A stable free tetragermacyclobutadiene incorporating fused-ring bulky EMind groups

Katsunori Suzukia, Yasuyuki Numatab, Naoko Fujitab, Naoki Hayakawab, Tomoharu Tanikawab, Daisuke Hashizumec, Kohei Tamaoa, Hiroyuki Fuenod, Kazuyoshi Tanakade and Tsukasa Matsuo*ab
aFunctional Elemento-Organic Chemistry Unit, RIKEN Advanced Science Institute (ASI), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
bDepartment of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan. E-mail: t-matsuo@apch.kindai.ac.jp
cMaterials Characterization Support Unit, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
dDepartment of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
eFukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Nishihiraki-cho, Tanano, Sakyo-ku, Kyoto 606-8103, Japan

Received 9th December 2017 , Accepted 15th January 2018

First published on 15th January 2018

The first free cyclobutadiene (CBD) germanium analogue was obtained as room-temperature stable dark red crystals via the reaction of the bulky EMind-substituted 1,2-dichlorodigermene with lithium naphthalenide. The cyclic 4π-electron antiaromaticity is essentially stabilized by the polar Jahn–Teller distortion in the germanium CBD producing a planar rhombic-shaped charge-separated structure.

While a variety of unsaturated germanium compounds such as digermenes,1,2 germaaromatics,3 trigermaallene,4 tetragerma-1,3-butadiene,5 and digermynes6 have been successfully isolated over several decades by taking advantage of the kinetic stabilization with the appropriate bulky substituents,7 free neutral molecules of pergerma-[n]annulenes, (GeR)n, (n is an even number equal to or greater than 4) have not been isolated and remained elusive, which would provide further insights into the aromaticity and antiaromaticity in small rings. Theoretical studies of the parent molecule of tetragerma-cyclobutadiene, (GeH)4, have indicated that a non-classical type of puckered D2d structure is the most stable among the cyclic isomers.8 Experimentally, only the tetragerma-tetrahedrane, Ge4(tBu3Si)4, was isolated among the valence isomers of Ge4R4, employing the steric protection with the isotropically-bulky tBu3Si groups.9 There have been two reports regarding the coordination-stabilized tetragermacyclobuta-diene species (Fig. 1).10,11 One is Ge4(tBu2MeSi)4 stabilized as a ligand by the complexation with transition metals (I and II), in which the Ge4 ring adopts a slightly folded square-planar geometry.10 The other is the zwitterionic base-stabilized tetragermacyclobutadiene derivative (III).11 Thus, the production of a free tetragermacyclobutadiene, germanium CBD, remains a challenge.12
image file: c7cc09443d-f1.tif
Fig. 1 Coordination-stabilized tetragermacyclobutadienes (I–III) and tetrasilacyclobutadiene (1) with EMind groups.

In 2011, we reported the first cyclobutadiene (CBD) silicon analogue, tetrasilacyclobutadiene, i.e., tetrasila-[4]annulene, Si4(EMind)4 (1), stabilized by the fused-ring bulky EMind groups (EMind = 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl).13,14 The four-membered Si4 ring in 1 exhibits a planar rhombic charge-separated structure as a consequence of the polar Jahn–Teller (J–T) distortion to counteract the antiaromaticity with a cyclic 4π-electron system.15 This result is in sharp contrast to the fact that the carbon CBD derivatives are mainly stabilized by the covalent J–T distortion producing a rectangular-shaped C4 ring with two isolated C[double bond, length as m-dash]C double bonds.16,17 An intriguing question arises whether the heavier congeners, germanium, tin, or lead, of the silicon CBD also display a planar diamond-like structure.18 We now report the synthesis and characterization of the first free CBD germanium analogue, Ge4(EMind)4 (2), supported by the bulky EMind groups.

As shown in Scheme 1, after several attempts to obtain the EMind-substituted 1,2-dihalodigermene as a suitable precursor for the tetragermacyclobutadiene 2, we have succeeded in the preparation of 1,2-dichlorodigermene, (EMind)ClGe = GeCl(EMind) (3), as yellow crystals in 54% yield via a room-temperature ligand redistribution reaction between the diarylgermylene, (EMind)2Ge: (4), and GeCl2·dioxane in toluene.19,20 The molecular structure of 3 has been characterized using X-ray crystallography (Fig. 2), displaying a highly trans-bent geometry with an E configuration; the trans-bent angle between the Ge1–Ge1* vector and the Cl1–Ge1–C1 plane is 44.02(12)°, which is similar to those reported for the 1,2-dihalodigermenes (36.8–46.0°).19,21 The Ge[double bond, length as m-dash]Ge bond length of 2.3853(7) Å is in the range of those of the dihalodigermenes (2.363–2.5087 Å).19,21

image file: c7cc09443d-s1.tif
Scheme 1 Synthesis of 2.

image file: c7cc09443d-f2.tif
Fig. 2 Molecular structure of 3. The thermal ellipsoids are shown at the 50% probability level. All hydrogen atoms, disordered chlorine, germanium and carbon atoms are omitted for clarity.

The reaction of 3 with two equivalents of lithium naphthalenide (LiNaph) in THF resulted in the formation of the CBD germanium analogue, Ge4(EMind)4 2. After removal of any insoluble materials, the air- and moisture-sensitive dark red crystals of 2 were isolated in 21% yield via recrystallization from a mixture of hexane and toluene. Compound 2 showed a rather good thermal stability compared to 1. Thus, while the decomposition of 1 occurred in solution and even in the solid state at room temperature,13 the crystals of 2 are stable at room temperature under an argon atmosphere. The UV-vis-NIR spectrum of 2 in hexane showed intense absorption maxima at 458 nm (ε = 15[thin space (1/6-em)]000) and 510 nm (sh, ε = 7400) along with a weak absorption at 836 nm (ε = 150) (Fig. S12, ESI).

The molecular structure of 2 was clearly determined via a single-crystal X-ray diffraction analysis (Fig. 3). The heavy CBDs 1 and 2 are isomorphous and their structures are very similar to each other. The selected structural parameters of 2 are summarized in Table 1, together with those of 1 for comparison. The bond lengths and the related parameters (entries 1 and 2) demonstrate that the Ge4 planar rhombic skeleton (Ge–Ge = av. 2.430 Å) has about a 7% larger similar structure than the silicon analogue 1 (Si–Si = av. 2.283 Å), in parallel to about 8% larger Ge than Si [covalent radius: Ge 1.20(4) Å vs. Si 1.11(2) Å].22 The sum of the internal bond angles (entry 3) is 359.97°, indicating the completely planar structure. The Ge1 and Ge3 atoms have a planar trigonal geometry with the sum of the surrounding angles (∑Ge) of 360.0° just like sp2-hybridization, whereas the Ge2 and Ge4 atoms prefer a rather pyramidal structure with sp3-like hybridization; the sums of the surrounding angles are 334.0° for Ge2 and 327.7° for Ge4, which are slightly smaller than those observed in 1 (338.8° and 335.1°) (entry 4).13 The deviation angles (θ) between the Ge–C vector and the mean plane of the Ge4 ring are estimated to be 37.85(5)° for Ge2 and 40.57(5)° for Ge4, being somewhat larger than those corresponding to 1 [32.87(7)° and 35.80(7)°] (entry 5).13 These structural features may be ascribed to the delicate balance between the steric congestion of the bulky EMind groups and the less effective hybridization of the heavier group 14 elements and/or the crystal packing configuration. The space-filling model of 2 reveals that the four EMind groups fit together in a gear-like arrangement around the Ge4 core (Fig. S10, ESI).

image file: c7cc09443d-f3.tif
Fig. 3 Molecular structures of 2: (a) top view and (b) front view. All hydrogen atoms, disordered carbon atoms, and a hexane molecule are omitted for clarity.
Table 1 Comparison between 2 and 1
Entry   2 (E = Ge) 1 (E = Si)
a Not observed.
1 E–E/Å 2.4283(2), 2.4606(2), 2.4014(2), 2.4294(2) (av. 2.430) 2.2877(8), 2.2924(8), 2.2671(8), 2.2846(8) (av. 2.283)
2 E⋯E/Å 3.0175(5), 3.8089(6) 2.8280(8), 3.5843(7)
3 E–E–E/° 102.354(8), 77.329(7), 104.083(8), 76.202(7) (sum 359.97) 103.00(3), 76.76(3), 103.90(3), 76.32(3) (sum 359.98)
4 ∑E/° 360.0(Ge1), 334.0(Ge2), 360.0(Ge3), 327.7(Ge4) 360.0(Si1), 338.8(Si2), 360.0(Si3), 335.1(Si4)
5 θ 0.59(4), 37.85(5), 0.71(4), 40.57(5) 0.55(7), 32.87(7), 0.59(7), 35.80(7)
6 NMR/ppm a −52, −50, 300, 308
7 E–E/Å 2.3723, 2.3708, 2.3730, 2.3720 (av. 2.372) 2.355, 2.327, 2.296, 2.305 (av. 2.321)
8 E⋯E/Å 2.9926, 3.6810 2.921, 3.606
9 E–E–E/° 101.81, 78.20, 101.75, 78.20 (sum 359.96) 101.4, 77.2, 102.5, 78.8 (sum 359.9)
10 ∑E/° 360.0(Ge1), 331.2(Ge2), 360.0(Ge3), 330.9(Ge4) 360.0(Si1), 334.2(Si2), 360.0(Si3), 332.9(Si4)
11 θ 0.3, 40.1, 0.3, 39.9 0.9, 36.1, 0.6, 31.3
12 WBI(E–E) 1.087, 1.081, 1.080, 1.090 1.070, 1.091, 1.151, 1.179
13 WBI(E⋯E) 0.347, 0.212 0.372, 0.269
14 NPA +0.602(Ge1), +0.146(Ge2), +0.573(Ge3), +0.151(Ge4) +0.647(Si1), +0.144(Si2), +0.637(Si3), +0.167(Si4)
15 NICS NICS(1) = −3.1, NICS(−1) = −3.2 NICS(1) = −0.9
16 CASSCF 1.887(HONO), 0.116(LUNO) 1.849(HONO), 0.153(LUNO)

To elucidate the bonding and electronic properties in the cyclic Ge4 framework, DFT calculations were performed for 2 at the B3LYP/6-31G(d,p) level23 using the Gaussian 09 program package,24 and the data are listed in Table 1, together with those of 1 for comparison. The optimized closed-shell structure well reproduced the X-ray crystal structure (entries 7–11). The most stable isomer is a planar rhombic singlet (C1 symmetry) with the Ge–Ge bond lengths of 2.3708–2.3730 Å (av. 2.372 Å) and the internal bond angles of 101.81°, 78.20°, 101.75°, and 78.20° (Fig. S13, ESI).25 The Ge–Ge bond orders of 2 were calculated to be 1.080–1.090 on the basis of the Wiberg bond index (WBI) (entry 12) (Fig. S20, ESI).26 The diagonal bond orders are 0.347 for Ge1⋯Ge3 and 0.212 for Ge2⋯Ge4 (entry 13), indicating some bonding interactions mainly due to the bonding molecular orbitals in HOMO−1 (Fig. 4). The natural population analysis (NPA)27 charge distribution of 2 shows a clear charge separation in the Ge4 ring, while the Ge2 and Ge4 atoms are slightly positively charged (+0.146 and +0.151), the Ge1 and Ge3 atoms are highly positively charged (+0.602 and +0.573) (entry 14) (Fig. S20, ESI). These computational studies suggest the contribution of an alternately charge-separated canonical form 2, as is the case of 1. The NICS values of 2 at the center of the Ge4 ring were estimated to be negative values [NICS(1) = −3.1 and NICS(−1) = −3.2] at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d,p) level (entry 15), but the NICS-scan plots of the out-of-plane components indicated the nonaromatic character of 2 (Fig. S27, ESI).28 The electron occupation numbers of 2 in terms of the natural orbitals (NOs) were obtained via the complete active space self-consistent field (CASSCF) (4,4) calculation, to support the low diradical character of 2 with the occupation numbers of 1.887 and 0.116 in the HONO and LUNO (entry 16) (Fig. S28, ESI).29

image file: c7cc09443d-f4.tif
Fig. 4 Selected molecular orbitals of 2 together with the energy levels.

The four pertinent π-type molecular orbitals (π-MOs) of 2 are shown in Fig. 4. Each π-MO is very comparable to that calculated for 1.13 Thus, while the HOMO consists of the out-of-phase orbitals (lone-pair-like orbitals) localized on the pyramidal Ge2 and Ge4 atoms with a back-lobe interaction, the LUMO represents the π*-like MO between the 4p-orbitals on the planar Ge1 and Ge3 atoms. The HOMO−1 is totally bonding with the delocalization of the electrons over all four Ge atoms, thereby being responsible for the diagonal interactions. The all-antibonding LUMO+1 has two nodal planes with the adjacent out-of-phase orbitals. The absorption wavelengths were determined to be 434 nm (HOMO → LUMO+1) and 495 nm (HOMO−1 → LUMO) based on the TD-DFT calculations (Fig. S24, ESI), which are in good qualitative agreement with the experimental value (458 nm). The weak peak observed at 836 nm is assignable to the symmetrically forbidden HOMO → LUMO transition, which is calculated to be 1008 nm.

There is currently a common view that the polar J–T distortion stabilizes the cyclic 4π-electron antiaromaticity in the heavy CBDs producing a planar rhombic-shaped charge-separated singlet state. The remarkable difference in the bonding and structure between the carbon CBDs and the heavy CBDs may be ascribed to the weaker π-bonds originating from the less effective p-orbital overlap with the larger inter-element distances between the heavier group 14 elements. The availability of the room-temperature stable free CBD germanium analogue would provide a new clue for the full understanding of the nature of the chemical bonding in the heavy [n]annulenes, as well as for various reactivities arising from the unsaturated small-ring Ge4 framework.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Electronic supplementary information (ESI) available: Experimental procedures, characterization for new compounds including NMR spectra, X-ray crystallographic analysis and theoretical calculations. CCDC 1505261 and 1580780. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc09443d

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