Efforts to understand the degree of carboxylate substitution in cis-[Pt(S(O) Me2)(guanidine)Cl2]: Synthetic, Structural and DFT Studies

The reaction of N,N ',N ''-tris(2-anisyl)guanidine, 1 with one equiv. of PhI(OAc)2 in acetonitrile under reflux condition for 6 h afforded a rigid 2-aminobenzimidazole type guanidine, 2 in 83% yield. The separate reactions of 2 with one equiv. of cis-[PtX2(S(O)Me2)2] (X = Cl and OC(O)CF3) in toluene under reflux condition for 8 h afforded mono adducts, cis-[PtX2(K1N-2)(S(O)Me2)] (X = Cl (3) and OC(O)CF3 (4)) in 89% and 78% yields, respectively. The reaction of 3 with two equiv. of AgOC(O)CF3 in CH2Cl2 at RT for 24 h also afforded 4 in 82% yield. The reaction of 3 with one and two equiv. of AgOC(O)Me in CH2Cl2 at RT for 24 h afforded cis-[PtCl(OC(O) Me)(K1N-2)(S(O)Me2)] (5) in 78% and 89% yields, respectively. Compounds 2-5 were characterised by elemental analyses, IR, NMR (1H, 13C{1H} and 19F) spectroscopies while 4 and 5 were also characterised by 195Pt {1H} NMR spectroscopy. Molecular structures of 2-5 were determined by single crystal X-ray diffraction (SCXRD). Complexes 3-5 exist as cis isomer in the solid state as revealed by SCXRD but as a mixture of cis (major) and trans (minor) isomers in solution as revealed by NMR spectroscopy. The reasons for the formation of bis (trifluoroacetato) complex, 4 versus mono-acetato complex, 5 irrespective of the molar ratio of the starting materials was rationalised based on the availability of greater amount of trifluoroacetate anion than acetate anion in solution from their respective Ag+ sources as deduced from DFT calculations.

Automated summary

In this study, a rigid 2-aminobenzimidazole type guanidine and its platinum complexes were synthesized through a series of reactions and characterized by various physical and chemical methods. The reaction mechanism was also explained through DFT calculations.