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Reduction of NO to NO Catalyzed by a Mn–Substituted KegginType Polyoxometalate: A Density Functional Theory Study Meng-Xu Jiang, and Chun-Guang Liu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 22, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Reduction of NO to N2O Catalyzed by a Mn–Substituted Keggin-Type Polyoxometalate: A Density Functional Theory Study

Meng–Xu Jiang and Chun-Guang Liu*

College of Chemical Engineering, Northeast Dianli University, Jilin City, 132012, P. R. China

ABSTRACT

Reaction mechanism corresponding to reduction of NO to N2O catalyzed by mono– transition–metal–substituted Keggin–type polyoxometalates (POMs) has been studied by using a density functional theory (DFT) method with the M06L functional. Compared with Fe, Co, Zn-substituted POM complexes, a Mn-substituted POM complex possesses good feature for activation of NO molecule because of considerable absorption energy and significant charge transfer from metal center to NO molecule. And the effective interaction between NO ligand and the Mn center mainly comes from an overlap of the π* orbital of the NO molecule with dxz and dz2 orbital of Mn center. Three possible reaction pathways for reduction of NO to N2O catalyzed by Mn-substituted POM complex have been considered based on a dimer mechanism. The calculated free energy profile indicates that the reaction pathway undergoing a cis-(NO)2 conformation is the favorable routes because of a low free 1

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energy barrier of 4.24 kcal mol-1. This work would be very useful to guide the design of nitric oxide reduction catalyst based on inorganic POM complexes in fiend of environmental catalysis.

1. INTRODUCTION Polyoxometalates (POMs) are a unique class of inorganic metal oxo clusters.1-3 They have found a variety of potential applications in medicine science,4 materials chemistry,1,5,6 and catalysis,3,7 etc. Mono–transition–metal–substituted Keggin–type POMs are important and typical structure in the field of POM catalysis.8-14 Those type POMs are always recognized as “inorganic metalloporphyrin” because of the analogous

molecular

geometries

and

catalytic

behaviors

relevant

to

metalloporphyin.9,15-17 Such as, an mono–manganese–substituted Keggin–type POM displayed very good activity and high selectivity for the epoxidation of alkenes that compared well to that of its counterpart the manganese tetrakis (2,6-dichlorophenyl) porphyrin.18,19

Transition metal nitrosyl complexes have attracted considerable attentions because of their potential applications in diminishing or removal of the concentration of NO in exhaust gases emitted by the fossil fuel combustion and industrial manufacture, such as, coal-fired power plants, automotive engine, and producing of organonitrogen compounds from NO in reactions catalyzed by transition metal.20-33 In the past four 2


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decades, much effect has been devoted to understand the way in which NO binds to the transition metal center.34-38 A lot of transition metal nitrosyl complexes have been reported. Nitrosyl iron porphyrin derivatives, where central transition metal was iron(II) atom, have been extensive studied in this field for simulating catalytic reduction mechanism transferred NO into N2O that happened on heme protein and nitric oxide reductase.39-47 Similar to nitrosyl metalloporphyrin, the transition metal nitrosyl POM complexes also have been successfully synthesized.9,15,16,48 Proust and co-workers16 used (n-Bu4N)2[Mo5O13(OMe)4(NO){Na(MeOH)}] 3MeOH as suitable nitrosyl precursors to reacted with Keggin–type ployanions [PM12O40]3- (M=W, Mo) for the synthesis of [PW11O39{Mo(NO)}]4- and [PMo11O39{Mo(NO)}]4- in acetonitrile in the presence of n-Bu4NOH. According to the IR spectroscopy study, they proposed that the coordinated NO molecule was activated because of an intramolecular electron transfer.

Kuznetsova

and

co-works48

have

been

successfully

prepared

[PW11O39{Fe(NO)}]5- by reaction between NO and [PW11O39{Fe(H2O)}]5-. Filipek9 reported the simply synthesis routes of [SiW11O39Rulll(NO)]6-, which was directly obtained by [SiW11O39RuIII(H2O)]5- aqueous reacted with NO molecules or NH2OH·HCl in 333 K temperature. All these works indicates that mono–transition– metal substituted Keggin–type POMs have the promising prospect for the development of efficient, stable, and durable inorganic catalyst for NO reduction. In the present paper, we performed density functional theory (DFT)11-14 method to probe possible reaction mechanism corresponding to reduction of NO to N2O 3



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catalyzed by mono–transition–metal–substituted Keggin–type POMs. Such results provide a fundamental understanding the structure–property relationships for Keggin– type POMs, and are useful to explore the potential applications of POMs in the field of environmental catalysis49-52 to effectively reduction of NO molecule.

2. COMPUTATIONAL METHOD All molecular geometries were optimized by using DFT method with the local meta–GGA exchange correlation functional M06L.53 This M06L functional is developed with incorporate spin kinetic energy density into the exchange and correlation functionals in a balanced way. Many advantages of this functional have been reflected in the calculations of thermochemistry, thermochemical kinetics, noncovalent interactions, and vibrational frequencies. The accuracy of the M06L functional in prediction of geometry structure, vibrational frequency, thermochemistry, etc. of POMs was proved in our previous work.14 The 6–31G(d)54,55 basis set was adopt for all main elements and scalar relativistic effective core potential of LANL2DZ56 was used for the metal elements for our studied POM systems. All of the structures discussed in the present work are minima or transition state on the corresponding potential energy surfaces, as confirmed by the correct number of imaginary frequencies. Berny Geometric Optimization (TS) method was preformed to obtain the energy surface profile. Meanwhile, the vibrational mode of imaginary

4


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frequency were analyzed and conformed that each transition state is consistency of the searched reaction path. The single point calculations on optimized geometries were performed using the same functional with the polarization and diffusion basis sets 6– 31+G(d) for main group atoms and the relativistic energy–consistent pseudopotential basis set SDD57 for transition metal atoms to further refine the electronic energy.

All geometry optimizations were performed without any symmetry constrains. The absorption energy (Ead) of NO molecule over mono–transition–metal substituted POM ligand was calculated by

Ead = Ecomplex – (EPOM + ENO) where Ecomplex, EPOM, and ENO are the total energies of the transition metal nitrosly POM complex, POM fragment, and NO molecule, respectively. Natural bond orbital (NBO)58 analysis was performed to assign the atomic charges and Wiberg bond indices (WBI) at the same levels. All calculations were implemented with the Gaussian 09 package.59

3. RESULTS AND DISCUSSION

3.1. Transition Metal Substituted Effects Table 1. The calculated absorption energy (kcal mol-1) of the series of transition metal nitrosyl POM complexes 5



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Ead

Mn–POM

Fe–POM

Co–POM

Zn–POM

spin multiplicity

3

4

3

2

N–end

–32.05

–44.34

–25.90

–9.61

O–end

–20.71

–32.57

–14.36

–5.80

In the present paper, catalytically important metal atoms Mn, Fe, Co, and Zn have been introduced into Keggin–type POM framework to form the mono–transition– metal–substituted Keggin–type POM nitrosyl derivatives, [PW11O39MII(NO)]5− (M = Mn, Fe, Co, Zn). It is well–known that coordination of NO molecule to transition metal center always forms nitrosyl metal complex with N– or O–end coordinated models. And thus both coordinated models of NO ligand in the series of transition metal nitrosyl POM complexes in diverse spin states have both been optimized at M06L/GEN levels (6-31G(d) basis sets on main group atoms and LANL2DZ basis sets on metal atoms). According to our DFT–M06L calculations, the ground state of all transition metal nitrosyl POM complexes studied here have been found (see Supporting Information Table S1) by comparison of electronic and zero-point energies in different spin multiplicity state. Both the N–end and O–end coordinated models of Mn– and Co–POM nitrosyl complexes are stable in triplet spin multiplicity state, Fe– POM nitrosyl complexes with N–end and O–end coordinated model have quartet spin multiplicity ground state and Zn–POM nitrosyl complexes all settle in double spin multiplicity state for both N–end and O–end coordinated model. 6


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Based on the optimized ground state geometries, we have calculated the absorption energy of NO molecule (Ead) over the series of transition metal nitrosyl POM complexes in both coordinated models, the calculated Ead values by used M06L functional with 6-31+G(d) basis sets on main group atoms and SDD basis sets on metal atoms have been listed in Table 1. For the both coordinated models, the calculated Ead values decrease in the following order: Fe–POM < Mn–POM < Co– POM < Zn–POM. For each transition metal center, the calculated Ead absolute value of transition metal nitrosyl POM complex with N–end coordinated model is larger than that of the O–end model. Due to the considerable adsorption energy, we will focus on N–end coordinated models in the following discussion. O dN-O

N Ob

∠M-N-O

dM-N

Oc W

d2

M

d3

d1

W

d4

d5

∠Oµ-M-Oµ

p

Oa

a

POM b

Figure 1. Molecular structure of completed Keggin-type POM (a) and metal−nitrosyl POM complexes bonded with N-end (b). Oa denoted the oxygen of the tetrahedral PO43- unitis single oxygen atom of NO molecule, Ob are four donor oxygen atoms that 7



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bridge the substituted metal and W atoms, Oc are corner oxygen atoms of the Keggin structure.

It is well-known that the Keggin-type POM contains a cage of tungsten atoms linked by oxygen atoms with tetrahedral phosphate group (see Figure 1a). Oxygen atoms form four physically distinct bonds. The Oa atom is relative to oxygen atom of tetrahedral phosphate group, the Ob atom corresponds to oxygen atom bridging the two tungsten atoms, and the Oc atom represents oxygen atom at the corners of the Keggin structure. Transition metal center is stabilized by four Ob atoms coming from the surface oxygen of Keggin–type POM ligand (see Figure 1b). Due to the very large distance between metal and Oa atoms, the M–Oa interaction can be viewed as a weakened bonding interaction, which provides a unsaturated metal atom in the pseudo–octahedral coordinated sphere.13,14,17 And thus the transition metal center has an additional ability to bind to a NO molecule.

The key geometric parameters of the series of POM complexes with N–end coordinated model in different spin multiplicity ground state optimized by M06L/GEN levels (6-31G(d) basis sets on main group atoms and LANL2DZ basis sets on metal atoms) studied here have been listed in Table 2.

Table 2. Geometric parameters (bond Length in Å, angles in degree), vibrational frequencies (in cm−1) , and NBO partial charges (e) on the selected atoms of the transition metal nitrosly POM complexes

8


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Param

MnII-POM

FeII-POM

CoII-POM

ZnII-POM

Geometric Parameters spin multiplicity

3

4

3

2

M-N

1.837

1.864

1.889

2.557

WBI(M-N)

1.058

0.864

0.817

0.123

N-O

1.220

1.199

1.191

1.173

WBI(N-O)

1.629

1.717

1.815

1.962

∠M-N-O

134

137

121

131

Av. M-O distance

1.933

2.026

2.026

2.072

∠Oµ-M-Oµ

163

165

170

179

ν(NO)

1626

1732

1723

1856

Partial charges M

0.731

0.986

0.882

1.333

Total charge of NO

-0.301

-0.208

-0.113

-0.084

N

0.061

0.078

0.138

0.161

O

-0.362

-0.286

-0.251

-0.235

The M–N bond distance is the most parameter for evaluation of the interaction strength between the transition metal center and NO molecule. According to our DFT–M06L calculations, the M–N bond distance increases in following order: Mn– POM < Fe–POM < Co–POM < Zn–POM. It is well–known that the bond distance is closely associated with the interaction of atoms and the atomic radius. In the present paper, the bonding nature also has been checked by using NBO analysis. The calculated WBI values for these M–N bonds also have been listed in Table 2. The 9



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calculated WBI values of the M-N bond decrease in the order of Mn–POM (1.058) > Fe–POM (0.864) > Co–POM (0.817) > Zn–POM (0.123). All these results indicate a very strong Mn–N bonding interaction.

Activated NO molecule is always directly reflected by the change of N–O bond distance when compared with the free NO molecule. As shown in Table 2, the optimized N–O bond distance decreases in the following order: Mn–POM (1.220 Å) > Fe–POM (1.199 Å) > Co–POM (1.191 Å) > Zn–POM (1.173 Å). Compared with the optimized N–O distance of the free NO molecule, 1.161 Å, the N–O distance of our studied POM systems is all larger than that of free NO molecule, especially, in the Mn–POM nitrosyl complex, N–O distance increases to 1.220 Å, the discrepancy arrive in 0.06 Å. The calculated WBI values of N–O bond decrease in the order of Zn–POM (1.962) > Co–POM (1.815) > Fe–POM (1.717) > Mn–POM (1.629), confirming the weakened N=O double bond nature. According to NBO analysis, the charge of NO molecule in these POM complexes is negative, indicating a charge reorganization or transfer from the transition metal center to the coordinated NO molecule. As shown in Table 2, the calculated NBO partial charge on NO moiety increases in the following order: Mn–POM(–0.301 e) < Fe–POM(–0.208 e) < Co– POM(–0.113 e) < Zn–POM(–0.084 e). It can be found that the charge transfer from transition metal atom to the coordinate NO molecule would contribute to the activation of NO molecule. Therefore, the Mn–POM nitrosyl complex display the longest N-O distance among all POM nitrosyl complexes studied here because of a 10


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significant charge transfer.

Experimentally, IR spectra is an efficient detection method for the small structural differences of molecular system. We have calculated the IR spectra of the series of the transition metal nitrosyl POM complexes at M06L/GEN levels in this work (6-31G(d) basis sets on main group atoms and LANL2DZ basis sets on metal atoms). The calculated results also have been listed in Table 2. The IR spectra of an known POM complex [PW11O39{FeII(NO)}]5– have been reported by Kuznetsova et al48. A high– frequency and strong IR bond at 1730 cm-1 is assigned to the stretch vibrations of the coordinated NO moiety based on their experimental measurement. Our DFT–M06 calculations well reproduced this result. The calculated stretch vibrations of the coordinated NO moiety appears at 1732 cm-1. The discrepancy between experimental and theoretical results is only 2 cm-1, indicating reliability of our DFT calculations for these POM system.

Due to the elongation of N–O bond in metal nitrosyl complex, the IR stretch vibration of N–O moiety always decreases relevant to the free NO molecule. As shown in Table 2, the calculated NO stretch vibrations of the series of transition metal nitrosyl POM complexes increase in the following order: Mn–POM (1626 cm-1) < Co–POM (1723 cm-1) < Fe–POM (1732 cm-1 ) < Zn–POM (1856 cm-1). Compared with the DFT–derived IR absorption band of free NO molecule 1977cm-1, the results are all small because of an relevant long N–O bond distance.

11



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McCleverty60 proposed that there are three main bonding models in metal nitrosyl complexes with N–end coordination, two terminal models: linear M–N–O groups, bent M–N–O groups; one bridging model. In addition, because NO molecule has a single electron, NO always plays the role of an electron donor, giving NO+, or an electron acceptor, giving NO– and even N2O22– in metal nitrosyl complexes. The NO ligand is usually represented as coordinated NO– unit when the bond angle of M–N–O unit is about 120°. As shown in Table 2, the optimized bond angles of M–N–O unit in the series of transition metal nitrosyl POM complexes (∠M–N–O) are in the range from 121° to 137°, indicating the terminal model with bent M–N–O groups (type ii), and the coordinated NO molecule plays the role of an electron acceptor, which is well in agreement with the NBO charge analysis.

3.2. The Mn–POM Nitrosyl Complexes

α-spin

HOMO-3

HOMO-2

HOMO-2

HOMO-1

HOMO

LUMO

β-spin

HOMO

12


LUMO

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Figure 2. Frontier molecular orbitals of the Mn–POM nitrosyl complex

Due to the significant charge transfer from metal center to NO molecule relevant to other metals studied here, the Mn–POM nitrosyl complex [PW11O39{MnIINO}]5– has been employed as an example to analyze the electron structural information in this work. The frontier molecular orbitals (FMOs) of this Mn–POM complex have been listed in Figure 2. Due to the Mn center is in a pseudo–octahedral–coordinated sphere and the bent arrangement of the Mn–N–O unit, the symmetry adapted dxz orbital is responsible for the Mn–NO bonding interaction. As shown in Figure 2, the α– HOMO–2 and β–HOMO are dxz–like orbital, which are made from an overlap of the π* orbital of the NO molecule with dxz orbital of Mn center. Based on this orbital topology, activation of NO molecule in this metal nitrosyl POM complex mainly comes from the donation of electron from the symmetry–adapted dxz orbital of Mn center into molecular orbitals that are antibonding with respective to NO molecule. We also note that a special topology of α–HOMO, which consists of π* orbital of the NO molecule and dz2 orbital of the Mn center. It is well–known that NO has a 1σ22σ23σ24σ21π45σ22π1 configuration. Thus this orbital interaction arises from a mixing between the occupied 2π orbital of NO ligand and unoccupied dz2 orbital of Mn center, and has the Mn–NO bonding orbital character. This special orbital topology is caused by the bent arrangement of the Mn–N–O unit. This orbital mixing pattern results in an electron transfer from the NO ligand to Mn center. The dxz–like orbital (α–HOMO–2 and β–HOMO) having metal to NO ligand electron transfer is 13



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doubly occupied, and dz2–like orbital (α–HOMO) with an electron transfer from NO ligand to metal is singly occupied, and thus a total result lead to electron transfer from metal to NO ligand, which is consistent with the electron acceptor character of NO unit in a bent terminal coordinated model. In addtion, the α–HOMO–3 and β– HOMO–2 are dyz–like orbital, which provide relatively weak interaction for the Mn– NO bonding. Because of a bent arrangement of Mn–N–O unit, the overlap between the π* orbital of the NO molecule and the dyz orbital of Mn center is not effective relevant to dxz– or dz2–like orbital. Mn element has the rich redox chemistry. Oxidation state of Mn atom can change in the range from +2 to +7. In the present paper, a multistep redox possess corresponding to MnII→MnIII→MnIV→MnV of the Mn–POM nitrosyl complexes has been considered. An important character of the transition metal center is that, they have the capability of adopting different spin states as a function of the ligand environment. In the present paper, the redox species in this multistep possess with various spin states have been optimized at M06L/6-31G(d) levels (LANL2DZ basis sets on metal atom) in various spin states. The calculated relative energies of these redox species in different spin states are summarized in Table S3. According to our DFT calculations, the MnII species is found to have a triplet ground state, and the MnIII species prefer the low–spin doublet ground state. For the MnIV and MnV species, they have the high-spin quintet and quartet ground state, respectively.

The optimized key geometrical parameters of these redox species in their ground 14


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state have been listed Table S4. For the MnII→MnIII oxidized process, a detailed comparison indicates that the M–N distance decreases from 1.837 to 1.664 Å (∆r(MnII/III−N) = 0.173 Å) and the N–O distance decreases from 1.220 to 1.178 Å (∆r(N−O(MnII/III) = 0.042 Å), this indicate that both the MnIII–N and N–O bonds have been strengthen. It is worth also emphasizing that the∠Mn–N–O increases from 134 o to 179 o in this one-electron-oxide possess, indicating a geometrical transformation from the bent to linear bonding model of Mn–N–O unit. The FMO diagram of the MnIII species in its ground state has been listed in Figure 3. According to our DFT–M06L calculations, α–HOMO–3, α–HOMO–4, β–HOMO, and β–HOMO–1 orbitals of the MnIII species are responsible for the Mn–NO bonding interaction, and both orbitals made from an overlap of the π* orbital of the NO molecule with the symmetry adapted dxz and dyz orbitals of MnIII center. The calculated NBO partial charge of NO moiety in MnIII species is positive. As mentioned above, the α–HOMO of MnII species is mainly localized on the NO unit and Mn center. And the ground state of MnII and MnIII species is triplet (two spin single electrons) and doublet (one spin single electron), respectively. These results indicate that the MnII→MnIII oxidized process involves ejection of an electron from the α–HOMO of MnII species, and thus increasing the charge of NO unit from negative to positive. This indicates that there is no electron transfer from the metal center to NO ligand to activate NO molecule, thus the MnIII species has a strong N–O bond relevant to the MnII species. In addition, the calculated vibrational frequency of 15



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NO ligand in MnIII-nitrosyl POM complex shown the value is 1883 cm-1, compared to 1626 cm-1 of NO ligand in MnII-nitrosyl POM complex, indicating a strong N–O bond in MnIII species relevant to MnII species.

α-spin

HOMO-4

HOMO-3

HOMO

LUMO

HOMO-2

HOMO-1

HOMO

LUMO

β-spin

Figure 3. Frontier molecular orbital of the MnIII-nitrosyl POM complex For the MnIII→MnIV oxidized process, the optimized Mn–N distance increases from 1.664 to 2.214 Å, ∆r(MnIII/IV−N) = 0.550 Å; and N–O bond distance decreases from 1.178 to 1.152 Å, ∆r(N−O) = 0.026 Å. These results indicate a weak interaction between NO molecule and the MnIV center. And for the MnIV→MnV oxidized process, the optimized Mn–N distance increases from 2.214 to 2.705 Å, ∆r(MnIII/IV−N) = 0.491 Å; and N–O bond distance decreases from 1.152 to 1.130 Å, ∆r(N−O) = 0.022 Å. The results show that a very weak interaction between MnV center and NO molecule.

16


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It is well–known that the POMs have excellent redox stability because it can accept one or several electrons with minimal structural changes. We have checked the geometric parameters of POM unit in this multistep redox process. As expected, there are no significant structural changes. By contrast, this multistep redox possess corresponding to MnII→MnIII→MnIV→MnV of the Mn–POM nitrosyl complexes mainly affects the geometric structure of the Mn–N–O unit. The high valent MnIV and MnV species possess the weak Mn–NO bonding interactions relevant to MnII and MnIII species according to our DFT–M06L calculations, indicating the poor stability. It is well–known that valence d orbital energy levels move down with an increasing oxidation state of the metal in the d–block. Thus the energies of high valent MnIV and MnV species could be expected to be higher than the MnII and MnIII species, and the energy gap between the symmetry–allowed dxz and dyz orbital of Mn center and the π* antibonding orbital of NO ligand increases significantly, they do not overlap each other to give an effective Mn–NO bonding orbital, and thus the weak interaction between Mn center and NO molecule.

3.3. The Dimer–Adsorbed MnII-POM Complexes

17



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O

1.428 N

N

1.251

1.254

1.246

O

N

O

Mn

O

O

O

O

1.252

O

1.451

2.320

2.504

2.487

N

O

O O

1.246

1.406 N

1.243

N

2.342 Mn

O

O

O

O

POM

POM

a. Eads= - 52.71 Kcal/mol

b. Eads= - 54.93 Kcal/mol

Mn

O

O

O

POM

c. Eads= -57.53 Kcal/mol

Figure 4. The optimized geometric structures and calculated adsorption energies of (NO)2 dimer on MnII-POM complex: a. Trapezoid OadNNOad species b. Tran-(NO)2 species c. Cis-(NO)2 species. The bond distances are in angstroms (Å) There are two possible mechanisms for NO reduction based on the studies of various catalytic system, especially in noble metals and platinum-group metals bulks or clusters heterogeneous catalysis. Namely, (i) direct dissociation mechanism, NO molecule decomposes into the N and O atoms on the active sites of catalyst, which combines with second NO to form N2O or another N atom to form N2.61-63 The (ii) dimer mechanism, which is found in the studies of catalytic reduction mechanism for transfer NO into N2O on heme protein and nitric oxide reductase. 41,42 The difference between two mechanisms is the number of active site, multiple active sites for direct dissociation mechanism and single active site for dimer mechanism. The MnII-POM complex can be viewed as a single active site species. The direct dissociation of NO molecule is not possible for our studied system because the MnII-POM complex can 18


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not provide spare site to absorb the decomposed N and O atoms. Thus, we will focus on the dimer mechanism in the following discussions.

As the key intermediate in dimer mechanism, the dimer, (NO)2, was firstly characterized by Dinerman and Ewing using IR spectroscopy in gas phase63. It was subsequently found in many noble metal and platinum-group metal catalytic systems during NO reduction.25,61,64 In the present paper, three possible molecular geometries of the dimer–adsorbed MnII-POM complexes have been considered, they are trapezoid OadNNOad (a), tran–(NO)2 (b), and cis–(NO)2 species (c), respectively (see Figure 4). The calculated adsorption energies and optimized key structural parameters of the three dimer–adsorbed MnII-POM complexes obtained by our DFT/M06L calculations are also listed in Figure 4. It can be found that the calculated adsorption energy is –52.71, –54.93 and –57.53 kcal/mol for trapezoid OadNNOad, tran–(NO)2, and cis–(NO)2 species, respectively. Interestingly, the adsorption energy of the dimer is significantly larger than that of the NO monomer (Eads. = –32.05 kcal mol–1 for Mn– POM nitrosyl complex, [PW11O39{MnIINO}]5–) (see Table 1,). It indicates that these dimer–adsorbed MnII-POM complexes are thermodynamic feasible species relevant to the NO monomer complex. Meanwhile, the adsorption energies also shown that the cis–(NO)2 species is more stable than other species. As shown in Figure 4, two NO molecules have been excellently coupled with each other in the three dimer–adsorbed complexes, the N–N bond distance is 1.428, 1.406, and 1.451Å for trapezoid OadNNOad, tran–(NO)2, and cis–(NO)2 species, respectively. 19



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Compared to the N–N bond distance of the free dimer in gas phase 2.080 Å, indicating that the N–N bond have been strengthen in the series of MnII–POM complexes. The calculated WBI value of the N–N bond is 1.170, 1.177 and 1.108 for trapezoid OadNNOad, tran-(NO)2, and cis-(NO)2 species, respectively, which indicates that the N–N single–bond feature. The NBO charge analysis shown the total net charges on the (NO)2 moiety are –0.707, –0.664 and –0.736 for trapezoid OadNNOad, tran-(NO)2, and cis-(NO)2 species, respectively. This charge transfer is remarkable when compared with the monomer–adsorbed complex, which is two times as large as partial charges of the NO moiety in monomer complex (–0.301). The negative charge on the (NO)2 moiety shows an electron transfer from the MnII center into (NO)2 dimers. Donation of electron from the d orbitals of MnII center into molecular orbitals of (NO)2 that are antibonding with respective to N–O bond and bonding corresponding to N–N bond, and thus leads to a strong coupled of two NO molecules in the three dimer–adsorbed MnII–POM complexes.

3.4. The Dimer Reaction Mechanism

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Mn

N2 3.466

N1

1.428

22.63

1.754

Mn

TS1 POM

2.504

2.487

O2

1.632

O2

O1

2.940

O1 N2 1.254

1.251

POM

1.111

N1

Kcal/mol 2NO + OH2

N2 +

Mn

1.277

0 POM

N1

1.150

N2

RC1 1.986

-24.12

O2

1.702

O2

O1 1.983

1.751

O1 path I

-5.85 IM1

Mn

1.854

Mn

POM

AD1 POM -56.18 P1

Figure 5. Calculated potential free energy surface profile from NO to N2 passed by trap-(NO)2 species The question we are now concerned about is that the possible reaction pathways for the dimer mechanism, which is corresponding to transfer (NO)2 dimer into N2O or N2 molecule. As mentioned above, three dimer–adsorbed MnII–POM complexes have been considered based on our DFT–M06L calculations, and thus three possible reaction pathways (path I about the trapezoid OadNNOad species, Path II corresponding to the tran–(NO)2 species, and path III relative to the cis–(NO)2 species) are discuss in there. On the other hand, because the mono–transition–metal– substituted Keggin–type POM is always the low–valent Keggin–type aquametal 21



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derivatives according to the synthetic chemistry of POMs. Thus, MnII–POM aquametal derivative, [PW11O39{MnIIH2O}]5– was arranged as the starting reactant for the these reaction pathways. All reaction pathways of dimer mechanism are design in phase at room temperature and 1bar pressure.

We firstly discuss the reaction pathway I corresponding to the trapezoid OadNNOad species because of its unique adsorption performance relevant to path II and III. Especially, the final product of this pathway is N2 molecule and not N2O molecule exist. The calculated reaction energy profile for this pathway has been listed in Figure 5. It can be found that the formation of trapezoid OadNNOad species is exothermic by –24.12 kcal mol–1, indicating trapezoid OadNNOad species is a thermodynamically stable intermediate. Along this reaction pathway, trapezoid OadNNOad species undergoes a transition state (TS1) with a very high energy barrier (∆G1⧧= 46.75 kcal mol–1) to formation of the N2 molecule (IM1) according to our DFT–M06L calculations. For TS1, it was confirmed with an imaginary frequency of 562.95i cm-1. The vibration motion of this imaginary frequency mainly focused on cleavage of two N–O bonds, and strengthen of N–N bond of trapezoid OadNNOad species, which indicates a releasing dinitrogen molecule trend from the trapezoid OadNNOad species. Due to the very high barrier for the transition state TS1, this reaction pathway is not possible in the moderate reaction conditions.

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Kcal/mol 2NO

+ O1

OH2 1.284

Mn

N1

O1

POM

− H2O

O1

Mn

RC1

N2

1.378

1.251

POM

1.726

N1

2.602

POM

Mn

-15.16 TS3

POM -24.58

-26.30

-30.28 AD3

O2

-35.59 1.245

O1 1.242 3.166

N1 2.342

Mn

POM

O

IM2

N2 1.451

POM

N1 2.869

POM

O2

3.365

1.266

1.576

Mn

1.246

Mn

1.237

1.744

TS4

N2

N1

2.320

O1

-26.04

TS2

AD2

1.252

O2 N2

path II

1.406

Mn

O2

RC2 path III

2.993

1.266

2.621

-15.15

O1

N2

O2

+ NO

N1

0

1.211

1.741

O1

1.266

N1

1.300

N2 O2

1.138

3.103

O 1.659

3.742

N

-43.21 P2

Mn

1.279

2.236

1.190

N

POM

Mn

POM

Figure 6. Calculated reaction energy profile from NO to N2O passed by tran–(NO)2 species (path II) and cis–(NO)2 species (path III) For the tran–(NO)2 (path II) and cis–(NO)2 (path III) dimer reaction mechanism, the calculated reaction energy profiles have been compared in Figure 6. The path II 23



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presented the reaction route that undergoes the tran–(NO)2 dimer (AD2). As shown in Figure 6, we consider that the tran–(NO)2 dimer is formed step by step, which means the first NO molecule pre–adsorption and coordinates to the Mn center with N1 atom (RC2) and then, the second NO molecule attack to the first NO molecule to form AD2. It can be found that two reaction steps all are exothermic, indicates the readily formation of tran–(NO)2 dimer POM complexes. However, it should be mention that, our calculated vibrational frequencies results by DFT/M06L method shown that this (NO)2 dimer configuration (AD2) exhibit a tiny imaginary frequency of 14.42i cm-1, which indicates the instability of tran-(NO)2 dimer in our catalytic system. It would be converted into the more stable configuration IM2 through a transition state (TS2) with a minimal energy barrier (∆G2⧧= 1.72 Kcal/mol). This process is exothermic by –9.29 kcal mol–1 related to AD2, indicating a readily process. The TS2 was confirmed by imaginary frequency of 84.92i cm-1. The vibration motion of this imaginary frequency is relative to the cleavage of Mn–N1 bond (2.624Å) and the forming of Mn–O2 bond (2.602Å). For the most stable IM2, the dimer moiety was adsorbed on the Mn center via O2–end coordination model, the distance of N–N bond decreased from 1.406 Å of AD2 to 1.300 Å of IM2, and the bond length of O2–N2 increased from 1.246Å of AD2 to 1.279Å of IM2, which indicates that the N–N bonding interaction was strengthened and the N–O bonding interaction was weakened. Subsequently, IM2 undergoes the second transition state (TS3) with a relatively high energy barrier (∆G3⧧= 19.99 kcal mol-1) to formed a N2O molecule in P2, finally. TS3 was confirmed

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with an imaginary frequency of 433.87i cm-1 relevant to the cleavage of N2–O2 bond (1.741 Å) and the forming of the Mn=O2 bond (1.726Å). The released energy of N2O for this step is –7.62 kcal mol. In P2, Mn=O2 double bond decreases to 1.659Å, and thus produce a high–valent Mn–oxo species, [PW11O39MnIV=O]5-. Path III shown another reasonable route for N2O formed mechanism, which goes though cis–(NO)2 dimer (AD3). It also consists of transfer the (NO)2 into the N2O molecule and generation of a high–valent Mn–oxo species. As shown in Figure 6, this pathway is similar to that of path II, where the cis–(NO)2 dimer is formed by a step reaction following in the first NO molecule. Compared with tran–(NO)2 dimer POM complex, the formation of cis–(NO)2 dimer POM complex is more delighted thermodynamically to occurs in this reaction pathway without any energy barrier. After across a transition state (TS4), the tran–(NO)2 dimer POM complex undergoes a transfer from the (NO)2 dimer species into the N2O molecule. This process only needs to overcome a tiny energy barrier (∆G4⧧= 4.24 kcal mol–1). Finally, the N2O is formed, simultaneously, generation of a high–valent Mn–oxo complex, [PW11O39MnIV=O]5-. This step from the cis–(NO)2 dimer to formed N2O molecule is release energy of 12.93 kcal mol–1. The TS4 was confirmed by imaginary frequency of 374.58i cm-1 relative to the forming of Mn=O1 bond (1.744Å) and the cleavage of O1–N1 bond (2.869Å). It can be found that the overall reaction of path II and path III starting from the MnII–POM aquametal derivative and two NO molecules is exothermic by –43.21 kcal mol–1. This suggests path II and path III could happen though thermodynamic 25



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driving force for this reaction studied here.

Taking into account above results, the following three points support that path III is the most reasonable and favorable reaction route for reduction of NO into N2O. (i) the stability of adsorption structure, it has been shown the most stable structure of (NO)2 dimers was cis–(NO)2 dimer species (–52.71, –54.93 and –57.53 kcal mol–1 for trap– (NO)2 dimer, tran–(NO)2 and cis-(NO)2 dimer, respectively). (ii) Thermodynamics, the calculated results shown path II and III are both exothermal for the formation of N2O molecule. By contrast, path I is endothermal for the formation of N2 molecule, and thus it is thermodynamically unfavorable. (iii) Kinetics, as illustration in Figure 5 and 6, the energy barriers for the rate-determining step of three pathways are 46.75, 19.99, and 4.24 kcal mol–1 for path I, II and III, respectively. It can be distinctly conclude that the decomposition of cis–(NO)2 into N2O only need overcome a tiny barrier of 4.24 kcal mol–1 in path III. Previous experimental studies65-67 reported by Neumann and co-workers showed that the transition–metal–substituted Keggin–type POMs can react with N2O to give the high–valent metal–oxo POM species, which has capability of epoxidation of alkenes. For our studied catalytic system, the N2O and [PW11O39MnIV=O]-5 are easy produced, and thus the [PW11O39MnIV=O]-5 may be utilized to decomposition of N2O further. Those works have been reported in our literature.68

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4. CONCLUSIONS In this paper, the molecular geometry, electronic structure, redox properties of a series of metal–nitrosyl POM complexes [PW11O39MII(NO)]5− (M = Mn, Fe, Co, Zn) have been firstly explored by our DFT/M06L calculations. The results indicate that Mn, Fe, Co-nitrosyl POM complexes display similar geometric structure, terminal model with a bent M–N–O unit. The calculated absorption energy of the NO molecule upon the metal center shown that the Mn-and Fe–nitrosyl POM complexes exhibited strong interaction between the metal atom and NO molecule. NBO analysis indicates that Mn-nitrosyl POM complex has the strongest M–N and the weakest N–O bonding interactions among all studied POM complexes in here. The FMOs analysis indicates that this M–N bonding interaction mainly comes from an effective overlap of the π* orbital of the NO molecule and dxz and dz2 orbital of Mn center, and thus leads to an electron transfer from the Mn center to NO ligand. According to a dimer mechanism, three possible reaction pathways for reduction of NO to N2O catalyzed by Mn-substituted POM complex have been considered, we found the most feasible and favorable route is the pathway III, which goes through a dimer cis-(NO)2 configuration.

ASSOCIATED CONTENT

Supporting Information 27



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Page 28 of 40

The following file is available free of charge on the ACS Publications website. Total energies of Keggin-type POM derivatives [PW11O39MII(NO)]5− (M = Mn, Fe, Co, Zn) and [PW11O39MnII/III/IV/V(NO)] n− with

various

spin

state

based

on

M06L/6-31G(d)/LANL2DZ calculations. The basis set and functional effects. The geometric parameters, vibrational frequencies and partial charges of various manganese-substituted POM complexes also list. Cartesian coordinates of most relevant structures reported in this paper.

AUTHOR INFORMATION

Corresponding Author *

Tel.: 86 0432 64606919. Fax: 86 0432 64606919. E-mail addresses: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21373043).

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O Catalyzed by a Mn-Substituted Keggin-Type ... - ACS Publications

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