Magnetic Properties of High-Nuclearity Fe_x-oxo (x = 7, 22, 24) Clusters Analyzed by a Multipronged Experimental, Computational, and Magnetostructural Correlation Approach

Abstract

The synthesis, structure, and magnetic properties of three related iron(III)-oxo clusters are reported, [ F e 7 O 3 ( O 2 C P h ) 9 ( m d a ) 3 ( H 2 O ) ]( 1 ) , [Fe 2 2 O 1 4 (OH) 3 (O 2 CMe) 2 1 (mda) 6 ](ClO 4 ) 2 (2), and [Fe 24 O 15 (OH) 4 (OEt)(O 2 CMe) 21 (mda) 7 ](ClO 4 ) 2 (3), where mdaH 2 is N-methyldiethanolamine. 1 was prepared from the reaction of [Fe 3 O(O 2 CPh) 6 (H 2 O) 3 ](NO 3 ) with mdaH 2 in a 1:2 ratio in MeCN, whereas 2 and 3 were prepared from the reaction of FeCl 3 /NaO 2 CMe/mdaH 2 in a 2:∼13:2 ratio and FeCl 3 / NaO 2 CMe/mdaH 2 /pyridine in a 2:∼13:2:25 ratio, respectively, both in EtOH. The core of 1 consists of a central octahedral Fe III ion held within a nonplanar Fe 6 loop by three μ 3 -O 2− and three μ 2 -RO − arms from the three mda 2− chelates. The cores of the cations of 2 and 3 consist of an A:B:A three-layer topology, in which a central Fe 6 (2) or Fe 8 (3) layer B is sandwiched between two Fe 8 layers A. The A layers structurally resemble 1 with the additional Fe added at the center to retain virtual C 3 symmetry. The central Fe 6 layer B of 2 consists of a {Fe 4 (μ 4 -O) 2 (μ 3 -OH) 2 } 6+ cubane with an Fe on either side attached to cubane O 2− ions, whereas that of 3 has the same cubane but with an {Fe 3 (μ 3 -O)(μ-OH)} unit attached on one side and a single Fe on the other. Variable-temperature dc and ac magnetic susceptibility studies revealed dominant antiferromagnetic coupling in all complexes leading to ground-state spins of S = 5 / 2 for 1 and S = 0 for 2 and 3. All Fe 2 pairwise exchange parameters (J ij ) for 1−3 were estimated by two independent methods: density functional theory (DFT) calculations using broken symmetry methods and a magnetostructural correlation previously developed for high-nuclearity Fe III /O complexes. The two approaches gave satisfyingly similar J ij values, and the latter allowed rationalization of the experimental ground states by identification of the spin frustration effects operative and the resultant relative spin vector alignments at each Fe III ion.

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

“…The obtained J MSC / J DFT values can be seen to separate into two groups: (i) weak ones typically in the 0 to −10 cm –1 range characteristic of bis- or tris-monoatomically bridged Fe 2 pairs, i.e., {Fe­(O) 2 Fe} and {Fe­(O) 3 Fe} units, respectively, as a result of the resultant small Fe–O–Fe angles; and (ii) strong ones in the −25 to −40 cm –1 range characteristic of Fe 2 pairs with a single monatomic bridge, {Fe­(O)­Fe}, and thus larger Fe–O–Fe angles. These J i , j ranges are consistent with what we have observed previously for other Fe III /oxo clusters. ,, The tris-oxo-bridged Fe 2 pairs have particularly small Fe–O–Fe angles of 88.2° and 88.1° for the Fe10–Fe11 and Fe6–Fe7 pairs, respectively, and lead to the greatest differences in the values obtained by the three methods. In a previous study of an Fe 24 cluster, we even obtained weak ferromagnetic values for tris-oxo-bridged Fe 2 pairs .…”

“…In a previous study of an Fe 24 cluster, we even obtained weak ferromagnetic values for tris-oxo-bridged Fe 2 pairs. 17 Given the very small angles, we feel that the MSC values in Table 5 are the more reliable for these interactions. Unfortunately, we are unaware of any dinuclear Fe III compounds with three monatomic oxo bridges to allow comparisons of the sign and magnitude of reliable experimental J i,j values with those from J MSC and J DFT calculations using various functionals.…”

“…These J i,j ranges are consistent with what we have observed previously for other Fe III /oxo clusters. 17,34,60 The tris-oxo-bridged Fe 2 pairs have particularly small Fe−O−Fe angles of 88.2°and 88.1°for the Fe10−Fe11 and Fe6−Fe7 pairs, respectively, and lead to the greatest differences in the values obtained by the three methods. In a previous study of an Fe 24 cluster, we even obtained weak ferromagnetic values for tris-oxo-bridged Fe 2 pairs.…”

“…The MSC calculations employed eq developed by Mitchell and Christou, where r is the average Fe–O bond length and φ is the Fe–O–Fe bond angle. We have previously employed this MSC for several high-nuclearity Fe III /O clusters with carboxylate and/or arsinate ligands and metal nuclearities of up to 24, and the present work is its first use for nuclearities of >30. , J i , j = ( 1.23 × 10 9 ) ( − 0.12 + 1.57 cos ⁡ φ + cos 2 ⁡ φ ) normale − 8.99 r …”

“…The first example was an Fe 10 wheel, and since then many others have been reported and now comprise a large family with various Fe x nuclearities. − They are also excellent model systems for the study of 1D magnetism and quantum effects, e.g., coherent tunneling . There are also a multitude of other Fe III /oxo clusters of various structural topologies, but it is surprising that very few of them possess an Fe nuclearity of >20. − This is an area where the creative development of new synthetic procedures for high-nuclearity Fe III /O clusters would be of great benefit, as they have been in Mn cluster chemistry where Mn 70 and Mn 84 oxo clusters have been known for many years. , …”