Asymmetric paramagnetic bimetallocenes of nickel and cobalt

Abstract

Reaction of tetramethylpentafulvalene dianion (2) with (C₅Me₅)Ni(acac) (acac = acetylacetonate) gave the asymmetric tetradecamethylbinickelocene Ni′Ni″ (M′ and M″  = penta- and nonamethylmetallocenyl, respectively). When (C₅Me₅)Co(acac) was used in the reaction, Co′-C₅Me₄, a tetramethylpentafulvalene stabilized by a (C₅Me₅)Co fragment, was obtained. The same reaction, followed immediately by oxidation with one equivalent of [Cp₂Fe]⁺[PF₆]⁻ and two equivalents of AgNO₃, gave the tetradecamethylbicobaltocenium mono- and dications, Co′Co″⁺ and Co′⁺Co″⁺, respectively. Two different metals were introduced in a tetradecamethylbimetallocene by first synthesizing a pentamethylnickelocene, which was coupled to a tetramethylcyclopentadiene (Ni′-C₅Me₄H). When this was deprotonated, the expected anion Ni′-[C₅Me₄]⁻ rearranged to Ni″-[C₅H₄]⁻. MO calculations demonstrate that the rearrangement occurs through the shift of a [(C₅Me₅)Ni]⁺ fragment from the non-methylated to the tetramethylated part of 2. Reaction of Ni″-[C₅H₄]⁻ with (C₅Me₅)Co(acac) gave the mixed-metal compound Co′Ni″.

Ni′Ni″ and Co′Ni″ have four and three unpaired electrons, respectively. They are shown by temperature-dependent ¹H NMR spectra to couple antiferromagnetically; data fits yield J  =  −195 and −174 cm⁻¹, respectively. All paramagnetic compounds gave strongly shifted ¹H and ¹³C NMR signals. The experimental shifts were converted to contact shifts that reflect the spin distribution within the molecules. The spin density proved to be delocalized from a given paramagnetic metallocene to the adjacent metallocene, regardless of whether it was diamagnetic or paramagnetic. In the latter case this led to antiferromagnetic coupling. The spin distribution was analyzed by means of MO calculations.

Cyclic voltammetry shows Co′⁺Co″⁺, Ni′Ni″ and Co′Ni″ to undergo electron transfers that introduce up to one negative and four positive charges. The redox potentials proved to depend mainly on methylation and charge localization.