ADVANCES IN INORGANIC CHEMISTRY, VOL, 52 HIGH-NUCLEARITY PARAMAGNETIC 3d-METAL COMPLEXES WITH OXYGEN- AND NITROGEN-DONOR LIGANDS R I C H A R D E. P. W I N P E N N Y Department of Chemistry, The University of Manchester Manchester M13 9PL, United Kingdom I. Introduction A. Single-Molecule Magnets B. Strategies for Making Polynuclear Cages II. Serendipitous Assembly III. Survey of the 3d Metals A. Vanadium B. Chromium C. Manganese D. Iron E. Cobalt F. Nickel G. Copper IV. Families of Cages A. Wheels a n d Metallocrowns B. Cubanes C. Trigonal Prisms V. Conclusions References I. Introduction It is probably fair to claim that we understand the magnetic and spectroscopic properties of mononuclear complexes of the 3d metals. Stretching a point, we could also claim we understand the preparative chemistry of mononuclear c o m p l e x e s ~ h o w to replace ligand A with ligand B and how to prepare complex X. Exploitation of known physical behavior, e.g., constructing devices based on spin-crossover complexes, remains a goal, b u t such studies build on a base of fundamental Copyright © 2001 by AcademicPress All rights of reproduction in any form reserved. 2 WINPENNY understanding rather than extending it. One or two slightly esoteric questions remain to be answered, b u t for most purposes we can be confident we know which properties a specific combination of a single 3d-metal center and ligands is likely to display. The work of Cotton, Chisholm, and others has created a huge body of knowledge concerning dinuclear species containing m e t a l - m e t a l bonds (1). Once species of higher nuclearity are considered, our knowledge falls off precipitately as the polymetallic complexes get larger. Fully characterized examples of 3d-metal cages become rare beyond hexanuclear species. The exceptions are studies of carbonyl cages by Dahl and others, and the chalcogenide cages reported by Fenske. The paucity of examples is illustrated in Table I, where the number of crystal structures on the Cambridge Structural Database is listed against nuclearity for the 3d metals. Here, efforts to address this gap in the chemical literature are reviewed. A. SINGLE-MOLECULEMAGNETS Initial investigations demonstrate that such cage complexes show physical phenomena of fundamental importance, many of which may have potential application. The magnetic behavior of 3d-metal cages has proved particularly exciting. The most important observation, made in 1993, was that a Mn12 cage showed hysteresis in a magnetization against field plot, and that this hysteresis was of molecular origin (2, 3). The cage in question is the first example of a so-called single-molecule magnet (SMM), and although other examples have since been reported, this remains the most fully studied example. The dodecanuclear manganese cages have the formula [MnmOm (O2CR)16(H20)4], which can be made with a variety of carboxylates, and have gr