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From Borapyramidane to Borole Dianion Vladimir Ya. Lee, Haruka Sugasawa, Olga A. Gapurenko, Ruslan M. Minyaev, Vladimir I. Minkin, Heinz Gornitzka, and Akira Sekiguchi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03473 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018
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From Borapyramidane to Borole Dianion Vladimir Ya. Lee,*1 Haruka Sugasawa,1 Olga A. Gapurenko,2 Ruslan M. Minyaev,2 Vladimir I. Minkin,2 Heinz Gornitzka,3 and Akira Sekiguchi*1 1
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305–8571, Ibaraki, Japan; 2Institute of Physical and Organic Chemistry, Southern Federal University, Rostov on Don 344090, Russian Federation; 3LCC-CNRS, Université de Toulouse, CNRS, UPS, Toulouse, France Supporting Information Placeholder ABSTRACT: Non-classical pyramidanes with their in-
verted tetrahedral configuration of the apical atom are among the most challenging synthetic targets in cluster chemistry. In this Communication, we report on the synthesis and structure of the first representative of pyramidal compounds with the group 13 element at the apex, namely, chloroborapyramidane 2. Reduction of 2 with excess of lithium metal unexpectedly produced the cageopening product, borole dianion derivative {32– + • [Li(thf) ]2}, a 6π-electron aromatic system.
Structural isomerism gives rise to a wealth of structural diversity in organic chemistry. This phenomenon is of particular importance for polycyclic and polyhedral compounds, for which the number of imaginable isomers dramatically increases. Thus, in organic chemistry, a great variation within the series of C5R6 derivatives represents a benchmark case of the structural isomerism for polycyclic compounds. Among these C5R6 isomers, cyclopentadiene A (Chart 1, E = CR2) is most fundamental and best-known, whereas isomeric bicyclo[2.1.0]pentene (so-called, “housene”) B (Chart 1, E = CR2) and tricyclo[2.1.0.02,5]pentane C (Chart 1, E = CR2) are encountered much less frequently. In the family of their boron analogues, that is the cyclic RʹB[C4R4] isomers, the boracyclopentadienes A (commonly coined as boroles) are undoubtedly most famous (Chart 1, E = BRʹ).1 Highly strained 5-borabicyclo[2.1.0]pentenes, “borahousenes” B (Chart 1, E = BRʹ) and tricyclic 5boratricyclo[2.1.0.02,5]pentanes C (Chart 1, E = BRʹ) are more exotic, and actually derivatives of such type are yet to be isolated. However, among a variety of the RʹB[C4R4] isomers, one was definitely overlooked, namely, highly strained borapyramidane D (Chart 1, E = BRʹ),2 although its very existence could be assumed given the well-recognized electron-deficiency of boron leading to electron delocalization in a variety of boron compounds. Even more so, based on the Wade’s rule,3 the compound of the type
RʹB[C4R4] should be classified as a nido-cluster, for which a predicted structure has a square-pyramidal shape. E
R
R
R
E
R
R
R
A
E
R
R R B
E
R
R
R R
R
R
R D
C
Chart 1. Structural isomers E[C4R4].
As the synthetically feasible route for still unprecedented borapyramidanes, one can propose interaction of the boron halides with the readily available cyclobutadiene dianion derivative 12–• [Li(thf)+]2,4 that has already been proven to be an indispensable source for a variety of clusters containing both main group elements5 and transition metals.6 And indeed reaction of 12–• [Li(thf)+]2 with BCl3 in hexane formed the desired chloroborapyramidane 2, as the first pyramidane with the second row element at the apex, isolated as pale-yellow crystals in moderate yield of 51% (Scheme 1).7 It is worth mentioning that borapyramidane 2 is isoelectronic to the still unprecedented highly challenging pyramidal cation [HC(C4H4)]+ 2a. Cl Li(thf) SiMe3 B BCl3 C C C SiMe3 C C C Me3Si hexane / r.t. Me3Si SiMe3 C Me3Si C SiMe3 Li(thf) Me3Si
12–●[Li(thf)+]2
2
Scheme 1. Synthesis of 2. As the manifestation of its high molecular symmetry, 2 uniformly exhibited only one signal in all of its NMR spectra. The 11B NMR resonance of 2 was observed at –38.5 ppm.8 Of particular importance is the resonance of the skeletal carbon atoms in 2, +101.8 ppm, which is quite comparable with those of the cyclobutadiene dianion alkali and alkaline-earth metal salts (+104.1 for {12– + 4 2– 2+ 5a • [Li(thf) ]2} and +106.4 for {1 • [Mg(thf)3 ]} ). This implies that the basal C4-ring in 2 still retains a substan-
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tial amount of the cyclobutadiene dianion character characteristic for the ionic Li- and Mg-salts. That is, the overall structure of 2 can be alternatively viewed as the ionic combination of the dication [ClB]2+ and cyclobutadiene dianion [(C4(SiMe3)4]2–: 2+ 2– [ClB] ←[(C4(SiMe3)4] . Accordingly, we probed potential aromaticity of 2: calculated NICS(1)zz9 = –7.9 (– 7.4) for 2 at the B3LYP/Def2TZVP (DFT1) and TPSSh/Def2TZVP (DFT2) levels of theory.10 These negative values are indicative of the diatropic ring current operative within the carbon base, albeit its extent is not great (for comparison: NICS(1)zz for the aromatic [12–• 2Li+] is –17.0 (–15.8) at the DFT1(DFT2) levels of theory). Crystal structure of 2 is shown in Figure 1.7 Cl1 B1 C2
C1 C1# C3
Figure 1. Crystal structure of 2 (ORTEP view, hydrogen atoms are omitted). Selected bond lengths (in Å): C1–C2 = 1.502(2), C1–C3 = 1.493(2), B1–C1 = 1.643(2), B1–C2 = 1.634(2), B1–C3 = 1.653(2), B1–Cl1 = 1.766(2).
In line with the above proposed delocalized structure, 4-membered carbon ring in 2 features nearly identical C–C bonds (1.493(2)–1.502(2) Å) that are intermediate between the standard single (1.54 Å) and double (1.34 Å) bond length values. The carbon base is nearly perfectly square-planar with the internal bond angles ranging from 89.38(8)° to 90.48(11)° and the sum of the internal bond angles of 359.09° (with a quite negligible folding of merely 0.4°). Pyramidal boron-carbon bonds in 2 of 1.634(2)–1.653(2) Å are longer than the sum of the single bond covalent radii of B and C atoms (1.60 Å)11. Such B–C bonds are highly polarized towards the carbon base being classified as the polar covalent, in a sharp contrast to the basal C–C bonds which represent the pure covalent interactions (for the results of the AIM calculations, see the SI).10 According to our Natural Resonance Theory (NRT) computations, the pyramidal B–C bonds are characterized by the remarkably lower bond order and lower covalent contribution to the overall structure, compared with those of the basal C–C bonds [NRT bond order/covalent character in %, at DFT1(DFT2)]: 0.63/59 (0.64/60) (B–C) and 1.03/99 (1.03/99) (C-C) (for the model ClB[C4H4]). Apical boron is tetrahaptocoordinated to the C4-base, being at the same time bound to chlorine, which makes boron hypervalent and pentacoordinate overall. Alt-
hough pentacoordination at B in organoboron compounds is precedented, it is still uncommon with only a few examples currently known.12 What is particularly striking, in all these previous reports pentacoordinate boron quite expectedly features trigonal-bipyramidal (TBP) configuration, whereas in borapyramidane 2 the geometry at boron is not traditional TBP any more, but unprecedented distorted square-pyramidal (SP) instead with the central boron atom being pulled above the center of the four-membered ring carbon base. This must be recognized as the novel coordination type for pentacoordinate boron. Definitely, such unusual SP geometry at boron is dictated by the peculiar pyramidal structure of 2, in which boron is forced to bind to the planar C4-base on the one hand and to the chlorine atom sitting just above boron on the other hand. Boron-chlorine bond of 1.766(2) Å in 2 is notably shorter than the sum of the single bond covalent radii of boron and chlorine atoms of 1.84 Å11. This B–Cl bond is only marginally longer than that in BCl3 (1.75(2) Å),13 with the latter being described as possessing some degree of the multiple bond character B(δ–)=Cl(δ+) due to the π-back donation from Cl to B.14 This was further supported by the B–Cl NRT bond order (1.06 at both DFT1 and DFT2) which value being even greater than that for the basal C–C bonds of 1.03. Square-pyramidal structure of chloroborapyramidane 2 is highly reminiscent of that of pentaborane B5H9, a milestone in the polyhedral boranes family.15 However, if tetragonal-pyramidal shapes are common in the latter, such square-pyramidal motif with the apical boron atom is still unprecedented among organoboron compounds. Interestingly, borapyramidane 2 was exclusively formed with no traces of its structural isomers of the types A–C, especially cyclopentadiene-type borole A (Chart 1, E = BRʹ).16 Our computations showed that the relative stability of borole A and borapyramidane D (E = BCl, R = SiMe3) (Chart 1) strongly depends on the computational level used, being in favor of the former at DFT1 and in favor of the latter at DFT2 [relative energies in kcal mol–1 at DFT1(DFT2) levels]: borole A [0] < borapyramidane D [+7.1(–2.7)] < boratricyclo[2.1.0.02,5]pentane C [+12.9(+3.8)]. It should be noted that the difference in the relative stabilities of borole and borapyramidane at DFT1 and DFT2 methods is not that great, being only 9.8 kcal/mol, which points to a lack for the clear thermodynamic preference of either isomer. This in turn might imply that the overall reaction course is not controlled by the thermodynamics but rather by the kinetics of the reaction driven by the ease of formation of the final product. That is the preference for the exclusive formation of pyramidane is likely to be caused by the relative ease of its formation which does not require expansion of the C4-ring, unlike the case for formation of the five-membered ring boroles that are
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further destabilized by their 4π-electron antiaromaticity.1a,b With chloroborapyramidane 2 in hands, we then challenged its reduction as a promising way to generate still unknown anionic borapyramidane (Chart 2, G) that is isoelectronic with already reported neutral pyramidanes of the group 14 elements F (Chart 2)5b-d or cationic pyramidanes of the group 15 elements E (Chart 2)5e. [M]+
[A]– P R R
B
Sn R R
E
R R
R F
R
R R
R G
R
Chart 2. Isoelectronic species with the apical main group elements: E - cationic phosphapyramidane (group 15 element) with the couteranion [A]–, F - neutral stannapyramidane (group 14 element), and G - anionic borapyramidane (group 13 element) with the countercation [M]+ (E5e and F5b,d are reported compounds, G – hypothetical compound).
LUMO+8 (+1.5 eV, ΔE(LUMO+8)–LUMO = 1.7 eV) (Figure 2). Accordingly, the first reduction step preferentially involves C4-base of chloroborapyramidane 2, followed by the ring expansion forming a dianionic 6π-electron five-membered BC4-ring 32–, thus leaving the B–Cl bond intact. The overall reduction process is apparently driven by the aromatization of the final product. This hypothesis was further substantiated by the calculated enthalpy for the formation of the borole dianion salt 32–• [Li+]2 and borapyramidane anion derivative LiB[C4R4] of the type G (Chart 2, R = SiMe3, M = Li) upon the reduction of chloroborapyramidane 2 with lithium. Thus, formation of the former species (2 + 2Li → 32–• [Li+]2) is thermodynamically much more favorable than formation of the latter (2 + 2Li → LiB[C4R4] + LiCl): –124.6 (– 123.3) vs. –47.0 (–46.9) kcal mol–1 at the DFT1 (DFT2) calculation levels.
Given the presence of a B–Cl bond in 2, which is typically a subject for a reduction with alkali metals,17 one would expect ready reductive cleavage of this bond thus generating borate anion of the type G (Chart 2). However, by contrast, reduction of 2 with an excess of lithium powder resulted in practically quantitative formation (by NMR spectroscopy) of the dilithium salt of the chloroborole dianion {32–• [Li(thf)+]2} as the cage-opening product (Scheme 2).7,18 Figure 2. Principal molecular orbitals of 2 (DFT1).
Cl B C SiMe3 C Me3Si C Me3Si C SiMe3 2
Li (excess) THF / r.t.
Li(thf) SiMe3 Me3Si C C B Cl C Me3Si C SiMe3 Li(thf) {32–●[Li(thf)+]2}
Scheme 2. Synthesis of {32–• [Li(thf)+]2}. Because of the lowering molecular symmetry on going from 2 to {32–• [Li(thf)+]2}, the latter exhibited two sets of signals in its NMR spectra (1H-, 13C-, and 29Si NMR), in contrast to the starting 2 featuring only one set of signals. Particularly important is the 7Li NMR resonance of {32–• [Li(thf)+]2} observed at extraordinary high-field at –6.73 ppm, undoubtedly caused by the diatropic ring current (NICS(1)zz9 for {3a2–• [Li(thf)+]2} = –27.9 (–27.7) at DFT1(DFT2).10 Puzzling at the first glance selective reduction of the carbon base in 2 in the presence of otherwise reactive B– Cl bond17 can be realized upon consideration of its molecular orbitals.10 Namely, LUMO (–0.2 eV) of the optimized structure of 2 is largely localized on the C4-base (fully antibonding π-orbital), whereas vacant σ*-orbital of the B–Cl bond represents remarkably higher-lying
The crystal structure of {32–• [Li(thf)+]2} is shown in Figure 3.7 The five-membered BC4-ring in 3a2– is practically planar with the sum of the internal bond angles of 540°, and both lithium cations situated above and below the borole ring being pentahaptocoordinated to that ring. All C–C bonds in 32– are nearly identical being within the narrow range of 1.453(4) – 1.466(5) Å, whereas both B–C bonds are also very similar to each other (1.515(5) and 1.513(5) Å). B atom is sp2-hybridized featuring a planar geometry with the sum of the bond angles of 360°. B–Cl bond in {32–• [Li(thf)+]2} of 1.841(4) Å perfectly matches the value of the sum of the single bond covalent radii of B and Cl atoms (1.84 Å),8 thus being markedly longer than that in the starting 2 (1.766(2) Å).
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C1 B1
C2 C3
Cl1
Li1/Li2 C4
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Figure 3. Crystal structure of {32–• [Li(thf)+]2} (ORTEP view, Li-coordinated thf molecules and hydrogen atoms are omitted). Selected bond lengths (in Å): B1–C1 = 1.515(5), B1–C4 = 1.513(5), C1–C2 = 1.465(5), C2–C3 = 1.453(4), C3–C4 = 1.466(5), B1–Cl1 = 1.841(4), Li1–B1 = 2.236(7), Li2–B1 = 2.227(8), range of the Li1/Li2–skeletal C atom bond distances = 2.127(7)–2.234(7).
Featuring halogen moiety at the boron center, chloroborapyramidane 2 is a prospective target for its further functionalization potentially leading to a diverse family of B-substituted borapyramidanes. On the other hand, as the boron analogues of very useful aromatic cyclopentadienyl anion Cp–, boryl dianion 32– may serve as the novel ligand for transition metal complexes, especially for the multiple-decker sandwiches. Both abovementioned possibilities are currently under investigation in our laboratories. ASSOCIATED CONTENT Supporting Information. Experimental section (synthetic procedures, spectroscopic and crystallographic data for 2 and {32–• [Li(thf)+]2}), computational data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Authors
*
[email protected] *
[email protected] Notes
The authors declare no competing financial interests.
A CKNOWLEDGMENT This work was financially supported by the JSPS KAKENHI Grant program (Nos. JP15K05413 and JP16K05682) from the Ministry of Education, Science, Sports, and Culture of Japan, and by the Russian Ministry of Education and Science for Research (Project Part, Project 4.844.2017/4.6). R. M. M. thanks the Russian Ministry of Education and Science (No. 4.6863.2017/6.7).
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5. (a) Takanashi, K.; Inatomi, A.; Lee, V. Ya.; Nakamoto, M.; Ichinohe, M.; Sekiguchi, A. Eur. J. Inorg. Chem. 2008, 1752. (b) Lee, V. Ya.; Ito, Y.; Sekiguchi, A.; Gornitzka, H.; Gapurenko, O. A.; Minkin, V. I.; Minyaev, R. M. J. Am. Chem. Soc. 2013, 135, 8794; (c) Lee, V. Ya.; Ito, Y.; Gapurenko, O. A.; Sekiguchi, A.; Minkin, V. I.; Minyaev, R. M.; Gornitzka, H. Angew. Chem. Int. Ed. 2015, 54, 5654. (d) Lee, V. Ya.; Gapurenko, O. A.; Ito, Y.; Meguro, T.; Sugasawa, H.; Sekiguchi, A.; Minyaev, R. M.; Minkin, V. I.; Herber, R. H.; Gornitzka, H. Organometallics 2016, 35, 346. (e) Lee, V. Ya.; Sugasawa, H.; Gapurenko, O.; Minyaev, R. M.; Minkin, V. I.; Gornitzka, H.; Sekiguchi, A. Chem. Eur. J. 2016, 22, 17585. 6. Takanashi, K.; Lee, V. Ya.; Sekiguchi, A. Organometallics 2009, 28, 1248. 7. Details of experimental procedures, spectroscopic and crystallographic data for compounds 2 and {32–• 2[Li(thf)]+} are given in the Supporting Information. 8. The shielding of apical boron atoms in 2 is not that spectacular compared to the neutral stannapyramidane and cationic phosphapyramidane, for which the record high-field signals were observed for the apical Sn (–2441.5 ppm) and P (–542.3 ppm) atoms (see refs. 5b and 5e, respectively). 9. Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Org. Lett. 2006, 8, 863. 10. All computations were performed with the GAUSSIAN 09 suite of programs (see the SI for details). 11. Pyykkö, P.; Atsumi, M. Chem. Eur. J. 2009, 15, 186. 12. (a) Lee, D. Y.; Martin, J. C. J. Am. Chem. Soc. 1984, 106, 5745. (b) Yamashita, M.; Yamamoto, Y.; Akiba, K.; Nagase, S. Angew. Chem. Int. Ed. 2000, 39, 4055. (c) Yamashita, M.; Yamamoto, Y.; Akiba, K.; Hashizume, D.; Iwasaki, F.; Takagi, N.; Nagase, S. J. Am. Chem. Soc. 2005, 127, 4354. (d) Nakatsuji, J.; Moriyama, Y.; Matsukawa, S.; Yamamoto, Y.; Akiba, K. Main Group Chem. 2006, 5, 277. (e) Yano, T.; Yamaguchi, T.; Yamamoto, Y. Chem. Lett. 2009, 38, 794. (f) Nakatsuji, J.; Yamamoto, Y. Bull. Chem. Soc. Jpn. 2010, 83, 767. (g) Hirano, Y.; Kojima, S.; Yamamoto, Y. J. Org. Chem. 2011, 76, 2123. (h) Dou, C.; Saito, S.; Yamaguchi, S. J. Am. Chem. Soc. 2013, 135, 9346. 13. (a) Atoji, M.; Lipscomb, W. N. J. Chem. Phys. 1957, 27, 195. (b) Spencer, C.; Lipscomb, W. N. J. Chem. Phys. 1958, 28, 355. 14. In line with this hypothesis, hybridization of boron forming B–Cl bond is unconventional sp1.96 indicative of its high s-character. 15. (a) Dulmage, W. J.; Lipscomb, W. N. Acta Cryst. 1952, 5, 260. (b) Larcher, J. F.; Linnett, J. W. Theor. Chim. Acta 1968, 12, 127. (c) Greatrex, R.; Greenwood, N. N.; Rankin, D. W. H.; Robertson, H. E. Polyhedron 1987, 6, 1849. (d) Lawler, K. A.; Hoffmann, R. Inorg. Chem. 1996, 35, 1431. 16. This is also in a sharp contrast to the case of the heavier group 15 elements which trihalides EX3 reacted with 12–• 2[Li(thf)]+ with exclusive formation of the housene-type structures B (Chart 1), see ref. 5e (for E = P–Cl) and also: Lee, V. Ya.; Ota, K.; Ito, Y.; Gapurenko, O. A.; Sekiguchi, A.; Minyaev, R. M.; Minkin, V. I.; Gornitzka, H. J. Am. Chem. Soc. 2017, 139, 13897 (for E = Sb–F). 17. Yamashita, M. Bull. Chem. Soc. Jpn. 2011, 84, 983. 18. Literature precedents of the isolable borole dianions: (a) Herberich, G. E.; Hostalek, M.; Laven, R.; Boese, R. Angew. Chem. Int. Ed. Engl. 1990, 29, 317. (b) Herberich, G. E.; Eigendorf, U.; Englert, U. Chem. Ber. 1993, 126, 1397. (c) So, C.-W.; Watanabe, D.; Wakamiya, A.; Yamaguchi, S. Organometallics 2008, 27, 3496. (d) Braunschweig, H.; Chiu, C.-W.; Wahler, J.; Radacki, K.; Kupfer, T. Chem. Eur. J. 2010, 16, 12229. See also a review: (e) Wei, J.; Zhang, W.-X.; Xi, Z. Chem. Sci. 2018, 9, 560.
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Graphical Abstract [for the TOC use]:
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