Synthesis and Characterization of Cu₂(I,I), Cu₂(I,II), and Cu₂(II,II) Compounds Supported by Two Phthalazine-Based Ligands: Influence of a Hydrophobic Pocket

The copper coordination chemistry of two phthalazine-based ligands of differing steric bulk was investigated. A family of dinuclear complexes were prepared from reactions of [Cu2(bdptz)(MeCN)2](OTf)2, 1(OTf)2, where bdptz = 1,4-bis(2,2‘-dipyridylmethyl)phthalazine. Treatment of 1(OTf)2 with NaO2CCH3 afforded the class I mixed-valent compound [Cu2(bdptz)2](OTf)3, 2(OTf)3, by disproportionation of Cu(I). Compound 2(OTf)3 displays an electron paramagnetic resonance spectrum, with g∥ = 2.25 (A∥ = 169 G) and g⊥ = 2.06, and exhibits a reversible redox wave at −452 mV versus Cp2Fe+/Cp2Fe. The complex [Cu2(bdptz)(μ-OH)(MeCN)2](OTf)3, 3(OTf)3, was prepared by chemical oxidation of 1 with AgOTf, and exposure of 1 to dioxygen afforded [Cu2(bdptz)(μ-OH)2]2(OTs)4, 4(OTs)4, which can also be obtained directly from [Cu(H2O)6](OTs)2. In compound [Cu2(bdptz)(μ-vpy)](OTf)2, 5(OTf)2, where vpy = 2-vinylpyridine, the vpy ligand bridges the two Cu(I) centers by using both its pyridine nitrogen and the olefin as donor functionalities. The sterically hindered compounds [Cu2(Ph4bdptz)(MeCN)2](OTf)2, 6(OTf)2, and [Cu2(Ph4bdptz)(μ-O2CCH3)](OTf), 7(OTf), were also synthesized, where Ph4bdptz = 1,4-bis[bis(6-phenyl-2-pyridyl)methyl]phthalazine. Complexes 1−7 were characterized structurally by X-ray crystallography. In 6 and 7, the four phenyl rings form a hydrophobic pocket that houses the acetonitrile and acetate ligands. Complex 6 displays two reversible redox waves with E1/2 values of +41 and +516 mV versus Cp2Fe+/Cp2Fe. Analysis of oxygenated solutions of 6 by electrospray ionization mass spectrometry reveals probable aromatic hydroxylation of the Ph4bdptz ligand. The different chemical and electrochemical behavior of 1 versus 6 highlights the influence of a hydrophobic binding pocket on the stability and reactivity of the dicopper(I) centers.