Transmission of Magnetic Interactions through an Organometallic Coupler: A Novel Family of Metallocene-Substituted alpha-Nitronyl Aminoxyl Radicals.

The capability of metallocene bridges as new organometallic magnetic couplers is evaluated by studying the family of diradicals 2 (M = Fe, Ru) consisting of two purely organic alpha-nitronyl aminoxyl radicals connected by a 1,1'-metallocenylene bridge. Preliminary studies performed with 2-metallocenyl-alpha-nitronyl aminoxyl monoradicals 1 (M = Fe, Ru, Os), as reference compounds, show the presence of a small spin density on the central metal of the metallocenes. This fact makes the metallocene units effective bridges to transmit magnetic interactions by a spin polarization mechanism. The study of the magnetic properties of diradicals 2 in the solid state and in diluted frozen solutions reveals the existence of an intramolecular antiferromagnetic exchange interaction between the radical subunits whose strength is highly dependent on the molecular conformation adopted by the diradical. As shown by crystal data and by ESR measurements, an intramolecular hydrogen bond between the two radical units forces the molecule to adopt a cisoid molecular conformation, which determines that the magnetic interaction occurs by a direct through-space interaction between the two SOMOs of the two radical units along with the classical spin polarization mechanism through the sigma-bonds of the metallocene unit. Lattice constants for both structures are as follows: 1 (M = Fe), C(17)H(21)FeN(2)O(2), a = 7.170(1) Å, b = 10.135(2) Å, c = 10.683(2) Å, alpha = 88.88(3) degrees, beta = 83.42(3) degrees, gamma = 79.75(3) degrees, triclinic, P&onemacr;, Z = 2; 2 (M = Fe), C(24)H(32)FeN(4)O(4), a = 11.848(3) Å, b = 11.785(2) Å, c = 17.728(4) Å, beta = 106.25(2) degrees, monoclinic, P2(1)/n, Z = 4.