Photophysical, catalytic, and theoretical investigations of kinetically stable [Cu(2,2′-biquinoline)(PR3)2]+ and [Cu(neocuproine)(PR3)2]+ complexes

Four complexes were synthesized and characterized with the formula [Cu(N^N)(PMePh2)2]ClO4 (4 and 6) or [Cu(N^N)(dppe)]ClO4 (5) and 7){N^N = 2,2′-biquinoline, neocuproine}. Crystal structures of the neocuproine-containing complexes (5 and 7) were obtained and described. The methyldiphenylphosphine-containing complexes are stable in solutions (compared to PPh3) due to: (1) slightly better σ-donation compared to PPh3 and (2) smaller Tolman cone angle of ∼136° (compared to ∼145° in PPh3). In the other hand, [Cu(N^N)(dppe)]ClO4 complexes were demonstrating evidence of forming [Cu(N^N)2]+ in solutions, highlighting the importance of the P-Cu-P angle. The angle P-Cu-P is around 123° in stable complex 5 compared to ∼92° in 7. Absorption spectra of 4 and 5 exhibit low intensity broad shoulder in the visible light domain assigned as MLLCT by computational studies. While [Cu(2,2′-biquinoline)(PPh3)2]+ cation was losing catalytic activity in Castero-Stephens cross-coupling over time as indicated by the low conversion rate, complexes 4 and 5 found to be active and achieved complete conversion of the starting materials.