Shu-An Li

Carboxyester hydrolysis promoted by dizinc(II) macrocyclic polyamine complexes with hydroxyethyl pendants: a model study for the role of the serine alkoxide nucleophile in zinc enzymes

A novel dinuclear Zn(II) complex with hexaaza macrocycle bearing two hydroxyethyl pendants (L1), 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxyethyl)-tricyclo[22,2,2,211,13]triaconta-1,11,13,24,27,29-hexaene), was synthesized as a model for alkaline phosphatase. The Zn(II) complex [Zn2L1(CH3CO2)2](ClO4)2(H2O)2 was isolated as a colorless crystal, triclinic, space group P-1. Both Zn(II) ions adopt the geometry of a distorted trigonal bipyramid in a pentacoordinated environment with Zn⋯Zn distance of 8.74 A. The solution complexation study has revealed that the alcoholic hydroxyl group of the complex Zn2L1 exhibits a low pKa value at 25 °C. Zn(II)-bound alkoxides, which act as reactive nucleophiles toward the hydrolysis of 4-nitrophenyl acetate in 10% (v/v) CH3CN at 25 °C, with I = 0.10 M (NaNO3) and pH 9.0, have shown a second-order rate constant of 0.32 ± 0.01 M−1s−1, a value that is approximately 13 times greater than the value for the corresponding dinuclear Zn(II) complex formed by a hexaaza macrocycle without hydroxyethyl pendants (L2). The pH–rate profile for Zn2L1 gave a sigmoidal curve. A possible mechanism has been proposed: the Zn(II)–RO− function acts as a nucleophile in the first step of the hydrolytic mechanism, to give an acetyl intermediate, which is subsequently hydrolyzed to acetate by a Zn(II)–OH− group.

Synthesis, structure and properties of an imidazolate-bridged dicopper complex of a novel macrocycle with two alcohol-pendants as an active site model of Cu, Zn-SOD

Abstract The imidazolate-bridged dicopper complex of a macrocyclic ligand with two hydroxyethyl pendants L, L=3,6,9,17,20,23-hexaza-6,20-bis(2-hydroxyethyl)-tricyclo[23.3.1.1.11,15]triaconta-1(29),11(30),12,14,25,27-hexaene, has been synthesized as a possible model for Cu,Zn-SOD. Single-crystal X-ray analysis revealed the geometry of the imidazolate-bridged CuCu center, and the distance between the two Cu(II) ions is 5.8135(9) A. The two metals are each pentacoordinated with geometries between trigonal-bipyramid (TBP) and tetragonal-pyramid (TP). Magnetic measurement of the imidazolate-bridged dicopper complex exhibited the presence of an antiferromagnetic exchange interaction with a coupling constant of −2J=72.8 cm−1. Investigation on the pH-dependent ESR of the complex in frozen 50% aqueous DMSO solution at 110 K revealed the existence of the imidazolate-bridged dicopper structure in solution mainly in the range 5.5≤pH≤12. The SOD activity of the complex has been evaluated by the nitro blue tetrazolium assay and the complex catalyzed the dismutation of superoxide at pH 7.8 with an IC50 value of 0.36 μmol dm−3.

The encapsulation of [Cu(en) 2 ] 2+ (en=ethylenediamine) by a novel three-dimensional Cu II –Mo IV bimetallic porous coordination polymer containing zigzag-ladder structure

Abstract The neutral three-dimensional cyano-bridged bimetallic coordination polymer, {[Cu(en)2][Cu(en)][Mo(CN)8]}n·4nH2O (en=ethylenediamine), has been synthesized and characterized. X-ray study shows that the asymmetric unit of the three-dimensional Cu(II)–Mo(IV) polymeric assembly consists of two half [Cu(en)2]2+ cations, a [Cu(en)]2+ cation and a [Mo(CN)8]4− ion, together with four water molecules. The latter Cu site is coordinated by two N atoms from an en ligand and by three cyanides in a distorted square-pyramidal environment. The Cu(II) ions of the two [Cu(en)2]2+ cations on the centers of inversion reside in an elongated octahedral geometry, and one of the [Cu(en)2]2+ cations is encapsulated in a ‘box’ formed by two pairs of co-lateral Cu2Mo2(CN)4 12-membered macrocyclic units. The Mo(VI) atom is coordinated by eight cyanides in an irregular square antiprism. Five of these acting as bridging units connect the Mo and the three Cu atoms to form an infinite three-dimensional porous network containing a zigzag-ladder structure.

A novel binucleating macrocyclic ligand with two alcohol pendants.

A novel binucleating 24-membered macrocyclic ligand, 6,20-bis(2-hydroxyethyl)-3,6,9,17,20,23-hexazatricyclo[23.3.1.1(11,15)]triaconta-1(29),11(30),12,14,25,27-hexaene (L), was synthesized and crystallized as the tetrahydrobromide salt, i.e. 6,20-bis(2-hydroxyethyl)-6,20-diaza-3,9,17,23-hexaazoniatricyclo[23.3.1.1(11,15)]triaconta-1(29),11(30),12,14,25,27-hexaene tetrabromide tetrahydrate, C28H50N6O2(4+)*4Br(-)*4H2O. A crystallographic inversion center is located in the macrocyclic cavity and the two hydroxyethyl pendants are on opposite sides of the macrocyclic plane. The benzene rings of the macrocycle are parallel to each other and a pi-pi-stacking interaction exists between the benzene rings of adjacent macrocycles, which are separated by 3.791 (9) A. An infinite intermolecular hydrogen-bond network stabilizes the crystal.

A new dicopper(II) complex with macrocyclic hexaaza bearing hydroxyethyl pendants

Abstract A new dicopper(II) complex with 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxyethyl)-tricyclo[22,2,2,211,14]triaconta-1,11,13,24,27,29-hexaene(L), was synthesized and characterized. The crystal structure of [Cu2L(N3)2](ClO4)2·2H2O has shown that both Cu2+ ions are in penta-coordination environment with distorted trigonal bipyramid consisting of three N atoms from diethylenetriamine moieties, one N atom from azide and one O atom from hydroxyethyl pendant, and the separation of Cu⋯Cu is 8.11 A. The two aromatic rings are close to each other, the shortest distance being 3.71 A for C11⋯C21, and the benzene ring planes from a dihedral angle of 70.9° in molecule. ES-MS spectra showed that the main species, [Cu2(L–H)N3]2+ and [Cu2(L–2H)]2+, were formed by deprotonation of the hydroxyethyl pendants and releasing azides from the Cu(II) complex stepwise.

The crystal structure of a new double hydroxyethyl-bridged dizinc(II) complex with macrocyclic ligand

Abstract A new dizinc(II) complex with 6,20-bis(2-hydroxyethyl)-3,6,9,17,20,23-hexaaza-tricyclo [23,3,1,111,15]-triaconta-1(29),11(30),12,14,25,27-hexaene (L) was synthesized and characterized. The crystal structure of [Zn2H−2L](ClO4)2 has indicated that both Zn2+ ions are in penta-coordination environment with distorted trigonal bipyramid consisting of three N atoms from diethylenetriamine moieties, one internal O atom of the hydroxyethyl pendant, one intermolar O atom from another hydroxyethyl pendant, and the separation of Zn⋯Zn is 3.05 A. The two aromatic rings are parallel to each other, with the centroid–centroid distance 9.21 A. ES–MS spectra showed the main species of [Zn2H−2L]2+, [Zn2H−2L(H2O)2]2+, and [Zn2H−2L(ClO4)]+. The hydrolysis of 4-nitrophenyl acetate catalyzed by [Zn2H−2L]2+ in 10% (v/v) CH3CN at 25 °C, with I=0.10 (NaNO3) and pH 9.0, has shown a second-order rate constant of 0.021 M −1 s −1 .

A novel imidazolate-bridged heterodinuclear Cu(II)Zn(II) complex derived from a unique macrocyclic ligand with two hydroxyethyl pendants

[(CuimZnL-2H)(CuimZnL-H)](ClO4)3, the first imidazolate-bridged Cu(II)-Zn(II) complex of a unique single macrocyclic ligand with two flexible hydroxyethyl pendants, L (L = 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxyethyl)tricyclo[22.2.2.2(11,14)]triaconta-1,11,13,24,27,29-hexaene) has been obtained, in which the macrocyclic ligand with two hydroxyethyl arms possesses a markedly different conformation compared to its dicopper analogue.

Synthesis and properties of a dizinc complex of a novel macrocyclic ligand with two alcohol-pendants: a model for alkaline phosphatase

A novel 24-membered macrocyclic compound having two hydroxyethyl pendants L, 3,6,9,17,20,23-hexazatricyclo[23.3.1.1.11,15]triconta-1(29),11(30),12,14,25,27-hexaene-6,20-bis(2-hydroxyethyl), has been synthesized as a dinucleating ligand. A new complex [Zn2H−2L]Br2(H2O)2 has been synthesized as a model to mimic the active center of alkaline phosphatase. The bridging coordination of the two alkoxides to two ZnII ions was confirmed by the crystal structure of [Zn2H−2L]Br2(H2O)2. The protonation constants of L as well as complexation constants of L with ZnII have been determined at 298.1 K by means of potentiometric titration. [Zn2H−2L]2+ and [Zn2H−2L(OH)]+ are the dominant species in aqueous solution at 7 < pH < 9. The kinetics of promoted hydrolysis of p-nitrophenyl acetate has also been studied; the second-order rate constant is 0.018 M−1 s−1 at pH = 9.0.

Crystal structure of a 26-membered hexaaza macrocycle bearing hydroxyethyl pendants containingp-xylyl spacers

The new macrocyclic compound bearing hydroxyethyl pendant arms containing p-xylyl spacers, 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxyethyl)-tricyclo[22,2,2,211,14] triaconta-1,11,13,24,27,29-hexaene, crystallizes in the monoclinic space group P21/c with a = 8.700(2) Å, b = 18.301(4) Å, c = 11.766(2) Å, β = 108.57(2)°. The two hydroxyethyl pendants are at the opposite sides of the macrocyclic plane. Benzene rings in macrocycle are parallel and exist the π-π weak interaction with distance of 4.28 Å. The crystal packing of the macrocycle is stabilized by the hydrogen bonds.