Synthesis and characterization of cobalt(II)–salicylate complexes derived from N4-donor ligands: Stabilization of a hexameric water cluster in the lattice host of a cobalt(III)–salicylate complex

Selective oxygenation of C-H and CC bonds with H2O2 by high-spin cobalt(II)-carboxylate complexes

Four cobalt(II)-carboxylate complexes [(6-Me₃-TPA)Co^II(benzoate)](BPh₄) (1), [(6-Me₃-TPA)Co^II(benzilate)](ClO₄) (2), [(6-Me₃-TPA)Co^II(mandelate)](BPh₄) (3), and [(6-Me₃-TPA)Co^II(MPA)](BPh₄) (4) (HMPA = 2-methoxy-2-phenylacetic acid) of the 6-Me₃-TPA (tris((6-methylpyridin-2-yl)methyl)amine) ligand were isolated to investigate their ability in H₂O₂-dependent selective oxygenation of C-H and CC bonds. All six-coordinate complexes contain a high-spin cobalt(II) center. While the cobalt(II) complexes are inert toward dioxygen, each of these complexes reacts readily with hydrogen peroxide to form a diamagnetic cobalt(III) species, which decays with time leading to the oxidation of the methyl groups on the pyridine rings of the supporting ligand. Intramolecular ligand oxidation by the cobalt-based oxidant is partially inhibited in the presence of external substrates, and the substrates are converted to their corresponding oxidized products. Kinetic studies and labelling experiments indicate the involvement of a metal-based oxidant in affecting the chemo- and stereo-selective catalytic oxygenation of aliphatic C-H bonds and epoxidation of alkenes. An electrophilic cobalt-oxygen species that exhibits a kinetic isotope effect (KIE) value of 5.3 in toluene oxidation by 1 is proposed as the active oxidant. Among the complexes, the cobalt(II)-benzoate (1) and cobalt(II)-MPA (4) complexes display better catalytic activity compared to their α-hydroxy analogues (2 and 3). Catalytic studies with the cobalt(II)-acetonitrile complex [(6-Me₃-TPA)Co^II(CH₃CN)₂](ClO₄)₂ (5) in the presence and absence of externally added benzoate support the role of the carboxylate co-ligand in oxidation reactions. The proposed catalytic reaction involves a carboxylate-bridged dicobalt complex in the activation of H₂O₂ followed by the oxidation of substrates by a metal-based oxidant.

Oxidative C-N bond cleavage of (2-pyridylmethyl)amine-based tetradentate supporting ligands in ternary cobalt(ii)-carboxylate complexes

Three mononuclear cobalt(ii)-carboxylate complexes, [(TPA)Co^II(benzilate)]⁺ (1), [(TPA)Co^II(benzoate)]⁺ (2) and [(iso-BPMEN)Co^II(benzoate)]⁺ (3), of N4 ligands (TPA = tris(2-pyridylmethyl)amine and iso-BPMEN = N¹,N¹-dimethyl-N²,N²-bis((pyridin-2-yl)methyl)ethane-1,2-diamine) were isolated to investigate their reactivity toward dioxygen. Monodentate (η¹) binding of the carboxylates to the metal centre favours the five-coordinate cobalt(ii) complexes (1-3) for dioxygen activation. Complex 1 slowly reacts with dioxygen to enable the oxidative decarboxylation of the coordinated α-hydroxy acid (benzilate). Prolonged exposure of the reaction solution of 2 to dioxygen results in the formation of [(DPA)Co^III(picolinate)(benzoate)]⁺ (4) and [Co^III(BPCA)₂]⁺ (5) (DPA = di(2-picolyl)amine and HBPCA = bis(2-pyridylcarbonyl)amide), whereas only [(DPEA)Co^III(picolinate)(benzoate)]⁺ (6) (DPEA = N¹,N¹-dimethyl-N²-(pyridine-2-ylmethyl)-ethane-1,2-diamine) is isolated from the final oxidised solution of 3. The modified ligand DPA (or DPEA) is formed via the oxidative C-N bond cleavage of the supporting ligands. Further oxidation of the -CH₂- moiety to -C([double bond, length as m-dash]O)- takes place in the transformation of DPA to HBPCA on the cobalt(ii) centre. Labelling experiments with ¹⁸O₂ confirm the incorporation of oxygen atoms from molecular oxygen into the oxidised products. Mixed labelling studies with ¹⁶O₂ and H₂O¹⁸ strongly support the involvement of water in the C-N bond cleavage pathway. A comparison of the dioxygen reactivity of the cobalt complexes (1-3) with those of several other five-coordinate mononuclear complexes [(TPA)Co^II(X)]^+/2+ (X = Cl, CH₃CN, acetate, benzoylformate, salicylate and phenylpyruvate) establishes the role of the carboxylate co-ligands in the activation of dioxygen and subsequent oxidative cleavage of the supporting ligands by a metal-oxygen oxidant.

A family of complexes with N-scorpionate-type and other N-donor ligands obtained in situ from pyrazole derivative and zerovalent cobalt. Physicochemical and cytotoxicity studies

In this paper we described one pot synthetic pathways which generated in situ three complexes which contain three different ligands: N,N,N-tris(3,5-dimethylpyrazolylmethyl)amine (L¹), urotropine (L²) and 3,5-dimethylpyrazole (L³).