Incorporating radical ligands into metal complexes is one of the emerging trends in the design of single-molecule magnets (SMMs). While significant effort has been expended to generate multinuclear transition metal-based SMMs with bridging radical ligands, less attention has been paid to mononuclear transition metal-radical SMMs. Herein, we describe the first α-diiminato radical-containing mononuclear transition metal SMM, namely, [κ2-PhTttBu]Fe(AdNCHCHNAd) (1), and its analogue [κ2-PhTttBu]Fe(CyNCHCHNCy) (2) (PhTttBu = phenyltris(tert-butylthiomethyl)borate, Ad = adamantyl, and Cy = cyclohexyl). 1 and 2 feature nearly identical geometric and electronic structures, as shown by X-ray crystallography and electronic absorption spectroscopy. A more detailed description of the electronic structure of 1 was obtained through EPR and Mössbauer spectroscopies, SQUID magnetometry, and DFT, TD-DFT, and CAS calculations. 1 and 2 are best described as high-spin iron(II) complexes with antiferromagnetically coupled α-diiminato radical ligands. A strong magnetic exchange coupling between the iron(II) ion and the ligand radical was confirmed in 1, with an estimated coupling constant J < -250 cm-1 (J = -657 cm-1, DFT). Calibrated CAS calculations revealed that the ground-state Fe(II)-α-diiminato radical configuration has significant ionic contributions, which are weighted specifically toward the Fe(I)-neutral α-diimine species. Experimental data and theoretical calculations also suggest that 1 possesses an easy-axis anisotropy, with an axial zero-field splitting parameter D in the range from -4 to-1 cm-1. Finally, dynamic magnetic studies show that 1 exhibits slow magnetic relaxation behavior with an energy barrier close to the theoretical maximum, 2|D|. These results demonstrate that incorporating strongly coupled α-diiminato radicals into mononuclear transition metal complexes can be an effective strategy to prepare SMMs.

The phenyltris[(tert-butylthio)methyl]borate ligand, [PhTt tBu], has been studied extensively as a platform for coordination, organometallic, and bioinorganic chemistry, especially with 3d metals. While [PhTt tBu]Co(3,5-DBCatH) (3,5-DBCatH is 3,5-di-tert-butylcatecholate), a CoII–monoanionic catecholate complex, was successfully isolated to model the active site of cobalt(II)-substituted homoprotocatechuate 2,3-dioxygenase (Co-HPCD) [Wang et al. (2019). Inorg. Chim. Acta, 488, 49–55], its iron(II) counterpart, [PhTt tBu]Fe(3,5-DBCatH), was not accessible via similar synthetic routes. Switching the nucleophile from catecholate to alkoxide or aryloxide, however, led to the successful isolation of three highly air-sensitive FeII–alkoxide and –aryloxide complexes, namely, (triphenylmethoxo){tris[(tert-butylsulfanyl)methyl]phenylborato-κ3 S,S′,S′′}iron(II), [Fe(C21H38BS3)(C19H15O)], (2), (2,6-dimethylphenolato){tris[(tert-butylsulfanyl)methyl]phenylborato-κ3 S,S′,S′′}iron(II), [Fe(C21H38BS3)(C8H9O)], (3), and bis{μ-tris[(tert-butylsulfanyl)methyl]phenylborato-κ3 S,S′:S′′}bis[(phenolato-κO)iron(II)] toluene disolvate, [Fe2(C21H38BS3)2(C6H5O)2]·2C7H8, (4). In the solid state, compounds (2) and (3) are monomeric, with [PhTt tBu] acting as a tridentate ligand. In contrast, compound (4) crystallizes as a dimeric complex, wherein each [PhTt tBu] ligand binds to an iron centre with two thioethers and binds to the other iron centre with the third thioether. The molecular structures of (2)–(4) demonstrate a diversity in the binding modes of [PhTt tBu] and highlight its potential use for assembling multinuclear complexes. In addition, the successful isolation of (2)–(4), as well as the structural information of a [PhTt tBu] modification product, namely, bis{μ-tris[(tert-butylsulfanyl)methyl](2-oxidophenolato)borato-κO,O′,S,S′:O′}dicobalt(II), [Co2(C21H37BO2S3)2], (5), obtained from the reaction of [PhTt tBu]CoCl with potassium monoanionic catecholate, shed light on the origin of the instability of [PhTt tBu]Fe(3,5-DBCatH).

Inspired by the active site structure of Co-HPCD (HPCD = homoprotocatechuate 2,3-dioxygenase), in which the catechol binds to the cobalt(II) center in its monoanionic form in a bidentate fashion, we report the synthesis and characterization of [PhTt(tBu)]Co(3,5-DBCatH) [PhTt(tBu) = phenyltris(tertbutylthiomethyl)borate; 3,5-DBCatH = 3,5-di-tert-butyl-catecholate monoanion], the first example of a well-characterized mononuclear cobalt(II) complex of a monoanionic catecholate ligand. The net reaction of hydrogen atom transfer (HAT) occurs immediately when [PhTt(tBu)]Co(3,5-DBCatH) is exposed to O-2, generating a previously reported five-coordinate Co(II)-semiquinonate complex, [PhTt(tBu)]Co(3,5-DBSQ) (3,5-DBSQ = 3,5-di-tert-butyl-1,2-semiquinonate). At low temperature, O-2 uptake by [PhTt(tBu)]Co(3,5-DBSQ) was observed by electronic absorption spectroscopy. Upon warming to room temperature, [PhTt(tBu)]Co(3,5-DBSQ) decays slowly under an O-2 atmosphere releasing muconic anhydride, an intradiol cleavage product, in 16% yield as previously reported. Kinetic measurements reveal that the decay of [PhTt(tBu)]Co(3,5-DBSQ) under excess O-2 exhibits pseudo-first-order behavior. Mechanistic considerations involving Co(III)-superoxo and Co(III)-alkylperoxo intermediates are presented based on the above-mentioned data and the metal ion effect on the O-2 reactivity.

The iron(II) semiquinonate character within the iron(III) catecholate species has been proposed by numerous studies to account for the O-2 reactivity of intradiol catechol dioxygenases, but a well-characterized iron(II) semiquinonate species that exhibits intradiol cleaving reactivity has not yet been reported. In this study, a detailed electronic structure description of the first iron(II) o-semiquinonate complex, [PhTttBu]Fe(phenSQ) [PhTt(tBu) = phenyltris(tertbutylthiomethyl)borate; phenSQ = 9,10-phenanthrenesemiquinonate; Wang et al. Chem. Commun. 2014, 50, 5871-5873], was generated through a combination of electronic and Mossbauer spectroscopies, SQUID magnetometry, and density functional theory (DFT) calculations. [PhTt(tBu)]Fe(phenSQ) reacts with O-2 to generate an intradiol cleavage product, diphenic anhydride, in 16% yield. To assess the dependence of the intradiol reactivity on the identity of the metal ion, the nickel analogue, [PhTt(tBu)](phenSQ), and its derivative, [PhTtBu]Ni(3,5-DBSQ) (3,5-DBSQ = 3,5-di-tert-butyl-1,2-semiquinonate), were prepared and characterized by X-ray crystallography, mass spectrometry, H-1 NMR and electronic spectroscopies, and SQUID magnetometry. DFT calculations, evaluated on the basis of the experimental data, support the electronic structure descriptions of [PhTt(tBu)]Ni(phenSQ) and [PhTt(tBu)]Ni(3,5-DBSQ) as high-spin nickel(II) complexes with antiferromagnetically coupled semiquinonate ligands. Unlike its iron counterpart, [PhTt(tBu)](phenSQ) decomposes slowly in an O-2 atmosphere to generate 14% phenanthrenequinone with a negligible amount of diphenic anhydride. [PhTt(tBu)]Ni(3,5-DBSQ) does not react with O-2. This dramatic effect of the metal-ion identity supports the hypothesis that a metal(III) alkylperoxo species serves as an intermediate in the intradiol cleaving reactions. The redox properties of all three complexes were probed using cyclic voltammetry and differential pulse voltammetry, which indicate an inner-sphere electron-transfer mechanism for the formation of phenanthrenequinone. The lack of O-2 reactivity of [PhTt(tBu)]Ni(3,5-DBSQ) can be rationalized by the high redox potential of the metal-ligated 3,5-DBSQ/3,5-DBQ couple.

A series of five-coordinate M-II-semiquinonate (M = Fe, Mn, Co) complexes were synthesized and characterized, including the first example of a mononuclear Fe-II-semiquinonate. Intermediates were observed in the reactions of M-II-phenSQ (M = Fe, Co) with O-2. Evidence for the relevance of these intermediates to the intradiol catechol dioxygenases was obtained by characterization of the oxidized semiquinone-derived product, muconic anhydride, resulting from the reaction of [PhTt(tBu)] Co-II(3,5-DBSQ) with O-2.

A series of enantiomerically pure mono- and bis-aryl tert-butyl sulfoxides were synthesised to promote the enantioselective allylation of aldehydes with allyltrichlorosilane. Moderate to good yields and modest to high enantioselectivities were achieved. The absence of nonlinear effect, spacer effect, promoter loading and concentration effect indicate that only one molecule of aryl tert-butyl sulfoxide is involved in the stereodetermining step.

Enzymatic ring-opening polymerization of cyclic phosphate was achieved in this study. NMR spectra experiments showed that the polyphosphate prepared by CL-catalyzed ring-opening polymerization had hydroxy and phosphoric acid end-groups at both termini. The influence of different reaction factors such as reaction temperature, enzyme concentration and polymerization time was studied. Six phosphates with different alkyl groups were polymerized in bulk by using three different enzymes. The result indicated that the length of alkyl groups made no marked influence on D-p but affected the yield greatly. The longer the length of alkyl groups, the stronger the lipophilicity of the polyphosphates is. Porcine pancreas lipase (PPL) and candida rugosa lipase (CL) exhibited better activity than akaline phosphatase (AP) in this polymerization reaction.