Polyiodides of amino acids. L-Proline triiodides

Two new salts of l-proline containing triiodide anions were obtained and investigated: (l-ProH···l-Pro)(I 3 ) (I) and [(l-ProH) 3 (l-Pro)] (I 3 ) 3 (II). Both compounds crystallize in the polar monoclinic space group P2 1 . Crystal structure determinations showed that (I) contains a dimeric cation formed by an O-H···O hydrogen bond with an O···O distance of 2.458(4) Å, while (II) features a peculiar tetrameric cation [l-ProH···(l-Pro-H-l-Pro)···l-ProH], where (l-Pro-H-l-Pro) is a pseudocentrosymmetric dimer with a very short hydrogen bond with an O···O distance of 2.427 Å. Infrared spectra of both crystals were registered and interpreted based on their structures. Electronic band structures were determined by quantum chemical calculations. The CASTEP code was used to calculate the band structures, total and partial density of states (TDOS, PDOS). Bandgaps were also measured by the diffuse re�ectance method.


Introduction
The study of polyiodides is of great scienti c and practical interest [1,2].The incorporation of iodine (I 2 ) molecules and triiodide (I 3 − ) anions into the structure of halogenobismuthate salts allows for a signi cant narrowing of the bandgap [3,4].
Recently, we have begun searching for new amino acid salts with halogenobismuthate anions as potential materials for solar energy conversion [5].Amino acid salts containing polyiodide anions and iodine molecules can serve as starting materials and are also interesting in their own right.Previously, two salts of amino acids with a triiodide anion were obtained: (l-AlaH•••l-Ala) (I 3 ) [6] and (GlyH)(I 3 ) [7].Based on these ndings, the search for new amino acid salts containing polyiodide anions and iodine molecules started [8].Recently a paper was published on (BetH)(I 3 ) which contains a centrosymmetric dimeric (BetH•••BetH) cation of the (A + •••A + ) type [9].

Experimental Materials and synthesis
As initial reagents we used l-proline (98.5-101.0%,Ph.Eur., USP, PanReac AppliChem ITW reagents), hydriodic acid (54.6% w/w, distilled, Vekton JSC), crystalline iodine ("high purity", Reakhim), acetonitrile (Ph.Eur., for UHPLC, PanReac AppliChem ITW reagents), and medical ethanol (95%, from a commercial source).Crystals were obtained at room temperature by slow evaporation of the appropriate solutions.Crystals of (I) were formed from a mixture containing l-proline, HI and I 2 in a stoichiometric 2:1:1 M-ratio taking 1.000 g of l-proline, 1.59 ml of hydriodic acid, and 2.849 g of iodine, and then adding 2 ml of ethanol.Trying to obtain (l-ProH)(I 3 ) from a 1:1:1 M-ratio (1.000 g of l-proline, 1.233 ml of hydriodic acid and 2.205 g of iodine) with 2 ml of acetonitrile, the crystals of (II) were formed.Subsequently, crystals of (II) were also synthesized from a solution with a stoichiometric 4:3:3 M-ratio of components, i.e., taking 1.000 g of l-proline, 0.92 ml of hydriodic acid and 1.653 g of iodine, and adding 2 ml of ethanol.Crystals formed within two weeks.

Structure determination
Appropriate single crystals were manually selected for homogeneous extinction, mounted on MiTeGen loops with silicone grease and used for single crystal X-ray data collections on a Bruker APEX II diffractometer equipped with a CCD area detector, an Incoatec Microfocus Source IµS (30 W, multilayer mirror, Mo-K α ) and an Oxford Cryosystems Cryostream 800 Plus LT device.
Data were collected at 200 K up to 65° 2θ full sphere by combining several runs of frames recorded at crystal-detector distances of 40 mm and 2° (I) and 1.5° (II) scan widths.Further data processing was done with the Bruker APEX4 software suite [11].The absorption was corrected by evaluation of multi-scans.The structures were solved by direct methods [12].Subsequent difference Fourier syntheses and least-squares re nements yielded the positions of the remaining atoms using the SHELX software [13] implemented in the ShelXle GUI tool [14].Non-hydrogen atoms were re ned with independent anisotropic displacement parameters.Optical measurements UV-Vis diffuse re ectance data were collected using an Agilent Cary 60 UV-Vis spectrophotometer equipped with a Remote Diffuse Re ectance Accessory (DRA).The 100% re ectance baseline was collected using a white PTFE reference.Data were recorded at room temperature (spectral range 200-1000 nm, scanning rate 10 nm/s, data interval 1.00 nm) for nely powdered crystalline samples.

Results and discussion
The salt (l-ProH•••l-Pro)(I 3 ) (I) crystallizes in the monoclinic polar space group P2 1 (Table 1).The asymmetric unit contains one formula unit (Fig. 1).Selected bond lengths and the angle in the triiodide anion are listed in Table 2, while geometric parameters of hydrogen bonds are provided in Table 3.The packing diagram in the structure of (I) is shown in Fig. 2. The intramolecular bond lengths of protonated l-ProH and zwitterionic l-Pro have typical values and are similar to those of (l-ProH•••l-Pro)(I) (Table 2).The bond lengths and the angle of the I 3 − ion are as expected (see [1,2]).
Table 2 Selected bond lengths and the angle of I 3 − ion (in Å and °) for C1-O1 The dimeric cation (l-ProH•••l-Pro) in the structure of (I) is formed due to a strong hydrogen bond O1A-H1A 3).This value is close to that of (l-ProH•••l-Pro)(I) (2.454(2) Å) [10].In (I), carbonyl atoms have a trans-arrangement relative to the hydrogen bond in contrast to (l-ProH•••l-Pro)(I), where they are in a cis-arrangement.The NH 2 + groups of A-and B-moieties form hydrogen bonds with the nearest oxygen atoms, but not with the triiodide anion, while there are ve C-H•••I type contacts (Table 3).Some of them may be considered as weak hydrogen bonds.Notably, the triiodide anions in the structure of (I) are not connected to each other by halogen bonds.
Table 3 Hydrogen bond parameters (in Å and °) for (l-ProH The salt [(l-ProH) 3 (l-Pro)](I 3 ) 3 (II) also crystallizes in the monoclinic polar P2 1 space group (Table 1).Selected bond lengths and angles are listed in Table 4, while the geometric parameters of hydrogen bonds are provided in Table 5.The salt (II) with a formal composition of [(l-ProH) 3 (l-Pro)](I 3 ) 3 has an unusual structure.In Fig. 3 5).
The infrared spectra of (I) and (II) are shown in Fig. 5.A tentative assignment of peaks is given in Table 6.
In the high-frequency region of (I), one can expect absorptions caused by stretching modes of NH 2 + , CH, and CH 2 groups.The stretching mode ν(OH) of the protonated A-moiety is assumed to be in the low-frequency region [19].The strong absorption band at 3123 cm − 1 is assigned to ν(NH), while the peaks at 3004, 2975, and 2943 cm − 1 to ν(CH).The peak at 1707 cm − 1 is characteristic for ν(C = O) of a carboxylic group, while the rather strong one at 1577 cm − 1 is assigned to the deformation vibrations of NH 2 + groups.Peaks at 1446, 1419, 1376, 1346, and 1321 cm − 1 are assigned to the deformation vibrations of CH 2 groups.The peak at 1419 cm − 1 may also be caused by ν s (COO − ).Those in the 1200 − 800 cm − 1 range are superimposed with a broad absorption centered ca.990 cm − 1 which is likely caused by ν(OH) stretching mode (see [19]).
In the high-frequency region of (II), one can expect absorptions caused by stretching modes of NH 2 + , CH, CH 2 groups as well as OH groups of carboxyl groups of A-and D-l-prolinium cations.The strong absorption band near 3000 cm − 1 is assigned to ).We assign it to the (l-Pro-H-l-Pro) dimeric cation.The weakness is likely caused by its pseudocentrosymmetric nature.The bands at 1552 and 1448 cm − 1 are also characteristic for proline and originated from δ(NH 2 ) and δ(CH 2 ), respectively.In the lower range there are peaks due to other deformation vibrations of CH 2 and NH 2 groups, as well as ring vibrations.The characteristic feature is also the absorption band at 1242 cm − 1 , which we attribute to the ν(C-OH) of the carboxyl groups of A-and D-l-prolinium cations.More interesting is that these peaks are superimposed on half of the broad band in the 1400 − 500 cm − 1 region.Such a feature in this region is characteristic for ν(OH) of very strong hydrogen bonds in dimeric cations.We assign it to the ν(OH) of the (l-Pro-H-l-Pro) dimeric cation.For strong [19] has approximately linear character: .427 Å the expected value of ν(OH) is ca.460 cm − 1 , which corresponds well to the center of the mentioned broad band.
( The grid parameters for calculating electronic properties were 3×3×2 (I) and 2×3×1 (II), k-point set in the Brillouin region for crystals.In a Kohn-Sham computation, the approximate functional used to determine the exchange-correlation energy () has a signi cant impact on the accuracy of the nal ndings.The electronic structure simulations were performed based on the DFT theory by OTFG (On-the-y generation) ultrasoft pseudopotentials.The relativistic treatment was Koelling-Harmon, energy range -10 eV, separation compares 0.005 1/Å.Band energy tolerance is within 1.0 x 10 −5 eV per atom.The DOS and PDOS were calculated.
The inhomogeneous electron densities in solids and the slow valence electron density uctuations in space make using the generalized gradient approximation (GGA) in the PBE scheme for computing electronic characteristics an excellent method.
We calculated the energy band structures with the directions with high rst Brillouin zone equilibrium points, including Z→G→Y→A→B→D→E→C for both (I) and (II) crystals.A direct transition energy for (I, II) crystals, which appears between the highest valence band value and the lowest conduction band value of the Brillion region at the symmetry point B, is 2.281 eV (I) and 1.641 eV (II), and an indirect band gap at Y→G range is 1.631 eV (II) (Fig. 6).For the (l-ProH•••l-Pro)(I 3 ) (I) crystal indirect transition was not observed.
As is known, the density of states (DOS) of a system describes the number of states occupied at each energy level in statistical and solid-state physics.Composition of the calculated energy bands can be resolved with the help of partial density of states (PDOS) and total density of states (TDOS) diagrams.

Bandgap measurements
The type of transition selected based on DFT-calculations and the bandgap were estimated from the UV-Vis diffuse re ectance data (Fig. 9) using Tauc expression and Kubelka-Munk function [20][21][22].The bandgap for the direct transition (I) is 2.04 eV while for the indirect one it is 1.51 eV (II).Infrared spectra of (I, II) were registered and interpreted based on their structures.The presence of short hydrogen bonds in dimeric cations is re ected in the spectra.

Conclusions
The calculations have been carried out using the density functional theory (DFT) based CASTEP (Cambridge Serial Total Energy Package) software, along with the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [16-18].

NH 2 +
groups form ve N-H•••I type hydrogen bonds with anions and two N-H•••O type hydrogen bonds (excluding one intramolecular bond) with the carbonyl oxygen atoms O2A and O2D of neighboring A-and D-l-prolinium cations.Additionally, there are ve C-H•••I type short contacts that can be considered weak hydrogen bonds.All iodine atoms that form N-H•••I and C-H•••I contacts are terminal atoms of triiodide anions.

Figures 7
Figures7 and 8demonstrate the total density of states for the valence and conduction bands.In these gures, the zero tick mark on the energy scales (the top of the valence band) indicates the position of the Fermi level.To obtain a measure of the contribution of different atomic states to the band structure, as well as to their possible hybridizations, a comprehensive analysis of the partial density of states was carried out.From the supercell calculations, the PDOS for the different elements O (2s 2 , 2p 4 ), N (2s 2 , 2p3 ) and I (5s 2 4d 10 5p 5 ) in the (l-ProH•••l-Pro)(I 3 ) (I) and [(l-ProH) 3 (l-Pro)](I 3 ) 3 (II) crystals are extracted and shown in Figs.7 and 8.These diagrams allow us to conclude that the main contribution near the edge of the conduction band for both crystals is made by the I (5p) states, which hybridize with the N (2p) and O (2p) states.In the presence of states I (5p), the band gap for (I) is E g = 2.281 eV, and for (II) it is E g = 1.631 eV.Salt (II) has a complicated composition [(l-ProH) 3 (l-Pro)](I 3 ) 3 containing a peculiar tetrameric cation [l-ProH•••(l-Pro-H-l-Pro)•••l-ProH].In addition, the presence of triiodide anions with their supramolecular halogen bonds leads to a decrease in the bandgap compared to the salt (l-ProH•••l-Pro)(I 3 ) (I).

Figure 7 Total
Figure 7