Two new organic-selenate salts: syntheses and crystal structures of bis(di-iso-propylammonium) selenate and di-n-butylammonium hydrogenoselenate

2.1 General

H₂SeO₄ was purchased from Merck while i-Pr₂NH, n-Bu₂NH and SnPh₃OH were acquired from Aldrich Chemicals and used without any further purification. Infrared spectra were recorded on a Bruker Vector 22 spectrometer equipped with a Specac Golden Gate™ ATR device. ¹H, ¹³C{¹H} and ⁷⁷Se NMR spectra were recorded on Bruker Avance 300 and 500 MHz spectrometers in D₂O. ⁷⁷Se shifts (δ, ppm) are converted to the scale downfield from Me₂Se. ¹H and ¹³C{¹H} NMR spectra of 1 and 2 are depicted as Supporting Information in Figs. S1–S2 and S3–S4, respectively. Elemental analyses were performed at the Institut de Chimie Moléculaire, Université de Bourgogne Franche-Comté, Dijon, France.

2.2 Synthesis of [i-Pr₂NH₂]₂[SeO₄] (1)

At first, the iso-propylammonium hydrogenoselenate salt, [i-Pr₂NH₂][HSeO₄], was obtained by partially neutralizing an aqueous selenic acid solution (40%) with di-iso-propylamine (98%). A limpid solution was rapidly obtained, which after evaporation for 3 days at 80°C gave a white powder. The isolation of 1 occurred by adding dropwise an ethanolic solution of triphenyltin hydroxide [0.634 g (1.722 mmol) in 15 mL of ethanol] to 0.300 g (0.862 mmol) of [i-Pr₂NH₂][HSeO₄], previously collected as a powder and dissolved in 15 mL of ethanol. A clear solution was obtained after 2 h stirring and was then submitted to a slow solvent evaporation at room temperature. Some days later, colorless crystals characterized as [(Ph₃Sn)₂O₄Se] were first obtained. The remaining solution resubmitted to a slow solvent evaporation led after 2 days to the formation of new colorless crystals, suitable for an X-ray crystallographic analysis and characterized as 1 (37% yield). – IR (ATR, cm⁻¹): 2961, 2932, 2873, 2795, 2454, 1458, 1159, 1079, 1031, 934, 900, 814, 788, 733, 704, 533. – ¹H NMR (D₂O, ppm): δ=1.26 (d, J=6,6 Hz, CH₃ isopropyl), 3.23 (sept, J=6,5 Hz, 2H, CH isopropyl). – ¹³C{¹H} NMR (D₂O, ppm): δ=18.95 (CH₃ isopropyl), 47.83 (CH isopropyl). – ⁷⁷Se NMR (D₂O, ppm): δ=1049. – Anal. calcd. for C₁₂H₃₂N₂O₄Se (348.15 g·mol⁻¹): C 41.49, H 9.29, N 8.06; found: C 41. 32, H 9.52, N 7.99.

2.3 Synthesis of [n-Bu₂NH₂][HSeO₄] (2)

H₂SeO₄ acid (40%) (0.302 g, 2.071 mmol) dissolved in 10 mL of ethanol was mixed at 60°C with 0.350 g (2.071 mmol) of n-Bu₂NH dissolved in 10 mL of ethanol. The solution was stirred for 2 h then filtered and submitted to a slow solvent evaporation at room temperature. After 1 week, colorless single crystals, suitable for an X-ray crystallographic analysis, were obtained and characterized as 2 (42% yield). – IR (ATR, cm⁻¹): 3119, 2967, 2934, 2866, 2457, 1565, 1454, 1260, 1047, 933, 892, 872, 844, 767, 737, 706. – ¹H NMR (D₂O, ppm): δ=0.69 (t, J=7.3 Hz, 6H, CH₃ butyl), 1.14 (m, C_γH₂ butyl), 1.41 (m, C_βH₂ butyl), 2.78 (m, C_αH₂ butyl) – ¹³C{¹H} NMR (D₂O, ppm): δ=12.69 (CH₃ butyl), 19.02 (C_γH₂ butyl), 27.39 (C_βH₂ butyl), 47.15 (C_αH₂ butyl). – ⁷⁷Se NMR (D₂O, ppm): δ=1043. – Anal. calcd. for: C₈H₂₁NO₄Se (275.06 g·mol⁻¹): C 35.04, H 7.72, N 5.11; found: C 35.12, H 7.57, N 5.21.

2.4 X-ray crystallography

The X-ray crystallographic data for 1 were collected using a Bruker-Nonius KappaCCD detector with an Oxford Cryosystems low-temperature apparatus operating at T=115 K. Data were measured using φ and ω scans using MoK_α radiation (λ=0.71073 Å, X-ray tube, 50 kV, 32 mA). The total number of runs and images was based on the strategy calculation from the program Collect [11]. Cell parameters were retrieved using the Scalepack software [12] and refined using Denzo [12]. Using Olex2 [13], the structures were solved with Superflip [14], [15], [16] structure solution program, using charge flipping algorithm method. The models were refined with Shelxl using least-squares minimization [17].

The X-ray crystallographic data for 2 were collected using a Bruker D8 Venture triumph Mo diffractometer equipped with an Oxford Cryosystems low-temperature apparatus operating at T=100 K. Data were measured using MoK_α radiation (X-ray tube, 50 kV, 30 mA). The total number of runs and images was based on the strategy calculation from the program Apex2 [18]. Cell parameters were retrieved and refined using the Saint software [19]. Data reduction was performed using the Saint software, which corrects for Lorentz polarization [19]. Using Olex2 [13], the structure was solved with the Shelxs structure solution program [17], using the direct solution method. The models were refined with Shelxl using least squares minimization [17].

Programs used for the representation of the molecular and crystal structures: Olex2 [13] and Mercury [20]. The Crystallographic data and experimental details for structural analyses of 1 and 2 are summarized in Table 1. Selected bond lengths and angles for 1 and 2 are listed in the caption of Figs. 2 and 5, respectively.

CCDC 1533769 (1) and 1533770 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 Synthesis

Compound 1 was isolated from a two steps procedure. Aqueous solutions of selenic acid (H₂SeO₄) and di-iso-propylamine (i-Pr₂NH) were first mixed together at room temperature leading to the formation of [i-Pr₂NH₂][HSeO₄] collected as a white powder (Eq. 1). In the past, related ammonium hydrogen selenates were already reported and structurally isolated by Baran [21] and Zakharov [22]. An equimolar amount of SnPh₃OH in ethanolic solution was then added. Single crystals of the known complex, [(Ph₃Sn)₂O₄Se] [6], grew first from the limpid solution. Subsequently, upon continuing the slow evaporation, new colorless single crystals were collected from the supernatant solution and were characterized as [i-Pr₂NH₂]₂[SeO₄] (1). Similarly, compound 2 was isolated by mixing equimolar ethanolic solutions of selenic acid (H₂SeO₄) and di-n-butylamine (n-Bu₂NH) at room temperature (Eq. 2). In a previous study, crude [n-BuNH₂][HSeO₄] led to the complex [n-Bu₂NH₂]₃[SnPh₃(SeO₄)₂] in the presence of Ph₃SnOH [9]. The yields of the reactions reported herein are 37% and 42%, respectively.

3.2 FT-IR spectroscopy

Crystals of 1 and 2 were first investigated by FT-IR spectroscopy in ATR mode. Both spectra, depicted in Fig. 1, show strong similarities. Thus and by comparison with the literature [21], [23], [24], [25], absorption bands contained in the regions A and B can be assigned to ammonium cations (N–H and C–H bonds) and selenate anions (Se–O bonds, ν₁ and ν₃ vibrational modes), respectively. In spectrum (b), the sharp band at 533 cm⁻¹ can be assigned to the ν₄(Se–O) vibrational mode. The intense bands located at 1565 cm⁻¹ (Fig. 1b), 1454 cm⁻¹ (Fig. 1b) and 1458 cm⁻¹ (Fig. 1a) are attributed to C–H bending vibrations, whereas those at 1159 cm⁻¹ (Fig. 1a) and 1260 cm⁻¹ (Fig. 1b) could correspond to C–N elongation bands. In spectrum (b), the sharp band at 706 cm⁻¹ supports the presence of a hydrogenoselenate derivative (Se–O(H) bond) [21], which is also corroborated by the broad absorptions observed above 3000 cm⁻¹, assigned to ν(O–H) vibrations.

Fig. 1:

IR specta (ATR mode) of (a) [i-Pr₂NH₂]₂[SeO₄] (1) and (b) [n-Bu₂NH₂][HSeO₄] (2).

3.3 Crystal and molecular structures

3.3.1 The structure of compound 1

An X-ray crystallographic analysis on suitable crystals solved unambiguously the structure of 1, which consists of a SeO₄²⁻ anion and two noncoordinating surrounding [i-Pr₂NH₂]⁺ cations. 1 crystallizes in space group P2₁/n. An Ortep view, together with selected bonds lengths and angles, is shown in Fig. 2. The SeO₄²⁻ anion describes a slightly deformed tetrahedral geometry with Se–O distances comprised between 1.6326(11) and 1.6416(10) Å. These distances characterize a double bond character and are in the average generally observed for selenate anions [26]. The O–Se–O angles vary from 108.64(6)° to 110.08(6)°. The N–C bond lengths and the C–N–C angles recorded for the di-iso-propylammonium cations are similar to those reported previously for comparable salts [27]. [SeO₄]²⁻ and [i-Pr₂NH₂]⁺ cations are linked together through NH···O hydrogen bonds. Distances are listed in Table 2. All oxygen atoms of [SeO₄]²⁻ anions are involved. Each [i-Pr₂NH]²⁺ is in interaction with two distinct [SeO₄]²⁻ anions. This results in the formation of a two-dimensional array leading to the propagation of layers in which [SeO₄]²⁻ are aligned (Fig. 3). By focusing on the internal structure of the layers, two types of self-assemblies, exhibiting macrocycles and involving both cations and anions, are detected: (i) a 12-membered hydrogen bonded ring and (ii) a 36-membered hydrogen bonded ring. As shown in Fig. 4, hydrogen bonded rings are fused together. Interestingly, the structure and the packing of the molecules in 1 are comparable to those previously observed by Okuniewski for the bis(di-iso-propylammonium) thiosulfate, (i-Pr₂NH₂)₂S₂O₃ [28].

Fig. 2:

Ortep view of 1 showing 30% probability ellipsoids for the non-H atoms and the crystallographic numbering scheme adopted [atom color code: C, gray; H, white; N, blue; O, red; Se, turquoise]. Selected bond lengths and angles [Å, deg]: Se–O1 1.6385(11), Se–O2 1.6343(12), Se–O3 1.6416(10), Se–O4 1.6326(11), N2–C10 1.505(2), N2–C7 1.500(2), N1–C1 1.510(2); N1–C4 1.501(2); O1–Se–O3 110.08(6), O4–Se–O1 109.50(6), O4–Se–O3 108.64(6), O4–Se–O2 109.60(8), O2–Se–O1 109.10(7), O2–Se–O3 109.91(6), C4–N1–C1 117.69(13), C7–N2–C10 117.62(12).

Table 2:

Geometry of the hydrogen bonds in the structure of 1.^a

D	H	A	d(D–H) (Å)	d(H–A) (Å)	d(D–A) (Å)	D–H–A (deg)
N2	H2A	O1	0.91	1.85	2.7342(16)	164.9
N2	H2B	O41	0.91	1.82	2.7251(16)	172.8
N1	H1A	O32	0.91	1.84	2.7185(16)	161.8
N1	H1B	O2	0.91	1.82	2.6986(17)	160.3

1. ^aSymmetry codes: ¹3/2−x, 1/2+y, 3/2−z; ²1−x, −y, 1−z.

Fig. 3:

Mercury view of 1 along the crystallographic b axis showing the propagation of the layers resulting from the hydrogen bond network between [SeO₄]²⁻ anions and [(i-Pr₂NH₂)]⁺ cations [atom color code: C, gray; N, blue; O, red; Se, orange]. Hydrogen atoms are omitted for clarity. Hydrogen bonds are represented by blue dotted lines.

Fig. 4:

Arrangement of hydrogen bonded rings in 1 [atom color code: H, white; N, blue; O, red; Se, orange]. Hydrogen bonds are represented by blue dotted lines. i-Pr groups are omitted for clarity].

3.3.2 The structure of compound 2

A view of the asymmetric unit of 2 with selected bonds lengths and angles is shown in Fig. 5. Compound 2 consists of one [n-Bu₂NH₂]⁺ cation and one [HSeO₄]⁻ anion. Three Se–O distances are in the range of 1.614(4) to 1.630(4) Å. The fourth is longer [1.728 (4) Å], consistent with the presence of a Se–OH group. The [HSeO₄]⁻ anion describes a deformed tetrahedral geometry with the O–Se–O angles varying from 104.5(2) to 113.1(2)°. Values of the N–C bond lengths and C–N–C angles within [n-Bu₂NH₂]⁺ are similar of those reported previously for this cation [29]. Each [HSeO₄]⁻ anion is in hydrogen bonding with two cations via two Se–O···H(N) contacts and also doubly linked with a neighboring anion, thus acting as hydrogen donor and acceptor through Se–OH···O(Se) and Se–O···H(O) interactions, respectively. The resulting dimerization of [HSeO₄]⁻ is shown in Fig. 5. The distances of these interactions are listed in Table 3. From a supramolecular point of view, this results in the formation of an inorganic layer shown in Fig. 6. The n-butyl chains are positioned below and above the plane of the layers. Figure 7 shows an enlarged view of a layer showing the presence of ribbons, which are associated with each other via anion-anion hydrogen interactions.

Fig. 5:

View of 2 showing 30% probability ellipsoids for the non-H atoms and the crystallographic numbering scheme adopted [atom color code: C, gray; H, white; N, blue; O, red; Se, turquoise; symmetry codes: (i) 1−x, 1−y, 1−z]. Hydrogen bonds are represented by red dotted lines. Selected bond lengths and angles [Å, deg]: Se–O1 1.617(4), Se–O2 1.728 (4), Se–O3 1.614(4), Se–O4 1.630(4), N–C1 1.492(7), N–C5 1.458(7); O1–Se–O2 107.3(2), O1–Se–O4 113.1(2), O3–Se–O1 112.6(2), O3–Se–O2 104.5(2), O3–Se–O4 112.8(2), O4–Se–O2 105.7(2), C5–N–C1 114.0(5).

Table 3:

Geometry of the hydrogen bonds in the structure of 2.^a

D	H	A	d(D–H) (Å)	d(H–A) (Å)	d(D–A) (Å)	D–H–A (deg)
O2	H2	O41	0.84	1.80	2.617(6)	162.7
N	HA	O32	0.91	2.21	2.863(6)	128.4
N	HB	O1	0.91	2.05	2.810(6)	139.7

1. ^aSymmetry codes: ¹1−x, 1−y, 1−z; ²+x, −1+y, +z.

Fig. 6:

View of 2 along the crystallographic b axis showing the propagation of the layer resulting from the hydrogen bonding network between neighboring [HSeO₄]⁻ anions, and also with surrounding [n-Bu₂NH₂]⁺ cations [atom color code: C, gray; N, blue; O, red; Se, orange]. Hydrogen atoms are omitted for clarity. Hydrogen bonds are represented by blue dotted lines.

Fig. 7:

Hydrogen bonding network in 2 [atom color code: H, white; N, blue; O, red; Se, orange]. Hydrogen bonds are represented by blue dotted lines. n-Bu groups are omitted for clarity.

3.4 ⁷⁷Se NMR spectroscopy

⁷⁷Se NMR spectroscopy is a useful probe generally employed to complete the characterization of selenium compounds. The values of ⁷⁷Se chemical shifts reported in the literature show a large range of about 3300 ppm, from +2400 to −900 ppm [30], [31]. Selenate derivatives are usually observed in the range of 1020–1050 ppm. ⁷⁷Se NMR spectra recorded for 1 and 2 in D₂O show one signal each at 1049 and 1043 ppm (Fig. 8), respectively, which is consistent with the formulae. A comparison with some related selenate salts is given in Table 4.

Fig. 8:

⁷⁷Se NMR spectra of (a) 1 and (b) 2 (D₂O, 300 K).