Tautomeric fluxional process in metal complexes. Insight into cobalt(III) and pentamethylcyclopentadienylrhodium(III) complexes of 2-mercaptopyrimidine ligand

Studies have been carried out on the unusual molecular nonrigidity of Co-III(PymS)(3) and Cp*Rh-III(PymS)(2) (PymS: 2-mercaptopyrimidine). The crystal structure of Co-III(PymS)(3) approximates to an octahedral mer isomer with 4-membered N-S chelating fashion. For Cp*Rh-III(PymS)(3), one PymS ligand bonds to the rhodium ion in an S-monodentate mode (Rh-S(2)= 2.366(1) Angstrom) while the other ligand chelates to the metal ion in an N,S-bidentate mode (Rh-S(1)= 2.414(1); Rh-N(1) = 2.103(3) Angstrom). Even though the conformations and configurations of both complexes are still retained in solution, an unusual nonrigidity for the protons of the PymS region is observed in the solution. The broad proton signals of Co-III(PymS)(3) exhibit a temperature-dependence in the range of -40 similar to 40 degrees C with a free energy of activation Delta G* = 64.49 kJ/mol (40 degrees C). For Cp*Rh-III(PymS)(2), such a fluxionality has been markedly observed in solution. This fluxional behavior can be explained in terms of "ligand tautomerism" in metal complexes containing potential tautomeric forms.