Synthesis, reactions and structure of a hydroxo-bridged dinuclear Zn(II) complex: modeling the hydrolytic zinc enzymes

A Conformationally Flexible Macrocyclic Dipyrrin Tetramer and Its Unsymmetrically Twisted Luminescent Zinc(II) Complex

A macrocyclic dipyrrin tetramer containing flexible m-phenylene linkages and its tetranuclear zinc(II) complex were synthesized. The obtained complex has an unsymmetrical figure-of-eight structure because of the conformational flexibility of the macrocyclic framework. The first μ-hydroxo- and μ-acetato-bridged dinuclear zinc(II) dipyrrin complex structure is realized in the twisted macrocyclic complex. Furthermore, the complex exhibited an efficient emission in toluene and chloroform.

(Aceto-nitrile-κN)aqua-[N,N'-bis-(pyridin-2-yl-methyl)ethane-1,2-di-amine-κ4N,N',N'',N''']zinc(II) perchlorate

The structure of the title compound, [Zn(C14H18N4)(C2H3N)(H2O)](ClO4)2, contains a six-coordinate cation consisting of the tetra-dentate bis-picen ligand, coordinated water, and coordinated aceto-nitrile, with the latter two ligands adopting a cis configuration. There are two formula units in the asymmetric unit. Both cations show almost identical structural features with the bis-picen ligand adopting the more common cis-β conformation. One of the four perchlorate anions is disordered over two positions, with occupancies of 0.9090 (15) and 0.0910 (15). There is extensive inter-ionic hydrogen bonding between the perchlorate anions and O-H and N-H groups in the cations, including a bifurcated hydrogen bond between an N-H group and two O atoms of one perchlorate anion. As a result of this extended hydrogen-bond network, the ions are linked into a complex three-dimensional array.

Bis{2-[(2-furylmeth-yl)imino-meth-yl]-5-meth-oxy-phenolato-κN,O}zinc(II)

In the title complex, [Zn(C(13)H(12)NO(3))(2)], the Zn(II) ion is located on a twofold rotation axis and is coordinated by two bidentate Schiff base ligands in a distorted tetra-hedral environment. The complex mol-ecules are stacked in columns along the b axis through C-H⋯O hydrogen bonds.

(m-Phenylenedimethylene)diammoniump-nitrophenylphosphate perchlorate

The title compound, C₈H₁₄N₂²⁺·C₁₂H₈N₂O₈P⁻·ClO₄⁻, was formed by the reaction of α,α-bis-m-xylenediamine and sodium bis-p-nitro­phenyl­phosphate in the presence of Zn(ClO₄)·6H₂O in methanol solution. The two amine groups of the m-xylenediammonium ion are each protonated and each hydrogen-bonded to two O atoms of the phosphate anion, which acts as a 1,3-bridge. The ammonium groups are arranged matched face to face and each pair is doubly bridged by two perchlorate ions through hydrogen bonding. In addition, there are also weak C—H⋯O inter­actions. Both the N—H⋯O and C—H⋯O inter­actions are contained in a channel down the a axis. The perchlorate oxygen atoms are disordered over two positions with site occupancy factors of ca 0.7 and 0.3.

An Unusual Zinc-Promoted Decomposition of a Bis(2-{pyrid-2-yl}ethyl)amine Derivative

The complex [ZnBr(2)L(1)] (L(1) = ferrocenylmethyl-bis[2-{pyrid-2-yl}ethyl]amine) contains bidentate L(1) by crystallography and NMR spectroscopy and decomposes during recrystallization from CH(2)Cl(2)/pentane via both C-N bond cleavage and formation steps. Other [MBr(2)L(1)] (M = Co, Ni, Cu) compounds do not undergo this reaction and contain L(1) bound in the more usual tridentate coordination mode.

Reactivity of mu-hydroxodizinc(II) centers in enzymatic catalysis through model studies

The stable dinuclear complex [Zn2(BPAM)(mu-OH)(mu-O2PPh2)](ClO4)2, where BPAN = 2,7-bis[2-(2-pyridylethyl)-aminomethyl]-1,8-naphthyridine, was chosen as a model to investigate the reactivity of (mu-hydroxo)dizinc(II) centers in metallohydrolases. Two reactions, the hydrolysis of phosphodiesters and the hydrolysis of beta-lactams, were studied. These two processes are catalyzed in vivo by zinc(II)-containing enzymes: P1 nucleases and beta-lactamases, respectively. The former catalyzes the hydrolysis of single-stranded DNA and RNA. beta-Lactamases, expressed in many types of pathogenic bacteria, are responsible for the hydrolytic degradation of beta-lactam antibiotic drugs. In the first step of phosphodiester hydrolysis promoted by the dinuclear model complex, the substrate replaces the bridging diphenylphosphinate. The bridging hydroxide serves as a general base to deprotonate water, which acts as a nucleophile in the ensuing hydrolysis. The dinuclear model complex is only 1.8 times more reactive in hydrolyzing phosphodiesters than a mononuclear analogue, Zn(bpta)(OTf)2, where bpta = N,N-bis(2-pyridylmethyl)-tert-butylamine. Hydrolysis of nitrocefin, a beta-lactam antibiotic analogue, catalyzed by [Zn2(BPAN)(mu-OH)(mu-O2PPh2)](ClO4)2 involves monodentate coordination of the substrate via its carboxylate group, followed by nucleophilic attack of the zinc(II)-bound terminal hydroxide at the beta-lactam carbonyl carbon atom. Collapse of the tetrahedral intermediate results in product formation. Mononuclear complexes Zn(cyclen)-(NO3)2 and Zn(bpta)(NO3)2, where cyclen = 1,4,7,10-tetraazacyclododecane, are as reactive in the beta-lactam hydrolysis as the dinuclear complex. Kinetic and mechanistic studies of the phosphodiester and beta-lactam hydrolyses indicate that the bridging hydroxide in [Zn2(BPAN)(mu-OH)(mu-O2PPh2)](ClO4)2 is not very reactive, despite its low pKa value. This low reactivity presumably arises from the two factors. First, the briding hydroxide and coordinated substrate in [Zn2(BPAN)(mu-OH)(substrate)]2+ are not aligned properly to favor nucleophilic attack. Second, the nucleophilicity of the bridging hydroxide is diminished because it is simultaneously bound to the two zinc(II) ions.

Artificial dinuclear phosphoesterases

Many elegant dinuclear transition metal and lanthanide complexes that efficiently hydrolyze phosphate esters including RNA and DNA have been reported recently. In most cases, the dinuclear complexes are much more reactive than the corresponding mononuclear metal complexes. Furthermore, structural and kinetic data indicate that substantial rate acceleration for phosphate diester cleavage is obtained by bridging the dinuclear metal centers with the two phosphoryl oxygens. Interestingly, such bridging structures have recently been implicated in several metalloenzyme catalyzed phosphate hydrolyses.