Bulky β-Diketones Enabling New Lewis Acidic Ligand Platforms

The synthesis of a sterically encumbered β-diketone ligand (^Aracac) substituted with 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃ is described. Coordination complexes of the type M(^Aracac)₂Cl(solv) (M = Ti, V, Cr; solv = THF, CH₃CN) were prepared by the reaction of ^Aracac with MCl₃ (M = V, Cr) or with TiCl₄ to generate Ti(^Aracac)₂Cl₂, followed by reduction. These complexes show a trend of alternating the cis/trans geometric preference with increasing dⁿ electron count (n = 0, 1, 2, 3), which is rationalized in part by the unusual ability of β-diketonates to behave as either a weak π donor or a π acceptor in the cis and trans geometries, respectively. In this way, the bis-β-diketonate platform can accommodate the varying electronic demands of the coordinated metal ion. These results demonstrate the ability to limit the coordination of β-diketonates on metal complexes for the first time, providing a chemically robust and coordinatively versatile platform for mechanistic investigations, metal functionalization, and improved catalyst design.