Salvador Conejero

Ruthenium Nanoparticles Stabilized by N-Heterocyclic Carbenes: Ligand Location and Influence on Reactivity

We thank V. Colliere and L. Datas (UPS-TEMSCAN) and P. Lecante (CNRS-CEMES) for TEM/HR-TEM and WAXS facilities, respectively. CNRS and ANR (Siderus project ANR-08-BLAN-0010-03) are also thanked for financial support. P.L. is grateful to the Spanish Ministerio de Educacion for a research contract. Financial support from the Junta de Andalucia (project no. FQM-3151) and the Spanish Ministerio de Ciencia e Innovacion (projects CTQ2010-17476 and CONSOLIDER-INGENIO 2010 CSD2007-00006, FEDER support) is acknowledged. O.R.-W. thanks the Spanish Ministerio de Ciencia e Innovacion for a research grant.

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.

Cycloisomerization versus Hydration Reactions in Aqueous Media: A Au(III)-NHC Catalyst That Makes the Difference

A novel water-soluble Au(III)-NHC complex has been synthesized and successfully applied in the intramolecular cyclization of γ-alkynoic acids into enol-lactones under biphasic toluene/water conditions, thus representing a rare example of an active and selective catalyst for this transformation in aqueous media. Remarkably, competing alkyne hydration processes were not observed, even during the desymmetrization reaction of challenging 1,6-diyne substrates. In addition, after phase separation, the water-soluble Au(III) catalyst could be recycled 10 times without loss of activity or selectivity.

True and masked three-coordinate T-shaped platinum(II) intermediates

Summary Although four-coordinate square-planar geometries, with a formally 16-electron counting, are absolutely dominant in isolated Pt(II) complexes, three-coordinate, 14-electron Pt(II) complexes are believed to be key intermediates in a number of platinum-mediated organometallic transformations. Although very few authenticated three-coordinate Pt(II) complexes have been characterized, a much larger number of complexes can be described as operationally three-coordinate in a kinetic sense. In these compounds, which we have called masked T-shaped complexes, the fourth position is occupied by a very weak ligand (agostic bond, solvent molecule or counteranion), which can be easily displaced. This review summarizes the structural features of the true and masked T-shaped Pt(II) complexes reported so far and describes synthetic strategies employed for their formation. Moreover, recent experimental and theoretical reports are analyzed, which suggest the involvement of such intermediates in reaction mechanisms, particularly C–H bond-activation processes.

Tuning N-Heterocyclic Carbenes in T-Shaped PtII Complexes for Intermolecular CH Bond Activation of Arenes†

Small change matters: T-shaped Pt(II) complexes with less flexible substituents, than, for example, isopropyl or tert-butyl groups, on N-heterocyclic carbene (NHC) ligands allow for C-H bond activation reactions of aromatic compounds (see scheme; BAr(f)(4)(-) =tetrakis[(3,5-trifluoromethyl)phenyl]borate; F yellow, Pt red). NHC substituents that are not highly branched prevent agostic interactions and reduce the barriers to achieve the C-H bond cleavage.

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.

Reactivity of Cationic Agostic and Carbene Structures Derived from Platinum(II) Metallacycles

This paper describes the formation of new platinacyclic complexes derived from the phosphine ligands PiPr2 Xyl, PMeXyl2 , and PMe2 Ar Xyl 2 (Xyl=2,6-Me2 C6 H3 and Ar Xyl 2=2,6-(2,6-Me2 C6 H3 )2 -C6 H3 ) as well as reactivity studies of the trans-[Pt(C^P)2 ] bis-metallacyclic complex 1 a derived from PiPr2 Xyl. Protonation of compound 1 a with [H(OEt2 )2 ][BArF ] (BArF =B[3,5-(CF3 )2 C6 H3 ]4 ) forms a cationic δ-agostic structure 4 a, whereas α-hydride abstraction employing [Ph3 C][PF6 ] produces a cationic platinum carbene trans-[Pt{PiPr2 (2,6-CH(Me)C6 H3 }{PiPr2 (2,6-CH2 (Me)C6 H3 }][PF6 ] (8). Compounds 4 a and 8 react with H2 to yield the same 1:3 equilibrium mixture of 4 a and trans-[PtH(PiPr2 Xyl)2 ][BArF ] (6), in which one of the phosphine ligands participates in a δ-agostic interaction. DFT calculations reveal that H2 activation by 8 occurs at the highly electrophilic alkylidene terminus with no participation of the metal. The two compounds 4 a and 8 experience C-C coupling reactions of a different nature. Thus, 4 a gives rise to complex trans-[PtH{(E)-1,2-bis(2-(PiPr2 )-3-MeC6 H3 )CHCH}] (7) that contains a tridentate diphosphine-alkene ligand, through agostic CH oxidative cleavage and C-C reductive coupling steps, whereas the C-C coupling reaction in 8 involves classical migratory insertion of its [PtCH] and [PtCH2 ] bonds promoted by platinum coordination of CO or CNXyl. The mechanisms of the CC bond-forming reactions have also been investigated by computational methods.

Solution dynamics of agostic interactions in T-shaped Pt(II) complexes from ab initio molecular dynamics simulations

Transition metal complexes forming agostic interactions have been extensively surveyed. However, the dynamic behaviour of these interactions is less documented though it could be crucial in chemical processes. For this purpose, ab initio molecular dynamics simulations (AIMD) of some representative T-shaped Pt(II) complexes (quantum mechanics) have been performed in an explicit dichloromethane solvent (molecular mechanics). The dynamics of the agostic interaction in solution strongly depends on the complex, going from stiff to flexible on-off agostic interactions at the time scale of the simulations (about 15 ps). Such behaviour can only be observed by using AIMD methods in solution.

Orbital-Like Motion of Hydride Ligands around Low-Coordinate Metal Centers†

Hydrogen atoms in the coordination sphere of a transition metal are highly mobile ligands. Here, a new type of dynamic process involving hydrides has been characterized by computational means. This dynamic event consists of an orbital-like motion of hydride ligands around low-coordinate metal centers containing N-heterocyclic carbenes. The hydride movement around the carbene-metal-carbene axis is the lowest energy mode connecting energy equivalent isomers. This understanding provides crucial information for the interpretation of NMR spectra.

Indenyl?ruthenium(ii) allenylidene complexes containing terpenic substituents as precursors of optically active terminal alkynes: scope and limitationsPart of this work was presented at the 20th International Conference on Organometallic Chemistry held in Corfu, Greece on 7?12 July 2002: see ref. 22.

The optically active allenylidene complex [Ru{CCC(C9H16)}(η5-C9H7)(PPh3)2][PF6] (C(C9H16) = (1R,4S)-1,3,3-trimethyl-bicyclo[2.2.1]hept-2-ylidene) 1 regio- and stereoselectively reacts with unhindered anionic nucleophiles to yield the neutral σ-alkynyl derivatives [Ru{CCC(C9H16)R}(η5-C9H7)(PPh3)2] (R = H 2a, CN 2b, Me 2c, CCPh 2d). Protonation of 2a–d with HBF4·Et2O affords the cationic vinylidene complexes [Ru{CC(H)C(C9H16)R}(η5-C9H7)(PPh3)2][BF4] 3a–d, which can be easily demetalated, by treatment with acetonitrile, yielding the corresponding chiral acetylenic compounds HCC(C9H16)R 4a–d. The novel optically active indenyl–ruthenium(II) allenylidene complexes [Ru{CCC(C9H16)}(η5-C9H7)(PPh3)2][PF6] (C(C9H16) = (1R,4R)-1,7,7-trimethyl-bicyclo[2.2.1]hept-2-ylidene) 9 and [Ru{CCC(C9H14)}(η5-C9H7)(PPh3)2][PF6] (C(C9H14) = (1S,5S)-4,6,6-trimethyl-bicyclo[3.1.1]hept-3-en-2-ylidene) 10 have been prepared by activation of propargylic alcohols derived from the natural ketones (+)-camphor and (−)-verbenone, respectively, with [RuCl(η5-C9H7)(PPh3)2] 6. Treatment of 9 with anionic nucleophiles generates the neutral σ-enynyl complex [Ru{CCC(C9H15)}(η5-C9H7)(PPh3)2] 11 (C(C9H15) = (1R,4R)-1,7,7-trimethyl-bicyclo[2.2.1]hept-2-en-2-yl).