Structures and Physical Properties of Chemically Reduced

Article pubs.acs.org/Organometallics

Structures and Physical Properties of Chemically Reduced Diindenosiloles and Their π‑Extended Derivatives Ryo Takano,† Ryo Shintani,*,†,⊥ Aiko Fukazawa,§ Shigehiro Yamaguchi,*,§,∥ Koji Nakabayashi,‡ Shin-ichi Ohkoshi,*,‡ and Kyoko Nozaki*,† †

Department of Chemistry and Biotechnology, Graduate School of Engineering and ‡Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan § Department of Chemistry, Graduate School of Science and ∥Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan S Supporting Information *

ABSTRACT: Diindenosilole Si1, a silicon-bridged fulvalene derivative, was successfully reduced to its dianions using various alkali metals, and the structures were characterized by X-ray crystallographic analysis. Radical anions of Si1 as well as dianions of π-extended Si3 could also be synthesized, and the structural and physical properties were systematically compared. It was also found that the NMR spectra of dianions Si12− and Si32− show countercation and temperature dependency for their signal broadness, indicating the possible existence of thermal interconversion between closed-shell singlet states and open-shell triplet states.

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INTRODUCTION Bicyclopentadienylidene, commonly known as fulvalene or pentafulvalene, has been extensively investigated both theoretically and experimentally for more than half a century mainly due to the facile redox nature to and from its aromatic dianionic species.1 Indeed, the parent fulvalene is typically synthesized by the 2e− oxidation of its dianion,2 which is generated by double deprotonation of dihydrofulvalene in solution.3 However, although the redox process between fulvalene and its dianion is known to be reversible, 2e− reduction of fulvalene or its derivatives has been surprisingly rarely investigated. In fact, except for the direct reductive synthesis of dinuclear ruthenium complexes using substituted fulvalenes with Ru3(CO)12,4 only one example has been reported on the chemical reduction of substituted fulvalene by sodium metal to generate the corresponding dianion in solution, with no description on its structure and physical properties.5 Related studies, electrochemical reduction of quinoidal fused oligothiophenes6 and selenophene7 and reductive cyclization of 6,6-dialkynylfulvenes,8 have also been reported for in situ generation of dianionic species of fulvalene derivatives. With regard to the structural characterization of alkali metal salts of dianionic fulvalene derivatives, only a lithium salt of phenylene-bridged (Z)-1,1′-biindenylidene has been reported so far through reductive cyclization of tribenzocyclotriyne.9 The deficiency of detailed studies on 2e− reduction of fulvalene derivatives could be mainly due to the instability of both the starting fulvalenes and their dianionic species, despite the widespread description of reversible redox properties between fulvalene and its dianion in many organic chemistry textbooks. In addition to 2e− redox between fulvalene and its dianion, the intermediate radical anion is also a subject of interest to © XXXX American Chemical Society

understand the redox process, but only one report on the generation and ESR characterization of fulvalene radical anion in solution has been made with no structural investigation.10 For fulvalene derivatives, redox behaviors of quinoidal fused oligothiophenes including their radical anions have been reported based on the spectroscopic analyses in situ.6 As a new structural motif of silicon-bridged π-conjugated compounds, we recently developed a synthetic method of a series of thermally stable ladder-type fused oligosiloles possessing a quinoidal structure capped with indenylidene termini by using a rhodium-catalyzed stitching reaction (Scheme 1).11 These compounds can also be described as diindenosiloles (or benzo-fused silicon-bridged fulvalenes) and their π-extended derivatives, and they were found to show reversible 2e− reduction properties by cyclic voltammetry (CV) analysis.11a In this context, herein we describe our investigation on the structures and physical properties of reduced forms of Scheme 1. Recently Developed Rhodium-Catalyzed Stitching Reaction to Give Quinoidal Fused Oligosiloles

Special Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: April 7, 2017

A

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between neutral Si1 and Li2Si1, indicating that the primary role of the silicon moiety is to keep the molecules in plane without significant electronic effects.11b Compound Si1 could also be reduced by other alkali metals such as sodium and potassium, and corresponding products Na2Si1 and K2Si1 were isolated in 76 and 98% yield, respectively. As shown in Figure 2, the crystal packing structure of Na2Si1 turned out to be quite different from that of Li2Si1.12 Two sodium ions are intercalated between two Si12− with no coordination of THF molecules to create a one-dimensional columnar structure. More precisely, sodium ions between two antiparallel Si12− are sandwiched by two cyclopentadienyl moieties with Na−Cp centroid = 2.37 Å, whereas sodium ions between two tilted Si12− (by ca. 32°) are coordinated by one cyclopentadienyl ring with Na−Cp centroid = 2.36 Å and one benzene ring with Na−benzene centroid = 2.40 Å. As summarized in Table 1, the structure of dianion Si12− of Na2Si1 was found to be almost identical to that of Li2Si1 in terms of each bond distance and the overall planarity (angle of two benzene mean planes of M2Si1: 2.2° for M = Li and 8.5° for M = Na). The crystal structure of K2Si1 was also obtained in the form of a one-dimensional column consisting of K(thf)· K·Si1 (Figure 3).12 Unlike Li2Si1 and Na2Si1, dianion Si12− of K2Si1 is significantly distorted (angle of two benzene mean planes = 25.5°), and two potassium ions sandwiched by the “convex” face of two Si12− are coordinated by two cyclopentadienyl rings with K−Cp centroid = 2.83 and 2.87 Å and one THF molecule with K−O = 2.72 Å. However, two potassium ions between the “concave” face of two Si12− are coordinated by one cyclopentadienyl ring with K−Cp centroid = 2.78 Å and by ring-fusion moiety of the indenyl structure with K−center of ring fusion = 2.78 Å. Despite the distorted structure of Si12−, the bond distances within the dianion are quite similar to those for Li2Si1 and Na2Si1 (Table 1). To gain further understanding of the structural features of discrete dianion Si12−, we treated K2Si1 with [2.2.2]cryptand to give quantitatively [K([2.2.2]cryptand)]2Si1 (eq 2). The X-ray

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RESULTS AND DISCUSSION Synthesis and Structures of Anionic Species of Si1. Synthesis and Structures of M2Si1. Initially, we employed diindenosilole Si1 as the substrate and treated it with 4.3 equiv of lithium in THF at room temperature under inert atmosphere (eq 1). The original orange solution of Si1 immediately turned

dark brown within a minute and gradually changed to dark green and then to dark red. The desired anionic species Li2Si1 was isolated in 85% yield as an air-sensitive red solid. The structure of Li2Si1 was confirmed by X-ray crystallographic analysis in the form of dimeric [Li(thf)2·Li·Si1]2 (Figure 1).12

Figure 1. X-ray crystal structure of [Li(thf)2·Li·Si1]2. Hydrogen atoms are omitted for clarity.

structure of this compound revealed that potassium ions were trapped by [2.2.2]cryptand as expected and that dianion Si12− was not coordinated by the metal cations (Figure 4).12 As shown in Table 1, the bond distances of this dianion were essentially the same as the ones observed for metal-coordinated M2Si1 (M = Li, Na, and K) described above with the angle of two benzene mean planes = 8.4°. These structural data were well-reproduced by the DFT calculation of isolated dianion Si12− as shown in Table 1. Synthesis and Structures of MSi1. In addition to dianion Si12−, we could synthesize the corresponding radical anion Si1•− by controlling the stoichiometry of alkali metal. For example, the reaction of Si1 (1.1 equiv) with 1.0 equiv of potassium in THF at room temperature gave NMR-silent and ESR-active KSi1 in 37% isolated yield (eq 3 and Figure 5). The observed hyperfine splitting patterns by the hydrogen nuclear spins in the ESR spectrum could be well-reproduced by the

Two lithium ions are sandwiched by two antiparallel dianions Si12− through coordination by the cyclopentadienyl rings (Li− Cp centroid = 1.96 and 2.02 Å). The other two lithium ions are located on the outer side of a cyclopentadienyl ring of each Si12− (Li−Cp centroid = 2.01 Å) with coordination of two THF molecules to each ion (Li−O = 1.94−1.96 Å). Regarding the structure of dianion Si12−, compared with that of neutral Si1, double bond character of C2−C3, C4−C5, and C6−C7 is significantly reduced, and C−C bond distances within cyclopentadienyl rings are 1.42−1.47 Å with C4−C5 = 1.48 Å (Table 1). In contrast, C10−C11 and C12−C13 bond distances are 1.37 Å, whereas C9−C10, C11−C12, and C1− C13 bond distances are 1.43 Å, displaying significant bond alternation of the fused benzene rings. It is also worth noting that the Si−C3 and Si−C6 bond distances are quite similar B

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Table 1. Selected Bond Distances (Å) for Si1,11a M2Si1 (M = Li, Na, K), [K([2.2.2]cryptand)]2Si1, and [K([2.2.2]cryptand)] Si1, and Comparison with Calculated Values

C1−C2 C2−C3 C3−C4 C4−C5 C5−C6 C6−C7 C7−C8 C4−C9 C1−C9 C9−C10 C10−C11 C11−C12 C12−C13 C1−C13 Si−C3 Si−C6

Si1

Li2Si1

Na2Si1

K2Si1

[K([2.2.2]cryptand)]2Si1

Si12−a

[K([2.2.2]cryptand)]Si1b

Si1•−c

1.476(2) 1.356(2) 1.487(2) 1.368(2) 1.486(2) 1.359(2) 1.476(2) 1.467(2) 1.417(2) 1.391(2) 1.395(2) 1.386(2) 1.394(2) 1.387(2) 1.876(2) 1.876(2)

1.424(2) 1.432(2) 1.449(2) 1.481(2) 1.443(2) 1.417(2) 1.420(2) 1.430(2) 1.465(2) 1.426(2) 1.369(2) 1.427(2) 1.368(2) 1.427(2) 1.877(2) 1.870(2)

1.421(4) 1.423(4) 1.445(4) 1.479(4) 1.446(4) 1.417(4) 1.426(4) 1.426(4) 1.468(4) 1.426(4) 1.382(4) 1.422(4) 1.377(4) 1.416(4) 1.873(3) 1.878(3)

1.421(7) 1.418(6) 1.432(6) 1.474(6) 1.450(6) 1.420(6) 1.426(7) 1.425(6) 1.473(6) 1.431(7) 1.378(7) 1.401(8) 1.388(7) 1.415(6) 1.869(5) 1.867(5)

1.413(4) 1.418(4) 1.446(4) 1.475(4) 1.441(4) 1.419(4) 1.412(4) 1.424(4) 1.464(4) 1.414(4) 1.387(4) 1.407(5) 1.378(4) 1.408(4) 1.864(3) 1.862(3)

1.418 1.418 1.440 1.481 1.440 1.418 1.418 1.425 1.476 1.416 1.393 1.424 1.394 1.413 1.861 1.861

1.444(7) 1.393(7) 1.461(7) 1.426(7) 1.461(6) 1.385(7) 1.439(7) 1.444(7) 1.448(7) 1.400(7) 1.392(7) 1.400(7) 1.388(7) 1.405(7) 1.877(5) 1.873(5)

1.445 1.389 1.461 1.429 1.461 1.389 1.445 1.446 1.448 1.407 1.396 1.410 1.397 1.403 1.872 1.872

a c

Calculated at the B3LYP/6-31G+(d) level of theory as closed-shell singlet state under vacuum. bValues for one out of two independent molecules. Calculated at the B3LYP/6-31G+(d) level of theory under vacuum.

Figure 3. X-ray crystal structure of [K(thf)·K·Si1]n. Hydrogen atoms are omitted for clarity.

Figure 2. X-ray crystal structure of [Na2Si1]n. Hydrogen atoms are omitted for clarity.

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Figure 6. X-ray crystal structure of [K([2.2.2]cryptand)]Si1. Hydrogen atoms are omitted for clarity.

well-reproduced by the DFT calculation of isolated radical anion Si1•−. Physical Properties of Anionic Species of Si1. With a series of anionic species derived from Si1 in hand, we decided to compare the properties of Si1, Si1•−, and Si12− computationally as well as experimentally. The calculated frontier orbitals for these species are shown in Figure 7. As we

Figure 4. X-ray crystal structure of [K([2.2.2]cryptand)]2Si1·THF. Hydrogen atoms are omitted for clarity.

Figure 7. Calculated frontier orbital profiles of Si1 (left), Si1•− (middle), and Si12− (right) at the B3LYP/6-311++G(d,p) level of theory under vacuum. The geometry was optimized at the B3LYP/631G+(d) level of theory. Only the α orbital profiles are shown for Si1•−.

Table 2. NICS(1) Values for Si1, Si1•−, and Si12− Figure 5. Observed ESR spectrum of KSi1 in 2-methyltetrahydrofuran at 300 K (top) and simulated ESR spectrum with g = 2.00268, aH = 10.45 MHz (6H), aH = 5.05 MHz (2H), aH = 3.65 MHz (2H), aH = 1.96 MHz (2H), and aH = 1.55 MHz (2H) (bottom).

simulation as shown in the bottom of Figure 5. For the structural characterization, we also prepared [K([2.2.2]cryptand)]Si1 by conducting a similar reaction in the presence of [2.2.2]cryptand (eq 4), and we could successfully obtain its

A ring B ring

Si1

Si1•−

Si12−

−9.01 −0.32

−7.72 −5.63

−6.21 −11.3

previously reported,11a the HOMO of neutral Si1 lies on the πorbitals of the conjugated system, whereas the LUMO is mainly on the π*-orbitals. Introduction of one electron to Si1 gives radical anion Si1•−, and the orbital profile of its SOMO resembles that of LUMO for neutral Si1. Addition of one more electron then leads to dianion Si12−, and its HOMO also has a profile similar to that of LUMO for neutral Si1.15 To evaluate the distribution of electrons in Si1•− and Si12−, NICS(1) values were calculated for the terminal benzene ring and the fused cyclopentadienyl ring of Si1, Si1•−, and Si12−. As shown in Table 2, aromaticity of the cyclopentadienyl ring significantly increases in the order of Si1, Si1•−, and Si12−, and aromaticity of the benzene ring gradually decreases in the same order, indicating that aromatic stabilization of the cyclopentadienyl moieties is the key for the acceptance of electrons

X-ray crystal structure,12 representing the first structural characterization of a fulvalene radical anion derivative.13 As shown in Figure 6, the potassium ion was again trapped by [2.2.2]cryptand with no coordination to Si1•−. We note that almost all the C−C bond distances within the diindenosilole framework were found to be between those of neutral Si1 and dianion Si12− as summarized in Table 1.14 These data were also D

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Organometallics in the event of reduction of Si1. This trend was also supported by the electrostatic potential maps of these compounds (Figure 8). Thus, the negative potentials mainly reside on the benzene

Figure 9. Aromatic region of variable-temperature 1H NMR spectra of Li2Si1 in THF-d8. Figure 8. Electrostatic potential maps for Si1, Si1•−, and Si12−.

rings for neutral Si1, and more delocalization toward the adjacent cyclopentadienyl rings is observed for radical anion Si1•− with lower potentials in overall. For dianion Si12−, cyclopentadienyl moieties show even lower potentials, which is consistent with the observed high aromaticity of these rings. The countercation of Si12− (Li+, Na+, K+, or K+([2.2.2]cryptand)) not only affects the crystal packing structure as discussed in the previous section, but also influences the UV− vis absorption spectrum in the solution state, although the coordination modes under dilute conditions have not been investigated in detail. (See the Supporting Information for details.16) As also mentioned in the previous section, radical anions KSi1 and [K([2.2.2]cryptand)]Si1 are NMR-silent due to the presence of an unpaired electron, and this was confirmed by the ESR spectrum of KSi1 (Figure 5). For dianion Si12−, we found that sharpness of the NMR signals depends on the countercation as well as on the temperature. Thus, relatively sharp 1H NMR signals were observed for Li2Si1 in THF-d8 at 313 K, but the signals became gradually broader by lowering the temperature as shown in Figure 9.17 However, Na2Si1 and K2Si1 showed the opposite temperature dependency. Thus, both Li2Si1 and Na2Si1 displayed similarly broad 1H NMR signals at room temperature, but unlike Li2Si1, the signals of Na2Si1 became broader by raising the temperature and sharper by lowering the temperature (Figure 10). For K2Si1, we could not observe 1H NMR signals at room temperature except for the broadened peaks corresponding to isopropyl groups on the silicon. Other peaks started to appear by lowering the temperature, and the 1H NMR spectrum at 183 K showed sharp signals (Figure 11). Furthermore, no significant signals could be observed for [K([2.2.2]cryptand)]2Si1 in the aromatic region regardless of the monitored temperatures between 193 and 293 K. A possible origin of these observations is the existence of thermal interconversion between closed-shell singlet state and open-shell triplet state for Si12−, although we do not have conclusive evidence at this stage.18,19 At the least, the countercation and temperature dependency of Na2Si1 and K2Si1 observed here is in accordance with that of benzenoid polycyclic dianions reported in the literature,

Figure 10. Aromatic region of variable-temperature 1H NMR spectra of Na2Si1 in THF-d8.

Figure 11. Aromatic region of variable-temperature 1H NMR spectra of K2Si1 in THF-d8.

which was attributed by ESR analysis to the existence of thermally accessible excited triplet states for dianions of singlet ground states.17a In contrast, the opposite temperature E

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Table 3. Selected Bond Distances (Å) for Si311a and Li2Si3 and Comparison with Calculation Values

dependency observed for Li2Si1 is quite unusual and needs further investigation for detailed understanding. Synthesis, Structures, and Properties of M2Si3. Generation of dianionic species is not limited to Si1 and could be extended to longer π-conjugation systems as well. Thus, treatment of Si3 with 8.4 equiv of lithium in THF at room temperature gave Li2Si3 in 76% yield (eq 5). The

C1−C2 C2−C3 C3−C4 C4−C5 C5−C6 C6−C7 C7−C8 C8−C9 C9−C10 C10−C11 C11−C12 C4−C13 C1−C13 C13−C14 C14−C15 C15−C16 C16−C17 C1−C17

structure was confirmed by X-ray crystallographic analysis in the form of [Li(thf)2]2Si3 as shown in Figure 12.12 Two lithium

a

Si3

Li2Si3

Si32−a

1.473(2) 1.361(2) 1.482(2) 1.373(2) 1.478(2) 1.375(2) 1.478(2) 1.373(2) 1.482(2) 1.361(2) 1.473(2) 1.466(2) 1.416(2) 1.387(2) 1.396(2) 1.383(3) 1.387(3) 1.386(2)

1.433(5) 1.410(4) 1.444(4) 1.448(4) 1.385(4) 1.473(4) 1.397(4) 1.464(4) 1.441(4) 1.403(4) 1.433(5) 1.420(4) 1.456(4) 1.417(5) 1.367(5) 1.420(5) 1.374(5) 1.407(5)

1.431 1.405 1.444 1.454 1.401 1.470 1.401 1.454 1.444 1.405 1.431 1.429 1.462 1.413 1.393 1.419 1.395 1.409

Calculated at the B3LYP/6-31G+(d) level of theory.

Figure 12. X-ray crystal structure of Li2(thf)4Si3. Hydrogen atoms are omitted for clarity.

ions are coordinated by the cyclopentadienyl rings (Li−Cp centroid = 1.97 and 2.04 Å) along with coordination of two THF molecules to each ion (Li−O = 1.93−2.05 Å). Regarding the structure of dianion Si32−, terminal indenyl moieties are quite similar to those of Li2Si1, and the central five-membered silacycle is closer to a regular silole structure, rather than a quinoidal structure observed for neutral Si3, based on the change of bond distances of C5−C6 and C7−C8 from 1.48 Å to 1.38−1.40 Å and C6−C7 from 1.38 to1.47 Å (Table 3). The overall crystal structure of dianion Si32− was also in good agreement with that obtained by the DFT calculation of isolated Si32−. Reduction of Si3 was successfully achieved by using potassium as well to give K2Si3 in 92% yield, although we have not obtained its X-ray crystal structure. As was the case for M2Si1, the sharpness of the 1H NMR signals of M2Si3 was found to be dependent on the alkali metals. Sharp signals could be observed for Li2Si3 at 293 K, which gradually became broader by lowering the temperature (Figure 13).17 In contrast, K2Si3 showed the opposite temperature dependency. Thus, sharp 1H NMR signals were observed at a low temperature such as 233 K, and the signals became broader by raising the temperature as shown in Figure 14. The reversal of temperature dependency between Li2Si3 and K2Si3 matches with the trend observed for Li2Si1 and K2Si1 described above.

Figure 13. Aromatic region of variable-temperature 1H NMR spectra of Li2Si3 in THF-d8.

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CONCLUSIONS We successfully reduced diindenosilole Si1, a silicon-bridged fulvalene derivative, to form its dianions using various alkali metals and structurally characterize them by X-ray crystallographic analysis. Radical anions of Si1 as well as dianions of πextended Si3 could also be synthesized and the structural and physical properties were systematically compared. The NMR spectra of dianions Si12− and Si32− were found to show countercation and temperature dependency for their signal broadness, indicating the possibility of existence of thermal interconversion between closed-shell singlet states and openshell triplet states, although further investigations are still F

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Procedure for Equation 3. KSi1. A mixture of Si1 (18.4 mg, 49.9 μmol) and potassium (1.8 mg, 46 μmol) in THF (2.0 mL) was stirred for 24 h at room temperature. The resulting mixture was filtered through a PTFE membrane (pore size: 0.20 μm) and concentrated under vacuum. The residue was recrystallized from THF/pentane to afford KSi1 as a brown solid (6.8 mg, 17 μmol; 37% yield). Elemental analysis was unsuccessful due to the high instability against air and moisture. Procedure for Equation 4. [K([2.2.2]cryptand)]Si1. A mixture of Si1 (18.4 mg, 49.9 μmol), [2.2.2]cryptand (22.7 mg, 60.3 μmol), and potassium (2.4 mg, 62 μmol) in THF (2.0 mL) was stirred for 20 h at room temperature. The resulting mixture was filtered through a PTFE membrane (pore size: 0.20 μm) and concentrated under vacuum. The residue was recrystallized from THF/hexane to afford [K([2.2.2]cryptand)]Si1 as a dark green solid (27.2 mg, 35.0 μmol; 70% yield). Elemental analysis was unsuccessful due to the high instability against air and moisture. Procedures for Equation 5. Li2Si3. A mixture of Si3 (16.6 mg, 25.7 μmol) and lithium (1.5 mg, 0.22 mmol) in THF (2.0 mL) was stirred for 46 h at room temperature. The resulting mixture was filtered through a PTFE membrane (pore size: 0.20 μm) and concentrated under vacuum. The residue was recrystallized from THF/pentane to afford Li2(thf)1.5Si3 as a dark blue solid (15.2 mg, 19.6 μmol (99 wt % pure with pentane); 76% yield). 1 H NMR (THF-d8, 20 °C): 7.42 (d, 3JHH = 7.8 Hz, 2H), 7.00 (d, 3 JHH = 7.7 Hz, 2H), 6.20 (t, 3JHH = 6.9 Hz, 2H), 6.09 (t, 3JHH = 6.9 Hz, 2H), 3.73−3.54 (m, 6H), 2.49 (s, 6H), 1.89−1.74 (m, 8H), 1.45−1.24 (m, 16H), 1.19 (d, 3JHH = 7.2 Hz, 12H), 0.92 (d, 3JHH = 5.7 Hz, 12H). 13 C NMR (THF-d8, 25 °C): δ 150.0, 146.2, 133.8, 129.2, 126.9, 126.5, 118.8, 117.3, 112.6, 110.0, 109.9, 68.0, 26.2, 21.2, 20.8, 20.4, 15.8, 14.3, 13.6. K2Si3. A mixture of Si3 (16.2 mg, 25.1 μmol) and potassium (2.5 mg, 64 μmol) in THF (2.0 mL) was stirred for 46 h at room temperature. The resulting mixture was filtered through a PTFE membrane (pore size: 0.20 μm) and concentrated under vacuum. The residue was recrystallized from THF/pentane to afford K2Si3 as a blue solid (17.3 mg, 23.2 μmol (97 wt % pure with pentane); 92% yield). 1 H NMR (THF-d8, −75 °C): 7.44 (d, 3JHH = 8.0 Hz, 2H), 7.10 (d, 3 JHH = 7.8 Hz, 2H), 6.33 (t, 3JHH = 7.1 Hz, 2H), 6.21 (t, 3JHH = 7.0 Hz, 2H), 2.51 (s, 6H), 1.88−1.75 (m, 2H), 1.45−1.24 (m, 16H), 1.20 (d, 3 JHH = 7.0 Hz, 12H), 0.99−0.77 (m, 12H). 13C NMR (THF-d8, −60 °C): δ 149.6, 147.2, 133.4, 127.9, 126.4, 126.2, 118.1, 117.7, 114.0, 111.5, 111.1, 21.2, 20.8, 20.4, 15.9, 14.3, 13.4.

Figure 14. Aromatic region of variable-temperature 1H NMR spectra of K2Si3 in THF-d8.

necessary. The present studies can also be regarded as the first systematic investigations on the preparation and the properties of anionic fulvalene derivatives.

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EXPERIMENTAL SECTION

Procedures for Equation 1. Li2Si1. A mixture of Si1 (18.4 mg, 49.9 μmol) and lithium (1.5 mg, 0.22 mmol) in THF (2.0 mL) was stirred for 44 h at room temperature. The resulting mixture was filtered through a PTFE membrane (pore size: 0.20 μm) and concentrated under vacuum. The residue was recrystallized from THF/pentane to afford Li2(thf)1.5Si1 as a red solid (21.2 mg, 42.4 μmol (98 wt % pure with pentane); 85% yield). 1 H NMR (THF-d8, 25 °C): 7.91 (d, 3JHH = 6.9 Hz, 2H), 7.13 (d, 3 JHH = 6.8 Hz, 2H), 6.31 (bs, 2H), 6.21 (bs, 2H), 3.70−3.57 (m, 6H), 2.47 (s, 6H), 1.85−1.73 (m, 6H), 1.42−1.22 (m, 2H), 1.22−1.02 (m, 12H). 13C NMR (THF-d8, 25 °C): δ 129.2, 123.4, 121.6, 119.3, 119.0, 113.7, 112.2, 106.0, 68.0, 26.2, 20.4, 14.7, 13.9. Na2Si1. A mixture of Si1 (18.4 mg, 49.9 μmol) and sodium (3.0 mg, 0.13 mmol) in THF (2.0 mL) was stirred for 45 h at room temperature. The resulting mixture was filtered through a PTFE membrane (pore size: 0.20 μm) and concentrated under vacuum. The residue was recrystallized from THF/pentane to afford Na2Si1 as a red solid (15.8 mg, 38.1 μmol; 76% yield). 1 H NMR (THF-d8, −80 °C): 7.77 (d, 3JHH = 8.2 Hz, 2H), 7.04 (d, 3 JHH = 8.0 Hz, 2H), 6.21 (t, 3JHH = 7.0 Hz, 2H), 6.06 (t, 3JHH = 7.0 Hz, 2H), 2.52 (s, 6H), 1.31−1.05 (m, 14H). 13C NMR (THF-d8, −80 °C): δ 131.6, 127.0, 120.0, 119.4, 118.8, 118.2, 112.2, 109.8, 106.9, 20.8, 15.6, 14.6. K2Si1. A mixture of Si1 (18.5 mg, 50.2 μmol) and potassium (4.8 mg, 0.12 mmol) in THF (2.0 mL) was stirred for 42 h at room temperature. The resulting mixture was filtered through a PTFE membrane (pore size: 0.20 μm) and concentrated under vacuum. The residue was recrystallized from THF/hexane to afford K2Si1 as a red solid (22.0 mg, 49.2 μmol; 98% yield). 1 H NMR (THF-d8, −80 °C): 7.75 (d, 3JHH = 8.2 Hz, 2H), 7.01 (d, 3 JHH = 7.9 Hz, 2H), 6.18 (t, 3JHH = 7.1 Hz, 2H), 6.03 (t, 3JHH = 7.0 Hz, 2H), 2.49 (s, 6H), 1.37−1.19 (m, 2H), 1.19−1.00 (m, 12H). 13C NMR (THF-d8, −80 °C): δ 131.6, 126.7, 123.6, 120.5, 118.7, 118.1, 112.1, 109.4, 106.7, 20.7, 15.4, 14.3. Procedure for Equation 2. [K([2.2.2]cryptand)]2Si1. A mixture of K2Si1 (11.3 mg, 25.3 μmol) and [2.2.2]cryptand (20.2 mg, 53.7 μmol) in THF (2.0 mL) was stirred for 22 h at room temperature. The resulting mixture was diluted with pentane (2.0 mL), and the supernatant was removed by decantation. The remaining precipitate was washed with pentane and dried under vacuum to afford a dark green solid (30.2 mg, 25.2 μmol; 100% yield). Only broad NMR spectra could be obtained even at −80 °C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00260. UV−vis absorption and emission spectra, theoretical calculations, and NMR spectra (PDF) Calculated structural data (MOL) Accession Codes

CCDC 1538081−1538086 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, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail: G

[email protected]. [email protected]. [email protected]. [email protected]. DOI: 10.1021/acs.organomet.7b00260 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics ORCID

Rogachev, A. Y.; Petrukhina, M. A. Angew. Chem., Int. Ed. 2011, 50, 8090−8094. (e) Amsharov, K. Y.; Krämer, Y.; Jansen, M. Angew. Chem., Int. Ed. 2011, 50, 11640−11643. (f) Zabula, A. V.; Spisak, S. N.; Filatov, A. S.; Grigoryants, V. M.; Petrukhina, M. A. Chem. - Eur. J. 2012, 18, 6476−6484. (g) Zabula, A. V.; Sumner, N. J.; Filatov, A. S.; Spisak, S. N.; Grigoryants, V. M.; Petrukhina, M. A. Eur. J. Inorg. Chem. 2012, 4675−4683. (h) Rudebusch, G. E.; Espejo, G. L.; Zafra, J. L.; Peña-Alvarez, M.; Spisak, S. N.; Fukuda, K.; Wei, Z.; Nakano, M.; Petrukhina, M. A.; Casado, J.; Haley, M. M. J. Am. Chem. Soc. 2016, 138, 12648−12654. (14) A similar trend was suggested for quinoidal fused oligothiophenes by theoretical calculation. See ref 6a. (15) Dianion Si12− was calculated as closed-shell singlet state. Calculation as open-shell triplet state under vacuum gives higher energy. See the Supporting Information for details. (16) For an example of counter cation effects on UV−vis absorption spectra in solution, see Araki, T.; Wakamiya, A.; Mori, K.; Yamaguchi, S. Chem. - Asian J. 2012, 7, 1594−1603. (17) The changes of chemical shifts at different temperatures may be due to the temperature-dependent aggregation, complexation, and/or solvation, but we do not have any conclusion at this stage. Similar observations were reported in lithium salt of 1,2-benzanthracene dianion and sodium salt of spirosilabifluorene dianion: (a) Minsky, A.; Meyer, A. Y.; Poupko, R.; Rabinovitz, M. J. Am. Chem. Soc. 1983, 105, 2164−2172. (b) Zabula, A. V.; Rogachev, A. Y.; Guzei, I. A.; West, R. Organometallics 2013, 32, 3760−3768. (18) For examples of ESR active dianions due to a thermally accessible triplet state: (a) Ref 17a. (b) Ebata, K.; Setaka, W.; Inoue, T.; Kabuto, C.; Kira, M.; Sakurai, H. J. Am. Chem. Soc. 1998, 120, 1335−1336. (c) Gherghel, L.; Brand, J. D.; Baumgarten, M.; Müllen, K. J. Am. Chem. Soc. 1999, 121, 8104−8105. (d) Eisenberg, D.; Quimby, J. M.; Ho, D.; Lavi, R.; Benisvy, L.; Scott, L. T.; Shenhar, R. Eur. J. Org. Chem. 2012, 2012, 6321−6327. (19) For examples of dianions with NMR signal broadening assigned as a thermally accessible triplet state: (a) Baumgarten, M.; Gherghel, L.; Wagner, M.; Weitz, A.; Rabinovitz, M.; Cheng, P.-C.; Scott, L. T. J. Am. Chem. Soc. 1995, 117, 6254−6257. (b) Aprahamian, I.; Preda, D. V.; Bancu, M.; Belanger, A. P.; Sheradsky, T.; Scott, L. T.; Rabinovitz, M. J. Org. Chem. 2006, 71, 290−298. (c) Ref 17b. (d) Spisak, S. N.; Zabula, A. V.; Filatov, A. S.; Petrukhina, M. A. J. Organomet. Chem. 2015, 784, 69−74.

Ryo Shintani: 0000-0002-3324-9393 Shigehiro Yamaguchi: 0000-0003-0072-8969 Shin-ichi Ohkoshi: 0000-0001-9359-5928 Kyoko Nozaki: 0000-0002-0321-5299 Present Address ⊥

Division of Chemistry, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support has been provided in part by a Grant-in-Aid for Challenging Exploratory Research (15K13688, the MEXT, Japan) and the Asahi Glass Foundation.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00260 Organometallics XXXX, XXX, XXX−XXX