Electrochemical host–guest interactions in a disordered oligosilyl coordination polymer

Abstract

We synthesize and study the charge transfer properties of a oligosilyl coordination polymer formed from zirconium clusters. Although the product lacks long range order, spectroscopic and computational evidence suggest that the metal–ligand bond is formed, and the principle crystallographic reflections closely match those simulated from inter-node spacings matching that of the ligand. The porous polymer allows for the incorporation of guest molecules as demonstrated by the intercalation of 7,7,8,8-tetracyanoquinodimethane (TCNQ). Charge transfer is predicted from DFT and experimentally observed by infrared spectroscopy, solid-state ²⁹Si nuclear magnetic spectroscopy, and voltammetry.

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Introduction

Silicon-containing materials have been widely explored for semiconducting applications, including as solar absorbers, transistor arrays in computer chips, and for energy storage.¹ The design of silicon materials has been rooted in a top-down approach that involves etching,² lithography,³ and other bulk processing techniques.⁴ While these methods are ideal for processability, there is continued interest in developing new silicon-containing molecular materials that afford bottom-up syntheses, providing opportunities for compositional and topological control.^5,6

Multitopic oligosilane based linkers afford one route to incorporate the silane core into materials via bottom-up synthesis, Scheme 1.^7–16 Compared to conventional carbon-based materials, Si atoms can participate in σ-conjugation within the Si–Si chain via primary, geminal, and vicinal interactions.^17,18 The same Si-orbitals are affected by varying chain length and conformation.^19,20 To this end, the electronic properties of oligo- and polysilanes have been studied,^21,22 but there has been limited examples of their incorporation into extended frameworks (Scheme 1); we reported the first oligosilane Cu-based MOF in 2020.¹⁰ In addition, prior studies largely concerned the formation of metal–organic frameworks (MOFs) with late transition metals, which have inherent instability at the metal–carboxylate interface. To mitigate this, some effort has been placed in forming framework materials using group IV metals, Zr from Scheme 1, but the linker itself was not an oligosilane.⁹

Scheme 1 Si-containing linkers have been included in frameworks comprised of Zn, Cd, Zr, and Cu. In this work, we combine oligosilane linkers, Si₂Me₄(C₆H₄CO₂H)₂, with zirconium nodes to yield a disordered coordination polymer.

Building upon the report in ref. 9, we sought to develop a material containing [Zr₆O₄(OH)₄]¹²⁺ clusters, linked together using an oligosilane from ref. 10. We hypothesized that these building units should give rise to a stable material in which the Zr cluster would not partake in the band edges, thereby allowing us to study any deviation in electronic properties of the ligand as a function of ligation to the cluster. This is important because while there are studies of post-synthetic doping via TCNQ inclusion,^23–25 there are fewer examples of ligand-mediated redox,²⁶ and even fewer examples of how these properties deviate due to the assembly of the material.

Like carbon-based aliphatic linkers, oligosilane-containing linkers are inherently flexible. When paired with Zr clusters, it proved difficult to isolate single crystals. As this paper will describe, these materials typically form disordered coordination polymers (dCPs) with crystallographic reflections matching the expectation for a trans-connected series of Zr clusters. While this presents some challenges for topological characterization, most framework materials feature highly localized electronic structure; i.e., the electronic properties of the framework do not primarily depend on the 3D ordering, but rather are dictated by the free motifs themselves. Since these oligosilanes are unlikely to form aggregate, and the trans configuration is more stable (due to the increased σ conjugation, see computational details in ESI‡), we surmise that materials constructed therefrom should feature electronic properties dictated by the free trans-linker itself.

Here, we expand upon the library of materials that feature oligosilane linkers [lin-Si-2, Si₂Me₄(C₆H₄CO₂)₂] and present Zr-dCP-Si-2, an electroactive disorder coordination polymer whose redox properties are dictated by the linker. Using a combined spectroscopic, electrochemical, and computational approach, we demonstrate that the oligosilyl linkers bind to the Zr clusters and form an extended porous network. The material maintains optical and electron chemical properties akin to the linker itself, which modulates the redox potential of redox active guests. Electronic structure calculations support the hypothesis that the metal clusters do not impact the band edges, bolstering a central design principle of framework chemistry: electronic properties are generally informed by the isolated components of the material.

Results and discussion

To assess the redox chemistry of a framework material comprising the lin-Si-2 linker, we synthesized an oligosilane dCP containing redox inert Zr nodes. These nodes, commonly found in UiO-66, MOF-808, and NU-1000, act as a skeleton to suspend the oligosilane in a trans-conformer as required for σ-conjugation.²⁷ While some MOFs do crystalize with flexible linkers,^28–32 (e.g. UiO-66 derivatives,²⁸ COK-69³²) we expected disorder due to the increased size of the linker (affecting the crystallization kinetics)³³ paired with the 12-connected node. We varied the solvent conditions and ratio of linker to modulators in an effort to improve crystallinity,³⁴ which yielded products ranging from gels to semi-crystalline powder. The most crystalline sample was obtained using 24 equivalents of formic acid per ZrCl₄, denoted here as Zr-dCP-Si-2 (Scheme 1 and Table S1, ESI‡), and featured a type II N₂ adsorption isotherm with a surface area of 197.5 m² g⁻¹. Despite numerous attempts, we were unable to obtain crystals large enough for single crystal X-ray crystallography.

Powder XRD (PXRD) of the as-synthesized product show dominant peaks resolution at 2θ = 5.2°, 7.4°, and 10.4°. (Fig. 1d). A more resolved PXRD was obtained of the suspend solid upon sonication, which was attributed to the separation of large amorphous components from the polycrystalline solid. From a 1D computational model system, we were able to simulate some expected reflections associated with repeating structural motifs in the experimental system (structure provided in Fig. S8, ESI‡). Although there are rare reports of MOFs formed from Zr₈, Zr₉, and Zr₁₂ nodes,^35–41 we used the most commonly reported [Zr₆O₄(OH)₄]¹²⁺ node for our calculations. Using the periodic model, the inter-cluster spacing was manually adjusted to simulate their separation. At internode separations equal to the length of the rigid DFT predicted size of the trans-oriented linker, we predict a reflection of 2θ = 5.0°, aligning with the experimental reflection at 5.2°. We attribute this reflection to internodal separation, with the deviation being due to imperfect trans connectivity, a characteristic that is expected with flexible linkers.

Fig. 1 (a) ATR-IR of Zr-dCP-Si-2. (b) Thermogravimetric analysis from 20 °C to 800 °C under argon atmosphere. Distinct peaks in the derivative curve correspond to the loss of solvent, modulator, and organic components of the linker. (c) ¹H NMR of digested Zr-Si-2 dCP. dCP was soaked in concentrated NaOH in D₂O overnight. The aromatic protons of the linker are at 7.82 ppm and 7.94 ppm and the formate proton is at 8.56 ppm. Integral values are denoted in red. (d) PXRD of Zr-dCP-Si-2 after sonication compared to DFT-simulated pXRD.

To assess the stoichiometry and thermal stability of Zr-dCP-Si-2, the dCP was heated to 800 °C at a rate of 5 °C min⁻¹ and the mass was measured via thermogravimetric analysis (TGA, Fig. 1b). There were three main areas of mass loss corresponding to (i) solvent trapped within the pores, (ii) modulator bound to the node, and (iii) decomposition of the organic the linker, in order of increasing temperature. The mass remaining at 800 °C is attributed to ZrO₂ from the metal nodes and SiO₂, which was confirmed by TGA of the pure linker (Fig. S4, ESI‡).

Since the monocarboxylate modulators competitively bind to the node and the synthesis requires a high ratio of modulators compared to linker,^42,43 this is an indication that the rate of lin-Si-2 ligation is relatively slow. To determine its composition, ¹H NMR of the base-digested dCP revealed integration signals corresponding to a ratio of 2.29 : 1 formate:lin-Si-2 (Fig. 1c). Elemental analysis through inductively coupled plasma optical emission spectrometry (ICP-OES) showed a Zr : Si molar ratio of 1 : 2. Assuming the presence of Zr₆ nodes, we deduce the composition of Zr-dCP-Si-2 to be [Zr₆O₄(OH)₄] [Si₂Me₄(C₆H₄CO₂)₂]_6−x[HCO₂]_2x, where x is between 3 and 4, highlighting the disordered nature of the coordination polymer.

The oligosilane has several characteristic attenuated total reflectance infrared (ATR-IR) signals, which can be used to assess the local environment of the linker. The bands at 1400 cm⁻¹, 1250 cm⁻¹, and 1100 cm⁻¹ corresponding to COO⁻, Si–CH₃, and Si–Ph, respectively (Fig. 1a).⁸ The binding of lin-Si-2 linker to the Zr clusters is readily observed using the same technique (Fig. S3, ESI‡), showing a red shift in vibrational frequency associated with the increased reduced mass of the Zr cluster compared to the protonated linker. This result was also predicted from our DFT models, which suggested that the linker C=O stretch should red-shift by ∼100 cm⁻¹ upon ligation to the Zr cluster, qualitatively aligning with experiment (Fig. S8, ESI‡). From our prior work, we have shown the red shifting of the carbonyl stretch indicates metal ligand bond formation, as the carbonyl is now oscillating near a heavy metal node.⁴⁴

Given the extended Zr-dCP-Si-2 features both the oligosilane and Zr cluster, we turn our interest to its emergent electronic properties. Taking inspiration from MOFs with host–guest interactions between electron donors and acceptors,⁴⁵ we incorporated electron accepting 7,7,8,8-tetracyanoquinodimethane (TCNQ) as a probe to elucidate the reducing behavior of the dCP. TCNQ was sublimed into the dCP by heating to 80 °C under static vacuum, yielding Zr-dCP-Si-2@TCNQ. Upon sublimation into the dCP, we observe a color change of the powder from a pale orange/yellow to a green solid.

To better understand the nature of these color shifts, we developed a series of DFT models to predict electronic gaps and relative band alignments, Fig. 2. We computed the energetics of the linker, the formate-passivated Zr₆ cluster, and free TCNQ (Fig. 2) and their electron energies were aligned to the vacuum level. A molecular fragment of the coordination polymer was computed—a single linker bound to one position on the Zr cluster—and the resulting electron energies were then aligned. By comparison, the lin-Si-2 linker comprises the highest energy electron, both in the free linker and dCP cluster, and maps to the general prediction that the properties of the polymer should be dictated by the linker itself. The conduction band of the cluster and, separately, the linker were found to be approximately 2.0 V from vacuum, indicating a predicted degree of orbital in the conduction band. Through alignment with TCNQ, the TCNQ LUMO is predicted to sit mid-gap relative to the dCP conduction band (−5.1 eV), suggesting that charge transfer from the linker to TCNQ is the most favorable electron transfer pathway. These results are in line with the experimental observation (vide infra) that the lin-Si-2 is oxidized in the presence of TCNQ and also closely match previous experimental reports.^46,47

Fig. 2 DFT computed electronic structure of the components and model disordered coordination polymer. The material gleans its properties from the linker (lin-Si-2), which is predicted to sit in a type I semiconductor offset relative to the Zr node. The LUMO of TCNQ sits mid gap relative to the valence band edge and acts as an electron acceptor from the framework. Upon reduction of TCNQ, the highest energy electron sits negative 0.3 eV relative to the framework LUMO, resulting in a metastable TCNQ-framework charge transfer complex.

We measured the diffuse-reflectance (DR) UV-vis spectra of the samples, and their corresponding Tauc plots for both the solid state (Fig. 3a) and solution state (Fig. S10, ESI‡). The resulting band gaps were calculated from the DR-UV-Vis spectra. The bandgap of 4.07 eV for lin-Si-2 is essentially preserved upon polymer formation; the bandgap of Zr-dPC-Si-2 is 3.96 eV. TCNQ has a gap of 3.02 eV (in close agreement with the transport gap, ∼2.9 eV),⁴⁶ which shifts ∼0.1 eV to 2.95 eV when incorporated into Zr-dPC-Si-2@TCNQ (Table S2, ESI‡). We note that the under estimation of the TCNQ gap by DFT is a well-studied problem related to the treatment of electron exchange, but qualitatively does not change our analysis.

Fig. 3 (a) Tauc plot of TCNQ (light blue), lin-Si-2 (pink), Zr-dCP-Si-2 (purple), and Zr-dCP-Si-2@TCNQ (dark blue) with corresponding fits extrapolated to the x-axis. The x-intercept corresponds to the band gap. (b) ATR-IR of TCNQ nitrile stretch before (light blue) and after (purple) intercalation into the dCP. The shift in the nitrile stretch frequency from 2222 cm⁻¹ to 2183 cm⁻¹ corresponds to a single electron reduction of TCNQ.

With these relative alignments in mind, low concentrations of thermally promoted charge transfer from the dCP to TCNQ is expected. The extent of charge transfer interactions was then measured by attenuated total reflectance-infrared spectroscopy (ATR-IR). As the neutral TCNQ is reduced to radical anion (and subsequently to a dianion), the C–N bond frequency red-shifts due to the reduced conjugation in the aromatic ring. We observe a change from 2222 cm⁻¹ to 2183 cm⁻¹ in Zr-dCP-Si-2@TCNQ, suggesting a single electron reduction of TCNQ in the framework (Fig. 3b). This is in close agreement with our DFT predicted C–N stretching frequency shift from 2247 cm⁻¹ to 2196 cm⁻¹ upon a single electron oxidation of molecular TCNQ (see ESI‡ for computational details). Previous literature also reports a similar magnitude of this change in the stretch frequency.⁴⁸

The interaction between the lin-Si-2 linker and TCNQ is also observed by solid-state NMR. The ²⁹Si spectra of the oligosilane and of Zr-dCP-Si-2 both have a peak centered at −11.2 ppm, indicating that the chemical environment of Si is unchanged when bound within the dCP (Fig. 4). Once the TCNQ is sublimed into Zr-dCP-Si-2, there is a 1.3 ppm upfield shift of the ²⁹Si peak to −12.5 ppm. Predicting chemical shifts in ²⁹Si NMR is non-trivial;⁴⁹ thus, we hesitate to remark on the direction of the shift other than the small change in peak position indicates a change in the Si chemical environment in the framework upon TCNQ intercalation.

Fig. 4 Solid-state ²⁹Si MAS NMR. lin-Si-2, Zr-dCP-Si-2, and Zr-dCP-Si-2 intercalated with TCNQ. Samples were referenced to tetrakis(trimethylsilyl)silane. The Si peak is preserved from linker to dCP and shifts −1.3 ppm upon intercalation with TCNQ.

Finally, the electrochemical properties of the ligand and guest–host framework is comprehensively investigated using cyclic voltammetry (Fig. 5). A three-electrode system was used with glassy carbon serving as the working electrode, Pt as the counter, and Ag wire with glass frit as the reference electrode with a supporting electrolyte of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) in DMF.

Fig. 5 Cyclic voltammograms of (a) lin-Si-2, (b) Zr-dCP-Si-2, and (c) TCNQ with and without lin-Si-2 and (d) TCNQ and Zr-dCP-Si-2@TCNQ. dCP samples were drop cast onto the glassy carbon electrode as a slurry of 2 mg mL⁻¹ of 1 : 2 graphene oxide: dCP in isopropanol. All CVs were taken in 0.1 M TBAPF₆ in DMF at a scan rate of 100 mV s⁻¹ and referenced to an external Fc/Fc⁺ reference. The gray dashed line indicates the E_1/2 position of the TCNQ features, and the red dashed line indicates the E_1/2 position after interaction with the respective species. The asterisk (*) indicates the oxidation of the silyl group in the linker as reported previously.²⁰

All CVs were first scanned anodically, starting at the open circuit potential. In the CV of lin-Si-2 (Fig. 5a), we observe two oxidative peaks at 0.54 V and 1.04 V vs. Fc/Fc⁺. We attribute the peak at 1.04 V (marked with an asterisk) to the oxidation of the oligosilane; a peak at the same location is observed in the brominated version of the linker and agrees with previous literature values (Fig. S14, ESI‡).²⁰ In the CV of Zr-dCP-Si-2 that was deposited on a carbon electrode, the silane oxidative feature (*) is notably retained (Fig. 5b), although with a minor anodic shift of 40 mV, asserting that the electrochemical behavior of the oligosilane is retained within the dCP. The features at 0.56 V and −2.06 V are attributed to the reduction of carboxylates present in the modulators of the dCP. The small reversible feature at 0 V corresponds to oxygen redox due to a minor leak in the cell.

We sought to understand how the intercalation of TCNQ affects its two-electron redox couple. TCNQ exhibits two reversible redox features at E_1/2 = −0.70 V for the TCNQ²⁻/TCNQ˙⁻ transition and E_1/2= −0.095 V for the TCNQ˙⁻/TCNQ transition (blue dashed lines, Fig. 5c and d). When lin-Si-2 and TCNQ are dissolved in solution (equimolar, 10 mM), we observe a shift of 95 mV in the E_1/2 of the TCNQ²⁻/TCNQ˙⁻ and 105 mV in the E_1/2 of the TCNQ˙⁻/TCNQ (green dashed lines, Fig. 5c). The minor peak in the reduction at −0.7 V is likely residual TCNQ that has not complexed with lin-Si-2. In the intercalated Zr-dCP-Si-2@TCNQ (Fig. 5d), we observe identical shifts in the two TCNQ transitions. These negative shifts in E_1/2 suggest there is increased electron density on the TCNQ upon interaction with the oligosilane linker, thereby making the complexed TCNQ more readily oxidized. The similar magnitude of changes in E_1/2 between lin-Si-2 and Zr-dCP-Si-2 with TCNQ, further suggesting that the Zr nodes serve primarily as a structural support in the dCP and minimally affect electronic structure of the oligosilane linker.

Conclusion

Together, we have presented a comprehensive assessment of the composition and physical properties of a disordered coordination polymer comprising Zr clusters supported by oligosilane ditopic linkers. Our bottom-up approach resulted in the retention of the oligosilane electronic properties upon material formation. Coupling the structural and DFT analysis, we then demonstrated that the metal cluster does not affect the properties of the linker, as evidenced by the electrochemical behaviour of Zr-dCP-Si-2@TCNQ composite. Together, the study demonstrates that oligosilane linkers are expected to retain their properties upon incorporation into a framework and should motivate their implementation in future bottom-up approaches.

Author contributions

R. I. A. designed and conducted the experimental procedures and analyzed the data. B. G. D. and C. H. H. conducted the theoretical calculations. V. S. T. designed the experimental procedures and analyzed the data. The manuscript was written through contributions from all authors.

Data availability

The data supporting this article have been included as part of the ESI.‡

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Science Foundation for the CAREER Award (DMR-1945114) and CAREER Award (CHE-2237345). We also are thankful for a Major Research Instrumentation grant (MRI-2018176), which enabled the acquisition of a solid-state 500 MHz Nuclear Magnetic Resonance spectrometer. This work used Expanse at SDSC through allocation CHE230104 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296. We thank Prof. Rebekka Klausen (Johns Hopkins University) for insightful discussions and initiating our interest in this area. We thank Dr David Burns and Dr Kyle Barcus for synthesis and crystallization advice, Dr Carter Gerke for his assistance with electrochemical experiments and SEM, Mr Sreyas Chintapalli for his help with UV-Vis spectroscopy, and Dr Jonathan Catazaro (Johns Hopkins University) for his assistance in solid-state NMR experiments. We also thank Mr Ian Dillingham and Dr Carter Gerke for proofreading this manuscript. R. I. A. dedicates her contribution to this paper to her grandmother who passed away as this project was being completed. V. S. T and C. H. H. are grateful for the support from the Camille and Henry Dreyfus Foundation.

Notes and references

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Footnotes

† R. I. A. dedicates her contribution to this paper to her grandmother and namesake who passed away as this project was being completed.

‡ Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc03032j

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