2.1.2. Synthesis of [PbBr2(L2)]n∞·0.5nH2O (**2**)

Compound **2** was synthesized by adopting the procedure similar to that of **1** using Ea (0.24 mL, 4 mmol) and HBr (0.15 mL, 4 mmol) in place of solid Ea·HCl, and dry Pb(NO3)2 instead of its solution. Orange prisms of **2** suitable for X-ray crystallography were formed within a week after addition of <sup>i</sup> PrOH (3 mL) to the orange solution. Yield based on Pb(NO3)2: 31%. FT–IR (ν, cm−1): 3392br, 3180br, 3086, 3062, 3016, 2930, 2900, 2872, 1654, 1590, 1568, 1440, 1314, 1224, 1154, 1108, 1048vs, 1006, 988, 912, 874, 782s, 634, 570, 510, 408. 1H NMR (400 MHz, DMSO-*d*6): <sup>δ</sup> (ppm) 8.76 (m, 1H, py), 8.50 (s, 1H, −N=CH−py), 8.00 (m, 1H, py), 7.98 (m, 1H, py), 7.62 (m, 1H, py), 4.40 (m, 1H, OH), 3.76 (m, 2H, CH2), 3.71 (m, 2H, CH2). Analysis calculated for C8H11Br2N2O1.5Pb (526.20): C, 18.26; H 2.11; N 5.32%. Found: C 18.45; H 2.15; N 5.47%.

## 2.1.3. Synthesis of [PbBr2(L3)]2 (**3**)

Compound **3** was synthesized by adopting the procedure similar to that of **2** using En (0.26 mL, 4 mmol) in place of Ea. Yellow plate-like crystals of **3** suitable for X-ray crystallography were formed within a week after addition of <sup>i</sup> PrOH (3 mL) to the orange solution. Yield based on Pb(NO3)2: 38%. FT–IR (ν, cm−1): 3076, 3060, 3008, 2920, 2898, 1662, 1652, 1586s, 1564, 1474, 1434, 1374, 1306s, 1218, 1148, 1104, 1036, 1000s, 986, 942, 782, 748, 626, 506, 406. 1H NMR (400 MHz, DMSO-*d*6): δ (ppm) 8.99 (m, 2H, py), 8.86 (s, 2H, −N=CH−py), 8.01 (m, 4H, py), 7.60 (m, 2H, py), 4.17 (s, 4H, =NCH2CH2N=). Anal. Calcd for C14H14Br2N4Pb (605.30): C, 27.78; H 2.33; N 9.26%. Found: C 28.04; H 2.45; N 9.38%.

#### *2.2. Single Crystal Structure Determination of 1–3*

Crystallographic data for the structures were collected on an Oxford Diffraction Gemini (**1**) and Bruker D8 Quest diffractometers (**2**, **3**) using Mo *Kα* (*λ* = 0.71073 Å) radiation. Following analytical absorption corrections and solution by direct methods, the structures were refined against *F*<sup>2</sup> with full-matrix least-squares using the program SHELXL-2019/2 [20]. Anisotropic displacement parameters were employed for the non-hydrogen atoms. The (N22, N23A) cation in **1** was modelled as being disordered over two sets of sites with site occupancies constrained to 0.701(5) and its complement. The hydroxyl and water molecule hydrogen atoms (**1**, **2**) were located from the experimental data and refined with O−H distances restrained to their idealized values. Other hydrogen atoms were added at calculated positions and refined by the use of a riding model with isotropic displacement parameters based on those of the parent atom. Details of the data collection and processing, structure solution and refinement are summarized in Table 1, while the selected bond lengths and

angles are presented in Table 2. CCDC 2217143, 2223148 and 2223147 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data-request/cif.


**Table 1.** Crystallographic parameters and refinement data for **1**−**3**.

#### *2.3. Theoretical Calculations*

The ORCA 5.0.3 package [21,22] was used for all DFT calculations. Range-separated ωB97M-V functional [23] with the ZORA-def2-TZVPP basis sets [24] and SARC/J auxiliary basis set [25,26] were used for all atoms except of Pb, for which the SARC-def2-TZVPP [25] basis set was applied. *AutoAux* keyword [27] was used to generate other auxiliary basis sets in all cases. The zero-order regular approximation (ZORA) [28] was used because of the presence of heavy elements. The crystal field was accounted for by means of the conductorlike polarizable continuum model (C-PCM) [29] with *ε* = infinity. The SCF optimization convergence criteria were settled with *VeryTightSCF* keywords, and integration grids of high density (*Defgrid3* keyword) were employed. Analysis of bond critical points and non-covalent interactions indexes was performed using the Multiwfn 3.8 program [30]. Visualization of the reduced density gradient [31] isosurfaces was performed by means of VMD 1.9.4a53 program [32].

#### **3. Results and Discussion**

#### *3.1. Synthesis and Characterization*

The mechanism of the formation of a substituted imidazo[1,5-*a*]pyridinium cation has been suggested by us before [8]. In the present study, the primary addition of Ea·HCl to FA aqueous solution yields 2-(methyleneamino)ethanol (Scheme 2). The subsequent 2-PCA attack on the protonated Schiff base A initiates condensation with the intermediate B. The intramolecular nucleophilic attack onto the aldehyde carbon atom by the amine nitrogen brings the product of cyclization C, which experiences an irrevocable dehydration reaction affording the aromatic heterocyclic cation [L1]+. The dehydration of the five-membered ring results in the formation of two double bonds and the delocalization of the positive charge on N atoms in a resonance structure. The reaction does not require the presence of metal ions.

**Scheme 2.** Proposed mechanism of the formation of [L1]+cation.

The lead chloride hybrid perovskite **1** formed by self-assembly from the in situ prepared monovalent organic cations, Pb2+ and chloride ions at an overall 1PbCl2:4CH2O: 4Ea·HCl:4(2-PCA) mole ratio. It is insoluble in alcohols and water and slightly soluble in *N*,*N*-dimethylformamide (DMF) and dimethylsulfoxide (DMSO); it is indefinitely stable in air. Similar reaction procedures that used Ea or En combined with HBr did not afford the respective imidazo[1,5-*a*]pyridinium cations, presumably due to the insufficient acid strength.

The IR spectra of **1**−**3** confirmed the presence of aromatic rings, alkyl and other functional groups (Figures S1–S3). The spectrum of the hybrid compound **1** demonstrates a distinctive pattern, which was suggested to characterize the imidazo[1,5-*a*]pyridinium skeleton [8,11,12]: the sharp very strong peaks attributed to aromatic C–H vibrations (3116–3060 cm−1), medium intensity sharp bands at 1654 and 1544 cm−<sup>1</sup> associated with heterocyclic ring stretching, an intense absorption at 1150 cm−<sup>1</sup> and three peaks in the region of out-of-plane C–H bending (796, 750 and 636 cm<sup>−</sup>1). The very intense band, due to ν(O–H) vibration at 3400 cm−1, dominated the spectrum; another intense absorption was ascribed to C–O stretching (1076 cm<sup>−</sup>1). In the case of compounds **2** and **3**, the aromatic ν(C– H) bands above 3000 cm−<sup>1</sup> are much weaker. The spectrum of compound **2** is dominated by a peak at 1048 cm−<sup>1</sup> due to ν(C–O), while the absorption of moderate intensity due to O–H stretching (3392 cm<sup>−</sup>1) is broad. ν(C=N) + ν(C=C) stretching frequencies of the metal coordinated Schiff bases are observed at 1654, 1590 cm−<sup>1</sup> (**2**) and 1652, 1586 cm−<sup>1</sup> (**3**).

The compounds are easily distinguishable by their 1H NMR spectra that demonstrate the sets of signals, as well as the correct alkyl/aromatic proton ratios expected for [L1]<sup>+</sup> cations and the Schiff base ligands. In the room temperature (r.t.) spectrum of compound **1** in DMSO-*d*6, CH protons in the imidazolium ring appear as singlets at δ = 10.01 and 8.32 ppm, the alkyl groups protons are observed as multiplets at 4.67 and 3.92 ppm. The imine protons in **2** and **3** are detected at 8.50 and 8.86 ppm, respectively, while the protons of CH2 groups appear as two multiplets at 3.76 and 3.71 (**2**) and one multiplet at 4.17 ppm (**3**). The hydroxyl protons in **1** and **2** are observed distinctly at 5.23 and 4.40 ppm, respectively.

#### *3.2. Structural Description of 1–3*

The asymmetric unit of the lead chloride hybrid **1** is comprised of two [L1]+ cations, [Pb2Cl6] 2– anionic fragment and water molecule of crystallization (Figure 1). Two crystallographically non-equivalent cations configurations are very similar; the (N22, N23A) cation is disordered over two sites, with occupancies of 0.701(5) and 0.299(5)-. The pyridinium rings bond distances of their fused heterocyclic cores are uneventful; the N−C bond lengths in the imidazolium entities fall in the range 1.331(7)–1.414(6) Å. The N atoms are planar, showing a sum of three angles of 360◦. In the cores, the five- and six-membered rings are nearly coplanar, with the respective dihedral angles of 1.35, 1.73 and 2.98◦.

**Figure 1.** Asymmetric units and principal atom labelling for [L1]2n[Pb2Cl6]n∞·nH2O (**1**) and [PbBr2(L2)]n∞·0.5nH2O (**2**) with the 50% probability ellipsoids. The major component of the disordered (N22, N23A) cation of **1** is shown.

The distorted octahedral environment of Pb1 atom in the anionic motif consists of two terminal, two *μ*2-Cl and two *μ*3-Cl atoms, while the distorted octahedron around Pb2 is built of two *μ*2-Cl and four *μ*3-Cl ligands (Figure 2a). The metal–chloride distances vary in the range 2.6509(14)–3.1516(14) Å. The average value of 2.907 Å is reasonable for Pb(II) ions in chloroplumbates(II) with strong ionic Pb–Cl bonds [33,34]; the *cis* and *trans* angles at the metal atom fall in the ranges of 77.50(4)–107.33(5)◦ and 162.86(5)–177.20(4)◦, respectively (Table 2), showing a strong degree of distortion of the metal polyhedra. The PbCl6 octahedra are joined by three-edge- and five-edge-sharing into a 1D twin lead(II) chloride wire of 10.344 Å width, lying in the *a*-axis direction (Figure 2a,b). The bulkier 2-(2-hydroxyethyl)- 2H-imidazo[1,5-*a*]pyridinium cation in **1**, if compared to its methyl analogue [8], expectedly templated the formation of a low-dimensional anionic perovskite frame, however, with a different type of a double chain. This set of connectivity modes within a double chain was not observed in the lead(II) chloride hybrid compounds in accordance with our survey of the Cambridge Structural Database [35]. The type of edge-sharing within a double haloplumbate(II) chain shown in the inset of Figure 2b is more frequent and found in, for example, 1D lead chloride perovskites with substituted diammonium and piperidinium dications [6,36].




**Table 2.** *Cont.*

[a] Symmetry codes: <sup>1</sup> *<sup>x</sup>* − 1, *<sup>y</sup>*, *<sup>z</sup>*; <sup>2</sup> −*x*, −*<sup>y</sup>* + 1, −*<sup>z</sup>* + 1; <sup>3</sup> −*<sup>x</sup>* + 1, −*<sup>y</sup>* + 1, −*<sup>z</sup>* + 1; <sup>4</sup> 1/2 − *<sup>x</sup>*, *<sup>y</sup>*, 1/2 + *<sup>z</sup>*.

In the crystal, non-equivalent [L1]+ cations of **1** form separate stacks parallel the *a-*axis with the fused heterocyclic cores of neighboring entities being strictly coplanar (Figure 2c). Centrosymmetically related trans-oriented [L1]<sup>+</sup> cations are stacked with varying levels of offset, showing the ring centroid distances of 3.456, 3.651 Å for (N12, N13A) and 3.839, 5.258 Å for (N22, N23A) cations enabling the possibility of weak π-bonding [37]. Hydrogen bonds of O−H···O/Cl and C−H···O/Cl types with the involvement of the OH group of the organic cation and water molecule (Table 3) strengthen the hybrid salt structure. The introduction of OH group on the cation enabled a stronger interaction between cationic and anionic counterparts in **1** through additional conventional hydrogen bonding, which is absent in the lead chloride hybrid with methyl derivative of [L1]+ cation [8]. The new type of a double chain realized in **1** may also be considered a result of such a modification of the organic cation.

**Figure 2.** (**a**) Fragment of the 1D twin offset chloroplumbate(II) chain in **1**. The symmetry codes have the same numbering as those in Table 2; (**b**) polyhedral representation of the same fragment; the inset shows a common type of edge-sharing within a double haloplumbate(II)wire; (**c**) fragment of the crystal packing of **1** demonstrating spatial arrangement of the organic and inorganic counterparts (the major component of the disordered (N22, N23A) cation is shown).

**Table 3.** Geometry of hydrogen bonds for **<sup>1</sup>**−**<sup>3</sup>** (Å and ◦) [a].



**Table 3.** *Cont.*

[a] Symmetry codes: <sup>1</sup> –*x* + 1, –*y*, –*z* + 1; <sup>2</sup> –*x*, –*y*, –*z* + 1; <sup>3</sup> *x* – 1, *y*, *z*; <sup>4</sup> –*x* + 1, –*y*, –*z*; <sup>5</sup> –*x* + 1, –*y* + 1, –*z*; <sup>6</sup> –*x* + 1, –*y* + 1, –*<sup>z</sup>* + 1; <sup>7</sup> –*x*, –*<sup>y</sup>* + 1, –*<sup>z</sup>* + 1; <sup>8</sup> −*<sup>x</sup>* + 1/2, *<sup>y</sup>*, *<sup>z</sup>* + 1/2; <sup>9</sup> −*<sup>x</sup>* + 1/2, *<sup>y</sup>*, *<sup>z</sup>* + 1/2; <sup>10</sup> −*x,* −*<sup>y</sup>* + 1*,* −*z*; <sup>11</sup> −*<sup>x</sup>* − 1/2*, y, z +* 1/2; <sup>12</sup> −*<sup>x</sup>* − 1/2, *<sup>y</sup>*, *<sup>z</sup>* − 1/2; <sup>13</sup> *<sup>x</sup>*, −*<sup>y</sup>* + 3/2, *<sup>z</sup>* − 1/2; <sup>14</sup> *<sup>x</sup>* – 1, *<sup>y</sup>* + 1, *<sup>z</sup>*; <sup>15</sup> *<sup>x</sup>* + 1, *<sup>y</sup>* – 1, *<sup>z</sup>*; <sup>16</sup> *<sup>x</sup>* + 1, *<sup>y</sup>*, *<sup>z</sup>*. [b] H bonds for the major component of the disordered (N22, N23A) cation are given.

Lead halide hybrid **2**, which crystallizes in the orthorhombic space group *Pccn*, consists of a 1D monobromo-bridged lead(II) polymer and water molecules of crystallization. The asymmetric unit of **2** is shown in Figure 1. In the double-sided, organically decorated chain structure, the nearest six-coordinate environment of each metal atom is formed by one tridentate Schiff base, one terminal and two *μ*2-Br ligands (Figure 3a,b). The neutral Schiff base ligand chelates the Pb1 ion by the N1 atom of the pyridyl ring, the imine N2 atom and O1 atom of the ethanol group with an average value of the Pb1−N distances of 2.57 Å, and the Pb−O1 bond [2.839(7) Å] being significantly elongated (Table 2). The metal distances to terminal and bridging bromide atoms are similar, with an average of 3.03 Å falling in the usual range for 1D lead(II) bromide systems [38,39]. The additional Pb···Br2{1/2 − *x*, y, −1/2 + *z*} contact of 3.378 Å is appreciably larger than the sum of the Shannon ionic radii of the octahedral lead(II) cation and bromide anion [*r*(Pb2+) + *r*(Br−) = 3.15 Å] and is barely significant in the construction of the bromoplumbate(II) chain. The bond angles at the Pb1 atom vary from 61.8(2) to 123.3(2)◦ and from 146.36(15) to 160.70(3)◦ (Table 2), evidencing the severe distortion of the metal polyhedron. The closest Pb···Pb separation within the chain is 4.4144(6) Å. In the crystal, two bromoplumbate(II) chains related by a crystallographic two-fold axis form a hydrophilic channel to host water molecules held by O–H···O hydrogen bonds (Figure 3c, Table 3). Numerous C–H···Br contacts result in a 3D supramolecular architecture.

Complex **3** belongs to the triclinic space group *P*¯ı; the dimeric molecule is located on an inversion centre (Figure 4). The geometry of Pb1 atom is that of an irregular pentagonal bipyramid, the equatorial plane consisting of four nitrogens from the tetradentate chelate L3 and the centrosymmetrically related bromide ligand Br1{1 − *x*, 1 − *y*, 1 − *z*} of the dimer, with two other bromides—Br1 and Br2—occupying apical positions. The Pb–N bond distances within the range of 2.621(6)–2.804(6) Å (Table 2) agree well with those of two other known Pb(II) complexes with L3 [40,41]. The Pb1–Br1{1 − *x*, 1 − *y*, 1 − *z*} bond is elongated at 3.2525(8) Å, in contrast to the average Pb1–Br1/Br2 bond length of 3.014 Å. The *cis* angles at the lead atom vary from 60.4(2) to 95.75(12)◦; the *trans* angles fall in the range 121.93(19)−176.39(16)◦. In the dimer, the Pb···Pb' separation is 4.7239(5) Å.

**Figure 3.** (**a**) Fragment of the double-side organically decorated bromoplumbate(II) chain in **2** [symmetry code: <sup>i</sup> 1/2 <sup>−</sup> *<sup>x</sup>*, *<sup>y</sup>*, 1/2 + *<sup>z</sup>*]; (**b**) polyhedral representation of the same fragment; (**c**) fragment of the crystal packing viewed down the *c*-axis showing spatial arrangement of two bromoplumbate(II) chains to form a hydrophilic channel filled with water molecules.

**Figure 4.** (**a**) Molecular structure and principal atom labelling of [PbBr2(L3)]2 (**3**), ellipsoids are shown at the 50% probability level [symmetry code: <sup>i</sup> <sup>1</sup> <sup>−</sup> *<sup>x</sup>*, 1 <sup>−</sup> *<sup>y</sup>*, 1 <sup>−</sup> *<sup>z</sup>*]; (**b**) the same molecule in a polyhedral form; (**c**) fragment of the 3D supramolecular network in **3** stabilized with C–H···Br hydrogen bonds (shown in blue, H atoms omitted).

In the solid state, the dimeric molecules of **3** are stacked identically, forming columns parallel to the *ab* plane, with the Pb2Br4 moieties in the column being strictly coplanar (Figure 4c). The minimal Pb···Pb distance in the lattice is 8.1348(6) Å; no effective πoverlap is observed. C–H···Br hydrogen bonding (Table 3) consolidates an extended supramolecular 3D network structure (Figure 4c). The contacts strength depends on their direction in the crystal lattice. In the *ab* plane, the interactions are represented by four distinct crystallographically independent C–H···Br hydrogen bonds with *d*(C···Br) < 3.9 Å (Table 3). Every dimeric molecule of **3** forms 16 respective contacts with the four nearest molecules in the *ab* plane. The dimeric molecules along the *c* axis are bridged by weaker C–H···Br hydrogen bonds with *d*(C···Br) > 3.9 Å (Table 3).

#### *3.3. Hirshfeld Surface and QTAIM Analyses*

The set of intermolecular interactions in complex **1** can be viewed as a combination of contacts between crystallographically independent (N12, N13A) and (N22, N23A) [L1]<sup>+</sup> cations, negatively charged [Pb2Cl6]n 2n– chains and water molecules. The HS of both [L1]<sup>+</sup> cations surrounding a fragment of the [Pb2Cl6]n 2n– chain is shown in Figure 5. Here, and further on, the colours on the surface correspond to shortened contacts (red), van der Waals (vdW) interactions (white) and contacts with longer distances (blue).

**Figure 5.** Hirshfeld surface mapped over *d*norm of (N12, N13a) and (N22, N23A) [L1]<sup>+</sup> cations for compound **1** (view along the *a* axis, the major component of the disordered (N22, N23A) cation is shown).

Both [L1]+ cations reveal involvement in several distinct hydrogen bonds (Table 3), which are identifiable in the fingerprint plots (Figure 6). The plots are characterized by broad segments of H···H interactions (Figure 6), while the C···C ones are much less abundant (5.4 and 4.3% for (N12, N13A) and (N22, N23A) independent cations, respectively, Figure S4).

The weak interactions between the stacked cations were evaluated by the analysis of reduced density gradient (RDG) [31] of the electron density calculated at the ωB97M-V/ZORA-def2-TZVPP level using crystallographic atomic coordinates (Figure 7). In line with the HS analysis results, the non-covalent contacts in the tetramer {(N12, N13A)-[L1]4·2H2O}4+ are mostly characterized by the broad vdW areas between stacked imidazo[1,5-*a*]pyridine moieties. These findings indicate that [L1]<sup>+</sup> cations in their stacks are too offset to engage in appreciable π-bonding. At the same time, the chain of (N12, N13A)-[L1]+ cations is supported by the strong attractive interactions involving water molecules (O121–H121···O01W), for which the electron density at the bond critical

point—*ρ*BCP(**r**) (3, −1) between the H121 and O01W atoms—is 0.02504 a.u. According to the dependence for charged assemblies reported in [42], the obtained value of *ρ*BCP(**r**) corresponds to 39.3 kJ mol−<sup>1</sup> of binding energy (BE) between the (N12, N13A)-[L1]<sup>+</sup> cation and water molecule. However, as the BE vs. *ρ*BCP(**r**) dependences [42] were developed for assemblies with a single overall charge (in contrast to 4+ charge of the tetramer), the obtained binding energy value has only indicative significance.

**Figure 7.** Reduced density gradient surfaces (isovalue of 0.5) showing vdW interactions (green area) between [L1]<sup>+</sup> cations and strong hydrogen bonds with participation of water molecules in the crystal structure of **1** (the major component of the disordered (N22, N23A) cation is shown). The colour scheme corresponds to the sign(λ2)*ρ*(**r**) function with the second largest eigenvalue of the Hessian of electron density, λ2, at respective points: negative (blue, strong attraction), nearly zero (green, vdW interaction), positive (red, strong repulsion).

The monomeric fragment [PbBr2(L2)] of the 1D chain in complex **2** was used to construct the Hirshfeld surface (Figure 8). The fingerprint plots disclose several distinct non-covalent contacts mostly H···O and H···Br in nature (Figures 8 and S5). The strongest hydrogen bonds between O1 and O1W atoms are reflected by sharp "peaks" having the shortest *d*<sup>e</sup> and *d*<sup>i</sup> distances in the fingerprint plots (Figures 8 and S5). The *ρ*BCP(**r**) electron density at BCP between H1 and O1W atoms was estimated as 0.01986 a.u., from which the binding energy of the respective contact can be evaluated as –15.4 kJ mol−<sup>1</sup> (according to the equation for non-charged assemblies [42]). This energy strength can be classified as "weak-to-medium" [42]. The RGD isosurfaces of the vdW contacts between the aromatic groups in **2** are shown in Figure 9.

**Figure 8.** Top left: fragment of the crystal structure of **2** viewed down the *c* axis showing the HS of the fragment [PbBr2(L2)]. Top right: selected fingerprint plots. Bottom: contributions of the specific Xi···Xe contacts.

**Figure 9.** Reduced density gradient surfaces (isovalue of 0.5) showing vdW interactions (green area) between aromatic fragments and strong hydrogen bonds involving water molecule for compound **2**.

The dimeric molecules of **3** create a network of non-covalent interactions, of which the highest contribution (40.9%) to the HS is from numerous H···H contacts (Figures 10 and S6). The latter form a broad surface with no distinct directions (Figure 11), except of the C8–H8A···H8A1−C8<sup>1</sup> ( <sup>1</sup> =2– *x*, –*y*, –*z*) interaction between methylene groups [*d*(C···C) = 4.379(15) Å] and a pair of weak interactions between pyridine rings C13–H13A···H14A–C14<sup>1</sup> and C14–H14A···H13A–C13<sup>1</sup> [ <sup>1</sup> =2– *x*, 1 – *y*, 1 – *z*; *d*(C···C) = 4.072(18) Å]. As can be seen from the particular Hirshfeld surface, the Br···H and H···Br contacts are mostly located in the *ac* plane (Figure 11). Since the molecules of **3** has no electrical charge, the strengths of these contacts can be estimated [42] from the *ρ*BCP(**r**) electron density with sufficient precision (Table 4).

**Figure 10.** Left: Hirshfeld surface of the dinuclear molecule of **3** surrounded by the closest molecules (view along the *b* axis). Right: selected fingerprint plots of the respective surface. Bottom: contributions (in %) of specific Xi···Xe interactions.

**Figure 11.** Hirshfeld surface of the dinuclear molecule of **3** highlighting regions of specific interactions.


**Table 4.** Electron density at the H···Br BCP and respective binding energies for selected intermolecular contacts in the structure **3** [a].

[a] binding energies were estimated according to the equation for non-charged assemblies [42]; [b] symmetry operations have the same numbering as those in Table 3.

Inspection of the geometry and Hirshfeld surface suggests the existence of the contact C8–H8B···C61 [ <sup>1</sup> = 1 – x, 1 – y, –z; *<sup>d</sup>*(C···C) = 3.722(10) Å, ∠(C–H···C) = 161.1◦] between methylene CH2 group and imine carbon atom of the neighboring molecules. However, despite the hydrogen atom being clearly oriented towards C6 one, the respective bond critical point is located rather between H8B and C51 atoms (Figure 12). Further analysis of the RDG isosurfaces disclosed the existence of a broad non-covalent interaction region between H8B donor and C41, C51 and C6<sup>1</sup> ( <sup>1</sup> = 1 – x, 1 – y, –z) atoms as acceptors containing 0.0054 electrons (Figure 12). The negative *q*bind index [43] of this domain (–0.00046383) accounts for a weak attractive interaction. The aromatic moieties of the ligand L3 in the molecule of **3** contact each other through vdW interactions (Figure S7).

**Figure 12.** Top: plots of the Laplacian of the electron density in the real space, <sup>∇</sup>2*ρ*(r), for selected weak contacts. Bottom left: 2D cut plane of the RDG function for the same projection as in the top left plot. Bottom right: 3D isosurface of the RDG (isovalue of 0.5) illustrating non-covalent interactions in the H8B—C41/C51/C61 groups of atoms (1 = 1 – x, 1 – y, –z).

#### *3.4. Optical Properties of 1–3*

The optical diffuse reflectance spectra of all the compounds were measured using powder samples at r.t. (Figure 13a). According to the analysis of the spectra by the Tauc plot based on the assumption of direct band gap, [*F*(*R*)*h*ν] <sup>2</sup> vs. *h*ν (where *F*(*R*) denotes the Kubelka–Munk function, *h*ν is photon energy in eV), the optical band gap values of **1**, **2** and **3** were estimated to be 3.36, 3.13 and 2.96 eV, respectively (Figure 13b). The values of **1** and **2** are comparable to those of reported 1D hybrid lead perovskites, such as [H2bpp]Pb2Cl6 (3.55 eV), [H2bpp]Pb2Br6 (3.30 eV; bpp = 1,3-bis(4-pyridyl)-propane) [38], [2,6-dmpz]3Pb2Br10 (3.12 eV; 2,6-dmpz = 2,6-dimethylpiperazine), [hex]PbBr3 (3.41 eV; hex = hexamethyleneimine), [hep]PbBr3 (3.50 eV; hep = heptamethyleneimine) [39]. Using density functional theory calculations, [2,6-dmpz]3Pb2Br10, [hex]PbBr3 and [hep]PbBr3 were found to be direct band gap semiconductors [39]. Their valence band maximum (VBM) is composed of mixed Br-p and Pb-s orbitals, while empty Pb-p orbitals constitute conduction band minimum (CBM). In the case of two H2bpp hybrid lead halides, the VBM was derived from hybridization of Pb-6p and Cl-3p or Br-4p orbitals, and CBM mainly originated from the C-2p/N-2p states of the bicyclic organic cations [38].

**Figure 13.** Diffuse reflectance spectra measured using powder samples of **1**–**3** at r.t. (**a**) and the Tauc plots in the range 2.5–4 eV based on the assumption of direct band gap calculated from the reflectance spectra of **1**–**3** (**b**).

Within the series of 1D and 2D lead bromide hybrids [39], the electronic structure was suggested to depend on the connectivity mode of PbBr6 octahedra, where the cornerconnected compound [4-amp]PbBr4 [4-amp-4-(aminomethyl)piperidine] exhibits the smallest band gap value (2.93 eV). The face-sharing compounds [hex]PbBr3 and [hep]PbBr3 possess larger band gaps than [2,6-dmpz]3Pb2Br10 with the edge- and corner-connecting structure. The band gaps successively going up from "corner-sharing" to "edge-sharing" and further "face-sharing" have also been observed in the lead iodide perovskite-derived organic–inorganic hybrids [44]. The band gaps of edge-sharing **1** and corner-sharing **2** perovskites follow the general trend.

#### **4. Conclusions**

In this study, we aimed to explore the templating effect of imidazo[1,5-*a*]pyridiniumbased cation with hydroxyl functionality on the dimensionality of the lead halide anionic framework and connectivity modes of the [PbHal6] 4– octahedra, as well as H-bonding interactions in the resulting hybrid lead halide perovskite. To this aim, Ea·HCl was used in the developed protocol for the synthesis of substituted imidazo[1,5-*a*]pyridinium cations by the acid catalyzed oxidative cyclocondensation of FA, amine and 2-PCA. The obtained in

situ [L1]+ cation directed the formation of the 1D chloroplumbate [L1]2n[Pb2Cl6]n∞·nH2O (**1**), revealing a new type of the [Pb2Cl6]<sup>∞</sup> twin chain constructed from three-edge- and fiveedge-sharing PbCl6 octahedra. In the case of [PbBr2(L2)]n∞·0.5nH2O (**2**) and [PbBr2(L3)]2 (**3**), the introduction of HBr—the necessary acid component and source of bromide anions directly to the reaction media subverted the expected oxidation cyclization and instead facilitated the amine-aldehyde condensation between 2-PCA and Ea or En, respectively. The isolated hybrid lead bromide **2** features a chain of corner-sharing PbBr3N2O octahedra, decorated with the tridenate Schiff base ligands on both sides. In the 0D dimer **3**, two irregular PbBr3N4 pentagonal bipyramids are connected by edge sharing. The Hirshfeld surface and QTAIM analyses disclosed the presence of numerous weak interactions between molecular and polymeric fragments in the structures of **1**–**3**. The band gaps of **1** (3.36 eV) and **2** (3.13 eV) with different connectivity follow the general trend of edge-sharing perovskites exhibiting larger band gaps than corner-sharing structures.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/cryst13020307/s1, Figure S1: IR spectrum of [L1]2n[Pb2Cl6]n∞·nH2O (**1**); Figure S2: IR spectrum of [PbBr2(L2)]n∞·0.5nH2O (**2**); Figure S3: IR spectrum of [PbBr2(L3)]2 (**3**); Figure S4: Selected fingerprint plots of the (N12, N13A) and (N22, N23A, major component) [L1]+ cations in the crystal structure of **1** showing Xi···Xe interactions (where X = Cl, O, C or H); Figure S5: Selected fingerprint plots of the fragment [PbBr2(L2)] in the crystal structure of **2** showing Xi···Xe interactions (where X = Br, O, C or H); Figure S6: Selected fingerprint plots of the dinuclear molecule of **3** showing Xi···Xe interactions (where X = Br, N, C or H); Figure S7: Reduced density gradient surfaces (isovalue of 0.5) showing weak vdW interactions (green area) between aromatic fragments of the ligands L3 in the molecular structure of **3** (lead and bromine atoms were excluded from calculations).

**Author Contributions:** Conceptualization, O.Y.V.; methodology, O.Y.V. and D.S.N.; investigation, O.Y.V., E.A.B., O.V.N. A.N.S. and D.S.N.; writing—original draft preparation, O.Y.V. and D.S.N.; writing—review and editing, O.Y.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was performed with the support of the Ministry of Education and Science of Ukraine (project 22BP037-13, grant for the perspective development of the scientific direction 'Mathematical sciences and natural sciences' at the Taras Shevchenko National University of Kyiv) and the Fundação para a Ciência e Tecnologia (FCT), Portugal (projects UIDB/00100/2020, UIDP/00100/2020, and LA/P/0056/2020 of Centro de Química Estrutural, contracts IST-ID/086/2018 and IST-ID/117/2018).

**Data Availability Statement:** Crystallographic data for the structures reported can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data-request/cif (accessed on 27 January 2023) quoting the deposition numbers CCDC 2217143 (**1**), 2223148 (**2**) and 2223147 (**3**).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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