Structure and Vibrational Spectra of Pyridine Solvated Solid Bis(Pyridine)silver(I) Perchlorate, [Agpy₂ClO₄]·0.5py

Abstract

A hemipyridine solvate of bis(pyridine)silver(I) perchlorate, [Agpy₂ClO₄]·0.5py (compound 1) was prepared and characterized by single crystal X-ray analysis and vibrational spectroscopy (R and low-temperature Raman). Compound 1 was prepared via the trituration of [Agpy₂ClO₄] and 4[Agpy₂ClO₄]·[Agpy₄]ClO₄ (as the source of the solvate pyridine) in a mixed solvent of acetone:benzene =1:1 (v = v) at room temperature. The monoclinic crystals of compound 1 were found to be isomorphic with the analogous permanganate complex (a = 19.1093(16) Å, b = 7.7016(8) Å, c = 20.6915(19) Å, β = 105.515(7)°; space group: C2/c). Two [Agpy₂]⁺ cations formed a dimeric unit [Agpy₂ClO₄]₂, and each silver ion was connected to two ClO₄⁻ anions via oxygen atoms. The Ag∙∙∙Ag distance was 3.3873(5) Å, the perchlorate ions were coordinated to silver ions, and the Ag∙∙∙O distances were 2.840(2) Å and 2.8749(16) Å in the centrosymmetric rectangle of Ag-O-Ag-O. The stoichiometric ratio of the monomer [Agpy₂ClO₄] and the solvent pyridine was 1:0.5. The guest pyridine occupied 527.2 ų, which was 18.0% of the volume of the unit cell. There was no additional residual solvent-accessible void in the crystal lattice. The solvate pyridine was connected via its a-CH to one of the O atoms of the perchlorate anion. Correlation analysis, as well as IR and low-temperature Raman studies, were performed to assign all perchlorate and pyridine vibrational modes. The solvate and coordinated pyridine bands in the IR and Raman spectra were not distinguishable. A perchlorate contribution via Ag-O coordination to low-frequency Raman bands was also assigned.

1. Introduction

The ligand–anion reactions of transition metal tetraoxometallates containing reducing ligands (ammonia, pyridine or urea) [1,2,3,4,5], including silver complexes containing ammonia and pyridine [6,7,8,9,10,11], are intensively studied areas of coordination chemistry. The interest in these compounds is generated by the structural diversity of [Agpy]_nXO₄ complexes (n = 2, 2.4, 2.5, and 4, X = Cl, Mn, and Re, respectively), including variability in the silver coordination number, solvate formation, mixed crystal formation and the presence/absence of Ag-Ag argentophilic and pyridine α-C-H∙∙∙O-XO₃ hydrogen bond formation [12,13]. The effects of these interactions on the coordination modes, topology and crystal engineering characteristics of Ag^I complexes have enormous importance [14,15,16,17,18,19,20].

Among the 12 possible members of the four series of the [Agpy_n]XO₄ compounds (n = 2, 2.4, 2.5 and 4, X = Cl, Mn, and Re, respectively), 10 compounds have been isolated until now [9,10,11,12,13]. The existence of [Agpy_2.5]XO₄ compounds with 0.5 pyridine solvate content (X = Cl (compound 1) and Mn (compound 2)) were recently discovered [12,13]. Among the four existing [Agpy_n]ClO₄ complexes, three have already been structurally characterized (n = 2 [21], 2.4 [22], and 4 [23] for compounds 3, 4 and 5, respectively). Earlier attempts to synthesize the missing member of the series, with n = 2.5, were not successful. In this paper, filling the gap, we report on the synthesis and structural characterization of [Agpy₂ClO₄]·0.5py) (compound 1) in order to explore the possible enhanced structural variability of this solvate compared with the permanganate analog. Compound 1 was prepared at a satisfactory purity to perform vibrational spectroscopic characterization including correlation analysis.

2. Results and Discussion

2.1. Preparation of [Agpy₂ClO₄]·0.5py (Compound 1)

The reaction of silver nitrate with aq. pyridine solutions generally resulted in a mixture of compounds 3 and 4. When we used 6 equivalents of pyridine to silver at a 36.5% pyridine concentration by cooling the reaction mixture to 10 °C, a mixture of compound 1, compound 3 and compound 4 was formed (1:3:15 ratio), but all of our efforts to isolate pure compound 1 in this reaction route failed [13]. The 1:1 mixture of compounds 3 and 4, prepared in a similar way applying an Ag:py ratio of 1:3 and a pyridine concentration of 1.25% at 10 °C, however, proved to be a good starting material in the preparation of compound 1 with trituration in a 1:1 (v/v) acetone–benzene mixture at room temperature [13], during which colorless block-shaped crystals suitable for single-crystal X-ray study were grown in 2 days. The solid phase left back was compound 1, whereas the solid phase isolated by the evaporation of the mother liquor was a mixture of compounds 1 and 3. The solid compound 1 that formed was only stable in contact with the acetone–benzene mother liquor. Its powder XRD results (Figure S1) agreed well with the calculated one from the single crystal measurement.

2.2. Structure of [Agpy₂ClO₄]·0.5py (Compound 1)

The monoclinic crystals of compound 1 were found to be isomorphic with the analogous permanganate complex (compound 2) [13]. Their space group was C2/c, a = 19.1093(16) Å, b = 7.7016(8) Å, c = 20.6915(19) Å, β = 105.515(7)°, V = 2934.2(5) ų, T = 103(2) K, Z = 4, and D_x = 1.834 Mg/m³) (Table S1). These values were very close to those found for compound 2 (

Table 1. Comparison of main cell parameters and inter- and intramolecular distances of [Agpy₂ClO₄]·0.5py (compound 1) and [Agpy₂MnO₄]·0.5py (compound 2).

	Compound 1	Compound 2
Temperature	103(2) K	293(2) K
Crystal system	monoclinic	monoclinic
Space group	C2/c	C2/c
Unit cell dimensions
a	19.1093(16) Å	19.410(1) Å
b	7.7016(8) Å	7.788(1) Å
c	20.6915(19) Å	21.177(1) Å
β	105.515(7)°	104.20(1)°
Cell volume	2934.2(5) Å3	3103.4(1) Å3
Anion	ClO4− not disordered	MnO4− disordered 66:34
KPI	69.8%	67.4%
Pyridine	527.2 Å3, 18.0%	604.5 Å3, 19.5%
Ag∙∙∙Ag	3.3873(5) Å	3.4213(9) Å
Ag-O2	2.840(2) Å	2.548(14) Å
Ag-O2′	2.8749(16) Å	2.745(9) Å
Cα21-H21α∙∙∙O2XO4	2.355 Å	2.602 Å
	2.701 Å	2.770 Å
π−π N1∙∙∙N1	3.745 Å	3.782 Å
π−π N1∙∙∙N2	3.666 Å	3.740 Å

). The cell volume of compound 1 was 5.5% smaller than the cell volume of compound 2, as was expected. An opposite tendency was observed in the case of isomorphic [4Agpy₂XO₄.Agpy₄XO₄] compounds (X = Cl and Mn) [10,13]. This means that not only the size of anion but also other conformational effects have influence on the compactness of the crystal lattices of [Agpy_n]XO₄ (X = Cl, Mn) compounds.

The asymmetric unit was found to contain the [Agpy₂ClO₄] moiety, as well as half a pyridine, arranged by a twofold axis. The Z’_pyridine = 0.5 (Z = 4) symmetry codes to generate equivalent atoms C31i and C32i were −x + 1, y, −z + 1/2 + 1 (Figure 1). Two pyridine molecules were shown to coordinate to the Ag⁺ cation. The Ag-N distances were 2.159(2) Å and 2.161(2) Å, only a bit shorter than in the permanganate complex (2.166(esd) Å and 2.174(esd) Å, respectively).

Two [Agpy₂]⁺ cations were found to form a dimeric unit [Agpy₂ClO₄]₂ (Figure 2a), where silver ions were connected by the O2 oxygen atoms of two ClO₄⁻ anions. The Ag∙∙∙Ag distance was 3.3873(5) Å, whereas the Ag∙∙∙O distances were 2.840(2) Å and 2.8749(16) Å in the centrosymmetric rectangle of Ag-O-Ag-O (Figure 2b and Table 1).

The C_α-H_{(pyridine ligand)}∙∙∙O_{(perchlorate)} intramolecular hydrogen bonds were shown to support complex formation (

Table 2. Hydrogen bond interactions (distances and angles are given) in compound 1.

D-H∙∙∙A	D-H [Å]	H∙∙∙A [Å]	D∙∙∙A [Å]	D-H∙∙∙A [o]	Symmetry Operation
C21-H21∙∙∙O3	0.93	2.360	3.228(3)	156	intra
C15-H15∙∙∙O4	0.93	2.701	3.621(3)	170	intra
C24-H24∙∙∙O4	0.93	2.598	3.261(3)	129	x, 1−y, −1/2 + z
C13-H13∙∙∙O1	0.93	2.600	3.376(3)	141	−1/2 + x, 1/2 + y, z
C23-H23∙∙∙O1	0.93	2.595	3.504(3)	166	x, 1−y, −1/2 + z
C32pyr-H32∙∙∙O4	0.93	2.64	3.555(3)	169	x, 1 + y, z

). The H…O distance in C21-H21∙∙∙O3 was 2.355 Å. On the other side of the silver cation, the C15-H15∙∙∙O4 distance was long and the H∙∙∙O distance was 2.701 Å (2.770 Å in compound 2), as O4 could not be close to the pyridine due to steric reasons (Figure 2b).

The atomic coordinates, anisotropic displacement parameters, molecular distances/angles, and hydrogen bond distances are given in Tables S1–S6.

The conformation of the [Agpy₂]⁺ cations in compounds 1 and 2 were found to be similar (rmsD = 0.1008 and maxD = 0.1572 Å) [24]. However, the geometry of the whole complex molecule was considerably different due to the effect of the exchange of the anion from MnO₄⁻ to ClO₄⁻ (Figure 3).

The structural dissimilarities induced conformational variability of the [Agpy₂]⁺ cation with the same anion (perchlorate) in the crystal structures of [Agpy₂ClO₄]∙0.5py (compound 1), [Agpy₂ClO₄] (compound 3), and [Agpy₄]ClO₄∙4[Agpy₂ClO₄] (compound 4) (Figure 3). The rmsD values were found to be 0.1334 and 0.1248 and the maxD values were found to be 0.2344 and 0.2019 when the conformations of [Agpy₂]⁺ cations in compound 1 were compared to those in compound 3 [21] or compound 4 [22].

The Ag⁺ cation was shown to have trigonal-bipyramid coordination in compound 1 when we also considered the oxygen atoms at a distance of around 3 Å. Two pyridine nitrogen atoms were found in the axial positions, while O2, O4, and a symmetry-generated O2 were shown to occupy the equatorial plane (Figure 4a). Via the longest Ag-O4 coordination bond, the dimers formed a chain along the c crystallographic axis (Figure 4b). Each oxygen atom of the perchlorate anion was shown to have a distinctive role in the formation of the crystal lattice. O1 forms hydrogen bonds with the pyridine para-hydrogens (C13-H and C23-H; see Figure 4c), which construct a chain of dimers in the ac diagonal. O2 takes part in the formation of the dimer (Figure 2). O3 is in a hydrogen bond with Cα of the ligand pyridine, stabilizing the dimer (Figure 4c). O4 is part of the 1D chain of the coordination network; it is in a weak intradimer hydrogen bond with a ligand pyridine Cα-H15, and it was shown to be close to a neighboring pyridine meta C24-H24. Finally, O4 was found to form a weak hydrogen bond C32-H32∙∙∙O4 to the solvent pyridine (Table 2). The position of the complex dimers in their 1D chain was alternating, with an angle of 88.60° concerning the rings containing N1 and N2 (Figure 4c).

The neighboring dimers were found to be connected by the weak C13-H13∙∙∙O1, C23-H23∙∙∙O1 and C24-H24∙∙∙O4 intermolecular interactions (a), and the almost perpendicular alternating positions of the dimers of compound 1 were shown to be built up along the ac diagonal (b) (Figure 4c). π∙∙∙π interactions were shown to contribute to the stability of the crystal lattice of compound 1 (Figure 5). The distance between the aromatic rings containing N1 and N2 was found to be 3.6657(14) Å within the dimer, while the distance between the aromatic rings of neighboring dimers containing N1 and N1 was calculated as 3.7449(14) Å. The angle of ring centroids in the column was 151.93°.

The stoichiometric ratio of the monomer [Agpy₂ClO₄] and the solvent pyridine was calculated as 1:0.5. The guest pyridine occupied 527.2 ų, which was 18.0% of the volume of the unit cell (Figure 6). There was no additional residual solvent-accessible void in the crystal lattice. The solvate pyridine was found to be connected to the O4 atom of the perchlorate anion with the α-CH. The Kitaigorodskii packing coefficient was found to be 69.8% [25], and it dropped to 59.5% after the removal of the solvent from the crystal lattice. The packing arrangement in the crystals of compound 1 viewing from the a, b and c crystallographic axes is shown in Figure S2, which includes the solvent pyridine.

The packing arrangement in crystals of compound 1 viewed along the a, b and c crystallographic axes is shown in Figure 7, which includes the solvent pyridine.

2.3. Vibrational Spectroscopic Analysis of [Agpy₂ClO₄]·0.5py (Compound 1)

Based on the single-crystal X-ray study (see preceding paragraphs), compound 1 was found to have a monoclinic cell (space group C2/c, Z = 4 for the full unit cell) but with Z = 2 for the primitive cell. We were able to perform a correlation analysis for the perchlorate ion, the isolated silver cation (only translational modes are taken into consideration), and two kinds of pyridine, coordinated (C₁ site) and non-coordinated solvate pyridine (C₂ site). The correlation diagrams for the internal and the external vibrational modes of perchlorate ions, pyridines, and translational modes of silver ion are given in Figure 8 and Figures S3–S7.

We found nine internal vibrational modes of the ClO₄⁻ anions at C₁ sites. Among them, the E mode was doubly and the F2 modes were triply degenerate. The A_u and B_u modes of the factor group were IR active, whereas the A_g and B_g were Raman active. Altogether, 18 IR (9Au and 9Bu) and 18 Raman (9A_g and 9B_g) vibrational degrees of freedom could be expected. Correlation diagrams for the external vibrations of the ClO₄⁻ anions at C₁ sites showed that a total of 24 such degrees of freedom (12 of translational and 12 of rotational origin) exist. The bands associated with these modes are expected in the far-IR region (12 bands, 6A_u and 6B_u) and in the low-frequency part of the Raman spectra (12 bands, 6A_g and 6B_g). R and T denote hindered rotations and hindered translations of the anions (ESI Figures S3–S5 and S7), respectively. No predictions of the intensities (either here or later) could a priori be made.

A total of 12 vibrational degrees of freedom were found to exist (four Ag⁺ ions per primitive cell). All g-modes were active in Raman scattering, and all u-modes were IR active.

Each mode from the local (site) group was shown to split into four components in the factor group. There were 27A_g, 27B_g, 27A_u, and 27B_u modes giving rise to 108 vibrational degrees of freedom. We observed two types of pyridine rings at C₁ sites, so the number of modes had to be doubled, i.e., 54A_g, 54B_g, 54A_u and 54B_u. Each external mode from the local (site) group split into four components in the factor group. There were 6A_g, 6A_u, 6B_g, and 6B_u external modes giving rise to 24 vibrational degrees of freedom (12 of translational and 12 of rotational origin). There were two types of pyridine rings at C₁ sites, so the number of modes had to be doubled, i.e., 12Ag, 12B_g, 12A_u and 12B_u modes.

The correlation tables for the 27 internal and 12 external vibrational modes of the coordinated and non-coordinated pyridine rings at the C₁ and C₂ sites are given at Figure 8a,b and Figures S3 and S4. Among the abovementioned modes of pyridine rings (that is 99A_g, 102B_g, 99A_u, and 102B_u), 1A_u and 2B_u were found to be acoustic modes. Since these acoustic modes under the k = 0 approximation had a frequency of 0, the 99A_g, 102B_g, 98A_u and 100B_u bands, at most, were expected due to fundamental transitions that appeared in the Raman (201) and the IR (198) spectra. Bands due to second-order transitions could further complicate the spectral picture, especially in cases where conditions for Fermi resonance are fulfilled.

2.4. Assignment of the Vibrational Modes in the Spectra of Compound 1

To support the assignment of the spectral bands, density functional theory (DFT) calculations were performed on the pyridine ligand, the dimer of Agpy₂ClO₄ shown in Figure 2, as well as the dimer coordinated by two py ligands at the O4 atoms, referred to a “dimer and 2 py” from here on. The geometries optimized at the M05/LANL2DZ and 6–31G(d,p) level (see the Section 3) are listed in Tables S7 and S8. In

Table 3. Assignment of the pyridine normal modes in the IR and Raman spectra of compound 1. * Overlapped with Cl-O modes.

C2v	ν	Vibrational Mode	Compounds				Pyridine [26,27,28]		DFT Calculations
			1		4 [8]				Pyridine	Agpy2ClO4 Dimer	Dimer and 2 py
			IR	Raman	IR	Raman	IR	Raman
A1 (in- plane)	1	C-H stretch	3120	3150	3094	3082	3076	3076	3261	3268–3272	3258–3269
	2	C-H stretch	3074	3085	3062	3066	3060	3060	3236	3236–3242	(3218–3243) b
	3	C-H stretch	3046	3068	3043	3030	3030	3030	3191	3223, 3229	(3218–3238) b
	4	Ring stretch	1574	1586	1592 1571	1602 1571	1578	1578	1652	1668–1670	1660–1673
	5	Ring stretch	1488	1488 1485	1482	1483	1483	1483	1479	1515–1520	1519–1524
	6	C-H wag	1222	1224	1215	1215	1217	1217	1246	1247–1257	1244–1267
	7	C-H wag	1069 a	1062 a	1079	1072	1071	1071	1094	1088, 1096, 1097 b	1095, 1097 1103
	8	Ring bend	1040 a	1042 a	1034	1034	1031	1031	1002	1024–1027	1006, 1024, 1026, 1029, 1030
	9	Ring breathing	880	863	-	865	858	858	1046	1024–1027	1043–1049
	10	Ring bend	619 a	621 a	-	-	601	601	606	630–636	610, 611, 632–640
A2 (out of plane)	11	C-H wag	983	992	983	985	982	982	1001	1020–1023	(1004–1040) c
	12	C-H wag	881	-	885	878	887	887	897	897–903	898–929
	13	Ring bend	-	-	-	-	-	-	375	389–395	389–405
B1 (out of plane)	14	C-H wag	1014	1013	1003	1010	997	997	1005	1011–1015	(1005–1016) c
	15	C-H wag	949	929 *	944	-	-	-	966	988–991	973–996
	16	C-H wag	751	-	751	-	-	-	713	714–717	715–729
	17	Ring bend	704, 692, 674	-	700	-	-	-	763	765–770	767–783
	18	Ring bend	-	-	-	-	-	-	412,	416–420	415–425
B2 (in- plane)	19	C-H stretch	3074	3068	3063	3066	3066	3066	3253	3256–3263	(3248–3253) b
	20	C-H stretch	3006	3009, 3028	3043	3032	3030	3030	3188	3223–3225	(3218–3236) b
	21	Ring stretch	1574	1571	1571	1574	1577	1577	1651	1648–1650	1644–1654
	22	Ring stretch	1447	1446	1440	1444	1443	1443	1473	1482–1483	1477–1487
	23	C-H wag	1373	1380	1384	1355	-	-	1370	1375–1380	1377–1389
	24	Ring stretch	1233	1230	1232	1227	1227	1227	1333	1334–1336	1335–1341
	25	C-H wag	1155	1165	1155	1155	-		1163	1169–1170	1167–1177
	26	C-H wag	1068 a	1065 *, 1072 *	1064	1072	1068		1082	1079, 1094, 1095, 1097 b, 1098, 1104	(1167–1177)
	27	Ring bend	651	649	-	649	654		668	660–663	660–667

^a Overlapped with Cl-O modes; ^b A₁ and B₂ modes are coupled; ^c A₂ and B₁ modes are coupled.

, we present the vibrational frequencies obtained for the pyridine modes.

The symmetric stretching and bending modes of perchlorate ions (ν₁ and ν₂) were not shown to be IR active for the regular tetrahedral perchlorate ion, but these modes did appear in the IR spectrum of compound 1 because of the distortions (ESI Figure S6) (see correlation analysis above). The weak singlet band was shown to belong to the symmetric stretching mode and appeared at 929 cm⁻¹ in the IR spectrum (Figure 9). Accordingly, an intensive band appeared at 930 cm⁻¹ in the Raman spectra of compound 1 (Figure 10). The DFT calculations on the dimer predicted a doublet at 960 and 961 cm⁻¹ for this mode, in which the symmetric vibrations of the two perchlorate units were coupled in-phase and anti-phase, the latter displaying some intensity. The deviation in the frequencies is within the expected range considering the capabilities of DFT methods.

The antisymmetric stretching band was shown to be an intensive mode in the IR spectra. A triplet-like band system appeared with maxima at 1041, 1055 and 1069 cm⁻¹. This complex band may also include pyridine vibrational modes such as the ν₈(A₁) ring bending of pyridines; ν₇ (A₁) and ν₁₄(B₂) C-H wags, both for coordinated and solvate pyridine; and the ν₂^a,b(Cl-O) and ν₄(Cl-O) (458 plus 619 cm⁻¹ and 474 plus 619 cm⁻¹) combination bands at the higher wavenumber side of the complex band system (Figure 9). The coupling with the pyridine C-H wagging was supported by the DFT calculations. For the dimer, the DFT prediction for this mode was a doublet at 1079 (w) and 1088 cm⁻¹ (vs), again at larger frequencies than in the experiment. In the former, the antisymmetric vibrations of the two perchlorate ions were found to be coupled in-phase with each other and with the ν₇ (A₁) wagging mode of pyridine, while in the latter, the perchlorate vibrations were found to be in anti-phase and coupled with the ν₁₄(B₂) C-H wag. A large intensity was predicted for the latter mode. For the dimer and 2py unit, in which the solvate pyridine was simulated by py molecules connected to the O₄ atoms, the asymmetric perchlorate stretch mode was shown to be involved in a series of strong transitions at 1092, 1112, 1117, 1167 and 1172 cm⁻¹. These modes contain large contributions from the two mentioned pyridine in-plane C-H wagging modes, in some cases involving all six pyridine rings. According to the DFT calculations, it cannot be excluded that the triplet-looking structure involves perchlorate stretching modes and that some may be associated with pyridine wags.

In the Raman spectrum, the weak bands of ν_as(Cl-O) components were expected and observed (ESI Figure S7). The most intense Raman peak appeared at 1013 cm⁻¹, which probably belonged to ν₁₇ (B₂) pyridine C-H wag mode, whereas a less intense band at 992 cm⁻¹ probably belonged to the same mode of the solvate pyridine (Figure 10a). The irregular shape singlet at 1043 cm⁻¹ may have belonged to ν₃(Cl-O). The symmetric bending mode δ_s(Cl-O) was shown to have a doublet at 458 and 474 cm⁻¹ in the IR spectra (Figure S6b) and at 456 and 484 cm⁻¹ in the Raman spectra (Figure 10a). The corresponding DFT frequencies were 436 and 444 cm⁻¹. The antisymmetric bending mode (δ _as(F₂) (601 and 607 cm⁻¹ according to DFT) is a possible component of the band that appeared at 619 cm⁻¹ in the IR spectrum (Figure 9b), but this band was shown to coincide with the pyridine ν₁₀ (A₁) ring bending mode (629 cm⁻¹ according to DFT), which causes a change in the shape of this band. The ν₁₀ mode of coordinated pyridine was not found in the IR spectra of compounds 2 and 3 [10], but this mode was assigned at 619 cm⁻¹ in the Raman spectrum of [Agpy₄]ReO₄ [9].

No splitting due to the triplet nature of δ_as(Cl-O) could be observed in the IR spectrum. The δ_s^a,b(Cl-O) and δ_as(Cl-O) combination band components completely coincided with the band system containing ν_as(Cl-O) and other modes between 1120 and 1000 cm⁻¹ (Figure 9a) (1080,1088,1187,1189 according to DFT). The Raman spectra contained a weak band at 621 cm⁻¹ that belonged to δ_s(Cl-O) (Figure 10a). The assignment of the normal modes of the perchlorate ion confirmed the appearance of weak but characteristic perchlorate ion combination and overtone bands [10] in the IR spectrum of compound 1, e.g., the ν_s and ν_as^a,b,c band at 1983, 1994 and 2014 cm⁻¹ and the 2δ_s^a(Cl-O) band (2 × 458 cm⁻¹) at 916 cm⁻¹. However, the 2δ_s^b(Cl-O) band (2 × 474 cm⁻¹) completely coincided with the ν₁₅ band of the pyridine rings (Figure S6d). Very weak overtones of 2δ_s at 917 cm⁻¹ and 968 cm⁻¹ were observed in the Raman spectrum of compound 1 (Figure 10a). The combination bands of δ_s^a,b(Cl-O) and δ_as(Cl-O) also appeared at 1070 and 1107 cm⁻¹, although the band at 1070 cm⁻¹ may have contained the ν₇ and ν₂₆ pyridine CH wag modes as well. The ν_as(Cl-O) and ν_s(Cl-O) combination bands were not found in the Raman spectrum.

The detailed assignment of pyridine vibrational modes in the IR and Raman spectra of compound 1 is summarized in Table 3, and these modes were also compared with the assignments for compound 4 and free pyridine. The frequencies obtained with DFT were generally close to the experimental results except for the high-frequency C-H stretches and the ring breathing, for which the calculated frequency was 1002 cm⁻¹ for the pyridine molecule instead of the experimental 858 cm⁻¹; interestingly, however, the approximately 20 cm⁻¹ shift in the complex was well-reproduced.

The other normal modes belonging to the cation, especially the Ag-N modes, were mainly found in the far-IR region, so only the Raman data could be evaluated.

The band in the IR spectrum of compound 1 at 412 cm⁻¹ is characteristic of the metal-coordinated pyridine ring [10,13,29]. The Raman spectrum was found to contain the ν_s (Ag-N) and ν_as(Ag-N) band of the Agpy₂⁺ cation at 166 and 231 cm⁻¹, and the contributions of the Ag-O modes appeared as a shoulder at 150 cm⁻¹ and a badly resolved peak at 218 cm⁻¹ [29]. The band at 107 cm⁻¹ was previously assigned as the ν_s (Ag-N) band for the Agpy₄⁺ cation and as the band belonging to the Ag-O contribution of the perchlorate ion in the Raman and IR spectra of [Agpy₄]ReO₄ [9] and compound 4 (4(Agpy₂ClO₄·[Agpy₄]ClO₄) [11,29]. [Agpy₄]ReO₄ was not shown to contain coordinated (perchlorate or perrhenate) anions, and compound 1 was not shown to contain the Agpy₄⁺ cation. Thus, the band at 107 cm⁻¹ in the Raman spectrum (Figure S7b) of compound 1 could only be assigned as the perchlorate contribution via Ag-O coordination, in accord with Bowmaker’s results regarding compound 4 [29].

3. Materials and Methods

Chemical-grade sodium perchlorate, silver nitrate and pyridine (Deuton-X Ltd., Érd, Hungary) were used in the synthesis experiments following the methods given in [13]. Compound 1 was identified by its powder X-ray diffractogram and chemical analysis (Ag, pyridine and perchlorate content) according to the methods given in [13,30].

All X-ray powder diffraction measurements were conducted on a Philips PW-1050 Bragg-Brentano parafocusing goniometer (Cu anode; 40 kV and 35 mA tube power) supplied with a secondary beam graphite monochromator and a proportional counter. All scans were recorded in the step mode. The XRD diffraction patterns were evaluated with a full profile fitting technique.

The diffraction pattern of the colorless block shape single crystal of compound 1 with the size of 0.50 mm × 0.30 mm × 0.30 mm was measured on a RIGAKU RAXIS Rapid II diffractometer at 103(2) K using MoK_α radiation. A crystal of compound 1 was mounted on a loop with parathon oil. Cell parameters were determined by least squares using 53,600 reflections (3.055° ≤ θ ≤ 27.445°). Intensity data were collected on a RIGAKU RAXIS-RAPID II diffractometer (graphite monochromator; Mo-Kα radiation, λ = 0.71073 Å) at 103(2) K in the range 3.141 ≤ θ ≤ 27.463 [25]. A total of 58,650 reflections were collected, of which 3347 were unique (R(int) = 0.0305, R(σ) = 0.0117); intensities of 3257 reflections were greater than 2σ(I). Completeness to θ was 0.998. Multi-scan absorption correction was applied to the data (the minimum and maximum transmission factors were 0.717 and 1.000).

The structure was solved by direct methods [31]. Anisotropic full-matrix least-squares refinement [32] on F² for all non-hydrogen atoms yielded R₁ = 0.0253 and wR₂ = 0.0600 for 1332 reflections [I > 2σ(I)] and R₁ = 0.0265 and wR₂ = 0.0606 for all (3347) intensity data (number of parameters = 191; goodness-of-fit = 1.205; and the maximum and mean shift/esd were 0.001 and 0.000, respectively). The maximum and minimum residual electron density in the final difference map were 0.916 and −0.623 e.Å⁻³, respectively. The weighting scheme applied was w = 1/[σ²(F_o²) + (0.02228.5193P)² + 8.5193P] where P = (F_o² + 2F_c²)/3.

Hydrogen atomic positions were calculated from assumed geometries. Hydrogen atoms were included in structure factor calculations, but they were not refined. The isotropic displacement parameters of the hydrogen atoms were approximated from the U(eq) value of the atom they were bonded to.

Crystal data and details of structure determination and refinement are listed in Table 1 and Table S1. Atomic coordinates and equivalent isotropic displacement parameters, hydrogen coordinates and equivalent isotropic displacement parameters, anisotropic displacement parameters, bond lengths and angles, and intermolecular interactions of compound 1 are presented in Table 2 and Tables S1–S6. CCDC-2191559 (compound 1) contains 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 (accessed on 19 July 2022).

The FT-IR spectrum of compound 1 was recorded on the Jasco FT-IR-4600 system described in [32]. The apparatus was equipped with a single reflection diamond ATR accessory (incident angle 45°). The measurements were conducted in the 4000–400 cm⁻¹ region with a resolution of 4 cm⁻¹, and 64 individual spectra were coadded. An ATR correction (OPUS v6.0, Bruker Optik Gmbh, Rudolf-Plank-Str. 27, 76275 Ettlingen, Germany) was performed on the raw spectra.

The Raman measurements of freshly prepared compound 1 were performed on a Horiba Jobin/Yvon LabRAM microspectrometer with an external Nd-laser source (532 nm, ~40 mW, Olympus BX-40 optical microscope) at 203 and 243 K with a Linkam THMS600 temperature-controlled microscope stage). The laser beam was focused (objective of 50×), and a D2 intensity filter was used to prevent thermal decomposition due to laser power. The confocal hole of 1000 µm and a 1800 groove mm⁻¹ grating monochromator were used in a confocal system and for light dispersion. The spectra were measured between 4000 and 100 cm⁻¹ with a resolution of 3 cm⁻¹. Exposure time was 60 s.

Quantum chemistry calculations were performed with the Gaussian 09 suite of programs [33]. The structure and vibrational spectra of characteristic fragments of the elementary cell were determined using density functional theory. We used the B3LYP [34], CAM-B3LYP [35] and M05 functionals [36]. For systems involving transition metals, M05 is the most appropriate, so we report the results obtained with this functional. The difference of the structural parameters obtained with the three functionals showed the usual, relatively small differences. The LANL2DZ basis set and pseudopotential were used for the Ag atom. For the remaining atoms the 6–31 G(d,p) basis set was employed. The presence of the polarization function proved to be essential for the chlorine atom. It is worth noting that when the LANL2DZ basis set was used for all atoms, the geometry optimization led to a formal Cl-O bond rupture within the perchlorate groups, because of the lack of d orbitals in the hypervalent Cl atom.

4. Conclusions

Compound 1 ([Agpy₂ClO₄]·0.5py) was successfully prepared via the trituration of [Agpy₂ClO₄] and 4[Agpy₂ClO₄]∙[Agpy₄]ClO₄ (as the source of solvate pyridine) mixture in a mixed solvent of acetone:benzene = 1:1 (v = v) at room temperature. The monoclinic crystals of compound 1 were found to be isomorphic with the analogous permanganate complex. Two [Agpy₂]⁺ cations were shown to form a dimeric unit [Agpy₂ClO₄]₂, and silver ions were found to be connected to two ClO₄⁻ anions via oxygen atoms. The Ag∙∙∙Ag distance was calculated as 3.3873(5) Å, whereas the Ag∙∙∙O distances were calculated as 2.840(2) Å and 2.8749(16) Å in the centrosymmetric rectangle of Ag-O-Ag-O. The stoichiometric ratio of the monomer [Agpy₂ClO₄] and the solvent pyridine was 1:0.5. The guest pyridine occupied 527.2 ų, which was 18.0% of the volume of the unit cell. There was no additional residual solvent-accessible void in the crystal lattice. The solvate pyridine was connected via its α-CH to one of the O atoms of the perchlorate anion. The IR and Raman bands were assigned for all of the perchlorate and pyridine vibrational modes based on correlation analysis and quantum chemistry calculations.