Crystal structure and thermal decomposition of the Silanimine tBu₂Si=N-SitBu₃·thf

1 Introduction

In the past few decades, the reactivity of unsaturated silicon compounds has been extensively studied. In this period, several examples of stable compounds with Si=N double bonds (silanimines) have been synthesized and structurally characterized [1–18]. Ab-inito calculations have shown that the parent silanimine H₂Si=N–SiH₃ possesses an almost linear Si–N–Si skeleton with a short Si=N double bond and a short Si–N single bond [11]. Additionally, these calculations have revealed a negative charge of 1.71 on the N center of H₂Si=N–SiH₃. It is interesting to note that the molecular structure of the first characterized silanimine tBu₂Si=N–SitBu₃ (1) [1], which bears bulky tBu groups features nearly the same Si=N–Si skeleton as that calculated for H₂Si=N–SiH₃. Apparently, only electronic effects, namely the difference in electronegativity between Si and N, determine the molecular structure of this type of silanimines.

In 1986, Wiberg and co-workers reported the first synthesis of 1 by the reaction of 1 equivalent of the silyl azide tBu₂SiClN₃ with 1 equivalent of the silanides Na[SitBu₃] [19, 20] in the weakly polar solvent Bu₂O (Scheme 1) [1]. However, when this reaction was carried out in thf instead of Bu₂O as solvent, the sodium amide tBu₂SiCl–NNa–SitBu₃ was formed rather than the silanimine 1 [21]. In this context, it should also be noted that the 2: 1 reaction of tBu₂ClSiN₃ with Na[SitBu₃] [19, 20] yielded the [3+2] cycloadduct of tBu₂ClSiN₃ with 1 [22, 23].

Scheme 1

Synthesis of the silanimine tBu₂Si=N–SitBu₃ (1). (i) –N₂, –NaCl, in Bu₂O, –78 °C. (ii) + thf.

Recently, we have reported that the tBu-substituted, donor-free silanimines 1 and tBu₂Si=N–SiPhtBu₂ [24] are thermolabile compounds that already decompose in benzene at r.t. [25]. In the course of their thermolysis, β-H elimination takes place to form isobutene. The purpose of the work described in this paper is to examine the thermal and chemical behavior of the thf-complexed silanimine 1(thf) and to make a comparison with the reactivity of its methyl-substituted congener Me₂Si=N–SitBu₃ · thf (2(thf)). Additionally, an X-ray crystal structure determination of the thf-complexed silanimine 1(thf) was performed to compare its structural parameters with those of the parent silanimine 1.

2 Results and Discussion

Following the approach of Wiberg et al. [1], we first prepared the donor-free silanimine 1 by the reaction of tBu₂SiClN₃ with Na[SitBu₃] [19, 20] in the weakly polar solvent Bu₂O. Subsequent treatment of a benzene solution of 1 with thf quantitatively gave the thf-complexed silanimine 1(thf) (Scheme 1). By storing a saturated benzene solution for 1 week at r.t. single crystals of a new polymorph of 1(thf) were obtained. This polymorph crystallized in the monoclinic space group C2/c.

To assess the stability of 1(thf), we conducted a series of thermolysis experiments. When a benzene solution of 1(thf) was heated in the absence of any silanimine trapping agent, no reaction was observed up to 140 °C. However, after heating the reaction mixture to 140 °C for 24 h, we found 16 % decomposition of 1(thf) (70 % after 6 days and 100 % after 11 d at 140 °C), and the ene reaction product of 1 with isobutene, H₂C=C(CH₂SitBu₂–NH–SitBu₃)₂ (3) was formed along with other compounds (cf. Ref. [25]) (Scheme 2). Based on these findings, we concluded that in the thermolysis of 1(thf) the formation of 3 occurs via the donor-free silanimine 1. Interestingly, benzene solutions of 1(thf) can be stored for long periods at r. t. without noticeable decomposition of the silanimine. By contrast, degradation of donor-free 1 takes place in benzene already at r. t. to give 3 [25].

Scheme 2

Decomposition of the thf-complexed silanimines R₂Si=N–SitBu₃ · thf (1(thf), R = tBu; 2(thf), R = Me). (i) -tBuHSiNSitBu₃, in benzene, 11 d at 140 °C. (ii) -H₂C=CH₂, in benzene, 19 h at 110 °C.

As shown in Scheme 2, the thermolysis reaction of 2(thf) in benzene at 110 °C yielded the silanimine dimer 4 in 51 % and the vinyl ether 5 in 49 % yield. It is worth to mention that in the thermolysis of the corresponding silanimine amine adduct Me₂Si=N–SitBu₃ · NMe₂Et the silyl amines EtNMeCH₂SiMe₂–NH–SitBu₃ and EtNMeCH(Me₂Si–NH–SitBu₃)₂ are formed by Stevens rearrangement [25].

The thf-complexed silanimine 1(thf) (Fig. 1) crystallizes together with a half molecule benzene in the asymmetric unit. The selected bond lengths and angles are listed in the caption of Fig. 1.

Fig. 1

Molecular structure of 1(thf) in the solid state. Selected bond lengths (Å) and bond angles (deg): Si(1)–N(1) 1.6025(16), Si(1)–O(1) 1.9151(15), Si(1)–C(21) 1.917(2), Si(1)–C(11) 1.922(2), Si(2)–N(1) 1.6714(16), Si(2)–C(41) 1.952(2), Si(2)–C(31) 1.954(2), Si(2)–C(51) 1.960(2); N(1)–Si(1)–O(1) 104.36(7), N(1)–Si(1)–C(21) 117.05(9), O(1)–Si(1)–C(21) 100.13(8), N(1)–Si(1)–C (11) 118.30(9), O(1)–Si(1)–C(11) 99.85(9), C(21)–Si(1)–C(11) 113.02(10), N(1)–Si(2)–C(41) 109.11(9), N(1)–Si(2)–C(31) 109.82(8), C(41)–Si(2)–C(31) 109.72(9), N(1)–Si(2)–C(51) 111.44(8), C(41)–Si(2)–C(51) 108.48(9), C(31)–Si(2)–C(51) 108.24(9), Si(1)–N(1)–Si(2) 172.47(11).

The present structure is that of a pseudo polymorph of the silanimine · thf adduct, which was reported by Wiberg et al. [1]. Whereas this adduct crystallizes in the primitive monoclinic space group P2₁/m with Z′ = 0.5, the structure which is presented in Fig. 1 has the monoclinic C-centered space group C2/c with Z′ = 1. Since no coordinates and therefore no structural parameters of the literature-known polymorph (monoclinic space group P2₁/m) are available, no comparison of structural parameters of the two polymorphs can be made.

As shown in Table 1, the thf-complexed silanimine 1(thf) displays a Si–N–Si angle somewhat smaller than 180°, and smaller than that in the donor-free silanimine 1, but significantly larger than those in the thf adduct 2(thf) [1: Si–N–Si = 177.8(2)°; 1(thf): Si–N–Si = 172.47(11)°; 2(thf): Si–N–Si = 161.0(6)°, 161.5(6)°]. The Si=N double bond [1.6025(16) Å] in 1(thf) is longer than the bond of the parent silanimine 1 and those of the thf adduct 2(thf) [1: Si=N = 1.568(3) Å; 1(thf): Si=N = 1.6025(16) Å; 2(thf): Si=N = 1.574(10), 1.588(9) Å]. As depicted in Table 1, the N–Si single bonds in the parent silanimine 1 and in the silanimine adducts 1(thf) and 2(thf), however, are remarkably shorter than those found in related supersilylated nitrogen compounds.

While the chemical behavior of the thf-supported silanimines 1(thf) resembles that of the corresponding donor-free silanimine 1 (e.g., ene reaction with acetone or insertion reaction with water as trapping agent [26]), the use of the thermally stable thf-complex 1(thf) instead of the parent silanimine 1 appears to be particularly advantageous for preparative chemistry.

3 Experimental section

The solvents thf, pentane, benzene, and C₆D₆ were stirred over sodium/benzophenone and distilled prior to use. 1 [1] and 2(thf) [1] were prepared according to the published procedures. All other starting materials were purchased from commercial sources and used without further purification. The NMR spectra were recorded on Bruker AM 250, DPX 250, Avance 400, and Avance 500 spectrometers. NMR chemical shifts are reported in ppm. Abbreviations d = doublet; dd = doublet of doublets. For quantitative separation, the dried filtrate was redissolved and separated by HPLC (Reprosil-Pur C18-AQ, 250 mm × 20 mm, 10 μm, Maisch GmbH, Germany; Sykam S3310 UV detector, λ = 254 nm; Sykam Refractive Index Monitor RI2000) with isocratic elution.