Activation and Oxidation of Mesitylene C–H Bonds by (Phebox)Iridium(III) Complexes

Cyclometallated complexes as catalysts for C-H activation and functionalization

The development of novel catalysts for C-H activation reactions with increased reactivity and improved selectivities has been attracting significant interest over the last two decades. More recently, promising results have been developed using tridentate pincer ligands, which form a stable C-M bond. Furthermore, based on mechanistic studies, the unique catalytic role of some metallacyclic intermediate species has been revealed. These experimental observations have subsequently translated into the rational design of advanced C-H activation catalysts in both Ru- and Ir-based systems. Recent breakthroughs in the field of C-H activation catalysed by metallacyclic intermediates are thus discussed.

Design of robust 2,2'-bipyridine ligand linkers for the stable immobilization of molecular catalysts on silicon(111) surfaces

The attachment of the 2,2'-bipyridine (bpy) moieties to the surface of planar silicon(111) (photo)electrodes was investigated using ab initio simulations performed on a new cluster model for methyl-terminated silicon. Density functional theory (B3LYP) with implicit solvation techniques indicated that adventitious chlorine atoms, when present in the organic linker backbone, led to instability at very negative potentials of the surface-modified electrode. In prior experimental work, chlorine atoms were present as a trace surface impurity due to required surface processing chemistry, and thus could plausibly result in the observed surface instability of the linker. Free energy calculations for the Cl-atom release process with model silyl-linker constructs revealed a modest barrier (14.9 kcal mol-1) that decreased as the electrode potential became more negative. A small library of new bpy-derived structures has additionally been explored computationally to identify strategies that could minimize chlorine-induced linker instability. Structures with fluorine substituents are predicted to be more stable than their chlorine analogues, whereas fully non-halogenated structures are predicted to exhibit the highest stability. The behavior of a hydrogen-evolving molecular catalyst Cp*Rh(bpy) (Cp* = pentamethylcyclopentadienyl) immobilized on a silicon(111) cluster was explored theoretically to evaluate differences between the homogeneous and surface-attached behavior of this species in a tautomerization reaction observed under reductive conditions for catalytic H2 evolution. The calculated free energy difference between the tautomers is small, hence the results suggest that use of reductively stable linkers can enable robust attachment of catalysts while maintaining chemical behavior on the electrode similar to that exhibited in homogeneous solution.

Catalytic Urea Synthesis from Ammonium Carbamate Using a Copper(II) Complex: A Combined Experimental and Theoretical Study

The synthesis of urea fertilizer is currently the largest CO₂ conversion process by volume in the industry. In this process, ammonium carbamate is an intermediate en route to urea formation. We determined that the tetraammineaquacopper(II) sulfate complex, [Cu(NH₃)₄(OH₂)]SO₄, catalyzed the formation of urea from ammonium carbamate in an aqueous solution. A urea yield of up to 18 ± 6% was obtained at 120 °C after 15 h and in a high-pressure metal reactor. No significant urea formed without the catalyst. The urea product was characterized by Fourier transform infrared (FT-IR), powder X-ray diffraction (PXRD), and quantitative ¹H{¹³C} NMR analyses. The [Cu(NH₃)₄(OH₂)]SO₄ catalyst was then recovered at the end of the reaction in a 29% recovery yield, as verified by FT-IR, PXRD, and quantitative UV-vis spectroscopy. A precipitation method using CO₂ was developed to recover and reuse 66 ± 3% of Cu(II). The catalysis mechanism was investigated by the density functional theory at the B3LYP/6-31G** level with an SMD continuum solvent model. We determined that the [Cu(NH₃)₄]²⁺ complex is likely an effective catalyst structure. The study of the catalysis mechanism suggests that the coordinated carbamate with [Cu(NH₃)₄]²⁺ is likely the starting point of the catalyzed reaction, and carbamic acid can be involved as a transient intermediate that facilitates the removal of an OH group. Our work has paved the way for the rational design of catalysts for urea synthesis from the greenhouse gas CO₂.

2-Pyridone-stabilized iridium silylene/silyl complexes: structure and QTAIM analysis

Iridium(iii) complexes of the general formula [Ir(X)(κ2-NSiiPr2)2] (NSiiPr2 = (4-methyl-pyridine-2-yloxy)diisopropylsilyl; X = Cl, 3; CF3SO3, 5; CF3CO2, 6) have been prepared and fully characterized, including X-ray diffraction studies and theoretical calculations. The presence of isopropyl substituents at the silicon atom favours the monomeric structure found in complexes 3 and 5. The short Ir-Si bond distances (2.25-2.28 Å) indicate some degree of base-stabilized silylene character of the Ir-Si bond in 3, 5 and 6 assisted by the 2-pyridone moiety. However, the shortening of these Ir-Si bonds might be a consequence of the constrained 2-pyridone geometry, and consequently the silyl character of these bonds can not be excluded. A DFT theoretical study on the nature of the Ir-Si bonds has been performed for complex 3 as well as for four other iridium complexes finding representative examples of different bonding situations between Ir and Si atoms: silylene, base-assisted silylene (both with an anionic base and with a neutral base), and silyl bonds, using the topological properties of the electron charge density. The results of these studies show that the Ir-Si bonds in Ir-NSiiPr2 complexes can be considered as an intermediate between the base-stabilized silylene and silyl cases, and therefore they have been proposed as 2-pyridone-stabilized iridium silylene/silyl bonds.

Ir-catalyzed selective reduction of CO2 to the methoxy or formate level with HSiMe(OSiMe3)2

Ir-NSi-based catalysts allow controlling the selective reduction of CO₂ with HSiMe(OSiMe₃)₂ to afford methoxysilane or silyl formate.

Adsorption of mesitylene via mesoporous adsorbents

Mesitylene (or 1,3,5-trimethylbenzene) is a volatile organic compound emitted from various industrial processes, e.g., spray coating. Its emissions have become a critical issue because mesitylene is toxic and cannot be removed using traditional adsorbents, e.g., zeolite (H-ZSM-5; the diameter of mesitylene molecules is greater than the pore size of H-ZSM-5). Hence, an adsorbent with a large pore size, MCM-41, is used in this study to investigate its adsorption capacity for mesitylene and compare with that of H-ZSM-5. Experimental results reveal that MCM-41 without Al₂O₃ exhibits a good adsorption capacity (184 mg/g) for the gas stream containing 100 ppm of mesitylene at a relative humidity of 10%. The adsorption kinetics is well described by the Freundlich isotherm. Furthermore, experimental results reveal that MCM-41 is effective for the adsorption of low concentrations (10 ppm) of mesitylene. In addition, adsorption-desorption tests revealed that the sample MCM-41-AS is stable to sustain the adsorption capacity after 10 adsorption-desorption cycles. After 10 adsorption-desorption cycles, MCM-41-AS retains 92.4% of its initial adsorption capacity (170 vs. 184 mg/g). Finally, MCM-41 and H-ZSM-5 in series are effective for the simultaneous removal of mesitylene and toluene in the gas stream.

IMPLICATIONS

This study aims to improve the performance of adsorbent for mesitylene, which is typically applied in the spray-coating industry. The zeolite MCM-41-AS is selected as a candidate for the investigation. Experimental results reveal that MCM-41-AS exhibits a good adsorption capacity for mesitylene and that it can be integrated with H-ZSM-5-25 for the simultaneous adsorption of mesitylene and toluene.

Computational Studies of Carboxylate-Assisted C-H Activation and Functionalization at Group 8-10 Transition Metal Centers

Computational studies on carboxylate-assisted C-H activation and functionalization at group 8-10 transition metal centers are reviewed. This Review is organized by metal and will cover work published from late 2009 until mid-2016. A brief overview of computational work prior to 2010 is also provided, and this outlines the understanding of carboxylate-assisted C-H activation in terms of the "ambiphilic metal-ligand assistance" (AMLA) and "concerted metalation deprotonation" (CMD) concepts. Computational studies are then surveyed in terms of the nature of the C-H bond being activated (C(sp²)-H or C(sp³)-H), the nature of the process involved (intramolecular with a directing group or intermolecular), and the context (stoichiometric C-H activation or within a variety of catalytic processes). This Review aims to emphasize the connection between computation and experiment and to highlight the contribution of computational chemistry to our understanding of catalytic C-H functionalization based on carboxylate-assisted C-H activation. Some opportunities where the interplay between computation and experiment may contribute further to the areas of catalytic C-H functionalization and applied computational chemistry are identified.

Chlorination of (Phebox)Ir(mesityl)(OAc) by Thionyl Chloride

Pincer (Phebox)Ir(mesityl)(OAc) (2) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)) complex, formed by benzylic C-H activation of mesitylene (1,3,5-trimethylbenzene) using (Phebox)Ir(OAc)2OH2 (1), was treated with thionyl chloride to rapidly form 1-(chloromethyl)-3,5-dimethylbenzene in 50% yield at 23 °C. A green species was obtained at the end of reaction, which decomposed during flash column chromatography to form (Phebox)IrCl2OH2 in 87% yield.