Zinc complex, dinuclear

A structurally characterized example of a dinuclear zinc complex with a bridging phosphate monoester was provided by Kitajima and co-workers using the tris(pyrazolyl)borate ligand system. The P—O bond in a tris- or bis-phosphate ester is cleaved by a hydroxo zinc complex resulting in the monoester compound.443... 

Bimetallic zinc complexes formed with hexaazamacrocycles were studied in the hydrolysis of activated carboxyesters. Potentiometric titration demonstrated the dominant presence of a dinuclear hydroxo bridged species at pH >7. /)-Nitrophenyl acetate is hydrolyzed with no loss of catalytic activity for at least 2.7 catalytic cycles 4... 

The crystal structure of the C-functionalized imidazole derivative of 1,5,9-triazcyclododecane (75) shows a five-coordinate zinc with four /V-donors from ligand and chloride in a distorted trigonal bipyramidal arrangement. The Zn—N imidazole bonds are the shortest at 2.025(3) A.678 Deprotonation of the imidazole group resulted in a bridging imidazolate to form dinuclear zinc complexes. The pATa of 10.3 varies from the pATa of bound water with similar ligands (as low as 7.3) and the complex is not catalytic for the hydrolysis of esters. 

An 18-membered N3O3 macrocycle could not be synthesized with transition metal templates but the N402 analog (85) was successfully synthesized with a zinc template. This gave dinuclear /r-hydroxy bridged zinc complexes including a rare example of a single unsupported... 

The zinc complex of a 14-membered CA-N2S2 dibenzo macrocycle with two pendent pyridyl-methyl groups was studied (L = 8,ll-bis(2-pyridylmethyl)5,6,7,8,9,10,16,17-octahydro-diben-zo[e,m][l,4]dithia[8,ll]diazacyclotetradecine). In solution, [ZnL]2+ takes up atmospheric carbon dioxide and transforms to the dinuclear complex [(//-C03)(ZnL)2](C104)2. The structures of both complexes have been elucidated. The zinc atoms, which are situated outside the macrocyclic ring, are unsymmetrically bridged by the //-carbonato group.739... 

The deprotonation of 2-pyridylmethylamine with dimethylzinc, Scheme 76, afforded methylzinc-2-pyridylmethy-lamide 116, which crystallizes as a trimer, but exists in solution in a dimer-trimer equilibrium. A dinuclear zinc complex bearing a diamino(tetrapyridyl) ligand 117, which must have been formed in a complex sequence of reactions, was isolated in low yield from a THF solution of 116. 

Catalysis of hydrolysis of P-lactams by dinuclear zinc complexes... 

Furthermore, experimental results showed that sterically hindered monoether-salen dinuclear magnesium complex 54 initiates the polymerization of lactide at 25 °C in 50 min with a 96% conversion, while its zinc complexes 55 required 4 h at 60 °C to reach 95% conversion, with the ratio [LA]o/[I]o = 100 (Fig. 10) [79]. 

ESR spectra obtained from magnetically dilute samples of the silver(U) complexes doped into the corresponding dimeric zinc complex could be attributed to both homo- (Ag,Ag) and hetero- (Ag,Zn) dinuclear species. In the Ag,Ag dimer the g values were almost equal to those for the Ag,Zn dimer however, the hyperfine coupling constants A were only about half. From this it was concluded that the structures of these two species were very similar.542... 

A crystal structure of a dinuclear zinc complex of the porphin precursor, l,2,3,7,8,12,13,17,18,19-decamethylbiladiene-a,c (174) has been described1190,1191 and the octaethyhsobacteriochlorin complex (175) has been studied as a model for siroheme enzymes.1192... 

The bis-prolinol-derived phenol (156) has been designed to facilitate the formation of heterodinuclear complexes based upon the large difference in pKH of the phenolic OH group and the tertiary OH groups. The first examples of its application involve hydroxyacetophenones (157) as donors in the asymmetric Michael addition to nitroalkene acceptors (158) the best stereocontrol was observed with a zinc- magnesium dinuclear complex, where enantiomeric excesses ranged up to 92% for the major anti diastereoisomer (159).212... 

Following the detailed mechanistic study using BDI zinc complexes, Coates subsequently reported a dinucleating BDI ligand the p-oxo-5-diimine (BODDI) ligands (Fig. 20) [149]. The reaction of the corresponding dinuclear zinc ethyl complexes with acetic acid afforded zinc acetate systems (with a Zn-Zn distance of 3 A), but no catalytic activity was reported. 

Trost and coworkers recently reported that these dinuclear zinc complexes catalyze Mannich reactions with unmodified aromatic hydroxy ketones as donors with excellent enantioselectivity [18]. Mannich-type reactions between an N-para-meth-oxyphenyl (PMP)-protected a-ethyl glyoxalate and hydroxyacetophenone in the presence of a catalytic amount of catalyst 5a afford the desired N-PMP protected amino acid derivative in 76 % yield with a dr of 7 1 and 95 % ee (Eq.5). 

The phenol substituted [9]aneN3 derivative H2L22 (Fig. 24) also forms a dinuclear zinc complex ion with the formula [Zn2(HL22)20H], in which one phenol from each ligand is deprotonated. The two zinc atoms are linked by one /ii-hydroxo bridge supported by two hydrogen bonds between the phenol on one macrocycle and the phenolate of the second. The Zn—Zn distance is 3.55 A (106). 

Fig. 16 The mechanism implicated by kinetic data by which a dinuclear zinc complex catalyzes the cyclization of RNA analogs. The pKa of the Zn-coordinated water was found to be 8.0 once deprotonated, the resulting coordinated hydroxide acts as a base to deprotonate the secondary alcohol, which acts as the nucleophile in a subsequent step.

With the use of l3C NMR techniques, the formation of the related zinc(II) dinuclear complexes of the formula [Zn2(PZ)]2 +, has been recently evidenced by treatment of ZnCl2 with the ligands H2PZ14, H2PZ15, and H2PZ16 (206, 207). 

Scheme 39 Proposed mechanism for enantioselective hydrophosphonylation of enones catalyzed by a dinuclear zinc complex...

The dinuclear zinc complex (7.61) developed by Trost and coworkers functions as an alternate bifunctional catalyst that is highly effective in the direct aldol reaction of aliphatic aldehydes with aryl methyl ketones and has also been used... 

TamUselvi, A, M Nethaji and G Mugesh (2006). Antibiotic resistance Mono- and dinuclear zinc complexes as metaUo- 8-lactamase mimics. Chemistry — A European... 

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