Re(i)-nitroxide complexes

Abstract

Spin-labeled cyrhetrenes [(NNCp)Re(CO)₃] and [(INCp)Re(CO)₃], where NNCp is nitronyl nitroxide 2-(η⁵-cyclopentadienyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and INCp is the corresponding imino nitroxide, were synthesized and characterized by EPR, CV, XRD, magnetochemistry and quantum chemistry methods. The correlations between different arrangements of paramagnetic centers and the magnetic exchange interactions for three polymorphs of [(NNCp)Re(CO)₃] were studied. It was concluded that high kinetic stability of nitroxide-substituted cyrhetrenes is a promising feature of compounds for the creation of multifunctional contrast agents.

Scheme 1

Fig. 1. Experimental EPR spectra for [(NNCp)Re(CO)₃] (a, blue curve) and [(INCp)Re(CO)₃] (b, red curve) recorded at room temperature in a degassed and diluted toluene solutions and their simulation (solid thin black curves).

Fig. 2. CV curves of oxidation and reduction of [(NNCp)Re(CO)₃] (blue line), [(INCp)Re(CO)₃] (red line) 5.0 × 10⁻³ M solutions in MeCN (a GC disk electrode d = 1.7 mm, supporting electrolyte 0.1 M Bu₄NBF₄/MeCN, the scan rate 100 mV s⁻¹, T = 298 K).

Scheme 2

Fig. 3. Molecular structure and the shortest intermolecular distances (Å) of I (a), II (b), III (c) and [(INCp)Re(CO)₃] (d). The H atoms and the geminal CH₃ groups are omitted for clarity.

Fig. 4. Experimental dependences μ_eff(T) for the polymorphs of nitronyl nitroxide [(NNCp)Re(CO)₃] (a: I – dark cyan triangles; II – dark blue squares; III – blue cycles) and imino nitroxide [(INCp)Re(CO)₃] (b: red squares). Solid thick lines – fitted data based on optimized parameters (Table 2).