Influence of Second Coordination Sphere Hydroxyl Groups on the Reactivity of Copper(I) Complexes

We report the enhanced reactivity of hydroxyl substituted CuN3+ derivatives, where N3 = tris­(picolinyl)­methane (tripic) and related derivatives, upon deprotonation of the O–H functionality. The work capitalizes on new methodology for incorporating hydroxyl groups into the second coordination sphere of copper centers. The key synthetic methodology relies on Pd-catalyzed coupling reactions of dilithiated 6-methyl-2-pyridone with bromopyridyl derivatives. These building blocks allow the preparation of tridentate N3 ligands with OH and OMe substituents flanking the fourth coordination site of a tetrahedral complex. Coupling of these tridendate ligands gives the corresponding hydroxy- and methoxy-functionalized bistripodal ligands. [Cu­[bis­(2-methylpyrid-6-yl)­(2-hydroxypyrid-6-yl)­methane]­(NCMe)]+ ([Cu­(2H)­(NCMe)]+) oxidizes readily in air to afford the mixed valence Cu1.5 dimer ([Cu2(2)2]+). Formation of [Cu2(2)2]+ is accelerated in the presence of base and can be reversed with a combination of decamethylferrocene and acid. The reactivity of [Cu­(2H)­(NCMe)]+ with dioxygen requires deprotonation of the hydroxyl substituent: neither [Cu­(tripic)­(NCMe)]+ nor the methoxy-derivatives displayed comparable reactivity. A related mixed valence dimer formed upon oxidation of the dicopper­(I) complex of a tetrahydroxy bis­(tridentate) ligand, [Cu2(6H4)­(NCMe)2]2+. The dicopper­(I) complex of the analogous tetramethoxy N6-ligand, [Cu2(5)­(NCMe)2]2+, instead reversibly binds O2. Deprotonation of [Cu­(2H)­(CO)]+ and [Cu­(2H)­(NCMe)]+ afforded the neutral derivatives Cu­(2)­(CO) and Cu2(2)2, respectively. The dicopper­(I) derivative Cu2(2)2 can be reoxidized, reprotonated, and carbonylated. The silver­(I) complex, [Ag­(2H)­(NCMe)]­BF4, forms an analogous neutral dimer (Ag2(2)2) upon deprotonation of the hydroxyl group. The structures of ligand 2H, [Cu2(5)­(NCMe)2]+, [Cu2(2)2]+, [Cu2(6H2)]+, [Ag­(2H)­(NCMe)]­BF4, and Ag2(2)2 were confirmed by single crystal X-ray diffraction.