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.

Study of the Coordination Modes of Hybrid NNCp Cyclopentadienyl/Scorpionate Ligands in Ir Compounds.

A new coordination mode for the hybrid scorpionate/cyclopentadienyl ligand bpzcp, [bpzcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethylcyclopentadienyl] is observed in iridium complexes. The reaction of the lithium precursor, [Li(bpzcp)(THF)], with a range of [IrCl(diene)]2 compounds leads to an unprecedented binding mode of the hybrid scorpionate/cyclopentadienyl ligand as η5-Cp-coordinated and the formation of Ir(I) derivatives [Ir(η5-Cp-bpzcp)(η4-cod)] (1), [Ir(η5-Cp-bpzcp){η4-CH2═C(Me)C(Me)═CH2}] (2), [Ir(η5-Cp-bpzcp)(η2-coe)2] (3), and [Ir(η5-Cp-bpzcp)(η2-CH2═CH2)2] (4). The Ir(I) complex 4 reacts with CO or bromine to afford the compound [Ir(η5-Cp-bpzcp)(CO)2] (5) and the 18e- Ir(III) complex [Ir(κ-N-η5-Cp-bpzcpBr2)Br2] (6), respectively. Reaction of the iridium compounds (2-4) with CuI or [PdCl2(CH3CN)2] yields the heterobimetallic iridium-copper or iridium-palladium complexes [Ir(η5-Cp-bpzcp){η4-CH2═C(Me)C(Me)═CH2}(µ-bpzcp){CuI(κ2-NN-bpzcp)}] (7), [Ir(η5-Cp-bpzcp)(η2-coe)2}(µ-bpzcp){CuI(κ2-NN-bpzcp)}] (8), [Ir(η5-Cp-bpzcp)(η2-CH2═CH2)2}(µ-bpzcp){CuI(κ2-NN-bpzcp)}] (9), [Ir(η5-Cp-bpzcp)(coe)2}(µ-bpzcp){PdCl2(κ2-NN-bpzcp)}] (10), and [Ir(η5-Cp-bpzcp)(η2-CH2═CH2)2(µ-bpzcp){PdCl2(κ2-NN-bpzcp)}] (11). All products were characterized by spectroscopic methods and the X-ray crystal structures of 1, 2, 3, 4, and 6 were also established.

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.

Reaction of [TpRh(C2 H4 )2 ] with Dimethyl Acetylenedicarboxylate: Identification of Intermediates of the [2+2+2] Alkyne and Alkyne-Ethylene Cyclo(co)trimerizations.

The reaction between the bis(ethylene) complex [TpRh(C2 H4 )2 ], 1, (Tp=hydrotris(pyrazolyl)borate), and dimethyl acetylenedicarboxylate (DMAD) has been studied under different experimental conditions. A mixture of products was formed, in which TpRh(I) species were prevalent, whereas the presence of trapping agents, like water or acetonitrile, allowed for the stabilization and isolation of octahedral TpRh(III) compounds. An excess of DMAD gave rise to a small amount of the [2+2+2] cyclotrimerization product hexamethyl mellitate (6). Although no catalytic application of 1 was achieved, mechanistic insights shed light on the formation of stable rhodium species representing the resting state of the catalytic cycle of rhodium-mediated [2+2+2] cyclo(co)trimerization reactions. Metallacyclopentene intermediate species, generated from the activation of one alkyne and one ethylene molecule from 1, and metallacyclopentadiene species, formed by oxidative coupling of two alkynes to the rhodium centre, are crucial steps in the pathways leading to the final organometallic and organic products.

A Diels-Alder reaction triggered by a [4 + 3] metallacycloaddition.

The Tp(Me2)Ir(III) complex 1-OH2 (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate), which contains a labile molecule of water and an iridium-bonded alkenyl moiety (-C(R)═C(R)-(R=CO2Me)) as part of a benzo-annulated five-membered iridacycle, reacts readily with the conjugated dienes butadiene and 2,3-dimethylbutadiene to afford the corresponding Diels-Alder products. Experimental and DFT studies are in accordance with an initial [4 + 3] cyclometalation reaction between the diene and the five-coordinated 16-electron organometallic fragment 1 (generated from 1-OH2 by facile water dissociation). The reaction can be extended to a related TpIr(III) complex (Tp = hydrotris(pyrazolyl)borate) that also features a labile ligand (i.e., 2-THF).

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.

Experimental evidences in favour of the hydroxylamine→nitrene-water tautomerization on the coordination sphere of Ir(III) centres.

And, to round off … A series of Ir(III) 5-membered metallacycles with an Ir-CH2 bond, react with aq. NH2OH with formation of hydride 6-membered iridacyclic complexes, which contain an Ir-NH=CH- imine functionality (see scheme).

Reactivity studies of iridium pyridylidenes [Tp(Me2)Ir(C(6)H(5))(2)(C(CH)(3)C(R)NH] (R=H, Me, Ph).

The reactivity of a series of iridiumpyridylidene complexes with the formula [Tp(Me2) Ir(C6 H5 )2 (C(CH)3 C(R)NH] (1 a-1 c) towards a variety of substrates, from small molecules, such as H2 , O2 , carbon oxides, and formaldehyde, to alkenes and alkynes, is described. Most of the observed reactivity is best explained by invoking 16 e(-) unsaturated [Tp(Me2) Ir(phenyl)(pyridyl)] intermediates, which behave as internal frustrated Lewis pairs (FLPs). H2 is heterolytically split to give hydridepyridylidene complexes, whilst CO, CO2 , and H2 CO provide carbonyl, carbonate, and alkoxide species, respectively. Ethylene and propene form five-membered metallacycles with an IrCH2 CH(R)N (R=H, Me) motif, whereas, in contrast, acetylene affords four-membered iridacycles with the IrC(CH2 )N moiety. C6 H5 (CO)H and C6 H5 CCH react with formation of IrC6 H5 and IrCCPh bonds and the concomitant elimination of a molecule of pyridine and benzene, respectively. Finally the reactivity of compounds 1 a-1 c against O2 is described. Density functional theory calculations that provide theoretical support for these experimental observations are also reported.

Chlorido[1-(2-oxidophen-yl)ethyl-idene][tris-(3,5-dimethyl-pyrazol-1-yl)hydro-borato]iridium(III) chloro-form monosolvate.

In the title compound, [Ir(C15H22BN6)(C8H7O)Cl]·CHCl3, the Ir atom is formally trivalent and is coordinated in a slightly distorted octa-hedral geometry by three facial N atoms, one C atom, one O atom and one Cl atom. The Ir=Ccarbene bond is strong and short and exerts a notable effect on the trans-Ir-N bond, which is about 0.10 Šlonger than the two other Ir-N bonds. The chloro-form solvent mol-ecule is anchored via a weak C-H⋯Cl hydrogen bond to the Cl atom of the Ir complex mol-ecule. In the crystal, the constituents adopt a layer-like arrangement parallel to (010) and are held together by weak inter-molecular C-H⋯Cl hydrogen bonds, as well as weak Cl⋯Cl [3.498 (2) Å] and Cl⋯π [3.360 (4) Å] inter-actions. A weak intra-molecular C-H⋯O hydrogen bond is also observed.

(Butane-1,4-di-yl)(trimethyl-phosphane-κP)[tris-(3,5-dimethyl-pyrazol-1-yl-κN (2))hydro-borato]iridium(III).

In the mononuclear title iridium(III) complex, [Ir(C4H8)(C15H22BN6)(C3H9P)], which is based on the [tris-(3,5-dimethyl-pyrazol-1-yl)hydro-borato]iridium moiety, Ir[Tp(Me2)], the Ir(III) atom is coordinated by a chelating butane-1,4-diyl fragment and a trimethyl-phosphane ligand in a modestly distorted octa-hedral coordination environment formed by three facial N, two C and one P atom. The iridium-butane-1,4-diyl ring has an envelope conformation. This ring is disordered because alternately the second or the third C atom of the butane-1,4-diyl fragment function as an envelope flap atom (the occupancy ratio is 1:1). In the crystal, mol-ecules are organized into densely packed columns extending along [101]. Coherence between the mol-ecules is essentially based on van der Waals inter-actions.

Building a parent iridabenzene structure from acetylene and dichloromethane on an iridium center.

Parenthood: The reaction of [TpIr(C2H4)2] (1) (Tp=hydrotris(pyrazolyl)borate) with acetylene in CH2 Cl2 affords a 1:1 mixture of the "parent" metallabenzene 2 (that is, all the ring carbon centers are CH units) and the ß-Cl substituted vinyl species 3. Generation of 2 is by the coupling of an iridacyclopentadiene (formed from two acetylene molecules at the Ir center) with the dichloromethane-derived chlorocarbene ":C(H)Cl" and a subsequent α-Cl elimination event.

Tautomerisation of 2-substituted pyridines to N-heterocyclic carbene ligands induced by the 16†e- unsaturated [Tp(Me2)Ir(III)(C6H5)2] moiety.

The complex [Tp(Me2)Ir(C(6)H(5))(2)(N(2))] reacts with several 2-substituted pyridines to generate N-heterocyclic carbenes resulting from a formal 1,2-hydrogen shift from C(6) to N. In this paper we provide a detailed report of the scope and the mechanistic aspects (both experimental and theoretical) of the tautomerisation of 2-substituted pyridines.

C-H bond activation reactions of ethers that generate iridium carbenes.

Two important objectives in organometallic chemistry are to understand C-H bond activation reactions mediated by transition metal compounds and then to develop efficient ways of functionalizing the resulting products. A particularly ambitious goal is the generation of metal carbenes from simple organic molecules; the synthetic chemist can then take advantage of the almost unlimited reactivity of this metal-organic functionality. This goal remains very difficult indeed with saturated hydrocarbons, but it is considerably more facile for molecules that possess a heteroatom (such as ethers), because coordination of the heteroatom to the metal renders the ensuing C-H activation an intramolecular reaction. In this Account, we focus on the activation reaction of different types of unstrained ethers, both aliphatic and hemiaromatic, by (mostly) iridium compounds. We emphasize our recent results with the Tp(Me2)Ir(C(6)H(5))(2)(N(2)) (1.N(2)) complex (where Tp(Me2) denotes hydrotris(3,5-dimethylpyrazolyl)borate). Most of the reactivity observed with this system, and with related electronically unsaturated iridium species, starts with a C-H activation reaction, which is then followed by reversible alpha-hydrogen elimination. An alpha-C-H bond is, in every instance, broken first; when there is a choice, cleavage of the stronger terminal C(sp(3))-H bonds is always preferred over the weaker internal C(sp(3))-H (methylene) bonds of the ether. Nevertheless, competitive reactions of the unsaturated [Tp(Me2)Ir(C(6)H(5))(2)] iridium intermediate with ethers that contain C(sp(3))-H and C(sp(2))-H bonds are also discussed. We present theoretical evidence for a sigma-complex-assisted metathesis mechanism (sigma-CAM), although for other systems oxidative addition and reductive elimination events can be effective reaction pathways. We also show that additional unusual chemical transformations may occur, depending on the nature of the ether, and can result in C-O and C-C bond-breaking and bond-forming reactions, leading to the formation of more elaborate molecules. Although the possibility of extending these results to saturated hydrocarbons appears to be limited for this iridium system, the findings described in this Account are of fundamental importance for various facets of C-H bond activation chemistry, and with suitable modifications of the ancillary ligands, they could be even broader in scope. We further discuss experimental and theoretical studies on unusual alkene-to-alkylidene equilibria for some of the products obtained in the reactions of iridium complex 1.N(2) with alkyl aryl ethers. The rearrangement involves reversible alpha- and beta-hydrogen eliminations, with a rate-determining metal inversion step (supported by theoretical calculations); the alkylidene is always favored thermodynamically over the alkene. This startling result contrasts with the energetically unfavorable isomerization of free ethene to ethylidene (by about 80 kcal mol(-1)), showing that the tautomerism equilibrium can be directed toward one product or the other by a judicious choice of the transition metal complex.

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.

Experimental and computational studies on the iridium activation of aliphatic and aromatic C-H bonds of alkyl aryl ethers and related molecules.

Reaction of the Ir(III) complex [(Tp(Me2))Ir(C(6)H(5))(2)(N(2))] (1N(2)) with ortho-cresol (2-methylphenol) occurs with cleavage of the O-H and two C(sp(3))-H bonds of the phenol and formation of the electrophilic hydride alkylidene derivative [(Tp(Me2))Ir(H){=C(H)C(6)H(4)-o-O}] (2). The analogous reaction of 2-ethylphenol gives a related product 3. Both 2 and 3 have been shown to be identical to the minor, unidentified products of the already reported reactions of 1 with anisole and phenetole, respectively. Thus, in addition to the route that leads to the known heteroatom-stabilized hydride carbene [(Tp(Me2))Ir(H){=C(H)OC(6)H(4)-o-}] (B), anisole can react with 1 with cleavage of the O-CH(3) bond and formation of a new carbon-carbon bond. In contrast, only C-H bond-activation products with structures akin to B result from 1N(2) and 3,5-dimethylanisole (complex 8) or 4-fluoroanisole (9). Using anisole as a model, a computational study of the triple C-H bond activation (two aliphatic C-H bonds plus an ortho-metalation reaction) that is responsible for the formation of these heteroatom-stabilized hydride carbenes has been undertaken.

Synthetic, mechanistic, and theoretical studies on the generation of iridium hydride alkylidene and iridium hydride alkene isomers.

Experimental and theoretical studies on equilibria between iridium hydride alkylidene structures, [(Tp(Me2))Ir(H){=C(CH(2)R)ArO}] (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate; R = H, Me; Ar = substituted C(6)H(4) group), and their corresponding hydride olefin isomers, [(Tp(Me2))Ir(H){R(H)C=C(H)OAr}], have been carried out. Compounds of these types are obtained either by reaction of the unsaturated fragment [(Tp(Me2))Ir(C(6)H(5))(2)] with o-C(6)H(4)(OH)CH(2)R, or with the substituted anisoles 2,6-Me(2)C(6)H(3)OMe, 2,4,6-Me(3)C(6)H(2)OMe, and 4-Br-2,6-Me(2)C(6)H(2)OMe. The reactions with the substituted anisoles require not only multiple C-H bond activation but also cleavage of the Me-OAr bond and the reversible formation of a C-C bond (as revealed by (13)C labeling studies). Equilibria between the two tautomeric structures of these complexes were achieved by prolonged heating at temperatures between 100 and 140 degrees C, with interconversion of isomeric complexes requiring inversion of the metal configuration, as well as the expected migratory insertion and hydrogen-elimination reactions. This proposal is supported by a detailed computational exploration of the mechanism at the quantum mechanics (QM) level in the real system. For all compounds investigated, the equilibria favor the alkylidene structure over the olefinic isomer by a factor of between approximately 1 and 25. Calculations demonstrate that the main reason for this preference is the strong Ir-carbene interactions in the carbene isomers, rather than steric destabilization of the olefinic tautomers.

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.

Unusual fragmentation of CH2Cl2 by an Ir(III) centre bonded to a doubly metalated Tp(Ms) Ligand (Tp(Ms) = hydrotris(3-mesitylpyrazol-1-yl)borate).

The Ir(III) compound Tp(Ms'')Ir(N2), that contains a pentadentate, doubly metalated 3-mesityl substituted tris(pyrazolyl)borate ligand, induces the cleavage of C-H and C-Cl bonds of CH2Cl2 to yield a highly electrophilic chlorocarbene Ir=C(H)Cl complex.

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.