Thiosemicarbazonates of palladium(II): The presence of methyl/phenyl substituents (R2) at C2 carbon atom induces C–H activation of R1 rings of thiosemicarbazones {R1R2C2N3–N2H–C1(S)–N1HR3}

A series of palladium(II) complexes involving C–H and N–H bond activations of the R1 rings of thiosemicarbazones {R1(R2)C2–N3–N2(H)–C1(S)–N1HR3; R1, R2: thiopheneyl, Me, H2L1 (R3 = Me), H2L2 (R3 = Ph); R1, R2: Ph, Me, H2L3 (R3 = Me), H2L4 (R3 = Ph); R1, R2: Ph, Ph, H2L5 (R3 = Me), H2L6 (R3 = Ph) and R1, R2: pyrrole, Me, H2L7 (R3 = Me), H2L8 (R3 = Ph)} are described. Methyl group (R2) at C2 carbon in H2L1 and H2L2 induced C–H bond activation of the thiopheneyl ring and formed cyclometallated complexes, [Pd(κ3–C4,N3,S–L)(PPh3)] (L = L1, 1; L2, 2). Similarly, phenyl rings (R1) in H2L3, H2L4, H2L5, H2L6 at C2 carbon have shown C–H activations forming cyclometallated complexes, [Pd(κ3–C4,N3,S–L)(PPh3)] (L = L3, 3; L4, 4; L5, 5; L6, 6). However, pyrrole ring did not exhibit similar C–H activation behavior, rather involved N–H activation and formed complexes, [Pd(κ3–N4,N3,S–L)(PPh3)] {L = L7, 7; L8, 8}. All these complexes have been characterized with the help of analytical data, spectroscopic techniques (IR, 1H and 31P NMR), and single crystal X-ray crystallography (1, 2, 4, 5, 7 and 8). The thiosemicarbazone ligands behave as dinegative C4, N3, S-chelating in 1–6 and N4, N3, S-chelating in complexes 7 and 8. Interestingly, complexes 2, 4 and 8, with phenyl substituent at N1 atom, have two independent molecules in their respective crystal lattices.

The presence of methyl and phenyl groups (R2) at C2 carbon in place of H induced activation of C–H bonds of the thiopheneyl and phenyl rings (R1) of N1–substituted thiosemicarbazone ligands, {R1C2(R2)N3–N2H–C1(S)N1HR3, R2, R3 = Me, Ph}, in their complexes 1–6. The pyrrole rings at C2 did not show similar cyclometallation and instead formed Pd–N bond (7, 8).Figure optionsDownload as PowerPoint slideHighlights
► The thiopheneyl and phenyl rings at C2 carbon of thiosemicarbazone are cyclopalladated in complexes.
► The pyrrole ring after deprotonation (–NH– group) is bonded to palladium through its nitrogen atom.
► The metalation is controlled by the electronic and steric factors of the substituents at C2 carbon.