Cobalt -cyclen

In Czamik s model compound 6 both cobalt(III) ions are presented in the same molecule [33]. Two (cyclen)Co(IH) units are covalently linked by a 1,4-dibutylbenzene spacer. By 6 the activated phosphodiester BNPP at pH 7.0 and 25 °C is hydrolyzed 3.2 times faster than by 2 equiv of (cyclen)Co(III) (7) under the same conditions. A more than 107-fold rate enhancement over the spontaneous hydrolysis of BNPP is observed. The reaction mechanism may be similar to that proposed for compound 2. 

The long-running dispute over the mechanism of base hydrolysis of cobalt(III)-ammine and -amine complexes, SVj2 versus SVjlCB (better termed Dcb), was several years ago resolved in favor of the latter (73). Recent activity on reactions of this type has concentrated on attempting to locate the precise site of deprotonation of the complex, an exercise successfully accomplished for the complexes syn,anti-[Co (cyclen)(NH3)2]3+ and syn,[Pg.80]

While there have been a considerable number of structural models for these multinuclear zinc enzymes (49), there have only been a few functional models until now. Czamik et al. have reported phosphate hydrolysis with bis(Coni-cyclen) complexes 39 (50) and 40 (51). The flexible binuclear cobalt(III) complex 39 (1 mM) hydrolyzed bis(4-nitro-phenyl)phosphate (BNP-) (0.05 mM) at pH 7 and 25°C with a rate 3.2 times faster than the parent Coni-cyclen (2 mM). The more rigid complex 40 was designed to accommodate inorganic phosphate in the in-temuclear pocket and to prevent formation of an intramolecular ju.-oxo dinuclear complex. The dinuclear cobalt(III) complex 40 (1 mM) indeed hydrolyzed 4-nitrophenyl phosphate (NP2-) (0.025 mM) 10 times faster than Coni-cyclen (2 mM) at pH 7 and 25°C (see Scheme 10). The final product was postulated to be 41 on the basis of 31P NMR analysis. In 40, one cobalt(III) ion probably provides a nucleophilic water molecule, while the second cobalt(III) binds the phosphoryl group in the form of a four-membered ring (see 42). The reaction of the phosphomonoester NP2- can therefore profit from the special placement of the two metal ions. As expected from the weaker interaction of BNP- with cobalt(in), 40 did not show enhanced reactivity toward BNP-. However, in the absence of more quantitative data, a detailed reaction mechanism cannot be drawn. 

Stoichiometric complexes of the zinc(II)-cyclene monomer with tetraacetylriboflavin and creatinine were prepared and co-polymerised with ethylene glycol dimethyl acrylate. After exhaustive extraction of the material which removed impurities and the templates, a functional polymer was formed. Control polymers without template and with cobalt(II) instead of zinc(II) were also prepared. 

Another example described in Section 9.5.3 reports the synthesis and screening of a synthetic receptor library aimed at transition metal binding. Other similar examples have been reported recently. Burger and Clark Still (211) prepared ionophoric, cyclen-based libraries decorated by amino acid units and screened them for their ability to complex copper and cobalt ions Malin et al. (212) identified novel hexapeptidic technetium-binding sequences from the screening of cellulose-bound libraries and... 

The effectiveness of the binuclear complex 11 (Fig. 13), with two mononuclear cyclen-cobalt(III) units linked together by an anthra-cenyl spacer (cyclen = 1,4,7,10-tetraazacyclododecane), was compared with the monomer in the hydrolysis of phosphate monoesters (354). The reaction assisted by this rigid binuclear complex, having a phosphate-sized pocket, was 10 times faster than that promoted in the presence of two equivalents of the single cyclen-Co complex. In these experiments the substrate concentration was 25 pM and the total cobalt concentration was 2 mM at 25°C and neutral pH (354). No such cooperativity could be noted using a diester substrate because the pseudo-first-order rate constants were similar for both 11 and the mononuclear complex. With 11 as catalyst, an overall rate enhancement of 10 was achieved over the uncatalyzed hydrolysis of paranitrophenyl phosphate monoester as substrate. 

Two different binuclear copperdi) complexes have been prepared recently, one with a bridging phenoxy ligand having two bis-benzi-midazole arms (12, Fig. 14), and the second having a bis-cyclen-naphthalene ligand (13, Fig. 15) (352, 353). Both of them show bimetallic cooperativity for the hydrolysis of phosphate diesters, contrary to studies with the dinuclear cobalt complex (354). The pseudo-first-order rate constants for hydrolysis of the para-nitrophenylphosphate ester of propylene glycol by bis-benzimidazole-based copper complexes... 

A binuclear cobalt(III) complex with two cyclic tetrammine (cyclen) ligands effectively catalyzes the hydrolysis of plasmid DNA. Rate enhancements of 10 at a concentration of 1 M are obtained (Hettich, 1997). 

Examination of the structure of this ligand in detail reveals how it is ideal for its purpose (Fig. 17). Two vacant adjacent coordination positions are required cyclen was chosen because it has long been known that that ligand can only bind to cobalt(III) in a folded tetradentate manner, thereby leaving two vacant coordination positions. The donor atoms of the pendant carboxyl group must not be capable of reaching the metal ion, but they must be able to interact with a bound carboxylate derivative the meta-benzyl carboxylic acid group provides such an orientation. Further, this choice of structural moiety opens the way for facile synthesis of the desired product. 

Cobalt. NMR spectroscopy has been used to study water exchange in [Co(tren)X(OH2)]2-", [Co(cyclen)X(OH2)]2-", and [Co(N-Me-cyclen)X(OH2)]2- -. 37 Rhodium. The rate constants and activation parameters for water exchange on [(H20)4Rh( i-0H)2Rh(0H2)4]" have been determined using NMR spectroscopy as a function of temperature and pressure. ... 

---