Scandium Alkyl and Hydride Complexes Supported by a Pentadentate Diborate Ligand: Reactions with CO2 and N2O

Alkyl and hydrido scandium complexes of the dianionic pentatdentate 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 k² formato and acetate complexes, respectively. The mechanisms were studied via density function theory and distinct transition states for insertion of CO₂ into the Sc-R (R = H, CH₃) were found, with the insertion into the 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 k¹ formato or acetate complexes that are only slightly less stable than the observed k² derivatives. The k¹ compounds can therefore be trapped by treating the k² isomers with tris-pentafluorophenyl borane.