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

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 = R = (NNHC1)-N-3-H-2(S)(NH2)-H-1 = 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(H-1, C-13, Hg-199) spectroscopy and X-ray crystallography of complexes 1, 5 and 6. The H-1 and C-13 NMR data suggest that the (NH)-H-2 group is deprotonated during reaction with phenylmercury(II) acetate and co-ordination occurs via the N-3,S atoms in a chelating mode. The Hg-199 NMR data suggest symmetrisation phenomenon for complexes 3 and 5, 2[HgPhL] reversible arrow HgPh2 + [HgL], which is supported also by H-1 and C-13 NMR data. The delta(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 approximate to cyclohexanone > cyclopentanone > pyrrole and the Lewis basicity of the thiosemicarbazones varies in the opposite order. The H-1 and C-13 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) Angstrom 6] and one weak bond [Hg-N-3 2.489(6) 1, 2.611(7) 5, 2.492(9) Angstrom 6], with C-Ph-Hg-S bond angles of 162.9(2), 174.2(3), 165.8(3)degrees respectively. The weak intermolecular interactions via Hg ... N-2 [3.001(6) Angstrom] in 1 and via Hg ... S in 5 [3.518(3)Angstrom] and 6 [3.528(3)Angstrom] form centrosymmetric dimers and Hg formally acquires four-co-ordination with two strong (Hg-C, Hg-S), one weak (Hg ... N-3) and one secondary (Hg ... N-2 or S) bonds. The preferred dimer formation via N-2 nitrogen in 1, rather than via sulfur atoms (5 and 6) despite Hg ... S affinity represents an unusual bonding mode. From the low-temperature H-1 NMR studies of some selected complexes, the energy barrier (Delta GT(c)*, T-c is coalescence temperature) to rotation of the amino group about the C-1-N-1 bond was calculated and correlated with bonding parameters of the thioamide group in the solid state.