Synthesis and reactivity of molybdenum(0) complexes containing sterically expanded arene ligands

Abstract

Synthesis of a series of sterically expanded arenes, containing one or more 1,2,3,4-tetrahydro-1,1,4,4-tetramethyl substituents, was accomplished in high yield utilizing classic Friedel–Crafts alkylation conditions. Metallation of these arenes with molybdenum was accomplished via reflux of Mo(CO)6 in a mixed solvent combination in the presence of the ligand. The (η6-arene)Mo(CO)3 complexes were isolated in reasonable yields (45–60%) and characterized through a combination of 1H and 13C NMR spectroscopy, X-ray crystallography, IR spectroscopy, elemental analysis, and mass spectrometry. The crystallographic data reveal the ability to tune the steric profile of the arene through judicious ligand choice. Ligand electronics can also be modified to some degree, though to a lesser extent than the steric congestion about the metal center. Computational analysis of the arene complexes corroborate the observed trend of increasing electron donation by the arene as the steric bulk increases. Reactivity of selected Mo complexes highlight the dramatic change in stability substitution imparts on the compounds, as the least sterically congested complex undergoes fastest arene exchange while the bulkiest ligand results in a compound indefinitely stable in neat arene under identical conditions. Attempts at determining whether this substitution chemistry is driven by thermodynamic or kinetic factors were explored computationally and align with systems that have been examined previously. These studies introduce a family of tunable, sterically congested arene ligands that may find value as new supports in the preparation of a range of metal–arene complexes.