engleski

The mononuclear MoO22+ complexes with stereochemically rigid 4-aminobenzhydrazide based ligands

A range of dioxidomolybdenum(VI) complexes has displayed very high catalytic activities and exquisite selectivity. The dioxidomolybdenum(VI) mononuclear complexes with 4- aminobenzoylhydrazone ligands (salicylaldehyde (H2L1), 3- methoxysalicylaldehyde (H2L2), or 4- methoxysalicylaldehyde 4-aminobenzoylhydrazone (H2L3) being of the type cis-[MoO2L1-3(D)] ; D = MeOH, EtOH, H2O, dmf] have been prepared and structurally characterized. The crystal structure analysis of mononuclear complexes (Fig.1.) reveals distorted octahedral Mo(VI) coordination by ONO donor atoms from dianionic tridentate 4- aminobenzoylhydrazone ligands, two oxido oxygen atoms from the MoO22+ moiety and the oxygen atom from coordinated alcohol molecules (D = MeOH, EtOH).The molybdenum oxido groups of the MoO22+ moiety show the expected mutual cis configuration and are located trans to the imine nitrogen atom and coordinated alcohol molecule. The bond distances range around Mo follows trend: Mo-Ot < Mo-O(phenolate) < Mo- O(hydrazone part) < Mo-N< MoO(ROH ; R=Me, Et). The bicyclic system of two chelate rings is slightly folded along the axis of molybdenum- imino bond and by the offset of metal centre apart from chelate planes towards the terminal oxygen atom. The most deviation from chelate rings coplanarity is shown by the phenolate oxygen atom due to its participation in intermolecular interactions. The complexes were tested for catalytic epoxidation of cyclooctene under eco-friendly reaction conditions by using aqueous tert-butyl hydroperoxide (TBHP) as an oxidant. Due to trans effect of oxido oxygen atom, Mo-O(D) bond is the longest bond distance within molybdenum coordination sphere and it expected to be the point of maximum catalytic reactivity of the complexes. It is previously shown that pentacoordinated molybdenum systems activate oxidizing agent TBHP through adduct formation assisted by the O–H…O hydrogen bond. Another explanation of the substituent influence is p-π conjunctive effect with methoxy group due to increasing electron density of the phenyl ring.

molybdenum complexes ; 4-aminobenzoylhydrazone ligands

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