Homo- and hetero-polymetallic exchange coupled metal-oximates

The strategy of using 'metal oximates' building blocks as ligands to synthesize various homo- and hetero-metal paramagnetic complexes is reviewed. The modular approach with 'metal-complexes', in this case 'metal-oximates', as ligands enables the synthesis of symmetrical, asymmetrical, and linear complexes, viz. MAMB, MAMBMA, MAMBMBMA (M-A, M-B being two different metal ions). Even the synthesis of asymmetric heterotrinuclear complexes with MAMBMC cores and of M-A(mu(3)-O)(2)M-B butterfly cores has been achieved. This article is focused on such exchange coupled polymetallic, both homo- and hetero-metallic, systems containing the bridging core M-N-O-M' with an aim to delineate the exchange mechanism through the bridging oximates. The uniqueness of oximates providing diatomic NO-bridging is demonstrated by several series of isostructural complexes with different terminal metal ions like Fe(III), Mn(III), Mn(IV) and Cr(III). Such isostructural series are not available for any other bridging ligands. A qualitative rationale for the trend and nature of exchange interactions between the spin carriers has been provided. This article confirms the essentially a nature of the exchange interaction transmitted through the diatomic NO-bridging ligand and the applicability of Goodenough - Kanamori rules, in general, to predict the nature of exchange interactions for different heterometal compounds. The pi-conjugated system of the oxime ligand, delocalized over the whole bridging groups and perpendicular to the plane of the oxime ligand, appears to play an important role in the antiferromagnetic interaction between the terminal metal ions, separated by as much as similar to 7 Angstrom. A comparison of the complexes MAMBMA containing a diamagnetic central ion (M-B) clearly indicates the participation of the central metal ion in the transmission of the antiferromagnetic exchange interaction between the terminal paramagnetic MA ions. Competing exchange interactions, or spin frustration in the general sense of the term, in polynuclear complexes may lead to ground-state variability. This type of variable ground-state has been observed for several complexes. The strategy of irregular spin-state structure resulting from a particular spin topology has been found to be more effective, as expected, in obtaining 'high spin' molecules than the common strategy of obtaining ferromagnetically coupled systems through involvement of symmetry-related strict orthogonality of the magnetic orbitals of the interacting metal centers. Of particular interest for molecular magnetism is the small but significant effect of bridging ligands like carboxylato anions for cooperation with the ancillary ligand, viz. the oxime ligands, to build up high-nuclearity metal clusters. (C) 2003 Elsevier Science B.V. All rights reserved.