Metal-carborane multidecker sandwiches as building blocks for metallopolymers and nanostructured materials

The ability of cyclic planar [RR'C2B3H2X](4-) carborane ligands (R, R' = alkyl, aryl, SiMe3, H; X = alkyl, Cl, Br, I, H) to bind tightly to transition and main-group metals on both sides of the C2B3 ring plane allows the construction of a wide range of polymetallic multidecker sandwich complexes involving different metals, metal oxidation states, and molecular architectures. Although these ligands are isoelectronic and isosteric analogues of C5H5- they form stronger covalent bonds to metal centers, and can stabilize many robust, isolable structural types that are not readily accessible (or are completely unknown) in metallocene chemistry. Many of these metallacarborane sandwich complexes are paramagnetic and exhibit extensive electron-delocalization that can be tuned by redox action or by the introduction of appropriate substituents to the carborane ligands or the metal centers. This remarkable versatility suggests that there is considerable potential for the creation of novel polymeric and solid state materials that can be tailored to have desired electronic, optical, or other properties. This review summarizes recent efforts in the designed syntheses of oligomeric and polymeric systems and the systematic exploration of structure property relationships.