Binuclear zinc complexes

Fig. 8a-c 90.52 MHz CP/MAS NMR spectra of the polycrystalline binuclear zinc complexes with dialkyl-substituted dithiocarbamate ligands, [Zn2(S2CNR2)4] (number of signal transients/MAS frequency (Hz)) a R=C3Hy (512/4500) b R=f-C3H7 (1024/5500) c R=C4H9 (2200/3000)... 

The binuclear zinc complex [(LH+)2Zn2(Gly-Gly)2](C104)4 (Fig. 31, L = 1-methyl-4-(6-amino-2-pyridylmethyl)-piperazine) was characterized by X-ray crystallography.168 In this structure, a water molecule is positioned close (O(water) C(carbonyl) 3.050 A)... 

Binuclear zinc complexes have been shown to mediate the hydrolysis of the activated peptide model substrate L-leucine- -nitroanilide (LNA, Fig. 32) with the reactions being followed spectroscopically by the formation of /7-nitroaniline ( max 400 nm).165-167 This model substrate has also been used in studies of ApAP.170 The hydrolysis of LNA mediated by the zinc complex [(bomp-)]Zn2(CH3C02)2]BPh4 (Fig. 33a H(bomp) 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol) in water/DMF (6 4) occurs via a reaction that is first-order in complex and substrate, with a second-order rate constant k = 2.3(1) x 10 3M 1 s-1 at 25 °C. At best, a yield of 65% for a single turnover reaction was obtained, indicating that... 

Fig. 33 Binuclear zinc complexes that mediate the hydrolysis of A-/ -nitrophenyl- L-leucine in water-containing solutions.

As with amide hydrolysis reactions, extensive mechanistic studies of metal-mediated phosphate monoester hydrolysis reactions have been performed using exchange inert mononuclear Co(III) complexes.9,188-202 To date, only a few zinc complexes that promote phosphate monoester hydrolysis have been reported. A binuclear zinc complex supported by a macrocyclic cryptate ligand (Fig. 36)... 

Binuclear zinc complexes wherein each metal center is supported by a tripodal tetradentate ligand environment involving a bridging phenolate donor are reactive toward tris(4-nitrophenyl) phosphate and bis(4-nitrophenyl) phosphate in ethanol/ water.234 The pH-rate profile studies of these reactions are consistent with a mechanism that involves the formation of a reactive mononuclear zinc hydroxide species via cleavage of the binuclear solid-state structure. This mononuclear Zn-OH species is proposed to act as the nucleophile toward the phosphate ester substrate. 

The rate of hydrolysis of bis(4-nitrophenyl) phosphate is approximately 10-fold higher for a binuclear Zn2(OH)2 complex of the L22 ligand (Fig. 52) versus a mononuclear zinc hydroxide complex of the L23 ligand at 308 K.105 In this system, a monohydroxide binuclear zinc complex (Zn2(p-OH)) does not promote phosphate diester hydrolysis. The 10-fold rate enhancement found for the reaction involving the binuclear Zn2(OH)2 complex was explained via cooperative interaction between the zinc centers. As shown in Fig. 54, the reaction is proposed to take place via attack of a terminal Zn-OH moiety on a phosphate diester substrate that is interacting with both zinc centers. This suggests a cooperative role for the two zinc centers in the phosphate diester hydrolysis reaction. 

Fig. 54 Proposed reaction pathway of bis(4-nitrophenyl) phosphate hydrolysis promoted by a binuclear zinc complex of the L22 macrocyclic ligand.

A similar mechanistic pathway to that shown in the lower portion of Fig. 58 is proposed for tris(4-nitrophenyl) phosophate hydrolysis promoted by binuclear zinc complexes of phenol-based ligands, albeit no kinetic or mechanistic studies were reported for these reactions.243... 

The [(BPAN)Zn2( i-0H)( i-Ph2P02)](C104)2 complex also promotes the hydrolysis of bis(4-nitrophenyl) phosphate.140 This reaction proceeds via initial coordination of the substrate to the binuclear zinc complex (K = 86 + 32 M-1). Studies of the effect of pH on this reaction revealed a kinetic pKa of 7.06 + 0.05, which is consistent with deprotonation of a bridging water molecule. Thus, a bridging hydroxide is the... 

Binuclear zinc complexes of pyrazolate-based chelate ligands (Fig. 61) exhibit differing bis(4-nitrophenyl) phosphate hydrolysis reactivity depending on the nature of the supporting chelate ligand.153 Plots of the pseudo first-order rate constant... 

Fig. 61 Binuclear zinc complex of pyrazolate-based chelate ligands.

The 2-hydroxypropyl 4-nitrophenyl phosphate (HPNP) transesterification reactivity (Fig. 40, bottom) of binuclear zinc complexes of the dianionic ICIMP and trianionic BCIMP ligands (Fig. 62) have been compared.245 Notably, both of these ligands contain imidazole and carboxylate donors akin to protein-derived residues that are found as ligands to the zinc centers in phosphate-ester-hydrolyzing enzymes such as phosphotriesterase. The initial rate for HPNP transesterification promoted by [(ICIMP)Zn2(Ph2Ac)]C104 in 50% acetonitrile-water solution (tris buffer) was determined to be five times faster than the reaction promoted by [(BCIMP)Zn2(Ph2Ac)]. 

A binuclear zinc complex of the 2,6-bis [(2-pyridylmethyl)(2-hydroxyethyl) amino]methyl -4-methylphenol ligand (L33, Fig. 63) catalyzes the hydrolysis of bis(4-nitrophenyl) phosphate in aqueous buffer solution to yield 4-nitrophenol and 4-nitrophenyl phosphate.246 Analysis of the pH-rate profile revealed a sigmoidalshaped curve with a pKa = 7.13. This value is similar to a deprotonation event characterized by potentiometric titration and has been assigned to deprotonation of... 

Binuclear zinc complexes of triazacyclononane-containing ligands having different bridge structures (Fig. 71) exhibit second-order rate constants for HPNP transesterification that are only 3-5 times larger than that exhibited by the mononuclear [(L360H)Zn]2+ complex.256 This behavior is contrasted by that of... 

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