Unpredictable Dynamic Behaviour of Ruthenium Chelate Pyrrole Derivatives

Reaction of [Ru(H)₂(CO)(PPh₃)₃] 1 with an equimolar amount of pyrrole-2-carboxylic acid (H) leads to the homoleptic chelate derivative k²(O,O)-[RuH(CO)(HL₁)(PPh₃)₂] 2. Prolonged acetonitrile refluxing promotes an unusual k²(O,O)- → k²(N,O)- dynamic chelate conversion, forming a neutral, stable, air- and moisture- insensitive, solvento-species k²(N,O)-[Ru(MeCN)(CO)(L₁)(PPh₃)₂] 3. Analogously, reaction of 1 with the pyrrole-2-carboxyaldehyde (HL) affords k²(N,O)-[RuH(CO)(HL₂)(PPh₃)₂] 4, 5, as a couple of functional isomers. Optimized reaction conditions such as temperature and solvent polarity allow the isolation of dominant configurations. Structure 5 is a pyrrolide Ru-carbaldehyde, obtained from cyclization of the pendant CHO function, whereas species 4 can be viewed as an ethanoyl-conjugated Ru-pyrrole. Derivatives 3-5 were characterized by single crystal X-ray diffraction, ESI-Ms, IR, and NMR spectroscopy, indicating distinct features for the Ru-bonded pyrrolyl groups. DFT computational results, coplanarity, bond equalization, and electron delocalization along the fused five-membered rings support aromatic features. In accordance with the antisymbiotic trans-influence, both the isolated isomers 4 and 5 disclose CO ligands opposite to N- or O-anionic groups. The quantitative Mayer bond order evidences a stabilizing backbonding effect. Antibacterial and antifungal trials on Gram-positive (Staphylococcus aureus), Gram-negative (Escherichia coli), and Candida albicans were further carried out.