Margarita Paneque

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.

Synthesis and characterization of some new organometallic complexes of nickel(II) containing trimethylphosphine

Abstract [Ni(cod)2]2 (cod = 1,5-cyclooctadiene) oxidatively adds C6H5CH2Cl in the presence of 1 or 2 equiv of PMe3 affording [Ni(η3-CH2C6H5)Cl(PMe3)] or trans-[Ni(η1-CH2C6H5)Cl(PMe3)2], respectively. Variable temperature NMR studies carried out with a mixture of these two complexes indicate that both are involved in a fast equilibrium which interchanges the two types of benzylic ligands. The above mentioned compounds decompose in the presence of excess PMe3 with the formation of [NiCl2(PMe3)3], [Ni(PMe3)4] and the reductive elimination product (C6H5CH2)2. In addition, the synthesis and spectral characterization of the new complexes trans-[Ni(C6H5)X(PMe3)2] (X = Cl, Br), trans-[Ni(2,4,6-C6H2Me3)X(PMe3)2] (X = Cl, Br) and [(η5-C5H5)Ni(2,4,6-C6H2Me3)(PMe3)], are also described.

C–H bond activation reactions by TpMe2Ir(III) centres. Generation of Fischer-type carbenes and development of a catalytic system for H/D exchange

The unsaturated [TpMe2Ir(C6H5)2] fragment, readily generated from [TpMe2Ir(C6H5)2(N2)], or from [TpMe2Ir(C2H4)2] and C6H6, is able to induce the regioselective cleavage of two sp3 C–H bonds of anisole, with formation of a Fischer-type carbene, 1. The process involves additionally ortho-metallation of the anisole aromatic ring, hence three C–H bonds are sequentially broken, the last one in the course of an α-H elimination reaction. Phenetole (ethyl phenyl ether) gives an analogous product, 3, despite the possibility of competitive α- or β-H eliminations in the last step. For C6H5NMe2, two hydride-carbenes, 5a and 5b, are produced. In the latter, the aniline phenyl ring is also metallated, but the former contains a C6H5 aryl group and a C6H5N(Me)CH carbene ligand. The same Ir(III) fragment, viz. [TpMe2Ir(C6H5)2], alternatively generated from C6H6 and [TpMe2Ir(η4-CH2C(Me)C(Me)CH2)], accomplishes the efficient, catalytic H/D exchange between C6D6 (used as the deuterium source) and a variety of organic and organometallic molecules that contain C–H bonds of different nature.

Tautomerisation of 2-Substituted Pyridines to N-Heterocyclic Carbene Ligands Induced by the 16 e- Unsaturated [TpMe2IrIII(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.

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. The computational output was of help to propose a reliable mechanistic picture of the process.

Generation and reactivity of sterically hindered iridium carbenes. Competitive α- vs.β-hydrogen elimination in iridium(III) alkyls

Some tris(pyrazolyl)borate complexes of iridium are able to cleave regioselectively the two α-CH bonds of ethers (cyclic and non-cyclic) and aliphatic amines, RCH2X (X = OR′, NR2′), with formation of Fischer-type carbene complexes, [Ir]C(X)R. The last step of these rearrangements, namely an α-H elimination from an alkyl intermediate, [Ir]–CH(X)R, takes place even when β-H atoms are present. Migratory insertion reactions of hydride or alkyl ligands onto highly electrophilic iridium alkylidenes have also been investigated. It has been found that an in situ generated [Ir]–C2H5+ species yields the corresponding [Ir](H)(CHCH3)+ derivative, that is, the α-H elimination product, at a rate faster than that of formation of the isomeric hydride ethene complex derived from β-H elimination.

Reactivity Studies of Iridium Pyridylidenes [TpMe2Ir(C6H5)2(C(CH)3C(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.

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.

Organometallic derivatives of Ni(II) with poly(pyrazolyl)borate ligands

Abstract The reaction of the alkyl or aryl derivatives Ni(R)X(PMe 3 ) 2 (R=CH 2 SiMe 3 , CH 2 CMe 3 , C 6 H 5 ; X=Cl, Br) with the potassium salt of the Bp ligand (Bp=dihydrobis(pyrazolyl)borate anion) forms the corresponding compounds BpNi(R)(PMe 3 ). In contrast, the reaction of the aryl derivatives Ni(C 6 H 4 - p -X)Br(PMe 3 ) 2 (X=H, Me, OMe, NMe 2 ) with the Bp tBu anion (Bp tBu =dihydrobis(3- t -butylpyrazolyl)borate) proceeds with formation of complexes of composition Bp tBu Ni(C 6 H 4 - p -X)(PMe 3 ) 2 , in which the polydentate ligand is bound to the metal through only one pyrazolyl group. The Tp anion leads to only aryl derivatives; the phenyl complex TpNi(C 6 H 5 )(PMe 3 ) has been obtained, and the reaction of the alkyl complex Ni(CH 2 CMe 2 Ph)Cl(PMe 3 ) 2 with KTp furnishes the aryl TpNi(C 6 H 4 - o -Bu t )(PMe 3 ), by means of a rearrangement of the neophyl ligand. The Tp ligand in these complexes is bonded in the η 2 fashion, although an X-ray analysis carried out for TpNi(Ph)(PMe 3 ) reveals the existence of an important Ni…N interaction with the third pz ring. Upon reaction with the bulky hydrotris(3- t -butylpyrazolyl)borate anion, the aryl derivatives Ni(C 6 H 4 - p -X)Br(PMe 3 ) 2 (X=H, Me, OMe, NMe 2 ) form complexes of composition Tp tBu Ni(C 6 H 4 - p -X)(PMe 3 ) 2 , in which the polydentate ligand is once more bound to the metal through only one pyrazolyl group. These complexes represent the first examples of η 1 coordination of poly(pyrazolyl)borate-type ligands. The acyl and aroyl complexes BpNi(COR)(PMe 3 ) (R=CH 2 SiMe 3 , CH 2 CMe 3 ) and TpNi(COPh)(PMe 3 ) have been obtained by carbonylation of the parent compounds. The aroyls Tp tBu Ni(COC 6 H 4 - p -X)(PMe 3 ) 2 have also been obtained from the derivatives Ni(COC 6 H 4 - p -X)Br(PMe 3 ) 2 although they evolve CO slowly in solution. An X-ray analysis carried out with Tp tBu Ni(C 6 H 4 - p -Me)(PMe 3 ) 2 confirms the η 1 -coordination mode of the Tp tBu ligand, which was deduced from NMR studies.