Supramolecular Materials: Metal–Quinonoid Complexes

Simple quinones such as hydroquinone, resorcinol, and catechol can be complexed to a transition metal through the aromatic π-system. This review focuses on the manganese and rhodium complexes of hydroquinone (H2Q), which bind to the metal fragment through all six aromatic carbons (η6) to afford (H2Q)Mn(CO)3+ and (H2Q)Rh(COD)+. Both of these undergo facile deprotonation to give hydrogen-bonded semiquinone complexes, which can be deposited on HOPG surfaces. A second deprotonation occurs with concomitant electron transfer to the metal to give anionic η4-benzoquinone complexes that bind to other metals through the quinonoid oxygens. In this way, self-assembly to metal–organic framework polymer materials results with the (benzoquinone)MLn− moiety serving as the spacer. The resultant polymers are porous and can have tetrahedral, octahedral, and square pyramidal geometries. In the case of rhodium as the metal, the quinonoid complexes are excellent catalysts for carbon–carbon coupling from aryl boronic acids, and activated olefins and aldehydes. The rhodium species also forms self-supporting coordination networks in the presence of main group alkoxides, and these function as catalysts for acetylene polymerization. The catalysts can also be deposited strongly on silica gel supports. An exciting application of this chemistry is the derivatization of the surfaces of magnetic nanoparticles (MNPs). In the case of superparamagnetic FePt and ferrite nanoparticles (NPs), the quinonoid–metal complexes bind to the surface and function as centers for the growth of coordination polymers. This provides multiple catalytic centers that are subject to magnetic decantation, and constitutes the first generalized methodology for the combination of organometallics with NP surfaces. Keywords: quinones; metal–organic frameworks; rhodium catalysis; Suzuki–Miyaura coupling; self-supporting nanocatalysts; surface-functionalized metallic nanoparticles