Tetravalent Group 14 Derivatives of a Bulky Aminocarbazole

Stepwise metalation and metathesis reactions with a bulky aminocarbazole were conducted to prepare derivatives of tetravalent group 14 elements. These were regarded as putatively suitable precursors for the formation of doubly bonded group 14/group 15 molecules such as imino species. Starting from an N-aminocarbazole, deprotonation with benzyl potassium formed the corresponding solvent-free dimeric amide. Metathesis reactions with EBr4 (E=Si, Ge, Sn) afforded RN(H)EBr3. Deprotonation of RN(SiMe3)H with benzyl potassium afforded the solvent-free monomeric amide RN(SiMe3)K which was then treated with SiCl4, GeBr4 and SnI4. Both obtained series of compounds, RN(H)EBr3 and RN(SiMe3)EX3, were characterized by multinuclear NMR spectroscopy and SCXRD studies.


Introduction
Among the multiple bond systems involving heavy group 14 elements, an example for a polar bond is the double bond to nitrogen atoms. [1] Several examples of stable silaimines are known and structurally characterised, but only few methods are known for the synthesis of imino compounds of group 14 elements.
Wiberg and Müller reported on the first silaimine t Bu 2 Si=NÀ Si t Bu 3 which was obtained by metathesis of t Bu 2 SiClN 3 with NaSi t Bu 3 and subsequent elimination of N 2 . [2] This method, however, was not extended to other examples. More general access is provided by the reaction of silylenes with organoazides and concomitant elimination of N 2 . [3][4][5] A similar reaction was found by So and coworkers who treated the disilyne [PhC (N t Bu) 2 Si] 2 with bis(Dipp)carbodiimide (Dipp = 2,6-bis(diisopropyl)phenyl) and obtained the silylenylsilaimine [PhC(N t Bu) 2 Si {NDipp}-Si(N t Bu) 2 CPh]. [6] Recently, Lips induced extrusion of DippN(SiMe 3 ) 2 from [Dipp(SiMe 3 )NSi] 4 by carbene addition which afforded a cyclic Si 4 unit with both a silylone and silaimine moiety. [7] The seemingly most generally applicable synthetic route to silaimines is salt elimination as reported by Klingebiel who investigated the lithiation of aminofluorosilanes. [8][9][10] Examples for the even heavier germaimines and stannaimines are scarce and these studies were spearheaded by Meller. Germaimines could be generated by treatment of Ge[N(SiMe 3 ) 2 ] with silyl-and arylazides. [11,12] Just one example of a stannaimine is known which is stable only stable up to À 30°C and was obtained by the reaction of Sn[N(SiMe 3 ) 2 ] with DippN 3 . The reactivity towards a variety of small molecules such as alcohols was studied in detail. [13] None of the known imino compounds feature two halide substituents on the heavy tetrel, consequently we made it our long-term goal to synthesise such compounds of the general formula RNEX 2 (E = Si, Ge, Sn; X = Cl, Br, I; R = bulky carbazole). Conceivable precursors for these compounds would be RN(H) EX 3 and RN(SiMe 3 )EX 3 . As certainly some degree of kinetic stabilisation by bulky substituents is required for the imino compounds, we envisaged utilisation of a carbazolyl substituent (R, Scheme 1). This is the logical continuation of work done previously in our group, where transamination reactions of the N-aminocarbazole RNH 2 were studied. [16] Employing a carbazolyl substituent R renders the RN(H)EX 3 and RN(SiMe 3 )EX 3 compounds formal derivatives of hydrazine. Hydrazine derivatives with distinct group 14 substituents, (Me 3 E) 2 NÀ N(EMe 3 ) 2 (E = C, Si, Ge, Sn), were reported by Wiberg and Veith in 1971, but nothing is known about their suitability for elimination reactions. [14] Results and Discussion

Syntheses
Starting from the N-aminocarbazole RNH 2 (1, Scheme 1), stepwise deprotonation and metathesis reactions were planned. As the base of choice, benzyl potassium was employed. The deprotonation of RNH 2 with BzK in toluene afforded [RN(H)K] 2 (2), which forms an unsymmetric dimer in the solid state with no further solvent molecule coordinated to the potassium atom. The amide 2 was then subjected to metathesis reactions with SiBr 4 , GeBr 4 and SnBr 4 in toluene. In all three cases, the reactions proceeded smoothly, and evaporation of all volatiles, extraction with n-hexane and recrystallisation afforded the desired compounds RN(H)EBr 3 (3E, E = Si, Ge, Sn) as colourless, yellow and red compounds, respectively.
All attempted reactions to induce elimination of HBr by addition of metal bases such as t BuLi, BzK, Na[N(SiMe 3 ) 2 ] or organobases such as NHCs or P 4 -phosphazene base were unsuccessful in the regard either no abstraction reaction was observed or the N-N bond broke to afford carbazolides. Consequently, a milder approach was studied, aiming to exploit the elimination of Me 3 SiX from RN(SiMe 3 )EX 3 .
The amide 2 was again a useful starting material for this path of reactivity (Scheme 2). Metathesis with Me 3 SiCl afforded RN(SiMe 3 )H (4) in good yields as colourless crystalline compound. The subsequent deprotonation requires a coordinating donor solvent such as THF to proceed, and with BzK in toluene no reaction was observed. Similarly, attempts of double deprotonation of RNH 2 (1) even in THF never produced RNK 2 but only [RN(H)K] 2 (2).
The silylated amide RN(SiMe 3 )K (5) was generated in THF solution, subsequently the solvent was evaporated, and the brown residue was treated with n-hexane. Immediately, a yellow powder formed. Again, all volatiles were removed in vacuo and the residue was crystallized from warm n-hexane (50°C). The crystal structure and the NMR spectra confirm that no donor solvent is coordinated to the K atom. This silyl amide was then employed in metathesis reactions with EBr 4 (E = Si, Ge, Sn). Surprisingly, only the reaction with GeBr 4 afforded the desired product RN(SiMe 3 )GeBr 3 (6 A) as yellow crystalline material, while all efforts with SiBr 4 and SnBr 4 , regardless of drying and degassing the involved chemicals, only afforded quantitative amounts of RN(SiMe 3 )H. In case of the tin compound, traces of THF were seen as a possible cause as SnCl 4 and SnBr 4 form sparingly soluble adducts with THF, so in situ (in THF) prepared RN(SiMe 3 )K (5) had to be avoided. But even though 5 was prepared solvent-free, but still no desired product was observed. However, when SnI 4 was used in toluene, near quantitative formation of RN(SiMe 3 )SnI 3 (6 C) was apparent from an 1 H NMR spectrum of the reaction mixture. The product could be extracted with n-hexane and obtained as dark red crystalline material in moderate yield. A similar problem occurred in case of metathesis of RN(SiMe 3 )K with SiX 4 , but here no insoluble precipitates are formed. After many variations one experiment was successful: On small scales of 30-40 mg, the reaction of RN (SiMe 3 )K with SiCl 4 in hexane at 50°C for 24 h lead to quantitative consumption of RN(SiMe 3 )K and only marginal amounts of RN(SiMe 3 )H. From the champagne coloured reaction mixture, colourless crystals could be isolated, in which approx. 92 % RN(SiMe 3 )SiCl 3 cocrystallised with 8 % of the known RSiCl 3 . [15] These two compounds could not be separated. No other solvents and none of the other silicon halides produced similar results, and larger scaled reactions repeatedly only afforded RN(SiMe 3 )H.

NMR spectroscopy
The NMR spectra of the aminocarbazole derivatives reveal that for RN(H)K (2), RN(H)EBr 3 (3 Si, 3 Ge, 3 Sn), RN(SiMe 3 )H (4) and RN(SiMe 3 )K (5), the rotation around the NÀ N axis is dynamic on NMR time scale. For instance, in both the 1 H and 13 C NMR spectra, the tertbutyl groups on the arenes give rise to only one set of resonances with a 2 : 1 intensity ratio compared to the carbazole tertbutyl groups. In contrast, in the RN(SiMe 3 )EX 3 molecules (6 A, 6 B, 6 C), this rotation is hindered and consequently, there are two sets of resonances for the arene tertbutyl groups in both 1 H and 13 C NMR spectra. The 1 H NMR resonance of the NH atom of RNH 2 (1) shifts from 3.47 to 1.98 upon deprotonation with benzyl potassium in toluene (2). Metathesis with group 14 halides then causes a shift to higher field with larger effect the heavier the group 14 element is (3 Si 4.99, 3 Ge 5.76, 3 Sn 6.48 ppm, Table 1). A similar effect is observed for the 1 H NMR resonance of the SiMe 3 group of RN (SiMe 3 )H (4): Upon deprotonation it is shifted from À 0.70 ppm to higher energy in 5 À 0.42 ppm) and metatheses with the group 14 halides then cause an downfield shift again (6 A À 0.06, 6 B À 0.04, 6 C À 0.10 ppm). The 15 N NMR resonances could provide information about the correlated with the changes in the chemical environment upon introduction of the tetravelent group 14 elements. However, the carbazole-N NMR resonance is only marginally influenced by different substitution of the amino-N and always found in the range of 124 to 147 ppm, with the exception of the amides 2 and 5 (166.9 and 161.5 ppm). The more telling resonance of the amino-N could Scheme 2. Synthesis of silylated aminocarbazolyl group 14 compounds (R = bulky carbazolyl substituent). not be observed for 6 C because of dynamics which influenced the 1 H-15 N HMBC experiment. The resonance shifts to higher field when heavier group 14 elements are attached to the N atom. This is a trend which is observed both in the RN(H)-and RN(SiMe 3 )-containing series. The 29 Si NMR resonance for the SiBr 3 moiety of 3 Si is found slightly downfield shifted compared to SiBr 4 but comparable to DippN(SiMe 3 )SiBr 3 (3 Si À 63.4, cf. SiBr 4 À 92, DippN(SiMe 3 )SiBr 3 À 63.0 ppm) [17,18] and the corresponding resonance of the SiCl 3 moiety in 6 A (À 22.5 ppm) is at nearly the same frequency as in SiCl 4 (À 20 ppm), [17] DippN (SiMe 3 )SiCl 3 (À 27.5 ppm) [19] or Ar*N(SiMe 3 )SiCl 3 (À 26.9 ppm). [20] Similarly, the 119 Sn NMR resonance is observed at higher energy for the aminocarbazole derivatives 3 Sn and 6 C (3 Sn À 383.4, 6 C À 1141.4 ppm) compared to the free tetrahalides (SnBr 4 À 638, SnI 4 À 1701 ppm). [21]

Crystal structures
The molecular structures of all compounds were determined by single crystal X-ray diffraction ( Table 2). As expected for 2 the potassium amide without donor solvents present, there are contacts between K and arene-C atoms between 3.0 and 3.5 Å indicative of π interactions (Figure 1). Within the series of aminocarbazolyl compounds perhaps the most unusual feature is the slightly shortened NÀ N bond in 3 Si, 3 Ge and 3 Sn (1.400(2), 1.390(5) and 1.397(5) Å), compared to both the other compounds in the series as well as to the sum of covalent radii of 1.42 Å [22] and other main group molecules containing the hydrazine structural motif which display NÀ N bond lengths in the range of 1.42-1.46 Å, [23][24][25][26][27][28] even in the instance of the previously reported divalent aminocarbazolyl compounds. [16] The other bond metrics compare well to known bulky aryl (silyl)amido trihalosilanes, such as DippN(SiMe 3 )SiCl 3 , DippN (SiMe 3 )SiBr 3 and Ar*N(SiMe 3 )SiCl 3 . [18][19][20] It is interesting to note that in 6 A and 6 C there is no disorder of the SiMe 3 and SiCl 3 or SnI 3 group with swapped positions. The SiCl 3 group of 6 A points into the cavity formed by the arenes, while in 6 C the SnI 3 group points out of the cavity (Figure 2). In contrast, in 6 B the SiMe 3 and GeBr 3 moiety are disordered with swapped positions. The NÀ E (E = Si, Ge, Sn) distances fall within the expected ranges.
In the sequence of 3Si, 3Ge and 3Sn, progressively smaller NNE (E = Si, Ge, Sn) angles are observed which is expected when considering the larger EX 3 groups interact more strongly with the flanking arenes and are consequently pushed out of the provided pocket. [a] GeBr 3 and SiMe 3 moiety are positionally disordered.

Conclusion
We prepared six derivatives RN(H)EBr 3 and RN(SiMe 3 )EX 3 (E = Si, Ge, Sn; X = Cl, Br, I) of a bulky N-aminocarbazole R by stepwise deprotonation and metathesis reactions. These compounds appeared to be suitable precursors for the synthesis of imino compounds of the heavier group 14 elements, RNEX 2 , but to date all efforts were futile. However, the goal warrants further efforts directed towards facilitating the elimination of HX or Me 3 SiX from these formal hydrazine derivatives. Alternatively, other bulky substituents will have to be employed.

Experimental Section
General considerations: NMR spectra were acquired on a Bruker Avance 400 MHz spectrometer. Reported chemical shifts are referenced to the 1 H and 13 C NMR resonances of the deuterated solvent. [29] Coupling constants J are given in Hertz as positive values regardless of their real individual sign. 1