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 B<sub>2</sub>Pz<sub>4</sub>Py are reported. The key starting material (B<sub>2</sub>Pz<sub>4</sub>Py)ScCl is readily prepared and alkylated with organolithium reagents RLi (R = CH<sub>3</sub>, CH<sub>2</sub>SiMe<sub>3</sub>, CH<sub>2</sub>SiMe<sub>2</sub>Ph, CH<sub>2</sub>CH<sub>2</sub>CH<sub>3</sub> and CH<sub>2</sub>CHMe<sub>2</sub>) 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 (B<sub>2</sub>Pz<sub>4</sub>Py)ScCl and NaHBEt<sub>3</sub> in 80% yield and was found to be more stable by 28 kcal mol<sup>-1</sup> 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 (B<sub>2</sub>Pz<sub>4</sub>Py)ScR react with water to form the bridging oxo dimer (B<sub>2</sub>Pz<sub>4</sub>Py)ScOSc(B<sub>2</sub>Pz<sub>4</sub>Py). The reactivity of the hydrido and methyl complexes towards carbon dioxide was explored; heating to 80˚C results in the formation of k<sup>2</sup> formato and acetate complexes, respectively. The mechanisms were studied via density function theory and distinct transition states for insertion of CO<sub>2</sub> into the Sc-R (R = H, CH<sub>3</sub>) were found, with the insertion into the Sc-CH<sub>3</sub> being more enthalpically difficult (by 18 kcal mol<sup>-1</sup>) than insertion into Sc-H. The slow rate of reaction between [(B<sub>2</sub>Pz<sub>4</sub>Py)ScH]<sub>2</sub> and CO<sub>2</sub> is attributed to the barrier associated with dimer dissociation. In both insertion reactions, the kinetic products are k<sup>1</sup> formato or acetate complexes that are only slightly less stable than the observed k<sup>2</sup> derivatives. The k<sup>1</sup> compounds can therefore be trapped by treating the k<sup>2</sup> isomers with <i>tris</i>-pentafluorophenyl borane.