Mixed valency in polynuclear Mn ^II /Mn ^III , Mn ^III /Mn ^IV and Mn ^II /Mn ^III /Mn ^IV clusters: a foundation for high-spin molecules and single-molecule magnets

Abstract

Mixed-valent Mn/O dinuclear and polynuclear molecular compounds containing Mn III are almost without exception trapped valence. Large differences between the strengths of the exchange interactions within Mn II Mn III , Mn III Mn III and Mn III Mn IV pairs lead to situations where Mn III Mn IV 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 Mn III centres, such molecules will function as single-molecule magnets (molecular nanomagnets). Dissection of the structures and exchange interactions within a variety of mixed-valent Mn x cluster molecules with metal nuclearities of Mn 4 , Mn 12 and Mn 25 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 Mn III JT axes. Such an understanding has provided a stepping stone to the identification of a 'magnetically soft' Mn 25 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.

Mixed valence: origins and developments

“…Likewise, finite class 2 clusters containing Mn(II) and Mn(III), for example, have combinations of ferro-and antiferromagnetic intracluster magnetic exchange, which yield very high spin ground states so that the clusters behave as nano-sized superparamagnets, as described here by Christou (Stamatatos & Christou 2008). When analogous situations are found in infinite lattice oxides, such as perovskites, we find many fascinating phenomena arising from the different distributions of the two oxidation states, coupled with the interaction between localized and delocalized electrons.…”

‘Metal Complexes as Ligands’ for the Synthesis of Coordination Polymers: A MnIII Monomer as a Building Block for the Preparation of an Unprecedented 1-D {MnIIMnIII}n Linear Chain

“…The room-temperature χ M T value of 7.52 cm 3 Kmol −1 is very close to the theoretical value of 7.375 cm 3 Kmol −1 for two non-interacting Mn II and Mn III ions (g = 2.00) of the 1-D repeating unit. The χ M T product remains almost constant in the 300-100 K range, and upon further cooling, it steadily decreases down to a value of 4.85 cm 3 Kmol −1 at 2 K. The overall magnetic response of 2 is suggestive of the presence of weak antiferromagnetic exchange interactions between the metal ions, as expected for a system where the alternating Mn II and Mn III ions are solely bridged by a syn-anti carboxylate group [38,43].…”

Two-dimensional molecular magnets with weak topological invariant magnetic moments: mathematical prediction of targets for chemical synthesis

“…The magnetic properties of a different Mn 12 complex were also reported to be affected when the molecule was adsorbed on a gold surface with a self-assembled monolayer [22]. Other studies have further shown that significant structural deformations and changes in the spin centre-spin centre coupling strengths occur when Cr 7 Ni and Mn 6 MMs are adsorbed on gold surfaces [23][24][25]. On the other hand, some experiments have achieved surface-adsorbed MMs with intact magnetic properties.…”

“…. can be described by the Ky Fan metric [41], 6) which shows that for any fixed > 0, there exists an integer n 0 such that for all k > n 0 ,…”

“…MMs possess unique magnetic properties that span both classical and quantum domains, making them of great fundamental interest, and the elaborate structures of large MMs make them challenging targets for synthetic chemistry [5]. Figure 1 shows the structure of the prototypic example of an MM, Mn 12 16 12 (acetate) molecule a total electron spin of 10 [2,6]. The structure of MMs is often described in terms of the arrangement of spin centres in the molecule.…”