TAUTOMERIZATIONS, PROTONATIONS, AND ELECTROPHILIC ADDITIONS OF ETA-2-COORDINATED PYRROLES

A series of complexes is synthesized of the form [Os(NH3)5(2,3-eta-2-pyrrole)]2+, with pyrrole and various alkylated pyrroles, In contrast to the free ligands, these complexes can be protonated chemo- and stereoselectively at the beta-carbon, away from the metal, to produce pyrrolium species whose acidities range in pK(a) from 4.2 to 7.5. In the presence of a weak base, two of these 3H-pyrrolium species can be converted to the corresponding 2H-pyrrolium tautomer, [Os(NH3)5(3,4-eta-2-2H-pyrrolium)]3+. In the case of L = 2,5-dimethylpyrrole, the 2H-pyrrolium species can be deprotonated at nitrogen (pK(a) = 7.9), rendering a neutral 2H-pyrrole ligand. When L = 1-methylpyrrole, the 2H-pyrrolium species can be deprotonated at the alpha-carbon, generating an unstable azomethine ylide complex bound through C3 and C4. Rapid rearrangement of this species yields the neutral 1H-pyrrole complex, [Os(NH3)5(2,3-eta-2-1-methylpyrrole)]2+ (pK(ep) = 7.8 (overall process)). Through consideration of pK(a) and electrochemical data, the pyrrole/pyrrolenine isomerization energy (DELTA-G-degrees) is found to decrease by about 16 kcal on osmium(II), to the point where these tautomers become virtually isoergic. In contrast, the pyrrole/pyrrolenine equilibrium is largely unaffected by coordination to Os(III), in comparison to the free ligand.