Cyanide Ligands Research Articles

Compromise between conjugation length and charge-transfer in nonlinear optical η 5-monocyclopentadienyliron(II) complexes with substituted oligo-thiophene nitrile ligands: Synthesis, electrochemical studies and first hyperpolarizabilities

A systematic series of η 5-monocyclopentadienyliron(II) complexes with substituted oligo-thiophene nitrile ligands of general formula [FeCp(P_P)(NC{SC 4H 2} n NO 2)][PF 6] (P_P = dppe, (+)-diop; n = 1–3) has been synthesized and characterized. The electrochemical behaviour of the new compounds was explored by cyclic voltammetry. Quadratic hyperpolarizabilities ( β) of the complexes with dppe coligands have been determined by hyper-Rayleigh scattering (HRS) measurements at two fundamental wavelengths of 1.064 and 1.550 μm, to uncover the two-photon resonance effect and to estimate static β values. The obtained overall results are found to be better than for the related η 5-monocyclopentadienyliron(II) complexes with p-benzonitrile derivatives. Although an increase of the resonant β at 1.064 μm with increasing number of thiophene units in the conjugated ligand was found (up to 910 × 10 −30 esu), the static values β 0 remain practically unchanged, as shown by the 1.550 μm measurements. Combined with the electrochemical and spectroscopic data (IR, NMR, UV–vis), this remarkable evolution of β shows that the increase of conjugation length is balanced by a decrease in charge-transfer efficiency.

Structural and Photophysical Properties of Adducts of [Ru(bipy)(CN)4]2- with Different Metal Cations: Metallochromism and Its Use in Switching Photoinduced Energy Transfer

We show in this paper how the 3MLCT luminescence of [Ru(bipy)(CN)4]2-, which is known to be highly solvent-dependent, may be varied over a much wider range than can be achieved by solvent effects, by interaction of the externally directed cyanide ligands with additional metal cations both in the solid state and in solution. A series of crystallographic studies of [Ru(bipy)(CN)4]2- salts with different metal cations Mn+ (Li+, Na+, K+, mixed Li+/K+, Cs+, and Ba2+) shows how the cyanide/Mn+ interaction varies from the conventional "end-on" with the more Lewis-acidic cations (Li+, Ba2+) to the more unusual "side-on" interaction with the softer metal cations (K+, Cs+). The solid-state luminescence intensity and lifetime of these salts is highly dependent on the nature of the cation, with Cs+ affording the weakest luminescence and Ba2+ the strongest. A series of titrations of the more soluble derivative [Ru(tBu2bipy)(CN)4]2- in MeCN with a range of metal salts showed how the cyanide/Mn+ association results in a substantial blue-shift of the 1MLCT absorptions, and 3MLCT energies, intensities, and lifetimes, with the complex varying from essentially non-luminescent in the absence of metal cation to showing strong (phi = 0.07), long-lived (1.4 micros), and high-energy (583 nm) luminescence in the presence of Ba2+. This modulation of the 3MLCT energy, over a range of about 6000 cm-1 depending on the added cation, could be used to reverse the direction of photoinduced energy transfer in a dyad containing covalently linked [Ru(bipy)3]2+ and [Ru(bipy)(CN)4]2- termini. In the absence of a metal cation, the [Ru(bipy)(CN)4]2- terminus has the lower 3MLCT energy and thereby quenches the [Ru(bipy)3]2+-based luminescence; in the presence of Ba2+ ions, the 3MLCT energy of the [Ru(bipy)(CN)4]2- terminus is raised above that of the [Ru(bipy)3]2+ terminus, resulting in energy transfer to and sensitized emission from the latter.

New insights on the electronic and molecular structure of cyanide-ligated iron(III) porphyrinates.

The preparation and characterization of several new cyano-ligated six-coordinate low-spin iron(III) porphyrinates are reported. The synthesis and structure of the new bis(cyanide) derivative K(222)][Fe(TMP)(CN)2] (TMP = tetramesitylporphyrinate) is described. Three mixed-ligand species of the general form [Fe(Porph)(CN)(L)], where L = 1-methylimidazole or pyridine, have also been prepared and structurally characterized. All complexes have been studied with EPR spectroscopy in frozen solution and in microcrystalline form. In some cases, especially those of the bis(cyanide) derivative above and the previously reported [Fe(TPP)(CN)2](-), there are significant differences in the EPR spectra as a result of the state change. These spectral differences can be correlated with changes in the electron configuration that are the likely result of a differing environment of the coordinated cyanide ligands; the core conformation and electronic structure of the porphyrin ligand are unlikely to play a role. All four new complexes and [Fe(TPP)(CN)2](-) have been studied by Mössbauer spectroscopy with variable-temperature and applied magnetic-field measurements. The sign of the quadrupole splitting value has been established as negative. These measurements have allowed us to give estimates of the energy difference between the two close-lying dpi (dxz and dyz) orbitals. These splitting values range from approximately 267 cm-1 for [Fe(TPP)(CN)2](-) to approximately 614 cm(-1) for [Fe(TPP)(CN)(Py)].

Redox-Switched Complexation/Decomplexation of K+ and Cs+ by Molecular Cyanometalate Boxes

The reaction of [N(PPh(3))(2)][CpCo(CN)(3)] and [Cb*Co(NCMe)(3)]PF(6) (Cb* = C(4)Me(4)) in the presence of K(+) afforded {K subset[CpCo(CN)(3)](4)[Cb*Co](4)}PF(6), [KCo(8)]PF(6). IR, NMR, ESI-MS indicate that [KCo(8)]PF(6) is a high-symmetry molecular box containing a potassium ion at its interior. The analogous heterometallic cage {K subset[Cp*Rh(CN)(3)](4)[Cb*Co](4)}PF(6) ([KRh(4)Co(4)]PF(6)) was prepared similarly via the condensation of K[Cp*Rh(CN)(3)] and [Cb*Co(NCMe)(3)]PF(6). Crystallographic analysis confirmed the structure of [KCo(8)]PF(6). The cyanide ligands are ordered, implying that no Co-CN bonds are broken upon cage formation and ion complexation. Eight Co-CN-Co edges of the box bow inward toward the encapsulated K(+), and the remaining four mu-CN ligands bow outward. MeCN solutions of [KCo(8)](+) and [KRh(4)Co(4)](+) were found to undergo ion exchange with Cs(+) to give [CsCo(8)](+) and [CsRh(4)Co(4)](+), both in quantitative yields. Labeling experiments involving [(MeC5H4)Co(CN)(3)]- demonstrated that Cs(+)-for-K(+) ion exchange is accompanied by significant fragmentation. Ion exchange of NH(4+) with [KCo(8)](+) proceeds to completion in THF solution, but in MeCN solution, the exclusive products were [Cb*Co(NCMe)(3)]PF(6) and the poorly soluble salt NH(4)CpCo(CN)(3). The lability of the NH(4+)-containing cage was also indicated by the rapid exchange of the acidic protons in [NH(4)Co(8)](+). Oxidation of [MCo(8)](+) with 4 equiv of FcPF(6) produced paramagnetic (S = 4/2) [Co(8)](4+), releasing Cs(+) or K(+). The oxidation-induced dissociation of M(+) from the cages is chemically reversed by treatment of [Co(8)](4+) and CsOTf with 4 equiv of Cp(2)Co. Cation recognition by [Co(8)] and [Rh(4)Co(4)] cages was investigated. Electrochemical measurements indicated that E(1/2)(Cs(+))--E(1/2)(K(+)) approximately 0.08 V for [MCo(8)](+).

The reaction of manganese carbonyl with tert-butylisocyanide: Synthesis and characterization of cis- and trans-[Mn I( tBuNC) 4(CN)(CO)

The reaction of Mn 2(CO) 10 with tert-butyl isocyanide in the presence of 10 bar of carbon monoxide leads to the formation of cis- and trans-[Mn( tBuNC) 4(CN)(CO)], 1a and 1b, in good yields together with [Mn( tBuNC) 6]CN ( 2), as a minor product. Nevertheless, the reaction pathway highly depends on the reaction conditions. An interesting side-product is obtained, if chloroform is used during the workup procedure. Compound 3 is composed of cationic [Mn( tBuNC) 5(CO)] units as well as dinuclear anionic [Mn( tBuNC) 4(CO)(μ–CN)MnCl 3] moieties. If no additional CO pressure is applied to the system, the organic product N, N′-di- tert-butyl-3,5-bis- tert-butylimino-4-phenyl-cyclopent-1-ene-1,2-diamine ( 4), is formed in considerable amount. Compound 4 most probably is produced via a double benzylic C–H activation of the solvent toluene and the oligomerization of four isocyanide moieties. The reaction of 1b with Co(NO 3) 2 leads to the isolation of the trinuclear cyanide bridged coordination compound {[Mn( tBuNC) 4) (CO) (μ–CN)] 2Co(NO 3) 2}, 5, in which the cobalt atoms are tetrahedrally surrounded by the two cyanide ligands and the η 1-coordinated nitro groups. In contrast to the reaction of 1b, treatment of the dicyano complexes cis- or trans-[Ru( tBuNC) 4(CN) 2] with Co(NO 3) 2 results in the formation of the coordination polymers {[Ru( tBuNC) 4(CN) 2]Co(NO 3) 2} n , 7 ( trans) and 9 ( cis). All new compounds are characterized by X-ray diffraction experiments.

Structure of cyano-bridged Eu(III)—Co(III) bimetallic assembly and its application to photophysical verification of photomagnetic phenomenon

AbstractNew crystal structure of Eu(DMF)4(H2O)3Co(CN)6·H2O (DMF = N,N′-dimethylformamide) (Eu-Co) has been determined to be monoclinic, P2(1)/n, a = 19.796(12) Å, b = 8.862(11) Å, c = 17.525(10) Å, β = 96.26(5)°, V = 3056(5) Å3, Z = 4. The Eu(III) ion adopts an antiprismatic eight-coordination and forms a cyano bridge with r(Eu-N) = 2.496(7) Å and Θ(Eu-N-C) = 165.7(7)° to the Co(III) ion. The complex exhibits some common features with the Eu-Fe complex. Diffuse reflectance electronic spectra and magnetic susceptibility of Eu-Cr, Eu-Mn, Eu-Fe, and Eu-Co complexes were compared. By substituting the metal ions, both electronic and structural features affected the charge transfer bands and superexchange interactions concerning cyanide ligands. In addition, only Eu-Co exhibited 5 D 0 → 7 F 2 and 5 D 0 → 7 F 1 luminescence bands at 16300 cm−1 and 16900 cm−1, respectively at 298 K (λex = 360 nm (27000 cm−1)), because quenching by cyano-bridged ions did not prevent Eu(III) ion from exhibiting emission. Thus, only Eu-Co may be suitable for verification of an assumption of mechanism concerning drastic photoinduced magnetic changes for Nd-Fe. Merely small decrease of magnetization was observed for Eu-Co after UV light irradiation at 2.0 K. This result was attributed to slight structural changes around cyano bridges without transitions of spin states.

A third type of hydrogenase catalyzing H2 activation

The activation of molecular hydrogen is of interest both from a chemical and biological viewpoint. The covalent bond of H(2) is strong (436 kJ mol(-1)). Its cleavage is catalyzed by metals or metal complexes in chemical hydrogenation reactions and by metalloenzymes named hydrogenases in microorganisms. Until recently only two types of hydrogenases are known, the [FeFe[-hydrogenases and [NiFe[-hydrogenases. Both types, which are phylogenetically unrelated, harbor in their active site a dinuclear metal center with intrinsic CO and cyanide ligands and contain iron-sulfur clusters for electron transport as revealed by their crystal structures. Fifteen years ago a third type of phylogenetically unrelated hydrogenase was discovered, which has a mononuclear iron active site and is devoid of iron-sulfur clusters. It was initially referred to as "metal free" hydrogenase, but was later renamed iron-sulfur cluster-free hydrogenase or [Fe[-hydrogenase. In this review, we introduce first the [FeFe[-hydrogenases and [NiFe[-hydrogenases, and then focus on the structure and function of the iron-sulfur cluster-free hydrogenase (Hmd) and show that this enzyme contains an iron-containing cofactor. The low-spin iron is complexed by two intrinsic CO-, one sulfur- and one or two N/O ligands and has one open coordination site, which is proposed to be the location of H(2) binding.

Hydrogen-bonded assemblies of ruthenium(ii)-biimidazole complex cations and cyanometallate anions: structures and photophysics

The complex cations [RuL2(H2biim)]2+ (L=bipy, 4,4'-tBu2-bipy) interact with cyanometallate anions via a chelating hydrogen-bonding interaction between the two N-H donors of the complex cation and the N lone pair of one cyanide ligand in the complex anion; the anion hexacyanoferrate(III) quenches the Ru(II)-based luminescence in CH2Cl2 solution by photoinduced electron-transfer within the H-bonded assembly, whereas hexacyanocobaltate(III) enhances the Ru(II)-based luminescence.

Three-component coordination networks based on [Ru(phen)(CN)4]2? anions, near-infrared luminescent lanthanide(iii) cations, and ancillary oligopyridine ligands: structures and photophysical properties

A series of cyanide-bridged coordination networks has been prepared which contain [Ru(phen)(CN)4](2-) anions, Ln(III) cations, and additional oligopyridine ligands (1,10-phenanthroline, 2,2':6',2'''-terpyridine or 2,2'-bipyrimidine) which coordinate to the Ln(III) centres. Five structural types have been identified and examples of each type of structure are described: these are hexanuclear Ru4Ln2 clusters; two-dimensional Ru-Ln sheets with a honeycomb pattern of edge-linked Ru6Ln6 hexagons; one-dimensional chains consisting of two parallel cross-linked strands in a ladder-like arrangement; simple single-stranded chains of alternating Ru/Ln components; and a one-dimensional 'chain of squares' in which Ru2Ln2 squares are linked by bipyrimidine bridging ligands which connect to the Ln(III) corners of adjacent squares in the sequence. The 3MLCT luminescence characteristic of the [Ru(phen)(CN)4](2-) units is quenched in those networks containing Ln(III) which have low-lying near-infrared luminescent f-f states [Pr(III), Nd(III), Er(III), Yb(III)], with sensitised Ln(III)-based near-IR luminescence generated by d --> f energy-transfer. The rate of d --> f energy-transfer, and hence the degree of quenching of the 3MLCT luminescence from the [Ru(phen)(CN)4](2-) units, depends on the availability of f-f levels of an appropriate energy on the Ln(III) centre, with Nd(III) (with a high density of low-lying f-f states) being the most effective energy-acceptor and Yb(III) (with a single low-lying f-f state) being the least effective. Rates of d --> f energy-transfer to different Ln(III) centres could be determined from both the residual (partially quenched) lifetimes of the 3MLCT luminescence, and--in the case of the Yb(III) networks--by a rise-time for the sensitised near-IR luminescence. The presence of the 'blocking' polypyridyl ligands, which reduced the number of cyanide and water ligands that would otherwise coordinate to the Ln(III) centres, resulted in increases in the Ln(III)-based emission lifetimes compared to networks where these blocking ligands were not used.

Synthesis, electrochemistry and luminescence of [Pt{4′-(R)trpy}(CN)]+(R = Ph, o-CH3C6H4, o-ClC6H4or o-CF3C6H4; trpy = 2,2′:6′,2″-terpyridine): crystal structure of [Pt{4′-(Ph)trpy}(CN)]BF4·CH3CN

The synthesis and characterization of [Pt{4'-(R)trpy}(CN)]X (R = Ph, X = BF(4) or SbF(6); R = o-CH(3)C(6)H(4), X = SbF(6); R = o-ClC(6)H(4), X = SbF(6); or R = o-CF(3)C(6)H(4), X = SbF(6)) are described where trpy = 2,2':6',2''-terpyridine. Single crystals of [Pt{4'-(Ph)trpy}(CN)]BF(4).CH(3)CN were grown by vapour diffusion of diethyl ether into an acetonitrile solution of [Pt{4'-(Ph)trpy}(CN)]BF(4). An X-ray crystal structure determination of the solvated complex confirms the near linear coordination of the cyanide ligand to the platinum centre. The cation is almost planar as evidenced by a twist of only 1.9 degrees of the phenyl group out of the plane of the terpyridyl moiety. Cyclic voltammograms were recorded in DMF/0.1 M TBAH for the [Pt{4'-(R)trpy}(CN)](+) cations. Two quasi-reversible one-electron reduction (cathodic) waves are observed with E(1/2) values that show the trend expected for an increasingly lower energy of the trpy-based LUMO of the complex i.e., [Pt{4'-(Ph)trpy}(CN)](+) approximately [Pt{4'-(o-CH(3)C(6)H(4))trpy}(CN)](+) < [Pt{4'-(o-ClC(6)H(4))trpy}(CN)](+) < [Pt{4'-(o-CF(3)C(6)H(4))trpy}(CN)](+). All the [Pt(4'-(R)trpy}(CN)](+) cations are photoluminescent in dichloromethane. Emission by [Pt{4'-(Ph)trpy}(CN)](+) is from an excited state with largely (3)MLCT orbital parentage, but with some intraligand (3)pi-pi* character mixed-in (tau = 0.1 micros). In contrast, the other three cations display emission that appears exclusively intraligand (3)pi-pi* in origin (tau approximately 0.8 micros). Emission spectra have been recorded in a low concentration frozen DME {1 : 5 : 5 (v/v) DMF-MeOH-EtOH} glass. For the R = o-CH(3)C(6)H(4), o-ClC(6)H(4) and o-CF(3)C(6)H(4) cations the envelope of vibronic structure and energies of the vibrational components are essentially the same as that recorded in dichloromethane. However, for the [Pt{4'-(Ph)trpy}(CN)](+) cation, there is a blue-shift in the energies of the vibrational components as compared to that recorded in dichloromethane, as well as a change in the envelope of vibronic structure to a more "domed" pattern; this has been interpreted in terms of a higher percentage of intraligand (3)pi-pi* character in the emitting state for the glass. Increasing the concentration of the glass invariably leads to aggregation of the cations and the consequent development of new low energy bands, such that at 0.200 mM broad peaks centred at ca. 650 and 700 nm dominate the spectrum; these bands are assigned to excimeric (3)pi-pi* and (3)MMLCT emission, respectively.

Exchange coupling and contribution of induced orbital angular momentum of low-spinFe3+ions to magnetic anisotropy in cyanide-bridgedFe2M2molecular magnets: Spin-polarized density-functional calculations

Electronic structure and intramolecular exchange constants are calculated for three cyanide-bridged molecular magnets, ${[T{p}^{\ensuremath{\star}}{\mathrm{Fe}}^{III}{(\mathrm{C}\mathrm{N})}_{3}{M}^{II}{(\mathrm{DMF})}_{4}]}_{2}{(\mathrm{O}Tf)}_{2}∙2\mathrm{DMF}$ $({M}^{II}=\mathrm{Mn},\mathrm{Co},\mathrm{Ni})$ (abbreviated as ${\mathrm{Fe}}_{2}{\mathrm{Mn}}_{2}$, ${\mathrm{Fe}}_{2}{\mathrm{Co}}_{2}$, and ${\mathrm{Fe}}_{2}{\mathrm{Ni}}_{2}$) that have been recently synthesized, within a generalized-gradient approximation in spin-polarized density-functional theory (DFT). Here $T{p}^{\ensuremath{\star}}={[{\mathrm{C}}_{3}{(\mathrm{C}{\mathrm{H}}_{3})}_{2}\mathrm{H}{\mathrm{N}}_{2}]}_{3}\mathrm{B}\mathrm{H}$, $\mathrm{O}Tf={\mathrm{O}}_{3}{\mathrm{SCF}}_{3}$, and $\mathrm{DMF}=\mathrm{HCON}{(\mathrm{C}{\mathrm{H}}_{3})}_{2}$. Due to strong ligand fields present in the ${[T{p}^{\ensuremath{\star}}{\mathrm{Fe}}^{III}{(\mathrm{C}\mathrm{N})}_{3}]}^{\ensuremath{-}}$ units, the ${\mathrm{Fe}}^{3+}$ ions exhibit a low ground-state spin of $S=1∕2$. Our calculations show that the metal ions in the ${\mathrm{Fe}}_{2}{\mathrm{Mn}}_{2}\phantom{\rule{0.3em}{0ex}}$ molecule interact antiferromagnetically via cyanide ligands, while those in the ${\mathrm{Fe}}_{2}{\mathrm{Co}}_{2}$ and ${\mathrm{Fe}}_{2}{\mathrm{Ni}}_{2}$ molecule interact ferromagnetically. The calculations also suggest that the smallest gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for ${\mathrm{Fe}}_{2}{\mathrm{Mn}}_{2}$, ${\mathrm{Fe}}_{2}{\mathrm{Co}}_{2}$, and ${\mathrm{Fe}}_{2}{\mathrm{Ni}}_{2}$ are 0.12, 0.03, and $0.33\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Based on the calculated electronic structures, the second-order magnetic anisotropy is computed including single-electron spin-orbit coupling within a DFT formalism. In comparison to a prototype single-molecule magnet ${\mathrm{Mn}}_{12}$, the three cyanide-bridged molecular magnets are found to bear substantial transverse magnetic anisotropy that becomes 15%--36% of molecular longitudinal anisotropy. Spin-orbit coupling arising from the low-spin ${\mathrm{Fe}}^{3+}$ and high-spin ${\mathrm{Co}}^{2+}$ ions induces significant orbital angular momentum that contributes to the total magnetic anisotropy of the three cyanide-bridged molecular magnets. The induced orbital angular momentum is 4--8 times those calculated for ${\mathrm{Mn}}_{12}$. The total magnetic anisotropy present in the three molecular magnets is due to competition between the magnetic anisotropy of the ${\mathrm{Fe}}^{3+}$ and of the ${M}^{2+}$ ions. In the ${\mathrm{Fe}}_{2}{\mathrm{Mn}}_{2}$ and ${\mathrm{Fe}}_{2}{\mathrm{Ni}}_{2}\phantom{\rule{0.3em}{0ex}}$ molecules, the anisotropy is primarily due to the ${\mathrm{Fe}}^{3+}$ ions, while in the ${\mathrm{Fe}}_{2}{\mathrm{Co}}_{2}\phantom{\rule{0.3em}{0ex}}$ molecule, the single-ion anisotropy of the ${\mathrm{Co}}^{2+}$ ions counters the ${\mathrm{Fe}}^{3+}$ contributions. These results are supported by previously reported magnetic measurements.

Tetraphenylphosphonium aquatetracyanonitridorhenate(V) pentahydrate

In the title complex, (C24H20P)2[Re(CN)4N(H2O)]·5H2O, the Re atom is coordinated by one nitride, one aqua and four cyanide ligands in a distorted octa­hedral geometry. The Re atom is 0.329 (3) Å from the plane formed by the C atoms of the cyanide ligands.

Comparing the Electronic Properties of the Low-Spin Cyano−Ferric [Fe(N4)(Cys)] Active Sites of Superoxide Reductase and P450cam Using ENDOR Spectroscopy and DFT Calculations

Superoxide reductase (SOR) and P450 enzymes contain similar [Fe(N)4(SCys)] active sites and, although they catalyze very different reactions, are proposed to involve analogous low-spin (hydro)peroxo-Fe(III) intermediates in their respective mechanisms that can be modeled by cyanide binding. The equatorial FeN4 ligation by four histidine ligands in CN-SOR and the heme in CN-P450cam is directly compared by 14N ENDOR, while the axial Fe-CN and Fe-S bonding is probed by 13C ENDOR of the cyanide ligand and 1Hbeta ENDOR measurements to determine the spin density delocalization onto the cysteine sulfur. There are small, but notable, differences in the bonding between Fe(III) and its ligands in the two enzymes. The ENDOR measurements are complemented by DFT computations that support the semiempirical equation used to compute spin densities on metal-coordinated cysteinyl and shed light on bonding changes as the Fe-C-N linkage bends. They further indicate that H bonds to the cysteinyl thiolate sulfur ligand reduce the spin density on the sulfur in both active sites to a degree that exceeds the difference induced by the alternative sets of "in-plane" nitrogen ligands.

Piezoelectric Quartz Crystal Microbalance Sensor for Trace Aqueous Cyanide Ion Determination

Using selective reaction chemistry, our present research has developed an online, real-time sensor capable of monitoring toxic cyanide at both drinking water standard and environmental regulatory concentrations. Through the use of a flow cell, aqueous samples containing cyanide are reacted with a gold electrode of a piezoelectric crystal to indirectly sense cyanide concentration by the dissolution of metallic gold. The quartz crystal is an AT-cut wafer sandwiched between two neoprene O-rings within the liquid flow cell. The presence of cyanide in solution results in the selective formation of a soluble dicyano-gold complex according to the Elsner reaction: 4Au + 8CN- + 2H2O + O2 <=> 4Au(CN)2- + 4OH-. The resulting loss of gold from the electrode is detected by the piezoelectric crystal as a resonant frequency change. Since free cyanide is a weak acid (pKa = 9.3), available protons compete for cyanide ligands. Therefore, increased sample pH provides higher sensitivity. The detection limits at pH 12 are 16.1 and 2.7 ppb for analysis times of 10 min and 1 h, respectively. The incorporation of the flow cell improves both analyte sensitivity and instrument precision, with an average signal intensity drift of only 5% over a 2-h analysis. The calibrations show excellent linearity over a variety of cyanide concentrations ranging from low ppb to hundreds of ppm. This detection method offers the advantage of selectively detecting cyanides posing a biohazard while avoiding detection of stable metal cyanides. This aspect of the system is based on competitive exchange of available metals and gold with cyanide ligands. Stable metal cyanide complexes possess a higher formation constant than cyanoaurate. This detection system has been configured into a flow injection analysis array for simple adaptation to automation. Anions commonly found in natural waters have been examined for interference effects. Additionally, the sensor is free from interference by aqueous cyanide analogues including thiocyanate. The developed detection system provides rapid cyanide determinations with little sample preparation or instrument supervision.

The Iron-Sulfur Cluster-free Hydrogenase (Hmd) Is a Metalloenzyme with a Novel Iron Binding Motif

The iron-sulfur cluster-free hydrogenase (Hmd) from methanogenic archaea harbors an iron-containing cofactor of yet unknown structure. X-ray absorption spectroscopy of the active, as isolated enzyme from Methanothermobacter marburgensis (mHmd) and of the active, reconstituted enzyme from Methanocaldococcus jannaschii (jHmd) revealed the presence of mononuclear iron with two CO, one sulfur and one or two N/O in coordination distance. In jHmd, the single sulfur ligand is most probably provided by Cys176, as deduced from a comparison of the activity and of the x-ray absorption and Mössbauer spectra of the enzyme mutated in any of the three conserved cysteines. In the isolated Hmd cofactor, two CO, one sulfur, and two nitrogen/oxygen atoms coordinate the iron, the sulfur ligand being most probably provided by mercaptoethanol, which is absolutely required for the extraction of the iron-containing cofactor from the holoenzyme and for the stabilization of the extracted cofactor. In active mHmd holoenzyme, the number of iron ligands increased by one when one of the Hmd inhibitors (CO or KCN) were present, indicating that in active Hmd, the iron contains an open coordination site, which is proposed to be the site of H2 interaction.

Generation of Gas-Phase VO2+, VOOH+, and VO2+−Nitrile Complex Ions by Electrospray Ionization and Collision-Induced Dissociation

Cationic metal species normally function as Lewis acids, accepting electron density from bound electron-donating ligands, but they can be induced to function as electron donors relative to dioxygen by careful control of the oxidation state and ligand field. In this study, cationic vanadium(IV) oxohydroxy complexes were induced to function as Lewis bases, as demonstrated by addition of O2 to an undercoordinated metal center. Gas-phase complex ions containing the vanadyl (VO2+), vanadyl hydroxide (VOOH+), or vanadium(V) dioxo (VO2+) cation and nitrile (acetonitrile, propionitrile, butyronitrile, or benzonitrile) ligands were generated by electrospray ionization (ESI) for study by multiple-stage tandem mass spectrometry. The principal species generated by ESI were complexes with the formula [VO(L)n]2+, where L represents the respective nitrile ligands and n=4 and 5. Collision-induced dissociation (CID) of [VO(L)5]2+ eliminated a single nitrile ligand to produce [VO(L)4]2+. Two distinct fragmentation pathways were observed for the subsequent dissociation of [VO(L)4]2+. The first involved the elimination of a second nitrile ligand to generate [VO(L)3]2+, which then added neutral H2O via an association reaction that occurred for all undercoordinated vanadium complexes. The second [UO(L)4]2+ fragmentation pathway led instead to the formation of [VOOH(L)2]+ through collisions with gas-phase H2O and concomitant losses of L and [L+H]+. CID of [VOOH(L)2]+ caused the elimination of a single nitrile ligand to generate [VOOH(L)]+, which rapidly added O2 (in addition to H2O) by a gas-phase association reaction. CID of [VONO3(L)2]+, generated from spray solutions created by mixing VOSO4 and Ba(NO3)2 (and precipitation of BaSO4), caused elimination of NO2 to produce [VO2(L)2]+. CID of [VO2(L)2]+ produced elimination of a single nitrile ligand to form [VO2(L)]+, a V(V) analogue to the O2-reactive V(IV) species [VOOH(L)]+; however, this V(V) complex was unreactive with O2, which indicates the requirement for an unpaired electron in the metal valence shell for O2 addition. In general, the [VO2(L)2]+ species required higher collisions energies to liberate the nitrile ligand, suggesting that they are more strongly bound than the [VOOH(L)2]+ counterparts.

Nitrile ligands for controlled synthesis of alkynyl-ruthenium based homo and hetero bimetallic systems

Wire like mono- and poly-nuclear molecules based on alkynyl ruthenium complexes whose core unit is trans-[Ru(C C-R)(R′C N)(dppe) 2][PF 6] are readily formed in soft conditions. The electronic dual character of the metallic unit, donor through the acetylide moiety, acceptor versus the nitrile ligand is exemplified through electrochemical studies of a series of ethynylferrocene and cyanoferrocene derivatives. A single crystal X-ray diffraction analysis of the [(dppe) 2(PhC C)Ru(N C-C 6H 4-C N)Ru(C CPh)(dppe) 2][2PF 6] bimetallic complex 5 shows that the global structure of such complexes consists of wire type dimetallic units. With the availability of this versatile, direct, and simple route, a new class of extended rigid rod systems of nanometric size with multilevel electron transfers is readily accessed.

The Uncommon Reactivity of Dihapto-Coordinated Nitrile, Ketone, and Alkene Ligands When Bound to a Powerful π-Base

A series of complexes of the form TpM(NO)(L‘)(η2-L) are prepared (where L is a nitrile, ketone, or alkene, M = W or Mo, L‘ = PMe3, MeIm, Tp = hydridotris(pyrazolyl)borate). These complexes are subjected to various electrophiles (e.g., alkyl halides, Brønsted acids) to systematically probe the ability of the π-basic transition metal to affect the reactivity of the dihapto-coordinated ligand. Alkylation is observed at the heteroatom of the ketone and the nitrile, but depending on the reagent, alkylation of the nitrosyl ligand or addition to the complexed π-bond also occurs. The structures of several nitrilium complexes and a rare example of an η2-acetonium complex are also reported.

Hetarylamidines derived from PtIV-mediated coupling of nitriles with aminoheterocycles

The trans-[PtCl4(EtCN)2] complex is involved in coupling reactions (CH2Cl2, 30–35 °C, 10–15 min) with 2-amino derivatives of heterocyclic compounds (2-NH2Het, where Het is pyridyl, 4-methylpyridyl, 5-methylpyridyl, 6-methylpyridyl, or thiazolyl) to form the trans-[PtCl4{ N(H)=C(Et)NHHet}2] complexes (1–5) with κ1(N)-coordinated hetarylamidine ligands. The reaction with the use of 2-aminopyrimidine (2-NH2Pym) produces the [PtCl4(2-NH2Pym)2] complex (6) as a result of complete replacement of the nitrile ligands in the [PtCl4(EtCN)2] complex. The compositions and structures of compounds 1–6 were confirmed by elemental analysis (C, H, N), IR spectroscopy, 1H, 13C{1H}, and 195Pt{1H} NMR spectroscopy, and FAB mass spectrometry. Complex 1 was additionally characterized by X-ray diffraction.

Protonation equilibria of [formula omitted] complexes possessing electron donating sites on the diimine ligand

The protonation equilibria of [Ru(LL)(CN) 4] 2− complexes, where LL = 4,4′-dicarboxy-2,2′-bipyridine (dcb) and 2,3-bis(2-pyridyl)-pyrazine (bppz), have been investigated by spectrophotometric titration and 13C NMR spectroscopy in the acidic region. Two protonation steps with p K a values of 2.0 and 0.5 and the absorption spectra of protonated species are distinguished for [Ru(bppz)(CN) 4] 2− in water. The 13C NMR spectra unambiguously prove the first protonation step occurring on the free pyridyl nitrogen of bppz. The second protonation step occurring on the cyanide ligand under pH = 2 results in a blue shift of the MLCT absorption maximum. The weakly emitting excited state of [Ru(bppz)(CN) 4] 2− is presumably more acidic than the ground state and a complete quenching of luminescence is observed with a diffusion controlled quenching rate of 4 ± 0.5 × 10 10 s −1. Four separate protonation steps with p K a values of 3.2, 2.2, 1.1, −0.2 and the absorption spectra of the different protonated complex of [Ru(dcb)(CN) 4] 4− have also been determined. The protonation of carboxyl groups gives rise to an upfield shift of 13C NMR lines (6.1 and 4.8 ppm) of the C4 and COO − carbon atoms. The chemical shift of cyanide ligands are also affected by the protonation of carboxyl group and 1.5 ppm upfield shift was measured between pH = 4 and 2. The emission intensity and apparent lifetimes of [Ru(dcb)(CN) 4] 4− decrease to ca. 10% of the original values at pH = 2, while the position of emission maxima shifts from 666 to 715 nm. The change of emission intensity versus pH suggests that the monoprotonated form of [Ru(dcbH)(CN) 4] 3− is a weaker emitter than [Ru(dcbH 2)(CN) 4] 2−.