Cobalt complexes with redox-active anthraquinone-type ligands

The strong in vitro and vivo cytotoxicity of three new cobalt(II) complexes with 8-methoxyquinoline

Three new cobalt(II) complexes, [Co(MQL)₂Cl₂] (CoCl), [Co(MQL)₂Br₂] (CoBr), and [Co(MQL)₂I₂] (CoI), bearing 8-methoxyquinoline (MQL) have been designed for the first time. MTT assays showed that CoCl, CoBr, and CoI exhibit much better antiproliferative activities than cisplatin toward cisplatin-resistant SK-OV-3/DDP and SK-OV-3 ovarian cancer cells, with IC₅₀ values of as low as 0.32-5.49 μM. Further, CoCl and CoI can regulate autophagy-related proteins in SK-OV-3/DDP cells and, therefore, they can induce primarily autophagy-mediated cell apoptosis in the following order: CoCl > CoI. The different antiproliferative activities of the MQL complexes CoCl, CoBr, and CoI could be correlated with the lengths of their Co-X bonds, which adopted the following order: CoI > CoBr > CoCl. The 8-HOMQ complexes CoCl (ca. 60.1%) and CoI (ca. 48.8%) also showed potent in vivo anticancer effects after 15 days of treatment. In summary, the MQL ligand highly enhances the antiproliferative activities of cobalt(II) complexes in comparison to other previously reported 8-hydroxyquinoline metal complexes.

Tunable carbocation-based redox active ambiphilic ligands: synthesis, coordination and characterization

The synthesis of novel redox active ambiphilic ligands L1-L3 and their coordination chemistry to first-row late transition metal halides (M = Co and Ni) is reported. The heterocyclic carbocation scaffolds act as Lewis acid moieties while the pyridine anchor acts as the coordinating Lewis base. The high synthetic tunability of this ligand scaffold allows for control of its rigidity and electronic properties. Anion exchange and coordination of a chloride anion to the metal center was observed resulting in the formation of [MCl3]- metallate. Upon coordination to the pyridine anchor, the metallate centers adopt a canonical tetrahedral geometry, resulting in an overall neutral complex best described as a zwitterionic metallate trichloride bound to a cationic ligand. Characterization techniques including single crystal X-ray diffraction, cyclic voltammetry, and UV-Vis absorption spectroscopy were employed to better understand the structural and chemical properties of the ligands and metal complexes. A possible weak interaction between one of the chlorides and the carbenium moiety in the ligand is observed in crystals of both of the Co(ii) and Ni(ii) complexes with ligand L1. Density functional theory (DFT) calculations support that this electrostatic interaction for complexes 2a and 2b exists only in the solid state.

Electrochemical properties and catalytic reactivity of cobalt complexes with redox-active meso-substituted porphycene ligands

The cobalt complexes of meso-aryl substituted porphycenes were synthesized and characterized. The reduction potentials of the complexes were shifted to the positive side depending on the strength of the electron-withdrawing properties of the meso-substituents, while the optical properties, such as the absorption spectra of these complexes, were similar. This suggests that the energy levels of the molecular orbitals of the complexes were changed by the meso-substituents while the gaps of the orbitals were not significantly changed. The one-electron reduction of the complex did not afford the Co(I) species, but the ligand-reduced radical anion, which was characterized by electrospectrochemistry. The generated ligand-reduced species reacted with alkyl halides to form the Co(III)-alkyl complex. As a result, the reduction potential of the electrolytic reaction could be directly controlled by the substituents of the porphycene. The catalytic reaction with trichloromethylbenzene was also performed and it was found that the ratio of the obtained products was changed by the reduction potentials of the catalyst, [Formula: see text]. the cobalt porphycenes.

The (Dioximate)NiII/I2 System: Ligand Oxidation and Binding Modes of Triiodide Species

Reinvestigation of (o-benzoquinonedioximate)₂Ni/I₂ systems demonstrated that the reaction itself and also the crystallization conditions dramatically affect the identity of generated species. Crystallization (CHCl₃, 20-25 °C) of the nickel(II) dioximate complex [Ni(bqoxH)₂] (bqoxH₂ = o-benzoquinonedioxime) with I₂ in the 1:(1-10) molar ratios of the reactants led to several (o-benzoquinonedioximate)₂Ni derivatives and/or iodine adducts [Ni(I)(bqoxH)(bqoxH₂)]·³/₂I₂, [Ni(I₃)(bqoxH)(bqoxH₂)]·[Ni(bqoxH)₂], and [Ni(I₃)(bqox^•-)(bqoxH₂)]·I₂; the latter one, featuring the anion-radical bqox^•- ligand, is derived from the formal (-2H⁺/1e^-)-oxidation of bqoxH₂. In these three adducts, various types of noncovalent interactions were identified experimentally and their existence was supported theoretically. The [Ni(I₃)(bqox^•-)(bqoxH₂)]·I₂ adduct exhibits simultaneous semicoordination and coordination patterns of the triiodide ligand; this is the first recognition of the semicoordination of any polyiodide ligand to a metal center. The semicoordination noncovalent contact Ni···I₃ (3.7011(10) Å) is substantially longer that the Ni-I₃ coordination bond (2.8476(9) Å), and the difference in energies between these two types of linkages is 8-12 kcal/mol.

Cobalt complexes with hemilabile o-iminobenzoquinonate ligands: a novel example of redox-induced electron transfer

The tetracoordinated square-planar CoIII complex (imSQC(O)Ph)CoIII(APC(O)Ph) (1) bearing a radical anion and the closed-shell o-amidophenolate forms of the functionalized o-aminophenol H2LC(O)Ph were synthesized. The intermediate spin state (SCo = 1) CoIII center was found for compound 1. The cyclic voltammogram of derivative 1 contains two oxidative processes and one reductive redox process as well as an additional multi-electron wave at high negative potentials above -2 V, which can involve both the ligand and metal center. One-electron oxidation of 1 by silver triflate produces the [(imSQC(O)Ph)CoII(imQC(O)Ph)]OTf·2toluene (2) derivative with the trigonal prismatic coordination environment of the metal arising from the additional coordination of -C(O)Ph hemilabile groups. This is a first example of a trigonal prismatic coordination polyhedron in cobalt-based complexes featuring o-iminobenzoquinone ligands. The trigonal prismatic geometry achieved by the unique flexibility of the ligand allows metal-to-ligand redox-induced electron transfer (RIET). Chemical oxidation of complex 1 promotes the reduction of CoIII to CoII in compound 2 due to the redox-active nature of o-iminobenzoquinonate ligands. Remarkably, this is the first example of RIET in cobalt-based derivatives with this type of ligand. The oxidative states of the ligands and cobalt ion in both complexes were unequivocally established according to the X-ray data collection by using the utility of "metric oxidation state" (MOS). The spin states of the metal centers were unambiguously determined by density functional theory. The strong antiferromagnetic exchange via metal-ligand interactions is dominant in compounds 1 and 2, giving the doublet (S = 1/2) and triplet (S = 1) ground spin state, respectively.