A reactive and versatile fourteen-electron copper(I) fragment for binding carbon monoxide, isocyanides, and phosphines: synthesis, crystal structure, and reactivity of (2-methylquinolin-8-olato)copper(I) derivatives

Abstract

The synthesis of the 14-electron copper(I) moiety [Cu(2Me-quin)](2Me-quin = 2-methylquinolin-8-olate) having a bent arrangement of the donor atom is reported. The reaction of CuCl with Na(2Me-quin) led to the disproportionation of copper(I) to copper(II) and copper metal, while in the presence of CO, a thermally very stable carbonyl complex [{Cu(2Me-quin)(CO)}₄](2) was isolated [ν(CO)(Nujol) 2 050 cm^–1]. Complex (2) has a tetranuclear cubane-like structure, with S₄ symmetry, in which the coppers are bridged by the oxygen atom of the quinolinolate anion [av. Cu–O 2.04, Cu ⋯ Cu, 3.615, Cu–C 1.76 Å). Carbon monoxide can be replaced in the tetranuclear structure by p-MeC₆H₄NC to form [{Cu(2Me-quin)(p-MeC₆H₄NC)}₄](4)[ν(CN)(Nujol) 2 145 cm^–1], which was also formed directly from CuCl, Na(2Me-quin), and p-MeC₆H₄NC. Reaction of PPh₃ and dppe (Ph₂PCH₂CH₂PPh₂) replaced CO and led to monomeric complexes [Cu(2Me-quin)(PPh₃)₂](5) and [Cu(2Me-quin)(dppe)](6). Complex (2) was used as a source of dicopper(I), by treating it with dppm (Ph₂PCH₂PPh₂) which caused loss of CO and led to the formation of [Cu₂(2Me-quin)₂(µ-dppm)](7). The two Cu(2Me-quin) fragments share the oxygen atom of the quinolinate ligand, forming a dimetallic fragment [Cu ⋯ Cu 2.790(2)Å], where the five-co-ordination of each copper(I) is completed by one of the P donor atoms from the dppm ligand. Compound (2) catalyses the reduction of nitrosobenzene to azoxybenzene and nitroalkanes to oximes and nitriles under a pressure of carbon monoxide (60–100 atm) at temperatures from 80 to 110 °C. Compounds (2), (5), and (7) were characterized by single-crystal X-ray analysis: (2), space group P2₁/n(monoclinic): a= 15.591(3), b= 18.558(3), c= 15.850(3)Å; β= 90.08(2), Z= 4, R 0.069 (R′= 0.076) for 1 414 observed reflections; (5), space group Pna2₁(orthorhombic), a= 27.478(4), b= 12.519(2), c= 11.031(3)Å, Z= 4, R 0.059 for 1 920 observed reflections; (7), space group P(triclinic), a= 12.018(4), b= 17.540(4), c= 11.793(4)Å, α= 90.82(3), β= 119.02(2), γ= 78.00(3)°, Z= 2, R 0.051 (R′= 0.054) for 3 876 observed reflections.