η5-Semiquinone and η4-Quinone Complexes of Manganese Tricarbonyl. Intermolecular Hydrogen Bonding in the Solid State and in Solution

Cationic manganese tricarbonyl complexes containing η 6 -hydroquinone ( 1a ), η 6 -catechol ( 2a ), and η 6 -resorcinol ( 3a ) ligands are readily deprotonated in DMSO with triethylamine or other weak bases to afford stable (η 5 -semiquinone)Mn(CO) 3 ( 1b − 3b ) and (η 4 -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 η 4 -quinone salts Na[ 1c − 3c ]·3H 2 O. The crystal structure of Na[ 2c ]·3H 2 O shows the sodium cation to be bonded to just one quinone oxygen. IR and crystal structure data of both η 5 -semiquinone and η 4 -quinone complexes provide clear evidence of significant contributions to the overall structure from more polar η 6 -resonance forms.