Cobalt(III) alkylperoxy complexes. Synthesis, x-ray structure, and role in the catalytic decomposition of alkyl hydroperoxides and in the hydroxylation of hydrocarbons

Abstract

Novel cobalt(II1) alkylperoxy complexes with the general formula Co(R"BPI)(OCOR')(OOR) [type I, R" = H, Me; BPI = 1,3-bis(2'-pyridylimino)isoindoline, R' = Me, Ph, t-Bu; R = t-Bu, CMe2Ph] and Co(BPB)(OOt-Bu)(4-Mepy) [type 11, BPB = N,N'-bis(2'-pyridinecarboxamide)-1,2-benzene] were synthesized from the reaction of alkyl hydroperoxides with their Co(I1) precursor, and characterized by physicochemical methods. The X-ray structure of Ib (R" = H, R' = Ph, R = t-Bu) revealed a distorted octahedral environment with a chelating carboxylate moiety and an apically bonded tert-butylperoxy group. The reactivity of cobalt(II1) alkylperoxy complexes toward hydrocarbons was found to be dependent on their thermal decomposition rate, with type I complexes being the most reactive ones. Saturated hydrocarbons are oxidized by Co(II1) tert-butylperoxy complexes into alcohols, ketones, and tert-butylperoxy products. The hydroxylation reaction preferentially occurs at the more nucleophilic C-H bonds with extensive epimerization at the hydroxylated carbon atom. Hydrocarbons having labile allylic or benzylic C-H bonds react beginning at room temperature to give a large amount of allylic or benzylic tert-butyl peroxide. Olefins having no allylic hydrogen atom are preferentially transformed into epoxides. The catalytic hydroxylation of alkanes by t-BuOOH in the presence of complexes 1-111 has the same characteristics as the stoichiometric reaction. The homolytic decomposition and reactivity of cobalt(II1) alkylperoxy complexes is discussed in the context of the Haber-Weiss mechanism of alkyl hydroperoxide decomposition and of hydrocarbon hydroxylation by first-row transition-metal peroxides.

Investigations into the Coordination Chemistry of 1,3-Bis(2’-benzimidazolyl­imino)isoindoline

“…These data provide important structural insight for DII based ligand systems and build on the synthetic work from Linstead, Speier, and others. [1,[25][26][27][28][29][30][31][32][33][34][35] These compounds may have useful applications in formation of new phthalocyanine-like chelates and resultant metal complexes and all of the compounds presented show significant hydrogen bonding in the solid-state. Our group will continue to explore the chemistry of DII in an effort to produce interesting ligand systems and new metal complexes.…”

Pyrazolate‐Based Cobalt(II)‐Containing Metal–Organic Frameworks in Heterogeneous Catalytic Oxidation Reactions: Elucidating the Role of Entatic States for Biomimetic Oxidation Processes

“…Usually, cobaltA C H T U N G T R E N N U N G (III) peroxo complexes of, for example, N,N'-ethylenebis(salicylimine) (salen) or porphyrin complexes are kinetically inert due to their low-spin d 6 configuration, thus a large number of such complexes have been isolated and characterised at room temperature. [63] However, due to steric hindrance and constraints upon the possible distortion of individual coordination environments of the cobalt centres by the 3D network, coordination of an additional ligand would constitute an energetically unfavourable situation, and thus a strained or entatic state as described above. A similar stabilisation is also seen in structurally related tetrahedral cobalt of tridentate scorpionate ligands comprising bulky substituents at the pyrazolate moieties.…”

Heterogeneous Catalytic Oxidation by MFU‐1: A Cobalt(II)‐Containing Metal–Organic Framework

“…In contrast to numerous reports on the oxidation, catalytic or otherwise, of cyclohexane by cobalt(II) complexes, [24] the corresponding transformation of cyclohexene has been explored to lesser extent. [25] The experimental data demonstrate that oxidation of cyclohexene is fast, and several turnovers are possible in the presence of MFU-1 (Figure 3 a), whereas the reaction occurring in absence of catalyst under the same conditions is almost negligible (ca.…”

“…To our knowledge, MFU-1 is the first example for which this feature has been unequivocally demonstrated. The fact that tert-butyl-2-cyclohexenyl-1-peroxide is the main reaction product suggests a reaction pathway in which freely diffusing peroxy radicals are generated by reductive cleavage of tertbutyl hydroperoxo ligands coordinating to cobalt(III) metal centers [24] (see the Supporting Information). UV/Vis and XPS measurements provide further evidence for the presence of cobalt(III) after long catalytic runs, whereas only cobalt(II) is present initially (Supporting Information, Figures S9, S17).…”