Electronic Properties of π‑Conjugated Nickel Bis(dithiolene) Networkand Its Addition Reactivity with EthyleneQing Tang and Zhen Zhou*

Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry(Ministry of Education), Computational Centre for Molecular Science, Institute of New Energy Material Chemistry, NankaiUniversity, Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, P. R. China

ABSTRACT: Density functional theory computations wereperformed to study the structure and electronic properties ofthe recently achieved two-dimensional nickel bis(dithiolene)single-layer sheet, as well as its cycloaddition activity withethylene. The nickel bis(dithiolene) monolayer has a planarand porous network, and electrically acts as an indirect bandgap semiconductor. Its lowest unoccupied state mainlydistributes onto p orbitals of S atoms, which makes S atoms active to accept electrons from the occupied p orbitals of CCbonds in ethylene. This is indeed confirmed by our computations, and the formation of the cis-interligand S,S′-adduct with anactivation energy barrier of about 21 kcal/mol is energetically favored. When an extra electron is added, the addition productbecomes destabilized and would dissociate into ethylene and nickel bis(dithiolene) anion. Therefore, with a controllable manner,the nickel bis(dithiolene) monolayer can bind and release ethylene molecules in neutral and reduction conditions, respectively.Because of its unique chemical reactivity, this newly achieved nickel bis(dithiolene) sheet can be potentially developed as anelectrocatalyst in olefin separation and purification.

1. INTRODUCTIONSeparation and purification of light olefins (alkenes) frommixtures including alkanes (olefin/paraffin separation) is animportant petrochemical process in chemical industry since thepetroleum feedstocks heavily rely on olefin-based compounds.The traditional method based on cryogenic distillation thatrequires low temperature and high pressure is energy-extensiveand operation-expensive.1 As an alternative to cryogenicdistillation, the separation scheme based upon reversiblechemical complexation of olefins with transition metal saltssuch as copper(I) and silver(I) is energetically more efficient.2,3

However, these metal-based systems can be easily poisoned anddeactivated by the olefin impurities such as H2, CO, H2S, andC2H2.Several studies, which included X-ray crystallographic

characterization of the reaction products, disclosed that metalbis(dithiolene) complexes have high cycloaddition reactivitytoward strained and cyclic alkenes (such as norbornadiene),where the olefin binds to ligand S atoms rather than themetal.4−6 This reaction can be mechanically viewed as theelectrophilic addition of bis(dithiolene) complexes to the CCbond, with each CC bond donating two electrons into thedithiolene complex. On the basis of this reaction chemistry, in2001, Wang and Stiefel7 developed an efficient method forolefin purification by reacting light olefins (ethylene andpropylene) with nickel bis(dithiolene) complexes [Ni(S2C2R2)2(R = CF3, CN)], where the produced S,S′-interligand adductswould subsequently release alkene and nickel bis(dithiolene)anion upon electrochemical reduction. The generated nickelbis(dithiolene) anion can also be readily converted back to itsneutral complex by electrochemical oxidation, and thus a rapid

binding and release of olefins can be electrochemicallycontrolled. This renders nickel bis(dithiolene) a promisingelectrocatalyst capable of fixing and releasing the alkenechemicals, and its addition reaction with olefins has initiatedsubstantial excitement in recent years.8−13

Note that the previous studies mainly focused on the nickelbis(dithiolene) complexes at a molecular level. Particularly, in aquiet recent report, Kambe et al.14 have synthesized a highly π-conjugated two-dimensional (2D) nickel bis(dithiolene) sheetby means of a regulated gas−liquid interfacial reaction betweenbenzenehexathiol (BHT) and nickel(II) acetate (Ni(OAc)2)components. Atomic force microscopy and scanning tunnelingmicroscopy indicated that such as-prepared nickel bis-(dithiolene) sheet featured a hexagonally ordered and planarmonolayer. Note that the atomically thin sheet-based 2Dmaterials such as graphene15−17 and graphene analoguematerials (including single or few layer BN,18−23 metaloxides,24,25 chalcogenides,26−28 carbides and nitrides,29−33 andcoordination or covalent polymers34−36) have attractedtremendous interests in recent years due to their novelproperties and great potential for high-quality and large-areananodevices.37 The newly achieved nickel bis(dithiolene)monolayer thus also adds to the 2D material family, andunderstanding its structural and chemical properties would becritical to the development of this material from the verybeginning. Very recently, this 2D nickel bis(dithiolene)framework has been predicted computationally to be a 2D

Received: May 22, 2013Revised: June 17, 2013Published: June 19, 2013

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© 2013 American Chemical Society 14125 dx.doi.org/10.1021/jp405055f | J. Phys. Chem. C 2013, 117, 14125−14129

organic topological insulator.38 Since the 2D nickel bis-(dithiolene) framework can be regarded as the polymerizedform of nickel bis(dithiolene) complex Ni[S2C2(C4S4H4)]2, it isexpected that this sulfur-containing polymer should alsodemonstrate chemical reactivity with alkenes. Exploring themechanism of alkene addition to the nickel bis(dithiolene)polymer would be of critical importance for developing novelapplications.For the further development and technological applications,

a systematic understanding of this periodic nickel bis-(dithiolene) framework from a theoretical perspective wouldbe highly desirable. Herein, in this work, using densityfunctional theory (DFT) computations, we first investigatedthe structural and electronic properties of the 2D nickelbis(dithiolene) sheet. Also, we investigated the binding ofethylene, the simplest alkene, to the nickel bis(dithiolene)sheet, and put emphasis on the reaction energies and transitionstates associated with this ligand-based reaction. Note thatethylene was selected here because it occupies the largestvolume feedstock of olefins.

2. COMPUTATIONAL DETAILS

DFT computations were performed by using the plane-wavetechnique as implemented in CASTEP code.39 A 400 eV cutoffenergy was used for the plane-wave basis set, and ultrasoftpseudopotentials were employed for all ions.40 The exchange-correlation energy is described by the generalized gradientapproximation (GGA) in the form of Perdew, Burke, andErnzerhof (PBE).41 We applied the 2D periodic boundarycondition for the 2D nickel bis(dithiolene) sheet, and a vacuumspace of 12 Å along the z direction was adopted to avoid theinteractions between two sheets in nearest-neighboring unitcells. The convergence threshold for optimization procedurewas set to be 10−4 eV in energy, 0.03 eV/Å in force, and 1 ×10−3 Å in atomic displacement. For the case of 2D nickelbis(dithiolene) sheet, the Brillouin zone was represented byMonkhorst−Pack special k-point mesh of 4 × 4 × 1 forgeometry optimizations, while a larger grid (12 × 12 × 1) wasused for electronic structure computations. Since DFT oftenprovides an unsatisfactory description for the stronglycorrelated transition-metal system with localized d subshells(Ni in this study), we considered the DFT+U approach forelectronic structure computations, in which a mixture of theHubbard model theory and DFT is required. The most popularimplementation is known as LDA+U, as has been integrated inthe CASTEP code.42 The LDA+U method, adopting asimplified and rotationally invariant approach as an implemen-tation for the GGA-PBE functional, was applied in ourcomputations. The only external parameter needed for thisapproach is the effective value of the on-site Coulombparameter, Ueff, for each affected orbital. In this regard, a Ueffvalue of 3 eV was used for the d orbital of Ni atoms, and thesevalues have been tested and used in previous theoreticalstudies.43 The reaction pathways for ethylene addition onto thenickel bis(dithiolene) monolayer were searched by using acombined linear synchronous transit (LST) and quadraticsynchronous transit (QST) method.44

3. RESULTS AND DISCUSSION

3.1. Nickel Bis(dithiolene) Sheet. First, we studied thegeometric and electronic properties of the 2D nickel bis-(dithiolene) sheet. The nickel bis(dithiolene) framework is

inherently cross-linked by Ni−S covalent bonds forming thesquare-planar Ni−S coordination, and structural relaxationsuggests that such a framework with hexagonal symmetry(Figure 1) is perfectly planar without any buckling, which is

very similar to the flat graphene or BN monolayer. The in-planelattice parameter of the unit cell (dashed rhombus in Figure 1)is optimized to be 14.62 Å, in good agreement with theexperimental value (14−15 Å)14 and the recent computationalresult (14.63 Å).38 Moreover, the nickel bis(dithiolene) sheet isa porous material with a large pore size of 11.92 Å, and theoptimized C−C, C−S, and C−Ni bond lengths are 1.437(1.421) , 1.707, and 2.138 Å, respectively.Despite the presence of magnetic Ni atoms, the extended

nickel bis(dithiolene) network is ground-state nonmagnetic,and this system is a semiconductor with an indirect band gap of0.20 eV at the DFT+U level (Figure 2a), slightly larger than thevalue of 0.16 eV at the GGA level, consisting of the highestoccupied state along the K → M point and the lowestunoccupied state at the Γ point. This computed band gap ismuch smaller than those of traditional semiconductors such assilicon, ZnO, and GaN. In order to understand the electronic

Figure 1. Optimized structure of 2D nickel bis(dithiolene) sheet. Thedashed rhombus denotes the unit cell, and the gray, yellow, and blueballs indicate the C, S, and Ni atoms, respectively.

Figure 2. (a) Band structure and the corresponding projected densityof states (PDOS) of 2D nickel bis(dithiolene) sheet. The red dashedline indicates the Fermi level. Top and side views of the charge densitydistribution of the lowest conduction band (b) and the highest valenceband (c).

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properties of the nickel bis(dithiolene) sheet in more detail, wecomputed the projected density of states (PDOS) of thisframework. The PDOS of 2D nickel bis(dithiolene) sheetsuggests that the valence band near the Fermi level mainlyderives from the occupied p orbitals of the C and S atoms andthe d orbitals of the Ni atoms, while the conduction band nearthe Fermi level mostly comes from the unoccupied p orbitals ofS atoms. Furthermore, the charge density distribution of thehighest valence band is shown in Figure 2c, which is mainlycontributed by the pz atomic orbitals of C (S) atoms and thedxz (dyz) atomic orbitals of Ni atoms, exhibiting π-type bondingcharacter. The charge density distribution of the lowestconduction band is shown in Figure 2b, which is mainlycontributed by the pz atomic orbitals of S atoms, exhibiting π*-type antibonding character. This indicates that the chargedensity analysis is in good consistency with the PDOS results,and the nickel bis(dithiolene) sheet, particularly the unoccupiedp orbitals of S atoms, probably has the chemical reactivity toaccept electrons from the occupied p orbitals of otherstructures such as CC bonds in alkenes.3.2. Ethylene Addition to Nickel Bis(dithiolene)

Monolayer. In addition to the electronic properties asdiscussed above, it is also of special interest to explore thepotential use of nickel bis(dithiolene) monolayer as the catalyst.Note that the study of reactions between alkenes and nickelbis(dithiolene)-based inorganic complexes has yielded tremen-dous insight, where they react to form S,S′-adducts. As anextended 2D nickel bis(dithiolene) polymer, it is accordingly ofboth fundamental and technological attraction to investigatethe cycloaddition reactivity of this novel material with alkenes.It this section, we concentrate on the binding of the simpleethylene molecule to the nickel bis(dithiolene) sheet.When an ethylene molecule is added onto the nickel

bis(dithiolene) surface, they may possibly form an intraligandadduct where the two reactive S atoms come from the sameBHT ligand (Figure 3a) or form an interligand adduct where

the two reactive S atoms come from the two neighboringligands (Figure 3b,c). In terms of the interligand additionproducts, both the twisted cis-interligand (Figure 3b) and thenormal cis-interligand configurations (Figure 3c) are examined.Structurally, these resulting nickel bis(dithiolene)-ethylenecomplexes appear to be strained, with the two attacking Satoms achieving the buckled tetrahedral rather than the planartrigonal stereochemistry. As a consequence, the nickel bis-

(dithiolene) sheet becomes slightly curved, with a corrugatedheight of 1.053 Å (intraligand), 1.287 Å (twisted cis-interligand), and 0.930 Å (cis-interligand), respectively. Bycomparing the structural parameters of the pristine nickelbis(dithiolene) monolayer and free ethylene species with theirbinding complexes, we found that the C−C bond in ethyleneincreases by about 0.21 Å from a double bond (1.328 Å) to atypical single bond (1.53−1.54 Å), and the new C−S bondforming between ethylene and the ligand (1.84−1.86 Å) is alittle longer than the C−S bond (1.71−1.77 Å) within thenickel bis(dithiolene) network. The computed reactionenergies, defined as the energy difference between the finalproducts and the separated reactants, suggest that theformation of nickel bis(dithiolene)−ethylene complexes isenergetically facile and exothermic, and the cis-interligandadduct has a higher exothermic energy (−5.01 kcal/mol) thanits intraligand (−3.37 kcal/mol) or twisted cis-interligand(−3.53 kcal/mol) counterpart.To further understand the reaction mechanism, we searched

the reaction pathways for the formation of addition products,and here we mainly focused on the preferred cis-interligandconfiguration. In Figure 4, the cis-interligand adduct can formby the direct addition of ethylene with nickel bis(dithiolene)moiety by overcoming an energy barrier of 21.07 kcal/mol(transition state 1, TS1). Alternatively, the initial reactants canfirst form the twisted cis-interligand adduct by overcoming anenergy barrier of 21.93 kcal/mol (TS2), which then isomerizesto give the thermodynamically more favorable cis-intergandstructure with an energy barrier of 14.33 kcal/mol (TS3). Thus,the twisted cis-interligand structure, if forms, would readilytransform to the normal cis-interligand structure upon thermalannealing. Here, the transition states were examined throughfrequency analyses, and they all have one imaginary frequency(239 cm−1 for TS1, 290 cm−1 for TS2, and 202 cm−1 for TS3).Finally, we investigated the effect of reduction on the stability

of the nickel bis(dithiolene)−ethylene system, and only thestable cis-interligand adduct is considered. When the system isreduced (herein the reduction process is simulated by injectingan extra electron into the neutral system), the relative freeenergies corresponding to the anions of the reactant (nickelbis(dithiolene) sheet and ethylene) and the cis-interligandadduct alter drastically. Specifically, the cis-interligand adductanion becomes now less stable than the reactant anion (with anenergy increase of the former by about 7.32 kcal/mol), ratherthan more stable as it is for the neutral species. Besides, similarbehavior has also been found for the twisted cis-interligandadduct, wherein the twisted cis-interligand anion is energeticallyhigher than the reactant anion by ∼9.01 kcal/mol. Thesecomputed relative energies indicate that the dissociation ofnickel bis(dithiolene)−ethylene complexes to form ethyleneand nickel bis(dithiolene) anion is strongly favored. Therefore,the reduction can be used to drive the recycle process, in whichthe initially formed adducts can then release the ethylenemolecules upon reduction, and the obtained nickel bis-(dithiolene) anion can also be oxidized to recover its neutralform, making nickel bis(dithiolene) sheet a reusable separationcatalyst to reversibly bind and release ethylene. In practicalexperiments, the oxidation or reduction of the nickelbis(dithiolene) species in its anion or neutral form can bepossibly modulated via an electrochemical process, in which theelectron is removed or added by switching the electric potentialat a given electrode. Noteworthy, compared with thosepreviously studied nickel bis(dithiolene) complexes at the

Figure 3. Top and side views of the optimized geometries for theconsidered ethylene addition products: intraligand adduct (a), twistedcis-interligand adduct (b), and normal cis-interligand adduct (c). Thecorresponding structure parameters and relative reaction energies areindicated below each configuration.

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single-molecule level,8−13 the extended 2D nickel bis-(dithiolene) monolayer at the nanoscale should potentiallyact as the more efficient catalyst for olefin purification, giventhat it has higher surface area and holds more active sites forethylene binding.

4. CONCLUSIONSIn summary, with DFT computations, we have systematicallyinvestigated the structural and electronic properties of theexperimentally achieved nickel bis(dithiolene) single-sheetpolymer and afterward explored the reactivity of this novelmaterial with ethylene cycloaddition. The nickel bis(dithiolene)monolayer is perfectly planar and behaves as a narrow-band gapsemiconductor. Its lowest unoccupied electron states are mainlycomposed of the p orbitals of S atoms, making S atoms asreaction sites to accept electrons from the occupied p orbitalsof CC bonds. Indeed, our computations reveal that thenickel bis(dithiolene) monolayer shows high reactivity towardethylene, wherein the formation of cis-interligand S,S′-adduct isthermodynamically preferred. When an extra electron is addedinto the reaction system, the addition product becomesdestabilized and its dissociation into ethylene and nickelbis(dithiolene) anion turns a spontaneous process. Therefore,the nickel bis(dithiolene) monolayer can effectively bind andrelease ethylene molecules in neutral and electron-rich(reduction) conditions, respectively. This endows nickelbis(dithiolene) sheet with great prospect as the reaction carrierand catalyst in olefin separation or purification. We hope thatour predictions will motivate further experimental andtheoretical studies on this novel 2D coordination polymer,and its possible applications in nanoelectronics and catalysts arealso expected.

■ AUTHOR INFORMATIONCorresponding Author*(Z.Z.) E-mail: zhouzhen@nankai.edu.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by NSFC (21073096) in China. Thecomputations were performed on Magic Cube at ShanghaiSupercomputer Center.

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Figure 4. Energy profile for the reaction of ethylene with nickel bis(dithiolene) network to form the cis-interligand product. The relative freeenergies are shown in brackets with the energy unit of kcal/mol. The structure parameters corresponding to the transition states (TS1, TS2, andTS3) are also indicated.

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