Polyoxometalate Molecular Magnets DOI: 10.1002/anie.200903541 Electronic Control of Spin Coupling in Keplerate-Type Polyoxo- molybdates** Bogdan Botar, Arkady Ellern, Raphael Hermann, and Paul Kögerler* The structuraland functional diversity ofpolyoxometalate (POM) chemistry [1] renders POMs ideal model platforms for a range of applications that require a systematic variation of the molecularelectronic properties, such as homogeneous catalysis or molecular magnetism. [2, 3] The interaction between itinerant and localized d electrons in such systems is crucial and determines, for example,electron storage properties, optical excitations, or magnetic interactions. However,these processes remain poorly understood, as are the formation mechanisms of the more complex POMs. Nanosized poly- oxomolybdates in particular form a wide spectrum of mixed- valent Mo V/VI clusters in which the localization or delocaliza- tion (Robin–Day class I to III) of the 4d electrons primarily depend on the linking modes of the MoO n polyhedra (corner-, edge-,or face-sharing). Furthermore,these structures can incorporate various unusual spin polytopes comprising 3d [4] or 4 f [5] ions that interlink or replace POM building blocks, that is, structuralfragmentswith characteristic connectivities that define the structural chemistry ofpolyoxomolybdates and -tungstates. [6] These spin polytopes exhibit magnetic phenom- ena that were observed in discrete molecules for the first time, such asmetamagnetic spin-phase transitions, which are a consequence of high-symmetry spin frustration. [7] Of particular importance are the magnetically function- alized derivatives of structural polyoxomolybdate archetypes that were first identified by Mller and co-workers. Keplerate structures, in which 30 mono-or dinuclearlinker groups connect12 pentagonal C 5v -symmetric {(Mo)Mo 5 } building blocks to form a spherical cluster of approximate I h symme- try, [8] can integrate Fe 3+ , [9] Cr 3+ , [10] or VO 2+[11] magnetic linker groups,thus creating a spin icosidodecahedron, that is, a molecularanalogue of a Kagom lattice. [12] Although a stepwise exchange of these spin centers is an advantage of polyoxometalate chemistry compared to classical polynuclear coordination complexes, [13, 14] to date,no strategies exist that allow the control and tuning of the intramolecular magnetic exchange energies that are mediated by the (diamagnetic) POM fragments. [15] We recently identified a syntheticprocedure forthe isolation ofKeplerate structures based on the reaction of multivalent heterometal centers with larger polyoxomolybate(V/VI)units that coexist in a reaction solution ata certain pH.This approach yielded Keplerate clusters of the type {Mo 72 Mo 8 V 22 }, [16] {Mo 72 V 15 Fe 7 Mo 8 },and {Mo 72 V 11 Fe 11 Mo 8 }, [14] which were obtained as kinetic products within minutes to a few hours following addition of KCl. Herein, we demonstrate for an analogous reaction system, how novel {Mo 72 Fe 30 } Keplerate species with partially reduced {(Mo)Mo 5 } building blocks(Robin–Day class III) can be identified,based on Raman spectroscopy time profiles of acidified aqueous molybdate reaction solutions. Crucially,in these systems the 4d electron density on the still diamagnetic POM fragments strongly influences the magnetic exchange. A representative of this class of compounds with 16 4d electrons per cluster anion was isolated as K 13 Na 3 {VO(H 2 O) 5 } 3 - [{Mo 6 O 21 (H 2 O) 3 (SO 4 )} 12 (Fe(H 2 O) 2 ) 30 ]·120 H 2 O·6 K 2 SO 4 (1 = K 13 Na 3 {VO(H 2 O) 5 } 3 1 a·120 H 2 O·6 K 2 SO 4 ), and its formation was spectroscopically monitored. The composition of such reaction solutions is complex, particularly within pH ranges in which structural types such as spherical Keplerate clusters and molybdenum blue-type ring clusters coexist. [17] However, resonance Raman spectroscopy (the excitation line at 1064 nm coincides with Mo VI ! Mo V IVCT transitions) allows us to follow the dominant reduced molybdate species and their specific reactions (Figure 1). Addition of vanadylsulfate and a large excess of ferrous sulfate to an acidified molybdate solution (pH 1.8) affords a dark blue solution within 5–10 min, the corresponding Raman spectrum ofwhich has a line pattern in the 200–500 cm 1 region that is characteristicfor molybdenum bluering clusters. [18] This pattern is retained with little change over the following 24 h. Interestingly, at this stage, the addition of a large excess of KCl results in the precipitation of Keplerate speciesthat are thus present in equilibrium with the molybdenum blue-type ring clusters. The crystalline product was identified spectroscopically and crystallographically as Keplerate clusters {Mo 72 Mo V x Fe III y V IV z } in which [VO- (H 2 O)] 2+ , [Fe(H 2 O) 2 ] 3+ , and [MoO(H 2 O)] 3+ groups are stat- istically distributed overthe 30 linker positionsof the Keplerate spheres. The exclusive precipitation of Keplerate cluster anions (and not of ring clusters) is caused by the the specific affinity [*] Dr. B. Botar, Prof. Dr. P. Kgerler Institut fr Festkrperforschung, Forschungszentrum Jlich 52425 Jlich (Germany) Fax: (+ 49) 2461-61-2620 E-mail: p.koegerler@fz-juelich.de Dr. A. Ellern Dept. of Chemistry, Iowa State University, Ames, IA 50011 (USA) Dr. R. Hermann Dept. of Physics, Universit de Lige, 4000 Sart-Tilman (Belgium) Prof. Dr. P. Kgerler Institut fr Anorganische Chemie, RWTH Aachen University 52074 Aachen (Germany) Fax: (+ 49) 241-80-92642 E-mail: paul.koegerler@ac.rwth-aachen.de [**] We thank Dr. M.T. Sougrati for help with recording the Mssbauer spectra. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200903541. Communications 9080 2009 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim Angew. Chem.Int.Ed.2009,48,9080 –9083