A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N₃O Macrocyclic Ligand

Abstract

The sluggish oxidants, [FeIV(O)(TMC)(CH3CN)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [FeIV(O)(d12-TMCN)(OTf)]+ (3d; d12-TMCN = 1,4,7,11-tetra-d3-methyl-1,4,7,11-tetraazacyclotetradecane), are transformed into a highly reactive oxidant, [FeIV(O)(TMCO)(OTf)]+ (1; TMCO = 4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane), upon replacement of a -NMe donor in the TMC and TMCN ligands by an O-atom. A rate enhancement of 5 – 6 orders of magnitude in both H-atom and O-atom transfer reactions is observed upon oxygen incorporation into the macrocyclic ligand and can be explained based upon the higher electrophilicity of the iron center and the higher availability of the more reactive S = 2 state in 1. This rationalizes nature’s preference for using O-rich ligand environments for the hydroxylation of strong C-H bonds in enzymatic reactions.

A Pseudotetrahedral Terminal Oxoiron(IV) Complex: Mechanistic Promiscuity in C−H Bond Oxidation Reactions

“…Herein we report the synthesis and characterization of the S= 2 pseudotetrahedral [Fe IV (O)( t Bu 3 tacn)] 2+ ( 2 , t Bu 3 tacn [6] =1,4,7‐tri‐ tert ‐butyl‐1,4,7‐triazacyclononane) complex, which exhibits spectroscopic and reactivity properties distinct from the oxoiron(IV) cores in TBP or O h geometries. In particular, in direct contrast to the vast majority of previous oxoiron(IV) cores, [3a–g, 5a–e] where the reactivity with substrates containing C−H bonds is controlled by the C−H bond dissociation energies (BDE C‐H ), complex 2 demonstrates a mechanistic promiscuity in its C−H oxidation reactions. Sterically less hindered C−H bonds are oxidized via a conventional direct hydrogen atom abstraction (HAA; Scheme 2) mechanism that is characterized by large deuterium kinetic isotope effects ( KIE s), which are greater than the semi‐classical limit of 7, implying a significant contribution of hydrogen tunnelling [7] .…”

“…Interestingly, the reactivity trend is reversed in reactions with 9,10‐dihydroanthracene (DHA), where 2 exhibits the least reactivity. Furthermore, when the logarithms of the statistically corrected second order rate constants ( k 2 ’) were plotted vs. the BDE C‐H values of the substrates (Figure 2 A, Figure S20A), the linear correlation typically observed for oxoiron(IV) cores [3a–i, 5] is found to be not valid for 2 . While the respective log ( k 2 ) values associated with 2 for the oxidation of 1,4‐CHD, 1,3‐cyclohexadiene (1,3‐CHD), ethylbenzene, cyclohexene and toluene fall on a line (Figure 2 A, black points), xanthene, DHA, indene and fluorene substrates (Figure 2 A, inset) deviate from this pattern and exhibit significantly lower rates than predicted by the linear relationship.…”

“…The oxidative reactivity of 2 (Figures S9–S18; Table S6) has been investigated with several substrates in oxygen atom transfer (OAT) and HAA reactions and the second order rate constants derived from these studies in CH 2 Cl 2 are compared with three of the most reactive high‐valent Fe‐oxo intermediates reported to date (namely the [(TQA)Fe IV (O)(CH 3 CN)] 2+ (TQA=tris(2‐quinolylmethyl)amine), [5e] [(Me 3 NTB)Fe IV (O)] 2+ (Me 3 NTB=tris((N‐methyl‐benzimidazol‐2‐yl)methyl)amine) [3c] and [(TMCO)Fe IV (O)(CH 3 CN)] 2+ (TMCO=4,8,12‐trimethyl‐1‐oxa‐4,8,12‐triazacyclotetradecane) [3f] complexes (Table 2). In reactions with ethylbenzene, 1,4‐cyclohexadiene (1,4‐CHD), and toluene, 2 is a stronger oxidant than [(TMCO)Fe IV (O)(CH 3 CN)] 2+ , but comparable to [(TQA)Fe IV (O)(CH 3 CN)] 2+ and [(Me 3 NTB)Fe IV (O)] 2+ .…”

A bioinspired oxoiron(iv) motif supported on a N2S2 macrocyclic ligand

“…This drastic downshift in KIE is unique for sulfur substitution and is not observed in the oxygen substituted oxoiron(IV) center. 34 Understanding this lowering of KIE will require further experimental and computational work. However the significant effect of the equatorial sulfur ligation on the physical and chemical properties…”

Dealing with Spin States in Computational Organometallic Catalysis

“…Recently, together with the groups of Ray and Nam, we reported a modification of the TMC ligand that was observed to be six orders of magnitude faster. [199] One of the NMe groups of the TMC ring was replaced by oxygen (TMCO), which changes the ligand field, and lowers the s* acceptor orbital; simultaneously, the spin state changed from a triplet state (S=1) for [Fe IV (O)(TMC)] 2+ to quintet for [Fe IV (O)(TMCO)] 2+ (with the S12g DFA). The change in spin state makes that a spin-switch is no longer needed to reach the spin state with the lowest barrier along the oxidation pathway; however, most importantly, the barrier reduced dramatically (ca.…”