Preparation and Degradation of Rhodium and Iridium Diolefin Catalysts for the Acceptorless and Base-Free Dehydrogenation of Secondary Alcohols

Abstract

Rhodium and iridium diolefin catalysts for the acceptorless and base-free dehydrogenation of secondary alcohols have been prepared, and their degradation has been investigated, during the study of the reactivity of the dimers [M(μ-Cl)(η 4 -C 8 H 12 )] 2 (M = Rh ( 1 ), Ir ( 2 )) and [M(μ-OH)(η 4 -C 8 H 12 )] 2 (M = Rh ( 3 ), Ir ( 4 )) with 1,3-bis(6′-methyl-2′-pyridylimino)isoindoline (HBMePHI). Complex 1 reacts with HBMePHI, in dichloromethane, to afford equilibrium mixtures of 1 , the mononuclear derivative RhCl(η 4 -C 8 H 12 ){κ 1 - N py -(HBMePHI)} ( 5 ), and the binuclear species [RhCl(η 4 -C 8 H 12 )] 2 {μ- N py , N py -(HBMePHI)} ( 6 ). Under the same conditions, complex 2 affords the iridium counterparts IrCl(η 4 -C 8 H 12 ){κ 1 - N py -(HBMePHI)} ( 7 ) and [IrCl(η 4 -C 8 H 12 )] 2 {μ- N py , N py -(HBMePHI)} ( 8 ). In contrast to chloride, one of the hydroxide groups of 3 and 4 promotes the deprotonation of HBMePHI to give [M(η 4 -C 8 H 12 )] 2 (μ-OH){μ- N py , N iso -(BMePHI)} (M = Rh ( 9 ), Ir ( 10 )), which are efficient precatalysts for the acceptorless and base-free dehydrogenation of secondary alcohols. In the presence of KO t Bu, the [BMePHI] − ligand undergoes three different degradations: alcoholysis of an exocyclic isoindoline-N double bond, alcoholysis of a pyridyl-N bond, and opening of the five-membered ring of the isoindoline core.

Silyl-Osmium(IV)-Trihydride Complexes Stabilized by a Pincer Ether-Diphosphine: Formation and Reactions with Alkynes

“…In agreement with the concerted character of the oxidative addition of the C−H bond, the hydride ligand H(01) and the metalated carbon atom C( 1 9) bond length of 1.424(9) Å is notably longer than those usually observed for free C−C double bonds of around 1.34 Å. 42 In accordance with the strong addition of this bond to the metal center, the resonances corresponding to C(8) and C( 9) are observed at significant high fields, 41.0 and 36.3 ppm, respectively, in the 13 C{ 1 H} NMR spectrum in benzene-d 6 . In a consistent manner, the signals due to the associated hydrogen atoms appear at 3.67 [C(8)H], and 2.49 and 1.84 [C(9)H 2 ] ppm in the 1 H NMR spectrum, which displays the hydride resonance at −7.15 ppm as a doublet of doublets with H−P coupling constants of 33.0 and 37.4 Hz.…”

“…The coordination causes a significant elongation of the double bond, as expected for the Chatt–Dewar–Ducanson bonding model. Thus, the C(8)–C(9) bond length of 1.424(9) Å is notably longer than those usually observed for free C–C double bonds of around 1.34 Å . In accordance with the strong addition of this bond to the metal center, the resonances corresponding to C(8) and C(9) are observed at significant high fields, 41.0 and 36.3 ppm, respectively, in the 13 C­{ 1 H} NMR spectrum in benzene- d 6 .…”

Homogeneous catalysis with polyhydride complexes

“…73 Isoindoline HBMePI displays non innocent character, participating in the formation of the metal-alkoxide bond, in contrast to tricyclohexylphosphine. This molecule, which undergoes metal-promoted degradation in basic media, 74 experiences N-H and C-H bond activations mediated by the pentahydride IrH 5 (P i Pr 3 ) 2 (10) 75 and the hexahydride OsH 6 -(P i Pr 3 ) 2 (11), 76 to form the homo-binuclear tetrahydride 12 and the hetero-binuclear pentahydride 13, via the iridiumdihydride derivative 14 (Scheme 6). Structural parameters, obtained from the X-ray structure analysis of these compounds, and its behavior towards alcohols affording the ionic alkoxides 15 and 16 suggest that the donor atoms of the bridge have a direct participation in the heterolytic O-H bond activation of the alcohols.…”

Alkynyl Ligands as Building Blocks for the Preparation of Phosphorescent Iridium(III) Emitters: Alternative Synthetic Precursors and Procedures

“…The distances Ir–C(1) of 1.992(10) and 1.998(9) Å, C(1)–N(2) of 1.326(11) and 1.294(11) Å, and N(2)–C(9) of 1.336(12) and 1.389(11) Å, which are intermediate between single and double bonds, suggest that there is electron delocalization in the bond sequence Ir(1)–C(1)–N(2). 19 However, the values of the nuclear independent chemical shift (NICS) computed at the center of the five-member ring and out of plane at 1 Å above and below the center (−1.7, −1.2, and −1.4 ppm) are scarcely negative, pointing out very poor aromaticity. The 1 H and 13 C{ 1 H} NMR spectra of 11 and 12 , at room temperature, in dichloromethane- d 2 are congruous with Figure 7 .…”