Nitrous oxide activation by picoline-derived Ni-CNP hydrides.

Oxygen atom transfer (OAT) from N2O to the Ni-H bond of proton-responsive picoline-derived CNP nickel complexes has been investigated both experimentally and theoretically. These Ni-CNP complexes efficiently catalyse the reduction of N2O with pinacolborane (HBpin) under mild conditions.

Ruthenium nanoparticles stabilized by 1,2,3-triazolylidene ligands in the hydrogen isotope exchange of E-H bonds (E = B, Si, Ge, Sn) using deuterium gas.

A series of ruthenium nanoparticles (Ru·MIC) stabilized with different mesoionic 1,2,3-triazolylidene (MIC) ligands were prepared by decomposition of the Ru(COD)(COT) (COD = 1,5-cyclooctadiene; COT = 1,3,5-cyclooctatriene) precursor with H2 (3 bar) in the presence of substoichiometric amounts of the stabilizer (0.1-0.2 equiv.). Small and monodisperse nanoparticles exhibiting mean sizes between 1.1 and 1.2 nm were obtained, whose characterization was carried out by means of transmission electron microscopy (TEM), including high resolution TEM (HRTEM), inductively coupled plasma (ICP) analysis and X-ray photoelectron spectroscopy (XPS). In particular, XPS measurements confirmed the presence of MIC ligands on the surfaces of the nanoparticles. The Ru·MIC nanoparticles were used in the isotopic H/D exchange of different hydrosilanes, hydroboranes, hydrogermananes and hydrostannanes using deuterium gas under mild conditions (1.0 mol% Ru, 1 bar D2, 55 °C). Selective labelling of the E-H (E = B, Si, Ge, Sn) bond in these derivatives, with high levels of deuterium incorporation, was observed.

Catalytic Nitrous Oxide Reduction with H2 Mediated by Pincer Ir Complexes.

Reduction of nitrous oxide (N2O) with H2 to N2 and water is an attractive process for the decomposition of this greenhouse gas to environmentally benign species. Herein, a series of iridium complexes based on proton-responsive pincer ligands (1-4) are shown to catalyze the hydrogenation of N2O under mild conditions (2 bar H2/N2O (1:1), 30 °C). Among the tested catalysts, the Ir complex 4, based on a lutidine-derived CNP pincer ligand having nonequivalent phosphine and N-heterocyclic carbene (NHC) side donors, gave rise to the highest catalytic activity (turnover frequency (TOF) = 11.9 h-1 at 30 °C, and 16.4 h-1 at 55 °C). Insights into the reaction mechanism with 4 have been obtained through NMR spectroscopy. Thus, reaction of 4 with N2O in tetrahydrofuran-d8 (THF-d8) initially produces deprotonated (at the NHC arm) species 5NHC, which readily reacts with H2 to regenerate the trihydride complex 4. However, prolonged exposure of 4 to N2O for 6 h yields the dinitrogen Ir(I) complex 7P, having a deprotonated (at the P-arm) pincer ligand. Complex 7P is a poor catalytic precursor in the N2O hydrogenation, pointing out to the formation of 7P as a catalyst deactivation pathway. Moreover, when the reaction of 4 with N2O is carried out in wet THF-d8, formation of a new species, which has been assigned to the hydroxo species 8, is observed. Finally, taking into account the experimental results, density functional theory (DFT) calculations were performed to get information on the catalytic cycle steps. Calculations are in agreement with 4 as the TOF-determining intermediate (TDI) and the transfer of an apical hydrido ligand to the terminal nitrogen atom of N2O as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC- or the P-CH2 pincer methylene linkers.

Reduction of N2O with hydrosilanes catalysed by RuSNS nanoparticles.

A series of RuSNS nanoparticles, prepared by decomposition of Ru(COD)(COT) with H2 in the presence of an SNS ligand, have been found to catalyse the reduction of the greenhouse gas N2O to N2 employing different hydrosilanes.

Ammonia-Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNNH Complexes.

Metal complexes incorporating proton-responsive ligands have been proved to be superior catalysts in reactions involving the H2 molecule. In this contribution, a series of IrIII complexes based on lutidine-derived CNNH pincers containing N-heterocyclic carbene and secondary amino NHR [R = Ph (4a), tBu (4b), benzyl (4c)] donors as flanking groups have been synthesized and tested in the dehydrogenation of ammonia-borane (NH3BH3, AB) in the presence of substoichiometric amounts (2.5 equiv) of tBuOK. These preactivated derivatives are efficient catalysts in AB dehydrogenation in THF at room temperature, albeit significantly different reaction rates were observed. Thus, by using 0.4 mol % of 4a, 1.0 equiv of H2 per mole of AB was released in 8.5 min (turnover frequency (TOF50%) = 1875 h-1), while complexes 4b and 4c (0.8 mol %) exhibited lower catalytic activities (TOF50% = 55-60 h-1). 4a is currently the best performing IrIII homogeneous catalyst for AB dehydrogenation. Kinetic rate measurements show a zero-order dependence with respect to AB, and first order with the catalyst in the dehydrogenation with 4a (-d[AB]/dt = k[4a]). Conversely, the reaction with 4b is second order in AB and first order in the catalyst (-d[AB]/dt = k[4b][AB]2). Moreover, the reactions of the derivatives 4a and 4b with an excess of tBuOK (2.5 equiv) have been analyzed through NMR spectroscopy. For the former precursor, formation of the iridate 5 was observed as a result of a double deprotonation at the amine and the NHC pincer arm. In marked contrast, in the case of 4b, a monodeprotonated (at the pincer NHC-arm) species 6 is observed upon reaction with tBuOK. Complex 6 is capable of activating H2 reversibly to yield the trihydride derivative 7. Finally, DFT calculations of the first AB dehydrogenation step catalyzed by 5 has been performed at the DFT//MN15 level of theory in order to get information on the predominant metal-ligand cooperation mode.

Hydrogenation/dehydrogenation of N-heterocycles catalyzed by ruthenium complexes based on multimodal proton-responsive CNN(H) pincer ligands.

Ru complexes based on lutidine-derived pincer CNN(H) ligands having secondary amine side donors are efficient precatalysts in the hydrogenation and dehydrogenation of N-heterocycles. Reaction of a Ru-CNN(H) complex with an excess of base produces the formation of a Ru(0) derivative, which is observed under catalytic conditions.

Hydroboration of carbon dioxide with catechol- and pinacolborane using an Ir-CNP* pincer complex. Water influence on the catalytic activity.

Iridium complexes based on deprotonated lutidine-derived CNP* pincers 2a/2b selectively catalyzed the hydroboration of CO2 under mild conditions (1-2 bar CO2, 30 °C) to methoxyborane using HBcat (TOF up to 56 h-1) and to the formate level with HBpin (TOF up to 1245 h-1). Interestingly, an intriguing, positive water effect on the reaction rates has been observed. NMR spectroscopy and ESI-MS analysis of the hydroboration reactions have shown the formation of ligand-protonated [Ir(CNP)(CO)(BR2)H][B(R2)2] (R2 = catecholate, pinacolate) derivatives under catalytic conditions. Control experiments, however, have demonstrated that these derivatives are not catalytically competent species in the hydroboration of CO2.

Activation of Small Molecules by the Metal-Amido Bond of Rhodium(III) and Iridium(III) (η5-C5Me5)M-Aminopyridinate Complexes.

We report the synthesis and structural characterization of five-coordinate complexes of rhodium and iridium of the type [(η5-C5Me5)M(N^N)]+ (3-M+), where N^N represents the aminopyridinate ligand derived from 2-NH(Ph)-6-(Xyl)C5H3N (Xyl = 2,6-Me2C6H3). The two complexes were isolated as salts of the BArF anion (BArF = B[3,5-(CF3)2C6H3]4). The M-Namido bond of complexes 3-M+ readily activated CO, C2H4, and H2. Thus, compounds 3-M+ reacted with CO under ambient conditions, but whereas for 3-Rh+, CO migratory insertion was fast, yielding a carbamoyl carbonyl species, 4-Rh+, the stronger Ir-Namido bond of complex 3-Ir+ caused the reaction to stop at the CO coordination stage. In contrast, 3-Ir+ reacted reversibly with C2H4, forming adduct 5-Ir+, which subsequently rearranged irreversibly to [Ir](H)(═C(Me)N(Ph)-) complex 6-Ir+, which contains an N-stabilized carbene ligand. Computational studies supported a migratory insertion mechanism, giving first a ß-stabilized linear alkyl unit, [Ir]CH2CH2N(Ph)-, followed by a multistep rearrangement that led to the final product 6-Ir+. Both ß- and α-H eliminations, as well as their microscopic reverse migratory insertion reactions, were implicated in the alkyl-to-hydride-carbene reorganization. The analogous reaction of 3-Rh+ with C2H4 originated a complex mixture of products from which only a branched alkyl [Rh]C(H)(Me)N(Ph)- (5-Rh+) could be isolated, featuring a ß-agostic methyl interaction. Reactions of 3-M+ with H2 promoted a catalytic isomerization of the Ap ligand from classical κ2-N,N' binding to κ-N plus η3-pseudoallyl coordination mode.

Functionalization of 3-Iridacyclopentenes.

Members of a series of iridacyclopentenes of composition [TpMe2 Ir(k2 -C,C-CH2 CR'=CRCH2 )(CO)] (TpMe2 =hydrotris(3,5-dimethylpyrazolyl)borate; R=R'=H, 1; R=Me, R'=H, 2; R=R'=Me, 3) have been subjected to common organic chemistry procedures for hydrogenation, cyclopropanation, epoxidation, water addition through hydroboration, cis-dihydroxylation, and ozonolysis. The stability of metallacycles 1-3, imparted by the presence of the co-ligands TpMe2 and CO, directs the reactivity towards the C=C double bonds, and furthermore the stereochemistry of the products formed is strongly dictated by the steric demands of the TpMe2 ligand. While the products obtained in some of the above-mentioned reactions are the expected ones from an organic chemistry point of view, in other cases the results differ from the outcomes of similar reactions carried out with the all-carbon counterparts.

Synthesis, structure and reactivity of Pd and Ir complexes based on new lutidine-derived NHC/phosphine mixed pincer ligands.

Coordination studies of new lutidine-derived hybrid NHC/phosphine ligands (CNP) to Pd and Ir have been performed. Treatment of the square-planar [Pd(CNP)Cl](AgCl2) complex 2a with KHMDS produces the selective deprotonation at the CH2P arm of the pincer to yield the pyridine-dearomatised complex 3a. A series of cationic [Ir(CNP)(cod)]+ complexes 4 has been prepared by reaction of the imidazolium salts 1 with Ir(acac)(cod). These derivatives exhibit in the solid state, and in solution, a distorted trigonal bipyramidal structure in which the CNP ligands adopt an unusual C(axial)-N(equatorial)-P(equatorial) coordination mode. Reactions of complexes 4 with CO and H2 yield the carbonyl species 5a(Cl) and 6a(Cl), and the dihydrido derivatives 7, respectively. Furthermore, upon reaction of complex 4b(Br) with base, selective deprotonation at the methylene CH2P arms is observed. The, thus formed, deprotonated Ir complex 8b reacts with H2 in a ligand-assisted process leading to the trihydrido complex 9b, which can also be obtained by reaction of 7b(Cl) with H2 in the presence of KOtBu. Finally, the catalytic activity of Ir-CNP complexes in the hydrogenation of ketones has been briefly assessed.

Synthesis and Reactivity toward H2 of (η(5)-C5Me5)Rh(III) Complexes with Bulky Aminopyridinate Ligands.

Electrophilic, cationic Rh(III) complexes of composition [(η(5)-C5Me5)Rh(Ap)](+), (1(+)), were prepared by reaction of [(η(5)-C5Me5)RhCl2]2 and LiAp (Ap = aminopyridinate ligand) followed by chloride abstraction with NaBArF (BArF = B[3,5-(CF3)2C6H3]4). Reactions of cations 1(+) with different Lewis bases (e.g., NH3, 4-dimethylaminopyridine, or CNXyl) led in general to monoadducts 1·L(+) (L = Lewis base; Xyl = 2,6-Me2C6H3), but carbon monoxide provided carbonyl-carbamoyl complexes 1·(CO)2(+) as a result of metal coordination and formal insertion of CO into the Rh-Namido bond of complexes 1(+). Arguably, the most relevant observation reported in this study stemmed from the reactions of complexes 1(+) with H2. (1)H NMR analyses of the reactions demonstrated a H2-catalyzed isomerization of the aminopyridinate ligand in cations 1(+) from the ordinary κ(2)-N,N' coordination to a very uncommon, formally tridentate κ-N,η(3) pseudoallyl bonding mode (complexes 3(+)) following benzylic C-H activation within the xylyl substituent of the pyridinic ring of the aminopyridinate ligand. The isomerization entailed in addition H-H and N-H bond activation and mimicked previous findings with the analogous iridium complexes. However, in dissimilarity with iridium, rhodium complexes 1(+) reacted stoichiometrically at 20 °C with excess H2. The transformations resulted in the hydrogenation of the C5Me5 and Ap ligands with concurrent reduction to Rh(I) and yielded complexes [(η(4)-C5Me5H)Rh(η(6)-ApH)](+), (2(+)), in which the pyridinic xylyl substituent is η(6)-bonded to the rhodium(I) center. New compounds reported were characterized by microanalysis and NMR spectroscopy. Representative complexes were additionally investigated by X-ray crystallography.

Dihydrogen catalysis of the reversible formation and cleavage of C-H and N-H bonds of aminopyridinate ligands bound to (η(5) -C5 Me5 )Ir(III.).

This study focuses on a series of cationic complexes of iridium that contain aminopyridinate (Ap) ligands bound to an (η(5) -C5 Me5 )Ir(III) fragment. The new complexes have the chemical composition [Ir(Ap)(η(5) -C5 Me5 )](+) , exist in the form of two isomers (1(+) and 2(+) ) and were isolated as salts of the BArF (-) anion (BArF =B[3,5-(CF3 )2 C6 H3 ]4 ). Four Ap ligands that differ in the nature of their bulky aryl substituents at the amido nitrogen atom and pyridinic ring were employed. In the presence of H2 , the electrophilicity of the Ir(III) centre of these complexes allows for a reversible prototropic rearrangement that changes the nature and coordination mode of the aminopyridinate ligand between the well-known κ(2) -N,N'-bidentate binding in 1(+) and the unprecedented κ-N,η(3) -pseudo-allyl-coordination mode in isomers 2(+) through activation of a benzylic C-H bond and formal proton transfer to the amido nitrogen atom. Experimental and computational studies evidence that the overall rearrangement, which entails reversible formation and cleavage of H-H, C-H and N-H bonds, is catalysed by dihydrogen under homogeneous conditions.

Photochemical Reactions of Fluorinated Pyridines at Half-Sandwich Rhodium Complexes: Competing Pathways of Reaction.

Irradiation of CpRh(PMe3)(C2H4) (1; Cp = η5-C5H5) in the presence of pentafluoropyridine in hexane solution at low temperature yields an isolable η2-C,C-coordinated pentafluoropyridine complex, CpRh(PMe3)(η2-C,C-C5NF4) (2). The molecular structure of 2 was determined by single-crystal X-ray diffraction, showing coordination by C3-C4, unlike previous structures of pentafluoropyridine complexes that show N-coordination. Corresponding experiments with 2,3,5,6-tetrafluoropyridine yield the C-H oxidative addition product CpRh(PMe3)(C5NF4)H (3). In contrast, UV irradiation of 1 in hexane, in the presence of 4-substituted tetrafluoropyridines C5NF4X, where X = NMe2, OMe, results in elimination of C2H4 and HF to form the metallacycles CpRh(PMe3)(κ2-C,C-CH2N(CH3)C5NF3) (4) and CpRh(PMe3)(κ2-C,C-CH2OC5NF3) (5), respectively. The X-ray structure of 4 shows a planar RhCCNC-five-membered ring. Complexes 2-5 may also be formed by thermal reaction of CpRh(PMe3)(Ph)H with the respective pyridines at 50 °C.

Aldehyde-assisted hydrogen transfer during the formation of hydride-iridafurans from alkynes and aldehydes.

The bis(ethylene) Ir(I) complex [Tp(Me(2))Ir(C(2)H(4))(2)] (1; Tp(Me(2))=hydrotris(3,5-dimethylpyrazolyl)borate) reacts with two equivalents of aromatic or aliphatic aldehydes in the presence of one equivalent of dimethyl acetylenedicarboxylate (DMAD) with ultimate formation of hydride iridafurans of the formula [Tp(Me(2))Ir(H){C(R(1))=C(R(2))C(R(3))O}] (R(1)=R(2)=CO(2) Me; R(3)=alkyl, aryl; 3). Several intermediates have been observed in the course of the reaction. It is proposed that the key step of metallacycle formation is a C-C coupling process in the undetected Ir(I) species [Tp(Me(2))Ir{η(1)-O-R(3)C(=O)H}(DMAD)] (A) to give the trigonal-bipyramidal 16e(-) Ir(III) intermediates [Tp(Me(2))Ir{C(CO(2)Me)=C(CO(2)Me)C(R(3))(H)O}] (C), which have been trapped by NCMe to afford the adducts 11 (R(3)=Ar). If a second aldehyde acts as the trapping reagent for these species, this ligand acts as a shuttle in transfering a hydrogen atom from the γ- to the α-carbon atom of the iridacycle through the formation of an alkoxide group. Methyl propiolate (MP) can be used instead of DMAD to regioselectively afford the related iridafurans. These reactions have also been studied by DFT calculations.

Synthesis and reactivity of molybdenum imido alkylidene bis-pyrazolide complexes.

Reaction of Li(3,5-R(2)-pyrazolide) (R = tBu or Ph, dXpz) with Mo(NAr)(CHCMe(2)Ph)(OTf)(2)(DME) yields Mo(NAr)(CHCMe(2)Ph)(dXpz)(2) in good yield. These complexes react with alcohols or the surface silanols of silica, to yield bis-alkoxy and surface mono-siloxy alkene metathesis catalysts, respectively.

Metal fragment isomerisation upon grafting a d(2) ML4 perhydrocarbyl Os complex on a silica surface: origin and consequence.

Grafting of Os bisalkylidene complexes, [Os(=CHtBu)(2)(CH(2)tBu)(2)], on a silica partially dehydroxylated at 700 degrees C selectively yields an alkylidyne complex [([triple bond]SiO)Os([triple bond]CtBu)(CH(2)tBu)(2)] according to mass balance analysis, IR and solid state NMR spectroscopies, but 70% of the silanols remains unreacted. Grafting corresponds to a replacement of one alkyl by a siloxy ligand and induces the isomerisation of the metal fragment from a bis-alkylidene to an alkyl alkylidyne. Molecular (B3PW91) and periodic DFT-calculations show that the bis-alkylidene is the energetically favoured isomer for the perhydrocarbyl complex, while the alkyl alkylidyne isomer is more stable when one of the alkyl ligands is replaced by a siloxy ligand. The change of the nature of the ligand is accompanied with a change of geometry: from a distorted tetrahedral structure for [Os(=CHtBu)(2)(CH(2)tBu)(2)] to a butterfly-geometry with apical siloxy and alkylidyne ligands for [([triple bond]SiO)Os([triple bond]CtBu)(CH(2)tBu)(2)]. Finally, DFT calculations show that grafting occurs via a sigma-bond metathesis between the silanol and a metal-alkyl bond and not through the typical addition of the silanol onto the alkylidene ligand.

Well-defined silica-supported Mo-alkylidene catalyst precursors containing one or substituent: methods of preparation and structure-reactivity relationship in alkene metathesis.

The monosiloxy surface complexes [([triple bond]SiO)Mo([triple bond]NAr)(=CHCMe(2)R')(OR)] (R' = Me or Ph; OR = OtBu, OCMe(CF(3))(2) or OAr) are obtained by grafting onto SiO(2-(700)) either symmetric Mo-alkylidene derivatives [Mo([triple bond]NAr)(=CHCMe(2)R')(OR)(2)] or asymmetric derivatives, that is, with two different pendant ligands, one amido and one alkoxy/aryloxy, [Mo([triple bond]NAr)(=CHCMe(2)R')(OR)(NC(6)H(8))]. The formation of these complexes was confirmed by mass-balance analysis, and infrared (IR) and NMR spectroscopies. These systems are highly efficient catalyst precursors for the metathesis of acyclic alkenes; the best results were seen when OR=OCMe(CF(3))(2). Nevertheless, they display poor performances in ring-closing metathesis, possibly due to the rigidity of the metal center (as evidenced by NMR spectroscopy), which therefore slows the rate of the metathesis.

Dramatic enhancement of the alkene metathesis activity of Mo imido alkylidene complexes upon replacement of one tBuO by a surface siloxy ligand.

[(triple bond SiO)Mo(triple bond NAr)(=CHCMe2R)(OtBu)], a well-defined silica supported alkene metathesis catalyst precursor, shows a dramatic enhancement of activity and selectivity compared to [Mo(triple bond NAr)(=CHCMe2R)(OtBu)2] and [(triple bond SiO)Mo(triple bond NAr)(=CHCMe2R)(CH2tBu)], respectively.

Access to well-defined isolated Fe(II) centers on silica and their use in oxidation.

Well-defined Fe(II) isolated sites are obtained by reaction of diaryl-N,N'-diazadiene bis(neosilyl) iron (1) with an aerosil silica, SiO(2-(700)). This system can be used as a precursor for the catalytic oxidation of cyclohexene into cyclohexene oxide, cyclohexenol and cyclohexenone in the presence of H(2)O(2).