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  • DaltonTransactions

    COMMUNICATION

    Cite this: DOI: 10.1039/c8dt02197j

    Received 29th May 2018,Accepted 29th June 2018

    DOI: 10.1039/c8dt02197j

    rsc.li/dalton

    Coordination-induced reversible electricalconductivity variation in the MOF-74 analogueFe2(DSBDC)

    Lei Sun, a Christopher H. Hendon b and Mircea Dinc *a

    Inner-sphere changes at the open Fe centers in Fe2(DSBDC)

    (DSBDC4 = 2,5-disulfidobenzene-1,4-dicarboxylate), as caused by

    coordination and release of solvent molecules, lead to reversible

    structural and electrical conductivity changes. Specifically, coordi-

    nation of N,N-dimethylformamide (DMF) to the open Fe sites

    improves the room-temperature electrical conductivity by three

    orders of magnitude. Supported by additional density functional

    theory calculations, we attribute the electrical conductivity

    enhancement to partial electron transfer from Fe to DMF, which

    generates hole carriers and improves the charge carrier density in

    Fe2(DSBDC).

    Introduction

    Traditionally raising interest due to their highly porousnature,1 metalorganic frameworks (MOFs) offer an excellentplatform for investigating and tuning electrical, magnetic, andoptical processes in organic-based materials, some of whichhave already led to applications in lithium-ion batteries,24

    thermoelectrics,5,6 and supercapacitors,7 among others. Theelectronic properties may be modulated either inherently,through changes in chemical composition, or through hostguest interactions. Guest molecules may cause structural vari-ation,8,9 tune electrical conductivity,10,11 induce spin statetransitions,12,13 modulate luminescence wavelength,14,15 andeven participate in guest-guest interactions.16 These phenom-ena are of fundamental interest and form the basis of variousapplications including chemiresistive sensors,11,17,18 fluo-rescence sensors,19 and cooperative adsorption.13,16

    Although several strategies are effective for increasing theelectrical conductivity in MOFs,20,21 among the most promis-ing, and certainly more tractable synthetically, are those thatinvolve post-synthetic doping. Guest molecules introducedafter the formation of a MOF may tune either the charge mobi-lity or the charge density in the skeleton of a given material,allowing in either case continuous enhancement of electricalconductivity over several orders of magnitude. Chemically, thedopant molecules may engage with the MOF either throughouter-sphere electron transfer (i.e. redox reactivity)2225 orthrough inner-sphere reactivity. The latter involves binding ofguest molecules to coordinatively unsaturated metal centers toform charge transport pathways and/or to inject charge car-riers.10 The inner sphere mechanism is proposed to be operat-ive, for instance, in TCNQ-infiltrated Cu3(benzene-1,3,5-tricar-boxylate)2 (TCNQ = 7,7,8,8-tetracyanoquinodimethane),wherein TCNQ molecules coordinate to Cu sites and generateincreasingly efficient charge percolation pathways and million-fold increase in the electrical conductivity of the parentMOF.10

    Herein, we show that simple coordination of solvent mole-cules such as DMF modulates the electrical conductivity ofFe2(DSBDC)

    26 by three orders of magnitude. Fe2(DSBDC) is astructural analogue of the well-known MOF-74 series M2(2,5-dihydroxybenzene-1,4-dicarboxylate) (M2(DOBDC), M = Mg,Mn, Fe, Co, Ni, Cu, Zn)2734 where phenoxide groups on theDOBDC ligand have been replaced by thiophenoxide. Each Featom in Fe2(DSBDC) is coordinated by two trans thiophenoxidegroups, three meridionally coordinated carboxylate groups,and one DMF molecule. The latter can be removed to yieldcoordinatively unsaturated Fe sites. The Fe and thiophenoxideS atoms form (FeS) chains, which are bridged byDSBDC4 ligands to form a three-dimensional framework con-taining one-dimensional hexagonal pores (Fig. 1). Previously,we reported that the regular hexagonal pores of DMF-filledFe2(DSBDC) (Fe2(DSBDC)(DMF)2x(DMF)) distort significantlywhen the unbound guest DMF molecules are replaced by di-chloromethane (DCM) and the material is activated 80 C togive Fe2(DSBDC)(DMF)2.

    26 The distortion is reversible: soaking

    Electronic supplementary information (ESI) available: Experimental details,computation details, thermogravimetric analysis profiles, powder X-ray diffrac-tion patterns, BET surface area analysis. See DOI: 10.1039/c8dt02197j

    aDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge,

    Massachusetts 02139, USA. E-mail: mdinca@mit.edubMaterial Science Institute, Department of Chemistry and Biochemistry, University of

    Oregon, Eugene, Oregon 97403, USA

    This journal is The Royal Society of Chemistry 2018 Dalton Trans.

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    www.rsc.li/daltonhttp://orcid.org/0000-0001-8467-6750http://orcid.org/0000-0002-7132-768Xhttp://orcid.org/0000-0002-1262-1264http://crossmark.crossref.org/dialog/?doi=10.1039/c8dt02197j&domain=pdf&date_stamp=2018-07-06http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/http://dx.doi.org/10.1039/C8DT02197Jhttp://pubs.rsc.org/en/journals/journal/DT

  • Fe2(DSBDC)(DMF)2 in DMF regenerates the regular hexagonalpores and original structure. We extend the structural and elec-trical properties study of this system here by investigating thefully desolvated material, Fe2(DSBDC), which shows divergentbehaviour from its solvated congeners.

    Results and discussion

    Fe2(DSBDC)(DMF)2x(DMF) was synthesized according to a pre-viously reported procedure.26 Owing to the sensitivity of thismaterial to both water and air, all subsequent manipulationswere performed in a dry, oxygen-free atmosphere.Thermogravimetric analysis (TGA) of this material revealedtwo consecutive weight loss events at 210335 C and320560 C, respectively (Fig. S1). The first weight loss stepcan be correlated to the loss of both bound and unbound DMFmolecules, while the second likely corresponds to framework

    decomposition. Indeed, powder X-ray diffraction (PXRD) con-firmed that heating Fe2(DSBDC)(DMF)2x(DMF) at 200 Cunder vacuum for 2 days causes amorphization (Fig. S2). Inan attempt to activate the material without decomposition, weexchanged DMF by soaking the as-synthesized MOF in lower-boiling methanol (MeOH), to produce Fe2(DSBDC)(MeOH)2x(MeOH). Near-complete displacement of DMF byMeOH was confirmed by infrared (IR) spectroscopy, whichrevealed significantly decreased intensity of the CvO stretchat 1654 cm1 and appearance of a MeOH CO stretching bandat 1008 cm1 as well as a broad OH stretching band at3300 cm1 (Fig. 2). Although Fe2(DSBDC)(MeOH)2x(MeOH)remains crystalline when kept in liquid MeOH (Fig. 3), it alsodecomposes upon desolvation (Fig. S2), possibly because of astronger interaction between MeOH and the framework. Wetherefore sought to replace methanol with a weaker interactingsolvent and soaked the methanol-exchanged material in tetra-hydrofuran (THF) to produce Fe2(DSBDC)(THF)2x(THF).IR spectroscopy once again confirmed near-quantitativeexchange, with complete disappearance of the bands at 1008and 3300 cm1 and rise of characteristic CH stretching bandsfor THF at 2969 and 2866 cm1, which are blue-shifted fromthe CH stretching bands for MeOH at 2938 and 2831 cm1.Note that direct replacement of DMF by THF is not successful,presumably because THF is too poorly coordinating andcannot displace Fe-bound DMF, thus requiring the intermedi-acy of MeOH.

    TGA of Fe2(DSBDC)(THF)2x(THF) confirmed that THF canbe removed even below 100 C, much lower than the decompo-sition temperature of approximately 270 C (Fig. S3).Therefore, to remove all solvent and produce completely acti-vated MOF, Fe2(DSBDC)(THF)2x(THF) was heated at 170 Cunder dynamic vacuum (

  • formula that matched that of fully desolvated Fe2(DSBDC),whose IR spectrum also indicates the complete absence ofDMF, MeOH, or THF characteristic bands (Fig. 2 and ESI).Most tellingly, an N2 adsorption of Fe2(DSBDC) exhibits a typeI isotherm with a saturation uptake of approximately 160 cm3

    of N2 per g at 77 K (Fig. 4) and an apparent BrunauerEmmettTeller (BET) surface area of 624 m2 g1 (211 m2

    mmol1) (Fig. S4). Although smaller than the value observedfor related Mn2(DSBDC) (329 m

    2 mmol1)35 and MOF-74 ana-logues (287416 m2 mmol1),30,32,33 possibly due to a furtherstructural distortion (vide infra), the apparent surface area ofFe2(DSBDC) is significantly larger than that of Fe2(DSBDC)(DMF)2 (83 m

    2 g1, 40 m2 mmol1) (Fig. 4),36 as would beexpected upon removal of pore-blocking DMF molecules.

    Although Fe2(DSBDC) is crystalline, its PXRD patterndiverges from those of both Fe2(DSBDC)(DMF)2x(DMF) and

    Fe2(DSBDC)(DMF)2: peaks at 6.6 and 11.3 corresponding tothe [210] and [300] planes in the original regular hexagonallattice are intact, but a new peak at 10.5 appears (Fig. 3).Qualitatively, this confirms that the hexagonal symmetry andpore shape is retained in Fe2(DSBDC), but the lattice suffers adistortion along the crystallographic c direction, which runsparallel to the (FeS) chains. Unfortunately, the relativelylow crystallinity of the activated material does not offersufficient quality for further structural refinement. However,soaking Fe2(DSBDC) in DMF or DCM produces materialswhose PXRD patterns are identical to th