Phthalazine-based dinucleating ligands afford dinuclear centers often encountered in metalloenzyme active sites

Antimalarial Activity of Highly Coordinative Fused Heterocycles Targeting β-Hematin Crystallization

The β-hematin formation is a unique process adopted by Plasmodium sp. to detoxify free heme and represents a validated target to design new effective antimalarials. Most of the β-hematin inhibitors are mainly based on 4-aminoquinolines, but the parasite has developed diverse defense mechanisms against this type of chemical system. Thus, the identification of other molecular chemical entities targeting the β-hematin formation pathway is highly needed to evade resistance mechanisms associated with 4-aminoquinolines. Herein, we showed that the highly coordinative character can be a useful tool for the rational design of antimalarial agents targeting β-hematin crystallization. From a small library consisting of five compound families with recognized antitrypanosomatid activity and coordinative abilities, a group of tetradentate 1,4-disubstituted phthalazin-aryl/heteroarylhydrazinyl derivatives were identified as potential antimalarials. They showed a remarkable curative response against Plasmodium berghei-infected mice with a significant reduction of the parasitemia, which was well correlated with their good inhibitory activities on β-hematin crystallization (IC₅₀ = 5-7 μM). Their in vitro inhibitory and in vivo responses were comparable to those found for a chloroquine reference. The active compounds showed moderate in vitro toxicity against peritoneal macrophages, a low hemolysis response, and a good in silico ADME profile, identifying compound 2f as a promising antimalarial agent for further experiments. Other less coordinative fused heterocycles exhibited moderate inhibitory responses toward β-hematin crystallization and modest efficacy against the in vivo model. The complexation ability of the ligands with iron(III) was experimentally and theoretically determined, finding, in general, a good correlation between the complexation ability of the ligand and the inhibitory activity toward β-hematin crystallization. These findings open new perspectives toward the rational design of antimalarial β-hematin inhibitors based on the coordinative character as an alternative to the conventional β-hematin inhibitors.

Mononuclear gold(iii) complexes with diazanaphthalenes: the influence of the position of nitrogen atoms in the aromatic rings on the complex crystalline properties

A series of mononuclear gold(iii) complexes of the general formula [AuCl3(diazanaphthalene)], where diazanaphthalene is quinazoline (qz, 1), phthalazine (phtz, 2), 1,5-naphthyridine (1,5-naph, 3), 1,6-naphthyridine (1,6-naph, 4) or 1,8-naphthyridine (1,8-naph, 5), were prepared and fully characterized. The complexes 1-5 consist of discrete monomeric species with the Au(iii) cation in a square planar coordination geometry surrounded by three chloride anions and one diazanaphthalene ligand. Crystallographic studies indicate the presence of an extended 4 + 1 or 4 + 2 geometry around the square planar [AuCl3(diazanaphthalene)] center due to Au⋯Cl and Au⋯N interactions. The crystal structures of these complexes are controlled by a variety of intermolecular interactions that utilize the amphiphilic properties of the coordinated chloride anions and involve C-H groups, π-electrons, and an uncoordinated nitrogen atom of the diazanaphthalene ligand. The usual offset π-stacking between the N-heteroaromatic ligands appears to be completely hindered between the 1,5-naph fragments and significantly weakened between the 1,6-naph and 1,8-naph in their respective complexes 3, 4 and 5, for which the average molecular polarizability (α) values are the lowest in the series. It is remarkable that the [AuCl3(benzodiazine)] complexes 1 and 2 form centrosymmetric crystals, but the [AuCl3(naphthyridine)] complexes 3-5 assemble into non-centrosymmetric aggregates, making them potential alternatives to the previously studied systems for application in various fields by taking advantage of their polarity.

Assembly of a mononuclear ferrous site using a bulky aldehyde-imidazole ligand

A new iron(II) complex has been prepared and characterized. [Fe(TrImA)2(OTf)2] (1, TrImA = 1-Tritylimidazole-4-carboxaldehyde). The solid state structure of 1 has been determined by X-ray crystallography. Compound 1 crystallizes in monoclinic space group P21/c, with a = 10.8323(18) Å, b = 8.1606(13) Å and c = 24.818(4) Å. The iron center is coordinated to two imidazole groups, two pendant aldehyde-derived carbonyl oxygens and two triflate oxygens. The complex is high spin between 300 and 20 K as indicated by variable field variable temperature magnetic measurements. A fit of the magnetic data yielded g = 2.17 and D = 4.05 cm-1. A large HOMO-LUMO gap energy (4.49 eV) exists for 1 indicating high stability.

The nature of the Au–N bond in gold(iii) complexes with aromatic nitrogen-containing heterocycles: the influence of Au(iii) ions on the ligand aromaticity

Aromatic character of nitrogen-containing rings is decreased upon gold(iii) coordination.

Dinuclear first-row transition metal complexes with a naphthyridine-based dinucleating ligand

A series of dinuclear and tetranuclear first-row transition metal complexes were synthesized with the dinucleating ligand 2,7-bis(di(2-pyridyl)fluoromethyl)-1,8-naphthyridine (DPFN). The coordination pocket and rigidity of the DPFN ligand enforces pseudo-octahedral geometries about the metal centers that contain chloro, hydroxo, and aqua bridging ligands forming a "diamond" shaped configuration with metal-metal distances varying from 2.7826(5) to 3.2410(11) Å. Each metal center in the dinuclear complexes has an additional open coordination site that accommodates terminal ligands in a syn geometry of particular interest in catalyst design. The complexes are characterized by electronic spectroscopy, electrochemistry and potentiometric titration methods.

Evolution of strategies to prepare synthetic mimics of carboxylate-bridged diiron protein active sites

We present a comprehensive review of research conducted in our laboratory in pursuit of the long-term goal of reproducing the structures and reactivity of carboxylate-bridged diiron centers used in biology to activate dioxygen for the conversion of hydrocarbons to alcohols and related products. This article describes the evolution of strategies devised to achieve these goals and illustrates the challenges in getting there. Particular emphasis is placed on controlling the geometry and coordination environment of the diiron core, preventing formation of polynuclear iron clusters, maintaining the structural integrity of model complexes during reactions with dioxygen, and tuning the ligand framework to stabilize desired oxygenated diiron species. Studies of the various model systems have improved our understanding of the electronic and physical characteristics of carboxylate-bridged diiron units and their reactivity toward molecular oxygen and organic moieties. The principles and lessons that have emerged from these investigations will guide future efforts to develop more sophisticated diiron protein model complexes.

Synthesis and Coordination Chemistry of Doubly-Tridentate Tripodal Pyridazine and Pyrimidine-Derived Ligands: Structural Interplay Between M2L and M2L2 (M = Ni and Pd) Complexes and Magnetic Properties of Iron(II) Complexes

The coordination chemistry of three bridging doubly-tridentate ligands, including the known compound 3,6-bis(di-2-pyridylmethyl)pyridazine (1), which is structurally similar to 1,4-bis(di-2-pyridylmethyl)phthalazine (2), and two pyrimidine-linked compounds 4,6-bis(di-2-pyridylmethyl)pyrimidine (3), and 4,6-bis(di-2-pyridylamino)pyrimidine (4), was investigated with FeII, NiII, and PdII metal salts. Ligands 3 and 4 were synthesized in one-pot reactions from easily obtained starting materials; compound 3 was synthesized from di-2-pyridylmethane and 4,6-diiodopyrimidine in 48% yield, while ligand 4 was prepared by reacting di-2-pyridylamine with 4,6-dichloropyrimidine in 27% yield. During the synthesis of 4, an additional compound, 4-chloro-6-(di-2-pyridylamino)pyrimidine (5), with only one tridentate binding site was obtained in 30% yield. Reactions of 1, 3, and 4 with FeII or NiII salts gave two types of complexes, either discrete M2L or M2L2 assemblies. The PdII complexes obtained were also characterized as discrete M2L complexes. The compounds were characterized by a combination of NMR and IR spectroscopy, microanalysis and X-ray crystallography. Noticeable differences in the structures obtained for NiII coordination complexes with the carbon-linked (3) and nitrogen-linked (4) ligands were observed, whereby the nitrogen linker adopted a trigonal planar geometry and prevented tridentate facial coordination of the octahedral metal centres. The magnetic properties of dinuclear FeII complexes of 1 were examined to see if they showed spin-crossover effects, a feature recently observed by others in other dinuclear helicate complexes, but the complexes remained high-spin at all temperatures between 300 and 2 K.

Synthesis and reactivity of a C3-symmetric trinuclear zinc(ii) hydroxide catalyst efficient at phosphate diester transesterification

Inspired by trinuclear Zn(II) sites in enzymatic systems, a ligand system containing three preorganized (2-pyridyl)methyl piperazine moieties anchored onto a rigid C3-symmetric triphenoxymethane platform has been developed for preorganizing three zinc ions into an environment conducive to intramolecular interaction. Zinc(II) binding by this ligand has been analyzed by means of potentiometric measurements in 50% (v/v) CH3CN-H2O solutions. Subsequently a C3-symmetric trinuclear Zn(II) hydroxide complex of the C3-symmetric ligand was synthesized and fully characterized using NMR spectroscopy and X-ray crystallography. This complex induces a 16,900-fold rate enhancement in the catalytic cyclization of the RNA model substrate, 2-hydroxypropyl-p-nitrophenyl phosphate (HPNP, pH 6.7, 25 degrees C) over the uncatalyzed reaction with multiple catalyst turnovers. The observed differences in the pH-rate profile can be attributed to the varying concentration of various trinuclear zinc species. The trinuclear Zn(II) catalyst exhibits a higher hydrolytic activity compared to its mononuclear analogue. The reactivity and structural features of this trinuclear Zn(II) complex will be discussed.

Anion-controlled nuclearity in nickel complexes with potentially dinucleating, poly(oxime) amine ligands

Two new ligands consisting of bis(oxime) amine units tethered by a bridge have been synthesized. Their nickel chloride and nickel nitrate complexes have also been synthesized and characterized by X-ray crystallography, FTIR, mass spectrometry, and elemental analysis. One of these ligands, L1 (N,N,N',N'-tetra(1-propan-2-onyl oxime)-diamino-m-xylene), is always dinucleating, while the other ligand, L2 (N,N,N',N'-tetra(1-propan-2-onyl-oxime)-1,3-diaminopropane), shows an unusual anion dependence on the nuclearity. When nickel chloride is used, the ligand acts in a dinucleating manner and coordinates two nickels; however, when nickel nitrate is used, the ligand acts in a monodentate fashion and coordinates only one nickel. Once the mononuclear complex is formed, it is not possible to add a second nickel if Ni(NO(3))(2) is used as the nickel source; it is possible, however, to add a second nickel if NiCl(2) is used as the nickel source. The dinuclear complex can be converted to the mononuclear one by either using silver nitrate to exchange the chloride anions for nitrates or by dissolving the complex in water. Ni(2)(L1)Cl(4)(DMF)(2).DMF: orthorhombic, P2(1)2(1)2(1), a = 12.2524(11) A, b = 16.6145(15) A, c = 20.1234(19) A, V = 4096.5(6) A(3), Z = 4. [Ni(2)(L2)Cl(4)(DMF)](2).2DMF: triclinic, P-1, a = 12.5347(5) A, b = 12.5403(5) A, c = 14.3504(6) A, alpha = 67.348(1) degrees , beta = 69.705(1) degrees , gamma = 81.549(1) degrees , V = 1952.25(14) A(3), Z = 1. Ni(L2).(NO(3))(2): monoclinic, P2(1)/n, a = 9.6738(3) A, b = 30.2229(9) A, c = 15.8238(5) A, beta = 97.995(1) degrees , V = 4581.4(2) A(3), Z = 8.

Modeling non-heme iron proteins

Synthetic modeling studies of non-heme iron proteins continue to contribute to our understanding of the mechanism of these proteins. Recently, mononuclear Fe(IV)=O complexes have been prepared and characterized to model the same species that are proposed to be the reactive intermediates in reactions involving mononuclear non-heme iron proteins. Generation of such species for the oxidation of organic substrates has also been demonstrated. Other advances include successful modeling of the structural and functional aspects of diiron non-heme proteins with the use of terphenyl-based carboxylate ligands and the development of several iron-based reagents that catalyze oxidation reactions with the use of various oxidants, including dioxygen.