The syntheses, crystal structures, and magnetic characterizations of three new hexanuclear iron(III) compounds are reported. Known [Fe(6)O(2)(OH)(2)(O(2)CBu(t))(10)(hep)(2)] (1) is converted to new [Fe(6)O(2)(OH)(O(2)CBu(t))(9)(hep)(4)] (3) when treated with an excess of 2-(2-hydroxyethyl)-pyridine (hepH). Similarly, the new compound [Fe(6)O(2)(OH)(2)(O(2)CPh)(10)(hep)(2)] (2), obtained from the reaction of [Fe(3)O(O(2)CPh)(6)(H(2)O)(3)] with hepH, is converted to [Fe(6)O(2)(OH)(O(2)CPh)(9)(hep)(4)] (4) when treated with an excess of hepH. This can be reversed by recrystallization from MeCN. The cores of the four Fe(6) complexes all comprise two triangular [Fe(3)(mu(3)-O)(O(2)CR)(3)(hep)](+3) units connected at two of their apices by two sets of bridging ligands. However, 1 and 2 differ slightly from 3 and 4 in the precise way the two Fe(3) units are linked together. In 1 and 2, the two sets of bridging ligands are identical, consisting of one mu-hydroxo and two mu-carboxylate groups bridging each Fe(2) pair, i.e., a (mu-OH(-))(mu-O(2)CR(-))(2) set. In contrast, 3 and 4 have two different sets of bridging ligands, a (mu-OH(-))(mu-O(2)CR(-))(2) set as in 1 and 2, and a (mu-OR(-))(2)(mu-O(2)CR(-)) set, where RO(-) refers to the alkoxide arm of the hep(-) chelate. Variable-field and -temperature dc magnetization measurements establish that 1 and 2 have S = 5 ground states and significant and positive zero-field splitting parameters (D), whereas 3 and 4 have S = 0 ground states. This dramatic difference of 10 unpaired electrons in the ground state S values for near-isomeric compounds demonstrates an acute sensitivity of the magnetic properties to small structural changes. The factors leading to this have been quantitatively analyzed. The semiempirical method ZILSH, based on unrestricted molecular orbital calculations, was used to obtain initial estimates of the Fe(2) pairwise exchange interaction constants (J). These calculated values were then improved by fitting the experimental susceptibility versus T data, using a genetic algorithm approach. The final J values were then employed to rationalize the observed magnetic properties as a function of the core topologies and the presence of spin frustration effects. The large difference in ground state spin value was identified as resulting from a single structural difference between the two types of complexes, the different relative dispositions (cis vs trans) of two frustrated exchange pathways. In addition, use of the structural information and corresponding J values allowed a magnetostructural correlation to be established between the J values and both the Fe-O bond distances and the Fe-O-Fe angles at the bridging ligands.

Molecules that exhibit magnetic bistability, commonly referred to as single-molecule magnets (SMMs), are of high interest because of their unusual physical properties and potential applications in quantum computing.[1] In contrast to traditional magnets, which are characterized by the bulk ordering of spins within a 3D solid, SMM behavior arises from the intrinsic magnetic properties of the individual molecules. The signature of an SMM is a slow paramagnetic relaxation of the magnetization evidenced by a frequency-dependent outof-phase (c 00 m ) ac susceptibility signal and hysteretic behavior. The presence of an appreciable thermal barrier for reversing the magnetization is related to the combination of a large total spin ground state (S) and negative uniaxial anisotropy (D). 4 ], which displays a ground state of S = 10.[2] Since this discovery, a number of oxo-bridged clusters containing V, [3] Mn, [4][5][6] Fe, [7] Co, [8] and Ni [9] have also been shown to display SMM behavior. In all cases, the molecules that have longer paramagnetic relaxation times have values of S > 3 or higher and a negative D value.In considering the prospects for preparing non-oxide SMMs, we turned our attention to chemistry inspired by the face-centered Prussian-blue solids. The recognition that bimetallic Prussian-blue solids exhibit spontaneous magnetization at temperatures as high as 376 K [10] prompted an extrapolation of this chemistry to the realm of magnetic

Molecules that exhibit magnetic bistability, commonly referred to as single-molecule magnets (SMMs), are of high interest because of their unusual physical properties and potential applications in quantum computing.[1] In contrast to traditional magnets, which are characterized by the bulk ordering of spins within a 3D solid, SMM behavior arises from the intrinsic magnetic properties of the individual molecules. The signature of an SMM is a slow paramagnetic relaxation of the magnetization evidenced by a frequency-dependent outof-phase (c 00 m ) ac susceptibility signal and hysteretic behavior. The presence of an appreciable thermal barrier for reversing the magnetization is related to the combination of a large total spin ground state (S) and negative uniaxial anisotropy (D). 4 ], which displays a ground state of S = 10.[2] Since this discovery, a number of oxo-bridged clusters containing V, [3] Mn, [4][5][6] Fe, [7] Co, [8] and Ni [9] have also been shown to display SMM behavior. In all cases, the molecules that have longer paramagnetic relaxation times have values of S > 3 or higher and a negative D value.In considering the prospects for preparing non-oxide SMMs, we turned our attention to chemistry inspired by the face-centered Prussian-blue solids. The recognition that bimetallic Prussian-blue solids exhibit spontaneous magnetization at temperatures as high as 376 K [10] prompted an extrapolation of this chemistry to the realm of magnetic

Alcoholysis of preformed tetranuclear and hexanuclear iron(III) clusters has been employed for the synthesis of four higher-nuclearity clusters. Treatment of [Fe(4)O(2)(O(2)CMe)(7)(bpy)(2)](ClO(4)) with phenol affords the hexanuclear cluster [Fe(6)O(3)(O(2)CMe)(9)(OPh)(2)(bpy)(2)](ClO(4)) (1). Reaction of [Fe(6)O(2)(OH)(2)(O(2)CR)(10)(hep)(2)] (R = Bu(t) or Ph) with PhOH affords the new "ferric wheel" complexes [Fe(8)(OH)(4)(OPh)(8)(O(2)CR)(12)] [R = Bu(t) (2) or Ph (3)]. Complexes 2 and 3 exhibit the same structure, which is an unprecedented type for Fe(III). In contrast, treatment of [Fe(6)O(2)(OH)(2)(O(2)CBu(t))(10)(hep)(2)] with MeOH leads to the formation of [Fe(10)(OMe)(20)(O(2)CBu(t))(10)] (4), which exhibits the more common type of ferric wheel seen in analogous complexes with other carboxylate groups. Solid-state variable-temperature magnetic susceptibility measurements indicate spin-singlet ground states for complexes 2 and 4. The recently developed semiempirical method ZILSH was used to estimate the pairwise exchange parameters (J(AB)) and the average spin couplings S(A)[empty set].S(B)[empty set] between the Fe(III) centers, providing a clear depiction of the overall magnetic behavior of the molecules. All exchange interactions between adjacent Fe(III) atoms are antiferromagnetic.