Studies in the coordination chemistry of certain transition metals

A number of new adducts of the metal-halogen cluster Mo₆Cl₈⁴⁺ have been prepared and characterized. Monodentate ligands generally yield complexes of the type [(Mo₆Cl₈)Cl₄2L]; these compounds show only low ionization in donor solvents and are amorphous to X-rays. On the other hand, bidentate ligands form crystalline derivatives of composition [(Mo₆Cl₈)Cl₄(L)₂]Cl₂ or [(Mo₆Cl₈)Cl₃L]Cl. The noteworthy features of these adducts is the characteristic six-fold coordination to the cluster, irrespective of ligand type. It was found that rhenium trichloride and tribromide had similar visible absorbtion spectra to the Re₃Cl₁₂³- anion and presumably comparable trinuclear structures, Re₃x₉. This was substantiated by the isolation of the halogene-anions Re₃Cl₁₁²-, Re₃Br₁₂³-, Re₃Br₁₁²- and Re₃Br₁₀- from these tribaludes. The composition of these anions can be correlated with the cation size and charge, and their formation suggests that the terminal in plane halogen atoms are extremely labile, Radiochemical exchange and reaction with thiocyanate ion confirmed this deduction and demonstrated that the six out of plane halogen atoms also are relatively labile, as compared with the kinetically inert bridging halogens. Thus the basic unit which determines the chemical and physical properties if these clusters is Re₃x₃. The niobium and tantalum polynuclear halides, Nb₆Cl₁₄7H₂0 and Ta₆Cl₁₄7H₂0, do not coordinate as easily as the cluster Mo₆Cl₈⁴⁺ with monodentate and bidentate ligand, and the products which were obtained, namely M₆Cl₁₄(pyr)₂ and M₆Cl₁₄(pyr-N-oxide) were less amenable to study. Finally, the common bonding, Structural and spectral features of these metal-halogen clusters have been discussed

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

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 H2O2 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, H2O2 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 1H 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 H2O2 required decreases from 6–8 equiv. with the pyridin-2-yl based ligands to 1–1.5 equiv. with pyridine-2-carboxylic acid.

Manganese catalyzed cis-dihydroxylation of electron deficient alkenes with H2O2

A practical method for the multigram scale selective cis-dihydroxylation of electron deficient alkenes such as diethyl fumarate and N-alkyl and N-aryl-maleimides using H2O2 is described. High turnovers (>1000) can be achieved with this efficient manganese based catalyst system, prepared in situ from a manganese salt, pyridine-2-carboxylic acid, a ketone and a base, under ambient conditions. Under optimized conditions, for diethyl fumarate at least 1000 turnovers could be achieved with only 1.5 equiv. of H2O2 with d/l-diethyl tartrate (cis-diol product) as the sole product. For electron rich alkenes, such as cis-cyclooctene, this catalyst provides for efficient epoxidation.

Oxidation of Alkenes with H2O2 by an in-Situ Prepared Mn(II)/Pyridine-2-carboxylic Acid Catalyst and the Role of Ketones in Activating H2O2

A simple, high yielding catalytic method for the multigram scale selective epoxidation of electron-rich alkenes using near-stoichiometric H2O2 under ambient conditions is reported. The system consists of a Mn(II) salt (<0.01 mol %), pyridine-2-carboxylic acid (<0.5 mol %), and substoichiometric butanedione. High TON (up to 300 000) and TOF (up to 40 s−1) can be achieved for a wide range of substrates with good to excellent selectivity, remarkable functional group tolerance, and a wide solvent scope. It is shown that the formation of 3-hydroperoxy-3-hydroxybutan-2-one from butanedione, and H2O2 in situ, is central to the activity observed.