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Mechanistic Details of Ru-bispyridylborate complex catalyzed dehydrogenation of ammonia-borane: The Role of pendant Boron Ligand in catalysis Sourav Bhunya, Lisa Roy, and Ankan Paul ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02616 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

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Mechanistic details of Ru-bispyridylborate complex Catalyzed Dehydrogenation of Ammonia-borane: The Role of Pendant Boron Ligand in Catalysis †

Sourav Bhunya, Lisa Roy and Ankan Paul*

Raman Centre for Atomic, Molecular and Optical Sciences, Indian Association for the Cultivation of Science, Kolkata 700 032, India. †Current address: Department of Molecular Theory and Spectroscopy, Max Planck Institute for Chemical Energy Conversion, Stiftstraβe 3436, Mülheiman der Ruhr, 45470 Germany. ABSTRACT: The role of pendant boron ligands in ammonia-borane (AB) dehydrogenation have been investigated using hybrid density functional theory for two very efficient Ruthenium based catalysts, developed by Williams and co-workers (J. Am. Chem. Soc. 2011, 133, 14212–14215). Our findings reveal that the catalytic action initiates through opening of the labile metal-ligand bridging group associated with the boron based pendant ligand arm for both the catalysts. In case of the hydroxyl bridged catalyst, the ligand (B-OH moiety) backbone plays an active role along with the metal center to perform concerted dehydrogenation of ammonia-borane by overcoming free energy activation barrier of 24.3 kcal/mol and this dehydrogenation step is the rate determining step of the catalytic cycle. However, for the trifluoroacetate bridged complex, H2 is released in a stepwise fashion with active participation of the solvent. It involves formation of a

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boronium cation with rate determining free energy activation barrier of 23.7 kcal/mol for the solvent assisted BH bond breaking step while the pendant boron ligand acts as a spectator. Overall, our detailed theoretical study illustrates that the chemical nature of the pendant boron ligand is decisive in the AB dehydrogenation pathway. Further computational investigations indicate that higher equivalents of hydrogen from AB is released by the dual participation of free NH2BH2 and the Ru-catalysts. KEYWORDS (Ammonia-borane, Chemical Hydrogen Storage, Solvent participation, Metalligand cooperativity, boronium cation). INTRODUCTION: Metal-based catalysis with spectator ligands is a well-known phenomenon.1 However, bond activation via metal-ligand cooperativity where the ligand itself actively participates in the catalytic cycle has garnered much attention recently.2-8 In chemical and biological catalysis cooperative ligands take part directly in bond activation processes and undergo reversible structural transformations.2-10 Thus in many instances, it is the metal ligand synergistic cooperativity that facilitates catalysis. Transition metal catalysts enriched with ligands "cooperating" in catalytic activity are much popular now a days in the context of "green catalysis".11 So development of synthetic strategy for such "cooperative" catalysts and understanding their mode of action at the atomic level is important. Humongous amount of effort has been devoted to design homogeneous transition metal catalyst based on this principle for small molecule activation, hydrogen production by dehydrogenation of amine-boranes, N containing heterocycles, amines, alkane, alcohol etc and for performing organic transformations to yield the building block of natural products, pharmaceutical drugs, and functional materials.2-9


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Both experimentally and theoretically, attempts have been made extensively to identify the nature of metal ligand cooperativity of the known homogeneous transition metal catalysts that would expedite the design of new effective catalysts for the associated processes.2-9 In recent times pincer based metal complexes have generated a lot of interest as they largely exploit metal-ligand cooperativity for hydrogenation of organic compounds, water splitting and several organic transformations.2b-h Milstein and his co-workers have developed several Ru complexes which can hydrogenate CO2 derivatives, while others are responsible for splitting of water.2e Theoretical studies from the groups of Hall, Wang and Milstein have revealed the unique feature of these catalysts, where both the metal center and the associated ligand participate in the chemical processes that is heavily reliant on the de-aromatization and aromatization of the pincer ligand backbone.9a-p The other most effective and investigated metal ligand framework is metal-imido and metal-amido complexes which are used exclusively for CH bond activation and hydrogenation of unsaturated substrates respectively.2i These reactions involve reversible hydrogenation and dehydrogenation of the metal-nitrogen bond.2i Another type of transition metal complexes which features weak M

B interactions has been identified

to work well for heterodiatomic bond cleavage and small molecule activation.2m,3i,9q-t


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Scheme 1. Series of Boron pendant Ru complexes which are synthesized by Williams's group. Triflate anion is present as counter anion with these cationic Ru complexes.

In 2011,Williams's group reported the catalytic dehydrogenation of ammonia borane to yield polyborazylene with their in-house developed ruthenium based complexes.12 Scheme 1 illustrates the various Ru-bispyridylborate complexes (1, 1a and 1b) which catalyze dehydrogenation of AB (NH3BH3, 2).12 This is a promising development in the context of hydrogen storage where ammonia-borane has been proposed to be a potential candidate.4 Thereby, Williams' Rubispyridylborate complexes have generated a lot of interest owing to their resistance towards aerial oxidation and release of more than two equivalent of hydrogen (4.6 wt% H2) from AB.12 It is reported that among the three catalytic congeners with hydroxyl (1), acetate (1a) and trifluoroacetate (1b) linkages between Ru and B atom, 1b shows a faster catalysis compared to 1 and 1a. Hence, the authors claim that dehydrogenation of NH3BH3 occurs through the dual participation of the metal and its associated ligand.12 Additionally, the kinetic isotope effects indicate that the rate determining step of the catalytic cycle for complex 1 involves simultaneous BH and NH hydrogen cleavage.12 In an attempt to provide evidence for dual site cooperativity, a later contribution from the same laboratory have reported a similar type of Ruthenium bis(pyrazolyl)borate complex (1c) with a bridging imine group, which catalyzes hydrogenation of nitrile to produce primary amine in presence of sacrificial methanol and sodium borohydride as the dihydrogen source.13 Based on their experimental observations they concluded that boron and ruthenium centers work in concert: ruthenium as an activating group and boron as a hydride donor.13


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Despite several efforts from Williams and his coworkers, the mechanistic underpinnings of the Ru-bispyridylborate complex catalyzed dehydrogenation of AB (NH3BH3, 2) still remain elusive. What's more important is to explain the origin of metal ligand cooperativity. To gain insight about the role of pendant boron ligand in this regard, we have studied the ammonia-borane dehydrogenation processes catalyzed by complex 1 and 1b using density functional theory. Our computational results show that the synergistic coordination between Ru and ligand B-OH plays a key role in the dehydrogenation of 2 by complex 1, whereas complex 1b does not follow the metal ligand cooperative route for the dehydrogenation process. To be specific, complex 1b dehydrogenate 2 by step wise BH and NH activation via ion pair formation with the assistance provided by the ether solvent. In case of complex 1 the metal ligand cooperative pathway involves the reversible protonation and deprotonation of the ligand backbone (B-OH moiety). Additionally, the role of the Ru complexes in case of higher equivalent of hydrogen removal from ammonia-borane is yet to be spelled out. Earlier we have shown that for bulky imine, Nheterocyclic carbene and Ir(POCOP)H2 complex, borazine and polyborazylene is formed from successive dehydrogenation of cyclotriborazane (CTB) and B-(cyclodiborazanyl)-borohydride (BCDB) by the transition metal complex (Scheme 2a). Additionally, there are instances when in a metal free pathway CTB and BCDB undergo removal of hydrogen by concerted proton and hydride transfer to the in situ generated free aminoborane (NH2BH2), resulting in the formation of NH3BH3 (Scheme 2b).14 We have reported in our previous endeavor that the latter route is operative only when the dehydrogenation barrier of CTB and BCDB by the catalyst (metal catalyzed pathway shown in Scheme 2a) is significantly higher compared to the aminoborane triggered transfer dehydrogenation route (metal free pathway shown in Scheme 2b).14 Hence, for determination of actual route for 2nd equivalent hydrogen release from NH3BH3 we have


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compared the first dehydrogenation activation barriers of CTB and BCDB by both the Ru complexes and free NH2BH2.

Scheme 2. Two possible pathways for releasing higher equivalent of H2 from ammonia-borane. H2 release from CTB and BCDB by (a) direct action of transition metal catalyst and (b) metal free pathway via catalytic action of free NH2BH2.

COMPUTATIONAL DETAILS: All theoretical calculations are done using Gaussian09 package.15 Geometries of all the intermediates and transition states were optimized with the hybrid exchange B3LYP functional16 in conjunction with the empirical dispersion correction (the B3LYP-D variant, invoked with the keyword "Empirical Dispersion") as implemented in Gaussian09.15 During optimization Pople’s 6-31+G(d,p) basis set is used for all atoms except Ru which was described by the LANL2DZ basis set which is a combination of an effective core potential LANL2 for 28 core electrons and a


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valence double-ζ basis set for 16 valence electrons. All intermediates and transition states are optimized in condensed phase using the SMD solvent model.17 The actual solvent used for the dehydrocoupling is the mixture of diglyme and benzene in 11:1 ratio. We have used diglyme (ε = 7.2) as solvent for our theoretical calculation which can be considered to be a good approximation. Furthermore, harmonic frequency calculations at the same level of theory enabled us to characterize transition states (with one imaginary normal mode) and intermediates (all real normal modes). Although we have performed all geometry optimizations in presence of a polarizable continuum, entropy of each solute is obtained through Sakur-Tetrode equation which essentially treats the molecules as ideal gas.18a Although the transition of solute molecules from gas phase to condensed phase has little effect on the vibrational and rotational entropies, the translational entropy encounters a significant change. So we have accounted for the reduced entropy of every species in solution by computing solution entropy of each solute species scaled by an empirical factor 0.5 times of its entropy obtained from the rigid-rotor model.18b-d The free energy corrections after applying the scaling factor to the Sakur -Tertrode equation derived entropy were added to the total energy computed within the SMD model. This approximation is based on experimental results and used frequently for determination of free energies in case of predicting reaction pathways.19 The rate determining barriers were also calculated through an equivalent approach using the CPCM model (see S1 in the supporting information). Alternatively, reaction barriers were further obtained from difference of free energies estimated from single point solvent phase computations at the SMD/CPCM levels of theory with equivalent corrections to entropies of the involved species (see S1 in the supporting information). Extensive comparison of the reaction free energy barriers shows that the effect of entropic scaling on the barriers used


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for the computations using the SMD model is by and large similar to those obtained from the other approaches. The working equation for determining solvent phase free energy is supplied in supporting information. Solvent phase free energies were estimated for 343.15 K and 1 atmospheric pressure by using the "Temp" keyword available in Gaussian09 package. In the following text we have discussed the relative stabilities and reaction barriers in terms of solvent phase free energies at the B3LYP-D (SMD) level of theory. We have also checked the rate determining free energy barriers using different functionals to calibrate our used theoretical methods (see S2 in the supporting information).

RESULTS AND DISCUSSION: To understand the nature of metal ligand cooperativity and the origin of difference in the rate for the ammonia-borane dehydrogenation by the two cationic Ru-bispyridylborate complexes (1 and 1b), we first computed the overall potential energy surface of the dehydrogenation process by DFT calculations. I. Ammonia-boranedehdyrogenation by Catalyst 1. The computed ground state structure of 1 is shown in Figure 1. Theoretically predicted bond lengths of the cationic complex show good agreement with the crystal structure (see Figure1). The optimized structure of 1 show that the B atom has tetrahedral geometry. Moreover, NBO analysis of 1 shows that O atom and the hydrogen present in the bridging hydroxyl group of catalyst 1 are negatively (-0.826 au) and positively (+ 0.535 au) charged respectively. Due to the presence of this bipolarity, introduction of an ammonia-borane molecule in close proximity to the catalyst 1 give rise to an intermediate 3, formation of which is slightly exothermic (ΔG = 3.5 kcal/mol) with regard to the separated reactants. Optimized geometry of 3 (see Figure S1)


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Figure 1. Optimized structure of cationic hydroxyl bridged Ru-complex (1). Important bond distances of our theoretically predicted structure of 1 are compared with the crystallographic information supplied by Williams's group12. All CH bonds are omitted for clarity.


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Scheme 3. Schematic representation of the catalytic cycle involving proton triggered concerted dehydrogenation of NH3BH3 by catalyst 1. Metal coordinated three acetonitrile are omitted for clarity. The pendant boron ligand has been modified for clarity (shown in inset). In later schemes the same model has been used for pendant boron ligand.

suggests the presence of a strong hydrogen bond between lone pair of O atom in the bridging hydroxyl group and NH proton of 2 (bond distance is 1.88 Å) and a dihydrogen bond between OH proton of catalyst and BH hydride of ammonia-borane (bond distance is 2.18 Å).

Proton triggered concerted dehydrogenation of ammonia-borane by 1. Presence of a strong dihydrogen bonding between O-H proton of catalyst and B-H hydride of ammonia-borane inintermediate 3 indicates that the hydride present in ammonia-borane can be protonated by the O-H hydrogen and simultaneously the bridging oxygen atom can abstract another N-H proton to regenerate the catalyst. We located a transition state (Ts1) (see Scheme 3 and Figure S1) corresponding to the proton shuttling mechanism to produce H2 and NH2BH2 with a free energy activation barrier of 28.3 kcal/mol with regard to 3, the most stable intermediate in the reaction sequence.


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Scheme 4. Catalytic cycle for ammonia-borane dehydrogenation by cationic catalyst 1 through metal ligand participation by opening of Ru-OH-B bridge. Metal coordinated three acetonitrile groups are omitted for clarity. Concerted dehydrogenation via Metal Ligand Cooperativity. Williams et al. suggested a probable dual site participation of both the metal center and the boron atom in case of ammoniaborane (2) dehydrogenation by the Ru-complexes.12 So we checked the possibility of opening of the metal-hydroxyl bridge linkage by the influence of substrate NH3BH3 (2), which is hydrogen bonded with the hydroxyl O atom near the metal center. We did find a transition state, Ts1', involving the cleavage of the Ru-O bond along with the simultaneous activation of B-H (of 2)

Figure 2. Free energy profile of ammonia-borane dehydrogenation process by 1 through synergistic activation of metal and ligand.


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with free energy activation barrier of 18.8 kcal/mol resulting in the formation of intermediate 4 (See Scheme 4 and Figure S2 in the supporting information). Once Ru-O bond is cleaved, 3 converts to 4, which consists of a 3c-2e interaction between the B-H hydride of ammonia-borane with the metal center. Additionally NH proton of ammonia-borane is hydrogen bonded with the oxygen atom of the OH group ligated to the boron center (see Scheme 4 and Figure S2). Formation of 4 is endothermic in nature by 9.4 kcal/mol corresponding to the lowest intermediate in the reaction sequence i.e. 3. Optimized structure of 4 (see Figure S3) shows that N-H (hydrogen bonded with B-OH) and B-H (activated by metal) bond length of 2 bound to the catalyst is 1.05 and 1.28 Å respectively. Dual activation of B-H hydride and N-H proton of 2 bound to the catalyst in intermediate 4 by Ru center and O atom of ligand center prompted us to consider a probable concerted proton and hydride transfer from 2 to the respective centers on the Ru complex. Starting from 4, we have found that concerted hydride and proton shift from ammonia-borane moiety to metal and ligand fragment (B-OH) occurs via Ts2 (see Scheme 4 and Figure S2) with activation barrier of 24.3 kcal/mol. Removal of hydride and proton from ammonia-borane (2) and transfer to the metal and ligand site respectively leads to the formation of intermediate 5 (see Scheme 4 and Figure S2) where Ru-H is hydrogen bonded to the proton of H2O ligated to the boron center. Formation of 5 is an endothermic process with regard to 3 (∆G = 17.2 kcal/mol). In the following process, intermediate 5 is converted to an isoenergetic species, 5', where the proton of H2O molecule which participates in H-bonding is from 2 (see Scheme 4 and Figure S2). Compared to 1, B-O bond distance is increased after protonation in 5 and 5'. The acidic proton from the coordinated H2O molecule is transferred to the hydride of Ru center through Ts3 to yield 6, where H2 is bound to metal center in an ɳ2 fashion and B-O bond is shortened (see Scheme 4 and Figure S2). Free energy activation barrier of Ts3 is 18.0 kcal/mol


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and formation of 6 is endothermic by 19.2 kcal/mol. Thus the calculated barrier for transfer of proton from the coordinated water molecule to the metallic hydride turns out to be 19.2 kcal/mol. Consequently in a direct H2 release pathway via Ts4, the catalyst is regenerated starting from intermediate 6. Simultaneous dihydrogen release from the metal center and reformation of the Ru-O bond occurs with a free energy activation barrier of 28.2 kcal/mol (see Scheme 4 and Figure S2). Regeneration of the catalyst (1) and dihydrogen release is a thermo-neutral process as compared to the lowest energy intermediate, 3 (see Figure 2). Additionally, the catalytic channel can proceed through an alternative avenue from intermediate 6. Highly endothermic intermediate 6 is stabilized by 4.9 kcal/mol by forming a hydrogen bonded adduct (7) with the NH protons of an approaching ammonia-borane molecule via the-OH ligand on the B center (see Scheme 4 and Figure S3). Thereafter, simultaneous loss of dihydrogen from the metal center and Ru-O bond restoration takes place by passing through Ts5 and by surmounting a free energy activation barrier of 21.5 kcal/mol with respect to the most stable intermediate 3 in the reaction pathway (see Scheme 4, Figure 2 and Figure S3). After dihydrogen release and Ru-O bond is restored via Ts5, hydrogen bonded complex of 2 and 1 i.e. 3 is obtained (see Scheme 4). Our computed free energy barriers of Ts4 and Ts5 clearly show that dihydrogen is released via Ts5 where an additional molecule of 2 participates in the stabilization of high energy intermediate 6. So our computed free energy pathway for dehydrogenation of 2 by catalyst 1 using metal ligand cooperativity shows that the rate determining barrier is decided by the initial concerted transfer of hydride (to the metal center) and proton (to the –OH group of the pendant ligand) from the substrate to the catalyst (via Ts2) associated with an overall activation energy of 24.3 kcal/mol. Solvent assisted Stepwise Dehydrogenation. Alongside with the metal-ligand coordinated simultaneous removal of hydrogen from the substrate to the complex, we have also investigated


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a possible stepwise B-H and N-H activation of 2 by the catalyst 1, aided by the nucleophilic solvent molecule. In a recent article, Conejero and coworkers reported that boronium cation [(NHMe2)2BH2]+ and anionic Pt-H complex is formed as a result of B-H bond activation of amine-borane by a Pt-complex with assistance from the ether solvent.20 After that the in situ formed boronium cation, which is enriched with an N-H acidic proton, protonates the metallic hydride of the complex, leading to release of H2.20 We investigated a similar possibility of formation of boronium cation from 4 employing explicit solvent molecule (see Scheme 5).

Scheme 5. Solvent assisted step wise ammonia-borane dehydrogenation by catalyst 1. Relative free energy of intermediates and transition states are given in the unit of kcal/mol. Metal coordinated three acetonitrile are omitted for clarity.

However, to reduce computational cost, instead of using the actual bulky solvent moiety (i.e. diglyme) we have used a simpler variety, ethyl methyl ether. We have found that a cationic intermediate 8 (which consists of hydrogen bonding interaction between a boronium cation and a


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neutral Ru-H complex) is obtained by the nucleophilic attack of one of the lone pairs of oxygen atom (of ethyl methyl ether) at the B atom (of ammonia borane) via Ts6 (see Scheme 5 and Figure S4). The free energy barrier required to carry out this solvent assisted hydride transfer is estimated to be ~29 kcal/mol. Also, formation of this cationic intermediate, 8, is endothermic in nature by 19.8 kcal/mol with respect to the separated reactants, 3 and ethyl methyl ether. Optimized structure of 8 (see Figure S4) display two strong hydrogen bonding interactions: (i) between one N-H proton of boronium cation and O atom of B-OH ligand fragment and (ii) between the hydride present in the Ru center with another N-H proton of the boronium cation. In the succeeding step, through Ts7 (See Scheme 5 and Figure S4), N-H proton (δ+) of the in situ generated boronium cation is found to get deprotonated by the Ru-H hydride (δ-), forming H2 bound to the Ru center. This proton transfer process is attainable overcoming a large free energy barrier (30.3 kcal/mol). After deprotonation of the boronium cation, the resultant intermediate 6 releases H2 (bound to the metal center in a ɳ2 fashion) through Ts5 as shown previously (see ammonia-borane assisted H2 release pathway in Scheme 4). We have also speculated another possibility where the N-H proton is transferred to O atom present in the ligand fragment (B-OH) in a similar solvent assisted pathway. However, in spite of repeated attempts we were unable to identify any transition state responsible for the protonation of B-OH moiety by boronium cation. Hence, the rate determining barrier of this catalytic cycle involving boronium cation formation is predicted to be 30.3 kcal/mol which is associated with the deprotonation of the boronium cation at the neutral metallic hydride.


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Scheme 6. Solvent assisted B-O bond cleavage does not occur in case of boron pendant cationic Ru complex 1. Metal coordinated three acetonitrile are omitted for clarity.

We have also explored the possibility of ammonia-borane dehydrogenation by opening of B-O bond of the catalyst 1. Earlier Vedernikov and coworkers have shown that bridging methyl group between boron and Pt center of boron pendant Pt complexes can be reversibly exchanged by the nucleophilic attack of an ether solvent.21 This prompted us to investigate a similar cleavage of the B-O bond of catalyst 1 (shown in Scheme 6) with formation of a Ru-OH moiety. However, we found that unlike Pt complexes which undergo alkyl group migration between Pt and B, similar exchange of bridging oxo group between Ru and the pendant B atom of 1 is not feasible even by explicit solvent molecule participation. Overview of the mechanism of dehydrogenation of 2 by complex 1. We have examined three different types of ammonia-borane dehydrogenation pathways for complex 1: (a) proton triggered concerted pathway (shown in Scheme 3), (b) metal ligand cooperative pathway (shown in Scheme 4) and (c) solvent assisted stepwise B-H and N-H activation (shown in Scheme 5). The first pathway does not involve any direct participation of the metal center in the catalysis. The last two pathways involve one common initial step which is ammonia-borane triggered opening of Ru-O bond. Previously, ligand dissociation before substrate coordination to create a vacant active site on the metal in proximity to the substrate has been shown for C-H bond activation of benzene/aryl species catalyzed by Ir/Pd acetates and other complexes.22 In case of the second pathway, dehydrogenation of ammonia-borane by catalyst 1 occurs through


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synergistic coordination of metal and bispyridylborate ligand. More precisely, B-OH bond present in ligand framework of catalyst 1 is protonated and deprotonated reversibly during the catalytic cycle. Pathway (c) involves participation of solvent to yield a boronium cation and neutral metal hydride which releases H2 by a proton transfer from boronium cation to the metal hydride. In the last two pathways H2 release is facilitated by hydrogen bonding stabilization provided by additional ammonia-borane molecule. After considering all the possible pathways which leads to the formation of NH2BH2 and H2 from 2 by catalyst 1, it is evident that the metal ligand cooperative pathway involving active participation of another ammonia-borane molecule for opening and closing of Ru-O linkage has the lowest rate determining free energy activation barrier (see Table 1) and the most acceptable route. If we consider the metal ligand cooperative pathway, then the actual catalytic species is intermediate 3 as the dehydrogenation process initiates from this complex and it is regenerated on completion of the catalytic dehydrogenation cycle. Also, our in depth computational analysis reveal that the rate determining transition state (Ts2) along the aforementioned pathway has a kinetic isotope effect (KIE) for both N-H and B-H hydrogen in AB (ND3BH3: 1.33,NH3BD3: 1.32,ND3BD3: 2.18) which is in satisfactory agreement with the experimentally observed KIE.12

Table1. Rate determining free energy activation barriers of different possible pathways of ammonia-borane dehydrogenation by catalyst 1. Rate determining

Rate determining Free Energy

transition state

activation barrier (kcal/mol)


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Proton Catalyzed concerted

Ts1

28.3

Metal ligand cooperative pathway

Ts2

24.3

Solvent assisted pathway

Ts7

30.3

dehydrogenation pathway

Thus our detailed theoretical analysis confirms the synergistic involvement of metal center and pendant boron ligand in deciding the mechanistic conduit and lowering the overall activation energy for ammonia-borane dehydrogenation by hydroxyl bridged Ru-pendant boron catalyst (1). Williams et al. provided more evidence in support of the involvement of pendant boron ligand in the catalysis for these Ru-bispyridylborate complexes by reporting higher catalytic activity in case of trifluoroacetate bridged complex 1b, another Ru based catalytic congener of 1.12 Hence, we have also investigated the mechanistic details of ammonia-borane dehydrogenation by catalyst 1b which is accounted in the following section.

II. Ammonia-borane dehdyrogenation by Catalyst 1b. Like catalyst 1, catalyst 1b also forms a hydrogen bonded complex, 9, in presence of ammonia-borane (2) (see Scheme 7). Formation


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Scheme 7. Reaction pathway for opening of Ru-O bond and activation of BH hydride of NH3BH3 (2) in case of catalyst 1b. Metal coordinated three acetonitrile are omitted for clarity.

of 9 is exothermic by 2.1 kcal/mol in terms of free energy. Optimized geometry of 9 (see Figure S5) shows a hydrogen bond between O atom of boron bound trifluoroacetate group and NH proton of 2. As 1b does not have any proton like complex 1, the proton triggered concerted dehydrogenation of 2 is not possible. So initially we have investigated the metal cooperative pathway similar to catalyst 1. We found that BH hydride of 2 can displace the Ru-O bond present in 9 and form a hydride shared complex (10) with the metal through Ts8 (see Scheme 7 and Figure S5), akin to the splitting of Ru-O bond found earlier for complex 1 (see Scheme 4). Simultaneous opening of Ru-O bond and B-H activation has a free energy activation barrier of 19.6 kcal/mol. Formation of 10 is endothermic by 4.0 kcal/mol compared to the most stable intermediate 9 in the potential energy surface. Optimized geometry of 10 (see Figure S5) shows that one of the B-H bond of ammonia-borane is elongated due to the electrophilicity of the metal center while one N-H proton has a hydrogen bonding with the O atom of the B bound trifluoroacetate group. Next we have evaluated the possibility of concerted dehydrogenation of 2 by metal center and the boron bound trifluoroacetate group. But despite several attempts we failed to locate any transition state, where BH hydride of 2 is transferred to the metal center and NH proton of 2 is transferred to the O atom of trifluoroacetate group, which is directly bound to the ligand B atom. However, we have found that 10 can be transformed into a geometrical isomer 10' (see Figure S5) where NH proton of 2 is hydrogen bonded with the carbonyl O atom instead of B bound O atom of trifluoroacetate group. Formation of 10' is thermoneutral when compared to 9. We have found that BH hydride is transferred to Ru and NH proton is shifted to


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the carbonyl O atom of trifluoroacetate group (bound to the pendant B atom of ligand framework) via a concerted transition state Ts9 (optimized geometry is shown in Figure S5). The possibility of ammonia-borane dehydrogenation occurring by synergistic metal ligand cooperation was found to be improbable due to high activation barrier of 44.8 kcal/mol associated with Ts9 along this reaction path.


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Scheme 8. Schematic representation of the catalytic cycle of ammonia-borane dehydrogenation by solvent assisted ion pair formation pathway in case of catalyst 1b.

Figure 3. Schematic representation of potential energy surface of the ammonia-borane dehydrogenation by 1b through solvent assisted ion pair formation.


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In line with the pathways studied for complex 1, we have checked the viability of the solvent assisted BH hydride transfer process to boronium cation formation from 10 and thereby H2 release by the reactivity of the in situ formed neutral Ru-H and boronium cation (Detail mechanistic pathway is shown in Scheme 8 and the relative free energy profile associated with this pathway is shown in Figure 3). Initiated from 10, the calculated transition state of solvent assisted BH hydride transfer to the metal center and final product (consists of neutral metal hydride and boronium cation) are Ts10 and 11 respectively (see Scheme 8 and Figure S6). As shown in Figure 3, this step of the reaction requires a free energy activation barrier of 23.7 kcal/mol and is endothermic by 14.0 kcal/mol. Following the formation of 11, a proton transfer occurs from the boronium cation [(MeOEt)BH2NH3]+ to the Ru-H and as a result forms Ru bound dihydrogen in ɳ2 fashion (see Scheme 8 and Figure S6). Free energy activation barrier of this proton transfer transition state, Ts11, is 7.0 kcal/mol and formation of Ru-H2 complex (12) is thermo neutral with respect to 11(see Scheme 8 and Figure S6). Additionally during proton transfer free NH2BH2 is formed in the reaction mixture. Then H2 elimination from 12via Ts12 with an estimated free energy of 23.2 kcal/mol yields 13', an isomer of catalyst 1b (see Scheme 8). Starting with 12, formation of 13' and H2 is exothermic by 10.7 kcal/mol. Optimized structure of 13' (see Figure S6) shows that B and Ru center is bridged by the same O atom of the trifluoroacetate group, whereas B and Ru center coordinated by two different O atom of trifluoroacetate group in case of 1b. 13' can easily convert to another isoenergetic geometric isomer 13 (see Scheme 8). Among these three different geometrical isomers, 1b, 13' and 13, first one is the most stable compared to the other two by~ 4.5 kcal/mol. It is therefore likely that intermediate 13 isomerizes to the reactant 1b, via transition state, Ts13 (see Scheme 8), which is accessible at a moderate free energy barrier of 13.8 kcal/mol. As a result of this linkage


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isomerization process catalyst 1b is regenerated to carry out further dehydrogenations of ammonia-borane as discussed earlier in this section. As 13' is a geometrical isomer of 13, it will have to climb similar free energy barrier to isomerize to 1b. Catalysis by opening of B-O bond with the assistance of ether solvent. An alternative pathway (shown in Scheme 9), in which solvent assisted cleavage of B-O bond of the catalyst 1b


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Scheme 9. Schematic representation of the catalytic cycle of ammonia-borane dehydrogenation by solvent assisted B-O bond opening and subsequent concerted dehydrogenation pathway in case of catalyst 1b.

occurs was also investigated. B-O bond length in the optimized geometries of two geometrical isomers, 1b and 13, are 1.60 and 1.65 Å respectively (see Figure S5 and S7). So, the longest B-O bond length of 13 suggests that B-O bond dissociation by ether solvent is more facile for this isomer compared to 1b. In the preceding section we have shown that the reverse reaction of 1b to 13 via Ts13 (see Scheme 8) is a viable pathway owing to the moderate free energy of activation (~ 15 kcal/mol). Thus one may hypothesize that the B-O bond of 13 gets cleaved in presence of solvent molecule to yield 14 (see Scheme 9 and Figure S7), where trifluoroacetate anion is bound to the metal center and solvent molecule is attached to the pendant boron atom. Afterwards ammonia-borane forms a hydrogen bonded complex with trifluoroacetate moiety and subsequently displaces Ru-O(C=O)CF3 bond to yield a Ru-H(B) bond. Interestingly the dissociated trifluoroacetate anion group linger around the metallic complex while hydrogen bonded to the NH protons of the metal bond ammonia-borane molecule (see Scheme 9). As a result of activation of BH and NH hydrogens of NH3BH3, concerted hydride and proton transfer occurs to the Ru center and trifluoroacetate anion to form dihydrogen bonded Ru-H and trifluoroacetic acid which could form 13 after dihydrogen elimination (see Scheme 9). The rate determining step of the whole catalytic cycle is the Ru-O bond cleavage by the B-H hydride of NH3BH3 and it turns out to be 28.5 kcal/mol (Please see S4 in the Supporting Information for further details).


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Table 2. Comparison of free energy activation barriers for all possible mechanistic routes for the formation of NH2BH2 and H2 by dehydrogenation of NH3BH3 in presence of catalyst 1b.

Rate determining

Rate determining Free Energy

transition state

activation barrier (kcal/mol)

Solvent assisted boronium cation

Ts10

23.7

Metal ligand cooperative pathway

Ts9

44.8

Solvent assisted B-O bond opening

Ts15

28.5

formation pathway

pathway

Overview of the mechanism of dehydrogenation of 2 by complex 1b. After considering all the possible mechanistic conduits leading to the dehydrogenation of ammonia-borane by complex 1b, we conclude that the solvent assisted pathway involving a boronium cation is the most acceptable one (see Table 2). In case of complex 1b the role of the pendant boron ligand is only limited to serve the substrate responsive opening to create a vacant coordination site in the metal center for ammonia-borane activation. Unlike the previous example of complex 1, pendant boron ligand (B-COOCF3) of 1b does not directly take part in the catalysis. Probably the difference in reactivity of the two complexes stems from the low proton affinity of the B-COCF3 moiety of catalyst 1b compared to that of B-OH moiety of catalyst 1 which renders the metal ligand cooperative catalytic channel to be inactive for catalyst 1b during ammonia-borane


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dehydrogenation. The role of the nucleophilic ether solvent is crucial for boronium cation formation and subsequent dihydrogen release process as it acts both as a good nucleophile and asa good leaving group respectively in those steps. In the preceding section we have identified the mechanistic principle of ammonia-borane dehydrogenation by the Ru-bispyridylborate complexes, 1 and 1b. Williams and coworkers have reported that 1 and 1b forms selectively borazine and polyborazylene as main BN dehydrocoupled product.12 After dehydrocoupling of ammonia-borane by metal ligand cooperative pathway (for complex 1, shown in Scheme 4) and solvent assisted boronium cation formation pathway (for complex 1b, shown in Scheme 8), NH2BH2 is released in the solution. The interaction of NH2BH2 with these complexes determine the fate of these free NH2BH2 (i.e. whether NH2BH2 oligomerizes to CTB and BCDB via solvent assisted oligomerization pathway or produce linear polyaminoborane via metal catalyzed polymerization route).23 In case of complex 1 the bifunctional site of the metal complex is generated in presence of NH3BH3 and closed after dihydrogen release, so it will be inert towards any complex formation with NH2BH2 which initiate metal catalyzed oligomerization process. Similar conclusions can be drawn for complex 1b. Also, we have found that opening of Ru-O bond (of catalyst 1) by NH2BH2 involves higher free energy activation barrier compared to the RDB of solvent catalyzed CTB and BCDB formation (see S5 in the supporting information for detailed results). These results indicate that the free NH2BH2 generated in the solution (after 1st equivalent hydrogen release from NH3BH3) will produce CTB and BCDB as main BN oligomer. So the additional hydrogen molecules will be released by subsequent dehydrogenation of CTB and BCDB. In the next section we have demonstrated the role of complex 1 and 1b in removal of higher equivalent of H2 from NH3BH3.


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III. Removal of 2nd equivalent hydrogen from ammonia-borane by catalyst 1 and 1b. We previously reported that any transition metal catalyst, which trigger the release of more than one equivalent of hydrogen from 2, do so by dehydrogenating the in situ generated BN oligomeric species, like cyclotriborazane (CTB), B-(cyclodiborazanyl)-aminoborohydride (BCDB) in two possible ways14 (see Scheme 2) (i) metal catalyzed pathway: direct dehydrogenation of CTB, BCDB by transition metal complex to release H2 or (ii) metal free pathway: dehydrogenation of CTB, BCDB by in situ generated "free NH2BH2" to yield NH3BH3. Transition metal catalysts perform dehydrogenation of those NH3BH3 to complete H2 removal and NH2BH2 regeneration. We have considered both pathways to identify the actual mechanism of 2nd equivalent hydrogen release from ammonia-borane (2) in case of catalyst 1 and 1b. We have theoretically computed the free energy activation barrier of first dehydrogenation of CTB, BCDB by both the complexes (1 and 1b) and compared them with the free energy barrier required for NH2BH2 to carry out the same dehydrogenation process (see S6 in the supporting information). The comparison is summarized in the table below.

Table 3. Comparison of free energy barriers for 1stdihydrogen transfer from BN heterocycles CTB and BCDB via metal free and metal catalyzed pathway for both the Ru-catalyst (1 and 1b). (* These barriers are calculated by accounting the thermodynamic stabilization obtained from the hydrogen bonded intermediate formation by the catalyst and the substrate (CTB and BCDB))

Saturated BN

Free energy barrier of 1stdihydrogen transfer

heterocycle used for

(kcal/mol)


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dehydrogenation

Metal free pathway*

Metal catalyzed pathway

Complex 1

Complex 1b

BCDB

14.0

25.8

CTB

19.5

32.5

BCDB

11.6

20.3

CTB

17.6

27.0

Comparing the rate determining free energy activation barriers associated with the first dehydrogenation of CTB and BCDB by NH2BH2 and cationic catalysts (1 and 1b) associated with the transfer dehydrogenation pathway (involving free NH2BH2) and metal catalyzed pathway respectively, we have found that former has a low energy route (see Table 3). So 2nd equivalent hydrogen of NH3BH3 is released through the combined catalytic activity of NH2BH2 and cationic Ru-catalysts(1 and 1b) (metal free pathway in Scheme 2). Our studies reveal that the Ru-catalysts have little role in dehydrogenation of BN oligomeric species like CTB and BCDB to form Borazine and higher oligomers like Polyborazylene as the transition metal catalysts do not directly dehydrogenate those BN oligomeric species. Hence, our results suggest that an ideal catalyst for ammonia-borane dehydrogenation is one which only performs the dehydrogenation of NH3BH3 and does not interact with the in situ generated BN oligomers which leads to low equivalent of hydrogen release channel such as polyaminoborane formation pathway14,23b. Presence of low barrier channels for catalyzed linear polyaminoborane formation would impede the performance of a catalyst with respect to higher equivalents of hydrogen removal from


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ammonia-borane as it gets rid of NH2BH2, the presence of which is crucial for formation of borazine and polyborazylene. CONCLUSION: The mechanism of 1st and 2nd equivalent of H2 release from ammonia-borane and cyclic BN oligomers (CTB and BCDB) respectively by two Ru-bispyridylborate complexes, 1 and 1b, has been portrayed computationally. We have reached to the conclusions which are mentioned below from our obtained results of this in depth theoretical studies(i) For both the complexes ammonia-borane first forms a hydrogen bonded complex with the bridging O atom via NH proton (3 and 9 for complex 1 and 1b respectively). Then a simultaneous Ru-O bond opening and BH activation process takes place (via Ts1' and Ts8 for complex 1 and 1b respectively). The uniqueness of these catalysts is that the active site of the complex is generated during the course of the reaction in presence of substrate and after completing the dehydrogenation cycle the active complexes restores back into their inactive form. Probably due this unique substrate selectivity these catalysts are highly effective in case of ammonia-borane dehydrogenation. Due to this substrate selectivity these complexes does not interact with free NH2BH2 to yield linear polyaminoborane and results higher equivalent of dihydrogen release. (ii) Complex 1 dehydrogenates ammonia-borane via concerted dehydrogenation with the involvement of both metal center and ligand B-OH moiety (via Ts2). After dehydrogenation RuH and B-OH2 is produced. This later generates the metal bound H2 after intermolecular proton transfer. Proximity to another NH3BH3 molecule triggers dihydrogen release from the highly endothermic metal bound H2 complex in a low barrier route.


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(iii) Complex 1b does not follow the metal ligand cooperative pathway. Due to low proton affinity of the B-(OCOCF3) group, reversible protonation and deprotonation of the ligand fragment is not possible. (iv) Complex1b dehydrogenates ammonia-borane by stepwise BH and NH activation. Initially solvent assisted hydride transfer from B to Ru produces an ion pair 11, which consists of dihydrogen bonded Ru-H and a boronium cation (NH3BH2-glyme)+. Thereafter protonation of Ru-H by the newly generated boronium cation leads to the formation of a dihydrogen bound metal complex. H2 release from Ru center produces an isomer of the catalyst (13) which latter regenerates the catalyst via a low energy isomerization process (via Ts13). Thereby it can be concluded that solvent plays a key role in the catalytic cycle of complex 1b. (v) Dehydrogenation of 2 catalyzed by catalyst 1 has rate determining barrier of 24.3 kcal/mol associated with Ts2, concerted proton hydride transfer to ligand and metal center. Whereas, the same catalytic dehydrogenation is achieved by catalyst 1b by overcoming a free energy barrier of 23.7 kcal/mol for Ts10 which happens to be the solvent assisted hydride transfer pathway. So RDB of the two catalytic cycles indicate that catalysis by complex 1b will be faster compared to 1. Our theoretical results are in satisfactory agreement with the experimental results obtained by Williams and coworkers. (v) We have also found that higher equivalent of hydrogen from ammonia-borane is released via the metal free pathway (see Scheme 2b) involving transfer dehydrogenation of CTB and BCDB by free NH2BH2 and subsequent dehydrogenation of NH3BH3. So the requirement of these transition metal catalysts in ammonia-borane dehydrocoupling process is for performing the transformation of NH3BH3 to NH2BH2 and H2 only. ASSOCIATED CONTENT


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Supporting Information. Details of methodology adapted for determination of free energy barrier in solution phase. Rate determining barriers for catalyst 1 at different level of theory. Optimized geometries of important intermediates and transition states. Details of catalytic cycle related to ammonia-borane dehydrogenation by complex 1b via opening of B-O bond with the assistance of ether solvent. Interaction of NH2BH2 with the catalyst to determine the fate of free NH2BH2. Details of 2nd equivalent H2 removal from NH3BH3. Coordinates of important optimized intermediates and transition States. This material is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. Funding Sources S.B. wants to thank CSIR-India for his research fellowship. A.P. would like to thank DST, India, for providing financial support through “Fast track” project (No. SR/FT/CS-118/2011). REFERENCES 1. (a) Beller, M., Bolm, C., Eds. Transition Metals for Organic Synthesis; Wiley-VCH: Weinheim, 2004; (b) Choi, J.; Roy MacArthur, A. H.; Brookhart, M; Goldman, A. S. Chem. Rev.2011, 111, 1761–1779; (c) Diez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev.2009,


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