A DFT study exploring the catalytic efficacy of 13-TMC bound MnIII-O2/OOH and MnIV/V-O species towards cyclohexene epoxidation–Deciphering the effect of additives

Abstract

The epoxidation of cyclohexene using the metal-based oxidants, viz. 13-TMC bound MnIII-O2 (1), MnIII-OOH (2), MnIV-O (3) and MnV-O (4) are computed in the absence and presence of additives. The thermodynamic stability of these oxidants is examined based on the formation free energy (ΔGR.E) from its precursor (i.e.2 from 1; 3 & 4 from 2). In the absence of additives, 2hs/ls (ΔGR.E = -3.8/-14.0 kcal/mol) is found to be thermodynamically more stable than 3hs/ls (ΔGR.E = +18.1/+10.6 kcal/mol) and 4is/ls (ΔGR.E = +88.3/+76.5 kcal/mol). Also, due to the lesser ΔGR.E associated with 2hs, the interconversion of 2hs and 1hs is energetically feasible. Therefore, in the absence of additives, MnIII-OOH (2hs/ls) and MnIII-O2 (1hs/ls) are proposed as the active oxidants to mediate cyclohexene epoxidation. Among the proposed oxidants, the epoxidation efficacy of 2hs is higher than 2ls and 1hs/ls by +9.5 to +33.6 kcal/mol. Further, the effect of additive (X) is investigated in 2hs (X = AcOH (2ahs), H2O (2bhs), H2SO4 (2chs)). In contrast to 4, the incorporation of additive significantly enhances the thermodynamic stability of MnIV-O˙/MnV-O (4ais-4cis, ΔGR.E = +2.0 to +11.9 kcal/mol). Herein, the 4ais-4cis (S = 1) is thermodynamically more preferred over 4ahs-4chs (S = 2) by +6.3 to +10.0 kcal/mol. Notably, the formation of 4ais-4cis is found to be kinetically feasible, which is evidenced by the ΔG‡ of ∼ +13.4 kcal/mol via additive-assisted OOH cleavage step from 2ahs-2chs. Therefore, the computed energetics suggest that in the presence of additives, MnIII-OOH (2ahs-2chs) and MnIV-O˙(4ais-4cis) are the plausible oxidants to medicate cyclohexene epoxidation. Importantly, the epoxidation efficacy of additive incorporated 2ahs (ΔG‡ = +10.0 kcal/mol) is found to be higher than 2hs by 50 %. Similarly, the additive incorporated 4ais-4cis exhibits facile epoxidation (ΔG‡ = ∼ +1.3 kcal/mol) with the rate-determining OOH cleavage step. Herein, the comparison of the ΔG‡ of 4ais-4cis with that of 2hs clearly shows that the epoxidation efficacy of 4ais-4cis is also superior over 2hs (by 42 %). Overall, the computed results tend to propose that the incorporation of additives facilitates the epoxidation efficacy of 13-TMC bound Mn-complex by invoking both MnIII-OOH and MnIV-O˙ as active oxidants.