Metal-Free Catalytic Hydrogenolysis of Chlorosilanes into Hydrosilanes with "Inverse" Frustrated Lewis Pairs.

The challenging metal-free catalytic hydrogenolysis of silyl chlorides to hydrosilanes is unlocked by using an inverse frustrated Lewis pair (FLP), combining a mild Lewis acid (Cy2 BCl) and a strong phosphazene base (BTPP) in mild conditions (10 bar of H2 , r. t.). In the presence of a stoichiometric amount of the base, the hydrosilanes R3 SiH (R=Me, Et, Ph) are generated in moderate to high yields (up to 95 %) from their chlorinated counterparts. A selective formation of the valuable difunctional monohydride Me2 SiHCl is also obtained from Me2 SiCl2 . A mechanism is proposed based on stoichiometric experiments and DFT calculations; it highlights the critical role of borohydride species generated by the heterolytic splitting of H2 .

Selective Reduction of Secondary Amides to Imines Catalysed by Schwartz's Reagent.

The partial reduction of amides is a challenging transformation that must overcome the intrinsic stability of the amide bond and exhibit high chemoselective control to avoid overreduction to amine products. To address this challenge, we describe a zirconium-catalysed synthesis of imines by the reductive deoxygenation of secondary amides. This reaction exploits the excellent chemoselectivity of Schwartz's reagent (Cp2 Zr(H)Cl) and utilises (EtO)3 SiH as a mild stoichiometric reductant to enable catalyst turnover. The reaction generally proceeds with high yields (19 examples, 51 to 95 % yield) and tolerates a variety of functional groups (alkene, ester, nitro, etc.). Stoichiometric mechanistic investigations suggest the regeneration of the active [Zr]-H catalyst is achieved through the metathesis of Si-H and Zr-OR σ-bonds.

Metal-Free Catalytic Hydrogenolysis of Silyl Triflates and Halides into Hydrosilanes.

The metal-free catalytic hydrogenolysis of silyl triflates and halides (I, Br) to hydrosilanes is unlocked by using arylborane Lewis acids as catalysts. In the presence of a nitrogen base, the catalyst acts as a Frustrated Lewis Pair (FLP) able to split H2 and generate a boron hydride intermediate capable of reducing (pseudo)halosilanes. This metal-free organocatalytic system is competitive with metal-based catalysts and enables the formation of a variety of hydrosilanes at room temperature in high yields (>85 %) under a low pressure of H2 (≤10 bar).

A Copper(I)-Catalyzed Sulfonylative Hiyama Cross-Coupling.

An air-tolerant Cu-catalyzed sulfonylative Hiyama cross-coupling reaction enabling the formation of diaryl sulfones is described. Starting from aryl silanes, DABSO and aryliodides, the reaction tolerates a large variety of polar functional groups (amines, ketones, esters, aldehydes). Control experiments coupled with DFT calculations shed light on the mechanism, characterized by the formation of a Cu(I)-sulfinate intermediate via fast insertion of a SO2 molecule.

Uranyl(VI) Triflate as Catalyst for the Meerwein-Ponndorf-Verley Reaction.

Catalytic transformation of oxygenated compounds is challenging in f-element chemistry due to the high oxophilicity of the f-block metals. We report here the first Meerwein-Ponndorf-Verley (MPV) reduction of carbonyl substrates with uranium-based catalysts, in particular from a series of uranyl(VI) compounds where [UO2(OTf)2] (1) displays the greatest efficiency (OTf = trifluoromethanesulfonate). [UO2(OTf)2] reduces a series of aromatic and aliphatic aldehydes and ketones into their corresponding alcohols with moderate to excellent yields, using iPrOH as a solvent and a reductant. The reaction proceeds under mild conditions (80 °C) with an optimized catalytic charge of 2.3 mol % and KOiPr as a cocatalyst. The reduction of aldehydes (1-10 h) is faster than that of ketones (>15 h). NMR investigations clearly evidence the formation of hemiacetal intermediates with aldehydes, while they are not formed with ketones.

Activation of SO2 by N/Si+ and N/B Frustrated Lewis Pairs: Experimental and Theoretical Comparison with CO2 Activation.

The guanidine 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and the substituted derivatives [TBD-SiR2 ]+ and TBD-BR2 reacted with SO2 to give different FLP-SO2 adducts. Molecular structures, elucidated by X-ray diffraction, showed some structural similarities with the analogous CO2 adducts. Thermodynamic stabilities were both experimentally evidenced and computed through DFT calculations. The underlying parameters governing the relative stabilities of the different SO2 and CO2 adducts were discussed from a theoretical standpoint, with a focus on the influence of the Lewis acidic moiety.

Carbonylation of C-N Bonds in Tertiary Amines Catalyzed by Low-Valent Iron Catalysts.

The first iron catalysts able to promote the formal insertion of CO into the C-N bond of amines are reported. Using low-valent iron complexes, including K2 [Fe(CO)4 ], amides are formed from aromatic and aliphatic amines, in the presence of an iodoalkane promoter. Inorganic Lewis acids, such as AlCl3 and Nd(OTf)3 , have a positive influence on the catalytic activity of the iron salts, enabling the carbonylation at a low pressure of CO (6 to 8 bars).

U(III)-CN versus U(IV)-NC coordination in tris(silylamide) complexes.

Treatment of the metallacycle [UN*2(N,C)] [N* = N(SiMe3)2; N,C = CH2SiMe2N(SiMe3)] with [HNEt3][BPh4], [HNEt3]Cl, and [pyH][OTf] (OTf = OSO2CF3) gave the cationic compound [UN*3][BPh4] (1) and the neutral complexes [UN*3X] [X = Cl (3), OTf (4)], respectively. The dinuclear complex [{UN*(µ-N,C)(µ-OTf)}2] (5) and its tetrahydrofuran (THF) adduct [{UN*(N,C)(THF)(µ-OTf)}2] (6) were obtained by thermal decomposition of 4. The successive addition of NEt4CN or KCN to 1 led to the formation of the cyanido-bridged dinuclear compound [(UN*3)2(µ-CN)][BPh4] (7) and the mononuclear mono- and bis(cyanide) complexes [UN*3(CN)] (2) and [M][UN*3(CN)2] [M = NEt4 (8), K(THF)4 (9)], while crystals of [K(18-crown-6)][UN*3(CN)2] (10) were obtained by the oxidation of [K(18-crown-6)][UN*3(CN)] with pyridine N-oxide. The THF adduct of 1, [UN*3(THF)][BPh4], and complexes 2-7, 9 and 10 were characterized by their X-ray crystal structure. In contrast to their U(III) analogues [NMe4][UN*3(CN)] and [K(18-crown-6)]2[UN*3(CN)2] in which the CN anions are coordinated to the metal center via the C atom, complexes 2 and 9 exhibit the isocyanide U-NC coordination mode of the cyanide ligand. This U(III)/U(IV) differentiation has been analyzed using density functional theory calculations. The observed preferential coordinations are well explained considering the electronic structures of the different species and metal-ligand bonding energies. A comparison of the different quantum descriptors, i.e., bond orders, NPA/QTAIM data, and energy decomposition analysis, has allowed highlighting of the subtle balance between covalent, ionic, and steric factors that govern the U-CN/NC bonding.

U-CN versus Ce-NC coordination in trivalent complexes derived from M[N(SiMe3)2]3 (M = Ce, U).

Reactions of [MN*3] (M = Ce, U; N* = N(SiMe3)2) and NR4CN (R = Me, Et, or (n)Bu) or KCN in the presence of 18-crown-6 afforded the series of cyanido-bridged dinuclear compounds [NEt4][(MN*3)2(µ-CN)] (M = Ce, 2a, and U, 2b), [K(18-crown-6)(THF)2][(CeN*3)2(µ-CN)] (2'a), and [K(18-crown-6)][(UN*3)2(µ-CN)] (2'b), and the mononuclear mono-, bis-, and tris(cyanide) complexes [NEt4][MN*3(CN)] (M = Ce, 1a(Et), and U, 1b(Et)), [NMe4][MN*3(CN)] (M = Ce, 1a(Me), and U, 1b(Me)), [K(18-crown-6)][MN*3(CN)] (M = Ce, 1'a, and U, 1'b), [N(n)Bu4]2[MN*3(CN)2] (M = Ce, 3a, and U, 3b), [K(18-crown-6)]2[MN*3(CN)2] (M = Ce, 3'a, and U, 3'b), and [N(n)Bu4]2[MN*2(CN)3] (M = Ce, 4a, and U, 4b). The mono- and bis(cyanide) complexes were found to be in equilibrium. The formation constant of 3'b (K3'b) from 1'b at 10 °C in THF is equal to 5(1) × 10(-3), and -ΔH3'b = 104(2) kJ mol(-1) and -ΔS3'b = 330(5) J mol(-1) K(-1). The bis(cyanide) compound 3a or 3b was slowly transformed in solution into an equimolar mixture of the mono- and tris(cyanide) derivatives with elimination of N(n)Bu4N*. The crystal structures of 1a(Me), 1b(Me), 1'a·toluene, 1'b·toluene, 2'a, 2'b, 3a, 3'a, 3'b, 3'a·2benzene, 3'b·2benzene, 4a·0.5THF, and 4b·Et2O were determined. Crystals of the bis(cyanide) uranium complexes 3'b and 3'b·2benzene are isomorphous with those of the cerium counterparts 3'a and 3'a·2benzene, but they are not isostructural since the data revealed distinct coordination modes of the CN group, through the C or N atom to the U or Ce metal center, respectively. This differentiation has been analyzed using density functional theory calculations. The observed preferential coordination of the cyanide and isocyanide ions toward uranium or cerium in the bis(cyanide) complexes is corroborated by the consideration of the binding energies of these groups to the metals and by the comparison of DFT optimized geometries with the crystal structures. The better affinity of the cyanide ligand toward U(III) over Ce(III) metal center is related to the better energy matching between the 6d/5f uranium orbitals and the cyanide ligand ones, leading to a non-negligible covalent character of the bonding.

Efficient disproportionation of formic acid to methanol using molecular ruthenium catalysts.

The disproportionation of formic acid to methanol was unveiled in 2013 using iridium catalysts. Although attractive, this transformation suffers from very low yields; methanol was produced in less than 2% yield, because the competitive dehydrogenation of formic acid (to CO2 and H2) is favored. We report herein the efficient and selective conversion of HCOOH to methanol in 50% yield, utilizing ruthenium(II) phosphine complexes under mild conditions. Experimental and theoretical (DFT) results show that different convergent pathways are involved in the production of methanol, depending on the nature of the catalyst. Reaction intermediates have been isolated and fully characterized and the reaction chemistry of the resulting ruthenium complexes has been studied.

Regioselective and stereospecific deuteration of bioactive aza compounds by the use of ruthenium nanoparticles.

An efficient H/D exchange method allowing the deuteration of pyridines, quinolines, indoles, and alkyl amines with D2 in the presence of Ru@PVP nanoparticles is described. By a general and simple procedure involving mild reaction conditions and simple filtration to recover the labeled product, the isotopic labeling of 22 compounds proceeded in good yield with high chemo- and regioselectivity. The viability of this procedure was demonstrated by the labeling of eight biologically active compounds. Remarkably, enantiomeric purity was conserved in the labeled compounds, even though labeling took place in the vicinity of the stereogenic center. The level of isotopic enrichment observed is suitable for metabolomic studies in most cases. This approach is also perfectly adapted to tritium labeling because it uses a gas as an isotopic source. Besides these applications to molecules of biological interest, this study reveals a rich and underestimated chemistry on the surface of ruthenium nanoparticles.

Revisiting the chemistry of the actinocenes [(η8-C8H8)2An] (An = U, Th) with neutral Lewis bases. Access to the bent sandwich complexes [(η8-C8H8)2An(L)] with thorium (L = py, 4,4'-bipy, tBuNC, R4phen).

In stark contrast to uranocene, (Cot)2Th reacts with neutral mono- or bidentate Lewis bases to give the bent sandwich complexes (Cot)2Th(L) (L = py, 4,4'-bipy, tBuNC, phen, Me4phen). DFT calculations in the gas phase show that, for both U and Th, formation of the bent compound (Cot)2An(L) should be facile, the linear and bent forms being close in energy.

Catalyst-free depolymerization of polycaprolactone to silylated monoesters and iodide derivatives using iodosilanes.

The homogeneous depolymerization of polycaprolactone (PCL) with excess iodotrimethylsilane (Me3SiI) proceeds without catalysts and selectively afforded I(CH2)5CO2SiMe3 or a mixture of I(CH2)5CO2SiMe3 and I(CH2)5CO2I depending on the solvent (CH2Cl2, MeCN). The latter mixture can undergo methanolysis or hydrolysis into the valuable ester I(CH2)5CO2Me or the acid I(CH2)5CO2H. In contrast, SiH2I2 depolymerized PCL into the fully deoxygenated species I(CH2)6I and n-hexane.

Reductive depolymerization of polyesters and polycarbonates with hydroboranes by using a lanthanum(III) tris(amide) catalyst.

The homogeneous reductive depolymerization of polyesters and polycarbonates with hydroboranes is achieved with the use of an f-metal complex catalyst. These polymeric materials are transformed into their value-added alcohol equivalents. Catalysis proceeds readily, under mild conditions, with La[N(SiMe3)2]3 (1 mol%) and pinacolborane (HBpin) and shows high selectivity towards alcohols and diols, after hydrolysis.

Exploring the uranyl organometallic chemistry: from single to double uranium-carbon bonds.

Uranyl organometallic complexes featuring uranium(VI)-carbon single and double bonds have been obtained from uranyl UO(2)X(2) precursors by avoiding reduction of the metal center. X-ray diffraction and density functional theory analyses of these complexes showed that the U-C and U=C bonds are polarized toward the nucleophilic carbon.

Iodide, azide, and cyanide complexes of (N,C), (N,N), and (N,O) metallacycles of tetra- and pentavalent uranium.

In contrast to the neutral macrocycle [UN*(2)(N,C)] (1) [N* = N(SiMe(3))(3); N,C = CH(2)SiMe(2)N(SiMe(3))] which was quite inert toward I(2), the anionic bismetallacycle [NaUN*(N,C)(2)] (2) was readily transformed into the enlarged monometallacycle [UN*(N,N)I] (4) [N,N = (Me(3)Si)NSiMe(2)CH(2)CH(2)SiMe(2)N(SiMe(3))] resulting from C-C coupling of the two CH(2) groups, and [NaUN*(N,O)(2)] (3) [N,O = OC(═CH(2))SiMe(2)N(SiMe(3))], which is devoid of any U-C bond, was oxidized into the U(V) bismetallacycle [Na{UN*(N,O)(2)}(2)(µ-I)] (5). Sodium amalgam reduction of 4 gave the U(III) compound [UN*(N,N)] (6). Addition of MN(3) or MCN to the (N,C), (N,N), and (N,O) metallacycles 1, 4, and 5 led to the formation of the anionic azide or cyanide derivatives M[UN*(2)(N,C)(N(3))] [M = Na, 7a or Na(15-crown-5), 7b], M[UN*(2)(N,C)(CN)] [M = NEt(4), 8a or Na(15-crown-5), 8b or K(18-crown-6), 8c], M[UN*(N,N)(N(3))(2)] [M = Na, 9a or Na(THF)(4), 9b], [NEt(4)][UN*(N,N)(CN)(2)] (10), M[UN*(N,O)(2)(N(3))] [M = Na, 11a or Na(15-crown-5), 11b], M[UN*(N,O)(2)(CN)] [M = NEt(4), 12a or Na(15-crown-5), 12b]. In the presence of excess iodine in THF, the cyanide 12a was converted back into the iodide 5, while the azide 11a was transformed into the neutral U(V) complex [U(N{SiMe(3)}SiMe(2)C{CHI}O)(2)I(THF)] (13). The X-ray crystal structures of 4, 7b, 8a-c, 9b, 10, 12b, and 13 were determined.

Formation of uranium(IV) oxide clusters from uranocene [U(eta(8)-C8H8)2] and uranyl [UO2X2] compounds.

Uranocene [U(eta(8)-C(8)H(8))(2)] reacted in refluxing pyridine with 2 equiv of the uranyl(VI) compound [UO(2)(OTf)(2)] or [UO(2)I(2)(THF)(3)] (OTf = O(3)SCF(3); THF = tetrahydrofuran) or 1 equiv of the uranyl(V) complex [UO(2)(py)(2.3)K(OTf)(2)] to afford the hexanuclear uranium(IV) oxide cluster [U(6)O(8)(OTf)(8)(py)(8)] (1) or [U(6)O(8)I(8)(py)(10)] (3). Complexes 1 and 3 were easily isolated in good yield because they were deposited as microcrystalline powders with the release of free cyclooctatetraene as a unique byproduct. A similar reaction in THF gave [U(6)O(8)(OTf)(8)(THF)(8)] (2), whose isolation was impeded by polymerization of the solvent. With complexes [U(eta(8)-C(8)H(8))(2)] and [UO(2)(OTf)(2)] in a molar ratio of 4:3 or in the presence of excess uranocene, only crystals of the mono(eta(8)-C(8)H(8)) aggregate [U(6)O(8)(eta(8)-C(8)H(8))(OTf)(6)(py)(8)] (4) were obtained. The crystal structures of 1 x 2 py, 2 x 2 THF, 3 x py, and 4 x py were determined. Whereas uranocene and uranyl(VI) compounds are generally viewed as the most robust uranium compounds, they were found to fuse together into hexanuclear assemblages. These studies revealed the unusual four-electron reductive capacity of a uranium(IV) complex, in particular uranocene, which, through the loss of its two C(8)H(8) ligands, induces the activation and reduction of the strong U=O bonds of the uranyl moiety.

The bis metallacyclic anion [U(N{SiMe3}2)(CH2SiMe2N{SiMe3})2]-.

A series of bis metallacyclic compounds [M(THF)(x)UN*(CH(2)SiMe(2)N{SiMe(3)})(2)](n) [M = Na (2), Li (3), or K (4), N* = N(SiMe(3))(2)] were isolated from reactions of UCl(4) or [UN*(3)Cl] with MN* or by treatment of [UN*(2)(CH(2)SiMe(2)N{SiMe(3)})] (1) or [UN*(3)] with MN*, MH, or LiCH(2)SiMe(3) in tetrahydrofuran (THF). Crystals of 2a x 1/6n-pentane (x = 0), 2b (x = 1), 2c (x = 2), and 4b (x = 1) were obtained by crystallization of 2 and 4 from pentane, and [Na(18-crown-6)(THF)][UN*(CH(2)SiMe(2)N{SiMe(3)})(2)] (2d) and [Na(15-crown-5)][UN*(CH(2)SiMe(2)N{SiMe(3)})(2)] (2e) were formed upon addition of the crown ether. The crystal structures of 2a-2e and 4b exhibit the same [UN*(CH(2)SiMe(2)N{SiMe(3)})(2)] units which are linked to Na or K atoms via methylene or methyl groups, giving either tight cation-anion pairs (2d and 2e) or one-dimensional (1D) or two-dimensional (2D) polymeric compounds with Na or K atoms in bridging position between methylene groups of adjacent units. Reaction of 2 with CO gave the double insertion derivative [Na(2)(THF)U(2)N*(2)(OC{=CH(2)}SiMe(2)N{SiMe(3)})(4)] (5b) and [Na(15-crown-5)UN*(OC{=CH(2)}SiMe(2)N{SiMe(3)})(2)] (5c) in the presence of the crown ether. Thermal decomposition of 5b gave [Na(2)(THF)U(OC{=CH(2)}SiMe(2)N{SiMe(3)})(3)](2) (6), the product of CO insertion into the putative tris metallacycle [Na(2)(THF)(x)U(CH(2)SiMe(2)N{SiMe(3)})(3)]. The crystal structures of 5b, 5c, and 6 show the interaction of the Na atoms with the exocyclic C=CH(2) bonds. Diffusion of CO(2) into a THF solution of 2 led to the formation of [Na(THF)(x)UN*(OC{O}CH(2)SiMe(2)N{SiMe(3)})(2)] (7) which crystallized from pyridine/pentane to give [Na(THF)(2)(py)(2)UN*(OC{O}CH(2)SiMe(2)N{SiMe(3)})(2)] x 0.5 py (8 x 0.5 py), the first crystallographically characterized complex resulting from CO(2) insertion into a M(CH(2)SiMe(2)N{SiMe(3)}) metallacycle. Compound 2 reacted with I(2) to give [UN*(CH(2)SiMe(2)N{SiMe(3)})(N{SiMe(3)}SiMe(2)CH(2)I)] (9) which would represent a new type of so-called "pendulum" systems resulting from a degenerate sigma bond metathesis reaction of U-C and C-I bonds.

Investigating the electronic structure and bonding in uranyl compounds by combining NEXAFS spectroscopy and quantum chemistry.

The nature of the reactivity of the "yl" oxygens has been a subject of constant interest for a long time in uranyl chemistry. Thus, the electron-donor ability of the equatorial ligands plays an important role in the nature of the uranyl U=O bond. In this paper, a combination of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and both ground-state and time-dependent density functional theory (DFT) calculations have been used to examine the effect of equatorial plane ligation on the U=O bonding in two uranyl complexes: [UO(2)(py)(3)I(2)] and [UO(2)(CN)(5)][NEt(4)](3). By coupling experimental data and theory, spectral features observed in the oxygen K-edge NEXAFS spectra have been assigned. Despite the inert character of the U=O bond, we observe that the electron-donating or withdrawing character of the equatorial ligands has a measurable effect on features in the NEXAFS spectra of these species and thereby on the unoccupied molecular orbitals of {UO(2)}(2+).