Introduction

Alkylammonium hexahalogenidometallates (RnNH4−n)2[MX6] (1 ≤ n ≤ 4 ; R = alkyl; M = Si and X = F, Cl) show phase transitions that involves changes in hydrogen bonding as well as re-orientational motions of the alkylammonium cations [1,2,3,4]. Hexafluoridosilicates such as ((C2H5)NH3)2SiF6, ((CH3)3NH)2SiF6, (C6H5NH3)2SiF6, (CH3(CH2)2NH3)2SiF6, (CH3(CH2)3NH3)2SiF6 and ((C2H5)2NH2)2SiF6 were studied by X-ray, DSC, Infrared and Raman spectroscopy and computational analyses [5,6,7,8,9,10]. Furthermore, there is broad scope of hydrogen bonded hexahalogenidometallates with tetravalent metal centers: tin(IV) [11,12,13,14,15], iridium(IV) [16], osmium(IV) [17], ruthenium(IV) [18], platinum(IV) [19]. It has also been shown that some hexahalogenidometallate anions (Sn, Ir) predominantly act as fourfold hydrogen bond acceptor, whereas the dialkylammonium counter cations doubly act as hydrogen bond donors. The combination of these ions yields layered structures [16]. The structure refinement in this class of compounds is hindered by the presence of pseudosymmetry problems. In detail such problems may arise in this class of compounds because the heavy halogenidometallate anions show a highly symmetrical arrangement, whereas the lighter organic counter cations may break this symmetry [16]. In these and comparable cases a careful refinement, which may be supported by the use of restraints is needed [20,21,22].

We here present the synthesis, crystal structure and infrared spectra of diethylammonium hexachloridoplumbate(IV), ((C2H5)2NH2)2[PbCl6]. An assignment of the observed infrared bands to the corresponding vibrational modes, based essentially on the previous works [5, 6, 10] is carried out.

Results and Discussion

Crystal Structure

The asymmetric unit of the title structure contains one diethylammonium (deaH) cation located at a general position and one-half of a hexachloridoplumbate(IV) anion located on an inversion center. Each anion acts as fourfold hydrogen bond donor, whereas each deaH cation donates two hydrogen bonds. Consequently, a two-dimensional framework is constructed roughly perpendicular to the [–1 0 1] direction (Fig. 1). The hydrogen bonded rings that are formed can be classified with the graph set descriptor R8 8(24) [23,24,25] as shown in Fig. 2. The pseudosymmetry element of a face-centering is clearly visible.

Fig. 1
figure 1

View along [−101] of the hydrogen bonded polymeric layer of the title structure

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Fig. 2
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Constructor graph [25] for the hydrogen bonded topology of the title structure

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The title structure is at least homeotypic to (deaH)2[SnCl6] [12] and (deaH)2[PtCl6] [19] (Table 1), as in an isotypic relation the atomic coordinates should be the same. The two-dimensional hydrogen bonded framework, which is present in these three structures is also frequently encountered in other compounds of this class. The exchange of the deaH cation by another cation leads to structurally closely related compounds. With (dipH)2[SnCl6] (dipH = diisopropylammonium) a homotypic structure is literature known (Table 1). Other compounds feature the same hydrogen bonding pattern but crystallize in a different way (Table 2).

Table 1 Space groups and lattice parameters of the isotypic (deaH)2[M(IV)Cl6] structures and a homeotypic (dipH)2[SnCl6] (dipH diisopropylammonium) structure (Z′=1 for all entries)
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Table 2 Space groups and lattice parameters of two structures, which are structurally related to the title compound (dipH diisopropylammonium; Z′ = 2)
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An inquiry to the Cambridge structural database showed, that there are at least two more structurally related compounds. These two isotypic tetrakis(dimethylammonium) hexachloridometallate chloride double salts (dmaH)4[MCl6]Cl (M = W [27], Ru [28]; dimethylammonium (dmaH)) contain the aforementioned two dimensional hydrogen bonded framework built from hexachloridometallate(III) anions and dimethylammonium cations which are-due to the oxidation state of the metal - are formally negatively charged. Two more dimethylammonium cations and the chloride anion form the neighbouring anionic layers.

In all these structures discussed before the hexahalogenidometallate anions show roughly the same sub structure, which is an arrangement of the four anions with an identical orientation and a fifth, symmetry related one between them with a different orientation (Fig. 3). In the title structure and its isotypic ones (Table 1) the hydrogen bonded layers are located in the diagonal of the unit cell. Consequently, the shortest distances of [PbCl6]2− anions in the title structure are 8.4697(3), 10.7284(3) and 13.109(4) Å (Figs. 1, 3). These values are similar for all isotypic structures given in Table 1 and can also be found in some of the lattice metrices of the structures of (dipH)2[IrCl6], (dmaH)4[WCl6]Cl and (dmaH)4[RuCl6]Cl (Tables 2, 3). It is worth mentioning that the formal antitype [29] of these hexahalogenidometallates salts has been reported. In the structure of (dimethylphosphoryl)methanammonium nitrate [30] dimeric units of cations act as fourfold hydrogen bond donor, whereas the nitrate anion acts as a twofold hydrogen bond acceptor.

Fig. 3
figure 3

The packing of the title structure is shown with a view along [001]. For the cations only the nitrogen atoms are shown. (Color figure online)

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Table 3 Space groups and lattice parameters of two isotypic, layered structures which contain hydrogen bonded {(dmaH)2[MCl6]}+ layers (dmaH dimethylammonium, M Ru(III), W(III))
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Symmetry Considerations of Internal Vibrations of (C2H5)2NH2 + Cation

The free (C2H5)2NH2 + cation may be taken to belong to the C2v symmetry point group. It has 45 internal vibrational modes which are described by the following representation: 14 A1 (IR, Ra) + 9 A2 (Ra) + 12 B1 (IR, Ra) + 10 B2 (IR, Ra). Theses cationic modes correspond mainly to the vibrations of the NH2, CH2 and CH3 groups and to those of the cationic skeletal C–C–N+ and C–N+–C. For an isolated cation, the vibrational modes of A1, B1 and B2 symmetries are infrared active.

Within the (deaH)2[PbCl6] crystal which may be belonging to P21/n (\({\text{C}}_{{2{\text{h}}}}^{5}\)) space group, the (C2H5)2NH2 + cations have the C1 sites symmetry that is lower than C2v. Then as a result of two formula units per primitive unit cell, i.e. four cations per cell, each of the A1, A2, B1 and B2 vibrational modes should split into four components by Davydov splitting effect (Table 4). In the crystal (C2h group factor), which is centrosymmetric, the ungerade modes of symmetry Au and Bu are Infrared active. This implies that all the characteristic (C2H5)2NH2 + infrared bands should be observable for this salt. Details are discussed in the Experimental section of this contribution.

Table 4 Correlations diagramme (C 2v --->C1--->C 2h ): vibrational modes of (C2H5)2NH2 +
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Experimental

Synthesis

Crystals of (deaH)2[PbCl6] were prepared in ice-cold hydrochloric acid solution containing stoichiometric amounts of PbO2 and (C2H5)2NH2Cl. To a HCl solution still in the ice bath, PbO2 was slowly added with constant stirring and a yellow solution obtained. The solution was left in the ice bath for a few minutes to allow it to settle. After the yellow solution was decanted into the ice-cold (C2H5)2NH2Cl solution (still in the ice bath) with stirring; the mixture was allowed to sit in the ice bath for many minutes to complete precipitation of (deaH)2[PbCl6].

Powder Diffraction

The crystalline powder was characterized by X-ray diffraction. XRD data were collected with a STOE STADI-P diffractometer working with CuKα1 radiation. Independent from using the parameters of the probably isostructural (deaH)2[SnCl6] [12] indexing was carried out using the program TOPAS [31]. The solution was adopted from the literature known (deaH)2[SnCl6] [12] in the monoclinic space group P21/n. The pattern was refined with the program TOPAS [31] (classical Rietveld method) using a TCHZ pseudo-Voigt function [32] for peak modelling. Chebyshev polynomials were used for shaping the background. Preferred orientation has been minimized by using spherical harmonics [33] (Fig. 4).

Fig. 4
figure 4

Blue line measured pattern; Red line: simulated pattern; Grey line difference pattern; vertical bars Bragg-Positions (deaH)2[PbCl6])

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The structure was refined by the Rietveld method using the [(C2H5)2NH2]+ ion and [PbCl6]2− ion with restrained bond length and angels (Pb–Cl 2.4–2.7 Å and 86–94°; C–C 1.45–1.55 Å and 110–115°; C–N 1.4–1.5 Å and 110–115°). Final refined bond length and angles are shown in Table 5. These soft rigid bodies stabilize the refinement [34]. The necessity for these restrained refinements is based on the fact that the diffraction pattern is dominated by the [PbCl6]2− ions. Consequently, the orientation of the cation is badly defined. The orientation of the diethylammonium cation was checked via pattern fitting. The best fitting was found building the shortest H···Cl hydrogen bonds between N–H···Cl as shown in Fig. 1 (N···Cl distances: 4.41–3.45 Å). Rotating the cation to get shorter C–H···Cl hydrogen bonds carried out only a local minimum-energy configuration with the best result. Therefore we decided on a chemical reasonable orientation, which is almost the same as those, which was reported for (deaH)2[SnCl6] [12]. Observed, calculated and difference pattern of the final refinement of the title structure are represented in Fig. 4.

Table 5 Selected bond length [Å] and bond angles [°]
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Crystal data and structure refinement parameters are given in Table 6.

Table 6 Crystal data and structure refinement parameters of (deaH)2[PbCl6])
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The Infrared Spectrum of (deaH)2[PbCl6]

The infrared spectrum of (deaH)2[PbCl6] was recorded at room temperature, in the 600–4000 cm−1 spectral range, on a VERTEX 70 FTIR spectrometer (BRUKER Optics) by using the ATR MIRACLE DIAMANT technique. The spectral resolution was 4 cm−1 (20 scans).

The infrared spectrum of this compound is presented in Fig. 5. Several characteristic bands corresponding to cationic (C2H5)2NH2 + vibrational modes were observed in the infrared spectrum of this compound. The assignment of the majority of infrared bands (Tables 7, 8), especially in the (C2H5)2NH2 + cationic spectral ranges (600–4000 cm−1) is made on the basis of literature data of some alkylammonium compounds [3,4,5,6,7, 10] and the factor group analysis.

Fig. 5
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Infrared spectrum of (deaH)2[PbCl6]

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Table 7 Assignment at room temperature of (deaH)2[PbCl6] infrared bands (cm−1)
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Table 8 Assignment of the combination and overtone bands observed on the IR spectrum of (deaH)2[PbCl6] at room temperature
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In the 3500–2700 cm−1 spectral region, appeared generally the N–H and C–H stretching modes. So, the strong band located at 3142 cm−1 is assigned to the asymmetric modes of NH2 group νas(NH2); while the two medium bands observed at 3089 and 3004 cm−1 may be due to the symmetric νs(NH2) modes. The C–H stretching modes give rise in the infrared spectrum of this compound to five bands. The band of medium intensity observed at 2806 cm−1 and the two weak features occurred at 2838 and 2885 cm−1 are assigned to the symmetric stretching modes νs(CH2) and νs (CH3); while the two weak bands observed at 2969 and 2990 cm−1 correspond to the asymmetric νas (CH2) and νas (CH3) stretching modes.

The bands observed at lower frequencies of NH and CH stretching spectral region (2700–2000 cm−1) have been considered as being due to overtones and combinations modes. These bands with appreciable intensities may be intensified by Fermi resonance as has been stated for several hybrid compounds in the literature [3,4,5, 7]. The observation of these non-fundamental modes in this IR spectral region may be an indication of the formation of appreciable N–H···Cl hydrogen bonds in the (deaH)2[PbCl6] crystal. This is in agreement with the X-ray results that showed that the stabilisation of the structure of this compound by shorter hydrogen bonds [2.37(3) Å]. An attempt of assigning bands observed at 2650 and 2100 cm−1 to corresponding combination and overtone modes is given in Table 8.

The bands appeared in the 1650–600 cm−1 range correspond generally to asymmetric and symmetric bending modes of CH3, CH2 and NH2 groups, to CH3 and CH2 rocking modes, to CH2 wagging, to scissoring (δas CH2) vibrations and to the skeletal stretching and bending modes of the diethylammonium cations. So, the band of strong intensity observed at 1571 cm−1 is assigned to δ (+NH2). The band of medium intensity appeared at 1635 cm−1 may be due to δ (H2O) deformation of moisture that is present in the compound itself. The TGA curve (cooling) confirms that (deaH)2[PbCl6] is hygroscopic and the elimination of moisture water was made in the 30–80 °C temperature domain. The two strong bands observed at 1465 and 1456 cm−1 were assigned to δas (CH2) or δas (CH3) modes, whereas the two features of strong intensity at 1424 and 1387 cm−1 were attributed to δs (CH3) modes or CH2 wagging ones. The bands corresponding to CH2 wagging, CH3 and CH2 rocking modes, the skeletal stretching and bending modes were all observed between 1400 and 600 cm−1 (Tables 7, 8).

The observation of some non-fundamental bands with appreciable intensities are interesting since they are indicative of the formation of moderate to strong N–H–Cl hydrogen bonding between (C2H5)2NH2 + cations and [PbCl6]2− anions. But, this makes the assignment of some characteristic cationic bands, in the NH and CH stretching spectral region, more difficult since the hydrogen bonding may be the origin of such deviations of some frequencies values in this region.

Conclusion

The X-ray powder diffraction pattern of the title compound deaH2PbCl6 shows monoclinic symmetry. A detailed analysis of the powder diffraction pattern verified that the title compound is isotypic to deaH2MCl6 (M = Sn, Pt). Even though, alkylammonium hexahalogenidometallates of lead(IV) are well known for decades [35], a search in the Cambridge Structural database proved that there is no structure containing a [Pb(IV)Cl6]2− anion reported until now and even the [Pb(II)Cl6]4− containing structures are rare [36,37,38]. This is surprising as hexahalogenidometallates are well known to be useful tectons for the construction of hybrid solids [39]. The main structural feature of the title structure is a hydrogen bonded two-dimensional framework, which is discussed on the basis of a constructor graph. In all the discussed layered structures each cation donates two hydrogen bonds, whereas each halogenidometallate accepts four hydrogen bonds. The isotypic structures deaH2MCl6 (M = Sn, Pt, Pb) and other structurally related ones seem to be dominated by this hydrogen bonding motif.