Impact of ring size on the C-H activation reactivity of iron(IV)-oxo complexes: A computational study with TMC macrocycles

Here, a DFT investigation has been presented to demonstrate the relevance of the macrocyclic ligand ring size of the high-valent Fe(IV)O complex-catalyzed CH activation process. A nonheme iron-oxo compound bound with extensively studied macrocycle tetramethylcyclam (TMC) with varying ring size measures in terms of n = 12, 13, 14, 15, and 16 in [Fe(IV)O(n-TMC)(CH3CN)](2+) has been considered as the oxidant and dihydroanthracene as the general substrate. Counterions, as always, have been considered to avoid the self-interaction error in these DFT calculations. Computations were carried out to determine the effect of the axial ligand, acetonitrile, on the CH activation reactivity. It has been discovered that the complexes without axial ligands turned out to be more reactive compared to their axially coordinated counterparts. The most intriguing finding, however, has been that reactivity increased steadily with ring size increments, giving us the trend 12 < 13 < 14 < 15 < 16. Behind this typical pattern of reactivity, several factors played a role, including the energy of the electron acceptor orbital, which sequentially decreases, and the distortion energy to achieve the transition state, which also decreases as we move on from n = 12 to 16. The triplet-quintet energy difference of the oxidants also has a part to play, as it decreases with increased ring size, with the quintet becoming more and more dominant. The current studies have also been able to corroborate the experimental data that was published regarding Fe(IV)O(13-TMC) (without axial syn form) having a higher CH activation reactivity than Fe(IV)O(14-TMC) (with axial anti form). On the whole, this computational exploration gives us a reactivity pattern based on the ring size commutes and can lead to successful experimental results if pursued based on this reaction.