Cyanide Ligands Research Articles

Syntheses, thermal analyses, crystal structures, FT-IR and Raman spectra of 2D [Zn(NH3)2(μ–ampy)M′(μ–CN)2(CN)2]n (M′ = Ni(II), Pd(II) or Pt(II)) complexes

Three new cyano-bridged heteronuclear polymeric complexes, [Zn(NH3)2(μ–ampy)Ni(μ–CN)2(CN)2]n (1), [Zn(NH3)2(μ–ampy)Pd(μ–CN)2(CN)2]n (2) and [Zn(NH3)2(μ–ampy)Pt(μ–CN)2(CN)2]n (3) (ampy=4-aminomethylpyridine) have been synthesized and characterized by vibrational spectroscopy (FT-IR and Raman), thermal (TG, DTG and DTA) and elemental analyses. The crystal structures of complexes 1 and 2 have been determined by the X-ray single crystal diffraction technique. Complexes 1 and 2 crystallize in the triclinic system with the space group P1¯. Structural studies reveal that the Ni(II) or Pd(II) ions are four coordinate with four cyanide–carbon atoms in a square planar geometry and the Zn(II) ion exhibits a distorted octahedral coordination geometry completed by the six N atoms from two ammine, one ampy and two cyano ligands. The adjacent metal centers are bridged by bis-monodentate cyano ligands to form a one-dimensional linear chain. These chains are linked by ampy ligands into a 2D sheet structure. The 2D units are connected together via intramolecular C–H⋯N and intermolecular N–H⋯N hydrogen bonding to form 3D supramolecular networks. In additional, there are also interactions between the Ni(II) or Pd(II) ion and the aromatic π-system. Vibrational spectral data indicate the presence of two ν(CN) bands for the complexes, which can be assigned to the terminal and bridging cyanide ligands. Decomposition reactions take place in the temperature range 40–900°C in a static air atmosphere.

One-dimensional square- and ladder-type architectures incorporating octacyanometallates of molybdenum(V) and tungsten(V)

Reactions of tptz complexes of Cu(II), Co(II), and Fe(III) with [MV(CN)8]3− (M=Mo, W) anions produced three new heterobimetallic chain compounds (2–4). The mer-capped building block [CuII(tptz)(CF3SO3)2(CH3OH)]·2H2O (1) (tptz=2,4,6-tris(2-pyridyl)-1,3,5-triazine) was also isolated. The structures of 2, 3, and 4 consist of linked squares composed of alternating [MV(CN)8]3− and [M(tptz)]2+/3+ units bridged by cyanide ligands. Both {[CuII(tptz)MoV(CN)8]−·H3O+·CH3OH}∞ (2) and {[FeIII(tptz)WV(CN)8]·2CH3OH}∞ (4) exhibit an infinite ladder-type motif of edge-sharing squares, whereas {[CoII2(tptz)2(H2O)2W2V(CN)16CoII(H2O)4]·2H2O}∞ (3) is composed of molecular squares bridged by [Co(H2O)4]2+ cations. The magnetic behavior of 2 is dominated by ferromagnetic coupling between Cu(II) and Mo(V) spin centers. The isostructural compound 4 exhibits weak antiferromagnetic coupling between Fe(III) and W(V) spin centers. In the case of 3, magnetic studies indicate the presence of ferromagnetic interactions between Co(II) and W(V) centers and the ac data reveal a weak out-of-phase signal at low temperature which suggests single-chain magnet behavior.

Cis-Tetrakis(μ-N-phenylacetamidato)-κ4N:O;κ4O:N-bis[(benzonitrile-κN)rhodium(II)](Rh—Rh)

The complex molecule of the title compound, [Rh2{N(C6H5)COCH3}4(C6H5CN)2], exhibits crystallographically imposed centrosymmetry. The four acetamide ligands bridging the dirhodium core are arranged in a 2,2-cis manner, with two N atoms and two O atoms coordinating to the unique RhII atom cis to one another. The Neq—Rh—Rh—Oeq torsion angles on the acetamide bridges vary between 1.62 (4) and 1.78 (4)°. The Rh—Rh bond length is 2.4319 (3) Å. The axial nitrile ligand completes the distorted octahedral coordination sphere and shows a non-linear coordination with an Rh—N—C bond angle of 167.14 (15)°, while the N—C bond length is 1.135 (3) Å.

Optimization of Triazine Nitriles as Rhodesain Inhibitors: Structure–Activity Relationships, Bioisosteric Imidazopyridine Nitriles, and X‐ray Crystal Structure Analysis with Human Cathepsin L

The cysteine protease rhodesain of Trypanosoma brucei parasites causing African sleeping sickness has emerged as a target for the development of new drug candidates. Based on a triazine nitrile moiety as electrophilic headgroup, optimization studies on the substituents for the S1, S2, and S3 pockets of the enzyme were performed using structure-based design and resulted in inhibitors with inhibition constants in the single-digit nanomolar range. Comprehensive structure-activity relationships clarified the binding preferences of the individual pockets of the active site. The S1 pocket tolerates various substituents with a preference for flexible and basic side chains. Variation of the S2 substituent led to high-affinity ligands with inhibition constants down to 2 nM for compounds bearing cyclohexyl substituents. Systematic investigations on the S3 pocket revealed its potential to achieve high activities with aromatic vectors that undergo stacking interactions with the planar peptide backbone forming part of the pocket. X-ray crystal structure analysis with the structurally related enzyme human cathepsin L confirmed the binding mode of the triazine ligand series as proposed by molecular modeling. Sub-micromolar inhibition of the proliferation of cultured parasites was achieved for ligands decorated with the best substituents identified through the optimization cycles. In cell-based assays, the introduction of a basic side chain on the inhibitors resulted in a 35-fold increase in antitrypanosomal activity. Finally, bioisosteric imidazopyridine nitriles were studied in order to prevent off-target effects with unselective nucleophiles by decreasing the inherent electrophilicity of the triazine nitrile headgroup. Using this ligand, the stabilization by intramolecular hydrogen bonding of the thioimidate intermediate, formed upon attack of the catalytic cysteine residue, compensates for the lower reactivity of the headgroup. The imidazopyridine nitrile ligand showed excellent stability toward the thiol nucleophile glutathione in a quantitative in vitro assay and fourfold lower cytotoxicity than the parent triazine nitrile.

Crystal structure and thermal properties of a square-planar NiIIcomplex of cyanide and a tricyclic bis-amidine ligand formedin situunder solvothermal conditions

The reaction of N(1),N(1')-(ethane-1,2-diyl)bis(propane-1,3-diamine) (bapen), K2[Ni(CN)4]·H2O and dimethylformamide in the presence of Gd(NO3)3·6H2O under solvothermal conditions yielded yellow crystals of dicyanido(2,3,4,6,7,9,10,11-octahydropyrimido[2',1':3,4]pyrazino[1,2-a]pyrimidine)nickel(II) hemihydrate, [Ni(CN)2(C10H16N4)]·0.5H2O, (I), the crystal structure of which is composed of [Ni(CN)2(pdpm)] molecules (pdpm is 2,3,4,6,7,9,10,11-octahydropyrimido[2',1':3,4]pyrazino[1,2-a]pyrimidine) on general positions linked by O-H···N hydrogen bonds to water molecules located on twofold axes. This structural unit is further linked by nonclassical C-H···N interactions to form a warped two-dimensional net perpendicular to the unit-cell b axis. The nets are stacked, with C-H···O contacts joining successive units. The Ni(II) cation is coordinated with square-planar geometry by a chelating pdpm ligand and two cyanide ligands in mutually cis positions. Complex (I) is stable up to 360 K, at which point dehydration takes place; the ligands start to decompose at 558 K.

Tetraammine-2κ4C-μ-cyanido-1:2κ2C:N-tricyanido-1κ3C-copper(II)palladium(II)

The title compound, [Cu(NH3)4-(μ2-NC)—Pd(CN)3], is a binuclear copper(II)palladium(II) complex, in which the CuII coordination is defined by four ammine ligands and one bridging cyanide ligand. The Cu—N bond lengths in the base of the resulting CuN5 pyramid are in the range 2.016 (3)–2.024 (3) Å and the apical Cu—N( C) distance is 2.385 (4) Å. Based on the τ parameter, the shape of the coordination polyhedron is tetra­gonal–pyramidal (τ = 0). All atoms of the square-planar tetracyanidopalladate(II) moiety and the CuII ion are located on a mirror plane. The electroneutral mol­ecules inter­act by N—H⋯N hydrogen bonds, resulting in the formation of a three-dimensional network.

Effect of Solvent Polarity on the Vibrational Dephasing Dynamics of the Nitrosyl Stretch in an FeII Complex Revealed by 2D IR Spectroscopy

The vibrational dephasing dynamics of the nitrosyl stretching vibration (ν(NO)) in sodium nitroprusside (SNP, Na2[Fe(CN)5NO]·2H2O) are investigated using two-dimensional infrared (2D IR) spectroscopy. The ν(NO) in SNP acts as a model system for the nitrosyl ligand found in metalloproteins which play an important role in the transportation and detection of nitric oxide (NO) in biological systems. We perform a 2D IR line shape study of the ν(NO) in the following solvents: water, deuterium oxide, methanol, ethanol, ethylene glycol, formamide, and dimethyl sulfoxide. The frequency of the ν(NO) exhibits a large vibrational solvatochromic shift of 52 cm(-1), ranging from 1884 cm(-1) in dimethyl sulfoxide to 1936 cm(-1) in water. The vibrational anharmonicity of the ν(NO) varies from 21 to 28 cm(-1) in the solvents used in this study. The frequency-frequency correlation functions (FFCFs) of the ν(NO) in SNP in each of the seven solvents are obtained by fitting the experimentally obtained 2D IR spectra using nonlinear response theory. The fits to the 2D IR line shape reveal that the spectral diffusion time scale of the ν(NO) in SNP varies from 0.8 to 4 ps and is negatively correlated with the empirical solvent polarity scales. We compare our results with the experimentally determined FFCFs of other charged vibrational probes in polar solvents and in the active sites of heme proteins. Our results suggest that the vibrational dephasing dynamics of the ν(NO) in SNP reflect the fluctuations of the nonhomogeneous electric field created by the polar solvents around the nitrosyl and cyanide ligands. The solute solvent interactions occurring at the trans-CN ligand are sensed through the π-back-bonding network along the Fe-NO bond in SNP.

Zn–Co Double Metal Cyanides as Heterogeneous Catalysts for Hydroamination: A Structure–Activity Relationship

Zn–Co double metal cyanide (DMC) materials are effective heterogeneous catalysts for intermolecular hydroaminations. Using the reaction of 4-isopropylaniline with phenylacetylene as a test, the effect of different catalyst synthesis procedures on the catalytic performance is examined. The best activities are observed for double metal cyanides with a cubic structure and prepared with a Zn2+ excess, and for nanosized particles prepared via a reverse emulsion synthesis. Detailed study of the active Zn2+ sites in the cubic material by EXAFS gives evidence for coordinative vacancies around the Zn, with four cyanide ligands in close proximity of the Zn. The substrate scope of the hydroaminations was successfully expanded to both aromatic and aliphatic alkynes and other aromatic and aliphatic amines. Even with styrenes the reaction proceeded with aromatic amines. The DMC catalysts are truly heterogeneous, possess a high thermal stability and are perfectly reusable.

Low-Dimensional 3d–4f Complexes Assembled by Low-Spin [FeIII(phen)(CN)4]− Anions

The synthesis, crystal structure, and magnetic properties of four new mixed 3d-4f complexes with formulas [{Fe(III)(phen)(CN)(4)}(4)Gd(2)(III)(bpym)(NO(3))(2)(H(2)O)(4)]·2CH(3)CN·2H(2)O}(n) (1), [{Fe(III)(phen)(CN)(4)}(4)Tb(2)(III)(bpym)(H(2)O)(8)]·(NO(3))(2)·2CH(3)CN}(n) (2), [{Fe(III)(phen)(CN)(4)}(4)Sm(III)(bpym)(NO(3))(2)(H(2)O)(5)]·2CH(3)CN}(n) (3), and [{Fe(III)(phen)(CN)(4)}(2)Pr(2)(III)(bpym)()(NO(3))(4)(H(2)O)(2)](n) (4) (phen = 1,10-phenanthroline and bpym = 2,2'-bipyrimidine) are discussed here. Compounds 1-3 are isomorphous and their structure consists of neutral ladder-like motifs where the rungs are made up by bpym-bridged dilanthanide(III) cations and the rods are defined by [Fe(phen)(CN)(4)](-) units adopting a bis-monodentate coordination mode through two of its four cyanide ligands. The electroneutrality in this family is achieved by either a chelating [at the Gd(III) (1) and Sm(III) (3)] or free [at the Tb(III) (2)] nitrate group and a peripheral [Fe(phen)(CN)(4)](-) entity, which act as a monodentate ligand across one of its four cyanide groups toward the rare-earth cation (1-3). Compound 4 exhibits a neutral two-dimensional structure where (μ-bpym)bis[diaquadi(nitrato-κ(2)-O,O')praseodymium(III)] fragments are interlinked through [Fe(phen)(CN)(4)](-) units adopting a tris-monodentate coordination mode across three of its four cyanide groups. Each iron(III) ion in 1-4 is six-coordinate with two nitrogen atoms from a chelating phen and four cyanide-carbon atoms building a somewhat distorted octahedral environment. The trivalent rare-earth cations are 9- (1-3) and 10-coordinate (4) having in common two nitrogen atoms from a bidentate bpym and three (1-3)/two (4) cyanide nitrogens, the coordination environment being completed by chelating nitrate (1, 3, 4) and water molecules (1-4). Magnetic susceptibility measurements in the 1.9-300 K temperature range show the occurrence of antiferromagnetic interactions in 1 through both the single cyanide- and the bis-bidentate bpym ligands. A weak ferromagnetic interaction is observed for 3 whereas very weak, if any, magnetic interactions would occur in 2 and 4, with the spin-orbit coupling of the low-spin iron(III) ion and the ligand field effects of the Tb(III) (2) and Pr(III) (4) masking their visualization.

Solution NMR characterization of magnetic/electronic properties of azide and cyanide-inhibited substrate complexes of human heme oxygenase: Implications for steric ligand tilt

Solution 2D 1H NMR was carried out on the azide-ligated substrate complex of human heme oxygenase, hHO, to provide information on the active site molecular structure, chromophore electronic/magnetic properties, and the distal H-bond network linked to the exogenous ligand by catalytically relevant oriented water molecules. While 2D NMR exhibited very similar patterns of two-dimensional nuclear Overhauser spectroscopy cross peaks of residues with substrate and among residues as the previously characterized cyanide complex, significant, broadly distributed chemical shift differences were observed for both labile and non-labile protons. The anisotropy and orientation of the paramagnetic susceptibility tensor, χ, were determined for both the azide and cyanide complexes. The most significant difference observed is the tilt of the major magnetic axes from the heme normal, which is only half as large for the azide than cyanide ligand, with each ligand tilted toward the catalytically cleaved α-meso position. The difference in chemical shifts is quantitatively correlated with differences in dipolar shifts in the respective complexes for all but the distal helix. The necessity of considering dipolar shifts, and hence determination of the orientation/anisotropy of χ, in comparing chemical shifts involving paramagnetic complexes, is emphasized. The analysis shows that the H-bond network cannot detect significant differences in H-bond acceptor properties of cyanide versus azide ligands. Lastly, significant retardation of distal helix labile proton exchange upon replacing cyanide with azide indicates that the dynamic stability of the distal helix is increased upon decreasing the steric interaction of the ligand with the distal helix.

1-Aminocyclopropane-1-carboxylic acid oxidase reaction mechanism and putative post-translational activities of the ACCO protein

1-Aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO) catalyses the final step in ethylene biosynthesis converting ACC to ethylene, cyanide, CO2, dehydroascorbate and water with inputs of Fe(II), ascorbate, bicarbonate (as activators) and oxygen. Cyanide activates ACCO. A 'nest' comprising several positively charged amino acid residues from the C-terminal α-helix 11 along with Lys158 and Arg299 are proposed as binding sites for ascorbate and bicarbonate to coordinately activate the ACCO reaction. The binding sites for ACC, bicarbonate and ascorbic acid for Malus domestica ACCO1 include Arg175, Arg244, Ser246, Lys158, Lys292, Arg299 and Phe300. Glutamate 297, Phe300 and Glu301 in α-helix 11 are also important for the ACCO reaction. Our proposed reaction pathway incorporates cyanide as an ACCO/Fe(II) ligand after reaction turnover. The cyanide ligand is likely displaced upon binding of ACC and ascorbate to provide a binding site for oxygen. We propose that ACCO may be involved in the ethylene signal transduction pathway not directly linked to the ACCO reaction. ACC oxidase has significant homology with Lycopersicon esculentum cysteine protease LeCp, which functions as a protease and as a regulator of 1-aminocyclopropane-1-carboxylic acid synthase (Acs2) gene expression. ACC oxidase may play a similar role in signal transduction after post-translational processing. ACC oxidase becomes inactivated by fragmentation and apparently has intrinsic protease and transpeptidase activity. ACC oxidase contains several amino acid sequence motifs for putative protein-protein interactions, phosphokinases and cysteine protease. ACC oxidase is subject to autophosphorylaton in vitro and promotes phosphorylation of some apple fruit proteins in a ripening-dependent manner.

Syntheses, structures and catalase-like activities of mono-, tri- and pentanuclear Cu(II), Cu(II)-Co(II) complexes with cyanide and ONO tridentate oxime ligand

Syntheses, structures and catalase-like activities of mono-, tri- and pentanuclear Cu(II), Cu(II)-Co(II) complexes with cyanide and ONO tridentate oxime ligand

Stereospinomers of pentacoordinate iron porphyrin complexes: the case of the [Fe(porphyrinato)(CN)]− anions

A computational study of the Fe(II) porphyrinato complexes [Fe(tmp)CN](-) and [Fe(tmp)(CN)2](2-) in the high- (S = 2) and low-spin (S = 0) states (tmp = tetramethylporphyrinato) unravels the reasons for a thermally accessible high-spin for the tetraphenylporphyrinato analogue of the former. It is shown that the different coordination geometry of the high-spin stereospinomer is in large part responsible for its stability, while a single strong-field cyanide ligand contributes to the stability of the low-spin stereospinomer. The results are compared with the isoelectronic complexes [Ru(tmp)CN](-) to evaluate the influence of the two-electron terms on the relative stability of the two stereospinomers. Shape and geometry ranges for the two spin states of [Fe(II)(porphyrinato)(ligand)] complexes are established and structural data for the deoxygenated form of hemo- and myoglobins are analyzed from that point of view.

Human Indoleamine 2,3-Dioxygenase Is a Catalyst of Physiological Heme Peroxidase Reactions: IMPLICATIONS FOR THE INHIBITION OF DIOXYGENASE ACTIVITY BY HYDROGEN PEROXIDE

The heme enzyme indoleamine 2,3-dioxygenase (IDO) is a key regulator of immune responses through catalyzing l-tryptophan (l-Trp) oxidation. Here, we show that hydrogen peroxide (H(2)O(2)) activates the peroxidase function of IDO to induce protein oxidation and inhibit dioxygenase activity. Exposure of IDO-expressing cells or recombinant human IDO (rIDO) to H(2)O(2) inhibited dioxygenase activity in a manner abrogated by l-Trp. Dioxygenase inhibition correlated with IDO-catalyzed H(2)O(2) consumption, compound I-mediated formation of protein-centered radicals, altered protein secondary structure, and opening of the distal heme pocket to promote nonproductive substrate binding; these changes were inhibited by l-Trp, the heme ligand cyanide, or free radical scavengers. Protection by l-Trp coincided with its oxidation into oxindolylalanine and kynurenine and the formation of a compound II-type ferryl-oxo heme. Physiological peroxidase substrates, ascorbate or tyrosine, enhanced rIDO-mediated H(2)O(2) consumption and attenuated H(2)O(2)-induced protein oxidation and dioxygenase inhibition. In the presence of H(2)O(2), rIDO catalytically consumed nitric oxide (NO) and utilized nitrite to promote 3-nitrotyrosine formation on IDO. The promotion of H(2)O(2) consumption by peroxidase substrates, NO consumption, and IDO nitration was inhibited by l-Trp. This study identifies IDO as a heme peroxidase that, in the absence of substrates, self-inactivates dioxygenase activity via compound I-initiated protein oxidation. l-Trp protects against dioxygenase inactivation by reacting with compound I and retarding compound II reduction to suppress peroxidase turnover. Peroxidase-mediated dioxygenase inactivation, NO consumption, or protein nitration may modulate the biological actions of IDO expressed in inflammatory tissues where the levels of H(2)O(2) and NO are elevated and l-Trp is low.

Preparation, structural characterisation, and magnetic properties of [Cu(men)2][Cu2Cd2Cl2(CN)6] (men = N-methylethane-1,2-diamine)

AbstractThe novel complex [Cu(men)2][Cu2Cd2Cl2(CN)6] (I) was isolated from the aqueous-ethanol system containing CuCl2, men (men = N-methylethane-1,2-diamine) and K2[Cd(CN)4] in the presence of dilute hydrochloric acid and chemically and spectroscopically characterised. The crystal structure of I consists of [Cu2I(CN)6] and [Cd2Cl2(CN)6] building units bridged by cyanide ligands and forms a three-dimensional skeleton with cavities. [Cu(men)2]2+ cations in which two men ligands are chelated (mean Cu-N is 2.033(6) Å) are located in the cavities. The coordination polyhedron around the Cu(II) atoms is formed as a tetragonal bipyramidal by two weaker axial Cu-Cl bonds (2.8642(12) Å) with chlorido ligands from the skeleton. The Cu(I) and Cd(II) atoms in the skeleton exhibit tetra-(CuC4 chromophore) and penta-coordination (CdN3Cl2), respectively. The temperature-dependent susceptibility measurements indicate a Curie-Weiss-like behaviour and the presence of weak anti-ferromagnetic interaction.

Combining Spectroscopy and Theory to Evaluate Structural Models of Metalloenzymes: A Case Study on the Soluble [NiFe] Hydrogenase from Ralstonia eutropha

Hydrogenases catalyse the reversible cleavage of molecular hydrogen into protons and electrons. While most of these enzymes are inhibited under aerobic conditions, some hydrogenases are catalytically active even at ambient oxygen levels. In particular, the soluble [NiFe] hydrogenase from Ralstonia eutropha H16 couples reversible hydrogen cycling to the redox conversion of NAD(H). Its insensitivity towards oxygen has been formerly ascribed to the putative presence of additional cyanide ligands at the active site, which has been, however, discussed controversially. Based on quantum chemical calculations of model compounds, we demonstrate that spectroscopic consequences of the proposed non-standard set of inorganic ligands are in contradiction to the underlying experimental findings. In this way, the previous model for structure and function of this soluble hydrogenase is disproved on a fundamental level, thereby highlighting the efficiency of computational methods for the evaluation of experimentally derived mechanistic proposals.

A structural phase transition coupled to the Fe3+ spin-state crossover in anhydrous RbMn[Fe(CN)6

Linkage isomerism is the coexistence of iso-compositional molecules or solids differing by connectivity of the metal to a ligand. In a crystalline solid state, the rotation is possible for asymmetric ligands, e.g., for cyanide ligand. Here we report on our observation of a phase transition in anhydrous RbMn[Fe(CN)6] (nearly stoichiometric) and on the effect of linkage isomerism ensuing our interpretation of the results of Mossbauer study in which we observe the iron spin state crossover among two phases involved into this transition. The anhydrous RbMn[Fe(CN)6] can be prepared via prolonged thermal treatment (1 week at at 80 °C) of the as-synthesized hydrated RbMn[Fe(CN)6]·H2O. The latter compound famous for its charge-transfer phase transition is a precursor in our case. As the temperature is raising above 80 °C (remaining below 100 °C) we observe RbMn[Fe(CN)6] that inherited its F-43 m symmetry from RbMn[Fe(CN)6]·H2O transforming to a phase of the Fm-3 m symmetry. In the latter, more than half of Fe3 + ions are in high-spin state. We suggest a plausible way to explain the spin-crossover that is to allow the linkage isomerism by rotation of the cyanide ligands.

A Palladium(II) Center Activates Nitrile Ligands toward 1,3-Dipolar Cycloaddition of Nitrones Substantially More than the Corresponding Platinum(II) Center

Palladium(II)-coordinated NCR(1) (R(1) = Et (1), NMe(2) (2), Ph (3)) species react smoothly with acyclic nitrones such as the ketonitrones Ph(2)C═N(O)R(4) (R(4) = p-MeC(6)H(4) (4), p-ClC(6)H(4) (5)) and the aldonitrone p-MeC(6)H(4)CH═N(O)Me (6) in the corresponding nitrile media. This reaction proceeds as a consecutive two-step intermolecular cycloaddition to give the mono- and bis-2,3-dihydro-1,2,4-oxadiazole complexes [PdCl(2)(R(1)CN){N(a)═C(R(1))ON(R(4))C(b)(R(2)R(3))}]((a-b)) (7a-13a; R(2), R(3) = Ph; R(4) = C(6)H(4)Me-p, R(1) = Et (7), NMe(2) (8), Ph (9); R(4) = C(6)H(4)Cl-p, R(1) = Et (10), NMe(2) (11), Ph (12); R(2) = H, R(3) = C(6)H(4)Me-p, R(4) = Me, R(1) = NMe(2) (13)) and [PdCl(2){N(a)═C(R(1))ON(R(4))C(b)(R(2)R(3))}(2)]((a-b)) (7b-13b), respectively. Inspection of the obtained data and their comparison with the previous results indicate that the Pd(II) centers provide substantially greater activation of RCN ligands toward the 1,3-dipolar cycloaddition than the relevant Pt(II) centers. The palladium(II)-mediated 1,3-dipolar cycloaddition of ketonitrones to nitriles is reversible. All complexes were characterized by elemental analyses (C, H, N), high-resolution ESI-MS, and IR and (1)H and (13)C{(1)H} NMR spectroscopy. The structure of trans-7b was determined by single-crystal X-ray diffraction. Metal-free 5-NR'(2)-2,3-dihydro-1,2,4-oxadiazoles (7c-13c) were liberated from the corresponding (2,3-dihydro-1,2,4-oxadiazole)(2)Pd(II) complexes by treatment with 1,2-(diphenylphosphino)ethane, and the heterocycles were characterized by high-resolution ESI(+)-MS and (1)H and (13)C{(1)H} spectroscopy.

Vacancy‐Induced Deformation in a CoFe Prussian Blue Analogue – A Theoretical Investigation

AbstractThe impact of the number and positions of Rb ions in the RbCo[Fe(CN)6] Prussian blue analogue is analyzed by means of complete active space self‐consistent field (CASSCF) and subsequent second‐order perturbation theory (CASPT2) calculations performed upon embedded cluster models. It is shown that the geometries and corresponding electronic structures of the monomeric [CoIII(NC)5(OH2)]2– and [CoII(NC)5(OH2)]3– units are differently affected when the apical cyanide ligand is substituted by a water molecule. The CoIII ion moves away from the equatorial plane by ca. 0.15 Å, whereas the CoII is almost not sensitive to the environmental change. Furthermore, we find that this phenomenon is rather independent of the positions and numbers of the nearest‐neighbour alkali ions. The Co ion displacement directly controls the overlap between the bridging cyanide and metal ion orbitals, a scenario which might be favorable to trigger electron transfer in the photomagnetic dimeric CoFe units of the CoFe Prussian blue analogues.

Computational study of the electronic structure and magnetic properties of the Ni–C state in [NiFe] hydrogenases including the second coordination sphere

[NiFe] hydrogenases catalyze the reversible formation of H(2). The [NiFe] heterobimetallic active site is rich in redox states. Here, we investigate the key catalytic state Ni-C of Desulfovibrio vulgaris Miyazaki F hydrogenase using a cluster model that includes the truncated amino acids of the entire second coordination sphere of the enzyme. The optimized geometries, computed g tensors, hyperfine coupling constants, and IR stretching frequencies all agree well with experimental values. For the hydride in the bridging position, only a single minimum on the potential energy surface is found, indicating that the hydride bridges and binds to both nickel and iron. The influence of the second coordination sphere on the electronic structure is investigated by comparing results from the large cluster models with truncated models. The largest interactions of the second coordination sphere with the active site concern the hydrogen bonds with the cyanide ligands, which modulate the bond between iron and these ligands. Secondly, the electronic structure of the active site is found to be sensitive to the protonation state of His88. This residue forms a hydrogen bond with the spin-carrying sulfur atom of Cys549, which in turn tunes the spin density at the nickel and coordinating sulfur atoms. In addition, the unequal distribution of spin density over the equatorial cysteine residues results from different orientations of the cysteine side chains, which are kept in their particular orientation by the secondary structure of the protein.