Metal-Ligand Proton Tautomerism, Electron Transfer, and C(sp3)-H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex.

The para-N-pyridyl-based PCP pincer proligand 3,5-bis(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine (pN-tBuPCP-H) was synthesized and metalated to give the iridium complex (pN-tBuPCP)IrHCl (2-H). In marked contrast with its phenyl-based congeners, e.g., (tBuPCP)IrHCl and derivatives, 2-H is highly air-sensitive and reacts with oxidants such as ferrocenium, trityl cation, and benzoquinone. These oxidations ultimately lead to intramolecular activation of a phosphino-t-butyl C(sp3)-H bond and cyclometalation. Considering the greater electronegativity of N than C, 2-H is expected to be less easily oxidized than simple PCP derivatives; cyclic voltammetry and DFT calculations support this expectation. However, 2-H is calculated to undergo metal-ligand-proton tautomerism (MLPT) to give an N-protonated complex that can be described with resonance forms representing a zwitterionic complex (with a negative charge on Ir) and a p-N-pyridylidene (a remote N-heterocyclic carbene) Ir(I) complex. One-electron oxidation of this tautomer is calculated to be dramatically more favorable than direct oxidation of 2-H (ΔΔG° = -31.3 kcal/mol). The resulting Ir(II) oxidation product is easily deprotonated to give metalloradical 2• which is observed by NMR spectroscopy. 2• can be further oxidized to give cationic Ir(III) complex, 2+, which can oxidatively add a phosphino-t-butyl C-H bond and undergo deprotonation to give the observed cyclometalated product. DFT calculations indicate that less sterically hindered analogues of 2+ would preferentially undergo intermolecular addition of C(sp3)-H bonds, for example, of n-alkanes. The resulting iridium alkyl complexes could undergo facile ß-H elimination to afford olefin, thereby completing a catalytic cycle for alkane dehydrogenation driven by one-electron oxidation and deprotonation, enabled by MLPT.

Ruthenium Complexes of a Triphosphorus-Coordinating Pincer Ligand: Ru-P Ligand-Substituent Exchange Reactions Driven by Large Variations of Bond Energies.

The reaction of [(p-cymene)RuCl2]2 with the triphosphine ligand bis(2-di-tert-butylphosphinophenyl)phosphine (tBuPHPP) results in an unusual exchange reaction in which a chloride ligand and a phosphorus-bound H atom are exchanged ("H-P/Ru-Cl exchange") to give the (chlorophosphine)ruthenium hydride complex (tBuPClPP)RuHCl [1Cl-HCl; tBuPClPP = bis(2-di-tert-butylphosphinophenyl)chlorophosphine]. Density functional theory calculations indicate that the presumed initial product of metalation, (tBuPHPP)RuCl2 (1H-Cl2), undergoes an H-P/Ru-Cl exchange via sequential P-to-Ru α-H migration to give the intermediate (tBuPPP)RuHCl2, followed by Ru-to-P α-Cl migration to give the observed product 1Cl-HCl (crystallographically characterized). Dehydrochlorination of 1Cl-HCl under a H2 atmosphere gives (tBuPClPP)RuH4 (1Cl-H4), which then can undergo a second dehydrochlorination and addition of H2 to give (tBuPHPP)RuH4 (1H-H4). This reaction may proceed via the reverse of the intramolecular exchange by 1H-Cl2, i.e., loss of H2 from 1Cl-H4 to give 1Cl-H2, which could undergo Cl-P/Ru-H exchange to give (tBuPHPP)RuHCl (1H-HCl). Accordingly, the thermodynamics of Cl-P/Ru-H exchange are found to be highly dependent on the nature of the ancillary anionic ligand (H or Cl), which is not directly involved in the exchange. The origin of this thermodynamic dependence can be explained in terms of the high stability of complexes (RPXPP)RuHCl (X = H, Cl; R = Me, tBu), in which the hydride is approximately trans to a vacant coordination site and the central phosphine group is approximately trans to the weak-trans-influence chloride ligand. This conclusion has general implications for five-coordinate d6 complexes, both pincer- and nonpincer-ligated.

Lewis Structures and the Bonding Classification of End-on Bridging Dinitrogen Transition Metal Complexes.

The activation of dinitrogen by coordination to transition metal ions is a widely used and promising approach to the utilization of Earth's most abundant nitrogen source for chemical synthesis. End-on bridging N2 complexes (µ-η1:η1-N2) are key species in nitrogen fixation chemistry, but a lack of consensus on the seemingly simple task of assigning a Lewis structure for such complexes has prevented application of valence electron counting and other tools for understanding and predicting reactivity trends. The Lewis structures of bridging N2 complexes have traditionally been determined by comparing the experimentally observed NN distance to the bond lengths of free N2, diazene, and hydrazine. We introduce an alternative approach here and argue that the Lewis structure should be assigned based on the total π-bond order in the MNNM core (number of π-bonds), which derives from the character (bonding or antibonding) and occupancy of the delocalized π-symmetry molecular orbitals (π-MOs) in MNNM. To illustrate this approach, the complexes cis,cis-[(iPr4PONOP)MCl2]2(µ-N2) (M = W, Re, and Os) are examined in detail. Each complex is shown to have a different number of nitrogen-nitrogen and metal-nitrogen π-bonds, indicated as, respectively: W≡N-N≡W, Re═N═N═Re, and Os-N≡N-Os. It follows that each of these Lewis structures represents a distinct class of complexes (diazanyl, diazenyl, and dinitrogen, respectively), in which the µ-N2 ligand has a different electron donor number (total of 8e-, 6e-, or 4e-, respectively). We show how this classification can greatly aid in understanding and predicting the properties and reactivity patterns of µ-N2 complexes.

Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity.

The selective functionalization of alkanes and alkyl groups is a major goal of chemical catalysis. Toward this end, a bulky triphosphine with a central secondary phosphino group, bis(2-di-t-butyl-phosphinophenyl)phosphine (tBuPHPP), has been synthesized. When complexed to iridium, it adopts a meridional ("pincer") configuration. The secondary phosphino H atom can undergo migration to iridium to give an anionic phosphido-based-pincer (tBuPPP) complex. Stoichiometric reactions of the (tBuPPP)Ir complexes reflect a distribution of steric bulk around the iridium center in which the coordination site trans to the phosphido group is quite crowded; one coordination site cis to the phosphido is even more crowded; and the remaining site is particularly open. The (tBuPPP)Ir precursors are the most active catalysts reported to date for dehydrogenation of n-alkanes, by about 2 orders of magnitude. The electronic properties of the iridium center are similar to that of well-known analogous (RPCP)Ir catalysts. Accordingly, DFT calculations predict that (tBuPPP)Ir and (tBuPCP)Ir are, intrinsically, comparably active for alkane dehydrogenation. While dehydrogenation by (RPCP)Ir proceeds through an intermediate trans-(PCP)IrH2(alkene), (tBuPPP)Ir follows a pathway proceeding via cis-(PPP)IrH2(alkene), thereby circumventing unfavorable placement of the alkene at the bulky site trans to phosphorus. (tBuPPP)Ir and (tBuPCP)Ir, however, have analogous resting states: square planar (pincer)Ir(alkene). Alkene coordination at the crowded trans site is therefore unavoidable in the resting states. Thus, the resting state of the (tBuPPP)Ir catalyst is destabilized by the architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.

Mechanisms of Electrochemical N2 Splitting by a Molybdenum Pincer Complex.

Molybdenum complexes supported by tridentate pincer ligands are exceptional catalysts for dinitrogen fixation using chemical reductants, but little is known about their prospects for electrochemical reduction of dinitrogen. The viability of electrochemical N2 binding and splitting by a molybdenum(III) pincer complex, (pyPNP)MoBr3 (pyPNP = 2,6-bis(tBu2PCH2)-C5H3N)), is established in this work, providing a foundation for a detailed mechanistic study of electrode-driven formation of the nitride complex (pyPNP)Mo(N)Br. Electrochemical kinetic analysis, optical and vibrational spectroelectrochemical monitoring, and computational studies point to two concurrent reaction pathways: In the reaction-diffusion layer near the electrode surface, the molybdenum(III) precursor is reduced by 2e- and generates a bimetallic molybdenum(I) Mo2(µ-N2) species capable of N-N bond scission; and in the bulk solution away from the electrode surface, over-reduced molybdenum(0) species undergo chemical redox reactions via comproportionation to generate the same bimetallic molybdenum(I) species capable of N2 cleavage. The comproportionation reactions reveal the surprising intermediacy of dimolybdenum(0) complex trans,trans-[(pyPNP)Mo(N2)2](µ-N2) in N2 splitting pathways. The same "over-reduced" molybdenum(0) species was also found to cleave N2 upon addition of lutidinium, an acid frequently used in catalytic reduction of dinitrogen.

Understanding Terminal versus Bridging End-on N2 Coordination in Transition Metal Complexes.

Terminal and bridging end-on coordination of N2 to transition metal complexes offer possibilities for distinct pathways in ammonia synthesis and N2 functionalization. Here we elucidate the fundamental factors controlling the two binding modes and determining which is favored for a given metal-ligand system, using both quantitative density functional theory (DFT) and qualitative molecular orbital (MO) analyses. The Gibbs free energy for converting two terminal MN2 complexes into a bridging MNNM complex and a free N2 molecule (2ΔGeq°) is examined through systematic variations of the metal and ligands; values of ΔGeq° range between +9.1 and -24.0 kcal/mol per M-N2 bond. We propose a model that accounts for these broad variations by assigning a fixed π-bond order (BOπ) to the triatomic terminal MNN moiety that is equal to that of the free N2 molecule, and a variable BOπ to the bridging complexes based on the character (bonding or antibonding) and occupancy of the π-MOs in the tetratomic MNNM core. When the conversion from terminal to bridging coordination and free N2 is associated with an increase in the number of π-bonds (ΔBOeqπ > 0), the bridging mode is greatly favored; this condition is satisfied when each metal provides 1, 2, or 3 electrons to the π-MOs of the MNNM core. When each metal in the bridging complex provides 4 electrons to the MNNM π-MOs, ΔBOeqπ = 0; the equilibrium in this case is approximately ergoneutral and the direction can be shifted by dispersion interactions.

Formation of Enamines via Catalytic Dehydrogenation by Pincer-Iridium Complexes.

Di-isopropylphosphino-substituted pincer-ligated iridium catalysts are found to be significantly more effective for the dehydrogenation of simple tertiary amines to give enamines than the previously reported di-t-butylphosphino-substituted species. It is also found that the di-isopropylphosphino-substituted complexes catalyze dehydrogenation of several ß-functionalized tertiary amines to give the corresponding 1,2-difunctionalized olefins. The di-t-butylphosphino-substituted species are ineffective for such substrates; presumably, the marked difference is attributable to the lesser crowding of the di-isopropylphosphino-substituted catalysts. Experimentally determined kinetic isotope effects in conjunction with DFT-based analysis support a dehydrogenation mechanism involving initial pre-equilibrium oxidative addition of the amine α-C-H bond followed by rate-determining elimination of the ß-C-H bond.

Polar molecules catalyze CO insertion into metal-alkyl bonds through the displacement of an agostic C-H bond.

The insertion of CO into metal-alkyl bonds is the key C-C bond-forming step in many of the most important organic reactions catalyzed by transition metal complexes. Polar organic molecules (e.g., tetrahydrofuran) have long been known to promote CO insertion reactions, but the mechanism of their action has been the subject of unresolved speculation for over five decades. Comprehensive computational studies [density functional theory (DFT)] on the prototypical system Mn(CO)5(arylmethyl) reveal that the polar molecules do not promote the actual alkyl migration step. Instead, CO insertion (i.e. alkyl migration) occurs rapidly and reversibly to give an acyl complex with a sigma-bound (agostic) C-H bond that is not easily displaced by typical ligands (e.g. phosphines or CO). The agostic C-H bond is displaced much more readily, however, by the polar promoter molecules, even though such species bind only weakly to the metal center and are themselves then easily displaced; the facile kinetics of this process are attributable to a hydrogen bonding-like interaction between the agostic C-H bond and the polar promoter. The role of the promoter is to thereby catalyze isomerization of the agostic product of CO insertion to give an [Formula: see text]-C,O-bound acyl product that is more easily trapped than the agostic species. This ability of such promoters to displace a strongly sigma-bound C-H bond and to subsequently undergo facile displacement themselves is without reported precedent, and could have implications for catalytic reactions beyond carbonylation.

Electrochemical C-H bond activation via cationic iridium hydride pincer complexes.

A C-H bond activation strategy based on electrochemical activation of a metal hydride is introduced. Electrochemical oxidation of ( tBu4 PCP)IrH4 ( tBu4 PCP is [1,3-( t Bu2PCH2)-C6H3]-) in the presence of pyridine derivatives generates cationic Ir hydride complexes of the type [( tBu4 PCP)IrH(L)]+ (where L = pyridine, 2,6-lutidine, or 2-phenylpyridine). Facile deprotonation of [( tBu4 PCP)IrH(2,6-lutidine)]+ with the phosphazene base tert-butylimino-tris(pyrrolidino)phosphorane, t BuP1(pyrr), results in selective C-H activation of 1,2-difluorobenzene (1,2-DFB) solvent to generate ( tBu4 PCP)Ir(H)(2,3-C6F2H3). The overall electrochemical C-H activation reaction proceeds at room temperature without need for chemical activation by a sacrificial alkene hydrogen acceptor. This rare example of undirected electrochemical C-H activation holds promise for the development of future catalytic processes.

H2 Addition to Pincer Iridium Complexes Yielding trans-Dihydride Products: Unexpected Correlations of Bond Strength with Bond Length and Vibrational Frequencies.

R4PONOP-Ir-Me (R1) and R4POCOP-Ir-CO (R2), R = tBu or iPr, are known to undergo acid-catalyzed oxidative addition of H2 that yields octahedral products with two hydrides in a trans-configuration. We use density functional theory to study the free energies (Δ Gtrans) and equilibrium isotope effects (EIEtrans) for H2/D2 addition to R1, R2, and related complexes for R = tBu, iPr, and Me. For a given R, reaction of R1 is ∼5 kcal/mol more exergonic than R2. For a given subclass of complexes, Δ Gtrans is more exergonic for the smaller R. The computed values of Δ Gtrans vary between +5.1 and -17.4 kcal/mol. EIEtrans varies between 0.78 and 1.22. Counterintuitively, it is the less-exergonic reactions that afford products with shorter Ir-H bonds, greater symmetric and asymmetric trans-Ir-(H)2 stretching vibrational frequencies, and more inverse EIEtrans. This disparity is amplified in Me4PONOP-Os-CO, where Δ Gtrans is -35.2 kcal/mol, yet the Os-H bonds are long, and the Os-H vibrational frequencies are low as compared with the Ir-H bonds, and EIEtrans is high (1.20). Attempts are made to account for the inverted bond strength-bond length correlation based on the hydricity of the products and the total negative charge on the trans-Ir(H)2 unit as computed using the Quantum Theory of Atoms in Molecules.

Selective Dehydrogenative Coupling of Ethylene to Butadiene via an Iridacyclopentane Complex.

An iridium complex is found to catalyze the selective dehydrogenative coupling of ethylene to 1,3-butadiene. The key intermediate, and a major resting state, is an iridacyclopentane that undergoes a surprisingly facile ß-H elimination, enabled by a partial dechelation (κ3-κ2) of the supporting 3,5-dimethylphenyl-2,6-bis(oxazolinyl) ligand.

Protonation and electrochemical reduction of rhodium- and iridium-dinitrogen complexes in organic solution.

Protonation and reduction of pincer-ligated Rh- and Ir-N2 complexes have been studied by NMR spectroscopy and cyclic voltammetry to assess the capability of these complexes to activate or reduce N2. Protonation, which is a prerequisite to electrochemical reduction, results in a cationic metal-hydride that loses N2 under an atmosphere of Ar. Reduction of the metal-hydride results in fast disproportionation of an unobserved transient Ir2+ species. These studies suggest that the regioselectivity of initial protonation is a strong determinant for the ability of a system to facilitate the reduction of N2.

Dehydrogenation of Alkanes and Aliphatic Groups by Pincer-Ligated Metal Complexes.

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.

Large-Scale Selective Functionalization of Alkanes.

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.

Catalytic Dehydrogenative C-C Coupling by a Pincer-Ligated Iridium Complex.

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 ß-vinyl and ortho-aryl positions, to give an iridium(III) metalloindene intermediate; this intermediate then adds a ß-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 ß-vinyl and ortho-methyl C-H bonds affords dimethylindene, demonstrating that the dehydrogenative coupling is not limited to C(sp2)-H bonds.

ß-Hydride Elimination and C-H Activation by an Iridium Acetate Complex, Catalyzed by Lewis Acids. Alkane Dehydrogenation Cocatalyzed by Lewis Acids and [2,6-Bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl]iridium.

NaBArF4 (sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate) was found to catalyze reactions of (Phebox)IrIII(acetate) (Phebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complexes, including (i) ß-H elimination of (Phebox)Ir(OAc)(n-alkyl) to give (Phebox)Ir(OAc)(H) and the microscopic reverse, alkene insertion into the Ir-H bond of (Phebox)Ir(OAc)(H), and (ii) hydrogenolysis of the Ir-alkyl bond of (Phebox)Ir(OAc)(n-alkyl) and the microscopic reverse, C-H activation by (Phebox)Ir(OAc)(H), as indicated by H/D exchange experiments. For example, ß-H elimination of (Phebox)Ir(OAc)(n-octyl) (2-Oc) proceeded on a time scale of minutes at -15 °C in the presence of (0.4 mM) NaBArF4 as compared with a very slow reaction at 125 °C in the absence of NaBArF4. In addition to NaBArF4, other Lewis acids are also effective. Density functional theory calculations capture the effect of the Na+ cation and indicate that it operates primarily by promoting κ2-κ1 dechelation of the acetate anion, which opens the coordination site needed to allow the observed reaction to proceed. In accord with the effect on these individual stoichiometric reactions, NaBArF4 was also found to cocatalyze, with (Phebox)Ir(OAc)(H), the acceptorless dehydrogenation of n-dodecane.

Experimental and computational study of alkane dehydrogenation catalyzed by a carbazolide-based rhodium PNP pincer complex.

A rhodium complex based on the bis-phosphine carbazolide pincer ligand was investigated in the context of alkane dehydrogenation and in comparison with its iridium analogue. (carb-PNP)RhH2 was found to catalyze cyclooctane/t-butylethylene (COA/TBE) transfer dehydrogenation with a turnover frequency up to 10 min-1 and turnover numbers up to 340, in marked contrast with the inactive Ir analogue. TONs were limited by catalyst decomposition. Through a combination of mechanistic, experimental and computational (DFT) studies the difference between the Rh and Ir analogues was found to be attributable to the much greater accessibility of the 14-electron (carb-PNP)M(i) fragment in the case of Rh. In contrast, Ir is more strongly biased toward the M(iii) oxidation state. Thus (carb-PNP)RhH2 but not (carb-PNP)IrH2 can be dehydrogenated by sacrificial hydrogen acceptors, particularly TBE. The rate-limiting segment of the (carb-PNP)Rh-catalyzed COA/TBE transfer dehydrogenation cycle is found to be the dehydrogenation of COA. Within this segment, the rate-determining step is calculated to be (carb-PNP)Rh(cyclooctyl)(H) undergoing formation of a ß-H agostic intermediate, while the reverse step (loss of a ß-H agostic interaction) is rate-limiting for hydrogenation of the acceptors TBE and ethylene. Such a step has not previously been proposed as rate-limiting in the context of alkane dehydrogenation, nor, to our knowledge, has the reverse step been proposed as rate-limiting for olefin hydrogenation.

Assessment of the Electronic Factors Determining the Thermodynamics of "Oxidative Addition" of C-H and N-H Bonds to Ir(I) Complexes.

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 (σ(X), π(X)) 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.

Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product.

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 (iPr4)PCP, 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 ((iPr4)PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of ((iPr4)PCP)IrH2, followed by 3,2-ß-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 ((tBu4)PCP)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.