Cobaltocenium hexafluorophosphate

Since the solubility of long-chain alkenes is higher in ionic liquids than in water, there is much interest in finding effective ionic liquid catalysts for the hydroformylation reaction. Bis-phosphines have proved to be particularly useful in hydroformylation reactions and a bis-phosphine with a charged cobaltocenium backbone, analogous to l,l/-bis(diphenylphosphino)ferrocene (dppf), has been developed specifically for use in ionic liquid hydroformylation reactions [26], In combination with [Rh(CO)2(acac)], the 1, I -bis(diphenylphosphino)cobaltocenium hexafluorophosphate ligand dissolved in [bmim][PF6] effectively catalyses the hydroformylation of 1-octene as shown in Scheme 8.11. 

In an attempt to better understand the mechanism for encapsulation of cobaltocenium inside 46 we performed careful titrations of this redox-active cation with variable concentrations of host 4. To our surprise, we quickly realized that only 2-3 equivalents were necessary to fully shut down the electrochemical response of cobaltocenium.51 This result is in strong contrast to the NMR data, which clearly indicate a 6 1 [host/guest] stoichiometry for full encapsulation. There are two important differences between these two types of experiments. In the NMR experiments, solutions were prepared in pure, deuterated CD2CI2 and the only solutes present are cobaltocenium hexafluorophosphate (ca. 1 mM) and host 4 (0-8 mM). In the electrochemical experiments, solutions were prepared in isotopically unenriched CH2CI2 also containing 0.1 M tetradodecylammonium bromide as supporting electrolyte. The concentrations of cobaltocenium hexafluorophosphate and host 4 were similar to those used in the NMR experiments. It clearly became evident that the nature of the supporting electrolyte, especially the nature of its anion, was crucial to... 

Besides EXAFS and related techniques, electrochemical methods have been used extensively to characterize metals in ILs. Lagunas et al. studied the electrochemistry of the ferricenium/ferrocene couple in Bmim-based ILs [194, 195], In a similar study, Rogers et al. studied ferrocene and cobaltocenium hexafluorophos-phate in a range of common ILs, including phosphonium ILs. The authors report solubilities and diffusion coefficients for a variety of concentrations and suggest that both ferrocene and cobaltocenium hexafluorophosphate are useful and reliable internal standards for use in ILs [196],... 

A first ligand system especially designed for the use in ionic liquids vras described in 2000 by Salzer et al. [160]. Cationic ligands with a cobaltocenium backbone were successfully used in the biphasic, Rh-catalyzed hydroformylation of 1-octene. l,T-Bis(diphenylphosphino)cobaltocenium hexafluorophosphate (cdpp) proved to... 

In the ligand series synthesized, l,l -bis(diphenylphosphino)cobaltocenium hexafluorophosphate (5) proved to be particularly suitable. In [BMIM]]PF,5], high catalyst activity (810 h ), high selectivity for aldehydes (94%) and for the n-product (n/i = 16.2), and above all no detectable catalyst leaching were demonstrated. The analogous cobaltocenium (6), in which an alkyl bridge was placed between the cyclopentadienyl and the phosphorus atom, yields much less positive effects (e.g., TOF = 66 h ). This result can be explained by the electronic influence of the central... 

Three tracers incorporating the cobaltocenium fragment have been described and utilized in MIA. These are the tracers for amphetamine, a psycho-stimulant, and for diphenylhydantoin and phenobarbital, whose therapeutic activity is outlined above. The general procedure is to use peptide coupling to conjugate an amine, whether pre-existing or added to the analyte, with the acid chloride of cobaltocenium hexafluorophosphate 34 prepared according to the method of Sheats [47] (Scheme 8.16). 

A cob alto cenium derivative working as a substrate of the enzyme alkaline phosphatase (AP), namely [(4-hydroxyphenyl)amino]-carbonyl]cobaltocenium hexafluorophosphate 42 was prepared starting from [(4-phenylphosphate) amino]-carbonyl] colbatocenium hexafluorophosphate 41 following the general procedure of Silverberg et al. [52]. Compound 41 was obtained as described by Beer et al. [53] as shown in Scheme 8.17 [54]. 

Table 3.5 Redox potentials of cobaltocenium hexafluorophosphate vs. Fc + in different organic media...

Figure 12.5 Cyclic voltammograms for the reduction of 1 mM cobaltocenium (Cp2Co+) hexafluorophosphate and oxidation of 1 mM ferrocene (Cp2Fe) in acetonitrile recorded at a band electrode (width = 4.6 / m) at a scan rate of 10 mV s 1. The supporting electrolyte is tetrabutylammonium hexafluorophosphate at (A) 0.02 M, (B) 0.2 mM, (C) 2.0 mM, and (D) 20 mM. [From Ref. 68, reprinted with permission of the copyright holder.]...

In the case of amphetamine, the coupling is carried out directly from its primary amine [48]. Labeling of diphenylhydantoin was carried out by coupling onto 3-(2-aminoethyl)-5,5-diphenylhydantoin 36 [49], the latter amino derivative having been prepared by the method of O Neal [50]. In the case of phenobarbital, labeling is effected by coupling between m-aminophenobarbital 38 and cobaltocenium carboxylic acid hexafluorophosphate 39 in the presence of DCC [51]. Preparation of m-aminophenobarbital 38 was by electrochemical reduction of the nitro derivative 14 [51], whose synthesis is outlined in Scheme 8.9. 

The hexameric arene capsules 397 and 401 (Scheme 3.86) have been studied in [82] as potent caging ligands for the encapsulation of cobaltocenium cation. According to NMR data, these ligands form 1 1 cage complexes that result in substantial changes in the CVs the reversible wave characteristic of the free cobaltocenium cation disappears after caging. Tetraalkylammonium halides do not affect this encapsulation process. In the case of hexafluorophosphate, tetrafluo-roborate, and perchlorate salts of these cations. 

---