Published: March 29, 2011 r2011 American Chemical Society 3849 dx.doi.org/10.1021/ic200256j | Inorg. Chem. 2011, 50, 3849–3851 COMMUNICATION pubs.acs.org/IC Raising the Spin of Fe III 7 Disklike Clusters: The Power of Molecular Spin Frustration Shreya Mukherjee, Rashmi Bagai, Khalil A. Abboud, and George Christou* Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States b S Supporting Information ABSTRACT: Two clusters with a new type of Fe III 7 disklike structure have been prepared; in contrast to other Fe III 7 disks, they possess high ground-state spins (S = 15 / 2 and 21 / 2 ), which have been rationalized by analysis of the spin-frustra- tion patterns. M olecules possessing large numbers of unpaired electrons (i.e., large ground-state spin, S) represent a fascinating subarea of metal cluster chemistry of importance to many fields. 1,2 For example, when coupled with significant easy-axis magnetoanisotropy, such molecules function as single-molecule magnets (SMMs), providing a molecular approach to nanoscale magnetism. 3,4 In contrast, when they possess little or no anisot- ropy, they are of interest as components for molecule-based magnetic refrigeration based on the magnetocaloric effect. 5 Also, of course, there is the fundamental desire to understand how the signs and relative magnitudes of the many exchange interactions within a cluster yield its high S value. We recently studied the origin of the S = 11 ground state of Mn 7 (4Mn II , 3Mn III ) complexes with a disklike structure 6,7 and showed it to be due to spin-frustration effects from competing exchange interactions of both ferromagnetic (F) and antiferro- magnetic (AF) nature. The relative magnitude of the various exchange parameters (J) suggested that complexes with the maximum S = 16 might be attainable, and they were successfully prepared from ligand-induced structural perturbations that al- tered the relative magnitude of the competing interactions. We then wondered whether spin modification might also be possible in the Fe III 7 disklike complexes that we and others had studied, 810 such as [Fe 7 O 3 (O 2 CR) 9 (mda) 3 (H 2 O) 3 ](1; mdaH 2 = N-methyl- diethanolamine), which possesses a buckled Fe 6 loop around a central Fe atom and an S = 5 / 2 ground state. As in our Mn 7 work, the first priority was to identify the origin of the S = 5 / 2 ground state of 1 because we expected all interactions now to be AF. We did this by determining the J values using a published magnetostructural correlation originally developed for dimers that employs both the FeO distances and FeOFe angles. 11 The J values (Figure 1) are indeed all AF but are of two types: relatively strong (20 to 39 cm 1 ) and weak (8.89 cm 1 ). The ground state can thus be rationalized (Figure 1) as comprising an antiparallel alignment of spins controlled by strong interactions and a parallel alignment of spins controlled by weak ones (Fe1Fe2 and its symmetry partners); i.e., these AF interactions are completely frustrated. This does not offer hope for experimentally changing the S = 5 / 2 ground state of 1 via small ligand-induced perturbations, in contrast to Mn 7 . 6,7 With strong AF interactions both between Fe atoms of the outer ring (J oo ) and between them and the inner Fe (J io ), where o = outer and i = inner, it would clearly take a major modification to affect the ground state, and it is not obvious how to target this. However, this was achieved through happenstance when we recently made a new type of Fe 7 disk while exploring Fe III chemistry with N,N 0 -bis(2-hydroxyethyl) ethylenediamine (heenH 2 ) and 2-(2-pyridylmethyl)aminoethanol (paeoH). This gives much higher ground-state spins. We have previously used heenH 2 in Fe chemistry but not paeoH. 12 In the present work, the reaction of FeCl 2 and heenH 2 (1:1) in refluxing MeOH gave upon cooling [Fe 7 O 3 (OMe) 3 - (heen) 3 Cl 4.5 (MeOH)(H 2 O) 1.5 ]Cl 1.25 [FeCl 4 ] 1/4 (2), isolated as orange needles of 2 3 2MeOH 3 1 / 2 H 2 O in 10% nonoptimized yield after 7 days. Similarly, the reaction of Fe(ClO 4 ) 3 , paeoH, and NEt 3 (1:3:1) in MeOH gave [Fe 7 O 3 (OH) 3 Cl(paeo) 6 ](Cl) (ClO 4 ) 4 (3) as orange crystals of 3 3 2Me 2 CO 3 1 / 2 Et 2 O in 14% nonoptimized yield. The cations of 2 and 3 have almost identical Fe 7 cores (Figure 2) 13 consisting of a near-planar Fe III 6 hexagon linked to a central Fe III ion by three μ 3 -O 2 ions and lying 1.437 Å (2) or 1.484 Å (3) above the Fe 6 plane. In 2, each heen 2 is η 2 :η 1 :η 1 :η 2 :μ 3 , chelating to an outer Fe and bridging to neighboring Fe atoms on either side. In 3, each of the now six paeo  groups is η 2 :η 1 :η 1 :μ 2 , chelating to one Fe and bridging to only one neighbor. Additional bridges between outer Fe atoms are by three μ 2 -OMe  (2) or μ 2 -OH  (3) groups, and terminal ligation at three outer Fe atoms in 2 is by a Cl  and either H 2 O or MeOH. Ligation at the central Fe is completed by a terminal Cl  ion. The main difference between 2/3 and prior Fe 7 disks is the tetrahedral geometry of the central Fe, which also rationalizes the near-planar Fe 6 hexagon. In both 2 and 3, the cations are surrounded by two types of anions and by solvate molecules; the [FeCl 4 ]  anion in 2 forms no interactions with the cation, directly or via solvent molecules, and thus is at best only very weakly exchange- coupled to the cation. Received: February 6, 2011