Synthesis, Structural Characterization, Conformational and Topological Classiﬁcation of Different Salts in the 2,2-Dimethylpropane-1,3-diamine/HCl/H 2 O-System

: The reaction of 2,2-dimethylpropane-1,3-diamine ( dmpn ) with an excess of concentrated aqueous hydrochloric acid yielded colorless crystals of 2,2-dimethylpropane-1,3-diaminium dichloride, dmpn H 2 Cl 2 ( 1 ), in addition to small amounts of a monohydrate, dmpn H 2 Cl 2 · H 2 O ( 2 ). The compounds were studied via X-ray crystallography, IR and Raman spectroscopy, NMR spectroscopy and thermal analysis. Single crystal structure determinations on 1 and 2 showed that dmpn H 2 Cl 2 exists in two polymorphic forms, 1a and 1b . The crystal structure of 1b showed to be much more complex than that of 1a . In the crystal structure of 2 , four ( dmpn H 2 ) 2+ cations and eight chloride anions form a cage constructed by N − H ··· Cl hydrogen bonds. In the center of these cages water dimers with a O ··· O distance of 2.776 (8) Å are present. In addition, a conformational analysis of the 2,2-dimethylpropane-1,3-diaminium cation was performed. The results are compared to the experimental ﬁndings of 1a , 1b , 2 and other related hydrogen bonded salt structures from the Cambridge crystallographic structure database (CCDC). Last, a topological classiﬁcation of the solid-state structures of 1a and 2 was performed and the simpliﬁed topological networks are discussed.


Introduction
Generally, 2,2-dimethylpropane-1,3-diamine (dmpn) is easily accessible through the reduction of 1,3-dinitro-2,2-dimethylpropane with iron in high yields [1]. The synthetic protocol of dmpn has been optimized and was applied for patent in 1973 [2]. With transition metals dmpn forms stable complexes [3][4][5][6]. Consequently, dmpn and ligands with a similar topology have a broad scope application such as their use as bridging ligands in metal organic frameworks and in coordination polymers [7,8]. Recently we reported the interesting finding that dmpn is able to capture CO 2 spontaneously by the formation of (3-ammonio-2,2-dimethyl-propyl)carbamate dihydrate [9]. The carbamate releases its CO 2 on heating which might add some applications.

Synthesis
All chemicals were obtained from commercial sources and used as purchased. 10 mL of conc. hydrochloric acid (~120 mmol) was added dropwise to 3.07 g of 2,2dimethylpropane-1,3-diamine (30 mmol). After complete neutralization of the amine by the hydrochloric acid, the HCl concentration in the obtained solution was approximately 6 mol•l -1 . This clear, colorless solution was left in a fume hood for two weeks until the liquid had completely evaporated. Yield: 4.83 g = 92% (calc. for C5H16N2Cl2), colorless solid. A CHN analysis was performed with a vario MICRO cube [22]. Analysis calculated for C5H16N2Cl2 [%]: C 34.30, H 9.21, N 16.00; found: C 34.12, H 9.09, N 15.74. 1 H-and 13 C{ 1 H}-NMR-spectra were recorded with a Bruker Avance III spectrometer and analyzed with the MestReNova software package [23]. 1 46.83 (C). Single crystals of 2,2-dimethylpropane-1,3-diaminium dichloride (1a and 1b) and of 2,2-dimethylpropane-1,3-diaminium dichloride monohydrate (2), suitable for crystal structure analysis were selected directly from the bulk material. The main phase turned out to be 1a, while 1b and 2 appear as by-products in small quantities. Since the crystals could not be clearly distinguished from each other morphologically, several single crystals were checked via X-ray diffraction and these were separated into three fractions 1a, 1b and 2 by hand based on their lattice parameters and used for further analysis. The ratio of 1a to the sum of 1b and 2 is estimated to be five to ten. Figure 1. Formation of the three salts in the 2,2-dimethylpropane-1,3-diamine/HCl/H 2 O-system described in this work: 2,2-dimethylpropane-1,3-diaminium dichloride (1a and 1b), dmpnH 2 Cl 2 , and 2,2-dimethylpropane-1,3-diaminium dichloride monohydrate (2), dmpnH 2 Cl 2 ·H 2 O.

Synthesis
All chemicals were obtained from commercial sources and used as purchased. 10 mL of conc. hydrochloric acid (~120 mmol) was added dropwise to 3.07 g of 2,2-dimethylpropane-1,3-diamine (30 mmol). After complete neutralization of the amine by the hydrochloric acid, the HCl concentration in the obtained solution was approximately 6 mol·L −1 . This clear, colorless solution was left in a fume hood for two weeks until the liquid had completely evaporated. Yield: 4.83 g = 92% (calc. for C 5 H 16 N 2 Cl 2 ), colorless solid. A CHN analysis was performed with a vario MICRO cube [22]. Analysis calculated for C 5 46.83 (C). Single crystals of 2,2-dimethylpropane-1,3-diaminium dichloride (1a and 1b) and of 2,2-dimethylpropane-1,3-diaminium dichloride monohydrate (2), suitable for crystal structure analysis were selected directly from the bulk material. The main phase turned out to be 1a, while 1b and 2 appear as by-products in small quantities. Since the crystals could not be clearly distinguished from each other morphologically, several single crystals were checked via X-ray diffraction and these were separated into three fractions 1a, 1b and 2 by hand based on their lattice parameters and used for further analysis. The ratio of 1a to the sum of 1b and 2 is estimated to be five to ten.

IR-and Raman Spectroscopy
IR spectra of 1a, 1b and 2 were recorded with a PerkinElmer Spectrum Two FT-IR spectrometer, equipped with a LiTaO 3 detector (4000-350 cm −1 ) and an universal ATR unit [24,25]. IR spectrum of 1a/1b [cm IR spectra of 1a, 1b and 2 were recorded with a PerkinElmer Spectrum Two FT-IR spectrometer, equipped with a LiTaO3 detector (4000-350 cm −1 ) and an universal ATR unit [24,25]. IR spectrum of 1a/1b [cm Assignments were made according to [27,28]. The bands and lines below 1400 cm −1 are less instructive and are not listed, with the exception of the characteristic CH2 rocking mode. The CH2 rocking mode at ~720 cm -1 is IR inactive (see Raman spectra).
A comparison to the IR and Raman spectra of dmpn can be found in Figure 2. IR and Raman spectra of dmpn (blue lines), 1a (green lines) and 2 (red lines). The IR bands and the Raman lines between 3400 and 3200 cm −1 in the spectra of dmpn are caused by NH-valence modes of the free amine and are no longer detected in the product spectra. The aminium groups in 1a and 2 are involved in electrostatically supported hydrogen bonds and the bands of their NHvalence modes are broad and significantly shifted to lower wave numbers, overlapping with the CH-valence modes between 2800 and 3000 cm −1 . The presence of water in 2 is indicated by the band of the OH-valence modes between 3600 and 3300 cm −1 . The respective signal in the Raman spectrum is of very low intensity.

Thermal Analysis
Compounds 1a and 2 were characterized by DSC/TGA thermal analysis. DSC curves were recorded under N2 atmosphere with a heating/cooling rate of 5 K•min −1 in the temperature range of 25-200 °C with a Mettler-Toledo DSC 1 STAR e system. The DSC curves of 1a and 2 (see Figure 3) show a similar appearance except for a series of endothermic features in the range of 55-135 °C, mostly related to the loss of water. The mass of 2 decreases by ~10% (calc. 9.3% for C5H16N2Cl2 H2O). Both compounds, 1a and 2, Figure 2. IR and Raman spectra of dmpn (blue lines), 1a (green lines) and 2 (red lines). The IR bands and the Raman lines between 3400 and 3200 cm −1 in the spectra of dmpn are caused by NH-valence modes of the free amine and are no longer detected in the product spectra. The aminium groups in 1a and 2 are involved in electrostatically supported hydrogen bonds and the bands of their NH-valence modes are broad and significantly shifted to lower wave numbers, overlapping with the CH-valence modes between 2800 and 3000 cm −1 . The presence of water in 2 is indicated by the band of the OH-valence modes between 3600 and 3300 cm −1 . The respective signal in the Raman spectrum is of very low intensity.

Thermal Analysis
Compounds 1a and 2 were characterized by DSC/TGA thermal analysis. DSC curves were recorded under N 2 atmosphere with a heating/cooling rate of 5 K·min −1 in the temperature range of 25-200 • C with a Mettler-Toledo DSC 1 STAR e system. The DSC curves of 1a and 2 (see Figure 3) show a similar appearance except for a series of endothermic features in the range of 55-135 • C, mostly related to the loss of water. The mass of 2 decreases by~10% (calc. 9.3% for C 5 H 16 N 2 Cl 2 H 2 O). Both compounds, 1a and 2, show two further endothermic features in their heating curves at~138 • C and~145 • C (peak maxima). The cooling curves each show two exothermic features at~140 • C and~125 • C (peak maxima). The shoulder at~120 • C in the cooling curve of 1a cannot be clearly assigned (see Figure 3). These observations suggest at least two reversible phase transitions and a common high-temperature phase of the substances. A TGA curve of the 'dried' material from the DSC measurements was recorded under N 2 atmosphere with a heating rate of 5 K·min −1 in the temperature range of 20-600 • C with a Netsch STA 449C device. A mass loss of~98% in the range of 250-400 • C (T onset = 250 • C) is compatible to the complete decomposition of the sample. Solids 2022, 3, FOR PEER REVIEW 4 show two further endothermic features in their heating curves at ~138 °C and ~145 °C (peak maxima). The cooling curves each show two exothermic features at ~140 °C and ~125 °C (peak maxima). The shoulder at ~120 °C in the cooling curve of 1a cannot be clearly assigned (see Figure 3). These observations suggest at least two reversible phase transitions and a common high-temperature phase of the substances. A TGA curve of the 'dried' material from the DSC measurements was recorded under N2 atmosphere with a heating rate of 5 K•min -1 in the temperature range of 20-600 °C with a Netsch STA 449C device. A mass loss of ~98% in the range of 250-400 °C (Tonset = 250 °C) is compatible to the complete decomposition of the sample.

Crystal Structure Analysis
The single crystal diffraction data of 1a, 1b, and 2 were collected with a Xcalibur Eos diffractometer. Indexing, unit cell refinement data collection and data processing followed the standard procedures of the CrysAlisPRO software package [29]. A multi-scan absorption correction was applied for 1a and 2 with the SCALE3 ABSPACK routine of CrysAlisPRO [29], and an analytical absorption correction was applied for 1b [30]. The structure solution with Direct Methods and the structure refinement by full-matrix leastsquares calculations on F 2 were carried out with programs of the SHLEX family [31][32][33]. After all non-hydrogen atoms have been located, H atoms bound to C and N were included in idealized positions using a riding model with bond lengths constrained to 0.96, 0.97, and 0.89 Å for CH3, CH2 and NH3, respectively. In addition, the CH3 and NH3 groups were allowed to rotate around the adjacent C-C bond. The displacement parameters Uiso(H) of the H atoms bound to C and N were set to 1.5Ueq, 1.2Ueq , and 1.5Ueq of the parent atoms for CH3, CH2 and NH3, respectively. Crystals of 1b and 2 suffered from twinning and were refined as 2-component twins with the HKLF5 option of SHELXL. The fractions of second individual contribution were refined to 0.335 and 0.404 for 1b and 2, respectively. Molecular graphics were generated with DIAMOND [34].

Crystal Structure Analysis
The single crystal diffraction data of 1a, 1b, and 2 were collected with a Xcalibur Eos diffractometer. Indexing, unit cell refinement data collection and data processing followed the standard procedures of the CrysAlisPRO software package [29]. A multi-scan absorption correction was applied for 1a and 2 with the SCALE3 ABSPACK routine of CrysAlisPRO [29], and an analytical absorption correction was applied for 1b [30]. The structure solution with Direct Methods and the structure refinement by full-matrix leastsquares calculations on F 2 were carried out with programs of the SHLEX family [31][32][33]. After all non-hydrogen atoms have been located, H atoms bound to C and N were included in idealized positions using a riding model with bond lengths constrained to 0.96, 0.97, and 0.89 Å for CH 3 , CH 2 and NH 3 , respectively. In addition, the CH 3 and NH 3 groups were allowed to rotate around the adjacent C−C bond. The displacement parameters U iso (H) of the H atoms bound to C and N were set to 1.5U eq , 1.2U eq , and 1.5U eq of the parent atoms for CH 3 , CH 2 and NH 3 , respectively. Crystals of 1b and 2 suffered from twinning and were refined as 2-component twins with the HKLF5 option of SHELXL. The fractions of second individual contribution were refined to 0.335 and 0.404 for 1b and 2, respectively. Molecular graphics were generated with DIAMOND [34]. Crystal data, data collection and structure refinement details for 1a, 1b and 2 are given in Table 1.

Structural Commentary
Compound 1a crystallizes in the monoclinic space group C2/c (Z = 8). The asymmetric unit contains one dmpn cation [(dmpnH 2 ) 2+ ] and two chloride anions (all in general positions). The structure forms layers along the crystallographic a-axis (see Figure 4). Each layer consists of strands, each of which contains two symmetry related cations arranged around a center of inversion. Both aminium groups of the cations and the chloride anions are involved into a N−H···Cl hydrogen-bonded network. The geometrical parameters of all hydrogen bonds are given in Table 2. All N···Cl distances are in narrow ranges of 3.146 (1)-3.180 (1) Å [35]. There are others in the range of 3.272 (1)-3.336 (1) Å which are significantly weaker. Obviously, the cations show an unsymmetrical hydrogen bonding pattern. The aminium group including N1 forms three hydrogen bonds to chloride anions within the same strand. The aminium group including N2 forms one hydrogen bond to a chloride anion within the same strand and one hydrogen bond to the adjacent strand, causing the connection between the strands. No N−H···Cl hydrogen bonds could be identified between adjacent layers along the crystallographic a-axis. Consequently, they are connected by van der Waals forces and non-classical C−H···Cl hydrogen bonds.
Compound 2 crystallizes in the triclinic space group P1 (Z = 2). The asymmetric unit contains one cation [(dmpnH 2 ) 2+ ] and two chloride anions (all in general positions). Four symmetry related formula units each form a cavity with two water molecules inside (see Figure 4). These two water molecules build a dimer arranged around a center of inversion in the middle of the cavity. They are connected by a classical O−H···O hydrogen bond with a O1···O2 distance of 2.776 (8) Å. Thus, the cavity is too large for only one water dimer the structure refinement suggests that there are two energetically identical positions for this water dimer which are each half occupied. In general, eight hydrogen bonds caused by aminium groups, and three hydrogen bonds caused by the water molecules can be identified. The geometrical parameters of these hydrogen bonds are given in Table 1 are involved into a N-H•••Cl hydrogen-bonded network. The geometrical parameters of all hydrogen bonds are given in Table 2. All N•••Cl distances are in narrow ranges of 3.146 (1)-3.180 (1) Å [35]. There are others in the range of 3.272 (1)-3.336 (1) Å which are significantly weaker. Obviously, the cations show an unsymmetrical hydrogen bonding pattern. The aminium group including N1 forms three hydrogen bonds to chloride anions within the same strand. The aminium group including N2 forms one hydrogen bond to a chloride anion within the same strand and one hydrogen bond to the adjacent strand, causing the connection between the strands. No N-H•••Cl hydrogen bonds could be identified between adjacent layers along the crystallographic a-axis. Consequently, they are connected by van der Waals forces and non-classical C-H•••Cl hydrogen bonds.
Compound 2 crystallizes in the triclinic space group P1 (Z = 2). The asymmetric unit contains one cation [(dmpnH2) 2+ ] and two chloride anions (all in general positions). Four symmetry related formula units each form a cavity with two water molecules inside (see Figure 4). These two water molecules build a dimer arranged around a center of inversion in the middle of the cavity. They are connected by a classical O-H•••O hydrogen bond with a O1•••O2 distance of 2.776 (8) Å. Thus, the cavity is too large for only one water dimer the structure refinement suggests that there are two energetically identical positions for this water dimer which are each half occupied. In general, eight hydrogen bonds caused by aminium groups, and three hydrogen bonds caused by the water molecules can be identified. The geometrical parameters of these hydrogen bonds are given in Table 1    Symmetry codes for 1a:

Conformational Analysis
A theoretical conformational analysis of the 2,2-dimethylpropane-1,3-diaminium cation was performed. As shown in Figure 5, six conformations can be postulated for the rotation of the amino group including N1 around the N1C1-C2C3 bond. The eclipsed conformations with the dihedral torsion angle Θ at 0 • (syn), +120 • and −120 • (=+240 • ) are conformers with maximum energies. The staggered conformations with Θ at +60 • (gauche) and −60 • (=+300 • ) as well as 180 • (anti) are conformers with minimum energies. Assuming that the two amino groups do not interact with each other, the same ideas can be applied for the rotation of the amino group including N2 around the C1C2−C3N2 bond.

Conformational Analysis
A theoretical conformational analysis of the 2,2-dimethylpropane-1,3-diaminium cation was performed. As shown in Figure 5, six conformations can be postulated for the rotation of the amino group including N1 around the N1C1-C2C3 bond. The eclipsed conformations with the dihedral torsion angle Θ at 0° (syn), +120° and −120° (=+240°) are conformers with maximum energies. The staggered conformations with Θ at +60° (gauche) and −60° (=+300°) as well as 180° (anti) are conformers with minimum energies. Assuming that the two amino groups do not interact with each other, the same ideas can be applied for the rotation of the amino group including N2 around the C1C2−C3N2 bond. In order to get a qualitative impression of the energy landscape for the rotation of the two amino groups in the (dmpnH 2 ) 2+ cation, quantum chemical calculations have been performed. The Turbomole [36] suite of programs was used at the density functional theory (DFT) level with B3-LYP functional and the def-SV(P) basis set in the gas phase. Constrained geometry optimizations have been performed on a 2-dimensional grid. The N1C1-C2C3 angle has been scanned from 0 to 360 • in steps of 5 • , the C1C2−C3N2 angle from 20 to 180 • in steps of 5 • (10 • in the range of 20-50 • ). The values for this angle from 180 to 340 • have been obtained by symmetry. The scan reveals the anti-anti-conformation [Θ 1 /Θ 2 : 180 • /180 • (aa)] to be the global minimum (see Figure 6). In this conformation, the atoms N1, C1, C2, C3, and N2 are all in-plane. Barriers of 21 kJ·mol −1 (read from the grid) lead to four equivalent local minima for the anti-gauche-conformations [Θ 1 /Θ 2 : 180 • /+60 • (ag), 180 • /−60 • (a-g), +60 • /180 • (ga), −60 • /180 • (−ga)]. These correspond to one of the amino groups being turned up or down from the C1-C2-C3 plane. An unconstrained optimization of these structures yields angles of 173 • /76 • and an energy of 13 kJ·mol −1 above the conformational ground state. Inspecting the four gauche-gauche-combinations [Θ 1 /Θ 2 : +60 • /+60 • (gg), +60 • /−60 • (g-g), −60 • /+60 • (-gg), −60 • /−60 • (-g-g)], two pairs of equivalent structures can be found. The first pair corresponds to turning the two amino groups in opposite directions. An optimization puts this conformation at 80.1 • /80.1 • , 25 kJ·mol −1 above the conformational ground state. The barrier connecting these g/g minima to the a/g minima is at 31 kJ·mol −1 above the conformational ground state or only 6 kJ·mol −1 above the g/g minimum. The second pair corresponds to both amino groups turned in the same direction. Here, no minimum is found on the grid but only a saddle point at 100 • /270 • , 52 kJ·mol −1 above the conformational ground state. Solids 2022, 3, FOR PEER REVIEW 8 Figure 5. Newman projections for possible conformations of the 2,2-dimethylpropane-1,3-diaminium cation resulting by the rotation of the amino groups including N1 and N2 around the C1-C2 bond and the C2-C3 bond, respectively. The anti-periplanar conformation is expected to be the most stable conformation with minimum energy.

Topological Classification
The aminium groups together with the chloride ions form supramolecular hydrogen bonded networks in their crystal structures. Using the ToposPro software package [37] the topology of these networks can be classified using the concept of the simplified underlying net [38]. Figure 7a shows the simplified network of 1a, which can be assigned to the class cbs/CrB (chromium boride type) [37,39,40]. Three different types of interconnected strands can be identified, which are colored orange, blue and pink for discussion. Each strand consists of distorted rectangles, each formed by two N and two Cl atoms. The arrangement guaranties alternating occurrence of aminium groups and chloride anions. Thus, each N atom is surrounded by four Cl atoms and vice versa. The rectangles within a strand are folded in a stair-like manner. The orange-colored strands are crossed by the pink-and the blue-colored strands, while pink-and blue-colored strands never cross (see Figure 7a). Figure 7b shows the simplified network of 2. The topological net can be assigned to the class tcs (thorium chromium silicide type) [41][42][43][44][45][46][47]. It consists of two different types of dodecahedra, which are colored orange and blue for discussion. These polyhedra show a significant distortion of their hexagonal faces, whose angles deviate from the ideal value of 120 • . While the orange-colored polyhedra are stretched along the crystallographic c-axis with an N···Cl···N angle of 147.2 • , the angle for the blue-colored polyhedra is 108.1 • . The individual disorder of the water dimers contained in the blue-colored polyhedra seems to be determined by the fact that this type of polyhedra is elongated along the crystallographic b-axis. Within one layer, polyhedra of the same type form a columnar connection pattern, where the connection is established by the hexagonal faces along the crystallographic a-axis. Along the crystallographic c-axis, the polyhedron types alternate and are connected by further hexagonal faces (see Figure 7b). seems to be determined by the fact that this type of polyhedra is elongated along the crystallographic b-axis. Within one layer, polyhedra of the same type form a columnar connection pattern, where the connection is established by the hexagonal faces along the crystallographic a-axis. Along the crystallographic c-axis, the polyhedron types alternate and are connected by further hexagonal faces (see Figure 7b). The topological nets can be assigned to the classes cbs/CrB (chromium boride type) and tcs (thorium chromium silicide type) in the case 1a and 2, respectively.

Conclusions
In this work, two new salts in the dmpn/HCl/H2O-system have been synthesized and characterized by crystal structure analysis, spectroscopic methods, thermal analysis, and topological classification. Thus, the series of reported halide salts of dmpn has been extended. Phases 1a and 2 show reversible phase transformations at the same temperatures, indicating that a common high-temperature phase of them exists. This could hold potential for further research. A comparison of the crystal structures of 1a, 1b, 2 and further salts from the CSD [18], revealed that the energetically favorable anti-anti-conformer is not always the best option for the 2,2-dimethylpropane-1,3-diaminium cation in the solid state [11][12][13][14][15][16][17]19]. The conformational analysis, which was supported by quantum chemical calculations, verified that the anti-anti-conformation represents the global minimum on the potential energy surface. In some cases, the anti-gauche-and the gauche-gauche-conformation are preferred (see Table 3), obviously due to the formation of hydrogen bonds within the corresponding solid-state structures. The most interesting finding is certainly the caging of water dimers in 2. This particular property of the dmpn/HX/H2O-system may enable the caging of other small molecules such as Br2, IF, IBr, and ICl. By using aliphatic diamines with extended alkyl chains, it might be possible to enlarge the cavity observed in 2 to cage polyiodide chains, such as [I6] 2− [48].  The topological nets can be assigned to the classes cbs/CrB (chromium boride type) and tcs (thorium chromium silicide type) in the case 1a and 2, respectively.

Conclusions
In this work, two new salts in the dmpn/HCl/H 2 O-system have been synthesized and characterized by crystal structure analysis, spectroscopic methods, thermal analysis, and topological classification. Thus, the series of reported halide salts of dmpn has been extended. Phases 1a and 2 show reversible phase transformations at the same temperatures, indicating that a common high-temperature phase of them exists. This could hold potential for further research. A comparison of the crystal structures of 1a, 1b, 2 and further salts from the CSD [18], revealed that the energetically favorable anti-anti-conformer is not always the best option for the 2,2-dimethylpropane-1,3-diaminium cation in the solid state [11][12][13][14][15][16][17]19]. The conformational analysis, which was supported by quantum chemical calculations, verified that the anti-anti-conformation represents the global minimum on the potential energy surface. In some cases, the anti-gaucheand the gauche-gauche-conformation are preferred (see Table 3), obviously due to the formation of hydrogen bonds within the corresponding solid-state structures. The most interesting finding is certainly the caging of water dimers in 2. This particular property of the dmpn/HX/H 2 O-system may enable the caging of other small molecules such as Br 2 , IF, IBr, and ICl. By using aliphatic diamines with extended alkyl chains, it might be possible to enlarge the cavity observed in 2 to cage polyiodide chains, such as [I 6 ] 2− [48].