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General H2 Activation Modes for Lewis AcidTransition Metal Bifunctional Catalysts Yinwu Li, Cheng Hou, Jingxing Jiang, Zhihan Zhang, Cunyuan Zhao, Alister J. Page, and Zhuofeng Ke ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02395 • Publication Date (Web): 27 Jan 2016 Downloaded from http://pubs.acs.org on January 28, 2016
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General H2 Activation Modes for Lewis Acid‐Transition Metal Bifunc‐ tional Catalysts Yinwu li,1 Cheng Hou,1 Jingxing Jiang,1 Zhihan Zhang,1 Cunyuan Zhao,1 Alister J. Page2 and Zhuofeng Ke1,* 1
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yatsen University, Guangzhou 510275, P. R. China 2 Newcastle Institute for Energy and Resources, The University of Newcastle, Callaghan 2308, NSW, Australia ABSTRACT: A general mechanism for H2 activation by Lewis acidtransition metal (LA-TM) bifunctional catalysts has been presented via DFT studies on a representative nickel borane system, (PhDPBPh)Ni. There are four typical H2 activation modes for LA-TM bifunctional catalysts: 1) the cis homolytic mode, 2) the trans homolytic mode, 3) the synergetic heterolytic mode, and 4) the dissociative heterolytic mode. The feature of each activa-
tion mode has been characterized by key transition state structures and natural bond orbital analysis. Among these four typical modes, (PhDPBPh)Ni catalyst most prefers the synergetic heterolytic mode (G‡ = 29.7 kcal/mol), however the cis homolytic mode cannot be totally disregarded (G‡ = 33.7 kcal/mol). In contrast, the trans homolytic mode and dissociative heterolytic mode are less feasible (G‡ = ~42 kcal/mol). The general mechanistic picture presented here is fundamentally important for the development and rational design of LA-TM catalysts in the future. KEYWORDS: H2 activation, Lewis acid, Lewis base, transition metal, nickel, mechanism, density functional theory, homolytic, synergetic, heterolytic, hydrogenation
1. INTRODUCTION Hydrogen is one of the most important reproducible green energy resources.1 Catalytic H2 activation is widely used in the energy storage,1,2 hydrogenation,3 dehydrogenation,4 hydrodesulfurization,5 hydrodenitrogenation6 and many other transformations. The most challenging aspect of H2 activation is H-H bond cleavage, which arises because of the strength of the H-H bond.7‐11
Figure 1. The development of TM catalysts, Lewis base-TM catalysts, and Lewis acid-TM catalysts for H2 activation. Transition metals (TMs) are particularly important in the development of homogenous catalysts for H2 activation, due to their ability for d-* back-donation to the H-H bond. The early well-known examplar TM catalyst is Wilkinson's catalyst (RhCl(PPh3)3), discovered in 1966, which is used for the acti-
vation of H2 and the hydrogenation of alkenes.12 Since then, a number of other TM catalysts with various ligands have been developed for the activation of hydrogen.7‐17 “Traditional” TM catalysts (since 1960s, Figure 1), generally subject H2 to oxidative addition onto their relatively electron rich TM center, via a homolytic cleavage mechanism (Figure 2).17,18 Later, another type of catalysts, Lewis base-transition metal (LB-TM) bifunctional catalysts were developed in 1980s (Figure 1).19‐21 LB-TM feature a ligand bearing a Lewis base site, which assists H2 activation.22 The archetypal LB-TM catalysts are the Noyori catalysts.23-25 The LB-TM bifunctional catalysts generally cleave H2 via a heterolytic mechanism, in which the metal center accepts the hydride and the assisting Lewis base attracts the proton (Figure 2). Main group elements with lone pair(s), such as nitrogen, oxygen, or sulfur, are usually utilized as the assisting Lewis base. LB-TMs based on many transition metals, such as Fe,26 Co,27 Rh,28,29 Ir,28,30,31 and Ru28,31-34 etc. have been developed in the past few decades. Very recently, a new family of Lewis acid TM (LA-TM) bifunctional systems has emerged. LA-TMs comprise an electron-rich late transition metal and a Lewis acid ligand35,36 with an electron deficient Lewis acid site to catalyze the activation
of H2. As far as we know, the TM and LB-TM systems have been widely studied for H2 activation, while the LATM catalytic systems did not emerge until 2010s (Figure 1).
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Proposed Four H 2 Activation Modes for LA-TM Catalysts
Figure 2. H2 activation mechanisms for TM catalysts, LB-TM catalysts, and LA-TM catalysts. In former studies, reversible binding of H2 in LA-TM system has been found.37,38 In 2011, Owen et al. reported that H2 could be activated by the rhodium borane catalytic system at 358.15 K.39 In 2012, Peter’s group synthetized a nickel borane complex, which catalyzed H2 activation at room temperature and low pressure, demonstrating the potential of LA-TM catalysts in H2 activation and hydrogenation.40 Erker and coworkers reported the hydrogen activation by an intermolecular zirconocene borane pair under mild conditions,41 and suggested a mechanism of heterolytic splitting of H2 similar to frustrated Lewis pairs.42 To date, a number of LA-TM catalytic systems have been developed, including Fe,43 Co,44 Ni,40 Ru,45 and Pt46 complexes. In these catalytic system, earth-abundant transition metals in first row47 such as Fe, Co, and Ni has attracted more interests, and boron is usually used as the Lewis acid site in ligands.48,49
Figure 3. H2 activation catalyzed by the (PhDPBPh)Ni system. In contrast to traditional TM and LB-TM catalysts, the H2 activation mechanism for LA-TM systems still remains to be fully understood. In a former study, Peter’s group found that H2 coordinates with (PhDPBPh)Ni forming a tetrahedral complex.50 Then the H2 cleavage leads to a dihydride product (Figure 3). Two potential mechanisms were proposed: H2 homolytic cleavage on the nickel center, and H2 heterolytic cleavage on the Ni-B bond. The heterolytic cleavage was suggested to be preferred due to its lower activation free energy.
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Sakaki’s group studied the different electronic effect51 between homolytic and heterolytic, and suggested that boron play an important role in the H2 cleavage in this LA-TM system.52 Herein, we investigate the mechanism of LA-TM catalyzed H2 activation using the representative (PhDPBPh)Ni nickel borane catalyst reported by Peter et al.,40 and present the first general mechanistic picture for LA-TM catalyzed H2 activation. We are particularly interested in the following pertinent questions concerning H2 activation with LA-TM catalysts: 1) Are there other types of H2 activation mechanisms for LA-TM catalysts? 2) Does the assistance of Lewis acid occur during H2 cleavage, or after cleavage to stabilize the hydride? 3) Does the LATM interaction remain during H2 cleavage? With these questions in mind, a comprehensive theoretical study was carried out on the (PhDPBPh)Ni system to unveil the general mechanism of H2 Activation by LA-TM catalysts. In this research, we presented four typical H2 activation modes for LA-TM catalysts (Figure 2): 1) The cis homolytic mode to a cis dihydride intermediate. 2) The trans homolytic mode to a trans dihydride species. 3) The synergetic heterolytic mode, where the H2 heterolytically cleaves onto the LA-TM bond. 4) The dissociative heterolytic mode, where the separate LA and TM activate H2 in a dissociative manner. These results are expected to provide a general mechanistic framework for future development and rational design of LATM catalysts.
2. COMPUTATIONAL DETAILS All structures were optimized in gas-phase by density functional theory (DFT),53 using the M06-L54 functional with basis sets I (BSI, lanl2dz55 for metal atom and 6-31G (d, p) for nonmetal atoms) in the gas phase. It should be noted that the optimizations were performed in the gas-phase, with the ideal gas conditions at 0 K. The optimized structure is taken as a single molecular cluster, which may be different from the reaction conditions in a solution. Frequency analysis calculations for optimized structures were performed to characterize the structures to be minima (no imaginary frequency) or transition states (one imaginary frequency). IRC calculations were carried out to confirm the connection between two minima for each transition state. Based on M06-L/BSI optimized geometries, the energy results were further refined by calculating the single point energy at the M06-L/BSII level of theory (BSII designates SDD56 for metal atom and 6-311++G**57 for nonmetal atoms). The bulky solvation effect of benzene (= 2.3) were simulated by SMD56 continuum solvent mode at the M06-L/BSII level of theory. The standard Gibbs free energy in solution is calculate by the equation (1), °
°
∆
°
ln 24.5
(1)
Eo(gas)
Where the is the gas-phase electronic energy, ZPE is the zero-point energy, Gsolv is the solvation free energy in benzene, and Go298K is the additional thermal contribution to Gibbs free energy at 298.15 K and 1 atm. The final term accounts for the free energy change from an ideal gas of 1 atm (24.5 L,
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298.15 K) to 1M solution. All natural bond orbital (NBO) analyses were performed using the NBO 3.0 package,58 and all electronic structure calculations were performed using Gaussian 09 program.59 The 3D optimized structures were displayed by CYLview visualization program.60
3. RESULTS AND DISCUSSION H2 cleavage initiates from intermediate (PhDPBPh)Ni(H2) (Figure 4), where the H2 is activated by nickel center through -coordination. This (PhDPBPh)Ni(H2) complex is located to be a tetrahedral structure, in agreement with previous study.50 The boron in (PhDPBPh)Ni(H2) is sp3 hybridized, indicating that the d10 Ni0 center contributes a pair of d electrons to boron’s empty p orbital, as expected for a typical LA-TM system. As shown in Figure 4, the H2 dative interaction and the d(Ni)σ*(H2) back donation both play an important role in H2 activation. The H–H bond length has been elongated to 0.824 Å in the (PhDPBPh)Ni(H2) complex, which is the evidence for d-σ* back donation.
Ph
Ph
Figure 4. Optimized ( DPB )Ni(H2) structure (left); dative interaction and nickel d-σ* back donation (right).
Figure 5. Mechanism of the out-of-plane (cis) homolytic mode for the LA-TM catalyzed H2 activation (PPh2 is simplified as P for clarity; M = nickel).
Cis Homolytic Mode. The mechanism of the cis homolytic mode of LA-TM catalyzed H2 activation is depicted in Figure 5. This mechanism involves two steps: 1)the H2 cleavage and 2) the hydride isomerized to final product PC. In the first step, as shown in Figure 5, H2 is cleaved via a pyramidal transition
state 1-TS, which exhibits out-of-plane cis cleavage character, i.e. the boron atom, is out of the H2 cleavage plane (M-H-H). The Hb-Ni-Ha-B and P2-Ni-Ha-B dihedral angles in 1-TS1 are 93° and 95°, respectively (Figure 6). Thus, boron severs as a Lewis acid to stabilize 1-TS1 by interacting with the axial filled dz2 orbital of nickel center. The calculated activation free energy of this step is 27.1 kcal/mol. IRC analysis found that this H2 cleavage via 1-TS1 leads to the cis dihydride intermediate 1-IM, which has a pseudo pyramidal structure (Figure 6). In 1-IM, the boron atom bridges to Ni-Hb Bond, where the NiHa distance is 1.473 Å, and the B-Ha and B-Ni distances are 1.363 Å, 2.397 Å, respectively. Such pyramidal intermediates, in which the boron Lewis acid binds to the M-H bond and the M-(μ2-H)-B orients in the metal dz2 orientation, have been observed previously in related systems.61-63 This cis dihydride NiII intermediate is less stable than RC by 27.4 kcal/mol. In the second step, hydride Ha transfers trans to Hb, via the isomerization transition state 1-TS2, resulting in the square planar product PC. The activation free energy for this process is 33.7 kcal/mol relative to RC. In PC, Ha is trans to Hb with the Hb-Ni-Ha = 166°. The Ni-B distance is 2.402 Å, the NiHa is 1.468 Å, and the Ni-Hb is 1.637 Å. These values indicate that Ha moves closer to boron atom and bridges between B and Ni, forming a Ni-(μ2-H)-B structure. Compared with the Ni-Ha bond, the Ni-Hb bond in Ni-(μ2-H)-B is shorter, which is in accord with classical Ni-(μ2-H)-B structures.40 The out-of-plane H2 cleavage observed in the cis homolytic mode can be well illustrated by NBO analysis. In 1-TS1, the Ha-Hb distance is 1.821 Å, and so this bond is essentially broken. Furthermore, the two localized Ni-H σ bonds are nearly totally formed, as can be seen in the orbital occupancies in Figure 7. NBO analysis along the reaction coordinates provides more details for the changes in orbitals in this cis homolytic H2 activation process, as depicted in Figure 7. Prior to H2 cleavage, the dx2-y2(Ni) and dxy(Ni) orbitals contribute prominently to H2 activation. The dxy(Ni) orbital occupancy drops from 1.87 to 1.83 (see SI), before increasing to 1.94, due to back-donation to the σ*(H2) orbital. More importantly, the dx2y2(Ni) occupancy drops continuously from 1.67 to 1.58, until it is eventually oxidized in generating the σ(Ni-H) bonds. Meanwhile the σ(H2) bond vanishes at the transition state region. The H2 cleavage driven by the dx2-y2(Ni) and dxy(Ni) orbitals is indicative of a typical homolytic process, where the Ni center is oxidized from Ni0 to NiII and the boron stays out of the xy plane. In the later stage of H2 cleavage, the Ni-H orbital interacts with boron’s empty p* orbital, generating a Ni-(μ2-H)-B structure. In the subsequent isomerization step, Ha moves to the trans position of Hb with the assistance of phosphorus. As shown in Figure 7, the σ(P-H) bond generates temporarily during isomerization, simultaneously with an increase in the dx2-y2(Ni) occupancy. In the late stage of isomerization, the Ni-Hb bond forms again as the σ(P-H) orbital becomes unoccupied. The cis homolytic mode induces significant changes in coordination geometry along the H2 activation pathway, from tetrahedral RC to pyramidal transition state 1-TS1, and finally to a square planar PC. Therefore, a cis homolytic mode may be prefered for LA-TM catalysts with structural flexibility.
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Figure 6. Key structures for the mechanism of cis homolytic mode (each phenyl group in phosphines is visualized as one carbon atom for clarity).
ligand field may lead to large deformation energy due to the existence of filled anti-bonding orbital (dz2) in the transition state. We cannot totally exclude that in some LA-TM cases, without a filled anti-bonding dz2 orbital, the trans homolytic mode may be possible, and the functionality of the Lewis acid is primarily to help stabilize the hydride in the product.
Figure 7. NBO analysis along the cis homolytic reaction pathway of H2 activation.
Trans Homolytic Mode. The mechanism of the trans homolytic mode for H2 activation is depicted in Figure 8. In this mode, H2 is cleaved via a pyramidal transition state 2-TS, in which the boron atom lies in the Ni-H-H cleavage plane. The dihedral angle Ha-Ni-Hb-B is 174°, as shown in Figure 8. The dihedral angle P1-Ha-P2-B is -8.6°, indicating that Ha lies in the Ni-P1-P2 plane, while Hb is located axial to the NiP1-P2 plane during the H2 cleavage. In 2-TS, the Ha-Hb distance is 1.964 Å, whereas the Hb-B distance is 3.122 Å, suggesting a homolytic H2 cleavage without the direct cooperation of boron. However, the boron atom remains interacting with metal center datively (dNi-B = 2.272 Å). This mode that directly leads to the trans dihydride product PC, has a high activation free energy of 41.5 kcal/mol. Therefore, this mode is unlikely to be feasible for the (PhDPBPh)Ni(H2) system, which is in good agreement with a previous study of the (PhDPBPh)Ni system.50 NBO analysis provides further insight into the in-plane character of the trans homolytic mode (Figure 9). From RC to 2-TS, dxz(Ni) orbital occupancy drops from 1.87 to 1.67, while the dz2(Ni) orbital occupancy decreases from 1.67 to 1.54. This indicates that dxz(Ni) and dz2(Ni) orbitals are involved in the H2 cleavage to generate new σ(Ni-H) bonds. Figure 9 also shows that the σ(H2) occupancy drops from 1.77 to 0 and σ(Ni-Hb) appears before 2-TS. dz2(Ni) orbital occupancy disappears after 2-TS, and this is accompanied with the formation of the σ(Ni-Ha) bond. The interchange of σ(Ni-Ha) and σ(B-H) bonds, and the disappearance of p*(B) orbital occupancy indicate the formation of the Ni-(μ2-H)-B bridge structure observed in the product PC. There is a significant change in the coordination geometry during the trans homolytic mode. The change of
Figure 8. Mechanism and 2-TS structure of the trans homolytic mode for the LA-TM catalyzed H2 activation (P denotes PPh2 for clarity; M = nickel).
Figure 9. NBO analysis along the trans homolytic reaction pathway of H2 activation.
Synergetic Heterolytic Mode: The transition state (3-TS) for synergetic heterolytic H2 cleavage is shown in Figure 10. This transition state exhibits a quasi-tetrahedral structure, with a B-Ni-Ha-Hb dihedral angle of 163°. This is similar to the in-plane character of homolytic transition state 2-TS. However,
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in the heterolytic 3-TS, the HaHb distance is 1.228 Å, indicating a relatively early transition state compared to 1-TS1 and 2TS. The HbB distance in 3-TS is 1.505 Å, shorter than that in 2-TS (3.122 Å). This means that the boron Lewis acid interacts synergetically with the Hb atom during H2 cleavage. The activation free energy of this mode is 29.7 kcal/mol, which is comparable to previous results (∆G‡ = 28.9 kcal/mol) for the (PhDPBiPr)Ni model.50
Figure 10. Mechanism and key structure for the synergetic heterolytic mode of LA-TM catalyzed H2 activation (P denotes PPh2 for clarity; M = nickel).
boron atom plays a very important role in the H2 cleavage. On the other hand, the dz2(Ni) occupancy drops from 1.67 to zero at 3-TS. As the dz2(Ni) orbital becomes unoccupied, another σ(Ni-H) bond is formed. Boron and nickel thus act synergeticallyin this H2 activation mode, leading to a tetrahedral transition state in which H2 cleavage is heterolytic, with the boron associated with both the nickel center and the Hb atom. It is reasonable to anticipate that this synergetic heterolytic mode is more likely to be preferred, if the LA-TM catalyst has an appropriate LATM distance and the penalty in the weakening of LA-TM dative bond is insignificant. Dissociative Heterolytic Mode. This dissociative heterolytic H2 activation involves two steps (Figure 12): 1) the insertion of H2 into the B-Ni bond; and 2) the H2 heterolytic cleavage by the dissociative boron atom and nickel center. In the first step, H2 inserts into the B-Ni bond via transition state 4-TS1 to generate tetrahedral intermediate 4-IM, which is saturated with a solvent molecule, benzene. As show in Figure 13, H2 is activated in 4-IM by the dissociative boron and nickel centers (dHa-Hb = 0.839 Å), which facilitates the subsequent H2 cleavage. As compared to RC, the Ni-B distance increases from 2.210 to 3.557 Å in 4-IM. The activation free energy of the first step is 34.7 kcal/mol. This mechanism is similar to that observed with frustrated Lewis pairs catalysts.2,11,42,64 In the second step, H2 is cleaved by dissociative Ni and B centers via transition state 4-TS2. In 4-TS2, dH-H is 1.189 Å, and the NiH and BH distances are 1.483 and 1.276 Å, respectively. The Ha-Ni-B-Hb dihedral angle is -8°, indicating that H2 cleavage occurs in the Ni-H-H-B plane. The activation free energy for the dissociative heterolytic H2 cleavage is 42.2 kcal/mol.
Figure 11. NBO analysis along the synergetic heterolytic reaction pathway of H2 activation. The distinctive synergetic character for this heterolytic H2 activation mode can be revealed by NBO analysis (Figure 11). From RC to 3-TS, σ(Ha-Hb) occupancy drops slightly from 1.77 to 1.64 before disappearing at 3-TS. At the same time, the occupation of the LP(Hb) orbital increases to ~ 1, which clearly indicates heterolytic H2 cleavage. After 3-TS, the LP(Hb) orbital becomes unoccupied, because the LP(Hb) interacts with the LP*(B) to generate the Ni-(μ2-H)-B bridge bond in PC. Thus the assistance of boron atom plays a very important role in the H2 cleavage. After 3-TS, the LP(Hb) orbital becomes unoccupied, and this is accompanied by the LP(Hb) interacting with the LP*(B) orbital. This interaction generates the Ni-(μ2-H)-B bridge bond in PC, wherein the assistance of
Figure 12. Mechanism of the dissociative heterolytic mode for the LA-TM catalyzed H2 activation (P denotes PPh2 for clarity;
M = nickel).
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Figure 13. Key structures for the mechanism of dissociative heterolytic mode (each phenyl group in phosphines is visualized as one carbon atom for clarity).
Figure 14. NBO analysis along the dissociative heterolytic reaction pathway of H2 activation.
This dissociative heterolytic mode provides an alternative avenue for LA-TM catalyzed H2 activation. NBO analysis was performed to gain further insight into this interesting mode (Figure 14). In the insert step, σ(Ha-Hb) drops from 1.76 to 1.62 to generate the intermediate 4-IM. NBO analysis shows that σ(Ha-Hb) is activated by LP*(B) in the 4-IM,while σ*(HaHb) is activated by dz2(Ni). In the H2 cleavage step, σ(Ha-Hb) drops from 1.63 to 1.58 before ultimately disappearing to generate a σ(B-Hb) bond. This is accompanied by the LP*(B) orbital becoming unoccupies. Concomitantly, dz2(Ni) drops from 1.73 to 1.58 before being oxidized to generate a σ(Ni-Ha) bond. H2 activation by this mode proceeds via a dissociative LA and TM structure, and therefore requires the LA-TM dative interaction being overcome. This could potentially be achieved by using a frustrated LA-TM catalyst, or by weakening the LATM directly. Overview of the Mechanism for (PhDPBPh)Ni Catalyzed H2 Activation. Having presented detailed mechanistic aspects of these four general H2 activation modes for LA-TM catalysts, we now discuss their relative feasibility for the [(PhDPBPh)Ni] system. The potential free energy surfaces for all H2 activation modes are summarized in Figure 15.
Figure 15. Free energy profiles of the four H2 activation modes catalyzed by the [(PhDPBPh)Ni] complex (P denotes PPh2 for clarity).
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Although these four types of H2 activation modes are, in general, possible LA-TM catalysts, the trans homolytic mode and dissociative heterolytic mode are less feasible for the [(PhDPBPh)Ni] system. As shown in Figure 15, Both modes have an activation free energy of around 42 kcal/mol relative to RC (2-TS, G‡ = 41.5 kcal/mol; 4-TS2, G‡ = 42.2 kcal/mol). In the case of the trans homolytic mode, the transition state has high activation free energy arises for two reasons. 1)The electronic configuration of metal center changes from d10 to d8 during H2 cleavage. The anti-bonding orbital dz2(Ni) is filled in pyramidal transition state 2-TS (Figure 8), and this leads to a large deformation energy for structure changing from tetrahedron to pyramid.65 2) The boron atom doesn’t assist the H2 cleavage (dB-H distance in 2-TS is too long). As for the dissociative heterolytic mode, it is less feasible because it requires overcoming the boron-nickel dative interaction, which is expected to be very strong in the [(PhDPBPh)Ni] system. This is evidenced by the fact thatthe intermediate 4-IM is already higher than RC by 34.5 kcal/mol after the dissociation of boron-nickel bond. The synergetic heterolytic mode and the cis homolytic mode are more favored for (PhDPBPh)Ni catalyzed H2 activation. The respective rate-determining activation free energies, relative to RC, are 33.7 and 29.7 kcal/mol (Figure 15). The synergetic heterolytic mode is relative more preferred by 4.0 kcal/mol. However, the cis homolytic mode is still kinetically competitive, with a lower H2 cleavage transition state (1-TS1, G‡ = 27.1 kcal/mol) than that of the synergetic heterolytic mode (3TS, G‡ = 29.7 kcal/mol). Due to the thermodynamic instablility of the cis dihydride intermediate 1-IM, its subsequent isomerization to the trans dihydride product PC requires overcoming slightly higher activation free energy (1-TS2, G‡ = 33.7 kcal/mol). Therefore, the synergetic heterolytic mode is the most plausible mechanism for (PhDPBPh)Ni catalyzed H2 activation, while the cis homolytic mode cannot be totally excluded. The synergetic heterolytic mode is favored because the H2 cleavage can be cooperatively assisted by Lewis acid (boron) and transition metal (nickel). Furthermore, the (PhDPBPh)Ni complex has an insignificant change in coordination geometry during the synergetic heterolytic H2 cleavage. Of these two homolytic activation modes, the cis homolytic mode is more favorable than the trans homolytic mode on account of electronic factors. Firstly, there is no filled anti-bonding d(Ni) orbitals in 1-TS1 (Figure 6), which is a very late transition state that can be regarded as a NiII species. Evidence can be seen in Figure 7 that the dx2 -y2 (Ni) is totally oxidized to be an empty anti-bonding orbital. Secondly, the filled dz2(Ni) orbital can be stabilized by the empty boron p orbital.
4. CONCLUSION In summary, a general H2 activation mechanism for the newly emerging LA-TM bifunctional catalysts has been elucidated. There are four typical H2 activation modes: (1) the cis homolytic mode through a cis dihydride intermediate; (2) the trans homolytic mode where H2 cleaves in the axial position to trans dihydride product; (3) the synergetic heterolytic mode, in which H2 cleaves cooperatively assisted by both the Lewis
acid and transition metal; and (4) the dissociative heterolytic mode, in which the Lewis acid and transition metal assist H2 cleavage in dissociative manner. The feature of each activation mode has been generalized by key transition state and natural bond orbital analysis. For the (PhDPBPh)Ni system, the calculated activation free energy is 33.7 kcal/mol for the cis homolytic mode, 41.5 kcal/mol for the trans homolytic mode, 29.7 kcal/mol for the synergetic heterolytic mode, and 42.2 kcal/mol for the dissociative heterolytic mode, respectively. Thus, the synergetic heterolytic mode is found to be the most plausible mechanism for (PhDPBPh)Ni catalyst, although the cis homolytic mode cannot be totally excluded. In contrast, the trans homolytic mode and dissociative heterolytic mode are less feasible for the [(PhDPBPh)Ni] system. Considering the structural diversity of LA-TM catalysts, it can be expected that other LA-TM systems may differ from [(PhDPBPh)Ni] and activate H2 via different scenarios. These results should provide a general mechanistic framework for the development and rational design of LA-TM catalysts in the future.
ASSOCIATIVE CONTENT Supporting Information. Complete ref. 59; details of orbital occupancy calculations, and Cartesian coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the NSFC Fundation (Grants 21203256 and 21473261), the Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2015A030306027). Computing facilities were supported in part by the high performance grid computing platform of Sun Yat-sen University, the Guangdong Province Key Laboratory of Computational Science and the Guangdong Province Computational Science Innovative Research Team. We thank Prof. Jonas C. Peters for very helpful discussions.
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