Mixed valency in polynuclear MnII/MnIII, MnIII/MnIV and MnII/MnIII/MnIV clusters: a foundation for high-spin molecules and single-molecule magnets.

Mixed-valent Mn/O dinuclear and polynuclear molecular compounds containing MnIII are almost without exception trapped valence. Large differences between the strengths of the exchange interactions within MnIIMnIII, MnIIIMnIII and MnIIIMnIV pairs lead to situations where MnIIIMnIV interactions, the strongest of the three mentioned and antiferromagnetic in nature, dominate the intramolecular spin alignments in trinuclear and higher nuclearity mixed-valent complexes and often result in molecules that have large, and sometimes abnormally large, values of molecular spin (S). When coupled to a large molecular magnetoanisotropy of the easy-axis-type (negative zero-field splitting parameter, D), also primarily resulting from individual Jahn-Teller distorted MnIII centres, such molecules will function as single-molecule magnets (molecular nanomagnets). Dissection of the structures and exchange interactions within a variety of mixed-valent Mnx cluster molecules with metal nuclearities of Mn4, Mn12 and Mn25 allows a ready rationalization of the observed S, D and overall magnetic properties in terms of competing antiferromagnetic exchange interactions within triangular subunits, resulting spin alignments and relative orientation of MnIII JT axes. Such an understanding has provided a stepping stone to the identification of a 'magnetically soft' Mn25 cluster whose groundstate spin S value can be significantly altered by relatively minor structural perturbations. Such 'spin tweaking' has allowed this cluster to be obtained in three different forms with three different groundstate S values.