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

The iridium(I)-aminophosphane complex [Ir{κ,,'-(SiNP-H)}(cod)] has been prepared by reaction of [IrCl(cod)(SiNP)] with KCHCOO. DFT calculations show that this reaction takes place through an unexpected outer sphere mechanism (SiNP = SiMe{N(4-CHMe)PPh}; SiNP-H = CHSiMe{N(4-CHMe)PPh}). The reaction of [IrCl(cod)(SiNP)] or [Ir{κ,,'-(SiNP-H)}(cod)] with diverse oxidants has been explored, yielding a range of iridium(III) derivatives.

β-()-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-BuImCHPyCHOMe)] and [Rh(cod)(κC,N-BuImCHPyCHOMe)]PF, have been prepared. Carbonylation of the cationic compound yields the dicarbonyl complex [Rh(CO)(κC,N-BuImCHPyCHOMe)]PF whereas carbonylation of the neutral compound affords a mixture of di- and monocarbonyl neutral complexes [RhBr(CO)(κC-BuImCHPyCHOMe)] and [RhBr(CO)(κC,N-BuImCHPyCHOMe)]. These complexes efficiently catalyze the hydrosilylation of 1-hexyne with HSiMePh with a marked selectivity towards the β-()-vinylsilane product.

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)(bis-NHC)], have been prepared from the appropriate carboxylate- or hydroxy-functionalized bis-imidazolium salts. The related complexes [Ir(cod)(NHC)] 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.

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

A series of mixed bis-NHC rhodium(I) complexes of type RhCl(η-olefin)(NHC)(NHC') have been synthesized by a stepwise reaction of [Rh(-Cl)(η-olefin)] with two different NHCs (NHC = N-heterocyclic carbene), in which the steric hindrance of both NHC ligands and the η-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)(η,η-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) [Cu(μ-Br)(BuImCHpyCHL)] (L = OMe, NEt, NHBu) 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 AgO. An X-ray diffraction analysis has revealed that [Cu(μ-Br)(BuImCHpyCHNEt)] is a linear coordination polymer formed by bimetallic [Cu(μ-Br)] units linked by the lutidine-based NHC-py-NEt ligand, which acts as a heteroditopic ligand with a 1κ-2κ coordination mode. We propose that the polymeric compounds break down in the solution into more compact tetranuclear [Cu(μ-Br)(BuImCHpyCHL)] compounds with a coordination mode identical to the functionalized NHC ligands.

Cyclometalated iridium complexes based on monodentate aminophosphanes.

Monodentate aminophosphanes HNP [NH(4-tolyl)PPh] and SiMeNP [SiMeN(4-tolyl)PPh] react with [Ir(μ-Cl)(cod)] affording tetra- or pentacoordinate complexes of formula [IrCl(L)(cod)] (L = HNP, = 1, 2; L = SiMeNP, = 1). The reaction of [IrCl(SiMeNP)(cod)] with carbon monoxide smoothly renders [Ir(CO)(SiMeNP)][IrCl(CO)]. The reaction of HNP or SiMeNP with [Ir(CHCN)(cod)][PF] yields the cyclometalated iridium(III)-hydride derivatives [IrH{κ,-NR(4-CHCH)PPh}(cod)(CHCN)][PF] (R = H, SiMe) as a result of the intramolecular oxidative addition of the tolyl C-H bond to iridium.

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

The iridium(III) hydride compound [IrH{κ,,'-(SiNP-H)}(CNBu)][PF] (1PF) was obtained by reaction of [Ir(SiNP)(cod)][PF] with CNBu as the result of the intramolecular oxidative addition of the SiCH-H bond to iridium(I) [SiNP = Si(CH){N(4-tolyl)PPh}, SiNP-H = CHSi(CH){N(4-tolyl)PPh}]. The mechanism of the reaction was investigated by NMR spectroscopy and DFT calculations showing that the pentacoordinated intermediate [Ir(SiNP)(cod)(CNBu)][PF] (2PF) forms in the first place and that further reacts with CNBu, affording the square planar intermediate [Ir(SiNP)(CNBu)][PF] (3PF) that finally undergoes the intramolecular oxidative addition of the SiCH-H bond. The reactivity of 1PF was investigated.

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

The dinuclear complex [Rh(μ-Cl)(η -coe)(IPr)] 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 κ 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.

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 CO and the dehydrogenation of formic acid and N-heterocycles, the functionalization of CO 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,η-ICou)(cod) or IrCl(κC, η-BzICou)(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)] 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.

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

Iridium(I) N-heterocyclic carbene complexes of formula Ir(κ O,O'-BHetA)(IPr)(η -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(κ O,O'-BHetA)(η -coe) complexes with IPr. These complexes react with 2-vinylpyridine to afford the hydrido-iridium(III)-alkenyl cyclometalated derivatives IrH(κ O,O'-BHetA)(κ N,C-C H N)(IPr) through the iridium(I) intermediate Ir(κ O,O'-BHetA)(IPr)(η -C H N). The cyclometalated IrH(κ O,O'-acac)(κ N,C-C H 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 %).

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

A series of Rh(κ -BHetA)(η -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)(η -coe)] with the corresponding anionic BHetA species. The Rh -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 with complete selectivity for the gem-isomer.

Synthesis and reactivity at the Ir-Tpm platform: from κ-N coordination to κ-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.

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*MCl{(MeIm)CHCOO}] and [M(diene){(MeIm)CHCOO}] (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)CHCOO]Br and suitable metal precursor. The solid-state structure of both types of complexes shows a boat-shaped six-membered metallacycle derived of the κC,C' coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the bowsprit position in the Cp*M complexes, whereas in the M(diene) complexes it is at the flagpole position of the metallacycle.

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 PdCl(NCPh) affording di- or mononuclear derivatives with formula trans-[Pd(μ-Cl)Cl(L)] (1a, L = L1; 1b, L = L2) or trans-PdCl(L) (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 Pd(μ-Cl) core (θ 46°).

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)(IPr)(py)][BF] (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 EtSiH, PhMeSiH, PhMeSiH, PhSiH and (EtO)SiH. Examples that show unexpected selectivity patterns that stem from the presence of aromatic substituents in hydrosilanes are also presented.

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.

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).

Oxidation and β-Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N-Heterocyclic Carbene Ligands.

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for CC bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)-cyclooctadiene complexes having a NHC ligand with a O- or N-functionalised wingtip efficiently catalysed the oxidation and β-alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH3 )(cod)(MeIm(2- methoxybenzyl))][BF4 ] (cod=1,5-cyclooctadiene, MeIm=1-methylimidazolyl) having a rigid O-functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF0 ) of 1283 h(-1) , and also in the β-alkylation of 2-propanol with butan-1-ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan-2-ol.

Efficient Rhodium-Catalyzed Multicomponent Reaction for the Synthesis of Novel Propargylamines.

[{Rh(μ-Cl)(H)2 (IPr)}2 ] (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazole-2-ylidene) was found to be an efficient catalyst for the synthesis of novel propargylamines by a one-pot three-component reaction between primary arylamines, aliphatic aldehydes, and triisopropylsilylacetylene. This methodology offers an efficient synthetic pathway for the preparation of secondary propargylamines derived from aliphatic aldehydes. The reactivity of [{Rh(μ-Cl)(H)2 (IPr)}2 ] with amines and aldehydes was studied, leading to the identification of complexes [RhCl(CO)IPr(MesNH2 )] (MesNH2 = 2,4,6-trimethylaniline) and [RhCl(CO)2 IPr].

Intramolecular C-H oxidative addition to iridium(I) triggered by trimethyl phosphite in N,N'-diphosphanesilanediamine complexes.

The reaction of [Ir(SiNP)(cod)][PF6] ([1][PF6]) and of IrCl(SiNP)(cod) (5) (SiNP = SiMe2{N(4-C6H4CH3)PPh2}2) with trimethyl phosphite affords the iridium(iii) derivatives of the formula [IrHClx(SiNP-H){P(OMe)3}2-x]((1-x)+) (x = 0, 3(+); x = 1, 6) containing the κ(3)C,P,P'-coordinated SiNP-H ligand (SiNP-H = Si(CH2)(CH3){N(4-C6H4CH3)PPh2}2). The thermally unstable pentacoordinated cation [Ir(SiNP){P(OMe)3}(cod)](+) (2(+)) has been detected as an intermediate of the reaction and has been fully characterised in solution. Also, the mechanism of the C-H oxidative addition has been elucidated by DFT calculations showing that the square planar iridium(i) complexes of the formula [IrClx(SiNP){P(OMe)3}2-x]((1-x)+) (x = 0, 4(+); x = 1, 7) should be firstly obtained from 2(+) and finally should undergo the C-H oxidative addition to iridium(i) via a concerted intramolecular mechanism.

Tuning PCP-Ir complexes: the impact of an N-heterocyclic olefin.

A new PCP-type ligand based on an N-heterocyclic olefin (NHO) scaffold has been prepared. The flexibility of this ligand, which is able to adopt facial coordination modes in Ir(I) or meridional in Ir(III) complexes, can be attributed to the dual nature (ylide-olefin) of the NHO scaffold. This results in a rare case of olefin "slippage" that is supported by X-ray crystallography and DFT calculations.

A bimetallic iridium(ii) catalyst: [{Ir(IDipp)(H)}2][BF4]2 (IDipp = 1,3-bis(2,6-diisopropylphenylimidazol-2-ylidene)).

A new Ir(ii) complex [{Ir(μ-κCNHC,η(6)Dipp-IDipp)(H)}2][BF4]2 has been prepared and fully characterised. This complex acts as a catalyst for the hydroalkynylation of imines according to an unprecedented diiridium-mediated mechanism.