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Syntheses and molecular structures of some di(amidino)monosilanes

  • Markus Bös , Marcus Herbig EMAIL logo , Uwe Böhme EMAIL logo and Edwin Kroke
Published/Copyright: September 2, 2021
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Main Group Metal Chemistry
From the journal Main Group Metal Chemistry Volume 44 Issue 1

Abstract

The syntheses of three different amidinosilanes of the type Me2Si[N=C(Ph)R]2 with R = pyrrolidino, morpholino, and diethylamino and one derivative with the composition R2Si[N=C(Ph)R]2 with R = morpholino are reported. These compounds were prepared in one-pot syntheses including three consecutive steps. All products are analysed by single crystal X-ray diffraction, NMR, and Raman spectroscopy. The Si–N=C–N units of these compounds show characteristic structural features and cause a significant high field shift of the 29Si NMR signals.

Keywords: organosilane; silylation; amidines; crystal structure

1 Introduction

Amidines can be considered as derivates of carboxamides (Figure 1a), wherein the oxygen atom is replaced by an imino group. The resulting compounds have the general structure (Figure 1b). They combine the properties of an amide and of an azomethine double bond. Anions of amidines show unique properties as allyl analogue ligands. Therefore, a large number of amidinate complexes (Figure 1c) of main group and transition metal elements is known (Banjeree et al., 2020, 2021; Edelmann, 1994; Kissounko et al., 2006; Roesky et al., 1988; So et al., 2006). Beside the various applications as ligand molecules in coordination chemistry (Azhakar et al., 2012; Barker and Kilner, 1994; Hao et al., 2000; Roesky et al., 1988;) amidines are interesting for their biological activity (Fry et al., 1994), as precursors for the synthesis of heterocycles (Szczepankiewicz and Suwinski, 1998), and as strong non nucleophilic bases, especially the compounds DBU and DBN (Savoca and Urgaonkar, 2006a, 2006b).

Figure 1 General formulae of carboxamide (a), amidine (b), amidinate complexes (c), and amidinosilanes prepared in this work (d).
Figure 1

General formulae of carboxamide (a), amidine (b), amidinate complexes (c), and amidinosilanes prepared in this work (d).

Since about two decades the research of our group is concerned with different types of silicon-nitrogen compounds like silicon carbodiimides (Balan et al., 2000; Kim et al., 1999; Lippe et al., 2009; Nahar-Borchert et al., 2003; Riedel et al., 1998), amides (Böhme et al., 2000, 2003; Meinel et al., 2014), silazanes (Kroke et al., 2000; Lehnert et al., 2006, 2007; Li et al., 2001), salen complexes (Mucha et al., 1998, 1999; Wagler et al., 2004), spirosilanes (Herbig et al., 2021), and lactamomethylsilanes (Herbig et al., 2019a). The Si–N bond of aminosilanes allows useful transformations, e.g., the insertion of heteroallenes like CO2 and isocyanates to form urea and carbamate derivatives (Herbig et al., 2018a, 2018b, 2019b; Kraushaar et al., 2014, 2017). Herein we present the synthesis and characterization of silylated amidines like in Figure 1d.

Silylated amidines can be synthesised by the reaction of lithiated aminosilanes (Scherer and Hornig, 1968; Yao et al., 2009) or lithium bis(trimethylsilyl)amide (Boeré et al., 1987; Sanger, 1973; Volkis et al., 2006) with a nitrile without α-hydrogen atom, like benzonitrile. To stabilise the intermediates, TMEDA (N,N,N′,N′-tetramethylethylenediamine) can be added (Yao et al., 2009). Additionally, a nitrile can react with a lithiated amine (without silyl moiety) and the product reacts with a chlorosilane to yield silylated amidines by the elimination of lithium chloride (Scherer and Hornig, 1968). More than one amidino moiety can be introduced at one silicon atom, using salt elimination starting with lithiated amidines (Scherer and Hornig, 1968). Another route for the preparation of silylated amidines is the amine-amidine exchange between an amidine and an aminosilane (Beckert et al., 1993). Silylated amidines are used for the synthesis of amidinato complexes (Borgholte et al., 1991; Fenske et al., 1989; Hartmann et al., 1989; Nelkenbaum et al., 2005).

Based on the above-mentioned studies we prepared a set of bifunctionalised monosilanes bearing two amidino substituents (Figure 2), followed by spectroscopic and structural analyses, which are reported here.

Figure 2 Amidinosilanes synthesised in this work.
Figure 2

Amidinosilanes synthesised in this work.

2 Results and discussion

The new amidinomonosilanes, shown in Figure 2, were synthesized by a preparative approach described by Scherer and Hornig (Scherer and Hornig, 1968). The synthesis procedure was optimised with different batches for the synthesis of 2. Optimisation parameters were the use of TMEDA, the solvent (diethylether or THF), and the reflux time. The best procedure is described in the experimental part of this work.

The optimised preparation is a one-pot procedure consisting of three reactions (see also Scheme 1):

  1. The formation of a lithium amide by the reaction of an amine with nBuLi. TMEDA is added in this step to increase the reactivity of the nBuLi and reduce the reaction time. Additionally, intermediates are stabilised and less side products are formed (Yao et al., 2009).

  2. The formation of a lithiated amidine by the reaction of the lithium amide with benzonitrile.

  3. The silylation with dimethyldichlorosilane and elimination of lithium chloride.

Scheme 1 Preparative approach for amidinosilanes 1–3 in this work (HNR2 = pyrrolidine, morpholine, diethylamine).
Scheme 1

Preparative approach for amidinosilanes 1–3 in this work (HNR2 = pyrrolidine, morpholine, diethylamine).

Many side reactions can occur due to the use of highly reactive organolithium reagents, resulting in product mixtures, which can be purified by recrystallisation. The synthesis in diethylether with TMEDA was found to be superior to the reaction in THF and/or with/without TMEDA in terms of the amount of side products. Nevertheless, the lithiated species are only slightly soluble in diethylether. During the attempt to synthesize an amidinosilane with four Si-N(amidine) bonds, the low solubility of the lithiated morpholine leads to a slow reaction in step 2, and unreacted lithium-morpholine reacts in step 3 of the procedure with the chlorosilane to form 4 (Scheme 2). This was somewhat surprising.

Scheme 2 Formation of 4 via a one-pot reaction and substitution of the chlorine atoms of SiCl4 with a lithium amide and a lithiated amidine in one step.
Scheme 2

Formation of 4 via a one-pot reaction and substitution of the chlorine atoms of SiCl4 with a lithium amide and a lithiated amidine in one step.

2.1 Spectroscopy

All products are analysed by NMR and Raman spectroscopy (see experimental section). Comparing the synthesised amidinosilanes with the corresponding aminosilanes, a shift of the 29Si NMR signals to higher shielding is observed (Table 1).

Table 1

29Si NMR chemicals shifts of the di(amidino)dimethylsilanes 1–3 in comparison with related aminosilanes Me2SiNR2

NR2 Me2 Si[N=C(Ph)NR2] Me2SiNR2
pyrrolidine −22.7 −8.1 (Herbig, 2019a)
diethylamine −21.9 −5.6 (Herbig, 2019a)
morpholine −20.5 −4.9 (Herbig, 2019a)

Compound 4 (29Si-NMR: δ = −57.4 ppm) has also a shift to higher shielding compared with tetrapyrrolidinosilane (29Si-NMR: δ = −43.3 ppm; Herbig et al., 2019a). Thus, each amidino group causes a shift of about 7 ppm towards higher shielding compared to the corresponding amino substituted silanes. The shift in shielding can be explained with the sp2 hybridisation of the nitrogen atom bound to the silicon atom and the +M effect of the amidine function. Both facts increase the basicity of the nitrogen atom which leads to higher shielding of the silicon atom.

In the Raman spectra bands between 1985 and 1580 cm−1 should be caused by the C=N stretching vibration (Socrates, 2004). All substances show bands around 1640 and 1599 cm−1 which could be assigned to this vibration. The CH3 deformation vibration of the Si bonded methyl moieties can be assigned to the bands around 1250 cm−1 in all Raman spectra (1290–1240 cm−1; Socrates, 2004).

2.2 Molecular Structures

Compound 1 crystallises in the triclinic space group P-1 with one molecule in the asymmetric unit. The molecular structure is shown in Figure 3, essential geometric parameters in Table 2. The compound shows the expected topology as it was predicted on basis of the NMR data. There is a disorder at one of the pyrrolidine rings. It was resolved by a split atom model for C10 with site occupation of 0.88/0.12. Bond lengths and angles are discussed below for compounds 1 and 2.

Table 2

Selected geometric parameters (Å, °) of 1 and 2

Compound 1 2
Si1–N1 1.724(1) Si1–N1 1.721(1)
Si1–N3 1.721(1) Si1–N3 1.726(1)
Si1–C23 1.873(1) Si1–C23 1.874(2)
Si1–C24 1.866(1) Si1–C24 1.867(2)
N1–C1 1.281(2) N1–C1 1.283(2)
C1–N2 1.364(2) C1–N2 1.370(2)
N2–C8 1.466(2) N2–C8 1.467(2)
N2–C11 1.468(2) N2–C10 1.465(2)
N3–C12 1.278(2) N3–C12 1.279(2)
N4–C12 1.366(2) N4–C12 1.372(2)
N4–C19 1.463(1) N4–C19 1.466(2)
N4–C22 1.467(2) N4–C21A 1.493(3)
N4–C21B 1.508(4)
N1–Si1–N3 109.47(5) N1–Si1–N3 110.99(6)
N1–Si1–C23 108.86(6) N1–Si1–C23 108.15(6)
N1–Si1–C24 106.10(6) N1–Si1–C24 105.52(6)
N3–Si1–C23 117.96(5) N3–Si1–C23 116.71(6)
N3–Si1–C24 105.68(6) N3–Si1–C24 106.43(6)
C23–Si1–C24 108.10(7) C23–Si1–C24 108.45(7)
C1–N1–Si1 131.24(9) C1–N1–Si1 131.7(1)
C12–N3–Si1 137.87(9) C12–N3–Si1 134.6(1)
C1–N2–C8 120.5(1) C1–N2–C8 119.3(1)
C1–N2–C11 126.9(1) C1–N2–C10 123.8(1)
C8–N2–C11 112.1(1) C8–N2–C10 116.6(1)
Sum of angles at N2 359.43 359.7
C12–N4–C19 121.12(9) C12–N4–C19 123.1(1)
C12–N4–C22 125.7(1) C12–N4–C21A 117.4(2)
C19–N4–C22 111.81(9) C19–N4–C21A 117.3(2)
C12–N4–C21B 119.7(2)
C19–N4–C21B 114.4(2)
Sum of angles at N4 358.67 357.83 (for A)
357.2 (for B)
Figure 3 Molecular structure of 1 (top) and 2 (bottom) including numbering scheme. The thermal displacement ellipsoids of the non-hydrogen atoms are drawn at the 50% probability level.
Figure 3

Molecular structure of 1 (top) and 2 (bottom) including numbering scheme. The thermal displacement ellipsoids of the non-hydrogen atoms are drawn at the 50% probability level.

Compound 2 crystallises in the monoclinic space group P21/c with one molecule in the asymmetric unit. The molecular structure is shown in Figure 3, essential geometric parameters in Table 2. The compound shows the expected topology. There is a disorder at one of the ethyl groups at N4. It was resolved by a split atom model for C21 and C22 with a site occupation of 0.62/0.38. The ethyl group C19–C20 shows also elongated thermal displacement ellipsoids, but it was not possible to resolve a disorder in this case.

Compounds 1 and 2 show very similar geometric features. Both silicon atoms are in distorted tetrahedral coordination geometry. The silicon nitrogen bonds vary between 1.721(1) to 1.726(1) Å. The silicon carbon bonds are in the normal range for Si–C(sp3) bonds (Kaftory et al., 1998). The C=N double bonds vary between 1.278(2) Å for N3–C12 in 1 and 1.283(2) Å for N1–C1 in compound 2. Bond angles at the imine nitrogen atoms N1 and N3 are between 131.24(9) and 137.87(9)°, which is to be expected for sp2 nitrogen atoms. Besides, the Si–N=C–N moieties in both compounds are nearly planar. Torsion angles Si–N=C–N vary between 170.5(1) and 178.1(1)°. The phenyl groups are orientated nearly perpendicular to the amidine units. The nitrogen atoms N2 and N4 are planarized with a sum of the bond angles around these atoms near 360° (Table 2). Compound 1 contains two pyrrolidine rings. These are in twist conformation, for details see Supporting Information. There are no hydrogen bonds in the crystal structures of 1 and 2.

Compound 3 crystallises in the trigonal space group P3121 with a half molecule in the asymmetric unit. The full molecule is generated by a 31 screw axes going through Si1. The molecular structure is shown in Figure 4, essential geometric parameters in Table 3. The compound shows the expected topology. There are no disorders.

Table 3

Selected geometric parameters (Å, °) of 3 and 4

Compound 3 4
Si1–N1 1.715(1) Si1A–N1A 1.7118(9)
Si1–C12 1.868(2) Si1A–N3A 1.7178(9)
N1–C1 1.273(2) N1A–C1A 1.279(1)
N2–C1 1.375(2) C1A–N2A 1.369(1)
N1–Si1–N1#1 102.00(9) N1A#2–Si1A–N1A 108.72(6)
N1–Si1–C12 107.46(8) N1A#2–Si1A–N3A#2 119.49(4)
N1#1–Si1–C12 116.07(8) N1A–Si1A–N3A#2 102.13(4)
N1–Si1–C12#1 116.07(8) N1A#2–Si1A–N3A 102.13(4)
N1#1–Si1–C12#1 107.46(8) N1A–Si1A–N3A 119.49(4)
C12–Si1–C12#1 108.0(1) N3A#2–Si1A–N3A 105.88(7)
C1–N1–Si1 136.1(1) C1A–N1A–Si1A 132.16(8)
C1–N2–C11 120.7(1) C1A–N2A–C8A 125.52(9)
C1–N2–C8 125.5(2) C1A–N2A–C11A 119.87(9)
C11–N2–C8 113.5(1) C8A–N2A–C11A 113.31(9)
Sum of angles at N2 359.68 Sum of angles at N2A 358.7
C12A–N3A–C15A 110.07(9)
C12A–N3A–Si1A 121.28(8)
C15A–N3A–Si1A 127.03(8)
Sum of angles at N3A 358.38

  1. Symmetry transformations used to generate equivalent atoms: #1 x-y,-y,-z+2/3; #2 -x+3/2,y,-z+1/2

Figure 4 Molecular structure of 3 including numbering scheme. The thermal displacement ellipsoids of the non-hydrogen atoms are drawn at the 50% probability level.
Figure 4

Molecular structure of 3 including numbering scheme. The thermal displacement ellipsoids of the non-hydrogen atoms are drawn at the 50% probability level.

Compound 4 crystallises in the monoclinic space group P2/n with two half molecules in the asymmetric unit. The full molecules are generated by twofold axes going through the silicon atoms. The molecular structure of one molecule is shown in Figure 5, essential geometric parameters in Table 3. Discussion of structural parameters is restricted here to this molecule in order to keep the discussion short. The other molecule shows very similar features.

Figure 5 Molecular structure of 4 including numbering scheme. The thermal displacement ellipsoids of the non-hydrogen atoms are drawn at the 50% probability level.
Figure 5

Molecular structure of 4 including numbering scheme. The thermal displacement ellipsoids of the non-hydrogen atoms are drawn at the 50% probability level.

The silicon atoms in compounds 3 and 4 are in distorted tetrahedral coordination geometry. The silicon amidine nitrogen bonds are at 1.715(1) Å in 3 and 1.7118(9) Å in 4 and are shorter than in the compounds 1 and 2. This can be explained for compound 4 with the presence of morpholino groups at silicon instead of methyl groups. This is in line with the finding that shorter bonds are found for compounds in which the iconicity of the bond is enhanced by the attachment of electronegative substituents to silicon (Kaftory et al., 1998). The C=N double bonds are at 1.273(2) Å in 3 and 1.279(1) Å in compound 4. Bond angles at the imine nitrogen atoms are at 136.1(1)° in 3 and 132.16(8)° in 4. The phenyl groups are orientated perpendicular to the amidine units. The nitrogen atoms N2 and N2A are planarized with sum of bond angles around these atoms near 360° (Table 3). Compound 4 contains additional morpholino groups at the silicon atom. The nitrogen atom N3A has a sum of bond angles of 358.38 which is also nearly planar. The Si–N=C–N units in compounds 3 and 4 are not planar. Torsion angles are −169.0(1)° for Si1–N1–C1–N2 in 3, −163.40(9) for Si1A–N1A–C1A–N2A in 4, and 173.62(9)° for Si1B–N1B–C1B–N2B in 4. All morpholino groups in 3 and 4 are in chair conformation (for details see Supplementary material). The crystal structure of 4 contains several intermolecular hydrogen bonds (please see Supplementary material).

3 Conclusions

Di(amidino)monosilanes can be synthesised in an effective one-pot synthesis. This includes three steps. It was possible to minimise side reactions by developing an optimised preparative procedure. The compounds 1–4 were isolated and obtained after recrystallization as pure compounds. Single crystal X-ray diffraction proved the identity of the products. The chemical shift in the 29Si NMR spectra tend to higher shielding compared with the corresponding aminosilanes due to the sp2 hybridisation of the nitrogen atom bonded to the silicon atom and the +M effect of the amidino moiety.

Experimental

All reactions were carried out under argon using Schlenk technique (Böhme, 2020; Herzog, 1964). Used chemicals and purification methods can be found in Supplementary material. NMR spectra were recorded in CDCl3 with TMS as internal standard either on a BRUKER DPX 400 spectrometer at 400.13, 100.61 and 79.49 MHz for 1H, 13C, and 29Si, respectively, or on a BRUKER AVANCE III 500 MHz spectrometer at 500.13, 125.76, and 99.36 MHz for 1H, 13C and 29Si, respectively. Raman spectra were measured with a FT-Raman spectrometer RFS/100S from BRUKER using an air-cooled Nd: YAG-laser with a wavelength of 1064 nm and a nitrogen-cooled germanium detector. Melting points were measured using a Polytherm A hot stage microscope from Wagner and Munz with an attached 52II thermometer from Fluke. Mass spectra were recorded on an Expression L CMS from Advion using ESI.

General synthesis procedure (compound 1 as example)

In a three necked round bottom flask with reflux condenser and dropping funnel 3.35 g pyrrolidine (47.10 mmol) were mixed with 0.30 g TMEDA (2.58 mmol) in about 60 mL of diethylether. The mixture was cooled in a dry ice/iso-propanol bath and 15.10 g nBuLi solution (2.3 M in n-hexane/cyclohexane, 47.1 mmol nBuLi) were added dropwise. After the addition the funnel was purged with 10 mL of diethylether and the mixture was allowed to warm up to room temperature and stirred for 30 min. The mixture was cooled in an ice/water bath and 4.84 g benzonitrile (46.97 mmol) in 15 mL of diethylether were added dropwise. The mixture turned yellowish. The funnel was purged with 5 mL of diethylether and the mixture was stirred 30 min at room temperature. 3.00 g dimethyldichlorosilane (22.25 mmol) dissolved in 10 mL of diethylether were added at room temperature to the solution. A white solid formed, and the funnel was purged with 5 mL of diethylether. After stirring for 45 min at room temperature, the solution was refluxed for 1 h. The white solid was filtered of, and all volatile compounds were removed under reduced pressure yielding an orange residue. After standing overnight with a small amount of CDCl3 (from taking the NMR sample), a green solid in a yellow solution is formed.

Dimethyl-di(pyrolidinobenzylamidino)silane (1)

Yield: 4.09 g (45%) green crystals – mp: 117°C – Raman: ν [cm−1] = 3055 (vs), 3040 (w), 3021 (vw), 2973 (s), 2953 (m), 2934 (w), 2893 (m), 2878 (m), 2849 (m), 1640 (w), 1599 (w), 1461 (vw), 1252 (vw), 1210 (vw), 1177 (vw), 1156 (vw), 1027 (vw), 1000 (m), 917 (vw), 656 (vw), 639 (vw), 616 (vw), 437 (vw), 325 (vw), 296 (vw), 259 (vw), 197 (vw), 161 (vw), 107 (m) – 1H-NMR (CDCl3, 400 MHz): δ [ppm] = −0.53 (s; 6 H; Si-CH3), 1.80 (s; 8 H, CH2), 2.8–3.70 (m; 8 H; CH2-N), 7.14–7.17 (m; 4 H; Ar), 7.24–7.28 (m; 6 H; Ar) – 13C-NMR (CDCl3, 100 MHz): δ [ppm] = −2.3 (Si-CH3), 124.5–125.4 (Ph), 138.6 (ispo-Ph), 157.5 (amidine) – 29Si-NMR (CDCl3, 79 MHz): δ [ppm] = −22.7 – MS: m/z = 405.392 ([M+H]+).

Dimethyl-bis(diethylaminobenzylamidino)silane (2)

This compound was synthesized using 3.42 g diethylamine (46.72 mmol) and 0.26 g TMEDA (2.23 mmol) in 40 mL of diethylether, 16.65 g nBuLi solution (2.5 M in n-hexane, 52.0 mmol nBuLi), 4.80 g benzonitrile (46.20 mmol) in 15 mL of diethylether and 3.02 g dimethyldichlorosilane (23.55 mmol) in 5 mL of diethylether. The crude product was an oil which was washed with n-pentane yielding a powder which crystallises from CDCl3 after two years.

Yield: 0.316 g yellowish hygroscopic crystals (3%) – mp: 62°C (decomp.) – Raman: ν [cm−1] = 3055 (vs), 2965 (s), 2932 (s), 2899 (s), 2874 (m), 1601 (w), 1576 (w), 1453 (w), 1409 (vw), 1366 (vw), 1283 (vw), 1196 (vw), 1179 (vw), 1158 (vw), 1092 (vw), 1081 (vw), 1029 (vw), 1000 (s), 709 (vw), 687 (vw), 647 (vw), 616 (vw), 575 (vw), 493 (vw), 435 (vw), 404 (vw), 377 (vw), 338 (vw), 300 (vw), 213 (vw), 201 (vw), 120 (m) – 1H-NMR (CDCl3, 400 MHz): δ [ppm] = −0.64 (s, SiMe2), 1.03 (m, CH3), 3.29 (m, CH2N), 7.07–7.25 (m, Ar), due to impurities no integral values can be given – 13C-NMR (CDCl3, 100 MHz): δ [ppm] = 2.4 (Si–CH3), 13.4 (CH3), 47.0 (CH2N), 126.9 (m-Ar), 127.7 (p-Ar), 128.5 (o-Ar), 140.2 (i-Ar), 160.2 (amidine) – 29Si-NMR (CDCl3, 79 MHz): δ [ppm] = −21.9 – MS: m/z = 409.425 ([M+H]+).

Dimethyl-di(morpholinobenzylamidino)silane (3)

This compound was synthesized using 4.06 g morpholine (46.61 mmol) and 0.31 g TMEDA (2.67 mmol) in 60 mL of diethylether, 15.20 g nBuLi solution (2.3 M in n-hexane/cyclohexane, 47.5 mmol nBuLi), 4.81 g benzonitrile (46.68 mmol) in 15 mL of diethylether and 3.01 g dimethyldichlorosilane (23.32 mmol) in 5 mL of diethylether. The crude product was an oil which was washed with n-pentane yielding a powder which crystallises from CDCl3 after two years.

Yield: 1.73 g (17%) yellow solid – mp: 114–117°C – Raman: ν [cm−1] = 3149 (vw), 3055 (vs), 3040(m), 3021 (w), 2973 (s), 2953 (s), 2934 (m), 2894 (s), 2879 (s), 2850 (s), 2662 (vw), 2630 (vw), 2581 (vw), 2558 (vw), 2183 (vw), 2061 (vw), 1640 (m), 1609 (m), 1599 (m), 1575 (w), 1488 (vw), 1461 (w), 1452 (w), 1402 (vw), 1402 (vw), 1337 (vw), 1253 (vw), 1225 (w), 1210 (vw), 1177 (vw), 1157 (vw), 1118 (w), 1075 (vw), 1027 (vw), 1000 (v), 973 (s), 940 (vw), 916 (vw), 857 (w), 769 (vw), 711 (vw), 676 (vw), 657 (w), 639 (w), 617 (w), 576 (vw), 537 (vw), 437 (vw), 390 (vw), 364 (w), 324 (w), 295 (w), 259 (w), 198 (w), 162 (w), 106 (s) – 1H-NMR (CDCl3, 400 MHz): δ [ppm] = −0.48 (s, 6 H, Si–CH3), 3.30 (br, 8 H, CH2–N), 3.57 (m, 8 H, CH2–O), 7.08 (m, 4 H, m-Ar), 7.27 (m, 6H, Ar) – 13C-NMR (CDCl3, 100 MHz): δ [ppm] = 1.8 (Si-CH3), 66.9 (morpholine), 126.9 (m-Ar), 127.6 (p-Ar), 127.8 (o-Ar), 138.6 (i-Ar), 160.4 (amidine) – 29Si-NMR (CDCl3, 79 MHz): δ [ppm] = −20.5.

Dimorpholino-di(morpholinobenzylaminidino)silane (4)

This compound was synthesized using 1.77 g morpholine (20.32 mmol) and 0.15 g TMEDA (1.29 mmol) in 50 mL of diethylether, 6.75 g nBuLi solution (2.3 M in n-hexane/cyclohexane, 21.1 mmol nBuLi), 2.20 g benzonitrile (21.35 mmol) in 7 mL of diethylether and 0.9 g tetrachlorsilane (5.29 mmol) in 5 mL of diethylether. After the addition of nBuLi a white solid is formed which was dissolved during the addition of tetrachlorosilane. After refluxing the solution for 2 h, a new solid was formed. The crude product was an oil which crystallises after the addition of CDCl3 overnight.

Yield: 0.42 g (14%) colourless crystals – mp: 229°C – Raman: ν [cm−1] = 3057 (s), 2994 (w), 2957 (vs), 2936 (w), 2890 (w), 2853 (m), 2818 (w), 2751 (vw), 2697 (vw), 1636 (w), 1601 (w), 1578 (vw), 1441 (vw), 1329 (vw), 1297 (vw), 1212 (vw), 1160 (vw), 1129 (vw), 1031 (vw), 1004 (m), 838 (vw), 668 (vw), 616 (vw), 508 (vw), 483 (vw), 261 (vw), 232 (vw), 153 (vw), 105 (m) – 1H-NMR (CDCl3, 400 MHz): δ [ppm] = 2.53 (m, 8H, CH2–N–Si), 3.27 (m, 16 H, amidine–morpholine), 3.62 (m, 8H, CH2–O), 7.17 to 7.32 (m, 10 H, Ar) – 13C-NMR (CDCl3, 100 MHz): δ [ppm] = 45.3 (CH2–N–Si), 67.1 and 68.5 (morpholines), 127.2 (m-Ar), 128.2 (Ar), 139.2 (i-Ar), 160.9 (amidine) – 29Si-NMR (CDCl3, 79 MHz): δ [ppm] = −57.4 – MS: m/z = 579.4 ([M+H]+).

Crystal structure determination

Single crystal X-ray diffraction of 1–4 was performed on a STOE IPDS-2T image plate diffractometer equipped with a low-temperature device with Mo-Kα radiation (λ = 0.71073 Å) using ω and ϕ scans. Software for data collection: X-AREA, cell refinement: X-AREA and data reduction: X-RED (Stoe & Cie, 2009). Preliminary structure models were derived by direct methods (Sheldrick, 2008) and the structures were refined by full-matrix least-squares calculations based on F2 for all reflections using SHELXL (Sheldrick, 2015). All hydrogen atoms were included in the models in calculated positions and were refined as constrained to the bonding atoms. Compound 3 crystallises in the chiral space group P3121 (No. 152). The molecule itself is not chiral, but the crystal structure has a polar 31 axis. The absolute structure parameter was refined to a value of 0.01(3). Further crystallographic data are listed in Table 4.

Table 4

Crystal data and structure refinement for 1–4

Compound 1 2 3 4
Formula C24H32N4Si C24H36N4Si C24H32N4O2Si C30H42N6O4Si
M (g.mol−1) 404.62 408.66 436.62 578.78
T (K) 153 153 153 153
Crystal system triclinic monoclinic trigonal monoclinic
Space group P-1 P21/c P3121 P2/n
a (Å) 8.3171(7) 18.8416(9) 8.6527(3) 18.4190(8)
b (Å) 8.8211(9) 7.3278(2) 8.6527(3) 9.5671(3)
c (Å) 17.8894(16) 17.2892(7) 27.6907(12) 18.9056(8)
α (°) 75.894(7) 90 90 90
β (°) 78.193(7) 98.429(4) 90 116.280(3)
γ (°) 63.192(6) 90 120 90
V (Å3) 1129.27(19) 2361.29(16) 1795.43(15) 2987.1(2)
Z 2 4 3 4
D (calc) (g·cm−3) 1.190 1.150 1.211 1.287
μ (mm−1) 0.121 0.116 0.125 0.124
F(000) 436 888 702 1240
Index ranges −10 ≤ h ≤ 10, −24 ≤ h ≤ 23, −11 ≤ h ≤ 10, −23 ≤ h ≤ 23,
−11 ≤ k ≤ 11, −9 ≤ k ≤ 9, −11 ≤ k ≤ 11, −12 ≤ k ≤ 12,
−23 ≤ l ≤ 23 −22 ≤ l ≤ 22 −35 ≤ l ≤ 35 −24 ≤ l ≤ 24
Reflections collected/unique 17027/5183 [R(int) = 0.0214] 27117/5224 [R(int) = 0.0356] 23666/2667 [R(int) = 0.0216] 41288/6859 [R(int) = 0.0358]
GOF on F2 1.070 1.057 1.146 1.064
Final R indices [I > 2sigma(I)] R1 = 0.0375, wR2 = 0.0932 R1 = 0.0369, wR2 = 0.0853 R1 = 0.0256, wR2 = 0.0649 R1 = 0.0323, wR2 = 0.0816
R indices (all data) R1 = 0.0433, wR2 = 0.0995 R1 = 0.0477, wR2 = 0.0933 R1 = 0.0276, wR2 = 0.0668 R1 = 0.0395, wR2 = 0.0879
Residual density (e Å−3) 0.567/–0.355 0.285/–0.231 0.221/–0.135 0.271/–0.299

Acknowledgement

The authors thank TU Bergakademie Freiberg (Freiberg, Germany) for financial support. E. Brendler (Institute for Analytical Chemistry, TU Bergakademie Freiberg) and B. Kutzner (Institute for Inorganic Chemistry) are acknowledged for help with the NMR measurements, K. Kraushaar (Institute for Inorganic Chemistry) for mass spectrometric and R. Moßig (Institute for Inorganic Chemistry) for Raman spectroscopic measurements.

  1. Funding information:

    Part of this work was performed within the research group “Chemical utilization of carbon dioxide with aminosilanes (CO2-Sil)” that is financially supported by the European Union (European regional development fund), the Ministry of Science and Art of Saxony (SMWK) and the Sächsische Aufbaubank (SAB).

  2. Author contributions:

    Markus Bös: methodology, formal analysis, investigation, data curation; Marcus Herbig: writing – original draft, writing – review and editing, formal analysis, visualization, project administration, investigation, data curation; Uwe Böhme: writing – original draft, writing – review and editing, formal analysis, visualization, investigation, data curation; Edwin Kroke: supervision, resources, funding acquisation, writing – review and editing.

  3. Conflict of interest:

    Authors state no conflict of interest.

  4. Data availability statement:

    CCDC 2075878 - 2075881 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

  5. Supplementary material:

    Supplementary material is available for this publication.

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Received: 2021-05-03
Accepted: 2021-07-01
Published Online: 2021-09-02

© 2021 Markus Bös et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/mgmc-2021-0024
Keywords for this article
organosilane; silylation; amidines; crystal structure
Creative Commons
BY 4.0

Articles in the same Issue

  1. Research Articles
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