Identical Spin Multi-State Reactivity Towards C-H Bond Activation in High-valent Fe/Mn-Oxo/Hydroxo Species
Activation of C-H bonds using an earth-abundant metal catalyst is one of the top challenges of chemistry where high-valent Fe/Mn-O/OH biomimic species play an important role. There are several open questions related to the comparative oxidative abilities of these species, and a unifying concept that could accommodate various factors influencing the reactivity is lacking. To shed light on these open questions, here we have used a combination of the DFT (B3LYP-D3/def2-TZVP) and ab initio
... SD(T); CASSCF/NEVPT2) calculations to study a series of high-valent metal-oxo/hydroxo species, [Mn+H3buea(X)] (M = Fe and Mn, n = II to V, X = O/OH; H3buea = tris[(N'-tert-butylureaylato)-N-ethylene)]aminato) towards the activation of dihydroanthracene (DHA). Detailed analysis unveils the following reactivity trend FeV=O > MnIII=O > MnIV=O > FeIII=O > MnV=O > FeII-OH > MnII-OH > MnIV-OH > FeIV-OH > FeIV=O > FeIII-OH > MnIII-OH and suggests that neither higher oxidation nor high-spin ground state yields superior reactivity. The secondary coordination sphere is found to play a vital role in controlling the reactivity wherein the H-bonding interactions reduce the crystal field strength, and this brings several excited states of the same spin multiplicity closer to the ground state resulting in the observation of identical spin multistate reactivity (ISMR) in MnIII/IV=O and FeII-OH species. For FeV=O species, strong ligand spin polarization was detected, diminishing the crystal field leading to the exhibition of ISMR reactivity. The ISMR is found to control the basicity of the oxo/hydroxo group as well as the redox potentials. Further, when pKa > 15, a PT-ET mechanism for C-H bond activation is detected, and a higher E1/2 value directs the reaction via the concerted HAT/PCET mechanism. On the other hand, for species that exhibit classical SSR/TSR reactivity, such as MnII-OH, FeIV=O, the secondary coordination sphere effect is found to be lethal. As the multireference character is absent in these species, they lack the electronic flexibility that ISMR species enjoy during the reaction, leading to sluggish/no reactivity for many species, including the popular FeIV=O species. As metalloenzymes' active sites have several H-bonding networks resembling the species studied here, this unlocks the possibility of having ISMR type reactivity for metalloenzymes to rationalize their superior catalytic abilities.