Mesoionic Dithiolates (MIDts) Derived from 1,3‐Imidazole‐Based Anionic Dicarbenes (ADCs)

Abstract Mesoionic dithiolates [(MIDtAr)Li(LiBr)2(THF)3] (MIDtAr={SC(NDipp)}2CAr; Dipp=2,6‐iPr2C6H3; Ar=Ph 3 a, 3‐MeC6H4 (3‐Tol) 3 b, 4‐Me2NC6H4 (DMP) 3 c) and [(MIDtPh)Li(THF)2] (4) are readily accessible (in≥90 % yields) as crystalline solids on treatments of anionic dicarbenes Li(ADCAr) (2 a‐c) (ADCAr={C(NDipp)2}2CAr) with elemental sulfur. 3 a‐c and 4 are monoanionic ditopic ligands with both the sulfur atoms formally negatively charged, while the 1,3‐imidazole unit bears a formal positive charge. Treatment of 4 with (L)GeCl2 (L=1,4‐dioxane) affords the germylene (MIDtPh)GeCl (5) featuring a three‐coordinated Ge atom. 5 reacts with (L)GeCl2 to give the Ge−Ge catenation product (MIDtPh)GeGeCl3 (6). KC8 reduction of 5 yields the homoleptic germylene (MIDtPh)2Ge (7). Compounds 3 a‐c and 4–7 have been characterized by spectroscopic studies and single‐crystal X‐ray diffraction. The electronic structures of 4–7 have been analyzed by DFT calculations.

dicarbene (ADC) II by the C4-deprotonation of I. [13] Later, they prepared a lithium dithiolene radical III by reacting II with S 8 . [14] The widespread applications of NHCs is largely attributed to their auspicious stereoelectronic features, in particular the σ-donor strength. [15] Mesoionic carbenes (iMICs), the so-called abnormal NHCs, are C4-(or C5) carbenes and are superior σdonors than C2-carbenes. [16] Unlike NHCs, stable iMICs are however rather scarce. The first free iMIC IV was reported by . [17] Very recently, we reported on C5protonated iMICs V, [18] which can be further deprotonated to give C4,C5-dicarbenes VI. We have also shown the suitability of these vicinal dicarbenes VI in accessing a variety of main-group heterocycles, [19] including diradicals. [20] Herein, we report on the high-yielding synthetic access to a new class of dithiolene ligands, namely the mesoionic dithiolate (MIDts) VII, derived from VI.
The desired precursors Li(ADC Ar ) (ADC Ar = {CN(Dipp)} 2 CAr); Dipp = 2,6-iPr 2 C 6 H 3 ; Ar = Ph 2 a, 3-MeC 6 H 4 (3-Tol) 2 b, 4-Me 2 NC 6 H 4 (DMP) 2 c) were prepared in situ (Scheme 1) [19,20] by the direct double deprotonation of the C2-arylated 1,3imidazolium bromides (IPr Ar )Br (IPr Ar = {HCN(Dipp)} 2 CAr; Ph 1 a-Br, 3-Tol 1 b-Br, DMP 1 c-Br) [21] with nBuLi. Treatment of a freshly prepared THF solution of 2 a-c with LiBr [22] and S 8 afforded compounds 3 a-c. Solid-state molecular structures of 3 a (Figure 2a), 3 b ( Figure S25 in Supporting Information), and 3 c ( Figure S26) reveal the presence of a cluster like S 2 Li 3 Br 2 unit. [23] Each four-fold coordinated lithium bears a THF molecule, while both the bromides serve as bridging ligands. The 1 H and 13 C NMR spectra of 3 a-c exhibit expected signals for the ADC Ar and THF moieties. The corresponding derivative 4 free from LiBr was prepared using 1 a-Cl instead of 1 a-Br to generate 2 a. This may be due higher lattice energy of LiCl than LiBr. 3 a-c and 4 are orange or yellow solids, accessible in � 90 % yields, and indefinitely stable under an inert gas (N 2 or Ar) atmosphere.  In 4, the four-coordinated lithium atom bears two THF molecules and binds to the sulfur atoms of the MIDt Ph to form a five-membered C 2 S 2 Li ring. The lithium atom in 4 is protruded out of the C 2 S 2 -plane and has a fold angle of 13.0(1)°between the LiS 2 and S 2 C 2 planes, which is slightly smaller than Robinson's radical anion III (14.2°). [14c] Notably, 4 has an additional electron than III. The C2À C3 bond length of 4 (1.383 (2) (13) Å) [9] and (IPr) = S (1.670(1) Å). [24] and compare well with those of thiophene (1.7126(5) Å) [10] and metal-dithiolates. [2] These features suggest the presence of a delocalized π-electron system over the C 2 S 2 Li ring.
The optimized structure of 4 at the PBE0-D3BJ/def2-TZVPP level of theory (see the Supporting Information) is in good agreement with its solid-state molecular structure ( Figure 2a). The Wiberg bond indices (WBIs) of the C2À C3 (1.32 au) and C2À S1/C3À S2 bonds (1.24 au) indicate that they have partial double character. Calculations reveal that the HOMO of 4 is a πtype orbital mainly located at the C 2 S 2 unit and has CÀ C and trans-annular S … S π-bonding character. The LUMO is primarily based on the phenyl group of ADC Ph (Figure 2b). The UV-visspectrum of 4 exhibits one main absorption (λ max. ) at 497 nm, which based on TD-DFT analysis may be assigned to the HOMO!LUMO transition (Table S10).
Robinson et al. isolated a square-planar germanium compound VIII by reacting III with (L)GeCl 2 (L = 1,4-dioxane) (Scheme 2a). Structural parameters suggest that VIII is a Ge(+ 4) compound. Thus, the ligand is 1e-reduced while germanium is 2e-oxidized in VIII with respect to III and (L)GeCl 2 . This finding prompted us to explore the reactivity of 4 with (L)GeCl 2 , given the fact that 4 already has one excess electron than III. Interestingly, treatment of 4 with (L)GeCl 2 in THF led to the clean formation of a heteroleptic germylene 5 as an orange solid (Scheme 2b). Further reaction of 5 with (L)GeCl 2 gave a mixed-valent Ge(+ 1)/Ge(+ 3) compound 6. The formation of GeÀ Ge catenation product 6 indicates formal insertion of GeCl 2 into the GeÀ Cl bond of 5.
We anticipated the formation of bis-germylene 7* on KC 8 reduction of 5. This reaction however led to the formation of a homoleptic germylene 7 instead of 7*, indicating reductive elimination of Ge 0 from 7*. This may be attributed to the electron-rich nature of MIDt entity. Notably, 7 may also be prepared directly by reacting 4 with (L)GeCl 2 in a 2 : 1 ratio (Scheme 2b). Compounds 5, 6 (orange), and 7 (red) are crystalline solids and indefinitely stable under an inert gas atmosphere. The observed 1 H and 13 C NMR signals for the ADC Ph unit of 5-7 are fully consistent with their solid-state molecular structures ( Figure 3).
Further insights into the electronic structures of 5-7 were obtained by computational calculations (see the Supporting Information). The optimized geometries of 5-7 are fully consistent with their X-ray diffraction structures ( Figure 3). The HOMO of 5 and 6 is a π-type orbital mainly located at the C 2 S 2 framework, while the LUMO is based primarily on the ADC unit ( Figure 4a). The HOMO-1 of 5 and 6 is a σ-type lone-pair orbital at the germanium atoms with a considerable contribution from the GeÀ S bonds. The HOMO of 7 is based on the C 2 S 2 Ge framework and has σ-type lone-pair orbital contribution at the germanium atom. The HOMO-1 of 7 is a π-type orbital largely located at the C 2 S 2 framework. Very recently, Gessner et al. reported a diylide-stabilized germylene Y 2 Ge (Y = Ph 3 PCSO 2 Tol) that has the highest lying HOMO (À 5.04 eV), also calculated at the PBE0-D3BJ/def2-TZVPP level of theory, among hitherto known germylenes. [28a] A comparison of HOMO and LUMO energies of 5-7 with literature known germylenes reveals that the energy of the HOMO of 7 (À 4.37 eV) is remarkably high and exceeds that of Y 2 Ge (À 5.04 eV). Moreover, the LUMO of 7 (À 1.55 eV) is lower in energy than that of Y 2 Ge (À 1.12 eV), resulting a very small HOMO-LUMO energy gap of 2.82 eV for 7.
The UV-vis-spectra of 5 and 6 each exhibits an absorption band (λ max ) at 384 and 385 nm ( Figure 5), respectively, which based on TD-DFT analyses may be assigned to HOMO!LUMO transitions (Table S11 and S12). The UV-vis spectrum of 7 shows absorption bands at 394 and 528 nm, which according to TD-DFT studies correspond to the HOMO-1!LUMO and HOMO! LUMO + 1 transitions, respectively (Table S13).
In conclusions, MIDts 3 a-c and 4 have been isolated as crystalline solids and characterized by spectroscopic and X-ray diffraction methods. The use of 4 as a ligand has been shown with germylenes 5, 6, and 7. Electronic structures of 4-7 have been analyzed by DFT calculations. The germylene 7 with a remarkably high-lying HOMO (À 4.37 eV) and low-lying LUMO (À 1.55 eV) has a promising prospective as ligands. Moreover, interesting reactivity of 7 based on its small HOMO-LUMO energy gap (2.82 eV) can be envisioned. Further reactivity studies of 4-7 with small molecules as well as ligands for maingroup and transition metal compounds are currently underway.