Autotandem Aromatization–Dearomatization Pathways for PNP-Ru

Article pubs.acs.org/Organometallics

Autotandem Aromatization−Dearomatization Pathways for PNP-RuIICatalyzed Formation of Imine and Hydrogen from Alcohol and Amine K. S. Sandhya and Cherumuttathu H. Suresh* Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum 695 019, India S Supporting Information *

ABSTRACT: Mechanism of autotandem catalytic reactions involving EtNH2 and EtOH promoted by the complex (PNP)Ru(CO)H (1) has been investigated at the TPSS level of DFT (PNP = C5H3N(CH2PtBu2)(CHPtBu2), C5H3N(CH2PisoPr2)(CHPisoPr2)). The reaction is identified by three mechanically distinct catalytic cycles. In cycle I, the EtOH adduct of 1 undergoes O−H proton transfer to the PNP ligand to yield the aromatic complex (PN′P)Ru(CO)(OEt)H (3). Subsequent dearomatization of PN′P leads to the formation of dihydrogen and the complex (PNP)Ru(CO)(OEt) (5). However, an outer-sphere mechanism involving the formation of a dihydrogen intermediate complex between 1 and EtOH is more facile for dihydrogen elimination (ΔG⧧ = 18.4 kcal/mol) than the ligand aromatization−dearomatization pathway (ΔG⧧ = 23.9 kcal/mol). In 5, the migration of a β-hydrogen from OEt to the unsaturated P arm of PNP forces removal of MeCHO and formation of the aromatic complex (PN′P)Ru(CO). Regeneration of the catalyst occurs by dearomatization of the PN′P ligand through proton migration from the P arm to the metal. In cycle II, aromatization of the amine adduct (PNP)Ru(CO)(EtNH2)H leads to (PN′P)Ru(CO)(EtNH)H, and subsequent reaction with MeCHO (formed in cycle I) yields the hemiaminal EtNHCH(CH3)OH. The direct reaction between aldehyde and amine can also yield the hemiaminal. In cycle III, the hemiaminal adduct of the catalyst undergoes aromatization at the PNP ligand via N−H proton migration, which simultaneously activates C−OH bond to produce the imine (EtNCHMe) and (PN′P)Ru(CO)(OH)H (17). Removal of water from 17 leads to dearomatization of PN′P and regeneration of the catalyst. Aromatization steps are characterized by proton migration from the coordinated ligands, viz. the O−H bond of the alcohol, the C−H bond of the alkoxide, the N−H bond of the amine, and the N−H bond of the hemiaminal, to the unsaturated P arm of PNP, and the respective ΔG⧧ values are 2.4, 15.7, 18.6, and 13.8 kcal/mol. The efficiency of this autotandem catalytic reaction is mainly attributed to metal−ligand cooperativity operating through several facile “aromatization−dearomatization” steps.

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INTRODUCTION Metal−ligand cooperation has been widely explored for the synthesis of organic molecules.1−20A metal−ligand bifunctional catalyst contains both a Lewis acid and a Lewis base functionality, which activate some of the bond-forming or bond-breaking stages of a reaction. For instance, the dehydrogenation of an alcohol in the presence of (hydroxylcyclopentadienyl)ruthenium(II) hydride or the hydrogenation of an aldehyde in the presence of transdihydride(diamine)ruthenium(II) takes place due to the bifunctional nature of the catalyst.12,21 Very recently, Milstein et al. developed a new mode of metal−ligand cooperation through aromatization−dearomatization of pyridine-based pincer PNN and PNP ligands under the influence of the coordination sphere of Ru(II), Fe(II), and Ir(II) hydride systems.22−26 These complexes can catalyze many reactions, such as coupling of alcohols with amines to amides and polyamides, alcohols to esters, alcohols to amines, alcohols to © 2013 American Chemical Society

ketones, ketones to alcohols and acetals, esters to alcohols, and amides to alcohols and amines.27−39 The pyridine portion of the PNN pincer ligand in the catalyst is nonaromatic (the metal−N bond is covalent) and contains an unsaturated R2PCH arm on the C2 position and a saturated R2NCH2− arm (R2PCH2− arm for the PNP ligand) on the C6 position (Figure 1). Aromatization−dearomatization processes involving the pincer ligand are proposed as the driving forces for the various kinds of reactivity patterns observed for these complexes, such as activation of C−H, N−H, O−H, and H− H bonds.40 For instance, sequential aromatization−dearomatization steps are proposed for the Milstein catalyst PNN-RuII for water splitting,41−43 and theoretical studies have shown the metal−ligand cooperativity in the catalysis.3,44,45 Protonation of the unsaturated arm of the PNN (or PNP) ligand leads to Received: February 19, 2013 Published: May 8, 2013 2926

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COMPUTATIONAL DETAILS

All calculations wre carried out with the Gaussian03 suite of programs with the density functional theory (DFT) methodology.49 The DFT functional used is TPSS (Tao−Perdew−Staroverov−Scuseria) for methyl-substituted pincer PNP systems.50 Previously Hall and coworkers have shown that the TPSS functional is good for conducting mechanistic studies on water-splitting reactions of the catalyst RuIIPNN, and this has inspired us to choose the same functional for the present studies.44 Our previous studies on the catalyst RuII-PNN also employed the TPSS functional. Very recently Wang et al. also successfully applied the TPSS functional in describing the mechanism of amide formation using the catalyst RuII-PNP.3 The basis set aug-ccpVDZ (augmented correlation consistent polarized valence double-ζ basis set) was selected for Ru with effective core potential to describe the 28 core electrons.51 The 6-31++G(d,p) basis set was used for all other atoms.52,53 All the transition states were confirmed by the presence of one imaginary frequency in the vibrational frequency analysis. Further, solvent effects were considered through single-point calculations with optimized geometries using the polarizable continuum method (PCM) as implemented in Gaussian 03.54 The selected alcohol and amine for the reaction are ethanol and ethylamine, respectively. All the reported energy parameters are solvent-corrected relative Gibbs free energy values, unless otherwise noted.

Figure 1. Models of PNN and PNP ligands: aromatic (top) and nonaromatic (bottom).

aromatization of the pyridine moiety, while removal of the proton from the saturated arm leads to dearomatization of the pincer ligand (Figure 1). During the aromatization−dearomatization processes, the oxidation state of the center metal does not change. Further, the cooperating pincer ligand changes the structure during reactant activation as well as product formation.46 Recently we have shown that the Milstein catalyst can also pass through an outer-sphere mechanism involving dihydrogen bonding between water and Ru−H (HO− Hδ+···Hδ−−Ru) to eliminate dihydrogen, and this mechanism avoided the aromatization of the pincer ligand.47 Although a clear mechanism has not yet been reported for imine and H2 production using the PNP-RuII complex 1, Milstein et al. have proposed a likely complex mechanism (Scheme 1)40,48 passing through the alkoxide intermediate A,

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RESULTS AND DISCUSSION Scheme 2 summarizes possible reactions in the autotandem catalytic cycle promoted by the PNP-RuII complex 1: Scheme 2. Summary of Auto-Tandem Catalytic Reactions Promoted by PNP-RuII Complex 1

Scheme 1. Mechanism of Imine Synthesis Proposed by Milstein and Co-Workers

generation of the hydrogen in cycle I, hemiaminal in cycle II, and imine and water in cycle III.55,56 Oxidative addition of an alcohol to 1 is the key step in cycle I, while hemiaminal formation in cycle II involves activation of one of the N−H bonds of the amine. In cycle III, the N−H and O−H bonds of the hemiaminal are further activated. The detailed mechanism of catalytic cycle I is presented in Figures 2, 3, and 5, and the catalytic cycles II and III are illustrated in Figures 6 and 7, respectively. In cycle I, at first the alcohol binds with the (PNP)Ru(CO)H catalyst 1 to form an adduct 2 (Figure 2). The binding energy for this reaction is 12.0 kcal/mol.45 The alcoholic O−H bond is 2.022 Å away from the unsaturated arm of the pincer ligand, and mild activation of this bond via TS1 (free energy of activation ΔG⧧ = 2.3 kcal/mol) leads to proton migration from O to C. This process converts the nonaromatic PNP ligand to the aromatic PN′P ligand, and the resulting complex is (PN′P)Ru(CO)(OEt)H (3). In the next step, one of the pyridinyl methylene hydrogen atoms migrates to the hydride ligand through TS2 (ΔG⧧ = 23.9 kcal/mol) and yields the dihydrogen complex (PNP)Ru(CO)(OEt)H2 (4). At this stage, the aromatic PN′P is converted back to the nonaromatic PNP. In 4, dihydrogen is strongly bonded to the metal (binding energy 17.4 kcal/mol and H−H distance 0.873 Å) and the removal (dissociation) of it can take place by passing through the high-energy transition state TS3 (ΔG⧧ = 20.2 kcal/mol) to yield the complex (PNP)Ru(CO)(OEt) (5). After the

the aldehyde intermediate B, the dihydride complex C, and the hemiaminal intermediate D. Dihydride complex C eliminates dihydrogen and regenerates the catalyst, while direct reaction between aldehyde and amine gives hemiaminal and subsequently imine. In this study our aim is to investigate the mechanism of PNP-RuII hydride catalyzed synthesis of imine using alcohol and amine. Probable aromatization−dearomatization pathways will be examined to understand the metal−ligand cooperativity in catalysis. Further, an outer-sphere mechanism will be discussed to assess the role of the PNP ligand. Catalystassisted imine and hemiaminal formation will also be considered. 2927

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Figure 2. Solvent-corrected free energy profile and the reaction mechanism of PNP-RuII-catalyzed hydrogen formation in cycle I.

Figure 3. Solvent-corrected free energy profile for dihydrogen elimination in cycle I.

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Figure 4. Solvent-corrected free energy profile and reaction mechanism for hydrogen migration and formation of 1 in cycle I.

profile in Figure 3 clearly suggests that, for the dihydrogen elimination, the outer-sphere mechanism which does not involve the aromatization−dearomatization of the pincer ligand is more favorable than the aromatization−dearomatization pathway given in Figure 2. From 5, formation of aldehyde (19) takes place when the βhydrogen atom of the alkoxy ligand migrates to the unsaturated P-arm of the PNP ligand via the transition state TS7 (Figure 4).58 A moderate ΔG⧧ value of 15.7 kcal/mol is observed for this step and produces the complex (PN′P)Ru(CO)(CH3CHO) (10). The release of the weakly bound aldehyde from 10 is highly exothermic (20.8 kcal/mol), leading to 11. The high stability of 11 can be attributed to conversion of the nonaromatic PNP ligand to the aromatic PN′P ligand with the removal of aldehyde. From 11, catalyst 1 will be regenerated if a hydrogen migration occurs from the saturated pyridinyl methylene carbon to the metal center. This part of the mechanism is investigated through a direct mechanism as well as using a water molecule. In the direct mechanism, TS8′ will be formed, which suggests ΔG⧧ = 29.6 kcal/mol, while in the water-assisted outersphere mechanism, TS8 suggests ΔG⧧ = 16.8 kcal/mol. Hence, the water-assisted mechanism is highly preferred for the last step of this catalytic cycle. The proposed mechanism by Milstein et al. is considered as a third alternative for the formation of dihydrogen (Figure 5). According to this mechanism, the Ru−PMe2 bond of A (Scheme 1; A is the same as complex 3 given in Figure 2) has to dissociate and this forces the elimination of β-hydrogen from alkoxide. The Ru−P bond dissociation is energetically unfavorable by 8.9 kcal/mol, and the ΔG⧧ value for β-hydrogen

completion of cycle I, the hydrogen atom of the alcohol becomes part of the PNP ligand while the hydride ligand and one of the pyridinyl methylene hydrogen atoms combine to form the dihydrogen. Though the strategy of “borrowing hydrogen” from the metal complex is not apparent in cycle I, the continuation of this reaction to the next turnover cycle may use the hydrogen of the alcohol now present in the PNP ligand. The reaction depicted in Figure 2 for cycle I has one aromatization and one dearomatization step. On the basis of our previous study on the reaction of water with the PNN-RuII pincer complex,47 an outer-sphere mechanism for dihydrogen elimination in cycle I can be considered (Figure 3). According to this, the protic solvent molecule EtOH can have a significant interaction with the negatively charged hydride ligand in 1. This interaction is a typical dihydrogen-bonded interaction57 (interaction energy Eint = 3.6 kcal/mol and H···H internuclear distance 1.718 Å) and leads to the formation of the complex 6. Further, this interaction drives EtOH to a strong coordination with the metal while passing through the transition state TS4 (ΔG⧧ = 9.2 kcal/mol) to yield 7. In 7, the coordinated alcohol has an activated O−H bond and as a result the O−H proton migrates to the hydride ligand via TS5 (ΔG⧧ = 18.4 kcal/mol) to yield the dihydrogen complex 8. The binding energy of dihydrogen in 8 is 8.1 kcal/mol, which is less stable than 5. The Ru−H2 and H−H distances are 1.696 and 0.855 Å, respectively. Removal of dihydrogen from 8 takes place through TS6 (ΔG⧧ = 13.2 kcal/mol) and also yields the alkoxy complex 9. 5 in Figure 2 and 9 in Figure 3 are isomers, as they differ mainly in the fluxional orientations of the CO and alkoxy ligands. Hence, their interconversion is expected to be easy. The reaction 2929

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Figure 5. Solvent-corrected free energy profile for the hydrogen formation by breaking of the P arm proposed by Milstein and co-workers.

Figure 6. Solvent-corrected free energy profile and reaction mechanism of PNP-RuII-catalyzed hemiaminal elimination in cycle II.

PN′P ligand. In C, the hydride ligands orient in the trans direction. Dihydrogen will be formed when H···H bond coupling occurs between a C−H bond of the saturated P arm

elimination is 22.9 kcal/mol. The aldehyde intermediate B thus formed eliminates the aldehyde, and the resulting metal dihydride complex C retains the pincer coordination from the 2930

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Figure 7. Solvent-corrected free energy profile and reaction mechanism of PNP-RuII-catalyzed imine and water formation in cycle III.

For the reaction given in cycle II, at first, the amine coordinates to the catalyst 1 and yields (PNP)Ru(CO)H(NH2Et) (12) (Figure 6). The binding energy of this amino adduct is 6.6 kcal/mol higher than the binding energy of the alcohol adduct 2. The metal−amine interaction activates the N−H protons in 12, and as a result one of the protons migrates to the unsaturated arm of the pincer ligand via TS9 (ΔG⧧ = 18.6 kcal/mol) to yield (PN′P)Ru(CO)H(NHEt) (13). Though the PNP ligand in 13 is aromatic, it is 11.0 kcal/mol less stable than 12. The amino nitrogen of 13 is nucleophilic and reacts with the carbonyl carbon of an incoming acetaldehyde molecule (19; formed in cycle I) via TS10 (ΔG⧧ = 16.7 kcal/mol) to yield the zwitterionic intermediate 14. This intermediate is quickly converted to the catalyst 1 (ΔG⧧ = 0.9 kcal/mol) by yielding the hemiaminal 20. The corresponding transition state TS11 shows the migration of the proton from the P arm to the anionic region of the zwitterions and leads to the formation of the hemiaminal-bound complex 15. The binding energy of this adduct is 11.8 kcal/mol. Subsequent release of hemiaminal from 15 is exergonic by 14.5 kcal/mol. If we consider the highest energy state TS10 as the rate-limiting state, the free energy barrier required for the hemiaminal formation can be estimated as 26.6 kcal/mol. Otherwise, if 13 is sufficiently long-lived to stabilize in the reaction medium, only ΔG⧧ = 16.7 kcal/mol will be needed to cross TS10, meaning that the highest ΔG⧧ barrier will be 18.6 kcal/mol (on the basis of TS9). Formation of hemiaminal (EtNHCH(CH3)OH) without the mediation of the metal complex is also possible. A direct nucleophilic attack of the primary amine on the carbonyl group of the aldehyde requires ΔG⧧ = 22.7 kcal/mol, and this process is exothermic by 5.4 kcal/mol (Supporting Information). Since

and one of the hydride ligands in C. A direct mechanism for this step of the reaction gives ΔG⧧ = 27.5 kcal/mol (TSC′) (Supporting Information), wherein the relative free energy of TSC′ is 19.2 kcal/mol. A water molecule or an alcohol molecule acting as a bridge between Ru−H and proton on the phosphorus arm may accelerate the liberation of H2. The relative free energies of C-H2O and C-CH3CH2OH are found to be 0.2 and 4.4 kcal/mol, respectively, while those of the corresponding transition states are 23.5 and 26.2 kcal/mol. This suggests that the effective activation barrier for the waterassisted mechanism is 31.7 kcal/mol and that of alcoholassisted mechanism is 34.5 kcal/mol. The water-assisted mechanism is depicted in Figure 5. In all these mechanisms, the resulting product is a dihydrogen σ complex (1-H2) which contains the dearomatized PNP ligand. Finally, the catalyst 1 is regenerated by dissociation of H2 from this complex. Among the three proposed mechanisms for cycle I (the mechanism given in Figures 2, 3, and 6), the most probable one for the formation of the H2 σ complex can be easily determined by comparing the corresponding ΔG⧧ values. In Figure 2 this step is 3 → TS2 → 4, and in Figure 3 it is 7 → TS5 → 8, while in Figure 5 it corresponds to C-H2O → TSC → 1-H2 and the respective ΔG⧧ values are 23.9, 18.4, and 23.3 kcal/mol. Hence, the outer-sphere mechanism given in Figure 3 can be chosen as the best for H2 elimination. If we follow this mechanism, the regeneration of the catalyst has to take place through the mechanism given in Figure 4, which needs ΔG⧧ = 16.8 kcal/ mol. Formation of the intermediate 11 (Figure 4) will facilitate this mechanism, because at this stage, the occurrence of the reverse path (11 → TS7 → 5) will be very unlikely (ΔG⧧ = 37.8 kcal/mol). 2931

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the pincer ligand, a direct nucleophilic reaction between EtNH2 and MeCHO is also possible and both mechanisms may coexist in the reaction. In cycle II, the coordinated hemiaminal on the catalyst undergoes a facile N−H proton migration, and this simultaneously activates its C−OH bond to produce imine.59 In this cycle, the pincer ligand passes through aromatization− dearomatization states. Overall, the efficiency of this autotandem catalytic reaction can be attributed mainly to metal−ligand cooperativity operating through several facile “aromatization−dearomatization” steps. Further, this mechanistic study clearly shows that this type of cooperative catalysis originally discovered by Milstein et al. is applicable to the activation of O−H, N−H, and C−H bonds of a variety of organic molecules.27,30

the formation of amine intermediate 13 is highly probable (Figure 6), it can be argued that the catalyst-assisted mechanism (ΔG⧧ = 16.7 kcal/mol) is more favorable than the direct mechanism. In the catalytic cycle III, hemiaminal (20) obtained from cycle II or from a direct reaction between amine and aldehyde binds with 1 to form the complex (PNP)Ru(CO)H(EtNHCH(CH3)OH) (16) (Figure 7). The binding energy of the hemiaminal is 7.6 kcal/mol. In 16, the hydrogen of the N−H bond of hemiaminal is in the close vicinity (2.411 Å) of the unsaturated carbon of the pincer ligand, which forces the N−H proton migration via TS12, leading to aromatization of the pincer ligand. During this process, the C−OH bond is also simultaneously activated and facilitates the elimination of the imine 21. The resulting complex is the hydrido hydroxo PNPRuII complex (PN′P)Ru(CO)H(OH) (17). This reaction is highly exergonic (31.9 kcal/mol) and needs only a small ΔG⧧ value of 13.7 kcal/mol. Regeneration of the catalyst is envisaged by eliminating a water molecule from 17 via TS13. During this reaction, a proton from the saturated P arm of the pincer ligand migrates to the hydroxy group to produce the water-bound complex (PNP)Ru(CO)H(H2O) (18; ΔG⧧ = 11.6 kcal/mol). Water is easily removed from 18 and regenerates the starting catalyst complex 1. The catalytic cycle given in Figure 7 clearly suggests that this reaction can take place only in the presence of a primary hemiaminal intermediate, because a β-hydrogen in the nitrogen is required for the formation of TS12. It may be noted that we have used a methyl substituent on phosphorus to reduce the computational cost of this work while in the actual experiments bulky isopropyl and tert-butyl substituents were used. To assess the reliability of the simplified model, we have optimized reactants and transition states of some important rate-determining steps of real molecules (the substituent on phosphorus is isopropyl) using the TPSS method. Steps involving TS7, TS8, and TSC have been selected for this study, and we found that ΔG⧧ = 16.2, 15.8, and 22.7 kcal/mol, respectively (Supporting Information). These values differ only by ±1 kcal/mol from those of the small models, suggesting that the results obtained using the simplified models are reliable.

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ASSOCIATED CONTENT

S Supporting Information *

Figures and tables giving optimized geometries, Gibbs free energy profiles, and energy data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-471-2515472. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the EMPOWER scheme (OLP132839) of the CSIR, Government of India. K.S.S. thanks the CSIR for an SRF fellowship. We also acknowledge the high-performance computing centers at CMSD, Hyderabad, CMMACS, Bangalore, and NCL, Pune.

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REFERENCES

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CONCLUSIONS We have explored the plausible mechanisms for PNP-RuIIcatalyzed one-pot production of hydrogen and imines through autotandem catalytic cycles using DFT. Amine has a higher tendency to bind with the catalyst than alcohol. Coordination of alcohol or amine or hemiaminal to the catalyst can initiate a proton migration from that ligand to the unsaturated P arm of the PNP pincer ligand. The driving force for this process comes from the aromatic stabilization of the newly formed PN′P pincer ligand. Proton migration from the coordinated alcohol is easier than proton migration from the strongly coordinated amine. The pincer ligand aromatization−dearomatization steps of cycle I can be bypassed if the alcohol undergoes an outersphere reaction initiated by the formation of a dihydrogen intermediate complex. This pathway is more facile for dihydrogen elimination than the aromatization−dearomatization pathway. Further, an outer-sphere mechanism is more probable than the proposed mechanism by Milstein and coworkers for dihydrogen elimination. The aldehyde formed in cycle I enters into cycle II to produce the hemiaminal EtNHCH(CH3)OH. Although hemiaminal formation can also be described by aromatization−dearomatization steps involving 2932

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dx.doi.org/10.1021/om4001428 | Organometallics 2013, 32, 2926−2933