Role of “S” Substitution on C–H Activation Reactivity of Iron(IV)–Oxo Cyclam Complexes: a Computational Investigation

Abstract

A comprehensive density functional theory (DFT) investigation has been presented in this article to address the role of equatorial sulfur ligation in C− H activation. A non-heme iron−oxo compound with four nitrogen atoms constituting the equatorially connected macrocyclic framework (represented as N 4 ) [Fe(IV)�O(THC)(CH 3 CN)] 2+ (THC = 1,4,8,4,8, has been considered as the base compound. Other complexes have been anticipated by the sequential replacement of this nitrogen by sulfur, that is, N 4 , N 3 S 1 , N 2 S 2 , N 1 S 3 , and S 4 . Counterions, as always, have been considered to avoid the self-interaction error in DFT. Generally, the anti-conformers (with respect to equatorial N−H and Fe�O) turned out to be the most stable. It was found that with the enrichment of the equatorial sulfur atom, reactivity increases successively, that is, we get the trend N 4 < N 3 S 1 < N 2 S 2 < N 1 S 3 < S 4. Our investigations have also verified the available experimental results where it has been reported that N 2 S 2 is more reactive than N 4 in their mixed conformation. In search of insights into this typical pattern of reactivity, the interplay of several factors has been recognized, such as the distortion energy which decreases for the transition states with the addition of sulfur; the spin density on the oxygen atom which increases implying that the radical character of abstractor increases on sulfur ligation; the energy of the electron acceptor orbital (the lowest unoccupied molecular orbital (σ z 2 *)) which decreases continuously with the sulfur substitution; and the triplet−quintet oxidant energy gap which decreases consistently with S enrichment in the equatorial position. The computational predictions reported here, if further validated by experiments, will definitely encourage the synthesis of sulfur-ligated bio-inspired complexes instead of the ones constituting nitrogen exclusively.

A density functional theory analysis of the C–H activation reactivity of iron(iv)-oxo complexes with an ‘O’ substituted tetramethylcyclam macrocycle

“…This is one of the prominent reasons for the increased reactivity with this hetero-substitution. 33,34 As depicted in Fig. 5, the linear relationship between activation energy and LUMO energy provides insight into reactivity.…”

“…33 We successfully established that hetero-substitution led to lower reaction barriers and hence opened the door to the use of alternative atoms in the vastly studied ferryl processes. 34 In a combined experimental and computational study performed by Prez et al , substitution of the N–CH 3 donor in TMC and TMCN macrocycles was studied to depict how this O substitution led to better reactivity patterns. The details of the structures are presented in Scheme 3.…”

Enhanced Reactivity through Equatorial Sulfur Coordination in Nonheme Iron(IV)–Oxo Complexes: Insights from Experiment and Theory

“…58 Their results show that the enrichment of sulfur atoms into the ligand scaffold resulted in the increased reactivity successively along the series N 4 < N 3 S < S 2 N 2 < S 3 N < S 4 . 58 To gain further insights into equatorial sulfur positioning in nonheme iron(IV)−oxo complexes, we report here a novel N 4 S-ligated iron(IV)−oxo complex (2b) and compare its structure and reactivity with the corresponding N 5 -ligated iron(IV)-oxo complex (1b); see Figure 1 for their structures. In particular, we report the synthesis and characterization of the stable N 5 -and N 4 S-ligated iron(IV)-oxo complexes.…”

“…Recently, Kaur and Mandal reported a thorough computational study on the role of equatorial sulfur substitution in the C−H activation reactivity of iron(IV)− oxo cyclam complexes. 58 Their results show that the enrichment of sulfur atoms into the ligand scaffold resulted in the increased reactivity successively along the series N 4 < N 3 S < S 2 N 2 < S 3 N < S 4 . 58 To gain further insights into equatorial sulfur positioning in nonheme iron(IV)−oxo complexes, we report here a novel N 4 S-ligated iron(IV)−oxo complex (2b) and compare its structure and reactivity with the corresponding N 5 -ligated iron(IV)-oxo complex (1b); see Figure 1 for their structures.…”

“…It was suggested that the rate enhancement may be due to the higher electrophilicity of the iron center as a result of the equatorial oxygen atoms as compared to N -methyl groups. Recently, Kaur and Mandal reported a thorough computational study on the role of equatorial sulfur substitution in the C–H activation reactivity of iron­(IV)–oxo cyclam complexes . Their results show that the enrichment of sulfur atoms into the ligand scaffold resulted in the increased reactivity successively along the series N 4 < N 3 S < S 2 N 2 < S 3 N < S 4 .…”