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 [Cu-2(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 [Cu-2(bdPtZ)(2)](OTf)(3), 2(OTf)(3), by disproportionation of Cu(I). Compound 2(OTf)(3) displays an electron paramagnetic resonance spectrum, with g(parallel to) = 2.25 (A(parallel to) = 169 G) and g(perpendicular to) = 2.06, and exhibits a reversible redox wave at -452 mV versus Cp2Fe+/Cp2Fe. The complex [Cu-2(bdptz)(mu-OH)(MeCN)(2)](OTf)(3), 3(OTf)(3), was prepared by chemical oxidation of 1 with AgOTf, and exposure of 1 to dioxygen afforded [Cu-2(bdptz)(mu-OH)(2)](2)(OTs)(4), 4(OTs)(4), which can also be obtained directly from [Cu(H2O)(6)](OTs)(2). In compound [Cu-2(bdptz)(mu-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 [Cu-2(Ph(4)bdptz)(MeCN)(2)](OTf)(2), 6(OTf)(2), and [Cu-2(Ph(4)bdptz)(mu-O2CCH3)](OTf), 7(OTf), were also synthesized, where Ph(4)bdptz = 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(l) centers.