Inner-Sphere Activation, Outer-Sphere Catalysis - ACS Publications

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Inner-Sphere Activation, Outer-Sphere Catalysis: Theoretical Study on the Mechanism of Transfer Hydrogenation of Ketones Using Iron(II) PNNP Eneamido Complexes Demyan E. Prokopchuk and Robert H. Morris* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *

ABSTRACT: We have studied the mechanism of transfer hydrogenation (TH) in silico using density functional theory (DFT) with the Fe(II) PNNP bis(eneamido) model complexes Fe(CO)(H2PCHCHNCH2CH2NCHCHPH2) based on compounds with the general formula Fe(CO)(R2PCH CHN((S,S)-CH(R′)CH(R′))NCHCHPR2) (R, R′ = alkyl, aryl). An initial activation period involving 1 equiv of isopropyl alcohol reduces the bis(eneamido) complex by a stepwise innersphere mechanism. This activation step is proposed to be slow because of the high barrier calculated for the inner-sphere transfer of hydride to an imine carbon on the ligand. This bis(eneamido) complex reacts with isopropyl alcohol to produce the active species, proposed to be the unsymmetrical amido-eneamido complex Fe(CO)(H2PCH2CH2NCH2CH2NCHCHPH2), which is within the catalytic cycle. The catalytic cycle propagates by addition of isopropyl alcohol to the Fe−amido half of the ligand to generate an FeH−NH unit. However, a stepwise outer-sphere mechanism has been calculated to transfer the proton and hydride in two discrete steps which are connected through a ground state involving an NH-stabilized alkoxide ion. The highest calculated barrier is hydride transfer in the activation period, while the second highest barrier involves hydride transfer during the catalytic cycle. The resting states during catalysis are the alkoxide complex Fe(CO)(OiPr)(H2PCH2CH2NHCH2CH2NCHCHPH2) and/or the amino−hydrido complex FeH(CO)(H2PCH2CH2NHCH2CH2NCH CHPH2), on the basis of their low relative free energies. Calculated kinetic isotope effect values are in rough agreement with the experimentally determined values, which also supports the proposed mechanism of catalysis. This data complements the experimental work recently published by our group (J. Am. Chem. Soc. 2012, 134, 12266−12280) and leads to a deeper understanding of how these highly active “green” catalysts operate under catalytic conditions.

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Recently, the field of ketone hydrogenation has experienced a shift away from precious-metal catalysts and toward iron-based catalysts.4 Our group discovered the first well-defined Fe(II) complexes that catalyze the transfer hydrogenation (TH) of prochiral ketones.5 These iron TH catalysts have the general formula trans-[Fe(L)(NCMe)(PPh2C6H4CHN((S,S)-C(R)HC(R)H)NCHC6H4PPh2)](BF4)2 (R = alkyl, aryl; L = CO, CNtBu) and contain a tetradentate [6.5.6] PNNP ligand with o-phenylene groups in the linkers between the diamine nitrogen and phosphorus atoms, forming two six-membered metallacycles and a five-membered metallacycle around the diamine backbone (A; Figure 1). These complexes exhibit high conversion, good ee, and moderate turnover frequencies for TH (>99% conversion, up to 96% ee, TOF up to 2600 h−1). Our second generation of Fe(II) complexes with the general formula trans-[Fe(CO)(L)(PR 2 CH 2 CHN((S,S)-C(R′)HC(R′)H)NCHCH2PR2)](BPh4)n (B; R, R′ = alkyl, aryl; L = CH3CN, Br−; n = 2, 1, respectively) contain a planar tetradentate [5.5.5] PNNP ligand without an o-phenylene linker

INTRODUCTION

The mechanism of transition-metal-catalyzed ketone hydrogenation has been thoroughly studied by experimental1 and computational2 means. It is widely accepted that hydrogen transfer, using either H2 or a sacrificial hydrogen donor such as isopropyl alcohol, can occur by an inner-sphere or outer-sphere mechanism and is usually classified by the hydride transfer step. Inner-sphere hydrogenation requires a vacant coordination site, with a metal-bound hydride or a sacrificial hydrogen donor transferring an H− equivalent to the metal-bound substrate. Proton transfer to the substrate can be facilitated by the ancillary ligand, protic solvent, or by the heterolytic splitting of H2 (if H2 is used as the reductant). During an outer-sphere hydrogenation mechanism, which does not require a vacant coordination site, the substrate does not bind to the metal; instead, a coordinated metal hydride is transferred from the metal complex directly to the substrate while a proton is acquired from an external protic solvent or the ancillary ligand. The latter scenario wherein the proton is transferred from the ancillary ligand is generally referred to as “bifunctional” or “cooperative” catalysis.3 © 2012 American Chemical Society

Received: June 27, 2012 Published: October 9, 2012 7375

dx.doi.org/10.1021/om300572v | Organometallics 2012, 31, 7375−7385

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Scheme 3. Proposed Mechanism Responsible for the Activation Period and Enolate Side Reactionsa

Figure 1. First-generation Fe(II) [6.5.6] (left) and second-generation Fe(II) [5.5.5] complexes (right) for the TH of ketones.

and are highly active and enantioselective TH catalysts (up to 98% conversion, up to 99% ee, TOF up to 28 000 h−1).6 The smaller metallacycles lead to a more rigid ligand, as opposed to the conformationally flexible [6.5.6] PNNP ligand, which can distort itself to form a pentadentate ferraaziridine complex in the presence of base.7 A typical transfer hydrogenation (TH) reaction catalyzed by iron PNNP complexes is shown in Scheme 1. Depending on Scheme 1. General Reaction Scheme for the Transfer Hydrogenation of Ketones Catalyzed by Iron PNNP Complexesa a

a

Relative free energies (G°, in kcal/mol) are also provided.

8 equiv of base with respect to [Fe].

the iron precatalyst used in the TH of acetophenone (AcPh) to 1-phenylethanol (PE), there is an activation period up to 10 min long before rapid catalysis.6c,e,8 Addition of the strong base potassium tert-butoxide (KOtBu) while using isopropyl alcohol (iPrOH) as the reaction solvent generates isopropoxide which acts as the base and reductant in the system, generating acetone (AcMe) as a byproduct. If the reaction of the iron precursors with isopropoxide were to directly produce the active species, the activation period would be negligible and rapid catalysis would occur without delay. In attempts to isolate the active species, the diimine precatalyst B was reacted with isopropoxide or tert-butoxide salts to form coordinatively unsaturated bis(eneamido) complexes of the type Fe(CO)(R2PCH CHN((S,S)-CH(R′)CH(R′))NCHCHPR2) (C; Scheme 2).8,9

Figure 2. Highest occupied molecular orbital (left) and lowest unoccupied molecular orbital (right) for complex 1.

Once isolated, base-free catalysis can be achieved using a threecomponent mixture which consists of AcPh as the substrate, iPrOH as the reductant/solvent, and C as the precatalyst at room temperature under an inert atmosphere. The order of addition and delay time in adding AcPh to C in iPrOH caused dramatic changes in activity.10 For the highly active systems of type C where R, R′ = Ph (CPh), the activation period could be avoided only af ter reacting CPh with iPrOH for 12 min prior to adding AcPh.8 Therefore, a reaction between CPh and iPrOH must occur before rapid catalysis takes place, which is responsible for the activation period. We recently reported that our first iron system A involves the formation of an Fe(0) complex7 and Fe nanoparticles11 during catalysis, but the complexes of this current study do not show any signs of nanoparticle formation.8 Thus, the mechanism was investigated on the basis of evidence that it is a homogeneous process. Now we present a detailed computational study using density functional theory and the model complex Fe(CO)(H2PCH CHNCH2CH2NCHCHPH2) to elucidate the mechanism of TH of ketones using Fe(II) eneamido precatalysts in conjunction with a detailed kinetic study recently conducted

Scheme 2. Diimine Precatalysts B and Their Reactivity with Strong Base To Generate C

These electron-rich complexes have been spectroscopically characterized and, in one case, structurally characterized. 7376


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Figure 3. Energy profile for the catalyst activation process and enolate side reactions. All energies are relative to 1, 3 iPrOH, and AcPh.

proton to the carbon atom adjacent to phosphorus (1 → 5). Increasing the concentration of AcPh has been experimentally shown to increase the duration of the activation period, presumably due to the formation of metal-bound enolates.8 Calculations clearly show that the kinetic product is the formation of alkoxido-NH complex 4 (G°⧧(TS1,4) = 11.2 kcal/ mol, G°(4) = 9.2 kcal/mol), while the thermodynamic product is the alkoxido-CH2 complex 5 (G°⧧(TS1,5) = 16.8 kcal/mol, G°(5) = −2.4 kcal/mol) (Figure 3). Reaction of AcPh with 1 is also feasible, resulting in the formation of enolate complexes 2 and 3; however, the energy required for their formation is higher (G°⧧(TS1,2) = 22.0 kcal/mol and G°⧧(TS1,3) = 25.9 kcal/mol, respectively). Nonetheless, formation of iron enolate complexes could prolong the activation period. Selected ground state and transition state geometries are shown in Figure 4. Alcohol-assisted proton transfer during ruthenium-12,2f and iron-catalyzed13 ketone hydrogenation has been calculated to lower the reaction barrier. We also investigated a “proton shuttle” mechanism, where an additional molecule of iPrOH assists in the proton transfer step going from 1 to 5 (see the Supporting Information). The calculated free energy barrier for alcoholassisted proton transfer toward the formation of 5 was found to be 18.9 kcal/mol, which is only 2.1 kcal/mol higher than TS1,5. This small energy difference also makes solvent-assisted proton transfer feasible on going from compound 1 to 5; probing the role of hydrogen-bonding solvent molecules such as iPrOH in this proton transfer event using explicit solvent modeling could uncover more information on the assistance of solvent in a “proton shuttle” type mechanism.2b Of the four scenarios presented above, compound 5 can react further by transferring a hydride equivalent to the imine carbon (G°⧧(TS5,6) = 22.8 kcal/mol), which is exergonic by 8.1 kcal/ mol after dissociation of AcMe. The product of this reaction is

by our group.8 Although this simple model complex is not suitable for addressing features governing enantioselectivity, it allows us to establish the fundamental mode of action of the catalyst. On the basis of the relative free energies of reactants, products, and their transition states, a plausible mechanism is presented that accounts for the experimentally observed activation period and rapid catalysis thereafter.

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RESULTS AND DISCUSSION Activation Period. We have based our studies on the initial presence of three species in solution during catalysis: AcPh, i PrOH (solvent and reductant), and the square-pyramidal achiral bis(eneamido) model complex 1 (Scheme 3); thus, all free energies are compared relative to 1, 3 equiv of iPrOH, and AcPh. Analysis of the molecular orbitals of 1 reveals that the HOMO is mainly a ligand-based nonbonding π-type orbital with electronic contributions from the eneamido nitrogen atoms and carbon atoms adjacent to phosphorus. The electronic contribution of the LUMO predominantly consists of an empty dz2-type orbital on iron (Figure 2). Low-lying bonding MOs in 1 reveal delocalization across the planar PNNP ligand plane, which distributes electron density about the metal center and eneamido atoms. This motif is represented by curved lines in Scheme 3 and other schemes throughout. For subsequent octahedral complexes that contain the eneamido moiety, lowlying orbitals delocalize electrons throughout the ene carbons and amido nitrogen but not the iron center. From this point, four reaction pathways were considered: enolate formation by which AcPh transfers a proton to nitrogen (1 → 2), enolate formation by which AcPh transfers a proton to the carbon atom adjacent to phosphorus (1 → 3), alkoxide formation by which iPrOH transfers a proton to nitrogen (1 → 4), and alkoxide formation by which iPrOH transfers a 7377


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Scheme 4 and Figure 5). As seen for 1, low-lying MOs in square-pyramidal 6 delocalize the electrons throughout the metal center and eneamido atoms. One equivalent of iPrOH approaches 6 with the O−H hydrogen atom above the amido nitrogen atom and the C−H atom pointing toward iron (G°(6aiPrOH) = −3.9 kcal/mol). Next, a transition state was found (TS6a,7) in which barrierless proton transfer to the amido nitrogen produces 7 (G° = −1.9 kcal/mol), a ground-state ion pair adduct in which the formally anionic oxygen atom is stabilized by the newly formed N−H moiety (N−H = 1.09 Å, O−H = 1.51 Å, Figure 6). In addition, the C−H bond pointing toward iron has been elongated (1.21 Å vs 1.10 Å for 6aiPrOH) and the Fe−H distance is short (1.81 Å). The bonding mode between iron and hydrogen can be classified as agostic but with an obtuse Fe−H−C angle of 145° and a strong electrostatic interaction between iron and hydrogen.14 The subsequent hydride transfer is exergonic and proceeds with only a 1.3 kcal/ mol free energy barrier (TS7,8) to form the ketone adduct 8AcMe. Hydride transfer from the NH-stabilized isopropoxide to 8 has been calculated to be the rate-limiting step (G°⧧(TS7,8) = −0.6 kcal/mol) but is only 1.2 kcal/mol higher in energy than the hydride transfer barrier from 8 to AcPh (vide infra). This difference is small and may not accurately predict which of these two steps is indeed rate limiting, considering that the model complex being used does not account for metal− substrate steric/electronic interactions and different density functionals report higher or lower electronic barriers (see Computational Details). Regardless of which step is rate limiting in reality, these barriers are much lower in energy than the 22.8 kcal/mol required for hydride transfer to the imine carbon in the activation period (TS5,6; Figure 3). After AcMe dissociation, the free energy of octahedral amino-hydrido complex 8 is −13.4 kcal/mol with respect to 1 plus all relevant small molecules. Complex 8 can now perform a stepwise outer-sphere proton/hydride transfer to AcPh through a sequence of events the reverse of that described for iPrOH. First, hydride transfer from iron to AcPh occurs to generate an NH-stabilized ion pair adduct (8AcPh → TS8,9 → 9); second, barrierless proton transfer from nitrogen to oxygen (9 → TS9,6 → 6PE) produces PE, regenerating intermediate 6 after product dissociation. The metrical parameters of the intermediates and transition states for hydrogen transfer from 8 to PE are very close to those calculated for iPrOH → 6 hydrogen transfer (Figure 6). Very similar stepwise proton-hydride transfer sequences containing NH-stabilized ion pair adducts have been calculated for transition-metal complexes with tridentate ligands incorporating a central amido donor. Grützmacher and co-workers have developed the catalyst Rh(trop2N)(PPh3) (trop2N = bis(5H-dibenzo[a,d]cyclohepten-5-yl)amide), a Rh(I) complex utilizing a tridentate diolefin amido ligand, and calculated the mechanism by which it catalyzes the transfer hydrogenation of ketones15 and the dehydrogenative coupling of primary alcohols with water, methanol, or amines.16 Bi and co-workers have calculated17 the mechanism of the transfer hydrogenation of ketones for the PNP iridium catalyst IrH3[(iPr2PC2H4)2NH], which was originally developed by Clarke et al.18 Gusev and co-workers have calculated NH-stabilized alkoxide adducts for the dehydrogenation of alcohols using the osmium catalysts OsH3[N(C2H4PiPr2)2] and OsH(CO)[N(C2H4PiPr2)2], which also incorporate PNP-type ligands.19 Of note, an OH-stabilized ion pair adduct has been identified using ab initio molecular dynamics simulations and an

Figure 4. Optimized structures and selected bond lengths (Å) of 1, TS1,2 (1120i cm−1), TS1,3 (1269i cm−1), TS1,4 (1409i cm−1), TS1,5 (1649i cm−1), 5, TS5,6 (869i cm−1), and 6.

the amido-eneamido complex 6, which is within the catalytic cycle (vide infra). This is the rate-limiting step in the reduction of 1 to 6 by iPrOH; we propose that the activation period seen during catalysis is due to the barrier associated with the hydride transfer during inner-sphere reduction of the PNNP ligand to generate 6. Catalytic Cycle. Our mechanistic studies regarding the catalytic cycle begin with 6, the structure reached after the activation period (G°(6) = −8.1 kcal/mol with respect to 1; 7378


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Scheme 4. Proposed Catalytic Cycle for the Transfer Hydrogenation of Ketones via Amido-Eneamido Complex 6a

a

Relative free energies (G°, in kcal/mol) are also provided.

basis of experimentally determined equilibrium constants and kinetic simulations.8 For the transfer of H− from iron to AcPh (TS8,9), the free energy barrier is about 12 kcal/mol if 10 and/ or 8 are indeed resting states, also close to the experimentally derived value of 14.3 kcal/mol. If calculations were performed using the full catalyst system, stereoelectronic factors (i.e., steric repulsion between the substrate and phenyl rings on catalyst) would be expected to increase the calculated free energy barriers and possibly lead to better agreement with experiment. The turnover frequency (TOF) of this catalytic cycle can be estimated from the calculated activation free energy barrier using conventional transition state theory.20 The calculated overall turnover-limiting step is hydride transfer from isopropoxide to iron (TS7,8), and two possible resting states are 10 and 8, giving an activation free energy between 12.8 and 13.4 kcal/mol (vide supra). This translates to a calculated TOF range (29 °C) of (1.1 × 107)−(4.2 × 106) h−1, which is 2−3 orders of magnitude higher than the experimentally observed maximum TOF of 5.5 × 104 h−1 at 29 °C.8 Although this

explicit solvent model for the Ru-catalyzed hydrogen transfer to formaldehyde using RuH(η6-C6H6)(OCH2CH2NH2).2b Metal-bound alkoxides also play an important role as resting states in the proposed catalytic cycle. The formation of 6 (Scheme 4 and Figure 5) makes it possible for the iPrOH O−H bond to add across the Fe−Namido bond, producing the alkoxido complex 10, which has a free energy of −14.0 kcal/mol, very similar to that of 8 (G° = −13.4 kcal/mol) plus all relevant small molecules. iPrOH coordinates to 6 such that the O−H bond sits atop the Fe−Namido bond (6biPrOH), which then goes through a barrierless transition state (TS6b,10) to generate 10. Presumably, either 10 or 8 are resting states during catalysis, which means that the activation energy for the entire catalytic process is approximately 13 kcal/mol if the rate-limiting step of transfer hydrogenation is either hydride transfer to iron from iPrOH (TS7,8) or from iron to AcPh (TS8,9). Given our simplified DFT models, this value is close to the activation free energy of 16.1 kcal/mol for the transfer of H− from iPrOH to iron, which is obtained from the estimated rate constants on the 7379


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Figure 5. Energy profile of hydrogen transfer from iPrOH to AcPh via amido-eneamido complex 6 (continued from Figure 3).

mechanism of transition-metal-catalyzed reactions.24 In a typical transfer hydrogenation reaction using the diimine precatalyst BPh (R, R′ = phenyl; Scheme 2), the solvent/reductant iPrOH was replaced with monodeuterated iPrOD-d ((CH3)2CHOD) or fully deuterated iPrOD-d8 ((CD3)2CDOD) followed by calculation of the KIEs at 29 °C.8 A primary KIE value of 1.3 ± 0.1 is obtained when iPrOD-d is used, while a primary KIE value of 2.5 ± 0.1 is obtained with iPrOD-d8 (Table 1): Since the concentrations of

discrepancy may seem dramatic, the calculated free energy barrier for the experimentally observed maximum TOF is 16 kcal/mol, only about 3 kcal/mol higher than the calculated activation free energy. A previous study has shown that when additional AcPh is added during catalysis as the rate decreases and the reaction progresses toward equilibrium, the rate increases again and rapid catalysis occurs for a brief period of time.6c An increased concentration of the product alcohol (PE) after prolonged catalysis would also make it possible to generate the alkoxido complex 11 in solution (Scheme 4 and Figure 5), which is even more stable than its isopropoxide analogue (G°(11) = −16.4 kcal/mol, G°(10) = −14.0 kcal/mol). Thus, it follows that an increased concentration of PE would also slow down catalysis, due to the larger activation barrier that is required (about 16 kcal/mol) to achieve productive catalytic turnovers. The formation of metal-bound alkoxides has experimental relevance in the field of ruthenium-catalyzed H2 hydrogenation catalysis.21 Bergens and co-workers have recently performed low-temperature intramolecular trapping experiments to investigate the reactions between a free RuH2(diamine)(diphosphine) complex and ketone to generate RuH(alkoxide)(diamine)(diphosphine).22 They conclude that bonding between the incoming ketone and Ru dihydride complex persists throughout the course of the reaction and formation of a free Ru amide complex and the product alcohol does not occur. On the basis of these findings, it may also be possible for alkoxide complexes 10 and 11 to undergo reversible intramolecular rearrangement to form the ion pair adducts 7/9 (Figure 5), though we have no computational evidence for this type of rearrangement. What is clear is that our DFT studies classify hydride transfer as occurring in the outer sphere via an NH-stabilized ion pair adduct; however, the thermodynamically favorable formation of Fe alkoxide complexes must also be taken into account. A detailed mechanistic DFT study concerning low barrier intramolecular rearrangement from an NH-stabilized ion pair adduct to form a Ru alkoxide during H2 hydrogenation is reported elsewhere.23 Kinetic Isotope Effect Calculations. The presence or absence of a kinetic isotope effect (KIE) can provide useful information to the experimentalist and theorist about the

Table 1. Experimental8 and Calculated KIE Values (1 atm, 29 °C) for Hydride Transfer during the Activation Period and Catalytic Cycle Using iPrOD-d and iPrOD-d8 kH/kD step 8

exptl 5 → TS5,6 7 → TS7,8 8AcPh → TS8,9

(CH3)2CHOD

(CD3)2CDOD

1.3 ± 0.1 0.9 0.9 1.1

2.5 ± 0.1 2.6 1.9 1.5

precatalyst and active species are constantly changing throughout catalysis, calculating separate KIEs during the activation period and rapid catalysis allows us to obtain more information about the origin of the experimental KIEs. To calculate the KIE for the activation period, we examined the rate-limiting hydride transfer step 5 → TS5,6. Selective deuteration of one hydrogen atom adjacent to phosphorus simulates a reaction with iPrOD-d, and deuterating seven additional hydrogen atoms on the isopropoxide moiety simulates a reaction with iPrOD-d8 (Figure 7; TS5,6). The calculated KIE using iPrOD-d is 0.9, suggesting that there is no significant isotope contribution (Table 1). However, a calculated KIE value of 2.6 was found when iPrOD-d8 was used, suggesting that the rate-limiting hydride transfer step contributes significantly and correlates with the observed value of 2.5 ± 0.1. These findings are consistent with the qualitative observation of a longer activation period when iPrOD-d8 is used.8 To calculate the KIE during catalysis, we examined the transfer of a hydride equivalent from the NH-stabilized ion pair adduct to iron (7 → TS7,8) and transfer of a hydride equivalent 7380


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isopropoxide moiety. When a hydride equivalent is transferred from iron to AcPh using iPrOD-d8, only four sites are deuterated, as shown in Figure 7 (TS8,9). Calculations using iPrOD-d for hydride transfer to/from iron reveal no significant kinetic isotope effects (0.9 and 1.1), while using iPrOD-d8 generated KIE values of 1.9 and 1.5 (Table 1). Bearing in mind that simple model complexes were used for KIE analysis, the calculated trends are in rough agreement with the experimental values. Using iPrOD-d reveals a minimal KIE during the activation period and rapid catalysis, while catalysis with iPrOD-d8 reveals KIE values greater than 1. On the basis of these results, we predict that the dominant contribution to the experimentally observed KIE when using iPrOD-d8 is hydride transfer during the activation period, which is also the largest surmountable barrier toward generating the active catalytic species. Formation of a Bis(amido) PNNP Complex and Modeling Its Catalytic Activity. Our group recently synthesized a saturated analogue of the diimine precatalyst B, the chiral Fe(II) PNNP diamine complex [Fe(CO)(Br)(PPh2CH 2 CH 2 NH((S,S)-C(Ph)HC(Ph)H)NHCH 2 CH 2 PPh 2 )][BF4] (D; Scheme 5), whose general structure is shown in Scheme 5.8 When complex D is used under standard catalytic conditions, the presence of an 8-fold excess of base presumably deprotonates both nitrogen atoms to generate the bis(amido) catalyst [Fe(CO)(PPh2CH2CH2N((S,S)-C(Ph)HC(Ph)H)NCH2CH2PPh2) (E). This complex can then accept a proton/ hydride equivalent from iPrOH and transfer it to AcPh, presumably across the Fe−N bond of the catalyst as described above for the amido-eneamido compounds. The bis(amino) complex D is a precatalyst for the TH of AcPh to PE; however, its activity is poor (10% conversion, 2 h, 82% ee, catalyst:base:substrate ratio of 1:8:6000)8 versus the analogous diimine precatalyst (90% conversion, 30 min, 82% ee, catalyst:base:substrate ratio of 1:8:2000),6b but the enantioselectivity remains unchanged. Since reduction of the PNNP ligand of bis(eneamido) complex 1 is thermodynamically favorable, we extended our calculations to include stepwise inner-sphere reduction of the eneamido portion of mixed amido-eneamido complex 6, yielding the bis(amido) complex 13 (Figure 8). The reduction proceeds in a way very similar to that described above for 1. First, iPrOH coordination and proton transfer generates the alkoxido complex 12 (6biPrOH → 12, G°⧧ = 16.0 kcal/mol) followed by hydride transfer and AcMe dissociation to afford the Fe(II) bis(amido) complex 13 (12 → 13, G°⧧ = 18.1 kcal/mol). Note that the hydride transfer free energy of TS12,13 is 4.7 kcal/mol lower in energy than TS5,6 (G°⧧ = 22.8 kcal/mol) in the activation period (vide supra) and 13 is exergonic by 10.6 kcal/ mol with respect to 1. This provides evidence that complete reduction of the ligand system is thermodynamically favorable and that “deactivation” or “decomposition” of precatalyst B after prolonged catalysis6b may in fact be the gradual formation of bis(amido) complexes such as 13 in solution. Our calculations model a plausible catalytic cycle using complex 13 (Figure 9), which also operates via a stepwise outersphere mechanism. Overall, the cycle is very similar to that of 6, with key differences being higher free energies of hydride transfer (G°⧧(TS14,15) = 1.0 kcal/mol, G°⧧(TS15,16) = 0.4 kcal/ mol) and lower free energies for hydrido-amino complex 15 and alkoxido complex 17 (−13.9 and −14.2 kcal/mol, respectively). However, the free energy of alkoxido complex 18 is 1.7 kcal/mol higher than that of the analogous eneamidoalkoxido complex 11. If the resting states are indeed complexes

Figure 6. Optimized structures and selected bond lengths (Å) of TS6a,7 (1143i cm−1), 7, TS7,8 (430i cm−1), 8, TS8,9 (407i cm−1), 9, TS9,6 (1096i cm−1), and TS6b,10 (483i cm−1).

from the FeH−NH complex to AcPh to generate the phenethoxide adduct (8AcPh → TS8,9). In order to simulate the use of monodeuterated iPrOD-d, only two sites were deuterated: the hydrogen atom adjacent to phosphorus (which is deuterated during the activation period) and the hydrogen attached to nitrogen (Figure 7; TS7,8 and TS8,9). To simulate the reaction with iPrOD-d8, eight additional sites are deuterated when a hydride equivalent is transferred to iron: the hydrogen atom adjacent to nitrogen (which is deuterated during the activation period) along with all hydrogen atoms on the 7381


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Figure 7. Transition states marked with potential deuteration sites for KIE calculations.

Scheme 5. Reaction of Bis(amino) Complex D8 with Base, Which Likely Generates Bis(amido) Complex E in Situ

validate that the proposed catalytic cycle depicted in Scheme 4 and Figure 5 is thermodynamically favorable.

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CONCLUSION

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COMPUTATIONAL DETAILS

With the aid of density functional theory, we have calculated the mechanism of transfer hydrogenation of ketones using the model bis(eneamido) precatalyst Fe(CO)(H 2 PCH CHNCH2CH2NCHCHPH2) which complement results from a kinetic study recently published by our group.8 A general mechanism is shown in Scheme 6. The largest calculated free energy barrier is hydride transfer from an iron-bound isopropoxide to an imine carbon on the ligand during a stepwise inner-sphere activation period. Overcoming this barrier transforms the bis(eneamido) complex into the mixed amidoeneamido compound Fe(CO)(H2PCH2CH2NCH2CH2NCH CHPH2), which we propose is the catalytically active species. The catalytic cycle operates via a stepwise outer-sphere mechanism where an H+/H− pair is transferred across the amido nitrogen and iron atom, respectively, with the highest barriers being hydride transfer to/from isopropyl alcohol/ acetophenone. Calculated kinetic isotope effect values are also in rough agreement with the experimentally determined values, which supports the proposed mechanism of catalysis. It is energetically feasible to reduce the ligand through another stepwise inner-sphere reduction to generate the symmetrical bis(amido) compound Fe(CO)(H2PCH2CH2NCH2−)2. This complex has been calculated to be active for the transfer hydrogenation of ketones and operates in a similar stepwise outersphere fashion but has higher hydride transfer barriers and a higher activation energy. We hope that this computational study, along with our recent kinetic investigations, leads to the rational design of other well-defined iron hydrogenation catalysts.

Figure 8. Energy profile to reduce the eneamido ligand portion of complex 6 (continued from Figure 5).

15 and 17/18, the activation free energy will be about 15−16 kcal/mol, which is consistent with the lower experimentally observed activity with E in comparison to a calculated activation energy of 12−13 kcal/mol for amido-eneamido catalyst 6. All intermediate and transition state geometries closely resemble those already discussed above. Other Considered Mechanisms. To rule out other productive pathways of transfer hydrogenation beginning with mixed amido-eneamido complex 6, two additional scenarios were investigated (Figure 10): first, hydride transfer to iron and proton transfer to the eneamido nitrogen to generate complex S5, and second, hydride transfer to iron and proton transfer to carbon adjacent to phosphorus to generate compound S8 (see the Supporting Information for additional details). The hydride transfer free energy when the proton is located on the eneamido nitrogen atom is 23.6 kcal/mol (TSS4*,S5), and the calculated free energy when the proton is transferred to carbon is 11.5 kcal/mol (TSS7,S8). These pathways are substantially higher in energy than hydride transfer to/from iPrOH across the iron−amido bond (G°⧧(TS7,8) = −0.6 kcal/mol) and

General Considerations. Density functional theory calculations were performed using Gaussian09.29 The M0626 hybrid functional was used for all calculations. All atoms were treated with the 6-31+ +G(d,p) basis set.27 A pruned (99,590) integration grid was used throughout (Grid=UltraFine). All NH-stabilized ion pair adducts were connected to their transition states by performing intrinsic reaction coordinate (IRC) calculations.28 The substituents on phosphorus and diamine were replaced with hydrogen atoms to reduce computational cost. Optimizations were performed in isopropyl alcohol (2-propanol) using the integral equation formalism polarizable continuum model (IEF-PCM)29 with radii and nonelectrostatic terms from the SMD solvation model.30 Kinetic isotope effect (KIE) values (1 atm, 29 °C) were calculated with the freqchk utility (supplied by Gaussian) using conventional transition state theory,2d,31 neglecting tunnelling/recrossing effects and assuming that the geometry of the protio and deuterio transition state remains the same. Basis set superposition error (BSSE) was examined by using the counterpoise method32 and computed to be 1−2 kcal/mol for weakly coordinating ketone/alcohol adducts 6aiPrOH, 8AcMe, 8AcPh, 7382


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Figure 9. Energy profile of hydrogen transfer from iPrOH to AcPh via bis(amido) complex 13 (continued from Figure 8).

Scheme 6. General Mechanism for the Transfer Hydrogenation of AcPh to PE Using Fe(II) [5.5.5] PNNP Bis(eneamido) Complexes as Precatalysts

Figure 10. Other calculated metal hydride complexes and transition states with higher hydride transfer free energies. See the Supporting Information for additional details. and 6PE; thus, it is expected that the BSSE for other ketone/alcohol adducts is within the same range. Open-shell triplet state optimizations were performed on five-coordinate complexes 1, 6, and 13; in comparison to their singlet state free energies, each structure was higher in energy by 6.4, 9.3, and 3.2 kcal/mol, respectively. Triplet state electronic energies were calculated for transition states TS5,6, TS8,9, TS12,13, and TS15,16 by using optimized singlet state geometries and calculating single-point energies; in comparison to their singlet state electronic energies, each structure was higher in energy by 38, 40, 34, and 36 kcal/mol, respectively. Therefore, odd-electron species were not further considered. Full vibrational and thermochemical analyses (1 atm, 298 K) were performed on optimized structures to obtain solvent-corrected free energies (G°) and enthalpies (H°). Optimized ground states were found to have zero imaginary frequencies, while transition

states were found to have one imaginary frequency. Threedimensional visualizations of calculated structures were generated by ChemCraft.33 7383


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Table 2. Electronic Energies of Key Transition States Using Various Density Functionals density functional

E(TS5,6)−E(5)

E(TS8,9)−E(8AcPh)

M06L TPSS B3LYP B3PW91 BMK HSE06 LC-ωPBE M06 MPW1k mPW1PW91 PBE0 TPSSh ωB97X-D

23.2 22.8 26.2 23.5 28.9 24.2 30.3 26.7 19.4 24.3 23.9 24.3 28.3

6.2 5.4 7.6 6.3 6.0 5.3 8.4 5.7 4.8 5.9 5.4 5.8 5.6

ASSOCIATED CONTENT

S Supporting Information *

Tables giving Cartesian coordinates and free energies for optimized structures, additional figures/discussion, and text giving the complete ref 25. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Energy Evaluation Using Other Density Functionals. To assess the validity of the calculated energies using the M0626 density functional, a variety of other common density functionals were also tested with the 6-31++G(d,p) basis set (Table 2). Single-point energies using M06/6-31++G(d,p) optimized structures were obtained for two key transition states: hydride transfer to the imine carbon in the activation period (5 → TS5,6, Figure 3) and hydride transfer from iron to AcPh in the proposed catalytic cycle (8AcPh → TS8,9, Figure 5). The following density functionals were chosen: pure M06L34 and TPSS,35 hybrid B3LYP, 36,37 B3PW91, 37,38 BMK, 39 HSE06 (also known as HSEh1PBE),40 LC-ωPBE,41 MPW1k,42 mPW1PW91,38,43 PBE0 (also known as PBE1PBE),44 TPSSh,35 and ωB97X-D.45 With regard to other research groups that have calculated NH-stabilized ion pair adducts, the MPW1k and PBE0 density functionals were used by Gusev,19 B3PW91 was used by Grützmacher,15,16 and B3LYP was used by Bi.17 Yang has recently reported a similar comparison of density functionals in conjunction with mechanistic studies of an Fe(II) PNP pincer complex for the hydrogenation of ketones.2g Our results show that energies for the imine reduction step (E(TS5,6)−E(5)) deviate no more than 4 kcal/mol from the barrier calculated using the M06 density functional (E = 26.7 kcal/mol), with the exception of MPW1k (19.4 kcal/mol, ΔE = 7.3 kcal/mol). Hydride transfer barriers from the complex 8AcPh to AcPh are all within 3 kcal/ mol from M06 (E = 5.7 kcal/mol). The LC-ωPBE density functional predicts the barriers will be highest for these two steps, while MPW1k predicts the lowest barriers. In general, we conclude that our choice of density functional is reasonable and does not alter our discourse.46

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Sciences and Engineering Research Council (NSERC) for research funding. D.E.P. also thanks the NSERC and Ontario Graduate Studies in Science and Technology (OGSST) scholarships for funding and Prof. Faraj Hasanayn for enlightening discussions. 7384


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NOTE ADDED AFTER ASAP PUBLICATION In the version of this paper published on Oct 9, 2012, there was an incorrect chemical formula given in the abstract. The version that appears as of Oct 11, 2012, has the correct formula.

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