Assembly of a trapped valent Ce^III/IV–TCNQ complex through metal–ligand redox cooperativity

Abstract

Complex (Cp₃Ce^IV)₂(TCNQ)(Ce^IIICp₃)₂ (1) was prepared by reducing neutral TCNQ⁰ (tetracyanoquinodimethane) with Cp₃Ce(THF). Two types of cerium centres with a dianionic TCNQ²⁻ moiety are present in 1, wherein each of the four cyano-groups are bound by a cation. Formation of this trapped-valent organocerium compound 1 is facilitated by metal–ligand redox cooperativity. Characterization of 1 was carried out using structural-, magnetometry-, and IR-spectroscopic analyses. Photophysical studies on this compound reveal Ce^III luminescence, and opens up avenues for promising multifunctional, mixed-valent lanthanide materials.

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Metal mixed valency is of interest in reticular chemistry to achieve long-range electronic coupling and associated physical properties.^1,2 Fine-tuning of metal frontier orbital overlap and associated redox potentials can afford discovery of emergent phenomena in molecule-based materials, tuning that is typically not possible in intermetallic nor other solid-state materials.^3–5 For example, redox active organics such as pyrazine have been shown to induce variable magnetic and conducting properties in materials comprising transition metal salts.^6–8 Lanthanide compounds that are otherwise dominated by the trivalent oxidation state can exhibit notable behaviours in formally divalent or tetravalent materials upon combining with suitable electron donors/acceptors.^9–11

Electron acceptors such as fullerenes, TCNQ (tetracyanoquinodimethane), TCNE (tetracyanoethylene) and others have been shown to assemble notable structures and associated, collective properties through charge transfer processes.^12–14 TCNQ and TCNE offer multiple accessible redox states and four terminal cyano-functionalities that bind metal coordination sites to assemble magnetic, conducting and multifunctional materials.^5,15,16 Mixed valent ferrocene-TCNQ materials comprising π–π interactions have also been assembled by charge transfer processes.¹⁷ On the other hand, redox assembly of lanthanide based TCNQ materials remain unexplored with the exception of ytterbocene, (C₅Me₅)₂Yb^II(THF)₂, which undergoes an unusual oxidative reorganization with TCNQ to form the Yb^III dimer, [(C₅Me₅)₂Yb^III(CH₃CN){(μ-CN)₂(C₆H₄)(CN)₂}]₂.¹⁸ A previous report of a ribbon-like coordination polymer of Ce^III and TCNQX₂ X = –Cl, –Br showed notable magnetic coercivity, but was not formed through redox processes.¹³

Among organometallic lanthanide complexes, cerium offers relatively mild and tunable Ce^III/IV oxidation potentials,^19,20 and isolable tetravalent compounds.^21–26 Tris(cyclopentadienyl) cerium(III) complexes have previously been shown to be a useful platform to access organocerium(IV) compounds: Cp₃Ce–X, X = –halides, –alkoxides, and –aryloxides.^27–29 As such, we expected that this platform could be used for molecule-based materials with multifunctional charge transfer properties if assembled with suitable organic acceptors. Our group recently reported that (C₅Me₄H)₃Ce reacts with C₆₀ to form [(C₅Me₄H)₃Ce]₂·C₆₀ where intermolecular interactions promoted the emergence of Mott insulator behaviour reminiscent of Ce₂O₃.³⁰ In the current work, we provide an initial demonstration that high valent, organocerium-based materials can be achieved with metal–ligand redox cooperativity using the organic acceptor TCNQ.

The first reduction of TCNQ⁰ reportedly occurs at E_1/2 = –0.34 V versus Fc in THF while the second reduction occurs at E_1/2 = –1.01 V.³¹ We set out to explore organocerium-based material by reducing TCNQ with Cp₃Ce(THF), the latter of which shows an oxidation potential of E_1/2 = –0.26 V versus Fc in THF.²⁹ Reaction of Cp₃Ce(THF) with TCNQ in toluene results in immediate precipitation of a brown solid. Using the tetramethylated precursor: (C₅Me₄H)₃Ce towards the goal of preparing a compound with increased solubility, does not show any color change or precipitation upon reaction with TCNQ⁰ in toluene. We postulate that the larger steric demand offered by three C₅Me₄H⁻ ligands prevent interaction with TCNQ⁰ to initiate the redox reaction. An optimized synthesis of the former reaction with Cp₃Ce(THF) produced a crystalline, brown-black material, suitable for single crystal X-ray diffraction analysis, in 71% yield (see ESI† for details). The isolated material was insoluble in toluene, DCM, and Et₂O at RT. Addition of THF and acetonitrile to the solid 1 produces yellow coloration, likely due to reductive decomposition. Notably, single crystal X-ray diffraction shows a neutral tetrameric complex (Cp₃Ce^IV)₂(TCNQ)(Ce^IIICp₃)₂ (1) with the TCNQ²⁻ ligand in the doubly reduced state (Scheme 1).

Scheme 1 Synthetic conditions for 1 from Cp₃Ce(THF) and TCNQ along with their respective redox potentials versus Fc in THF.

The X-ray diffraction data for 1 are shown in the Table 1. The tetrameric TCNQ²⁻ complex consists of two, structurally distinct, types of cerium centres. The Ce–N and Ce–Cp(centroid) bond distances at Ce(1) and Ce(3)* are 2.558(3) Å and 2.558(avg.) Å respectively, the latter of which is consistent with the structure of the previously reported Cp₃Ce^III–OC(NMe₂)₂, 2.576(avg.) Å.³² These same bond distances at Ce(2) and Ce(4)* are 2.414(3) Å and 2.458(avg.) Å, respectively, distances that more closely resemble those of Cp₃Ce^IVCl with Ce–Cp(centroid) distance 2.460(avg.) Å.²⁷ The redox state of the TCNQ²⁻ ligand in 1 was evaluated by examining the bond length c (Fig. 1), where a change to the benzenoid form upon reduction is expressed through an elongation of that bond compared to neutral TCNQ. In 1, this bond length is 1.468(3) Å which is longer than the 1.374(3) Å observed in the structure of neutral TCNQ.³³ The bond length c is also used in the Kistenmacher relationship calculation to estimate the charge distribution on TCNQ.³⁴ (see ESI†). For 1, the net calculated charge on TCNQ is −2.19.

Table 1 Bond length and cyano vibrational frequency comparison for 1 and reported TCNQ^0/˙^−/2− materials

	Bond ‘c’ (Å)	M–N (Å)	νCN (cm^−1)	M–N≡C (°)
1 (M = Ce)	1.469 (5)	2.414(3), 2.558(3)	2098, 2177	168.5(3), 159.9(3)
TCNQ^0 (ref. 35)	1.374(3)	n/a	2223	n/a
TCNQ˙^− (ref. 17, M = Ce)	1.400–1.432	2.654–2.666	2165, 2198	168.7(3), 149.0(3), 171.4(3)
TCNQ^2− (ref. 36, M = Zn)	1.472(5)	2.160(3)–2.167(3)	2121, 2194	159.26(3), 158.21(3)

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Fig. 1 Thermal ellipsoid plot of 1 shown at the 30% probability level. Ce^III centers are shown in orange and Ce^IV in purple color. The * indicates atoms at equivalent position (−x,1−y,1−z). Hydrogen atoms and co-crystalized toluene molecules are omitted for clarity.

A notable feature of 1 is the bond angles described by the atoms: Ce–N≡C. For the Ce(2) and Ce(4)* sites comprising the tetravalent oxidation state, more linear Ce(IV)–N≡C bond angles of 168.5(3)° are observed. The Ce(1) and Ce(3)* sites exhibit more bent Ce(III)–N≡C bond angles of 159.9(3)°, presumably due to the weaker association of the cyano groups with the cerium(III) cations. Overall, based on the structural investigation, 1 comprises two cerium(III) cations and two cerium(IV) cations and is best described as a trapped-valent complex, class I under the Robin–Day classification.³⁷

TCNQ vibrational signatures are sensitive to the extent of reduction of this moiety in molecules and materials. IR spectra were collected and compared with neutral TCNQ crystals and 1 in a KBr matrix. Compound 1 shows characteristic stretching frequencies associated with the cyano groups at 2098 and 2177 cm⁻¹ (Table 1) whereas neutral TCNQ shows a single feature at 2223 cm⁻¹. This evident decrease in frequency is attributed to 2e⁻ reduction of TCNQ moiety in 1 as reported previously.³⁸ Attempts to perform Raman measurements of 1 were unsuccessful due to a consistently poor signal-to-noise ratio.

Malischewski and co-workers recently reported that coordination of B(C₆F₅)₃ to TCNQ shifted the first reduction potential of the latter by 1.2 V to 0.9 V versus Fc in CH₂Cl₂.³⁹ We and others have previously shown such Lewis acid promoted potential shift (LAPPS) of organic acceptors using trivalent lanthanides.^35,40 For example, we demonstrated LAPPS by cerium coordination and concomitant reduction of para-benzoquinone using [{Li₃(thf)₄}{(BINOLate)₃Ce(thf)}].⁴¹ Similarly, 1 evidently assembles from a Lewis acid effect of Ce^III bonding that shifts the redox potentials of the Cp₃Ce donor and TCNQ acceptor, and relatively covalent bonding of Ce^IV coordination. The operative Lewis acid-assisted coordination significantly lowers the reduction potentials of TCNQ and concurrently reduces it to assemble the trapped valent cerium complex 1. This chemistry is reminiscent of transition metals based TCNQ reduction that has not been previously reported for lanthanides.⁴²

Variable temperature magnetic susceptibility data performed at 2 T and variable field magnetization saturation experiments performed at 2 K were recorded for 1 to further verify the redox assignment of the cerium cations and TCNQ (Fig. 2). For Cp₃Ce(THF), the χ_MT value is 0.68 emu K mol⁻¹ at 300 K and slowly decreases to 0.25 emu K mol⁻¹ at 2 K, similar to the reported value for Cp₃CeOP(NMe₂)₃.³⁶ For 1, the χ_MT value is 1.56 emu K mol⁻¹ at 300 K and slowly decreases to 0.49 emu K mol⁻¹ at 2 K. This data falls in the expected range for two non-interacting Ce^III ions (1.60 emu K mol⁻¹) and helps to confirm the presence of two Ce^III cations in 1. With decreasing temperature, the χ_MT value decreases slowly and drops to 0.50 emu K mol⁻¹ at 2 K. This decrease in the χT product is likely due to thermal depopulation of the cerium(III) Kramers doublets or weak antiferromagnetic coupling. A field dependent magnetization saturation experiment showed a value of M = 1.59μ_B at 2 K at an applied field of 7 T, which further supports the assignment of two cerium(III) centres in 1.⁴³

Fig. 2 Variable temperature χT measurements of Cp₃Ce(THF) (square markers) and 1 (circle markers) at 2T, variable field magnetization experiments (inset) conducted at T = 2 K.

To further interrogate the photophysical properties of 1, photoluminescence spectra were collected on (only poorly soluble) suspensions of the compound in toluene (Fig. 3). With excitation at 340 nm, a broad emission band was observed with λ_max = 414 nm. The excitation spectrum of 1, collected between 230 and 380 nm, contains a broad excitation band with λ_max = 343 nm and a prominent shoulder located at 360 nm. The separation of λ_max and the lower energy shoulder is 1377 cm⁻¹ which falls in the range of peak separations commonly observed for cerium(III) 5d → 4f transitions between the ²D excited state and ²F_5/2 and ²F_7/2 ground state manifold.⁴⁴ The Stokes shift for 1 was determined to be 54 nm (0.45 eV) which is comparable to those shifts reported for other luminescent cerium(III) compounds.⁴⁵ Given that the luminescence profile of 1 resembles those observed for reported Ce(III) 5d → 4f emissions, we tentatively attribute the observed luminescence features to electronic transitions at the cerium(III) sites of 1.

Fig. 3 Photoluminescence spectra of 1 (red) and Cp₃Ce(THF) (purple) as a suspension in toluene and a 0.005 mM solution respectively. The excitation spectra at 420 nm emission are shown as solid traces. The emission spectra at 340 nm excitation are shown as dashed traces.

Previous studies of the photophysical properties of neutral TCNQ revealed that it exhibits solvent dependent photoluminescent behaviour.⁴⁶ While TCNQ was not luminescent in toluene, the starting Cp₃Ce(THF) complex exhibited a similar luminescence profile to 1 and resembled the spectra previously reported in THF.⁴⁷ The excitation spectrum was essentially indistinguishable from that of 1 with an identical λ_max value and prominent shoulder at 343 and 360 nm respectively. Notwithstanding, the emission band of Cp₃Ce(THF) obtained upon excitation at 340 nm occurred at slightly higher energy with the λ_max = 403 nm. As such, Cp₃Ce(THF) exhibits a smaller Stokes shift than that of 1 (43 nm, 0.36 eV). Our group has previously demonstrated that the Stokes shift in cerium(III) emitters is correlated with the degree of nonradiative vibrational losses of energy in the emissive state(s).⁴⁸ The results therefore indicate that 1 undergoes greater vibrational relaxation in the ²D excited state compared with the mononuclear Cp₃Ce(THF) complex.

A synthetic pathway for cerium coordination-induced double reduction of TCNQ has been described. This strategy can be used to assemble molecule-based materials comprising reduced organics with otherwise poorly matched donor–acceptor redox potentials. Currently, studies of a range of these materials with various metals and Cp-substituents, and studies of their electronic structures towards multi-functional materials are underway in our laboratory.

We gratefully acknowledge the U.S. Department of Energy under Award DE-SC0020169 for support of this work. H. G. also acknowledges the Vagelos Institute for Energy Science and Technology for a graduate fellowship. And E. J. S. also thanks the University of Pennsylvania for financial support. We also thank the JASCO facility for Spectroscopic Excellence in the Chemistry Department at the University of Pennsylvania for access to the Raman spectrometer used in this work.

Data availability statement

Data supporting this article have been included as part of the ESI.† Crystallographic data for 1 have been deposited at the CCDC under accession number 2343799 and can be obtained from https://www.ccdc.cam.ac.uk/structures.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic Details, characterization, additional magnetometry, vibrational spectroscopy, and X-ray crystallographic data. CCDC 2343799. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc01478b

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