Symmetrisation, isomerism and structural studies on novel phenylmercury(II) thiosemicarbazonates: correlation of the energy barrier to rotation of the amino group with the bonding parameters of the thioamide group

Abstract

The reactions of phenylmercury(II) acetate with a series of alkyl, aryl and heterocyclic thiosemicarbazones in ethanol formed novel phenylmercury(II) derivatives of stoichiometry [HgPhL] [HL = RN³N²HC¹(S)N¹H₂ = cyclopentanone 1, cyclohexanone 2, benzaldehyde 3, 2-hydroxybenzaldehyde 4, 4-methoxybenzaldehyde 5, pyrrole-2-carbaldehyde 6, thiophene-2-carbaldehyde 7 or furan-2-carbaldehyde 8 thiosemicarbazone], characterised with the help of analytical data, physical properties, IR, far-IR, multinuclear NMR (¹H, ¹³C, ¹⁹⁹Hg) spectroscopy and X-ray crystallography of complexes 1, 5 and 6. The ¹H and ¹³C NMR data suggest that the N²H group is deprotonated during reaction with phenylmercury(II) acetate and co-ordination occurs via the N³,S atoms in a chelating mode. The ¹⁹⁹Hg NMR data suggest symmetrisation phenomenon for complexes 3 and 5, 2[HgPhL] ⇌ HgPh₂ + [HgL], which is supported also by ¹H and ¹³C NMR data. The δ(Hg) values reveal that shielding of Hg with the change of organic group in the thiosemicarbazones decreases in the order: 2-hydroxybenzene ⋙  furan > benzene > 4-methoxybenzene  thiophene ≈ cyclohexanone ≈ cyclopentanone > pyrrole and the Lewis basicity of the thiosemicarbazones varies in the opposite order. The ¹H and ¹³C NMR data reveal that 7 and 8 show isomerism. There are two strong [Hg–C 2.063(7) 1, 2.069(10) 5, 2.049(11) 6; Hg–S 2.382(2) 1, 2.357(3) 5, 2.377(3) Å 6] and one weak bond [Hg–N³ 2.489(6) 1, 2.611(7) 5, 2.492(9) Å 6], with C_Ph–Hg–S bond angles of 162.9(2), 174.2(3), 165.8(3)° respectively. The weak intermolecular interactions via Hg · · · N² [3.001(6) Å] in 1 and via Hg · · · S in 5 [3.518(3) Å] and 6 [3.528(3) Å] form centrosymmetric dimers and Hg formally acquires four-co-ordination with two strong (Hg–C, Hg–S), one weak (Hg · · · N³) and one secondary (Hg · · · N² or S) bonds. The preferred dimer formation via N² nitrogen in 1, rather than via sulfur atoms (5 and 6) despite Hg · · · S affinity represents an unusual bonding mode. From the low-temperature ¹H NMR studies of some selected complexes, the energy barrier (ΔG_Tc*, T_c is coalescence temperature) to rotation of the amino group about the C¹–N¹ bond was calculated and correlated with bonding parameters of the thioamide group in the solid state.