Untangling Complex Redox Chemistry in Zeolitic Imidazolate

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Untangling Complex Redox Chemistry in Zeolitic Imidazolate Frameworks Using Fourier Transformed Alternating Current Voltammetry Pavel Maximovich Usov, Alexandr N. Simonov, Alan M Bond, Michael J Murphy, and Deanna M. D'Alessandro Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01224 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 6, 2017

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Analytical Chemistry

Untangling Complex Redox Chemistry in Zeolitic Imidazolate Frameworks Using Fourier Transformed Alternating Current Voltammetry Pavel M. Usov,a Alexandr N. Simonov,b Alan M. Bond,b Michael J. Murphya and Deanna M. D’Alessandroa* a School of Chemistry, The University of Sydney, New South Wales, Australia 2006 b School of Chemistry and the ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria, Australia 3800 * D.M. D’Alessandro, Phone: +61 (2) 93513777, Fax: +61 (2) 93513329, Email: [email protected]

ABSTRACT: Two Zeolitic Imidazolate Frameworks, ZIF-67 and ZIF-8 were interrogated for their redox properties using Fourier transformed alternating current voltammetry which revealed that the 2-methylimidazolate ligand is responsible for multiple redox transformations. Further insight was gained by employing discrete tetrahedral complexes, [M(DMIM)4]2+ (DMIM = 1,2dimethylimidazole, M = CoII or ZnII) which have similar structural motifs to ZIFs. In this work we demonstrate a multidirectional approach that enables the complex electrochemical behavior of ZIFs to be unraveled.

Introduction 1

Metal-Organic Frameworks (MOFs) are a diverse class of 2and 3-dimensional microporous solids which have attracted considerable attention in the scientific community since their discovery more than 20 years ago.2,3 These materials have been shown to possess several unique properties including high porosity and near-infinite chemical and structural tunability4,5 which make them ideal candidates for a range of electrochemical applications such as batteries,6,7 electrocatalysts8,9 and porous conductors.10–13 An ongoing challenge in this respect is the design and synthesis of novel electroactive frameworks with improved performance capabilities such as redox stability and recyclability. For this purpose, understanding the parameters governing electron transfer within MOFs is crucial. D.c. cyclic voltammetry has successfully been employed to determine the mechanism of electron transfer in frameworks14 as this often mirrors the redox behaviour of the components. In addition, in situ spectroelectrochemical techniques such as UV-Vis-NIR and EPR have been utilised to gain deeper insights into the nature of redox transformations.15–21 MOFs containing redox-active ligands are highly suitable for steady state measurements due to the multiple redox states that are present within the material. Based on previous reports, two distinct mechanisms for electron transfer in MOFs have been proposed.22 The first involves surface-confined processes23 where only the redox centres on the surface of MOF particles undergo redox changes while the bulk of the material remains unaffected. This behaviour is particularly expected for low porosity and insulating frameworks where the charge has no means of propagating through the material. In contrast, the second mechanism postulates that electrons hop between redox centres through the particle. Strong evidence for this type of behaviour has been observed previously.14–24 An important requirement for the latter mech-

anism is diffusion of counter-ions within the pores of the framework in order to compensate for the charge variations. Zeolitic Imidazolate Frameworks (ZIFs) represent a subclass of MOFs which contain tetrahedral metal centres (CoII, ZnII and CdII) connected by imidazole derived ligands into zeolitelike structures.25–27 We previously showed that one member of the series, ZIF-67 (Co(MIM)2, MIM = 2-methylimidazolate) undergoes an electrochemically-induced transformation to Co3O4 in aqueous electrolyte solutions.28 The presence of hydroxide ions in the electrolyte medium led to displacement of the 2-methylimidazolate ligand from ZIF structure, thus precluding a determination of the intrinsic redox behaviour. In order to prevent ligand displacement, the electrochemical properties of the framework were investigated in a nonaqueous solvent, acetonitrile, using supporting electrolytes with relatively weak coordinating anions (ClO4- and PF6-). Attempts to access higher oxidation states of ZIF-67 using chemical oxidants such as I2, Br2, NOBF4 and (NH4)2[Ce(NO3)6] resulted in either loss of crystallinity or complete dissolution of the framework. Recently Awaga et. al.29 synthesised a redox-active MOF featuring both a redox-active ligand and a metal cluster which resulted in complex electrochemical behaviour. The peak potentials of each component under d.c. voltammetric conditions were well separated such that their electrochemical properties could be investigated independently. ZIF-67 is another example of a framework with potentially complex electrochemistry – both the ligand (MIM)30 and the metal (CoII) are redox active, and they are expected to have close lying redox potentials.31 Herein, we present an alternative approach to unravelling the redox transformation mechanism in ZIF-67 – a framework which possesses unstable higher oxidation states and is therefore unsuitable for interrogation using most of the commonly available in situ methods.32–34 The isostructural

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framework, ZIF-8 (Zn(MIM)2, MIM = 2-methylimidazole) offers a convenient point of comparison with ZIF-67 owing to the redox inactivity of the ZnII centres. Furthermore, discrete CoII and ZnII imidazolate complexes which mimic the structural motifs of tetrahedral metal centres in ZIFs were utilised as model compounds to further understand electrochemical properties of ZIF-67 and ZIF-8. Overall this work seeks to demonstrate the importance of a multidirectional approach for providing mechanistic insights into complex electrochemistry of MOFs. Experimental Section Materials All reagents and solvents were purchased from commercial sources and used without further purification. Co(NO3)2·6H2O, (n-C4H9)4NClO4 (electrochemical grade) and (n-C4H9)4NPF6 were purchased from Alfa Aesar; Co(ClO4)2·6H2O from Fluka AG and 2-methylimidazole (HMIM), 1,2-dimethylimidazole (DMIM), Zn(NO3)2·6H2O and Zn(ClO4)2·6H2O from Sigma Aldrich. [Co(DMIM)4](ClO4)2 was synthesised according to the literature procedure.31 Synthesis of Zeolitic Imidazolate Frameworks (ZIFs) ZIF-67 and ZIF-8 were synthesised following the documented procedures28 using Co(NO3)2·6H2O and Zn(NO3)2·6H2O as metal precursors, respectively. The synthesised ZIF powders were characterised by X-ray diffraction (Figure S1) and their purity was assessed by elemental analysis (ZIF-67: found C 42.7, H 4.22, N 25.1; calculated for Co(MIM)2·0.2H2O: C 42.7, H 4.67, N 24.9. ZIF-8: found C 41.2, H 4.20, N 23.8; calculated Zn(MIM)2·0.4H2O: C 41.2, H 4.69, N 24.0). Synthesis of [Zn(DMIM)4](ClO4)2 The synthetic procedure was identical to that for the Co analogue. A solution of Zn(ClO4)2·6H2O (3.72 g, 10 mmol) in CH3CN (15 mL) was added dropwise to a solution of DMIM (3.8 g, 40 mmol) forming a clear solution. After stirring for 10 min, diethyl ether (100 mL) was added to the reaction mixture resulting in precipitation of white solid. The powder was isolated using vacuum filtration, washed with excess diethyl ether and dried under vacuum (5.8 g, 90.6% yield based on Zn). ESI-MS (ESI+, CH3CN): m/z (%) 223.9 (98%), [Zn(DMIM)4]2+. Elemental analysis: found: C 37.0, H 4.94, N 17.1; calculated for [Zn(DMIM)4](ClO4)2: C 37.0, H 4.97, N 17.3. Synthesis of [Co(HMIM)4](ClO4)2 A solution of Co(ClO4)2·6H2O (3.66 g, 10 mmol) in CH3OH (15 mL) was added dropwise to a solution of HMIM (3.28 g, 40 mmol) forming a purple solution. After stirring for 10 min, diethyl ether (100 mL) was added to the reaction mixture resulting in precipitation of a purple solid. The powder was isolated using vacuum filtration, washed with excess diethyl ether and dried under vacuum (4.97 g, 84.8% yield based on Co). ESI-MS (ESI+, CH3CN): m/z (%) 193.4 (56%), [Co(HMIM)4]2+. Elemental analysis: found: C 32.6, H 4.20, N 18.8; calculated for [Co(HMIM)4](ClO4)2: C 32.8, H 4.13, N 19.1. Synthesis of [Zn(HMIM)4](ClO4)2 The synthetic procedure was identical to that for the Co analogue except that Zn(ClO4)2·6H2O (3.72 g, 10 mmol) was used as a metal precursor (4.0 g, 67.5% yield based on Zn). ESI-MS (ESI+, CH3CN): m/z (%) 195.9 (66%), [Zn(HMIM)4]2+. Ele-

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mental analysis: found: C 32.5, H 4.05, N 18.7; calculated for [Zn(HMIM)4](ClO4)2: C 32.4, H 4.08, N 18.9. Single-crystal X-ray diffraction Single-crystal X-ray diffraction measurements were conducted on an Oxford Supernova diffractometer with Cu–Kα radiation (λ = 1.54178 Å) at 150 K. Empirical absorption corrections were made using SCALE3 ABSPACK.35 Structure solutions were obtained using SHELXS program of the SHELXTL package and refined using SHELXL–201436 in WinGX.37 Atoms were refined anisotropically where possible. Hydrogen atoms H1N, H2N, H3N and H4N were identified from a Fourier difference map and allowed to refine with a N-H distance of 0.87(2) Å with isotropic displacement parameters set to 1.20 Ueq of the parent N atom. All other H atoms were placed in calculated positions with appropriate riding modes. Both compounds are twin by inversion with flack parameters of 0.460(20) and 0.387(5) for [Zn(HMIM)4](ClO4)2 and [Co(HMIM)4](ClO4)2 respectively. Single crystals of [Co(HMIM)4](ClO4)2 and [Zn(HMIM)4](ClO4)2 were obtained by slow (10 days) vapour diffusion of diethyl ether into saturated CH3CN solutions of each complex. Crystal Data for [Zn(HMIM)4](ClO4)2 (M =592.70): monoclinic, space group Cc (no. 9), a = 15.6929(3) Å, b = 16.0879(4) Å, c = 11.3047(2) Å, β = 121.482(3)°, V = 2433.95(12) Å3, Z = 4, T = 150(2) K, µ(CuKα) = 3.951 mm-1, Dcalc = 1.618 g / mm3, 44009 reflections measured (8.594 ≤ 2θ ≤ 152.646), 4665 unique (Rint = 0.0326, Rsigma = 0.0133) which were used in all calculations. The final R1 was 0.0330 (I > 2σ(I)) and wR2 was 0.0875 (all data). Crystal Data for [Co(HMIM)4](ClO4)2 (M =586.26): monoclinic, space group Cc (no. 9), a = 15.6924(2) Å, b = 16.0720(2) Å, c = 11.30760(10) Å, β = 121.3850(10)°, V = 2434.60(5) Å3, Z = 4, T = 150(2) K, µ(CuKα) = 8.075 mm-1, Dcalc = 1.599 g / mm3, 34638 reflections measured (8.592 ≤ 2θ ≤ 152.976), 5069 unique (Rint = 0.0458, Rsigma = 0.0253) which were used in all calculations. The final R1 was 0.0346 (I > 2σ(I)) and wR2 was 0.0920 (all data). CCDC 1048142-1048143 contains the supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Powder X-ray Diffraction Data were collected on a PANalytical X’Pert PRO MPD diffractometer with Cu-Kα (1.5406 Å) radiation. The application software was X’Pert Data Collector v2.2f, and the instrument control software was XPERT-PRO v1.9E. PXRD data were collected over the 5-50° 2θ range with a 0.02° step size and 2°/min scan rate. Powders were mounted onto reflective discs with a Si(510) surface which were placed into a BraggBrentano reflection transmission spinner attachment. Elemental Analysis CHN analyses were conducted at the Chemical Microanalysis Facility at the Department of Chemistry & Biomolecular Sciences, Macquarie University (Australia). Mass Spectrometry Mass spectra were acquired for CH3CN solutions of the synthesised compounds with a 100 µLmin-1 flow rate on a Finnegan LCQ MS Detector (ESI). An ESI spray voltage of 5 kV


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was applied with a heated capillary temperature of 473 K and a N2 sheath gas pressure of 60 psi. d.c. and Fourier transformed a.c. voltammetry Electrochemical experiments were undertaken in a three electrode configuration at ambient temperature (295 ± 1 K) using either an Epsilon electrochemical workstation (BASi) or a custom-made FT a.c. voltammetry instrument.38 Voltammetric studies were undertaken in acetonitrile solutions with either 100 mM (n-C4H9)4NClO4 or 100 mM (n-C4H9)4NPF6 added as the supporting electrolyte after deaeration by purging with high-purity argon for 15 min. For voltammetric studies in solution, a glassy carbon macrodisc electrode (GC; nominal diameter 1 mm) embedded in an isolating inert sheath (BASi) was employed as a working electrode. Prior to use, the surface of the working electrode was thoroughly polished with alumina powder (0.3 µm) on a wet polishing cloth (BASi), and washed repeatedly with water. Finally, the working electrode surface was flushed with acetone and dried in an Ar stream. For measurements with surface confined solids, working electrodes were prepared by drop-casting and drying of an CH3CN suspension of the powdered material onto a graphite paper strip (20 mm2) with a Pt wire current collector (0.075 mm diameter; connected using silver paint). High surface area Pt wire in contact with solvent (electrolyte) and Ag wire installed in the vicinity of the surface of the working electrode were used as auxiliary and quasi-reference electrodes, respectively. The potential of the quasi-reference electrode was calibrated vs. the reversible potential of the Fc0/+ couple measured in the same solution before or after the experiment. Unless otherwise stated, current data are normalised to the electrode surface area.

Results and Discussion Electrochemistry of surface confined ZIF-67 and ZIF-8 Oxidation of the surface confined ZIF-67 and ZIF-8 frameworks was first investigated using d.c. cyclic voltammetry in CH3CN with (n-C4H9)4NClO4 as the supporting electrolyte (Figure 1a). Both analogues exhibited qualitatively similar behaviour, with two oxidation processes (O1 and O2) observed at potentials above 0.7 V vs. ferrocene (Fc0/+) (hereinafter all potentials are referred to the reversible potential of the Fc0/+ process measured in the same medium), although the oxidation currents per unit surface concentration are higher for ZIF-67 than for ZIF-8. Comparison of the d.c. voltammograms obtained in the presence and in the absence of ZIFs on the electrode surface suggests that the O1 and O2 processes (Figure 1a) are superimposed on the underlying slow and irreversible oxidation of the frameworks commencing at potentials above ca. 1.1 V, while contributions from the background faradaic current, presumably associated with the oxidation of the electrode surface and/or solvent, is less significant. Qualitatively similar voltammetric behaviour displayed by both ZIFs examined suggests that the O1 and O2 found in d.c. voltammetry involve oxidation of 2-methylimidazolate ligands, rather than the metal centres. Efficient separation of the faradaic current associated with comparatively fast redox transformations of ZIFs (O1 and O2) from the underlying background current is possible via application of the method of Fourier transformed (FT) a.c. voltam-

metry, which also has the capacity to enhance the quality and quantity of the useful information available from the experiment.38–39 In the present study, the electrochemistry of ZIF-67 and ZIF-8 was probed for the first time with the use of the FT a.c. voltammetry to gain a deeper insight into the redox mechanisms of the frameworks. Juxtaposition of the a.c. voltammetric data obtained for oxidation of ZIF-67 and ZIF-8 (Figure 1b-c) confirms the qualitative similarity of their electrochemistry. The 4th and higher order a.c. harmonics are essentially devoid of the background current and allow clear resolution of two well-defined oxidation processes, which was not possible with the aperiodic (d.c.) or lower a.c. harmonic components (1st to 3rd) (Figures S2 and S3). The first oxidation process (O1) produces low-intensity distorted a.c. harmonic components at potentials ca. 0.7-0.8 V and is chemically irreversible (i.e. coupled to a fast and irreversible chemical transformation step) for both frameworks examined, as concluded from the absence of a corresponding reductive (R1) a.c. faradaic response during the reverse sweep of the d.c. potential (Figure 1b-c). The second oxidation process (O2) is also found at similar potentials for both ZIFs (ca. 1.3 V for ZIF-67 and ca. 1.2 V for ZIF-8), but in contrast to O1, demonstrates chemically reversible behaviour reflected by the similar magnitudes of the faradaic a.c. current measured during the forward (O2) and backward (R2) sweeps of the d.c. potential (Figures 1b-c, S2 and S3). The only major distinction between the a.c. voltammograms obtained with ZIF-67 and ZIF-8 is a chemically irreversible process Ox found at 0.15-0.20 V for the Zn-based framework (Figure 1b-c), but not for the Co-based one. This process also is not detected under d.c. voltammetric conditions as it makes a negligible contribution to the overall oxidation charge of ZIF-8 as compared to O1 and O2 (Figure 1a). It has not been possible to assign the Ox process to a particular redox transformation of the MIM-ligand nor to establish the mechanism of its inhibition by CoII, and all further analysis focuses on the main processes O1 and O2/R2.



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Figure 1. (a) D.c. cyclic voltammetry (sweep rate, v = 0.20 V s-1), (b) 4th and (c) 5th harmonic components (envelope presentation38) of a.c. cyclic voltammograms (a.c. frequency, f = 18 Hz; a.c. amplitude, ∆E = 0.15 V; v = 0.075 V s-1) obtained at 296 K for oxidation of ZIF-67 (solid; blue) and ZIF-8 (dashed; red) immobilised on a graphite electrode in contact with CH3CN (100 mM (n-C4H9)4NClO4). Voltammetric data obtained for a ZIF-free electrode are shown as solid grey curves. Panels b and c additionally show the theoretical data simulated for the mechanism shown in Scheme 1c (dash-dotted; purple) using the parameters given in Table S1 (Supporting Information). R1 and Red denoted with a strikethrough indicate the absence of these processes on the reverse potential scan. Currents are normalised to the geometric surface area of the electrode. Modelling of the FT a.c. voltammetric data for oxidation of ZIF-67 and ZIF-8

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Qualitative observations available from the FT a.c. voltammetric analysis of ZIF-67 and ZIF-8 suggest that both frameworks undergo significant structural rearrangement upon initial oxidation (process O1, Figure 1) with the formation of a new species capable of a comparatively fast and chemically reversible oxidation (process O2/R2, Figure 1). To provide further insights into the oxidation mechanism of ZIFs, several plausible schemes where all species are confined to the electrode surface (Scheme 1) were modelled using MECSim software,40 and the simulated a.c. voltammograms were compared to the experimental data. The Butler-Volmer formalism with charge transfer coefficients arbitrarily set as 0.50 was used to describe the electron transfer kinetics. Heuristic fitting of theory to experiment was confined to the 4th and 5th a.c. harmonic components to avoid ambiguities associated with the simulation of the background current. Precise determination of the surface concentration of the electrochemically active ZIF units (Γ) by using Faraday’s law and the charge derived from the d.c. voltammograms exemplified in Figure 1a was not possible. Thus, the Γ values used in simulations (Table S1) can be considered as estimates, and therefore, semiquantitative conclusions only can be made on the basis of the experimenttheory comparisons. All electrode mechanisms considered were based on two oxidation processes and different combinations of first order chemical transformations, all of which were assumed to be irreversible (backward rate constant, kb set at 10-20 s-1) on the basis of the experimental observations and chemical considerations. Although perfect experiment-theory agreement was not achieved (Figures 1b-c, S4), the examined models mimicked the experimental behaviour acceptably. The best fit was achieved with the mechanism shown in Scheme 1c. However, it is worth noting that this model has more adjustable parameters than those in Schemes 1a-b, and therefore has more opportunities to reproduce the experimental voltammograms with no guarantee of improved plausibility.41 Nevertheless, several important features were common for all schemes examined: (i) the initially oxidised forms of ZIF-67 and ZIF-8 are highly unstable and undergo fast structural transformation; (ii) the resulting species are either electroactive or undergo fast transformation into a new electroactive compound; (iii) the resulting O2 redox process is faster than the initial oxidation of ZIFs and is also coupled to a chemical transformation, which is however, comparatively slow. Importantly, all observations made on the basis of the a.c. voltammetric experimenttheory comparisons apply to both ZIF-67 and ZIF-8, and thus confirm the ligand-based nature of the processes O1 and O2/R2. Voltammetric studies on model compounds in solution All attempts to chemically oxidise the ZIFs resulted in their degradation, as confirmed by solid-state Vis-NIR spectroelectrochemical analysis which was previously used successfully to monitor the spectral changes associated with the electrochemically induced changes in the redox state of related compounds.15–21,32 Thus, a fundamentally different strategy involving the analysis of soluble model systems based on 2methylimidazole (HMIM) and 1,2-dimethylimidazole (DMIM) was employed to support the mechanism suggested above for oxidation of ZIF-67 and ZIF-8. We define these ligands as giving uncapped (HMIM) and capped (DMIM) complexes.


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2-Methylimidazole, the protonated form of the MIM ligand, was itself found to be a poor model system as it undergoes radical polymerisation upon oxidation (Scheme S1) to form polyimidazole which blocks the working electrode30 and precludes further useful measurements (Figure S5). The discrete tetrahedral imidazole-containing complexes [Co(HMIM)4](ClO4)2 and [Zn(HMIM)4](ClO4)2 (Scheme 2a), which contain similar coordination environments to those within ZIFs, provide improved model systems. The two complexes were synthesised and single-crystal X-ray diffraction confirmed (Figures S6 and S7) their structural similarity to the motifs within ZIF-67 and ZIF-8, respectively.

Scheme 1. Proposed mechanisms for the oxidation of ZIF-67 and ZIF-8: a) mechanism 1, b) mechanism 2 and c) mechanism 3

Scheme 2. Model complexes [M(L)4]2+, where M = Co2+ and Zn2+: (a) uncapped complexes, L = 2-methylimidazole (HMIM); (b) capped complexes, L = 1,2-dimethylimidazole (DMIM). D.c. voltammetry for oxidation of the uncapped model complexes in acetonitrile (100 mM (n-C4H9)4NClO4) (Figure 2) exhibited two resolved irreversible oxidation processes O1 and O2, similar to their corresponding frameworks. These observations confirm the ligand-based redox behaviour of [Co(HMIM)4]2+ and [Zn(HMIM)4]2+. The relative peak current intensities of the O1 and O2 processes are highly dependent on the scan rate: O1 is more prominent at lower v whereas O2 dominates at faster scan rates. Also, as with the voltammetry

of 2-methylimidazole, an insulating film was formed on the working electrode surface immediately after one d.c. voltammetric cycle indicating that an analogous polymerisation reaction occurred. On this basis, a mechanism for oxidation of [M(HMIM)4]2+ is proposed (Scheme S2), where the HMIM ligand undergoes a one electron oxidation to a radical cation species followed by fast deprotonation to produce a neutral imidazole radical, which can either polymerise or produce a cationic species which is stabilised by back-donation from the metal centre. At low v, the polymerisation pathway dominates and the O2 oxidation is suppressed due to deactivation of the electrode surface. In situ spectroscopic evidence for this mechanism was precluded due to blockage of the working electrode by polymerised species. Chemical oxidation with agents such as peroxydisulfate resulted in the formation of polymeric gels that were difficult to characterise.

Figure 2. D.c. cyclic voltammetry (v = 0.025 (a), 0.20 (b) and (c)) at 296 K for oxidation of 0.80 V s-1 3.1 mM [Co(HMIM)4](ClO4)2 (solid; blue) and 2.3 mM [Zn(HMIM)4](ClO4)2 (dashed; red) in CH3CN (100 mM (n-C4H9)4NClO4) using a GC electrode. Solid grey curves show the voltammograms obtained in the absence of electroactive species in the solution. Currents are normalised to the geometric surface area of the electrode.

Modifying the ligand to 1,2-dimethylimidazole offered model complexes in which the reactive nitrogen sites were blocked (capped) to polymerisation. Within the potential range examined, d.c. cyclic voltammograms for DMIM solutions in acetonitrile (100 mM (n-C4H9)4NClO4) were essentially featureless, which is consistent with the shift of the imidazole oxidation to more positive potentials upon addition of the methyl group at the nitrogen position.30 Probing the redox properties of the capped DMIM-based complexes by d.c. voltammetry revealed that both [Co(DMIM)4]2+ and [Zn(DMIM)4]2+ undergo a well-defined major oxidation (O1; Figure 3) superimposed on a smaller irreversible oxidation process (O1’). Notable differences in the background currents of a GC electrode obtained in the absence and in the presence of the DMIM-based complexes can be interpreted in terms of adsorption of an electroactive material on the electrode surface.



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ment of the ZIF takes place at the surface, the pores become blocked and the structure is no longer permeable. The newly formed surface phase X subsequently undergoes fast and chemically reversible oxidation. The mechanism applies equally to both ZIF-67 and ZIF-8 with only minor variations in voltammetric oxidation peak potentials.

Figure 3. D.c. cyclic voltammetry (v = 0.05 (a), 0.20 (b) and 0.80 V s-1 (c)) at 296 K for oxidation of 2.0 mM [Co(DMIM)4](ClO4)2 (solid; blue) and 2.9 mM [Zn(DMIM)4](ClO4)2 (dashed; red) in CH3CN (100 mM (n-C4H9)4NClO4) using a GC electrode. Solid grey curves show the voltammograms obtained in the absence of electroactive species in the solution. Currents are normalised to the geometric surface area of the electrode.

A significant contribution from diffusion control of the O1 peaks in Figure 3 was concluded from analysis of the dependence of the background corrected peak current (Ip) on the scan rate. Approximately linear log Ip – log v dependencies with slopes of 0.3-0.4 (v range from 0.05 to 1.6 V s-1) were obtained for both DMIM-complexes. Departure of the slope from 0.5 expected for an exclusively diffusion-controlled process can be explained by significant uncertainties in the background correction procedure, implications of the Ohmic losses and other complexities. One of the complexities is reflected by the barely resolved peak O1’ (Figure 3), which is better defined for [Zn(DMIM)4]2+, and is possibly associated with a surface confined species, as its relative intensity with respect to O1 increases at higher v. More importantly, the radical cation formed upon oxidation of 1,2-dimethylimidazole cannot liberate a proton to remove the excess positive charge, and therefore DMIM-based complexes have no means of undergoing a second oxidation process like uncapped [M(HMIM)4]2+. However, an alternative and also fast route for quenching the [M(DMIM)4]●3+ radical cations exists, as evidenced by the chemical irreversibility of the O1 oxidation process (Figure 3). Another implication of the inability of DMIM to polymerise is the significant positive shift in potential of the O1 d.c. voltammetric peaks with respect to those found with HMIM-based complexes and ZIFs.30 Mechanism for oxidation of ZIFs-67 and ZIF-8 The observations summarised above allow us to propose a mechanism for the oxidation of the ZIFs (Scheme 3). Initial oxidation of imidazolate anions within the framework to neutral imidazole radicals produces a highly labile oxidised ZIF structure, which undergoes fast and irreversible structural rearrangement to form electroactive phase X. The nature of this transformation is poorly understood, but is expected to lead to significantly altered connectivity and other properties of the framework. Once oxidation and ensuing structural rearrange-

Scheme 3. Proposed oxidation mechanism of ZIF-67 and ZIF8. The imidazole radical and X+ species are capable of further chemical transformations as shown in Scheme 1, but omitted here.

Conclusions In conclusion, the present work demonstrates that frameworks featuring complex electrochemical behaviour and unstable redox states can be investigated by employing a multidirectional approach. Coupled with the studies on discrete complexes as models for the redox-active motifs in ZIFs, d.c. and FT a.c. voltammetry provide avenues to assigning the redox processes in the frameworks. This work has provided deeper insights into the electrochemical behaviour of imidazolatebased ligands within framework materials, which until now has received relatively limited attention. It was found that the solvent plays a major role in determining the overall redox behaviour, however, the nature of this effect will require further study. The multidirectional approach paves the way for an improved understanding of the redox properties of MOFs as needed for the rational design of frameworks in electroactive applications. The redox stability of ZIFs could be improved through modification of the imidazolate linkers to stabilise the heterocycle-based radical. These materials could also find applications in sensing devices and electrocatalysis.

ASSOCIATED CONTENT Supporting Information Supporting information contains powder and single crystal X-ray diffraction data, d.c. and FT a.c. voltammograms of ligand, complexes and frameworks, parameters used in the simulation of a.c. voltammograms, and schemes for mechanism of oxidation of complexes.

AUTHOR INFORMATION Author Contributions The manuscript was written by P.M.U, A.S. and D.M.D. Experiments and modelling were performed by P.M.U., A.S. and M.M. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT Financial support from the Australian Research Council is gratefully acknowledged.

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Untangling Complex Redox Chemistry in Zeolitic Imidazolate

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