Axial vs. Equatorial Ligand Rivalry in Controlling the Reactivity of Iron(IV)-Oxo Species: Single-State vs. Two-State Reactivity

 

 

¹Ravi Kumar, ²Azaj Ansari, ¹Gopalan Rajaraman

 

¹ravikumar.gupta505[at]gmail.com, IIT Bombay, India

 

²Central University of Haryana, India

 

 

High-valent iron-oxo species are known for its very high reactivity and this aspect has been studied in detail over the years. Particularly the role of axial ligands in fine-tuning the reactivity of the iron(IV)-oxo species are studied in detail. The role of equatorial ligands in fine-tuning the reactivity of such species is rarely explored and is of prime importance in the development of non-heme chemistry. Here, we have undertaken a detailed DFT calculations on [(L^NHC)Fe^IV(O)(CH₃CN)]²⁺ (1) species (L^NHC=3,9,14,20-tetraaza-1,6,12,17-tetraazoniapentacyclohexanecosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene) in comparison to compound II of cytochrome P450 [(Porphyrin)Fe^IV(O)(SH)]^- (2) to probe this aspect. The electronic structures of 1 and 2 are found to vary significantly and this has led to a large variation in the reactivity. Particularly, strong equatorial ligand present in 1 destabilizes the quintet states significantly as compared to species 2. To fully understand the reactivity pattern of this species, we have modelled the hydroxylation of methane by species 1 and 2. Our calculations reveal that species 1 reacts via low-lying S=1, -pathway and generally available S=2, σ-pathway is not energetically accessible. In addition to possessing a significant barrier for C-H bond activation, the -OH rebound step is also computed to have a large barrier height leading to a marked difference in reactivity between these two species. Of particular relevance here is the observation of pure triplet state reactivity for species 1. Besides we have also attempted to test the role of axial ligands in fine-tuning the reactivity of species 1 and our results demonstrate that in contrast to heme systems, the axial ligands in 1 do not significantly influence the reactivity. This highlights the importance of designing the equatorial ligands to fine-tune reactivity of high-valent iron(IV)-oxo species.

Figure 1. Schematic representation of strong equatorial ligation suppress the two-state reactivity and suggest a way to fine-tune the reactivity of Fe^IV=O species using equatorial ligands.

 

References

1. Kumar, R.; Ansari, A.; Rajaraman, G. Chem. Eur. J. 2018, 24, 6818-6827.