η⁵-semiquinone and η⁴-quinone complexes of manganese tricarbonyl. Intermolecular hydrogen bonding in the solid state and in solution

Cationic manganese tricarbonyl complexes containing η⁶-hydroquinone (1a), η⁶-catechol (2a), and η⁶-resorcinol (3a) ligands are readily deprotonated in DMSO with triethylamine or other weak bases to afford stable (η⁵-semiquinone)Mn(CO)₃ (1b-3b) and (η⁴-quinone)-Mn(CO)₃^- (1c-3c) complexes. The relative acidity in DMSO follows the qualitative order 2a > 3a > 1a. NMR spectra of the semiquinones 1b-3b indicate dynamic, strong hydrogen bonding in solution. The crystal structures of 1b and 2b feature strong intermolecular hydrogen bonding, resulting in polymers with 1b and dimers with 2b. Deprotonation of 1a-3a or 1b-3b with sodium acetate in DMSO allowed the isolation and characterization of the η⁴-quinone salts Na[1c-3c]·3H₂O. The crystal structure of Na[2c]-3H₂O shows the sodium cation to be bonded to just one quinone oxygen. IR and crystal structure data of both η⁵-semiquinone and η⁴-quinone complexes provide clear evidence of significant contributions to the overall structure from more polar η⁶-resonance forms.