Comment

In recent years, there has been considerable interest in the study of the crystal structures of amino acid salts, as they are of interest from several points of view, e.g. as candidates for nonlinear optical crystals (Fleck, Becker et al., 2008; Kaminskii et al., 2009, and references therein). Within these salts, the amino acids can act as cationic, anionic or zwitterionic units. The most extensive data are found for salts of glycine (see, for example, Fleck, Held et al., 2008; Fleck, 2008, and references therein), but many other amino acid salts have been reported (e.g. Ghazaryan et al., 2011a). Apart from the 20 `standard' amino acids, several others have been utilized in the design of crystalline salts, e.g. -alanine (Sridhar et al., 2001; Baran et al., 1995), betaine (e.g. Wiehl et al., 2008; Kocadag et al., 2008) and ornithine (Via et al., 2006; Ghazaryan et al., 2011b). Our group has been studying sarcosine (Sar) in recent years and investigating its molecular and crystal structures, vibrational spectra and thermal expansion. This work has mainly concerned sarcosinium salts, i.e. compounds comprising sarcosine as cations combined with inorganic anions (see Table 1, and references therein), although there are also several structures of sarcosine as a zwitterion combined with neutral salts (see Table 2, and references therein). As can be seen, most of these salts represent combinations of sarcosine and metal halogenides. In addition, two salts are known in which sarcosine acts as an anion in combination with metal cations, namely bis(sarcosinato)copper dihydrate (space group P2₁/n; Krishnakumar et al., 1994) and bis(sarcosinato)nickel dihydrate (space group P; Guha, 1973). In this work, we focus on the structure of the title compound, (I), formed from sarcosine and a neutral salt (Sar₄·KI·2H₂O).

In (I), there are two crystallographically different sarcosine moieties (labelled A and B) located on general positions, and one K⁺ cation and one I^- anion, both of which are located on twofold axes (Wyckoff position 4e). Additionally, there is one water molecule on a general position, which participates in an extensive hydrogen-bond network stabilizing the structure (Table 3). The sarcosine molecules are zwitterions and their geometry agrees well with the usual values (Fig. 1). However, one feature deserves attention, namely the C-O bond lengths within the two carboxylate groups, which show some variation; in the deprotonated mesomeric state one would expect more or less identical distances. These differences between the somewhat longer C1-O1 bonds [1.2640 (19) and 1.2624 (17) Å] and the shorter C1-O2 bonds [1.2379 (18) and 1.2348 (17) Å] are due to the fact that both atoms O1A and O1B are acceptors of hydrogen bonds extending from the N atoms and the water molecule (Table 3), whereas atoms O2A and O2B are not. The sarcosinate anions act as both O:O'- and O:O-bidentate-bridging ligands, thus connecting adjacent irregular eight-coordinated K⁺ cations into infinite chains along [001]. Hydrogen bonding provides further intra-chain connections, except for the hydrogen bonds involving the water molecules, which connect adjacent chains into a three-dimensional framework. A packing diagram is shown in Fig. 2 and a detailed view of one polyhedron as part of the chain is shown in Fig. 3.

The IR and Raman spectra of (I) are shown in Fig. 4. As expected, they show the characteristic vibrations of zwitterionic sarcosine and solvent water molecules. Comparison of the spectra of pure sarcosine with those shown in Fig. 4 allows the assignment of the IR absorption band with peaks at 3477 and 3425 cm^-1 and the Raman band at 3441 cm^-1 to O-H stretching vibrations of the water molecule. The characteristic intense Raman lines at 3025-2939 cm^-1 and the respective IR peaks at 3022-2938 cm^-1 are caused by C-H stretching vibrations of the CH₂ and CH₃ groups. These peaks are superimposed on a broad band caused by stretching N-H vibrations of the NH₂⁺ groups and possibly by combination bands. In the region near 1600 cm^-1, there is a strong IR absorption band with peaks at 1640, 1621 and 1586 cm^-1. In the IR spectrum of sarcosine, there are two peaks at 1642 and 1600 cm^-1 in this region. On this basis, we assign the peaks at 1640 and 1621 cm^-1 to an asymmetric stretching vibration of the COO^- group and a deformation vibration of the NH₂⁺ groups, and the peak at 1585 cm^-1 to a deformation vibration of the water molecule. The IR absorption band with peaks at 1401 and 1379 cm^-1 is assigned to a symmetric stretching vibration of the COO^- group, and the peaks at 648 and 598 cm^-1 (and the respective Raman line at 601 cm^-1) are assigned to a deformation vibration of the same group. We assign the peaks at 1485 and 1462 cm^-1 (and the respective Raman lines at 1470 and 1418 cm^-1) to an asymmetric deformation vibration of CH₃ and a deformation vibration of CH₂ groups, while the peaks at 1306, 1298 and 1289 cm^-1 and the Raman line at 1298 cm^-1 are assigned to the wagging vibration of the CH₂ group. The IR peaks at 1167 and 1145 cm^-1, and the respective lines at 1173 and 1140 cm^-1, are assigned to a rocking vibration of the CH₃ group, in contrast with the peak at 695 cm^-1 and the Raman lines at 708 and 697 cm^-1 which are assigned to the rocking vibration of the CH₂ group. The position of the absorption peak at 1061 cm^-1 (the respective Raman line is at 1062 cm^-1) is characteristic of stretching vibrations of a C-N bond. The deformation vibration (CC=O) is reflected by the presence of an absorption peak at 499 cm^-1 (and Raman lines at 507 and 487 cm^-1), while the vibration (CNC) is expressed at 377 cm^-1 in the Raman spectrum. The Raman lines at 970, 926 and 900 cm^-1 and the respective IR absorption peaks are assigned to skeleton vibrations. These spectroscopic data are in excellent agreement with the structural data elucidated from the X-ray diffraction experiment.

As stated above, (I) belongs to the group of sarcosine metal halogenides, which represent the majority of the compounds of sarcosine with neutral salts (Table 2). A comparison of these sarcosine metal halogenide structures shows that, as found previously for similar compounds with glycine, the structural diversity is high, i.e. each salt represents a unique structure type (for a detailed classification, see Fleck, 2008). These can be arranged into groups according to the connectivity of the building units. In doing so, it becomes apparent that there are many striking similarities with the respective compounds of glycine. There are examples of isolated units, as found in Sar₂·ZnCl₂ (Subha Nandhini et al., 2001) and Sar⁺·ZnCl₃^- (Krishnakumar et al., 2001), which comprise tetrahedral four-coordinate Zn²⁺ cations with monodentate sarcosine ligands. The structure of the former is closely related to that of Gly₃·ZnCl₂ (Hariharan et al., 1989), which is of interest because of its physical properties (e.g. Fleck, Becker et al., 2008; Kaminskii et al., 2009), and that of the latter bears a close similarity to Gly·ZnCl₂·H₂O (Fleck, Held et al., 2008). Another example of a structure comprising isolated units is that of Sar₂·PtBr₂ (Sabo et al., 2005), which also has a glycine analogue, namely trans-(Gly^-)₂·Pt^IVCl₂²⁺ (Davies et al., 1995). Fig. 5 shows the similarities of the respective parts of these structures.

Nevertheless, most sarcosine metal halogenide crystal structures can be described as chains [as is the case for (I)], the majority of which have glycine analogues as well. Among the chains, there are two subtypes: (i) isolated polyhedra connected to chains via bridging sarcosine ligands and (ii) polyhedral chains with the ligands attached (Fig. 6). Examples of the first subtype are Sar₂·MnCl₂·2H₂O (Rzaczynska et al., 2002) (an analogue of Gly₂·MnCl₂; Narayanan & Venkataraman, 1975), Sar·MnCl₂·2H₂O (Silva et al., 2001) [an analogue of Gly·NiCl₂·2H₂O (Fleck & Bohatý, 2004) and Gly·MnCl₂·2H₂O (Clegg et al., 1987)] and Sar₃·CaCl₂ (Mishima et al., 1984) (similar to Gly₃·CeCl₃; Fleck, Held et al., 2008). It is noteworthy that the structures of Sar·MnCl₂·2H₂O and Gly·MnCl₂·2H₂O are so closely related that even the unit-cell parameters agree reasonably well (allowing for the additional CH₃ group of sarcosine), confirming that the crystal structures are not merely topologically similar.

Naturally, larger cations tend to form structures of the second subtype, i.e. polyhedral chains. These chains can be formed by edge-sharing polyhedra, as in Sar·CdCl₂ (Yamada et al., 1994) (similar to Gly·NaI·H₂O; Verbist et al., 1971), or face-sharing polyhedra, as in Sar·BaCl₂·4H₂O (Krishnakumar & Natarajan, 1995) or Sar₃·BaBr₂ (Trzebiatowska-Gusowska et al., 2009), both of which are roughly similar to Gly₂·BaCl₂·H₂O and Gly₂·SrCl₂·3H₂O (Narayanan & Venkataraman, 1975). It is interesting to note that analogues of glycine and sarcosine exist for all these examples. Compound (I) is a notable exception, as it is the first instance (for both sarcosine and glycine) with an amino acid-metal ratio of 4:1. Nevertheless, as far as connectivity is concerned, the structure is related to those of salts with large cations (compare Figs. 6e, 6f and 6k).

In conclusion, it has been shown that there are several sarcosine metal halogenide structures, most of which can be characterized by infinite chains as the building units, and all of which have analogues with similar glycine compounds (if the connectivity of polyhedra via organic molecules is considered). The title compound, Sar₄·KI·2H₂O, is exceptional in that no similar glycine compound exists, as the amino acid-metal ratio of 4:1 has not been observed so far in any reported glycine or sarcosine metal halogenide compound.

Experimental

The initial reagents were sarcosine (Sigma) and chemically pure KI (Reakhim). The title compound, (I), was obtained from an aqueous solution containing sarcosine and KI in a stoichiometric 4:1 molar ratio by slow evaporation at room temperature. However, crystals of (I) were obtained initially from a solution containing an equimolar ratio of sarcosine and KI. A 2:1 molar ratio leads to crystals with the same composition.

Fourier-transform Raman spectra were recorded using an NXR FT-Raman module on a Nicolet 5700 spectrometer (number of scans 512, power at the sample 0.15 W, resolution 4 cm^-1) at room temperature. The same spectrometer was used for measuring attenuated total reflection Fourier transform IR spectra (FT-IR ATR) (ZnSe prism, 4000-500 cm^-1, Happ-Genzel apodization, ATR distortion corrected; number of scans 32, resolution 4 cm^-1). Part of the IR spectrum in the region 500-400 cm^-1 was taken from FT-IR spectra recorded with a Nujol mull (4000-400 cm^-1; number of scans 32, resolution 2 cm^-1).

H atoms bonded to C atoms were refined using the riding model, with C-H = 0.96-0.97 Å. The positions of the H atoms bonded to O and N atoms were refined with bond-length restraints of 0.89 (2) and 0.82 (2) Å for N-H and O-H, respectively. For methyl and water H atoms, U_iso(H) = 1.5U_eq(C,O), and for methylene H atoms, U_iso(H) = 1.2U_eq(C), while for the amine H atoms, their U_iso(H) values were refined freely.

Data collection: APEX2 (Bruker, 2004); cell refinement: APEX2; data reduction: APEX2; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Bergerhoff et al., 1996); software used to prepare material for publication: SHELXL97.

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Supplementary data for this paper are available from the IUCr electronic archives (Reference: MX3086 ). Services for accessing these data are described at the back of the journal.

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