Tetrafluorenofulvalene: A Sterically Frustrated Open-Shell Alkene

doi:10.21203/rs.3.rs-2587889/v1

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Tetrafluorenofulvalene: A Sterically Frustrated Open-Shell Alkene

https://doi.org/10.21203/rs.3.rs-2587889/v1

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A typical π bond is weakened by oxidation and reduction, corresponding respectively to the removal of electrons from bonding orbitals and addition of electrons to antibonding orbitals, and by unpairing of the bonding electrons, e.g. in the triplet state. Here we describe tetrafluorenofulvalene (TFF), a twisted, open-shell alkene for which these general rules no longer hold. In particular, the bond in TFF becomes significantly stronger in the tri- and tetraanion, which can be generated by chemical reduction. While the triplet state of TFF does contain a weaker alkene bond than the singlet, in the quintet state the bond order increases significantly, yielding a flatter structure. The behavior of TFF originates from the doubly bifurcated topology of the underlying spin system and can be rationalized by the balancing effects of benzenoid aromaticity and spin paring.

Physical sciences/Chemistry/Organic chemistry/Structure elucidation

Physical sciences/Chemistry/Theoretical chemistry/Computational chemistry

Physical sciences/Chemistry/Physical chemistry/Optical spectroscopy/Spectrophotometry

The interplay between electronic conjugation and geometrical distortion of π bonds is of fundamental importance in organic chemistry because of its role in defining the properties of chromophores,^[1] organic semiconductors,^[2,3] chemical reagents,^[4] and molecular machinery.^[5] In Nature, double-bond isomerization forms the functional basis of the retinoid cycle responsible for human vision,^[6] and is fine-tuned by molecular distortions of the chromophore.^[7] In the laboratory, twistable double bonds have been incorporated into molecular switches and motors,^[5] switchable chromophores,^[2,8–10] and organic magnetic materials.^[11–17] The performance of such molecular devices critically depends on electronic and steric features of the π system containing the switchable bond. All of these applications have created considerable interest in the properties of distorted π systems, ranging from simple alkenes^[18,19] to complex polycyclic aromatics.^[20–23]

The equilibrium twist angle θ_eq and the isomerization barrier of an alkene are interrelated and can be controlled by changes of the bond order and by steric effects. In particular, the double bond can be weakened by both oxidation^[24,25] and reduction,^[2,26–28] as well as by introducing permanent twist to its ground state structure.^[29,30] In particular, highly distorted alkenes have been developed from 9,9'-bifluorenylidene (1a, Figure 1) as well as some closely related fused frameworks.^[31–34] The twist of 1a^[35] is increased by bulky substitution^[36,37,8] or ring fusion,^[38–41] as illustrated by 1b^[36] and 2,^[38–40] respectively. The large twist in 1b and 2 results in the elongation of the respective double bonds, reduction of electronic energy gaps, and in decreased isomerization barriers.

Simple alkenes become non-planar in the triplet state,^[42,43] while some of the intrinsically twisted systems have thermally accessible triplets (e.g. ³2). These observations suggest a possible strategy of controlling the alkene twist by increasing open-shell contributions to the ground state. We reasoned that such an influence can be studied by hybridizing a twisted alkene system with appropriately chosen stable oligoradicaloid motifs, such as those obtainable by indene fusion.^[44–46] This approach leads to proaromatic systems such as 3 (Figure 1),^[47] which derive their stability in the singlet and triplet states from open-shell contributions containing an increased number of Clar sextets.

By applying quadruple indene fusion to 9,9'-bifluorenylidene, we have now obtained the twisted hydrocarbon tetrafluorenofulvalene (TFF, 4), in which the properties of the center alkene bond show a uniquely complex dependence on the electronic state of the π system. A fully closed-shell (Kekulé-like) valence structure can be drawn for 4 (Figure 1), which however is devoid of any Clar sextets. In fact, only one double-bond localization pattern exists for 4, implying a purely olefinic (non-aromatic) character of the closed-shell contribution. Given the presence of six five-membered rings in 4, it is possible to draw a range of open-shell configurations, containing up to six radical centers and up to eight Clar sextets (4', Figure 1). Their relative contributions define the degree of spin pairing in the system, which in turn affects the central alkene bond. The twist of the alkene is further modified by changes of the oxidation level which span a range of seven redox states. We show that the behavior of the system can be rationalized by analyzing changes in its aromaticity and electron pairing using a simple, yet general valence bond model.

The mesityl-substituted TFF derivative 4a was obtained in a four-step procedure described in the Supplementary Information and completely characterized. In the solid state (Fig. 2A), the two diindenofluorenylidene (DIF) subunits of 4a form a twist angle θ_eq of 50.5°, which is larger than the corresponding torsion in 1a (42°), yet smaller than in the sterically congested 1b and 2. However, with a length of 1.431(7) Å, the central alkene bond a is significantly elongated in comparison with unhindered 9,9'-bifluorenylidene (BF) derivatives (1.36–1.38 Å),^{[35, 37, 48]} and is even longer than the corresponding bond in 2 (1.40–1.41 Å).^[40] Since the b bonds in 4a (1.422(3) Å) are also shorter than those in the reported unhindered BF systems (1.46–1.48 Å), the central double bond in 4a is weakened by conjugation within the DIF subunits. Interestingly, distance c, which corresponds to a formal single bond in the closed-shell configuration of 4a, is shorter than the formally double bond d (1.398(4) Å vs. 1.456(4) Å). Thus, the solid-state bond length pattern suggests considerable open-shell contributions to the electronic structure of TFF.

Compound 4a, which is NMR-silent in solution, revealed a persistent ESR signal in the solid samples of 4a, whose intensity decreased with the decreasing temperature. The temperature dependence of the ESR signal was modeled using the Bleaney–Bowers equation, yielding an estimate of the singlet–triplet gap ΔE_S–T of − 1.5(1) kcal/mol. A SQUID analysis confirmed that 4a is a ground-state singlet, with ΔE_S–T = − 3.1(1) kcal/mol calculated using the Bleaney-Bowers model. Absorption spectra of the deep-blue 4a contained several strong maxima in the 400 to 800 nm range and a weak band tailing beyond 1000 nm, corresponding to a relatively small energy gap of ca. 1.2 eV. In line with the latter observation, 4a displayed pronounced redox amphoterism in electrochemical experiments (Supplementary Fig. 1). In differential pulse voltammetry, three oxidation events were found at 0.01 V, 0.16 V, and 1.03 V (vs. Fc⁺/Fc in dichloromethane). Reduction of 4a occurred at − 1.07 V (2e), − 1.90 V (1e), and − 2.01 V (1e), respectively.

Titration of 4a with a one-electron oxidant, tris(4-bromophenyl)ammoniumyl hexachloroantimonate (BAHA, E_ox = 0.7 V in DCM) revealed consecutive formation of two NIR-absorbing species, with each step yielding near-perfect isosbestic points (Fig. 3A). The neutral 4a could be quantitatively recovered by reduction with KO₂. The two oxidation products were also generated electrochemically (Supplementary Figs. 15–16), and identified, respectively, as the radical cation [4a]^•+ (λ_max = 860 nm) and dication [4a]²⁺ (λ_max = 985 nm). The absorption of the radical cation features a shoulder at ca. 1200 nm, implying a smaller energy gap in [4a]^•+ than in [4a]²⁺. In contrast to the neutral 4a, the dication [4a]²⁺ is diamagnetic. In its ¹³C NMR spectrum, the C-11 resonance was identified at a highly downfield position of 184 ppm, indicating partial localization of the positive charge at this site. The structure of the dication was further elucidated in an X-ray diffraction analysis of a single crystal of the [4a][SbCl₆]₂ salt (Fig. 2B). In comparison with the neutral 4a, the a bond is somewhat lengthened (to 1.462(6) Å), while the θ_eq torsion increases to 54.5–55.8°. These changes suggest that the two-electron oxidation results in a moderate decrease of the a bond order.

Interestingly, even though a two-electron reduction event had been revealed in electrochemical measurements, titration of 4a with cobaltocene (CoCp₂, E_red = − 1.3 V) provided evidence for stepwise electron transfer (Fig. 3B'). Specifically, upon addition of up to 4 equiv of CoCp₂, we observed initial formation of the radical anion [4a]^•– (λ_max = 685, 920, 1155, ~ 1355 nm). Further addition produced the dianion [4a]^2– (λ_max = 760, 1120 nm), which could be oxidized back to 4a using diiodine. A broader range of anionic states of 4a was achievable by reduction with sodium naphthalenide (NaN), in the presence of 15-crown-5 (Figs. 3B and 3C). Initial spectra, observed in the range of 0–9 formal equivalents of added NaN corresponded to the sequential formation of [4a]^•– and [4a]^2–. On further addition of NaN (ca. 17 equiv), we observed the formation of a new species, with NIR absorptions extending beyond 2000 nm, which was presumed to be the radical trianion [4a]^•3–. When an even larger excess of NaN was added (up to 33 equiv), these characteristic bands disappeared, and the final spectrum had an absorption onset at ca. 1200 nm. Thus, the ultimate reduced product had a larger energy gap than all the preceding forms and was tentatively assumed to be the tetraanion [4a]^4–. Both [4a]^4– and the dianion [4a]^2– could be selectively generated on a larger scale and characterized using ¹H NMR (Supplementary Figures S2 and S7–S11).

Single crystals containing [4a]^2– and [4a]^4– anions were grown from THF solutions of 4a reduced with sodium metal in the absence of the crown ether additive. In particular, the tetraanion structure [Na(THF)₃]₄[4a] revealed a highly regular pattern of Na cations coordinated directly to the π system (Fig. 2D). The cations are bound near the fused edges of the BF core, with the shortest Na···C distances of 2.636(6) Å. Apparently, binding to the five-membered rings, which are presumed to carry a significant portion of the negative charge, is not feasible because of the steric protection by the Mes substituents. The a bond in the tetraanion is stronger than in other oxidation levels of 4a, as evidenced by its short length of 1.35(1) Å and the smaller θ_eq torsion of 40.9°. Partial reduction of 4a yielded crystals with a stoichiometry of [Na(THF)₆][Na(THF)₅]_0.74[4a]·8.3THF, indicating a mixed-valence character of 4a. Sodium occupancies indicate that the crystal contains mostly the dianion [4a]^2– with a ca. 26% admixture of the radical anion [4a]^•–. The structure is notable for the lack of Na···π coordination, reflecting the smaller negative charge residing in the π system. Specifically, two non-equivalent sodium sites were found: octahedral [Na(THF)₆]⁺, and disordered square-pyramidal [Na(THF)₅]⁺. The apparent geometry of 4a is averaged over the two contributing redox states (–1 and − 2) and features a relatively long bond a (1.436(6) Å) and a large θ_eq torsion (58.1°). These parameters may indicate weaker conjugation between the DIF subunits in [4a]^2– and [4a]^•– than in [4a]^4–.

Computational analysis. DFT calculations performed for the substituent-free TFF molecule 4b (R = H, Fig. 1) revealed significant variations of the key geometrical parameters as a function of the charge and multiplicity of the system (Supplementary Table 1). At the UCAM-B3LYP/6-31G(d,p) level of theory (hereafter denoted CAM), the equilibrium torsion, θ_eq, and the central bond distance a ranged from 33.7° and 1.372 Å in ⁵[4b] to 90.0° 1.477 Å in ³[4b]^4–, respectively. The a distance shows good correlation with θ_eq, indicating that both parameters can be used to quantify the strength of inter-subunit interaction in TFF. Relaxed potential energy surface (PES) scans along the θ coordinate revealed a complex dependence of the energy profile on the charge and multiplicity of 4b (Fig. 4, Supplementary Table 1). Specifically, all singlets and doublets have a twisted equilibrium geometry, characterized by θ_eq < 60°, and a transition state at θ = 90°. The singlet dication ¹[4b]²⁺ features the lowest twist barrier (ΔE_twist = ΔE_rel(90°) – ΔE_rel(θ_eq) = 0.8 kcal/mol) and the least acute θ_eq angle (59.2°). The ΔE_twist barriers increase in the order ¹[4b]²⁺ < ¹[4b]^2– < ²[4b]⁺ < ¹[4b] < ²[4b]^– < ²[4b]^3– < ¹[4b]^4– and correlate with a decrease of the respective θ_eq angles. The above sequence can thus be assumed to reflect an increase of the bond order a. PES scans of the triplets lie above the corresponding singlet scans at all θ angles: a single-well potential with θ_eq = 90° is predicted for ³[4b]^4–, while double-well potentials are found for ³[4b]²⁺ and ³[4b]^2–. The latter two species are significantly destabilized relative to the respective singlets, however, their twist barriers ΔE_twist are actually higher, implying that the a bond becomes stronger in these two triplet states. Conversely, the neutral triplet, ³[4b], has a shallow minimum at θ_eq = 58.2° with a low ΔE_twist barrier of 1.1 kcal/mol, implying a particularly weak a bond. This behavior could be considered typical of an alkene, however, in the quintet state ⁵[4b], the bond is predicted to be very strong, with a much higher twist barrier of 15.7 kcal/mol. This unusual response of the inner alkene in the neutral 4b to changes of spin multiplicity is further confirmed by the torsional dependence of a and b bond lengths (Supplementary Fig. 16). The triplet ³[4b] and quintet ⁵[4b] approach the limits of a pure single and double a bond, respectively, whereas the singlet ¹[4b] contains a strongly conjugated alkene with an intermediate bond order.

Adiabatic singlet–triplet gaps predicted for even-electron ions of 4b at the CAM level are relatively large (ca. − 9 to − 15 kcal/mol, Fig. 4, Supplementary Table 1), in line with the observed diamagnetism of [4a]²⁺, [4a]^2–, and [4a]^4–. For the neutral 4b, a significantly smaller gap was obtained (ΔE_ST = − 3.91 kcal/mol), relatively close to the experimental SQUID value, whereas the quintet state ⁵[4b] was predicted to have a much higher energy (ΔE_SQ = − 14 kcal/mol). Since CAM, as a single-reference method, is not fully suitable for quantitative assessment of spin-state energetics, we evaluated energies of the neutral ^m[4b] (m = 1, 3, 5) using two active-space methods, i.e. CAS-SCF(6,6)/6-31G(d,p) (denoted CAS), and the spin-flip approach^{[49, 50]} at the RAS(4,4)-SF-srB3LYP/cc-pVDZ level of theory (denoted RAS). RAS calculations, performed for the CAM-optimized minima and PES scans, indicate that the ST and SQ gaps may be smaller than predicted by the CAM level (ca. − 3 and − 7 kcal/mol, respectively, Supplementary Figure S18).

An analysis of natural orbital occupation numbers (NOONs) showed the neutral singlet ¹[4b] had a pronounced tetraradicaloid character (y₀ ≥ 0.98 and y₁ ≥ 0.29), with a possible smaller hexaradicaloid contribution (y₂^CAM = 0.12). The number of unpaired electrons obtained from the CAM-derived NOONs (n_U^CAM, Supplementary Table 1) is non-zero for all states except for ¹[4b]^4–, which is the only one with a purely closed-shell configuration. The n_U^CAM value of 3.15 obtained for the neutral singlet ¹[4b] is in fact higher than in the corresponding triplet state. Extensive mixing of open-shell configurations is indicated by the high values of n_U^CAM (i.e., exceeding m – 1) obtained for ²[4b]⁺ and ²[4b]^–. Since twisting of an alkene normally leads to π bond breaking, one could intuitively expect the n_U values to increase with increasing θ. Remarkably, however, no such general relationship is found for 4b (Supplementary Fig. 17). Paradoxically, n_U decreases with θ for the neutral singlet ¹[4b], as well as for ²[4b]^–, ²[4b]⁺, ¹[4b]^2–, and ¹[4b]²⁺, suggesting that in these species, electron pairing is actually enhanced by decoupling of the DIF subunits.

Nucleus-independent chemical shifts (NICS) revealed striking variations of magnetism in 4b caused by changes of its oxidation and spin state (Fig. 5 and Supplementary Fig. 23). Remarkably, while the triplet state ³[4b] is less aromatic than the singlet ¹[4b], the quintet ⁵[4b] state shows enhanced aromaticity. The NICS map obtained for the triplet is essentially identical with the map obtained for the diindenofluorenyl radical ²[DIF-H], consistent with the weak interaction between DIF subunits in ³[4b] found in the PES scan. Thus, the enhancement of aromaticity in the singlet and quintet originates from the stronger inter-subunit coupling in each of these two spin states. This conclusion is supported by the harmonic oscillator model of aromaticity (HOMA, Supplementary Table 2), which produced significantly higher indexes of rings B in ⁵[4b] (0.89) and ¹[4b] (0.71) than in ³[4b] (0.63). The doubly charged ¹[4b]²⁺ and ¹[4b]^2–, show opposite changes of their magnetism, being respectively para- and diatropic. The tetraanion ¹[4b]^4– experiences a dramatic increase in the aromaticity of rings C–D, confirming that it can be treated as a union of four fluorenyl anions. In line with this interpretation, the HOMA indices for rings B, C, and D, are 0.76, 0.42 and 0.76.

Valence structure of TFF. A unified description of the valence structure of TFF, which is valid for all oxidation levels, can be developed using the five partial canonical structures A through E*** shown in Fig. 6. These structures differ in (a) the number of Clar sextets, (b) the number of formally non-bonding sites (denoted with an asterisk), and (c) the order of the linking bond (single or double). For instance, there are no Clar sextets and no non-bonding sites in structure A, whereas structure B* features three sextets and one non-bonding site, i.e., either a cation, a radical, or an anion. Valence structures of TFF can be constructed from pairs of partial canonical contributors with matching linking bond orders, e.g. A + A or B* + B*. Each such structure can be characterized by the total number of sextets (N_CS) and the number of unpaired electrons (N_UE). Given that the stability of the structures is expected to increase for high N_CS values and low N_UE values, one can consider a simple stability metric ΔN = N_CS – N_UE.

Data obtained for the neutral TFF indicate that the triplet state ³[4b] is well approximated by the diradical structure B^•–B^•, containing a single bond between the DIF units. Similarly, the tetraradicaloid form D^••=D^•• provides an accurate representation of the quintet ⁵[4b]. Mixing of these two contributions in ¹[4b] can be proposed to explain the intermediate aromaticity, inter-subunit bond order, and high tetraradicaloid character of the singlet state. Interestingly, B^•–B^• and D^••=D^•• have the highest ΔN = 4 among all canonical forms, which explains their relative importance. Doubly charged TFF ions ¹[4b]²⁺ and ¹[4b]^2– can be similarly characterized with singly bonded structures B⁺–B⁺ and B^––B^–, respectively, however, small contributions of the doubly bonded forms D^•–=D^•– and D^•+=D^•+ need to be invoked to explain the non-zero n_U values and non-vanishing inversion barriers of these two species. The latter two forms should become dominant in the structures of respective triplets, ³[4b]²⁺ and ³[4b]^2–, explaining the high ΔE_twist values predicted for these species. Analogous contributions become even more relevant in the singly charged ²[4b]⁺ and ²[4b]^– (D^••=D^•– and D^••=D^•+, respectively), both of which have significant triradicaloid character (n_U > 2.5). Finally, the doubly bonded contributors fully dominate in the higher anions, ²[4b]^3– and ¹[4b]^4–, which feature the highest ΔE_twist barriers, and very low n_U values.

In this work, we showed how the sterically frustrated alkene bond in tetrafluorenofulvalene is affected by the open-shell nature of the π-conjugated system. The underlying network of radicaloid sites in TFF is neither linear nor cyclic and features a unique doubly bifurcated topology with a centrally positioned formal double bond. The strength of the alkene bond is controlled by mixing of oligoradicaloid configurations in the neutral singlet state, by electron unpairing in high-spin states, and by electron transfer in the oxidized and reduced forms of tetrafluorenofulvalene. Changes of the oxidation level, spanning seven consecutive states, result in profound alteration of the spectroscopic signatures of this unusual π system. The pivotal role of the central alkene bond in controlling the π conjugation in tetrafluorenofulvalene suggests that, by using similar design principles, it may be possible to create molecular organic materials that will change their spin state, redox potentials, and optical characteristics in response to mechanical stimuli.

Data availability

All relevant data are available within the paper and its Supplementary Information files. Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition nos. 2250914 (4a·C₆H₁₄), 2209267 ([4a]²⁺[SbCl₆^–]₂), 2209269 ([Na(THF)₆][Na(THF)₅]_0.74[4a]), and 2209268 ([Na(THF)₃]₄[4a]). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/.

Acknowledgements

Financial support from the National Science Center of Poland (UMO-2019/35/B/ST4/00401 to M.S.) is gratefully acknowledged. Theoretical calculations were performed using the resources provided by the Wrocław Center for Networking and Supercomputing. D.-K.D. and P.M.Z. acknowledge the U.S. Department of Energy for support under DE-SC0022241. D.-K.D. additionally acknowledges the U.S. National Science Foundation for their support through the Graduate Research Fellowship under grant DGE 1841052 as well as the University of Michigan Rackham Graduate School and Chemistry Department for their support through internal fellowships.

Author contributions

B.P. resynthesized 4a and its radical cation and obtained and characterized the anionic states. M.A. developed the initial synthesis of 4a and characterized the neutral and oxidized states. D.-K. D. and P.M.Z. carried out the RAS calculations and analyzed the results. P.J.C. performed electrochemical and ESR experiments. T.L. performed XRD analyses. C.J.G.G. carried out the SQUID magnetic measurements. M.S. conceived and directed the project, carried out DFT calculations and analyzed all data. The manuscript was written by M.S. with contributions from all authors.

Competing financial interests

The authors declare no competing financial interests.

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Tetrafluorenofulvalene: A Sterically Frustrated Open-Shell Alkene

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