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 E_1/2 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.