5,5'-Diamino-2,2'-bipyridine: a versatile building block for the synthesis of bipyridine/catechol ligands that form homo- and heteronuclear helicates

Herein we present an improved synthesis of 5,5'-diamino-2,2'-bipyridine (1) starting from the pyrrole-protected aminopyridine 4. By standard reactions 1 can easily be transformed into the imine- or amide-bridged dicatechol-bipyridine ligands L1-H4 and L2-H4. Whereas ligand L1 readily forms homodinuclear helicates [(L1)3Ti2]4-, the attempted formation of mono-, tri-, or even oligonuclear coordination compounds from this ligand did not work. However, the amide-connected ligand L2 affords mononuclear ([(L2-H4)PdCl2], [(L2-H4)3Zn]2+), dinuclear ([(L2)3Ti2]4-), and heterotrinuclear coordination compounds ([(L2)3Ti2Zn]2-).     

Cobalt Complexes with Pyrazole Ligands as Catalyst Precursors for the Peroxidative Oxidation of Cyclohexane: X‐ray Absorption Spectroscopy Studies and Biological Applications

[CoCl(μ‐Cl)(Hpz^Ph)₃]₂ ( 1 ) and [CoCl₂(Hpz^Ph)₄] ( 2 ) were obtained by reaction of CoCl₂ with HC(pz^Ph)₃ and Hpz^Ph, respectively (Hpz^Ph=3‐phenylpyrazole). The compounds were isolated as air‐stable solids and fully characterized by IR and far‐IR spectroscopy, MS(ESI+/?), elemental analysis, cyclic voltammetry (CV), controlled potential electrolysis, and single‐crystal X‐ray diffraction. Electrochemical studies showed that 1 and 2 undergo single‐electron irreversible Co^II→Co^III oxidations and Co^II→Co^I reductions at potentials measured by CV, which also allowed, in the case of dinuclear complex 1 , the detection of electronic communication between the Co centers through the chloride bridging ligands. The electrochemical behavior of models of 1 and 2 were also investigated by density functional theory (DFT) methods, which indicated that the vertical oxidation of 1 and 2 (that before structural relaxation) affects mostly the chloride and pyrazolyl ligands, whereas adiabatic oxidation (that after the geometry relaxation) and reduction are mostly metal centered. Compounds 1 and 2 and, for comparative purposes, other related scorpionate and pyrazole cobalt complexes, exhibit catalytic activity for the peroxidative oxidation of cyclohexane to cyclohexanol and cyclohexanone under mild conditions (room temperature, aqueous H₂O₂). In situ X‐ray absorption spectroscopy studies indicated that the species derived from complexes 1 and 2 during the oxidation of cyclohexane (i.e., Ox‐ 1 and Ox‐ 2 , respectively) are analogous and contain a Co^III site. Complex 2 showed low in vitro cytotoxicity toward the HCT116 colorectal carcinoma and MCF7 breast adenocarcinoma cell lines.