Cobaltocenium receptors

A new class of ferrocene (also cobaltocenium) receptors which sense anions both spectrally and electrochemically, is based upon attachment of metallocene moieties to a porphyrin skeleton. The porphyrin fragment can be simultaneously complexed by zinc [120]. 

A great variety of molecular hosts, mostly devised as anion receptors, contain cobaltocenium moieties. These can be combined with polyoxo macrocyclic ethers to give redox responsive coronands, 69, with electrochemical sensitivity to anion complexation [121]. 

The cobaltocenium fragment is involved in anion recognition and complexation, whereas the crown ether moiety can interact with alkali metal ions. The result is that the anion recognition properties can be switched on and off through the absence or presence of potassium cation simultaneously bound by the crown. 

The receptor containing two crown ether macrocycles forms a sandwich 1 1 complex with potassium, 70, and a 1 2 complex with sodium, 71, because of the different recognition properties of the crown towards the two cations [121]. 

Another variety is the ester type of receptors, containing carboxycobaltocenium units esterified with various diphenols, 72 and 73, which introduce aromatic spacers [122]. Such compounds accept Br guests and shift the cobaltocenium reduction wave towards more negative potentials. In this instance anion recognition and fixation seem to be based simply on electrostatic interactions. 

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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... 

Anion receptors incorporating cobaltocenium have been studied extensively due to the combination of an accessible redox couple and potential favourable electrostatic interactions of the cationic organometallic metallocene complex with anions. The first anion receptor based on this species was reported by Beer and co-workers in 1989 [6]. The macrocyclic bis-cobaltocenium receptor 1 was shown to bind (via electrostatic interaction) and to electro chemically sense bromide in acetonitrile solvent media. 

A tetra-cobaltocenium receptor 4 has been synthesised that is supported by a porphyrin skeleton [9]. This exhibits parallel photo- and electrochemical sensing of anions with a trend in selectivity of Cl >Br NOg. Proton NMR titrations in CD3CN showed chloride and bromide to be bound in 1 1 stoichiometry with stability constants of 860 and 820 M"1, respectively, whereas nitrate exhibited weaker binding with K=190 M"1. In the UV-vis spectrum of 4 in acetonitrile solution the Soret band (Amax=425 nm) of the porphyrin was signif-... 

The incorporation of crown ether units into a cobaltocenium receptor has been shown to allow the switchable sensing of anions. Proton NMR titrations of receptor 8 in CD3CN solution gave log K values of 3.1 for chloride and 3.0 for... 

Receptor 93 incorporates a zinc porphyrin backbone with four ferrocene amides [65]. This shares the design of the cobaltocenium receptor 4, except that now a zinc atom occupies the centre of the porphyrin. The Lewis acid metal centre provides an additional binding site for anion recognition. In dichloro-methane solution no significant anion-induced shifts in the lH NMR signals of the amide protons were seen in the free-base precursor of 93, whereas the... 

The first ester functionalized cobaltocenium receptor (74) developed by us in 1989, was based solely on electrostatic interactions (as discussed earlier) (9, 186). It was therefore decided to append the cobaltocenium moiety with secondary amide functionalized arms . In this manner, the receptors became more resistant to hydrolysis (a recurrent problem with ester-based systems), and also incorporated neutral hydrogen-bond donors capable of coordinating anions. [An interaction between a secondary amide (acetamide) and the bromide anion was physically investigated (by IR spectroscopy) as long ago as 1961 (211)]. We made the first report of this novel type of receptor (91 and 92) in 1992 (212), which was, in fact, the first class of inorganic anion receptor that incorporated hydrogen-bonding functionalities. 

Aza-crown substituted cobaltocenium receptors (96) were also prepared and the electrostatic effect of the presence of alkali metal guests in the crown ether rings on the anion-binding process was investigated (215). Evidence for a cooperative anion-binding effect on the addition of sodium ions (due to electrostatic attraction) was presented. The ability of methylated pyridine substituents to electrostatically enhance the anion-binding process has also been reported (216). 

Cuadrado. I. del Barrio, J.L. A polymerizable pyn ole-cobaltocenium receptor for the electrochemical recognition of anions in solution and immobilized onto 125. electrode surfaces. Inorg. Chem. Commun. 2002.5. 288-... 

Beer. P.D. Hesek, D. Kingston. J.E. Smith, D.K. Stokes, S.E. Drew, M.G.B. Anion recognition by redox-responsive ditopic bis-cobaltocenium receptor molecules including a noVel calix[4]arene derivative that binds a dicarboxylate dianion. Organometallics 1995. 14. 3288. 

Beer, P.D. Hazlewood, C. Hesek, D. Hodacova. J. Stokes. S.E. Anion recognition by acyclic redox-responsive amide-linked cobaltocenium receptors. J. Chern. Soc. Dalton Trans. 1993, 8, 1327. 

Inclusion of pyridinium moieties in cobaltocenium receptors, 79 and 80, was found to enhance their emion-binding capacity (compared with analogous pyridyl complexes). All the receptors shown recognize the X (halide), HS04 and H2PO4" anions and form 1 1 complexes [129]. 

Among ditopic cobaltocenium receptors, 81, those containing alkyl spacers form 1 1 host-guest complexes with halide anions, whereas those containing larger, aromatic spacers form 2 1 complexes. The selectivity and complexing capacity of the receptors decreases with increasing distance between the cobaltocenium units... 

With a chiral cobaltocenium receptor chiral selectivity and recognition towards camphor-10-sulfonate was observed [132, 133] raising hopes for future use of such systems in the separations of optically active compounds. These species also have receptor properties towards Cl , Br , acetate, and tosylate anions. 

The two-to-one stoichiometry contrasts to the related cobaltocenium receptor 2.78. Reduction of the Co(iii) centre to Co(ii) and re-oxidation with Cu(OH)2 in non-polar solvents gives a 1 1 complex with OH that adopts a sj/n-conformation, with an OH anion chelated between the Lewis-acid groups (Figure 2.16). This... 

Indeed, in Figure 74, both CP and Br would be more strongly bound to the cobaltocenium receptor in the presence of the complexed Na. ... 

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