Reprogramming Kinetic Phase Control and Tailoring PoreEnvironments in CoII and ZnII Metal−Organic FrameworksPublished as part of the Crystal Growth & Design virtual special issue IYCr 2014 - Celebrating the InternationalYear of Crystallography

Damien Rankine, Tony D. Keene,⊥ Christian J. Doonan,* and Christopher J. Sumby*

School of Chemistry & Physics, The University of Adelaide, Adelaide, Australia 5005

*S Supporting Information

ABSTRACT: Metal−organic frameworks (MOFs) 1-Co and1-Zn ([M3(L)(H2L)(DMF)(DABCO)], where M = Co andZn), which are based on trimeric nodal clusters(MTdMOhMTd), have been synthesized from the ligands 2,2′-dihydroxy-1,1′-biphenyl-4,4′-dicarboxylic acid (H4L) and 1,4-diazabicyclo[2.2.2]octane (DABCO). High temperature syn-thesis (150 °C) led to the formation of 1-Co, but an identicalreaction mixture gave exclusively 2-Co ([Co(H2L)(DMF)2])when reacted at 65 °C. Reactions at intermediate temperaturesgave a mixture of products confirming that 1-Co is thethermodynamic product and 2-Co is the kinetic product. Conditions used to form 2-Co at 65 °C were “reprogrammed” bydoping the reaction solution with ZnII to generate the thermodynamically favored phase (1-M) with a mixed CoII/ZnII

composition, 1-CoZn. Heterometallic mixtures of ZnII/CoII were explored for a range of starting metal ratios, showingpreferential incorporation of CoII over ZnII at 150 °C. Furthermore, coordination of CoII ions to the free diol moieties in 1-Znwas achieved by post-synthetic doping of 1-Zn with Co(NO3)2 in MeOH, generating Co@1-Zn. On the basis of pore sizedistributions and fluorescence emission spectroscopy, CoII was shown to bind to the diol moieties for all CoII-containing forms of1 during MOF synthesis but this does not occur for excess ZnII in 1-Zn. These synthetic conditions allow precise control overboth the internal pore dimensions and pore environment for variants of 1, leading to demonstrable improvements in the enthalpyof CO2 adsorption.

■ INTRODUCTION

The judicious selection of synthetic conditions is essential forgenerating metal−organic frameworks (MOFs) of predeter-mined structure metrics and crystal morphologies.1 Such finecontrol of MOF architectures is essential to the development ofnovel materials for size and shape selective gas and liquidseparations,2 and catalysis.3 In MOF materials the relationshipbetween structure and function is built on the principles ofreticular chemistry,4 whereby regular changes in both structuremetrics and pore environment can be achieved by the linearextension of organic linkers.5 This process is in competitionwith effects such as interpenetration which act to reduceavailable pore space. Changes in pore structure can eitherimprove or diminish a particular effect, depending on theapplication.6 Reduced pore sizes may be required forapplications involving recognition processes (i.e., selective gasadsorption,7 enantioselective sensing8), whereas larger poresizes are often preferred when high rates of reagent diffusion arerequired, for example, in heterogeneous catalysis. Analysis ofsuch structure/function relationships has proven to be aneffective method for the rational design of functional materials.Given the intimate structure/function relationships that are

observed for MOF materials, simple methods for “preprogram-

ming” their crystalline morphologies are of significant interest.Several methods for controlling the formation and/or phase ofMOF products of particular structure have been reported,including the form of the starting ligand and reagents,9

modification of solution chemistry,10 templating,11 thermody-namic and kinetic effects,12 pH,13 or crystal seeding methods.14

Each of these approaches seeks to influence the thermodynamicand/or kinetic parameters of MOF formation so thatreproducible, high yielding synthetic protocols for particularphases can be achieved.Here, we report the synthesis of three MOFs, 1-Zn, 1-Co

([M3(L)(H2L)(DMF)(DABCO)], where M = Zn and Co) and2-Co ([Co(H2L)(DMF)2]), with precise control over phaseformation via thermodynamic and kinetic methods (Scheme 1).Control of the reaction conditions allowed for phase-puresynthesis of 1-Co and 2-Co at 150 and 65 °C, respectively,from solutions that otherwise contained a mixture of phases atintermediate temperatures. Structural “reprogramming” of thekinetic product could be achieved at 65 °C by seeding a CoII-

Received: July 2, 2014Revised: August 27, 2014Published: September 8, 2014

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containing reaction solution with ZnII. Notably, this simplestrategy, which is akin to the “soldiers and sergeants” approachof Meijer,15 does not lead to a structural analogue of 2-Co, butto mixed-CoII/ZnII analogues of 1, denoted 1-CoZn. 1-CoZncould be synthesized from reaction solutions that encompasseda range of ZnII/CoII ratios with defined final metalcompositions. In addition, the successful incorporation ofsecondary metals ions into 1-Zn at the uncoordinated diolmoieties was undertaken via post-synthetic metalation16 withCoII, generating Co@1-Zn. Notably, these post-syntheticallymetalated MOFs (1-M) showed a marked preference at thediol moiety for coordination of Co over Zn and anenhancement in the MOF’s affinity for CO2.

■ EXPERIMENTAL SECTIONGeneral Experimental Methods. Unless otherwise stated, all

reagents were commercially obtained and used without furtherpurification. 2,2′-Dihydroxybiphenyl-4,4′-dicarboxylic acid (H4L) wassynthesized using literature procedures.9a Infrared (IR) spectra wererecorded on a Perkin−Elmer Fourier transform infrared (FT−IR)spectrometer on a zinc-selenide crystal. The Campbell microanalyticallaboratory at the University of Otago, Dunedin performed allelemental analyses. Thermogravimetric analysis (TGA) was performedon a Perkin−Elmer STA-6000 under a constant flow of N2 (20 L/min)at a temperature ramp rate of 5 °C/min. N2 adsorption isotherms at 77K were recorded on a Micromeritics ASAP 2020 adsorption analyzer.The Brunauer−Emmett−Teller (BET) method17 was used fordetermining surface areas from N2 isotherms at 77 K. Pore sizedistribution plots were calculated from N2 isotherms at 77 K using thedensity functional theory (DFT) method through the MicromeriticsASAP 2020 software. Isosteric heats of adsorption were calculatedusing the Virial method. UV−visible spectroscopy was performed on aCary 5000 spectrophotometer equipped with a Harrick Praying Mantisdiffuse reflectance attachment. Samples were dispersed in dried KBrprior to loading. Energy dispersive spectroscopy (EDS) was performedon a Philips XL30 field emission scanning electron microscope(FESEM) at 10 keV and further analyzed using the program EDAXGenesis. Samples surfaces were coated in carbon prior to EDS analysisto reduce surface charging and improve resolution.Synthesis of Metal−Organic Frameworks. General Procedure

for the Synthesis of 1. To a solution of H4L (1.0 mL, 0.1 M in DMF)was added M(NO3)2·6H2O (1.0 mL, 0.1 M) followed by DABCO (1.0mL, 0.055 M DMF solution). The resulting mixture was sealed in a 20mL scintillation vial and heated at 150 °C for 18 h.[Co3(L)(H2L)(DABCO)(DMF)], 1-Co. Dark purple crystals (30.8 mg,

68%). FT−IR (cm−1): 1655, 1588, 1540, 1410−1340 (br.), 1243,1023. Analysis calc. for [1-Co]·3/10[Co(DMF)4]·2DMF·H2O: C47.88, H 4.94, N 7.43; Found C 47.53, H 5.08, N 7.67%.[Zn3(L)(H2L)(DABCO)(DMF)], 1-Zn. Colorless crystals (29.4 mg,

63%). FT−IR (cm−1): 1648, 1591, 1541, 1410−1340 (br.), 1252,1022. Analysis calc. for [1-Zn]·2H2O·DMF: C 40.18, H 4.01, N 5.07;Found C 40.10, H 4.44, N 5.44%.“Reprogramming” Procedure for the Synthesis of [1-CoZn]

Analogues. To a solution of H4L (1.0 mL, 0.1 M in DMF) was

added Co(NO3)2·6H2O (0.25 mL, 0.1 M in DMF) and Zn(NO3)2·6H2O (0.75 mL, 0.1 M in DMF), followed by DABCO (1.0 mL, 0.055M DMF solution). The resulting mixture was sealed in a 20 mLscintillation vial and heated at 65 °C for 18 h yielding pale purplecrystals of [1-Co/Zn].

Standard Procedure for the Synthesis of 1-CoZn Analogues, [1-CoZn] (1:1). To a solution of H4L (0.5 mL, 0.1 M in DMF) wereadded Co(NO3)2·6H2O (0.25 mL, 0.1 M), Zn(NO3)2·6H2O (0.25mL, 0.1 M), and DABCO (0.5 mL, 0.055 M DMF solution). Theresulting mixture was sealed in a 20 mL scintillation vial and heated at150 °C for 18 h. 1-CoZn: Pale purple crystals (25.4 mg, 56%).

Standard Conditions for the Synthesis of [Co(H2L)(DMF)2], 2-Co.To a solution of H4L (1.0 mL, 0.1 M in DMF) were added Co(NO3)2·6H2O (1.0 mL, 0.1 M), DABCO (1.0 mL, 0.055 M DMF solution),and EtOH (0.5 mL). The resulting mixture was sealed in a 20 mLscintillation vial and heated at 65 °C for 8 h. 2-Co: Light pink crystals(26.1 mg, 62%). FT−IR (cm−1): X. Analysis calc. for [2-Co]·1/3H2O·

3/4DMF: C 47.68, H 5.03, N 7.35; Found C 47.30, H5.23, N 7.84%.

Procedure for the Synthesis of Co@[1-Zn]. In a scintillation vial,as-synthesized 1-Zn was washed with fresh DMF (3 × 5 mL) over 3 hand then washed with fresh MeOH (3 × 5 mL) over 3 h. The MeOHwas decanted and a solution of Co(NO3)2 (30 mg, mmol) in MeOH(2 mL) was added. The vial was then heated at 60 °C for 18 h. Theresulting pale pink crystals were washed with MeOH (3 × 5 mL) overa 3 h period and then left to soak overnight. [1-Zn]·4/5[Co(MeOH)4]·21/2MeOH: C 46.36, H 4.50, N 3.80; Found C 46.07, H 4.28, N4.13%.

X-ray Crystallography. Crystals were mounted under paratone-Noil on a plastic loop. X-ray diffraction data were collected with Mo-Kαradiation (λ = 0.7107 Å) using an Oxford Diffraction X-calibur singlecrystal X-ray diffractometer at 150(2) K. Data sets were corrected forabsorption using a multi-scan method, and structures were solved bydirect methods using SHELXS-9718 and refined by full-matrix least-squares on F2 by SHELXL-86,19 interfaced through the program X-Seed.20 Data were recorded at the Australian Synchrotron (Clayton,VIC) performed on the MX1 beamline (set to the Mo-Kα wavelength,λ = 0.7107 Å) equipped with an ADSC Quantum 210r detectorinterfaced through the program BluIce21 and collected by scanning180° through phi at 150(4) K. Collected data were processed andsolved using XDS.22 Refinement procedures were as described above.

In general, all non-hydrogen atoms were refined anisotropically, andhydrogen atoms were included as invariants at geometrically estimatedpositions, unless specified otherwise in additional details (seeSupporting Information (SI)). Details of data collections and structurerefinements are given below. CCDC numbers 1010726, 1010727,1010725, and 1010728 contain the supplementary crystallographicdata for the structures 1-Zn, 1-Co, 1-Zn·MeOH (SI), and 2-Co,respectively. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

The structures of 1-Co, 1-Zn, and 2-Co possess large voidscontaining diffuse electron density peaks that could not be adequatelymodeled as solvent. The SQUEEZE routine of PLATON23 wasapplied to the collected data, resulting in reductions in both R1 and

Scheme 1

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wR2 (SI, Table S2.1). Electron density removed from the pores of 1-Co resulted in the equivalent of 12.8 DMF molecules per unit cell,equating to 3.2 DMF molecules per formula unit (131 e−, 1-Co·3.2DMF). For 1-Zn, the results are similar, with the equivalent of 13.6DMF molecules found per unit cell, or 3.4 DMF molecules performula unit (136 e−, 1-Zn·3.4DMF). 2-Co was found to contain theequivalent of 4.8 DMF molecules per unit cell, or 1.6 DMF moleculesper formula unit (64 e−, 2-Co·1.6DMF).Powder X-ray Diffraction (PXRD). Unless otherwise stated,

PXRD data were collected on a Rigaku Hiflux Homelab system usingCu−Kα radiation with an R-Axis IV++ image plate detector. Sampleswere mounted on plastic loops using paratone-N and data werecollected by scanning 90° in phi for 120 s exposures. The data wasconverted into xye format using the program DataSqueeze. SimulatedPXRD patterns were generated from the single crystal data usingMercury 2.4.

■ RESULTS AND DISCUSSIONSynthesis and Structure of 1-Zn, 1-Co, and 2-Co.

Solvothermal reactions of H4L with DABCO and M(NO3)2·6H2O, where M = Co or Zn, in DMF at 150 °C yielded 3Disostructural MOFs [Co3(H2L)(L)(DABCO)(DMF)] (1-Co)as dark purple crystals, or [Zn3(H2L)(L)(DABCO)(DMF)](1-Zn) as colorless crystals. Analysis of the MOF crystalstructures shows that the diol ligand L is present in two distinctstructural motifs; a fully deprotonated ligand (L) thatcoordinates through all available oxygen donors; and as adoubly deprotonated ligand (H2L) that binds to the metal ionsvia the carboxyl groups only (Figure 1c). For the latter case thisleaves the non-coordinating dihydroxy moiety exposed in the

pores of the MOF, but due to rotational flexibility of LH2 aboutits biaryl axis, the pendant −OH groups of LH2 were found tobe disordered over two positions in the crystal structure. Inaddition to L and LH2 organic links, coordination of thetrimeric MTdMOhMTd cluster (Figure 1a) is completed by abridging DABCO ligand and a single coordinated DMFmolecule. Each of these clusters is connected to eight adjacentnodes to form the extended 3D network.Each of the three MII atoms is unique in the structure and

assumes a Td−Oh−Td geometrical arrangement within thecluster, with M[1, Td] adopting a slightly distorted Tdgeometry, which is attributed to lattice-imposed constraintsand a long carboxylate O−M interaction (2.328 and 2.514 Å forCo−O and Zn−O, respectively). The difference in thecoordination environment between M[1, Td] and M[3, Td] issmall, differing only in either coordination or bridging by one ofthe carboxylate donors, for M[1, Td] and M[3, Td], respectively.To the best of our knowledge, this trimeric MII cluster is aunique metal node in MOFs,24 with previously reported MOFscontaining trimetallic SBUs exhibiting a six-coordinate (Oh)geometry over all three positions of their trimeric units,24a,c,d,25

compared to the uncommon Td−Oh−Td arrangement26

observed in the structure of 1.The structure of 1 has channels along the a-, b- and c-axes

(Figure 1b, SI Figures S2.1−2.4) with a maximum porediameter of 12.5 Å. This open framework architecture inspiredus to probe the permanent porosity of 1-Zn and 1-Co. N2adsorption measurements were performed at 77 K on activatedsamples of 1-Co and 1-Zn and are shown in Figure 1d. Bothisotherms are best described as Type-1 in shape with BETsurface areas of 968 m2/g and 957 m2/g, for 1-Zn and 1-Co,respectively (SI, Tables S3.1−3.2). The bulk phase purity ofeach material was assessed using PXRD methods with Le Bailrefinement, modeled against simulated patterns obtained fromsingle X-ray crystal data (SI, Figures S4.3−4.4). Additionally,soaking as-synthesized 1-Zn in MeOH enabled solventexchange of the coordinated DMF, giving 1-Zn·MeOH thatwas confirmed by X-ray diffraction methods (SI, Table S2.2)Solvothermal reactions of H4L with DABCO and Co(NO3)2·

6H2O in DMF under milder synthetic conditions at 65 °Cyielded 2-Co as large pink crystals. X-ray crystallographicanalysis revealed that the crystals possess a 3D, kagome-like nettopology with the formula [Co(H2L)(DMF)2], 2-Co (SI,Figure S2.6), analogous to the NiII and MgII structurespreviously reported by our group.27 Confirmation of 2-Cophase-purity was undertaken by Le Bail refinement on theexperimental PXRD data (SI, Figure S4.5).

Thermodynamic vs Kinetic Control of CrystallizationProducts.We found that the optimum synthetic conditions for1-Zn required reaction in sealed solvothermal vessels attemperatures between 130 and 150 °C. However, 1-Zn couldstill be synthesized under solvothermal conditions, at temper-atures as low as 65 °C (SI, Figure S4.1). In contrast, 1-Co wasformed at 150 °C, but a different phase, 2-Co, formed from lowtemperature synthesis (65 °C). In order to investigate therelationship between temperature and phase formation, wesynthesized the CoII MOFs at selected temperatures over therange of 65−150 °C. In the case of reaction at 100 °C, amixture of phases were generated, as revealed by the formationof both dark purple and pale pink crystalline materials and twosets of diffraction peaks by PXRD. Patterns collected for 1-Co/2-Co crystallization products at 65, 85, 100, 120, and 150 °Crevealed a precise thermodynamic contribution to the

Table 1. Selected X-ray Crystallography Data andRefinement Parameters

1-Co 1-Zn 2-Co

compound [Co3(H2L)(L)(DABCO)(DMF)]

[Zn3(H2L)(L)(DABCO)(DMF)]

[Co(H2L)(DMF)2]

formula C37H31N3O13Co3 C37H31N3O13Zn3 C20H22N2O8Cocrystal system monoclinic monoclinic trigonalspace group P21/c P21/c P3221a/Å 17.6122(5) 17.5414(2) 17.3720(4)b/Å 18.1405(4) 18.1004(2)c/Å 18.3816(5) 18.4729(2) 8.6547(6)α/° 90 90 90β/° 91.826(3) 91.8580(10) 90γ/° 90 90 120.0V/Å3 5869.8(3) 5862.18(11) 2261.9(19)ρ/g cm−3 1.021 1.047 1.051Z 4 4 3T/K 150(2) 150(2) 150(2)μ/mm−1 0.883 1.263 0.604reflectionscollected

60156 65966 10564

uniquereflections(Rint)

11531 (0.0468) 11322 (0.0494) 3534 (0.0420)

reflectionsI > 2σ(I)

9064 9616 2637

data/restraints/parameters

11531/0/561 14323/12/551 10564/0/143

goodness of fit(S)

1.037 1.098 0.993

R1/wR2[I > 2σ(I)]

0.0440/0.1193 0.0362/0.0985 0.0449/0.1108

R1/wR2 (alldata)

0.0576/0.1251 0.0500/0.1031 0.0640/0.1189

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formation of 1-Co vs 2-Co (Figure 2a). Thermodynamic versuskinetic control of product formation was evident by PXRD(Figure 2b), whereby low temperatures drove the formation ofthe kinetic product 2-Co at 65 and 85 °C; a mixture of the 1-Co and 2-Co phases formed at 100 °C, distinguishable as 2-Coby 010 peaks at 5.8° in 2θ, whereas phase-pure 1-Co wasformed under thermodynamic control at 120 and 150 °C. Inthe case of 1-Zn, only one phase was observed under identicalconditions (SI, Figure S4.1), indicating the 1-Zn phase is thethermodynamically favorable product over an identical temper-ature range.Given the preference that reactions containing ZnII show for

forming a single phase, 1-Zn (with no evidence for a kagome-type 2-Zn phase), over the entire temperature range assessed,we were motivated to study the ability of small amounts of ZnII

to “reprogram” the reaction conditions that yielded thekinetically favored topology 2-Co. Accordingly, 0.25 equiv ofZn(NO3)2 was added to the reaction solution used to form 2-Co at 65 °C. The resultant MOF was a mixed CoII−ZnIIanalogue of the thermodynamically favored topology 1,denoted 1-CoZn, confirmed by PXRD (Figure 2c) and EDS.This represents a reversal, or “reprogramming”, of thermody-namic control for an otherwise kinetically driven process.12d,28

While the mechanism for this process has not been fullydetermined, it is evident that incorporation of ZnII, which doesnot form a 2-Zn kagome phase, drives formation of thethermodynamic phase, 1, at temperatures that would normallyyield the kinetic product 2-Co.2-Co proved to be less stable than the previously reported

NiII or MgII structural analogues.27a Despite employing a range

of activation methods, 2-Co did not maintain its porositysubsequent to solvent removal, and furthermore PXRD analysisshowed that the framework had decomposed upon activation.Nevertheless, DMF molecules coordinated to the metal nodecould be exchanged via solvent exchange with MeOH.Desolvation under a vacuum exhibited a distinct change inUV−visible absorbance of 2-Co (SI, Figure S5.1) that wasreflected in a noticeable change in the color of the crystals froma light pink to a dark blue color that is indicative of a well-known shift in coordination sphere from Oh Co

II to Td CoII.

This observed desolvation process is analogous to that of theNiII analogue, [Ni(H2L)(DMF)2]; however, in this case theprocess was irreversible. This is likely due to both a morestabilized pseudo-Td CoII complex compared to NiII, andlimited access to the metal by pore solvents, resulting in astabilization of the desolvated MOF. Although DABCO is not astructural component of 2-Co, its removal from the syntheticprocedure resulted in a severe reduction in the rate of synthesisand yield. In these reactions it is likely that DABCO increasesthe pH of the solution leading to an increase in the rate ofMOF formation by rapid deprotonation of the carboxylate and/or hydroxyl moieties on the ligand.

Expansion of Mixed-Metal Analogues of 1. To assessthe role of the starting metal ion ratio for the synthesis of mixedCoII/ZnII MOFs, we prepared a series of reaction solutions ofvarying metal concentration, using a reaction temperature of150 °C. In place of solutions of a single metal salt, mixtures ofCoII and ZnII were used with tight control over solutionstoichiometry (ligand/metal/DABCO, 1:1:0.55) and reactionconcentration (0.1 M). To monitor the effect of heterometallic

Figure 1. (a) Structural representations of the node in 1-Co. (b) Extended structure of 1-Co viewed down the crystallographic b-axis. (c) Two formsof the ligand, H2L (left) and L (right). Co − purple, C − gray, N − blue, and O − red. (d) N2 adsorption isotherm at 77 K of 1-Co (purple) and 1-Zn (dark cyan).

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mixtures on final MOF composition, we looked at starting Co/Zn ratios of 1:10, 1:5, 1:3, 1:2, 1:1, 2:1, 3:1, 5:1, and 10:1. Atlow concentrations of CoII the crystals appear light purple yetincrease in intensity to a dark purple/blue color as theconcentration of CoII increases. To ascertain the bulk phase,each sample was analyzed by PXRD experiments, where closeinspection of the peak positions and intensities confirmed thatin each case phase-pure samples of 1 (Figure 3a) wereobtained; N2 adsorption measurements, to confirm mixedanalogues maintain permanent porosity (Figure 3b); solid-stateUV−visible spectroscopy, to probe any change in geometry ofthe CoII centers (Figure 3c); and EDS for determination ofelemental composition (Figure 3d). From UV−visible data theformation of solid solutions was observed in all Co/Znmixtures, whereby a random distribution of both CoII and ZnII

is present in the MOF nodes, with limited evidence that mightindicate site-specific occupation of Oh or Td sites within thetrimetallic node. In addition, EDS confirmed the ratio of CoII/ZnII present in the products formed to have a bias for CoII.Backscattered electron (BSE) analysis showed a homogeneousdispersion of the metal ions throughout the batch of crystals,effectively ruling out co-crystallization of discrete 1-Co and 1-Zn crystallites.

Selective Coordination of CoII over ZnII at theNoncoordinating Diol Moieties. To understand theenhanced incorporation of CoII over ZnII in the mixed metalMOFs, we investigated whether the CoII ions were beingincorporated at the noncoordinated diol moieties within 1, inaddition to the structural node. This might explain the greaterpercentage of CoII found in EDS studies, particularly observedwhen very low amounts of Co were used in the startingmixture. Accordingly, 1-Zn crystals were soaked in varioussolutions containing excess CoII salts in order to generate Co-doped 1-Zn. Selective incorporation became evident duringmetalation trials on 1-Zn, whereby CoII ions (CoCl2 orCo(NO3)2) coordinating larger solvent molecules in solution(i.e., DMF or i-PrOH) were either unable to permeate thepores of the crystals or unable to coordinate the diol moieties inthe MOF nodes due to steric constraints. This was observed bya lack of permanent coloring in CoII soaked crystals of 1-Zn.Undertaking metalation in MeOH or EtOH displayednoticeable incorporation of CoII into the MOF, as observedby a permanent pink coloration to the crystals. 1-Zn soaked inEtOH or MeOH solutions containing CoCl2 resulted in darkblue discoloration of 1-Zn samples within several hours at roomtemperature and subsequent decomposition of the MOF.However, soaking 1-Zn in a MeOH solution of Co(NO3)2 at

Figure 2. (a) Synthetic scheme showing formation of 1-Co and 1-Zn (left) at 150 °C as the thermodynamic product, formation of 2-Co (top right)at 65 °C as the kinetic product, and reprogramming to yield 1-CoZn at 65 °C (note: structural representation of 1-CoZn only). (b) PXRD patternsfor MOFs synthesized at various temperatures under standard synthetic conditions. Simulated patterns for 1-Co and 2-Co are shown for reference.Note the appearance of both phases in the pattern derived from the 100 °C synthesis. (c) PXRD patterns collected for MOFs formed at 65 °C fromsolutions containing monometallic, Co, and dimetallic, Co + Zn, synthetic mixtures of 2-M or 1-M phase, respectively.

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Figure 3. Analysis of 1-CoZn analogues containing varying amounts of Co and Zn. (a) PXRD patterns for analogues of 1 synthesized at 150 °C frommixed metal solutions. The ratio of each metal in the reaction solution is given next to each pattern. (b) N2 adsorption isotherm at 77 K of 1-CoZn(∼65:35 Co−Zn by EDS, dark cyan). (c) Normalized solid-state UV−visible spectra of selected 1-Co/1-Zn analogues. Elemental ratio representsthat used in the reaction solution. (d) EDS of 1-CoZn mixtures. Percentage of Co given as the concentration in the initial reaction solution and inthe resulting MOFs.

Figure 4. (a) Coordination preferences for CoII and ZnII in the MOFs formed from H4L. (b) Fluorescence emission spectra generated with anexcitation wavelength of λexc = 265 nm. (c) Pore size distributions generated from 77 K N2 isotherms.

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65 °C overnight caused 1-Zn crystals to become a light pinkcolor with little to no loss in crystallinity of the original phase,denoted as Co@1-Zn. Thus, it seems that the non-coordinateddiol moieties of L within the pores of 1 proved to be accessiblebinding sites for secondary metal ions. The observationsupports the hypothesis that metalation, perhaps duringsynthesis, may account for the higher incorporation of CoII insamples of 1 formed from mixed metal salt starting materials.For 1-Zn soaked in a methanolic solution containing 10

equiv of Co(NO3)2 per free diol, EDS experiments showed anaverage 4:1 ratio of Zn to Co, and combined with the pale pinkcolor of the crystals, effectively ruled out significant metalexchange at the metal nodes in 1-Zn. The equivalent of 1 freediol ligand per node would give a Zn/Co ratio of 3:1 atmaximum CoII loading; thus the experimental EDS resultsequate to a CoII loading of ∼75%. An obvious restriction to100% metalation in this case may result from steric crowding ofthe pores as the loading of CoII increases. The pink coloring inCo@1-Zn is highly indicative of an Oh CoII geometry,presumably comprised of one or two hydroxide donors fromthe diol moiety with the remaining free coordination sitesoccupied by solvent.Analysis of pore size distributions, generated from 77 K N2

adsorption isotherms, of 1-Co, 1-Zn, 1-CoZn, and Co@1-Zn(Figure 4c), shows a step-wise loss of the larger poreenvironment in 1 (in the region 8−10 Å). Moderate loss ofthis larger pore is observed for 1-Co, with even greaterreduction seen for 1-CoZn, and then complete loss of this poredimension in Co@1-Zn. At the same time the importance of apore dimension centered around 7 Å increases. As this wasshown to occur in all MOFs containing CoII, it is most likelyattributed to the coordination of CoII cations at free diolmoieties within the pores. Interestingly, coordination of freediols is not observed in 1-Zn, indicating a preference fortrimeric node formation over diol coordination by Zn (Figure4a). This was supported by fluorescence spectroscopy.Excitation of 1-Zn, at λexc = 265 nm, resulted in a broadfluorescence emission band at λ = 430 nm, indicatingnoncoordinated diol moieties are still present in the pores ofthe structure (Figure 4b), similar to previously reportedfluorescence emission spectra for this ligand in alkali-earthmetal MOFs.27b This is quenched for those MOFs with Cocoordinated diol moieties: 1-Co, 1-CoZn, and Co@1-Znindicating the diol sites are occupied in these forms of 1.Interestingly, the changes in pore environment upon

metalation for Co@1-Zn generate an increase in the enthalpyof CO2 adsorption (Figure 5b), likely due to an increase in thepolar groups lining the interior pore surface. This is despiteobserving reductions in BET surface area from 957 m2/g, for 1-Zn, to 801 m2/g, for Co@1-Zn (SI, Figure S3). Thisobservation correlates well with previous reports, wherebyinteractions between the quadrupole moment of CO2 withhighly polarized organic or inorganic moieties, within the poresof MOFs, result in increases to the enthalpy of CO2adsorption.29 In particular, marked variations in CO2adsorption enthalpy have been observed between first rowtransition metals.30 This material can be activated further byheating at 140 °C for 1 h, generating ΔCo@1-Zn, whichdisplays an even higher enthalpy of CO2 adsorption, yet with alower total CO2 uptake and BET surface area of ∼100 cm3/g(Figure 5a) and 689 m2/g (SI, Table S3.5), respectively. Fromthe pore size distribution of ΔCo@1-Zn (SI, Figure 3.3), asmall increase in the size of the ∼7 Å pore is observed

compared to Co@1-Zn. However, PXRD patterns collected onsamples of Δ Co/Zn indicated the material was amorphous;thus the increase in CO2 enthalpy cannot be assigned to aprecise structural feature of the MOF.These observations rationalize the apparent selectivity for

CoII over ZnII in analogues of 1, whereby the increase is due toincorporation of CoII both at the structural node of the MOFand at the exposed diol moieties. As steric constraints limitedpost-synthetic metalation using Co(NO3)2 in DMF, which arethe standard reaction conditions for synthesis of 1, we cansurmise that coordination of CoII to the ligand diol moietiesoccurs during the process of MOF assembly. The differingextents of metalation for 1-Co and 1-CoZn compared to Co@1-Zn can therefore be related to the difference in coordinationpreference and rate of MOF synthesis for ZnII and CoII. Thismay account for the incorporation of more CoII (1-CoZn), orless CoII (1-Co), at the diol moieties. Furthermore, aninteresting comparison can be noted between samples of 1-CoZn made by the “reprogramming” method (65 °C) andthose prepared at 150 °C. At 150 °C Co is the preferred metaldue to a combination of its inclusion in the MOF node and inthe diol sites of LH2. However, at low temperature an oppositeobservation was made. For the “reprogramming” syntheticapproach, at low concentrations of Co in the reaction solution(25% Co in the starting solution) a close correspondence wasobserved with the ratio of Co/Zn in the MOF (based on EDS;see SI, Figure S5.8). Surprisingly, when higher concentrationsof Co in the reaction solution (75:25 Co-Zn) were used only

Figure 5. (a) CO2 adsorption isotherms collected at 273 K. (b) Heatsof adsorption curves from CO2 isotherms at 273 and 293 K,determined using the Virial method. Cyan − 1-Zn, green − Co@1-Zn,and purple − ΔCo@1-Zn.

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35% of the metal composition in the MOF was shown to beCo, confirmed again by EDS and PXRD (SI, Figure S4.2). Thissuggests, first, that the 1-M phase forms faster with Zn at 65 °C,and second that there is a limiting process occurring at thistemperature disfavoring Co incorporation (i.e., its preferencefor octahedral coordination) and resulting in a nonlinearrelationship between the Co concentration in reaction solutionand the Co/Zn ratio in the MOF.

■ CONCLUSIONSFor 1-M and 2-Co, two methods have been developed anddemonstrated to control phase pure MOF synthesis, as well asthe precise modification of pore environment by coordinationat secondary noncoordinating dihydroxyl sites. Three novelframeworks, 1-Co, 1-Zn, and 2-Co, have been synthesized, withthermal control over the phase-purity and synthesis of 1-Co vs2-Co. Formation of 2-Co under kinetic control was reversed bythe incorporation of ZnII salts into the MOF reaction solution,generating a mixed Co-Zn form of 1 at 65 °C, denoted 1-CoZn. This amounts to using an additive to reprogram theproduct distribution, presumably by seeding the formation ofthe thermodynamic product at low temperatures. Synthesis ofmixed-metal MOFs (1-CoZn) was expanded to include a rangeof starting Co/Zn ratios during MOF synthesis, resulting in amarked preference for CoII in the MOF products by EDS. Post-synthetic metalation, at noncoordinating diol moieties, in 1-Znwith Co(NO3)2 in MeOH formed Co@1-Zn, with a CoII

occupancy of ∼0.75 per free diol moiety. Analyzing theincorporation of CoII into 1-Zn revealed coordination of CoII

was occurring at these free diol moieties in all cases where CoII

was present, with varying amounts of incorporation. Thesesynthetic approaches have provided a precise method for themodification of pore dimensions and pore environment inanalogues of 1.14c While these conditions have providedintimate control over a set of MOFs formed from H4L, the“reprogramming” approach utilized may be able to beemployed to direct the formation of other systems wherecompeting phases are present.

■ ASSOCIATED CONTENT*S Supporting InformationX-ray crystallography, gas adsorption data, powder X-raydiffraction, spectroscopic and structural characterization. Thismaterial is available free of charge via the Internet at http://pubs.acs.org/.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: christian.doonan@adelaide.edu.au.*E-mail: christopher.sumby@adelaide.edu.au. Fax: +61 8 83134358. Tel: +61 8 8313 5770, +61 8 8313 7406.Present Address⊥(T.D.K.) School of Chemistry, University of Southampton,University Road, Southampton, SO17 1BJ, UK.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research is supported by the Science and IndustryEndowment Fund (SIEF). C.J.D. and C.J.S. would like toacknowledge the Australian Research Council for fundingFT100100400 and FT0991910, respectively. D.R. wishes to

acknowledge the support of an Australian Postgraduate Award.T.D.K. wishes to acknowledge the support of a Marie CurieInternational Incoming Fellowship within the 7th EuropeanCommunity Framework Program (Grant PIIF-GA-2011-300462). Collection of X-ray diffraction data for [1-Zn]·MeOH was undertaken on the MX1 beamline at the AustralianSynchrotron, Victoria, Australia. The authors wish to thank Dr.Deanna M. D’Alessandro at the University of Sydney for herassistance with solid-state UV−visible spectroscopy experi-ments and for helpful discussions.

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