The alkyl group is the most common component of organic molecules and the most difficult to selectively functionalize. The development of catalysts for dehydrogenation of alkyl groups to give the corresponding olefins could open almost unlimited avenues to functionalization. Homogeneous systems, or more generally systems based on molecular (including solid-supported) catalysts, probably offer the greatest potential for regio- and chemoselective dehydrogenation of alkyl groups and alkanes. The greatest progress to date in this area has been achieved with pincer-ligated transition-metal-based catalysts; this and related chemistry are the subject of this review. Chemists are still far from achieving the most obvious and perhaps most attractive goal in this area, the dehydrogenation of simple alkanes to yield alkenes (specifically monoenes) with high yield and selectivity. Greater progress has been made with tandem catalysis and related approaches in which the initial dehydrogenated product undergoes a desirable secondary reaction. Also reviewed is the substantial progress that has been made in the closely related area of dehydrogenation of alkyl groups of substrates containing heteroatoms.

Great progress has been made in the past several decades concerning C–H bond functionalization. But despite many significant advances, a commercially viable large-scale process for selective alkane functionalization remains an unreached goal. Such conversions will require highly active, selective, and long-lived catalysts. In addition, essentially complete atom-economy will be required. Thus, any reagents used in transforming the alkanes must be almost free (e.g., O2, H2O, N2), or they should be incorporated into the desired large-scale product. Any side-products should be completely benign or have value as fuels (e.g., H2 or other alkanes). Progress and promising leads toward the development of such systems involving primarily molecular transition metal catalysts are described.

The pincer-iridium fragment (iPrPCP)Ir (RPCP = κ3-2,6-C6H3(CH2PR2)2) has been found to catalyze the dehydrogenative coupling of vinyl arenes to afford predominantly (E,E)-1,4-diaryl-1,3-butadienes. The eliminated hydrogen can undergo addition to another molecule of vinyl arene, resulting in an overall disproportionation reaction with 1 equiv of ethyl arene formed for each equivalent of diarylbutadiene produced. Alternatively, sacrificial hydrogen acceptors (e.g., tert-butylethylene) can be added to the solution for this purpose. Diarylbutadienes are isolated in moderate to good yields, up to ca. 90% based on the disproportionation reaction. The results of DFT calculations and experiments with substituted styrenes indicate that the coupling proceeds via double C–H addition of a styrene molecule, at β-vinyl and ortho-aryl positions, to give an iridium(III) metalloindene intermediate; this intermediate then adds a β-vinyl C–H bond of a second styrene molecule before reductively eliminating product. Several metalloindene complexes have been isolated and crystallographically characterized. In accord with the proposed mechanism, substitution at the ortho-aryl positions of the styrene precludes dehydrogenative homocoupling. In the case of 2,4,6-trimethylstyrene, dehydrogenative coupling of β-vinyl and ortho-methyl C–H bonds affords dimethylindene, demonstrating that the dehydrogenative coupling is not limited to C(sp2)–H bonds.

•Pincer-iridium complexes catalyze dehydrogenation of n-pentane to give piperylene.•Less-bulky pincer ligands are much more effective (e.g. iPr4PCP vs. tBu4PCP).•Catalyst effectiveness correlates with effectiveness for alkane dehydroaromatization.Conjugated dienes are desirable reagents for several important applications. We report that sterically uncrowded PCP-pincer iridium complexes, including precursors of (iPr4PCP)Ir and (Me2tBu2PCP)Ir, catalyze the transfer dehydrogenation of pentane, using high concentrations of t‐butylethylene (TBE) as hydrogen acceptor, to give high yields of 1,3-pentadiene (piperylene). Piperylene yields are ca. 100-fold greater than those obtained with the more widely used di(t‐butyl)phosphino substituted pincer iridium catalysts. This represents, to our knowledge, the first reported high-yield synthesis of dienes via the dehydrogenation of n-alkane using molecular catalysts. To our knowledge, this is the first reported high-yield synthesis of dienes achieved via the dehydrogenation of n-alkane using molecular catalysts.Download high-res image (62KB)Download full-size image

We report that pincer-ligated iridium catalysts for alkane dehydrogenation can operate in tandem with zeolite catalysts for arene–alkene coupling, to effect the overall intramolecular dehydrocoupling of alkyl–H and aryl–H bonds (i.e., the dehydrocyclization of alkyl benzene). Thus, zeolite and soluble iridium cocatalysts in refluxing pentylbenzene (205 °C) gave high yields of 1-methyl-1,2,3,4-tetrahydronaphthalene. Subsequent dehydrogenation and isomerization affords 1- and 2-methylnaphthalene and 2-methyl-1,2,3,4-tetrahydronaphthalene. Total yields of cyclized product as high as 5.4 M (94%) have been obtained, corresponding to 6800 turnovers per mol Ir. Turnover numbers for the tandem-catalyzed dehydrocyclization are much greater than those obtained for simple dehydrogenation by Ir catalysts (to give olefins) in the absence of zeolite.Keywords: alkane dehydrogenation; C−C coupling; C−H activation; dehydrogenative cross-coupling; pincer ligand; tandem catalysis; zeolite

The species (R4PCP)Ir are found to effect a double C–H activation addition of biphenyl or phenanthrene to give the corresponding cyclometalated complexes (biphenyl-2,2′-diyl and phenanthrene-4,5-diyl, respectively), which have been characterized spectroscopically and crystallographically. The rate-determining step of the overall reactions is calculated to be the 14-electron (R4PCP)Ir(I) fragment undergoing addition of the sterically hindered C–H bond positioned ortho to the interaryl ring C–C bond. The resulting Ir(III) aryl hydride undergoes a subsequent second C–H addition to give a cyclometalated Ir(V) dihydride complex. This C–H addition to Ir(III) is calculated to be very facile: e.g., a barrier as low as ΔG⧧ = 5.9 kcal/mol in the case of (tBu4PCP)Ir(H)(o-phenanthrenyl). The computational results are fully consistent with, and facilitate explaining, the experimental observations. (tBu4PCP)Ir(NBE) adds an unhindered (m or p) C–H bond of biphenyl or phenanthrene (following loss of NBE) to give an observable Ir(III) aryl hydride. At ambient temperature these species slowly (ca. 24 h) convert to the cyclometalated complexes; the presumed o-C–H addition intermediate is never present at concentrations sufficiently high to be observed. In contrast, in the case of (iPr4PCP)Ir, which is much less hindered than (tBu4PCP)Ir, the reaction with biphenyl does not lead to any observable mono-C–H addition intermediate; this is consistent with a relatively rapid addition of the o-C–H bond followed by an even faster second C–H addition (cyclometalation) by the resulting Ir(III) aryl hydride. Intermolecular double C–H addition has also been explored computationally. Addition of benzene to the Ir(III) species (R4PCP)Ir(H)Ph to afford (R4PCP)Ir(H)2Ph2 is calculated to have a very low barrier for the sterically uncrowded (Me4PCP)Ir species. We propose that the very facile kinetics of Ir(III)/Ir(V) C–H additions/eliminations has significant implications for C–C coupling and other catalytic reactions.

We report the transfer-dehydrogenation of gas-phase alkanes catalyzed by solid-phase, molecular, pincer-ligated iridium catalysts, using ethylene or propene as hydrogen acceptor. Iridium complexes of sterically unhindered pincer ligands such as iPr4PCP, in the solid phase, are found to give extremely high rates and turnover numbers for n-alkane dehydrogenation, and yields of terminal dehydrogenation product (α-olefin) that are much higher than those previously reported for solution-phase experiments. These results are explained by mechanistic studies and DFT calculations which jointly lead to the conclusion that olefin isomerization, which limits yields of α-olefin from pincer–Ir catalyzed alkane dehydrogenation, proceeds via two mechanistically distinct pathways in the case of (iPr4PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir–H bond of (iPr4PCP)IrH2, followed by 3,2-β-H elimination. The use of ethylene as hydrogen acceptor, or high pressures of propene, precludes this pathway by rapid hydrogenation of these small olefins by the dihydride. The second isomerization pathway proceeds via α-olefin C–H addition to (pincer)Ir to give an allyl intermediate as was previously reported for (tBu4PCP)Ir. The improved understanding of the factors controlling rates and selectivity has led to solution-phase systems that afford improved yields of α-olefin, and provides a framework required for the future development of more active and selective catalytic systems.

A study of electronic factors governing the thermodynamics of C–H and N–H bond addition to Ir(I) complexes was conducted. DFT calculations were performed on an extensive series of trans-(PH3)2IrXL complexes (L = NH3 and CO; X = various monodentate ligands) to parametrize the relative σ- and π-donating/withdrawing properties of the various ligands, X. Computed energies of oxidative addition of methane to a series of three- and four-coordinate Ir(I) complexes bearing an ancillary ligand, X, were correlated with the resulting (σX, πX) parameter set. Regression analysis indicates that the thermodynamics of addition of methane to trans-(PH3)2IrX are generally strongly disfavored by increased σ-donation from the ligand X, in contradiction to widely held views on oxidative addition. The trend for oxidative addition of methane to four-coordinate Ir(I) was closely related to that observed for the three-coordinate complexes, albeit slightly more complicated. The computational analysis was found to be consistent with the rates of reductive elimination of benzene from a series of isoelectronic Ir(III) phenyl hydride complexes, measured experimentally in this work and previously reported. Extending the analysis of ancillary ligand energetic effects to the oxidative addition of ammonia to three-coordinate Ir(I) complexes leads to the conclusion that increasing σ-donation by X also disfavors oxidative addition of N–H bonds to trans-(PH3)2IrX. However, coordination of NH3 to the Ir(I) center is disfavored even more strongly by increasing σ-donation by X, which explains why the few documented examples of H–NH2 oxidative addition to transition metals involve complexes with strongly σ-donating ligands situated trans to the site of addition. An orbital-based rationale for the observed results is presented.

A pincer iridium(III) complex, (Phebox)Ir(OAc)2OH2 (1) (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)), selectively cleaves the benzylic C–H bond of mesitylene to form an isolable iridium mesityl complex, (Phebox)Ir(mesityl)(OAc) (3), in >90% yield. The trifluoroacetate analogue, (Phebox)Ir(OCOCF3)2OH2 (2), was synthesized to compare with complex 1 for C–H activation, and (Phebox)Ir(mesityl)(OCOCF3) (4) was synthesized by ligand exchange of complex 3. Both complexes 1 and 2 catalyze H/D exchange between mesitylene and D2O at 180 °C, exclusively at the benzylic position; 2 gave a higher turnover number (11 TO) than 1 (6 TO) in 12 h. Using d4-acetic acid as the source of deuterium, up to 92 turnovers of benzylic H/D exchange of mesitylene were obtained with complex 1. (Phebox)Ir(OCOCF3)2OH2 catalyzed the benzylic C–H oxidation of mesitylene using Ag2O as a terminal oxidant at 130 °C, to form 3,5-dimethylbenzaldehyde and 3,5-dimethylbenzoic acid in 35% ± 4% yield (5.1 ± 0.6 TO). DFT calculations were used to investigate two possible pathways for the catalytic oxidation of mesitylene: (1) C–H activation followed by oxy-functionalization and (2) Ir-oxo formation followed by outer-sphere C–H hydroxylation. Results of calculations of the C–H activation pathway appear to be the more consistent with the experimental observations.

While the addition of C–H bonds to three-coordinate Ir(I) fragments is a central theme in the field of C–H bond activation, addition to square planar four-coordinate complexes is far less precedented. The dearth of such reactions may be attributed, at least in part, to kinetic factors elucidated in seminal work by Hoffmann. C–H additions to square planar carbonyl complexes in particular are unprecedented, in contrast to the extensive chemistry of oxidative addition of other substrates (e.g., H2, HX) to Vaska’s Complex and related species. We report that Bronsted acids will catalyze the addition of the alkynyl C–H bond of phenylacetylene to the pincer complex (PCP)Ir(CO). The reaction occurs to give exclusively the trans-C–H addition product. Our proposed mechanism, based on kinetics and DFT calculations, involves initial protonation of (PCP)Ir(CO) to generate a highly active five-coordinate cationic intermediate, which forms a phenylacetylene adduct that is then deprotonated to give product.

New carbazolide-based iridium pincer complexes (carbPNP)Ir(C2H4), 3a, and (carbPNP)Ir(H)2, 3b, have been prepared and characterized. The dihydride, 3b, reacts with ethylene to yield the cis-dihydride ethylene complex cis-(carbPNP)Ir(C2H4)(H)2. Under ethylene this complex reacts slowly at 70 °C to yield ethane and the ethylene complex, 3a. Kinetic analysis establishes that the reaction rate is dependent on ethylene concentration and labeling studies show reversible migratory insertion to form an ethyl hydride complex prior to formation of 3a. Exposure of cis-(carbPNP)Ir(C2H4)(H)2 to hydrogen results in very rapid formation of ethane and dihydride, 3b. DFT analysis suggests that ethane elimination from the ethyl hydride complex is assisted by ethylene through formation of (carbPNP)Ir(H)(Et)(C2H4) and by H2 through formation of (carbPNP)Ir(H)(Et)(H2). Elimination of ethane from Ir(III) complex (carbPNP)Ir(H)(Et)(H2) is calculated to proceed through an Ir(V) complex (carbPNP)Ir(H)3(Et) which reductively eliminates ethane with a very low barrier to return to the Ir(III) dihydride, 3b. Under catalytic hydrogenation conditions (C2H4/H2), cis-(carbPNP)Ir(C2H4)(H)2 is the catalyst resting state, and the catalysis proceeds via an Ir(III)/Ir(V)/Ir(III) cycle. This is in sharp contrast to isoelectronic (PCP)Ir systems in which hydrogenation proceeds through an Ir(III)/Ir(I)/Ir(III) cycle. The basis for this remarkable difference is discussed.

Aryl alkyl ethers, which are widely used throughout the chemical industry, are typically produced via the Williamson ether synthesis. Olefin hydroaryloxylation potentially offers a much more atom-economical alternative. Known acidic catalysts for hydroaryloxylation, however, afford very poor selectivity. We report the organometallic-catalyzed intermolecular hydroaryloxylation of unactivated olefins by iridium “pincer” complexes. These catalysts do not operate via the hidden Brønsted acid pathway common to previously developed transition-metal-based catalysts. The reaction is proposed to proceed via olefin insertion into an iridium–alkoxide bond, followed by rate-determining C–H reductive elimination to yield the ether product. The reaction is highly chemo- and regioselective and offers a new approach to the atom-economical synthesis of industrially important ethers and, potentially, a wide range of other oxygenates.

A pincer-ligated iridium complex, (PCP)Ir (PCP = κ3-C6H3-2,6-[CH2P(t-Bu)2]2), is found to undergo oxidative addition of C(sp3)–O bonds of methyl esters (CH3–O2CR′), methyl tosylate (CH3–OTs), and certain electron-poor methyl aryl ethers (CH3–OAr). DFT calculations and mechanistic studies indicate that the reactions proceed via oxidative addition of C–H bonds followed by oxygenate migration, rather than by direct C–O addition. Thus, methyl aryl ethers react via addition of the methoxy C–H bond, followed by α-aryloxide migration to give cis-(PCP)Ir(H)(CH2)(OAr), followed by iridium-to-methylidene hydride migration to give (PCP)Ir(CH3)(OAr). Methyl acetate undergoes C–H bond addition at the carbomethoxy group to give (PCP)Ir(H)[κ2-CH2OC(O)Me] which then affords (PCP-CH2)Ir(H)(κ2-O2CMe) (6-Me) in which the methoxy C–O bond has been cleaved, and the methylene derived from the methoxy group has migrated into the PCP Cipso–Ir bond. Thermolysis of 6-Me ultimately gives (PCP)Ir(CH3)(κ2-O2CR), the net product of methoxy group C–O oxidative addition. Reaction of (PCP)Ir with species of the type ROAr, RO2CMe or ROTs, where R possesses β-C–H bonds (e.g., R = ethyl or isopropyl), results in formation of (PCP)Ir(H)(OAr), (PCP)Ir(H)(O2CMe), or (PCP)Ir(H)(OTs), respectively, along with the corresponding olefin or (PCP)Ir(olefin) complex. Like the C–O bond oxidative additions, these reactions also proceed via initial activation of a C–H bond; in this case, C–H addition at the β-position is followed by β-migration of the aryloxide, carboxylate, or tosylate group. Calculations indicate that the β-migration of the carboxylate group proceeds via an unusual six-membered cyclic transition state in which the alkoxy C–O bond is cleaved with no direct participation by the iridium center.

We report on the synthesis and reactivity of rhodium complexes featuring bulky, neutral pincer ligands with a “POP” coordinating motif, tBuxanPOP, iPrxanPOP, and tBufurPOP (tBuxanPOP = 4,5-bis(di-tert-butylphosphino)-9,9-dimethyl-9H-xanthene; iPrxanPOP = 4,5-bis(diisopropylphosphino)-9,9-dimethyl-9H-xanthene; tBufurPOP = 2,5-bis((di-tert-butylphosphino)methyl)furan). The (POP)Rh complexes described in this work are, in general, more reactive than their (PNP)Rh and (PCP)Rh analogues, which allows for the generation of several new species under relatively mild conditions. Thus, monomeric (POP)RhCl complexes oxidatively add H2 to form (POP)Rh(H)2Cl, from which the coordinatively unsaturated hydride complexes (POP)Rh(H)2+ and (tBuxanPOP)Rh(H) can be obtained. In the case of the new ligand tBufurPOP, a major kinetic product of the reaction with H2 is, surprisingly, the trans dihydride, i.e. trans-(tBufurPOP)Rh(H)2Cl; this is most likely attributable to reversible decoordination of one of the pincer coordinating groups, followed by addition of H2 to a highly reactive three-coordinate species. Ethylene is hydrogenated by (tBuxanPOP)Rh(H)2+ at 25 °C, but propylene is not, even at elevated temperatures. Ethylene undergoes insertion into the Rh–H bond of (tBuxanPOP)RhH; this reaction is reversible, allowing for an experimental determination of the equilibrium constant for this hydrometalation. The less bulky iPrxanPOP ligand affords a dihydride complex which functions as a modestly active alkane dehydrogenation catalyst, the first such example for a cationic pincer complex of any metal.

Both the bisphosphine and bisphosphinite pincer complexes (tBu4PCP)IrH2 and (tBu4POCOP)IrH2 can cocatalyze alkane metathesis in tandem with olefin metathesis catalysts, but the two complexes have different resting states during catalysis, suggesting that different steps are turnover-limiting in each case. This led to the hypothesis that a complex with intermediate properties would be catalytically more active than either of these two species. Accordingly, “hybrid” phosphine–phosphinite pincer ligands (PCOP) and the corresponding iridium complexes were synthesized (3c–e). In tandem with olefin-metathesis catalyst MoF12, (tBu4PCOP)IrH2 displays significantly higher activity for the metathesis of n-hexane than does (tBu4PCP)IrH2 or (tBu4POCOP)IrH2. (tBu2PCOPiPr2)IrH4 (3d) is even more active (>30-fold more active than (tBu4POCOP)IrH2) and affords nearly 4.6 M alkane products after 8 h at 125 °C.Keywords: alkane metathesis; dehydrogenation; homogeneous catalysis; iridium; pincer complexes

Methods for the conversion of both renewable and non-petroleum fossil carbon sources to transportation fuels that are both efficient and economically viable could greatly enhance global security and prosperity. Currently, the major route to convert natural gas and coal to liquids is Fischer–Tropsch catalysis, which is potentially applicable to any source of synthesis gas including biomass and nonconventional fossil carbon sources. The major desired products of Fischer–Tropsch catalysis are n-alkanes that contain 9–19 carbons; they comprise a clean-burning and high combustion quality diesel, jet, and marine fuel. However, Fischer–Tropsch catalysis also results in significant yields of the much less valuable C3 to C8n-alkanes; these are also present in large quantities in oil and gas reserves (natural gas liquids) and can be produced from the direct reduction of carbohydrates. Therefore, methods that could disproportionate medium-weight (C3–C8) n-alkanes into heavy and light n-alkanes offer great potential value as global demand for fuel increases and petroleum reserves decrease.This Account describes systems that we have developed for alkane metathesis based on the tandem operation of catalysts for alkane dehydrogenation and olefin metathesis. As dehydrogenation catalysts, we used pincer-ligated iridium complexes, and we initially investigated Schrock-type Mo or W alkylidene complexes as olefin metathesis catalysts. The interoperability of the catalysts typically represents a major challenge in tandem catalysis. In our systems, the rate of alkane dehydrogenation generally limits the overall reaction rate, whereas the lifetime of the alkylidene complexes at the relatively high temperatures required to obtain practical dehydrogenation rates (ca. 125 −200 °C) limits the total turnover numbers. Accordingly, we have focused on the development and use of more active dehydrogenation catalysts and more stable olefin-metathesis catalysts. We have used thermally stable solid metal oxides as the olefin-metathesis catalysts. Both the pincer complexes and the alkylidene complexes have been supported on alumina via adsorption through basic para-substituents. This process does not significantly affect catalyst activity, and in some cases it increases both the catalyst lifetime and the compatibility of the co-catalysts.These molecular catalysts are the first systems that effect alkane metathesis with molecular-weight selectivity, particularly for the conversion of Cnn-alkanes to C2n–2n-alkanes plus ethane. This molecular-weight selectivity offers a critical advantage over the few previously reported alkane metathesis systems. We have studied the factors that determine molecular-weight selectivity in depth, including the isomerization of the olefinic intermediates and the regioselectivity of the pincer-iridium catalyst for dehydrogenation at the terminal position of the n-alkane.Our continuing work centers on the development of co-catalysts with improved interoperability, particularly olefin-metathesis catalysts that are more robust at high temperature and dehydrogenation catalysts that are more active at low temperature. We are also designing dehydrogenation catalysts based on metals other than iridium. Our ongoing mechanistic studies are focused on the apparently complex combination of factors that determine molecular-weight selectivity.

Theoretical methods (B3LYP, M06, and CCSD(T)) have been used to study the kinetics and thermodynamics of ethyl migratory insertion in a series of square-planar [(X∧Y)Ni(ethyl)(ethylene)] complexes (X∧Y = anionic bidentate ligand). The results are discussed qualitatively using general trans-influence arguments. When X ≠ Y, the reactions of the two possible isomers have been compared. The results reveal that when one of the coordinating groups exerts a strong trans influence (STI) and the other a weak trans influence (WTI), as in a STI∧WTI chelating ligand such as a phosphino-enolate (P∧O), one of the two isomers has an activation energy for ethylene insertion (i.e., ethyl migration) that is much less than that calculated for symmetrical bidentate ligands of either the WTI∧WTI or STI∧STI types. Specifically, a low activation energy is found when an ethyl group, coordinated trans to the STI group, migrates to the ethylene coordinated trans to the WTI group. The converse pathway in the STI∧WTI system, wherein ethyl migrates from a position trans to a WTI group, encounters a very high barrier. However, the kinetic barrier to isomerization (prior to migration) is sufficiently low to allow repeated insertions to proceed via the low-barrier pathway, in which an alkyl group in effect migrates from the position trans to the STI group to the position trans to the WTI group. The overall barrier (isomerization plus insertion) for an [(STI∧WTI)Ni(ethyl)(ethylene)] complex is less than that calculated for insertion in a WTI∧WTI analogue. Ethylene dissociation from [(X∧Y)Ni(ethyl)(ethylene)] leads to an intermediate exhibiting a Ni–ethyl β-agostic bond. Unexpectedly, the data reveal that increased trans influence exerted by the ligand trans to the ethyl α-carbon results in a strengthening of the β-agostic interactions. The [(STI∧STI)Ni(ethyl)] species, therefore, have a surprisingly low energy agostic resting state. As a result, ethylene binding to [(STI∧STI)Ni(ethyl)] is predicted to be endoergic; this contributes to an overall barrier to catalytic ethylene insertion which is greater than that calculated for (STI∧WTI)Ni-based species. These results may explain, at least in part, the favorable role of STI∧WTI chelating ligands in nickel-catalyzed olefin oligomerization. They likely also have bearing on factors influencing the activity of late-transition-metal catalysts for olefin oligomerization and polymerization more generally.

When a pincer-ligated iridium complex with a phosphinite substituent in the para-position of the aromatic backbone is immobilized on γ-alumina, it becomes a highly effective supported catalyst for the transfer-dehydrogenation of alkanes. The nature of the interaction between the organometallic complex and the support was investigated using solid-state 31P MAS NMR spectroscopy, solution-state 1H and 31P{1H} NMR spectroscopy, IR and GC/MS analysis of extracted reaction products. The phosphinite substituent is cleaved from the pincer ligand by its reaction with hydroxyl groups on the γ-alumina surface, resulting in covalent anchoring of the complex via the aryl ring. A similar reaction occurs on silica, allowing for ready grafting onto this support as well. A strategy for anchoring homogeneous catalysts on hydroxyl-terminated oxide supports though the selective cleavage of [POR]-containing ligand substituents is suggested.

The transfer dehydrogenation of several ketones by (PCP)IrH2 (PCP = κ3-C6H3-2,6-(CH2PtBu2)2) (1) has been observed. Catalytic turnover was inhibited in most cases by the formation of stable metallacycles or the O–H oxidative addition of phenolic products. Catalytic transfer dehydrogenation of 3,3-dimethylcyclohexanone was achieved, giving the corresponding α,β-enone. The transfer dehydrogenation reaction of cycloheptanone with 1 was found to generate a surprisingly stable PCP-iridium troponyl hydride (9), which is stabilized by conjugation and possibly represents an unusual bicyclo[5.2.0]troponyliridium metalloaromatic structure. Complex 9 was found to catalyze the dimerization of tropone to give a fused tricyclic dihydrodicycloheptafuranol. A mechanism for this reaction is proposed wherein the coordinated troponyl group nucleophilically attacks a free tropone molecule.Graphical abstractThe transfer dehydrogenation of several ketones by (PCP)IrH2 has been observed. Catalytic turnover was inhibited in most cases by the formation of stable metallacycles or phenolic O–H addition products. Transfer dehydrogenation of cycloheptanone gave an iridium troponyl complex (9), which may represent an unusual bicyclic metalloaromatic species. Complex 9 catalyzes a novel dimerization of tropone.Research highlights► The transfer dehydrogenation of several ketones by (PCP)IrH2 has been observed. ► Catalytic transfer dehydrogenation of 3,3-dimethylcyclohexanone was achieved. ► Most dehydrogenated ketones added to (PCP)Ir to form metallacycles or O–H addition products. ► Dehydrogenation of cycloheptanone gave an unusual bicyclo[5.2.0]troponyliridium hydride. ► Catalytic dimerization of tropone gave a novel fused tricyclic dihydrodicycloheptafuranol.

The iridium pincer complexes (PCP)IrH4 (1; PCP = [κ3-1,3-(CH2PtBu2)2C6H3]) and (POCOP)IrH4 (2; POCOP = [κ3-1,3-(OPtBu2)2C6H3]) have proven to be effective catalyst precursors for dehydrogenation of alkanes. The complex (POCOP)IrH2 has also been applied successfully as a catalyst for release of H2 from ammonia borane. Investigation of the “tetrahydride” forms of these complexes by solution NMR methods suggests their formulation as dihydrogen/dihydride species. This is in contrast to the solid state structure of 1, determined by neutron diffraction (at 100 K), which indicates a compressed tetrahydride structure with only weak H−H interactions. Complex 1 (C24H47IrP2) crystallizes in the space group P42, tetragonal, (Z = 2) with a = 11.7006 (19) Å, c = 9.7008(27) Å, and V = 1328.1(5) Å3. Electronic structure calculations on 1 and 2 indicate that the global minima on the potential energy surfaces in the gas phase are tetrahydride structures; however, the dihydrogen/dihydride forms are only slightly higher in energy (1−3 kcal/mol). A dihydrogen/dihydride species is calculated to be the global minimum for 2 when in solution. The barriers to interconversion between the tetrahydride and dihydrogen/dihydride species are almost negligible.

The adamantyl-substituted pincer-ligand precursor AdPCP-H [(AdPCP = κ3-C6H3-2,6-(CH2PAd2)2); Ad = 1-adamantyl] has been synthesized by the reaction of 1,3-dibromoxylene with di-1-adamantylphosphine in the presence of triethylamine. Treatment of AdPCP-H with [Ir(COD)Cl]2 (COD = 1,5-cyclooctadiene) affords the pincer-ligated complex (AdPCP)IrHCl, which was crystallographically characterized. Dehydrohalogenation of (AdPCP)IrHCl either with LiBEt3H or with KOtBu, under hydrogen atmosphere, yields the hydrides (AdPCP)IrH2 and (AdPCP)IrH4. (AdPCP)IrH2 catalyzes dehydrogenation of alkanes with a level of activity comparable to that of the previously reported (tBuPCP)IrH2, while it is thermally much more robust than the tBuPCP analogue, as well as iPrPCP or tBuPOCOP pincer complexes.

(PCP)Ir (PCP = κ3-C6H3-2,6-[CH2P(t-Bu)2]2) is found to undergo oxidative addition of the methyl−oxygen bond of electron-poor methyl aryl ethers, including methoxy-3,5-bis(trifluoromethyl)benzene and methoxypentafluorobenzene, to give the corresponding aryloxide complexes (PCP)Ir(CH3)(OAr). Although the net reaction is insertion of the Ir center into the C−O bond, density functional theory (DFT) calculations and a significant kinetic isotope effect [kCH3OAr/kCD3OAr = 4.3(3)] strongly argue against a simple insertion mechanism and in favor of a pathway involving C−H addition and α-migration of the OAr group to give a methylene complex followed by hydride-to-methylene migration to give the observed product. Ethoxy aryl ethers, including ethoxybenzene, also undergo C−O bond cleavage by (PCP)Ir, but the net reaction in this case is 1,2-elimination of ArO−H to give (PCP)Ir(H)(OAr) and ethylene. DFT calculations point to a low-barrier pathway for this reaction that proceeds through C−H addition of the ethoxy methyl group followed by β-aryl oxide elimination and loss of ethylene. Thus, both of these distinct C−O cleavage reactions proceed via initial addition of a C(sp3)−H bond, despite the fact that such bonds are typically considered inert and are much stronger than C−O bonds.

“PCP”-pincer-ligated iridium complexes have been found to be highly effective catalysts for the dehydrogenation of alkanes. We report a computational and experimental study of the effect on catalytic activity resulting from systematically varying steric crowding by the substitution of methyl groups for the phosphino tert-butyl groups of (R4PCP)Ir (R4PCP = κ3-C6H3-2,6-(CH2PR2)2; R = tBu or Me). DFT calculations for (R4PCP)Ir species (R4 = tBu4 or tBu3Me) indicate that the rate-determining step in the n-alkane/1-alkene transfer dehydrogenation cycle is β-H elimination by (R4PCP)Ir(n-alkyl)(H). It is calculated that the transition state for this step is ca. 10 kcal/mol lower for (tBu3MePCP)Ir than for (tBu4PCP)Ir (relative to the corresponding free (R4PCP)Ir). However, this catalytically favorable effect is calculated to be partially offset by the strong binding of 1-alkene to (tBu3MePCP)Ir in the resting state, so the overall barrier is thus lower by only ca. 4 kcal/mol. Further Me-for-tBu substitutions have a smaller effect on the transition states, and the calculated energy of the olefin-bound resting states is lowered by comparable amounts; therefore these additional substitutions are predicted to have little overall favorable effect on catalytic rates. (tBu3MePCP)IrH4 has been synthesized and isolated, and its catalytic activity has been investigated. It is indeed found to be a more active catalyst precursor than (tBu4PCP)IrH4 for alkane transfer dehydrogenation. (tBu2Me2PCP)IrH4 was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than (tBu3MePCP)IrH4. However, synthetic precursors of (tBu2Me2PCP)IrH4 tended to yield dinuclear clusters, while complex mixtures were observed during catalysis that were not amenable to characterization. It is therefore not clear if the lesser catalytic activity of (tBu2Me2PCP)Ir vs (tBu3MePCP)Ir derivatives is due to the energetics of the actual catalytic cycle or due to deactivation of this catalyst via the facile formation of clusters.

Tandem dehydrogenation–olefin-metathesis catalyst systems, comprising a pincer-ligated iridium-based alkane dehydrogenation catalyst and a molybdenum-based olefin-metathesis catalyst, are reported to effect the metathesis-cyclooligomerization of cyclooctane and cyclodecane to give cycloalkanes with various carbon numbers, predominantly multiples of the substrate carbon number, and polymers.

We report a structural, spectroscopic, and computational study of two tBuPNP (2,6-bis(di-tert-butyl-phosphinomethyl)pyridine) complexes of iron, (tBuPNP)FeCl2 (1) and (tBuPNP)Fe(CO)2 (3). Complex 1, (tBuPNP)FeCl2, also independently synthesized by Milstein, has unusually long iron−ligand bond distances. DFT calculations show that these are clearly attributable to its high-spin electronic structure, and in particular to occupancy of the strongly antibonding dx2−y2 orbital. The crystal structure of 3 reveals two unusual aspects. (1) The geometry around the iron atom in 3 is much closer to square pyramidal (SQP; apical CO) than to trigonal bipyramidal (TBP), although five-coordinate Fe(0) complexes are generally expected to be TBP; moreover, Chirik et al. have reported that (iPrPNP)Fe(CO)2 has essentially a perfect TBP structure (iPrPNP = 2,6-bis(di-isopropyl-phosphinomethyl)pyridine). (2) The apical carbonyl ligand in 3 deviates significantly from linearity (Fe−C−O = 171.9°). Additionally, complex 3 is intensely blue in color, which is unusual for an Fe(0) complex and also significantly different from the red color of Chirik’s (iPrPNP)Fe(CO)2 species. Results from DFT calculations reproduce and explain these structural and spectroscopic aspects as well as the contrast between 3 and its iPrPNP analogue.

The isomerization of olefins by complexes of the pincer-ligated iridium species (tBuPCP)Ir (tBuPCP = κ3-C6H3-2,6-(CH2PtBu2)2) and (tBuPOCOP)Ir (tBuPOCOP = κ3-C6H3-2,6-(OPtBu2)2) has been investigated by computational and experimental methods. The corresponding dihydrides, (pincer)IrH2, are known to hydrogenate olefins via initial Ir–H addition across the double bond. Such an addition is also the initial step in the mechanism most widely proposed for olefin isomerization (the “hydride addition pathway”); however, the results of kinetics experiments and DFT calculations (using both M06 and PBE functionals) indicate that this is not the operative pathway for isomerization in this case. Instead, (pincer)Ir(η2-olefin) species undergo isomerization via the formation of (pincer)Ir(η3-allyl)(H) intermediates; one example of such a species, (tBuPOCOP)Ir(η3-propenyl)(H), was independently generated, spectroscopically characterized, and observed to convert to (tBuPOCOP)Ir(η2-propene). Surprisingly, the DFT calculations indicate that the conversion of the η2-olefin complex to the η3-allyl hydride takes place via initial dissociation of the Ir–olefin π-bond to give a σ-complex of the allylic C–H bond; this intermediate then undergoes C–H bond oxidative cleavage to give an iridium η1-allyl hydride which “closes” to give the η3-allyl hydride. Subsequently, the η3-allyl group “opens” in the opposite sense to give a new η1-allyl (thus completing what is formally a 1,3 shift of Ir), which undergoes C–H elimination and π-coordination to give a coordinated olefin that has undergone double-bond migration.

We report on the synthesis and reactivity of rhodium complexes featuring bulky, neutral pincer ligands with a “POP” coordinating motif, tBuxanPOP, iPrxanPOP, and tBufurPOP (tBuxanPOP = 4,5-bis(di-tert-butylphosphino)-9,9-dimethyl-9H-xanthene; iPrxanPOP = 4,5-bis(diisopropylphosphino)-9,9-dimethyl-9H-xanthene; tBufurPOP = 2,5-bis((di-tert-butylphosphino)methyl)furan). The (POP)Rh complexes described in this work are, in general, more reactive than their (PNP)Rh and (PCP)Rh analogues, which allows for the generation of several new species under relatively mild conditions. Thus, monomeric (POP)RhCl complexes oxidatively add H2 to form (POP)Rh(H)2Cl, from which the coordinatively unsaturated hydride complexes (POP)Rh(H)2+ and (tBuxanPOP)Rh(H) can be obtained. In the case of the new ligand tBufurPOP, a major kinetic product of the reaction with H2 is, surprisingly, the trans dihydride, i.e. trans-(tBufurPOP)Rh(H)2Cl; this is most likely attributable to reversible decoordination of one of the pincer coordinating groups, followed by addition of H2 to a highly reactive three-coordinate species. Ethylene is hydrogenated by (tBuxanPOP)Rh(H)2+ at 25 °C, but propylene is not, even at elevated temperatures. Ethylene undergoes insertion into the Rh–H bond of (tBuxanPOP)RhH; this reaction is reversible, allowing for an experimental determination of the equilibrium constant for this hydrometalation. The less bulky iPrxanPOP ligand affords a dihydride complex which functions as a modestly active alkane dehydrogenation catalyst, the first such example for a cationic pincer complex of any metal.

Tandem dehydrogenation–olefin-metathesis catalyst systems, comprising a pincer-ligated iridium-based alkane dehydrogenation catalyst and a molybdenum-based olefin-metathesis catalyst, are reported to effect the metathesis-cyclooligomerization of cyclooctane and cyclodecane to give cycloalkanes with various carbon numbers, predominantly multiples of the substrate carbon number, and polymers.

When a pincer-ligated iridium complex with a phosphinite substituent in the para-position of the aromatic backbone is immobilized on γ-alumina, it becomes a highly effective supported catalyst for the transfer-dehydrogenation of alkanes. The nature of the interaction between the organometallic complex and the support was investigated using solid-state 31P MAS NMR spectroscopy, solution-state 1H and 31P{1H} NMR spectroscopy, IR and GC/MS analysis of extracted reaction products. The phosphinite substituent is cleaved from the pincer ligand by its reaction with hydroxyl groups on the γ-alumina surface, resulting in covalent anchoring of the complex via the aryl ring. A similar reaction occurs on silica, allowing for ready grafting onto this support as well. A strategy for anchoring homogeneous catalysts on hydroxyl-terminated oxide supports though the selective cleavage of [POR]-containing ligand substituents is suggested.