Stepwise Iodide-Free Methanol Carbonylation via Methyl Acetate Activation by Pincer Iridium Complexes.

Iodide is an essential promoter in the industrial production of acetic acid via methanol carbonylation, but it also contributes to reactor corrosion and catalyst deactivation. Here we report that iridium pincer complexes mediate the individual steps of methanol carbonylation to methyl acetate in the absence of methyl iodide or iodide salts. Iodide-free methylation is achieved under mild conditions by an aminophenylphosphinite pincer iridium(I) dinitrogen complex through net C-O oxidative addition of methyl acetate to produce an isolable methyliridium(III) acetate complex. Experimental and computational studies provide evidence for methylation via initial C-H bond activation followed by acetate migration, facilitated by amine hemilability. Subsequent CO insertion and reductive elimination in methanol solution produced methyl acetate and acetic acid. The net reaction is methanol carbonylation to acetic acid using methyl acetate as a promoter alongside conversion of an iridium dinitrogen complex to an iridium carbonyl complex. Kinetic studies of migratory insertion and reductive elimination reveal essential roles of the solvent methanol and distinct features of acetate and iodide anions that are relevant to the design of future catalysts for iodide-free carbonylation.

Selective Transformation of CO2 to CO at a Single Nickel Center.

Carbon dioxide conversion mediated by transition metal complexes continues to attract much attention because of its future potential utilization as a nontoxic and inexpensive C1 source for the chemical industry. Given the presence of nickel in natural systems that allow for extremely efficient catalysis, albeit in an Fe cluster arrangement, studies that focus on selective CO2 conversion with synthetic nickel species are currently of considerable interest in our group. In this Account, the selective conversion of CO2 to carbon monoxide occurring at a single nickel center is discussed. The chemistry is based on a series of related nickel pincer complexes with attention to the uniqueness of the coordination geometry, which is crucial in allowing for particular reactivity toward CO2. Our research is inspired by the efficient enzymatic CO2 catalysis occurring at the active site of carbon monoxide dehydrogenase. Since the binding and reactivity toward CO2 are controlled in part by the geometry of a L3Ni scaffold, we have explored the chemistry of low-valent nickel supported by PPMeP and PNP ligands, in which a pseudotetrahedral or square-planar geometry is accommodated. Two isolated nickel-CO2 adducts, (PPMeP)Ni(η2-CO2-κ C) (2) and {Na(12-C-4)2}{(PNP)Ni(η1-CO2-κ C)} (7), clearly demonstrate that the geometry of the nickel ion is crucial in the binding of CO2 and its level of activation. In the case of a square-planar nickel center supported by a PNP ligand, a series of bimetallic metallacarboxylate Ni-µ-CO2-κ C, O-M species (M = H, Na, Ni, Fe) were synthesized, and their structural features and reactivity were studied. Protonation cleaves the C-O bond, resulting in the formation of a nickel(II) monocarbonyl complex. By sequential reduction, the corresponding mono- and zero-valent Ni-CO species were produced. The reactivities of three nickel carbonyl species toward various iodoalkanes and CO2 were explored to address whether their corresponding reactivities could be controlled by the number of valence d electrons. In particular, a (PNP)Ni(0)-CO species (13) shows immediate reactivity toward CO2 but displays multiple product formation. By incorporation of a -CMe2- bridging unit, a structurally rigidified acriPNP ligand was newly designed and produced. This ligand modification was successful in preparing the T-shaped nickel(I) metalloradical species 9 exhibiting open-shell reactivity due to the sterically exposed nickel center possessing a half-filled d x2- y2 orbital. More importantly, the selective addition of CO2 to a nickel(0)-CO species was enabled to afford a nickel(II)-carboxylate species (22) with the expulsion of CO(g). Finally, the (acriPNP)Ni system provides a synthetic cycle in the study of the selective conversion of CO2 to CO that involves two-electron reduction of Ni-CO followed by the direct addition of CO2 to release the coordinated CO ligand.

Direct CO2 Addition to a Ni(0)-CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO.

Addition of CO2 to a low-valent nickel species has been explored with a newly designed acriPNP pincer ligand (acriPNP- = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). This is a crucial step in understanding biological CO2 conversion to CO found in carbon monoxide dehydrogenase (CODH). A four-coordinate nickel(0) state was reliably accessed in the presence of a CO ligand, which can be prepared from a stepwise reduction of a cationic {(acriPNP)Ni(II)-CO}+ species. All three Ni(II), Ni(I), and Ni(0) monocarbonyl species were cleanly isolated and spectroscopically characterized. Addition of electrons to the nickel(II) species significantly alters its geometry from square planar toward tetrahedral because of the filling of the dx2-y2 orbital. Accordingly, the CO ligand position changes from equatorial to axial, ∠N-Ni-C of 176.2(2)° to 129.1(4)°, allowing opening of a CO2 binding site. Upon addition of CO2 to a nickel(0)-CO species, a nickel(II) carboxylate species with a Ni(η1-CO2-κC) moiety was formed and isolated (75%). This reaction occurs with the concomitant expulsion of CO(g). This is a unique result markedly different from our previous report involving the flexible analogous PNP ligand, which revealed the formation of multiple products including a tetrameric cluster from the reaction with CO2. Finally, the carbon dioxide conversion to CO at a single nickel center is modeled by the successful isolation of all relevant intermediates, such as Ni-CO2, Ni-COOH, and Ni-CO.

A T-Shaped Nickel(I) Metalloradical Species.

A T-shaped NiI complex was synthesized using a rigid acridane-based pincer ligand to prepare a metalloradical center. Structural data displays a nickel ion is embedded in the plane of a PNP ligand. Having a sterically exposed half-filled dx2-y2 orbital, this three-coordinate NiI species reveals unique open-shell reactivity including the homolytic cleavage of various σ-bonds, such as H-H, N-N, and C-C.

Fabrication of Hierarchical, Porous, Bimetallic, Zeolitic Imidazolate Frameworks with the Incorporation of Square Planar Pd and Its Catalytic Application.

Bimetallic zeolitic imidazolate frameworks (ZIFs) containing two different metal ions can exhibit superior performances when applied in heterogeneous catalysis. Herein, we present a facile one-pot synthesis method for PdCo-ZIFs with various Pd/Co ratios, where Pd(II) ions are successfully incorporated into the Co node sites of the ZIF structure. The local structure of the bimetallic ZIFs was comprehensively investigated by pore-structure, X-ray absorption fine structure, and in situ CO adsorption Fourier transform infrared analyses. The results demonstrated that the framework comprises different coordination geometries of Co (tetrahedral) and Pd (square planar) ions connected by the benzimidazolate ligand. Notably, the inherently nonporous, 2D Co-ZIF structure was transformed into a hierarchical porous structure, and the PdCo-ZIFs exhibited a significantly increased concentration of defects and distorted Co sites. Based on these results, the catalytic performances of the synthesized ZIFs in the cycloaddition of CO2 to epoxides were evaluated under a cocatalyst and solvent-free conditions. The PdCo-ZIFs exhibited significantly higher catalytic activity (maximum turnover frequency, TOF = 2501 h-1) than Co-ZIF (TOF = 65 h-1) and Pd-ZIF (no activity), which revealed that the undercoordinated Co sites with distorted structure are the active sites rather than the incorporated Pd ions. This study provides a facile one-pot method for synthesizing bimetallic ZIFs with mixed-coordination modes, hierarchical porous structures, and modified defect concentrations, which would expand the library of structurally diverse bimetallic ZIFs toward various applications.

Nickel-catalyzed ester carbonylation promoted by imidazole-derived carbenes and salts.

Millions of tons of acetyl derivatives such as acetic acid and acetic anhydride are produced each year. These building blocks of chemical industry are elaborated into esters, amides, and eventually polymer materials, pharmaceuticals, and other consumer products. Most acetyls are produced industrially using homogeneous precious metal catalysts, principally rhodium and iridium complexes. We report here that abundant nickel can be paired with imidazole-derived carbenes or the corresponding salts to catalyze methyl ester carbonylation with turnover frequency (TOF) exceeding 150 hour-1 and turnover number (TON) exceeding 1600, benchmarks that invite comparisons to state-of-the-art rhodium-based systems and considerably surpass known triphenylphosphine-based nickel catalysts, which operate with TOF ~7 hour-1 and TON ~100 under the same conditions.

Immobilization of a trimeric ruthenium cluster in mesoporous chromium terephthalate and its catalytic application.

Molecular trimeric ruthenium carboxylate clusters (Ru3 clusters) have been introduced into the pore channels of mesoporous metal-organic framework chromium terephthalate [MIL-101(Cr)] by employing a facile two-step post-synthetic strategy in which diamine hooks anchored on the framework metal nodes of the MOF are used to covalently immobilize the Ru3 clusters. The catalytic activity of the isolated Ru3 clusters in the pore channels of the MOF was significantly improved compared to the bulk counterpart.

Ni(0)-promoted activation of Csp2 -H and Csp2 -O bonds.

A dinickel(0)-N2 complex, stabilized with a rigid acridane-based PNP pincer ligand, was studied for its ability to activate C(sp2)-H and C(sp2)-O bonds. Stabilized by a Ni-µ-N2-Na+ interaction, it activates C-H bonds of unfunctionalized arenes, affording nickel-aryl and nickel-hydride products. Concomitantly, two sodium cations get reduced to Na(0), which was identified and quantified by several methods. Our experimental results, including product analysis and kinetic measurements, strongly suggest that this C(sp2)-H activation does not follow the typical oxidative addition mechanism occurring at a low-valent single metal centre. Instead, via a bimolecular pathway, two powerfully reducing nickel ions cooperatively activate an arene C-H bond and concomitantly reduce two Lewis acidic alkali metals under ambient conditions. As a novel synthetic protocol, nickel(ii)-aryl species were directly synthesized from nickel(ii) precursors in benzene or toluene with excess Na under ambient conditions. Furthermore, when the dinickel(0)-N2 complex is accessed via reduction of the nickel(ii)-phenyl species, the resulting phenyl anion deprotonates a C-H bond of glyme or 15-crown-5 leading to C-O bond cleavage, which produces vinyl ether. The dinickel(0)-N2 species then cleaves the C(sp2)-O bond of vinyl ether to produce a nickel(ii)-vinyl complex. These results may provide a new strategy for the activation of C-H and C-O bonds mediated by a low valent nickel ion supported by a structurally rigidified ligand scaffold.

Decarbonylative ether dissection by iridium pincer complexes.

A unique chain-rupturing transformation that converts an ether functionality into two hydrocarbyl units and carbon monoxide is reported, mediated by iridium(i) complexes supported by aminophenylphosphinite (NCOP) pincer ligands. The decarbonylation, which involves the cleavage of one C-C bond, one C-O bond, and two C-H bonds, along with formation of two new C-H bonds, was serendipitously discovered upon dehydrochlorination of an iridium(iii) complex containing an aza-18-crown-6 ether macrocycle. Intramolecular cleavage of macrocyclic and acyclic ethers was also found in analogous complexes featuring aza-15-crown-5 ether or bis(2-methoxyethyl)amino groups. Intermolecular decarbonylation of cyclic and linear ethers was observed when diethylaminophenylphosphinite iridium(i) dinitrogen or norbornene complexes were employed. Mechanistic studies reveal the nature of key intermediates along a pathway involving initial iridium(i)-mediated double C-H bond activation.

Cation-controlled catalysis with crown ether-containing transition metal complexes.

Transition metal complexes that incorporate crown ethers into the supporting ligands have emerged as a powerful class of catalysts capable of cation-tunable reactivity. Cations held in the secondary coordination sphere of a transition metal catalyst can pre-organize or activate substrates, induce local electric fields, adjust structural conformations, or even modify bonding in the primary coordination sphere of the transition metal. This Feature Article begins with a non-comprehensive review of the structural motifs and catalytic applications of crown ether-containing transition metal catalysts, then proceeds to detail the development of catalysts based on "pincer-crown ether" ligands that bridge the primary and secondary coordination spheres.

Carbon dioxide binding at a Ni/Fe center: synthesis and characterization of Ni(η1-CO2-κC) and Ni-µ-CO2-κC:κ2O,O'-Fe.

The degree of CO2 activation can be tuned by incorporating a distinct electronic coordination environment at the nickel center. A mononuclear nickel carboxylate species (Ni-CO2, 3) and a dinuclear nickel-iron carboxylate species (Ni-CO2-Fe, 5) were prepared. The structure of 3 reveals a rare η1-κC binding mode of CO2, while that of 5 shows bridging CO2 binding (µ2-κC:κ2O,O') between the nickel and iron, presented as the first example of a nickel-µ-CO2-iron species. The structural analyses of 3 and 5 based on XRD and DFT data reveal a higher degree of CO2 activation in 5, imparted by the additional interaction with an iron ion.

Efficient synthesis of frutinone†A and its derivatives through palladium-catalyzed C - H activation/carbonylation.

Frutinone A, a biologically active ingredient of an antimicrobial herbal extract, demonstrates potent inhibitory activity towards the CYP1A2 enzyme. A three-step total synthesis of frutinone A with an overall yield of 44 % is presented. The construction of the chromone-annelated coumarin core was achieved through palladium-catalyzed CH carbonylation of 2-phenolchromones. The straightforward synthetic route allowed facile substitutions around the frutinone A core and thus rapid exploration of the structure-activity relationship (SAR) profile of the derivatives. The inhibitory activity of the synthesized frutinone A derivatives were determined for CYP1A2, and ten compounds exhibited one-to-two digit nanomolar inhibitory activity towards the CYP1A2 enzyme.