Four mixed-valent (Mn(IV)Mn(III)(6)Mn(II)(6)) tridecanuclear Mn clusters [Mn(13)O(8)(OH)(6)(ndc)(6)] (1), [Mn(13)O(8)(OEt)(5)(OH)(ndc)(6)] (2), [Mn(13)O(8)(O(2)CPh)(12)(OEt)(6)] (3), and [Mn(13)O(8)(OMe)(6)(ndc)(6)] (4) are reported, where ndcH(2) is 1,8-naphthalenedicarboxylic acid. This is the first use of the latter in Mn chemistry. Complexes 1-3 are essentially isostructural and possess a central core composed of three layers. The middle layer consists of a Mn(II)(6) hexagon containing a central Mn(IV) atom, and above and below this are Mn(III)(3) triangular units. These core Mn atoms are held together by a combination of O(2-), RO(-), or HO(-) bridging groups. The overall metal topology is an unusual one, with the overall geometry being a metal-centered cuboctahedron (heptaparallelohedron). Variable-temperature, solid-state dc, and ac magnetization studies were carried out on complexes 1-4 in the 5.0-300 K range. Compound 1 was found to possess an S = 9/2 ground-state spin, whereas 2, 3, and 4 have an S = 11/2 ground state. Fitting of the magnetization (M) versus field (H) and temperature (T) data by matrix diagonalization and including only axial zero-field splitting, D, gave D = -0.14 cm(-1) for 1. High-frequency EPR studies were carried out on single crystals of 1.xDMF, and these confirmed D to be very small, that is, 1 is essentially isotropic. The combined work demonstrates the ligating ability of 1,8-naphthalenedicarboxylate, notwithstanding its robust organic backbone and the restricted parallel disposition of its two carboxylate moieties, and its usefulness in the synthesis of new polynuclear Mn(x) clusters. The work also demonstrates a sensitivity of the ground-state spin in this Mn(13) family of complexes to relatively small structural perturbations, while the high-frequency EPR study demonstrated the magnetically isotropic nature of the Mn(13) core.