Diverse Hydrogen-Bonded Structural Motifs in 1,4-Diazabicyclo[2.2.2]octane N,N’-Dioxide Salts with Oxoanions

Abstract

Four new hybrid inorganic–organic salts of 1,4-diazabicyclo[2.2.2]octane N,N’-dioxide (odabco) with the formulae odabco·2HNO₃ (1), odabco·HClO₄ (2), odabco·H₂SO₄ (3) and odabco·2H₂SO₄ (4) were synthesized and characterized by single-crystal x-ray diffraction (XRD), powder XRD and infrared spectroscopy. Compound 1 is based on the isolated (H₂odabco)²⁺ fragments, representing 0D structure. 2 is based on cationic 1D {Hodabco}_nⁿ⁺ hydrogen-bonded chains. Compound 3 comprising a strongly coordinated sulfate anion consists of two-component hydrogen-bonded {-H₂odabco-SO₄₋} fragments forming uncharged 1D helical chains. 4 contains both {-HSO₄-}_n^n– and {-HSO₄-H₂odabco-HSO₄₋} hydrogen bonding motifs, resulting in a 3D polymeric network. A comparative structural analysis of both the obtained and the previously reported cognate structures was performed to rationalize the impact of the degree of odabco protonation and the anion donor ability on the structural features and dimensionalities of odabco-based hydrogen-bonded lattices.

1. Introduction

1,4-diazabicyclo[2.2.2]octane N,N’-dioxide (odabco) moiety contains quite sterically unhindered oxygen atoms with a high donor ability (Scheme 1a) [1,2,3,4]. Therefore, odabco can easily act as both a ligand in metal complexes [5,6,7,8] and a protonated cation in salts with inorganic [9,10,11] or organic anions [12,13]. Diverse types of such alicyclic moiety mobility (Scheme 1b–d) provide structural phase transitions in odabco-based compounds; hence, they can precede their functional properties, e.g., piezo-, para- and ferroelectricity [10,12,14]. In addition, low UV/vis absorption of the saturated odabco molecule is a valuable feature in the design of optical metal–organic framework materials [15]. Thus, the synthesis and acquisition of a structural library of new odabco-based compounds are of great interest for their possible applications as ligand sources and paraelectric-type materials. This work reports the preparation, crystal structures and characterization of four new protonated 1,4-diazabicyclo[2.2.2]octane N,N’-dioxide salts containing oxoanions of readily available nitric, sulfuric and perchloric acids, namely, odabco·2HNO₃ (1), odabco·HClO₄ (2), odabco·H₂SO₄ (3) and odabco·2H₂SO₄ (4). A comprehensive structural analysis of the obtained virtual library, appended by the previously reported structures odabco·HNO₃ [16] and odabco·2HClO₄·2H₂O [10], was performed. Depending on both the degree of odabco protonation and the donor strength of the anions, the presented salts form structures of diverse dimensionalities and modes of hydrogen bonding.

2. Results and Discussion

2.1. Crystal Structures

Compound odabco·2HNO₃ (1) crystallizes in monoclinic symmetry with the P2₁/n space group. The asymmetric unit of 1 consists of one twice-protonated odabco cation and two nitrate anions. Both nitrates are hydrogen-bound as donors to (H₂odabco)²⁺ acceptor moiety with the corresponding O(NO₃)…O(odabco) distances of 2.55 and 2.56 Å (Figure 1a). Weak π–π stacking interactions between the neighboring nitrates are notable, with 3.27 Å as the closest intermolecular N…O distance and 3.24 Å interplanar distance. The closest O(NO₃)…C(odabco) distance is 3.22 Å, representing no strong interactions between the nitrate and odabco aliphatic core. Furthermore, no significant contact appears between the closest organic cations due to the chess-like packing of (H₂odabco)²⁺ and (NO₃^–)₂ pairs in 1’s crystal structure (Figure 1b). Therefore, in terms of the hydrogen bonding system, 1 can be considered as a zero-dimensional structure.

The comparison of 1 to the previously reported structure odabco·HNO₃ [16] reveals different types of strong intermolecular interactions. In odabco·HNO₃, a monoprotonated organic cation forms slightly entangled one-dimensional polymeric chains {-Hodabco-}_nⁿ⁺, in which neighboring monomeric moieties are bound to each other through hydrogen bonds with 2.44 Å O(odabco)…O(odabco) intermolecular distances. A nitrate anion appears not to be included into the system of hydrogen bonds. Such a principal difference between the structures of mono- and diprotonated odabco nitrates can be explained by the higher donor strength of odabco oxygen compared to the nitrate, illustrating a presence of a high negative charge on the odabco oxygen (see Scheme 1a).

The odabco·HClO₄ compound (2) crystallizes in monoclinic symmetry with the P2₁/n space group. The asymmetric unit consists of one monoprotonated odabco cation and one perchlorate anion. The organic moieties of 2 are hydrogen-bonded one to another in a similar way to the abovementioned odabco·HNO₃ [16] manner by strong hydrogen bonds forming 1D chains (Figure 2a) with the corresponding O(odabco)…O(odabco) intermolecular distances of 2.45 Å. The perchlorate anion is, in the same way, not included in the system of hydrogen bonds, only weakly contacting to the odabco aliphatic core (Figure 2b) with the shortest C(odabco)…O(ClO₄) distance of 2.93 Å. The torsion angles H-O-O-H within {-Hodabco-}_nⁿ⁺ polymeric chains are 136° in odabco·HNO₃ and 144° odabco·HClO₄ (2, see Figure 2a), respectively, showing an almost anion-independent similarity of the chain geometry and entanglement.

An analysis of the previously reported structure of bis-perchlorate odabco·2HClO₄·2H₂O [10] was also carried out for full representativity. This compound is based on the twice-protonated odabco cations, which are hydrogen-bonded as acceptors to water donor molecules. The corresponding O(odabco)…O(H₂O) distances are 2.57 Å, indicating the presence of a relatively strong hydrogen bond. Perchlorate anions are connected only as the donors with the water acceptor through weak hydrogen contacts, with O(H₂O)…O(ClO₄) distances of 2.96 Å and 2.97 Å, showing the much poorer binding energy of the perchlorate anion compared to the water, again consistent to the low donor ability of this anion. No polymeric system of hydrogen bonds appears in odabco·2HClO₄·2H₂O. Therefore, in terms of the hydrogen bonding system, this structure is zero-dimensional, similarly to 1, besides the presence of water in the crystal structure of odabco·2HClO₄·2H₂O. In summary, poorly coordinating nitrate and perchlorate adopt quite a similar structure-forming behavior in the discussed odabco salts.

The odabco·H₂SO₄ compound (3) crystallizes in orthorhombic symmetry with the Pnna space group. The asymmetric unit consists of two independent halves of the odabco cation, both of which are protonated, and one sulfate anion. Sulfate is hydrogen-bound as a donor to the (H₂odabco)²⁺ acceptor with the corresponding O(SO₄)…O(odabco) distances of 2.48 and 2.50 Å. These cationic and anionic moieties alternate within one-dimensional chains (Figure 3a). The behaviour of SO₄^2– thereby drastically differs to the behaviour of nitrate and perchlorate described above, because, unlike those anions, sulfate is both a relatively strong donor and a possible bridge in the formation of hydrogen bonds. This feature results in the two-component hydrogen-bonded polymeric network. The described chains are twisted into helices around 2₁ screw axes (Figure 3b), which is apparently provided by the presence of low (ca. 53.2°) H-O-O-H torsion angles in the H₂odabco²⁺ cation along with the tetrahedral geometry (ca. 109.5° valence angle) of the sulfate anion. The chains are packed in a crystal structure without any strong interchain interactions, as the strongest intermolecular contact between the chains is represented by a 2.99 Å C(odabco)…O(SO₄) distance. Therefore, in terms of the hydrogen bonding system, 3 is a one-dimensional polymeric structure. Unlike the above-described 1D salts, odabco·HNO₃ and odabco·HClO₄, both comprising cationic {-Hodabco-}_nⁿ⁺ polymeric chains, 3 is based on uncharged {-H₂odabco-SO₄-}_n chains, thus having no additional counteranion in its crystal structure. Such a difference between 3 and the corresponding mononitrate and monoperchlorate is apparently attributed to the high donor ability of the sulfate anion, when compared to nitrate and perchlorate anions, making it able to be incorporated into the polymeric hydrogen bond system.

The odabco·2H₂SO₄ compound (4) crystallizes in monoclinic symmetry with the Cc space group. The asymmetric unit consists of one twice-protonated odabco cation and two monoprotonated sulfates. Each hydrosulfate anion is interlinked through hydrogen bonds, both with two neighbouring HSO₄^– and one (H₂odabco)²⁺, resulting in a complicated system of hydrogen bonds (Figure 4a). The O(SO₄)…O(SO₄) distances between hydrogen-bonded hydrosulfates are 2.53 and 2.58 Å. The O(SO₄)…O(odabco) distances to the hydrogen-bonded odabco cations are 2.55 and 2.60 Å, allowing us to speculate that slightly more strongly bound {-HSO₄-}_n^n– chains act as the main structural motif in 4. These 1D chains lying in two almost perpendicular directions are interlinked by the hydrogen-bonded organic cation (Figure 4b), which acts thereby as a bridge between the poly-hydrosulfate chains. The resulting polymeric structure therefore appears to be the three-dimensional framework, again uncharged, similarly to the monosulfate 3 described above.

2.2. Characterization

All the salts were successfully prepared by the reaction of readily available odabco·3H₂O₂ [1,8] with acids in different solvents. PXRD patterns of the synthesized samples 1–4 (Figures S1–S4) correspond well to the theoretical ones, taking into account some reasonable shifting of the diffraction peaks to lower angles due to performing SCXRD at low temperatures and the subsequent thermal expansion of the crystal lattices at room temperature, at which the PXRD data were obtained. The infrared spectrum of 1 (Figure S5) contains a very strong band at 1381 cm^–1, corresponding to the N-O valence vibration in the nitrate anion. The IR spectrum of 2 (Figure S6) contains a very strong band at 1096 cm^–1 and a strong band at 624 cm^–1, corresponding to Cl-O valence vibrations in the perchlorate anion. The infrared spectrum of 3 (Figure S7) contains a strong band at 1120 cm^–1 and a medium band at 620 cm^–1, corresponding to S-O valence stretchings in the sulfate anion. In 4, the corresponding bands are shifted down to 1073 cm^–1 and 576 cm^–1, respectively (see Figure S7), due to the protonation of the sulfate anion [17]. Very weak C(sp³)-H vibration bands of the odabco aliphatic core appear in the 2820–2930 cm^–1 region, accordingly to the previously published results [8,15]. Medium (1, 2) and weak (3, 4) split bands in the 3000–3050 cm^–1 region indicate an involvement of protonated odabco moieties in hydrogen bonding [12]. The IR spectrum of 4 also contains a medium band at 1284 cm^–1, which is absent in the spectrum of 3, indicating a presence of SOH plane bending oscillations [17] in the hydrosulfate-containing 4.

3. Materials and Methods

ATTENTION: care should be taken in all the procedures carried out with 1,4-diazabicyclo[2.2.2]octane N,N’-dioxide tris-(hydrogen peroxide) solvate (odabco·3H₂O₂) and odabco compounds with potentially explosive anions, such as nitrate and perchlorate.

3.1. Materials

Nitric acid (62% wt.) water solution, perchloric acid (65% wt.) water solution, sulfuric acid (98% wt.) and DMF (high purity grade) were commercially available and used as received. odabco⸱3H₂O₂ was synthesized according to the previously published procedure [8]. Distilled water was used in all the experiments.

3.2. Instruments

Infrared (IR) spectra were obtained in the 4000−400 cm⁻¹ range using a Bruker Scimitar FTS 2000 spectrometer in KBr pellets. Elemental CHNS analyses were carried out using a VarioMICROcube device. Powder X-ray diffraction (PXRD) data were acquired with a Shimadzu XRD-7000 diffractometer (Cu-Kα radiation, λ = 1.54178 Å) at room temperature. ¹H NMR spectra were recorded on a Bruker Advance 500 NMR spectrometer (500.13 Hz). The analyte solutions for NMR were prepared by dissolving ca. 5 mg of each salt in 0.8 mL of D₂O. Diffraction data for single crystals of 1–4 were collected with an automated Agilent Xcalibur diffractometer equipped with an AtlasS2 area detector and graphite monochromator (λ(MoKα) = 0.71073 Å). The CrysAlisPro program package [18] was used for the integration, absorption correction and determination of unit cell parameters. The dual space algorithm (SHELXT [19]) was used for the structure solution and the full-matrix least-squares technique (SHELXL [20]) was used for structure refinement. Anisotropic approximation was applied for all atoms, except hydrogens. Positions of hydrogen atoms of organic ligands were calculated geometrically and refined in the riding model. Details for single-crystal structure determination experiments and structure refinements are summarized in Appendix A,

Table A1. The crystallographic data and structure refinement details for 1–4.

	1	2	3	4
Chemical formula	C6H14N4O8	C6H13ClN2O6	C6H14N2O6S	C6H16N2O10S2
Mr, g/mol	270.21	244.63	242.25	340.33
Crystal system	Monoclinic	Monoclinic	Orthorhombic	Monoclinic
Space group	P21/n	P21/n	Pnna	Cc
Temperature, K	150	150	135	150
a, Å	6.6537(4)	7.8923(4)	11.6565(7)	9.5987(3)
b, Å	18.9836(11)	11.6883(4)	23.8750(14)	10.3468(3)
c, Å	8.5758(5)	10.6752(4)	7.0514(4)	12.7952(5)
α, °	90	90	90	90
b, °	94.448(6)	98.338(4)	90	102.635(4)
γ, °	90	90	90	90
V, Å3	1079.96(11)	974.35(7)	1962.4(2)	1239.99(7)
Z	4	4	8	4
F(000)	568	512	1024	712
D(calc.), g·cm–3	1.662	1.668	1.640	1.823
μ, mm–1	0.15	0.41	0.34	0.49
Crystal size, mm	0.53 × 0.30 × 0.29	0.70 × 0.60 × 0.34	0.68 × 0.26 × 0.15	0.53 × 0.35 × 0.26
θ range for data collection, °	2.2 < θ < 25.3	2.6 < θ < 25.4	3.4 < θ < 25.4	2.9 < θ < 25.4
No. of reflections: measured/independent/observed [I > 2σ(I)]	8569/ 1974/ 1730	6851/ 1783/ 1622	5377/ 1796/ 1564	9464/ 2296/ 2258
Rint	0.0213	0.0222	0.0168	0.0201
Index ranges	–7 ≤ h ≤ 8 –22 ≤ k ≤ 22 –10 ≤ l ≤ 18	–9 ≤ h ≤ 6 –14 ≤ k ≤ 14 –12 ≤ l ≤ 12	–12 ≤ h ≤ 14 –20 ≤ k ≤ 28 –8 ≤ l ≤ 6	–11 ≤ h ≤ 11 –12 ≤ k ≤ 12 –15 ≤ l < 15
Final R indices [I > 2σ(I)]	R1 = 0.0327 wR2 = 0.0818	R1 = 0.0308 wR2 = 0.0845	R1 = 0.0314 wR2 = 0.0792	R1 = 0.0218 wR2 = 0.0558
Final R indices (all data)	R1 = 0.0388 wR2 = 0.0853	R1 = 0.0343 wR2 = 0.0866	R1 = 0.0381 wR2 = 0.0824	R1 = 0.0222 wR2 = 0.0562
Goodness of fit on F2	1.073	1.068	1.060	1.098
Largest diff. peak, hole, e/Å3	0.22, –0.26	0.26, –0.39	0.28, –0.41	0.16, –0.31

. CCDC 2205241 (1), 2205243 (2), 2205239 (3), 2205240 (4) entries contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center at https://www.ccdc.cam.ac.uk/structures/.

3.3. Synthesis of Odabco·2HNO₃ (1)

In an evaporating cup, odabco∙3H₂O₂ (218 mg, 0.89 mmol), H₂O (5.0 mL) and HNO₃ (126 μL) were mixed. The evaporating cup with the solution was left at room temperature until the water completely evaporated, then the precipitated crystals were recrystallized from a minimal amount of water. The resulting colorless needles of 1 were filtered off and dried in air. The yield was 226 mg (95%). IR, ν/cm^–1: 3422 (s), 3373 (w), 3074 (w), 3054 (w), 3026 (m), 3017 (m), 2974 (w), 2880 (w), 2856 (w), 2834 (w), 2794 (w), 2726 (m), 2700 (m), 2633 (w), 2592 (w), 2549 (w), 2498 (w), 2424 (w), 2301 (w), 1997 (w), 1764 (w), 1625 (m), 1557 (w), 1534 (m), 1498 (m), 1444 (w), 1427 (m), 1384 (s), 1275 (s), 1183 (w), 1104 (m), 1033 (s), 917 (m), 849 (s), 823 (w), 719 (w), 656 (m), 472 (m). Elemental analysis (%): Found C, 26.7; H, 5.1; N, 20.4, calculated for odabco·2HNO₃ (C₆H₁₄O₈N₄): C, 26.7; H, 5.2; N, 20.7. ¹H NMR (D₂O, ppm): 4.154 (s).

3.4. Synthesis of Odabco·HClO₄ (2)

In an evaporating cup, odabco∙3H₂O₂ (218 mg, 0.89 mmol), H₂O (5.0 mL) and HClO₄ (82 μL) were mixed. The evaporating cup with the solution was left at room temperature until the water completely evaporated, then the precipitated crystals were recrystallized from a minimal amount of water. The resulting colorless crystals of 2 were filtered off and dried in air. The yield was 203 mg (94%). IR, ν/cm^–1: 3433 (s), 3045 (s), 3016 (w), 3006 (m), 2971 (w), 2912 (w), 2025 (m), 1640 (m), 1476 (m), 1376 (w), 1358 (w), 1144 (w), 1095 (m), 1042 (w), 1004 (w), 957 (w), 868 (m), 855 (m), 726 (s), 670 (s), 623 (s), 551 (s), 525 (m), 476 (m), 450 (s). No elemental CHNCl analysis was performed due to the high explosion hazard of the organic perchlorates. ¹H NMR (D₂O, ppm): 4.309 (s).

3.5. Synthesis of Odabco·H₂SO₄ (3)

In a glass vessel with a screw cap, odabco∙3H₂O₂ (218 mg, 0.89 mmol), DMF (5.0 mL) and H₂SO₄ (75 μL) were mixed. The mixture was treated in an ultrasonic bath for 10 min and kept at 100°C for 3 h. The resulting precipitate was filtered on a porous paper filter, washed with DMF, and dried in air to obtain colorless crystals of 3. The yield was 191 mg (91%). IR, ν/cm^–1: 3433 (m), 2049 (w), 3026 (w), 2980 (w), 2779 (w), 1718 (w), 1588 (w), 1470 (w), 1352 (w), 1119 (s), 939 (s), 855 (m), 621 (m), 592 (m), 483 (m). Elemental analysis (%): Found C, 28.2; H, 6.1; N, 11.4; S, 15.0, calculated for odabco·H₂SO₄·(Me₂NH₂HSO₄)_0.3 (C_6.6H_16.7O_7.2N_2.3S_1.3): C, 27.8; H, 5.9; N, 11.3; S, 14.6. ¹H NMR (D₂O, ppm): 4.205 (s).

3.6. Synthesis of Odabco·2H₂SO₄ (4)

In an evaporating cup, odabco∙3H₂O₂ (218 mg, 0.89 mmol), H₂O (5.0 mL) and H₂SO₄ (100 μL) were mixed. The evaporating cup with the solution was left at room temperature until the water completely evaporated, then the precipitated crystals were recrystallized from a minimal amount of water. The resulting colorless crystals of 4 were filtered off and dried in air. The yield was 258 mg (86%). IR, ν/cm^–1: 3435 (m), 3354 (m), 3033 (w), 3015 (w), 2996 (w), 2862 (w), 2822 (w), 2796 (w), 2746 (w), 2686 (w), 2612 (w), 2524 (w), 2466 (w), 1996 (w), 1746 (w), 1636 (w), 1593 (w), 1507 (s), 1470 (w), 1458 (m), 1451 (m), 1376 (w), 1334 (m), 1284 (m), 1181 (m), 1107 (w), 1069 (s), 1042 (w), 1007 (s), 887 (m), 850 (m), 652 (m), 613 (m), 578 (m), 456 (s). Elemental analysis (%): Found C, 21.4; H, 4.6; N, 8.2; S, 19.0, calculated for odabco·2H₂SO₄(C₆H₁₆O₁₀N₂S₂): C, 21.2; H, 4.7; N, 8.2; S, 18.9. ¹H NMR (D₂O, ppm): 4.280 (s).

4. Conclusions

To summarize, four new salts of 1,4-diazabicyclo[2.2.2]octane N,N’-dioxide (odabco) with different anions were synthesized and structurally characterized. A comparative analysis of the structural features for the new salts and two previously reported cognate structures was performed, revealing hydrogen bonding as a main structure-driving factor. Both the chemical nature and dimensionality of forming hydrogen bond networks were found to be anion-dependent. Perchlorate bearing the weakest donor ability among the applied anions is not included in strong hydrogen bonds in its odabco salts, which results in the low dimensionality (0D or 1D) of the forming structures. Nitrate, being a bit stronger than perchlorate, but still a weak donor, may be involved in hydrogen bonding to protonated odabco cations, with no remarkable changes in the general structural motifs and dimensionality of the lattices. Sulfate, bearing the strongest donor ability among the considered anions, is involved in hydrogen bond systems in a bridging manner, which results in an increase in the polymeric lattice dimensionality to 1D or 3D. The reported results contribute to the further structural design of odabco-based optical and paraelectric materials.