Synthesis, characterization and O2 reactivity of a bioinspired cobalt(II)-catecholate complex

Predicting valence tautomerism in diverse cobalt-dioxolene complexes: elucidation of the role of ligands and solvent

The ability of molecular switches to reversibly interconvert between different forms promises potential applications at the scale of single molecules up to bulk materials. One type of molecular switch comprises cobalt-dioxolene compounds that exhibit thermally-induced valence tautomerism (VT) interconversions between low spin Co(iii)-catecholate (LS-CoIII-cat) and high spin Co(ii)-semiquinonate (HS-CoII-sq) forms. Two families of these compounds have been investigated for decades but have generally been considered separately: neutral [Co(diox)(sq)(N2L)] and cationic [Co(diox)(N4L)]+ complexes (diox = generic dioxolene, N2L/N4L = bidentate/tetradentate N-donor ancillary ligand). Computational identification of promising new candidate compounds prior to experimental exploration is beneficial for environmental and cost considerations but requires a thorough understanding of the underlying thermochemical parameters that influence the switching. Herein, we report a robust approach for the analysis of both cobalt-dioxolene families, which involved a quantitative density functional theory-based study benchmarked with reliable quasi-experimental references. The best-performing M06L-D4/def2-TZVPP level of theory has subsequently been verified by the synthesis and experimental investigation of three new complexes, two of which exhibit thermally-induced VT, while the third remains in the LS-CoIII-cat form across all temperatures, in agreement with prediction. Valence tautomerism in solution is markedly solvent-dependent, but the origin of this has not been definitively established. We have extended our computational approach to elucidate the correlation of VT transition temperature with solvent stabilisation energy and change in dipole moment. This new understanding may inform the development of VT compounds for applications in soft materials including films, gels, and polymers.

Synthesis and Characterization of a Masked Terminal Nickel-Oxide Complex

In exploring terminal nickel-oxo complexes, postulated to be the active oxidant in natural and non-natural oxidation reactions, we report the synthesis of the pseudo-trigonal bipyramidal NiII complexes (K)[NiII (LPh )(DMF)] (1[DMF]) and (NMe4 )2 [NiII (LPh )(OAc)] (1[OAc]) (LPh =2,2',2''-nitrilo-tris-(N-phenylacetamide); DMF=N,N-dimethylformamide; - OAc=acetate). Both complexes were characterized using NMR, FTIR, ESI-MS, and X-ray crystallography, showing the LPh ligand to bind in a tetradentate fashion, together with an ancillary donor. The reaction of 1[OAc] with peroxyphenyl acetic acid (PPAA) resulted in the formation of [(LPh )NiIII -O-H⋅⋅⋅OAc]2- , 2, that displays many of the characteristics of a terminal Ni=O species. 2 was characterized by UV-Vis, EPR, and XAS spectroscopies and ESI-MS. 2 decayed to yield a NiII -phenolate complex 3 (through aromatic electrophilic substitution) that was characterized by NMR, FTIR, ESI-MS, and X-ray crystallography. 2 was capable of hydroxylation of hydrocarbons and epoxidation of olefins, as well as oxygen atom transfer oxidation of phosphines at exceptional rates. While the oxo-wall remains standing, this complex represents an excellent example of a masked metal-oxide that displays all of the properties expected of the ever elusive terminal M=O beyond the oxo-wall.

Iron(II)-alkoxide and -aryloxide complexes of a tris(thioether)borate ligand: synthesis, molecular structures, and implications on the origin of instability of their iron(II)-catecholate counterpart

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 Co^II-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 Fe^II-alkoxide and -aryloxide complexes, namely, (triphenylmethoxo){tris[(tert-butylsulfanyl)methyl]phenylborato-κ³S,S',S''}iron(II), [Fe(C₂₁H₃₈BS₃)(C₁₉H₁₅O)], (2), (2,6-dimethylphenolato){tris[(tert-butylsulfanyl)methyl]phenylborato-κ³S,S',S''}iron(II), [Fe(C₂₁H₃₈BS₃)(C₈H₉O)], (3), and bis{μ-tris[(tert-butylsulfanyl)methyl]phenylborato-κ³S,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).

Molecular and Electronic Structures and Single-Molecule Magnet Behavior of Tris(thioether)-Iron Complexes Containing Redox-Active α-Diimine Ligands

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-PhTt^tBu]Fe(AdNCHCHNAd) (1), and its analogue [κ²-PhTt^tBu]Fe(CyNCHCHNCy) (2) (PhTt^tBu = 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.

Cobalt Superoxo and Alkylperoxo Complexes Derived from Reaction of Ring-Cleaving Dioxygenase Models with O2

The syntheses and O₂ reactivities of active-site models of cobalt-substituted ring-cleaving dioxygenases are presented. The pentacoordinate cobalt(II)-aminophenolate complex, [Co(Tp^Me2)(^tBu2APH)], gives rise to two distinct dioxygen adducts at reduced temperatures. The first is a paramagnetic (S = 1/2) cobalt(III)-superoxo species that was characterized with spectroscopic and computational techniques. The identity of the second Co/O₂ adduct was elucidated by X-ray crystallography, which revealed an unprecedented cobalt(III)-alkylperoxo structure generated by O₂ addition to the metal ion and ligand. These results provide synthetic precedents for proposed intermediates in the catalytic cycles of O₂-activating cobalt enzymes.