Manuel L. Poveda

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.

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).

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.

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.

Preparation and properties of dinitrogen trimethylphosphine complexes of molybdenum and tungsten—II ☆: Synthesis and crystal structures of [MCl(N2)(PMe3)4] (M = Mo, W) and trans-[MoCl2(PMe3)4]

Abstract Sodium amalgam reduction of the complexes [MCl3(PMe3)3] (M = Mo, W) in tetrahydrofuran, under dinitrogen, yields dark red-brown suspensions from which red-orange crystals of composition trans-[MCl(N2)· (PMe3)4] can be collected. Spectroscopic and chemical evidence indicate the compounds are best formulated as mixtures of trans-[M(N2)2(PMe3)4] and trans-[MCl2(PMe3)4] species, but attempts to isolate the pure bis(dinitro derivatives have proved unsuccessful. Single crystals of analytical composition [MCl(N2)(PMe3)4] have been studied by X-ray crystallography, and the structure of trans-[MoCl2(PMe3)4] has been determined for comparison. trans-[MCl(N2)(PMe3)4] (M = Mo, W) and trans-[MoCl2(PMe3)4] are all isostructural, crystallizing in the tetragonal space group I 4 2 trans-[MoCl(N2)(PMe3)4] has a = 9.597(5), b = 12.294(6) A, Dc = 1.36g cm−3 Z = 2 and was refined to a final R value of 0.021 based on 319 independent observed reflections. The tungsten analogue has a = 9.573(4), b = 12.278(5) A, Dc = 1.63g cm−3 for Z = 2 and was refined to R = 0.19 with 322 independent observed reflections. trans-[MoCl2(PMe3)4] has cell parameters a = 9.675(5), b = 12.311(6) A Dc = 1.36 g cm−3 for Z = 2 and was refined to R = 0.043 with 316 independent observed reflections. In each case the metal atom resides on a crystallographic 4 2m position. For trans-[MoCl(N2)(PMe3)4] (M = Mo, W) the chlorine and dinitrogen ligands are disordered. M-N distances of 2.08(1) Ă (M = Mo) and 2.04(2) Ă (M = W) and M-Cl bond lengths of 2.415(8) A (M = Mo) and 2.46(1) A (M = W) are observed. In trans-[MoCl2(PMe3)4], where there is no disorder, the Mo-Cl distance is 2.420(6) A.

Carbon dioxide chemistry. The synthesis and properties of trans-(Mo(CO/sub 2/)/sub 2/(PMe/sub 3/)/sub 4/): the first stable bis(carbon dioxide) adduct of a transition metal

A method for the straight-forward, high-yield preparation of (Mo(CO/sub 2/)/sub 2/(PMe/sub 3/)/sub 4/) is described. Results of chemical and spectroscopic analysis demonstrate that the compound is the first stable bis CO/sub 2/ adduct of a transition metal. The yellow, moderately stable solid is undecomposed by heating in vacuo at approx.50/sup 0/C for 4-5 h, however, heating to temperatures of 70-80/sup 0/C results in quick decomposition. The stability of the compound is attributed to the delicate balance of steric and electronic effects. The remarkable strength of the MO-CO/sub 2/ bonds as compared to the same kind for other transition metals is believed to be due to the interior back-bonding from the Mo center to the coordinated CO/sub 2/ molecules and to the oxophilic nature of Mo.Synthese et etude de la stabilite du complexe qui serait due a un equilibre entre les effets sterique et electronique

Some trimethyl phosphine and trimethyl phosphite complexes of tungsten(IV)

Abstract Direct reduction of WCl 6 with PMe 3 in toluene at 120°C in a sealed tube affords the complexes [WCl 4 (PMe 3 ) x ] ( x = 2, 3). [WCl 4 (PMe 3 ) 3 ] abstracts oxygen from equimolar amounts of water in wet acetone or tetrahydrofuran to give [WOCl 2 (PMe 3 ) 3 ] in very high yields. This procedure has been successfully applied to the high yield synthesis of other known oxotungsten(IV) complexes, [WOCl 2 (PR 3 ) 3 ] (PR 3 = PMe 2 Ph and PMePh 2 ). Metathesis reactions of [WOCl 2 (PMe 3 ) 3 ] with NaX give [WOX 2 (PMe 3 ) 3 ] (X = NCO, NCS) and [WOX 2 (PMe 3 )] (X = Me 2 NCS 2 ). The synthesis of the trimethylphosphite analogue, [WOCl 2 (P(OMe) 3 ) 3 ], is also described and the structures of the new complexes assigned on the basis of IR and 1 H and 31 P NMR spectroscopy.

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.

Experimental and Theoretical Studies on Arene‐Bridged Metal–Metal‐Bonded Dimolybdenum Complexes

The bis(hydride) dimolybdenum complex, [Mo2(H)2{HC(N-2,6-iPr2C6H3)2}2(thf)2], 2, which possesses a quadruply bonded Mo2(II) core, undergoes light-induced (365 nm) reductive elimination of H2 and arene coordination in benzene and toluene solutions, with formation of the Mo(I)2 complexes [Mo2{HC(N-2,6-iPr2C6H3)2}2(arene)], 3⋅C6H6 and 3⋅C6H5Me, respectively. The analogous C6H5OMe, p-C6H4Me2, C6H5F, and p-C6H4F2 derivatives have also been prepared by thermal or photochemical methods, which nevertheless employ different Mo2 complex precursors. X-ray crystallography and solution NMR studies demonstrate that the molecule of the arene bridges the molybdenum atoms of the Mo(I)2 core, coordinating to each in an η(2) fashion. In solution, the arene rotates fast on the NMR timescale around the Mo2-arene axis. For the substituted aromatic hydrocarbons, the NMR data are consistent with the existence of a major rotamer in which the metal atoms are coordinated to the more electron-rich C-C bonds.

Formation of 1,2,3-η3-butadienyl derivatives by photochemical C-H activation of Ir(I)-η4-1,3-diene complexes containing tris(pyrazolyl) borate ligands

Abstract Several Ir(I) complexes of composition [Tp′Ir(diene)], where Tp′ = hydrotris(1-pyrazolyl)borate, Tp or hydrotris(3,5-dimethyl-lpyrazolyl)borate, Tp * and diene = conjugated diene, have been prepared by treating [Ir(μ-Cl)(coe) 2 ] 2 with the appropriate diene and then with KTp′. The series of related complexes [Tp * Ir[η 4 -2,3-RR′C 4 H 4 )](R, R′ = H, 3; R = Me, R′ = H, 4 ; R, R′ = Me, 5 ) has been chosen for comparative studies on photochemical C-H bond activation reactions. These have been demonstrated to occur only at the C-R and C-R′ moieties. Both vinylic and allylic (CH 3 ) activations have been found for compound 4 (R = Me, R′ = H), while 3 seems to undergo exclusively central C-H vinylic addition to the metal centre with only low efficiency. The dimethyl substituted diene ligand of 5 is readily activated at one of the methyl groups, yielding the hydrido-allyl complex [Tp * Ir(H)(η 3 -CH 2 C(C(Me)=CH 2 )CH 2 )] 11 . The thermal activation of C 6 H 6 by compound 5 to give the N 2 -bridged binuclear species [Tp * Ir(H)(C 6 H 5 )] 2 (μ-N 2 ) 12 is also reported.