Interligand Charge-Transfer Interactions in Electroactive Coordination

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Interligand Charge-Transfer Interactions in Electroactive Coordination Frameworks Based on N,N′‑Dicyanoquinonediimine (DCNQI) Robert W. Elliott,†,⊥ Pavel M. Usov,‡,⊥ Brendan F. Abrahams,† Bun Chan,‡,§ Richard Robson,*,† and Deanna M. D’Alessandro*,‡ †

School of Chemistry, The University of Melbourne, Victoria, Australia 3010 School of Chemistry, The University of Sydney, New South Wales, Australia 2006 § Graduate School of Engineering, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan ‡

S Supporting Information *

ABSTRACT: Coordination frameworks containing DCNQI2− (DCNQI = N,N′dicyanoquinonediimine ligand) are produced by deprotonation of DCNQIH2 in the presence of a metal center and a co-ligand. This approach has yielded twodimensional (2D) sheet compounds [Cd(DCNQI)(L)2] (where L = pyridine (py) or isoquinoline (isoquin)) that can be partially oxidized via solid-state electrochemical and in situ spectroelectrochemical methods to materials that contain DCNQI as its radical monoanion. The new frameworks display charge-transfer bands that are indicative of interligand charge-transfer interactions as supported by TDDFT computational calculations. The redox-state dependent spectral properties of the frameworks have been probed using a newly developed solid-state spectroelectrochemical cell. Coupled with computational calculations, the experimental data provide an understanding of the fundamental charge-transfer processes that may underpin long-range functional properties such as conductivity in framework materials.

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INTRODUCTION Coordination networks (otherwise known as metal−organic frameworks, or MOFs) that are electroactive are receiving considerable attention, because of the numerous applications that can be realized through redox-state manipulation.1−7 At a fundamental level, the charge-transfer interactions that occur within these infinite lattices open up fascinating possibilities for probing charge migration in three-dimensional (3D) coordination space. Examples of metal-based intervalence charge transfer (IVCT),8 ligand-based IVCT,9,10 donor−acceptor charge transfer,11,12 and π−π-type transitions13 have all been shown to manifest in interesting spectral and electrochemical properties of frameworks. By exploiting the long-range nature of certain charge-transfer interactions, highly sought after properties such as electrical conductivity can be realized.9,14−18 The potential anisotropy of the charge transfer may also be advantageous for device applications where the directionality of charge transport may be important for signal transduction.14,19 To date, [Cu(2,5-Me2DCNQI)2], where 2,5-Me2DCNQI is the 2,5-dimethyl derivative of the N,N′-dicyanoquinonediimine ligand, has been shown to exhibit the highest recorded conductivity in a 3D coordination polymer, with metallic-like conductivity of 5 × 105 S cm−1 at 3.5 K.20−26 Within the material, the 2,5-Me2DCNQI ligands act as 2-connecting linkers between a pair of Cu centers, which bind to four 2,5Me2DCNQI ligands in a tetrahedral arrangement. The overall © XXXX American Chemical Society

structure consists of seven interpenetrating networks, each with the topology of diamond. Although the formal charges on the Cu and 2,5-Me 2 DCNQI moieties are +1 and −1/2, respectively, noninteger charges have been calculated as +4/3 and −2/3, respectively.26 DCNQI itself has been used to generate a wide range of compounds,27,28 including chargetransfer complexes29−33 and radical anion salts,34,35 as well as metal-containing compounds.36−44 Other compounds related to [Cu(2,5-Me2DCNQI)2] but containing different 2,5substituted DCNQI ligands have also been shown to exhibit high electrical conductivities.21−25,28 Despite the excellent charge-transfer properties of DCNQI and its analogues, only a small number of DCNQI coordination polymers have been reported, with the DCNQI moiety commonly exhibiting a charge between 0 and −1. The dianionic form, DCNQI2−, is also particularly interesting, with respect to its capacity to facilitate charge-transfer interactions. For example, along with selected substituted derivatives, it has been shown to mediate magnetic and electronic coupling between RuII/III centers within dinuclear complexes.45−50 Special Issue: Applications of Metal Complexes with LigandCentered Radicals Received: January 15, 2018

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DOI: 10.1021/acs.inorgchem.8b00130 Inorg. Chem. XXXX, XXX, XXX−XXX

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highly twinned pale yellow crystals suitable for XRD (13.5 mg, 0.026 mmol, 26% yield). IR (KBr) νCN: 2135, ∼2100 cm−1. Anal. Calcd for Cd(C8H4N4)(C9H7N)2: C, 58.27%; H, 3.57%; N, 15.68%. Found: C, 57.64%; H, 3.26%; N, 16.08%. Details of X-ray Crystal Structure Refinements. Crystals of [Cd(DCNQI)(py)2] and [Cd(DCNQI)(isoquin)2] were removed from the mother liquor and transferred directly to paratone-N oil prior to being mounted on a goniometer in a cooled stream of nitrogen (130 K). Data were collected using an Oxford Diffraction SuperNova diffractometer. Each of the crystals were found to be twinned. Crystal structures were solved by direct methods (SHELXS) and refined using a full-matrix least-squares refinement based on F2 (SHELXL).53 Solution and refinement were performed within the WinGX system of programs.54 Crystallographic data are presented in Table 1. In the case

DCNQI can undergo reversible reduction processes to form a radical monoanion and a diamagnetic dianion. In the latter case, the rigid double bonds to the N atoms of the quinonoid form become single bonds and are subsequently able to undergo free rotation. Hence, either the anti or syn form may be found in compounds containing unsubstituted DCNQI2− units. These ligands can coordinate to two metal centers at the terminal N atoms to form a linear linker, or coordinate through all four N atoms to form a four connecting (usually planar) node. Within the compounds, the Ru centers are coordinated by the nitrile groups of the DCNQI2− ligands. Antiferromagnetic coupling over a range of 13 Å occurs through superexchange between the RuIII centers, involving the dp orbitals of the RuIII centers and the p HOMO of the DCNQI2− ligand. Mixed-valence RuII/III systems also displayed superexchange, with the degree of delocalization across the ligand dependent on the coordination environments of the Ru centers. In 2010, two coordination polymers containing DCNQI in its dianionic form were prepared using a synthetic procedure analogous to the one used to generate coordination polymers containing the dianion of the closely related electron acceptor TCNQ (7,7,8,8-tetracyanoquinodimethane).51 The protonated doubly reduced form of DCNQI, DCNQIH2, is deprotonated by a weak base in the presence of a metal ion. The present work was undertaken with the aim of generating electroactive frameworks containing the DCNQI2− ligand. Interconversion of these materials between their different redox states was of interest with respect to the potentially distinct electronic and optical features of the different states. The electrochemical properties of two new frameworks [Cd(DCNQI)(L)2], where L = pyridine (py) and isoquinoline (isoquin), have been investigated and compared with the properties of the free ligand. The redox state of the DCNQI ligand in the as-synthesized framework was determined using electron paramagnetic resonance (EPR), infrared (IR) spectroscopy, and ultraviolet−visible light−near-infrared (UV-visNIR) spectroscopy experiments. In order to elucidate the nature of the redox behavior of the [Cd(DCNQI)(L)2] materials, solid-state electrochemistry and UV-vis-NIR spectroelectrochemistry (SEC) were performed using a newly developed “second-generation” SEC cell.52 The data from solution-state UV-vis-NIR spectroelectrochemical measurements of DCNQI, coupled with DFT computational calculations, aided the assignments of the absorption bands in the framework. Based on these experiments, the nature of the redox processes in [Cd(DCNQI)L2], as well as interesting charge-transfer processes, have been elucidated. Understanding fundamental charge-transfer processes in multidimensional materials may underpin the realization of long-range functional properties, such as conductivity in frameworks.14,16,18

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Table 1. Crystal Details and Refinement for [Cd(DCNQI)(py)2] and [Cd(DCNQI)(isoquin)2] chemical formula a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z formula weight space group T (°C) Dcalcd (g cm−3) μ (cm−1) R(F0) [I > 2σ(I)] Rw(F02)

[Cd(DCNQI)(py)2]

[Cd(DCNQI)(isoquin)2]

C18H14CdN6 5.6387(4) 8.6030(8) 9.3712(11) 71.879(9) 79.112(8) 88.064(7) 424.12(7) 1 426.75 P1̅ −143 1.671 10.413 0.0858 0.2113

C26H18CdN6 5.6219(3) 9.3846(4) 10.8372(5) 104.127(4) 93.498(4) 100.466(4) 541.91(4) 1 526.86 P1̅ −143 1.614 8.283 0.0267 0.0678

of [Cd(DCNQI)(py)2], the pyridine ligand was found to be disordered over two distinct orientations, which were refined with complementary site occupancies. The occupancy of the largest fraction refined to 0.51(2). The atoms in both orientations were refined with anisotropic displacement parameters. Higher symmetry cell checks were performed by the PLATON program.55 Crystal absorption corrections were applied through CrysAlisPro56 using the Semiempirical from equivalents or Gaussian methods. Additional structural visualisation and analysis was performed with Mercury.57 Crystal structure diagrams were generated using CrystalMaker.58 Line drawings were produced using ChemSketch.59 For [Cd(DCNQI)(isoquin)2], the Cd center and the non-hydrogen atoms of the DCNQI ligand were clearly defined and were refined with anisotropic displacement parameters. The isoquinoline ligand was disordered over three distinct positions, the occupancies of which were refined to values of 0.492, 0.238, and 0.269. The atoms of the isoquinoline ligand were refined with isotropic displacement parameters. Further crystallographic data are presented in CIF files that have been deposited with the Cambridge Crystallographic Data Center (CCDC 1581271 [Cd(DCNQI)(py)2] and CCDC 1581272 [Cd(DCNQI)(isoquin)2]). General Physical Measurements. Powder XRD measurements were performed over the 2θ range of 5°−50° with a 0.02° step size and 2° min−1 scan rate on a PANalytical X’Pert Pro diffractometer fitted with a solid-state PIXcel detector (40 kV, 30 mA, 1° divergence and antiscatter slits, and 0.3 mm receiver and detector slits), using Cu Kα (λ = 1.5406 Å) radiation. Profile fits were performed using the Le Bail extraction method in Rietica.60 A histogram profile function with pseudo-Voigt peak shape61 and Finger, Cox, and Jephcoat asymmetry function62 was used. Fourier transform infrared (FT-IR) spectra were collected using a Bruker Vertex 80v system that was equipped with an attenuated total reflectance (ATR) attachment. The powdered samples

EXPERIMENTAL SECTION

Synthesis of [Cd(DCNQI)(py)2]. A solution of Cd(NO3)2·4H2O (29.9 mg, 0.097 mmol) and pyridine (0.1 mL) in MeOH (5 mL) was allowed to diffuse into a solution of DCNQIH2 (15.3 mg, 0.097 mmol) in DMF (1 mL) under a nitrogen atmosphere, giving highly twinned pale yellow crystals suitable for X-ray diffraction (XRD) (11.1 mg, 0.026 mmol, 27% yield). IR (KBr) νCN: 2136 (br) cm−1. Anal. Calcd for Cd(C8H4N4)(C5H5N)2: C, 50.13%; H, 3.39%; N, 19.49%. Found: C, 49.97%; H, 3.13%; N, 19.55%. Synthesis of [Cd(DCNQI)(isoquin)2]. A solution of Cd(NO3)2· 4H2O (29.9 mg, 0.097 mmol) and isoquinoline (0.1 mL) in MeOH (5 mL) was allowed to diffuse into a solution of DCNQIH2 (15.3 mg, 0.097 mmol) in DMF (1 mL) under a nitrogen atmosphere, giving B


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tions.66 The structures for the cluster models were taken from the crystal structure without modification. TD-DFT calculations were carried out at the BMK/6-31G(d) level,67 which we have evaluated and applied in our previous investigations into similar materials.12,68,69 Note that while this method provides a reliable means for qualitative comparison, the calculated electronic absorption frequencies with this method has been shown to be systematically blue-shifted in general. The exact magnitude of the shifts varies and typically amounts to thousands of cm−1.

were mounted onto the Ge crystal. For the solid-state UV-vis-NIR spectroscopy in the 5000−40000 cm−1 region, a Praying Mantis attachment was employed with the background collected from dry, finely ground BaSO4. Continuous wave solid-state EPR spectra were collected at room temperature, using a Bruker Elexsys 500 spectrometer that was equipped with an X-band microwave bridge. The spectra were referenced against strong pitch. Cyclic Voltammetry. All electrochemical experiments were undertaken in a three-electrode cell configuration at ambient temperature (295 ± 1 K). The DC cyclic voltammograms were measured using a BASi Epsilon electrochemical analyzer. The supporting electrolytes were 0.1 M (n-C4H9)4NPF6/CH3CN and 0.2 M LiClO4/ethylene glycol, which were degassed with high-purity argon. A glassy carbon disk (diameter 1 or 3 mm) was utilized as the 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 washed with acetone and dried under a stream of argon. For measurements with surface-confined solids, powdered samples were mounted onto a glassy carbon disk electrode using mechanical immobilization; the electrode was pressed against the powder, forming a weakly bound layer on its surface. High-surface-area platinum wire in contact with solvent (electrolyte) was used as a counter electrode. A silver wire installed in the vicinity of the surface of the working electrode was used as a quasi-reference electrode. The potential of the quasi-reference electrode was calibrated versus the reversible potential of the Fc0/+ couple measured in the same solution before or after the experiment. A similar cell configuration was utilized for the solution-state measurements, which were performed on a small amount of material dissolved in the supporting electrolyte. Solution-State UV-vis-NIR Spectroelectrochemistry (SEC). The solution state UV-vis-NIR spectra were measured in situ using a quartz optically transparent thin-layer electrochemical (OTTLE) cell (path length = 0.685 mm) mounted in a hollow block of Teflon. The spectra were collected at 295 ± 1 K, using a CARY Model 5000 spectrophotometer. The optical cuvette section of the OTTLE cell served as a working electrode compartment for which a high-surfacearea platinum gauze was utilized. The high-surface-area platinum coil and the Ag wire, which served as counter and reference electrodes, respectively, were immersed into the electrolyte in the upper section of the cell. The electrodes were separated by glass frits. In order to ensure that the redox process only occurred in the optical compartment, the platinum wire connecting the working electrode was insulated into the Teflon tubing. The background was collected on the analyte-free electrolyte with the working electrode in place; additional platinum gauze was placed into the reference beam. The potentials were applied using an eDAQ ecorder 410 potentiostat. The positions of redox peaks obtained from the cyclic voltammetry experiments were used as a guide. The attainment of a steady-state spectrum and the decay of the current to a constant minimum served as an indicator that the electron transfer process has reached a dynamic equilibrium. Solid-State Vis-NIR Spectroelectrochemistry (SEC). The solidstate diffuse reflectance Vis-NIR spectra of the redox-active species were collected in situ, using a CARY Model 5000 UV-vis-NIR spectrophotometer equipped with a Harrick Omni Diff Probe attachment interfaced to Varian WinUV software, as reported previously. A second-generation SEC cell was developed with modified design parameters from the first-generation cell.52 A full description of this design and experimental procedure is provided in the Supporting Information (Figure S1). Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) Calculations. Standard density functional theory (DFT) calculations were carried out using Gaussian 09.63 Generally, geometries were optimized with the B3-LYP/6-31+G(d,p) procedure. This methodology was chosen because of its robust performance in the prediction of vibrational frequencies.64,65 Following each geometry optimization, harmonic frequency analysis was carried out to confirm the nature of the stationary point as an equilibrium structure, as well as to obtain simulated Raman spectra. We applied a scale factor of 0.9648 to the vibrational frequencies, according to literature recommenda-

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RESULTS AND DISCUSSION Synthesis and Structure. Reaction of cadmium acetate, pyridine, and DCNQIH2 in MeOH/DMF afforded small twinned pale yellow crystals of formula [Cd(DCNQI)(py)2]. The Cd centers are all crystallographically equivalent and each is located on a center of inversion. Four N atoms from four different DCNQI ligands bind to each CdII center, as indicated in Figure 1a. These four N atoms form a square plane around the CdII center with two “elbow” N atoms trans to each other and two terminal N atom groups also trans to each other. Each DCNQI2− ligand adopts the anti configuration and is bound to four CdII centers through the two elbow N atoms and the two nitrile groups. The four CdII centers are located at the vertices of a parallelogram with dimensions of 5.64 Å × 9.98 Å, with an acute angle of 67.2°. In the resulting two-dimensional (2D) structure, both the CdII centers and the DCNQI2− ligands serve as 4-connecting nodes within a network that has the 4,4 topology. A pair of trans pyridine ligands extend above and below the plane of the Cd-DCNQI network to complete an octahedral coordination environment around each CdII center. The pyridine groups of parallel sheets interdigitate, as shown in Figure 1b. As a result of the interdigitation, there is no room for solvent molecules between the sheets, the mean planes of which are 8.18 Å apart. The structure of [Cd(DCNQI)(isoquin)2] is very similar to that of [Cd(DCNQI)(py)2], except that a pair of trans isoquinoline ligands are coordinated, instead of the pyridine ligands. The 4,4 Cd(DCNQI) network is almost identical with the four CdII centers at the vertices of a parallelogram of dimensions 5.62 × 10.03 Å with an acute angle of 67.0°. The isoquinoline ligands, which exhibit disorder, interdigitate in a manner similar to that observed for the pyridine analogue (Figure 1c) but the larger size of the ligand leads to a separation of the parallel sheets of 10.51 Å. In addition to elemental analysis, the bulk purity of [Cd(DCNQI)(py)2] and [Cd(DCNQI)(isoquin)2] was confirmed by PXRD analysis (see Figure S2 in the Supporting Information). The redox state of DCNQI in the as-synthesized frameworks was analyzed using EPR and IR spectroscopies. The DCNQI dianion used in the synthesis of [Cd(DCNQI)(L)2] is EPR silent; however, a distinct signal was observed in the EPR spectra of the pyridine and isoquinoline analogues (see Figure 2a). This signal could not be attributed to either CdII, because of its d10 configuration, or any of the axial ligands; hence, it was concluded that a small proportion of DCNQI ligands within the frameworks are present in the monoanionic, radical form. The radical signals corresponding to a g-factor of 2.008 closely match those values observed in other DCNQI-containing materials.70,71 IR spectroscopy provided further evidence for the presence of the DCNQI radical. The vibrational mode corresponding to the nitrile stretch (2000−2200 cm−1) is particularly sensitive to the redox state, with two distinct peaks observed in the IR spectra of [Cd(DCNQI)(L)2] (Figure S3 in C


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2197, 2140, and 2127 cm−1, respectively (see Figure S4 in the Supporting Information). The diffuse reflectance spectra of [Cd(DCNQI)(L)2] (L = py and isoquin) were measured between 5000 and 40 000 cm−1 (Figure 2b) and compared with those of the ligand itself (see Figure S5 in the Supporting Information). In the latter case, TD-DFT calculations were employed to gain insight into the origins of the spectral features for the ligand in its neutral, radical monoanion, and dianion states (see Figures S5 and S6 in the Supporting Information). Figure 2b shows the similarities between the spectroscopic features of both frameworks. An intense absorption band was observed in the UV region, which was tentatively assigned to the π → π* transition of DCNQI2−. In the case of [Cd(DCNQI)(isoquin)2], a low-energy shoulder band was detected at 26 000 cm−1, which was not present in the spectrum of the pyridine analogue. As such, it was attributed to the n → π* transition of isoquinoline. Our calculated spectra for models of pyridine and isoquinoline in the framework also suggests the presence of such a lower-energy band in the latter that is absent from the former (Figure S6 in the Supporting Information; also see Figure S7 for comparison with DCNQI). The energy of this transition has been red-shifted because of coordination of the ligand to CdII, where a similar effect was reported in the protonated isoquinolinium species.74 In addition, low-intensity absorption bands were observed between 14 000 and 18 000 cm−1, which featured a characteristic splitting pattern similar to that found in the spectrum of DCNQI• −.75 This result confirms the presence of the radical form, which is consistent with the results from EPR and IR spectroscopic measurements. Electrochemical Properties. The electrochemical properties of [Cd(DCNQI)(L)2] were investigated using solid-state cyclic voltammetry in 0.2 M LiClO4/ethylene glycol as a supporting electrolyte, and compared with that of the ligand itself (see Figure S9 in the Supporting Information). The resulting cyclic voltammograms (Figures 3a and 3b) reveal small oxidation peaks at 0.21 and 0.29 V vs Fc0/+ for the pyridine and isoquinoline analogues, respectively, upon the first cycle. Upon the return cycle, a pronounced reversible redox process appeared at E1/2 of 0.02 (L = py) and 0.03 V vs Fc0/+ (L = isoquin), which remained relatively stable for up to three cycles. Additional shoulder peaks were observed at higher potentials in both cases. The peak−peak separation (ΔE) of the major redox process was found to be 0.15 and 0.13 V for the pyridine and isoquinoline analogues, respectively, which was larger than that expected for a reversible one-electron process (0.059 V). The proposed oxidation mechanism for [Cd(DCNQI)(L)2] is outlined in Figure 3c. Since only one distinguishable redox couple was observed in the cyclic voltammograms, two possible oxidation pathways can occur. In the first instance, the oxidation peak could be attributed to the one-electron oxidation of DCNQI2− to its radical anion form. In this case, the second oxidation of the ligand was not observed within the measured potential window. Alternatively, the observed redox process could arise from the two-electron oxidation of DCNQI2−, generating DCNQI0, which bypasses the radical state. UV-vis-NIR spectroelectrochemical experiments (vide infra) confirmed the generation of the DCNQI• − monoradical anion, but not DCNQI0. The absence of a significant oxidation peak on the first cycle may be attributable to the DCNQI ligands on the surface of the particles existing in their radical state, because of autoxidation

Figure 1. X-ray crystal structures of [Cd(DCNQI)(L)2] (L = py and isoquin): (a) a sheet of [Cd(DCNQI)(py)2] viewed from a direction close to the b-axis; (b) the structure of [Cd(DCNQI)(py)2], showing the stacking of the sheets and the interdigitation of the pyridine groups; and (c) the structure of [Cd(DCNQI)(isoquin)2], showing the interdigitation of the isoquinoline groups. Only one orientation of each axial ligand (pyridine or isoquinoline) is shown.

the Supporting Information). It has been shown in the literature that the CN bond weakens upon reduction of DCNQI, with the peak at 2120 cm−1 corresponding to the radical form.72,73 The less-pronounced peak at 2080 cm−1 has been assigned to the nitrile stretch in the DCNQI dianion. Such an assignment is also consistent with our DFT spectra for the neutral, radical anion, and dianion forms of isolated DCNQI, with scaled (by 0.96) NCN stretching frequencies of D


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Figure 2. (a) X-band EPR spectra of [Cd(DCNQI)(L)2], L = py (blue) and isoquin (red). (b) Diffuse reflectance UV-vis-NIR spectra of [Cd(DCNQI)(L)2], L = py (blue) and isoquin (red).

Figure 3. Solid-state cyclic voltammograms of [CdII(DCNQI)(L)2] ((a) L = py and (b) isoquin) (ν = 200 mV s−1) measured in 0.2 M LiClO4/ ethylene glycol electrolyte. (c) Schematic of the possible electrochemical processes within the framework.

analogue was gradually increased to 0.5 V, at which point a significant spectral change began to occur (Figure 4a). The absorption band in the visible region attributed to the radical form of DCNQI increased in intensity, which indicated that DCNQI2− inside the framework structure underwent a oneelectron oxidation to DCNQI• −. The absorption increase above 24 000 cm−1 corresponded to a shift of the π → π* transition of DCNQI to lower energies upon generation of the radical state. Another spectral change was observed in the NIR region where a broad, low-intensity band appeared. The band does not correspond to DCNQI in any of its redox states, suggesting that it is unique to the framework. Increasing the applied potential further did not result in any new spectral features, indicating that oxidation to neutral DCNQI does not occur under these conditions. Gradually decreasing the potential to −0.2 V leads to a reversal of the changes to the spectrum, such that the original spectrum was almost fully recovered (see Figure 4b). This supports the reversibility of the oxidation of the compound, as observed in the CV experiments, as also observed qualitatively in photographs of the sample during the experiment (see Figure 4c). The sample of [Cd(DCNQI)(py)2] is initially beige in color and, upon oxidation, attains a dark blue coloration. Application of a reverse potential results in the regeneration of the original

in air, as observed previously (Figure 2a). The scan rate of the CV experiment is sufficiently rapid such that ligands near the surface of the material only are addressed electrochemically, with the bulk of the material remaining unchanged. Small shoulder peaks are observed at potentials slightly higher than the main oxidation and reduction peaks, and they may be due to electrochemical processes of the DCNQI2− ligands from beneath the surface layer. In order to maintain charge neutrality, the oxidation process requires the incorporation of a counteranion into the crystalline material. While inspection of Figures 1b and 1c clearly reveals the absence of intersheet spaces, it is possible to envisage the interdigitating sheets moving apart to accommodate an anion such as perchlorate.76 Such a process could facilitate the flow of counter-anions into the interior of the framework, allowing the oxidation of DCNQI2− ligands that are not located on the surface. Redox State Modulation and Computational Modeling of Electronic States. Solid-state UV-vis-NIR spectroelectrochemistry on the frameworks was performed using the newly designed second-generation SEC cell and compared with the solution-state spectroelectrochemical properties of the ligand itself (see Figure S10 in the Supporting Information). The potential applied to the powdered sample of the pyridine E


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Figure 4. (a, b) Solid-state UV-vis-NIR spectroelectrochemistry of [Cd(DCNQI)(py)2] measured in 0.2 M LiClO4/ethylene glycol electrolyte, using the second-generation SEC cell. Left and right panels show the raw spectra and difference spectra, respectively. The spectral changes were recorded as the potential was (a) increased from 0 to 0.5 V and (b) decreased from 0.5 V to −0.2 V. (Arrows denote the direction of the spectral change.) (c) Photographs of [Cd(DCNQI)(py)2] undergoing a color change upon application of various potentials in the second-generation UV-vis-NIR spectroelectrochemical cell.

color. However, some of the blue color is retained, and this could be due to the trapping of counterions within the framework. In order to clarify the nature of the new transition appearing in the NIR region, TD-DFT computations were conducted on small model systems for the framework (see Figure 5a, as well as Figures S11 and S12 in the Supporting Information). Models C and D were found to reproduce the spectra relatively closely. In these cases, both the neutral species that correspond to two CdII centers with two DCNQI2− ligands were examined, as well as the dication that formally comprises two CdII centers and two DCNQI• − radical anions. It was found that, for the dication, while both models gave bands corresponding to an isolated DCNQI• − radical anion, notably the characteristic SUMO − 1 to SUMO band at ∼17 000 cm−1, the spectrum of model D had an additional low-energy band at 14 447 cm−1 (Figure S11(d)). This is associated with a π−π-type HOMO − 1 to LUMO intermolecular charge transfer transition between the two DCNQI moieties (Figure 5b). Overall, the simulated spectrum when all models are taken into account (Figure S12 in the Supporting Information) reproduce the qualitative features of Figure 4 rather nicely. The oxidation mechanism of [Cd(DCNQI)(isoquin)2] was also investigated using the second-generation SEC under the same conditions as those described above for the pyridine analogue. As the applied potential was gradually increased to 0.75 V, several distinct spectral changes were detected (see Figure S13(a) in the Supporting Information). The radical band (14 000−18 000 cm−1) corresponding to minute amounts

Figure 5. (a) Model fragments (A−D) of [Cd(DCNQI)(L)2] frameworks (L = py and isoquin) used in TD-DFT calculations. (Black, blue, and cyan spheres represent C, N, and Cd, respectively.) (b) BMK/6-31G(d) molecular orbitals involved in the π−π-type transition (14 447 cm−1) of model D of (Cd-DCNQI)2• +.

of DCNQI• − present inside the framework increased in intensity, indicating oxidation of DCNQI 2−. Additional evidence for the generation of DCNQI radical form can be F


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obtained from an increase in absorption above 24 000 cm−1, which was most clearly visible in the difference spectrum. This absorption feature was attributed to the lowering of the π → π* transition energy upon formation of the radical species. It is important to note that, unlike the pyridine analogue, [Cd(DCNQI)(isoquin)2] did not exhibit any NIR absorption bands during oxidation. This finding can be explained by the lack of DCNQI• − radical required to facilitate interligand charge transfer, which can be inferred from the relatively small spectral change observed in the isoquinoline analogue. A similar conclusion can be drawn from the images of [Cd(DCNQI)(isoquin)2] at different potentials (see Figure S13(c)). Application of an anodic potential resulted in minor color changes that were not consistent with the formation of the dark blue coloration characteristic of DCNQI• −, suggesting that the framework underwent only a partial conversion. The incomplete spectral response can be rationalized by assessing the framework crystal structure, which consists of 2D layers largely held together by edge-to-face interactions between the axial ligands. Since isoquinoline is significantly larger than pyridine, the van der Waals interaction between isoquinoline ligands should be stronger and, thus, separation of the sheets, which is required for intercalation of the counterion upon oxidation, is less likely to occur for [Cd(DCNQI)(isoquin)2]. As a result, DCNQI2− moieties were less accessible for oxidation. A further increase in the applied potential did not result in any additional spectral transformations. Despite the absence of pronounced spectral and color changes, the redox process observed in the cyclic voltammogram of [Cd(DCNQI)(isoquin)2] can be assigned to the one-electron oxidation of DCNQI2− inside the framework, in a similar manner to the pyridine analogue. The reversibility of the redox transformation was confirmed by applying a cathodic potential (−0.1 V) which resulted in the regeneration of the starting spectrum (Figure S13(b)).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00130. Additional experimental details including solid-state VisNIR spectroelectrochemical cell; PXRD, FT-IR, electrochemical, spectroelectrochemical and UV-vis-NIR data; TD-DFT computational results (PDF) Accession Codes

CCDC 1581271 and 1581272 contain 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, U.K.; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R. Robson). *E-mail: [email protected] (D. M. D’Alessandro). ORCID

Brendan F. Abrahams: 0000-0003-2957-860X Bun Chan: 0000-0002-0082-5497 Deanna M. D’Alessandro: 0000-0002-1497-2543 Author Contributions ⊥

These authors contributed equally.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Australian Research Council is acknowledged for their funding of this work. Notes

The authors declare no competing financial interest.

CONCLUSIONS

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The new [Cd(DCNQI)(L)2] (L = py and isoquin) frameworks exhibit redox-state dependent spectral properties, including an interligand charge-transfer process in the pyridine analogue that has been supported using TD-DFT computational calculations. Electrochemical experiments on [Cd(DCNQI)(L)2] revealed the presence of a single redox process, and UV-vis-NIR spectroelectrochemistry (using a new “second generation” cell) unambiguously showed the formation of DCNQI• − radical within the frameworks upon application of an anodic potential. Interestingly, the presence of the DCNQI• − species in the assynthesized framework samples was also detected using EPR, IR, and UV-vis-NIR spectroscopic measurements. No indication of the neutral form of DCNQI was found during the SEC experiment, even though the free ligand is capable of up to two redox processes at relatively mild potentials. In summary, the complementary suite of solid-state electrochemical and spectroelectrochemical approaches used here has proven particularly valuable for examining the redox-state dependent properties of framework materials. Coupled with computational calculations, these methods have great utility for understanding fundamental charge-transfer processes that may underpin long-range functional properties, such as conductivity.

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the Australian Research Council, the Vibrational Spectroscopy Core Facility and the Sydney Nano Institute at The University of Sydney, and the provision of computational resources by the National Computational Infrastructure of Australia.

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Interligand Charge-Transfer Interactions in Electroactive Coordination

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