The unexpected role of pyridine-2-carboxylic acid in manganese based oxidation catalysis with pyridin-2-yl based ligands

Abstract

A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl groups have been reported previously to achieve both high turnover numbers and selectivity in the oxidation of alkenes and alcohols, using H(2)O(2) as terminal oxidant. Here we report our recent finding that these ligands decompose in situ to pyridine-2-carboxylic acid and its derivatives, in the presence of a manganese source, H(2)O(2) and a base. Importantly, the decomposition occurs prior to the onset of catalysed oxidation of organic substrates. It is found that the pyridine-2-carboxylic acid formed, together with a manganese source, provides for the observed catalytic activity. The degradation of this series of pyridyl ligands to pyridine-2-carboxylic acid under reaction conditions is demonstrated by (1)H NMR spectroscopy. In all cases the activity and selectivity of the manganese/pyridyl containing ligand systems are identical to that observed with the corresponding number of equivalents of pyridine-2-carboxylic acid; except that, when pyridine-2-carboxylic acid is used directly, a lag phase is not observed and the efficiency in terms of the number of equivalents of H(2)O(2) required decreases from 6-8 equiv. with the pyridin-2-yl based ligands to 1-1.5 equiv. with pyridine-2-carboxylic acid.

OO Bond Formation Mediated by a Hexanuclear Iron Complex Supported on a Stannoxane Core

“…Similar results were reported previously for related systems. [8] Slow evaporation of the solvent from a CH 3 CN solution of the yellow oil resulted in deep red crystals, which when analyzed by X-ray crystallography established the molecular structure as [Fe Figure S4, Table S7). [6] The hexanucleating ligand 1 when reacted with 6 eqv of FeA C H T U N G T R E N N U N G (OTf) 2 in CH 2 Cl 2 yielded the metalated species 2 as a yellowish-green powder (60 % yield).…”

O2 Activation and Double CH Oxidation by a Mononuclear Manganese(II) Complex

“…This was confirmed by the disappearance of the ESR signal associated with the presence of Mn II ( Figure S27). Thes tructure is complicated by disorder.T he CÀOb ond lengths of 1.425(9) f or the major component and 1.408 (11) for aminor disorder component are consistent with aC ÀOs ingle bond and are about 0.2 greater than the C = Ob onds in [MnBr 2 (hidpe) 2 ]. In the absence of ethyl acetate or when the reaction was performed in pure acetonitrile,n o crystals were obtained.…”

“…[7] Thec atalytic reduction of O 2 to H 2 O 2 by am anganese thiolate complex via an intermediate mono-m-hydroxo-bridged Mn III dimer was demonstrated by Duboc and co-workers. [10] CÀHo xidation at benzylic positions is known to occur in the presence of Mn and H 2 O 2 , [11] and some reactive intermediates have been spectroscopically observed at low temperatures. Whereas there are several reports on iron-based complexes, [9] examples with Mn have remained rare.…”

Molecular Iron-Based Oxidants and Their Stoichiometric Reactions

“…Alternatively if the amine group is linked to a methylpyridine or a methylbenzyl group, a cleavage reaction can occur with the formation of an amide complex, or it can perhaps further oxidized to a picolinate complex. Formation of stable picolinate complexes of transition metal ions derived from more elaborate ligands have been reported on several occasions [152,153,163,164]. There is no reason to assume that iron complexes will be immune to this particular ligand degradation.…”