Cobaltocenium salt

Cobaltocene is stable as a monomer under all conditions, showing no tendency to dimerize, as does the rhodium congener rhodocene. Cobaltocenium salts are easily prepared by several routes, including mild oxidation of cobaltocene, and are stable as cations. However, neutral electron-rich monomers are no longer stable if the aromaticity of one of the Cp rings is perturbed by interposed CH2 groups (e.g. CpCo(cyclohexadienyl)). Dimerization of a C-C bond adjacent to the jr-system frequently occurs (Scheme 28). The process can be followed electrochemically on reduction of the CpCo()] -E)+ cation to the neutral sandwich, which then dimerizes. Logically, the same dimerization is observed with so-called half-open cobaltocene, that is, a bis(pentadienyl)cobalt, which has one pentadienyl see... 

A transformation of some synthetic versatility is the oxidation of methyl groups in a methylcobaltocenium salt by Mn04 to give the respective carboxylic acid. In this way, mono- and dicarboxylatocobaltocenium salts have been prepared. These acids can be further functionalized and used as receptors for the selective recognition of anion guest species. A pyrrole-fimctionalized cobaltocenium salt has also been electropolymerized on an electrode surface this system displays anion sensing in solution and when immobilized. ... 

The system of Scheme 7 has also enabled a [Mo 02] center to be isolated in substance for the first time as the cobaltocenium salt, [CoC-P2][(L-N3)Mo02(SPh)]. No oxygen atom transfer chemistry has been observed to date for this species. This may be the reason that a [Mo 02] center has never been detected (via its characteristic EPR signal) 107) during turnover of any enzyme. 

Figure 12. Transient absorption spectra of (A) Mn(CO)5 and (B) Co(CO)4 radicals recorded on the microsecond time scale upon 532-nm CT excitation of the cobaltocenium salts of Mn(CO)s and Co(CO)4, respectively [118].

Herrmann recently introduced carbene-based ligands for the hydroformylation reaction (cf. Section 3.1.10) [267, 268]. The use of electron-poor phosphine-substituted cobaltocenium salts as ligands (26) for the biphasic hydroformylation has been investigated [269]. In ionic liquids this ligand enables the hydroformylation of 1 -octene at high catalyst activity and high selectivity to the -product without detectable catalyst leaching (cf. Section 3.1.1.2.2) [270]. 

Cobaltocene is reduced to a monoanion (E° = —1.9 V in MeCN) (491, 504) which is subject to electrophilic attack. Protonation by weak acids such as water or phenol occurs directly at the cyclopentadienyl ligand with no evidence for initial formation of metal hydrides (505). In dmf, the anion adds other electrophiles to give the substituted cyclopentadiene compounds [Co(r 4-C5H5 R)Cp]. Hydride abstraction by [CPh3] + then provides a route to substituted cobaltocenium salts (502, 506). 

Much work was then developed around the functionalization of the ligand for a better immobilization of the catalyst in the IL phase. The first study was described by Saltzer et al. in 2000 [32], The authors used electron-poor phosphine-substituted cobaltocenium salts as ligands for the biphasic hydroformylation of 1-octene (5 and 6). 

The intercalation of cobaltocene and its ions into a range of materials have been considered e.g. into Cd2P2S6. Cobaltocenium salts have been used as redox labels in Nation films. The photochemical properties of metallocene redox diodes has been reported. Several electrochemical investigations have been carried out on cobaltocenes or cobaltocene modified electrodes. Rigid rod paraphenylene acetylenes have been introduced into metallocenes for non linear optic use. The crystal structure of cobaltocene has been redetermined " and the crystal structure of cobaltocenium bis[(4,5-dimercapto-S4,S5)-l,3-dithiolate-2-thionato] nickelate has been reported. Finally, cobaltocenium metallacarboranide mixed-sandwich salts have been prepared. ... 

The principle of this competitive immunoassay without separation, developed by Degrand and Limoges, is based on electrochemical detection of a cationic electroactive label, a cobaltocenium salt and on the specific properties of Nafion , a polyanionic perfluorosulfonate possessing a strong affinity for small-sized cationic organic molecules. Its effect is to sort the molecules within a mixture according to their size and charge [48]. 

Mochida, T, Funasako, Y, Inagaki, T, Li, M.J., Asahara, K. and Kuwahara, D., Crystal structures and phase-transition dynamics of cobaltocenium salts with... 

ZiCi = 0 for all species i in a system, determine the steady-state ohmic drop as a function of overpotential for the reduction of a cobaltocenium salt (AX where A" " is cobaltocenium and X is its counter-ion) at a hemispherical electrode in the absence of any supporting electrolyte. Assume that all ions have equal diffusion coefficients. The relevant reaction is... 

Cobaltocenium tetrak/s(tetracarbonylcobaltio)bismuthate( — 1), [Cp2Co]-[Bi Co(CO)4 4], is a reddish-brown solid which must be stored under nitrogen.8 The [Me4N]+ salt has been isolated and structurally characterized.9 The Cp2Co+ salt is insoluble in hexane, toluene, and only partially soluble in diethyl ether. It dissolves in CH2C12, MeOH, acetone, MeCN, and THF. IR (CH2C12, cm-1) 2066(m), 2028(vs), 1969(s), and 1890(w). 

Cobaltocenium calix[4]arene receptors, characteristics, 12,475 Cobaltocenium-metallacarborane salts, preparation, 3, 23 Cobaltocenium receptors, characteristics, 12, 474 Cobalt phosphines, as supports, 12, 683 Cobalt-platinum nanoparticles, preparation, 12, 74 Cobalt-ruthenium clusters, as heterogeneous catalyst precursors, 12, 768... 

Metal-14 anions can add to activated ethylenic bonds8. Thus, the reaction of Ph3ELi (E = Ge, Sn) with cobaltocenium or decamethylcobaltocenium salts resulted in a... 

Similar photoinduced dimerizations and ligand substitutions in the presence of additives such as triphenylphosphine are observed with ion-pairs salts of Mn(CO)s and V(CO)6" with cobaltocenium or other cationic acceptors such as Ph2Cr", pyr-idinium, quinolinium, etc [118]. Most importantly, all photochemical transformations of the various carbonyl metallate salts are initiated by actinic light that solely excites the charge-transfer absorption bands of the contact ion pairs whereas local excitation of the separate ions is deliberately excluded. 

In contrast, the use of cobaltocenium bis(diphenyl)phosphine 3 results in increased activity and selectivity toward the n-aldehyde [23]. In an alternative approach, Rh2(OAc)4 was used as the catalyst in a phosphonium salt melt [24]. 

The diborylated cobaltocenium species [(CpBR2)2Co] + (R = t-Pr) is an interesting example that illustrates the different binding modes encountered for bifunctional Lewis acids.With hydroxy counterions, an oxygen-bridged complex (178), which represents an inverse chelate structure, was confirmed in the solid state and in solution (R = t-Pr, F). Salt-like structures on the other hand were observed with PFe as the counterion and a zwitterionic 1 1 complex (179) formed upon reaction of the diborylated cobaltocene with hexachloroethane. Low-temperature NMR studies show that the chloride rapidly exchanges position between the two Lewis-acidic boron centers. 

In non-aqueous electrolytes, the different properties of the solvated metal ions lead to different equilibrium and standard potentials. For comparing standard potentials, electrode reactions should be defined as reference systems with similar values in different solvents. Koepp, Wendt, and Strehlow suggested ferrocene/ferrocinium and cobaltocene/ cobaltocenium redox systems. The redox systems are bis-pentadienyl complexes of Fe +/Fe + and Co /Co , respectively. Gritzner and Kuta recommended ferrocene/ferrocinium and bis(biphenyl)Cr(l)/bis(biphenyl)Cr(0). Salt bridges with conventional cells should be avoided. Similar to aqueous electrolytes a reference to the physical potential scale is possible. Similar considerations hold for ionic melts and molten and solid electrolytes. 

Figure 31.1 shows the structures of some of the calixarene and calixpyrrole derivatives that were found to self-assemble into dimeric capsules (see structures 2-5). The tetraureacalix[4]arenes (2) were found to from dimeric capsules with internal volumes estimated to be in the range of 160-200 [lib]. These dimeric capsules were found to encapsulate solvent molecules like chloroform and benzene and small ammonium salts [11]. In addition, it was demonstrated with the aid of diffusion NMR [16] that such dimers have a high affinity for charged systems, and it was found that tropylium cation and cobaltocenium cation have higher affinities than benzene and ferrocene, respectively, for the cavity of the dimer of 2 [17]. These and other studies demonstrated that indeed diffusion NMR is an excellent means to probe encapsulation [17, 18], a fact that was instrumental in the study of the large hexameric capsules of resorcin[4]arenes and pyrogallol[4]arenes as will be demonstrated below [18-20]. 

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. 

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