Bulky β-Diketones Enabling New Lewis Acidic Ligand Platforms

Abstract

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.

Group 4 Organometallics Supported by Sterically Hindered β‐Diketonates

“…In each, the chlorides adopt a cis ‐chloride configuration, with the bulky m ‐terphenyl groups in opposing directions, and overall assuming a pseudo‐C 2 molecular geometry. This consistency is attributed to the electronic structure of the metals, rather than steric effects . The average M–O and M–Cl bond lengths decrease going from Zr to Hf, consistent with the lanthanide contraction (Table S1).…”

“…Accessing larger examples has been a synthetic challenge until the recent development of methods for incorporating the bulky meta ‐terphenyl group into the ligand platform . A first‐row transition metal survey using L 1 has identified that the ligand inhibits tris‐ or tetrakis‐chelation, enabling the isolation of kinetically‐stabilized bis‐acetylacetonate type compounds that are plausible active species in catalytic mechanisms , …”

“…Reduction of the Ti IV complex occurs about 700 mV more positive than its lower congeners, explaining why Ti IV uniquely suffers reduction by benzyl Grignard reagent. Similar chemical reduction of the dichloride complex has been previously demonstrated; however, attempted chemical reduction of the Zr and Hf dichloride complexes did not result in isolable products.…”

“…Single crystals of these complexes were grown and analysed by X‐ray diffraction in order to gain insight into their coordination geometries. Both Ti( L 1 ) 2 Cl and the dichloride have been described previously, and are included here for completeness of discussion. Crystal structures for the dichlorides M( L n ) 2 Cl 2 (M = Zr, Hf) can be found in the Supporting Information (Figures S1–S4).…”

“…form from Sigma Aldrich and further dried with and stored over 4 Å molecular sieves. Compounds H(L 1 ), H(L 2 ), and Ti(L 1 ) 2 Cl 2 were prepared according to known procedures , . NMR spectra were recorded on a Bruker Avance‐III 300 NMR spectrometer (300 MHz) or Bruker Avance‐III 400 NMR spectrometer (400 MHz).…”