-determining step of the activation. Complex 2 reacts with 1,1-diphenyl-2-propyn-1-ol and 1-phenyl-1-propyne. The reaction with the former affords Os{C≡CC(OH)Ph2}2{=C=CHC(OH)Ph2}{κ3-P,O,P-[xant(PiPr2)2]} (6), which catalyzes the conversion of the propargylic alcohol into (E)-2-(5,5-diphenylfuran-2(5H)-ylidene)-1,1-diphenylethan-1-ol, via (Z)-enynediol. In methanol, the hydroxyvinylidene ligand of 6 dehydrates to allenylidene, generating Os{C≡CC(OH)Ph2}2{=C=C=CPh2}{κ3-P,O,P-[xant(PiPr2)2]} (7). The reaction of 2 with 1-phenyl-1-propyne leads to OsH{κ1-C,η2-[C6H4CH2CH=CH2]}{κ3-P,O,P-[xant(PiPr2)2]} (8) and PhCH2CH=CH(SiEt3).

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 Cnitrile···O-H···Namidate 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.

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.

Reduction of Benzonitriles via Osmium-Azavinylidene Intermediates Bearing Nucleophilic and Electrophilic Centers.

The reduction of the N≡C bond of benzonitriles promoted by OsH6(PiPr3)2 (1) has been studied. Complex 1 releases a H2 molecule and coordinates 2,6-dimethylbenzonitrile to afford the tetrahydride OsH4{κ1- N-(N≡CC6H3Me2)}(PiPr3)2 (2), which is thermally stable toward the insertion of the nitrile into one of the Os-H bonds. In contrast to 2,6-dimethylbenzonitrile, benzonitrile and 2-methylbenzonitrile undergo insertion, via Os(η2-N≡CR) intermediates, to give the azavinylidene derivatives OsH3(═N═CC6H4R)(PiPr3)2 [R = H (3) or Me (4)]. The analysis by means of computational tools (EDA-NOCV) of the bonding situation in these compounds suggests that the donor-acceptor nature of the osmium azavinylidene bond dominates over the mixed electron-sharing/donor-acceptor and pure electron-sharing bonding modes. The N atom is strongly nucleophilic, whereas one of the hydrides is electrophilic. In spite of the different nature of these centers, the migration of the latter to the N atom is kinetically prevented. However, the use of water as a proton shuttle allows hydride migration, as a consequence of a significant decrease in the activation barrier. The resulting phenylaldimine intermediates evolve by means of orthometalation to give OsH3{κ2- N, C-(NH═CHC6H3R)}(PiPr3)2 [R = H (5) or Me (6)]. The presence of electrophilic and nucleophilic centers in 3 confers upon it the ability to activate σ-bonds, including H2 and pinacolborane (HBpin). The reaction with the latter gives OsH3{κ2- N, C-[N(Bpin)═CHC6H4]}(PiPr3)2 (7).

Evidence for a Bis(Elongated σ)-Dihydrideborate Coordinated to Osmium.

The formation and Atoms in Molecules (AIM) analysis of osmium(IV) and osmium(II) complexes containing dihydrideborate groups and primary aminoborane ligands are reported. Complex OsH6(P iPr3)2 (1) loses a hydrogen molecule and the resulting unsaturated OsH4(P iPr3)2 species coordinates 9-borabicycle[3.3.1]nonane (HBbn) and pinacolborane (HBpin) to give the dihydrideborate derivatives OsH3{κ2- H, H-(H2BR2)}(P iPr3)2 (BR2 = Bbn (2), Bpin (3)). The bonding situation in these compounds and in the related osmium(II) derivative Os(Bcat){κ2- H, H-(H2Bcat)}(CO)(P iPr3)2 (4) (HBcat = catecholborane) has been analyzed by the AIM method. The Laplacian distributions in the Os-H-B plane exhibit a four-membered cyclic topology possessing two Os-H and two B-H bond critical points associated with one OsHHB ring critical point, which resembles that found for B2H6. The tetrahydride OsH4(P iPr3)2 also coordinates catecholborane, which initially affords OsH3{κ2- H, H-(H2Bcat)}(P iPr3)2 (5). In contrast to 2 and 3, complex 5 reacts with a second molecule of HBcat to give the elongated σ-borane-{bis(elongated σ) -dihydrideborate}-osmium(II) derivative OsH(η3-H2Bcat)(η2-HBcat)(P iPr3)2 (6). Complexes 5 and 6 have been also analyzed via the AIM method. Complex 5 displays the same topology as complexes 2-4. However, the OsH2B unit of 6 shows, besides the Os-H and B-H bond critical points, an additional Os-B bond critical point, which is associated with a bond path running between these atoms. This double triangular topology is completed with the respective ring critical points. Reactions of 1 with dimethylamine-borane (H3B·NHMe2) and tert-butylamine-borane (H3B·NH2tBu) give OsH2(η2:η2-H2BNR2)(P iPr3)2 (NR2 = NMe2 (7), NH tBu (8)). The AIM analyses of 7 and 8 also reveal the occurrence of an Os-B bond critical point associated with a bond path running between those atoms. However, neither Os-H bond critical points nor bond paths are observed in the latter species.