Theoretical Studies on the Mechanism of Iridium-Catalyzed Alkene Hydrogenation by the Cationic Complex [IrH₂(NCMe)₃(P^<i>i</i>Pr₃)]⁺

A mechanistic DFT study has been carried out on the ethene hydrogenation catalyzed by the [IrH₂­(NCMe)₃­(P^<i>i</i>Pr₃)]⁺ complex (<b>1</b>). First, the reaction of (<b>1</b>) with ethene has been theoretically characterized, and three mechanistic proposals (<b>A</b>–<b>C</b>) have been made for an identification of the preferred pathways for the alkene hydrogenation catalytic cycle considering Ir­(I)/Ir­(III) and Ir­(III)/Ir­(V) intermediate species. Theoretical calculations reveal that the reaction path with the lowest energy starts at an initial ethene migratory insertion into the metal–hydride bond, followed by dihydrogen coordination into the vacancy. Ethane is formed via σ-bond metathesis between the bound H₂ and the Ir-ethyl moiety, being the rate-determining step, in agreement with the experimental data available. The calculated energetic span associated with the catalytic cycle is 21.4 kcal mol^–1. Although no Ir­(V) intermediate has been found along the reaction path, the Ir­(V) nature of the transition state for the proposed key σ-bond metathesis step has been determined by electron localization function and geometrical analysis

Intramolecular C–H Oxidative Addition to Iridium(I) in Complexes Containing a <i>N</i>,<i>N</i>′‑Diphosphanosilanediamine Ligand

The iridium­(I) complexes of formula Ir­(cod)­(SiNP)⁺ (<b>1</b>^<b>+</b>) and IrCl­(cod)­(SiNP) (<b>2</b>) are easily obtained from the reaction of SiMe₂{N­(4-C₆H₄CH₃)­PPh₂}₂ (SiNP) with [Ir­(cod)­(CH₃CN)₂]⁺ or [IrCl­(cod)]₂, respectively. The carbonylation of [<b>1</b>]­[PF₆] affords the cationic pentacoordinated complex [Ir­(CO)­(cod)­(SiNP)]⁺ (<b>3</b>⁺), while the treatment <b>2</b> with CO gives the cation <b>3</b>⁺ as an intermediate, finally affording an equilibrium mixture of IrCl­(CO)­(SiNP) (<b>4</b>) and the hydride derivative of formula IrHCl­(CO)­(SiNP–H) (<b>5</b>) resulting from the intramolecular oxidative addition of the C–H bond of the SiCH₃ moiety to the iridium­(I) center. Furthermore, the prolonged exposure of [<b>3</b>]­Cl or <b>2</b> to CO resulted in the formation of the iridium­(I) pentacoordinated complex Ir­(SiNP–H)­(CO)₂ (<b>6</b>). The unprecedented κ³<i>C</i>,<i>P</i>,<i>P</i>′ coordination mode of the [SiNP–H] ligand observed in <b>5</b> and <b>6</b> has been fully characterized in solution by NMR spectroscopy. In addition, the single-crystal X-ray structure of <b>6</b> is reported

Brønsted Acid/Base Driven Chemistry with Rhodathiaboranes: A Labile {SB₉H₉}–Thiadecaborane Fragment System

Reversible H₂ cleavage promoted by closo to nido transformations of [1,1-(PPh₃)₂-3-(NC₅H₅)-<i>closo</i>-1,2-RhSB₉H₈] (<b>2</b>)/[8,8,8-(PPh₃)₂(H)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>1</b>) is a cooperative action with application in catalysis; the treatment of <b>2</b> and [1,1-(PPh₃)(CO)-3-(NC₅H₅)-<i>closo</i>-RhSB₉H₈] (<b>3</b>) with either HCl or HOTf in CH₂Cl₂ affords the 11-vertex <i>nido</i>-rhodathiaboranes [8,8-(PPh₃)(Cl)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>4</b>) and [8,8,8-(PPh₃)(CO)(Cl)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>5</b>), respectively. In contrast, the reaction of <b>1</b> with triflic acid yields the salt [8,8-(PPh₃)₂(H)-9-(NC₅H₅)-<i>nido</i>-RhSB₉H₁₀][OTf] (<b>6</b>). These results illustrate the bifunctional nature of the clusters and their nido to closo redox flexibility, which open new routes for the tuning of the reactivity of these polyhedral compounds and widen their potential applications

Hydridorhodathiaboranes: Synthesis, Characterization, and Reactivity

The reaction between pyridine and [8,8-(PPh₃)₂-<i>nido</i>-8,7-RhSB₉H₁₀] (<b>1</b>) has given the opportunity to synthesize a new family of 11-vertex hydridorhodathiaboranes that feature boron-bound N-heterocyclic ligands. To explore the scope of this reaction, <b>1</b> has been treated with the methylpyridine isomers (picolines) 2-Me-NC₅H₄, 3-Me-NC₅H₄, and 4-Me-NC₅H₄, affording the picoline ligated clusters [8,8,8-(H)­(PPh₃)₂-9-(L)-<i>nido</i>-8,7-RhSB₉H₉], where L = 2-Me-NC₅H₄ (<b>3</b>), 3-Me-NC₅H₄ (<b>4</b>), 4-Me-NC₅H₄ (<b>5</b>). Thermal treatment of these <i>nido</i> clusters leads to dehydrogenation and the formation of <i>isonido</i>/<i>closo-</i>[1,1-(PPh₃)₂-3-(L)-1,2-RhSB₉H₈] (<b>9</b>–<b>11</b>). Compounds <b>3</b>–<b>5</b> react with ethylene to form [1,1-(η²-C₂H₄)­(PPh₃)-3-(L)-1,2-RhSB₉H₈] (<b>13</b>–<b>15</b>). Similarly, treatment of <b>3</b>–<b>5</b> with carbon monoxide produces [1,1-(CO)­(PPh₃)-3-(L)-1,2-RhSB₉H₈] (<b>17</b>–<b>19</b>). These series of η²-C₂H₄ and CO ligated 11-vertex <i>isonido</i>/<i>closo</i>-rhodathiaboranes result from the substitution of one PPh₃ ligand by ethylene or CO together with H₂ loss and a concomitant <i>nido</i> to <i>closo</i>/<i>isonido</i> cluster structural transformation. The reactivity of <b>3</b>–<b>5</b> with propene, 1-hexene, and cyclohexene under a hydrogen atmosphere is also reported and compared with the reactivity of the pyridine ligated analogue [8,8,8-(H)­(PPh₃)₂-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>2</b>). Low-temperature NMR studies have allowed the characterization of intermediates which undergo inter- and intramolecular exchange processes, depending on the nature of the N-heterocyclic ligand. The CO ligand enhances the nonrigidity of the cluster, opening mechanisms of H₂ loss

Hydridorhodathiaboranes: Synthesis, Characterization, and Reactivity

The reaction between pyridine and [8,8-(PPh₃)₂-<i>nido</i>-8,7-RhSB₉H₁₀] (<b>1</b>) has given the opportunity to synthesize a new family of 11-vertex hydridorhodathiaboranes that feature boron-bound N-heterocyclic ligands. To explore the scope of this reaction, <b>1</b> has been treated with the methylpyridine isomers (picolines) 2-Me-NC₅H₄, 3-Me-NC₅H₄, and 4-Me-NC₅H₄, affording the picoline ligated clusters [8,8,8-(H)­(PPh₃)₂-9-(L)-<i>nido</i>-8,7-RhSB₉H₉], where L = 2-Me-NC₅H₄ (<b>3</b>), 3-Me-NC₅H₄ (<b>4</b>), 4-Me-NC₅H₄ (<b>5</b>). Thermal treatment of these <i>nido</i> clusters leads to dehydrogenation and the formation of <i>isonido</i>/<i>closo-</i>[1,1-(PPh₃)₂-3-(L)-1,2-RhSB₉H₈] (<b>9</b>–<b>11</b>). Compounds <b>3</b>–<b>5</b> react with ethylene to form [1,1-(η²-C₂H₄)­(PPh₃)-3-(L)-1,2-RhSB₉H₈] (<b>13</b>–<b>15</b>). Similarly, treatment of <b>3</b>–<b>5</b> with carbon monoxide produces [1,1-(CO)­(PPh₃)-3-(L)-1,2-RhSB₉H₈] (<b>17</b>–<b>19</b>). These series of η²-C₂H₄ and CO ligated 11-vertex <i>isonido</i>/<i>closo</i>-rhodathiaboranes result from the substitution of one PPh₃ ligand by ethylene or CO together with H₂ loss and a concomitant <i>nido</i> to <i>closo</i>/<i>isonido</i> cluster structural transformation. The reactivity of <b>3</b>–<b>5</b> with propene, 1-hexene, and cyclohexene under a hydrogen atmosphere is also reported and compared with the reactivity of the pyridine ligated analogue [8,8,8-(H)­(PPh₃)₂-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>2</b>). Low-temperature NMR studies have allowed the characterization of intermediates which undergo inter- and intramolecular exchange processes, depending on the nature of the N-heterocyclic ligand. The CO ligand enhances the nonrigidity of the cluster, opening mechanisms of H₂ loss

Brønsted Acid/Base Driven Chemistry with Rhodathiaboranes: A Labile {SB₉H₉}–Thiadecaborane Fragment System

Reversible H₂ cleavage promoted by closo to nido transformations of [1,1-(PPh₃)₂-3-(NC₅H₅)-<i>closo</i>-1,2-RhSB₉H₈] (<b>2</b>)/[8,8,8-(PPh₃)₂(H)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>1</b>) is a cooperative action with application in catalysis; the treatment of <b>2</b> and [1,1-(PPh₃)(CO)-3-(NC₅H₅)-<i>closo</i>-RhSB₉H₈] (<b>3</b>) with either HCl or HOTf in CH₂Cl₂ affords the 11-vertex <i>nido</i>-rhodathiaboranes [8,8-(PPh₃)(Cl)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>4</b>) and [8,8,8-(PPh₃)(CO)(Cl)-9-(NC₅H₅)-<i>nido</i>-8,7-RhSB₉H₉] (<b>5</b>), respectively. In contrast, the reaction of <b>1</b> with triflic acid yields the salt [8,8-(PPh₃)₂(H)-9-(NC₅H₅)-<i>nido</i>-RhSB₉H₁₀][OTf] (<b>6</b>). These results illustrate the bifunctional nature of the clusters and their nido to closo redox flexibility, which open new routes for the tuning of the reactivity of these polyhedral compounds and widen their potential applications

Reactions of 11-Vertex Rhodathiaboranes with HCl: Synthesis and Reactivity of New Cl-Ligated Clusters

Reactions of [8,8,8-(H)­(PPh₃)₂-9-(Py)-<i>nido</i>-8,7-RhSB₉H₉] (<b>1</b>), [1,1-(PPh₃)₂-3-(Py)-<i>closo</i>-1,2-RhSB₉H₈] (<b>2</b>), and [1,1-(CO)­(PPh₃)-3-(Py)-<i>closo</i>-1,2-RhSB₉H₈] (<b>4</b>), where Py = Pyridine, with HCl to give the Cl-ligated clusters, [8,8-(Cl)­(PPh₃)-9-(Py)-<i>nido</i>-8,7-RhSB₉H₉] (<b>3</b>) and [8,8,8-(Cl)­(CO)­(PPh₃)-9-(Py)-<i>nido</i>-8,7-RhSB₉H₈] (<b>5</b>), have recently demonstrated the remarkable <i>nido</i>-to-<i>closo</i> redox flexibility and bifunctional character of this class of 11-vertex rhodathiaboranes. To get a sense of the scope of this chemistry, we report here the reactions of PR₃-ligated analogues, [8,8,8-(H)­(PR₃)₂-9-(Py)-<i>nido</i>-8,7-RhSB₉H₉], where PR₃ = PMePh₂ (<b>6</b>), or PPh₃ and PMe₃ (<b>7</b>); and [1,1-(PR₃)₂-3-(Py)-<i>closo</i>-1,2-RhSB₉H₈], where PR₃ = PPh₃ and PMe₃ (<b>8</b>), PMe₃ (<b>9</b>) or PMe₂Ph (<b>10</b>), with HCl to give Cl-ligated clusters. The results demonstrate that in contrast to the PPh₃-ligated compounds, <b>1</b>, <b>2</b>, and <b>3</b>, the reactions with <b>6</b>–<b>10</b> are less selective, giving rise to the formation of mixtures that contain monophosphine species, [8,8-(Cl)­(PR₃)-9-(Py)-<i>nido</i>-8,7-RhSB₉H₉], where PR₃ = PMe₃ (<b>11</b>), PMe₂Ph (<b>12</b>), or PMePh₂ (<b>15</b>), and bis-ligated derivatives, [8,8,8-(Cl)­(PR₃)₂-9-(Py)-<i>nido</i>-8,7-RhSB₉H₉], where PR₃ = PMe₃ (<b>13</b>) or PMe₂Ph (<b>14</b>). The {RhCl­(PR₃)}-containing compounds, <b>3</b>, <b>11</b>, <b>12</b>, and <b>15</b>, are formally unsaturated 12 skeletal electron pair (sep) clusters with <i>nido</i>-structures. Density functional theory (DFT) calculations demonstrate that the <i>nido</i>-structure is more stable than the predicted <i>closo</i>-isomers. In addition, studies have been carried out that involve the reactivity of <b>3</b> with Lewis bases. Thus, it is reported that <b>3</b> interacts with MeCN in solution, and it reacts with CO and pyridine to give the corresponding Rh-L adducts, [8,8,8-(Cl)­(L)­(PPh₃)-9-(Py)-<i>nido</i>-8,7-RhSB₉H₉], where L = CO (<b>5</b>) or Py (<b>20</b>). On the other hand, the treatment of <b>3</b> and <b>5</b> with Proton Sponge (PS) promotes the abstraction of HCl, as [PSH]­Cl, from the <i>nido</i>-clusters, and the regeneration of the parent <i>closo</i>-species, completing two new stoichiometric cycles that are driven by Brønsted acid/base chemistry

Metal–Nitroalkene and <i>aci</i>-Nitro Intermediates in Catalytic Enantioselective Friedel–Crafts Reactions of Indoles with <i>trans</i>-β-Nitrostyrenes

The half-sandwich aqua complex (<i>S</i>_Rh,<i>R</i>_C)-[(η⁵-C₅Me₅)­Rh­{(<i>R</i>)-Prophos}­(H₂O)]­[SbF₆]₂ (Prophos = propane-1,2-diylbis­(diphenylphosphane)) efficiently catalyzes the asymmetric reaction between <i>N</i>-methyl-2-methylindole and <i>trans</i>-β-nitrostyrenes (up to 94% ee). The metal–nitroalkene complex involved has been characterized by X-ray crystallography, and the <i>aci</i>-nitro intermediate complex has been spectroscopically detected. A plausible catalytic cycle is proposed

Enantioselective Catalytic Diels–Alder Reactions with Enones As Dienophiles

The aqua complexes (<i>S</i>_M,<i>R</i>_C)-[(η⁵-C₅Me₅)­M­(PROPHOS)­(H₂O)]­[SbF₆]₂ [PROPHOS = (<i>R</i>)-propane-1,2-diylbis­(diphenylphosphane); M = Rh (<b>1</b>), Ir (<b>2</b>)] are active catalysts for the asymmetric Diels–Alder reaction between ketones and dienes. At low temperatures, enantioselectivities of up to 89% ee are achieved. The intermediate Lewis acid–dienophile complexes (<i>S</i>_M,<i>R</i>_C)-[(η⁵-C₅Me₅)­M­(PROPHOS)­(MVK)]­[SbF₆]₂ (MVK = methyl vinyl ketone; M = Rh (<b>3</b>), Ir (<b>4</b>)) and (<i>S</i>_Ir,<i>R</i>_C)-[(η⁵-C₅Me₅)­Ir­(PROPHOS)­(EVK)]­[SbF₆]₂ (EVK = ethyl vinyl ketone (<b>5</b>)) have been isolated and characterized by analytical and spectroscopic means, including the determination of the crystal structure of the iridium complexes <b>4</b> and <b>5</b> by X-ray diffractometric methods. Structural parameters indicate that the dispositions of the coordinated dienophiles are controlled by the CH/π attractive interactions established between a phenyl group of the PROPHOS ligand and the α-vinyl proton of the ketones. Proton NMR parameters indicate that these interactions are maintained in solution. From these data, the stereoselectivity of the catalytic reaction is discussed

NH₃‑Promoted Ligand Lability in Eleven-Vertex Rhodathiaboranes

The reaction of the 11-vertex rhodathiaborane, [8,8-(PPh₃)₂-<i>nido</i>-8,7-RhSB₉H₁₀] (<b>1</b>), with NH₃ affords inmediately the adduct, [8,8,8-(NH₃)­(PPh₃)₂-<i>nido</i>-8,7-RhSB₉H₁₀] (<b>4</b>). The NH₃–Rh interaction induces the labilization of the PPh₃ ligands leading to the dissociation product, [8,8-(NH₃)­(PPh₃)-<i>nido</i>-8,7-RhSB₉H₁₀] (<b>5</b>), which can then react with another molecule of NH₃ to give [8,8,8-(NH₃)₂(PPh₃)-<i>nido</i>-8,7-RhSB₉H₁₀] (<b>6</b>). These clusters have been characterized in situ by multielement NMR spectroscopy at different temeperatures. The variable temperature behavior of the system demonstrates that the intermediates <b>4</b>–<b>6</b> are in equilibrium, involving ligand exchange processes. On the basis of low intensity signals present in the ¹H NMR spectra of the reaction mixture, some species are tentatively proposed to be the <i>bis</i>- and <i>tris</i>-NH₃ ligated clusters, [8,8-(NH₃)₂-<i>nido</i>-8,7-RhSB₉H₁₀] (<b>7</b>) and [8,8,8-(NH₃)₃-<i>nido</i>-8,7-RhSB₉H₁₀] (<b>8</b>). After evaporation of the solvent and the excess of NH₃, the system containing species <b>4</b>–<b>8</b> regenerates the starting reactant, <b>1</b>, thus closing a stoichiometric cycle of ammonia addition and loss. After 40 h at room temperature, the reaction of <b>1</b> with NH₃ gives the hydridorhodathiaborane, [8,8,8-(H)­(PPh₃)₂-<i>nido</i>-8,7-RhSB₉H₉] (<b>2</b>), as a single product. The reported rhodathiaboranes show reversible H₃N-promoted ligand lability, which implies weak Rh–N interactions, leading to a rare case of metal complexes that circumvent “classical” Werner chemistry