Mechanistic Pathways in the Thermal and Photochemical Conversion of (TTP)Mo(η ² ‐O ₂ ) ₂ to (TTP)MoO ₂ (TTP = Tetratolylporphyrin)

The reaction pathway for the thermal and photochemical oxygen dissociation from (TTP)Mo(η 2 ‐O 2 ) 2 ( 1a ) to produce cis ‐dioxo (TTP)MoO 2 ( 2a ) is probed experimentally and computationally. The reaction of 1a is initiated photochemically by absorption at the Soret band (445 nm), but no transient intermediate was detected in the microsecond time domain. The photochemical process for the 1a to monoperoxo intermediate (TTP)Mo(O 2 ) (3a) conversion is addressed via time‐dependent density functional theory (TD‐DFT) and is suggested to occur via excitation of singlet reactant 1a to an excited state singlet with weakened Mo–O 2 binding, causing O 2 dissociation to the monoperoxo 3a . DFT studies of the thermal reaction from (POR)Mo(η 2 ‐O 2 ) 2 ( 1b ) reveal that it dissociates O 2 via a superoxo‐like transition state (TS) (POR)Mo(η 1 ‐O 2 )(η 2 ‐O 2 )*, leading to the monoperoxo complex ( 3b ). 3b , both in the singlet and triplet state, is suggested to convert to the cis ‐dioxomolybdenum(VI) product 2b by a novel electronic reorganization. On the singlet surface, the 3b / 2b conversion proceeds via an asynchronous concerted process with an unsymmetrical TS and an activation energy of 19.5 kcal/mol. On the triplet surface, the 3b/2b conversion proceeds through a symmetrical maximum with an approximate barrier of 16.4 kcal/mol, followed by spin crossover to reach the singlet dioxo product 2b .