Communication pubs.acs.org/Organometallics
An Isolable Anionic Gallabenzene: Synthesis and Characterization Taichi Nakamura,† Katsunori Suzuki,*,† and Makoto Yamashita*,†,‡ †
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Bunkyo-ku, Tokyo 112-8551, Japan ‡ CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: An anionic gallabenzene was isolated and structurally characterized for the first time. The structure of gallabenzene in the crystal was isostructural with that of the recently reported anionic aluminabenzene. Structural parameters of the gallabenzene ring such as planar structure and unsaturated bond character without bond alternation were consistent with the structural criteria of the aromaticity. By DFT calculations of the gallabenzene, in addition to the presence of an ambiphilic contribution similar to the aluminum analogue, the plausible difference of the electronic structures between the galla- and aluminabenzenes due to the difference of electronegativity was also suggested. Scheme 1. Synthesis of the Anionic Gallabenzenea
C
ompared with the research on the heterobenzenes containing heavier group 14 and 15 elements in the third or higher rows of the periodic table,1 the examples of heavier group 13 element analogues such as alumina- and gallabenzenes have been limited in contrast to the well-known borabenzenes.2 As a sole example on the gallium analogues, Ashe et al. reported the generation and spectroscopic characterization of the anionic gallabenzene in solution (Figure 1a).3 However, the gallabenzene has not been isolated and
a
Reaction conditions: (a) GaCl3, toluene then pyridine; (b) MesLi, Et2O; (c) 1 equiv of DME, then recrystallization from toluene; (d) recrystallization from DME.
addition, recrystallization of 1Ga-Li(DME) from DME also gave single crystals of 1Ga-Li(DME)3. The molecular structures of 1Ga-Li(DME) and 1GaLi(DME)3 are shown in Figures 2, S3, and S4. The Li atom was located on the gallabenzene ring [Ga1−Li1 = 2.931(6) Å,
Figure 1. (a) Reported anionic gallabenzene and (b) mesomeric resonance structures alumina- and gallabenzenes.
structurally characterized. Recently, we have reported the synthesis of the first anionic aluminabenzene 1Al bearing aromatic and ambiphilic contributions A and B (Figure 1b) via aluminacyclohexadiene intermediate 2 developed by us (Scheme 1).4 Thus, the aluminacyclohexadiene 2 is expected to be a useful precursor for the synthesis of heterobenzenes bearing heavier group 13 elements. Here, we report the synthesis and isolation of the anionic gallabenzene 1Ga. An addition of GaCl3 to aluminacyclohexadiene 2 gave the corresponding gallacyclohexadiene 3. The following reaction of 3 with mesityllithium in Et2O gave 1Ga-Li(Et2O) (Scheme 1). Compound 1Ga-Li(Et2O) is sensitive toward air and moisture, but thermally stable at 190 °C under an inert atmosphere. Although X-ray crystallographic analysis of 1Ga-Li(Et2O) failed due to the low crystallinity, the treatment with one equivalent of DME and recrystallization from toluene afforded single crystals of 1Ga-Li(DME), suitable for X-ray analysis. In © 2015 American Chemical Society
Figure 2. Molecular structures of 1Ga-Li(DME) (A) and 1GaLi(DME)3 (B) (50% thermal ellipsoid probability). Hydrogen atoms are omitted for clarity. Received: April 14, 2015 Published: May 7, 2015 1806
DOI: 10.1021/acs.organomet.5b00310 Organometallics 2015, 34, 1806−1808
Communication
Organometallics
the lithium cation.9,10 Because the lithium cation in 1GaLi(DME) was located above the gallabenzene ring, the 7Li resonance of 1Ga-Li(DME) in C6D6 was observed at an upfield region (−4.8 ppm) due to the magnetic shielding by the aromatic ring current of 1Ga. In contrast, the 7Li resonance of 1Ga-Li(DME) in DME was observed at −1.9 ppm. The downfield shift of the resonance can be explained by the separation of the solvated lithium cation from gallabenzene. The structure of gallabenzene 1Ga would be describable as a resonance hybrid consisting of the aromatic A and ambiphilic B contributions similar to 1Al. The structure in the solid state is isostructural with 1Al. However, theoretical calculation suggests the electronic structure of 1Ga would be slightly different from that of 1Al probably due to the difference of the electronegativity. To get insight into the electronic structures of these heterobenzenes, detailed studies about the reactivity are in progress.
C−Li1 = 2.301(7)−2.702(7) Å] in 1Ga-Li(DME), whereas the Li cation of 1Ga-Li(DME) 3 was separated from the gallabenzene moiety due to the coordination by three DME molecules (Ga1−Li1 distances >7 Å). The sum of the internal angles of the gallabenzene ring was 720° for 1Ga-Li(DME)3, and the sum of the bond angles around the Ga1 atom is 360°, indicating the completely planar geometry of the six-membered ring. The C−C bond lengths in the gallabenzene ring of 1GaLi(DME)3 [1.406(4)−1.411(4) Å] were almost equal and between the typical carbon−carbon single and double bonds, comparable to those of benzene.5 The endocyclic Ga1−C1 and Ga1−C5 distances [1.927(3) Å] were shorter compared to the exocyclic Ga1−C6 single bond length of 2.004(3) Å. It is noteworthy that 1Ga is isostructural with the previously reported aluminabenzene 1Al.4 The bond distances including E−C bonds and bond angles of 1Al and 1Ga were identical probably due to the same covalent radii of gallium and aluminum atoms.5b According to the structural analysis, the structure of gallabenzene 1Ga is in agreement with the structural criteria of aromaticity, similar to 1Al. The structural optimization for 1Ga by the DFT calculations at the B3LYP/6-31G(d) level reproduced the experimentally observed structure.6 The calculated MOs in 1Ga showed the six π-MOs, and their lower three MOs were filled with six πelectrons (Figure S8) similar to those of benzene and 1Al.4 The bond order analysis based on the Wiberg bond index (WBI)7 suggested unsaturated characters for the gallabenzene ring. The values of Ga−C were estimated to be 0.81, which was 1.4 times larger than that of the exocyclic Ga−CMes single bond (0.61). The C−C bond orders of the ring (1.39−1.48) were consistent with the unsaturated character. The calculated NPA (natural population analysis)8 charge on the gallium atom was +1.32, while the charges on the five carbon atoms were calculated to be negative, consistent with the contribution of the ambiphilic resonance structure B in the gallabenzene ring as observed in 1Al. The WBI bond order and NPA charge distribution of 1Ga were slightly different from those in 1Al. The WBI values of the endocyclic Ge−C bond in 1Ga (0.81, 0.81) were slightly larger than those of the Al−C bond in 1Al (0.70, 0.71), while the cationic character of the Ga atom in 1Ga (+1.32) slightly decreased compared with that of the Al atom in 1Al (+1.58). These results suggested the contribution of ambiphilic resonance structure in 1Ga was slightly less significant in comparison with that in 1Al, probably due to the difference in the electronegativity of Ga (1.81) and Al (1.61) atoms,5b leading to more covalent Ga−C bonds. The nuclear independent chemical shifts (NICS)9 of 1Ga calculated at the B3LYP/6-311+G(2d,p) level were −2.4 for NICS(0) and −4.4 for NICS(1), slightly larger than those of 1Al (−1.5/−3.8). The result was possibly attributed to the stronger diatropic ring current in 1Ga relative to 1Al due to the more covalent Ga−C bonds as suggested by the WBI and NPA analysis. The observed NMR spectra of 1Ga-(DME) were also consistent with the presence of the aromatic contribution A, similar to those of a previously reported gallabenzene.3 In the 1 H NMR spectrum of 1Ga-Li(DME), the resonances for the protons attached to the gallabenzene ring were observed at 8.23 ppm for meta-protons and 6.01 ppm for para-protons in the aromatic region. The 13C resonances for the ortho-, meta-, and para-carbon atoms were also observed in the aromatic region (129.5, 148.2, and 103.0 ppm). The 7Li NMR chemical shifts of the lithium cation can be used to probe for evidence of aromaticity, as the diatropic ring current magnetically shields
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Details of experiments, crystallographic analysis (cif files), and DFT calculations (xyz file). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00310.
Corresponding Authors
*E-mail (K. Suzuki):
[email protected]. *E-mail (M. Yamashita):
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research on Innovative Areas “Stimuli-Responsible Chemical Species for the Creation of Functional Molecules” (No. 24109012) and “New Polymeric Materials Based on ElementBlocks” (No. 25102538) from MEXT, the Sasagawa Scientific Research Grant from the Japan Science Society, and CREST area “Establishment of Molecular Technology towards the Creation of New Functions” from JST. The computations were performed using Research Center for Computational Science, Okazaki, Japan. We thank Prof. T. Hiyama, Chuo University, for providing the X-ray diffractometer.
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REFERENCES
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DOI: 10.1021/acs.organomet.5b00310 Organometallics 2015, 34, 1806−1808
