Reaction of potassium germabenzenide with benzil: Unusual ring opening to form a unique polycyclic penta-coordinated germanate

1 Introduction

We have recently reported the synthesis and isolation of alkali metal (M⁺) 2-t-Bu-germabenzenide (M⁺·1 ⁻ ), the first example of a heavy phenyl anion, by the reaction of 1-Tbt-2-t-Bu-germabenzene (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) with the corresponding alkali metal arenide (Figure 1a) [1,2]. In this reaction, reductive dearylation reaction occurred, expelling the bulky substituent (Tbt) essentially introduced to stabilize the highly reactive neutral germabenzene. Despite the lack of such bulky substituent, compound 1 ⁻ is stable in the solid state and even in solution due to the electronic repulsion between the anionic moieties. Moreover, potassium 2-t-Bu-stannabenzenide, the Sn analog of K⁺·1 ⁻ , was also synthesized and isolated [3] by the treatment of an equilibrated mixture of the corresponding stannabenzene and its dimer [4] with KC₈.

Figure 1

(a) Synthesis of K⁺·1 ⁻ and (b) schematic representation of two different characteristics of 1 ⁻ .

In the course of examining their reactivity, some unique properties became apparent [5,6]. In particular, we have recently found that the reaction of K⁺·1 ⁻ with bromine-substituted double-bond compounds between silicon or germanium atoms replaces the germanium atom in the germabenzene ring of 1 ⁻ with an element derived from the double bond and that the removed germanium atom can be introduced into other molecules as single atomic germanium [7]. The key to their reactivity is the two-faced nature of 1 ⁻ , which retains sufficient aromaticity (+localized germyl anion) like the parent phenyl anion, while revealing the importance of the contribution of a divalent carbene-like canonical structure (+C₅-delocalized anion) negligible in the case of phenyl anion (Figure 1b). It should be noted that the aromatic contribution also implies reactivity as a reactive Ge═C double bond. However, it is difficult to evaluate these properties separately in terms of reactivity. Although [1 + 4] cycloaddition reactions with dienes would be useful to demonstrate their nature as a divalent species (for a review see the study of Mizuhata et al. [8]), the reaction of K⁺·1 ⁻ with 2,3-dimethylbuta-1,3-diene gave a very complicated mixture (possibly with concomitant reduction of the butadiene), which could not be studied in detail. Benzil (diphenylethanedione) is well known as another type of 4π unit to react with group 14 divalent compounds (for germylene [9] and stannylenes [10,11,12]). In order to distinguish the different types of reactivity of 1 ⁻ , we report herein that the reaction of K⁺·1 ⁻ with benzil proceeds cleanly to give polycyclic penta-coordinated germanate K⁺·2 ⁻ via [1 + 4] cycloaddition.

2 Results and Discussion

To a tetrahydrofuran (THF) solution of K⁺·1 ⁻ was added a THF solution of benzil (2 equiv.) at room temperature (Scheme 1). After stirring at the same temperature for 24 h, THF was removed and the residue was filtered and dissolved in THF-d ₈ in a glovebox filled with argon. The ¹H nuclear magnetic resonance (NMR) spectrum of the crude mixture under inert atmosphere showed the almost quantitative formation of K⁺·2 ⁻ .

Scheme 1

Reaction of K⁺·1 ⁻ with benzil to give K⁺·2 ⁻ .

Recrystallization of the crude mixture in a glovebox afforded single crystals of K⁺·2 ⁻ ·2thf suitable for X-ray crystallographic analysis. The crystal structure is shown in Figure 2a. As shown in Figure 2b, K⁺·2 ⁻ ·2thf shows a one-dimensional infinite structure along the c-axis in the form of an intervening K⁺(thf). Compound 2 ⁻ consists of 1 ⁻ and two benzil molecules, forming a penta-coordinated germanate. The germabenzene ring of 1 ⁻ was cleaved at the bond between Ge and C5 atoms, and the carbonyl moiety of benzil was inserted. The geometry around the germanium center of 2 ⁻ has a distorted trigonal bipyramid (TBP) structure. O2 and O4 can be regarded as being in the apical positions [∠O2–Ge–O4 = 153.89(13)°] and O1 and O3 in the equatorial positions [∠O1–Ge–O3 = 138.30(13)°]. As compared with the optimized structure of the acyclic model [MeGe(OMe)₄] ⁻ (calculated at B3LYP/6-31 G(d,p) level), the former is narrower (vs 170.6°) and the latter is wider (vs 124.1°) to a considerable extent. This indicates the core structure of 2 ⁻ gets close to O₄-based square pyramid (SP). In general, apical and equatorial bond lengths differ markedly in the TBP structure of a penta-coordinated compound. On the other hand, the apical/equatorial Ge–O bond lengths of 2 ⁻ [Ge–O2/O4 = 1.878(3)/1.896(3) Å, Ge–O1/O3 = 1.847(3)/1.847(3) Å] get closer than those of [MeGe(OMe)₄] ⁻ (apical: 1.909 Å, equatorial: 1.838 Å, calculated). Similar distortions (which can be called pseudo-SP structures) and bond-length equivalence have been reported as features of bis(catecholato)germanate derivatives [13,14], except for the systems with a rigid tetradentate ligand system such as porphyrin, but compounds with CGeO₄ cores with other substitution modes generally have TBP structures. We would like to emphasize that this system is a rare example of a non-rigid/non-bis(catecholato) system with a pseudo-SP structure and important for the verification of the ligand–structure relationship of germanates.

Figure 2

(a) Thermal ellipsoid plot (50%) of K⁺·2 ⁻ ·2thf. Hydrogen atoms and solvated THF were omitted for clarity. The central skeleton from another viewpoint was shown within a rounded rectangle. (b) Crystal packing of K⁺·2 ⁻ ·2thf.

Next, the formation mechanism of 2 ⁻ was investigated with the theoretical calculations (Figures 3 and 4). At the initial stage of the reaction, two interaction modes between 1 ⁻ and benzil are conceivable: the interaction between the carbonyl oxygen with the empty p orbital on the Ge atom and a nucleophilic attack to the carbonyl carbon by the Ge anion. The characteristic electronic structure of 1 ⁻ is that the orbital of the lone pair is highly stabilized. In the parent phenyl anion, the orbital of its lone pair is the highest occupied molecular orbital (HOMO), whereas in 1 ⁻ , the π orbital including the p orbital of the Ge atom is the HOMO [1]. This suggests that the former interaction is more favorable. In addition, both interactions give computationally stable structures 3 ⁻ and 4 ⁻ , and 3 ⁻ derived from Ge–O interaction is more stable by 3.4 kcal·mol⁻¹ than 4 ⁻ (Figure 3).

Figure 3

Energy diagram of initial stage of the reaction between germabenzenide 1 ⁻ and benzil.

Figure 4

(a) Plausible mechanism for the formation from 7 ⁻ of 2 ⁻ and (b) relative energies for the compounds regarding the addition of the second benzil.

From intermediate 3 ⁻ , two pathways via the interaction of the carbonyl carbon with the Ge or the adjacent carbon atom were found, but the latter (formal [2 + 2] cycloaddition between the germene (Ge═C) and carbonyl moieties) giving 5 ⁻ requires a high activation energy (25.7 kcal·mol⁻¹). The former gives the formal [1 + 2] cycloadduct 6 ⁻ , finally giving 1,3,2-dioxagermole (GeO₂C₂) 7 ⁻ through the bond recombination with a smaller barrier (9.5 kcal·mol⁻¹). From these results, it is reasonable to assume that the 1:1 adduct between 1 ⁻ and benzil is 7 ⁻ , indicating the divalent character in 1 ⁻ .

The possible formation mechanism from 7 ⁻ to 2 ⁻ is shown in Figure 4a. In 7 ⁻ , the contribution of the anion on C5 seems to be most favorable and it undergoes nucleophilic attack to the carbonyl of the second benzil molecule. The adduct can form 8 ⁻ and 8′ ⁻ depending on the relative position between 7 ⁻ and benzil, but we suppose that this process is reversible and converges to the path via 8 ⁻ leading to a more stable product by subsequent reactions. In the model systems without t-Bu group (Figure 4b), the negative ΔE _ZPE and positive ΔG ²⁹⁸ between 9 ⁻ + benzil and 10 ⁻ or 10′ ⁻ clearly suggested the reasonableness of the intermediates and reversibility. It should be noted that the suitable isomer 10 ⁻ is slightly more stable (0.6–0.7 kcal·mol⁻¹) than 10′ ⁻ . The rearrangement from 8 ⁻ to 2 ⁻ can be explained as metallo-ene reaction [15], and the Ge–C5 bond is cleaved at this stage.

In summary, we clearly demonstrated the reactivity of germabenzenide 1 ⁻ as a divalent germanium species in its reaction with benzil, which was supported by the theoretical calculations. Furthermore, the generated [1 + 4] cycloadduct 7 ⁻ reacted with an additional benzil molecule to give final product 2 ⁻ featuring a unique structure likely formed via metallo-ene-type reaction.

3 Experimental

Unless otherwise noted, all reactions were carried out under an atmosphere of argon. All manipulations were performed in a Yamato YGB1-CS glovebox. Solvents were purified by The Ultimate Solvent System (Glass Contour Company) [16]. Anhydrous tetrahydrofuran-d ₈ was obtained by bulb-to-bulb distillation from potassium mirror. Prior to use, Celite^® 545 was dried by heating (100°C) under vacuum for at least 24 h, while all glassware was oven-dried (120°C) for at least 3 h. Potassium 2-t-Bu-germabenzenide K⁺·1 ⁻ was prepared according to the literature procedure [1].

¹H and ¹³C NMR spectra were measured on a Bruker Ascend^TM 400 (¹H: 400 MHz, ¹³C: 100 MHz). For the ¹H NMR spectra, signals arising from residual partially hydrogenated –OC H (D)CD₂– in THF-d ₇ (3.58 ppm) were used as references, while the signals of –O C D₂CD₂– in THF-d ₈ (67.2 ppm) were used for the ¹³C NMR spectra. The multiplicity of the signals in the ¹³C NMR spectra was determined by distorsionless enhancement by polarization transfer (DEPT) techniques. For the assignments of the signals, one- or two-dimensional NMR methods were used. All melting points were determined on a BÜCHI melting point M-565 and are uncorrected.