Scandium complexes containing beta-diketiminato ligands with pendant phosphanyl groups: competition between Sc/γ-C [4 + 2] cycloaddition and Sc/P frustrated Lewis pair reactions.

Three cationic scandium aryloxide complexes, i.e., [LScOAr]+[B(C6F5)4]- (Ar = 2,6-tBu2C6H3; L = CH3C(2,6-iPr2C6H3N)C(R)C(CH3)(N(CH2)nCH2PPh2); 6a: n = 1, R = H; 6b: n = 1, R = CH3; 6c: n = 2, R = C6H5CH2), were prepared by a three-step synthetic route. The reactivities of tridentate NNP ligand supported scandium complexes 6 toward various unsaturated substrates were investigated. Treatment of 6a with non-conjugated small molecules such as phenyl isocyanates, phenyl isothiocyanate, and benzaldehyde resulted in formation of Sc/γ-C [4 + 2] cycloaddition products 7-9 because of nucleophilic reactivity at the β-diketiminato γ-carbon, with retention of ScP interactions. Analogous Sc/γ-C [4 + 2] cycloaddition reactions occurred for complex 6b with phenyl isocyanate and phenyl isothiocyanate, affording complexes 10 and 11, respectively. Complex 6c, with a bulky benzyl substituent at the γ position of the β-diketiminato ligand, showed typical Sc/P frustrated Lewis pair (FLP) reactivity toward phenyl isocyanate to give the 1,2-addition product 12. Complex 6c was efficient in the polymerization of γ-methyl-α-methylene-γ-butyrolactone (MMBL). The corresponding initiating species 13, which was formed by Sc/P FLP-type 1,4-addition, was isolated.