Mononuclear Nonheme High‐Spin (S=2) versus Intermediate‐Spin (S=1) Iron(IV)–Oxo Complexes in Oxidation Reactions

Abstract

Mononuclear nonheme high-spin (S=2) iron(IV)-oxo species have been identified as the key intermediates responsible for the C-H bond activation of organic substrates in nonheme iron enzymatic reactions. Herein we report that the C-H bond activation of hydrocarbons by a synthetic mononuclear nonheme high-spin (S=2) iron(IV)-oxo complex occurs through an oxygen non-rebound mechanism, as previously demonstrated in the C-H bond activation by nonheme intermediate (S=1) iron(IV)-oxo complexes. We also report that C-H bond activation is preferred over C=C epoxidation in the oxidation of cyclohexene by the nonheme high-spin (HS) and intermediate-spin (IS) iron(IV)-oxo complexes, whereas the C=C double bond epoxidation becomes a preferred pathway in the oxidation of deuterated cyclohexene by the nonheme HS and IS iron(IV)-oxo complexes. In the epoxidation of styrene derivatives, the HS and IS iron(IV) oxo complexes are found to have similar electrophilic characters.

Theoretical Identification of the Factors Governing the Reactivity of C−H Bond Activation by Non‐Heme Iron(IV)‐Oxo Complexes

“…Density functional approach has been found to predict a higher reactivity for high spin quintet iron‐oxo complexes, compared to the triplet congeners. However, achieving a high spin on a Fe(IV)=O model is pretty difficult and is reported to display similar reactivity as the low spin prototype . To be noted, that DFT calculations often falter to predict reliable relative energies which can have a span of ∼10 kcal/mol with selection of different exchange‐correlation functionals .…”

“…To be noted that non‐heme mononuclear iron enzymes feature a high‐spin ( S=2 ) electronic ground state while most of the synthetic mononuclear iron‐oxygen intermediates contain an intermediate spin ( S=1 ) ferryl center . Recent researches have utilized sterically encumbered scaffolds or coordination sphere tuning of the [FeO] 2+ core to achieve S=2 configuration on iron(IV)‐oxo complexes, examples being [Fe IV (O)TQA(NCMe)] 2+ (TQA=tris(2‐quinolylmethyl)amine) and [Fe IV (O)TMG 3 tren] 2+ (TMG 3 tren =1,1,1‐tris{2‐[N2‐(1,1,3,3‐tetramethylguanidino)]ethyl}amine) …”

NMR Reveals That a Highly Reactive Nonheme FeIV=O Complex Retains Its Six‐Coordinate Geometry and S=1 State in Solution

“…An alternative hypothesis invokes the presence of a thermal equilibrium between a more stable S =1 six‐coordinate species and a more reactive S =2 trigonal‐bipyramidal species that is accessible at higher temperature upon dissociation of a bound solvent . However, the thermal instability of 1 ( t 1/2 <1 min at 233 K in CH 3 CN) has made it difficult to obtain experimental support for the latter hypothesis. We have thus sought to identify reaction conditions that could increase the lifetime of 1 and allow its characterization in liquid solution by 1 H NMR spectroscopy.…”

“…[10] An alternative hypothesis invokes the presence of at hermale quilibrium between am ore stable S = 1s ix-coordinate speciesa nd am ore reactive S = 2t rigonal-bipyramidal species that is accessible at higher temperature upon dissociation of ab ound solvent. [12] However,t he thermal instability of 1 (t 1/2 < 1min at 233 Ki nC H 3 CN) [13] has made it difficultt oo btain experimental support for the latter hypothesis. We have thus sought to identify reaction conditions that could increaset he lifetime of 1 and allow its characterization in liquid solutionb y 1 HNMR spectroscopy.T his goal has been achieved by using [D 6 ]acetonea st he solvent (Scheme 1), whicha llows 1 to be characterized in solution at temperatures down to 193 K. Insights into the structure of 1 in liquid solution and its consequences are presented herein.Addition of 2equiv 2-(tBuSO 2 )-C 6 H 4 IO (ArIO) to as olution of [Fe II (Me 3 NTB)(CH 3 CN)](CF 3 SO 3 ) 2 in [D 6 ]acetonea t1 93 Ku nder N 2 generates 1,w hich exhibitsan ear-IRb and with a l max at 770 nm (Figure 1, left) that is identical to that reported in CH 3 CN and arises from d-d transitions of the Fe=Ou nit.…”

Synthesis, spectral, DFT calculations and antibacterial studies of Fe(III) complexes of new fluorescent Schiff bases derived from imidazo[4',5':3,4]benzo[1,2‐c]isoxazole

“…Iron is one of the essential mineral that can function as regulators, activators, transmitters, and controllers of various enzymatic reactions . In hence, the iron complexes have received considerable attention owing to their effective biological importance such as antibacterial antifungal antivirus antiproliferative and anticancer activity .…”