Hydrogen-bonding network of N,N′-bis­[2-(tert-butyl­dimethyl­siloxy)­ethyl]­ethyl­ene­di­ammonium dichloride

The title salt, C₁₈H₄₆N₂O₂Si₂²⁺·2Cl⁻, has been synthesized by reaction of N,N′-bis­(2-hy­droxy­ethyl)­ethyl­ene­di­amine with tert-butyl­dimethyl­silyl chloride. The zigzag backbone dication is located across an inversion centre and the two chloride anions are related by inversion symmetry. The ionic components form a supra­molecular two-dimensional network via N—H...Cl hydrogen bonding, which is responsible for the high melting point compared with the oily compound N,N′-bis­[2-(tert-butyl­dimethyl­siloxy)­ethyl]­ethyl­ene­di­amine.

S1. Comment

There are a variety of hydroxyl-group protection methods in organic synthesis (Greene & Wuts, 1991; Kocienski, 2005). Silyl ethers are usually used as protecting groups for alcohols. The reagent tert-butyldimethylsilyl chloride in a basic environment can react readily with alcohols to form tert-butyldimethylsilyl ether. This method is highly specific in the presence of amino or imino groups, and the subsequent deprotection is easy to achieve with fluoride.

In our efforts to protect the two hydroxyl groups of N,N'-bis(2-hydroxylethyl)ethylenediamine with tert-butyldimethylsilyl chloride accompanied by triethylamine, which has not been reported in the literature before, we unexpectedly obtained the title compound, (I). Although the hydroxyl groups were transformed to silyl ether groups as expected, the two imine groups were unfortunately protonated, despite excess triethylamine being used. A similar protection process normally keeps the imino group intact. Although in a few references the silyl ethers which contain an imine group are declared as hydrochloride salts (Nemoto et al., 2012; Zhai et al., 2009; Shin et al., 2008; Diaz-Oltra et al., 2008; Breccia et al., 2003; Herdeis & Telser, 1999; Herdeis & Schiffer, 1999; Fahrni & Pfaltz, 1998; Pohlmann et al., 1997), the structures have not been clearly established. There is only one crystallographic analysis of such a compound, namely (1R,2S)-(-)-2-benzylamino-1- tert-butyldimethylsiloxy-1-phenylpropane hydrochloride, which was in fact not obtained directly in the silyl ether protection process (Gorter & Brussee, 1992).

The diffraction data for (I) are of high quality and the amino H atoms can be located clearly in the difference Fourier map. As shown in Fig. 1, the cation of (I) lies across an inversion centre. The six methylene groups and the two amino groups between the two silyl O atoms are aligned as a zigzag line. The two terminal tert-butyldimethylsiloxy groups are positioned as in other silyl ether compounds (Yao & Lu, 2011; Dutta et al., 2011; Johansson et al., 2004; White & Hansen, 2002; Robinson & Donahue, 1992; Habich et al., 1988). Each chloride anion is located in the vicinity of the amino groups, forming three kinds of N—H···Cl hydrogen bond with three nearby parallel silyl ether dications (Table 1 and Fig. 2). As shown in Fig. 2, the connection of these N—H···Cl hydrogen bonds leads to two patterns of circles, which can be encoded as R₂²(4) and R₄²(8) according to graph-set theory for hydrogen-bond patterns in organic compounds (Etter, 1990). One amino group [Which?] is involved in an R₂²(4) and a neighbouring R₄²(8) ring. The infinite expansion of alternately linked rings form a one-dimensional straight belt. The other amino group [Which?] participates in another belt, so each silyl ether fragment is involved in two parallel belts. Due to the large steric effect of the silyl groups, neighbouring parallel silyl ether dicationic rods are aligned in a staggered arrangement. Parallel belts linked by the silyl ether rods constitute a two-dimensional network (Fig. 2). No significant interactions are found between the two-dimensional layers.

The crystal structure analysis of (I) has solved a significant chemistry problem for us. Before obtaining crystals of (I) and analysing its structure, we believed that the synthetic procedure for (I) yielded the expected N,N'-bis[2-tert-butyldimethylsiloxyethyl]ethylenediamine, (II), because the ESI-MS and ¹H NMR results for the solid product appeared to be in agreement with (II). But the fact that the product existed in the solid state and was quite soluble in water raised some suspicions, given the presence of the two large hydrophobic tert-butyldimethylsilyl groups. The chemical reactivity of the product was also not that expected for (II), for example it was unable to react with alkyl halides. The present X-ray diffraction analysis reveals that the product is in fact (I) and thus the questions raised were resolved. In order to keep the imine groups unprotonated while protecting the hydroxyl groups, we added 4-dimethylaminopyridine, which was found to be an efficient and selective catalyst for the silyation of alcohols by Chaudhary & Hernandez (1979). The desired compound, (II), was thus obtained successfully. It is oily, as are many other silyl ethers. In contrast, the melting point of (I) is quite high (452–454 K), due to the extensive hydrogen-bonding interactions. The absence of positive charges on the N atoms in (II) moves the corresponding chemical shifts for (II) upfield compared with those for (I) in the ¹H NMR spectroscopic data.

Related literature

For related literature, see: Breccia et al. (2003); Chaudhary & Hernandez (1979); Diaz-Oltra, Carda, Murga, Falomir & Marco (2008); Dutta et al. (2011); Etter (1990); Fahrni & Pfaltz (1998); Gorter & Brussee (1992); Greene & Wuts (1991); Habich et al. (1988); Herdeis & Schiffer (1999); Herdeis & Telser (1999); Johansson et al. (2004); Kocienski (2005); Nemoto et al. (2012); Pohlmann et al. (1997); Robinson & Donahue (1992); Shin et al. (2008); White & Hansen (2002); Yao & Lu (2011); Zhai et al. (2009).

S2. Experimental

Synthesis of (I). To a mixture of N,N'-bis(2-hydroxylethyl)ethylenediamine (0.74 g, 5 mmol) and triethylamine (1.21 g, 12 mmol) in dichloromethane (25 ml) in an ice bath was added dropwise a solution of tert-butyldimethylsilyl chloride in dichloromethane (25 ml) under a dinitrogen atomsphere. The resulting suspension was allowed to react for 48 h at room temperature. The solvent was then removed and the solid was washed thoroughly with n-hexane. Recrystallization from a solution in ethanol afforded colourless plate-like crystals of (I) (m.p. 452–454 K). ESI-MS: m/z 377.44 [100%, (M - H - 2Cl)⁺]. Spectroscopic analysis: ¹H NMR (Frequency?, d₆-DMSO, δ, p.p.m.): 3.88 (t, 4H, OCH₂C), 3.68 (t, 4H, OCH₂CH₂N), 3.32 (s, 4H, N(CH₂)₂N), 0.88 [s, 18H, SiC(CH₃)₃], 0.09 [s, 12H, Si(CH₃)₂].

Synthesis of (II). To a mixture of N,N'-bis(2-hydroxylethyl)ethylenediamine (2.31 g, 15.6 mmol), triethylamine (3.04 g, 30.1 mmol) and 4-dimethylaminopyridine (0.24 g, 1.97 mmol) in dichloromethane (40 ml) in an ice bath was added dropwise a solution of tert-butyldimethylsilyl chloride (5.15 g, 34.3 mmol) in dichloromethane (40 ml) under a dinitrogen atomsphere. After reaction overnight at room temperature the solution was washed with water (50 ml) and saturated sodium bicarbonate (50 ml). The organic phase was dried with anhydrous magnesium sulfate. Removal of the dichloromethane afforded a light-yellow oily product, which was purified by column chromatography (chloroform/methanol = 10/1 v/v). ESI-MS: m/z 377.40 [100%, (M + H)⁺]. Spectroscopic analysis: ¹H NMR (Frequency?, CDCl₃, δ, p.p.m.): 3.65 (t, 4H, OCH₂C), 2.66 (t, 4H, OCH₂CH₂N), 2.64 [s, 4H, N(CH₂)₂N], 2.40 (br, s, 2H, NH), 0.82 [s, 18H, SiC(CH₃)₃], 0.02 [s, 12H, Si(CH₃)₂].

S3. Refinement

Secondary H atoms attached to C atoms were positioned geometrically and allowed to ride on their parent atoms, with C—H = 0.99 Å and U_iso(H) = 1.2U_eq(C). N-bound H atoms were located in a difference Fourier map and treated in the riding-model approximation, with N—H = 0.90 Å and U_iso(H) = 1.5U_eq(N). For the methyl groups, Si—C—H and C—C—H angles (109.5°) were kept fixed while the torsion angle was allowed to refine, with the starting positions based on the circular Fourier synthesis averaged using the local threefold axis, and with C—H = 0.98 Å and U_iso(H) = 1.5U_eq(C).

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: APEX2 (Bruker, 2004); data reduction: APEX2 (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 2012); software used to prepare material for publication: PLATON (Spek, 2009).

Figures

	Fig. 1. The structure of (I), with the atomic numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. [Symmetry code: (i) 1 -x, 3 - y, 1 - z.]
	Fig. 2. A packing diagram for (I), showing the two-dimensional network induced by N—H···Cl hydrogen bonds (dashed lines). For clarity, methyl and butyl groups attached to Si atoms, and H atoms not involved in the hydrogen bonding, have been omitted.

2,2,3,3,14,14,15,15-Octamethyl-4,13-dioxa-7,10-diaza-3,14-disilahexadecane-7,10-diium dichloride

Crystal data

C18H46N2O2Si2^2+·2Cl^−	Z = 1
Mr = 449.65	F(000) = 246
Triclinic, P1	Dx = 1.134 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 6.2088 (4) Å	Cell parameters from 7080 reflections
b = 6.4128 (6) Å	θ = 6.5–54.8°
c = 18.7953 (10) Å	µ = 0.35 mm^−1
α = 88.989 (2)°	T = 150 K
β = 87.640 (2)°	Plate, colourless
γ = 61.736 (2)°	0.35 × 0.24 × 0.16 mm
V = 658.57 (8) Å^3

Data collection

Bruker APEXII CCD area-detector diffractometer	2859 independent reflections
Radiation source: fine-focus sealed tube	2483 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.041
ϕ and ω scans	θmax = 27.0°, θmin = 3.3°
Absorption correction: multi-scan SADABS (Sheldrick, 2005)	h = −7→6
Tmin = 0.887, Tmax = 0.946	k = −8→8
7608 measured reflections	l = −23→23

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.041	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.115	H-atom parameters constrained
S = 1.07	w = 1/[σ^2(Fo^2) + (0.0552P)^2 + 0.3038P] where P = (Fo^2 + 2Fc^2)/3
2859 reflections	(Δ/σ)max = 0.001
123 parameters	Δρmax = 0.69 e Å^−3
0 restraints	Δρmin = −0.36 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cl1	0.79662 (8)	0.84481 (8)	0.55047 (2)	0.03111 (15)
Si1	0.79366 (8)	0.74565 (8)	0.22119 (2)	0.02190 (15)
O1	0.8381 (3)	0.9500 (2)	0.26035 (6)	0.0296 (3)
N1	0.6726 (2)	1.2484 (2)	0.43581 (7)	0.0201 (3)
H1D	0.6347	1.1460	0.4594	0.024*
H1E	0.8256	1.2143	0.4467	0.024*
C1	0.4999 (3)	1.4930 (3)	0.45974 (8)	0.0226 (4)
H1A	0.3331	1.5366	0.4446	0.027*
H1B	0.5499	1.6057	0.4378	0.027*
C2	0.6667 (3)	1.2147 (3)	0.35779 (8)	0.0237 (4)
H2A	0.7075	1.3272	0.3314	0.028*
H2B	0.4998	1.2487	0.3454	0.028*
C3	0.8472 (4)	0.9641 (3)	0.33584 (9)	0.0290 (4)
H3A	1.0143	0.9261	0.3494	0.035*
H3B	0.8025	0.8503	0.3597	0.035*
C4	0.4906 (3)	0.7762 (4)	0.25201 (10)	0.0323 (4)
H4A	0.4769	0.7842	0.3041	0.048*
H4B	0.4758	0.6392	0.2358	0.048*
H4C	0.3597	0.9213	0.2323	0.048*
C5	1.0403 (3)	0.4452 (3)	0.24472 (10)	0.0313 (4)
H5A	1.0434	0.4272	0.2966	0.047*
H5B	1.1987	0.4261	0.2264	0.047*
H5C	1.0083	0.3245	0.2235	0.047*
C6	0.8039 (3)	0.8067 (3)	0.12290 (9)	0.0265 (4)
C7	0.6039 (4)	1.0575 (4)	0.10578 (11)	0.0411 (5)
H7A	0.6139	1.0886	0.0548	0.062*
H7B	0.6270	1.1730	0.1334	0.062*
H7C	0.4429	1.0709	0.1181	0.062*
C8	1.0549 (4)	0.7849 (4)	0.10001 (10)	0.0368 (5)
H8A	1.0573	0.8189	0.0490	0.055*
H8B	1.1836	0.6238	0.1094	0.055*
H8C	1.0835	0.8982	0.1271	0.055*
C9	0.7630 (4)	0.6268 (4)	0.08066 (10)	0.0382 (5)
H9A	0.7653	0.6604	0.0296	0.057*
H9B	0.6041	0.6385	0.0951	0.057*
H9C	0.8932	0.4666	0.0902	0.057*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0247 (2)	0.0366 (3)	0.0372 (3)	−0.0184 (2)	−0.00962 (18)	0.0106 (2)
Si1	0.0206 (2)	0.0233 (3)	0.0209 (2)	−0.0096 (2)	−0.00125 (18)	−0.00182 (18)
O1	0.0409 (7)	0.0328 (7)	0.0198 (6)	−0.0212 (6)	0.0019 (5)	−0.0061 (5)
N1	0.0193 (6)	0.0217 (7)	0.0201 (7)	−0.0103 (6)	−0.0023 (5)	−0.0003 (5)
C1	0.0245 (8)	0.0205 (8)	0.0212 (8)	−0.0092 (7)	−0.0021 (6)	−0.0018 (6)
C2	0.0240 (8)	0.0289 (9)	0.0182 (8)	−0.0124 (7)	−0.0025 (6)	−0.0015 (6)
C3	0.0349 (10)	0.0294 (9)	0.0205 (8)	−0.0129 (8)	−0.0023 (7)	−0.0053 (7)
C4	0.0252 (9)	0.0335 (10)	0.0338 (10)	−0.0106 (8)	−0.0010 (7)	0.0065 (8)
C5	0.0248 (9)	0.0298 (9)	0.0336 (10)	−0.0082 (7)	−0.0033 (7)	0.0015 (8)
C6	0.0253 (8)	0.0330 (9)	0.0209 (8)	−0.0135 (7)	−0.0014 (7)	−0.0028 (7)
C7	0.0420 (11)	0.0404 (12)	0.0301 (10)	−0.0107 (9)	−0.0047 (8)	0.0079 (8)
C8	0.0337 (10)	0.0543 (13)	0.0275 (9)	−0.0251 (10)	0.0012 (8)	0.0010 (9)
C9	0.0378 (11)	0.0500 (13)	0.0301 (10)	−0.0233 (10)	−0.0001 (8)	−0.0120 (9)

Geometric parameters (Å, º)

Si1—O1	1.6520 (14)	C4—H4B	0.9800
Si1—C4	1.866 (2)	C4—H4C	0.9800
Si1—C5	1.8667 (19)	C5—H5A	0.9800
Si1—C6	1.8865 (18)	C5—H5B	0.9800
O1—C3	1.429 (2)	C5—H5C	0.9800
N1—C1	1.485 (2)	C6—C9	1.534 (3)
N1—C2	1.490 (2)	C6—C7	1.535 (3)
N1—H1D	0.8999	C6—C8	1.541 (3)
N1—H1E	0.9000	C7—H7A	0.9800
C1—C1^i	1.518 (3)	C7—H7B	0.9800
C1—H1A	0.9900	C7—H7C	0.9800
C1—H1B	0.9900	C8—H8A	0.9800
C2—C3	1.513 (2)	C8—H8B	0.9800
C2—H2A	0.9900	C8—H8C	0.9800
C2—H2B	0.9900	C9—H9A	0.9800
C3—H3A	0.9900	C9—H9B	0.9800
C3—H3B	0.9900	C9—H9C	0.9800
C4—H4A	0.9800
O1—Si1—C4	109.19 (8)	Si1—C4—H4C	109.5
O1—Si1—C5	109.75 (8)	H4A—C4—H4C	109.5
C4—Si1—C5	109.46 (9)	H4B—C4—H4C	109.5
O1—Si1—C6	104.78 (8)	Si1—C5—H5A	109.5
C4—Si1—C6	111.92 (8)	Si1—C5—H5B	109.5
C5—Si1—C6	111.62 (8)	H5A—C5—H5B	109.5
C3—O1—Si1	122.76 (12)	Si1—C5—H5C	109.5
C1—N1—C2	112.73 (12)	H5A—C5—H5C	109.5
C1—N1—H1D	109.1	H5B—C5—H5C	109.5
C2—N1—H1D	108.9	C9—C6—C7	109.11 (16)
C1—N1—H1E	109.2	C9—C6—C8	108.86 (15)
C2—N1—H1E	108.9	C7—C6—C8	108.83 (17)
H1D—N1—H1E	107.9	C9—C6—Si1	109.51 (13)
N1—C1—C1^i	109.48 (16)	C7—C6—Si1	110.24 (12)
N1—C1—H1A	109.8	C8—C6—Si1	110.25 (12)
C1^i—C1—H1A	109.8	C6—C7—H7A	109.5
N1—C1—H1B	109.8	C6—C7—H7B	109.5
C1^i—C1—H1B	109.8	H7A—C7—H7B	109.5
H1A—C1—H1B	108.2	C6—C7—H7C	109.5
N1—C2—C3	110.79 (13)	H7A—C7—H7C	109.5
N1—C2—H2A	109.5	H7B—C7—H7C	109.5
C3—C2—H2A	109.5	C6—C8—H8A	109.5
N1—C2—H2B	109.5	C6—C8—H8B	109.5
C3—C2—H2B	109.5	H8A—C8—H8B	109.5
H2A—C2—H2B	108.1	C6—C8—H8C	109.5
O1—C3—C2	107.19 (14)	H8A—C8—H8C	109.5
O1—C3—H3A	110.3	H8B—C8—H8C	109.5
C2—C3—H3A	110.3	C6—C9—H9A	109.5
O1—C3—H3B	110.3	C6—C9—H9B	109.5
C2—C3—H3B	110.3	H9A—C9—H9B	109.5
H3A—C3—H3B	108.5	C6—C9—H9C	109.5
Si1—C4—H4A	109.5	H9A—C9—H9C	109.5
Si1—C4—H4B	109.5	H9B—C9—H9C	109.5
H4A—C4—H4B	109.5
C4—Si1—O1—C3	60.90 (15)	C4—Si1—C6—C9	−61.47 (15)
C5—Si1—O1—C3	−59.08 (15)	C5—Si1—C6—C9	61.61 (15)
C6—Si1—O1—C3	−179.05 (13)	O1—Si1—C6—C7	−59.61 (16)
C2—N1—C1—C1^i	174.94 (17)	C4—Si1—C6—C7	58.59 (17)
C1—N1—C2—C3	179.38 (15)	C5—Si1—C6—C7	−178.34 (15)
Si1—O1—C3—C2	−127.31 (14)	O1—Si1—C6—C8	60.57 (15)
N1—C2—C3—O1	−177.39 (14)	C4—Si1—C6—C8	178.77 (14)
O1—Si1—C6—C9	−179.67 (12)	C5—Si1—C6—C8	−58.15 (16)

Symmetry code: (i) −x+1, −y+3, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1D···Cl1	0.90	2.42	3.1604 (14)	140
N1—H1D···Cl1^ii	0.90	2.66	3.2391 (15)	123
N1—H1E···Cl1^iii	0.90	2.19	3.0797 (14)	168

Symmetry codes: (ii) −x+1, −y+2, −z+1; (iii) −x+2, −y+2, −z+1.