Crystal structure of hexaglycinium dodecaiodotriplumbate

The crystal structure of (GlyH)6(Pb3I12) is reported. Dimeric cations of type (A+⋯A+) for the amino acid glycine are observed for the first time.


Chemical context
Various inorganic and organic-inorganic hybrid materials are used in third-generation photovoltaic devices as solar energy converters (Peng et al., 2015;Ahmed et al., 2015;Zhou et al., 2018).
Our research group has been studying various amino acid salts for a long time (Fleck & Petrosyan, 2014), and we assumed that amino acids could be used to synthesize organicinorganic hybrid materials.After the successful synthesis of (GlyH)PbBr 3 (Tonoyan et al., 2024), efforts were focused on obtaining (GlyH)PbI 3 .
These compounds are also interesting for lead chemistry.Pb 2+ has an electronic configuration of [Xe]6s 2 4f 14 5d 10 .The 6s 2 electrons determine the stereochemistry of Pb II .Upon hybridization of the s and p orbitals, the stereochemically active 6s 2 electron pair occupies a position in the coordination sphere of the metal (hemidirected coordination).In this case, such hybridization does not occur, the 6s 2 electron pair has only s character and is stereochemically inactive (holodirected coordination) (Casas et al., 2006;Seth et al., 2018).As the lead ion has released its two 6p 2 electrons, �-hole interactions are possible.These interactions are known among elements of group IV and usually include the tetrel bonding interaction.In other words, the hemidirectional nature of lead(II) centers is the basic reason for different tetrel bonding interactions such as Pb� � �O (S, N, Cl, Br, I), which lead to the formation of supramolecular assemblies.
Instead of (GlyH)PbI 3 crystals, those of (GlyH) 6 (Pb 3 I 12 ) were formed unexpectedly.The [Pb 3 I 12 ] 6À anion is already known (Wang et al., 2015(Wang et al., , 2017;;Lemmerer & Billing, 2012); it has three lead centers, which can be stereochemically different.In the [Pb 3 I 12 ] 6À anion of {(tbp) 2 [Pb 3 I 12 ]} n obtained by Wang et al. (2015), the lead centers are holodirected, coordinated by six iodine atoms, and have an octahedral geometry.In (GlyH) 6 (Pb 3 I 12 ), the Pb1 center has a holodirected coordination and is bound to six I atoms, while the Pb2 centers with hemidirected coordination are linked to five I atoms.The anion described by Lemmerer & Billing (2012), as well as that reported by Wang et al. (2017) both correspond to our case considering the long Pb1-I6 distance [3.482 (1) A ˚]; however, these authors misinterpreted the coordination as holodirected or six-coordinate.

Structural commentary
The title salt (GlyH) 6 (Pb 3 I 12 ) crystallizes in the triclinic space group P1 with the asymmetric unit containing half of the formula unit.Selected bond lengths are given in Table 1 and the molecular structure is shown in Fig. 1.In (GlyH) 6 (Pb 3 I 12 ) the [Pb 3 I 12 ] 6À anion is discrete.The Pb1 center has a holodirected coordination with six I atoms, thus forming an octahedron.The two Pb2 centers have hemidirected coordinations with five I atoms, forming distorted tetragonal pyramids.These hemidirected lead ions have stereochemically active lone pairs.Despite this, any donor-acceptor, covalent or tetrel bonds are missing.The lead centers are connected with each other via Pb-I-Pb covalent bonds (Fig. 2).The anions are located parallel to each other, and the glycinium cations crosslink the entire structure through C-H� � �I, N-H� � �I and O-H� � �I hydrogen bonds (Fig. 3).

Database survey
A survey of the Cambridge Structural Database (CSD2023.2.0, version 5.45, November update; Groom et al., 2016) revealed several similar structures.Currently, the Cambridge Structural Database contains 23 entries for the [Pb 3 I 12 ] 6À anion, which can exist in both discrete and polymeric forms that also have different subtypes.In particular, the discrete type has three subtypes: when the middle lead atom of the trinuclear [Pb 3 I 12 ] 6À anion has one (Leng et al., 2023), two, or three (Wang et al., 2015;Yue et al., 2019;Zhang et al., 2022) bridging iodine atoms.When there are one or two bridging iodine atoms, the central lead center has a holodirected coordination and the outer lead atoms have a hemidirected coordination.The anions presented in these works (Lemmerer & Billing, 2012;Wang et al., 2017;Cheng et al., 2023) correspond to our case, where the central lead atom has two bridging iodine atoms and the lead centers have different stereochemistry: holodirected (six-coordinate) and hemidirected (five-coordinate).The polymeric [Pb 3 I 12 ] 6À anion can be linear (Liang et al., 2023) or cross-linked (Michael & Harald, 2018;Nazarenko et al., 2018;Passarelli et al., 2020).In summary, 15 [Pb 3 I 12 ] 6À anions from the 23 entries in the CSD are discrete, 7 are polymeric and one case is remarkable (Yao et al., 2022) with both a polymer and a discrete [Pb 3 I 12 ] 6À anion being present in the crystal structure.

Synthesis and crystallization
As initial reagents we used amino acid glycine (99%) and hydriodic acid (57% w/w, distilled, stabilized with <1.5% hypophosphorous acid, 99.95%).Initially, lead and hydriodic acid were taken in a 1:3 stoichiometric ratio.When the amount of acid in the solution decreases, the reaction between metal and acid slows and eventually almost stops (when no H 2 gas is released).At this point, the amount of obtained lead(II) iodide (PbI 2 ) and remaining acid (HI) was calculated (1:6 stoichiometric ratio).Next, the appropriate amount of glycine was added and mixed.The final stoichiometric ratio of Gly, PbI 2 and HI was 1:1:6.Instead of the desired compound (GlyH)PbI 3 , only (GlyH) 6 (Pb 3 I 12 ) was obtained.Light-red, needle-shaped crystals were obtained by solvent evaporation in a closed container, using silica gel as an absorber.(GlyH) 6 (Pb 3 I 12 ) is very hygroscopic: in the IR spectrum the absorption band at 3524 cm À 1 corresponds to the �(OH) stretching modes of the hygroscopic water molecules.The band with a peak at 3036 cm 2854 cm À 1 are assigned to �(CH) of the CH 2 groups, and the strong band at 1716 cm À 1 to �(C O) of the carboxyl groups.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were treated as riding on their parent atoms [C-H = 0.99 A ˚, N-H = 0.91 A ˚; U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (N)] except those of the carboxyl group, which were refined with the restraint U iso (H) = 1.5U eq (C).

Figure 3
Figure 3Parallel anions in the packing of (GlyH) 6 (Pb 3 I 12 ) viewed along the a axis.

Figure 4 FTIR
Figure 4FTIR spectrum of the title compound.

Table 3
Experimental details.