Awal Noor

Metal–Metal Distances at the Limit: A Coordination Compound with an Ultrashort Chromium–Chromium Bond

The nature of the chemical bond is of fundamental importance, and has always fascinated scientists. Metal–metal bonds are of particular interest, as bond orders greater than four are known and are of considerable current interest. The quest for the shortest metal–metal bond is strongly linked with the element chromium and has very recently been reinitiated after the first observation of a bond order greater than four for this metal in a stable compound. Soon afterwards, the shortest metal–metal bond with a chromium–chromium distance of 1.80 ( was observed in a dimeric chromium complex with such a high bond order. Detailed studies on ArCrCrAr complexes (Ar= aryl) performed at the same time showed that such small values can be obtained for this class of compounds as well. Some years ago, we started working with aminopyridinato complexes of chromium and herein report the synthesis and the (electronic) structure of a bimetallic Cr2 complex with a drastically shortened metal– metal distance. The very short metal–metal bond of only 1.75 ( results from a combination of Power3s concept for the stabilization of bond orders higher than four, Hein– Cotton3s principles on the realization of extremely short metal–metal bonds with bridging anionic ligands of type XYZ, and a minimization of additional metal–ligand interactions by optimal steric shielding (Scheme 1). The deprotonation of 1 with potassiumhydride leads to potassium [6-(2,4,6-triisopropylphenyl)pyridin-2-yl](2,4,6-trimethylphenyl)amide, which readily reacts with [CrCl3(thf)3] affording complex 2 (Scheme 2). Compound 2 can be isolated as a green crystalline material in good yield. In the H NMR spectrum, only broad signals can be observed, and magnetic susceptibility experiments show a magnetic moment meff(300 K)= 3.2 mB. When 1 is deprotonated with BuLi and allowed to react with CrCl2 in THF, the Cr II 2 complex 3 is obtained in good yield as a green crystalline material after removal of the solvent and subsequent extraction with

The ligand-based quintuple bond-shortening concept and some of its limitations

Herein, the ligand-based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium-diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride-ate complex gave a quintuply bonded bimetallic complex with a Cr-Cr distance of 1.7056 (12) Å. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C-C bond (1.704 (4) Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr(I) dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by-product. The formation of this by-product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over-reduction takes place, and a chromium-arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.

Cycloaddition Reactions of a Chromium–Chromium Quintuple Bond

Bonds with unusually high bond orders have fascinated chemists for nearly half a century. Molecules that have bond orders higher than four have been known for decades. They can be found in transient diatomic molecules like M2 (M= V, Nb, Cr, Mo). [3–6] The Cr2 molecule, the most popular example among them, has been synthesized in the gas phase by means of pulsed photolysis and by vaporisation of the metal. Furthermore, it can be isolated in an inert matrix. Unfortunately, the stability restricts its use in synthesis (chemistry). In 2005, the Power group reported on the first stable molecule with a quintuply bonded dichromium unit; for the definition of a quintuple bond please refer to a footnote in the paper. Shortly after, the Theopold group reinitiated the “race” for the shortest metal–metal bond and many groups participated in this still ongoing “hunt”. In 2009, the Tsai group reported on the first stable dimolybdenum complex with a quintuple bond. The new opportunity, now that stable compounds with such extreme bond orders are in hand, is not to make shorter and shorter metal–metal bonds, but is to study quintuple bond reactivity and to understand these bonds chemically. Only very few reports on quintuple bond reactivity have appeared in the literature so far. Quintuply bonded bimetallic complexes show a potential for small-molecule activation, particularly on a diatomic platform that can provide from two to eight (in principle even ten) electrons. These complexes feature not just low-valent metal centres, they are also coordinatively highly unsaturated. Alkyne and diene ligand have been successfully used as protecting ligands for low-valent Group 4 metallocenes by (for instance) the groups of Rosenthal, Erker, Beckhaus, and Mach. If one uses a diACHTUNGTRENNUNGatomic site of high bond order to activate such unsaturated molecules, cycloaddition reactions might be expected. We are interested in analogies of quintuple bonds to simple double and triple bonds. Here we report on reactions between a chromium–chromium quintuple bond and substituted acetylenes and dienes. The dichromium complex 1 reacts vigorously when acetylene is introduced into the reaction vessel and leads suddenly to black precipitation, which has been tentatively assigned to be polyacetylide material. However, 1 reacts smoothly in equimolar ratio with diphenylacetylene, phenylacetylene and trimethylsilylacetylene to give 2 a, 2 b and 2 c, respectively (Scheme 1). The formation of a 1:1 adduct is selective, even if 1 is treated with an excess of the corresponding acetylefnes. Compounds 2 a–2 c are diamagnetic. The

Adjusting the DNA Interaction and Anticancer Activity of Pt(II) N-Heterocyclic Carbene Complexes by Steric Shielding of the Trans Leaving Group.

Five platinum(II) complexes bearing a (1,3-dibenzyl)imidazol-2-ylidene ligand but different leaving groups trans to it were examined for cytotoxicity, DNA and cell cycle interference, vascular disrupting properties, and nephrotoxicity. The cytotoxicity of complexes 3a-c increased with the steric shielding of their leaving chloride ligand, and complex 3c, featuring two triphenylphosphanes, was the most efficacious, with submicromolar IC50 concentrations. Complexes 3a-c interacted with DNA in electrophoretic mobility shift and ethidium bromide binding assays. The cationic complex 3c did not bind coordinatively to DNA but led to its aggregation, damage that is not amenable to the usual repair mechanisms. Accordingly, it arrested the cell cycle of melanoma cells in G1 phase, whereas cis-dichlorido[(1,3-dibenzyl)imidazol-2-ylidene](dimethyl sulfoxide) platinum(II) 3a induced G2/M phase arrest. Complex 3c also disrupted the blood vessels in the chorioallantoic membrane of fertilized chicken eggs. Ex vivo studies using precision-cut tissue slices suggested the nephrotoxicities of 3a-c to be clinically manageable.

The shortest metal-metal bond.

The synthesis and isolation of stable bimetallic complexes that contain formally quintuply bonded transition metals is a novel and emerging field of science. Efforts have been undertaken in designing and tuning the ligands to achieve a very short (actually the shortest) metal-metal bond. The motivation for these efforts arose from the expectation that an increasing bond order may go along with a shortening of the bond length. In consequence, formally quintuply bonded bimetallics could have shorter metal-metal distances than quadruply bonded ones. A chromium homo-bimetallic complex with a Cr-Cr bond length of 1.7293(12) Å has been synthesized, and a formal bond order of five was assigned. This compound holds the record for the shortest metal-metal bond in a stable molecule to date. At this stage, there is no evidence that additional shortening is impossible.

Quintuple bond reactivity toward group 16 and 17 elements: addition vs insertion.

The low valent, coordinatively unsaturated, and formally quintuply bonded bimetallic aminopyridinato chromium complex 1 was investigated regarding its reactivity toward group 16 and 17 elements. Reaction of 1 with O(2) yielded a dimeric Cr oxo complex 2, a compound with a high formal oxidation state carrying both bridging and terminal oxo ligands. Reactions with the higher homologues of the group lead to the formation of dimeric Cr(II) complexes in which E(2)(2-) ligands were formed [E = S (3), Se (4), and Te (5)]. Here the quintuply bonded dichromium unit formally undergoes an addition reaction. Reaction of 1 with the homo diatomic molecules of the group 17 elements leads to products in which the Cr-Cr quintuple bond is inserted into the corresponding X(2) molecule [X = Cl (6), Br (7), and I (8)]. Complex 1 was also found to insert into the S-S and Se-Se bonds of 1,2-diphenyldisulfane or the corresponding selenium compound (complexes 9 and 10, respectively). All the compounds have been characterized by NMR and elemental analysis. Additionally, eight of the complexes have been characterized by X-ray analysis. The bimetallic Cr(II) complexes feature metal-metal distances between 1.8369(18) and 1.918(12) Å.

Syntheses, molecular structure, and electrochemical investigations of cobalt(II), copper(II), palladium(II), and zinc(II) complexes with 3-methylpyrazole

Mononuclear complexes of 3-methylpyrazole with general formulas (3-Mepz)4CuCl2 (1), (3-Mepz)4CoCl2 (2), (3-Mepz)2PdCl2 (3), and (3-Mepz)2ZnCl2 (4) were prepared by reaction of the corresponding MCl2 salt (M = Cu, Co, Pd, and Zn) with 3-methylpyrazole in appropriate amounts using acetonitrile as solvent at ambient temperature. The X-ray crystal structure determination reveals that 1 and 2 possess octahedral geometry, while 3 and 4 are square planar and tetrahedral, respectively. All the synthesized compounds have the MCl2 fragment, thus making the synthesized compounds attractive synthons for further transformation. The cyclic voltammograms of the synthesized complexes were obtained and the voltammetric signatures of 1, 2, and 4 showed a single irreversible pH-dependent cathodic peak, while 3 has two reversible cathodic peaks. Involvement of protons accompanying the electron transfer processes was ascertained from differential pulse voltammetric results, indicating peak potential shift as a function of pH. Graphical Abstract Complexes of M(II) with 3-methylpyrazole as ligand were prepared at ambient temperature using MCl2 metal salts (M = Cu, Co, Pd, and Zn). Reactions led to the formation of octahedral (Cu, Co), square-planar (Pd), and tetrahedral (Zn) mononuclear complexes. All reactions were straightforward and no appreciable amount of side products was detected. X-ray structures were determined for all the complexes. The cyclic voltammetric investigations indicate the involvement of proton during electron transfer reactions of these complexes.

FACILE SYNTHESIS, CHARACTERIZATION AND DFT CALCULATIONS OF 2-ACETYL PYRIDINE DERIVATIVES

Haroon ur Rashida, Adnan Shahzadb,d, Zarif Gulc, Ezzat Khanc,*, Muhammad Naveed Umarc,#, Muhammad Raza Shahd, Awal Noore and Sher Wali Khanf Department of Chemistry, Sarhad University of Science & Information Technology, Peshawar, Khyber Pakhtunkhwa, Pakistan Institute of Chemical Science, University of Swat, Swat, Khyber Pakhtunkhwa, Pakistan Department of Chemistry, University of Malakand, Chakdara, Dir (Lower), Khyber Pakhtunkhwa, Pakistan HEJ Research Institute, Sindh University, Karachi, Pakistan. Anorganishe Chemie II, Universität Bayreuth, D-95440 Bayreuth, Germany Department of Chemistry, Shaheed Benazir Bhutto University, Shringel, Upper Dir, Khyber Paktunkhwa, Pakistan

Synthesis Characterization and DFT Calculations of 2,5-Substituted Thiophene Derivatives

Thiophene-2,5-dicarbonyl dichloride was treated with pyrazole and 3-methylpyrazole (3-MePz) in the presence of trimethylamine. Under normal conditions at room temperature, thiophene-2,5-diylbis((1H-pyrazol-1-yl)methanone) (1) and thiophene-2,5-diylbis((3-methyl-1H-pyrazol-1-yl)methanone) (2) were afforded, respectively. Structure of compounds was deduced from the characteristic 1H and 13C-NMR data set. The structure of compound 2 was also confirmed by X-ray diffraction. Selected parameters of compound 1 and structural parameters of compound 2 were calculated by DFT using B3LYP/G-311 level of theory. The calculated data (bond lengths and angles) were found in close agreement with the experimental data. The compound 2 in solid state has planar structure, the free rotation of 3-methylpyrazolyl group has probably been restricted by some sort of intermolecular interactions. In contrast the gas phase optimized structure shows that 3-MePz rings stabilize themselves at maximum distance from each other.Graphical AbstractPyrazolyl and 3-methylpyrazolyl substituted compounds derived from thiophene-2,5-dicarbonyl dichloride were obtained. The X-ray structure of 3-methylpyrazolyl substituted derivative was established, the solid state data was also calculated by DFT. The experimental structure shows high degree of planarity while optimized structure allows free rotation of the substituents.

Coordination compounds of 4,5,6,7-tetrahydro-1H-indazole with Cu(II), Co(II) and Ag(I): structural, antimicrobial, antioxidant and enzyme inhibition studies

Abstract Coordination behavior of 4,5,6,7-tetrahydro-1H-indazole (H-Ind) with Cu(II), Co(II), and Ag(I) was studied. The ligand affords complexes bearing different geometries depending upon the metal and anion present in the starting salts. Five compounds with different structural perspectives, trans-[CuCl2(H-Ind)4] (1), trans-[CuBr2(H-Ind)4] (2), trans-[Cu(CH3COO)2(H-Ind)2] (3), trans-[CoCl2(H-Ind)4] (4), and [Ag(H-Ind)2]NO3 (5), were obtained. The ligand adopts tetrahydro-1H-indazole isomeric form in Cu(II) and Co(II) complexes and with Ag(I) ion the same ligand adopts tetrahydro-2H-indazole form. In the case of sterically demanding acetate counter ion in contrast to Cl or Br, the Cu(II) ion accepts two equivalents of the ligand and four-coordinated square planar complex was obtained. With AgNO3, the expected complex was obtained. The yield of reactions was >80% and all complexes were obtained as crystalline material from the reaction mixtures. Their structures were determined by X-ray diffraction and all complexes were tested for antibacterial (Enterobacter sakazkii, Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniea), antifungal (Aspergillus flavus, Aspergillus fumegatus, Aspergillus nigar, Fusarium oxysporium), and antioxidant (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH)) activities. The same were also tested as inhibitors against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) .