Reactivity of Ir(I)-aminophosphane platforms towards oxidants.

The iridium(I)-aminophosphane complex [Ir{κ3C,P,P'-(SiNP-H)}(cod)] has been prepared by reaction of [IrCl(cod)(SiNP)] with KCH3COO. DFT calculations show that this reaction takes place through an unexpected outer sphere mechanism (SiNP = SiMe2{N(4-C6H4Me)PPh2}2; SiNP-H = CH2SiMe{N(4-C6H4Me)PPh2}2). The reaction of [IrCl(cod)(SiNP)] or [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with diverse oxidants has been explored, yielding a range of iridium(III) derivatives. On one hand, [IrCl(cod)(SiNP)] reacts with allyl chloride rendering the octahedral iridium(III) derivative [IrCl2(η3-C3H5)(SiNP)], which, in turn, reacts with tert-butyl isocyanide yielding the substitution product [IrCl(η3-C3H5)(CNtBu)(SiNP)]Cl via the observed intermediate [IrCl2(η1-C3H5)(CNtBu)(SiNP)]. On the other hand, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with [FeCp2]X (X = PF6, CF3SO3), I2 or CF3SO3CH3 results in the metal-centered two-electron oxidation rendering a varied assortment of iridium(III) compounds. [Ir{κ3C,P,P'-(SiNP-H)}(cod)] reacts with [FeCp2]+ (1 : 2) in acetonitrile affording [Ir{κ3C,P,P'-(SiNP-H)}(CH3CN)3]2+ isolated as both the triflato and the hexafluorophosphato derivatives. Also, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with I2 (1 : 1) yields a mixture of iridium(III) derivatives, namely the mononuclear compound [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I, containing the η2,η3-cycloocta-2,6-dien-1-yl ligand, and two isomers of the dinuclear derivative [Ir2{κ3C,P,P'-(SiNP-H)}2(µ-I)3]I, the first species being isolated in low yield. DFT calculations indicate that [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I forms as the result of a bielectronic oxidation of iridium(I) followed by the deprotonation of the cod ligand by iodide and the protonation of the methylene moiety of the [Ir{κ3C,P,P'-(SiNP-H)}] platform by the newly formed HI. Finally, the oxidation of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] by methyl triflate proceeds via a hydride abstraction from the cod ligand, with the elimination of methane and the formation of the η2,η3-cycloocta-2,6-dien-1-yl ligand with the concomitant two-electron oxidation of the iridium centre. The crystal structures of selected compounds have been determined.

ß-(Z)-Selective alkyne hydrosilylation by a N,O-functionalized NHC-based rhodium(I) catalyst.

Neutral and cationic cyclooctadiene rhodium(I) complexes with a lutidine-derived polydentate ligand having NHC and methoxy side-donor functions, [RhBr(cod)(κC-tBuImCH2PyCH2OMe)] and [Rh(cod)(κ2C,N-tBuImCH2PyCH2OMe)]PF6, have been prepared. Carbonylation of the cationic compound yields the dicarbonyl complex [Rh(CO)2(κ2C,N-tBuImCH2PyCH2OMe)]PF6 whereas carbonylation of the neutral compound affords a mixture of di- and monocarbonyl neutral complexes [RhBr(CO)2(κC-tBuImCH2PyCH2OMe)] and [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)]. These complexes efficiently catalyze the hydrosilylation of 1-hexyne with HSiMe2Ph with a marked selectivity towards the ß-(Z)-vinylsilane product. Catalyst [RhBr(CO)(κ2C,N-tBuImCH2PyCH2OMe)] showed a superior catalytic performance, in terms of both activity and selectivity, and has been applied to the hydrosilylation of a range of 1-alkynes and phenylacetylene derivatives with diverse hydrosilanes, including HSiMe2Ph, HSiMePh2, HSiPh3 and HSiEt3, showing excellent ß-(Z) selectivity for the hydrosilylation of linear aliphatic 1-alkynes. Hydrosilylation of internal alkynes, such as diphenylacetylene and 1-phenyl-1-propyne, selectively affords the syn-addition vinylsilane products. The ß-(Z) selectivity of these catalysts contrasts with that of related rhodium(I) catalysts based on 2-picolyl-functionalised NHC ligands, which were reported to be ß-(E) selective. An energy barrier ΔG‡ of 19.8 ± 2.0 kcal mol-1 (298 K) has been determined from kinetic studies on the hydrosilylation of 1-hexyne with HSiMe2Ph. DFT studies suggest that the methoxy-methyl group is unlikely to be involved in the activation of hydrosilane, and then hydrosilane activation is likely to proceed via a classical Si-H oxidative addition.

Selective Oxidation of Glycerol via Acceptorless Dehydrogenation Driven by Ir(I)-NHC Catalysts.

Iridium(I) compounds featuring bridge-functionalized bis-NHC ligands (NHC = N-heterocyclic carbene), [Ir(cod)(bis-NHC)] and [Ir(CO)2(bis-NHC)], have been prepared from the appropriate carboxylate- or hydroxy-functionalized bis-imidazolium salts. The related complexes [Ir(cod)(NHC)2]+ and [IrCl(cod)(NHC)(cod)] have been synthesized from a 3-hydroxypropyl functionalized imidazolium salt. These complexes have been shown to be robust catalysts in the oxidative dehydrogenation of glycerol to lactate (LA) with dihydrogen release. High activity and selectivity to LA were achieved in an open system under low catalyst loadings using KOH as a base. The hydroxy-functionalized bis-NHC catalysts are much more active than both the carboxylate-functionalized ones and the unbridged bis-NHC iridium(I) catalyst with hydroxyalkyl-functionalized NHC ligands. In general, carbonyl complexes are more active than the related 1,5-cyclooctadiene ones. The catalyst [Ir(CO)2{(MeImCH2)2CHOH}]Br exhibits the highest productivity affording TONs to LA up to 15,000 at very low catalyst loadings.

Preparation of Mixed Bis-N-Heterocyclic Carbene Rhodium(I) Complexes.

A series of mixed bis-NHC rhodium(I) complexes of type RhCl(η2-olefin)(NHC)(NHC') have been synthesized by a stepwise reaction of [Rh(µ-Cl)(η2-olefin)2]2 with two different NHCs (NHC = N-heterocyclic carbene), in which the steric hindrance of both NHC ligands and the η2-olefin is critical. Similarly, new mixed coumarin-functionalized bis-NHC rhodium complexes have been prepared by a reaction of mono NHC complexes of type RhCl(NHC-coumarin)(η2,η2-cod) with the corresponding azolium salt in the presence of an external base. Both synthetic procedures proceed selectively and allow the preparation of mixed bis-NHC rhodium complexes in good yields.

Copper-Catalyzed Azide-Alkyne Cycloaddition (CuAAC) by Functionalized NHC-Based Polynuclear Catalysts: Scope and Mechanistic Insights.

Copper(I) [Cu2(µ-Br)2(tBuImCH2pyCH2L)] n (L = OMe, NEt2, NHtBu) compounds supported by flexible functionalized NHC-based polydentate ligands have been prepared in a one-pot procedure by reacting the corresponding imidazolium salt with an excess of copper powder and Ag2O. An X-ray diffraction analysis has revealed that [Cu2(µ-Br)2(tBuImCH2pyCH2NEt2)] n is a linear coordination polymer formed by bimetallic [Cu(µ-Br)]2 units linked by the lutidine-based NHC-py-NEt2 ligand, which acts as a heteroditopic ligand with a 1κC-2κ2 N,N' coordination mode. We propose that the polymeric compounds break down in the solution into more compact tetranuclear [Cu2(µ-Br)2(tBuImCH2pyCH2L)]2 compounds with a coordination mode identical to the functionalized NHC ligands. These compounds have been found to exhibit high catalytic activity in the Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. In particular, [Cu2(µ-Br)2(tBuImCH2pyCH2NEt2)]2 efficiently catalyzes the click reaction of a range of azides and alkynes, under an inert atmosphere at room temperature in neat conditions at a very low catalyst loading, to quantitatively afford the corresponding 1,4-disubstituted 1,2,3-triazole derivatives in a few minutes. The cycloaddition reaction of benzyl azide to phenylacetylene can be performed at 25-50 ppm catalyst loading by increasing the reaction time and/or temperature. Reactivity studies have shown that the activation of the polynuclear catalyst precursor involves the alkyne deprotonation by the NHC moiety of the polydentate ligand to afford a copper(I)-alkynyl species bearing a functionalized imidazolium ligand. DFT calculations support the participation of the dinuclear species [(CuBr)2(µ-tBuImCH2pyCH2NEt2)], resulting from the fragmentation of the tetranuclear compound, as the catalytically active species. The proposed reaction pathway proceeds through zwitterionic dinuclear intermediates and entails the active participation of both copper atoms, as well as the NHC moiety as an internal base, which activates the reacting alkyne via deprotonation.

Cyclometalated iridium complexes based on monodentate aminophosphanes.

Monodentate aminophosphanes HNP [NH(4-tolyl)PPh2] and SiMe3NP [SiMe3N(4-tolyl)PPh2] react with [Ir(µ-Cl)(cod)]2 affording tetra- or pentacoordinate complexes of formula [IrCl(L)n(cod)] (L = HNP, n = 1, 2; L = SiMe3NP, n = 1). The reaction of [IrCl(SiMe3NP)(cod)] with carbon monoxide smoothly renders [Ir(CO)3(SiMe3NP)2][IrCl2(CO)2]. The reaction of HNP or SiMe3NP with [Ir(CH3CN)2(cod)][PF6] yields the cyclometalated iridium(III)-hydride derivatives [IrH{κ2C,P-NR(4-C6H3CH3)PPh2}(cod)(CH3CN)][PF6] (R = H, SiMe3) as a result of the intramolecular oxidative addition of the tolyl C2-H bond to iridium. The straighforward formation of [IrH{κ2C,P-SiMe3N(4-C6H3CH3)PPh2}(cod)(CH3CN)]+ was observed when the reaction was monitored by NMR spectroscopy at 233 K, whereas a more complex reaction sequence was observed in the formation of [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(cod)(CH3CN)]+, including the formation of [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(HNP)(cod)]+ and [Ir(cod)(HNP)2]+. The "mixed" complex [IrH{κ2C,P-SiMe3N(4-C6H3CH3)PPh2}(HNP)(cod)]+ was obtained upon reaction of [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(cod)(CH3CN)][PF6] with SiMe3NP at 233 K. Finally, the reaction of [Ir(CH3CN)2(coe)2][PF6] with SiMe3NP or HNP resulted in the formation of [Ir(CH3CN)2(SiMe3NP)2][PF6] and [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(HNP)2(CH3CN)][PF6], respectively. Both the OC-6-35 and the OC-6-52 isomers of [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(HNP)2(CH3CN)]+ - featuring facial and meridional dispositions of the phosphorus atoms, respectively - were isolated depending on the reaction solvent. Several compounds described herein catalyse the dehydrogenation of formic acid in DMF, [IrCl(HNP)2(cod)] being the most active, with TOF1 min of about 2300 h-1 (5 mol% catalyst, 50 mol% sodium formate, DMF, 80 °C).

Synthesis and reactivity of an iridium complex based on a tridentate aminophosphano ligand.

The iridium(III) hydride compound [IrH{κ3C,P,P'-(SiNP-H)}(CNtBu)2][PF6] (1PF6) was obtained by reaction of [Ir(SiNP)(cod)][PF6] with CNtBu as the result of the intramolecular oxidative addition of the SiCH2-H bond to iridium(I) [SiNP = Si(CH3)2{N(4-tolyl)PPh2}2, SiNP-H = CH2Si(CH3){N(4-tolyl)PPh2}2]. The mechanism of the reaction was investigated by NMR spectroscopy and DFT calculations showing that the pentacoordinated intermediate [Ir(SiNP)(cod)(CNtBu)][PF6] (2PF6) forms in the first place and that further reacts with CNtBu, affording the square planar intermediate [Ir(SiNP)(CNtBu)2][PF6] (3PF6) that finally undergoes the intramolecular oxidative addition of the SiCH2-H bond. The reactivity of 1PF6 was investigated. On one hand, the reaction of 1PF6 with N-chlorosuccinimide or N-bromosuccinimide provides the haloderivatives [IrX{κ3C,P,P'-(SiNP-H)}(CNtBu)2][PF6] (X = Cl, 4PF6; Br, 5PF6), and the reaction of 5PF6 with AgPF6 in the presence of acetonitrile affords the solvato species [Ir{κ3C,P,P'-(SiNP-H)}(CH3CN)(CNtBu)2]2+ (62+) isolated as the hexafluorophosphate salt. On the other hand, the reaction of 1PF6 with HBF4 gives the iridium(III) compound [IrH(CH2SiF2CH3)(HNP)2(CNtBu)2][BF4] (7BF4) as the result of the formal addition of hydrogen fluoride to the Si-N bonds of 1+ [HNP = HN(4-tolyl)PPh2]. A similar outcome was observed in the reaction of 1PF6 with CF3COOH rendering 7PO2F2. In this case the intermediate [IrH{κ2C,P-CH2SiMeFN(4-tolyl)PPh2}(HNP)(CNtBu)2]+ (8+) was observed and characterised in situ by NMR spectroscopy. DFT calculations suggests that the reaction goes through the sequential protonation of the nitrogen atom of the Si-N-P moiety followed by the formal addition of fluoride ion to silicon. Also, the crystal structures of SiNP, 1PF6, 4PF6 and 7BF4 have been determined by X-ray diffraction measurements.

Rhodium-NHC-Catalyzed gem-Specific O-Selective Hydropyridonation of Terminal Alkynes.

The dinuclear complex [Rh(µ-Cl)(η2 -coe)(IPr)]2 is an efficient catalyst for the O-selective Markovnikov-type addition of 2-pyridones to terminal alkynes. DFT calculations support a hydride-free pathway entailing intramolecular oxidative protonation of a π-alkyne by a κ1 N-hydroxypyridine ligand. Subsequent O-nucleophilic attack on a metallacyclopropene species affords an O-alkenyl-2-oxypyridine chelate rhodium intermediate as the catalyst resting state. The release of the alkenyl ether is calculated as the rate-determining step.

Catalytic applications of zwitterionic transition metal compounds.

This frontiers article highlights recent developments on the application of transition metal-based zwitterionic complexes in catalysis. Recent applications of selected zwitterionic catalysts in polymerization reactions, including the carbonylative polymerization of cyclic ethers, carbon-carbon coupling reactions, the asymmetric hydrogenation of unfunctionalized olefins, and the hydrofunctionalization of alkenes are reviewed. In addition, advances in the field of hydrogenation/dehydrogenation reactions related to energy applications, including the hydrogenation of CO2 and the dehydrogenation of formic acid and N-heterocycles, the functionalization of CO2 with amines and hydrosilanes, and the valorization of polyfunctional bio-based feedstocks, such as the dehygrogenation of glycerol to lactic acid or the reduction of levulinic acid into γ-valerolactone, are also described.

Iridium(i) complexes bearing hemilabile coumarin-functionalised N-heterocyclic carbene ligands with application as alkyne hydrosilylation catalysts.

A set of iridium(i) complexes of formula IrCl(κC,η2-IRCouR')(cod) or IrCl(κC, η2-BzIRCouR')(cod) (cod = 1,5-cyclooctadiene; Cou = coumarin; I = imidazolin-2-carbene; BzI = benzimidazolin-2-carbene) have beeen prepared from the corresponding azolium salt and [Ir(µ-OMe)(cod)]2 in THF at room temperature. The crystalline structures of 4b and 5b show a distorted trigonal bipyramidal configuration in the solid state with a coordinated coumarin moiety. In contrast, an equilibrium between this pentacoordinated structure and the related square planar isomer is observed in solution as a consequence of the hemilability of the pyrone ring. Characterization of both species by NMR was achieved at the low and high temperature limits, respectively. In addition, the thermodynamic parameters of the equilibrium, ΔHR and ΔSR, were obtained by VT 1H NMR spectroscopy and fall in the range 22-33 kJ mol-1 and 72-113 J mol-1 K-1, respectively. Carbonylation of IrCl(κC,η2-BzITolCou7,8-Me2)(cod) resulted in the formation of a bis-CO derivative showing no hemilabile behaviour. The newly synthesised complexes efficiently catalyze the hydrosilylation of alkynes at room temperature with a preference for the ß-(Z) vinylsilane isomer.

Preparation of Butadienylpyridines by Iridium-NHC-Catalyzed Alkyne Hydroalkenylation and Quinolizine Rearrangement.

Iridium(I) N-heterocyclic carbene complexes of formula Ir(κ2 O,O'-BHetA)(IPr)(η2 -coe) [BHetA=bis-heteroatomic acidato, acetylacetonate or acetate; IPr=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-carbene; coe=cyclooctene] have been prepared by treating Ir(κ2 O,O'-BHetA)(η2 -coe)2 complexes with IPr. These complexes react with 2-vinylpyridine to afford the hydrido-iridium(III)-alkenyl cyclometalated derivatives IrH(κ2 O,O'-BHetA)(κ2 N,C-C7 H6 N)(IPr) through the iridium(I) intermediate Ir(κ2 O,O'-BHetA)(IPr)(η2 -C7 H7 N). The cyclometalated IrH(κ2 O,O'-acac)(κ2 N,C-C7 H6 N)(IPr) complex efficiently catalyzes the hydroalkenylation of aromatic and aliphatic terminal alkynes and enynes with 2-vinylpyridine to afford 2-(4R-butadienyl)pyridines with Z,E configuration as the major reaction products (yield up to 89 %). In addition, unprecedented (Z)-2-butadienyl-5R-pyridine derivatives have been obtained as minor reaction products (yield up to 21 %) from the elusive 1Z,3gem-butadienyl hydroalkenylation products. These compounds undergo a thermal 6π-electrocyclization to afford bicyclic 4H-quinolizine derivatives that, under catalytic reaction conditions, tautomerize to 6H-quinolizine to afford the (Z)-2-(butadienyl)-5R-pyridine by a retro-electrocyclization reaction.

Rhodium(I)-NHC Complexes Bearing Bidentate Bis-Heteroatomic Acidato Ligands as gem-Selective Catalysts for Alkyne Dimerization.

A series of Rh(κ2 -BHetA)(η2 -coe)(IPr) complexes bearing 1,3-bis-hetereoatomic acidato ligands (BHetA) including carboxylato (O,O), thioacetato (O,S), amidato (O,N), thioamidato (N,S), and amidinato (N,N), have been prepared by reaction of the dinuclear precursor [Rh(µ-Cl)(IPr)(η2 -coe)]2 with the corresponding anionic BHetA species. The RhI -NHC-BHetA compounds catalyze the dimerization of aryl alkynes, showing excellent selectivity for the head-to-tail enynes. Among them, the acetanilidato-based catalyst has shown an outstanding catalytic performance reaching unprecedented TOF levels of 2500 h-1 with complete selectivity for the gem-isomer. Investigation of the reaction mechanism supports a non-oxidative pathway in which the BHetA ligand behaves as proton shuttle through intermediate κ1 -HBHetA species. However, in the presence of pyridine as additive, the identification of the common RhIII H(C≡CPh)2 (IPr)(py)2 intermediate gives support for an alternative oxidative route.

Synthesis and reactivity at the Ir-MeTpm platform: from κ1-N coordination to κ3-N-based organometallic chemistry.

Reaction of [Ir(µ-Cl)(COE)2]2 (COE = cis-cyclooctene) with tris(3,5-dimethylpyrazol-1-yl)methane (MeTpm) affords [IrCl(κ1-N-MeTpm)(COD)] (1) (COD = 1,5-cyclooctadiene). The formation of 1 implies the transfer dehydrogenation of a COE ligand to give COD and COA (cyclooctane). A mechanistic proposal based on DFT calculations that explains this iridium promoted process has been disclosed. Additionally, reactivity studies have allowed the preparation and characterization, including determination of the molecular structures of a number of iridium complexes with the MeTpm ligand in κ1, κ2 or κ3-N coordination modes. Moreover, the first example of an Ir-cyclooctyl complex featuring hydride and carbonyl ligands, whose solid state structure has been determined by X-ray diffraction methods, is reported.

Zwitterionic Rhodium and Iridium Complexes Based on a Carboxylate Bridge-Functionalized Bis-N-heterocyclic Carbene Ligand: Synthesis, Structure, Dynamic Behavior, and Reactivity.

A series of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized bis-N-heterocyclic carbene ligand of formula [Cp*MIIICl{(MeIm)2CHCOO}] and [MI(diene){(MeIm)2CHCOO}] (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; M = Rh, Ir; MeIm = 3-methylimidazol-2-yliden-1-yl; diene = 1,5-cyclooctadiene (cod), norbornadiene (nbd)) were prepared from the salt [(MeImH)2CHCOO]Br and suitable metal precursor. The solid-state structure of both types of complexes shows a boat-shaped six-membered metallacycle derived of the κ2C,C' coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the bowsprit position in the Cp*MIII complexes, whereas in the MI(diene) complexes it is at the flagpole position of the metallacycle. The complexes [RhI(diene){(MeIm)2CHCOO}] (diene = cod, nbd) exist as two conformational isomers in dichloromethane, bowsprit and flagpole, that interconvert through the boat-to-boat inversion of the metallacycle. An inversion barrier of ∼17 kcal·mol-1 was determined by two-dimensional exchange spectroscopy NMR measurements for [RhI(cod){(MeIm)2CHCOO}]. Reaction of zwitterionic Cp*MIII complexes with methyl triflate or tetrafluoroboric acid affords the cationic complexes [Cp*MIIICl{(MeIm)2CHCOOMe}]+ or [Cp*MIIICl{(MeIm)2CHCOOH}]+ (M = Rh, Ir) featuring carboxy and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands, respectively. Similarly, complexes [MI(diene){(MeIm)2CHCOOMe}]+ (M = Rh, Ir) were prepared by alkylation of the corresponding zwitterionic MI(diene) complexes with methyl triflate. In contrast, reaction of [IrI(cod){(MeIm)2CHCOO}] with HBF4·Et2O (Et = ethyl), CH3OTf, CH3I, or I2 gives cationic iridium(III) octahedral complexes [IrIIIX(cod){(MeIm)2CHCOO}]+ (X = H, Me, or I) featuring a tripodal coordination mode of the carboxylate bridge-functionalized bis-NHC ligand. The switch from κ2C,C' to κ3C,C',O coordination of the bis-NHC ligand accompanying the oxidative addition prevents the coordination of the anions eventually formed in the process that remain as counterions.

Mechanistic insights into the tropo-inversion of the biphenyl moiety in chiral bis-amido phosphites and in their palladium(ii) complexes.

Chiral bis-amido phosphites L1 and L2 containing a diaminobiphenyl unit and a chiral alkoxy group derived from either (-)-menthol or 3-acetoxy deoxycholic methyl ester have been synthesised. Both L1 and L2 react with PdCl2(NCPh)2 affording di- or mononuclear derivatives with formula trans-[Pd(µ-Cl)Cl(L)]2 (1a, L = L1; 1b, L = L2) or trans-PdCl2(L)2 (2a, L = L1; 2b, L = L2) depending on the Pd : L molar ratio. The crystal structure of (M,P)-1a confirms the trans arrangement of the ligand L1 and shows an unusual puckering of the Pd2(µ-Cl)2 core (θ 46°). Both the ligands L1 and L2 and their complexes (1 and 2) are fluxional in solution as a consequence of the tropo-inversion of the diaminobiphenyl unit. For L1, L2, 1a and 2a a combined study including variable temperature 31P{1H} NMR spectroscopy and line shape analysis, Eyring plots and DFT calculations have shed light on the mechanism of the tropo-inversion.

A well-defined NHC-Ir(iii) catalyst for the silylation of aromatic C-H bonds: substrate survey and mechanistic insights.

A well-defined NHC-Ir(iii) catalyst, [Ir(H)2(IPr)(py)3][BF4] (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene), that provides access to a wide range of aryl- and heteroaryl-silanes by intermolecular dehydrogenative C-H bond silylation has been prepared and fully characterized. The directed and non-directed functionalisation of C-H bonds has been accomplished successfully using an arene as the limiting reagent and a variety of hydrosilanes in excess, including Et3SiH, Ph2MeSiH, PhMe2SiH, Ph3SiH and (EtO)3SiH. Examples that show unexpected selectivity patterns that stem from the presence of aromatic substituents in hydrosilanes are also presented. The selective bisarylation of bis(hydrosilane)s by directed or non-directed silylation of C-H bonds is also reported herein. Theoretical calculations at the DFT level shed light on the intermediate species in the catalytic cycle and the role played by the ligand system on the Ir(iii)/Ir(i) mechanism.

Efficient preparation of carbamates by Rh-catalysed oxidative carbonylation: unveiling the role of the oxidant.

The synthesis of a wide variety of carbamates from amines, alcohols and carbon monoxide has been achieved by means of a Rh-catalysed oxidative carbonylation reaction that uses Oxone as a stoichiometric oxidant. In-depth studies on the reaction mechanism shed light on the intimate role of Oxone in the catalytic cycle.

Rhodium-Catalyzed Dehydrogenative Silylation of Acetophenone Derivatives: Formation of Silyl Enol Ethers versus Silyl Ethers.

A series of rhodium-NSiN complexes (NSiN=bis (pyridine-2-yloxy)methylsilyl fac-coordinated) is reported, including the solid-state structures of [Rh(H)(Cl)(NSiN)(PCy3 )] (Cy=cyclohexane) and [Rh(H)(CF3 SO3 )(NSiN)(coe)] (coe=cis-cyclooctene). The [Rh(H)(CF3 SO3 )(NSiN)(coe)]-catalyzed reaction of acetophenone with silanes performed in an open system was studied. Interestingly, in most of the cases the formation of the corresponding silyl enol ether as major reaction product was observed. However, when the catalytic reactions were performed in closed systems, formation of the corresponding silyl ether was favored. Moreover, theoretical calculations on the reaction of [Rh(H)(CF3 SO3 )(NSiN)(coe)] with HSiMe3 and acetophenone showed that formation of the silyl enol ether is kinetically favored, while the silyl ether is the thermodynamic product. The dehydrogenative silylation entails heterolytic cleavage of the Si-H bond by a metal-ligand cooperative mechanism as the rate-determining step. Silyl transfer from a coordinated trimethylsilyltriflate molecule to the acetophenone followed by proton transfer from the activated acetophenone to the hydride ligand results in the formation of H2 and the corresponding silyl enol ether.

Double hydrophosphination of alkynes promoted by rhodium: the key role of an N-heterocyclic carbene ligand.

The regioselective double hydrophosphination of alkynes mediated by rhodium catalysts is presented. The distinctive stereoelectronic properties of the NHC ligand prevent the catalyst deactivation by diphosphine coordination thereby allowing for the closing of a productive catalytic cycle.

En route to phosphonato iridium(i) complexes: the decisive effect of an intramolecular hydrogen bond.

Pentacoordinated iridium(i) complexes of formula IrCl(SiNP)(tfbb) (1) and IrCl(HNP)2(tfbb) (2) (SiNP = SiMe2{N(4-C6H4CH3)PPh2}2; HNP = NH(4-C6H4CH3)PPh2) have been prepared and fully characterised. Both feature a distorted square pyramidal coordination polyhedron at the metal centre in the solid state and are fluxional in solution. Their reaction with trimethyl phosphite yields the derivatives [Ir(SiNP){P(OMe)3}(tfbb)]Cl ([3]Cl) and Ir{PO(OMe)2}(HNP)2(tfbb) (4). The course of the reaction between IrCl(HNP)2(tfbb) (2) and trimethyl phosphite was elucidated by NMR spectroscopy and DFT calculations, showing that the intermediate [Ir(HNP)2{P(OMe)3}(tfbb)](+) ((5+)) forms and further reacts with the chloride anion yielding the phosphonato derivative 4 and methyl chloride. The decisive role of the N-H group in the formation of the phosphonato ligand has been established by IR and NMR spectroscopic measurements and by DFT calculations.