Synthesis and magnetic properties of 1:1 salts containing the bis(η6-hexamethylbenzene)cobalt cation

Abstract

The salts [Co(η⁶-C₆Me₆)₂]⁺[A]^–(A = BPh₄, NbF₆, PF₆ or SbF₆) have been prepared from anhydrous CoCl₂. Solid-state magnetic susceptibility measurements on these salts indicate that they all obey the Curie–Weiss law in the temperature range 30–300 K with µ_eff= 2.7, 2.8, 2.8 and 2.6 µ_B with corresponding θ values of –1.4, 0.2, –4.1 and 0.4 K for the BPh₄^–, NbF₆^–, PF₆^– and SbF₆^– salts respectively. Below 20 K, the magnetic susceptibility for all the salts deviate substantially from the Curie–Weiss law. The experimental susceptibility data for the compounds [Co(η⁶-C₆Me₆)₂]⁺[BPh₄^]– and [Co(η⁶-C₆Me₆)₂]⁺[NbF₆]^– can be fitted to a model in which the ³A_2g triplet ground state assigned to the [Co(η⁶-C₆Me₆)₂]⁺ moiety exhibits a positive zero-field splitting of 17.8 and 20.5 cm^–1 respectively. In contrast, the magnetic susceptibilities for [Co(η⁶-C₆Me₆)₂]⁺[A]^–(A = PF₆ or SbF₆) cannot be fitted to this model in the range 2–30 K due to more significant intermolecular antiferromagnetic spin–spin interactions. All the salts were EPR silent at between 4 and 300 K both in the solid state and in frozen solution. Reaction of [Co(η⁶-C₆Me₆)₂]⁺[PF₆]^– with either [NBu₄][tcnq](tcnq = tetracyano-p-quinodimethane) or [NBu₄][C₃{C(CN)₂}₃][C₃{C(CN)₂}₃= tris(dicyano-methylene)cyclopropane] gave insoluble paramagnetic precipitates with µ_eff= 4.1 and 4.5 µ_B and θ values of –12.9 and –3.5 K respectively.