Molybdenum(VI) Dioxo Complexes with Tridentate Phenolate Ligands

Abstract

A series of new molybdenum(VI) dioxo complexes of the type [MoO₂ClL^X] with potentially monoanionic phenolate ligands L^X (L = 4,6-di-<i>tert</i>-butyl-2-{[(X)methylamino]methyl}-phenolate; L^OMe, where X = 2′-methoxyethyl; L^SEt, where X = 2′-ethylthioethyl; L^NEt2, where X = 2′-diethylaminoethyl; and L^NMe2, where X = 2′-dimethylaminoethyl) have been synthesized as models for molybdoenzymes. All molybdenum complexes were readily accessible by employing the η²-coordinate pyrazolate complex [MoO₂Cl(η²-<i>t</i>-Bu₂pz)]. The nitrogen ligand can easily be exchanged by the monoanionic phenolate ligands L^X in toluene at room temperature, leading to monosubstituted complexes <b>1</b>−<b>4</b> as yellow to red powders in good yields. Suitable single crystals for X-ray diffraction analysis of complexes <b>2</b> and <b>3</b> were obtained from a concentrated benzene solution. Both complexes reveal a six-coordinate molybdenum atom in a distorted octahedral surrounding, with a tridentate fac coordination of the ligand. Additionally, the complexes were characterized by elemental analysis; IR, UV/vis, and NMR spectroscopy; and mass spectrometry. For complexes <b>1</b> and <b>2</b>, only one isomer can be detected in solution, whereas complexes <b>3</b> and <b>4</b> reveal the formation of two isomers in a 1:1 ratio for <b>3</b> and in a 1:3 ratio for <b>4</b>. Optimized geometries and relative free energies of all possible isomers have been established by DFT calculations, indicating two isomers in solutions of <b>3</b> and <b>4</b> to be two types of facially coordinated complexes. Furthermore, this is supported by gauge-independent atomic orbital calculations of the ¹H NMR shifts with a high correlation of experimental and calculated shifts for the fac compounds. Oxygen atom transfer reactions of <b>1</b> to PMe₃ quickly form monooxo molybdenum compound <i>cis</i>,<i>mer</i>-[MoOCl₂(PMe₃)₃].