Metal‐Induced Tautomerization of p ‐ to o ‐Quinone Compounds: Experimental Evidence from Cu I and Re I Complexes of Azophenine and DFT Studies

Azophenine (7,8‐diphenyl‐2,5‐bis(phenylamino)‐ p ‐quinonediimine, L p ) reacts with [Cu(PPh 3 ) 4 ](BF 4 ) or [Re(CO) 5 Cl] to yield the (Ph 3 P) 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 [(N∩N)Re(CO) 3 Cl] confirm the spectroscopic results, showing considerably higher oscillator strengths of the MLCT transition for the o ‐quinonediimine complexes in comparison to compounds with N∩N=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 BF 4 − 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 ortho ‐quinonoid forms with and without Li + or Cu + are presented to discuss the relevance for metal‐dependent quinoproteins.