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

Reduction of nitrous oxide (NO) with H to N 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 (-) are shown to catalyze the hydrogenation of NO under mild conditions (2 bar H/NO (1:1), 30 °C). Among the tested catalysts, the Ir complex , 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.

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.

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)] compounds leads to an unprecedented binding mode of the hybrid scorpionate/cyclopentadienyl ligand as η-Cp-coordinated and the formation of Ir(I) derivatives [Ir(η-Cp-bpzcp)(η-cod)] (1), [Ir(η-Cp-bpzcp){η-CH═C(Me)C(Me)═CH}] (2), [Ir(η-Cp-bpzcp)(η-coe)] (3), and [Ir(η-Cp-bpzcp)(η-CH═CH)] (4). The Ir(I) complex 4 reacts with CO or bromine to afford the compound [Ir(η-Cp-bpzcp)(CO)] (5) and the 18e Ir(III) complex [Ir(κ-N-η-Cp-bpzcpBr)Br] (6), respectively.

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.

Functionalization of 3-Iridacyclopentenes.

Members of a series of iridacyclopentenes of composition [Tp Ir(k -C,C-CH CR'=CRCH )(CO)] (Tp =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 Tp 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 Tp 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](AgCl) complex 2a with KHMDS produces the selective deprotonation at the CHP 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).

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).

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.

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 β-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.

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).

(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).

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 Å longer than the two other Ir-N bonds.

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.

Catalytic amine-borane dehydrogenation by a PCP-pincer palladium complex: a combined experimental and DFT analysis of the reaction mechanism.

Catalytic dehydrogenation of ammonia-borane (NH(3)·BH(3), AB) and dimethylamine borane (NHMe(2)·BH(3), DMAB) by the Pd(II) complex [((tBu)PCP)Pd(H(2)O)]PF(6) [(tBu)PCP = 2,6-C(6)H(3)(CH(2)P(t)Bu(2))(2)] leads to oligomerization and formation of spent fuels of general formula cyclo-[BH(2)-NR(2)](n) (n = 2,3; R = H, Me) as reaction byproducts, while one equivalent of H(2) is released per amine-borane equivalent. The processes were followed through multinuclear ((31)P, (1)H, (11)B) variable temperature NMR spectroscopy; kinetic measurements on the hydrogen production rate and the relative rate constants were also carried out. One non-hydridic intermediate could be detected at low temperature, whose chemical nature was explored through a DFT modeling of the reaction mechanism, at the M06//6-31+G(d,p) computational level.

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).

Iridium(III) complexes with polypyridine ligands coordinated as N-heterocyclic carbenes. Synthesis, structure and photophysical properties.

Unsaturated [Tp(Me2)Ir(III)] fragments, readily generated from compounds [Tp(Me2)Ir(C(6)H(5))(2)(N(2))], (1a) and [Tp(Me2)Ir(η(4)-CH(2)=C(Me)C(Me)=CH(2)] (1b) (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate), induce the isomerisation of the polypyridines, 2,2'-bipyridine, 1,10-phenanthroline and 2,2':6'2''-terpyridine, to form complexes that contain the carbene tautomer of these ligands. For terpy, a binuclear compound has also been isolated, in which this molecule bridges two Ir(III) centres, thanks to its coordination as a bidentate N-heterocyclic carbene. The new compounds have been structurally authenticated by X-ray crystallography and their photophysical properties have been investigated.

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.

Synthesis and structural characterization of a binuclear iridium complex with bridging, bidentate N-heterocyclic carbene coordination of 2,2':6',2''-terpyridine.

Thermal activation of terpyridine in the presence of an unsaturated Ir(III) fragment stabilized by a hydrotris(pyrazolyl) borate ligand gives rise to mononuclear complex and binuclear , in which the polypyridine behaves, respectively, as a mono- or a bi-dentate N-heterocyclic carbene.

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).

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.