Synthesis, Structure, and Magnetic Properties of an Fe₃₆ Dimethylarsinate Cluster: The Largest “Ferric Wheel”

Abstract

The synthesis and characterization of a high-nuclearity FeIII/O/arsinate cluster is reported within the salt [Fe36O12(OH)6(O2AsMe2)63(O2CH)3(H2O)6](NO3)12 (1). The compound was prepared from the reaction of Fe(NO3)3·9H2O, dimethylarsinic acid (Me2AsO2H), and triethylamine in a 1:2:4 molar ratio in acetonitrile. The Fe36 cation of 1 is an unprecedented structural type consisting of nine Fe4 butterfly units of two types, three {FeIII 4(μ3-O)2} units A, and six {FeIII 4(μ3-O)(μ3-OH)} units B, linked by multiple bridging Me2AsO2 – groups into an Fe36 triangular wheel/loop with C 3 crystallographic and D 3 virtual symmetry that looks like a guitar plectrum. The unusual structure has been rationalized on the basis of the different curvatures of units A and B, the presence of intra-Fe36 hydrogen bonding, and the tendency of Me2AsO2 – groups to favor μ3-bridging modes. The cations stack into supramolecular nanotubes parallel to the crystallographic c axis and contain badly disordered solvent and NO3 – anions. The cation of 1 is the highest-nuclearity “ferric wheel” to date and also the highest-nuclearity Fe/O cluster of any structural type with a single contiguous Fe/O core. Variable-temperature direct-current magnetic susceptibility data and alternating-current in-phase magnetic susceptibility data indicate that the cation of 1 possesses an S = 0 ground state and dominant antiferromagnetic interactions. The Fe2 pairwise J i,j couplings were estimated by the combined use of a magnetostructural correlation for high-nuclearity FeIII/oxo clusters and density functional theory calculations using broken-symmetry methods and the Green’s function approach. The three methods gave satisfyingly similar J i,j values and allowed the identification of spin-frustration effects and the resulting relative spin-vector alignments and thus rationalization of the S = 0 ground state of the cation.

Semiempirical Magnetostructural Correlation for High-Nuclearity MnIII-Oxo Complexes: Accommodation of Different Relative Jahn–Teller Axis Orientations

“…This was also the case when the WG MSC was modified for application to dinuclear M/oxo complexes of other metal ions, including heterometallic compounds . Our polynuclear MSC, however, was developed to be more general because bis-monoatomically bridged {Fe 2 O 2 } units are common within higher nuclearity clusters. − On the basis of the above considerations, it was decided to target a polynuclear Mn III /oxo MSC using a parallel procedure to our Fe III /oxo MSC.…”

“…For Fe III −O−Fe III units, most of these situations are not possible, and the overall J ij is determined by multiple pathways involving the overlap of half-filled orbitals. This leads to AFM J ij of usually significant magnitude except when the Fe−O−Fe angles are small (typically ≪100°) and some pairs of orbitals are therefore near-orthogonal, 20−22 as occurs in bis-and tris-monoatomically bridged {Fe(μ-O) n Fe} (n = 2, 3) systems, 15,16 and also predicted by the Fe III /oxo MSC. 11 For Mn III −O−Mn III , however, the presence of an empty d orbital can lead to significant FM contributions to the overall J ij , which is still typically AFM (and weaker than for Fe III ) but sometimes even FM.…”

“…With such objectives in mind, we previously reported an MSC specifically developed for higher nuclearity Fe III –oxo clusters of any nuclearity and topology; this was an updated and more general version of one we had earlier constructed specifically for hexanuclear Fe III /oxo clusters with a particular structure . This new MSC has successfully predicted reliable J ij values for various high-nuclearity Fe III /oxo clusters, , currently up to Fe 24 and Fe 36 clusters. The obtained J ij were used to rationalize experimental ground state S values, simulate variable-temperature magnetic susceptibility data, and serve as reliable input values for magnetic susceptibility fits, when possible, to minimize false-fit problems and give reasonable fit values …”

Comparative Density Functional Theory Study of Magnetic Exchange Couplings in Dinuclear Transition-Metal Complexes

“…Using DFT, an intractable multireference problem can be reduced to the manageable task of extracting effective exchange-coupling constants between interacting magnetic centers in a pairwise manner as embodied in a nearest-neighbor spin-Hamiltonian representation, allowing DFT to tackle large nuclearity complexes with ease. 25 The overall accuracy of any DFT calculation is largely determined by the choice of the exchange−correlation (XC) functional. This is especially apparent for calculations of magnetic properties, in part because the differences in energy between two magnetic states are often small.…”

“…Density functional theory (DFT) with the so-called broken symmetry techniques − offers efficiency gains and potential for a reduced number of reference calculations. Using DFT, an intractable multireference problem can be reduced to the manageable task of extracting effective exchange-coupling constants between interacting magnetic centers in a pairwise manner as embodied in a nearest-neighbor spin-Hamiltonian representation, allowing DFT to tackle large nuclearity complexes with ease …”