Article pubs.acs.org/Organometallics
Theoretical Studies on the Mechanism of Iridium-Catalyzed Alkene Hydrogenation by the Cationic Complex [IrH2(NCMe)3(PiPr3)]+ Victor Polo,*,† Abdulaziz A. Al-Saadi,*,‡ and Luis A. Oro§,∥ †
Departamento de Química Física and Instituto de Biocomputación y Física de los Sistemas Complejos (BIFI), Universidad de Zaragoza, Pl. S. Francisco S/N 50009 Zaragoza, Spain ‡ Chemistry Department, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia § Instituto de Síntesis Química y Catálisis Homogénea (ISQCH)-Departamento de Química Inorgánica, CSIC-Universidad de Zaragoza, Pl. S. Francisco S/N 50009 Zaragoza, Spain ∥ Center of Research Excellence in Petroleum Refining and Petrochemicals, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia S Supporting Information *
ABSTRACT: A mechanistic DFT study has been carried out on the ethene hydrogenation catalyzed by the [IrH2(NCMe)3(PiPr3)]+ complex (1). First, the reaction of (1) with ethene has been theoretically characterized, and three mechanistic proposals (A−C) have been made for an identification of the preferred pathways for the alkene hydrogenation catalytic cycle considering Ir(I)/Ir(III) and Ir(III)/Ir(V) intermediate species. Theoretical calculations reveal that the reaction path with the lowest energy starts at an initial ethene migratory insertion into the metal−hydride bond, followed by dihydrogen coordination into the vacancy. Ethane is formed via σ-bond metathesis between the bound H2 and the Irethyl moiety, being the rate-determining step, in agreement with the experimental data available. The calculated energetic span associated with the catalytic cycle is 21.4 kcal mol−1. Although no Ir(V) intermediate has been found along the reaction path, the Ir(V) nature of the transition state for the proposed key σ-bond metathesis step has been determined by electron localization function and geometrical analysis.
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Scheme 1. Crabtree’s [Ir(COD)(py)(PCy3)]+, Pfaltz’s [Ir(COD)(PHOX)]+, and [IrH2(NCMe)3(PiPr3)]+ Catalysts
INTRODUCTION Homogeneous hydrogenation is without any doubt the most studied homogeneous catalytic reaction, and Wilkinson’s catalyst, RhCl(PPh3)3, is the prominent example of a homogeneous hydrogenation catalyst.1,2 Another interesting type of hydrogenation catalyst are cationic [M(diene)L2]+ (M = Rh, Ir) complexes, reported by Schrock and Osborn.3 Under hydrogen, diene is hydrogenated, generating reactive [MH2S2(PR3)2]+ species that, in some cases, can be isolated relatively easily from coordinating solvents (S) such as acetone or ethanol.3 The proposed catalytic cycles for the abovementioned rhodium-based catalytic systems involve fundamental organometallic reactions with the metal centers in oxidation states, I or III. Another remarkable hydrogenation catalyst is the Crabtree’s catalyst (Scheme 1a), [Ir(COD)(py)(PCy3)]+ (py = pyridine and COD = 1,5-cyclooctadiene), that, in dichloromethane as solvent, effectively hydrogenates tetrasubstituted alkenes.4 The related benzonitrile (bzn) derivatives, [Ir(COD)(bzn)(PR3)]+ (PR3 = PCy3 or chiral phosphines, neomenthyldiphenylphosphine, or phenyl-(o-methoxyphenyl)methylphosphine), are also very active catalysts for the hydrogenation of prochiral didehydro amino acid derivatives,5 although poor © 2014 American Chemical Society
enantiomeric excesses were obtained with chiral monodentate phosphines. The effectiveness of [Ir(COD)(N-donor)(PR3)]+ cations for the hydrogenation of hindered alkenes has been boosted toward remarkable enantioselectivites by replacing the N-donor/PR3 ligand pair with chiral N,P bidentate ligands such as phosphinooxazolidine (PHOX) in the Pfaltz’s catalyst (Scheme 1b), [Ir(COD)(PHOX)]+.6 The above-mentioned Received: April 7, 2014 Published: September 5, 2014 5156
dx.doi.org/10.1021/om500361e | Organometallics 2014, 33, 5156−5163
Organometallics
Article
path involving the reaction of [IrH(nPr)(NCMe)3(PiPr3)]BF4 with hydrogen via iridium(V) species could be an alternative pathway.10 According to Scheme 2, all the species detected contain the iridium center in oxidation states I and III, although, on the basis of some experimental observations, we have recently speculated about the possibility of a cycle involving Ir(III)/Ir(V) intermediates.10 Although three intermediate species have been identified working at three different temperatures,8 the mechanistic details for the ethene hydrogenation using this catalyst are still unknown. In the past few years, theoretical studies on alkene hydrogenation reactions proposed different possible mechanism through Ir(I)/Ir(III) or Ir(III)/Ir(V) intermediates.10 The asymmetric hydrogenation of alkenes by chiral bidentate rhodium and iridium catalysts is a subject of major interest. While the mechanism of rhodium asymmetric hydrogenation, via Rh(I)/Rh(III) intermediates, is well-established,2b,c,11 the mechanism of asymmetric alkene hydrogenation by [Ir(COD)(N∩P)]+ species has been the object of debate for a long time, and pathways involving Ir(I)/Ir(III) and Ir(III)/Ir(V) intermediates have been considered.6a In this context, it seems interesting to mention that there is increasing evidence of the participation of iridium(V) intermediates in iridium(III) complexes’ reactivity.12 Furthermore, although the Ir(I)/Ir(III) cycle was proposed by Dietiker and Chen13 based on gas phase MS, recent theoretical studies have shed light on the enantioselectivity of chiral Ir-(PHOX) catalysts, pointing out that the catalytic cycle passes through Ir(III) and Ir(V) intermediates. Two slightly different mechanistic proposals involving Ir(V) species were made (see Scheme 3), differing whether (a) the hydride migratory insertion14−17 or (b) dihydrogen metathesis18,19 is the lowest energetic pathway. Basically, the olefin and H2 are both coordinated to the metal, and the first step can be described as either migratory insertion of the hydride into the olefin and oxidative addition of H2 to the metal (a) or insertion of the olefin into the Ir bounded H2 (b). In both cases, the same trihydride Ir(V) intermediate is obtained and the product is released via a reductive elimination. Nevertheless, other pathways involving Ir(I)/Ir(III) cannot be excluded a priori, because the calculated energetic barriers are not excessively high and a change in the mechanism might happen with different catalysts, substrates, or reaction conditions. Burgess suggested an Ir(I)/Ir(III) cycle if two coordination positions are occupied by the substrate.20 Hopmann and Bayer16 found different mechanisms for the alkene or imine substrates using the Ir-(PHOX) catalyst. Neese and co-workers21 have provided solid theoretical support to these studies using high-level ab
[Ir(COD)(L)(PR3)]+ (L = N-donor or PR3) or [Ir(COD)(N∩P)]+ (N∩P = chiral chelating N,P-donors) complexes generate an active hydride catalyst by hydrogenation of the coordinate diene, yielding [IrH2S2(L)(PR3)]+ or [IrH2S2(P∩N)]+ species.2b,6c−e,7 We have previously reported that complex [Ir(COD)(NCMe)(PiPr3)]BF4, closely related to the Crabtree’s catalyst, reacts with hydrogen, in acetonitrile, to give rise to the formation of [IrH2(NCMe)3(PiPr3)]BF4 (Scheme 1c), which can be isolated as a white solid.7 This dihydrido-iridium(III) cationic complex containing one basic phosphine and three acetonitrile ligands in fac disposition has proved to be an excellent hydrogenation catalyst and has allowed the observation and characterization of reaction intermediates potentially involved in alkene homogeneous hydrogenations.8 Interestingly, no deactivation of the catalyst by trimer formation was observed, in contrast with the reported tendency of Crabtree’s type catalysts that form inactive hydride-bridged trimeric polyhydrides at low alkene concentrations.4,9 Scheme 2a Scheme 2. Reaction Intermediates Potentially Involved in Homogeneous Hydrogenation of Ethene (a) and Propene (b) by the [IrH2(NCMe)3(PiPr3)]+ Cationic Complex
shows the observed intermediates for the iridium-catalyzed ethane hydrogenation. At 233 K, the [IrH2(η2-C2H4)(PiPr3)(NCCH3)2]+ complex is observed by NMR spectroscopy. The progress along the catalytic cycle can be controlled by the temperature, as it is indicated in Scheme 2,8 allowing us to identify key reaction intermediates. The rate-determining step of the process seems to be the reductive elimination of ethane, which is the only reaction that requires temperatures above 273 K to proceed.8 When the alkene used was propene, a very stable allyl-hydrido complex, [IrH(η3-C3H5)(NCMe)2(PiPr3)]BF4, was fully characterized, suggesting that this compound could be just a resting state (Scheme 2b) and that a possible
Scheme 3. Alternative Mechanistic Pathways for the Ir(III)/Ir(V) Catalytic Cycle
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dx.doi.org/10.1021/om500361e | Organometallics 2014, 33, 5156−5163
Organometallics
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Scheme 4. Reaction Pathways Investigated in This Worka
initio calculations at the CCSD(T) level, pointing out the good performance of DFT methods considering dispersion interactions. Very recently, the elusive Ir-(PHOX) dihydride alkene complex has been experimentally characterized by Pfaltz and Gruber, showing that an additional molecule of H2 is required to obtain the hydrogenation product, supporting an Ir(III)/ Ir(V) cycle via an [Ir(H)2(alkene)(H2)(PHOX)]+ intermediate.22 The mechanistic details have been shown by DFT calculations, obtaining an excellent agreement with the experimental enantiomeric excess.23 Nowadays, computational studies can yield a full understanding of the reaction mechanisms even for complex chemical reactions involved in homogeneous catalysis. This knowledge is required for a rational improvement of organometallic catalysts for alkene hydrogenation. In view of the complexity of the mechanistic results obtained for the Ir-(PHOX) catalyst, we decided to conduct a full theoretical investigation using the [IrH2(NCMe)3(PiPr3)]+ complex in order to provide additional information into the alkene hydrogenation catalytic cycle using an alternative catalyst. In this work, first, the reactivity of the [IrH2(NCMe)3(PiPr3)]+ catalyst toward ethene will be considered in order to test the calculation level employed against the experimental data existing on this reaction and to shed light on the nature of the species taking part into the catalytic cycle. Second, alternative reaction mechanisms for the alkene hydrogenation process consistent with the experimental data will be proposed and calculated and the energetically preferred pathway will be established.
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a
Three possible mechanisms are proposed for C2H6 release: (A) reductive elimination through Ir(I)/Ir(III) species from 5, (B) through Ir(V) species from complex 5, and (C) through Ir(V) species from complex 3.
when the temperature is raised to 273 K, the diethyl complex [Ir(ethyl)2(NCMe)3(PR3)]+ (8) is formed. Two monohydride intermediates were detected between (5) and (8). These processes are reversible at 273 K, as observed by bubbling argon in a solution of (8). On raising the temperature to 298 K, butane formation from (8) was not observed, but ethane was released and the catalyst is regenerated (see Scheme 4). On the basis of these experimental data, the reaction pathway from 1 to 8 will be first determined computationally in order to identify all intermediates that could initiate the alkene hydrogenation and to test if the computational methodology employed for the calculations can reproduce successfully the experimental observations. Second, three reasonable pathways for the C2H6 formation are proposed, and they will be investigated for a complete characterization of the ethene hydrogenation reaction catalyzed by [IrH2(NCMe)3(PR3)]+. The proposed pathways can be described as follows: (A) direct reductive elimination from [Ir(ethyl)H(NCMe)3(PR3)]+ intermediate (5) involving only Ir(I) or Ir(III) intermediates; (B) from the same complex 5, substitution of acetonitrile by H2, and release of ethane by migratory insertion or metathesis mechanisms through Ir(V) species; and (C) the same proposal as (B), but starting from the [Ir(η2-C2H4)H(NCMe)3(PR3)]+ (3) intermediate. Reaction of [IrH2(NCMe)3(PR3)]+ Complex (1) with C2H4 at Low Temperatures. The full energetic profile for the reaction of [IrH2(NCMe)3(PR3)]+ (1) with ethene to yield the diethyl complex [Ir(ethyl)2(NCMe)3(PR3)]+ (8) is presented in Figure 1. The decoordination of one acetonitrile molecule trans to a hydride ligand to form the five-coordinated structure 2 increases the energy by 8.8 kcal mol−1. The generated vacancy can be occupied by an ethene molecule, yielding [Ir(η2-C2H4)H(NCMe)3(PR3)]+ (3), being the relative energy of 1.3 kcal mol−1. These results indicate that exchange of acetonitrile by olefin is also allowed and acetonitrile can be easily decoordinated from the metal, in particular, if there is a ligand with strong trans influence such as hydride. Therefore, the presence of low energy isomers for intermediates or transition states has to be taken into account in this reaction. While the relative energy of these isomers can be easily rationalized considering the trans influence of the ligands for minimum structures, it may not be evident for
COMPUTATIONAL DETAILS
All DFT theoretical calculations have been carried out using the Gaussian program package.24 The B3LYP method25 has been employed, including the D3 dispersion correction scheme developed by Grimme26 for both energies and gradient calculations and the “ultrafine” grid. The def2-SVP basis set27 has been selected for all atoms for geometry optimizations and calculation of Gibbs energy corrections at 298, 273, and 233 K temperatures for better comparison with experimental results. Final electronic energies have been improved by single-point calculations on the def2-SVP optimized structures using the def2-TVZP basis set. The iPr ligands of the phosphine group have been modeled by methyl groups. Exhaustive conformational searches have been carried out due to the large number of possible conformational isomers. All structures have been optimized considering solvent effects using the PCM continuum model for acetone.28 All reported energies are Gibbs energies relative to the catalyst [IrH2(NCMe)3(PiPr3)]+ and isolated reactants and including zero-point energy and solvent corrections. The nature of the stationary points has been confirmed by analytical frequency analysis, although negligibly low frequencies (
