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

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

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

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)

An Acyl-NHC Osmium Cooperative System: Coordination of Small Molecules and Heterolytic B–H and O–H Bond Activation

The hexahydride complex OsH6(PiPr3)2 (1) activates the C–OMe bond of 1-(2-methoxy-2-oxoethyl)-3-methylimidazolium chloride (2), in addition to promoting the direct metalation of the imidazolium group, to afford a five-coordinate OsCl(acyl-NHC)(PiPr3)2 (3) compound. The latter coordinates carbon monoxide, oxygen, and molecular hydrogen to give the corresponding carbonyl (4), dioxygen (5), and dihydrogen (6) derivatives. Complex 3 also promotes the heterolytic bond activation of pinacolborane (HBpin), using the acyl oxygen atom as a pendant Lewis base. The hydride ligand and the Bpin substituent of the Fischer-type carbene of the resulting complex 7 activate the O–H bond of alcohols and water. As a consequence, complex 3 is a metal ligand cooperating catalyst for the generation of molecular hydrogen, by means of both the alcoholysis and hydrolysis of pinacolborane, via the intermediates 7 and 6.Financial support from the MINECO of Spain (Projects CTQ2014-52799-P and CTQ2014-51912-REDC), the Diputación General de Aragón (E-35), and the European Social Fund (FSE) and FEDER. M.P.G. thanks the Spanish MINECO for her FPI fellowship. T.B. thanks the Spanish MINECO for funding through the Juan de la Cierva program