The article deals with applying NMR and EPR spectroscopy to the study of the active sites of catalysts in two important classes of reactions: enantioselective olefin epoxidation with hydrogen peroxide catalyzed by iron aminopyridine complexes and olefin polymerization in the presence of titanium and vanadium post-metallocene catalysts.

The structures of intermediates generated by the activation of 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridineiron(II) chloride ( 1 ) with various cocatalysts, methylalumoxane (MAO), trimethylaluminum (TMA), and TMA in combination with B(C 6 F 5 ) 3 and Ph 3 CB(C 6 F 5 ) 4 , is studied by 1 H and 2 HNMR spectroscopy. The 1/AlMe 3 system exhibits a higher catalytic activity in ethylene polymerization than the 1/MAO system. The activity of the latter decreases sharply with a decrease in the amount of AlMe 3 in MAO. Neutral Fe(II) complexes rather than cationic intermediates are suggested to be active components in both catalytic systems.

New data on the structure of polymethylaluminoxane[-Al(CH3)O-](n) (MAO), the nature of its Lewis acidity, and the mechanism of formation of the active centers of the metallocene catalysts during interaction between MAO and Cp2ZrMe2 obtained by spectroscopy (Al-27,O-17,H-1, and C-13 NMR and spin probe ESR) and density functional theory (DFT) calculations are considered. A three-dimensional structure of MAO (a cage structure) with four-coordinated aluminum atoms and three-coordinated oxygen atoms is found to he the most probable. Experimental evidence for the presence of relatively weak Lewis acid sites in MAO (about 1 site per 50 atoms of Al) is obtained for the first time. The acidity of MAO depends on the size of a MAO molecule n, and it increases with increasing n. A scheme for the formation and structure of Lewis acid sires formed upon interaction between MAO and Al2Me6, which is always present in the MAO samples is proposed, Cationic intermediates formed in the MAO/Cp2ZrMe2 system at high molar ratios Al/Zr = 200-4000 corresponding to real conditions of polymerization are characterized for the first time. A scheme of equilibria between them and the most probable precursors of the active centers in this system are proposed. A new mechanism for the formation of the [Cp2ZrMe](+)[MAO . Me](-) ionic pair involving the abstraction and occlusion of a methyl ligand in the bulk of a MAO molecule is proposed on the basis of quantum-chemical calculations.

NMR-spectroscopic studies of V-51, (MO)-M-95, Co-59, O-17, and H-1 nuclei present in coordination compounds that catalyze alkene and alkane oxidation with organic hydroperoxides are reviewed. Multinuclear NMR spectroscopy was shown to have advantages in structural studies of selective oxidation intermediates.

The mechanism of formation of propylene glycol monoacetate in the oxidation of propylene by molecular oxygen in a PdCl2/CuCl2/HOAc system was studied. The formation of olefin-palladium-cuprate complexes in the reaction solutions was established using NMR, EPR, and electron spectroscopy methods.

The superoxo complexes of palladium Pd(\(O_2 ^{\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{ - } } \)) and titanium Ti(\(O_2 ^{\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{ - } } \)) interact with spin traps (cyclic aldonitrones) to form spin adducts. The EPR parameters of these spin adducts differ from those of the spin adduct formed by reaction with the free superoxo complex ion, and depend on the nature of the metal.

EPR and NMR studies of solutions of KO 2 in halogenated hydrocarbons at 213-273 K have revealed paramagnetic centers differing form the superoxide ion 02-, the peroxy radical H02, and alkylhalogenoperoxy and alkylperoxy radicals RO='. These paramagnetic centers are probably molecular complexes of the superoxide ion 02with the molecules CHCI3, CH2C12, CH3CI , CHCI2F , CH=Br 2 and C~HCI 3. The complexes are formed by the two-center coordination of 02by interaction with a hydrogen atom (hydrogen bonding) and a halogen atom in the halogenated hydrocarbon.

Stoichiometric reaction of bis(acetylacetonato) cobalt(II) with cumene hydroperoxide in chloroform solutions containing added pyridine has been studied by the PMR method.

Synthesis and structure of (pyridine) bis(acetylacetonato) (1-methyl-1-phenylethylperoxo) cobalt(III) complex are described.

The intermediate formation of vanadium(5+) peroxo complexes was detected under conditions of the disproportionation of hydrogen peroxide in acetic acid catalyzed by ammonium metavanadate by51V NMR spectroscopy and the complex of vanadium(5+) ions with the dioxygen radical-anion was detected by ESR spectroscopy.

Oxidation of xylenes with Pd(II) complexes in acidic medium is possible according to two pathways: a) electrophilic substitution in the arene ring with the formation of an organometalic intermediate and b) one-electron transfer from the arene to the palladium atom with the intermediacy of the arene cation-radical. The relative contribution of each pathway depends on the nature of the arene and on the acidity of the medium.

I. V. Kozhevnikov, V. I. Kim, E. P. Talzi and V. N. Sidelnikov, J. Chem. Soc., Chem. Commun., 1985, 1392 DOI: 10.1039/C39850001392