(218c) Dihydrogen Catalysis Assisted by Molecular Hydrogen Reactions: Storage Linked Processes and Possible N2-Activation

ABSTRACT. Density functional theory is employed in the context of molecular hydrogen assisted processes in order to disclose two types of reactions: (i) symmetric-syn¬chro¬nous double hydrogen-atom abstraction, and (ii) H2-mediated H-transfer. The reaction analysis is specifically applied to the heterogeneous decomposition of diazene, the most important intermediate of N2-activation processes. KEYWORDS. Diazene, Dihydrogen Catalysis, Nitrogen Fixation, Nitrogenase, Hydrogen Storage. While developing a comprehensive potential energy surface for hydrazine, N2H4 at CCSD(T)/CBS and CBS-QB3 levels of theory to uncover chemical activation reactions of amidogen radicals (NH2), we have revealed a stereoselective reaction pathway (Eq.1) directly associated to the gas-phase decomposition of cis-diazene, the most important substrate of nitrogenase enzyme and a key intermediate in heterogeneous catalytic processes involving elemental nitrogen and hydrogen, with fairly low gas-phase activation barrier of 22.5 kcal mol-1 (Fig.1): 1,2 H2 + cis-N2H2 ↔ N2 + H2 + H2 (1) Please Insert Fig.1. Here This reaction (Eq. 1) is the simplest possible model for the symmetric-synchronous double hydrogen-atom abstraction reaction from an H-donor as cis-diazene molecule. or: D2 + cis-N2H2  2 DH + N2 D?notation we employed here for clarity, and note that such an isotopic scheme explains the observed formation of HD during N2-reduction in the presence of D2 (vide infra). 3 Intriguingly, the reverse N2-activation reaction (Eq.-1) faces a barrier of only 76 kcal mol-1 which is almost 50 kcal mol-1 lower than the analogous bimo¬le¬cu¬lar nitrogen fixation reaction (ca.125 kcal mol-1).4 The reverse reaction is termolecular and is consequently, significantly un¬favored entropically. The reaction becomes viable at very high pres¬sures5 but would be feasible if the molecular hydrogen is partially bonded to a hetero-surface and the H-H bond is both polarized and weakened. Natural Bond Orbital (NBO) analysis of the reaction (1) at B3LYP/6-311++G(2d,2p) level of theory shows that N-H bonds in isolated cis-N2H2 are polarized (natural charges on H and N atoms are QH= +0.14e and QN= -0.14e, respectively). However, the electron density in the transition state ring (Fig.1) is almost completely delocalized, with partial polarization of newly formed H-H bonds (D-H in Eq.1): QH= +0.08e and QN= -0.07e. Terminal hydrogen atoms (D atoms) posses small negative charges, QD = - 0.01e. Natural electron configuration of nitrogen atoms in TS is 2s0.782p1.77. As expected, 2p-orbital electrons are responsible for the partial electron donation from the isolated diazene (2s0.762p1.86) to the approaching hydrogen. Hence, promoting role of the initial activation of attacking H2 can be anticipated. In this report, we have studied analogous metalorganic systems and discuss several possible implications. The fixation of N2, generally considered as its reduction to NH3, is an important topic throughout the natural sciences studied over several decades; yet the detailed molecular mechanism remains an open issue.6-11 The N≡N bond is exceptionally strong (ca. 225 kcal mol-1) and hence the first step activating this triple bond is extremely unfavorable despite the overall process of ammonia synthesis being exothermic by ca 22 kcal mol-1. Recent theoretical calculations at G2M(MP2)//MP2/6-31G** level suggest4a that the lowest barrier associating H2 and N2 into iso-diazene is 125.4 kcal mol-1, consonant with the MRCI(+Q)/aug-cc-pVQZ//FVCAS/aug-cc-pVQZ value of 5.39 eV.4b Chain reactions are therefore considered as the dominant gas-phase and surface mediated mechanisms for hydrogenation of nitrogen.4 It is known, that very high pressures and temperatures (200 atm and 500oC) with iron-based catalysts are employed to hydrogenate nitrogen in the industrial Haber-Bosch process, which is believed to proceed via initial dissociative adsorption of reagents in a (Fe, or Ru based) catalyst surface. 4b In contrast, the activation of N2 occurs at ambient conditions in biological media, supported by enzymes, such as nitrogenase.6 The con¬ven¬¬tional bio-catalytic mechanism considers binding of N2 to the Fe-[H2] comp¬lex, reduction of N2 to diazene (diimide, N2H2) and hydrazine (N2H4) and release of NH3.5 Hydrazine as an inter¬me¬diate has been detected ex¬pe¬rimentally, while diazene, which is also proven to be a substrate for nitrogenase, 7 is not observed due to its high reactivity. 6b It is further important to note that molecular hydrogen is always produced when nitrogenase reduces N2 to NH3 (a turnover cycle); evolution of hydrogen by nitrogenase is uniformly observed, even under high nitrogen pressures (e.g., 50 atm).8 Most N2 fixing bacteria contain hydrogenase, the another relevant enzyme, to recycle excessive hydrogen.9 The equation for conversion of N2 on nitrogenase is considered as N2 + (6+2x)H+ +(6+2x)e-  2NH3 + xH2 (2) The value of x varies from 1.0 - 7.5 depending on the type of enzyme used; for main MoFe enzyme it is considered unity. 6,8,10 The nitrogenase also reduces other substrates such as C2H2, HCN and N2O to C2H4, CH4 + NH3 and N2 + H2O, respectively. Note that hydrogen is not evolved during enzymatic reduction of acetylene and interestingly, C2H2 is specifically converted to cis-C2H2D2 in presence of D2O.6 Iron plays a vital role in variety of enzymatic and heterogeneous catalytic processes.6,9,11 We have therefore, studied several simple heterogeneous models based on iron-organic complexes to further evaluate the relevance of assisted by molecular hydrogen mechanisms. Calculations were performed at the B3LYP hybrid DFT level12 using the Dunning-Hay double zeta basis set13 and Los Alamos effective core potential14 for Fe as implemented in Gaussian-03. 15 A Dresden/Stuttgart pseudo¬potential is also used to verify key results, not presented here. Different spin-less (S=0) and high-spin (S=4) neutral and ionic systems were considered for comparison. In this short report the reaction of hydrogen molecule with a straightforward (CH3)2FeH2 model as a source of activated hydrogen atoms will be addressed. A five (trigonal bipyramidal) and a four (tetrahedral) coordinated Fe-complexes are also adopted, to generally model some features of the organometallic catalytic active sites. Structures similar to the gas-phase transition state (Fig.1) are observed for reactions between the organome¬tallic¬ model (CH3)2FeH2 with diazene and CH2NH intermediates (Fig2a,b), where a hydride of a tran¬si¬tion metal serves as a donor of the dihydrogen, a ?H2-catalyst?: R2FeH2 + cis-N2H2 = R2Fe + H2 + H2 + N2 (3) R2FeH2 + CH2NH = R2Fe + H2 + H2 + HCN (4) In Fe-organic models the hydridic H-atoms are pre-activated, and this substantially reduces the activation energy. The barrier height for the catalytic decomposition reaction of cis-N2H2 (Eq.3) with R≡CH3 is fairly low (6.5 kcal mol-1) compared to the 22.5 kcal mol-1 barrier in the gas-phase. Thus, the ring-mechanism is expected to be functioning in both gas and condensed phase systems. Please Insert Fig.2. Here This mechanism dealing with different nitrogenase substrates and intermediates (such as diazene, HCN, N2O, etc) may serve as a simple molecular model involving requisite evolution of hydrogen (an attribute to the turnover cycle of nitrogenase enzyme) and could explain why the activation process is inhibited by H2.8 We demonstrate that a similar ring- transition state endures in different environments, in a variety of neutral and ionic systems. This dihydrogen catalysis phenomenon can be possibly applied to the decomposition processes of high hydrogen content compounds. It opens also a variety of new pathways re¬le¬vant to a number of processes, including remote transfer of H-atoms supported by molecular hydrogen. 16,18 We report one further, me¬¬cha¬nistically important mechanism, viz., a new H2-mediated H-transfer reactions, which we believe may occur virtually in any systems. We demonstrated this mechanism on a simple Fe-S-Fe backbone organometallic model with neutral CO ligands (Fig.3), which is a rather common moiety in biological systems.16 Fig.3a demonstrates a regenerative cycle for hydrogen transfer from an S-H group to the coordinated N2H-group, the possible intermediate of N2-hydro¬ge¬na¬tion. Here, the reactant H2 molecule is a transferor of an H atom from a bridged S to the distal N atom. It can be considered a catalyst as it is chemically unchanged at the end of reaction, when only one hydrogen isotope is involved. A similar process holds in the case of H-transfer from a Fe-H group in unsaturated tetrahedral Fe1 center to the axially located N2H group in the trigonal bipyramidal structure (Fig3b), while formed (and yet adsorbed) N2H2?group transfers back to the unsaturated Fe1 center. Note, that hydrogenation (protonation with further electron attachment) of a bridged sulfur atom in nitrogenase models is shown to facilitate activation of N2 adsorbed on the proximate Fe-centers.10,17