Bulky β-Diketones Enabling New Lewis Acidic Ligand Platforms.

The synthesis of a sterically encumbered β-diketone ligand (acac) substituted with 2,6-(2,4,6-MeCH)CH is described. Coordination complexes of the type M(acac)Cl(solv) (M = Ti, V, Cr; solv = THF, CHCN) were prepared by the reaction of acac with MCl (M = V, Cr) or with TiCl to generate Ti(acac)Cl, followed by reduction. These complexes show a trend of alternating the cis/trans geometric preference with increasing d 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.