Rh-POP Pincer Xantphos Complexes for C-S and C-H Activation. Implications for Carbothiolation Catalysis

The neutral Rh­(I)–Xantphos complex [Rh­(κ³-_P,O,P-Xantphos)­Cl]_<i>n</i>, <b>4</b>, and cationic Rh­(III) [Rh­(κ³-_P,O,P-Xantphos)­(H)₂]­[BAr^F₄], <b>2a</b>, and [Rh­(κ³-_P,O,P-Xantphos-3,5-C₆H₃(CF₃)₂)­(H)₂]­[BAr^F₄], <b>2b</b>, are described [Ar^F = 3,5-(CF₃)₂C₆H₃; Xantphos = 4,5-bis­(diphenylphosphino)-9,9-dimethylxanthene; Xantphos-3,5-C₆H₃(CF₃)₂ = 9,9-dimethylxanthene-4,5-bis­(bis­(3,5-bis­(trifluoromethyl)­phenyl)­phosphine]. A solid-state structure of <b>2b</b> isolated from C₆H₅Cl solution shows a κ¹-chlorobenzene adduct, [Rh­(κ³-_P,O,P-Xantphos-3,5-C₆H₃(CF₃)₂)­(H)₂(κ¹-ClC₆H₅)]­[BAr^F₄], <b>3</b>. Addition of H₂ to <b>4</b> affords, crystallographically characterized, [Rh­(κ³-_P,O,P-Xantphos)­(H)₂Cl], <b>5</b>. Addition of diphenyl acetylene to <b>2a</b> results in the formation of the C–H activated metallacyclopentadiene [Rh­(κ³-_P,O,P-Xantphos)­(ClCH₂Cl)­(σ,σ-(C₆H₄)­C­(H)CPh)]­[BAr^F₄], <b>7</b>, a rare example of a crystallographically characterized Rh–dichloromethane complex, alongside the Rh­(I) complex <i>mer</i>-[Rh­(κ³-_P,O,P-Xantphos)­(η²-PhCCPh)]­[BAr^F₄], <b>6</b>. Halide abstraction from [Rh­(κ³-_P,O,P-Xantphos)­Cl]_<i>n</i> in the presence of diphenylacetylene affords <b>6</b> as the only product, which in the solid state shows that the alkyne binds perpendicular to the κ³-POP Xantphos ligand plane. This complex acts as a latent source of the [Rh­(κ³-_P,O,P-Xantphos)]⁺ fragment and facilitates <i>ortho</i>-directed C–S activation in a number of 2-arylsulfides to give <i>mer</i>-[Rh­(κ³-_P,O,P-Xantphos)­(σ,κ¹-Ar)­(SMe)]­[BAr^F₄] (Ar = C₆H₄COMe, <b>8</b>; C₆H₄(CO)­OMe, <b>9</b>; C₆H₄NO₂, <b>10</b>; C₆H₄CNCH₂CH₂O, <b>11</b>; C₆H₄C₅H₄N, <b>12</b>). Similar C–S bond cleavage is observed with allyl sulfide, to give <i>fac</i>-[Rh­(κ³-_P,O,P-Xantphos)­(η³-C₃H₅)­(SPh)]­[BAr^F₄], <b>13</b>. These products of C–S activation have been crystallographically characterized. For <b>8</b> in situ monitoring of the reaction by NMR spectroscopy reveals the initial formation of <i>fac</i>-κ³-<b>8</b>, which then proceeds to isomerize to the <i>mer</i>-isomer. With the <i>para</i>-ketone aryl sulfide, 4-SMeC ₆H₄COMe, C–H activation <i>ortho</i> to the ketone occurs to give <i>mer</i>-[Rh­(κ³-_P,O,P-Xantphos)­(σ,κ¹-4-(COMe)­C₆H₃SMe)­(H)]­[BAr^F₄], <b>14</b>. The temporal evolution of carbothiolation catalysis using <i>mer</i>-κ³-<b>8</b>, and phenyl acetylene and 2-(methylthio)­acetophenone substrates shows initial fast catalysis and then a considerably slower evolution of the product. We suggest that the initially formed <i>fac</i>-isomer of the C–S activation product is considerably more active than the <i>mer</i>-isomer (i.e., <i>mer</i>-<b>8</b>), the latter of which is formed rapidly by isomerization, and this accounts for the observed difference in rates. A likely mechanism is proposed based upon these data

Rhodium Cyclopentyl Phosphine Complexes of Wide-Bite-Angle Ligands DPEphos and Xantphos

Rh(I) and Rh(III) complexes of tricyclopentylphosphine (PCyp 3), or its dehydrogenated variant PCyp 2(η 2-C 5H 7), partnered with wide-bite-angle chelating diphosphine ligands DPEphos and Xantphos have been prepared and characterized in solution and the solid state with the aim of studying their potential for reversible dehydrogenation of the PCyp 3 ligand. The complexes fac-[Rh(κ 3-P,O,P-L){PCyp 2(η 2-C 5H 7)}][BAr F4] (L = DPEphos, Xantphos) show pseudo-trigonal-bipyramidal structures in which the dehydrogenated phosphine alkene ligand acts in a chelating manner. Addition of H 2 to fac-[Rh(κ 3-P,O,P-DPEphos){PCyp 2(η 2-C 5H 7)}][BAr F4] resulted in an equilibrium mixture of hydride and hydride-dihydrogen complexes, fac-[Rh(κ 3-P,O,P-DPEphos)(H) 2(PCyp 3)][BAr F4] and [Rh(κ 2-P,P-DPEphos)(η 2-H 2)(H) 2(PCyp 3)][BAr F4], in which the DPEphos acts as a hemilabile ligand. For the more rigid Xantphos ligand two dihydride isomers, fac-[Rh(κ 3-P,O,P-Xantphos)(H) 2(PCyp 3)][BAr F4] and mer-[Rh(κ 3-P,O,P-Xantphos)(H) 2(PCyp 3)][BAr F4], are formed, which are also in equilibrium with one another. A van′t Hoff analysis of this mixture shows that enthalpically there is very little difference between the two geometries for this system, with the driving force for the preferred fac-geometry being entropic. Addition of MeCN to these hydrido complexes results in the central oxygen atom being displaced to form [Rh(κ 2-P,P-L)(PCyp 3)(H) 2(MeCN)][BAr F4], while removal of H 2 from the hydrido complexes (under vacuum or on addition of a hydrogen acceptor) forms the Rh(I) complexes [Rh(κ 3-P,O,P-L) (PCyp 3)][BAr F4], which are characterized as having square-planar geometries with meridonial coordination of the respective chelating phosphines. Dehydrogenation of the PCyp 3 ligand in these complexes to re-form the phosphine-alkene ligands does not occur, even under forcing conditions. © 2012 American Chemical Society

Amino-borane oligomers bound to a Rh(I) metal fragment.

Coordination complexes of previously observed intermediates, H(3)B.NMe(2)BH(2).NMe(2)H and [H(2)BNMeH](3), in the transition metal catalysed dehydrocoupling of H(3)B.NMe(2)H and H(3)B.NMeH(2) have been isolated and structurally characterised using the [Rh{PR'(2)(eta(2)-C(5)H(7))}](+) fragment. Their onward reactivity shows that further dehydrogenation is not a simple intramolecular process

Alkyl dehydrogenation in iridium tri-cyclopentyl phosphines

The iridium cyclooctadiene complex incorporating a tricyclopentyl phosphine ligand (PCyp3), Ir(η2:η2-C8H12)(PCyp3)Cl, has been prepared. Removal of the chloride from this complex using Na [BArF4] [ArF = C6 H3 (CF3)2] in CH2Cl2/arene solvent results in dehydrogenation (C-H activation followed by β-H transfer) of one of the alkyl phosphine rings and formation of the complexes [Ir (η6 - C6 H5 X) {PCyp2 (η2 - C5 H7)}] [BArF4] (X = H, F) which contain a hybrid phosphine-alkene ligand. These complexes are formed alongside another product (5-20% yield) that has been identified as [Ir (η2 : η2 - C8 H12) {PCyp2 (η2 - C5 H7)}] [BArF4], which can be prepared in high yield by an alternative, and slightly modified, route. This complex is with a minor isomer that has been tentatively identified as [Ir (η2 : η3 - C8 H11) (H) {PCyp2 (η2 - C5 H7)}] [BArF4], which results from allylic C-H activation of cyclooctadiene. Addition of H2 to [Ir (η2 : η2 - C8 H12) {PCyp2 (η2 - C5 H7)}] [BArF4] and its isomer in arene solvent (C6H5X, X = F, H) forms the dihydrido η6-arene Ir(III) complexes [Ir (H)2 (η6 - C6 H5 X) (PCyp3)] [BArF4]. In contrast, hydrogenation in CH2Cl2 alone results in the formation of Ir (H)2 (PCyp3) {η6 - (C6 H3 (CF3)2) BArF3} in which the [BArF4]- anion is now acting as a ligand through one of its aryl rings. The fluorobenzene complex [Ir (H)2 (η6 - C6 H5 F) (PCyp3)] [BArF4] can be cleanly converted to [Ir (η6 - C6 H5 F) {PCyp2 (η2 - C5 H7)}] [BArF4] by addition of the hydrogen acceptor tert-butylethene (tbe). © 2009 Elsevier B.V. All rights reserved

Catching the first oligomerization event in the catalytic formation of polyaminoboranes : H3B·NMeHBH2·NMeH2 bound to iridium

We report the first insertion step at a metal center for the catalytic dehydropolymerization of H(3)B center dot NMeH(2) to form the simplest oligomeric species, H(3)B center dot NMeHBH(2)center dot NMeH(2), by the addition of 1 equiv of H(3)B center dot NMeH(2) to [Ir(PCy(3))(2)(H)(2)(eta(2)-H(3)B center dot NMeH(2))] [BAr(4)(F)] to give [Ir(PCy(3))(2)(H)(2)(eta(2)-H(3)B center dot NMeHBH(2)center dot NMeH(2))] [BAr(4)(F)]. This reaction is also catalytic for the formation of the free linear diborazane, but this is best obtained by an alternative stoichiometric synthesis

Structural snapshots of concerted double E–H bond activation at a transition metal centre

Bond activation at a transition metal centre is a key fundamental step in numerous chemical transformations. The oxidative addition of element–hydrogen bonds, for example, represents a critical step in a range of widely applied catalytic processes. Despite this, experimental studies defining steps along the bond activation pathway are very rare. In this work, we report on fundamental studies defining a new oxidative activation pathway: combined experimental and computational approaches yield structural snapshots of the simultaneous activation of both bonds of a β-diketiminate-stabilized GaH2 unit at a single metal centre. Systematic variation of the supporting phosphine ligands and single crystal X-ray/neutron diffraction are exploited in tandem to allow structural visualization of the activation process, from a η2-H,H σ-complex showing little Ga–H bond activation, through species of intermediate geometry featuring stretched Ga–H and compressed M–H/M–Ga bonds, to a fully activated metal dihydride featuring a neutral (carbene-type) N-heterocyclic GaI ligand