Scandium alkyl and hydride complexes supported by a pentadentate diborate ligand: reactions with CO2 and N2O

Abstract

Alkyl and hydrido scandium complexes of the dianionic pentadentate ligand B₂Pz₄Py are reported. The key starting material (B₂Pz₄Py)ScCl is readily prepared and alkylated with organolithium reagents RLi (R = CH₃, CH₂SiMe₃, CH₂SiMe₂Ph, CH₂CH₂CH₃ and CH₂CHMe₂) to form alkyl derivatives in 61–93% yields. These compounds are very thermally stable and do not undergo sigma bond metathesis reactions with dihydrogen. The hydrido complex was prepared from (B₂Pz₄Py)ScCl and NaHBEt₃ in 80% yield and was found to be more stable by 28 kcal mol⁻¹ as a dimer, rather than a monomeric hydrido complex. However, the monomer is accessible through dissociation of the dimer at 80 °C. All of the compounds (B₂Pz₄Py)ScR react with water to form the bridging oxo dimer (B₂Pz₄Py)ScOSc(B₂Pz₄Py). The reactivity of the hydrido and methyl complexes towards carbon dioxide was explored; heating to 80 °C results in the formation of κ² formato and acetate complexes, respectively. The mechanisms were studied via density functional theory and distinct transition states for insertion of CO₂ into the Sc–R (R = H, CH₃) were found, with the insertion into Sc–CH₃ being more enthalpically difficult (by 18 kcal mol⁻¹) than insertion into Sc–H. The slow rate of reaction between [(B₂Pz₄Py)ScH]₂ and CO₂ is attributed to the barrier associated with dimer dissociation. In both insertion reactions, the kinetic products are κ¹ formato or acetate complexes that are only slightly less stable than the observed κ² derivatives. The κ¹ compounds can therefore be trapped by treating the κ² isomers with tris-pentafluorophenyl borane.