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44691-G3
New Strategies for Incorporating Magnetic Anisotropy into Single-Molecule Magnets

Matthew P. Shores, Colorado State University

      The synthesis of new single-molecule magnet (SMM) candidate materials requires the balancing of many factors—high nuclearity, magnetic coupling (J), large ground state spin (S) and magnetic anisotropy (D)—most of which compete with each other. Our projects in the second year of PRF support have focused on increasing D in exchange-coupled clusters by preparing molecules and ligand systems with topological anisotropy. The hypothesis is that enforcing molecular shape anisotropy will predictably drive magnetic anisotropy.

      (1) Transition metal ethynylbenzene chemistry. Our goal was to expand known Fe(III)-tris(ethynylbenzene—TEB) complexes[1] to achieve discotic SMMs. We prepared the Fe(III)-containing di- and trinuclear core complexes 1 and 2 by oxidation of their Fe(II) analogues (only the neutral dinuclear complex was known) (Figure 1). The magnetic exchange interactions displayed in 1 and 2 are as predicted: strong antiferromagnetic coupling in 1, and ferromagnetic interactions for 2 (J = –133 and +29 cm^–1 from MAGFIT;[2] spin Hamiltonian = –2J(S_i·S_j)). These values are promising for isolating high-spin ground states, and the presence of replaceable chloride ions means that 1 and 2 are poised for growth to higher nuclearity species, both necessary for the development of SMMs.

Figure 1. Structures of [(dmpe)₄Fe₂(TEB)Cl₂]²⁺ and [(dmpe)₆Fe₃(TEB)Cl₃]³⁺ at 40 % ellipsoids (no H); DC magnetic susceptibility data for 1 and 2 (H = 1000 Oe).

      (2) Uranium(IV) ethynylbenzene complexes. Exploiting known triamidoamine phenylacetylide complexes,[3] we began exploring uranium magnetochemistry. The purpose of our initial study was to check for exchange coupling between U(IV) species with trigonal bipyramidal coordination. If exchange was operative, then we would assess how the bridging geometry affected U-U interactions. We prepared the di- and trinuclear U(IV) ethynylbenzene complexes [(NN'₃)₂U₂(m-DEB)], [(NN'₃)₂U₂(p-DEB)], and [(NN'₃)₃U₃(TEB)] (3-5); they and the mono-phenylacetylide 6 were characterized by FT-IR, UV-Vis, and X-ray analyses (example of 3 in Figure 2). Magnetic susceptibility data appear to be consistent with a U(IV) non-magnetic ground state,[4] which we hoped to avoid with trigonal bipyramidal complex geometries. However, use of a subtraction scheme[5] suggests the presence of U-U magnetic interactions. Fits to the corrected data show weak ferromagnetic coupling for all three complexes; this is unexpected based on analogous transition metal complexes. We are performing electronic structure calculations in collaboration with A. Rappé (CSU) to better understand these results. This work has produced a new actinide-based system where we can expect to derive meaningful magnetostructural correlations.

Figure 2. Crystal structure of 3 with 40% ellipsoids (no H); DC magnetic susceptibility data for 3 and [(NN'₃)U(CCPh)] (6) (H = 1000 Oe).

      (3) Progress toward linear metal-cyanide clusters. The ultimate in topological anisotropy is represented by linear species. Our goal was the preparation of trinuclear complexes to test if covalent linkage of Schiff base complexes could improve magnetic anisotropy. We prepared the ether-linked bis(salpn) ligand 7, and metallated with manganese(III) acetate to form the Mn₂ complex 8, which was characterized by TOF-MS, ¹H NMR and FT-IR. Reactions of 8 with hexacyanometalates and trans-M(CN)₂ complexes are underway. We anticipate that the CN-bridged trinuclear complexes linked covalently at the ends—via rigid (not shown) or flexible (7) spacers—will allow the SMM candidates to maintain structural integrity when redissolved, something which has not been observed without the covalent linker.

Scheme 1. Synthesis of a bis-(salpn)Mn(III) complex bound to a sterically flexible spacer.

      Summary. We have synthesized 10 dia- and paramagnetic ethynylbenzene-bridged metallodendrimer building blocks, including several Co(III), Fe(II), Mn(III) and Cr(II) species (not discussed here due to space limitations) that are poised for further reactivity. The Fe(III)-containing species display predicted magnetic exchange along with sizable J couplings, encouraging our efforts to make higher nuclearity species. Our initial forays into the organometallic uranium magnetochemistry have produced new multinuclear complexes that show tantalizing evidence of U-U (ferro)magnetic interactions. Also, a series of ligands and transition metal Schiff base complexes are poised for assembly into linear metal-cyanide clusters. One paper has been submitted to JACS; three manuscripts are in preparation.

([1])     Weyland, T.; Costuas, K.; Mari, A.; Halt, J.-F.; Lapinte, C. Organometallics 1998, 17, 5569.

([2])     Schmitt, E. A. Ph.D. Thesis, University of Illinois at Urbana-Champaign, 1995.

([3])     (a) Boaretto, R.; Roussel, P.; Alcock, N. W.; Kingsley, A. J.; Munslow, I. J.; Sanders, C. J.; Scott, P. J. Organomet. Chem. 1999, 591, 174. (b) Le Borgne, T.; Riviere, E.; Marrot, J.; Thuery, P.; Girerd, J. J.; Ephritikhine, M. Chem. Eur. J. 2002, 8, 774.

([4])     (a) Almond, P. M.; Deakin, L.; Porter, M. J.; Mar, A.; Albrecht-Schmitt, T. E. Chem. Mater. 2000, 12, 3208-3213. (b) Schelter, E. J.; Morris, D. E.; Scott, B. L.; Thompson, J. D.; Kiplinger, J. L. " Inorg. Chem. 2007, 46, 5528-5536.

([5])     Edelstein, N. M.; Lander, G. H. The Chemistry of the Actinide and Transactinide Elements. Springer: 2006; Vol. 4, Ch. 20.

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