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

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 moiety⁵[7,8] or salts where a N-protonated pyridinium cation is counterbalanced by discrete³ 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 perturbed X–Y moiety 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.^1–4

3-(2-Pyridyl)-5,6-diphenyl-1,2,4-triazine[11,12] has been often reported as an efficient donor towards a variety of metal ions.[13,14,15,16] This notwithstanding the reactivity of this donor towards halogens or interhalogens has not been described to date. Pursuing our interest towards the reactivity of polypyridyl substrate towards ICl, ^1–4 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; Scheme 1).

2. Results

The reaction of 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (L; Scheme 1) with ICl in molar ratios 0.5:1, 1:1, 1:2, and 1:5 in CH₂Cl₂ yielded products featuring the same microanalytical (elemental analysis, melting point determination) and spectroscopic (FT-IR, FT-Raman) features. Single crystal X-ray diffraction analysis revealed the reaction product to be (HL)[ICl₂] (1).

Scheme 1. Molecular scheme of 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (L).

Scheme 1. Molecular scheme of 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (L).

Compound 1 crystallizes in the monoclinic space group P2₁/c with four molecules in the unit cell.

Figure 1. Ellipsoid plot of compound 1 with the numbering scheme adopted viewed along the b-axis. Displacement ellipsoids are drawn at 50% probability level.

Figure 1. Ellipsoid plot of compound 1 with the numbering scheme adopted viewed along the b-axis. Displacement ellipsoids are drawn at 50% probability level.

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, 19110 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 of the donor are rotated by 38.3(3) and 47.7(3)° relative to the plane of the triazine, and the pyridine by 12.6(3)°. Quite surprisingly, 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)°, respectively, for HEWLOL01¹²] and the corresponding values optimized at density functional theory (DFT)[19,20] level 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]. 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 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 in the FT-Raman spectrum (Figure 3), in excellent agreement with the corresponding values calculated at 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 [ICl₂]^– anions, for which the stretching and the bending modes were reportedly in the range 265–285 cm^–1 and 85–90 cm^–1, respectively.^1–4,[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 powder 5 mW). 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^18,19 calculations were carried out both on L, HL⁺, and [ICl₂]^– at DFT level with the commercial suite of programs Gaussian 16[25] by adopting the mPW1PW hybrid functional.[26] The def2SVP and def2TZVP 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 1:05 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). By slow air evaporation of the resulting mixture a crystalline precipitate was isolated and washed with light petroleum ether and dried under reduced pressure. 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): 3467m, 2362m, 1624m, 1612s, 1608m, 1538m, 1506m, 1445m, 1410s, 1375m, 1306m, 1227s, 1164s, 1005m, 943m, 783m, 766s, 755s, 736m, 692m, 660s, 539m, 534m, 510m, 443m, 416w cm^–1. FT-FIR (polytene pellet, 500–50 cm ^–1): 487m, 444m, 416m, 402s, 391m, 366m, 332m, 319m, 310m, 284m, 273s, 260s, 234s, 218s, 199s, 191s, 163m, 145m, 135s, 129s, 103w, 95m, 85m, 67s 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.