Reversible C(sp3)-Si Oxidative Addition of Unsupported Organosilanes: Effects of Silicon Substituents on Kinetics and Thermodynamics

An Operationally Unsaturated Iridium-Pincer Complex That C-H Activates Methane and Ethane in the Crystalline Solid-State

The known complex [Ir(tBu-PONOP)MeH][BArF4], 1[BArF4] [tBu-PONOP = κ3-2,6-(tBu2PO)2C5H3N); ArF = 3,5-(CF3)2(C6H3); J. Am. Chem. Soc. 2009, 131, 8603], is a robust precursor for in crystallo single-crystal to single-crystal (SC-SC) C-H activation of methane and ethane at 80 °C. This contrasts with the reported solution (CD2Cl2) behavior, where 1[BArF4] decomposes by methane loss. Crystalline 1[BArF4] is accessed as a single polymorph on a gram scale. A single-crystal neutron diffraction study locates the hydride. 13C{1H} SSNMR experiments on 1[BArF4], and its isotopologue [Ir(tBu-PONOP)(CD3)D][BArF4], d4-1[BArF4], suggest a rapid and reversible endergonic reductive bond formation is occurring in crystallo to access an Ir(I) σ-methane complex. Heating 1[BArF4] to 80 °C under high vacuum results in loss of methane and intramolecular C-H activation to form cyclometalated [Ir(cyclo-tBu-PONOP')H][BArF4], 2[BArF4], in a SC-SC reaction. This is reversible, and the addition of CH4 or CD4 to 2[BArF4] at 80 °C results in an equilibrium with 1[BArF4] or d4-1[BArF4], respectively. Complex 2[BArF4] is thus an operationally unsaturated source of 14-electron [Ir(tBu-PONOP)][BArF4], III, that undergoes C-H activation with methane. Periodic DFT studies, alongside isotope labeling experiments, link 1[BArF4] and 2[BArF4]/CH4 via a reductive elimination/oxidative addition pathway. Heating 2[BArF4] to 80 °C under N2 forms [Ir(tBu-PONOP)(κ1-N2)][BArF4], in a SC-SC transformation. Reaction with CO forms [Ir(tBu-PONOP)(CO)][BArF4] at room temperature. Calculations suggest reaction with N2 occurs via an associative process or competitively through III, while with CO only an associative process operates. Heating 2[BArF4] to 80 °C under an ethane atmosphere results in alkane dehydrogenation, via a SC-SC reaction, forming a ∼1:1 mixture of [Ir(tBu-PONOP)(η2-H2C═CH2)][BArF4], and [Ir(tBu-PONOP)H2][BArF4].

Cadmium-Doped and Pincer Ligand-Modified Gold Nanocluster for Catalytic KA2 Reaction

A gold nanocluster Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ (PNP=2,6-bis(diphenylphosphinomethyl)pyridine, SR=4-MeOPhS) consisting of an icosahedral Au₁₃ kernel, two Au₂ CdS₆ staple motifs, and two PNP pincer ligands has been designed, synthesized and well characterized. This cadmium and PNP pincer ligand co-modified gold nanocluster showed high catalytic efficiency in the KA² reaction, featuring high TON, mild reaction conditions, broad substrate scope as well as catalyst recyclability. Comparison of the catalytic performance between Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ and the structurally similar single cadmium (or PNP) modified gold nanoclusters demonstrates that the co-existence of the cadmium and PNP on the surface is crucial for the high catalytic activity of the gold nanocluster. This work would be enlightening for developing efficient catalysts for cascade reactions and discovering the catalytic potential of metal nanoclusters in organic transformations.

Metathesis between E-C(spn ) and H-C(sp3 ) σ-Bonds (E=Si, Ge; n=2, 3) on an Osmium-Polyhydride

The silylation of a phosphine of OsH6 (Pi Pr3 )2 is performed via net-metathesis between Si-C(spn ) and H-C(sp3 ) σ-bonds (n=2, 3). Complex OsH6 (Pi Pr3 )2 activates the Si-H bond of Et3 SiH and Ph3 SiH to give OsH5 (SiR3 )(Pi Pr3 )2 , which yield OsH4 {κ1 -P,η2 -SiH-[i Pr2 PCH(Me)CH2 SiR2 H]}(Pi Pr3 ) and R-H (R=Et, Ph), by displacement of a silyl substituent with a methyl group of a phosphine. Such displacement is a first-order process, with activation entropy consistent with a rate determining step occurring via a highly ordered transition state. It displays selectivity, releasing the hydrocarbon resulting from the rupture of the weakest Si-substituent bond, when the silyl ligand bears different substituents. Accordingly, reactions of OsH6 (Pi Pr3 )2 with dimethylphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane afford OsH5 (SiR2 R')(Pi Pr3 )2 , which evolve into OsH4 {κ1 -P,η2 -GeH-[i Pr2 PCH(Me)CH2 SiR2 H]}(Pi Pr3 ) (R=Me, OSiMe3 ) and R'-H (R'=Ph, Me). Exchange reaction is extended to Et3 GeH. The latter reacts with OsH6 (Pi Pr3 )2 to give OsH5 (GeEt3 )(Pi Pr3 )2 , which loses ethane to form OsH4 {κ1 -P,η2 -GeH-[i Pr2 PCH(Me)CH2 GeEt2 H]}(Pi Pr3 ).

Visible Light-Induced Hydrosilylation of Electron-Deficient Alkenes by Iron Catalysis

Herein, we reported a method for iron-catalyzed, visible-light-induced hydrosilylation reactions of electron-deficient alkenes to produce value-added silicon compounds. Alkenes bearing functional groups with different steric properties were suitable substrates, as were derivatives of structurally complex natural products. Mechanistic studies showed that chlorine radicals generated by iron-catalyzed ligand-to-metal charge transfer in the presence of lithium chloride promoted the formation of silyl radicals.