Metal-induced tautomerization of p- to o-quinone compounds:: Experimental evidence from CuI and ReI complexes of azophenine and DFT studies

Azophenine (7,8-diphenyl-2,5-bis(phenylamino)-p-quinonediimine, LP) reacts with [Cu(PPh3)(4)](BF4) or [Re(CO)(5)Cl] to yield the (Ph3P)(2)Cu+ or [(OC)(3)ClRe] complex of the tautomeric form 7,8-diphenyl-4,5-bis(phenylamino)-o-quinonediimine, L-o, as evident from structure determinations and from very intense metal-to-ligand charge transfer (MLCT) transitions in the visible region. Time-dependent DFT (TD-DFT) calculations on model complexes [(Nboolean ANDN)Re(CO),Cl] confirm the spectroscopic results, showing considerably higher oscillator strengths of the MLCT transition for the o-quinonediimine complexes in comparison to compounds with Nboolean ANDN=1,4-dialkyl-1,4-diazabutadiene. The complexes are additionally stabilized through hydrogen bonding between two now ortho-positioned NHPh substituents and one fluoride of the BF4- anion (Cu complex) or the chloride ligand (Re complex). DFT Calculations on the model ligands p-quinonediimine or 2,5-diamino-p-quinonediimine and their orthoquinonoid forms with and without Li+ or Cu+ are presented to discuss the relevance for metal-dependent quinoproteins.