Jack Cheng

Mono- and binuclear cobalt(II)-azido complexes of arylazoimidazole: synthesis, spectral characterization, electrochemistry and crystal structure

Abstract Upon setting up proper reaction conditions 1-alkyl-2-(arylazo)imidazoles (RaaiR′ where R=H (a), Me (b); R′=Me (1/3/5), Et (2/4/6) may react with Co(OAc)2·4H2O and NaN3 in methanolic solution to give two classes of azido complexes: mononuclear bis-chelated [Co(RaaiR′)2(N3)2] (3,4) and binuclear μ-(1,1) azido bridged mono-chelated [Co(RaaiR′)(μ-(1)N3)(μ-(1,1)N3]2 (5,6) complexes. They are characterized by UV–Vis, IR spectra, room temperature magnetism and they have been structurally confirmed by X-ray diffraction study of [Co(MeaaiMe)2(N3)2] (3b) and [Co{(μ-1)N3}(MeaaiMe)]2 (5b). The mononuclear complex (3b) is distorted octahedral whereas the binuclear azide bridged complex (5b) has the distorted square pyramidal geometry around the cobalt(II) centre. Redox properties of the complexes are examined by cyclic voltammetry and show a high potential Co(III)/Co(II) couple along with ligand reductions. EHMO calculation and comparison between the two types of complexes explain the spectral and redox properties of the complexes.

Studies on the zinc(II)-azoimine system. Single-crystal X-ray structure of Zn(MeaaiMe)Cl2·H2O and Zn(HaaiMe)2(NCS)2 (MeaaiMe=1-methyl-2-(p-tolylazo)imidazole, HaaiMe=1-methyl-2-(phenylazo) imidazole)

Abstract Dichloro zinc(II) complexes of 1-alkyl-2-(arylazo)imidazole, Zn(RaaiR′)Cl2·H2O (RaaiR′=p-RC6H4N=NC3H2NN(1)R′: R=H, Me, Cl and R′=Me, CH2CH3, CH2Ph) are characterized by elemental analysis, IR, UV–Vis and 1H NMR spectral data. The reaction of ZnCl2 and RaaiR′ (excess) in the presence of NH4CNS has synthesized Zn(RaaiR′)2(NCS)2. The single-crystal X-ray structure of dichloro-{1-methyl-2-(p-tolylazo)imidazole}zinc(II)-monohydrate (Zn(MeaaiMe)Cl2·H2O) suggests that the complex is distorted trigonal bipyramidal (TBP) around Zn(II) with a trigonal plane constituted from Cl(1), Cl(2), N(imidazole) (N(1)). Molecular packing shows a 1D chain via a hydrogen bonded eight membered ring. The X-ray structure of di-thiocyanato-bis-{1-methyl-2-(phenylazo)-imidazole}zinc(II) also shows a distorted TBP around Zn(II). The coordination sphere is ZnN3 type: where one of the ligands acts as a bidentate chelating agent and the second one coordinates via only the imidazole-N along with two N-centers from the NCS group.

Isolation and structural characterization of trans-β-{(C-meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)(diaquo)}nickel(II) perchlorate and cis-α-{(C-racemic-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)(diisothiocyanato)}nickel(II)

Abstract The β-[Ni(C-meso-1,7-CTH)](ClO4)2 (1,7-CTH=5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) isomer having trans-III (RRSS) conformation leads to the formation of trans-β-[Ni(C-meso-1,7-CTH)(H2O)2](ClO4)2 in 1:1 (v/v) H2O–MeOH mixture containing 1 M NaCl and exists as the equilibrium mixture of low spin square planar β-[Ni(C-meso-1,7-CTH)](Cl)2 and high spin octahedral trans-β-[Ni(C-meso-1,7-CTH)(H2O)2](ClO4)2 complexes. α-[Ni(C-rac-1,7-CTH)](ClO4)2 possesses trans-V (RRRR) conformation and forms cis-α-[Ni(C-rac-1,7-CTH)(NCS)2] with NaSCN in MeOH. The structures of trans-β-[Ni(C-meso-1,7-CTH)(H2O)2](ClO4)2 and cis-α-[Ni(C-rac-1,7-CTH)(NCS)2] complexes have been characterized by X-ray diffraction studies. The aquo complex exhibits three characteristic bands at approximately 688, 445 and 290 nm. cis-α-[Ni(C-rac-1,7-CTH)(NCS)2] shows a band corresponding to d–d transitions at approximately 583 nm.

A biophysical elucidation for less toxicity of Agglutinin than Abrin-a from the Seeds of Abrus Precatorius in consequence of crystal structure

X-ray crystal structure determination of agglutinin from abrus precatorius in Taiwan is presented. The crystal structure of agglutinin, a type II ribosome-inactivating protein (RIP) from the seeds of Abrus precatorius in Taiwan, has been determined from a novel crystalline form by the molecular replacement method using the coordinates of abrin-a as the template. The structure has space group P41212 with Z = 8, and been refined at 2.6 Å to R-factor of 20.4%. The root-mean-square deviations of bond lengths and angles from the standard values are 0.009 Å and 1.3°. Primary, secondary, tertiary and quaternary structures of agglutinin have been described and compared with those of abrin-a to a certain extent. In subsequent docking research, we found that Asn200 of abrin-a may form a critical hydrogen bond with G4323 of 28SRNA, while corresponding Pro199 of agglutinin is a kink hydrophobic residue bound with the cleft in a more compact complementary relationship. This may explain the lower toxicity of agglutinin than abrin-a, despite of similarity in secondary structure and the activity cleft of two RIPs.

M–NCO (M=Mn, Zn) complexes of 1-alkyl-2-(arylazo)imidazole: crystal structures of [Mn(MeaaiEt)2(NCO)2] and [Zn(HaaiMe)2(NCO)2] [HaaiMe=1-methyl-2-(phenylazo)imidazole and MeaaiEt=1-ethyl-2-(P-tolylazo)imidazole]

Reaction of M(OAc)2·4H2O (M = Mn, Zn) and 1-alkyl-2-(arylazo)imidazole [RaaiR′ where R = H (a), Me (b); R′ = Me (1/3/5), Et (2/4/6)] and NaNCO in methanol at 1 : 2 : 2 mol ratio gave [Mn(RaaiR′)2(NCO)2] (3, 4) and [Zn(RaaiR′)2(NCO)2](5, 6) complexes. The structures of these species were confirmed by single-crystal X-ray studies of 4b and 5a.

Hydrogen bonded networks based on Ni(II)-tetraazamacrocycle (tet-b) complexes: synthesis, isolation and structural characterization of α -[Ni(tet-b)(Cl)](ClO4) and α -[Ni(tet-b)(en)](ClO4)2

Five- and six-coordinate, α-[Ni(tet-b)(Cl)](ClO4) (1) and α-[Ni(tet-b)(en)](ClO4)2 (2) (tet-b = C-racemic-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) complexes have been synthesized and isolated from the reactions of α-[Ni(tet-b)](ClO4)2, which has trans-V (1R,4R,8R,11R,7S,14S or 1S,4S,8S,11S,7R,14R) conformation, with t-Bu4NCl and ethylenediamine (en), respectively. The complexes have been characterized by X-ray crystallography. The crystal structure of 1 shows a distorted trigonal bipyramidal (TBP) coordination geometry composed of four nitrogen atoms from tet-b and a chloro group with an N4Cl chromophore about the nickel(II) ion. The complex cations of 1 are assembled by the perchlorate ions via N–H ··· O hydrogen bonding to form 1-D zigzag chains along the [001] direction. The chains are linked through intermolecular hydrogen bonding where the coordinated chloro group of the complex cation forms two-center double hydrogen bonds with the adjacent N–H groups of the macrocyclic ligand along the [100] direction, resulting in a two-dimensional α-network. The crystal structure of 2 shows a distorted octahedral coordination environment consisting of four nitrogen atoms from tet-b and two from en with an N6 chromophore about nickel(II) ion. The crystal packing analysis shows that the complex cations, α-[Ni(tet-b)(en)]2+ are interconnected by perchlorate ions through conventional two-center (N)H ··· O, and bifurcated (N)H ··· O ··· H(N) hydrogen bonding.

Exchange interactions in a one-dimensional bromo-bridged copper(II) compound with a ladder-like structure

A deep green complex, [Cu(L)(μ-Br)Br]2 (L = 1,4-diazacycloheptane) was synthesized and its crystal structure and magnetic properties have been studied. The structure determination reveals that in the binuclear unit each copper(II) occupies a distorted square pyramidal geometry in which each bridging bromine atom is simultaneously ligated in axial and equatorial positions between two adjacent copper(II) centres. The binuclear units are packed along the [101] direction. Each dimer is connected to its two nearest neighbours by two Cu–(μ-Br)⋯H–N–Cu hydrogen bonding interactions resulting in an infinite 1D ladder-like chain. The magnetic data were fitted considering the interdimer interactions using different models to obtain precise results and the susceptibility vs. temperature curve for the complex exhibits a maximum at 12 K indicating antiferromagnetic behaviour. Magneto-structural correlations have also been carried out.