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Reversible Isomerizations between 1,4-Digermabenzenes and 1,4Digerma-Dewar-Benzenes: Air-stable Activators for Small Molecules Tomohiro Sugahara, Jing-Dong Guo, Daisuke Hashizume, Takahiro Sasamori, and Norihiro Tokitoh J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00129 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 1, 2019

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Journal of the American Chemical Society

Reversible Isomerizations between 1,4-Digermabenzenes and 1,4Digerma-Dewar-Benzenes: Air-stable Activators for Small Molecules Tomohiro Sugahara,1 Jing-Dong Guo,1,2 Daisuke Hashizume,3 Takahiro Sasamori,4* and Norihiro Tokitoh1,2 1

Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.

2

Integrated Research Consortium on Chemical Sciences, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.

3

RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

4

Graduate School of Natural Sciences, Nagoya City University, Yamanohata 1, Mizuho-cho, Mizuho-ku, Nagoya, Aichi 467-8501, Japan.

Supporting Information Placeholder ABSTRACT: The first examples of stable, crystalline,

and air-sensitive 1,4-digermabenzenes were isolated. These species photochemically isomerize into the corresponding air-stable digerma-Dewar-benzenes. More importantly, alkyl-substituted Dewar-type-1,4digermabenzenes can be considered as reversible “air-stable activators” for small molecules such as dihydrogen, carbon dioxide, and acetylene at room temperature. The regeneration of these activators can be accomplished via a thermal retroisomerization that afford the corresponding 1,4digermabenzenes.

ty toward self-oligomerization and/or valenceisomerization,4 kinetic stabilization can effectively permit the isolation of stable metalla- and 1,2dimetallabenzenes (Chart 1).4-7 Tbb Ge

in-situ interconversion

SiMe3

very high reactivity H–H, C=C, etc.

Ge Ge air/moisture sensitive not shelf-stable R R

activation of SiMe3 small molecules

Me 3Si

Classic aromatic compounds, i.e., [4n+2]πelectron cyclic conjugated systems such as benzene are widely known as intriguing chemical species with unusual electric, magnetic, and optical properties that have been applied to pharmaceuticals, organic electronic materials, and fluorescence probes.1 In contrast to the wealth of physical functionality of benzene, its chemical functionality is relatively limited due to considerable aromatic stabilization energy.2 However, replacing a C-H moiety of benzene with a main-group moiety (group 13/15 elements) significantly affects the physical and chemical properties of such cyclic-π-conjugated systems.3 Therefore, we are interested in the replacement of C–H or HC=CH moieties with moieties that contain heavier group-14 elements (E–R or RE=ER; E = Si, Ge, Sn, or Pb; R = organic substituent) in order to control the physical and chemical properties of the resulting metalla- and 1,2-dimetallabenzenes.4-7 Although the isolation of the heavier-element analogues of benzene is difficult due to their extremely high propensi-

R

Ge Tbb very high reactivity

Ge Tbb air/moisture stable “shelf-stable” Me 3Si

R Ge Ge

Tbb Ge

previously reported main-group-element-based activators of small molecules

tBu Tbb

Chart 1. Potential small-molecule activators containing low-coordinated Ge atoms such as digermynes, 1,2-digermabenzenes, and 1,4-digermabenzenes. Tbb Ge

HC

Ge 1

Tbb PhC

Ph

CPh

C H H C Ge Ge Ge–C insertion Tbb 4 Tbb H C

C H Ge–Ge insertion

Ph Tbb Ge

Ge Ge Tbb

CH

6

Tbb

Ge Tbb 5

Ge Ge Tbb

2

Tbb

Tbb HC

Ge

CH

Ge Tbb

3

Scheme 1. Synthesis and potential formation of digermabenzenes. Although hitherto isolated 1,2dimetallabenzenes exhibit considerable aromaticity,6-8 they also show extremely high reactivity toward

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air, moisture, and alkynes. Accordingly, not only the physical, but also the chemical properties of metallaand 1,2-dimetallabenzenes should be of great interest. While investigating the reactivity of the isolable digermyne TbbGeºGeTbb (1; Tbb = 2,6[CH(SiMe3)2]-4-tBu-phenyl),7 we observed that the reaction of 1 with acetylene resulted in the predominant formation of 1,2-digermabenzene 2 together with small amounts of 9,10-digermabarrelene 3 (Scheme 1).7b The mechanism for the formation of 1,2-dimetallabenzenes has been investigated on the basis of theoretical calculations,6c,7b whereby 1,2digermacyclobutadiene 4 was identified as a likely key intermediate for the final products (2 and 3). The Ge–C insertion of acetylene in 4, initiated by a ºCH···Ge interaction, was calculated to afford 2 via a pathway with a slightly smaller barrier relative to that of a Ge=Ge insertion of acetylene in 4 triggered by the CºC(π)···Ge interaction that gives 1,4digermabenzene 5, followed by a facile further [4+2]cycloaddition with acetylene to furnish 3. This proposed mechanism was experimentally corroborated by the isolation of 1,2-digermacyclobutadiene 6 from the reaction of 1 with PhCºCPh.7b,9 The aforementioned results of the theoretical calculations led us to speculate that the insertion of alkynes in the Ge=Ge bond of 4 could be an appropriate synthetic route to unprecedented 1,4-digermabenzenes.10 While 1,2-dimetallabenzenes can be obtained from the reaction of stable dimetallynes11 with alkynes,6,7 1,3- and 1,4-dimetallabenzenes have not yet been isolated.12,13 The generation of a 1,4-silabenzene from the photochemical isomerization of a 1,4-disilaDewar-benzene has been reported.14 The intriguing nature of the transiently generated 1,4-disilabenzene was demonstrated by e.g. trapping with butadiene and benzene. However, further investigations on its chemical/physical properties, its aromaticity, and its behavior in the activation of small molecules have not been carried out, most likely due to the instability of the photo-generated species.15 Herein, we report the synthesis of stable 1,4-digermabenzenes, their photochemical isomerization reactions, and their behavior in the context of main-groupelement-based activation of small molecules. Stirring a mixture of tolan and 1,2digermacyclobutadiene 6, which was generated by the reaction of digermyne 1 with tolan, for 3 days in C6D6 at 60 ˚C quantitatively afforded the corresponding tetraphenyl-1,4-digermabenzene 7 (20% isolated yield)16 (Scheme 2). The alkyl-substituted 1,4-digermabenzene 8 was obtained directly from digermyne 1. Treatment of a C6D6 solution of digermyne 1 with an excess of 3-hexyne at room tem-

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perature quantitatively afforded 1,4-digermabenzene 8. All attempts to isolate and/or observe the intermediate were unsuccessful, most likely due to the facile formation of 8. Single crystals of 7 and 8 (vide infra) exhibit a crystallographic center of symmetry. However, due to the severe disorder in the crystals, we could not draw meaningful conclusions on whether the symmetry actually reflects the intrinsic electron density or if it is due to pseudocrystallographic symmetry. In order to circumvent this obstacle, unsymmetrically-substituted 1,4digermabenzene 9 was synthesized in 16% isolated yield from the reaction of the isolated 1,2digermacyclobutadiene 6 with an excess of 2-hexyne in hexane at room temperature. R Tbb Ge

RC

Ge Tbb 1

PhC

CPh

CR

R = Ph: 60 ºC R = Et: r.t. PhC

r.t.

CPh 60 ˚C

Ph

Ph MeC Ge Ge

Tbb

r.t. Tbb

6

CnPr

R

Tbb Ge

Ge Tbb

R R 7: R = Ph 8: R = Et Ph

Ph

Tbb Ge

Ge Tbb

Me

nPr 9

Scheme 2. Synthesis of 1,4-digermabenzenes 7-9. The structural parameters of isolated 1,4digermabenzenes 7–9 were unambiguously determined by single-crystal X-ray diffraction analysis (Figure 1).17 1,4-Digermabenzenes 7–9 exhibit virtually planar [Ge2C4] rings, even though the Ge atoms slightly protrude the C4 planes in a transpyramidalized fashion including Ge–C4 angles of 4.0–9.0˚ (cf. Figure 1a). In addition, both Ge atoms are partially pyramidalized in a trans-pyramidalized fashion, which is reflected in the bond angle sums around the Ge atoms (SGe = 349˚-357˚). 1,4Digermabenzenes 7 and 8 contain crystallographic centers of symmetry at the center of the sixmembered [Ge2C4] ring, which would render a conclusion on the aromaticity based on the presence of bond alternation in the ring skeleton ambiguous. In the case of 9, which does not contain a crystallographic center of symmetry, the four Ge–C bond lengths (1.877(2), 1.878(2), 1.882(2), 1.871(2) Å) in its [Ge2C4] ring are similar to each other, suggesting a highly symmetric structure due to the cyclic conjugation of π-electrons as in aromatic rings. In addition, the Ge-C bond lengths fall in between typical values for Ge–C single and double bonds18 and are comparable to those of previously reported 1,2digermabenzenes [1.883(2)–1.920(2) Å].7 The C–C


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bond lengths in the [Ge2C4] ring [1.410(3) and 1.391(3) Å] fall within the typical range of C–C bonds in benzene rings. The structural features of 1,4digermabenzenes 7 and 8 are very similar to those of 9, suggesting that their highly symmetric structures should arise from the highly delocalized cyclic conjugation of π-electrons, and not from crystallographic pseudo-disorder. A combined consideration of the structural parameters suggests that the 1,4digermabenzenes contain six cyclic delocalized πelectrons on the [Ge2C4] rings, which is indicative of considerable aromaticity. Ge1 C1

(a)

C2 ΣGe/º

“Ge–C4 angle” C1 C2 Ge1 C1’ C2’

Ge1’

9.3º 1.890(3) Ge1 1.866(3) C1 357.3º C2’ 1.406(4) C2

C1’ Ge1’ 9.3º

(b)

4.2º 1.898(4) Ge1 1.887(3) C1 349.2º C2’

C2

C1

C1’

Ge1

C2’

Ge1’

1.376(4) C2

C1’ Ge1’ 4.2º

(c) C1C2 Ge1

C4 C3

Ge2

8.6º 1.877(2) Ge1 1.878(2) C1 355.4º C3 1.410(3) 1.391(3) C2 356.8º C4 1.882(2) Ge2 1.871(2) 9.0º

Figure 1. Molecular structures of 1,4-digerma– benzenes (a) 7, (b) 8, and (c) 9 (ORTEP drawings with thermal ellipsoids set to 50% probability; hydrogen atoms are omitted for clarity) together with selected bond lengths (Å; black), Ge-C4 bent angles (red), and angle sums around Ge (green).

H Ge

Ge H

10 Ge–C 4 angle = 12.6º ΣGe = 344.4º

HOMO LUMO 0.36 (18%) 1.64 (82%) Occupation Number of Electrons

Figure 2. Frontier orbitals of parent 1,4-digerma– benzene 10 calculated at the CASSCF(6,6)/6311G(2d,p) level of theory. The structural features of 7-9, including the experimentally observed bent geometries, could be

reproduced by theoretical structure optimizations at the TPSSTPSS-D3(BJ)/6-311G(2d,p) level of theory.19 The theoretically optimized structure of the parent 1,4-digermabenzene Ge2C4H6 (10) at the MP2/6311G(2d,p) or TPSSTPSS-D3(BJ)/ 6-311G(2d,p) level of theory suggested completely planar geometries. Conversely, at the CASSCF(6,6)/6-311G(2d,p) level of theory, bent structures were obtained for 10 (Figure 2), suggesting that a partial open-shell character of the 1,4-digermabenzenes should be crucial to explain the bent structures. The corresponding diradicalcharacter values (y0 = 0.36) were calculated using the Yamaguchi scheme.20 NICS(0) calculations21a for 7–9 furnished negative NICS(0) values of –4.2, –5.9, and –5.0, respectively, suggesting lower levels of aromaticity relative to benzene (–8.3 at the same level of theory). Similarly, NICSzz(r) scan calculations21b delivered NICSzz(1.4) = –13.5 (7), NICSzz(1.7) = –18.1 (8), and NICSzz(1.5) = –15.6 (9), indicating that the alkylsubstitution might expand the frontier π-orbitals in the 1,4-digermabenzene ring relative to 10 (NICS(0) = –5.0; NICS(1.4) = –17.3) based on the larger r values. The NICS(0) and NICSzz(1.2) values for a planar geometry (D2h) of 10 were much more negative (–6.6 and –19.6, respectively) than those for a bent structure; in other words, a bent geometry of 7-9 would partially reduce their aromaticity. 1,4-Digermabenzenes 7–9 are NMR active and give sharp signals in their 1H and 13C NMR spectra, suggesting negligible radical character in C6D6. The 13C NMR signals for the [Ge2C4] rings of 7–9 were shifted to low field [161.6–169.3 ppm], which is indicative of a ring current effect from the six π-electrons. The UV-vis spectra of 7–9 in hexane showed characteristic strong absorptions for their π-π* electron transitions (7: lmax(e) = 442 (10,000) and 511 (8,100); 8: 468 (16,000); 9: 448 (2,600)). Thus, it seems feasible to conclude that 7–9 retain their cyclic systems with six conjugated π-electrons in the crystalline state and in solution. Notably, 7–9 are photoresponsive when n-hexane solutions of 7–9 are exposed to LED light (410–490 nm).22 We observed facile photochemical isomerizations, resulting in the quantitative formation of the corresponding 1,4digerma-Dewar-benzenes 11–13 (Scheme 3).17,23 While the photochemical isomerization of 7 into 11 was thermally irreversible, i.e., a retro-conversion was not observed upon heating to 100 ˚C in toluene, 8 was regenerated quantitatively from heating 12 to 100 ˚C for 2 h in toluene. When a toluene solution of 13 was heated to 100 ˚C for 24 hours, 9 was regenerated in 55% yield (1H NMR yield). The thermal conversion of 11–13 can be reasonably interpreted in terms of the relative thermodynamic energies be-



tween the corresponding 1,4-digermabenzenes and the 1,4-digerma-Dewar-benzenes, i.e., 11 is by 6.9 kcal/mol more stable than 7, and 12 is by 2.2 kcal/mol less stable than 8.19 Since the thermodynamic energies of 9 and 13 are comparable, the thermal reaction of 13 would result in a partial conversion giving 9. It should be noted that 11–13 are inert toward air and moisture for at least two weeks, even though 7–9 are highly air- and moisturesensitive. R1 Tbb Ge R2

R1

h ν (410–490 nm) r.t. quant.

Ge Tbb R3

R1 Tbb Ge

toluene 100 ºC 0% (7) quant. (8) 55% (9)

7: R1 = R 2 = R 3 = Ph 8: R1 = R 2 = R 3 = Et 9: R1 = Ph, R 2 = Me, R 3 = nPr

R1 Ge Tbb

R2 R3 air stable 11: R1 = R 2 = R 3 = Ph 12: R1 = R 2 = R 3 = Et 13: R1 = Ph, R 2 = Me, R 3 = nPr

Scheme 3. Photoreactions of 1,4-digermabenzenes 7-9. Et

Et Et

Ge

Tbb

Et Ge Et Tbb Et 15 Et Et 8 O O r.t. Tbb CO2 air quant. Ge Et (1 atm) Et –CO2 Ge SiMe3 r.t. 60 ºC Et Et Et Tbb SiMe3 quant. Et quant. Tbb 16 tBu Ge Ge H HO O H Tbb SiMe3 Ge Et Et Et H2 SiMe3 Et (1 atm) Ge 14 Et Tbb r.t. Et quant. Et Et 17 H2 (1 atm) Tbb Ge Ge Tbb 100 ºC quant. Et Et 12 air-stable Tbb Ge

Ge Tbb

(1 atm) r.t. quant.

8

Scheme 4. Reactions of 1,4-digermabenzene 8 with air, acetylene, CO2, and H2. The extremely high reactivity of 7–9 toward air/moisture prompted us to examine their reactivity toward small inert molecules such as acetylene, CO2, and H2 (Scheme 4).24,25 Exposing 8 in C6D6 to air quantitatively afforded oxidized 14. When an nhexane solution of 8 was exposed to acetylene (1 atm), digermabarrelene 15 was quickly and quantitatively formed via a [4+2]cycloaddition. Exposure of 8 in hexane to CO2 (1 atm) quantitatively furnished [4+2]cycloadduct 16. Notably, the reaction of 8 with CO2 is reversible, i.e., heating a C6D6 solution of 16 to 60 ˚C afforded 8 via a retro-[4+2]cycloaddition.26 Moreover, 8 activates H2 and cleaves the H–H bond, even under moderate conditions, i.e., exposing an nhexane solution of 8 to H2 (1 atm, r.t., 30 min) afford-

ed the reduced product 17. Upon considering the photochemical and chemical reactivity of 1,4digermabenzene 8 in its entirety, we developed the idea to activate H2 with an air-stable compound. The reaction of air-stable 1,4-digerma-Dewar-benzene 12 with H2 (1 atm, 100 ˚C, toluene, 2 h) resulted in the quantitative formation of hydrogen-adduct 17, which should most likely be interpreted in terms of a thermal isomerization to give intermediate 8, which can activate H2. Thus, 1,4-digerma-Dewar-benzene 17 can be considered as an air-stable main-group-elementbased activator for small molecules that can be activated thermally. In summary, we have isolated the first stable 1,4-digermabenzenes (7–9), which exhibit central [Ge2C4] rings with a cyclic conjugation of six πelectrons, and indicate considerable levels of aromaticity despite their non-planar structures. 7–9 are photo-responsive and easily isomerize into 1,4digerma-Dewar-benzenes 11–13 upon exposure to visible light. In case of alkyl-substituted 1,4digermabenzenes 12 and 13, their thermal retroreaction regenerates the 1,4-digermabenzenes. 1,4Digermabenzene 8 is able to activate small molecules such as acetylene, CO2, and H2. 1,4-Digerma-Dewarbenzene 12 serves as a air-stable promotor for H2 that can be activated thermally. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental section (PDF), spectral data (PDF), crystallographic data (CIF), and theoretically optimized coordinates (XYZ) are included in the SI.

AUTHOR INFORMATION Corresponding Author [email protected] ORCID Takahiro Sasamori: 0000-0001-5410-8488 Jing-Dong Guo: 0000-0002-3388-6181. Daisuke Hashizume: 0000-0001-7152-4408 Norihiro Tokitoh: 0000-0003-1083-7245

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI grants JP15H03777 and JP15K13640 from MEXT (Japan), a Grant-in-Aid for Research at Nagoya City University, the Collaborative Research Program of the Institute for Chemical Research, Kyoto University, and the DAIKO FOUNDATION, as well as JURC, IRCCS, and KURCA (Kyoto University). The authors are grateful for the assistance of the Research Equipment Sharing Center at


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Nagoya City University. T. Sugahara gratefully acknowledges support from a grant-in-aid for JSPS fellows (JP16J05501). The authors would also like to thank Mr. Toshiaki Noda and Ms. Hideko Natsume (Nagoya University) for the expert manufacturing of customtailored glassware.

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Independent Chemical Shifts (NICS): Distance Dependence and Revised Criteria for Aromaticity and Antiaromaticity. J. Org. Chem. 2006, 71, 883–893. Horspool, W. M. Equipment and Techniques, 1984, 489–509, In Synthetic Organic Photochemistry, Springer, Boston, MA: the filter solution was prepared using 1) NaNO2 aq (1.0 M) 2) CuSO4 aq (1.5 M) 3) NH3 aq (4.0 equiv.). Attempted trapping reactions of the expected biradical intermediates in the presence of 9,10-dihydroanthracene under otherwise identical conditions were unsuccessful. (a) Spikes, G. H.; Fettinger, J. C.; Power, P. P. Facile Activation of Dihydrogen by an Unsaturated Heavier Main Group Compound. J. Am. Chem. Soc. 2005, 127, 12232–12233. (b) Chu, T.; Nikonov, G. I. Oxidative Addition and Reductive Elimination at Main-Group Element Centers. Chem. Rev. 2018, 118, 3608–3680. (c) Weetman, C.; Inoue, S. The Road Travelled: After Main-Group Elements as Transition Metals. ChemCatChem, 2018, 10, 4213–4228. For activation reactions of H2 and/or CO2 using previously reported 1,4-dimetallabenzene derivatives, see: a) Wu, D.; Kong, L.; Li, Y.; Ganguly, R.; Kinjo, R. 1,3,2,5Diazadiborinine featuring nucleophilic and electrophilic boron centres. Nat. Commun. 2015, 6, 7340. b) von Grotthuss, E.; Diefenbach, M.; Bolte, M.; Lerner, H.-W.; Holthausen, M. C.; Wagner, M. Reversible Dihydrogen Activation by Reduced Aryl Boranes as Main-Group Ambiphiles. Angew. Chem., Int. Ed. 2016, 55, 14067-14071. c) Wang, B.; Li, Y.; Ganguly, R.; Hirao, H.; Kinjo, R. Ambiphilic boron in 1,4,2,5-diazadiborinine. Nat. Commun. 2016, 7, 11871. d) Taylor, J. W.; McSkimming, A.; Guzman, C. F.; Harman, W. H. N-Heterocyclic Carbene-Stabilized Boranthrene as a Metal-Free Platform for the Activation of Small Molecules. J. Am. Chem. Soc. 2017, 139, 11032-11035. e) von Grotthuss, E.; Prey, S. E.; Bolte, M.; Lerner, H.-W.; Wagner, M. Selective CO2 Splitting by Doubly Reduced Aryl Boranes to Give CO and [CO3]2–. Angew. Chem., Int. Ed. 2018, 57, 16491-16495. Detailed DFT calculations revealed the mechanisms for the reactions of 8 with CO2 and H2; both reactions proceed with very small free energy barriers of 5.4 and 7.8 kcal/mol, respectively. The barrier for the retro-reaction of 17 (22.3 kcal/mol) is much higher than that of 16 (14.9 kcal/mol); cf. Supporting Information.


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