117897-01-5Relevant academic research and scientific papers
Dinuclear Iron(III) and Cobalt(III) Complexes Featuring a Biradical Bridge: Their Molecular Structures and Magnetic, Spectroscopic, and Redox Properties
Mondal, Dhrubajyoti,Majee, Mithun Chandra,Kundu, Sanchita,M?rtel, Max,Abbas, Ghulam,Endo, Akira,Khusniyarov, Marat M.,Chaudhury, Muktimoy
, p. 1004 - 1016 (2018)
Bis-bidentate ligand H4LB featuring two o-amidophenol noninnocent units was used to synthesize novel binuclear complexes [(LR)MIII(?LB?)MIII(LR)](ClO4)2, M = Fe (1) and Co (2, 3), with HLR (R = CH3, Cl) being the facially coordinating tetradentate coligands. Upon the synthesis, the fully reduced amidophenolate form of the ligand (LB)4- becomes oxidized, resulting in the formation of a rare example of a biradical (?LB?)2- bridge connecting two metal ions, as supported by X-ray crystallography. The electronic structures of the complexes have been probed by M?ssbauer spectroscopy, magnetic susceptibility measurements, and electron paramagnetic resonance (EPR) spectroscopy. Species 1 contains two high-spin Fe(III) ions (S = 5/2) each coupled strongly antiferromagnetically (|J| > 150 cm-1 ? = -2J?1?2) with a semiquinone ?-radical (S = 1/2) form of the bridging (?LB?)2- ligand. The effective S = 2 spins of each [Fe(III)+R?] monomeric unit are then weakly ferromagnetically coupled with J = +0.22 cm-1. Species 2 and 3 reveal very similar electronic structures: the low-spin Co(III) ion is diamagnetic, which leaves the two-spin carriers at the bridging (?LB?)2- biradical to display an isotropic EPR signal at g = 1.995 for 2 (1.993 for 3) in solution at room temperature and in the frozen state with no hyperfine structure. The weak half-field signal at g = 3.988 for 2 (3.978 for 3) was also observed at 17 K for the spin-forbidden |ΔMS| = 2 transition due to ferromagnetically coupled S = 1/2 spins (J = +47 cm-1) of the bridging biradical. The compounds show rich electrochemistry, displaying two (1) or four (2, 3) one-electron reversible processes. Normal and differential pulse voltammetry as well as constant potential coulometry, combined with EPR experiments, confirmed that the observed electron transfers are all localized at the bridging noninnocent (?LB?)2- ligand.
Functional Models for Catechol 1,2-Dioxygenase. The Role of the Iron(III) Center
Cox, David D.,Que, Lawrence
, p. 8085 - 8092 (1988)
A series of complexes, where L is a tetradentate tripodal ligand and DBC is 3,5-di-tert-butylcatecholate, has been prepared to serve as functional mimics for the catechol dioxygenases.The tripodal ligands, (RCH2)2N(CH2R')(NTA, R = R'= COO-; PDA, R =COO-, R'= 2-pyridyl; BPG, R = 2-pyridyl, R'= COO-; HDP, R = 2-pyridyl, R'= 2-hydroxy-3,5-dimethylphenyl), serve to tune the Lewis acidity of the ferric center.This tuning is manifested in differences in the energies of the catecholate-to-Fe(III) charge-transfer bands, the potentials of the semiquinone/catecholate couple, and the shifts of the DBC protons found for the complexes.These complexes react with O2 with high specifity to yield a product resulting from C1-C2 oxidative cleavage.Kinetic studies show that the rate-determining step involves the attack of dioxygen on the complex, and the rates of reaction of the complexes increase in the order of HDP, NTA, PDA, and BPG.This order in general follows the increase in Lewis acidity of the ferric center but more specifically correlates with increasing semiquinone character on the DBC ligand as indicated by the NMR shifts.As a further check, .2CH3OH (P21/n) was crystallographically characterized and compared with the previously reported 2-.The structures are similar; both are six-coordinate high-spin ferric complexes with unsymmetrically chelated DBC ligands (ΔrFe-O(DBC) = 0.1 angstroem for both).Taken together, these observations suggest that the substrate activation proposed for the dioxygenase mechanism results from the delocalization of unpaired spin density from the ferric center onto the coordinated catecholate arising from ligand-to-metal charge transfer.
