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.