Karen I. Goldberg

Characterization of a Rhodium(I) σ-Methane Complex in Solution

Methane Loosely Bound For the most part, molecular bonds involve sharing of electrons between two discrete atoms. In certain cases, however, a third atom can also attract a portion of the electron density without fully cleaving the bond. Such loose complexes between C-H bonds and transition metals are often invoked as short-lived intermediates in metal-catalyzed reactions of hydrocarbons, though they are rarely observed directly. Bernskoetter et al. (p. 553) glimpse this coordination motif for methane (CH4) and rhodium (Rh) using low-temperature nuclear magnetic resonance spectroscopy after protonation of an Rh-CH3 precursor. Kinetics measurements revealed a half-life of just over 80 minutes at −87°C. A loosely bound complex of rhodium and methane has been observed by low-temperature nuclear magnetic resonance spectroscopy. Numerous transition metal–mediated reactions, including hydrogenations, hydrosilations, and alkane functionalizations, result in the cleavage of strong σ bonds. Key intermediates in these reactions often involve coordination of the σ bond of dihydrogen, silanes (Si-H), or alkanes (C-H) to the metal center without full scission of the bond. These σ complexes have been characterized to varying degrees in solid state and solution. However, a σ complex of the simplest hydrocarbon, methane, has eluded full solution characterization. Here, we report nuclear magnetic resonance spectra of a rhodium(I) σ-methane complex obtained by protonation of a rhodium-methyl precursor in CDCl2F solvent at –110°C. The σ-methane complex is shown to be more stable than the corresponding rhodium(III) methyl hydride complex. Even at –110°C, methane rapidly tumbles in the coordination sphere of rhodium, exchanging free and bound hydrogens. Kinetic studies reveal a half-life of about 83 minutes at –87°C for dissociation of methane (free energy of activation is 14.5 kilocalories per mole).

Iridium Catalyzed Dehydrogenation of Substituted Amine Boranes: Kinetics, Thermodynamics and Implications for Hydrogen Storage.

Dehydrogenation of amine boranes is catalyzed efficiently by the iridium pincer complex (kappa (3)-1,3-(OP ( t )Bu 2) 2C 6H 3)Ir(H) 2 ( 1). With CH 3NH 2BH 3 (MeAB) and with AB/MeAB mixtures (AB = NH 3BH 3), the rapid release of 1 equiv of H 2 is observed to yield soluble oligomeric products at rates similar to those previously reported for the dehydrogenation of AB catalyzed by 1. Delta H for the dehydrogenation of AB, MeAB, and AB/MeAB mixtures has been determined by calorimetry. The experimental heats of reaction are compared to results from computational studies.

HOMOGENEOUS HYDROCARBON CH BOND ACTIVATION AND FUNCTIONALIZATION WITH PLATINUM

Publisher Summary The latest developments in the mechanistic understanding of platinum-catalyzed alkane oxidation have involved Pt complexes with chelating ligands having mainly—but not exclusively—nitrogen donors. This chapter focuses on recent studies of these chelated Pt systems. Kinetic studies, isotope labeling, modeling of intermediates, and computational methods have all been used to determine the key factors necessary for achieving facile C–H activation by Pt(II). A general understanding that the alkane must coordinate to a site within the square plane of the Pt(II) center to form a σ-complex has been reached. Oxidative addition of a C–H bond of the alkane in the σ-complex to generate a five-coordinate Pt(IV) alkyl hydride appears to be a viable mechanism for C–H activation and in many cases favored over an electrophilic pathway.

Reactions of Late Transition Metal Complexes with Molecular Oxygen

Limited natural resources, high energy consumption, economic considerations, and environmental concerns demand that we develop new technologies for the sustainable production of chemicals and fuels. New methods that combine the selective activation of C-H bonds of hydrocarbons with oxidation by a green oxidant such as molecular oxygen would represent huge advances toward this goal. The spectacular selectivity of transition metals in cleaving C-H bonds offers the potential for the direct use of hydrocarbons in the production of value-added organics such as alcohols. However, the use of oxygen, which is abundant, environmentally benign, and inexpensive (particularly from air), has proven challenging, and more expensive and less green oxidants are often employed in transition-metal-catalyzed reactions. Advances in the use of oxygen as an oxidant in transition-metal-catalyzed transformations of hydrocarbons will require a better understanding of how oxygen reacts with transition metal alkyl and hydride complexes. For alkane oxidations, researchers will need to comprehend and predict how metals that have shown particularly high activity and selectivity in C-H bond activation (e.g. Pt, Pd, Rh, Ir) will react with oxygen. In this Account, we present our studies of reactions of late metal alkyls and hydrides with molecular oxygen, emphasizing the mechanistic insights that have emerged from this work. Our studies have unraveled some of the general mechanistic features of how molecular oxygen inserts into late metal hydride and alkyl bonds along with a nascent understanding of the scope and limitations of these reactions. We present examples of the formation of metal hydroperoxide species M-OOH by insertion of dioxygen into Pt(IV)-H and Pd(II)-H bonds and show evidence that these reactions proceed by radical chain and hydrogen abstraction pathways, respectively. Comparisons with recent reports of insertion of oxygen into other Pd(II)-H complexes, and also into Ir(III)-H and Rh(III)-H complexes, point to potentially general mechanisms for this type of reaction. Additionally, we observed oxygen-promoted C-H and H-H reductive elimination reactions from five-coordinate Ir(III) alkyl hydride and dihydride complexes, respectively. Further, when Pd(II)Me(2) and Pt(II)Me(2) complexes were exposed to oxygen, insertion processes generated M-OOMe complexes. Mechanistic studies for these reactions are consistent with radical chain homolytic substitution pathways involving five-coordinate M(III) intermediates. Due to the remarkable ability of Pt(II) and Pd(II) to activate the C-H bonds of hydrocarbons (RH) and form M-R species, this reactivity is especially exciting for the development of partial alkane-oxidation processes that utilize molecular oxygen. Our understanding of how late transition metal alkyls and hydrides react with molecular oxygen is growing rapidly and will soon approach our knowledge of how other small molecules such as olefins and carbon monoxide react with these species. Just as advances in understanding olefin and CO insertion reactions have shaped important industrial processes, key insight into oxygen insertion should lead to significant gains in sustainable commercial selective oxidation catalysis.

Investigations of Iridium-Mediated Reversible C−H Bond Cleavage: Characterization of a 16-Electron Iridium(III) Methyl Hydride Complex

New iridium complexes of a tridentate pincer ligand, 2,6-bis(di-tert-butylphosphinito)pyridine (PONOP), have been prepared and used in the study of hydrocarbon C-H bond activation. Intermolecular oxidative addition of a benzene C-H bond was directly observed with [(PONOP)Ir(I)(cyclooctene)][PF(6)] at ambient temperature, resulting in a cationic five-coordinate iridium(III) phenyl hydride product. Protonation of the (PONOP)Ir(I) methyl complex yielded the corresponding iridium(III) methyl hydride cation, a rare five-coordinate, 16-valence electron transition metal alkyl hydride species which was characterized by X-ray diffraction. Kinetic studies of C-H bond coupling and reductive elimination reactions from the five-coordinate complexes have been carried out. Exchange NMR spectroscopy measurements established a barrier of 17.8(4) kcal/mol (22 degrees C) for H-C(aryl) bond coupling in the iridium(III) phenyl hydride cation and of 9.3(4) kcal/mol (-105 degrees C) for the analogous H-C(alkyl) coupling in the iridium(III) methyl hydride cation. The origin of the higher barrier of H-C(aryl) relative to H-C(alkyl) bond coupling is proposed to be influenced by a hindered rotation about the Ir-C(aryl) bond, a result of the sterically demanding PONOP ligand.

σ-Borane Complexes of Iridium : Synthesis and Structural Characterization

Reaction of NaBH4 with (tBuPOCOP)IrHCl affords the previously reported complex (tBuPOCOP)IrH2(BH3) (1) (tBuPOCOP = kappa(3)-C6H3-1,3-[OP(tBu)2]2). The structure of 1 determined from neutron diffraction data contains a B-H sigma-bond to iridium with an elongated B-H bond distance of 1.45(5) A. Compound 1 crystallizes in the space group P1 (Z = 2) with a = 8.262 (5) A, b = 12.264 (5) A, c = 13.394 (4) A, and V = 1256.2 (1) A(3) (30 K). Complex 1 can also be prepared by reaction of BH3 x THF with (tBuPOCOP)IrH2. Reaction of (tBuPOCOP)IrH2 with pinacol borane gave initially complex 2, which is assigned a structure analogous to that of 1 based on spectroscopic measurements. Complex 2 evolves H2 at room temperature leading to the borane complex 3, which is formed cleanly when 2 is subjected to dynamic vacuum. The structure of 3 has been determined by X-ray diffraction and consists of the (tBuPOCOP)Ir core with a sigma-bound pinacol borane ligand in an approximately square planar complex. Compound 3 crystallizes in the space group C2/c (Z = 4) with a = 41.2238 (2) A, b = 11.1233 (2) A, c = 14.6122 (3) A, and V = 6700.21 (19) A(3) (130 K). Reaction of (tBuPOCOP)IrH2 with 9-borobicyclononane (9-BBN) affords complex 4. Complex 4 displays (1)H NMR resonances analogous to 1 and exists in equilibrium with (tBuPOCOP)IrH2 in THF solutions.

C–H Functionalization

The conversion of a C H bond to a C Z bond where Z is an atom other than hydrogen is the most encompassing transformation in organic chemistry. Yet it has also proven to be one of the most challenging for synthetic chemists to accomplish selectively in the laboratory. In nature, this transformation is essential for metabolism, and individual enzymes have evolved to carry out very specific conversions. In industry, the C H to C Z conversion is a core component in energy, fine chemical, and pharmaceutical production, but often the methods used to carry out these conversions involve multiple steps, generate significant byproducts, and/or are energy intensive. Direct transformations of C H bonds have long been known from reactions in organic chemistry as unselective oxidation processes. However, more recent efforts by investigators around the world haveproduced refinements that offer high selectivity, and it is for these processes that the term “C H functionalization” is currently used. The C H bond has been called the “unfunctional” group. It is highly stable and generally resistant to reactions with acids and bases or electrophiles and nucleophiles. Early efforts in the area of C H functionalization have been successful in using functional groups that are attached to carbon to change the reactivity of the nearby C H bond and contribute to its cleavage. In contrast, more recent efforts have sought to perform transformations on C H bonds that are not activated by adjacent functional groups: selective C H functionalization of unactivated C H bonds. Selective transformations of C H bonds are even more challenging in complex molecules that have multiple C H bonds that are electronically similar. Thus, selectivity includes not only chemoselectivity but also regioselectivity and stereoselectivity. The pathway to activation and cleavage of the C H bond is central to understanding how selectivity can be achieved. In its early incarnation, the C H to C Z bond conversion was focused on free radical oxidative processes or carbene insertion reactions that, for the most part, were unselective. Advances in organometallic chemistry have informed us on how reactivity and selectivity in reactions with C H bonds could be modified to achieve high selectivity, and the outcome of these advances is revealed in the Accounts that are included in this Special Issue. However, the C H bond cleavage is only one aspect of functionalization. To achieve high selectivity, understanding of both the path for formation of the new C Z bond, and of how the catalyst can be recycled, is also needed. This Special Issue of Accounts covers the breadth of C H functionalizations that extends from the transformation of alkanes to the synthesis of complex molecules. New advances in method development are revealed. Strategies and successes in selective C H bond activation and cleavage are presented along with functionalization by dehydrogenation, insertions, or oxidations. Metal carbene chemistry is the basis for recent developments in highly regioand stereoselective carbenoid insertion reactions. Oxidative transformations that offer directed C H functionalization and utilization of dioxygen extend the dimensions of C H functionalization. We are grateful to our esteemed colleagues who have contributed to this issue. They have filled the pages that follow with their unique stories describing their remarkable strategies and achievements in catalysis, mechanistic investigations, synthetic methodologies, and applications, all directed to realizing selective functionalization of the “unfunctional” C H bond.

Preparation of a Dihydrogen Complex of Cobalt

Ammonia borane (AB) is an attractive candidate for the chemical storage of hydrogen. Recently, our research group reported that the complex [(pocop)IrH2] (pocop= k -C6H31,3-[OP(tBu)2]2) has facilitated the rapid release of H2 from AB under mild conditions. While this result is promising, iridium is too expensive for widespread application. Our efforts to extend the catalytic chemistry of [(pocop)IrH2] to Co have led to several intriguing complexes. Direct analogues of the Ir catalyst were obtained, thereby demonstrating the unusual ligation of H2 on a Co center. Herein we report the first cobalt–dihydrogen complex, which was characterized by NMR spectroscopy and studied by means of theoretical calculations. The metalation of pocop–H with Co, achieved with a method similar to procedures developed for Ir or Ni, did not proceed in satisfactory yield. An alternative approach that makes use of an iodinated pocop ligand was pursued, as described for a similar synthesis. Complex 1 was prepared in good yield by activation of the ligand with nBuLi and the addition of CoI2·THF (Scheme 1).

Intermolecular Hydroarylation of Unactivated Olefins Catalyzed by Homogeneous Platinum Complexes

The hydroarylation of olefins is a valuable C C bond forming reaction used to produce alkyl arenes. Olefin hydroarylation can be catalyzed by Lewis acids, but such reactions proceed through a Friedel–Crafts type mechanism involving an intermediary carbocation. Thus, these reactions give predominantly Markovnikov products, and ortho, meta, and para selectivity is determined by the substituents on the aromatic ring. In contrast, the use of transition-metal catalysts can afford different regioselectivities acting via a mechanism of arene C H bond activation and olefin insertion. While in the past transition-metal-catalyzed olefin hydroarylation reactions were primarily limited to activated arenes wherein a chelating functionality on the arene was available to assist and direct the C H bond activation step, recently Ir and Ru catalysts have demonstrated hydroarylation with unactivated arenes and olefins. 3] Mechanistic and computational studies on these Ir and Ru catalysts suggest that the hydroarylation does not proceed through a Friedel–Crafts type activation but through olefin insertion followed by oxidative hydrogen migration. In addition, selectivity for antiMarkovnikov over Markovnikov products (ca. 60:40) was observed. However, significantly higher selectivities and turnover numbers (TONs) are needed to make these processes economical, so a broadly tunable system that can be modified both sterically and electronically is likely needed. One promising metal for olefin hydroarylation is platinum. There is considerable precedent for both arene C H bond activation and olefin insertion at Pt ; 5] however, attempts at olefin hydroarylation with Pt have been disappointing. Selectivities consistent with an electrophilic Friedel–Crafts type pathway were observed using a mixed Ag–Pt catalyst system. With a related Pt catalyst, the hydroarylation of norbornene was reported, but other olefins were found to be unreactive. Finally, tridentate chelation of a tris(pyrazolyl)borate ligand stabilized a potential Pt intermediate preventing catalytic turnover. Herein, we describe the rational development of an effective Pt system for intermolecular hydroarylation with unactivated arenes and olefins and present mechanistic evidence consistent with a pathway involving aryl–olefin insertion and C H bond oxidative addition at Pt. While Markovnikov products are favored, anti-Markovnikov products are observed, and the mechanistic insight gained is promising for rational design of more selective and productive Pt catalysts for these reactions. We recently reported that thermolysis of the five-coordinate Pt complexes [(LX)PtMe3] {LX = dtbpp [3,5-di-tertbutyl-2-(2-pyridyl)pyrrolide] (1a) or dppp [3,5-diphenyl-2-(2pyridyl)pyrrolide] (1b)} at 85–100 8C in C6D6 in the presence of C2H4 (9–60 equivalents) led to the release of ethane and methane and formation of Pt complexes 2a or 2b, which contain a cyclometalated substituted pyrrolide group and C2H4 (Scheme 1). [9] This indicates that the (pyridyl)pyrrolide

Hydrogenolysis of Palladium(II) Hydroxide and Methoxide Pincer Complexes

Hydrogenolysis reactions of palladium(II) hydroxide and methoxide complexes to form water and methanol, respectively, and the corresponding palladium(II) hydride are reported. In the presence of water, 1 was found to exist in solution as a water-bridged dimer; however, kinetic studies suggest the reaction of 1 and H(2) proceeds exclusively through the hydroxide monomer to form the palladium(II) hydride and water. Computational studies suggest a four-center intramolecular proton transfer as opposed to an oxidative addition/reductive elimination pathway.