Tetraorganylstibonium halides of the general formula R4SbHlg, among which the most studied are phenyl derivatives of pentavalent antimony, are used in various fields of analytical chemistry and have a great synthetic potential, in particular, tetraalkylstibonium halides react with aldehydes to form addition products, the hydrolysis of which gives the corresponding alcohols [1–7]. Therefore, it is of interest to study the synthesis and structure of such compounds.

The reaction of pentaarylantimony with hydrohalic acids is the main method for obtaining tetraarylstibonium halides [3, 8]. One of the methods for the synthesis of tetraorganylstibonium halides [ArAlk3Sb]Hlg containing aryl and alkyl substituents at the antimony atom is the reaction of ArAlk2Sb with haloalkanes [9]. However, for a long time the attempts to attach alkyl halides to antimony(III) compounds with two or three aromatic substituents were unsuccessful [3, 8]. Therefore, trimethyloxonium borofluorides are applied as alkylating agents to synthesize the stibonium compounds [Аr3МеSb]Х (Аr = Ph, Tol, Mes, (3,4-Me)2C6H3,(2,4-Me)2C6H3, Х = BF4) [10–12].

For the first time, the addition of alkyl halides to triarylstibine was observed in reactions involving (2,6-(MeO)2C6H3)3Sb. To prove the structure of tetraorganylstibonium halides [Аr3RSb]Hlg [Аr = 2,6-(MeO)2C6H3; R = Me, Et, n-Bu, СН2СН=СН2; Hlg = Cl, Br, I], the authors present elemental analysis and NMR spectroscopy data [13]. We studied the alkylation of tris(2,6-dimethoxyphenylstibine) with iodoacetic acid ethyl ether and 1,4-diiodbutane. Structure of the compounds [Ar3RSb]n+In–, Ar = 2,6-(MeO)2C6H3; n = 1, R = Me, (CH2)4I, CH2C(O)OEt; n = 2, R = (CH2)4SbAr3 and [Ar3SbCH2C(O)OEt]2[Hg2I6], [Ar3MeSb]2[HgI4]∙DMSO was confirmed by the X-ray and NMR spectroscopy methods [14].

Thus, to date the possibility of triarylstibine alkylation with alkyl halides and ethyl iodoacetate has been shown only by the example of [2,6-(MeO)2C6H3]3Sb [13, 14]. In this regard, the study of the alkyl halides reactions with other antimony(III) fully substituted aromatic compounds and the determination of the structure of the resulting alkylaryl stibonium compounds is an urgent task.

The aim of this work was to study the alkylation of tris(4-N,N-dimethylaminophenyl)stibine with methyliodide, to develop the directed synthesis of complexes with the tris(4-N,N-dimethylaminophenyl)methylstibonium cation, and to study their structure and properties.

Triphenylstibine does not react with alkyl halides [3]. The presence of strong electron-donor groups N(Me)2 in phenyl substituents as a whole suggests that the basicity (and hence, nucleophilicity) of the tris(4-N,N-dimethylaminophenyl)stibine molecule increases compared to triphenylstibine, and, as a result, the possibility of alkylation with an antimony atom appears. Another nucleophilic center of the tris(4-N,N-dimethylaminophenyl) molecule capable of being subjected to an electrophilic attack by methyl is the amino group nitrogen atom. Its basicity decreases due to the p–π conjugation of its unshared electron pair with the benzene ring π-electrons. It is known that the nucleophilicity of atoms decreases along the N–P–As–Sb–Bi series, and methyliodide, as a soft reagent in nucleophilic reactions, prefers soft nucleophiles [8, 15]. It can be expected that the alkylation of the tris(4-N,N-dimethylaminophenyl)stibine molecule, containing two competing nucleophiles (nitrogen and antimony atoms), with methyliodide will proceed through the antimony atom to form tris(4-N,N-dimethylaminophenyl)methylstibonium iodide.

Alkylation of tris(4-N,N-dimethylaminophenyl)stibine with methyliodide was carried out in chloroform at room temperature. It was found that, regardless of the molar ratio of the reagents (from 1 : 1 to 1 : 10) and the experiment duration (24–120 h), alkylation proceeds through the antimony atom to form colorless crystals of the complex [(4-N,N-Me2C6H4)3MeSb]I 1 (Scheme 1).

Scheme
scheme 1

1.

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To determine the space structure of compound 1, single crystals suitable for X-ray diffraction analysis were obtained by slow crystallization from chloroform. We note that the tetraorganylstibonium derivatives containing the 4-N,N-Me2C6H4 substituent were not structurally characterized. According to the X-ray analysis data, complex 1 consists of I– and [(4-N,N-Me2C6H4)3MeSb]+ ions. The antimony atom in the cation has a distorted tetrahedral coordination. The values of the CSbC angles are in the range 106.0(1)°–114.6(1)° (Fig. 1). The main crystallographic characteristics, experimental data, and parameters of the structure refinement are given in Table 1.

Fig. 1.
figure 1

General view of the cation and anion of the compound 1 (CCDC 2067967).

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Table 1. Crystallographic characteristics of compounds 1 and 2, experimental data, and parameters of structure refinement
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Tetraphenylstibonium halides are stable during storage, non-volatile, and readily soluble in water, therefore they can be purified by recrystallization from an aqueous solution. At the same time, tetraalkylstibonium halides are hygroscopic and decompose in hot aqueous solutions to release alkyl halide and trialkylstibine [3]. Since complex 1 contains aryl and methyl substituents at the antimony atom, it is of interest to study its stability and the possibility of decomposition to triarylstibine.

Heating an aqueous solution of compound 1 at 80°C leads to the formation of an insoluble product, the structure of which was established by NMR spectroscopy, agrees with the elemental analysis and IR spectroscopy data, and corresponds to tris (4-N,N-dimethylaminophenyl)stibine. According to the 1H NMR data, the spectrum of the obtained compound contains signals of eighteen methyl protons in the region of 2.80 ppm, six aromatic protons in the region of 6.67 ppm, and six aromatic protons in the region of 7.21 ppm. The 13C NMR spectra contain signals in the regions of 113.6, 135.8, 137.1, and 153.9 ppm corresponding to the chemical shifts of carbon nuclei in the benzene ring, and the signal of carbon atoms of methyl fragments in the region of 39.1 ppm. The obtained result points to the decomposition of the stibonium salt with the formation of tris (4-N,N-dimethylaminophenyl)stibine (Scheme 1).

Organylantimony(III, V) halides, reacting with heavy metal halides (Hg, Cd, and Bi), form complex compounds in which they act as a ligand or a stibonium cation [14, 16, 17]. The reaction of complex 1 with mercury(II) iodide (1 : 1 mol) in DMSO was studied (Scheme 1). The reaction product [(4-N,N-Me2C6H4)3MeSb]2[Hg2I6]·2DMSO 2 is a crystal highly soluble in acetone, chloroform, and DMSO. The absorption bands in the IR spectra of compounds 1 and 2 were assigned in accordance with the data of [18, 19]. These spectra contain absorption bands at 2798 and 1433 cm–1, i.e. in the region of stretching and bending vibrations of the C-H bonds of the methyl group bound to the nitrogen atom. The band at 1594 cm–1 can be attributed to stretching vibrations of aromatic C–C bonds. In the IR spectrum of complex 2, a band of stretching vibrations ν(SO) is observed at 1016 cm–1, the shift of which to the long-wave region of the spectrum in comparison with the free DMSO molecule points to the DMSO coordination through the oxygen atom [20]. The IR spectroscopy data are consistent with the X-ray results.

In the crystal of compound 2, there is an additional Sb···O1S (2.831 Å) coordination between the antimony atoms of the stibonium cation and the oxygen of the solvate DMSO molecule (the sum of the van der Waals radii 3.58 Å [21]), which determines the stibonium cation configuration as being intermediate between tetrahedral and trigonal-bipyramidal (Figs. 2, 3). The O1S atoms of the DMSO molecule and С2 atoms of one of the aryl substituents are located in the pseudoaxial plane (the angle C2Sb1O1S is 171.97°). The pseudo-equatorial positions are occupied by the C1, C11, and C20 atoms of the methyl and two aryl substituents; the values of the CeqSbСeq angles are 107.21(15)°–119.50(17)°, and of the СaxSbСeq angles: 101.92(16)°–103.09(16)° (Figs. 2, 3).

Fig. 2.
figure 2

General view of the compound 2 anion and its cation coordinated with the DMSO molecule (CCDC 2059337).

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Fig. 3.
figure 3

Fragment of the crystal package of compound 2 (CCDC 2059337). The dotted line indicates the intermolecular contacts between the cation and the DMSO solvate molecule.

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The Sb–CМе bond lengths are 2.097(3) (1) and 2.104(4) Å (2), the Sb–CAr distances lie in the interval 2.078(2)–2.087(3) (1), 2.087(4)–2.091(4) Å (2) (the sums of the covalent radii of Sb and Csp3 or Csp2 atoms are 2.15 or 2.12 Å) [22]. In the trigonal-bipyramidal tetraphenylstibonium iodide molecule, the Sb–CPh bonds are longer: Sb–Cax 2.141(3) Å, Sb–Ceq 2.103(3)– 2.117(3) Å, and the Sb–I distance is 3.341 (1) Å [23]. The reduction of the Sb–CAr distances in compound 1 is caused by the presence of an amino group in the phenyl substituent, the nitrogen atom of which can participate in the p–π–d conjugation.

Nitrogen atoms of the amino groups in complexes 1 and 2 have the sp2 hybridization [the CNC angles are 116.7(4)°–121.7(3)° (1) and 116.9(4)°–120.9(4)° (2)]. Substituents at all the nitrogen atoms form planes; the maximum outgoing of a nitrogen atom from the corresponding root-mean-square planes does not exceed 0.02 Å. Conjugation of a free electron pair of the nitrogen atom with the aromatic ring leads to a decrease in the N–CAr bond lengths of 1.359(4)–1.374(6) (1), 1.373(5)–1.381(5) Å (2) compared to the N–CМе distance of 1.420(6)–1.454(4) (1), 1.445(6)–1.458(6) Å (2) (sum of the covalent radii 1.47 Å).

Centrosymmetric binuclear anions [Hg2I6]2– form tetrahedra joined along edges; the IHgI angles are 94.128(10)°–123.833(12)°. The Hg2I2 cycle is flat, the torsion angle I3Hg1I3аHg1а is 0.00(1)°. The terminal iodine atoms are more strongly bound to mercury atoms [distances 2.7063(4) and 2.7118(4) Å] than the two–coordinated bridging iodine atoms [Hg–I-μ2 2.8691(4) and 3.0011(4) Å]. These values are comparable with parameters of the [Hg2I6]2– anion of complexes containing {[(2,6-MeO)2C6H3]3SbCH2C(O)OEt}+, [Ph4Sb]+, and [p-Tol4Sb]+ cations, where these distances vary within the intervals 2.6874(4)–2.7222(3) Å for terminal and 2.8250(4)–3.0748(5) Å for bridging iodine atoms [14]. The Hg1I3Hg1аI3а and I1I2I1аI2а planes are almost perpendicular with the angle between them of 86.31°.

Iodine atoms participate in the formation of the crystal structure 1, forming weak I···H–СAr (2.985 Å) and I···H–СМе–Sb (2.991 Å) contacts. Potential coordinating centers—nitrogen atoms of amino groups—participate in weak intermolecular interactions with the DMSO molecule (N···H–CMe 2.628 Å) 2. The sum of the van der Waals radii of iodine (nitrogen) and hydrogen atoms is 3.08 (2.65) Å [21].

Thus, a method has been proposed for the synthesis of triarylalkylstibonium complexes [(4-N,N-Me2C6H4)3MeSb]I and [(4-N,N-Me2C6H4)3MeSb]2[Hg2I6]·2DMSO, which is based on the alkylation reaction of tris(4-N,N-dimethylaminophenyl)stibine at the antimony atom. The crystal structure of the complexes was confirmed by the X-ray analysis method. The coordination number of the antimony atom in the [(4-N,N-(Me)2C6H4)3MeSb]2[Hg2I6]·2DMSO solvate increases to five, which results in the cation configuration as intermediate between tetrahedral and trigonal-bipyramidal.

EXPERIMENTAL

The NMR spectra were recorded on a Bruker Avance AV-300 spectrometer with a proton resonance frequency of 300 MHz. For the 13C nucleus, a cross-polarization technique with suppression of dipole-dipole interactions and magic angle rotation was used. The duration of the 90° pulse for protons was 4 µs, the polarization transfer time was 500-3000 ms, the rotation speed was 5, 7, and 16 kHz, the sample diameter was 4 mm, the time between pulses was 20 s, and the number of accumulations was 512. The hydrogen signals were recorded using a single-pulse sequence, the inter-pulse delay was 3 s. The error in determining the chemical shift did not exceed 1 ppm. The spectra were recorded at 300 K. The IR spectra of the compounds were recorded on a Fourier spectrometer FSM 2202 in tablets with KBr. The elemental analysis was performed on a Carlo Erba CHN analyzer (model 1106).

The X-ray analysis was performed on a Bruker APEX II CCD diffractometer (MoKα-radiation, graphite monochromator, ω scanning). The structures were interpreted by the direct method and refined by the OLS in the anisotropic full-matrix approximation for F2. The positions of the hydrogen atoms (except for the hydrogen of the methyl group at the C1 atom in complex 2) were calculated geometrically and refined in the rider model (the parameters of the hydrogen atoms were calculated in each refinement cycle according to the coordinates of the corresponding carbon atoms). The hydrogen atoms of the methyl group at the С1 2 atom were identified from the difference Fourier synthesis of the electron density. The maximum value of the residual electron density peak in the vicinity of the C1 atom of complex 2 does not exceed 0.5 е. The X-ray analysis parameters of compounds 1 and 2 are deposited at the Cambridge Crystallographic Data Center (CCDC 2067967, 2059337). All calculations were carried out using the SHELX-97 and SHELXL software packages [24-26].

Tris(4-N,N-dimethylaminophenyl)methylstibonium iodide (1). A solution of 9.00 g (53.41 mmol) of methyliodide in 30 mL of chloroform was added drop by drop with stirring to 4.82 g (10.00 mmol) of tris(4-N,N-dimethylaminophenyl)stibine [3] in 50 mL of chloroform. The mixture was kept in the dark at room temperature in a closed ampoule for 48 h. The solvent was evaporated. The crystals were washed with diethyl ether (3×15 mL). Yield 6.18 g (99%), mp 220°C (decomp.). IR spectrum, ν, cm–1 3029 (CAr–H), 2951 [νas(СH3)], 2924 [νas(СH3)], 2853 [νs(СH3)], 2798 (NC–H), 1594 (СAr–СAr), 1460 [δ(СH3)], 1433 [δ(NC–H)], 1396 [δ(СH3)], 1202 (СNC), 1165 (СNC), 947 [δ(СH3)], 939 [δ(СH3)], 549 (Sb–С). Found, %: C 49.21; H 5.76; N 5.98. C25H33IN3Sb. Calculated, %: C 48.10; N 5.33; N 6.73.

Decomposition of tris(4-N,N-dimethylaminophenyl)methylstibonium iodide. A solution of 3.00 g of compound 1 in 30 mL of water was heated at 80°C for 0.5 h. The precipitate was filtered, washed sequentially with water (25 mL), diethyl ether (2×15 mL), dried, and recrystallized (chloroform–alcohol). Yield of tris(4-N,N-dimethylaminophenyl)stibine is 1.39 g (60%), mp 229°C (mp 229°C [3]). IR spectrum, ν, cm–1: 3056 (CAr–H), 2797 (NC–H), 1588 (СAr–СAr), 1199 [νs(СNC)], 1167 (СNC), 944 [δ(СH3)], 519 (Sb–С). 1H NMR spectrum, ppm: 7.21 d (6H, 2.6-H, J = 9.0 Hz), 6.67 d (6H, 3.5-H, J = 9.0 Hz), 2.80 s (18H, Me). Found, %: C 58.82; H 6.69; N 9.03. C24H30N3Sb. Calculated, %: C 58.93; H 6.82; N 9.16.

Bis[tris(4-N,N-dimethylaminophenyl)methylantimony] bis(µ2-iodo)tetraiododimercurate(II), solvate with dimethyl sulfoxide (2). A solution of 0.36 g (0.80 mmol) of mercury(II) iodide in 20 mL of DMSO was added to 0.50 g (0.80 mmol) of compound 1 dissolved in 20 mL of DMSO. The mixture was kept at room temperature for 12 h. The solvent was removed. Yield 0.81 g (87%), mp 163°C. IR spectrum, ν, cm–1: 3032 (CAr–H), 2952 [νas(СH3)], 2924 [νas(СH3)], 2853 [νs(СH3)], 2798 (NC–H), 1594 (СAr–СAr), 1458 [δ(СH3)], 1433 [δ(NC–H)], 1399 [δ(СH3)], 1197 (СNC), 1161 (СNC), 1016 (SO), 954 [δ(СH3)], 944 [δ(СH3)], 549 (Sb–С). Found, %: C 26.94; H 2.99; N 4.12. C54H78Hg2I6N6O2S2Sb2. Calculated, %: C 28.02; N 3.40, N 3.63.