Reactivity of Scandium β-Diketiminate Alkyl Complexes with Carbon Dioxide

The reactions of two highly air- and moisture-sensitive
scandium
bis­(alkyls) supported by a bulky β-diketiminato (nacnac) ligand
with carbon dioxide are described. [κ2-ArNC­(tBu)­CHC­(tBu)­NAr]­ScR2 (Ar = 2,6-iPr2C6H3; R = CH3, 1a; R = CH2SiMe3, 1b) react rapidly with CO2 to give mixtures of mono- and bis­(carboxylato) insertion
products 2a/2b and 3a/3b depending on the stoichiometry and conditions of the reaction.
Compound 2a (R = CH3) is a dimeric complex
with bridging acetato groups, as determined by X-ray crystallography.
These compounds were characterized by NMR spectroscopy, and 3a could be isolated in pure form. Treatment of these compounds
with excess CO2 resulted in addition to the central carbon
of the Sc­(nacnac) six-membered ring and displacement of the nitrogen
donors to yield dimeric scandium carboxylates 4a/4b; compound 4b was characterized by X-ray crystallography.
Reactions of the nacnac scandium cations formed upon abstraction of
one or two methides from 1a using B­(C6F5)3 with CO2 were also explored. Although
the products were qualitatively more thermally robust, eventually
ligand displacement occurred in these cationic acetato complexes as
well. Nevertheless, insertion products were characterizable in solution
using NMR spectroscopy. Overall, this study shows the facility with
which CO2 is taken up by scandium alkyls but that the nacnac
ligand framework is too reactive to support chemistry aimed at catalytic
conversion of CO2 into other products.