in Orienting and Locking Together [M2

Communication pubs.acs.org/crystal

Role of NEt4+ in Orienting and Locking Together [M2lig3]2− (6,3) Sheets (H2lig = Chloranilic or Fluoranilic Acid) to Generate Spacious Channels Perpendicular to the Sheets Christopher J. Kingsbury,† Brendan F. Abrahams,*,† Deanna M. D’Alessandro,‡ Timothy A. Hudson,† Ryuichi Murase,†,‡ Richard Robson,*,† and Keith F. White† †

School of Chemistry, University of Melbourne, Parkville, Victoria 3010, Australia School of Chemistry, University of Sydney, Camperdown, NSW 2006, Australia

‡

S Supporting Information *

ABSTRACT: In the presence of the Et4N+ cation the chloranilate dianion (can2−) associates with a range of divalent cations, M2+, to yield an isomorphous series of crystalline compounds of composition (Et4N)2[M2(can)3] (M = Mg, Mn, Fe, Co, Ni, Cu, and Zn). The fluoranilate dianion (fan2−) likewise affords the closely related (Et4N)2[Zn2(fan)3]. The structures of (Et4N)2[Zn2(can)3], (Et4N)2[Fe2(can)3], and (Et4N)2[Zn2(fan)3] were determined by single crystal X-ray diffraction. Powder X-ray diffraction indicates that all the members of the can2− series are isomorphous. The structure of (Et4N)2[Zn2(fan)3] is very closely related to the structures of the can2− compounds. The [M2(can)32−]n component is present as chicken-wire-like sheets with (6,3) topology. The Et4N+ cation binds sheet to sheet and aligns them so that the large holes within the sheets are arranged one above another, thereby generating spacious channels running perpendicular to the sheets. The solvent molecules present in the channels are ill-defined and easily removed. The (Et4N)2[M2(can)3] structure remains intact after desolvation. The void spaces are calculated to be ∼39% in the case of the can2− compounds and ∼43% in (Et4N)2[Zn2(fan)3]. Substantial amounts of CO2 are sorbed at 273 K by (Et4N)2[Zn2(can)3] and (Et4N)2[Zn2(fan)3]. Spectroscopic evidence supports the presence of at least some of the chloranilate in the radical trianion form in (Et4N)2[Fe2(can)3].

T

Pioneering structural work in the area of coordination polymers of 2,5-dihydroxybenzoquinone was carried out by Robl and co-workers 30 years ago;1 these authors reported the synthesis and structures of [Na(μ2-H2O)3]2(M2(dhbq)3)· 18H2O; M = Mn or Cd. The structure of the Mn compound is shown in Figure 1. The structures we describe below involve [M2(can)32−]n and [M2(fan)32−]n sheets closely analogous to the [(M2(dhbq)3)2−]n sheets seen in Figure 1. In our experience it has often been difficult to obtain coordination polymers of dhbq2−, can2−, or fan2− directly from the corresponding protonated forms in a homogeneous and crystalline form. One tactic to surmount these difficulties that we have developed is to use the slow air-oxidation of the related tetraphenol (i.e., C6H2(OH)4, C6Cl2(OH)4, or C6F2(OH)4) to generate the substituted quinone in situ.2 This technique has allowed us to obtain crystals of coordination polymers of better quality than could be obtained directly and it is used again in the work presented here. Slow diffusion in the presence of air of an acetone solution of the tetraphenol C6Cl2(OH)4 or C6F2(OH)4 into aqueous solutions of the metal nitrates or

he dianion of 2,5-dihydroxybenzoquinone (I, X = H; H2dhbq) has the potential to chelate and link metal centers and thus may serve as a robust bridging ligand in network materials. Coordination polymers involving the ligand and its various 3,6-substituted derivatives may provide materials with interesting and useful electronic properties arising from the access provided by these ligands to radical species carrying a −1 or −3 charge. The results presented below are concerned with coordination polymers of the chloranilate and fluoranilate dianions, hereafter can2− and fan2−, respectively, i.e., the twicedeprotonated forms of I in which X = Cl or F. An isomorphous series of compounds of composition (Et4N)2[M2(can)3] (M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is herein described in which the [M2(can)32−]n component is in the form of chicken-wirelike sheets with the (6,3) topology detailed below. Also reported is (Et4N)2[Zn2(fan)3] whose structure is closely related to those of (Et4N)2[M2(can)3].

Received: December 24, 2016 Revised: February 14, 2017 Published: February 28, 2017 © 2017 American Chemical Society

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DOI: 10.1021/acs.cgd.6b01886 Cryst. Growth Des. 2017, 17, 1465−1470

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Figure 1. (a) Structure of [Na(μ2-H2O)3]2[Mn2(dhbq)3]·18H2O viewed perpendicular to the (6,3) sheet. Isolated red circles represent lattice water molecules. Yellow centers represent Na+ ions. (b) Representation of the [Na+(H2O)3]n columns running perpendicular to the sheets in which, for simplicity, only the Mn centers are shown. Red centers represent water molecules in the column, each “bridging” two sodium cations. Hydrogen atoms have been removed for clarity.

Figure 2. Structure of (Et4N)2[Zn2(can)3]. (a) Two [Zn2(can)32−]n sheets, showing the positions of the N centers of the Et4N+ cations (blue spheres) on a line normal to the sheets, halfway between a metal center in the upper sheet and one in the lower sheet. The ethyl groups are omitted for clarity. (b) Representation of one of the orientations of the Et4N+ cations which are disordered around a threefold axis perpendicular to the sheets. Two sorts of ethyl groups are shown here. The ethyl groups omitted are related to those shown by a twofold axis. H···O and H···Cl interactions are represented by banded bonds.

sulfates together with Et4NBr gives a new structural class of crystalline compounds of composition (Et4N)2[M2(can)3] (M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) and (Et4N)2[Zn2(fan)3]. Crystals suitable for single crystal X-ray diffraction studies were obtained in the cases of (Et4N)2[M2(can)3]·solvate (M = Fe or Zn) and (Et4N)2[Zn2(fan)3]·solvate; X-ray powder diffraction confirms that all the can2− derivatives are isomorphous. (Et4N)2[Zn2(fan)3] has essentially the same structure. The structure of (Et4N)2[Zn2(can)3], which is representative of all the compounds reported here, is shown in Figure 2. As can be seen by comparison of Figure 2 with Figure 1, the geometrical arrangements within the [Zn2(can)32−]n sheets are very similar to those in the [Mn2(dhbq)32−]n sheets. In

(Et4N)2[Zn2(can)3], however, the nitrogen atoms of the Et4N+ cations are located, as shown in Figure 2a, halfway between metal centers from adjacent sheets. The M···N···M centers lie on a line perpendicular to the sheets. We surmise that the six 1466



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above another: (a) Λ centers may be stacked above Λ centers and Δ above Δ, or (b) metal centers of opposite hand may be stacked on top of one another. In the stacking mode observed for all the compounds reported here, the Et4N+ cations link metal centers of the same hand as is immediately apparent in Figure 2b. This dual role of the Et4N+ cation, orienting and cementing together anionic sheets, thereby generating potentially useful channels has been noted previously in other circumstances. The compound (Et4N)2[SnIVCaII(can)4]·2Me2CO·2H2O, whose structure is shown in Figure 3, consists of coordination polymer sheets of composition {[SnIVCaII(can)4]2−}n with the (4,4) topology.3 These are electrostatically glued together by (Et4N)+ cations. Figure 3b shows the way in which the Et4N+ cation is able to form eight significant C−H···O interactions with can2− oxygen atoms in a manner similar to that seen in (Et4N)2[Zn2(can)3]. The alignment of the (6,3) [M2(can)32−]n and [(M 2(fan) 3 ) 2−] n sheets reported in this paper is reminiscent not only of the Sn/Ca system described above, but also of the alignment of nonpolymeric, molecular squares to generate tubular structures.4 Harris and co-workers recently reported two isostructural compounds containing substituted ammonium cations together with [M2 (can) 32−] n sheets of (6,3) topology, namely, (Me2NH2)2(M2(can)3)·2H2O·6DMF (M = Zn and Fe).5 The (Me2NH2)+ cations resulted from hydrolysis of DMF, the solvent used for the reaction. The (Me2NH2)+ cation in these cases is playing a significantly different structural role from that of Et4N+ in the (Et4N)2[M2(can)3] series we report here. The (Me2NH2)+ cations are located inside the roughly hexagonal holes within a sheet, the result of which is that the sheet-tosheet separation of 8.7449(5) Å for (Me2NH2)2[Fe2(can)3]· 2H2O·6DMF is considerably less than that in (Et4N)2[Zn2(can)3] (10.1059(5) Å). For the same reason the estimated void space (∼20%) for (Me2NH2)2[M2(can)3] is approximately half that seen in (Et4N)2[Zn2(can)3]. In the case of (Me2NH2)2[M2(can)3] all the Fe centers are in the +III oxidation state and the charge per three ligands is −8. The solvated form, (Me2NH2)2[Fe2(can)3]·2H2O·6DMF, behaves as a magnet below 60 K, but upon desolvation the Tc drops to 20 K; the authors described this system as “a microporous magnet with solvent-induced switching from Tc = 26 to 80 K”.5 Other examples of anionic networks indicate that magnetic properties are significantly affected by countercations.6 With regard to the scope offered for interesting effects of guest molecules upon electronic properties of host networks, we draw attention to the fact that the void space in the (Et4N)2[M2(can)3] series is approximately twice that in solvent-free (Me2NH2)2[Fe2(can)3]. The difficulty in obtaining coordination polymers of dhbq2− and can2− in homogeneous, crystalline form is particularly acute in the case of iron. As mentioned above, generation of the appropriate dihydroxybenzoquinone in situ by the slow aerial oxidation of the corresponding tetraphenol has proven successful in some cases in providing coordination polymers of dhbq2− and can2− in a more satisfactorily crystalline form. An alternative tactic we have developed is to generate these anions in situ by hydrolysis of 2,5-diaminobenzoquinone or 2,5diamino-3,6-dichlorobenzoquinone. The use of this device led us to the discovery of a new structural class of compounds with compositions (Bu4N)2[M2(dhbq)3] (M = Mg, Mn, Fe, Co, Ni, Zn, and Cd) and (Bu 4N) 2[Mn2(can) 3].7 All of these compounds had structures consisting of two independent and

oxygen donors surrounding each metal center will bear much of the negative charge associated with the sheet and no doubt electrostatic attraction between the Et4N+ cation, and these nearby centers of negative charge will be the major force binding the system together. It appears, however, that specific C−H···O and C−H···Cl interactions (and C−H···F in the fluoranilate) constitute significant additional components of the binding. The Et4N+ cations are disordered around a threefold axis over three equivalent orientations, one of which is shown in Figure 2b. In any one orientation there are two types of ethyl groups, only one of each type being shown in Figure 2b; the ethyl groups omitted are related to those shown by a twofold axis. The two types of ethyl groups play different roles. As can be seen in Figure 2b, one type of ethyl group participates in two significant C−H···O interactions, and two C−H···Cl interactions with members of the upper sheet and in addition forms a C−H···Cl interaction to the sheet below. The second type of ethyl group participates in only one H···Cl interaction. It appears that the Et4N+ cation, as a result of these multiple C− H···O and C−H···Cl interactions, securely locks the two sheets together in the orientation seen in Figure 2b. One can envisage the three coordinated oxygen centers and the three associated chlorine atoms as forming a bowl with the chlorine atoms on the rim and the oxygen atoms at the bottom; the Et4N+ cation can then be seen to fit snugly between the lower bowl and the upper inverted bowl. These electrostatic and C−H···O, C−H···Cl (and C−H···F in the fluoranilate) interactions align the sheets so that the holes therein are stacked one above the other to generate spacious open channels unobstructed by counter-cations. The location of the Et4N+ cations seen in (Et4N)2[Zn2(can)3] is to be contrasted with that of the Na + cations in [Na(μ2H2O)3]2[M2(dhbq)3]·18H2O (M = Mn or Cd), in which case the channels are blocked by [Na+(H2O)3]n columns, as can be seen in Figure 1. The spacious channels are of considerable interest with regard to future work in which we shall study the introduction of various guests into intact networks. The solvent occupying these channels is highly disordered and easily lost. Elemental analysis of a sample of (Et4N)2[Fe2(can)3] that had been allowed to equilibrate with the atmosphere at room temperature prior to the analysis indicates less than two H2O molecules per (Et4N)2[Fe2(can)3] formula unit. Variable temperature powder XRD for (Et4N)2[Co2(can)3] indicates minor contraction of the unit cell upon desolvation; the unit cell volume for (Et4N)2[Co2(can)3] at 300 K (powder X-ray diffraction on a slurry in mother liquor) is 1686 Å3, which is to be compared with 1640 Å3 for a sample desolvated at 400 K. The void spaces in (Et4N)2[Zn2(can)3] and (Et4N)2[Zn2(fan)3] are estimated to be ∼39% and ∼43%, respectively. The sheet-to-sheet separation in [Na(μ2 H2O)3]2[Mn2(dhbq)3]·18H2O is 6.565 Å, whereas that for (Et4N)2[Zn2(can)3] is 10.106 Å, indicating the extent to which the location of the Et4N+ pushes the sheets apart. This expansion perpendicular to the sheets is no doubt a contributor to the large estimated void space. Within a (6,3) sheet, octahedral metal centers are linked together by bridging ligands such as can2− and fan2−. Individual metal centers are chiral as a consequence of each metal center being bound by three chelating ligands. Every Λ center is directly connected via can2− bridges to three Δ centers, each of which in turn is connected to three Λ centers. In principle, therefore, there are two distinct ways in which one such sheet can be stacked on top of another with metal centers aligned one 1467



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how the Et4N+ cation promotes the assembly of can2− and a range of M2+ cations into [M2(can)32−]n sheets with the (6,3) topology, whereas the Bu4N+ cation promotes the assembly of two interpenetrating 3D [M2(can)32−]n or [M2(dhbq)32−]n networks with the (10,3)-a topology. We note that the 2D [M2(can)32−]n coordination polymer with (6,3) topology is related in a simple manner to its 3D cousin with the same composition but with (10,3)-a topology, in that the absolute configurations at the chiral metal centers alternate in the former, whereas they are all the same in the latter.7 In 2015 Long and co-workers employed a variety of physical techniques to probe the properties of (Bu4N)2[Fe2(dhbq)3].8 The compound has a conductivity of 0.16 S/cm at 298 K, one of the highest yet observed for a coordination polymer. Mössbauer spectra indicate that all the iron centers are in the +III oxidation state. Given the [Fe2(dhbq)3]2− composition, this implies that a charge of −8 must be associated with every three ligands. UV−NIR spectra indicated a Robin-Day mixedvalence Class II/III electronic arrangementthe first coordination polymer of this type. Very recently, two aluminum coordination polymers with (6,3) sheet structures closely similar to those discussed above were reported by Stock and co-workers.9 The compounds were assigned the compositions (Me2NH2)3(Al4(dhbq)6)·3DMF and (Me2NH2)3(Al4(can)6)·9DMF. These compositions imply a charge of −15 associated with six ligands, i.e., half the ligands are formally in the −II state, and the others are in the −III radical state. EPR measurements support this. These compounds are further examples where the (Me2NH2)+ cations originated in hydrolysis of the reaction solvent, DMF. The preference of (Bu4N)2[Fe2(dhbq)3] and (Me2NH2)2[M2(can)3] to exist not in the Fe2+/(ligand)2− form that the composition suggests, but rather as Fe3+/ (ligand)3‑·, is in marked contrast to the electronic arrangement in Fe(can)(H2O)2, magnetic, and Mössbauer studies of which we reported 16 years ago.10 This compound contains 1D zigzag polymeric chains in which the Fe centers are 6-coordinate, with two chelated can2− ligands and two cis aqua ligands. The Mössbauer and magnetic studies leave no doubt that the iron centers in this case are in the high-spin FeII state, with weak ferromagnetic coupling between neighbors. We propose, on simple electrostatic grounds, that the iron center is sufficiently stabilized in the +III state when surrounded by three negatively charged ligands to make possible the electron transfer from FeII to (ligand)2− to generate FeIII/(ligand)3−; however, when there are only two such anionic ligands, together with two neutral aqua ligands, the driving force to achieve the +III oxidation state is relatively reduced and the system remains in the FeII/ (ligand)2− state. The positive charge per Fe center is estimated by the bond valence sum procedure,11 for structurally characterized coordination polymers in which Fe centers are linked by dhbq and can ligands affords interesting contrasts. The charges p e r Fe c o m p ut e d i n t h i s w a y a r e a s f o ll o w s : (Me2NH2)2[Fe2(can)3]·2H2O·6DMF, 3.0; (Bu4N)2[Fe2(dhbq)3], 3.0; [Fe(can)cis(H2O)2]·(H2O), 2.1; (Et4N)2[Fe2(can)3], 3.0. These numbers are roughly consistent with the experimental observations that the Fe centers are in the +III oxidation state in the first two compounds and in the +II state in the third; they also suggest that (Et4N)2[Fe2(can)3] promises interesting electronic properties, containing some Fe3+ and can3−· radical species.

Figure 3. Structure of (Et4N)2[SnIVCaII(can)4]·2Me2CO·2H2O. (a) A {[SnIVCaII(can)4]2−}n sheet. Magenta spheres represent Sn and blue spheres represent Ca. Note that all Sn centers within a sheet are necessarily of one hand and all Ca centers of the opposite hand. (b) The Et4N+ cation is disordered over two equivalent orientations, one of which is shown here. Every Et4N+ cation is bound by four C−H···O hydrogen bonds (H···O, 2.462, 2.506 Å) to a Ca center of one hand and by four other C−H···O hydrogen bonds (H···O, 2.601, 2.662 Å) to a Sn center of the opposite hand. A fourfold axis passes through Sn···N···Ca and the Et4N+ cation is disordered around this in two orientations. Hydrogen atoms of the methyl groups have been omitted for clarity.

interpenetrating 3D [M2(dhbq)32−]n or [M2(can)32−]n networks with the (10,3)-a topology. The (10,3)-a net is intrinsically chiral; in this series of compounds the two interpenetrating nets were of opposite hand. In a single network all metal centers have the same configuration. When negatively charged networks are assembling, the nature of the countercation can be decisive regarding the geometry and topology of the network formed. Thus, we have seen above 1468



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calculated as 2460 cm−1 (Supporting Information). The slightly broader bandwidth at half-height compared with the theoretical value further supports this classification. Optical band gaps of 1.94 and 0.79 eV have been deduced from Tauc plots for (NEt4)2[Zn2(can)3] and (NEt4)2[Fe2(can)3] (Supporting Information, Figures S4 and S5), strongly suggesting a significantly reduced electronic band gap in the iron compound compared with its zinc counterpart.22 Preliminary CO2 uptake measurements on (Et4N)2[Zn2(can)3] and (Et4N)2[Zn2(fan)3] reveal that substantial amounts of CO2 are adsorbed in the spacious channels, e.g., 0.352 g of CO 2 are adsorbed per gram of (Et4N)2[Zn2(can)3] at 273 K and 1844 kPa pressure and 0.403 g of CO2 are adsorbed per gram of (Et4N)2[Zn2(fan)3] at 273 K and 1139 kPa pressure. The isotherms indicate a stepwise adsorption process (Supporting Information, Figure S2). Stepwise uptake of guests by a coordination polymer is often attributed to the flexibility of the framework material; however, in this case the powder diffraction patterns of the completely desolvated materials closely matches that of the solvated material, indicating that the rigid framework structure is preserved regardless of whether the pores are filled or empty. Zhao and co-workers have proposed that stepwise adsorption could be explained by “sequential adsorption of sorbates on different adsorption sites”.23 Such an explanation may be applicable in this case, with the 1D channels offering two distinct sites where the CO2 could be located, one in the approximate plane of the anionic network and the other in the plane of the NEt4+ cations. Further investigations of the stepwise adsorption are planned. The large void spaces suggest that it will be possible to adsorb a wide range of guestsnot only gases, but a variety of small moleculeswith all the various metal derivatives (Et4N)2[M2(can)3] (M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) and (Et4N)2[Zn2(fan)3]. In conclusion, the results presented here underline the importance of exploratory synthetic work in the development of coordination polymers with new and useful properties. The work demonstrates the crucial role played by the countercation in determining the topology and geometry of the anionic [M2(can)32−]n, [M2(dhbq)32−]n, or [M2(fan)32−]n coordination polymer; Et4N+ promotes the formation of (6,3) nets whereas Bu4N+ promotes the formation of (10,3)-a nets. The structural and spectroscopic results provide a strong indication that electrical conductivity, magnetic, and Mössbauer spectroscopic studies on (Et4N)2[Fe2(can)3] will be informative. The demonstration of the uptake of substantial amounts of CO2 in (Et4N)2[Zn2(can)3] and (Et4N)2[Zn2(fan)3] and the fact that the networks remain intact after desolvation or guest removal offers considerable promise for future investigations of the sorption of a wide range of guests. Study of the variation in electrical conductivity, magnetic properties, and Mössbauer spectra of (Et4N)2[Fe2(can)3] as a function of the guest species may well prove interesting and productive.

In order to further investigate the redox states of the chloranilate ligands in (Et4N)2[Fe2(can)3] and (Et4N)2[Zn2(can)3] the solid state diffuse reflectance spectra were measured and are presented in Figure 4 where F(R)

Figure 4. Diffuse reflectance spectra of (NEt4)2[Fe2(can)3] (black), (NEt4)2[Zn2(can)3] (blue), and chloranilic acid (green). The inset shows the band in the near-infrared region with the underlying Gaussian bands obtained from deconvolution (black dotted lines) and the associated curvefit (red).

represents the Kubelka−Munk transform (see Supporting Information for experimental details). While the spectra of (NEt4)2[Zn2(can)3] and chloranilic acid are featureless in the near-infrared region, (NEt4)2[Fe2(can)3] exhibits a distinct Gaussian-shaped band centered at 7890 cm−1, with a bandwidth at half-height, Δν1/2, of 2620 cm−1 (see inset Figure 4 and Supporting Information). This band is tentatively ascribed to an Intervalence Charge Transfer Transition (IVCT) which is due to an intraligand mixed-valence interaction between chloranilate ligands that are present in both their can2− and can3− forms. The higher energy transitions in the visible and UV regions can be ascribed to contributions from intraligand π−π* interactions within the ligands themselves, in addition to π−d interactions. According to the theory pioneered by Hush,12,13 the characteristics of an IVCT band, specifically, its energy (νmax), intensity (F(R)max), and bandwidth at half-height (Δν1/2) can be quantitatively related to the activation barrier for electron transfer. Previous reports have also noted a link between the extent of electronic delocalization deduced from analysis of IVCT bands, and the conductivity of solids.8,14−19 In cases where the absolute extinction coefficient of an IVCT band cannot be obtained (as is common for diffuse reflectance spectra), comparison of the experimental Δν1/2 value with that theoretically derived from the Hush equation, Δν1/2° can be used to gain insight into the extent of electronic delocalization of the system. The magnitude of the Γ parameter, where Γ = 1 − (Δν1/2)/(Δν1/2°), can be used to classify a mixed-valence system within the Robin and Day classification scheme,14 where 0 < Γ < 0.1 for weakly coupled Class II systems, 0.1 < Γ < 0.5 for moderately coupled Class II systems, Γ ≈ 0.5 at the transition between Class II and Class III, and Γ > 0.5 for Class III systems. 2 0 , 2 1 The |Γ| value of 0.065 for the (NEt4)2[Fe2(can)3] system suggests that the material lies in the weakly coupled Class II regime, where Δν1/2° was

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01886. Syntheses, single crystal and powder crystallographic details, and gas adsorption and spectroscopic information (PDF) 1469



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Accession Codes

(10) Abrahams, B. F.; Lu, K. D.; Moubaraki, B.; Murray, K. S.; Robson, R. X-Ray diffraction and magnetic studies on a series of isostructural divalent metal chloranilates with zigzag polymeric chain structures and on a dinuclear iron(III) chloranilate. J. Chem. Soc., Dalton Trans. 2000, 1793−1797. (11) Gagné, O. C.; Hawthorne, F. C. Comprehensive derivation of bond-valence parameters for ion pairs involving oxygen. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2015, 71, 562−578. (12) Hush, N. S. Intervalence-transfer absorption. Part 2. Theoretical considerations and spectroscopic data. Prog. Inorg. Chem. 1967, 8, 391. (13) Hush, N. S. Homogeneous and heterogeneous optical and thermal electron transfer. Electrochim. Acta 1968, 13, 1005. (14) Robin, M. B.; Day, P. Mixed Valence Chemistry-A Survey and Classification. Adv. Inorg. Chem. Radiochem. 1968, 10, 247−422. (15) England, S. J.; Kathirgamanathan, P.; Rosseinsky, D. R. J. Chem. Soc., Perturbation calculation from the charge-transfer spectrum data of intervalence site-transfer D.C. conductivity in Prussian Blue. J. Chem. Soc., Chem. Commun. 1980, 840−841. (16) Behera, J. N.; D’Alessandro, D. M.; Soheilnia, N.; Long, J. R. Synthesis and Characterization of Ruthenium and Iron−Ruthenium Prussian Blue Analogues. Chem. Mater. 2009, 21, 1922−1926. (17) D’Alessandro, D. M. Exploiting redox activity in metal-organic frameworks: concepts, trends and perspective. Chem. Commun. 2016, 52, 8957−8971. (18) Leong, C. F.; Usov, P. M.; D’Alessandro, D. M. Intrinsically conducting metal−organic frameworks. MRS Bull. 2016, 41, 858−864. (19) D’Alessandro, D. M.; Kanga, J. R. R.; Caddy, J. S. Towards Conducting Metal-Organic Frameworks. Aust. J. Chem. 2013, 64, 718− 722. (20) Brunschwig, B. S.; Creutz, C.; Sutin, N. Optical transitions of symmetrical mixed-valence systems in the Class II−III transition regime. Chem. Soc. Rev. 2002, 31, 168−184. (21) D’Alessandro, D. M.; Keene, F. R. Current trends and future challenges in the experimental, theoretical and computational analysis of intervalence charge transfer (IVCT) transitions. Chem. Soc. Rev. 2006, 35, 424−440. (22) Tauc, J. Optical properties and electronic structure of amorphous Ge and Si. Mater. Res. Bull. 1968, 3, 37−46. (23) Li, L.; Tang, S.; Wang, C.; Lv, X.; Jiang, M.; Wu, H.; Zhao, X. High gas storage capacities and stepwise adsorption in a UiO type metal−organic framework incorporating Lewis basic bipyridyl sites. Chem. Commun. 2014, 50, 2304−2307.

CCDC 1524353−1524355 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Telephone: +61 3 8344 0341. Fax: +61 3 9347 5180. *E-mail: [email protected]. Telephone: +61 3 8344 6469. Fax: +61 3 9347 5180. ORCID

Brendan F. Abrahams: 0000-0003-2957-860X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the Australian Research Council (Discovery Project grants: DP120100670 and DP150100570). Part of this research was undertaken on the Powder Diffraction beamline at the Australian Synchrotron.

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REFERENCES

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