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Axial vs. Equatorial Ligand Rivalry in Controlling the Reactivity of Iron(IV)‐Oxo Species: Single‐State vs. Two‐State Reactivity
Author(s) -
Kumar Ravi,
Ansari Azaj,
Rajaraman Gopalan
Publication year - 2018
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201800380
Subject(s) - reactivity (psychology) , chemistry , ligand (biochemistry) , porphyrin , hydroxylation , stereochemistry , crystallography , photochemistry , receptor , organic chemistry , enzyme , medicine , biochemistry , alternative medicine , pathology
High‐valent iron‐oxo species are known for their very high reactivity, and this aspect has been studied in detail over the years. The role of axial ligands in fine‐tuning the reactivity of the iron(IV)‐oxo species has been particularly well studied. The corresponding role of equatorial ligands, however, has rarely been explored, and is of prime importance in the development of non‐heme chemistry. Here, we have undertaken detailed DFT calculations on [(L NHC )Fe IV (O)(CH 3 CN)] 2+ ( 1 ; L NHC =3,9,14,20‐tetraaza1,6,12,17‐tetraazoniapenta‐cyclohexacosane‐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, implying a large variation in their reactivities. In particular, the strong equatorial ligand present in 1 significantly destabilizes the quintet states as compared to species 2 . To fully understand the reactivity pattern of these species, we have modelled the hydroxylation of methane by both 1 and 2 . Our calculations reveal that 1 reacts via a low‐lying S =1 π pathway, and that the generally available S =2 σ pathway is not energetically accessible. In addition to having 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 1 . We have also attempted to test the role of axial ligands in fine‐tuning the reactivity of 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 equatorial ligands to fine‐tune reactivity of high‐valent iron(IV)‐oxo species.