The (Tetraazaannulene)copper‐Catalyzed Reduction of Sulfur(IV) Species. A Pulse‐Radiolysis and Theoretical Study of the Associated Reaction Mechanism

The monomeric species, [Cu II {H x (tmdnTAA)}] x + ( x =0, 1, and 2, and (tmdnTAA) 2− =[5,7,12,14]tetramethyldinaphtho[ b , i ][1,4,8,11]tetraaza[14]annulenato(2−)=7,9,18,20‐tetramethyldinaphtho[2,3‐ b  : 2′,3′‐ i ] [1,4,8,11]tetraazacyclotetradecinato(2−)) catalyze the electrochemical reduction of SO 2 (aq) in homogeneous solution. A pulse‐radiolytic study of the reaction of the [Cu I (tetraazaannulene)] complexes with S IV species revealed the formation of Cu I SO 2 adducts. The formation of Cu I SO 2 adducts prevents the disproportionation of the Cu I complexes and precedes the reduction of the S IV species to S 2 O $\rm{ _4^{2 - } }$ . DFT Calculations were carried out to establish the structure and stability of the Cu I SO 2 adducts. A good agreement between the calculated and recorded electronic spectrum of the Cu I SO 2 adducts corroborated the formation of species with SO 2 coordinated to Cu I through the O‐atom.