An unusual but informative synthesis and the crystal structure of [Co(tpaCO2)Cl](ClO4) (tpaCO2−=6-carboxylato-2- (pyridylmethyl)-bis(2-pyridylmethyl)amine)

Cleaner and stronger: how 8-quinolinolate facilitates formation of Co(III)-thiolate from Co(II)-disulfide complexes

The formation of Co(III)-thiolate complexes from Co(II)-disulfide complexes using the anionic ligand 8-quinolinolate (quin^-) has been studied experimentally and quantum chemically. Two Co(II)-disulfide complexes [Co₂(L^xSSL^x)(Cl)₄] (x = 1 or 2; L¹SSL¹ = 2,2'-disulfanediylbis(N,N-bis(pyridin-2-ylmethyl)ethan-1-amine; L²SSL² = 2,2'-disulfanedylbis (N-((6-methylpyridin-2-yl)methyl)-N-(pyridin-2-ylmethyl) ethan-1-amine) have been successfully converted with high yield to their corresponding Co(III)-thiolate complexes upon addition of the ligand 8-quinolinolate. Using density functional theory (DFT) computations the d-orbital splitting energies of the cobalt-thiolate compounds [Co(L¹S)(quin)]⁺ and [Co(L²S)(quin)]⁺ were estimated to be 3.10 eV and 3.07 eV, indicating a slightly smaller ligand-field strength of ligand L²SSL² than of L¹SSL¹. Furthermore, the orientation of the quin^- ligand in the thiolate compounds determines the stability of the thiolate complex. DFT computations show that the thiolate structure benefits from more electrostatic attraction when the oxygen atom of the quin^- ligand is positioned trans to the sulfur atom of the [Co(L¹S)]²⁺ fragment. Quin^- is the first auxiliary ligand with which it appeared possible to induce the redox-conversion reaction in cobalt(II) compounds of the relatively weak-field ligand L²SSL².

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.

Unraveling the origins of catalyst degradation in non-heme iron-based alkane oxidation

A series of iron(ii) complexes with tetradentate and pentadentate pyridyl amine ligands has been used for the oxidation of cyclohexane with hydrogen peroxide. Ligand degradation is observed under oxidising conditions via oxidative N-dealkylation.