Structural and functional evolution of metallacrowns.

This Perspective summarises the chemistry of the pyrazole ring, and reviews the metal coordination modes adopted by 1H-pyrazoles and their anions. Pyrazolide anions are probably the most versatile ligands in coordination chemistry, with 20 different terminal or bridging coordination modes having been identified so far. Metal cluster compounds supported by pyrazolido ligation are surveyed, concentrating on those reported during the past ten years. Highlights include the wide structural diversity in apparently simple main group pyrazolides; luminescence and charge-transfer complexes in coinage metal pyrazolide clusters; the use of robust metal pyrazolide clusters to construct liquid crystals, supramolecular materials and metal–organic frameworks; and supramolecular complexes formed by pyrazolide-supported metallacrowns.

Pyrazoles and pyrazolides—flexible synthons in self-assembly

In 1990, Fujita and co-workers described the first example of a rationally designed metallacycle, the molecular square [{Pd(en)(μ-4,4′-bipy)}4](NO3)8, prepared by self-assembly of the cis-protected square-planar Pd(II) precursor with two adjacent labile ligands [Pd(en)(ONO2)2] (en ) ethylenediamine) with the linear linker 4,4′-bipyridine (4,4′-bipy). Since then, supramolecular chemistry has produced an amazing number of fascinating 2D and 3D metal-mediated molecular architectures, including many macrocycles and cages. Several review articles have thoroughly covered this thriving field in recent years. Beside their shared structural beauty, some of these assemblies are finding applications as receptors and molecular vessels for trapping reactive intermediates, as well as for stoichiometric and catalytic reactions.

Trinuclear metallacycles: metallatriangles and much more.

Copper-dioxygen complex mediated C-H bond oxygenation: relevance for particulate methane monooxygenase (pMMO).

Advances in the coordination chemistry of nitrogen ligand complexes of coinage metals

Magnetic susceptibility and EPR studies show that trinuclear Cu II pyrazolato complexes with a Cu3(I3-X)2 core (X ) Cl, Br) are ferromagnetically coupled: JCu-Cu ) +286 cm -1 (X ) Cl), +31 cm -1 (X ) Br) The orderly transition from an antiferromagnetic to a ferromagnetic exchange among the Cu centers of Cu3(I3-X) complexes, X ) O, OH, Cl, Br, follows the change of the CuX-Cu angle from 120° to 80° The crystal structures of [Bu4N]2[Cu3(I3-Br)2(I-pz*)3Br3] (pz* ) pz (1a )o r 4-O 2N-pz (1b), pz ) pyrazolato anion, C3H3N2 1- ) are presented

Triangular, ferromagnetically-coupled CuII 3-pyrazolato complexes as possible models of particulate methane monooxygenase (pMMO).

A rigid trinuclear copper pyrazolato framework supports the solvolytic exchange of mu3-X by mu3-OR ligands (X = Cl and Br; R = alkyl group), converting the trinuclear ferromagnetically coupled S = 3/2 system to antiferromagnetically coupled S = 1/2 in the solid state. In contrast, we propose that, in alcoholic solutions, solvolysis results in unsymmetrical coordination of the Cu3 cluster, magnetically decoupling one Cu center from the other two. This disguises the intact triangular Cu(II)3 system as a mononuclear Cu(II) complex in its electron paramagnetic resonance spectrum.

Joshua Telser

Papers

Triangular, ferromagnetically-coupled CuII 3-pyrazolato complexes as possible models of particulate methane monooxygenase (pMMO).

Trinuclear, antiferromagnetically coupled CuII complex with an EPR spectrum of mononuclear CuII: Effect of alcoholic solvents