Chemistry of a Ni(II) Acetohydroxamic Acid Complex: Formation, Reactivity with Water, and Attempted Preparation of Zinc and Cobalt Analogues

Evaluating Metal-Ligand Interactions of Metal-Binding Isosteres Using Model Complexes

Bioisosteres are a useful approach to address pharmacokinetic liabilities and improve drug-like properties. Specific to developing metalloenzyme inhibitors, metal-binding pharmacophores (MBPs) have been combined with bioisosteres, to produce metal-binding isosteres (MBIs) as alternative scaffolds for use in fragment-based drug discovery (FBDD). Picolinic acid MBIs have been reported and evaluated for their metal-binding ability, pharmacokinetic properties, and enzyme inhibitory activity. However, their structural, electronic, and spectroscopic properties with metal ions other than Zn(II) have not been reported, which might reveal similarities and differences between MBIs and the parent MBPs. To this end, [M(TPA)(MBI)]⁺ (M = Ni(II) and Co(II), TPA = tris(2-pyridylmethyl)amine) is presented as a bioinorganic model system for investigating picolinic acid, four heterocyclic MBIs, and 2,2'-bipyridine. These complexes were characterized by X-ray crystallography as well as NMR, IR, and UV-vis spectroscopies, and their magnetic moments were accessed. In addition, [(Tp^Ph,Me)Co(MBI)] (Tp^Ph,Me = hydrotris(3,5-phenylmethylpyrazolyl)borate) was used as a second model compound, and the limitations and attributes of the two model systems are discussed. These results demonstrate that bioinorganic model complexes are versatile tools for metalloenzyme inhibitor design and can provide insights into the broader use of MBIs.

Catalytic reduction of dioxygen to water with a monomeric manganese complex at room temperature

There have been numerous efforts to incorporate dioxygen into chemical processes because of its economic and environmental benefits. The conversion of dioxygen to water is one such example, having importance in both biology and fuel cell technology. Metals or metal complexes are usually necessary to promote this type of reaction and several systems have been reported. However, mechanistic insights into this conversion are still lacking, especially the detection of intermediates. Reported herein is the first example of a monomeric manganese(II) complex that can catalytically convert dioxygen to water. The complex contains a tripodal ligand with two urea groups and one carboxyamidopyridyl unit; this ligand creates an intramolecular hydrogen-bonding network within the secondary coordination sphere that aids in the observed chemistry. The manganese(II) complex is five-coordinate with an N(4)O primary coordination sphere; the oxygen donor comes from the deprotonated carboxyamido moiety. Two key intermediates were detected and characterized: a peroxo-manganese(III) species and a hybrid oxo/hydroxo-manganese(III) species (1). The formulation of 1 was based on spectroscopic and analytical data, including an X-ray diffraction analysis. Reactivity studies showed dioxygen was catalytically converted to water in the presence of reductants, such as diphenylhydrazine and hydrazine. Water was confirmed as a product in greater than 90% yield. A mechanism was proposed that is consistent with the spectroscopy and product distribution, in which the carboxyamido group switches between a coordinated ligand and a basic site to scavenge protons produced during the catalytic cycle. These results highlight the importance of incorporating intramolecular functional groups within the secondary coordination sphere of metal-containing catalysts.

Nickel(II) complexes stabilized by bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine: Synthesis and characterization of complexes stabilized by a hydrogen bonding network

Hydrogen bonds in metalloproteins are key in directing reactivity yet to be achieved in synthetic systems. We have been developing a synthetic system that uses hydrogen-bonding interactions to modulate the secondary coordination around a transition metal ion. This was accomplished with the ligand bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine (H(2)pmb), which contains two carboxyamido units appended from pyridine rings. Several nickel complexes were prepared and structurally characterized. In particular, we found that the appended carboxyamido groups either provide intramolecular H-bond donors or can be converted to bind directly to a metal center. We established that the complex Ni(II)H(2)pmb(Cl)(2) can be sequentially deprotonated with potassium tert-butoxide, causing coordination of the carboxyamido oxygen atoms and concomitant loss of the chloro ligands. The chloro ligands were also removed with silver(I) salts-in the presence of acetate ions, the complex Ni(II)H(2)pmb(κ(2)-OAc)(κ(1)-OAc) was isolated, in which an intramolecular H-bonding network occurs between the H(2)pmb ligand and the coordinate acetato ligands.

Synthesis and characterization of the solid uranium(VI) dioxo-diacetohydroxamate complex

A novel dry synthesis for the uranium(VI) dioxo-diacetohydroxamate (UAHA) complex has been developed. The complex was generated in >80% yield by mechanically grinding solid uranyl acetate dihydrate (UAc) with solid acetohydroxamic acid in stoichiometric amounts. The resulting UO2(AHA)2·4H2O solid is purified by washing with acetone. The stoichiometry was confirmedviacolorimetric assays for U(VI) and AHA. The analogous ferric trisacetohydroxamate complex (FeAHA) was synthesized for comparison. A novel dry synthesis for the uranium(VI) dioxo-diacetohydroxamate (UAHA) complex has been developed. The complex was generated in >80% yield by mechanically grinding solid uranyl acetate dihydrate (UAc) with solid acetohydroxamic acid in stoichiometric amounts. The resulting UO2(AHA)2·4H2O solid is purified by washing with acetone. The stoichiometry was confirmedviacolorimetric assays for U(VI) and AHA. The analogous ferric trisacetohydroxamate complex (FeAHA) was synthesized for comparison.

The UAHA solid was extensively characterized by ultraviolet-visible (UV-vis), Fourier-transform infrared (FT-IR), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The compound did not fluoresce after laser excitation. Proton nuclear magnetic resonance (NMR) spectra were obtained of the complex in D2O, acidified acetonitrile-d3, and DMSO-d6. The solubility was determined over a range of solvents. It was determined that in the purified solid, two bidentate AHA molecules bind to uranylviathe carbonyl and hydroxamate oxygen atoms, a structure analogous to known ferric, nickel, and lanthanum AHA complexes. In an acidic environment, binding is monodentate through the hydroxamate oxygen. And in aqueous solution, the UAHA complex assumes both binding moieties, depending on the pH. This pH-dependent speciation change is demonstrated for the first time.

The easy synthesis and purification of UAHA enables researchers to strictly control reaction conditions; to eliminate interfering salts and water; and to study the complex in the solid-phase.

Glyoxalase I-type hemithioacetal isomerization reactivity of a mononuclear Ni(II) deprotonated amide complex

The synthesis, characterization, and hemithioacetal isomerization reactivity of a mononuclear Ni(II) deprotonated amide complex, [(bppppa-)Ni]ClO4.CH3OH (1, bppppa- = monoanion of N,N-bis-[(6-phenyl-2-pyridyl)methyl]-N-[(6-pivaloylamido-2-pyridyl)methyl]amine), are reported. Complex 1 was characterized by X-ray crystallography, 1H NMR, UV-vis, FTIR, and elemental analysis. Treatment of 1 with an equimolar amount of the hemithioacetal PhC(O)CH(OH)SCD3 in dry acetonitrile results in the production of the thioester PhCH(OH)C(O)SCD3 in approximately 60% yield. This reaction is conveniently monitored via 2H NMR spectroscopy. A protonated analogue of 1, [(bppppa)Ni](ClO4)2 (2), is unreactive with the hemithioacetal, thus indicating the requirement of the anionic chelate ligand in 1 for hemithioacetal isomerization reactivity. Complex 1 is unreactive with the thioester product, PhCH(OH)C(O)SCD3, which indicates that the pKa value for the PhCH(OH)C(O)SCD3 proton of the thioester must be significantly higher than the pKa value of the C-H proton of the hemithioacetal (PhC(O)CH(OH)SCD3). Complex 1 is the first well-characterized Ni(II) coordination complex to exhibit reactivity relevant to Ni(II)-containing E. coli glyoxalase I. Treatment of NiBr2.2H2O with PhC(O)CH(OH)SCD3 in the presence of 1-methylpyrrolidine also yields thioester product, albeit the reaction is slower and involves the formation of multiple -SCD3 labeled species, as detected by 2H NMR spectroscopy. The results of this study provide the first insight into hemithioacetal isomerization promoted by a synthetic Ni(II) coordination complex versus a simple Ni(II) ion.

Self-assembly and cytotoxicity study of waterwheel-like dinuclear metal complexes: The first metal complexes appended with multiple free hydroxamic acid groups

Two waterwheel-like dinuclear complexes [M(2)(PHA)(4)(H(2)O)(2)] (M = Cu(II) (1), Zn(II) (2); HPHA = phthal-hydroxamic acid) appended with four free hydroxamic acid groups, namely, free hydroxamic acid metal complexes (FHAMCs) have been synthesized and characterized. The crystal structure of complex 1 was determined by single crystal X-ray diffraction, which adopts the paddlewheel motif with four bidentate carboxylate ligands joining two Cu(II) ions. The relative cytotoxicities of compounds 1 and 2 against SMMC-7721 and HO-8910 cell lines are similar and more predominant than HPHA (IC(50): Cu(II)>Zn(II)>>HPHA). The synergic effect of the bound water molecules, multiple free hydroxamic acid groups and dimetal active sites with bridging carboxylate may have significant impacts on their pharmacological activity. As the prototype for a new class of hydroxamic acid derivatives, the self-assembly of FHAMCs presents a promising new strategy in designing multiple hydroxamic acids with remarkable bioactivities.

The Coordination Chemistry of FeCl3 and FeCl2 to Bis[2-(2,3-dihydroxyphenyl)-6-pyridylmethyl](2-pyridylmethyl)amine: Access to a Diiron(III) Compound with an Unusual Pentagonal-Bipyramidal/Square-Pyramidal Environment

Coordination of FeCl3 to the title ligand yields a mononuclear iron(III) complex 1, which was characterized by spectroscopic techniques and X-ray diffraction. The ligand is (kappa3-N) tridentate and the metal, which lies in a pseudo-octahedral environment, is bound to a phenolate group from the catechol substituent. The dichloroiron(II) complex 2 was easily obtained by metalation of the ligand with FeCl2 and characterized by various spectroscopic techniques. In their cyclic voltammograms both 1 and 2 display the same reversible FeII/FeIII wave at E1/2=10 mV (vs. SCE). Reduction of compound 1 with Zn/Hg yields 2', which displays identical properties to 2. Taken together, these findings indicate that in spite of the different oxidation state of the metal in 2, no major geometrical/structural change is observed at the metal center with respect to 1. The reaction of 2 with dioxygen in the absence of organic substrates proceeds extremely rapidly and yields compound 3, which is a diiron(III) derivative whose X-ray crystal structure is also reported. The possibility of a radical-based mechanism is discussed. Compound 3 displays an unusual geometry: one iron(III) center is seven-coordinate, whereas the other lies in a square-pyramidal environment. The two iron atoms are bridged by the catecholato substituents. To the best of our knowledge, 3 is the first example of a seven-coordinate iron(III) derivative with tris(2-pyridylmethyl)amine ligands.