Mononuclear Complexes Rates

No evidence of ruthenium metal formation was found in catalytic reactions until temperatures above about 265°C (at 340 atm) were reached. The presence of Ru metal in such runs could be easily characterized by its visual appearance on glass liners and by the formation of hydrocarbon products (J/1J) The actual catalyst involved in methyl and glycol acetate formation is therefore almost certainly a soluble ruthenium species. In addition, the observation of predominantly a mononuclear complex under reaction conditions in combination with a first-order reaction rate dependence on ruthenium concentration (e.g., see reactions 1 and 3 in Table I) strongly suggests that the catalytically active species is mononuclear. 

Even more efficient bimetallic cooperativity was achieved by the dinuclear complex 36 [53]. It was demonstrated to cleave 2, 3 -cAMP (298 K) and ApA (323 K) with high efficiency at pH 6, which results in 300-500-fold rate increase compared to the mononuclear complex Cu(II)-[9]aneN at pH 7.3. The pH-metric study showed two overlapped deprotonations of the metal-bound water molecules near pH 6. The observed bell-shaped pH-rate profiles indicate that the monohydroxy form is the active species. The proposed mechanism for both 2, 3 -cAMP and ApA hydrolysis consists of a double Lewis-acid activation of the substrates, while the metal-bound hydroxide acts as general base for activating the nucleophilic 2 -OH group in the case of ApA (36a). Based on the 1000-fold higher activity of the dinuclear complex toward 2, 3 -cAMP, the authors suggest nucleophilic catalysis of the Cu(II)-OH unit in 36b. The latter mechanism is comparable to those of protein phosphatase 1 and fructose 1,6-diphosphatase. 

The above complexes have been shown to mimic the second step of RNA hydrolysis as well, i.e. the-efficient cleavage of ribonucleoside 2, 3 -cyclic monophosphates [55] with bell-shaped pH-rate profile. With these substrates 37 showed much higher bimetallic cooperativity the dime/2 m0nomer ratios range between 64 and 457 for the different 2, 3 -NMPs used, while for 38 this ratio varies between 1 and 26. Since the mononuclear complexes have nearly the same activity toward the different 2, 3 -NMPs, these kinetic data indicate a notable base-selectivity of the dimer complexes. Since no correlation was observed with the size,... 

As shown by the cyclic voltammetric response in Fig. 10, the peak potential separation of the initial Mn(II,II) — Mn(II,III) electrode reaction is much larger than that of the other steps. This suggests significant inner-shell reorganization and a small rate of heterogenous electron transfer for oxidation of the fully reduced Mn(II,II) state. Similar kinetic sluggishness is observed for Mn(III)/Mn(II) electron-transfer reactions of some mononuclear complexes (see Sects 16.1.2 and 16.1.3). 

Figure 2.21 Cyclic voltammogram of a MeCN solution of [Cun(17)]2+ mononuclear complex. Supporting electrolyte 0.1 M [Bu4N]C104 scan rate 0.2 V/s internal reference electrode Fc + /Fc. Diagram adapted from Ref. 21.

Wall et al. built a binuclear copper(II) complex 43 in order to see acceleration of phosphodiester cleavage (52). With the substrate (50 p.M) shown, the reaction might be considered as a model for the first step of the hydrolysis of RNA, in which the alcohol function of the side chain intramolecularly attacks the Cun-activated phosphate as a nucleophile for a ring closure reaction. Compared to an analogous mononuclear complex 44 (at 1 mM), a rate constant ca. 50 times larger for 43 (at 1 mM) was observed at 25°C and pH 7, implying that the two metal ions probably cooperate. An analogous zinc(II) complex 45 was reported only as a structural model for the active site of phospholi-... 

In 1959, the coordinated mercaptide ion in the gold(III) complex (4) was found to undergo rapid alkylation with methyl iodide and ethyl bromide (e.g. equation 3).9 The reaction has since been used to great effect particularly in nickel(II) (3-mercaptoamine complexes.10,11 It has been demonstrated by kinetic studies that alkylation occurs without dissociation of the sulfur atom from nickel. The binuclear nickel complex (5) underwent stepwise alkylation with methyl iodide, benzyl bromide and substituted benzyl chlorides in second order reactions (equation 4). Bridging sulfur atoms were unreactive, as would be expected. Relative rate data were consistent with SN2 attack of sulfur at the saturated carbon atoms of the alkyl halide. The mononuclear complex (6) yielded octahedral complexes on alkylation (equation 5), but the reaction was complicated by the independent reversible formation of the trinuclear complex (7). Further reactions of this type have been used to form new chelate rings (see Section 7.4.3.1). 

Dinuclear dihydroxo-bridged complexes can often be obtained from the parent mononuclear complexes by the solid-state reaction Eq. (7). This was first reported by Werner (7, 11) and Dubsky (18), and it is generally the most convenient method for the preparation of dihydroxo-bridged complexes of Cr(III), Co(III), Rh(III), and Ir(III) with L4 = (NH3)4 or (en)2 [and (tn)2 in the case of chromium(III)] (67, 131, 133, 214 219). With the exception of the ammonia chromium(III) complex, these reactions are essentially quantitative and the rate of reaction follows the order chromium(III) > cobalt(III) > rhodium(III) >... 

Our research groups have investigated various b/s(thio/oxo phosphinic)diamido yttrium initiators (e.g. complex 5) [36, 54—56]. These complexes show excellent rates for LA ROP and reasonable control polymerization control is improved in the presence of exogenous alcohol. We have shown that the nuclearity of the initiator has an important influence over polymerization stereochemistry, with a mononuclear complex enabling high degrees of heteroselectivity (Fig. 12). 

Stability constants of complexes may be determined by (i) kinetic and (ii) equilibrium methods. In the present discussion, attention will be focused on mononuclear complexes and the fact that the activity coefficients of all the species can be held effectively constant by using suitable ionic media. The kinetic approach is applicable when (i) the rates of formation and dissociation of a complex are sufficiently slow and (ii) accessible to experimental measurement by suitable techniques. Using the law of mass equation... 

AsPh3) occurs by a primarily hgand-independent mechanism, probably CO dissociation. For more nucleophilic entering ligands (PBu3, CN-t-Bu), the hgand-dependent path still predominates. The CO dissociation rates from Ir4(CO)uL show the same trends with changes in L as mononuclear complexes and metal-carbonyl dimers. 

Up to now, these mononuclear complexes do not fulfill the requirements for an efficient in vitro or in vivo DNA or RNA hydrolysis, especially because the rate constant acceleration is too slow to allow the cleavage of phosphodiester bonds of DNA within hours or minutes under physiological conditions. Efforts were made to enhance their cleavage efficiency by tethering them to antisense oligonucleotides (see Section III,C,6). 

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. 

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