Molybdenum(VI) complexes with a chiral L-alanine bisphenol [O,N,O,O’] ligand. Synthesis, structure, spectroscopic properties and catalytic activity

Abstract

<p>Dioxidomolybdenum(VI) compound [MoO₂Cl₂(dmso)₂] reacts with a chiral tetradentate O₃N-type <em>l</em>-alanine bisphenol ligand precursor (Et₃NH)H₂L^ala to form an oxidochloridomolybdenum(VI) complex [MoOCl(L^ala)] (<strong>1</strong>) as two separable geometric isomers with phenolate groups in <em>cis</em> or <em>trans</em> positions. The single crystal X-ray and NMR analyses of <em>cis</em>- and <em>trans</em>-<strong>1</strong> reveal that the complexes are formed of monomeric molecules, in which the ligand has a tetradentate coordination through three oxygen donors and one nitrogen donor. The reaction of Na₂MoO₄·2H₂O with the same ligand precursor in an acidic methanol solution leads to the formation of an anionic dioxido complex (Et₃NH)[MoO₂(L^ala)] (<strong>2</strong>) with a <em>trans</em> coordination of the tetradentate ligand. <em>Trans</em>-<strong>1</strong> and <strong>2</strong> were studied as active catalysts for olefin epoxidation: <em>i.e.</em> styrene, cyclohexene, S(–)-limonene and (–)-α-pinene using H₂O₂ and tBuOOH as oxidants.<br></p>