Conceptual Extension of the Degradation-Transformation of N-Heterocyclic Carbenes: Unusual Rearrangements on Osmium

The range of processes of degradation-transformation of NHC ligands in the coordination sphere of a transition metal has been enlarged. The NHC-acyl ligand of the complex Os{¿2-C, C-[C(O)CH2ImMe]}Cl(PiPr3)2 (1) undergoes a complex rearrangement promoted by internal alkynes to give Os{¿2-C, N-[CH2ImMe]}Cl(CO)(PiPr3)2 (2). Mechanistic studies have revealed that the degradation involves a catalytic alkyne-mediated deinsertion of CO from the acyl moiety to afford Os{¿2-C, C-[CH2ImMe]}Cl(CO)(PiPr3)2 (3), followed by a thermally activated stoichiometric 1, 2-methylene shift from N to C. The catalytic activity of the alkynes depends upon their substituents, decreasing in the sequence diphenylacetylene > 1-phenyl-1-propyne > 3-hexyne > 2-butyne. Phenylacetylene tautomerizes in the metal coordination sphere to afford the stable vinylidene Os{¿2-C, C-[C(O)CH2ImMe]}Cl(=C=CHPh)(PiPr3)2 (4), which experiences the coupling of the acyl moiety and the vinylidene ligand under a carbon monoxide atmosphere. The addition of HBF4·OEt2 to the resulting complex Os{¿2-C, C-[C(=CHPh)C(O)CH2ImMe]}Cl(CO)2(PiPr3) (5) leads to [Os{¿2-O, C-[O=C(CH=CHPh)CH2ImMe]}Cl(CO)2(PiPr3)]BF4 (6) containing an NHC-(a, ß-unsaturated ketone) ligand

Kinetic Analysis and Sequencing of Si-H and C-H Bond Activation Reactions: Direct Silylation of Arenes Catalyzed by an Iridium-Polyhydride

The saturated trihydride IrH3{¿3-P, O, P-[xant(PiPr2)2]} (1; xant(PiPr2)2 = 9, 9-dimethyl-4, 5-bis(diisopropylphosphino)xanthene) coordinates the Si-H bond of triethylsilane, 1, 1, 1, 3, 5, 5, 5-heptamethyltrisiloxane, and triphenylsilane to give the s-complexes IrH3(¿2-H-SiR3){¿2-cis-P, P-[xant(PiPr2)2]}, which evolve to the dihydride-silyl derivatives IrH2(SiR3){¿3-P, O, P-[xant(PiPr2)2]} (SiR3 = SiEt3 (2), SiMe(OSiMe3)2 (3), SiPh3 (4)) by means of the oxidative addition of the coordinated bond and the subsequent reductive elimination of H2. Complexes 2-4 activate a C-H bond of symmetrically and asymmetrically substituted arenes to form silylated arenes and to regenerate 1. This sequence of reactions defines a cycle for the catalytic direct C-H silylation of arenes. Stoichiometric isotopic experiments and the kinetic analysis of the transformations demonstrate that the C-H bond rupture is the rate-determining step of the catalysis. As a consequence, the selectivity of the silylation of substituted arenes is generally governed by ligand-substrate steric interactions

Square-Planar Alkylidyne-Osmium and Five-Coordinate Alkylidene-Osmium Complexes: Controlling the Transformation from Hydride-Alkylidyne to Alkylidene

Square-planar alkylidyne and five-coordinate alkylidene mixed iPr3P–Os–IPr (IPr = 1, 3-bis(diisopropylphenyl)imidazolylidene) complexes have been discovered and characterized, and their formation has been rationalized. The cationic five-coordinate hydride-alkylidyne compounds [OsHX(=CPh)(IPr)(PiPr3)]OTf (X = Cl (1), F (4); OTf = CF3SO3) undergo deprotonation with KOtBu to afford the trans-halide-alkylidyne square-planar derivatives OsX(=CPh)(IPr)(PiPr3) (X = Cl (2), F (5)). Oxidative addition of the C(sp)–H bond of phenylacetylene and methyl propiolate along the Cl–Os–CPh axis of 2 with the hydrogen atom directed to the alkylidyne leads to alkynyl-cis-hydride-alkylidyne intermediates, which rapidly evolve into the five-coordinate alkylidene complexes Os(C=CR)Cl(-CHPh)(IPr)(PiPr3) (R = Ph (6), CO2Me (7)) as a consequence of the migration of the hydride from the metal center to the Ca atom of the alkylidyne. Oxidative addition of the C(sp)–H bond of methyl propiolate along the X–Os–CPh axis of 2 and 5 with the hydrogen atom directed to the halide gives the alkynyl-trans-hydride-alkylidyne derivatives OsH(C=CCO2Me)X(=CPh)(IPr)(PiPr3) (X = Cl (8), F (9)). Complex 8 evolves into 7. However, complex 9 containing the stronger p-donor fluoride is stable. The oxidative addition of HCl to 2 selectively yields the cis-hydride-alkylidyne compound OsHCl2(=CPh)(IPr)(PiPr3) (10), which is also stable

Tuning the Nature and Formation of Bis(dihydrogen)-Osmium Species

The influence of chelate ligands in the formation and nature of bis(dihydrogen) units of OsH4 complexes has been studied. The classical trihydride OsH3{¿2-C, N-(C6H4-py)}(PiPr3)2 (1) reacts with HBF4·OEt2 to give the Kubas-type dihydrogen-elongated dihydrogen derivative [Os{¿2-C, N-(C6H4-py)}(¿2-H2)2(PiPr3)2]BF4 (2), as a result of the protonation of one of the hydride ligands. Triflate (OTf) displaces the Kubas-type dihydrogen and elongates the elongated dihydrogen ligand, which is converted into a compressed dihydride. Thus, the addition of 1 equiv of HOTf to 1 leads to Os(H···H){¿2-C, N-(C6H4-py)}(OTf)(PiPr3)2 (3). Similar to [OTf]-, acetone reacts with 2 to afford the related compressed dihydride [Os(H···H){¿2-C, N-(C6H4-py)}(¿1-OCMe2)(PiPr3)2]BF4 (4), whereas acetonitrile leads to a 1:8 mixture of the monohydride [OsH(CH3CN)3(PiPr3)2]BF4 (5) and the trihydride [OsH3(CH3CN)2(PiPr3)2]BF4 (6). Reactions of 2 with toluene and p-xylene yield the half-sandwich derivatives [Os{¿2-C, N-(C6H4-py)}(¿6-toluene)(PiPr3)]BF4 (7) and [Os{¿2-C, N-(C6H4-py)}(¿6-p-xylene)(PiPr3)]BF4 (8), respectively. The acyl oxygen atom of the C, C-chelate ligand of the trihydride OsH3{¿2-C, C-[C(O)CH2ImMe]}(PiPr3)2 (10; Im = imidazolylidene) provides reliable and effective protection of the hydride ligands against the protonation. Thus, the addition of HBF4·OEt2 or HOTf to 10 leads to the trihydride-hydroxycarbene cation [OsH3{¿2-C, C-[C(OH)CH2ImMe]}(PiPr3)2]+ (11). The hydroxycarbene-NHC ligand of the latter is unstable, undergoing an intramolecular 1, 3-hydrogen shift, which produces the rupture of the chelate ligand and the formation of the cis-hydride dihydrogen (12a) and trans-hydride-dihydrogen (12b) derivatives [OsH(¿2-H2)(CO)(Me2Im)(PiPr3)2]+ (12). The protonation of 11 with a second equivalent of HOTf affords the transient bis(Kubas-type dihydrogen) [Os(¿2-H2)2{¿2-C, C-[C(OH)CH2ImMe]}(PiPr3)2](OTf)2, which undergoes the displacement of a coordinated hydrogen molecule by a [OTf]- anion to give the Kubas-type dihydrogen [Os(OTf)(¿2-H2){¿2-C, C-[C(OH)CH2ImMe]}(PiPr3)2]OTf (13). The addition of MeOTf to 10 leads to [OsH3{¿2-C, C-[C(OMe)CH2ImMe]}(PiPr3)2]OTf (14) which, in contrast to 11, is stable. Similar to the latter, the protonation of 14 with HOTf yields the Kubas-type dihydrogen complex [Os(OTf)(¿2-H2){¿2-C, C-[C(OMe)CH2ImMe]}(PiPr3)2]OTf (15)

Evidence for a Bis(Elongated s)-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(PiPr3)2 (1) loses a hydrogen molecule and the resulting unsaturated OsH4(PiPr3)2 species coordinates 9-borabicycle[3.3.1]nonane (HBbn) and pinacolborane (HBpin) to give the dihydrideborate derivatives OsH3{¿2-H, H-(H2BR2)}(PiPr3)2 (BR2 = Bbn (2), Bpin (3)). The bonding situation in these compounds and in the related osmium(II) derivative Os(Bcat){¿2-H, H-(H2Bcat)}(CO)(PiPr3)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(PiPr3)2 also coordinates catecholborane, which initially affords OsH3{¿2-H, H-(H2Bcat)}(PiPr3)2 (5). In contrast to 2 and 3, complex 5 reacts with a second molecule of HBcat to give the elongated s-borane-{bis(elongated s)-dihydrideborate}-osmium(II) derivative OsH(¿3-H2Bcat)(¿2-HBcat)(PiPr3)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·NH2 tBu) give OsH2(¿2:¿2-H2BNR2)(PiPr3)2 (NR2 = NMe2 (7), NHtBu (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

Iridium-Promoted B-B Bond Activation: Preparation and X-ray Diffraction Analysis of a mer-Tris(boryl) Complex

The tris(boryl) complex Ir(Bcat)3{¿3-P,O,P-[xant(PiPr2)2]} has been prepared. Its X-ray diffraction analysis structure reveals that the boryl groups are disposed in a mer rearrangement despite of the very strong trans influence of the boryl ligands. An energy decomposition analysis method coupled to natural orbitals for chemical valence suggests that the p-backdonation from the metal to the pz atomic orbital of the boron atom decreases about 43% in the Ir-B bonds disposed mutually trans with regard to the other one

Ammonia Borane Dehydrogenation Promoted by a Pincer-Square-Planar Rhodium(I) Monohydride: A Stepwise Hydrogen Transfer from the Substrate to the Catalyst

The pincer d8-monohydride complex RhH{xant(PiPr2)2} (xant(PiPr2)2 = 9, 9-dimethyl-4, 5-bis(diisopropylphosphino)xanthene) promotes the release of 1 equiv of hydrogen from H3BNH3 and H3BNHMe2 with TOF50% values of 3150 and 1725 h–1, to afford [BH2NH2]n and [BH2NMe2]2 and the tandem ammonia borane dehydrogenation–cyclohexene hydrogenation. DFT calculations on the ammonia borane dehydrogenation suggest that the process takes place by means of cis-¿2-PP-species, through four stages including: (i) Shimoi-type coordination of ammonia borane, (ii) homolytic addition of the coordinated H–B bond to afford a five-coordinate dihydride-boryl-rhodium(III) intermediate, (iii) reductive intramolecular proton transfer from the NH3 group to one of the hydride ligands, and (iv) release of H2 from the resulting square-planar hydride dihydrogen rhodium(I) intermediate

Amide-Directed Formation of Five-Coordinate Osmium Alkylidenes from Alkynes

The amide-directed synthesis of five-coordinate osmium alkylidene derivatives from alkynes is reported. These types of complexes, which have been elusive until now because of the tendency of osmium to give hydride alkylidyne species, are prepared by reaction of the dihydride OsH2Cl2(PiPr3)2 (1) with terminal alkynes containing a distal amide group. Complex 1 reacts with N-phenylhex-5-ynamide and N-phenylhepta-6-ynamide to give OsCl2{=C(CH3)(CH2)nNH(CO)Ph}(PiPr3)2 (n = 3 (2), 4 (3)). The relative position of carbonyl and NH groups in the organic substrates has no influence on the reaction. Thus, treatment of 1 with N-(pent-4-yn-1-yl)benzamide leads to OsCl2{=C(CH3)(CH2)3NHC(O)Ph}(PiPr3)2 (4). The new compounds are intermediate species in the cleavage of the C-C triple bond of the alkynes. Under mild conditions, they undergo the rupture of the Ca-CH3 bond of the alkylidene, which comes from the alkyne triple bond, to afford six-coordinate hydride-alkylidyne derivatives. In dichloromethane, complex 2 gives a 10:7 mixture of OsHCl2{=C(CH2)3C(O)NHPh}(PiPr3)2 (5) and OsHCl2{=CCH(CH3)(CH2)2C(O)NHPh}(PiPr3)2 (6). The first complex contains a linear separation between the alkylidyne Ca atom and the amide group, whereas the spacer is branched in the second complex. In contrast to the case for 2, complex 4 selectively affords OsHCl2{=C(CH2)3NHC(O)Ph}(PiPr3)2 (7). In spite of their instability, these compounds give the alkylidene-allene metathesis, being a useful entry to five-coordinate vinylidene complexes, including the dicarbon-disubstituted OsCl2(=C=CMe2)(PiPr3)2 (8) and the monosubstituted OsCl2(=C=CHCy)(PiPr3)2 (9)

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

The d2-hexahydride complex OsH6(PiPr3)2 (1) promotes the activation of C-H bonds of 2, 2'-bipyridines and related heterocycles. The study of the same reactions with the deuteride counterpart OsD6(PiPr3)2 (1-d) reveals that the activation of the C-H bonds situated in the sterically less hindered positions is kinetically preferred. However, the isolated products are the result of the thermodynamic control of the reactions. Thus, reactions of 1 with 2, 2'-bipyridine, 6-phenyl-2, 2'-bipyridine, and 6-methyl-2, 2'-bipyridine give the "rollover cyclometalation" products OsH3{¿2-C, N-[C5(R)H2N-py]}(PiPr3)2 (R = H (2), Ph (3), Me (4)), whereas 3, 5-dimethyl-6-phenyl-2, 2'-bipyridine affords OsH2{¿3-C, N, C-[C5H3N-(Me)2py-C5H4]}(PiPr3)2 (5), containing a dianionic C, N, C-pincer ligand. The behavior of substrates pyridyl-benzimidazolium and -imidazolium is similar. Reaction of 1 with 3-methyl-1-(6-phenylpyridin-2-yl)-1H-benzimidazolium tetrafluoroborate leads to OsH3{¿2-C, C-[MeBzim-C5(Ph)H2N]}(PiPr3)2 (6), bearing an anionic Cpy, CNHC-chelate. On the other hand, 3-methyl-1-(6-phenylpyridin-2-yl)-1H-imidazolium tetrafluoroborate yields [OsH2{¿3-C, N, C-(MeIm-py-C6H4)}(PiPr3)2]BF4 (7), containing a monoanionic C, N, C-pincer with a NHC-unit coordinated in an abnormal fashion. The reactivity pattern of these substrates is also observed with the d4-iridium-pentahydride IrH5(PiPr3)2 (8), which has generated IrH2{¿2-C, N-[C5(R)H2N-py]}(PiPr3)2 (R = H, (9), Ph (10)) and IrH{¿3-C, N, C-[C5H3N-(Me2)py-C5H4]}(PiPr3)2 (11). The osmium(IV)-carbon bonds display a higher degree of covalency than the iridium(III)-carbon bonds. In contrast to 2, the metalated carbon atom of 9 undergoes the addition of a proton of methanol to give [IrH2{¿2-N, N-(bipy)}(PiPr3)2]BF4 (12)

Preparation and degradation of rhodium and iridium diolefin catalysts for the acceptorless and base-free dehydrogenation of secondary alcohols

Rhodium and iridium diolefin catalysts for the acceptorless and base-free dehydrogenation of secondary alcohols have been prepared, and their degradation has been investigated, during the study of the reactivity of the dimers [M(µ-Cl)(I4-C8H12)]2 (M = Rh (1), Ir (2)) and [M(µ-OH)(I4-C8H12)]2 (M = Rh (3), Ir (4)) with 1, 3-bis(6'-methyl-2'-pyridylimino)isoindoline (HBMePHI). Complex 1 reacts with HBMePHI, in dichloromethane, to afford equilibrium mixtures of 1, the mononuclear derivative RhCl(I4-C8H12){¿1-Npy-(HBMePHI)} (5), and the binuclear species [RhCl(I4-C8H12)]2{µ-Npy, Npy-(HBMePHI)} (6). Under the same conditions, complex 2 affords the iridium counterparts IrCl(I4-C8H12){¿1-Npy-(HBMePHI)} (7) and [IrCl(I4-C8H12)]2{µ-Npy, Npy-(HBMePHI)} (8). In contrast to chloride, one of the hydroxide groups of 3 and 4 promotes the deprotonation of HBMePHI to give [M(I4-C8H12)]2(µ-OH){µ-Npy, Niso-(BMePHI)} (M = Rh (9), Ir (10)), which are efficient precatalysts for the acceptorless and base-free dehydrogenation of secondary alcohols. In the presence of KOtBu, the [BMePHI]- ligand undergoes three different degradations: Alcoholysis of an exocyclic isoindoline-N double bond, alcoholysis of a pyridyl-N bond, and opening of the five-membered ring of the isoindoline core.