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

Abstract

Crystallization of ferrocene and ruthenocene substituted in the 1- and 1'-positions by two nitronyl nitroxide radicals gave the new crystal phases beta-1 (besides the known phase alpha-1), alpha-2, and beta-2 whose structures were determined by X-ray analysis. In beta-1 the radical moieties adopt transoid positions, whereas two different cisoid conformations are adopted by alpha-2 and beta-2. These conformations result from inter- and intramolecular hydrogen bonds, respectively. All compounds experience antiferromagnetic interactions, and J/k(B) values up to -7 K have been found by fitting the experimental magnetic susceptibilities to a modified Bleaney-Bowers equation. The solid diradicals alpha-1, beta-1, alpha-2, and beta-2 as well as the ferrocene 3, which was substituted by a unique nitronyl nitroxide, were investigated by (13)C and (1)H NMR spectroscopy with magic angle spinning. The carbon signals cover a range of 2000 ppm, and are well resolved such that the structure could be confirmed. Conversion of the signal shifts into spin densities disclosed the mechanisms by which spin delocalization from the nitronyl nitroxide substituents to the metallocene core occurs. The spin density distribution in alpha-1, beta-1, and 3 was also predicted by DFT calculations. There is good agreement between the experimental and theoretical trends of the spin delocalization. The magnetic interactions were discussed in the light of intramolecular spin transfer and its dependence on geometric constraints, demonstrating that the 1,1'-metallocenylene bridge is not a robust magnetic coupler.

The Organometallic Chemistry of Rh‐, Ir‐, Pd‐, and Pt‐Based Radicals: Higher Valent Species

“…Chemical and electrochemical oxidation of [M I (HBPz' 3 )(CO)(L)] (P z ¼ pyrazolyl) and [Rh I (HCPz' 3 )(CO)(L)] þ complexes has recently been described [L ¼ CO, PPh 3 , PCy 3 , P(NMe 2 ) 3 , P(p-tolyl) 3 , P(m-tolyl) 3 , P(OPh) 3 , AsPh 3 ], in which HBPz' 3 represents a tris-pyrazolylborate anion and HCPz' 3 represents the neutral carbon equivalent (Fig. 14) (42)(43)(44)(45)(46).…”

“…These species were analyzed by EPR spectroscopy in frozen solution (Table VI). Except for the P(OPh) 3 ligand, all frozen solution spectra revealed resolved N and Rh hyperfine patterns for the g z component. This, together with the g values g x , g y > 2 and g z $ 2 (see Fig.…”

“…Innocent Radical Ligands. Common ligands are sometimes substituted with noncoordinating persistent organic radical fragments, for example, nitronyl nitroxide radical fragments (3). For metal complexes containing these ligands, there is no real confusion about the oxidation state of the metal, and these radical ligands are thus not referred to as 'redox noninnocent'.…”

Restricting Magnetic Interaction Pathways in Polyoxometalate Salts of Cationic Nitronyl Nitroxide Free Radicals

“…3.6 Å (Figure 1). The overall structure can also be viewed as alternating layers of [W 6 O 19 ] 2-anions and cationic free radicals. The organic layers are very well isolated, the shortest distance between nitroxide functionalities being greater than 8 Å.…”

“…Usually, ferromagnetic interactions have been ascribed to intermolecular contacts between atoms carrying spin densities of opposite signs (McConnell I proposal) [5]. This model is an easy-to-use tool, since spin densities can be obtained experimentally (NMR or polarized neutron diffraction methods) in the solid state [6]. However, it has been shown that McConnell I model fails to predict the magnetic behaviour of numerous crystalline assemblies of NN radicals [7].…”