A series of new branched, dendritic, polynucleating, poly(aminophthalazine) ligands involving two (N-8) (O-P, C-P), three (N-12) (G-P), and four (N-16) (P-P) connected phthalazine groups has been synthesized and their copper coordination chemistry investigated. 6,6’-(1,2-phenylenedioxy)bis[1,4-bis(2’-pyridylamino)phthalazine] (C-P) and 6,6’-oxybis[1,4-bis(2’-pyridylamino)phthalazine] (O-P) bind four copper(II) centers in two well-separated dinuclear copper pairs in one molecular entity in the derivatives [Cu-4(C-P)(mu 2-OH)(2)(mu(2)-NO3)2(H2O)2(NO3)(2)](NO3)2 . 4H(2)O (1) {[Cu-4(O-P)(mu(2)-OH>2(mu(2)-NO3)(2)CH2O)(7)](2)(mu(2)-NO3)}(NO3)(7) . 6H(2)O (2). Variable-temperature magnetic studies indicate strong local antiferromagnetic coupling in these hydroxide-bridged compounds, with nonexistent coupling across the ligand framework itself. The crystal and molecular structure for 1 and 2 are reported. 1 crystallizes in the monoclinic system, space group P2(1)/c (No. 14), with a = 13.346(4) Angstrom, b = 37.71(1) Angstrom, c = 21.519(6) Angstrom, beta = 91.04(2)degrees, and Z = 4. The structure was refined to R = 0.063 and R(w) = 0.058 for 5958 unique reflections with I > 2.50 sigma(I). 2 crystallizes in the monoclinic system, space group C2/c, with a = 31.948(5) Angstrom, b = 30.900(8) Angstrom, c = 12.683(6) Angstrom, beta = 96.22(2)degrees, and Z = 8. A poorer structural refinement gave R = 0.121 and R(w) = 0.137 for 5634 unique reflections with I > 2.00 sigma(I).In 1 two chemically identical, but structurally slightly different, tetranuclear species exist per unit cell. Pairs of squase-pyramidal copper centers are bridged equatorially by hydroxides and phthalazine diazine units with Cu-OH-Cu angles in the range 110.4-116.7 degrees, and Cu-Cu separations in the range 3.122-3.188 Angstrom. In 2 pairs of six-coordinate copper centers are bridged equatorially by hydroxides and phthalazine groups with comparable dinuclear center dimensions to 1. Two tetranuclear complexes are linked by a bidentate nitrate bridge to form. an associated octanuclear species. External dimensions for both complexes are very large, bringing them well into the nanometer size range. Other polynuclear complexes involving three and four hydroxide-bridged copper(II) pairs have been synthesized and will be discussed in terms of their structural and magnetic properties.