A Remarkable Avenue in the Reactivity of Molecular Hydrogen

Dihydrogen Catalysis: A Remarkable Avenue in the Reactivity of Molecular Hydrogen

Rubik Asatryan and Eli Ruckenstein

Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York, USA Molecular hydrogen is the simplest and most abundant compound in the universe and is involved in numerous industrial chemical processes. In conventional chemistry, dihydrogen typically plays the role of a reductant and a reagent for homogeneous and heterogeneous hydrogenation processes such as the industrial and enzymatic ammonia formation, reduction of metallic ores and hydrogenation of unsaturated fats and oils. However, there are also processes in which molecular hydrogen participates as promoter, and even as catalyst. The catalytic role of the dihydrogen in free-valence migration in irradiated polymers and the interstellar isomerization of the formyl cation (protonated carbon monoxide) are well-documented examples of such processes. Recently, this issue has received new attention. Dihydrogen has been shown to play the role of a dehydrogenation catalyst (involving particularly metallocomplexes and inorganic materials), a relay (pass-on) transfer molecular agent and a transporter of protons. This review article, combined with original results, is focused on the mechanisms of the chemical processes where dihydrogen demonstrates catalytic behavior. We will call these processes (with somewhat broader meaning of the term) “dihydrogen catalysis” (DHC) which also includes the reactions mediated by transition metal dihydrides. Dihydrides are tentatively considered as pre-activated dihydrogen, coordinated to a metal center or implanted into a solid surface/support. DHC reactions are classiﬁed into ﬁve major reaction types: (i) dihydrogen-assisted relay transport of H-atoms (H2-RT); (ii) dihydrogen-assisted stepwise relay

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 transport of H-atoms or of a free valence (sH2-RT); (iii) dihydrogen-assisted proton transport (H2-PT); (iv) dihydrogen-assisted dehydrogenation (H2-DeH); and (v) pre-activated dehydrogenation (PA-DeH). The classiﬁcation of these mechanisms is

Received 10 January 2014; accepted 30 July 2014. Address correspondence to Rubik Asatryan, Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Amherst, NY 14226-4200. E-mail: [email protected] Color versions of one or more of the ﬁgures in the article can be found online at www. tandfonline.com/lctr. 403 404 R. Asatryan and E. Ruckenstein

based on a detailed analysis of numerous potential energy surfaces studied by DFT and ab initio methods in conjunction with available experimental data. The H2-RT, H2-DeH, and PA-DeH processes occur via cyclic transition states. The relay H2-RT transport involves the H-H-H triad linked to both H-donor and H-acceptor centers, whereas the transition state ring in the H2-DeH dehydrogenation processes involves a H-H-H-H tetrad with the dihydrogen catalyst located in the middle. The H2-PT mechanism provides the transport of a proton mediated by dihydrogen combined in + a triangular (H3 )-carrier unit. There are also practically important processes stimulated by dihydrogen such as the hydrogen spillover and hydrogen build-up in electronics, in which the catalytic role of dihydrogen is ambiguous, either because of the uncertainties in mechanisms, or pre- vailing traditional views. Some examples are brieﬂy discussed in the framework of the concept of dihydrogen catalysis, some being provided with theoretical support (in part calculated by us), and others being merely hypothesized to provide suggestions to an interested reader.

Keywords Hydrogen assisted reactions, Dehydrogenation, Hydrogen atom transfer, Proton transport, Two-state reactivity

1. INTRODUCTION

Molecular hydrogen (dihydrogen, H2) is the simplest and most abundant compound in the universe. It is involved in a variety of terrestrial and extraterrestrial processes and may become a future carrier of clean energy. Elemental hydrogen is an essential reactant in numerous industrial and labo- ratory synthesis, and an important ligand in coordination and organometallic chemistry (1–11). The hydrogenation is a cornerstone of the conventional homogeneous catalysis, in which H2 is oxidatively added to unsaturated substrates, such as unsaturated fats and oils (7–11). Large amounts of dihydrogen are involved in petroleum and chemical industries, in hydrodealkylation, hydrodesulfurization, hydrodenitriﬁcation (hydrodenitrogenation), and hydro- cracking processes (7,8). H2 is employed in the “upgrading” of fossil fuels and in the production of ammonia (6). In all these processes molecular hydrogen is typically viewed as a compound which undergoes catalysis—an agent for hydrogenation and reduction

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 processes. However, dihydrogen can itself stimulate and catalyze a variety of processes. Few of them are well documented, but many can be listed. The macroscopic behavior of H2 and intermediates are typically described using integrated reaction mechanisms and global kinetic schemes, mainly because of the lack of detailed elementary reaction mechanisms and the difﬁculties in identiﬁcation the actual role of dihydrogen. Thus, the development of elementary reaction mechanisms of the H2- assisted processes opens a new avenue in the chemical reactivity area of dihydrogen. Dihydrogen Catalysis 405

The promoting role of molecular hydrogen has recently received novel attention (5,12–27). Dihydrogen has been recognized to play the role of a conventional catalyst (5,15,19), pass-on transfer molecular agent (17,18), and transporter of protons (16,21–27). Some reaction rates are remarkably increased by dihydrogen, for instance the conversion of isoformyl cation (HOC+) to its more stable formyl isomer (HCO+)withHCO+/HOC+ abundant ratios ranging from 360–6000 (21–28). The reaction has been a critical proving ground for both theory and experiment since its detection in the cold interstellar clouds in 1983 (22). The observed isomerization rate constant of 3.8 × 10−10 cm3 s−1 at 25K (16b) suggests the + absence of any significant barrier on the lowest adiabatic H3CO potential energy surface, in sharp contrast to the high activation barrier of the unimolecular rearrangement (40 kcal mol−1)(28). The mystery of the formyl ion, the most easily detectable molecule in the interstellar medium, after CO, has been resolved only when the dihydrogen catalysis phenomenon was discovered (23,26). H2 reduces dramatically the isomerization barrier (by almost 40 kcal mol−1, vide infra). Thus, the process can occur under the cold interstellar conditions (16,21–27). Due to its high mobility and small size, the apolar dihydrogen facilitates also various condensed phase processes such as the migration of a free valence in irradiated polymers and oligomers. The process is believed to occur via the relay transport of H-atoms. The catalytic decay of macroradicals and the migration of a free valence in irradiated polyethylene have been the first recognized catalytic activities of H2 since the early 1960s (14). This article is focused on the chemical processes and mechanisms in which dihydrogen plays a promoting role and behaves as a catalyst; it contains also significant original results. Previously, we employed the term “Dihydrogen Catalysis” (DHC) for such processes with a somewhat broader meaning of the word “catalysis” by involving also some closely related processes (5,12,13,17). Here, we suggest for the first time a classification of the main DHC mechanisms and provide relevant examples. A significant part of the mechanistic details presented here are based on theoretical conclusions mainly because of the average character of the ear- Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 lier experimental results based on non selective measurements for direct and catalyzed by H2 processes. Many spectroscopic techniques applied to heterogeneous catalysis, with H2 being a co-catalyst, are not enough sensitive to distinguish the active surface species; particularly, those located on high-surface-area oxides with large band gaps. The rates of reactant consumption or product formation provide only average activities of the surface structures (18), thus preventing development of the experimentally-based detailed mechanisms for hydrogen mediated surface reactions. The traditional measures of activity failed, for 406 R. Asatryan and E. Ruckenstein

instance, to distinguish the activities of alumina-supported vanadium oxide species because of the intrinsic defects and the surface OH-groups. Only quite recently, Kim et al. (18) identified the structure-dependent reactivities of the vanadium oxide catalyst species and discriminated the role of the dihydrogen catalysis (H2-RT pathway, vide infra) using species-selective UV Raman spectroscopy in conjunction with DFT-calculations (see Section 2.2.2). The results suggest a synthesis strategy for the oxidative dehydrogenation catalysts to produce alkenes, chemicals of significant commercial importance. This article discusses only general features of the dihydrogen catalysis and provides several illustrative examples. The potential contributions of DHC to various practically important processes, as well as a detailed analysis of the organometallic DHC-reactions specified in the next section, will be the subject of a separate publication, which is currently in preparation. It will include particularly an ongoing cluster analysis of the interaction of H2 with Pd/SiO2 catalyst, which is relevant to the spillover processes, POSS-coated Pd- nanoparticle catalysis, and the growth of the palladium silicide nanowires, as well as a novel DHC-based mechanism for the initial-step activation of N2 by nitrogenase enzyme (12b).

2. CLASSIFICATION OF DIHYDROGEN CATALYSIS MECHANISMS

Five major reaction schemes are outlined, which involve dihydrogen as catalyst, presented in Eqs. (2.1.1)–(2.1.5) (dihydrogen is denoted as D2):

i. dihydrogen-assisted relay transport of H-atoms (H2-RT)

ii. dihydrogen-assisted stepwise-relay transport of H-atoms/free valence (sH2-RT) Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

iii. dihydrogen-assisted proton transport (H2-PT) Dihydrogen Catalysis 407

iv. dihydrogen-assisted dehydrogenation/hydrogenation (H2-DeH)

v. pre-activated dehydrogenation (PA-DeH)

where X and Y are H-donor and H-acceptor ligands, respectively, and M is a transition metal (TM) that involves ancillary ligands. The D2 notation is used to discriminate between the reactive hydrogen atoms. The H2-assisted relay and the stepwise relay H-transport mechanisms produce a proximal or a distal (remote) transfer of H-atoms (Eqs. (2.1.1) and (2.1.2)), described in detail in Sections 2.2 and 2.4. Typical examples of H2-catalyzed proton-transport reactions (2.1.3) are presented in Section 2.3. The dihydrogen assisted dehydrogenation mechanism (2.1.4) and its organometallic version (2.1.5) produce a reductive elimination of two hydrogen molecules; the mechanistic details of such processes are described in Sections 2.5 and 2.6, respectively. Equation (2.1.5) provides a tentative mechanism for the dehydrogenation of a substrate-assisted by pre-activated transition-metal-hydridic atoms. It is an organometallic version of reaction (2.1.4) that involves a dihydride/dihydrogen complex as donor of “activated”-H2. Some basic features of the above mechanisms have been examined previously (5,12–18,20,21,29–31). Here they are classiﬁed and systematized, additional mechanisms suggested, and distinct types of H2-stimulated reactions and the interrelations between mechanisms discussed. Characteristic transition state (TS) structures are presented in Scheme 1. Reactions (2.1.1), (2.1.4), and (2.1.5) occur via cyclic TS involving both hydrogen donor and acceptor centers. The ring-size of TS and the corresponding barrier heights depend on the nature of the constituents which can be neutral molecules or ions of various multiplicities, as well as ion-radicals. Reaction (2.1.1) involves a hydrogen-

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 atom-triad H-H-H as a common constituent (17). The transition state of + reaction (2.1.3) involves the HD2 -triangle as the catalytic proton- transport- ing unit, whereas reaction (2.1.2) constitutes a combination of two regular H-abstraction processes. The pre-activated dehydrogenation mechanism constitutes a particular topic of dihydrogen catalysis. It involves dihydrides and H2-complexes of transition metals with pre-activated dihydrogen units, and monohydrides as catalysts (Eq. (2.1.5)). This reaction mechanism constitutes a speciﬁc issue, 408 R. Asatryan and E. Ruckenstein

Scheme 1: Classiﬁcation of DHC reactions by transition state structures. The radical and ionic structures are chosen to represent key features of the sH2-RT and H2-PT processes. X and Y are hydrogen (proton) donors and acceptors, and M is a transition metal involving ancillary ligands.

and will be considered in a future publication. Some details can be found in Asatryan et al. (5). The termolecular bond-exchange reactions are prototypes of molecular hydrogen catalyzed reactions involving the cyclic transition states presented below.

2.1. Cyclic Hydrogen-Atom Exchange

The termolecular thermo-neutral reaction 3H2 ←→ 3H2 constitutes the simplest model of H2-assisted dehydrogenation reaction (2.1.4). Distinct products are formed in this reaction only when different hydrogen isotopes are involved (see Eq. (2.1.6)). The H-exchange reaction proceeds through a symmetric hexagonal transition state with D6h symmetry without breaking the H-H bonds (29). The hexagonal conﬁguration was found to be larger by 68.7 kcal mol−1 than −1 the isolated 3H2 asymptote and by about 39 kcal mol lower than the +

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 2H2 2H asymptote, which involves the dissociation of a hydrogen molecule. The calculations have been carried out at the DZPD+CI ab initio level (29,32).

Because of the substantial stability of the H6 conﬁguration compared to the H-H bond dissociation, LeSar and Herschbach suggested that the solid molecular hydrogen might undergo a high pressure transition to a new Dihydrogen Catalysis 409

phase involving termolecular complexes before the transition to the atomic or metallic phase (33a). This suggestion is in agreement with the structural features of the high-pressure hydrogen (33b,c), viz. the hexagonal configuration of hydrogen molecules in the graphene-like layer of the phase-IV hydrogen recently discovered by Gregoryanz and co-workers (33b). In addition, a proton transfer has been identified between the graphene-like and H2 molecular layers of phase-IV hydrogen accompanied by the rotation of the three molecular rings (33d,e). Based particularly on the spheroidal box model of LeSar and Herschbach (33f), Hoffmann and co-workers further evaluated the effect of pressure on intramolecular and intermolecular H-H distances in dense hydrogen using B3LYPDFTandMP2ab initio methods (34b). A numerical experiment based on the H6-model suggested a “chemical effect” involving an increase in the pop- ∗ ulation of H2 σ u orbitals upon compression and a depopulation of σ g orbitals acting to elongate the H-H bonds. The pressure effect has been compared to the effect of a transition metal in η -H2 metallocomplexes, which is particularly important in DHC catalysis, as indicated above (see Eq. (2.1.5) and ref. (5) for more details). Kinetic calculations for some termolecular reactions show that, despite the reduced probability of termolecular collisions (2.1.6) as compared with bimolecular ones, the six-center processes with vibrationally excited reactants are consistent with the experimental data (33g). A special form of “vibrational catalysis” may become possible if a substantial part of the energy needed to surmount the potential barrier goes into vibration of the H2 or D2 regenerated by the six-center reaction. A rapid vibration–vibration energy transfer in bimolecular collisions of these molecules with the ambient gas can raise the vibrational temperature significantly and hence accelerate the overall exchange rate. Herschbach suggested testing the termolecular mechanism by looking for this effect in future experiments (33g). The underlying physical principles behind the exchange reactions are the Woodward-Hoffmann orbital symmetry rules: the cycloaddition reactions involving 4n-electrons (H4-exchange) are forbidden as concerted processes whereas those involving 4n+2 electrons are thermally allowed (29,34a). The six-center, termolecular reaction of 3H2 is allowed, whereas the hydro- Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 gen exchange reaction of 2H2 is forbidden by the orbital correlation rules (29,32,35,36). The last statement is strictly relevant for H4 exchange via a D4h transition state, whereas the exchange is allowed through a C2h reaction coordinate and a rhombic transition state (D2h), as well as from trans to the rhombic arrangement (36). The H6 bond-exchange also obeys this rule only through the configuration interaction, not because of the typically utilized nodal properties (29). The degenerate H6 exchange process has been studied in numerous theoretical studies mainly directed to improve the consistency of the theory 410 R. Asatryan and E. Ruckenstein

with inconclusive experimental results on the activation energy of the H/D- exchange (29,32,35–41). A review of the earlier investigations can be found in references (17). Taylor et al. refined calculation of the 3H2-exchange barrier and obtained the value of 67.4 kcal mol−1 at the CASSCF level, and 66.5 kcal mol−1 when the multireference Davidson’s correction was employed (32). A somewhat higher value (68.8 kcal mol−1 for the activation enthalpy at 0K) was obtained by Schleyer and co-workers at the QCISD(T)/6-311++G(d,2p) level of the theory for MP2 geometries (37). As noted in (17), the popular moderate basis-set DFT method B3LYP/6-31G(d,p) predicts a barrier of 68.4 kcal mol−1, whereas the aug-cc-pVDZ correlation-consistent basis set reduces the barrier to about 61.0 kcal mol−1. The quite large “enthalpy of concert” (the difference of 28–30 kcal mol−1 between the H-H bond enthalpy and the concerted-hydrogen exchange activation barrier) has been assigned to the partial aromatic character of the benzene-like H6 ring (32,36–41). A degenerate bond-exchange also occurs in other similar homo- and het- eroligand systems such as (HF)3,(HCl)3 via similar hexagonal (albeit more polarized) transition states (41–43). Some of the low-barrier reactions have been in agreement with the liquid and gas-phase experiments and provide a basis for the explanation of the cyclic termolecular reactions (33,41–43). Following the seminal work of Fukui and coworkers on the dynamics of the double proton exchange in the formamidine-water system (43), many reactions and transition states have been identified in which a solvent (often water) becomes a positive participant (44,45,49). Morokuma and Muguruma demonstrated the catalytic role of the second H2O for the gas-phase reaction SO3+H2O(45). The intermolecular rearrangement process lowered remarkably the potential energy barrier, thus becoming a basic model of the double proton transfer mechanism in biochemical processes. The ionic dissociation of HCl has been proposed to occur via a proton relay reaction between water molecules subsequent to the H-transfer from HCl to a water molecule (46). In addition, a concerted low-energy transfer of two hydrogen atoms has been identified on the PES of pure water clusters, e.g., the (H2O)5 (47). Note that water can exhibit also a negative behavior on the course of the reactions. Recently, an anti-catalytic effect of water was suggested for Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 the vibrational-overton induced dehydration reaction of methanediol (48). The quantum yield of such a dehydration reaction exhibited a delayed threshold effect, so that the reaction does not occur until the photon energy is enough large compared to the energy barrier. Other solvents can also stimulate the H-exchange processes (49–54). The keto-enol tautomerization, for instance, is effectively catalyzed by the formic acid, as demonstrated by da Silva (49,50). The H-exchange can be regarded as dihydrogen catalysis if one of the constituents replaced by dihydrogen is reproduced in products. The products Dihydrogen Catalysis 411

would have different isotopic contents if deuterium or tritium (T2) are used as reagent. The double hydrogen-exchange indicated above can also occur when the water catalyst is replaced by molecular hydrogen. The presence of H2 generally reduces the polarity of the transition state and increases the role of the covalent contributions to bond formation. Dihydrogen, as will be shown throughout this article, has several unique advantages and in certain reactions is the most effective contributor. Table 1 presents various examples of bond-exchange processes involving H2 (17). The barrier heights of those reactions vary substantially with the elec- tronegativities of the reactive X- and Y-centers, the ionic and electronic states of reagents, and the type of substituent in the cyclic TS. There is a direct correlation between the electronegativity of the heavy atoms linked to the hydrogen-triad and the barrier height in homoligand systems (the top part of Table 1). The same effect can also be traced in mixed systems. The all-hydrogen 3H2 and the H2+2MgH2 systems are obvious exceptions. The transition state structures are similar to those in degenerate 6-membered ring multiple hydrogen exchange reactions mentioned above (e.g., 3H2, 3HCl). They differ only by the presence of molecular hydrogen (denoted D2 forclarity)asathird party. The transition states of some neutral homoligand systems such as D2+(H2S)2 are unstable. However, the replacement of one H2S ligand by the more basic ammonia makes this reaction possible (H# = 32.5 kcal mol−1, Table 1) because of the additional polarization of the TS. The triple hydrogen (D-D-H triad) bond angle θ, is in all cases around 90◦ except for the H6 reaction involving the symmetric hexagonal TS structure which has 120◦ internal angles (see Table 1)(17). A particularly striking case is the neutral MgH2-based reaction with the practically linear D-D-H bond (θ = 178.5 o, Table 1) and low barrier of activation (as low as 5.4 kcal mol−1). The concerted, symmetric structures are stabilized because of the high overlapping of the molecular orbitals in TS. In addition, the hydrogen triad is polarized as identiﬁed by the NBO population analysis of the transition state structures (17). The natural charges of the key atoms presented in Table 1 show the alternation of electron populations of ring-constituting atoms. Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 All central hydrogen atoms are negatively charged, q(D) =−0.13 e to −0.48 e, and are involved in the electrostatic stabilization of the TS. The X and Y heavy atoms in turn bear substantial negative charges and interact electrostatically with the hydrogen-triad. On the other hand, the electron density in (H2)3 trimer is completely delocalized—the NBO charges being practically zero. The higher barrier compared to that of the other systems containing electronegative atoms (64.5 kcal mol−1 vs. 25-38 kcal mol−1, Table 1)isdueto the absence of the electrostatic stabilization of the TS. Such a statement is in Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] at 10:18 29 September 2014

Table 1: Termolecular bond exchange models involving dihydrogen, adopted from Asatryan (17)a.

b b #, c −1 d −1 e − e − c − c − HnXH χ HYHm χ H kcal mol ν1, cm ,Deg. q(D), e q(H), e q(X), e q(Y), e

H2 2.20 H2 2.20 64.5 −2537.3 120 0.0 0.0 0.0 0.0 HF 3.98 HF 3.98 25.0 −2047.7 83.9 −0.165 0.372 −0.567 −0.567 H2O 3.44 H2O 3.44 32.5 −2020.4 87.4 −0.326 0.375 −0.944 −0.944 NH3 3.04 NH3 3.04 38.0 −1596.3 87.1 −0.478 0.362 −1.094 −1.094

412 CH4 2.55 CH4 2.55 106.9 −2508.4 90.9 −0.129 0.195 −0.945 −0.945 f f C2H6 2.55 C2H6 2.55 107.4 −2507.8 90.5 −0.132 0.200 −0.739 −0.739 h h C2H4 2.55 C2H4 2.55 81.2 −2174.7 96.3 −0.084 0.168 −0.507 −0.507 g CH4 2.55 NH3 3.04 76.5 −1840.4 88.0 −0.310 0.153 −1.068 −1.003 f NH3 3.04 H2S 2.58 32.5 −1477.3 95.1 −0.238 0.341 −0.979 −0.584 f CH3NH2 3.04 H2S 2.58 29.7 −1481.8 94.9 −0.247 0.333 −0.812 −0.584 NH2NH2 3.04 H2S 2.58 29.3 −1453.0 94.9 −0.250 0.108 −0.638 −0.582 NH3 3.04 CH3HS 2.58 35.2 −1460.4 95.4 −0.249 0.099 −0.988 −0.313 MgH2 1.66 MgH2 1.66 5.35 −570.2 178.5 0.067 −0.405 1.407 1.407 aTransition vectors are sketched only in one direction to show progress of H-atoms. bPauling Electronegativity of central atoms. cEnthalpies at B3LYP/6-31G(d,p) and NBO analysis at B3LYP/6-311++G(2d,2p) levels. dImaginary frequencies for TS. eNBO charges for two types of triple hydrogen atoms. fq(D) = 0.11 e− on the S-attached hydrogen atom. g- − q(C terminal) =−0.574 e . h − q(C terminal) =−0.315 e . Dihydrogen Catalysis 413

agreement with the lowering of the polarity of the TS for the 6-centered bond- exchange reactions (29,32,37). This also occurs with the 4-center elimination reactions of BH3NH3 and C2H6 (55). Intriguingly, the central D atom in 2MgH2+D2 system is almost electroneutral and the stabilization is achieved mainly through the electrostatic interactions between Mg and the proximal hydrogen atoms. The terminal H- atoms in all TS structures as well as the bridged hydrogen atoms are also positively charged. −1 The barrier is exceptionally high for CH4 (106.9 kcal mol ), somewhat higher than the C-H bond dissociation energy (BDE) in methane (105 kcal mol−1). Even a somewhat larger difference is seen for the next aliphatic homo- −1 −1 logue, C2H6 (107.4 kcal mol vs. BDE 101 kcal mol ). On the other hand, the barrier is signiﬁcantly reduced in the bond-exchange reaction between H2 and two ethylene molecules where the π-electrons of ligands are involved in the TS- interactions (81.2 kcal mol−1 vs. BDE at 111 kcal mol−1). As mentioned above, all termolecular bond exchange reactions listed in Table 1 occur via 6- membered ring transition state structures (33,35–39). However, the TS-ring can be extended by switching the cycle to a remote reaction center. Figure 1 illustrates a 7-membered ring TS generated via switching the bond with the proximal N-atom to the remote S-center. The exchange reaction mediated by H2 (D2) and ammonia results in the isomerization of sulfhydrylamine (the interstellar molecule NH2SH, known also as thiohydroxylamine) (17). The reverse reaction barrier is fairly low (25.0 kcal mol−1) with an exothermicity of −1 14.1 kcal mol . The product NH3S has a zwitterion character. Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

Figure 1: TS for the H2 (D2) and ammonia mediated isomerization of sulfhydrylamine via 7- membered-ring TS (exchange reaction NH3 + D2 + NH2SH → NH2D + HD + NH3S) resulting in a switch between the proximal N and a remote S-centers (17). 414 R. Asatryan and E. Ruckenstein

Degenerate hydrogen exchange reactions also occur during ion-pairing reactions assisted by Lewis acids in zeolites (40,42) as well as in the in-cage reactions of H2 catalyzed by fullerene (C60)(56).

2.2. Dihydrogen-Assisted Relay Transport of H-atoms (H2-RT) The transport of H-atoms via a H2-RT mechanism constitutes a particular case of degenerate multi-hydrogen exchange reaction identiﬁed for the ﬁrst time by Asatryan and co-workers (12,13,17). It has been tested for several organic, organometallic, and heterogeneous systems (5,17,18). The H2-RT mechanism produces an indirect (relay) transfer of a H-atom to a proximal or distal (remote) reaction center, which is often located on a hetero-organic ligand or TM, either in an intramolecular reaction (2.1.1) or intermolecular H-exchange reaction described above”

where X and Y are H-donor and H-acceptor centers. The D2 notation is used to discriminate between hydrogen atoms. A H/D isotope exchange may occur when deuterium (or tritium) gas is employed as catalyst. Dihydrogen acts as a pass-on-agent and a catalyst for the transport of a hydrogen atom or an associated-free valence (as it takes place in the irradiated polymers described in Section 2.4). The transfer of H-atoms (protons) from X to D2 and from D2 to Y occurs simultaneously, with different reaction rates, which depend mainly on the H-affinities for the corresponding centers, and the thermochemistry of the processes. Theoretically, an intermolecular H-atom transfer via a cyclic H-exchange may also occur, however it is a termolecular process of low probability at the ambient pressure unless the process occurs in solid state with fixed co- reagents. Therefore, the notation H2-RT will be used hereafter regarding reaction (2.1.1), if not specified otherwise. The intermolecular transformation can be more relevant for rigid systems such as irradiated polymers with trapped radicals. The corresponding stepwise mechanism is denoted in this article as sH2-RT reaction and is discussed in Section 2.4 along Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 with the closely related inter-chain disproportionation reaction of two alkyl macroradicals. Several advantages of dihydrogen-assisted H-atom relay transport reactions can be outlined by comparing with the simple (direct) H-transfer process. The forbidden by orbital symmetry 4-center 2 σ -bond (or a σ/π) exchange reaction becomes allowed by the Woodward-Hoffman rules when the molecular hydrogen is involved (34). The high aromaticity of the planar ring-TS is also expected to reduce the barrier heights (57). In addition, the extension of the TS-rings involving H2 decreases the ring-strain and reduces the barrier heights compared to the simple H-transfer reaction. Dihydrogen Catalysis 415

The small-size and neutral characteristics of the hydrogen molecule explain the higher mobility of H2 in condensed matter and its unique reactions. On the other hand, the exceptionally small size of H2 does not cope with the electronic and geometrical bonding capabilities of some reaction partners (58).

2.2.1. Homogeneous Models A series of simple termolecular and bimolecular “gas-phase”, and more intricate organometallic models have been examined in (17) using DFT and ab initio methods. All reactions contain a typical hydrogen-triad in transition states (Scheme 1). All 3H2-like termolecular reactions are very unfavorable entropically. Instead, H-transfer can take place in a bimolecular manner between two centers of a single molecule (Eq. (2.1.1)). Several examples of such H2-RT reaction in which one of the H-exchange units is replaced by a bifunctional (ambident) bulk ligand are presented in Tables 2 and 3. Figure 2 illustrates a simple bimolecular keto-enol transformation reaction via the relay transport of the H-atom of oxygen-(Y)-center to the sp2-hybridized terminal C-atom of vinyl alcohol (X-center) (17). The arrows represent the reaction coordinate vectors, the normal coordinate having an imaginary frequency. The methyl group and D2 molecule are concertedly transferring their H-atoms (protons) to the proximal D- and the (proton acceptor) O-center resulting in the formation of two new H-D and O-D bonds. The involvement of the π-electrons in TS (the increased orbital overlap), along with the decreased ring-strain, facilitate the reaction. The barrier is reduced by about 15 kcal mol−1, as compared with the uncatalyzed 4-electron center H-transfer reaction which possesses a barrier of 53.4 kcal mol−1 calculated at the same moderate basis-set B3LYP level (17). The uncatalyzed barrier at a higher G3SX level is even higher, 60 kcal mol−1 (49,50). The H2-RT-mechanism involves attractive interactions and polarization of the H-H covalent bond by protic and hydridic H-atoms connected to heteroatoms. The role of the proton acceptor center can be played by an electropositive transition metal or a π-bonded or even saturated carbon Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 atom. The constituent atoms of the hydrogen triad are typically located at about 1.0–1.2 Å distances from each other, whereas the distances between the terminal H and X,Y-atoms vary with the nature of the heavy atoms (typically between 1.2–1.5 Å) (17). A reminiscent of such inter-hydrogen connections is the dihydrogen bond A-H···H-B, where the H···H distance is known to be typically of 1.7–1.9 Å, much shorter than the normal H···H contact of ca. 2.4 Å (58–66). The classical H-bond involves a protonic hydrogen X-H (X = N,O) interacting with the basic lone-pair of an electronegative atom. Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] at 10:18 29 September 2014

Table 2: H/D-exchange reactions of simple hydrides of elements via H2-RT mechanism involving 4-membered ring TSa.

(X) (XH) # (TS) (XH) Dp Dd (HD) (H3) XH χ µ E ν1 µ qH q q q q qX

Second Period LiH 0.98 5.7 8.8 −1077.3 4.1 156.2 −0.25 −0.19 0.10 0.06 −0.28 0.25 BeH2 1.57 0.0 21.5 −1272.8 1.7 142.7 −0.07 −0.12 0.21 −0.05 −0.03 0.14 BH3 2.04 0.0 4.6 −1054.7 1.6 129.8 −0.03 0.02 0.15 0.05 0.19 0.08 CH4 2.55 0.0 106.5 −3118.8 1.5 40.0 0.11 0.24 −0.28 0.13 0.20 −0.45 NH3 3.04 1.6 67.6 −1139.0 6.4 52.1 0.23 0.37 −0.57 0.14 0.17 −0.67 H2O 3.44 2.0 68.7 −2797.7 1.8 57.5 0.27 0.16 −0.24 −0.11 0.08 −0.55 HF 3.98 1.9 61.5 −2609.2 2.5 58.8 0.32 0.22 −0.06 −0.10 0.38 −0.32 Third Period 416 NaH 0.93 6.0 19.3 −1423.0 6.5 165.4 −0.30 −0.27 0.06 0.03 −0.48 0.29 AlH3 1.61 0.0 22.3 −1366.1 0.4 149.5 −0.17 −0.14 0.13 0.03 −0.15 0.30 MgH2 1.66 0.0 27.0 −1407.3 0.3 153.3 −0.15 −0.21 0.16 −0.06 −0.26 0.50 SiH4 1.90 0.0 99.3 −2801.8 0.1 38.3 −0.08 0.18 −0.18 0.26 0.18 0.36 PH3 2.19 0.7 76.0 −2170.8 0.3 110.2 0.04 0.08 −0.07 −0.15 0.09 −0.10 H2S 2.58 1.2 71.2 −2637.8 2.1 55.3 0.11 0.20 0.11 −0.09 0.51 −0.22 HCl 3.16 1.3 61.4 −2363.7 3.6 57.2 0.19 0.25 0.00 −0.25 0.50 −0.19 Group 5A HON 3.04 2.6 32.2 −597.5 0.8 143.5 0.27 −0.03 −0.01 −0.30 −0.07 −0.31 HOP 2.19 1.8 58.6 −540.3 1.8 138.2 0.30 0.03 −0.02 −0.05 0.04 0.06 HOAs 2.18 1.4 52.8 −2364.4 1.0 68.8 0.30 0.21 −0.10 −0.09 0.32 0.42 Group 6A b H2OO 3.44 4.9 −−−−−−−−−−0.08 H2SO 2.58 4.7 28.0 −1511.1 2.7 121.3 0.33 0.04 −0.04 −0.29 0.04 −0.41 H2SeO 2.55 3.9 46.4 −1395.3 2.2 132.2 0.06 0.04 0.08 −0.02 0.16 0.75

a # −1 (XH) Calculated at B3LYP/6-311+G(2d,p) level. The parameters are: E - barrier height (kcal mol ), µ - dipole moment of X-H reagent (Debye); ν1 −1 (TS) (cm ), µ (Debye) - (Degree) - imaginary frequencies, dipole moments and reactive angles in TS, respectively. qDp and qDd are the Mulliken charges on distal and proximal hydrogen atoms of the hydrogen thread, respectively; q(HD) –difference in absolute charges of H-bonded atoms at TS. χ (X)- electronegativity of the heteroatom attached to H. bConverges to 5-membered ring TS of the H2-RT , due likely the absence of the d-AO on the oxygen atom attached to H. Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] at 10:18 29 September 2014

Table 3: Typical H2-RT reactions involving 4- to 7-membered ring transition statesa).

# TS XZ XAYH µ XYZE ν1 µ q(Z)reag q(X)prod q

4-Membered-Ring TS (H/D-Exchange via Inversion at X) HOO+ 2.6 O(O) −−15.4 −401.5 8.2 63.5 − 0.29 − HON 2.6 O(N) −−32.2 −597.5 0.8 143.5 − 0.04 − HOO− 2.9 O(O) −−35.4 −809.3 3.6 48.5 −−0.40 − HOO• 2.3 O(O) −−62.5 −1654.3 2.6 86.1 −−0.11 − HONO-trans 2.7 O(NO) −−70.8 −2421.3 2.9 63.8 −−0.14 − HCOOH 1.5 O(CHO) −−72.4 −2500.0 3.4 60.3 −−0.34 − 5-Membered-Ring TS (1,2-H-Shift) HOO− 2.9 O − O0.0−183.8 3.1 64.8 −0.81 −0.81 0

417 HON 2.6 N − O4.2−1298.8 0.9 98.0 −0.31 −0.07 −0.24 NHOH2 3.8 N(H) − O(H) 4.9 −1098.8 2.3 81.8 −0.66 −0.32 −0.24 HOO+ 2.6 O − O5.8−590.4 4.8 71.5 0.30 0.30 0 • H3NN 5.8 N(H2) − N 7.27 −1461.4 4.2 85.4 −0.55 −0.29 −0.26 CH3O(H)O 5.1 O(CH3) − O(H) 7.33 −1533.2 3.5 80.6 −0.56 −0.24 −0.32 H2OO 4.9 O(H) − O7.4−1506.1 3.1 81.2 −0.56 −0.28 −0.28 NH3NH 5.5 N(H2) − N(H) 7.5 −1333.4 4.4 82.4 −0.72 −0.44 −0.28 NNH2 3.6 N(H) − N23.0−1809.5 2.5 92.4 −0.35 −0.23 −0.15 HOO• 2.3 O − O25.3−1824.6 1.3 86.6 −0.18 −0.18 0 HN•OH 0.7 N(H) − O(H) 33.3 −2017.2 1.7 85.5 −0.34 −0.24 −0.10 HONO-trans 2.7 O − N(O) 42.2 −1984.5 2.0 83.6 −0.08 −0.17 0.09 HCOOH 1.5 O − C(O,H) 71.5 −1952.9 2.0 84.1 −0.01 −0.41 0.40 6-Membered-Ring TS (1,3-H-Shift) − HCO3 1.7 O C(O) O 18.0 −419.1 1.9 74.4 −0.69 −0.69 0 HOOO+b 1.8 O O O 23.8 −795.1 6.0 62.2 0.17 0.17 0 HN = CHNH2 2.7 N(H) C(H) N(H) 39.2 −1597.2 4.2 85.5 −0.47 −0.47 0 c CH2C(CH3)OH 3.1 O C(CH3)C(H2)40.5 −2006.4 2.2 93.5 −0.35 −0.43 0.08 HCOOH 1.5 O C(H) O 40.9 −2150.3 1.0 81.5 −0.41 −0.41 0 HNNN+• 2.4 N N N 41.4 −1052.8 3.3 91.7 0.37 0.37 0 c CH2 = CHOH 2.9 O C(H) C(H2)42.5 −2032.0 2.0 95.2 −0.23 −0.36 0.13 HONO-cis 1.6 O N O 46.1 −2226.3 1.6 76.3 −0.10 −0.10 0 (Continued) Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] at 10:18 29 September 2014

Table 3: (Continued).

# TS XZ XAYH µ XYZE ν1 µ q(Z)reag q(X)prod q

7-Membered Ring TS (1,4-H-Shift)

418 d H2-ase (17) − NDTN∼Fe 23.3 −1705.5 − 105.8 −− − +• Glycine 4.3 N(H2)CH2 C(O) O 26.5 −1543.5 2.6 94.9 −0.22 0.13 −0.35 e Glycine-H2O 14.4 N(H2)CH2 C(O) O 33.0 −1175.8 9.9 83.7 −0.73 −0.63 −0.10 + Glycine-H 7.8 N(H2)CH2 C(OH) O 36.7 −1565.0 5.4 77.8 −0.33 −0.55 0.22 f Glycine 1.9 N(H2)CH2 C(O) O 40.1 −1474.9 8.3 83.4 −0.36 − + g Glycine-H 7.8 N(H2)CH2 C(O) O(H) 44.1 −1300.8 4.6 95.2 −0.42 −

a) # TS TS vectors are sketched to show the transfer of H-atoms. Parameters are: µ - dipole moment, E -energybarrier;ν1, µ and the imaginary frequency, dipole moment and reactive angle of the TS structure, respectively. All calculations are at B3LYP/6-311+G(2d,p) level of theory, if not stated otherwise, b)See Eq. (2.3.3), c)See Fig. 2, d) See Fig. 6, e) Zwitterion form is stabilizes by water-solvent (PCM-IEF data), f) Zwitterion glycine converges to its molecular form bearing a cis-oriented OH group in the gas-phase. g) The TS could not be located. Dihydrogen Catalysis 419

Figure 2: TS for the dihydrogen-assisted keto-enol transformation of vinyl alcohol via H2-RT mechanism: CH2 = CHOH + D2→CH2D-CHO + HD (17).

Several H2-RT reactions that occur via 4- to 7-membered ring cyclic TS are listed in Tables 2 and 3. The simplest H2-RT model is the H2-mediated H/D-exchange reaction via 4-membered ring transition state partly discussed in (17). The process is accompanied by the inversion of the reactive center (see Table 2 and inserted TS-sketch). The periodical characteristics of the reactive (heavy) atom generally correlate with the corresponding barrier heights. The highest barrier occurs for the reactions of CH4 and SiH4 molecules, i.e., for hydrogenates of the borderline (mid-period) elements. As expected, the H/D-exchange barrier involving the activation of inert C-H bonds in methane is particularly high (about 40 kcal mol−1) compared to the barriers of the homologous nonmetallic systems involving the central atoms N, O, and F, and even higher than those for the reactions of the hydrides of the second period metallic elements (BH3,BeH2, and LiH). For instance, the H2-mediated H/D exchange via the inversion of NH3 bearing polar N-H bonds requires a much lower activation energy, lower by Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 about 37 kcal mol−1, than the dissociation energy of H-H and about 26 kcal mol−1 than the BDE of the N-H bond. The barrier height can be correlated with the electronegativity (χ X)ofthe central non metal atoms C, N, O, F and Si, P, S, and Cl (Table 2). The inverse relation is seen when the metallic character of the central atom is increased due to the increased polarity of the TS-ring which provides the stability of the transition states. The increase in metallic properties reduces dramatically the barriers. The same trend can be found for the hydrides of the third period elements. Again, the borderline SiH4 has the 420 R. Asatryan and E. Ruckenstein

highest barrier, whereas the extreme metallic and non metallic NaH and HCl—the lowest ones. Therefore, the barrier heights in simple thermoneutral systems are associated with the dipole moments of reagents and the effective (HX) charges on reactive H-atoms qH . The partial reduction of the TS ring-strain, expressed by the extension of the internal hydrogen-triad angle θ, generally correlates with the barrier heights. Methane and silane have the smallest positive charges of H-atoms and their TS structures possess the most folded H-H-H triads (the smallest θ angles: 40◦ and 38◦, respectively) being sharper on the PES with the highest imaginary frequencies of 3118.8 cm−1 and 2801.8 cm−1, respectively. It should be emphasized that the gradual changes in the metallic properties of the central atoms expressed by χ X correlates well with the effective charges on H atoms of reagents qH(XH). The lowest barrier (with the lowest ν1) among the hydrides of the second −1 row elements occurs for BH3 (as low as 4.6 kcal mol ). It might be considered # (HX), X as an exception based on the periodic changes in E , qH and χ . However, the barrier of its counterpart from the third period (AlH3) also has a lower barrier than that of MgH2, thus disordering the periodicity of the property– barrier-height relationships. The lowest barrier in this period has a H/D-exchange reaction catalyzed by NaH (lower by 3 kcal mol−1). Remarkably, such an anomaly is predicted (HX) by qH values (−0.15 e and −0.17 e for MgH2 and AlH3, respectively, vs. the corresponding barriers of 27.0 and 22.3 kcal mol−1). Signiﬁcant changes in TS structures caused by strong electrostatic interactions in compounds with heavier central atoms change the periodicity of E# - property relationships. This can be traced in reactions of 5A group compounds (Table 2), whereas the increased negative charge of H-donating atom reduces the barrier height, as in HON compared to HOP. The next homologous system HOAs provides a different picture, namely a more compact TS with much folded H-H-H triad (θ = ◦ −1 68.8 ), and increased ν1 (−2364.4 cm ). Perhaps this is due to the strong polarization of X-O bond of HOAs (qAs = 0.42e, qO =−0.72e vs. qN =−0.31e, qO = 0.04e, and qP = 0.06e, qO =−0.36e for HON and HOP, respectively). Importantly, in HOAs a relatively much more positive charge is located on (H3) the proton-transporting H3-unit ( q = 0.32e) in contrast to its negligible Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 values in TS for other two compounds (−0.07 and 0.04, respectively, for HON and HOP). The only systems with substantial presence of a positive charge on H3-units are H2S and HCl (0.51e and 0.50 e, respectively). They provide relatively lower barriers than that for SiH4 but still too high compared to the hydrogenates of the non-metallic elements of the same period which are characterized by negative q(H3) values. Thus, this is not exactly a proton-transport process but rather the transport of a H-atom. Note that TS could not be identiﬁed for H2OO, the water-oxide, to compare with the other two homologous systems H2SO and H2SeO involving the 6A-group elements. A lower barrier occurs for the reaction of H2SO compared Dihydrogen Catalysis 421

S to the homologous H2SeO with a much less polarized X-O bond (q =−0.41e, qO =−0.26e; qSe =+0.75e, qO =−087e). The barriers in the analogous reactions involving electron-rich substituents are reduced significantly (by more than 30 kcal mol−1,asshownat the bottom of Table 2). The effect can be related to the additive influence of the attached electronegative N-, O-, and S-atoms and the involvement of π-bond electrons. Interestingly, the hydrogen-triad in TS is pushed-back in such systems. For instance, it is located at a distance of 1.912 Å in the reaction of HON ◦ with the -angles as wide as 140.5 (cf. also MgH2 in Table 1). It is interesting to note that the proton affinities (PA) of reagents correlate with the barrier heights in homologous reactions except for CH4 discussed above (PA for CH4,NH3,H2O, and HF are 131.9, 204.1, 166.5, and 117.1 kcal mol−1, respectively (21), vs. barrier heights of 106.5, 67.6, 68.7, and 61.5 kcal −1 mol ). Note that the barriers for NH3 and H2O are generally close to each other and sensitive to the method used. A clearer trend is obtained for the third row elements. It should be emphasized that the above correlation with the electronegativity of reactive centers is relevant only for the considered thermoneutral reactions (Erxn = 0) involving simple hydrides of elements, the trends in substituted systems are more complex. The effect of the traditional organic chemistry X substituents forming simple X-O bonds on H/D-exchange can be traced to a series of XOH+D2 reactions (Fig. 3 and Table 2). A clear correlation exists between the barrier height and the excess charge on the oxygen atom in TS (Fig. 3): = − q O q O TS q O HOX , Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

Figure 3: Substituent effect on the barrier height in the H/D-exchange reactions: X-OH + D2 → X-OD + HD. q(O) is the difference between the effective Mulliken charge on the O-atoms of the reagent and TS, calculated at the B3LYP level (68). The TS-sketch and X = H reference level (reaction of H2O) are provided. 422 R. Asatryan and E. Ruckenstein

where q(O) is the difference between the charge density on the corresponding key-oxygen center of the reagents, q(O)HOX, and TS, q(O)TS. One can see that the electron-donating substituents generally increase the barrier (the species above H2O, which constitutes the reference system with X = H), whereas the electron withdrawing groups (acceptors) reduce the barrier (the bottom part of the graph). It should be emphasized that the barrier reduction even for the asymp- totic values of q(O) is not significant. At the crossing point with the ordinate axis (q(O) ∼0.45e), the barrier is ca.63 kcal mol−1. On the other hand, the barrier is dramatically reduced for the more reactive X –substituents that are conjugated with the in-ring oxygen atoms (Table 3). The effect is significant for instance, for the reactions of HON, HOO+ and HOO−, which involve the interactions of π-electrons of the X = O bonds with the reactive OH group. The same picture holds (albeit to a lesser extent) for the reactions of HON homologs HOP and HOAs (Table 2). However, these systems constitute separate classes of compounds with the corresponding relationships yet to be identified. Importantly, the halogens (as well as OH) do not provide stabilization of H2-RT- transition state. Instead, a modified mechanism is valid, which involves the displacement of a hydrogen atom (Eq. (2.2.1)). It can be considered as a novel “oxygen-pull-out” mechanism. The approaching H2(D2)-catalyst forces the H-atom of the OH-group back to the X center to form a HX molecule, and concertedly associates with the O-reactive center. As a result, the dihydrogen catalyst takes the O-atom out of the HOX molecule thus forming a [HX ... H2O] product-complex (or separate products at higher temperatures):

The analogous reaction of H2O2 with D2 (X = OH), apparently, produces two H2O molecules. The DFT-level barriers for reactions of D2 with HOF, HOCl, HOBr, and H2O2 are relatively low considering the Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 complex rearrangements, viz., 33.5, 45.3, 45.1, and 51.0 kcal mol−1, respectively. On the other hand, the transition state rings extended to two or more heavy atoms lead to a regular H2-RT mechanism with relay transport of the hydrogen atom between heavy atoms (see Table 3 and the insert TS structure). The process is accompanied by H/D-exchange if D2 is applied, as it occurs for all H2-RT reactions. This is in contrast to the H2-PT proton-transport reactions (Eq. (2.1.3)) which will be examined in Section 2.3. Dihydrogen Catalysis 423

The reduced ring-strain energy for the processes involving extended transition state rings explains the higher average barrier heights for the 4-membered-ring-TS reactions (cf. e.g., the different pathways for the same trans-HONO and the HCOOH reagents from different sections of Table 3). Remarkably, the energy barrier for the 5-membered ring of the H/D-exchange reaction of HOO− is practically zero (G# = 4.75 kcal mol-1), due likely to the high negative charge on the acceptor center (−0.81e) which develops additional columbic interactions with the electropositive H-atom. Dihydrogen catalyzes also the isomerization of various glycine-based species, particularly the glycine radical-cation via 7-membered-ring TS with the barrier height of 27 kcal mol−1 (Table 3). According to Simon et al., the gas- phase (direct) isomerization of glycine radical-cation via a 5-membered ring TS requires 39.9 kcal mol−1 activation energy at the B3LYP/6-31++G(d,p) level of theory (69), which is in good agreement with the analogous optimized MP2 and single-point CCSD(T) calculation results with the same basis set. Our calculations with a somewhat extended basis set (by adding a polarization function for the heavy atoms) results in a comparable value of 40.5 kcal mol−1. However, a closer look revealed that the TS identified in (69) seems is rather related to the H-transfer reaction from the carboxyl C-atom, than from the internal methyl group. The actual barrier height from CH2 to the double bonded O-atom (calculated in the present paper) is significantly lower at about 29.1 kcal mol−1, which is still higher by 2.6 kcal mol−1 than the barrier of the H2-RT catalyzed reaction (26.5 kcal mol−1). In an attempt to find a general relationship between the reaction rate and the properties of the isolates species one can consider the competition between the reagent’s H-accepting (Z) and product’s H-donating (X) centers in the transport of the hydrogen atom, viz. the hydrogen affinities of the corresponding centers (Table 3). The hydrogen affinities of those centers are expected to correlate with their effective charges in the isolated reagents q(Z)reag and products q(X)prod (see Fig. 4). Indeed, a clear correlation exists between the barrier height and qXZ pro- XZ XZ vided by q = q(Z)reag –q(X)prod. The correlation holds also for q = ln [q(Z)reag – q(X)prod] which, perhaps, is more appropriate to compare with reaction rates, although with somewhat lower R squared value (0.807 vs. 0.897). Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 The correlation presented in Fig. 4 includes all systems of Table 3 (independent of their TS-ring sizes) which have distinct H-donor and H- acceptor atoms. Such a relationship provides a simple tool to seek an effective H/D-exchange reaction catalyzed by H2/D2. The correlation involves even H2-RT reactions of neutral and protonated glycine (NH2CH2COOH and + NH3CH2COOH , respectively). Moreover, the calculated parameters with the solvent effect included in the isomerization reaction of neutral glycine to its zwitterion counterpart (noted in Table 3 as Glycin-H2O), also are well fitted by the linear relationship. 424 R. Asatryan and E. Ruckenstein

Figure 4: Barrier heights of H2-RT reactions versus the hydrogen afﬁnity of the H-accepting center (data from Table 3) expressed through qXZ, which is the difference between the effective charges on Z and X hydrogen-accepting centers in the isolated reagents and products, calculated at the B3LYP level (68).

2.2.2. Heterogeneous Models Recently, the H2-RT mechanism has been successfully applied by Curtis and coworkers to the more complex “pure inorganic” reaction of dihydrogen with alumina-supported vanadium oxide (18). The H2-assisted reduction provides the key step in explaining the sequence of reactivities of surface complexes based on the combined results from the species-selective UV resonance, visible Raman measurements, and DFT calculations (18). The reaction pathway involving bond breaking of H2 by the vanadyl oxygen has been found to be similar in bidentate, tridentate, and molecular monomer structures on a θ-alumina surface (Fig. 5). The reduction of H2 on these structures starts with a reaction at the V = O bond. The bidentate and tridentate structures have a reaction pathway that begins with the H2 reacting asymmetrically with the vanadyl oxygen to break the H-H bond followed by the breaking of the vanadium-oxygen double bond to Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

Figure 5: Schematics of the three vanadyl species on θ-Al2O3 characterized by DFT and species-selective Raman spectroscopy (adopted from Kim et al. (18)). H atom at the non-surface oxygen atom of the molecular form is omitted. Dihydrogen Catalysis 425

Figure 6: Minimum energy pathway of molecular vanadium oxide monomer reacting with hydrogen. The free energies at 450 ◦C and 0.04 atm. pressure in kcal mol−1 are relative to the molecular reactant. The reduction occurs by a hydrogen-assisted hydrogen transfer and desorption of H2 to form a water-desorbed structure (adopted from Kim et al. (18)).

form a hydroxyl group bound to vanadium and a weakly bound hydrogen radical (as a side note, this mechanism can be considered another conﬁrmation for the stepwise sH2-RT type pathways, Eq. (2.1.2), see Section 2.4). The pathway is completed by the formation of a second oxygen-hydrogen bond that results in an aqua ligand bound to vanadium. The aqua ligand can desorb, leading to an overall endothermic reaction at the temperatures considered in Kim et al. (18). Dissociative addition of H2 to bidentate and tridentate monomers requires the barriers of 53 and 70 kcal mol−1, respectively, calculated at B3LYP/6-31G(d,p) DFT level. Importantly, the most favorable reaction for the molecular monomer is the dihydrogen-assisted hydrogen atom relay transport (H2-RT, Eq. (2.1.1)) from Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 the surface hydroxyl (at the Al-O-V bridge) to H2, followed by the breaking of the O-H and H-H bonds and formation of a three-centered H-H-H transition state, with an intermediate barrier of 57 kcal mol−1 (Fig. 6). The net result of this reaction is the hydrogen exchange with loss (regeneration) of H2 and transformation of V = O to V-O-H. The subsequent hydrogen abstraction from a surface hydroxyl to the newly created V-O-H group leads to the formation of an aqua ligand. It should be emphasized, that only the involvement of H2-RT dihydrogen catalysis mechanism allowed providing consistent results and to explain the 426 R. Asatryan and E. Ruckenstein

ordering of the reactivities of monomers observed experimentally (18). The H2 reducibility sequence for the three alumina-bound monomeric species has been found to be in the order bidentate > “molecular”> tridentate. The structure-activity relationship established in Kim et al. (18) pro- poses a synthesis strategy to obtain vanadium oxide structures with greatly enhanced activity for oxidative dehydrogenation reactions that would provide more efﬁcient catalysts for the production of alkenes.

2.2.3. Organometallic and Biomimetic Models (A) Biocatalysis The H2-assisted organometallic reactions have been suggested to play a signiﬁcant role in biocatalytic processes (5,12,13,17). The H2-RT mechanism has been identiﬁed in a variety of simple models and the mimics of hydrogenase and nitrogenase enzymes (5,17). Figure 7 illustrates a relay H-transport in a truncated Fe-only model of hydrogenase containing CO- and CN-ligands, the dithiomethylamine (DTN) linker, µ-SCH2NHCH2S, as well as SCH3-group to model the S[(Fe4S4)-Cys] ligand (17). The calculations for a Fe(II)Fe(II)-hydrogenase model with a protonated bridged DTN and/or CH3S-groups (Q = -2, S = 0), suggested that the approaching H2-molecule (D2) mediates a relay H-transfer of the protonated bridge (NH2-group) to the distal Fe-center. Dihydrogen forms a 7-membered semi-rigid ring transition state, transfers an atom to Fe, captures a H-atom from the amino group of the DTN-linker, and releases back the isotopically mixed HD. Thus, the dihydrogen plays the role of a relay (pass-on) agent in the remote transfer of the H-atom of protonated DTN-bridge (NH2-group) to the distal Fe-center as it occurs in simple “gas- phase” models described above. The barrier is fairly low (23 kcal mol−1) compared to the other gas-phase models of hydrogenase (5,17,70,71), and the small models presented in Table 3. −1 The process is also favorable thermodynamically (Hrxn =−24 kcal mol ). The relay H-transport (H-export) can also occur to a ligand with the additional expansion of the TS-ring (e.g., to the C-atom of the CO-group). The spin-state plays an important role in changing the mechanism and redirecting the process (vide infra). Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 The decomposition of non-hydridic intermediates of nitrogenase (various levels of activated enzyme) has been suggested to occur via a H2-RT- mechanism (5). Figure 8 illustrates the degradation mechanism of a simple non-hydridic intermediate model of nitrogenase (HS)Fe(SH2)NNH. Dihydrogen again serves as a relay-transporter of a terminal hydrogen atom (bound to NN) to the iron center of a diazenido complex (Eq. (2.2.2)) (17): Dihydrogen Catalysis 427

Figure 7: Dihydrogen catalyzed relay transfer mechanism (H2-RT ) for the model [FeFe]- hydrogenase (Q =−2, S = 0). The approaching dihydrogen, denoted as D2, produces the relay transport of a H-atom of the protonated NH2 bridgehead to the distal Fe-atom accompanied by a H/D exchange (17). The truncated parts of the enzyme, viz. cysteine and (Fe4S4] cubane, are also presented in the Scheme for completeness.

The destruction of the HNN-group concurrently generates a hydridic ligand attached to the iron center via cyclic TS (Fig. 8). The initial oxidation state of the iron center remains unchanged during +2 reaction (Fe ). A weak linkage occurs between the newly generated N2-moiety Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 and the iron-center of the product (RNN<0.001 Å). It should be noted that reaction (2.2.2) is not an electron consuming process to generate HD as in reductive elimination processes, but provides a redis- tribution of the electron density accompanied by a change in the oxidation number of a metal center in polynuclear processes (see (5) for more details and other relevant examples). The H2-RT interconversion process usually occurs between a hydridic and a protic H-atom (proton-hydride exchange). However, it may also occur between two protic H-atoms (e.g., N2H and S centers) to form N2 and 428 R. Asatryan and E. Ruckenstein

Figure 8: Dihydrogen- (D2) assisted relay H-transport mechanism (H2-RT ) for the degradation of the non hydridic (HS)Fe(SH2)NNH intermediate (quintet state). The schematic drawing illustrates the net reaction involving a resonance-hybrid reagent bearing a non innocent NNH radical group (the corresponding Mulliken spin-densities are provided in the scheme), adopted from Asatryan (5).

SH moieties, as noted in (17) for simple models with higher activation barriers.

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 (B) Models Involving Transition Metals other than Iron The organometallic H2-RT-processes can occur with systems containing various transition metals such as iridium and palladium. An interconversion of two iridium-aminopyridinate complexes supported by dihydrogen (1 and 2 in Fig. 9), has been recently observed experimentally and studied theoretically by Valpuesta et al. (19). The H2-assisted 1 ←→ 2 interconversion process (also quoted as “dihydrogen catalysis”) leads to the reversible formation and cleavage of an amido N-H bond and of a benzylic C-H bond of the pyridine 2,6-Me2C6H3 Dihydrogen Catalysis 429

Figure 9: H2 catalyzed reversible isomerization of complexes 1 and 2 (19).

substituent. Following the DHC-concept, a relay transport of H-atoms mediated by dihydrogen could be expected to occur between the hydridic and protic atoms (proton-hydride exchange), as well as between the metal center and either the N-atom of the amido group or the C-atom of the CH3 group. Our preliminary illustrative calculations using the same B3LYP/LanL2DZ method employed in (19) indeed demonstrated the relevance of H2-RT mechanism to such processes (vide infra). The dihydrogen-supported prototropic rearrangement and interconversion of two iridium-aminopyridinate complexes have been originally explained through the intermediary formation of the iridium dihydride or H2-complexes. Two mechanisms have been studied theoretically by Valpuesta et al. (19), without or with participation of dihydrogen. Mechanism (i) initiates an intramolecular oxidative addition of a methyl C-H bond to Ir followed by the hydride migration onto the metal-bound amido nitrogen, whereas mechanism (ii) involves an initial dihydrogen addition to the metal, followed by the same sequence of processes: hydride migration and methyl CH oxidative addition plus dihydrogen elimination (19). The overall barrier for mechanism (i) has been found to be too high (G= is ca. 48 kcal·mol−1 at the B3LYP/LanL2DZ level) to be compatible with the experimental observations. The more favored mechanism (ii) has been found to be controlled by a free energy barrier height of 28.7 kcal mol−1 in the second half of the process involving the hydride Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 migration. Importantly, the hydrogenation has been found to be endoergic by 15 kcal mol−1. Note that an alternative direct reaction involving a non-hydridic H-transfer between the amido and methyl groups would allow avoiding the coordination of H2 to the Ir-center and its further splitting - oxidative addition, accompanied by hydride formation. The system described in Fig. 9 is too complex to be calculated directly at the PES level. Figure 10 illustrates a truncated model-cation used for the relative elaboration of two (direct and H2-asssisted) competitive pathways. We used the same B3LYP/LanL2DZ DFT method to be able to compare the results. 430 R. Asatryan and E. Ruckenstein

Figure 10. A suggested in current article alternative H2-RT - mechanism for H-atom transfer of the Ir-hydride to amido nitrogen center relevant to the observed in (19)H2-catalyzed reversible isomerization of complexes 1 and 2 using a truncated model (cf. Fig. 9); see text for clariﬁcations.

The electron donor di-isopropyl phenyl group at N was replaced by a donor methyl group. In addition, taking into account that the cyclopentadienyl ligand usually does not get involved chemically, it was replaced by a H atom—the simplest way to saturate a dangling bond in catalysis modeling. Although such truncations can affect the absolute energies, largely because of the ligand field and steric effects, as well as the effect of conjugation in ligands, this allowed us to study the relative reactivities based on the full PES of two competitive processes under the same conditions. The calculations demonstrated that the H-atom relay mechanism H2- RT is indeed possible between Ir-H and N-centers (the controlling step in the original mechanism). The free-energy barrier height for the hydrogen assisted H-atom transfer from Ir to N of the amino group was found to be −1 16.4 kcal mol , which is significantly lower than the original H2-catalyzed barrier height of 28.7 kcal mol−1 calculated in (19) for the second, rate-limiting step that involves H2-addition/H-transfer (scrambling) reaction, not to men- tion the much higher barrier of 48 kcal mol−1 for the uncatalyzed reaction Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 (vide infra). It should be emphasized that we could not locate a DHC-type TS for the H-transfer of the benzylic CH3 group to Ir (the first reaction step). Assuming that this is not the result of the model limitations, a possible scenario can be suggested involving the direct, first step reaction, as described by Valpuesta et al. (19), followed by the H2-RT lower-energy-barrier reaction illustrated in Fig. 10, i.e., via a mixed mechanism involving both the simple H-atom transfer mechanism suggested in (19)andtheDHC(H2-RT) processes proposed in the current work. More detailed explorations and extended models are required to obtain more quantitative results. Dihydrogen Catalysis 431

Dihydrogen is also known to stimulate the syn←→ anti isomerization of the ﬁve-coordinate iridium monohydride complexes involving a pincer biPSi (or PSiP) ligand (152). This is an extreme case of the fast anti←→ syn exchange in the presence of dihydrogen, a reagent that does not form any detectable new product with three obtained isomers but renders the isomerization. This is believed to constitute another example of the possible action of H2 via a DHC mechanism without involving any Ir-H/H2 atom exchange (scrambling). Notably, the scrambling mechanism has been ruled out by experimentalists (152). The reaction follows a second order kinetics with respect to hydrogen (kobs = k(anti←→ syn)[H2]) without involving any H2/H(-Ir) hydrogen atom scrambling. Because of the various possibilities for proton/hydride (and H2) exchange processes that involve Ir, N, and unsaturated (benzylic) C-centers, some relevant DHC processes can be expected to occur for the H2 activation by (PNP)Ir(C6H5)2 complex accompanied by the dearomatization/ aromatization of the pyridine ring (153). An interesting topic with possible implications of DHC-mechanisms is the dihydrogen-induced formation of iridium-metal clusters from supported mononuclear metal complexes. The process typically involves reduction, migration of metal species on the support surface, and metal-metal bond formation processes, as demonstrated particularly by Gates and coworkers (154). To understand the chemistry of supported metal-cluster formation, and their interactions with H2, one needs to identify how the support and ligands on a metal inﬂuence the processes and what is the exact role of the dihydrogen. In this regard, the hydrogen spillover concept employed by Gates and coworkers to explain their results (154), is terra incognita for DHC purposes still full of controversies, especially for the transfer of hydrogen species onto the non reducible support surfaces (155). Our ongoing work on spillover of hydrogen based on combined Pd/SiO2 clusters (Pd4 tetrahedral cluster located on top of the cyclic silica consisting of three SiO4-linkages) indicates the main role of the H2-catalyst (the transporter of the spillover-hydrogen species), and the adsorbed (capped on palladium vertexes) hydrogen molecules (151). Note also that H2-RT mechanism is expected to operate in other practically important palladium-linked processes such as the hydrogen assisted Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 growth of vertically aligned Pd2Si nanowires (156), uptake of hydrogen atoms by a scavenger—the hydrogenation of an unsaturated ligand attached to the POSS-coated palladium catalyst—to eliminate the hydrogen build-up in electronics (157), etc.

2.3. Dihydrogen-Assisted Proton-Transport (H2-PT). Comparison with Alternative H-atom Transfer Mechanisms The hydrogen assisted proton-transport (Eq. (2.1.3)) has been originally suggested for cationic systems (22–27). However, it occurs also in neutral 432 R. Asatryan and E. Ruckenstein

and even anionic systems, as shown below (Table 4) even though the H2-PT mechanism generally implies a proton-transport:

In contrast to the above relay transport of H-atoms (H2-RT), a reactive + proton is involved in the transport of a H3 -unit - a triangle H-H-H triad. The transported hydrogen is effectively attracted by the heavy atoms of both reaction centers as opposed to the other two H-atoms (belonging to H2/D2- catalyst) which remain undissociated during the reaction. The isomerization of isoformyl ion HOC+ to its more stable HCO+ isomer (Eq. (2.3.1a,b)) is a classical example of dihydrogen catalyzed proton-transport reaction (16, 22–27):

The facile isomerization has been proven to take place in the interstellar + + medium with the “abundant ratio” of [HCO ]/[HOC ] = 160–360 at H2-gas densities > 105 cm−3 per cm−3 (22). The abundance of isoformyl HOC+ in these models is grounded on the isomerization rate of the HOC+ to the lowest energy + formyl isomer, HCO assisted by H2. This reaction is one the most studied in the theory of DHC processes (16,23–27). The current view, which is based on the PES and kinetic studies, indicates that the process has a low barrier of activation. A higher stabilization of the H2 complex occurs for the isoformyl ion than the much more stable + +/

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 HCO . This explains the high abundance ratio of the isomer ions HCO HOC+ (∼330) in the dense interstellar clouds (24). The barrier for the uncatalyzed reaction (2.3.1a) is ca. 40 kcal mol−1, calculated at CCSD(T)/TZ2P level by Martin et al. (28). It becomes incredibly small −1 (almost zero kcal mol )whenH2 is involved (Eq. (2.3.1b)) (27). The abundant ratio derived from kinetic studies (the ratio of the differential formation and depletion rates (23)) appears to be very sensitive to the zero-point vibration energies (ZPE) of TS and tunneling corrections. It becomes closer to the experimental 160–360 value at lower barrier heights. The barrier height calculated Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] at 10:18 29 September 2014

Table 4: Comparisons between three H-transfer mechanisms in ions and radical-Ionsa.

HAT H2-PT H2-RT # # # Reagent µR Product µP Erxn Edir E µTS CPT E µTS CRT

1,2-H-Shift HOO+ 2.6 HOO+ 2.56 0.0 49.5 13.1 7.0 36.4 5.8 4.8 43.7 HON+• 2.3 HNO+• 3.64 −0.2 39.4 3.4 7.4 36.0 −0.1 6.1 39.5 HOC+ 2.6 HCO+ 3.73 −38.5 35.0 0.6 8.3 34.4 −− − HOS+ 1.7 HSO+ 3.07 −5.3 48.3 34.4 5.6 13.9 20.9 3.0 27.4 HOP+• 1.6 HPO+• 3.72 24.2 46.5 41.0 5.0 5.5 38.8 4.6 7.7 HOSi+ 0.3 HSiO+ 5.85 64.5 94.2 84.5 7.3 9.7 −− − + + NH3NH2 3.0 NH2NH3 3.0 0.0 43.5 43.2 1.9 0.4 33.2 1.4 10.4 +• +• H2NN 3.1 trans-N2H2 0.0 −2.5 48.7 31.6 4.9 17.1 28.8 4.1 19.9 +• •+ CH3OH 1.5 CH2OH2 1.9 −0.5 30.3 27.1 2.0 3.2 26.1 3.0 4.2 + + NH2OH2 2.7 NH3OH 3.7 −25.2 25.8 23.7 1.8 2.1 16.4 1.5 9.4 + + HNOH 0.3 H2NO 3.5 −16.9 49.5 15.5 2.5 34.0 −− − HNN+ 3.2 HNN+ 3.2 0.0 45.3 14.3b 8.2 30.9 −− − + + 433 NH2NH 2.6 NHNH2 2.6 0.0 63.8 −−−36.5 0.5 27.3 HOSe+ 1.7 HSeO+ 2.8 9.2 52.3 −−−30.7 2.9 21.6 +• +• H2OO 3.8 trans-H2O2 0.0 −22.9 34.0 −−− 5.7 4.0 28.3 +• +• HNO2 3.0 trans-HONO 2.7 −34.3 22.5 −−− −− − Triplet Cations 3HOC+ 1.8 3HCO+ 2.4 −5.2 39.4 23.0 5.5 16.4 15.9 4.0 23.5 3 + 3 + CH2OH 0.9 CH3O 2.5 −11.9 28.2 21.4 4.0 6.9 19.5 3.8 8.7 Anions − − HCO3 2.7 HCO3 2.7 0.0 23.2 −−−18.0 1.9 5.2 HOO− 2.9 HOO− 2.9 0.0 16.9 15.4 1.2 1.5 −1.7 3.1 18.6 1,3-H-Shift HNNN+• 2.4 HNNN+• 2.4 0.0 76.3 −−−41.4 3.3 34.9 +• +• CH3OOH 0.5 CH2O-OH2 1.1 −49.8 49.1 49.6 2.3 −0.5 −− − +• +• NH2CH2COOH 4.3 NH2CHC(OH)2 1.0 −20.4 29.1 29.3 2.8 −0.2 26.5 2.5 2.6 HOOO+ 1.8 HOOO+ 1.8 0.0 36.3 22.7 6.4 13.6 23.8 6.0 12.5 1,4-H-Shift + + NH3CH2COOH 7.8 NH2CH2C(OH)2 4.2 9.2 8.1 −−−36.7 5.4 −28.6 + + c NH3CH2COOH 7.8 NH2CH2CO + H2O − 29.2 29.3 −−−44.1 4.6 −14.8

a) HAT- simple (direct) H-transfer; H2-PT and H2-RT - dihydrogen assisted proton and relay H-atom transport, respectively. All calculation have been b) + + carried out at B3LYP/6-311+G(2d,p) level, except for glycine cations calculated at B3LYP/6-31G(d,p) level. H3 -ring for HNN is orthogonal to the c) + + NN bond. Elimination of H2O is accompanied by the spontaneous decomposition of NH2CH2CO thus forming H2O + NH2CH2 + CO product set. 434 R. Asatryan and E. Ruckenstein

at CCSD(T)/aug-p-VTZ level even changes sign from -1.5 to +1.4 kcal mol−1 when the zero-point-energy correction is added to the equilibrium value (27). Reaction (2.3.1b) belongs to the class of processes in which a catalyst facilitates the proton transport via complexation, process initially described by Bohme as a catalyzed proton-transport (87), and more generally referred to as ion-transport catalysis by Radom and coworkers (21,30,67). It is also called H2-escorted isomerization reaction (27). This constitutes a classic example of a mechanism referred to here as dihydrogen assisted proton-transport (H2-PT) as a particular case of the more general DHC-catalysis (Eqs. (2.1.1)–(2.1.5)). In the transition state, dihydrogen is bound to a hydrogen atom (proton) of the + + HCO in a H3 triangle (Eq. (2.3.1b)). Such a TS lies below the energy of separated reagents (16a). The energetic lowering of H2-assisted (catalyzed) reaction + is a result of the stabilization of the H3 complex in TS. It is therefore expected to occur for systems with higher dipole moments to provide better catalysts for PT-reactions. However, as shown by Chalk and Radom (30), such a relationship is not straightforward. An important role is played by the residual portion of the molecule-substrate, which competes for the possession of H+. The barriers in degenerate proton-transport reactions of NNH+ cation with different transporter-catalysts considered in (30) are lowered compared to their values for the isolated ion. The most effective result is due to the interaction with species (-catalysts) having proton affinities lower than molecular nitrogen (N≡N). As shown in (30), H2 as a catalyst also reduces the barrier. However, its PA is close to that of dinitrogen. The barrier for the uncatalyzed proton-transfer reaction recalculated by us at the B3LYP/6-311+G(d,p) level of the theory is 45.3 kcal mol−1 in agreement with the barrier of 43.5 kcal mol−1 calculated by Chalk and Radom with the modified G2 method (15). Thus, the H2 catalyst reduces the barrier by more than 30 kcal mol−1 (see Table 4). The vibrational-mode analysis of TS shows + that H3 is transferred as a combined proton transporting unit between the + two N-atoms, as in the isomerization of HCO (Eq. (2.3.1b)). Similarly, the + + sum of the Mulliken charges on H3 also indicates a proton transfer ( qH3 = + 0.89e), and the coupling of H by H2 catalyst in the TS. The interaction with species that have higher proton affinities leads to an additional lowering of the barrier, but the transfer to the neutral molecule X Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 constitutes in such cases a more energetically favorable process (15). Toro-Labbé et al. analyzed the reaction force, electronic chemical potential, and the reaction electronic flux along the intrinsic reaction coordinate of dihydrogen catalyzed isomerization of HCO+ and found them important for the characterization of the entrance and exit regions where CO is coupled with + + H and the TS region where H2 is coupled with H (16a). The H2-PT mechanism can occur in some other cationic, radical and neutral species listed in Tables 4–5 also involving the 1,3-H-shift reactions, which were not previously identified. Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] at 10:18 29 September 2014

Table 5: Comparisons between the three H-transfer mechanisms in neutral molecules and radicalsa.

HAT H2-PT H2-RT # # # Reagent µR Product µP Erxn Edir E µTS C E µTS C

1,2-H Shift HNO2 2.7 t-HONO 2.1 −6.9 47.9 −− −42.2 2.0 5.7 NNH2 3.6 t-N2H2 0.0 −20.3 50.0 51.0 2.1 −1.0 23.0 2.5 27.0 NH3NH 5.5 NH2NH2 2.0 −22.7 18.8 −− − 7.5 4.4 11.4 HON 2.6 HNO 1.7 −40.3 32.1 −− − 4.2 0.9 27.9 CH2NH3 5.5 CH3NH2 1.4 −63.9 15.2 −− − 3.7 4.6 11.5 CH3O(H)O 1.7 CH3OOH 5.1 −44.0 5.6 6.2 3.6 −0.6 7.3 3.5 −1.7 H OO 4.9 HOOH 1.8 −45.5 4.6 5.3 3.2 −0.7 7.3 3.1 −2.7

435 2 CH2OH2 3.1 CH3OH 1.8 −81.8 0.3 −− − 4.9 3.4 −5.3 1,3-H Shift c-HONO 1.6 c-HONO 1.6 0.0 −−−−46.1 1.58 − CH2CHOH 1.0 CH3CHO 2.9 −11.1 54.8 −− −42.5 2.0 12.3 CH2C(OH)CH3 0.6 (CH3)2C = O 3.1 −12.9 51.3 −− −40.5 2.2 10.8 HCOOH 1.5 HCOOH 1.5 0.0 −−−−40.4 1.0 − NH2CHNH 2.7 NH2CHNH 2.7 0.0 43.6 −− −39.2 4.2 4.4 Radicals HNN• 2.0 HNN• 2.0 0.00 45.2 46.1 1.7 −0.89 −− − • • HNOH 0.7 H2NO 3.2 9.9 43.4 44.2 1.6 −0.81 33.4 1.7 10.1 HOO• 2.3 HOO• 2.3 0.00 37.8 −− −29.3 1.3 8.5 HCC• 0.8 HCC• 0.8 0.00 20.8 21.5 1.0 −0.24 −− − • • NNH3 5.8 NH2NH 2.7 −59.1 18.2 −− − 7.3 4.2 10.9

a) HAT- uncatalyzed (direct) H-atom transfer; H2-PT and H2-RT - dihydrogen assisted proton and relay H-atom transport reactions, respectively. All calculation are at B3LYP/6-311+G(2d,p) level. 436 R. Asatryan and E. Ruckenstein

2.3.1. Comparison between the Ionic Mechanisms Table 4 presents catalytic reactions of various ionic and ion-radical substrates in comparison with the simple H-transfer (HAT)andrelay H-transport (H2-RT) mechanisms. Intriguingly, topologically the same “proton-transport” mechanism occurs − even in systems involving negatively charged anions such as HO2 , whereas − the only possible (located) mechanism for H-transfer reaction for HCO3 anion is the relay transport H2-RT pathway. Some correlations exist between the barrier height and various calculated parameters for systems along the Periodic Table, similar to those found for the H2-RT reactions. The reactions of molecules containing more metallic elements (left side of C and Si elements in the Periodic Table, as well as the halogens) are not involved in the discussion, because they tend either to form a 4-membered ring TS (H/D-exchange) at the O-center (as in HOB and HOBe, Table 2)or + collapse with the moving away of the H3 -groups, or undergo more complicated rearrangements. Based on the supremacy of the proton-exchange mechanism in HOC+,it was expected (at least for cations) the dominant role of the proton-transfer mechanism H2-PT compared to the relay H-transport mechanism H2-RT. However, the comparative data presented in Table 4 indicate that such a con- clusion is accurate at best for large TS-ring systems (vide infra), whereas in all 1,2 H-shift reactions the relay transport barriers are persistently lower than those for proton-transport (H2-PT) processes. For some systems of Table 4 only one type of reaction could be identified attributed with a high catalytic efficiency. The high CRT is seen when the search failed for the H2-PT transition state, and, correspondingly, the high CPT is observed when the TS could not be located for the H2-RT pathway. The phenomenon can be related to the intrinsic characteristic of a system, not the one caused by the method limitation and needs to be explored further. Table 4 indicates that there is a correlation between the thermochemistry of the homologous reactions and the barrier heights in accord with the Bell-Evans-Polanyi (BEP) principle (72,73). For this reason, systems involving atoms of the same periods are presented with separate entry-blocks in Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 Table 4. The gradual changes in properties of reagents and products become determining factors for homologous systems. It is important to emphasize that the reaction energy Erxn actually represents the difference between the hydrogen affinities (HA) of two H-donor and H-acceptor centers, which is expected to be correlated with the corresponding barrier height. Comparison of the data of Table 4 demonstrates that the barrier for sim- # ple H-transfer reaction E dir correlates well with the barrier height for Dihydrogen Catalysis 437

both H-transport mechanisms and for compounds involving the second row elements (HOO+,HON+,andHOC+). However, the correlation breaks for the third row elements because of significant changes in the TS structures as indicated by the dipole moments and imaginary frequencies of TS for HPO+ and HOS+). Figure 11 shows a good correlation between the barrier heights for both H-transport mechanisms. The data from Table 4 were used for this correlation diagram. It demonstrates that the two mechanisms are closely related and that the factors that determine the H-transfer barriers, such as the characteristics of the terminal (reactive) H-donating and H-accepting centers, are common. So far we have examined the H2-PT reactions involving only the 1,2- H-shift processes (the only available in the literature) which occur along an individual bond (as it occurs in HCO+ reaction 2.3.1). However, a proton can be transported to a more distant reactive center as it occurs in some H2-RT reactions (5). Several examples of 1,3- and 1,4-H-shift processes that follow the H2-PT mechanism have been included in Table 4 for comparison. All three H-transfer mechanisms (viz., the simple H-transfer, proton- transfer and relay H-transfer) were identified for HOOO+ and glycine radical- +• + cation [NH2CH2COOH] . The alternative H-transport reactions for HOOO are expressed as Eqs. (2.3.2) and (2.3.3) for illustration purposes. Remarkably, the reaction of HOOO+ is the only reaction identified so far in which the proton-transport mechanism is preferred energetically to the relay transport. It can also occur in other 1,3-shift processes because the extended 4-membered TS cycle decreases the barrier height compared to the reaction via the 3- membered TS which has a higher ring-strain. The H2-RT mechanism is also +• −1 somewhat preferred by [NH2CH2COOH] (by 2.8 kcal mol ,seeTable 4). Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

Figure 11: Correlation between the H-transport barrier heights (in kcal mol−1) in ionic systems resulted from alternative mechanisms H2-RT and H2-PT (data from Table 4). 438 R. Asatryan and E. Ruckenstein

The barriers for both reaction series (including the regular H-transfer processes, HAT) are related to the reaction energies via the Bell-Evans-Polanyi principle (72,73).

The analysis of the TS vibrational modes shows that the D2-catalyst is better described in some reactions (such as protonated hydrazine, diazene) as almost a spectator, rather than a proton-transporter due to the insigniﬁcant stabilization (or even destabilization) of the TS by the H2-catalyst. The spectator reagents (ligands) are known in other hydrogen containing systems as well. Such a phenomenon, for instance, was reported by Radom and coworkers for the hydrogenation reactions of acrolein (CH2 = CH-CH = O) and propenimine (CH2 = CH-CH = NH) (15). In summary, in all cases in which both H-transport mechanisms occur, the H2-RT mechanism provides a more effective catalysis than the H2-PT mechanism. This can be attributed to the more effective interactions with two alternative H-donor and H-acceptor reaction centers. Some 1,3-H-shift processes where the TS-ring extension reduces the ring-strain energy can be regarded as exceptions. Reactions of radical-cations are generally similar to those of cations (Table 4). For instance, The H2-PT isomerization of methanol radical- +• cation (CH3OH ) to its distonic isomer methyleneoxonium radical-cation • + (C H2OH2 ) that contains both a radical and an ionic site on different atoms of

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 the same molecule, occurs in a manner similar to that presented by Eq. (2.31b) for the HCO+ cation. The isomers are separated by a large barrier and as a result, they have been identiﬁed experimentally (31). However, the barrier is −1 −1 decreased only by 3.2 kcal mol when H2 /D2 is involved (31.8 kcal mol vs. 35.0 kcal mol−1 for the uncatalyzed reaction at the B3LYP/6-31G(d,p) level. It seems interesting to examine in the future if the separation of the two radical and ionic centers in distonic systems affect the catalytic efﬁciency of the H-transport via the H2-PT mechanism. Note that the “proton-transport” concept (analogous to H2-PT reaction) has been argued in literature by Fridgen and Parnis (31). They particularly Dihydrogen Catalysis 439

suggested to consider such reactions as H-atom transport processes on the basis of the analysis of NBO charges in TS of the catalyzed by N2,Ar,Krand +• • + Xe isomerization of CH3OH to C H2OH2 (31).

2.3.2. Effect of Multiplicity The multiplicity affects signiﬁcantly the reaction pathways. In contrast + to the isomerization of the singlet HCO ,inwhichtheH2-catalyst remains unchanged (undissociated) during reaction (operates only via the H2-PT pathway), the triplet 3HCO+ undergoes also a relay transport of a H-atom via the H2-RT mechanism. + Again, a cationic H3 triangle is formed in the transition state, at the top of the molecule. However, the triangle is now turned upward with the D- H edge parallel to the C-O bond, as it occurs in the above H2-RT reactions (Section 2.2). The switch of the mechanism is attributed to changes in the transporting unit. Instead of a proton, the H-atom becomes a (relay-) transporting unit through a H-H-H triad (the total charge of the H3-unit being only qH3 =+0.59e). The barrier for H2-RT reactions is lower than that for the triplet H2-PT reaction (E# = 15.9 kcal mol−1 vs. 23.0 kcal mol−1, respectively) but it is signiﬁcantly higher than that of its singlet counterpart. Thus, the higher multiplicity allows both pathway (H2-PT to H2-RT)to occur due to the smaller polarity of the triplet state TS. This can have important implications and should be taken into account in the relevant kinetic studies. 3 + 3 + The isomerization of the triplet methoxy cation CH3O to its CH2OH isomer also involves both a typical H-atom relay transport and proton transport features (see Table 4 and Fig. 12) in addition to the simple, direct isomerization reaction which is illustrated in Fig. 12, for comparison purposes. As shown by the arrows representing the H2-RT reaction coordinate vector, the C-bound H-atom and the distal D-atom of the D2 are synchronously transferred to the D2 molecule and O-atom, respectively, to generate HD and 3 + + CH2OH products (Eq. (2.3.5)). Moreover, the H3 -triangle, which carries a + higher positive charge in TS ( qH3 =+0.65e) is shifted towards the more Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 electronegative oxygen atom (Fig. 12). The positive charge is mostly located on the catalyst (D2)atoms(∼+0.6e). This indicates the transfer of a H-atom rather than a proton, in accord with the suggestion of Fridgen and Parnis concerning the H2-PT isomerization of methanol radical-cation (31)(vide infra). The barrier for the uncatalyzed isomerization reaction (28.2 kcal mol−1) is reduced by about 9 kcal mol−1, somewhat larger than that via the H2-PT −1 3 + reaction (ca. 7 kcal mol . Thus, the triplet CH2OH undergoes a H2-RT relay 3 + transport reaction to yield the triplet CH2OD isomer, whereas the product of the H2-PT reaction remains isotopically unchanged. 440 R. Asatryan and E. Ruckenstein

Figure 12: The uncatalyzed (HAT ) and catalyzed by dihydrogen (H2-RT and H2-PT ) mechanisms for the isomerization of the triplet hydroxymethyl to the methoxy cation.

In contrast to these two H-atom(proton)-transport reactions of the triplet 3 + 1 + spin state CH2OH ,thesinglet CH2OH undergoes only the hydrogen assisted dehydrogenation (H2-DeH) reaction, viz., the hydrogen elimination + reaction to produce a singlet HCO and two H2 molecules (see Section 2.5 below). Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

2.3.3. Alternative H-transfer Mechanisms for Neutral Molecules and Radicals The H2-PT and H2-RT mechanisms are concurrently functioning in neutral closed-shell and radical systems (Table 5). Table 5 also contains for comparison purposes the simple uncatalyzed (direct) H-transfer (HAT) results. As expected for electroneutral molecular Dihydrogen Catalysis 441

systems, the dihydrogen in H2-PT processes plays a negligible catalytic role unlike the analogous cationic reactions described above. The analysis of the TS-vibration modes shows that the H2-PT mechanism for neutral systems is better described as a simple H-transfer reaction in presence of the spectator H2, rather than the transport of a proton. In contrast, the H2-RT mechanism performs well. The catalytic reaction efficiency repre- # # # # sented by CRT = E RT -E dir, where E RT and E dir are the barriers of the H2-RT and HAT reactions, respectively, varies within a large range. The barrier heights generally correlate three alternative H-transfer pathways and follow the BEP-principle (72,73). In thermoneutral reactions (Erxn = 0.0), the effective orbital overlap becomes a more important issue. # A reasonable correlation between E and Erxn can be seen for the 1,2-H- shift reactions via the H2-RT mechanism (the available CRT > 0 for systems on the top of Table 5). A separate, somewhat shifted correlation can be traced for systems with negative CRT (bottom of the first entry block in Table 5), which generally obey the same rules. A H2-PT transition state for the 1,3-H shift could not be stabilized. The H- atom in electron-rich systems, such as cis-HONO and HC( = O)OH, is pushed- out from the ONO/OCO reaction areas. The TS could not be stabilized even in the ionized cis-HONO+• when the total electronic density decreases and the electrostatic interactions are expected to be reduced. On the other hand, TS was identified for the HOOO+ cation (Table 4). The three H-transfer mechanisms on radicals are in reasonable agreement with each other as follows from Table 5. However, no energy gain via DHC occurs in the H2-PT reactions of radicals, as opposed to the corresponding H2- RT processes, which provide significant reduction of the barriers, similar to that for closed-shell molecules.

2.4. Dihydrogen-Assisted Relay-Transport of H-atoms/Free Valence in Irradiated Polymers In contrast to the gas- and liquid-phase reactions, the main role in the reactivity of solid systems (particularly, the crystalline polymers) is played by the transport of the reacting species—molecules and radicals (14). An intermedi-

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 ary H2-RT-reaction mechanism involving a combined-stepwise relay transport of H-atoms (sH2-RT) has been hypothesized more than half a century ago to explain the transport of free valence in irradiated polymers (74–86,88–94,102– 106,108–112). It can be regarded as an alternative to the H2-RT-reaction (2.1.1) for solid state systems with rigid long-range order. Using the reaction schemes suggested in this paper, the sH2-RT process can be presented as two successive direct H-abstraction reactions (2.1.2a) and (2.1.2b), or their analogs for ions and ion-radicals. 442 R. Asatryan and E. Ruckenstein

The catalytic behavior of dihydrogen in the decay of free radicals in irradiated (or treated thermally) polyethylene (PE) has been recognized and experimentally confirmed in the early 1960th (14,77–81). It has been suggested that it involves a cascade of (relay) H-transfer acts (14,80,82,83). The migration of the free-valence (H-atoms) was explained to occur via a mechanism called “relay transfer of valence” (see, e.g., Emanuel and Buchachenko (14)and Ungar (81)). The process can occur either via a concerted H2-RT mechanism (2.1.1) or a stepwise relay transport of H-atoms (free valence) sH2-RT through the combination of two half-reactions. The first half-reaction (2.1.2a) represents a conventional H-abstraction of H2(D2) by a radical reagent. The second half-reaction (2.1.2b) involves a H-abstraction of the intermediate molecule by an in situ generated H(D)-atom, so that the initial bound to the H-atom reagent never becomes free (14,78,80)(vide infra). A related mechanism has been identified by Kim et al. (18) while considering the interaction of molecular hydrogen with surface vanadium oxide species. The inter-chain processes between semi-isolated reagents may provide also conditions for the concerted intermolecular H2-RT processes to occur via termolecular H-exchange reaction (see Section 2.4.2).

2.4.1. Dihydrogen Catalysis of Free Radical Migration and Decay in Irradiated Polymers Since the earliest observations of Dole and coworkers (77,78) and Ormerod (80), it is known that the hydrogen gas catalyzes the decay of alkyl • macroradicals (-CH2C HCH2-) in irradiated polymers and oligomers (see also Emanuel and Buchachenko (14) and Dole et al. (76)). Varshavskii et al. demonstrated that the irradiation in presence of D2 produces HD (84). Dole and Cracco found that HD was produced also when deuterium gas was added shortly after the irradiation of polyethylene (PE) Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 (78), when radicals have a certain chance to survive. In addition, Voevodskii • et al. suggested that the alkyl radicals (R1 ) are transformed into allyl ones in the presence of double bonds (86). To reach double bonds, the alkyl radicals have been proposed to migrate in the polymers (75,86). Based on the exchange of deuterium gas with PE subsequent to its irradiation, Dole and Cracco suggested (78) that free radical migration occurs via the chain reactions (2.4.1) and (2.4.2): Dihydrogen Catalysis 443

where R• represents an alkyl free radical. They observed (indirectly) that the decay rate of the alkyl free radical was increased with increasing deuterium gas pressure. More decisive evidence regarding the DHC has been found by Ormerod (80) by plotting the second order rate constant for radicals k1 as a linear function of the build-up-pressure of hydrogen

where p is the hydrogen pressure (in mm Hg). He suggested explaining this effect via migration processes assisted by H2. The catalytic effect of dihydrogen has been found essentially the same in the freeze-dried and hot-dried single crystal samples, although no hydrogen solubility has been detected in the freeze-dried samples. The next important step was the decomposition of the alkyl radical decay in γ -irradiated single crystals of PE by Kusumoto et al. into two components: fast and slow decays (90). The slow decay was attributed to the reactions of isolated radicals to be dependent on the crystals thickness, while the fast decay was assumed to take place on the fold surface and crystal defects (see also Ungar (81)). Using this scheme, Dole and coworkers (91,92) interpreted their results on the decay of alkyl radicals in PE of various morphologies (their conversion into allyl radicals via Eq. (2.4.4)) at various hydrogen gas pressures

according to a ﬁrst-order kinetics

where c is the radical concentration. The conversion of radicals occur monomolecularly with two rate constants: almost half of the radicals with ini- ◦ Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 tial concentration cf are rapidly converted at the rate constant kf, whereas ◦ another cs half of radicals is converted slowly with a ks rate constant. The rate constants differ by almost an order of magnitude and both increase pro- portionally with the proportion of amorphous phase. It was concluded that hydrogen acts by means of a surface catalysis. Since H2 catalyzes both the fast and slow decay reactions (Fig. 13), both reactions must occur primarily in the amorphous surface layers. Thus, both types of radicals—fast and slow—are 444 R. Asatryan and E. Ruckenstein

Figure 13: Effect of dihydrogen on the fast and slow ﬁrst-order decay constants of alkyl radicals (adopted from Johnson et al. (91)).

present in PE crystallites, and their conversion to allyl radicals occurs via successive identical (H-) transfer reactions, such as reactions (2.4.1) and (2.4.2), i.e., through a “relay” transfer of a H-atom (cf. Eqs(2.1.2a) and (2.1.2b)). As a result, the free valence migrates in the polymer (H-atom transfer occurs in the opposite direction to the valence transfer) until it meets a double bond (a structural defect in PE). Then, abstraction of a H-atom occurs, which Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 is located in the α-position to the double bond, to yield a localized allyl radical -CH•CH = CH-. The rate controlling step in the alkyl-allyl conversion is the H-transfer reaction, the rate of which varies being dependent on the chemical environment of the radicals (the region of the polymer). Notably, both ﬁrst- order reaction rate constants have been shown to increase linearly with the pressure of the ambient hydrogen gas (Fig. 13). All fast decaying radicals were suggested to be located in the amorphous regions and are immediately affected by the hydrogen dissolved in those Dihydrogen Catalysis 445

regions. Meanwhile, the slowly decaying radicals have ﬁrst to diffuse into the amorphous regions before reacting, at which time the catalytic effect of hydrogen comes into play. However, for both reactions they found almost the same −1 −1 activation energy (Ea)ofca.17 kcal mol in vacuum and 13 kcal mol in the presence of H2, which agreed with the earlier results of Waterman and Dole (85,89). The difference between the fast and slow reactions was attributed to the difference in the pre-exponential factors. In other words, Ea for the uncatalyzed reaction was found to be 17 kcal mol−1, whereas the barrier for the catalyzed by hydrogen reaction was reduced to 13 kcal mol−1. Later, Patel and Dole (92) determined the activation energies and Arrhenius pre-exponential factors A of both fast and slow decay reactions as a function of the fold period of the single crystal sample. For the fast reaction the barriers of 8.5; 13.9; 13.0 kcal mol−1 were found for fold periods of 112, 137, and 150 Å, respectively. The corresponding slow reaction barriers were 20.7; 16.5; 17.1 kcal mol−1. These results are comparable to those found earlier by Wen et al. (10.2, 17.4 kcal mol−1,forthefast and slow reactions, respectively) (93). Even though the dihydrogen catalysis has been mainly considered to take place by exchange processes represented by reactions (2.4.1) and (2.4.2), Ormerod suggested (80) that the hydrogen atoms never become free atoms and the overall process occurs via a concerted mechanism (2.4.6):

In fact, Ormerod was the ﬁrst to hypothesize the possibility of a dihydrogen assisted H-transfer mechanism (dihydrogen catalysis)(80) (see also Waterman and Dole (85)). Evidence in favor of this hypothesis for bound H-atoms during reaction (85) has been provided by Ormerod (80) as well as Dole and coworkers (85,89). It is expected the unbound (free) hydrogen atoms (i) to combine prefer- entially with R• to form RH or, (ii) to recombine with other free hydrogen atoms to yield H2. In the former case, a net loss of both free radicals and hydrogen pressure above the PE sample occur, whereas in the latter case, a net gain of molecular hydrogen is obtained. In addition, the combination of R• macroradicals with the atomic hydrogen is expected to reduce the cross-linking when

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 the hydrogen gas is introduced into the irradiation medium. However, neither of these possibilities was observed experimentally. Our theoretical calculations at the CCSD(T) ab initio level for the uncatalyzed model reaction (Eq. (2.4.7)) between the secondary propyl radical and propylene as precursors of the alkyl and allyl radicals predicts an activation barrier of 17.4 kcal mol−1 (95): 446 R. Asatryan and E. Ruckenstein

The involvement of H2 (H2-RT mechanism via Eq. (2.4.8)) reduces the barrier by about 3 kcal mol−1 (95), in full accord with the experimental data described above (85,89,92):

The catalyzed reaction occurs via a transition state denoted TS1 (not presented here) to distinguish it from TS2 for the interchain recombination of two isolated radicals (vide infra). The “isolated” H-abstraction reaction of the iso-propyl radical with molecular hydrogen (Eq. (2.4.9)) requires an activation barrier of 16.8 kcal mol−1, which is lower than E# for the direct reaction (2.4.7). Thus, the addition of H2 affects both the enthalpy and entropy of reaction, and increases the steric accessibility in rigid systems.

Consequently, the physical meaning of the H2-RT (sH2-RT) catalysis is that a slow (sterically less accessible) state is replaced by a dihydrogen mediated faster (sterically accessible, low E#) reaction—the relay transfer. This results in an increased number of relay transfer steps per unit time, hence in an accelerated recombination. There was an early attempt by Bartoš to evaluate the concerted mechanism based on the empirical BEBO model (94). The activation energy has been estimated to be 20.9 kcal mol−1 which was higher than that for the two-step process (14.8 kcal mol−1), and the two-step reaction mechanism was concluded as preferable. Perhaps, the results could be improved if the current value of 104 kcal mol−1 for BDE of the H-H bond were used, instead of the too large values of 110.6 kcal mol−1 employed in (94).

2.4.2. DHC in Recombination of Isolated (Trapped) Macroradicals. A Novel

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 Disproportionation Mechanism Waterman and Dole found that a fraction of alkyl radicals persist at room temperature following the electron beam irradiation (85,89). These radicals decay very slowly via a ﬁrst order kinetics to form allyl radicals. The process has been strongly catalyzed by molecular hydrogen (Fig. 14). On the other hand, about 35% of alkyl radicals regenerated from the allyl precursors (under UV radiation, at 77 K) has been shown to persist at room temperature with even a lower decay rate in the absence of dihydrogen. These residual radicals decay in the presence of dihydrogen by a second-order Dihydrogen Catalysis 447

Figure 14: Effect of hydrogen and deuterium gas pressure on recombination rate constants of alkyl radicals (adopted from Waterman and Dole (85)). Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

Figure 15: A novel transition state model (TS2) for dihydrogen mediated interchain recombination (disproportionation) of two alkyl radicals. The trapped active centers on different chains are represented by propyl radicals to produce CH3-CH2-CH2D + CH2 = CH-CH3 + HD (Eq. (2.4.10)). The TS-vibration modes and the interchain distance are highlighted (95).

kinetics without forming allyl radicals. The mechanism of this process has not been explained properly and still remains open. Figure 15 illustrates a transition state model proposed by us to account for the decay of the trapped (residual) alkyl radicals in the presence of H2. 448 R. Asatryan and E. Ruckenstein

The model is based on reactions (2.4.1) and (2.4.4) for the generation and recombination of such radicals, respectively, and involves disproportionation of two alkyl radicals (Eq. (2.4.10)) via the transition state TS2:

In fact, the DHC-reaction between two trapped radical centers located • on two different chains (either penultimate RCH2CH CH3 or interior • R’CH2CH CH2R types), leads to their recombination and provides a theoretical basis for the experimental work of the Smith and Jacobs (79) on reaction (2.4.11):

This could explain also why a decrease of these residual alkyl radicals increases the content of unsaturation without forming allyl radicals, observed by Waterman and Dole (85), being merely a second-order disproportionation reaction. It should be emphasized that the recombination of two alkyl radicals will simply form a hexane molecule if the process occurs in gas- or liquid-phases where the radical centers are capable of moving to each other, before D2 is acting. However, in crystals the two radicals can be located (trapped) in separate positions on different chains. The barrier height for the TS2 transition state (Fig. 15) is 17.9 kcal mol−1 at CCSD(T)/6-31G(d,p) and 18.2 kcal mol−1 at B3LYP/6-31G(d,p) levels, which are close to the experimental values for radical decay processes presented above. In summary, the disproportionation reaction (2.4.4) can be important in solid state processes being negligible in gas- and even in the liquid phase - cf. radiolysis of hexane in liquid (100), and gas-phases (101). Solid systems involve more rigid chains, which are inaccessible for direct interactions of radical centers, and the developed model can explain the observed catalytic role of the dihydrogen. It should be noted that an intrachain TS2-like H-abstraction reaction can

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 also occur mediated by radical pairs and clusters formed during irradiation (see, e.g., Emanuel and Buchachenko (14) and Ungar (81)). A 1,3-biradical seems to be the smallest possible pair of radicals. A recombination via TS3 (not presented here) requires the presence of either a diradical or cyclopropane moieties. The barrier height is somewhat reduced when a H-atom is removed from • the initial molecular reagent CH3CH2C H2 by transforming it into a diradical • • −1 CH2 CH2C H2. The activation energy is 14.6 kcal mol (95). An interesting issue is the growth and decay of polyene and polyenyl free radical groups upon radiation studied by Dole and coworkers (97). Theoretical Dihydrogen Catalysis 449

interpretations of polyene formation in radical polymerization processes can be found in (98)and(107). The formation of polyene (conjugated) fragments has been found to correlate with the calculated stabilities of model chain-carrier radicals and spin densities of the corresponding reactive centers. In en-yne systems (such as vinyl acetylene), the spin densities in different carbon-centers determine the possible chain-crossing features. The recombination of two alkyl radicals mediated by dihydrogen can also occur through a synchronous dehydrogenation reaction mediated by dihydrogen (Eq. (2.4.12)):

This process, however, is energy demanding and faces a high barrier of activation (52 kcal mol−1 calculated at the B3LYP level). The barrier can be reduced in ionic or ion-radical systems generated during radiation (102)(vide infra). The dihydrogen-catalyzed decay of macroradicals is a widespread process (81). DHC particularly occurs in isotactic polypropylene (103) and even in irradiated crystalline amino acids. In γ -irradiated crystalline L-leucine, for instance, H2 signiﬁcantly increases the rate of decay of the free radicals (104) (but the process is a second order diffusion controlled reaction). However, not all types of macroradical decays are catalyzed by H2 (105). In contrast to the strong catalytic effect of the dihydrogen on the alkyl-decay, no accelera- • tion of decay of allyl free radicals (R2 ) occur due to the presence of hydrogen (93), probably, because of the resonant stabilization of allyl free radicals. The absence of the DHC effect is expected when comparing the corresponding energy barriers (95): 8.9 kcal mol−1 vs. 22.8 kcal mol−1 and 9.3 kcal mol−1 vs. 25.6 kcal mol−1 (Benson and co-workers estimated it to be about 10 kcal mol−l (106)) depending on the type of substituents CH2 = CH-CH-(..D..D)-C2H5 (95). The H2-RT mechanism is effective also with ionic and ion-radical systems even though the experimentalists have ruled out the participation of ionic species in dihydrogen catalyzed processes in selected conditions (81). The ionic H2-RT mechanism can be relevant to processes in which H2 (D2) is introduced during irradiation (84). Intriguingly, the DHC-barriers in ionic systems are signiﬁcantly lower than those for molecular reactions. To conclude, the properties of solid polymers are dominantly controlled Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 by the transport of the reacting species—molecules and radicals—as opposed to the gas- and liquid-phase reactions (14). This leads to a new and unusual solid state reaction mechanisms and kinetics. There is a balance between the two limiting modes: diffusion and kinetic (see, e.g., Wen et al. (93)). All these issues require further exploration of the elementary reaction mechanisms of radical decay and migration of a free valence in organic solids. In particular, in the reactions described above, the stepwise relay H-transport mechanism (sH2-RT) is relevant for the migration of a H-atom via TS1 (E# = 12.82 kcal mol−1, which is close to the experimental result of 12.5 kcal mol−1 (99), while 450 R. Asatryan and E. Ruckenstein

the H2-RT mechanism prevails in macroradical recombination reactions (via TS2, Fig. 15).

2.5. Dihydrogen Assisted Dehydrogenation (H2-DeH)

2.5.1. Overview Dihydrogen-mediated dehydrogenation (H2-DeH, Eq. (2.1.4)) is a catalytic process similar to other DHC mechanisms. It involves the H2-reagent which is regenerated during reaction (albeit with a different isotopic content, if D2 is used).

This is the most investigated DHC-mechanism in literature (5,15,17,20,113–115). The reaction between cis-diazene (N2H2) and molecular hydrogen (Eq. (2.5.1)) constitutes a classical example of such a process (5,17). The stereoselective process occurs via a six-membered-ring TS, typical to this type of reactions (Fig. 16). This does not constitute a relay transport of H-atoms, but rather a bond- exchange reaction leading to the decomposition of a substrate (diazene) to elemental hydrogen and nitrogen (Eq. (2.5.1)) (17). It can be also regarded as an elementary model for the synchronous double H-atom transfer of a H-donor (cis-diazene) to a hydrogen molecule (Eq. (2.5.1)) (17): Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

Figure 16: TS for the H2-DeH reaction of cis-diazene (Eq. (2.5.1)), calculated at CCSD(T)/CBS level (20). Dihydrogen Catalysis 451

The process requires the relatively low activation energy of 22.5 kcal mol−1 calculated at the CBS-QB3 composite, and 26.4 kcal mol−1 at the CCSD(T)/CBS ab initio levels of the theory (20). The reverse reaction can be considered as a symmetry-allowed N2- activation process with a barrier of only about 76 kcal mol−1. Such a barrier is significantly lower (by almost 50 kcal mol−1) than the analogous, orbitally forbidden bimolecular nitrogen fixation reaction N2 + H2 → cis-N2H2 (ca.125 kcal −1 mol )(131). However, the reverse reaction is a termolecular process (N2 + H2 + H2 → H2 + cis-N2H2) with low probability to occur in the gas-phase under ambient conditions. It may occur at very high pressures or when the molecular hydrogen is activated by a TM or a hetero-surface (5). The gas-phase decomposition reaction (2.5.1) involves the most important substrates of the nitrogenase enzyme (see Asatryan et al. (5) and the references cited therein). The isotopic scheme explains the formation of HD observed during the turnover cycle of nitrogenase, the catalyzed reduction of nitrogen in the presence of molecular deuterium. A DHC- mediated mechanism for the first- step activation of N2 by nitrogenase enzyme has been suggested in a recent report (12b). It will be examined in detail in an upcoming article, along with the various H2-assisted metalorganic reactions, and other applications of DHC. The unique catalytic properties of dihydrogen have been particularly demonstrated by Radom and coworkers (15) when comparing the calculated 1,2-hydrogenation barriers of benzene vs. the acidities of different HX catalysts including dihydrogen (Fig. 17). The gas-phase acidity values correspond to the calculated at B3LYP/6-31+G(d) level of theory enthalpies at 0 K for the corresponding deprotonation reactions: HX → H+ + X− (15). Increased catalyst acidity decreases the barrier, generally observed for the catalytic 1,2-hydrogenation reactions. Figure 17 shows that the barrier Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

Figure 17: Calculated barriers vs. catalyst acidities (deﬁned as the enthalpies of deprotonation reactions) at 0K for the catalytic 1,2-hydrogenation of benzene: a triangle - H2,diamonds - ﬁrst row, squares - second or third row elements (adopted from Zhong et al. (15)). The units are in kJ mol−1. 452 R. Asatryan and E. Ruckenstein

decreases as the catalyst becomes more acidic, with the exception of the H2-catalyzed reaction. A more complex relationship has been found for the catalytic 1,4-hydrogenation. On the basis of both the low acidity and the low proton affinity of H2 it was expected for the H2-catalysis to have little effect on the barrier. However, the barrier is lowered significantly (33.5 kcal mol−1) reflecting the importance of the orbital symmetry effects in H2-DeH reactions along with the electrostatic interactions in TS. The H2-catalyzed 1,2-hydrogenation reaction has a lower barrier than many of the acid-catalyzed 1,2-hydrogenations with significantly higher acidities. The higher symmetry in the H2-catalyzed hydrogenation enables a concerted synchronous reaction, which leads to higher orbital overlap and a lower barrier. As emphasized by Radom and co-workers (Fig. 18)(15), the simple orbital correlation rules are working well for the dihydrogen catalysis reactions of simple aromatic compounds in predicting their relative intramolecular reactivities. The orbital correlation diagram in Fig. 18 shows that the 1,4- hydrogenation of benzene is formally symmetry allowed and is expected to have a lower barrier of activation, in contrast to the 1,2-hydrogenation, which is symmetry forbidden and is expected to have a higher barrier. Similar reactivity patterns for 1,2- and 1,4- positions were found by us for more complex H2-DeH reactions of tetralin (vide infra, Fig. 19)(116). The thermally forbidden 1,2-hydrogenation reactions ([6+2]-type) can be assisted by the catalyst (H2), which creates a thermally allowed [6+2+2] reaction and, hence, leads to a lowering of the reaction barrier. In contrast, the presence of a catalyst in thermally allowed 1,4-hydrogenations ([4+2]-type) hinders the reaction by converting the symmetry-favored reaction pathway to one that is symmetry-forbidden ([4+2+2]-type). Importantly, the presence of heteroatoms in conjugated systems (such as 1,4-hydrogenation of acrolein and propenimine considered in (15)), lowers the overall symmetry of the system, and hence the orbital symmetry considerations are less significant in polar asymmetric systems. Several examples are included in Table 6 for comparison (vide infra). Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 2.5.2. Dehydrogenation of Tetralin The energy-intensive dehydrogenation reactions using TM-based catalysts are common in the current industrial practice for alkene production. In this regard, the metal-free dehydrogenation provides a promising alternative. According to the present calculations (116), the molecular hydrogen can have a significant catalytic effect on the successive dehydrogenation of tetralin (TET), which is considered a key process in pyrolysis (117,118). The kinetics and product distribution in the pyrolysis of tetralin have been recently analyzed by Poutsma (117) and Bounaceur et al. (118). The Dihydrogen Catalysis 453

Figure 18: Orbital correlation diagrams for the 1,4- and 1,2-hydrogenation of benzene (15).

challenge in developing a mechanistic model for the pyrolysis of TET arises from a delicate balanced competition among several product-forming pathways (ring contraction to methylindan derivatives, successive dehydrogena- tions, hydrogenolytic ring-opening and side-chain cracking, and expulsion of ethylene to yield benzocyclobutene) that are very sensitive to tempera-

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 ture, concentration, and conversion level (117). It is important to understand the role of the 1,2-dihydronaphthalene impurity based on the dominant formation of H2 and the multiple initiation-, propagation-, and termination pathways. The most ubiquitous pathway, present under all conditions and dominant under several, is the dehydrogenation of tetralin to form 1,2-dihydronaphthalene (DHN), followed by the dehydrogenation of DHN to naphthalene (NP) (118). Figure 19 involves six dehydrogenation processes (Eqs. (2.5.2)–(2.5.7)) via the transition states TS1-TS4, calculated at B3LYP/6-31(d,p) level of theory 454 R. Asatryan and E. Ruckenstein

via 1,2-DHN

TS1a TS2a 0.00 +2H2 (2.5.2) 100.3 99.7 1,2-DHN +H2

TS3a +H2 TS4a +3H2 0.00 (2.5.3) 66.3 1,2-DHN +2H2 58.7

via 2,3-DHN

TS1b TS2b 0.00 +2H2 (2.5.4) 109.3 47.1 2,3-DHN +H2

TS3b +H2 TS4b (2.5.5) 0.00 +3H2 69.7 ? 2,3-DHN +2H2

via 1,4-DHN

TS1c TS2c +2H2 0.00 (2.5.6) 69.3 84.3 1,4-DHN +H2

TS3c +H2 TS4c (2.5.7) 0.00 +3H2 ? 1,4-DHN +2H2 47.5

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 Figure 19: Dehydrogenation of tetralin (TET): uncatalyzed (direct) reaction barriers versus H2- catalyzed reactions (116). The numbers next to arrows indicate the barriers in kcal mol−1.NP and DHN stand for naphthalene and dehydronaphthalene, respectively.

(116). TS1 and TS2 correspond to the uncatalyzed reactions, whereas TS3 and TS4 represent H2-catalyzed processes. The barrier heights are indicated next to the arrows. Sufﬁxes a, b,andc indicate the pathways via intermediates 1,2-DHN, 2,3-DHN, and 1,4-DHN, respectively. The effect of dihydrogen is signiﬁcant compared to the uncatalyzed reactions (34–41 kcal mol−1). The barrier for the uncatalyzed 1,4-hydrogenation is Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] at 10:18 29 September 2014

a Table 6: Dihydrogen hydrogen- (D2) assisted dehydrogenation reactions (H2-DeH)

R P # b TS #c d µ µ E ν1, µ Edir CDeH Erxn HXYH Debye XY Debye kcal mol−1 cm−1 Debye kcal mol−1 kcal mol−1 kcal mol−1

6-Membered Ring TS e CH3OOH 1.7 CH2OO 4.4 97.7 −1775.7 4.2 65.7 −32.1 51.9 f C2H4 0.0 C2H2 0.0 86.1 −1538.7 0.9 − 41.5 H2O2-gauche 1.8 O2 0.0 87.0 −1160.2 1.8 127.9 40.9 24.9 C2H6 0.0 C2H4 0.0 75.3 −1672.1 0.7 122.5 47.2 29.9 CH3NH2 1.4 CH2NH 2.1 68.3 −1752.8 1.7 101.4 33.1 23.4 HCOOH 1.5 CO2 0.0 65.7 −2042.8 2.9 67.1 1.4 −7.4 NH2NH2 2.0 N2H2-cis 3.0 64.5 −1691.6 0.5 107.3 42.9 22.7 CH3OH 1.8 CH2O 2.4 63.5 −1724.4 2.1 87.8 24.3 18.4 f 455 CH2NH 2.1 HCN 3.1 60.5 −1786.7 2.2 −−5.5 N2H2-cis 2.9 N2 0.0 20.5 −2012.7 1.2 89.2 68.5 −51.5 5-Membered Ring TS BeH2 0.0 Be 0.0 74.1 −931.5 1.8 106.3 32.2 44.4 CH2O 2.4 CO 0.1 60.8 −2115.2 0.3 80.1 19.3 0.8 MgH2 0.0 Mg 0.0 46.1 −1764.2 1.8 79.7 33.6 −3.8 f N2H2-iso 2.6 N2 0.0 25.1 −2049.2 1.0 −−51.5 Ions + + NH3OH 3.7 NH2O 3.6 78.5 −1078.1 3.1 138.4 59.9 63.0 + + NH3NH2 3.0 NH2NH 2.5 70.5 −1209.1 1.9 72.2 1.7 44.6 + + NH2NH 2.6 HNN 3.2 54.0 −1333.7 2.1 85.7 31.7 21.9 +g + CH2OH 2.1 HCO 3.7 48.8 −919.0 3.8 81.5 32.7 28.5 + + NH2O 3.6 NO 0.4 43.4 −1428.5 3.4 88.6 45.2 15.9 Ions Radicals +• +• f H2O2-cis 3.7 O2 0.0 53.9 −487.3 2.6 − 64.5 +• +• CH3NH2 1.4 CH2NH 0.7 59.9 −696.1 1.3 72.5 12.6 45.4 +• +• CH3SH 1.2 CH2S 1.7 53.2 −879.1 2.9 57.9 4.7 30.9 +• +• CH3OH 1.5 CH2O 3.1 44.9 −887.1 3.62 65.1 20.1 23.5

a) TS vectors are sketched to show formation of two HD molecules. All results are recalculated at the same B3LYP/6-311+G(2d,p) level for adequate comparisons. b) Vibrational modes describing the reaction coordinate. c) Uncatalyzed reaction barrier. d) Gain or loss in activation energies via catalysis. e) Criegee intermediate, carbonyl oxide zwitterion biradical. f) TS could not be identiﬁed. g) Data in Fig. 21 are at B3LYP/6- 311+G(d,p) level to be compared with results of Harvey et al. (136). 456 R. Asatryan and E. Ruckenstein

much lower (by about 15–31 kcal mol−1) than that for the 1,2-dehydrogenation, in accord with the orbital symmetry considerations. Both the uncatalyzed 1,4- hydrogenation and the catalyzed 1,2-hydrogenation are formally symmetry- allowed processes similar to that of benzene (Fig. 18). The barrier for H2-catalyzed 1,4-hydrogenation of tetralin is also significantly lower (by more than 11 kcal mol−1) than that for 1,2-hydrogenation, despite comparable reaction enthalpies. The remarkable effect of H2 catalysis can be attributed to the more efﬁcient orbital overlap for the synchronous H2- catalyzed reaction (15). On the other hand, as emphasized by Radom and coworkers (15), the orbital symmetry considerations are less signiﬁcant in polar asymmetric systems. The presence of heteroatoms in the 1,4-hydrogenation of acrolein and propenimine lowers the overall symmetry of the system, and the process is not controlled by orbital symmetry. The role of the H2-DeH reactions in tetralin is expected to be important especially in the pyrolysis of aromatic compounds and industrial hydroprocessing, where hydrogenation, dehydrogenation, and hydrogenolysis play a major role, and should be included in the relevant mechanistic considerations and kinetic models. Table 6 considers several H2-DeH processes, involving reactants of different polarities and ionic states. The barrier heights are clearly related to the reaction enthalpies by the Bell-Evans-Polanyi principle (72,73). The exception is the reaction of dihydrogen with the cis-oriented in TS hydrogen peroxide, perhaps due to the required additional rotation of the originally gauche conformation of H2O2 in the transition state. The most effective H2-DeH process among those considered in Table 6 molecular systems is the stereoselective reaction of cis- N2H2, involving two in-ring nitrogen atoms in TS and producing stable N≡N molecule. The barrier increases by 5 kcal mol−1 when the same products are formed from iso-N2H2 (H2NN) because of the increased ring-strain energy in the 5-membered ring TS, and the lack of in ring π-electrons. Another intriguing example includes H2-DeH reaction of formaldehyde.

2.5.3. Dehydrogenation of Formaldehyde Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 Photodissociation of formaldehyde has been the subject of numerous theoretical and experimental studies (119–132). It involves two decomposition and one isomerization channels: Dihydrogen Catalysis 457

whereas the isomerization of CH2O produces hydroxycarben, HCOH (125,126), • • the unimolecular decomposition generates H2+CO molecular and H +HC O radical products. Decomposition to molecular fragments (Eq. (2.5.8)) requires an activation energy of 79.2 kcal mol−1 determined from the Stark level- crossing spectroscopy data with the best ﬁt of the RRKM rate constants (127). According to (123), the molecular channel dominates at 28188 cm−1 (80.6 kcal mol−1) energies, whereas the threshold of the alternative radical decay pathway is 32,250 cm−1 (92.2 kcal mol−1). Moore and co-workers found that most of the available energy in the molecular reaction is released in translation and the rest of the energy is parti- tioned between a strong CO rotational excitation and a modest H2 vibrational excitation (130). It was explained by the skewed structure of the transition- state in which both hydrogen atoms are on the same side of the CO molecule (Eq. (2.5.8a)) with little energy in the CO vibrational excitation or in the rotational excitation of H2 (130,131):

A second molecular channel has also been suggested to explain the obtained unusual energy distribution in products at excitation energies above the threshold of the radical channel (130). The detected rotationally cold CO and vibrationally hot H2 were related to the opening of the radical channel via a distinct new pathway to the formation of molecular products via intramolecular hydrogen abstraction (H-atom “roaming” mechanism) (131). Dihydrogen catalyzes both the isomerization (relay H-transfer via the H2- RT mechanism) and dehydrogenation reactions (via the H2-DeH mechanism) (133). Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

The decomposition barrier calculated by us for uncatalyzed reaction (2.5.8a) at the B3LYP level using the cc-pVDZ correlation consistent basis set results in an activation energy of 78.1 kcal mol−1, which agrees well with 458 R. Asatryan and E. Ruckenstein

Figure 20; Transition state structure of the dihydrogen catalyzed molecular decomposition of formaldehyde (Eq. (2.5.11)) at B3LYP/cc-pVDZ level (133).

the above mentioned literature data, whereas the dihydrogen-catalyzed reaction requires a much lower barrier of activation (53.8 kcal mol−1). In fact, the H2-catalyst reduces the barrier for molecular decomposition reaction by more than 24 kcal mol−1. Remarkably, the transition state for the DHC reaction (2.5.11) has the symmetric structure presented in Fig. 20, in contrast to the skewed TS-structure of the direct decomposition reaction (Eq. (2.5.8a)) providing a distinct energy distribution of products. Obviously, such a signiﬁcant energy gain through the formation of a symmetric molecular decomposition TS should be taken into account in the kinetic modeling of combustion and pyrolysis of relevant processes. This expected to have even broader implications in industrial and H2-abundant interstellar processes.

2.5.4. Dehydrogenation of Triplet Cations In the previous section, the important role of the multiplicity providing the switching between H2-RT and H2-PT as well as the H2-DeH mechanisms is emphasized. Another relevant example is the possible isomerization of singlet + 3 + CH2OH to triplet CH3O catalyzed by dihydrogen (Fig. 21)(133). Such an isomerization is known to represent a key elementary reaction in the overall unimolecular decomposition of the triplet methoxy cation (134,135). It has been suggested to involve a spin-ﬂopping through two minimum energy crossing points (135). The conventional stepwise mechanism involves formation and Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 + [1,2]-elimination (dehydrogenation) of singlet CH2OH intermediate to yield 1 HCO + H2 products (Eq. (2.5.13)):

A key argument in favor of the conventional mechanism consists in the almost identical kinetic energy release from both methoxy and hydroxymethyl cation isomers observed in experiments (134). Whatever the actual mechanism Dihydrogen Catalysis 459

Figure 21: Sketch of the geometry of the transition state in dihydrogen catalyzed + dehydrogenation of singlet CH2OH (133).

of the overall process is, the unimolecular dehydrogenation (decomposition) of 1 + −1 CH2OH has a barrier height of 88.1 kcal mol , according to CCSD(T)/cc- p-VTZ//B3LYP/6-311+G(d,p) calculations of Harvey and coworkers (135). This value nearly coincides with that predicted by B3LYP/6-311+G(d,p) DFT method (88.2 kcal mol−1). + Remarkably, the dehydrogenation of singlet CH2OH catalyzed by dihydrogen appears to be much more facile than the uncatalyzed reaction. The reaction operates via the same H2-DeH mechanism and cyclic TS structure (Fig. 21) with an energy barrier for the catalyzed decomposition of + −1 singlet CH2OH as low as 48.7 kcal mol , calculated at the same DFT level employed in (135), for adequate comparison. Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 Thus, the barrier is reduced by more than 30 kcal mol−1 compared to the barrier of the uncatalyzed (direct) reaction:

This represents another elegant example of H2-catalyzed H2-DeH process yet to be implicated in the analysis of processes relevant to the decomposition of a methoxy cation and related systems. 460 R. Asatryan and E. Ruckenstein

2.6. DHC and Two-State Reactivity The participation of spin inversion in the rate-determining steps, known as the multi-state reactivity (MSR), is believed to be a key feature in a number of chemical (mainly TM containing) processes where reactants, intermediates and products have different multiplicities (136–147). The spin-crossing can dramatically affect the DHC-reaction mechanisms and rate constants. For some organometallic DHC-reactions, it has been shown to diminish the barriers. The spin-sensitive reactions have been examined in more detail in Asatryan et al. (5). Here, we consider only the reaction of FeO+ with dihydrogen for illustrative purposes. This is a simple model for oxidation of hydrocarbons by TM oxenids in liquid phase and in enzymatic systems (146,147). DHC is believed to play a role in this classical reaction. The mechanism of the reaction of FeO+ with dihydrogen is a well- documented example of the MSR-concept explored by two research groups (140–141). The overall process can be presented by Eq. (2.6.1), which involves + + + three intermediates: H2FeO ,HFeOH ,andFe(OH2) and two TS:

The main mechanistic puzzle of this reaction is its poor efficiency (one in every 600 collisions) despite the fact that it is a highly exothermic −1 process (Hrxn = -37 kcal mol ), orbitally unrestricted and spin-allowed (140,141,145,146). Formally, the reaction conserves the spin because the reactants and products have ground state of the same spin multiplicity, FeO+(6 +) and Fe+(6D). In addition to the inefficient - barrierless reaction, the process involves another very efficient reaction with a barrier of ca. 0.6 eV (140,141). Theoretical calculations using multireference CASPT2 (143) and single- reference CCSD(T) (144) methods predict close barrier heights for the last reaction on the sextet PES (about 0.55 eV). More controversial results were found for the relative reactivities of the ground sexted and first excited quar-

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 tet state reagents and intermediates to account for the inefﬁcient-barrierless reaction, possibly caused by spin-crossing. A current view on the entire reaction mechanism is best described by the energy proﬁles presented in Fig. 22 (142a), which basically employs the B3LYP calculation results using Watcher’s 8s6p4d basis on Fe and Dunning-Huzinaga triple-zeta basis for light atoms (Fig. 23)(145). The thermal process is considered to involve double crossing of high- and low-spin surfaces along the reaction coordinate at the minimum energy crossing points C1 and C2 (Fig. 22). Dihydrogen Catalysis 461

Figure 22: Two-state reactivity with double spin-crossing at points C1 and C2, adopted from Irigoras et al. (142a). Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

+ Figure 23: Sextet and quintet PES cross-Sections for FeO /H2 System calculated at B3LYP level (adopted from Schröder et al. (145)).

Dihydrogen catalysis provides an alternative pathway (2.6.2) to produce + Fe + H2O, illustrated in Fig. 24. The mechanism is based on the assump- tion that a second H2 can feasibly interact with intermediates of the reaction (2.6.1). FeO+ is known to possess three coordination sites around the iron 462 R. Asatryan and E. Ruckenstein Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014

+ + Figure 24: Comparison of the uncatalyzed FeO +H2→Fe +H2O reaction pathways with their DHC-counterparts involving a second H2-molecule: (a) - sextet and (b) - quintet spins state PESs, calculated at the B3PW91/6-311+G(2d,p) level. Dihydrogen Catalysis 463

center (148) and the formation of a multi-hydrogen intermediate is gener- + ally expected (58–62). Note that [FeO .H2] complex has not yet been detected experimentally. On the other hand, Bohme and coworkers observed, using the Selected-Ion Flow Tube (SIFT) technique, that FeO+ sequentially coordinates up to three H2O, N2O, and CO2 ligands at room temperature (148). The lack + of the [FeO .H2] intermediate has been explained by a possible collisional dissociation in the sampling region. According to the spin-coupling calculations of Danovich and Shaik (136), + the probability of a ﬁrst spin-crossover on PES H2+FeO to form a quartet HFeOH+ intermediate is very low. However, the next sextet barrier to ﬁnal −1 + products is high (∼25 kcal mol ), with the reverse reaction barrier to H2FeO even higher. This suggests that if the sextet HFeOH+ intermediate is formed it would be kinetically stable with a noticeable lifetime. The initial complex + + H2FeO can also be associated with H2 to form an intermediate (H2)2FeO . The combination of these factors could allow accumulating some equilibrium concentration of the intermediates to interact with another readily available H2 reagent to initiate a DHC process. Dihydrogen in this case acts as a catalyst:

We have identiﬁed all relevant transition states along the reaction paths (2.6.3) for both the quartet and sextet states, at the B3PW91/6-311+G(2d,p) level of theory (Fig. 24):

All DHC transition states, including the initial sextet-state barrier, are located below the entrance channel. The sextet-state reaction now proceeds without a barrier, in contrast to that in reaction (2.6.1), yet DHC-reaction would not be much efﬁcient because of higher entropic limitations. On the other hand, it can occur via chemical activation, as it occurs in gas-phase combustion processes (149,150) due to the substantial well-depth of the initial intermediate (Fig. 24a).

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 As an alternative, the sextet and quartet states can undergo intersystem crossing during DHC reaction in the same manner as it occurs in reaction (2.6.1). The overall process starts from the lower energy sextet reagents then switches to the sextet state before a product is formed (Eq. (2.6.3) and Fig. 24). The barrier for the uncatalyzed reaction (2.6.1) at B3PW91/6-311+G(2d,p) level of theory is 7.6 kcal mol−1 in accord with the literature data described above (cf. Figs. 23 and 24). In summary, the DHC mechanism can be considered as an alternative to the two-state reactivity processes to account for some peculiarities 464 R. Asatryan and E. Ruckenstein

+ of the FeO + H2 reaction. In particular, due to the spin-crossing and/or entropy limitations, the DHC-reaction can provide an explanation for the inefﬁcient-barrierless reaction observed experimentally.

3. CONCLUSIONS

Dihydrogen-assisted processes are involved in different types of H-atom transfer, dehydrogenation and hydrogenation reactions. In this article, we attempt to systematize all these processes in the framework of the concept of dihydrogen catalysis (DHC), and present detailed consideration of the basic DHC-mechanisms and their potential applications. The classification of H2-assisted reactions suggested in this paper involves five major types of elementary reaction schemes: (i) relay transport of H- atoms (H2-RT), (ii) stepwise relay transport of H-atoms (sH2-RT), (iii) proton transport (H2-PT), (iv) dehydrogenation (H2-DeH), and (v) Pre-activated dehydrogenation (PA-DeH). Our classification is strongly based on a detailed potential energy surface (PES) analysis using various theoretical protocols. All DHC mechanisms occur via cyclic transition states. Dihydrogen is generally included in a transition state ring, except for the H2-PT reactions in which the catalyst-dihydrogen is located out-of-the-ring, in close prox- imity. Most pathways are sensitive to the electronic environment and spin configuration of the reaction complexes. Both H2-RT and H2-PT mechanisms lead to H-transfer reactions and provide the same products. However, they lead to different products when hydrogen isotopes are involved. In contrast to the proton transport processes, the H-atom-relay transport results in H/D-exchange in the presence of D2. Thus, the H2-RT mechanism can explain various H/D-exchange processes in the simple “gas-phase”, intricate organometallics, as well as in heterogeneous processes. The H2-PT mechanism is most effectively functioning in cationic systems. However, it can operate also in less polarized systems where H2 plays mostly a spectator role. It occurs even in negatively charged system. The H2-DeH mechanism also occurs with reagents of different polarities and ionic states. The remarkable examples explored in this paper include

Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 the dihydrogen assisted dehydrogenation of formaldehyde and tetralin. Some organometallic examples are discussed in Asatryan et al. (5). A detailed analysis of the organometallic DHC-reactions, as well as the potential contributions of the DHC to various practically important processes, will be the subject of a separate article, which currently in preparation. The properties of solid systems are dominantly controlled by the transport of the reacting species. This leads to a new and unusual solid state reaction mechanisms and kinetics. The stepwise relay H-transport mechanism (sH2-RT) is relevant to the H-migration in irradiated polymers, while the H2-RT mechanism prevails in macroradicals’ recombination reactions. Dihydrogen Catalysis 465

All these issues require further exploration at the elementary reaction mechanisms levels. In addition, quantum tunneling may play a supporting role for low temperature DHC processes, where the entropy factor is less important. The effect of solvation is most important for some liquid-phase processes such as the reaction of the zwitterion form of glycine. The comprehensive PES results presented in this paper can be employed further to develop detailed kinetic models for a multitude of reactions. It should be emphasized that DHC-schemes are not isolated processes, but parts of general mechanisms that typically include also conventional (in some cases even faster) H-transfer, direct hydrogenation/dehydrogenation and radical reactions. Thus, they can dominate or not for a given process, including the conventional catalytic reactions. To distinguish their contributions, one needs to pay especial attention to the relative reactivities and detailed mechanisms in the frame of DHC concept and presented here theoretical background, which combines the results from different, rarely crossing areas. For adequate description of the mechanisms, a variety of examples are presented which are available in the literature or calculated in this article. Some suggestions are hypothetical merely to provide more incentive for the interested readers. Dihydrogen catalysis opens a variety of new pathways, which can be also relevant to a number of industrially significant processes, such as hydrogen assisted catalytic reduction (H2-SCR), water-gas-shift, spillover of hydrogen, dihydrogen catalyzed growth of metal-silicide nanowires, high pressure hydroprocessing (hydrodenitrification, hydrodesulfurization, hydrodeoxygena- tion), activation of CO, and others. This is a broad topic for future publications, which includes particularly the ongoing study on hydrogen spillover onto a non-reducible support based on the Pd4/SiO4 cluster analysis (151), as well as a new DHC-based mechanism for the first (rate-controlling) step activation of N2 by nitrogenase enzyme (12b).

FUNDING

This work was supported in part by the Ruckenstein fund (University at Buffalo) and the National Science Foundation under grant CBET-1330311. We acknowledge also the generous allocations of computing time from the Downloaded by ["University at Buffalo Libraries"], [Rubik Asatryan] 10:18 29 September 2014 Center for Computational Research - University at Buffalo (SUNY), and the High Performance Computing Center at New Jersey Institute of Technology.

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