Detailed Investigation of the Mechanism of Rh-Diphosphite Supported

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Detailed investigation of the mechanism of Rh-diphosphite SILP catalyzed 1-butene hydroformylation in the gasphase via combined kinetic and DFT modeling studies Simon Walter, Hanna Hahn, Robert Franke, Wolfgang Hieringer, Peter Wasserscheid, and Marco Haumann ACS Catal., Just Accepted Manuscript • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Detailed investigation of the mechanism of Rh-diphosphite SILP catalyzed 1-butene hydroformylation in the gas-phase via combined kinetic and DFT modeling studies

Simon Walter 1 ,6

1

1‡

, Hanna Hahn 2 , Robert Franke

3,4

, Wolfgang Hieringer 5 *, Peter Wasserscheid

, Marco Haumann 1 *

Friedrich-Alexander-Universität

Erlangen-Nürnberg

(FAU),

Lehrstuhl

für

Chemische

Reaktionstechnik (CRT), Egerlandstr. 3, 91058 Erlangen, Germany, corresponding author: [email protected] 2 Evonik Technology & Infrastructure GmbH, Paul-Baumann-Str. 1, 45772 Marl, Germany 3 Evonik Performance Materials GmbH, Paul-Baumann-Str. 1, 45772 Marl, Germany 4 Ruhr-Universität Bochum, Lehrstuhl für Theoretische Chemie, Universitätsstr. 150, 44780 Bochum, Germany 5 Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Lehrstuhl für Theoretische Chemie,

Egerlandstr.

3,

91058

Erlangen,

Germany,

corresponding

author:

[email protected] 6 Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen Catalysis Resource Center, Egerlandstr. 3, 91058 Erlangen, Germany ‡ Current address: DSM Nutritional Products, Hauptstr. 4, 4334 Sisseln, Switzerland

Keywords Hydroformylation, hydrogenation, isomerization, supported ionic liquid phase (SILP), kinetic modeling, density functional theory (DFT) Abstract A detailed kinetic investigation of the gas-phase continuous hydroformylation of 1-butene has been carried out. The supported ionic liquid phase (SILP) catalyst was based on a Rhdiphosphite, the ionic liquid [EMIM][NTf2] and silica support material. Based on the ACS Paragon Plus Environment

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established Wilkinson mechanism, the hyperbolic rate expressions were used to fit the experimental results. While the hydroformylation could be modeled with high accuracy, the hydrogenation and isomerization trends could not be reproduced by the given rate expressions. An alternative reaction mechanism was developed and allowed an excellent fit of experimental data by the new reaction rate expressions. Initial steps of the mechanism were studied using DFT calculations. Introduction The hydroformylation is the conversion of alkenes and syngas into aldehydes in the presence of a homogeneous transition metal catalyst. With a worldwide production capacity exceeding 10 Mio. tons per year it is the largest application of homogeneous catalysis in industry.[1] The formed aldehydes are important intermediates in the synthesis of bulk chemicals like plasticizers, detergents, alcohols, esters, and amines.

Scheme 1. Detailed reaction network for the hydroformylation of 1-butene. isomerization, alkene hydrogenation, hydroformylation, aldehyde hydrogenation, aldol condensation.

Due to the nature of the catalytic cycle, two possible isomers can be produced, either the linear n-aldehyde or the branched iso-aldehyde as shown in Scheme 1. Major applications have been developed for the linear aldehydes and the required selectivity is achieved by the use of sophisticated ligands around the central atom, usually rhodium.[2] The ligand 2,2'-

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((3,3'-di-tert-butyl-5,5'-dimethoxy-biphenyl-2,2'-diyl)bis(oxy))bis-(4,4,5,5-tetraphenyl-1,3,2dioxaphospholane) (BzP) has been tested in liquid phase hydroformylation by Selent et al. and yielded excellent activity and selectivity.[3] Recently, we have immobilized the Rh-BzP catalyst system via the supported ionic liquid phase (SILP) technology (see Figure 1) and tested in the gas-phase hydroformylation of mixed C4 feedstock.[4] In the SILP material, the catalyst complex is dissolved in a nanometer thin film of ionic liquid, which is dispersed on the inner surface of a porous support material.[5] SILP catalysts are macroscopically heterogeneous solid materials that can be applied in classical fixed-bed reactors.[6] Since ionic liquids, containing only cations and anions, have no detectable vapor pressure under common reaction conditions, SILP catalysis is advantageous for continuous gas-phase reactions. Successful examples have been reported for hydroformylation, carbonylation, metathesis, hydrogenation, hydraminomethylation, methoxycarbonylation, Friedel-Crafts alkylation and water-gas shift.[7-15] In addition, the fact that in continuous gas-phase reactions the catalytic system is operating under steady-state conditions allows the detailed investigation of the kinetics and the reaction network.


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Figure 1. Schematic illustration of the SILP concept with dissolved Rh-BzP catalyst complex.

Most studies on traditional liquid-phase hydroformylation reactions focused on optimizing the stereoselectivity of the aldehydes, mainly via ligand design.[1,16] In this respect, the wellknown Wilkinson mechanism is used as the basis for discussion of kinetic data and selectivity.[17] Side reactions like isomerization or hydrogenation of the alkene double bond were only moderately or not at all considered in kinetic modeling studies. There are currently only a few reports published on kinetic modeling the whole reaction network on hydroformylation reaction. Computational methods, usually employing density functional theory (DFT), have addressed several mechanistic aspects of hydroformylation in the last decade.[18] Very recently, a kinetic modelling study based on quantum-chemical data has highlighted the complexity of the reaction, showing that more than one step can be in control of the overall kinetics.[19] Markert et al. in 2013 and Kiedorf et al. in 2014 proposed a detailed mechanism for the Rhcatalyzed hydroformylation of 1-dodecene in a thermomorphic multicomponent solvent system.[20] Based on kinetic and perturbation experiments the authors suggested the extended Wilkinson mechanism for bidentate ligands as shown in Scheme 2. Starting from the Rhprecursor Rh(acac)(CO)2 (A*) the 18e species HRh(P-P)(CO)2 (A) is formed in the presence of the diphosphine (or diphosphite) ligand P-P and under syngas atmosphere. Dissociation of one CO ligand generates the coordinately unsaturated 16e species HRh(P-P)CO (B) which coordinated one alkene in a π-bond fashion in complex (C). In the hydroformylation cycle, the transfer of the bonded hydride to the alkene determines the regioselectivity of the cycle by either yielding complex (D) or (D*). The latter one will follow the same steps as (D) to produce the branched aldehyde. Addition of another CO lead to the formation of the 18e species (E), followed by migratory insertion of the CO into the Rh-alkyl bond, thereby forming complex (F). This 16e species can either add another CO ligand to form the dormant state (F*) or, upon oxidative addition of H2, form the 18e species (G). Out of this dihydride ACS Paragon Plus Environment

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species the linear aldehyde is released via reductive elimination and the active species (B) is regenerated.

Scheme 2. Extended Wilkinson cycle for the bidentate ligand modified Rh catalyzed hydroformylation. Adapted from [20].

Hydrogenation activity of the Rh catalyst can be explained by an additional cycle that starts from species (D) by oxidative addition of H2. The formed 18e complex (H) releases the alkane, regenerating the active species (B) in this reductive elimination step. Isomerization


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within the alkene fraction is explained by the hydride transfer within complex (C) to the C1 position, thereby generating the alkyl species (I) which is equal to (D*). Instead of addition of CO the β-hydride elimination yields the 18e species (J) with the now internal alkene being bound to the Rh in a π-fashion. Dissociation of this alkene regenerates the active 16e species (B). In this study, we apply the extended Wilkinson cycle to account for the observed hydroformylation, hydrogenation and isomerization activity of the diphosphite modified RhBzP catalyst in a SILP system. An alternative catalytic cycle for side reactions (hydrogenation and isomerization) is suggested based on kinetic modeling results. DFT calculations were carried out to confirm the viability of the initial steps (H2 oxidative addition vs. butene coordination) of the alternative catalytic Rh-cycle with various catalyst compositions. Experimental part and computational details Chemicals

All syntheses were carried out under inert atmosphere in a glovebox. Rh(acac)(CO)2 and the stabilizer bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate were purchased from Aldrich. The BzP ligand was synthesized according to the literature.[3] Silica gel 100 (0.063–0.200 mm) and dichloromethane (max. 0.004 Vol-% H2O) and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][NTf2], were purchased from Merck KGaA. The silica gel used was thermally treated at 450°C for 24 h. CO (99.97%) and H2 (99.999%) were purchased from Linde AG. SILP catalyst synthesis

For the preparation of the SILP catalyst, Rh(acac)(CO)2 (Sigma-Aldrich) was dissolved in water-free CH2Cl2 and stirred for 2 min. The BzP ligand was added in tenfold excess (BzP/Rh = 10:1). The catalyst complex solution was stirred for 5 min. and the ionic liquid was then added to the solution. In the case of the BzP modified Rh-SILP, the stabilizer bis(2,2,6,6ACS Paragon Plus Environment

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tetramethyl-4-piperidyl)sebacate

(Sigma-Aldrich)

was

added

in

fourfold

excess

(BzP/stabilizer = 1:4). After 5 min. of stirring the calculated amount of calcinated silica 100 (surface area: 361 m2 g-1, pore volume 1.02 cm3 g-1) was added to the solution. After a further 15 min. of stirring, CH2Cl2 was removed from the suspension by evaporation. The yellow powder obtained was stored under argon until further use. Kinetic experiments

A fixed bed reactor with an inner diameter of 4.6 mm and a length of 80 mm was used for evaluation of the kinetic data. The detailed reactor design has been published previously and can be found in the supporting information (SI).[21] 0.5 g of a SILP-catalyst was placed in the reactor. The tubular reactor was operated differentially. The conversion was kept below 10 % to minimize thermal deviations caused by the heat of the reactions. Therefore, the axial concentration gradient is small so that Equation (1) is suitable for calculation of the reaction rate:

,

̅ =

= ∑

(1)

All organic products were monitored by online GC using a Bruker 450-GC equipped with a HayeSep Q 80/100 mesh (0.5 m x 1/16´´ x 1.0 mm), a ShinCarbon micropacked CP Wax 52 CB column (25 m x 0.53 mm x 0.7 mm), an HP-AL/S column (50 m x 0.535 mm x 0.015 mm), a deactivated fused silica column (10 m x 0.32 mm), one thermal conductivity detector, and two flame ionization detectors. The injector temperature was 230 °C and the split ratio was adjusted to 20:1. Argon was used as carrier gas. The temperatures of the TCD were 230 °C for the filament and 200 °C for the housing. The two FIDs were heated to 250 °C. To detect the high-boiling products and achieve separation of the isomers the following temperature program was used: initial temperature 55 °C, initial time 4 min, heating ramp


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4.5°C min-1, final temperature 100 °C for 12 min, heating ramp 10°C min-1, final temperature 130 °C for 6 min. Computational details

Density functional theory (DFT) calculations on the initiating steps of the discussed catalytic cycles with various coordination spheres have been performed using the TURBOMOLE program package.[22] All data given here have been calculated with the generalized-gradient approximation functional due to Becke and Perdew, denoted BP here, [23] using RI techniques [24] implemented in TURBOMOLE. In addition, for one of the ligand spheres (X = H, see Results Section) all calculations have also been performed with the TPSSh metageneralized gradient approximation hybrid functional, [25,26] see the SI for a comparison of the resulting data with those from the BP functional. A good qualitative agreement between both functionals has been found for the present system. Hence, for all other ligand spheres, the more efficient BP functional has been used. All calculations use the standard def2-TZVP basis set for all atoms (i.e., Rh, H at Rh, O, P, Cbutene, CCO, N, S, Cl) except the C and H atoms in the backbone of the BzP ligand and the C and F atoms of NTf2 (which are usually remote from the Rh reaction center), where a smaller SV(P) basis set is used.[27] A effective core potential associated with the TZVP basis set was used on Rh.[28] Approximate solvent effects have been included via the COSMO model (dielectric constant ε = 12).[29,30] Geometry optimizations have been carried out without constraints using standard procedures as implemented in TURBOMOLE (SCF convergence 10-7 a.u., energy convergence for geometry optimizations at least 10-6 a.u.). Transition states were verified by frequency calculations and IRC calculations. For comparison, reaction energies and activation barriers from gas-phase calculations (i.e., without COSMO correction) are given in the Supporting Information (Scheme S2).


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Although no complete sampling of the full conformational space has been attempted, several relative orientations of the ligands and substrates have been tested in a prescreening process using the smaller SV(P) basis set on all atoms, and the lowest-energy conformers found were used for subsequent refinement (i.e., geometry optimization) at the final basis sets specified above. Please see the SI for Cartesian coordinates of the corresponding optimized geometries. Van der Waals dispersion corrections as well as zero-point vibrational and free energy corrections from gas phase formulae have not been included in the present study as the appropriateness of such corrections for catalytic processes in ionic liquid solutions is presently unclear. Results and discussion In our previous studies, it has been observed that the reaction of alkenes with syngas under the reaction conditions applied in hydroformylation results in a complex network. Beside isomerization and hydrogenation, the formed aldehydes are reacting in aldol addition and aldol condensation reaction to high boiling oligomers according to Scheme 1. Figure 2 shows the conversion of 1-butene and the selectivity to the corresponding products over the time on stream.


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Figure 2. Stability experiment for gas-phase h ydroformylation of 1-butene using Rh-BzP modified SILP catalyst. p a b s = 10 bar; T = 100 °C; V total = 25 ml min - 1 ; p 1 - b u t e n e = 2 bar; p H 2 = 4 bar; p C O = 4 bar; m S I L P - c a t = 0.5 g; precursor = Rh(acac)(COD);ligand = BzP; ionic liquid = [EMIM][NTf 2 ]; stabilizer = bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate; support = silica 100 (calc.); α = 10 vol%; w R h = 0.2 wt%; L : Rh = 10 : 1; St : L = 4 : 1. 1-Butene conversion (filled triangles), total aldehyde selectivity (filled squares), n-pentanal selectivity (open squares), butane selectivity (open diamonds), trans-2-butene selectivity (open triangles), cis-2-butene selectivity (open triangles inverted) and 2-prop yl-2-heptenal selectivit y (filled triangles tilted).

After an induction period of five hours, the system operates at stationary conditions. The conversion of 1-butene is about 12 %. The selectivity to the linear aldehyde is the highest with 65 – 67 %. The bidentate phosphite ligand is one of the best ligands concerning the stereo selectivity of the aldehydes as mentioned in previous articles. The stereo selectivity is above 99.5 %. More than 30 % of the converted 1-butene results in internal alkenes and the consequent alkane. Cis-2-butene is the main side product with 15 % selectivity. The favored production of the thermodynamically unfavored cis-2-butene at the elevated reaction temperature of 100 °C underlines the steric demand of the diphosphite ligand. The selectivity ACS Paragon Plus Environment

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to 2-propyl-2-heptenal is about 2 %. The consecutive reaction of the aldehydes could be catalyzed by acidic surface groups of the support, by the ionic liquid, by the basic stabilizer, or just thermodynamically driven by temperature. Higher boiling products than dimers of two aldehydes could not be detected by gas chromatography. It is know that the activity of the homogeneous catalyst complex used for hydroformylation depends on the concentration of the alkene, carbon monoxide and hydrogen.[31] The reaction order of the alkene is normally around 1 for diphosphite complexes while for carbon monoxide mostly negative or zero and for hydrogen zero or slightly positive orders are found.[32] Figure 3 depicts the role of the feed composition on the reaction rate for the corresponding products catalyzed by the Rh-diphosphite complex. It should be noted that, in contrast to the high conversion shown in Figure 2, all kinetic data were obtained from experiments at conversion levels of 1-butene well below 10 % (see Figures S2 to S4 in the SI for details).


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Figure 3. Variation of the feed comp osition in Rh-BzP-SILP catalyzed hydroformylation in a tubular reactor. Variation of the partial pressure of 1-butene (top) and variation of partial pressure of carbon monoxide and hydrogen (bottom). p a b s = 10 bar; T = 100 °C; V total = 25 ml min - 1 ; p 1 - b u te n e = 0,5 - 2 bar; p H 2 = 1 - 4 bar; p C O = 2 - 4 bar; p H e = 0,5 – 3 bar; m S I L P - c a t = 0,5 g; Rh(acac)(CO) 2 ; BzP; [EMIM][NTf 2 ]; stabilizer; Silica 100 (calc.); α = 10 vol%; w R h = 0,2 wt%; L : Rh = 10 : 1; St : L = 4 : 1. Reaction rates for n-pentanal (filled triangles), n-butane (open diamonds), trans-2-butene (open triangles) and cis-2-butene (open triangles inverted); grey symbols indicate p C O variation.

All reaction rates increase with increasing alkene concentration. A reaction order of 0.7 could be determined for the 1-butene in hydroformylation reaction, 0.9 for hydrogenation reaction, and 1.1 for the isomerization reactions. Changes in the 1-butene concentration show the biggest influence on the reaction rate of isomerization. The partial pressure of hydrogen shows also positive orders on the reaction rates for all products. The observed reaction orders at 100 °C are 0.2 for n-pentanal, 0.6 for n-butane, and 0.5 for cis- and trans-2-butene. Interestingly we find a dependency of the partial pressure of hydrogen on the rate of isomerization while there is no such dependency visible in the postulated mechanism by ACS Paragon Plus Environment

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Markert and Kiedorf in Scheme 2. The partial pressure of carbon monoxide has an extensive influence on the reaction rates of the hydrogenation and isomerization side reactions. The influence on the hydroformylation reaction is slightly negative in the investigated range. This behavior results in reaction orders of -0.1 for the product n-pentanal, -1.9 for n-butane, and -2 for the cis- and trans-2-butene. The determined reaction orders for hydroformylation are in good accordance with literature data for rhodium complexes containing diphosphite ligands (see Table 1) and give a first conclusion on the catalytic mechanism.[32]

Table 1. Summar y of reaction orders determined for Rh-SILP catalyzed h ydroformylation of 1butene.

Substrate

Hydroformylation

Hydrogenation

Isomerization

1-butene

0.7

1.0 a)

0.9

1.1

CO

-0.1

-0.7 a)

-1.9

-2.0

H2

0.2

0.2 a)

0.6

0.5

a) Literature data for 1-octene hydroformylation from Ref. [30].

To get a more detailed kinetic insight into the catalytic mechanism hyperbolic rate expressions are derived based on the extended catalytic cycle given in Scheme 2.[33] Considering that the rate-determining step (RDS) is initially unknown, all rate expressions were postulated assuming every step could be rate limiting (see SI). The established rate equations were fitted with a non-linear regression analysis to the evaluated data in Figure 3. The equations with the best fit to the data are given below. Parity plots indicate an excellent match between calculated and measured rates as shown in Figures S5 to S8 in the SI. For the hydroformylation reaction, the insertion of the alkene (step 3 in Scheme 2) was determined as rate-determining step in the catalytic cycle, named r3 in Equation 2. This is in accordance with literature, where the insertion of the alkene to the Rh-alkyl complex is also ACS Paragon Plus Environment

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postulated to be the RDS for diphosphite ligands.[34] The steric and bulky demand of the diphosphite ligand is believed to hamper the alkene coordination/insertion compared to the smaller molecules hydrogen and carbon monoxide.

=

!"!

(2)

#$% # !"!

Reaction rate r8 give the best prediction for the hydrogenation of butene to n-butane, suggesting that the oxidative addition of hydrogen in step 8 from the 16e species (D) to the 18e species (H) might be the RDS here.

& =

' !"! (

#$% # !"! # !"!

(3)

The dependency of partial pressure of 1-butene and hydrogen could be modeled very well for the hydrogenation applying Equation (3), whereas the dependency of carbon monoxide partial pressure shows lower reaction rates than measured in the experiment as shown in Figure 4.


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Figure 4. Measured and modeled reaction rates depending on the feed composition for 1-butene hydroformylation (top) and hydrogenation (bottom). 1-butene (filled squares), CO (circles vertical), H 2 (circles horizontal), ̶ modeled according to best fit rate expression.

The same poor fit for the carbon monoxide concentration is obtained when assuming the hydride transfer from species (C) to (I) to be the RDS for isomerization. The dependency on ACS Paragon Plus Environment

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the concentration of hydrogen partial pressure could not be modeled because of hydrogen not being present in the postulated extended catalytic cycle.

)* =

!"!

(4)

#$% # !"!

For both the formation of cis-2-butene and trans-2-butene the modelling of CO partial pressure yields lower reaction rates than measured in the experiment as shown in Figure 5.


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Figure 5. Measured and modeled reaction rates depending on the feed composition for formation of trans-2-butene (left) and cis-2-butene (right). 1-butene (filled squares), CO (open circles), H 2 (filled triangles), ̶ modeled according to best-fit rate expression.

The non-satisfactory modeling of the experimental results and the fact that numerical determination of the equilibrium constants K1 und K2 (see SI) does not result in the same order of magnitude suggests that the hydrogenation and isomerization reaction are catalyzed by an alternative cycle. Therefore, we postulated this alternative catalytic reaction mechanism for the side reactions based on the literature data. According to the mechanism for the hydrogenation of alkenes with a Rh-catalyst of Wilkinson type, we postulate an active species that can attach hydride as ligand, the ionic 18e species Rh(P-P)(CO)2X (A in Scheme 3), where X can be hydride or the anion of the ionic liquid or a negatively charged surface group of the support.


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Scheme 3. Postulated catalytic cycle for the hydrogenation and isomerization reaction by a homogeneous catalyst complex under reaction conditions applied in hydroformylation reaction.

After the dissociation of carbon monoxide in step 1 to form the 16e species Rh(P-P)(CO)X (B), the oxidative addition of hydrogen (step 13 in Scheme 3) is favored over the coordination of an alkene. After a second dissociation of carbon monoxide the terminal alkene can coordinate to the Rh(III)-complex (T). Hydrogen transfer will form the alkyl species (V), from which either the isomerized product can be generated via β-hydride elimination (step20 in Scheme 3) or the hydrogenated alkane is formed via complex (X). The steps 17 and 18 in the catalytic cycle could also take place in reverse order. To the best of our knowledge, no


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catalytic cycle for the hydrogenation and isomerization of alkenes under carbon monoxide pressure has been reported in the open literature. The additional hyperbolic reaction rates were derived from the alternative catalytic cycle in Scheme 3 and fitted to the measured results by non-linear regression analysis. For both the hydrogenation of 1-butene to n-butane and the isomerization to cis-2-butene and trans-2butene the insertion of 1-butene into the Rh-alkyl-complex (U) could be determined as the RDS, indicated as step 16 in Scheme 3.

)+ =

, - . !"! (

$% #$% # $% ( #. ( #

(5)

- . !"! (

With Equation (5) the strong dependence of the partial pressure of carbon monoxide could be modeled well. Because of the oxidative addition of hydrogen before the alkene coordination there is a dependency on hydrogen of the rate of isomerization that could also be measured in the experiments. In Figure 6 the measured and modeled results for hydrogenation and isomerization reaction (only trans-2-butene is shown, for cis-2-butene see SI) are summarized.


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Figure 6. Comparison of the measured reaction rates versus the modeled reaction rates obtained by the alternative reaction c ycle for hydrogenation (left) and isomerization (right) to trans-2butene. 1-butene (filled squares), CO (open circles), H 2 (filled triangles), ̶ modeled according to rate expression shown in Equation (5).


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The newly proposed catalytic cycle shown in Scheme 3 is thus in good agreement with the current experimental results, in contrast to the traditional mechanism (Scheme 2). We performed preliminary density-functional calculations to further corroborate parts of the new mechanistic proposal. Specifically, we have calculated the energetics of the initiating steps of both traditional and new mechanism, i.e., alkene addition (step 2 in Scheme 2) and H2 oxidative addition (step 13 in the new proposal, Scheme 3). The crucial question in the present investigation was to clarify if the initial H2 oxidative addition to species (B) postulated in Scheme 3 is energetically possible in comparison to the alkene addition step which is the first step in the traditional catalytic cycle. Moreover, in the present experimental system there is an additional question as to the nature of the catalyst, as mentioned above. In Scheme 3, the original Wilkinson catalyst complex is recovered for X = H. In the experimental setup studied here, one can alternatively think of X being an anion from the ionic liquid (i.e., X = NTf2), which is available in the IL solution in great excess. In addition, we have considered X = Cl which is the classical anion of the Wilkinson catalyst. The results of the corresponding DFT calculations are summarized in Scheme 4; cf. also Scheme S2 of the Supporting Information.


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Scheme 4: Reaction energies (∆E R ) and activation energies (∆E # ) for H 2 oxidative addition and butene coordination to the catalyst species (B), cf. Schemes 2 and 3, for neutral (top) and cationic (bottom) paths, in kJ mol - 1 , calculated using DFT with COSMO solvation correction (see the previous section for computational details); activation energies are onl y given for the oxidative addition of H 2 ; the butene coordination is assumed as a low barrier process (see text).

We start with the discussion of the traditional Wilkinson catalyst, i.e., X = H in Schemes 3, 4. A brief discussion of the initial CO dissociation from the precursor (A) (step 1 in Schemes 2,3) can be found in the SI. In the conventional mechanism (Scheme 2), butene coordination to the active catalyst species (B) is the first step in the catalytic cycle (step 2, Scheme 2). According to the present DFT calculations, it is an exothermic step (∆E = -19.9 kJ mol-1 in COSMO solution; -27.7 kJ mol-1 in gas phase). No attempts have been made at calculating the


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kinetic activation barrier for this step. Earlier computational studies report calculated olefin association energy barriers of 0.4 – 5.7 kJ mol-1 for monodentate ligands and up to 25-30 kJ mol-1 for bidentate ligands.[19,35,36] In the alternative catalytic cycle this step is replaced by the oxidative addition of H2 (Scheme 3, step 13; butene coordination takes place at a later stage). This step involves formation of an intermediary (H2)-complex (∆E = -4.4 / -6.5 kJ mol1

with COSMO / gas phase), where an essentially intact H2 molecule is weakly bound to the

Rh center (H2···Rh). (Such complexes are well known in transition metal chemistry.[37] See the SI for further details.) From there, oxidative addition of the attached H2 molecule to form the tri-hydride complex (species (S) in Scheme 3) is associated with a very low activation barrier of only 3.2 kJ mol-1 (3.5 kJ mol-1 in the gas phase). Complex (S) is lower in energy by -31.6 kJ mol-1 (-28.8 kJ mol-1 in gas phase) with respect to species (B). The present DFT calculations thus show that butene coordination (step 2 in Scheme 2) and the alternative H2 oxidative addition (step 13 in Schemes 3, 4) are both exothermic with roughly the same reaction energy. Both are presumably associated with a low barrier and are thus likely to proceed smoothly at the relevant temperatures of the catalytic process. As mentioned above, we also consider the possibility that the terminal hydride in species (A) is replaced by an anion which is present in the reaction mixture. We start with the case that the hydride has been replaced by a chloride anion, i.e., X = Cl. In this case, conventional butene coordination (step 2 in Scheme 2, but Rh-H replaced by Rh-Cl) is calculated as roughly thermoneutral or slightly exothermic (∆E = +2.6 kJ mol-1 in COSMO solution, -11.4 kJ mol-1 in gas phase, see Scheme 4 and the SI), less exothermic than for X = H (-19.9 kJ mol1

, -27.7 kJ mol-1, see above). In part, this can be explained by the increased steric demand of

the chloro-ligand in comparison to the hydrido-ligand. (This becomes apparent from the graphical representation of the optimized geometries; see Figure S10 in the SI.) The situation for the proposed oxidative addition step (step 13 in Scheme 3) is qualitatively different for X = Cl when compared with X = H (see above). For X = Cl, the formation of a dihydrogen ACS Paragon Plus Environment

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complex (H2···Rh) is calculated to be energetically uphill (∆E = +15.3 kJ mol-1 with respect to species (B); see the SI for further details). Hence the oxidative addition of H2 to species (B) can be considered as a direct, i.e., bimolecular reaction without involvement of an intermediary dihydrogen complex. The activation barrier for step 13 along this direct pathway is calculated to 36.7 kJ mol-1 (gas phase: 32.1 kJ/mol). While this is a barrier that can still be easily surmounted at the relevant temperatures (in particular, it is not expected to be the highest barrier in the overall process), it is significantly larger than that for X = H. The overall oxidative addition step is slightly exothermic (∆E = -1.6 kJ mol-1 in COSMO solution, -2.6 kJ mol-1 in gas phase). Finally, we consider the coordination of an [NTf2]- ion from the ionic liquid solvent to the Rh center in place of hydride (i. e., X = NTf2 in Schemes 3, 4; all energies with COSMO solvent correction). The first question to address is of course whether [NTf2]- can bind to the Rh center in a hypothetical species (B) with X = NTf2. Indeed, the present DFT calculations with the COSMO solvent model suggest that [NTf2]- is only very weakly bound by only 3.9 kJ mol-1 to the Rh center with respect to ionic dissociation. For comparison, the ionic dissociation of species (B) for X = Cl is calculated to 94.5 kJ mol-1 at the same level, i.e., the chloro-ligand is strongly bound to the Rh center in species (B) for X = Cl. The coordination of [NTf2]- to the Rh center in the hypothetical butene complex (species (C) in Scheme 2 with H replaced by NTf2, see Figure S10 of the Supporting Information for graphical representations) is calculated to be energetically even uphill by 11.3 kJ mol-1. The present, approximate DFT calculations thus do not suggest a strong interaction of [NTf2]- ions from the ionic liquid with the Rh center of the catalyst. On the other hand, one should consider that [NTf2]- will be present in the ionic liquid solution in great excess, thereby pushing all equilibria in favor of [NTf2]- coordination to the Rh center. To further analyze the situation for X = NTf2, we have therefore decided to consider two additional flavors of the catalytic cycles shown in Scheme 2, 3. We consider both cycles in ACS Paragon Plus Environment

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the presence of an NTf2 ligand near the Rh center and, alternatively, a cationic cycle where the axial anion ligand X is omitted altogether (see lower pathway in Scheme 4). In the latter case, all catalyst species are thus cationic (charge +1). The corresponding DFT calculations (with COSMO solution) in the presence of NTf2 suggest that butene coordination (step 2 in Scheme 2 with Rh-H replaced by Rh···NTf2) would be uphill by 34.0 kJ mol-1. Inspection of the corresponding optimized geometry, shown in Figure S10 of the SI, suggests that this is likely due to the high steric demand of the NTf2 ligand in the coordination sphere, which hampers the coordination of the butene ligand. In contrast, oxidative addition of H2 (step 13 in Scheme 3, X = NTf2) is calculated to be exothermic by 13.5 kJ mol-1. Apparently, the smaller hydrido-ligands can be accommodated by the Rh center even in the presence of the bulky NTf2 moiety, and the overall reaction energy for H2 oxidative addition is exothermic as for X = H, Cl. As expected, however, the activation barrier for bimolecular H2 oxidative addition becomes sizeable (22.9 kJ mol-1) in the presence NTf2. A hypothetical intermediary dihydrogen complex (H2···Rh) is calculated to be endothermic (+4.8 kJ mol-1 with respect to species (B)) and is thus not considered any further. The DFT calculations suggest that the presence of NTf2 in the coordination sphere of the present Rh catalyst hampers both butene coordination and oxidative addition of H2. Considering that NTf2 may only be weakly bound to the Rh center as discussed above, it is therefore worthwhile to also consider the situation where no anion is bound to the Rh center of the catalyst at all, i.e., the cationic pathway outlined above. The calculations predict that butene coordination (step 2 in Scheme 2, catalyst without axial and cationic) is uphill by +18.8 kJ mol-1 (cf. +34.0 kJ mol-1 in the presence of NTf2, cf. Scheme 4). The unfavorable energetics of olefin association in this case can partly be explained by the presence of an unusual η1-Ph-Rh interaction that apparently stabilizes the cationic (B’)+ (see the SI for atomic coordinates).


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The calculated bimolecular activation barrier for H2 oxidative addition to cationic species (B’)+ (step 13 in Scheme 3, X = ---) amounts to approximately 48.5 kJ mol-1, and thus is larger than the cases X = Cl and X = NTf2 (Scheme 4). The overall reaction energy is calculated to +17.4 kJ mol-1, i.e., H2 oxidative addition to cationic (B’)+ is predicted to be energetically uphill. This contrasts the other cases discussed before, and may be understood from the fact that oxidation of the Rh center in a cationic complex will be more difficult than in a neutral complex. Conclusions The reaction network of 1-butene hydroformylation, hydrogenation and isomerization in the presence of Rh-diphsophite modified SILP catalysts has been investigated. The product distribution between n-pentanal, n-butane, and 2-butene depend strongly on the inlet composition of 1-butene, hydrogen, and carbon monoxide. The determined reaction orders give a first insight into the reaction mechanism. Detailed investigations into the reaction cycle are given by modeling of the reaction network with hyperbolic reaction rate equations as well as preliminary density functional calculations on the steps initiating the cycle with various catalyst compositions. The kinetic modelling lead us to suggest a new mechanism for the isomerization and hydrogenation side reactions of the catalytic cycle. The present preliminary theoretical analysis which compared the initial steps only for both catalytic cycles shown in Schemes 2 to 4 with four different catalyst species (X = H, Cl, NTf2, ---) suggests the following: for the classical Wilkinson-type complex with X = H, both butene coordination and, alternatively, H2 oxidative addition should be possible, i.e., they do not face any significant energetic barriers. This means that our newly proposed catalytic cycle shown in Scheme 3, which proposes H2 oxidative addition as the initial step and which allows for a much better kinetic modelling of the measured reaction rates than the traditional mechanism (Scheme 2), is plausible judging from the present DFT analysis of the initial steps in the


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catalytic cycles. Further theoretical analysis will be necessary to support or refute the new mechanistic alternative. Supporting Information: gas-phase reactor setup, raw data for kinetic analysis, hyperbolic rate expressions, parameter estimation, parity plots, modeling of isomerization reaction, DFT calculations on Rh···H2 intermediates formation and CO dissociation from the catalyst precursor, functional dependence of the DFT results, optimized intermediate structures, Cartesian coordinates. Acknowledgements The authors gratefully acknowledge financial support from the Bundesministerium für Bildung und Forschung (BMBF, German Federal Ministry of Education and Research) under the HY-SILP project (no. 01RC1107A) and its framework program Research for Sustainable Development (FONA). In addition, the authors gratefully acknowledge the funding of the German Research Council (DFG), which, within the framework of its Excellence Initiative, supports the Cluster of Excellence “Engineering of Advanced Materials” (www.eam.unierlangen.de) at the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU). References [1] Franke, R.; Selent, D.; Börner, A.; Chem. Rev. 2012, 112, 5675-5732 and references therein. [2] van Leeuwen, P.W.N.M.; Claver, C.; Rhodium Catalyzed Hydroformylation, Springer, 2000. [3] Selent, D.; Franke, R.; Kubis, C.; Spannenberg, A.; Baumann, W.; Kreidler, B.; Börner, A.; Organometallics 2011, 30, 4509-4514. [4] Jakuttis, M.; Schönweiz, A.; Franke, R.; Wiese, K.-D.; Haumann, M.; Wasserscheid, P.;

Angew. Chem. Int. Ed. 2011, 50, 4492-4495.


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[5] Supported Ionic Liquids – Fundamentals and Applications (Fehrmann, R.; Riisager, A.; Haumann, M.; Eds.), Wiley-VCH, Weinheim, 2014. [6] Riisager, A.; Fehrmann, R.; Haumann, M.; Wasserscheid, P.; Eur.J. Inorg. Chem. 2006, 695-706. [7] a) Riisager, A.; Fehrmann, R.; Flicker, S.; van Hal, R.; Haumann, M.; Wasserscheid, P.;

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Detailed Investigation of the Mechanism of Rh-Diphosphite Supported

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