Nonheme Iron(III) Azide and Iron(III) Isothiocyanate Complexes: Radical Rebound Reactivity, Selectivity, and Catalysis

An Azide Adduct of Ti(II) with a Sterically Encumbering Trityl Azide and Independent Synthesis of a Bulky Ti(IV) Imido and Its Isomer

We report here the synthesis and characterization of a bulky titanium(IV) imido via the bimolecular deazotation of trityl azide by the Ti(II) precursor [(Tp3-tBu,5-Me)TiCl] (1, Tp3-tBu,5-Me = hydrido(tris(3-methyl-5-tert-butylpyrazolyl)borate)). The sterically encumbering trityl substituent on an azide group discourages the unimolecular deazotation pathway and allows the isolation and full characterization of a rare organic azide adduct of Ti, namely, the diazenylimide complex [(Tp3-tBu,5-Me)Ti(N3CPh3)Cl] (2). We show how free Ti(II) is a necessity to promote deazotation, thus allowing for a bimolecular conversion of 2 to a sterically crowded triphenylimido complex, [(Tp3-tBu,5-Me)Ti(NCPh3)Cl] (3). Complex 3 is the kinetic species formed from deazotation of 2, and mild thermolysis results in the formation of its isomer [{HB(pyztBu,Me)2(pyzMe,tBu)}Ti{NCPh3}Cl] (4), {HB(pyztBu,Me)2(pyzMe,tBu)} = hydrido(bis(3-methyl-5-tert-butylpyrazolyl)(5-methyl-3-tert-butylpyrazolyl)borate) via a 1,2-borotropic shift. This study demonstrates that unimolecular deazotation of 2 to 3 is significantly disfavored, and attributed to the steric bulk of the trityl substituent. By congesting the imido fragment, we also show mechanistically how conversion of 3 takes place to alleviate congestion by the CPh3 group to form 4. This process involves a rate-limiting 1,2-borotropic shift with activation parameters Ea = 30(1) kcal/mol, ΔH‡ = 30(1) kcal/mol, and ΔS‡ = 15(1) cal/mol·K as measured by VT-1H NMR spectroscopy.

Using machine learning methods to predict the diabatic bond dissociation energy of non-heme iron complexes

Bond dissociation energy (BDE) is an important property in chemical research. In the process of non-heme iron complex catalytic reactions, diabatic BDE has a significant impact on the selectivity of halogenation and hydroxylation reactions. Measuring or calculating BDE by using traditional experimental or theoretical methods is often expensive and complex, so we propose the first application of machine learning on non-heme iron complexes to predict and rationalize the diabatic BDEs of Fe-X and Fe-OH bonds in order to assist in the study of selectivity in non-heme iron complex catalytic reactions. We built a reliable and representative dataset containing over 600 types of non-heme iron complexes and used density functional theory (DFT) to calculate nearly 900 diabatic BDE for machine learning. In terms of model training, we used 2D molecular fingerprints and 3D descriptors as inputs to train the regression model. The results indicate that the ensemble algorithm combined with Morgan fingerprints can effectively predict the diabatic BDEs of non-heme iron complexes. Using the Gradient Boosting Regressor (GBR) model and Morgan fingerprints can achieve an accurate prediction of R2 = 0.791 and the mean absolute error (MAE) = 10.23 kcal mol-1. The incorporation of 3D descriptors significantly improves the predictive performance of molecular fingerprints other than Morgan fingerprints. Notably, the SOAP descriptor effectively captures key 3D molecular information, making it particularly advantageous for predicting isomers with large ΔBDE. However, when the ΔBDE of isomers in the dataset is small, Morgan fingerprints remain the more efficient choice.

Cluster model study of the mechanism and origins of enantio- and chemoselectivity in non-heme iron enzyme-catalyzed C-H azidation

The mechanisms and enantio- and chemoselectivities of non-heme iron enzyme-catalyzed C-H azidation were investigated using density functional theory (DFT) calculations. A detailed active site cluster model comprising 337 atoms was constructed, incorporating essential features of the first- and second-coordination spheres and substrate-binding pockets. The catalytic cycle involves N-F bond activation, hydrogen atom transfer (HAT), and radical rebound steps. DFT calculations suggest that the observed enantioselectivity arises from steric effects between the substrate and key active-site residues. Additionally, in the non-heme Fe(N3)F complex, the Fe-N3 bond, which has a lower diabatic bond dissociation energy, preferentially rebounds to form the azidation product.

Nonheme Mononuclear and Dinuclear Iron(II) and Iron(III) Fluoride Complexes and Their Fluorine Radical Transfer Reactivity

The nonheme iron(II) complexes containing a fluoride anion, FeII(BNPAPh2O)(F) (1) and [FeII(BNPAPh2OH)(F)(THF)](BF4) (2), were synthesized and structurally characterized. Addition of dioxygen to either 1 or 2 led to the formation of a fluoride-bridged, dinuclear iron(III) complex [Fe2III(BNPAPh2O)2(F)2(μ-F)]+ (4), which was characterized by single-crystal X-ray diffraction, 1H NMR, and elemental analysis. An iron(II)(iodide) complex, FeII(BNPAPh2O)(I) (3), was prepared and reacted with O2 to give the mononuclear complex cis-FeIII(BNPAPh2O)(OH)(I) (5). Addition of excess fluoride to 5 led to the formation of the oxo-bridged, dinuclear iron(III) complex [Fe2III(BNPAPh2O)2(F)2(μ-O)] (6), while the mononuclear iron(III)(fluoride) complex cis-FeIII(BNPAPh2O)(F)(Cl) (7) was prepared from the addition of excess F- to FeIII(BNPAPh2O)Cl2. The dinuclear complexes 4 and 6 were unreactive to fluorine radical transfer, but mononuclear 7 reacts with the radical substrate (p-MeO-C6H4)3C• to give the fluorine radical transfer products FeII(BNPAPh2O)(Cl) and (p-OMe-C6H4)3CF. These results show that a mononuclear FeIII(F) complex is capable of mediating fluorine radical transfer, even in the presence of second coordination sphere hydrogen bonds to the F- ligand. These findings are placed in context with what is known about the nonheme iron halogenases and related synthetic catalysts regarding their ability, or lack thereof, to mediate fluorine radical transfer reactions.

Biocatalytic Generation of Trifluoromethyl Radicals by Nonheme Iron Enzymes for Enantioselective Alkene Difunctionalization

The trifluoromethyl (-CF3) group represents a highly prevalent functionality in pharmaceuticals. Over the past few decades, significant advances have been made in the development of synthetic methods for trifluoromethylation. In contrast, there are currently no metalloenzymes known to catalyze the formation of C(sp3)-CF3 bonds. In this work, we demonstrate that a nonheme iron enzyme, hydroxymandelate synthase from Amycolatopsis orientalis (AoHMS), is capable of generating CF3 radicals from hypervalent iodine(III) reagents and directing them for enantioselective alkene trifluoromethyl azidation. A high-throughput screening (HTS) platform based on Staudinger ligation was established, enabling the rapid evaluation of AoHMS variants for this abiological transformation. The final optimized variant accepts a range of alkene substrates, producing the trifluoromethyl azidation products in up to 73% yield and 96:4 enantiomeric ratio (e.r.). The biocatalytic platform can be further extended to alkene pentafluoroethyl azidation and diazidation by altering the iodine(III) reagent. In addition, anion competition experiments provide insights into the radical rebound process for this abiological transformation. This study not only expands the catalytic repertoire of metalloenzymes for radical transformations but also creates a new enzymatic space for organofluorine synthesis.

Isothiocyanate-Based Microemulsions Loaded into Biocompatible Hydrogels as Innovative Biofumigants for Agricultural Soils

Biofumigation was proposed as an alternative to synthetic pesticides for the disinfection of agricultural soils, in view of the biocidal effect of isothiocyanates (ITCs) released by some vegetal species, like Brassicaceae. However, biofumigation also presents limitations; thus, a novel and viable alternative could be the direct introduction of ITCs into agricultural soils as components loaded into biodegradable hydrogels. Thus, in this work, ITCs-based microemulsions were developed, which can be loaded into porous polymer-based hydrogel beads based on sodium alginate (ALG) or sodium carboxymethyl cellulose (CMC). Three ITCs (ethyl, phenyl, and allyl isothiocyanate) and three different surfactants (sodium dodecylsulfate, Brij 35, and Tween 80) were considered. The optimal system was characterized with attenuated ATR-FTIR spectroscopy and differential scanning calorimetry to study how the microemulsion/gels interaction affects the gel properties, such as the equilibrium water content or free water index. Finally, loading and release profiles were studied by means of UV-Vis spectrophotometry. It was found that CMC hydrogel beads showed a slightly more efficient profile of micelles' release in water with respect to ALG beads. For this reason, and due to the enhanced contribution of Fe(III) to their biocidal properties, CMC-based hydrogels are the most promising in view of the application on real agricultural soils.

Catalyst and Medium Control over Rebound Pathways in Manganese-Catalyzed Methylenic C-H Bond Oxidation

The C(sp3)-H bond oxygenation of a variety of cyclopropane containing hydrocarbons with hydrogen peroxide catalyzed by manganese complexes containing aminopyridine tetradentate ligands was carried out. Oxidations were performed in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 2,2,2-trifluoroethanol (TFE) using different manganese catalysts and carboxylic acid co-ligands, where steric and electronic properties were systematically modified. Functionalization selectively occurs at the most activated C-H bonds that are α- to cyclopropane, providing access to carboxylate or 2,2,2-trifluoroethanolate transfer products, with no competition, in favorable cases, from the generally dominant hydroxylation reaction. The formation of mixtures of unrearranged and rearranged esters (oxidation in HFIP in the presence of a carboxylic acid) and ethers (oxidation in TFE) with full control over diastereoselectivity was observed, confirming the involvement of delocalized cationic intermediates in these transformations. Despite such a complex mechanistic scenario, by fine-tuning of catalyst and carboxylic acid sterics and electronics and leveraging on the relative contribution of cationic pathways to the reaction mechanism, control over product chemoselectivity could be systematically achieved. Taken together, the results reported herein provide powerful catalytic tools to rationally manipulate ligand transfer pathways in C-H oxidations of cyclopropane containing hydrocarbons, delivering novel products in good yields and, in some cases, outstanding selectivities, expanding the available toolbox for the development of synthetically useful C-H functionalization procedures.

Functional Model of Compound II of Cytochrome P450: Spectroscopic Characterization and Reactivity Studies of a FeIV-OH Complex

Herein, we show that the reaction of a mononuclear FeIII(OH) complex (1) with N-tosyliminobenzyliodinane (PhINTs) resulted in the formation of a FeIV(OH) species (3). The obtained complex 3 was characterized by an array of spectroscopic techniques and represented a rare example of a synthetic FeIV(OH) complex. The reaction of 1 with the one-electron oxidizing agent was reported to form a ligand-oxidized FeIII(OH) complex (2). 3 revealed a one-electron reduction potential of -0.22 V vs Fc+/Fc at -15 °C, which was 150 mV anodically shifted than 2 (Ered = -0.37 V vs Fc+/Fc at -15 °C), inferring 3 to be more oxidizing than 2. 3 reacted spontaneously with (4-OMe-C6H4)3C• to form (4-OMe-C6H4)3C(OH) through rebound of the OH group and displayed significantly faster reactivity than 2. Further, activation of the hydrocarbon C-H and the phenolic O-H bond by 2 and 3 was compared and showed that 3 is a stronger oxidant than 2. A detailed kinetic study established the occurrence of a concerted proton-electron transfer/hydrogen atom transfer reaction of 3. Studying one-electron reduction of 2 and 3 using decamethylferrocene (Fc*) revealed a higher ket of 3 than 2. The study established that the primary coordination sphere around Fe and the redox state of the metal center is very crucial in controlling the reactivity of high-valent Fe-OH complexes. Further, a FeIII(OMe) complex (4) was synthesized and thoroughly characterized, including X-ray structure determination. The reaction of 4 with PhINTs resulted in the formation of a FeIV(OMe) species (5), revealing the presence of two FeIV species with isomer shifts of -0.11 mm/s and = 0.17 mm/s in the Mössbauer spectrum and showed FeIV/FeIII potential at -0.36 V vs Fc+/Fc couple in acetonitrile at -15 °C. The reactivity studies of 5 were investigated and compared with the FeIV(OH) complex (3).

Selective Radical Transfer in a Series of Nonheme Iron(III) Complexes

A series of nonheme iron complexes, FeIII(BNPAPh2O)(Lax)(Leq) (Lax/eq = N3-, NCS-, NCO-, and Cl-) have been synthesized using the previously reported BNPAPh2O- ligand. The ferrous analogs FeII(BNPAPh2O)(Lax) (Lax = N3-, NCS-, and NCO-) were also prepared. The complexes were structurally characterized using single crystal X-ray diffraction, which shows that all the FeIII complexes are six-coordinate, with one anionic ligand (Lax) in the H-bonding axial site and the other anionic ligand (Leq) in the equatorial plane, cis to the Lax ligand. The reaction of FeIII(BNPAPh2O-)(Lax)(Leq) with Ph3C• shows that one ligand is selectively transferred in each case. A selectivity trend emerges that shows •N3 is the most favored for transfer in each case to the carbon radical, whereas Cl• is the least favored. The NCO and NCS ligands showed an intermediate propensity for radical transfer, with NCS > NCO. The overall order of selectivity is N3 > NCS > NCO > Cl. In addition, we also demonstrated that H-bonding has a small effect on governing product selectivity by using a non-H-bonded ligand (DPAPh2O-). This study demonstrates the inherent radical transfer selectivity of nonhydroxo-ligated nonheme iron(III) complexes, which could be useful for efforts in synthetic and (bio)catalytic C-H functionalization.