Ernesto Carmona

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.

Direct Bonds Between Metal Atoms: Zn, Cd, and Hg Compounds with Metal–Metal Bonds

The synthesis and characterization of [Zn(2)(eta(5)-C(5)Me(5))(2)], a stable molecular compound with a Zn-Zn bond and the first example of a dimetallocene structure, has opened a new chapter in the organometallic chemistry of zinc and in metallocene chemistry. The existence of two directly bonded zinc atoms demonstrates that the [Zn-Zn](2+) unit, the lightest Group 12 homologue of the well-known [Hg-Hg](2+) ion, can be stabilized by appropriate ligands. Activity in this area has increased enormously in the few years since the determination of the structure of this molecule. Numerous theoretical studies have been devoted to the investigation of the electronic, structural, and spectroscopic properties of this and related compounds, and new metal-metal coordination and organometallic compounds of zinc, cadmium, and mercury have been synthesized and structurally characterized. This Minireview gives an overview of activity in this field during the past three to four years.

Cleavage of Palladium Metallacycles by Acids: A Probe for the Study of the Cyclometalation Reaction

Useful mechanistic information about the cyclometalation reaction may be obtained by studying the reverse reaction, namely, the protonation of metallacycle 1 by acids of different coordinating anions (see scheme). Ar=3,5-(F3C)2C6H3; Tf=F3CSO2.

Carbon Monoxide and Isocyanide Complexes of Trivalent Uranium Metallocenes

The [Cp′3U] metallocenes contain substituted cyclopentadienyl ligands and UIII with f3 electron configuration. They are good π donors and bind π-accepting ligands (L) such as carbon monoxide and isocyanides to form the corresponding adducts [Cp′3U(L)] (see scheme). The π-donating capability of the [Cp′3U] fragments appears to be readily modulated by the substituents on the cyclopentadienyl ligand.

C−H Bond Activation of Benzene and Cyclic Ethers by TpIrIII Species

An unusual Fischer carbene derivative that, in addition, contains an alkyl and a hydride ligand is obtained by C−H bond activation of THF by the hydride–vinyl species [TpMe2IrH(CHCH2) (C2H4)]. This complex is also capable of activating the C−H bonds of benzene to give remarkably stable IrIII–N2 complexes (see illustration).

The chemistry of group 10 metalacycles

Abstract Metalacycles of Nickel, Palladium and Platinum in which the metal binds two carbon atoms form a rapidly growing class of compounds that find diverse applications in organic synthesis both in stoichiometric and catalytic reactions. We address herein the synthetic approaches used in the preparation of these complexes, and their chemical reactivity, with emphasis in processes such as the insertion of small unsaturated molecules into the metal–carbon bonds, which lead to the formation of organic products.

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.

Interconversion of Quadruply and Quintuply Bonded Molybdenum Complexes by Reductive Elimination and Oxidative Addition of Dihydrogen

Transition metal complexes that exhibit multiple metal–metal bonding are of fundamental importance in chemistry. Following decades of intense scrutiny of double, triple, and quadruple metal–metal bonds, in 2005 Power and co-workers made an outstanding discovery with the characterization of the first stable molecule with fivefold bonding between two chromium atoms. This report encouraged the search for further examples of quintuply bonded dimetal compounds and was followed by numerous experimental and computational studies. By and large, these studies have focused on chromium compounds. 2–8,11] However, Tsai and coworkers have extended their investigation of the Cr Cr quintuple bond 7,11] to analogous molybdenum complexes and have isolated two closely related compounds supported by monoanionic amidinate ligands, N,N’-disubstituted with 2,6-diisopropylphenyl groups. Recently, another compound with the Mo2 central unit reinforced by three amidinate ligands and a bridging lithium cation has also been reported by the same group of researchers. Identification of the above complexes with fivefold metalmetal bonding has prompted research directed toward understanding their bonding characteristics and the chemical reactivity of their central M M quintuple bond. So far, only a few reports have appeared in the literature concerning mainly the chemical properties of the Cr Cr quintuple bond. Thus, these complexes feature interesting reactivity toward unsaturated molecules like N2O, [19] alkenes, and alkynes, and are able to activate white phosphorous, yellow arsenic, and AsP3. [22] Similar to carbon carbon double and triple bonds, the Cr Cr quintuple bond of an aminopyridinate complex undergoes facile carboalumination, to generate the corresponding Cr Cr quadruple-bond derivative with formally monoanionic bridging CH3 and AlMe2 groups. [23] Disproportionation of Cr to Cr and Cr induced by the addition of [18]crown-6-ether to a Cr Cr quintuply bonded complex has also been demonstrated. During the preparation of this manuscript the reactivity of Mo Mo quintuply bonded compounds with alkynes has been shown. The above results illustrate the potential of quintuply bonded electron-rich M2 centers for bimetallic activation. Herein we describe that the quadruply bonded dimolybdenum dihydride complex [Mo2(H)2{HC(N-2,6-iPr2C6H3)2}2(thf)2] 2 undergoes reductive elimination of H2 from its [(H)Mo Mo(H)] core under UV irradiation (365 nm) to afford the known [Mo2{HC(N-2,6-iPr2C6H3)2}2], 3, [9] with fivefold Mo Mo bonding (Scheme 1B). Because a tetrahydrofuran (THF) solution of the latter reacts readily with H2 to reform 2, our results demonstrate that quadruply and quintuply bonded dimolybdenum complexes may interconvert by means of reductive elimination and oxidative

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.