Crystal structure and spectroscopic properties of aqua-dichlorido-{1,1'-[(pyridine-2,6-diyl-κN)bis(methyl-ene)]bis-(4-butyl-4,5-di-hydro-1H-1,2,4-triazole-5-thione-κN 2)}cobalt(II).

The structure of the title compound, [CoCl2(C19H27N7S2)(H2O)], at 173 K has monoclinic (C2/c) symmetry. We report here the synthesis, single-crystal structure, electrospray mass spectrum and NMR spectroscopy of a new six-coordinate cobalt(II) pincer complex. The pincer ligand, in this complex, which is novel, coordinates via three nitro-gen atoms (two triazole and one pyridine). The ligand is ambidentate and can coordinate via three nitro-gen atoms or two sulfur and one nitro-gen atoms. The cobalt(II) metal center has pseudo-octa-hedral geometry and based on the single-crystal structure, the pincer ligand coordinates in a meridional fashion with the metal and adjacent six-membered ring ligands all in a similar plane and forming two slightly distorted boat configurations. The other two coordinated monodentate ligands are one water mol-ecule and two chloride ions with four cobalt(II) complexes in the unit cell. The asymmetric unit of the complex is comprised of half the pyridine ring and water mol-ecule with the CoII atom at the center of the pincer situated about a twofold axis. The Co-N, Co-O, and Co-Cl bond lengths are consistent with single bonds. In the crystal, the complex forms a three-centre bifurcated weak hydrogen-bonding inter-action with a chlorine ion, forming one inter-molecular inter-action with the pincer group and a water mol-ecule and a second intra-molecular inter-action with a C-H group within the pincer group. Crystal packing is also highlighted with C 2 2(6)>aa>a ring motifs, forming a three-dimensional supra-molecular network structure. While some stacking of the pyridine rings in the unit cell is observed, there are no relevant π-π inter-actions in the crystal packing. The 1H and 13C{1H} NMR spectra of the complex are consistent with a plane of symmetry being present. The electrospray mass spectrum, which was collected in positive ion mode, showed the loss of one water mol-ecule and one chloride ligand from the complex. In the future, we plan to screen this cobalt(II) complex for electrocatalysis reactivity.

Preparation of SNS Cobalt(II) Pincer Model Complexes of Liver Alcohol Dehydrogenase.

Chemical model complexes are prepared to represent the active site of an enzyme. In this protocol, a family of tridentate pincer ligand precursors (each possessing two sulfur and one nitrogen donor atom functionalities (SNS) and based on bis-imidazole or bis-triazole compounds) are metallated with CoCl2·6H2O to afford tridentate SNS pincer cobalt(II) complexes. Preparation of the cobalt(II) model complexes for liver alcohol dehydrogenase is facile. Based on a quick color change upon adding the CoCl2·6H2O to acetonitrile solution that contains the ligand precursor, the complex forms rapidly. Formation of the metal complex is complete after allowing the solution to reflux overnight. These cobalt(II) complexes serve as models for the zinc active site in liver alcohol dehydrogenase (LADH). The complexes are characterized using single crystal X-ray diffraction, electrospray mass spectrometry, ultra-violet visible spectroscopy, and elemental analysis. To accurately determine the structure of the complex, its single crystal structure must be determined. Single crystals of the complexes that are suitable for X-ray diffraction are then grown via slow vapor diffusion of diethyl ether into an acetonitrile solution that contains the cobalt(II) complex. For high quality crystals, recrystallization typically takes place over a 1 week period, or longer. The method can be applied to the preparation of other model coordination complexes and can be used in undergraduate teaching laboratories. Finally, it is believed that others may find this recrystallization method to obtain single crystals beneficial to their research.

trans-Di-aqua-bis-(N,N,N'-tri-methyl-ethylenedi-amine)-nickel(II) dichloride.

In the title salt, [Ni(C5H12N2)2(H2O)2]Cl2, the asymmetric unit is comprised of half of the complex cation and a chloride ion with the NiII atom of the cation situated about a twofold rotation axis. The six-coordinate NiII atom of the cation is connected to four N atoms from two methyl-substituted ethyelenedi-amine ligands and two water mol-ecules in a slightly distorted octa-hedral environment. The five-membered chelate ring is in a slight envelope conformation. The crystal packing features O-H⋯Cl and N-H⋯Cl inter-molecular inter-actions with the Cl- ion forming weak bifurcated hydrogen bonds with nearby water mol-ecules and N-H inter-actions, leading to a three-dimensional supra-molecular network structure.

Studies of iron(II) and iron(III) complexes with fac-N2O, cis-N2O2 and N2O3 donor ligands: models for the 2-His 1-carboxylate motif of non-heme iron monooxygenases.

Enzymes in the oxygen-activating class of mononuclear non-heme iron oxygenases (MNOs) contain a highly conserved iron center facially ligated by two histidine nitrogen atoms and one carboxylate oxygen atom that leave one face of the metal center (three binding sites) open for coordination to cofactor, substrate, and/or dioxygen. A comparative family of [Fe(II/III)(N(2)O(n))(L)(4-n))](±x), n = 1-3, L = solvent or Cl(-), model complexes, based on a ligand series that supports a facially ligated N,N,O core that is then modified to contain either one or two additional carboxylate chelate arms, has been structurally and spectroscopically characterized. EPR studies demonstrate that the high-spin d(5) Fe(III)g = 4.3 signal becomes more symmetrical as the number of carboxylate ligands decreases across the series Fe(N(2)O(3)), Fe(N(2)O(2)), and Fe(N(2)O(1)), reflecting an increase in the E/D strain of these complexes as the number of exchangeable/solvent coordination sites increases, paralleling the enhanced distribution of electronic structures that contribute to the spectral line shape. The observed systematic variations in the Fe(II)-Fe(III) oxidation-reduction potentials illustrate the fundamental influence of differential carboxylate ligation. The trend towards lower reduction potential for the iron center across the [Fe(III)(N(2)O(1))Cl(3)](-), [Fe(III)(N(2)O(2))Cl(2)](-) and [Fe(III)(N(2)O(3))Cl](-) series is consistent with replacement of the chloride anions with the more strongly donating anionic O-donor carboxylate ligands that are expected to stabilize the oxidized ferric state. This electrochemical trend parallels the observed dioxygen sensitivity of the three ferrous complexes (Fe(II)(N(2)O(1)) < Fe(II)(N(2)O(2)) < Fe(II)(N(2)O(3))), which form µ-oxo bridged ferric species upon exposure to air or oxygen atom donor (OAD) molecules. The observed oxygen sensitivity is particularly interesting and discussed in the context of α-ketoglutarate-dependent MNO enzyme mechanisms.

An anion-dependent switch in selectivity results from a change of C-H activation mechanism in the reaction of an imidazolium salt with IrH5(PPh3)2.

Changing the counteranion along the series Br, BF4, PF6, SbF6 in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with IrH5(PPh3)2 to switch from chelating N-heterocyclic carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT) suggests that the AN path involves C-H oxidative addition to Ir(III) to give Ir(V) with little anion dependence. The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride. The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer, leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via Ir(III), not Ir(V), because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE NMR experiments support the formation of ion-pair in both the reactants and the products. 19F,1H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)).