Structure and magnetism of trinuclear and tetranuclear mixed valent manganese clusters from dicyanonitrosomethanide derived ligands

Abstract

Two new mixed valent clusters of manganese have been synthesised in which the starting pseudochalcogenide ligand, dicyanonitrosomethanide, (ONC(CN) 2 − , (dcnm −)) undergoes nucleophilic addition of methanol in complex 1 and water in complex 2 during coordination to the metal ions. Crystal structures show that complex 1 is a linear trinuclear Mn(II)Mn(III)Mn(II) compound, [Mn 3(mcoe) 6]NO 3·2H 2O, containing bridging oximate moieties from the chelating ligand methyl(2-cyano-2-hydroxyimino)ethanimidate ([ONC(CN)C(NH)OCH 3] −, mcoe −). Compound 2 has a planar rhomboidal (butterfly) arrangement of Mn(II)Mn(III)Mn(II)Mn(III) with the Mn(III) ions in ‘body’ positions bridged to the ‘wingtip’, seven coordinate Mn(II) ions by μ 3-oxo atoms and by the NO − oximato groups of the cyanoacetamidooximate chelating ligand, cao −, [ONC(CN)CONH 2] −. 2 has the formula (Me 4N) 2[Mn 4O 2(cao) 4(MeCN) 2(H 2O) 6](NO 3) 4·2H 2O. There are hydrogen bonded cluster–cluster interactions in both compounds. Detailed susceptibility and magnetisation measurements on 1 and 2 reveal intra-cluster antiferromagnetic coupling with a total spin ground state at the crossover point of S T=2 and 1 for 1, with other states very close in energy, and a rare sixfold degenerate set of S T levels, 0,1,2,3,4,5, lying lowest in the case of 2. In the latter case this is largely because of the large J 13 (Mn(III)Mn(III)); body–body) value (−46.0 cm −1) compared with the wing-body (Mn(II)Mn(III)) J 12 value of −2.5 cm −1. These coupling constants and S T states are compared with those of other recent examples of planar rhomboidal mixed valent clusters, some of which show ferromagnetic J values and very large S T ground states, with single-molecule magnetic behaviour exhibited in those cases. AC susceptibility studies show that complex 2 does not exhibit single-molecule magnetism behaviour.