Mary Grellier

Ruthenium-Catalyzed Hydrogenation of Nitriles: Insights into the Mechanism

Hydrogenation of benzonitrile into benzylamine is catalyzed under very mild conditions by the ruthenium bis(dihydrogen) complex RuH(2)(H(2))(2)(PCyp(3))(2), incorporating two tricyclopentylphosphines. Two key intermediates have been isolated, resulting from the activation of benzonitrile at early stages of activation, i.e., either N-coordination through the nitrile function or first hydrogenation with benzylimine formation, followed by, thanks to C-H activation, coordination at ruthenium as an orthometalated ligand.

Iron-Catalyzed C–H Borylation of Arenes

Well-defined iron bis(diphosphine) complexes are active catalysts for the dehydrogenative C-H borylation of aromatic and heteroaromatic derivatives with pinacolborane. The corresponding borylated compounds were isolated in moderate to good yields (25-73%) with a 5 mol% catalyst loading under UV irradiation (350 nm) at room temperature. Stoichiometric reactivity studies and isolation of an original trans-hydrido(boryl)iron complex, Fe(H)(Bpin)(dmpe)2, allowed us to propose a mechanism showing the role of some key catalytic species.

Pd Catalysed intramolecular coupling between tertiary amines and allylic groups; Synthesis of 3-hydro-1H-2-benzazepinium salts

Tertiary amines have been used as intramolecular nucleophiles in a Pd catalysed allylic substitution reaction leading to benzazepinium salts.

Motional heterogeneity in single-site silica-supported species revealed by deuteron NMR

The molecular dynamics of [-SiDMe(2)] grafted on two amorphous silica materials, mesoporous SBA and non-porous Aerosil, was investigated by deuteron ((2)H) solid-state NMR spectroscopy. Quadrupole echo (QE), quadrupole Carr-Purcell-Meiboom-Gill (QCPMG) and magic angle spinning (MAS) spectra were recorded as a function of temperature. These were analyzed to determine the rates and trajectories of molecular motion of the surface species. The dynamics were modelled as a composite two frame motion with independent rotations around the two Si-O bonds. In the first frame there are fast three-site jumps of the -SiDMe(2) group described by a single rate (k(1)) and unequal populations of the tetrahedral sites, such that the ratio D : Me : Me is around 1 : 4 : 4. In the second frame, the Si-O axis makes small step, nearest-neighbour jumps at a rate k(2) along an arc defined by the rim of a cone with a fixed half-angle. Both rates were found to be in the fast motional regime (k(1,2) > 10(10) s(-1)) throughout the experimentally accessible temperature range, 190-350 K. The experimental data are compatible only with models that include a distribution of arc lengths, lambda, in the second frame. The best fit of the simulations to the experimental data yields the distributions of the arc length. The results unequivocally demonstrate that even though the sites all have the same average environment, as reported by the isotropic chemical shifts, the dynamics of the grafted species are microscopically spatially heterogeneous with different molecules on the surface having different ranges of motional trajectories and populations. Furthermore, a clear difference in dynamic behavior is observed between the two silica supports, the motion being more constrained on the mesoporous SBA. This differential mobility is possibly due to differences in surface roughness and to the pore structure of SBA compared with the smoother surface of Aerosil.

Versatile coordination of 2-pyridinetetramethyldisilazane at ruthenium: Ru(II) vs Ru(IV) as evidenced by NMR, X-ray, neutron, and DFT studies.

The novel disilazane compound 2-pyridinetetramethyldisilazane (1) has been synthesized. The competition between N-pyridine coordination and Si-H bond activation was studied through its reactivity with two ruthenium complexes. The reaction between 1 and RuH(2)(H(2))(2)(PCy(3))(2) led to the isolation of the new complex RuH(2){(eta(2)-HSiMe(2))N(kappaN-C(5)H(4)N)(SiMe(2)H)}(PCy(3))(2) (2) resulting from the loss of two dihydrogen ligands and coordination of 1 to the ruthenium center via a kappa(2)N,(eta(2)-Si-H) mode. Complex 2 has been characterized by multinuclear NMR experiments ((1)H, (31)P, (13)C, (29)Si), X-ray diffraction and DFT studies. In particular, the HMBC (29)Si-(1)H spectrum supports the presence of two different silicon environments: one Si-H bond is dangling, whereas the other one is eta(2)-coordinated to the ruthenium with a J(SiH) value of 50 Hz. DFT calculations (B3PW91) were also carried out to evaluate the stability of the agostic species versus a formulation corresponding to a bis(sigma-Si-H) isomer and confirmed that N-coordination overcomes any stabilization that could be gained by the establishment of SISHA interactions. There is no exchange between the two Si-H bonds present in 2, as demonstrated by deuterium-labeling experiments. Heating 2 at 70 degrees C under vacuum for 24 h, leads to the formal loss of one equivalent of H(2) from 2 and formation of the 16-electron complex RuH{(SiMe(2))N(kappaN-C(5)H(4)N)(SiMe(2)H)}(PCy(3))(2) (3) formulated as a hydrido(silyl) species on the basis of multinuclear NMR experiments. The dehydrogenation reaction is fully reversible under dihydrogen atmosphere. Reaction of Ru(COD)(COT) with 3 equiv of 1 under a H(2) pressure led to the isolation of the new complex RuH{(SiMe(2))N(kappaN-C(5)H(4)N)(SiMe(2)H)}(3) (4) characterized as a hydridotrisilyl complex by multinuclear NMR techniques, X-ray and neutron diffractions, as well as DFT calculations. The (29)Si HMBC experiments confirm the presence of two different silicon atoms in 4, with a signal at -14.64 ppm for three dangling Si-Me(2)H fragments and a signal at 64.94 ppm (correlating with the hydride signal) assigned to three Si-Me(2)N groups bound to Ru. Comparison of DFT and neutron parameters involving the hydride clearly indicates an excellent correlation. The Si-H distance of approximately 2.15 A is much shorter than the sum of the van der Waals radii and typically in the range of a significant interaction between a silicon and a hydrogen atom (SISHA interactions). In 4, three dangling Si-H groups remain accessible for further functionalization.

Nature of Si–H Interactions in a Series of Ruthenium Silazane Complexes Using Multinuclear Solid-State NMR and Neutron Diffraction

Three new N-heterocyclic-silazane compounds, 1a-c, were prepared and employed as bidentate ligands to ruthenium, resulting in a series of [Ru(H){(κ-Si,N-(SiMe2-N-heterocycle)}3] complexes (3a-c) featuring the same RuSi3H motif. Detailed structural characterization of the RuSi3H complexes with X-ray diffraction, and in the case of triazabicyclo complex [Ru(H){κ-Si,N-(SiMe2)(C7H12N3)}3] (3a), neutron diffraction, enabled a reliable description of the molecular geometry. The hydride ligand of (3a) is located closer to two of the silicon atoms than it is to the third. Such a geometry differs from that of the previously reported complex [Ru(H){(κ-Si,N-(SiMe2)N(SiMe2H)(C5H4N)}3] (3d), also characterized by neutron diffraction, where the hydride was found to be equidistant from all three silicon atoms. A DFT study revealed that the symmetric and less regular isomers are essentially degenerate. Information on the dynamics and on the Ru···H···Si interactions was gained from multinuclear solid-state ((1)H wPMLG, (29)Si CP MAS, and 2D (1)H-(29)Si dipolar HETCOR experiments) and solution NMR studies. The corresponding intermediate complexes, [Ru{κ-Si,N-(SiMe2-N-heterocycle)}(η(4)-C8H12)(η(3)-C8H11)] (2a-c), involving a single silazane ligand were isolated and characterized by multinuclear NMR and X-ray diffraction. Protonation of the RuSi3H complexes was also studied. Reaction of 3a with NH4PF6 gave rise to [Ru(H)(η(2)-H -SiMe2)κ-N-(C7H12N3){κ-Si,N-(SiMe2)(C7H12N3)}2](+)[PF6](-)(4aPF6) which was isolated and characterized by NMR spectroscopy, X-ray crystallography, and DFT studies. The nature of the Si-H interactions in this silazane series was analyzed in detail.

Phosphinodi(benzylsilane) PhP{(o-C6H4CH2)SiMe2H}2: A Versatile “PSi2Hx” Pincer-Type Ligand at Ruthenium

The synthesis of the new phosphinodi(benzylsilane) compound PhP{(o-C6H4CH2)SiMe2H}2 (1) is achieved in a one-pot reaction from the corresponding phenylbis(o-tolylphosphine). Compound 1 acts as a pincer-type ligand capable of adopting different coordination modes at Ru through different extents of Si-H bond activation as demonstrated by a combination of X-ray diffraction analysis, density functional theory calculations, and multinuclear NMR spectroscopy. Reaction of 1 with RuH2(H2)2(PCy3)2 (2) yields quantitatively [RuH2{[η(2)-(HSiMe2)-CH2-o-C6H4]2PPh}(PCy3)] (3), a complex stabilized by two rare high order ε-agostic Si-H bonds and involved in terminal hydride/η(2)-Si-H exchange processes. A small free energy of reaction (ΔrG298 = +16.9 kJ mol(-1)) was computed for dihydrogen loss from 3 with concomitant formation of the 16-electron species [RuH{[η(2)-(HSiMe2)-CH2-o-C6H4]PPh[CH2-o-C6H4SiMe2]}(PCy3)] (4). Complex 4 features an unprecedented (29)Si NMR decoalescence process. The dehydrogenation process is fully reversible under standard conditions (1 bar, 298 K).

Probing highly selective H/D exchange processes with a ruthenium complex through neutron diffraction and multinuclear NMR studies.

Deuterium labeling is a powerful way to gain mechanistic information in biology and chemistry. However, selectivity is hard to control experimentally, and labeled sites can be difficult to assign both in solution and in the solid state. Here we show that very selective high-deuterium contents can be achieved for the polyhydride ruthenium phosphine complex [RuH2(H2)2(PCyp3)2] (1) (PCyp3 = P(C5H9)3). The selectivity of the H/D exchange process is demonstrated by multinuclear NMR and neutron diffraction analyses. It has also been investigated through density functional theory (DFT) calculations. The reactions are performed under mild conditions at room temperature, and the extent of deuterium incorporation, involving selective C-H bond activation within the cyclopentyl rings of the phosphine ligands, can easily be tuned (solvent effects, D2 pressure). It is shown that D2 gas can inhibit the C-H/C-D exchange process.

Step-by-step introduction of silazane moieties at ruthenium: different extents of Ru-H-Si bond activation.

The coordination of pyridine-2-amino(methyl)dimethylsilane ligands to ruthenium has afforded access to a family of novel complexes that display multicenter Ru-H-Si interactions according to the number of incorporated ligands. The new complexes Ru[κ-Si,N-(SiMe2)N(Me)(C5H4N)](η(4)-C8H12)(η(3)-C8H11) (1), Ru2(μ-H)2(H)2[κ-Si,N-(SiMe2)N(Me)(C5H4N)]4 (2), and Ru(H)[κ-Si,N-(SiMe2)N(Me)(C5H4N)]3 (3) were isolated and fully characterized. The complexes exhibit different degrees of Si-H activation: complete Si-H cleavage, secondary interactions between the atoms (SISHA), and η(2)-Si-H coordination. Reversible protonation of 3 leading to the cationic complex [RuH{(η(2)-H-SiMe2)N(Me)κ-N-(C5H4N)}{κ-Si,N-(SiMe2)N(Me)(C5H4N)}2](+)[BAr(F)4](-) (5) was also demonstrated. The coordination modes in these systems were carefully studied with a combination of X-ray and neutron diffraction analysis, DFT geometry optimization, and multinuclear NMR spectroscopy.

Redistribution at silicon by ruthenium complexes. Bonding mode of the bridging silanes in Ru2H4(μ-η2:η2:η2:η2-SiH4)(PCy3)4and Ru2H2(μ-η2:η2-H2Si(OMe)2)3(PCy3)2

The bis(dihydrogen) complex RuH2(η2-H2)2(PCy3)2 (1) reacts with 2 equiv. of H2SiMePh to produce a mixture of Ru2H4(μ-η2:η2:η2:η2-SiH4)(PCy3)4 (2) and RuH2(η2-H2)(η2-HSiPh3)(PCy3)2 (4) together with HSiMePh2, HSiMe2Ph and traces of HMe2SiSiMe2H as a result of redistribution at silicon. The bridging SiH4 ligand in 2 is coordinated to the two ruthenium via four σ-Si–H bonds in agreement with NMR, X-ray data (on 2, and 2′ the analogous PiPr3 complex) and DFT calculations. Each interaction involves σ-donation to a ruthenium and back-bonding from the other ruthenium. Elimination of SiH4 and formation of RuH2(CO)2(PCy3)2 (5), RuH2(tBuNC)2(PCy3)2 (6) or RuH(η2-H2)Cl(PCy3)2 (7) were observed upon the reaction of 2 with CO, tBuNC, CH2Cl2, respectively. No reaction occurred in the presence of H2, but H/D exchange was observed under D2 atmosphere. Another redistribution reaction at silicon can be obtained by adding 4 equiv. of HSi(OMe)3 to 2 to produce Si(OMe)4 and Ru2H2(μ-η2:η2-H2Si(OMe)2)3(PCy3)2 (3) displaying three bridging (μ-η2:η2 alkoxysilane) ligands. Complex 3 is characterized by multinuclear NMR spectroscopies and by a crystal structure. DFT calculations show that the model complex Ru2H2(μ-η2:η2-H2Si(OR)2)3(PR3)2 (R = H, Me) is a minimum on the potential energy surface, and support the dihydride formulation with three bridging H2Si(OMe)2 ligands coordinated to the two ruthenium through σ-Si–H bonds.