DFT Studies on the Reduction of Dinitrogen to Ammonia by a ... · 15250−15251). However, the mechanistic aspects involved in the catalytic cycle and the possibility of reducing

DFT Studies on the Reduction of Dinitrogen to Ammonia by aThiolate-Bridged Diiron Complex as a Nitrogenase MimicYi Luo,*,† Yang Li,† Hang Yu,† Jinfeng Zhao,† Yanhui Chen,† Zhaomin Hou,†,‡ and Jingping Qu*,†

†State Key Laboratory of Fine Chemicals, School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian116024, People's Republic of China‡Organometallic Chemistry Laboratory, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

*S Supporting Information

ABSTRACT: We have recently reported that the binuclear iron complexesCp*Fe(μ-SR1)2(μ,η

2-R2NNH)FeCp* (R1 = Me, Et; R2 = Me, Ph; Cp* = η5-C5Me5) as novel models of nitrogenase could effectively catalyze the N−N bondcleavage of hydrazines, including NH2NH2 (J. Am. Chem. Soc. 2008, 130,15250−15251). However, the mechanistic aspects involved in the catalytic cycleand the possibility of reducing N2 by these complexes have remainedunexplored. In the present study, DFT has been applied for modeling thebinding of Cp*Fe(μ-SEt)2FeCp* with N2 and the reduction of the N2 to two NH3 molecules at the diiron centers. Thecalculations of model system indicate that the hydrogenation (H+ + e−) energetically prefers to occur at the N atoms rather thanthe Fe or S atoms. The rate-determining step of the reduction of HNNH could be the isomerization of a μ,η2-HN−NH2moiety to a μ-HN−NH2 form with a NH-bridging feature, which occurred at the diiron centers. Such a transformation of thebinding mode of HN−NH2 might be driven by unequal charge populations on the two Fe atoms. The results show an event ofnet electron transfer from the ancillary ligands to the Fe atoms and the NHNH2 moiety during the rate-determining step. In viewof the experimental observations reported previously, the current computations suggest that the diiron complex Cp*Fe(μ-SEt)2FeCp* is possible to bind N2 and reduce it to NH3 via protonation/reduction. Such a reduction of N2 to NH3 at the diironcenters favorably occurs through the HNNH and HNNH2 forms rather than via the H2NNH2 unit.

■ INTRODUCTIONThe activation of nitrogenase substrates mediated by transition-metal complexes bearing sulfide ligands has attractedconsiderable attention,1 not only because of the biologicalinterest in nitrogenase but also its industrial potential inammonia synthesis from N2 and H2. Much effort has beenspent on understanding the transformation process from N2 toNH3 in biological systems,1e,2 and great progress has beenmade in this framework. It is generally considered that theactive site of nitrogenase is most likely the FeMo-cofactor(FeMoco), where N2 binds and is reduced.3 FeMoco has thestoichiometric formula MoFe7S9X and is bound to the proteinthrough the end Fe atom and the Mo atom. In FeMoco, six ofthe seven Fe atoms at the core form a prism and are only 3-foldcoordinated. The light atom X as a μ6 ligand located at thecenter of the prismatic cavity is presumed to be N, O, or C buthas not yet been identified. The three central sulfur atoms (μ2-S) bridge two Fe atoms each, and the remaining six sulfurs (μ3-S) bridge three Fe atoms each. It is also considered that diazene(NHNH) and hydrazine (NH2NH2) are nitrogenase-relavent substrates as partially reduced species of N2. Sincethe biological nitrogen fixation process is rather complicated, itis hard to directly obtain related mechanistic information. Thisis one reason only very limited knowledge from experiments isavailable for an understanding of the nitrogen fixationmechanism. An alternative approach is to design and synthesizenitrogenase model compounds and to investigate their

reactivity toward N2 or its partially reduced species such asNHNH and NH2NH2. On the basis of the structuralcharacters of FeMoco, much attention has been paid todeveloping sulfido-ligated transition-metal complexes. Forinstance, Coucouvanis et al. reported that the cubane-typeMFe3S4 (M = Mo, V) clusters showed catalytic activity towardthe reduction of hydrazine to ammonia.4 Hidai et al. reportedthe catalytic N−N bond cleavage of hydrazine by cubane-typeRuMo3S4 and Mo2M2S4 (M = Ir, Rh) complexes.5 In view ofintermetallic cooperation, efforts toward the activation of theN−N bond by binuclear metal complexes were also made. Forexample, Hidai et al. demonstrated a thiolate-bridged binuclearRu complex showing catalytic activity toward the disproportio-nation of hydrazine into NH3 and N2.

6 The groups of Lee7 andHolland8 reported that some diiron complexes bearing thiolateligands could activate the N−N bond. It was also reported thatthe electrochemical reduction of a dimolybdenum complexbearing the HNNR (R = Me, Ph) ligand in the presence ofan acid led to cleavage of the NN double bond to give anilineand an amido or ammine complex.9 However, these binuclearcomplexes could not reduce H2N−NH2 or HNNH to twoNH3 molecules in a catalytic manner.The other powerful approach to elucidate the mechanism of

N−N bond reductive cleavage related to nitrogen fixation in

Received: October 10, 2011Published: December 27, 2011

Article

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nitrogenase is theoretical calculation. A number of computa-tional studies have been reported to discuss the binding of N2

to iron−sulfur clusters modeling FeMoco and their electronicproperties.10 Dance10d and Rod et al.10f found that the on-topbonding of N2 to a single Fe atom was the most stable.However, calculations on the activation of the N−N triple bondof N2 or its reductive species, such as NHNH and H2N−NH2, mediated by iron−sulfur model complexes are relativelyless. The chemistry associated with N2 reduction is mostcommonly discussed in the context of Schrock11 and Chatt12

mechanisms (Scheme 1a), which were initially proposed formononuclear metal (Mo, W) complex systems. Despitedifferent theoretical approaches, Stavrev13 and Nørskov14

found that the hydrogenation of N2 by FeMoco modelcompounds occurred through the [M]−NH2NH2 (M =metal) intermediate, as shown in Scheme 1b. However, Kastnerand Blochl15 recently reported that the reduction of N2 to NH3

at the Fe sites of FeMoco favorably experience the NHNH andNHNH2 forms but not the NH2NH2 fashion. The reduction ofHNNH at dimolybdenum centers reported by McGrady16

was also found to go through the NHNH2 forms (Scheme 1c)rather than the NH2NH2 fashion before NH3 release.Inspired by the results that a series of thiolate-bridged

diruthenium complexes have diverse reactivities,1a,6,17 two ofthe authors have recently explored the iron analogues.18,19 Itwas found that, for the first time, a thiolate-bridged diironcomplex as a nitrogenase mimic possesses excellent catalyticactivity toward N−N bond reductive cleavage of hydrazines.19

In that work, the catalytic cycle was also proposed, as shown inScheme 2. In the case of R = Ph, complex A (Scheme 2) wasstructurally well characterized and could also catalyticallyreduce NH2NH2 (R = H in Scheme 2) to NH3.

19 As shownin Scheme 2, the HNNH moiety of Cp*Fe(μ-SEt)2(μ,η

2-HNNH)FeCp* (Cp* = η5-C5Me5,) could be reduced toammonia (R = H in A) and the species Cp*Fe(μ-SEt)2FeCp*(B) might be generated. However, the catalytic mechanism,including the reaction intermediates and whether B could bindN2, remained unknown. This stimulated us to further explorethe possibility of B to bind N2 and further reduce it and toinvestigate the reductive mechanism involved in the reactionobserved experimentally. Thiolate-bridged diiron clusters

bearing cis-HNNH ligands have recently also isolated andstructurally characterized, which show high activity toward thecatalytic cleavage of the N−N bond of hydrazines.20

During our DFT studies on the electronic structure andreactivity of rare earth metal complexes,21a−g,i,j we also becameinterested in computing a transition metal complex system.21h

In the present study, we performed DFT calculations to seewhether B shown in Scheme 2 can possibly bind N2 and furtherreduce it to HNNH, a moiety of A (R = H in Scheme 2).The mechanistic details of further reduction of HNNH at thediiron centers was also elucidated. The structures andenergetics associated with the reductive process were explored,from which the rate-determining step can be depicted. We alsofocused on examining the factors affecting the N−N bondactivation. The theoretical results obtained in this study areexpected to help experimentalists develop more efficientcatalysts for catalytic activation of the N−N bond of hydrazine(H2N−NH2), diazene (HNNH), and perhaps N2 as well.

■ COMPUTATIONAL DETAILSThe two-layer ONIOM(TPSSTPSS/6-31G*:UFF)22 calculations werecarried out for modeling the binding of Cp*Fe(μ-SEt)2FeCp* with N2and the reduction of N2 to the HNNH moiety. In the ONIOMcalculation, the methyl groups of η5-C5Me5 and μ2-SEt ligands areplaced in the outside layer treated by the UFF force field23 (low-levelcalculation), and the other atoms, including those in HNNH moiety

Scheme 1. Mechanisms of N−N Bond Reduction at the Metal Center(s)

Scheme 2. Previously Proposed Catalytic Cycle for N−NBond Reduction at Diiron Centers

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and proton, constitute the inner layer. In these calculations, the stericeffect of the methyl groups was considered for the N2 binding. Forsimplicity, however, the model compound CpFe(μ-SMe)2(μ,η

2-HNNH)FeCp (Cp = η5-C5H5) was adopted for pure DFT calculations tomodel the cleavage of the NN double bond of the HNNHmoiety at the diiron centers. The DFT calculation was performed withthe TPSS exchange and correlation functional and the all-electronbasis set 6-31G*. Normal-coordinate analyses were performed toverify the geometrically optimized stationary points and to obtain thethermodynamic data. To consider solvation effects, single-point self-consistent reaction field (SCRF) calculations based on the conductor-like polarizable continuum model (CPCM)24 were performed for thegas-phase optimized geometries from ONIOM or pure DFTcalculations. The CPCM model has been widely used for theinvestigation of solvation effects in various metal complex systems.25

The tetrahydrofuran was used as solvent, corresponding to theexperimental conditions.19 In the SCRF calculation, the molecularcavity was built up by using UFF radii, and the 6-311+G(d,p) basis setwas used for all atoms. To take into account the influence of enthalpyand entropy, the Gibbs free energy contributions from the gas-phasecalculations were added to give the final free energy in solution (ΔGsol,298.15 K, 1 atm). According to the experimental conditions,19

(LutH)BPh4 was used as the proton source and Cp2Co was used asthe reductant. The energies for the reduction and protonation stepsare therefore relative to the processes [Cp2Co] → [Cp2Co]

+ andLutH+ → Lut, respectively.26 The energy calculations of the twoprocesses adopted the same strategy described above. That is, theSCRF single-point calculation at the TPSSTPSS/6-311+G** level wasperformed at the TPSSTPSS/6-31G* geometry. The energy profilewas described by the relative free energies in solution. For eachstationary point, the most stable spin state was tested. The open-shellspecies was treated with an unrestricted manner, and the stabilities ofthe wave functions were tested. The broken symmetry was used duringthe optimization. All of the calculations were performed with theGaussian 09 program.27

■ RESULTS AND DISCUSSION1. Effect of Density Functional and Basis Set. Before

accessing the mechanistic aspect, we tested the effects of severaldensity functionals and basis sets on the geometry of the modelcompound CpFe(μ-SMe)2(μ,η

2-HNNH)FeCp and com-pared with its geometrical parameters with available crystaldata of Cp*Fe(μ-SMe)2(μ,η

2-PhNNH)FeCp* observedexperimentally. Since the B3LYP, TPSS, and BP86 functionalsare often used to compute iron-containing metal complexsystems, such functionals were also tested in this study. Theresults are shown in Table 1, where the atom labeling refers toChart 1. With the B3LYP functional, the pseudopotentialmethods (BS1−BS3) produced an average error of more than

0.02 Å in distances and greater than 3° in angles. However, theall-electron basis set (BS4 and BS5) with the same functionalgave better results. With the BS5 (6-31G* for all atoms), theTPSS and BP86 gave the best results: viz., an average error of0.01 in distances and less than 2° in angles (Table 1).Considering that the TPSS functional has been successfullyused for calculating an iron−sulfur cluster complex,28 the TPSSfunctional29 was selected in the present study.

2. N2 Binding and Its Partial Reduction to HNNH. Tosee whether the diiron complex Cp*Fe(μ-SEt)2FeCp* (B inScheme 2) binds N2 and reduces the N2 to the NHNH form,a moiety of A (R H, Scheme 2), we performed theTPSSTPSS/6-311+G**//ONIOM(TPSSTPSS/6-31G*:UFF)calculations. The computed energy profile and some importantstructures are shown in Figures 1 and 2, respectively. Thesuperscript of the labeling of the stationary point shows thecorresponding spin multiplicity and charge. For example, thelabeling m03 denotes the neutral species m0 with a spinmultiplicity of 3, and the labeling m1b3+ represents structurem1b with a multiplicity of 3 and charge of +1 (protonationproduct). As shown in Figure 1, among the three N2 complexesof Cp*Fe(μ-SEt)2FeCp*, m01 (S = 0 state) and m0a1 (S = 0state) are higher in energy then m03 (S = 1) by 8.70 and 7.34kcal/mol, respectively. The bare complex Cp*Fe(μ-SEt)2FeCp* has a singlet ground state, and its triplet state ishigher in energy by 3.62 kcal/mol. m01 and m0a1 show side-on/end-on and bridged end-on binding manners of N2,respectively. However, m03 shows an end-on binding fashionof N2. In fact, taking the structure of m01 or m0a1 as an initialstructure, geometrical optimization at the multiplicity of 3 (S =1 state) led to m03. In m03, the binding energy of N2 wascomputed to be 13.25 kcal/mol (containing a BSSEcorrection). Such a type of binding energy (BE) was calculatedas BE [E(N2) + E(cat)] − E(cat-N2), where E(N2), E(cat),and E(cat-N2) are the total electronic energies of N2, Cp*Fe(μ-SEt)2FeCp*, and the coordination complex Cp*Fe(N2)(μ-SEt)2FeCp*, respectively. These electronic energies were

Table 1. Comparison of Computed Geometrical Parameters of Model Compounda and Available Experimental Compounds(Interatomic Distances in Å and Angles in deg)

functional and basis setb Fe···Fe Fe1−S1 Fe1−S2 Fe1−N1 Fe2−N2 N−N Fe−S1−Fe Fe−S2−Fe av errorc

exptld 3.211 2.289 2.300 1.875 1.831 1.337 89.2 88.8B3LYP/BS1 3.178 2.342 2.341 1.896 1.848 1.287 85.6 85.7 0.04/3.4B3LYP/BS2 2.992 2.273 2.273 1.834 1.768 1.299 82.5 82.6 0.07/6.5B3LYP/BS3 3.175 2.335 2.333 1.896 1.844 1.290 85.8 85.9 0.03/3.2B3LYP/BS4 3.187 2.334 2.332 1.881 1.837 1.301 86.3 86.4 0.02/2.7B3LYP/BS5 3.194 2.339 2.337 1.879 1.836 1.300 86.3 86.5 0.03/2.6TPSS/BS5 3.199 2.310 2.309 1.863 1.832 1.328 87.7 87.9 0.01/1.2BP86/BS5 3.199 2.306 2.305 1.861 1.827 1.330 87.9 88.1 0.01/1.0

aModel compound: CpFe(μ-SMe)2(μ,η2-HNNH)FeCp. bBS1: 6-31G* for C, H, N, and S atoms; LanL2DZ and associated poseudopotential for

Fe atom. BS2: 6-31G* for C, H, N, and S atoms; LanL2DZ with outer p function31 and associated poseudopotential for Fe atom. BS3: 6-31G* for C,H, and N atoms; SDD and associated poseudopotential for S and Fe atoms, a single d polarization function (exponent of 0.65) augmented for Satom. BS4: 6-31G* for C, H, N, and S atoms; 6-311G* for Fe atom. BS5: 6-31G* for all atoms. cThe average error of interatomic distances (Å) andangles (deg, in italics). dTaken from the crystal structure of Cp*Fe(μ-SMe)2(μ,η

2-PhNNH)FeCp*.19

Chart 1

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computed at the level of TPSSTPSS/6-311+G**//TPSSTPSS/6-31G* theory. In a similar way, however, thebinding of the solvent molecule THF to the metal centers wascomputationally found to be energetically unfavorable, possiblydue to the steric effect of the THF molecule and theelectrostatic interaction between the binding atoms: viz.,Fe···O (see Figure S1 in the Supporting Information). TheHOMO−LUMO energy gap of m03 is 1.783 eV at theTPSSTPSS/6-311+G**//ONIOM(TPSSTPSS/6-31G*:UFF)level, which suggests the stability of m03.The N−N bond length (1.141 Å) in m03 is longer than that

(1.114 Å) in the free N2 molecule by 0.027 Å. This indicatesthat the N2 was activated by the diiron complex. The activationof the N2 moiety in m03 is also suggested by the NAO bondorder (1.70) of the N−N bond. The bond order of 1.70 of theN−N bond indicates that the triple bond of dinitrogen could bereduced at least to a double bond once the dinitrogen is boundto the Fe center. Such a reduction was also previously reportedfor a hydride-bridged diniobium complex system.30 The bondorder of Fe−Fe in m03 was computed to be 0.02, suggestingalmost no Fe−Fe bond interaction in this complex. However,the Fe−Fe bond order of 0.22 was found for the bare complexCp*Fe(μ-SEt)2FeCp*, suggesting a weak Fe−Fe interaction.An addition of one proton to a nitrogen atom of m03 led tom1b3+, which has an S = 1 state; the S = 0 state is 3.75 kcal/molhigher in energy. The subsequent reduction of m1b3+ led tom1b4 with an S = 3/2 state; the S = 1/2 state is 5.30 kcal/molhigher in energy. There are two competing pathways for theisomerization of m1b4 to m12 with the μ-η2-NNH moiety.One is directly through TS4, with an energy barrier of 11.81kcal/mol. The other one is a stepwise process, which occurredvia TS12, the intermediate m1a2, and the transition state TS2.The conversion of m1b4 to m1a2 is feasible, as suggested by theenergy barrier of 4.02 kcal/mol. The TS12 has an S = 1/2 state,and attempts to locate its higher spin state (S = 3/2) werefruitless. Although the stepwise process needs to overcome a

slightly larger energy barrier of 15.19 kcal/mol compared to theformer process (barrier of 11.81 kcal/mol), the relative energiesof TS4 (16.92 kcal/mol) and TS2 (15.86 kcal/mol) are similar(Figure 1). In fact, geometrical optimization of the lower spinstate structure (S = 1/2) of TS

4 led to TS2. A following intrinsicreaction coordinate, however, confirmed that the TS2 connectsm1a2 and m12. The m12 has an S = 1/2 ground state; the S =3/2 state is 12.36 kcal/mol higher in energy. In m1b4, a spindensity analysis shows that the Fe atom connecting to theNNH motif carries a spin population of 0.54, and another Featom has a spin density of 1.80. The NNH motif in m1b4 has aspin population of 0.44. In m1a2 (S = 1/2), however, the spinpopulation mainly locates on the Fe centers, as suggested bythe spin densities of 0.50 and 0.46 at the two Fe atoms,respectively. The structures and spin states of m1b4, m1a2, andm12 suggest that the bridging character of the NNH moietycould decrease the spin state to achieve a more stable structure:viz., m12. m12 could undergo protonation to give m22+ andsubsequent reduction to give m21. The m21 has an S = 0ground state; the S = 1 state is 28.31 kcal/mol higher in energy.The whole process for the reduction of dinitrogen to theNHNH moiety assisted by the diiron complex is exergonic by36.05 kcal/mol. Two possible structures, viz., m2a1 and m2b1,which could be obtained by protonation and subsequentreduction of m12 and m1a2, respectively, were also located(Figure 2). However, both of them with a NNH2 moiety arehigher in energy than m21 by 27.9 and 13.99 kcal/mol,respectively. This suggests that an intermediate with a NNH2

moiety (cf. Scheme 1a) was unlikely to be involved in thecurrent reaction system. m21 has the μ,η2-HNNH moietyand is actually the species A (R = H in Scheme 2). Asmentioned above, m21 was experimentally found to experienceNN double bond reductive cleavage and finally to give twoNH3 molecules.19 In this sense, our computational resultssuggest that the diiron complex Cp*Fe(μ-SEt)2FeCp*, which is

Figure 1. Computed energy profile for the protonation/reduction of dinitrogen to HNNH. The schematic representation of protonated species issimilar to that of the corresponding neutral species and therefore is not included in this figure. The same is true for Figures 2−4.

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computationally found to be a minimum, can possibly reduceN2 to ammonia via protonation and reduction processes.3. Reductive Cleavage of the NN Bond of the HN

NH Moiety. The reductive cleavage of the NN bond of theHNNH moiety at the diiron centers has been experimentallyobserved.19 The computed energy profile of such a process isshown in Figure 3. The free energies in solution are relative tothe complex 13 and the protonation (or reduction) processesillustrated in Computational Details (vide ante). The super-script of the labeling of the stationary point (Figure 3) showsthe corresponding spin multiplicity and charge, which is similarto that for stationary points in Figure 1. As shown in Figure 3,the starting complex has two almost isoenergetic spin states, S =0 (11) and S = 1 (13), with an energy difference of 0.15 kcal/mol. The protonation of 13 is endergonic by 13.48 kcal/mol,leading to 23+ with an S = 1 state; the S = 0 state is 4.34 kcal/mol higher in energy. The subsequent reduction of 23+ gives 22

with an S = 1/2 state; the S =3/2 state is 13.97 kcal/mol higher

in energy. The first protonation/reduction process is slightlyendergonic by 1.80 kcal/mol. We also explored the possibilitiesof protonation/reduction occurring at a Fe or S site in 13. Itwas computationally found that the resulting species CpFe(μ-SMe)2(μ,η

2-HNNH)Fe(H)Cp (S = 1/2 ground state) andCpFe(μ-SMe)(μ-S(H)Me)(μ,η2-HNNH)FeCp (S = 1/2

ground state) are higher in free energy by 11.97 and 17.95kcal/mol, respectively, in comparison with 22. This suggeststhat the first protonation/reduction process preferably occurredon a nitrogen atom, such as N1 (see Chart 1 for the atomlabeling), and led to the complex 22. The protonation/reduction occurring at the N1 atom slightly changed the corestructure of 22. For example, the N−N bond length of 1.457 Å(Figure 4) and the Fe···Fe distance of 3.245 Å (Table 2) arelonger than that in 13 (1.328 and 3.236 Å, respectively). In 22,the Fe−NH2 contact (2.012 Å) is also therefore longer than theFe−NH bond length (1.833 Å). Unlike the case for 13, the twoFe−S bonds in the Fe−S−Fe connection of 22 are no longerequal. The populations of Mulliken charge and spin density arealso changed during the protonation/reduction process (Table2). The 22 has unequal charge population on the two Fe atoms(0.45 and 0.58). The same is true for the two N atoms. It isfound that the unpaired electron of 22 (S = 1/2) is mainlydistributed on the Fe2 (0.79) and N2 (0.22) atoms (Table 2).22 may coexist with the NH-bridged structure 32, and their

interconversion occurs via the transition state TS[2-3]2 (Figure3). The conversion of 22 to 32 needs to overcome a free-energybarrier of 22.07 kcal/mol, and 32 is slightly higher in energythan 22 by 1.83 kcal/mol. However, the energetically favorableprotonation and subsequent reduction of 32, which led to 42+

Figure 2. Optimized stationary points for N2 binding and its partial reduction. The relative free energies in solution are given in kcal/mol. Thedistances are in Å and angles are in deg. The methyl groups of the Cp* ligand are omitted for clarity.

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and then to 41, could facilitate the conversion of 22 to 32 viaTS[2-3]2. The higher spin states (S = 3/2) of the threestationary points involved in this isomerization are higher inenergy by 3−18 kcal/mol (Figure 3). The 44+ (S = 3/2) and 43

(S = 1) are about 2.5 kcal/mol higher in energy compared withtheir corresponding lower spin states. An isomeric structure of32, viz., CpFe(μ-SMe)2(μ-NNH3)FeCp, which was obtained viahydrogen transfer in 32, was computationally found to be 23.70kcal/mol higher than 32. This suggests that the N−NH3 form(cf. the Chatt pathway in Scheme 1a) was unlikely to beinvolved in the reduction process of HNNH. Severalisomeric structures of 41 were also computationally located,viz. 4a−d, as shown in Chart 2. 4a,b, having newly formed Fe−H bonds, are higher in energy than 4c,d with newly formedHN−H and H2N−H bonds (Chart 2). However, all of themare significantly less stable by more than 24 kcal/mol incomparison with 41 with (μ2-NH)···NH3 moiety (Chart 2 andFigure 3). Therefore, the formation of 4c with (μ,η2-H2N−NH2) moiety (cf. the FeMoco pathway in Scheme 1b) also isunlikely to occur during the current reduction process.The Fe···Fe distance in 32 is shortened by 0.631 Å compared

with 22 (Table 2). The structure of TS[2-3]2 shows a Fe···Scontact of 2.548 Å, which is significantly longer than that(∼2.30 Å) in 22 and 32. This indicates that the Fe−S bond mayserve as a “switch” to assist the interconversion between 22 and32. It is noteworthy that 22 has an unequal charge populationon the two Fe atoms, and the same is true for the spin densityon the Fe atoms of 22 (Table 2). Such unequal populations maybe a driven force for the conversion of 22 to 32 having equalpopulations of both charge and spin density on the two Featoms. In 41, as the product of second protonation andsubsequent reduction processes, the N−N bond has completely

cleaved, and the resulting NH3 molecule to be released interactswith the bridging μ2-NH moiety via a hydrogen bond (seestructure 41 in Figure 4). Attempts to locate the N−N bondcleavage transition state during the second protonation/reduction processes were fruitless. Actually, relaxed scans ofthe N−N contact against the energy for the second protonationand reduction steps show no significant transition state regionon the potential energy surface considered (Figure 5). Thisresult suggests that the N−N bond cleavage has no energybarrier with respect to the current calculation. The release ofNH3 from 41 led to the intermediate 51 with an NH-bridgingfeature. The S = 1 state of this structure is slightly higher inenergy by 2.16 kcal/mol. The 51 underwent the third andfurther the fourth protonation/reduction processes to give 73

with the newly formed NH3 (Figure 3). The formation of 73 isexergonic by 80.65 kcal/mol. The binding energy of NH3 in 73

was computed to be −23.63 kcal/mol (containing BSSEcorrection), while the binding energy of HNNH in 13 wascomputed to be −103.58 kcal/mol. This suggests that therelease of NH3 from 73 via an access of HNNH isenergetically favorable, and the catalytic cycle could beachieved.As shown in Figure 3, the isomerization of 22 to 32 via TS[2-

3]2 needs to overcome an energy barrier of 22.07 kcal/mol,which is greater than the energy required for the firstprotonation step (13.48 kcal/mol). The former could betherefore the rate-determining step for the reduction of theHNNH moiety to two NH3 molecules. For this reason, wefurther analyzed the stationary points involved in the rate-determining step. To get more accurate results, such an analysisis based on single-point calculations (tpsstpss/6-311+G**) onthe optimized geometries. The results are shown in Tables 3

Figure 3. Computed energy profile for the reductive cleavage of the NN double bond.

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and 4. As shown in Table 3, the Wiberg bond indexes (WBI)for Fe···Fe contacts significantly increased from 0.06 in 22 to0.23 in 32. This suggests that the very weak interaction betweenthe two Fe atoms was strengthened by the bridging of the μ-

NH−NH2 moiety. However, the strength of the N−N bonddid not significantly change during the isomerization of 22 to32, as suggested by the similar bond indexes (∼1.0) in 22, TS[2-3]2, and 32. The WBI of Fe1···S1 in TS[2-3]2 (0.57) is smaller

Figure 4. Optimized stationary points involved in the HNNH reduction. The distances are in Å, and angles are in deg.

Table 2. Mulliken Charge, Spin Density, and Fe···Fe Distances (Å) in the Computed Stationary Pointsa

Mulliken charge spin density

stationary point Fe1 Fe2 N1 N2 Fe1 Fe2 N1 N2 Fe···Fe dist

13 (S = 1) 0.56 0.56 −0.54 −0.54 0.73 0.72 0.29 0.29 3.23623+ (S = 1) 0.53 0.60 −0.67 −0.60 1.10 0.86 −0.03 0.21 3.20922 (S = 1/2) 0.45 0.58 −0.64 −0.60 0.02 0.79 −0.01 0.22 3.245TS[2-3]2 (S = 1/2) 0.56 0.50 −0.61 −0.61 0.59 0.13 0.02 0.20 3.10232 (S = 1/2) 0.47 0.47 −0.54 −0.67 0.48 0.48 0.00 0.07 2.61442+ (S = 1/2) 0.51 0.51 −0.93 −0.74 0.45 0.44 0.00 0.02 2.48841 (S = 0) 0.51 0.51 −0.90 −0.79 2.74751 (S = 0) 0.53 0.53 −0.76 2.76263+ (S = 1) 0.48 0.48 −0.93 0.99 0.99 0.04 2.52662 (S = 1/2) 0.45 0.45 −0.92 0.48 0.48 0.07 2.62172+ S = 1/2) 0.34 0.47 −0.95 1.11 −0.06 0.00 2.54873 S = 1) 0.42 0.53 −0.93 2.01 0.00 0.00 3.3954a (S = 1) 0.49 0.50 −0.63 0.57 0.94 0.89 −0.02 0.10 3.6954b (S = 1) 0.47 0.57 −0.67 −0.56 1.06 0.88 −0.02 0.11 3.6314c (S = 0) 0.47 0.47 −0.66 −0.66 3.3034d (S = 1) 0.49 0.54 −0.96 −0.75 0.03 1.30 0.02 0.68 3.438

aAtom labeling referring to Chart 1. The data were computed at the level of TPSSTPSS/6-31G*.

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than that in 22 and 32 (0.76), suggesting that the Fe1−S1 bondserves as a “switch” during the isomerization of 22 to 32. Thebonding interaction between the Fe1 and N2 atoms is almostabsent in 22 but is observed in 32 (bond index of 0.66). Also,the Fe1···N1 bonding in 22 (bond index of 0.57) almostdisappeared in 32 (bond index of 0.03). On the other hand,Fe2···N2 bonding exists in both 22 and 32. These resultssupport the μ-NH−NH2 bridging character of 3

2. An analysis ofMulliken charge population suggests an event of net electrontransfer from the Cp and SMe2 ligands to the Fe atoms and theNHNH2 moiety during the conversion of 22 to 32 (Table 4).Also, the two SMe2 ligands served as major electron donors.This suggests that increasing electron density on sulfide andancillary Cp ligands may facilitate such an electron transferevent and therefore be beneficial to the subsequent hydro-genation of the NHNH2 moiety in 32.In light of the process discussed above, the reductive cleavage

of the HNNH double bond at the diiron centers could occuralong with the sequences [M]NHNH → [M]NHNH2 →([M]NH + NH3) → ([M]NH2 + NH3) → ([M] + 2NH3) (M= metal centers). The feature of this cleavage mechanism is thatthe release of the first NH3 molecule is via the protonation/reduction of the [M]NHNH2 intermediate. Such a process is incontrast to either the Chatt12 mechanism (Scheme 1a), wherethe release of the first NH3 molecule is directly from the

[M]NNH2 form, or the mechanism proposed for N2 reductionon the FeMoco model compounds (Scheme 1b),13,14 where therelease of the first NH3 molecule occurred through thehydrogenation of [M]NH2NH2 species. This is because the[M]NNH3 (cf. Chatt pathway in Scheme 1a) and [M]NH2NH2(cf. FeMoco pathway in Scheme 1b) species are higher inenergy than 32 and 41, respectively, and therefore was unlikelyto be involved in the reduction process (vide ante). Althoughthe reductive mechanism obtained in this study is similar to theSchrock route11 (Scheme 1a), where the release of the firstNH3 is via the hydrogenation of [M]NHNH2, our computationsuggests that the N2 reduction at the diiron centers occursthrough the [M]NHNH intermediate rather than the[M]NNH2 species (Figures 1 and 2) proposed in the Schrockmechanism11 and therefore follows the mechanism recentlyproposed for N2 reduction at the Fe sites of FeMoco.15 In thissense, it is of importance to take the thiolate-bridged diironcomplex investigated here as a nitrogenase model compound tostudy the mechanism of N−N bond reduction. According tothe computational results obtained in this study, one feature ofthe reduction of N2 to NH3 assisted by the diiron complex is tooccur via HNNH and HNNH2 forms rather than the H2NNH2unit. The other feature of such a process is that the rate-determining step could be the isomerization of (μ,η2-HN−NH2) moiety to the bridging form (μ-HN−NH2), and such anisomerization is reversible.

■ CONCLUSION

Computational studies have been carried out to investigateNN triple bond cleavage assisted by a thiolate-bridged diironcomplex. The crucial geometrical parameters of the modelcompound optimized at the level of TPSSTPSS/6-31G* theoryare in excellent agreement with available experimental data. Thecurrent computations suggest that the Cp*Fe(μ-SEt)2FeCp*can possibly bind N2 and reduce it to HNNH viaprotonation/reduction. The calculations of the model systemindicated that the subsequent reductive cleavage of HNNHat the diiron centers, which was involved in the catalytic processobserved experimentally, could follow the pathway HNNH →HNNH2 → (HN + NH3) → (H2N + NH3) → 2NH3. Thehydrogenation (H+ + e−) energetically prefers to occur at the Natoms rather than at the Fe or S atoms. The rate-determiningstep is the transformation of the binding mode of μ,η2-HN−NH2 to μ-HN−NH2 at the diiron centers. Such a trans-formation might be driven by unequal charge populations on

Chart 2. Isomeric Structures of 41a

aThe free energies in solution (kcal/mol) shown above are relative to13 and the corresponding protonation/reduction process.

Figure 5. Plot of energy vs N−N distance during N−N bond cleavage.

Table 3. Wiberg Bond Index of Some Bonds in Several Stationary Pointsa

stationary point Fe···Fe N−N Fe1···N1 Fe1···N2 Fe2···N1 Fe2···N2 Fe1···S1

22 0.06 1.01 0.57 0.03 0.03 0.89 0.76TS[2-3]2 0.12 1.03 0.52 0.28 0.04 0.69 0.5732 0.23 1.02 0.03 0.66 0.03 0.66 0.76

aAtom labeling refers to that in Chart 1. The data were obtained at the level of TPSSTPSS/6-311+G**//TPSSTPSS/6-31G*, and the same is truefor Table 4.

Table 4. Mulliken Charges on NHNH2, SMe, and CpLigands and the Two Fe Atoms of 22 and 32

stationary point NHNH2 (SMe)2 2Fe Cp

22 −0.094 −2.245 3.577 −0.76132 −0.570 −1.637 2.540 −0.535variation −0.476 0.608 −1.037 0.225

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the two Fe atoms. The calculation results indicate an event ofelectron transfer from the sulfide and ancillary ligands (Cp) tothe Fe atoms and the HNNH2 moiety during the change ofbinding mode of HNNH2. This suggests that increasingelectron density on the sulfide and ancillary ligands mayfacilitate the change of the binding mode of μ,η2-HN−NH2 toμ-HN−NH2 at the diiron centers and therefore be beneficial tothe subsequent hydrogenation of the NHNH2 moiety.

■ ASSOCIATED CONTENT*S Supporting InformationText giving the full citation of ref 27 and figures and tablesgiving the THF-coordinated complex, SCRF single-pointenergy, formal charges, and Cartesian coordinates of optimizedstationary points. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (Y.L.); [email protected] (J.Q.).

■ ACKNOWLEDGMENTSThis work was partially supported by the National NaturalScience Foundation of China (Nos. 21028001, 21174023,21137001, 20806012). Y.L. thanks the SEM Scientific ResearchFunding for ROCS. Z.H. acknowledges financial support fromChina’s Thousand Talents Program. We also thank the RICC(RIKEN Integrated Cluster of Clusters) and the Network andInformation Center of the Dalian University of Technology forcomputational resources.

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