2-(5,6-Diphenyl-1,2,4-Triazin-3-yl)pyridinium Dichloroiodate (I)

Abstract

2-(5,6-diphenyl-1,2,4-triazin-3-yl)pyridinium dichloroiodate (I) (1) was synthesized by reacting 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine with ICl in dichloromethane solution. The structural characterization of 1 by SC-XRD analysis was accompanied by elemental analysis, FT-IR, and FT-Raman spectroscopy measurements.

1. Introduction

Polypyridine donors react with interhalogens XY and dihalogens X₂ (X, Y = Cl, Br, I) to give a variety of products whose nature depend on the nature of the donor, the interhalogen, and the reaction conditions [1,2,3,4]. Among the possible reaction products, charge-transfer adducts containing the N–X–Y linear group [5,6], halonium compounds featuring a N–X⁺–N moiety [5,7,8], or salts where a N-protonated pyridinium cation is counterbalanced by discrete [3] or extended [9] (poly)halide anions can be mentioned. FT-Raman spectroscopy can provide useful information on the nature of the resulting products [1,3,9,10]. In simple CT-adducts, the adduct formation results in an elongation of the X–Y bond with respect to the free halogen/interhalogen [3,4]. When polyhalides are formed, the peculiar stretching vibrations of the interacting synthons, such as neutral (inter)halogens and tri(inter)halides, can be detected in the low-energy region of the FT-Raman spectrum [1,2,3,4].

3-(2-Pyridyl)-5,6-diphenyl-1,2,4-triazine [11,12] (Scheme 1) has often been reported as an efficient donor towards a variety of metal ions [13,14,15,16]. Notwithstanding, the reactivity of this donor towards halogens or interhalogens has not been described to date. Pursuing our interest towards the reactivity of the polypyridyl substrate towards ICl [1,2,3,4], we report here on the synthesis and structural and vibrational characterization of the novel salt 2-(5,6-diphenyl-1,2,4-triazin-3-yl)pyridinium dichloroiodate (I) (1).

2. Results

The reaction of 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (L; Scheme 1) with ICl in molar ratios 1:0.5, 1:1, 1:2, and 1:5 in CH₂Cl₂ yielded a crystalline product featuring the same microanalytical (elemental analysis, melting point determination) and spectroscopic (FT-IR, FT-Raman) features. Since crystals were collected before dryness, the reaction yield could not be determined. Single-crystal X-ray diffraction analysis revealed the reaction product to be (HL)[ICl₂] (1; Figure 1). Compound 1 crystallizes in the monoclinic space group P2₁/c with four molecules in the unit cell.

Crystal data for compound 1: C₂₀H₁₅Cl₂IN₄, (Mr = 509.16 g mol⁻¹) monoclinic, P2₁/c (No. 14), a = 17.7427(13) Å, b = 7.4326(5) Å, c = 15.5351(12) Å, β = 100.659(7)°, α = γ = 90°, V = 2013.3(3) ų, T = 120(2) K, Z = 4, Z’ = 1, µ(Mo Kα) = 1.868 mm⁻¹, 19,110 reflections measured, 4624 unique (R_int = 0.0302), which were used in all calculations. The final wR₂ was 0.0634 (all data) and R₁ was 0.0270 [F² ≥ 2 σ(F²)].

The asymmetric unit of compound 1 consists of a donor molecule protonated at the N4 pyridine nitrogen atom counterbalanced by a classical [17,18] [ICl₂]⁻ anion. Protonation of (poly)pyridyl donors as a result of the reaction with dihalogens or interhalogens is not unusual and it has been attributed to solvolysis or reaction with incipient moisture [1].

In the HL⁺ cation, the two phenyl rings and the pyridine rings of the donor are rotated by 38.3(3), 47.7(3), and 12.6(3)° with respect to the plane of the triazine ring, respectively. Notwithstanding no crystal structures featuring the HL⁺ cation have been deposited to date, these torsion angles can be compared to those featured by the two polymorphs (CSD codes HEWLOL and HEWLOL01) reported for L (phenyl torsions: 32.4(1) and 53.6(1)°; pyridine torsion 1.0(1)° for HEWLOL; [11] 32.2(2), 56.2(2), and 9.1(2)° for HEWLOL01 [12]) and the corresponding values optimized at the density functional theory (DFT) level [19,20] for L (phenyl torsion, 29.6 and 34.6°; pyridine torsion, 6.1°) and HL⁺ (phenyl torsion, 30.9 and 38.1°; pyridine torsion, 4.4°), suggesting that electronic factors rather than crystal packing interactions are responsible for the nonplanar conformation of the ligand.

The Cl2 terminal atom of the [ICl₂]⁻ anion is involved in a hydrogen bonding (HB) interaction with the pyridinium moiety (interaction a in Figure 2 and

Table 1. Intermolecular interactions of compound 1.

Interaction		dD–H (Å)	dH∙∙∙A (Å)	dD∙∙∙A (Å)	αD–H···A (°)
a	N4–H4∙∙∙Cl2	0.85(3)	2.39(2)	3.174(2)	153(2)
b	C20i–H20i∙∙∙Cl1	0.95	2.88	3.541(2)	128.2
c	C13ii–H13ii∙∙∙Cl2	0.95	2.70	3.610(3)	160.9
d	C12ii –H12ii ∙∙∙N3	0.95	2.66	3.291(3)	124.5
e	C14i–H14i···N2	0.95	2.62	3.425(3)	143.2

ⁱ = $1-x,-\frac{1}{2}+y, \frac{3}{2}-z$; ⁱⁱ =$x,\frac{3}{2}-y,-\frac{1}{2}+z.$

). The HB interaction results in a remarkable asymmetry of the [ICl₂]⁻ anion (Cl1–I1, 2.4856(8); Cl2–I1, 2.6005(6) Å; Cl1–I1–Cl2, 178.27(2)°). The Cl1 and Cl2 atoms further interact with the H20ⁱ and H13ⁱⁱ protons of different symmetry-related adjacent HL⁺ units (interactions b and c in Table 1 and Figure 2; ⁱ = $1-x,-\frac{1}{2}+y, \frac{3}{2}-z$; ⁱⁱ =$x,\frac{3}{2}-y,-\frac{1}{2}+z$). These interactions, along a set of weak C–H···N contacts (Table 1), generate the crystal packing, seen along the b-axis in Figure 2.

The FT-Raman spectrum of compound 1 shows the expected band due to the symmetric stretching mode peculiar to the [ICl₂]⁻ ion at 278 cm⁻¹ (Figure 3), in excellent agreement with the corresponding values calculated at the hybrid-DFT level (ν(σ_g) = 257, ν(σ_u) = 238 cm⁻¹). The π_u bending mode is calculated at 108 cm⁻¹ and can be envisaged both in the FT-Raman and in the FT-FIR spectra within the complex envelope of bands at about 100 cm⁻¹ (Figure S3). Notably, the wavenumbers of the stretching and bending modes are very close to those previously reported for different compounds featuring (poly)pyridinium cations balanced by classical [ICl₂]⁻ anions, for which the stretching and the bending modes were reportedly in the range of 265–285 cm⁻¹ and 85–90 cm⁻¹, respectively [1,2,3,4,21].

3. Materials and Methods

3.1. General

All the reagents and solvents were purchased from Sigma-Aldrich or Lancaster and used without further purification. Elemental analyses were performed with an EA1108 CHNS-O Fisons instrument. Infrared spectra were recorded on a Bruker IFS55 spectrometer at room temperature using a flow of dried air. Far-IR (500–50 cm⁻¹) spectra (resolution 2 cm⁻¹) were recorded as polythene pellets with a Mylar beam-splitter and polythene windows. Middle-IR spectra (resolution 2 cm⁻¹) were recorded as KBr pellets, with a KBr beam-splitter and KBr windows. FT-Raman spectra were recorded (resolution of 2 cm⁻¹) on a Bruker RFS100 FT-Raman spectrometer, fitted with an In–Ga–As detector (room temperature) operating with a Nd:YAG laser (excitation wavelength 1064 nm) with a 180° scattering geometry (excitation power 5 mW). The melting point was determined on a FALC mod. C apparatus. X-ray diffraction data for compound 1 were collected at 120(2) K by means of combined φ and ω scans with a Bruker Nonius KappaCCD area detector situated at the window of a rotating anode (graphite Mo-K_α radiation, λ = 0.71073 Å). The structure was solved with the ShelXT [22] solution program using dual methods and the model was refined with ShelXL [23] 2018/3 using full-matrix least-squares minimization on F². Olex2 1.5 [24] was used as the graphical interface. DFT [19,20] calculations were carried out both on L, HL⁺, and [ICl₂]⁻ at the DFT level with the commercial suite of programs Gaussian 16 [25] by adopting the mPW1PW hybrid functional [26]. The def2-SVP and def2-TZVP basis sets [27,28] were adopted for the atomic species of the L donor and the [ICl₂]⁻ ion, respectively. Vibrational frequencies were calculated at the optimized geometries. GaussView [29] and PyFreq [30] were used to investigate the Kohn–Sham molecular orbital composition and the vibrational frequencies.

3.2. Synthesis of 2-(5,6-Diphenyl-1,2,4-Triazin-3-yl)pyridinium Dichloroiodate (I) (1)

To 2 mL of a CH₂Cl₂ solution of 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (26 mg, 8.4 × 10⁻⁵ mol), a 0.054 M solution of ICl in the same solvent was added dropwise in L:ICl molar ratios ranging between 1:0.5 and 1:5 (0.8 mL, 4.2 × 10⁻⁵ mol; 1.6 mL, 8.4 × 10⁻⁵ mol; 3.1 mL, 1.7 × 10⁻⁴ mol; 7.8 mL, 4.2 × 10⁻⁴ mol). The solution was allowed to concentrate slowly in air. After a few days, the solid product was separated and washed with light petroleum ether and dried under reduced pressure. Due to the experimental synthetic procedure, the reaction yield could not be determined. Elemental analysis calcd. for C₂₀H₁₅N₄ICl₂: 47.18; H, 2.97; N, 11.00%. Found: C, 47.23; H, 2.47; N, 11.28%. M.p. 218 °C. FT-MIR (KBr pellet, 4000–400 cm⁻¹): 3467 m, 2362 m, 1624 m, 1612 s, 1608 m, 1538 m, 1506 m, 1445 m, 1410 s, 1375 m, 1306 m, 1227 s, 1164 s, 1005 m, 943 m, 783 m, 766 s, 755 s, 736 m, 692 m, 660 s, 539 m, 534 m, 510 m, 443 m, 416 w cm⁻¹. FT-FIR (polythene pellet, 500–50 cm⁻¹): 487 m, 444 m, 416 m, 402 s, 391 m, 366 m, 332 m, 319 m, 310 m, 284 m, 273 s, 260 s, 234 s, 218 s, 199 s, 191 s, 163 m, 145 m, 135 s, 129 s, 103 w, 95 m, 85 m, 67 s cm⁻¹. FT-Raman (500–50 cm⁻¹, 5 mW, relative intensities in parentheses related to the highest peak taken equal to 10.0): 278 (10.0), 163 (3.3), 131 (3.8), 94 (6.2), 73 (7.1) cm⁻¹.

4. Conclusions

2-(5,6-diphenyl-1,2,4-triazin-3-yl)pyridinium dichloroiodate (I) (1) was synthesized and fully characterized. Further studies are ongoing in our laboratories to investigate the reactivity of 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine towards different dihalogens and interhalogens and its potential role as a building block in the formation of extended supramolecular assemblies.