Six-membered [C,N] cyclopalladated sym N,N′,N″-tri(4-tolyl)guanidines: Synthesis, reactivity studies and structural aspects

Abstract

Six-membered [C,N] cyclopalladated sym N,N′,N″-tri(4-tolyl)guanidines, [(ArNH)2CNAr] (sym = symmetrical; Ar = 4-MeC6H4; LH24-tolyl) of the types [(C,N)Pd(μ-OC(O)R)]2 (1 and 2), [(C,N)Pd(μ-Br)]2 (3), cis-[(C,N)PdLBr] (4–7), and [(C,N)Pd(acac)] (8) were prepared in high yield by established methods with a view aimed at understanding the influence of the 4-tolyl substituent of the guanidine moiety upon the solution behaviour of 1–8. The composition of 1–8 was confirmed by elemental analysis, IR, and NMR spectroscopy, and mass spectrometry. The molecular structures of 1–6 were determined by single-crystal X-ray diffraction. Palladacycles 1–3 exist as a dimer in transoid conformation in the solid state while 4–6 exist as a monomer with cis configuration around the palladium atom as the Lewis base is placed cis to the Pd–C bond due to antisymbiosis. The NMR spectra of 1–8 revealed the presence of a single isomer in solution and this spectral feature is ascribed to the rapid inversion of the six-membered “[C,N]Pd” ring due to the presence of sterically less hindered and more symmetrical 4-tolyl substituent in the NAr unit of the guanidine moiety.