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

Abstract

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.

Magneto‐Structural Characterization of Metallocene‐Bridged Nitronyl Nitroxide Diradicals by X‐Ray, Magnetic Measurements, Solid‐state NMR Spectroscopy, and Ab Initio Calculations

“…Such a cisoid conformation enables through-space magnetic interaction between the two radical subunits along with the classical spin polarization mechanism through the s bonds of the metallocene unit. Similar results were found for diradical b-2, [11] which was crystallized from Et 2 O/nhexane, although at that time no crystals suitable for X-ray structure analysis were obtained.…”

“…[11] On the contrary, the two Cp rings adopt a transoid conformation characteristic of 1,1'-disubstituted metallocenes bearing conjugated substituents. Therefore, the crystal packing of diradical b-1 seems to be determined by weak intermolecular CÀH¥¥¥OÀN hydrogen bonds.…”

“…It substantiates previous results on the ferrocenebridged diradical a-1, for which the interaction in frozen solution is almost twelve times larger than in the crystal, a fact that had been attributed tentatively to conformational changes. [11] How do conformational changes affect intramolecular magnetic exchange interactions in metallocene-based nitronyl nitroxide diradicals? Considering that the magnetic interaction in compounds 1 and 2 can take place either by a classical spin polarization mechanism across the Cp rings and the metal of the metallocene unit, or by a through-space interaction between the two radical units, or by a combination of both mechanisms, two main conformational features may play a critical role in the intramolecular magnetic interactions.…”

“…[11] In that study, the spin density on the metallocene units linked to the a-carbon atom of the nitronyl nitroxide radicals was found to be very small. This was because the singly occupied molecular orbital (SOMO) of each nitronyl nitroxide subunit has a node at the a-carbon atom and, therefore, spin density on the metallocene is induced by spin polarization rather than being transmitted directly.…”

SNH reactions of ferrocenyllithium and azine N-oxides

“…An interest in heterocyclic ferrocene derivatives is due first of all to their unique photophysical, 1 magnetic, 2 and redox 3 properties along with the possibility of their application in analytical 4 and medicinal 5 chemistry, and as efficient catalytic reagents in asymmetric synthesis. 6 Heteroarylferrocenes are often synthesized by means of a "building on" of a heterocyclic subunit on the ferrocene matrix using substituents introduced before in the ferrocene structure.…”