Four mixed-valent ((MnMn6Mn6II)-Mn-IV-Mn-III) tridecanuclear Mn clusters [Mn13O8(OH)(6)(ndC)(6)] (1), [Mn13O8(OEt)(5)(OH)(ndC)(6)] (2), [Mn13O8(O2CPh)(12)(OEt)(6)] (3), and [Mn13O8(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-6(II) hexagon containing a central Mn-IV atom, and above and below this are Mn-3(III) triangular units. These core Mn atoms are held together by a combination of O2-, 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 (M 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.