José A. Olabe

Advances in the coordination chemistry of [M(CN)5L]n− ions (M=Fe, Ru, Os)

Abstract New developments in the synthesis, crystal and molecular structure, spectroscopy and kinetic properties of the [M(CN) 5 L] n − ions (M II,III =Fe, Ru, Os; L=H 2 O, NH 3 , amines, CN − , N -heterocyclic ligands, etc.), performed over the last decade, are reviewed. The properties of dinuclear complexes containing pentacyano-fragments bridged by pyrazine, cyanide or other bridging ligands, are also considered, with emphasis on mixed-valent systems. The influence of the solvent and other medium effects on the electronic structure, as well as on the modification of the reactivity of L upon coordination is addressed particularly. Some catalytic processes relevant to bioinorganic chemistry are discussed, involving either ligand oxidation (hydrazine, thiolates) or reduction (nitrosyl).

Three Redox States of Nitrosyl: NO+, NO., and NO−/HNO Interconvert Reversibly on the Same Pentacyanoferrate(II) Platform†

Not so elusive: [Fe(II)(CN)(5)(HNO)](3-) has been characterized spectroscopically after the two-electron reduction of nitroprusside (see scheme). The complex is stable at pH 6, slowly decomposing to [Fe(CN)(6)](4-) and N(2)O. It is deprotonated at increasing pH value with oxidation of bound NO(-) to [Fe(II)(CN)(5)(NO)](3-). [Fe(II)(CN)(5)(HNO)](3-) is the first non-heme iron-nitroxyl complex prepared in aqueous solution that is reversibly redox-active under biologically relevant conditions.

Kinetics and mechanism of ligand interchange in the [RuIII(edta)L] complexes; L=Cysteine and related thiolates

Complexes of the [RuIII(edta)SR]n series, with SR−= deprotonated cysteine, N- acetylcysteine, 2–mercaptoethanol, glutathione and penicilamine, were prepared from [Ru(edta)H2O]− and the corresponding RSH thiols, at pH=5.5. The complexes exhibit intense visible absorption bands at ca. 520nm (ε≅3500M−1 cm−1), associated with LMCT from the sulfur ligands bound to RuIII. The kinetics of the formation reactions were first order in [RuIII(edta)H2O]− and thiol reactants, with k1 values ca. 1–5×102 M−1s−1 (25°C) for all the sulfur ligands except penicilamine, which reacted slower by a factor of 10. Activation parameters suggest an associative mechanism, as for the coordination of other S- and N-bound ligands to [RuIII(edta)H2O]−. A reactivity decrease is apparent at low and high pHs (ranges 1–3 and 8–10, respectively), associated with acid-base equilibria involving the less reactive [RuIII(Hedta)H2O] and [RuIII(edta)OH]2− species. A significant rate increase was found for cysteine and penicilamine at ca. pH=8.0, because the thiol reactants deprotonate. The equilibrium constants for all the ligands showed that robust complexes were formed, with K=ca. 1×105 M−1 (25°C). The dissociation rate constants, k−1, were in the 10−3–10−4 s−1 range. The influence of nucleophilic and steric effects increasing and decreasing the formation rates, respectively, is discussed for the thiolate ligands, with adequate comparisons with other L species bound to [RuIII(edta)H2O]−.

Redox properties of ruthenium nitrosyl porphyrin complexes with different axial ligation: structural, spectroelectrochemical (IR, UV-visible, and EPR), and theoretical studies.

Experimental and computational results for different ruthenium nitrosyl porphyrin complexes [(Por)Ru(NO)(X)] ( n+ ) (where Por (2-) = tetraphenylporphyrin dianion (TPP (2 (-) )) or octaethylporphyrin dianion (OEP (2-)) and X = H 2O ( n = 1, 2, 3) or pyridine, 4-cyanopyridine, or 4- N,N-dimethylaminopyridine ( n = 1, 0)) are reported with respect to their electron-transfer behavior. The structure of [(TPP)Ru(NO)(H 2O)]BF 4 is established as an {MNO} species with an almost-linear RuNO arrangement at 178.1(3) degrees . The compound [(Por)Ru(NO)(H 2O)]BF 4 undergoes two reversible one-electron oxidation processes. Spectroelectrochemical measurements (IR, UV-vis-NIR, and EPR) indicate that the first oxidation occurs on the porphyrin ring, as evident from the appearance of diagnostic porphyrin radical-anion vibrational bands (1530 cm (-1) for OEP (*-) and 1290 cm (-1) for TPP (*-)), from the small shift of approximately 20 cm (-1) for nu NO and from the EPR signal at g iso approximately 2.00. The second oxidation, which was found to be electrochemically reversible for the OEP compound, shows a 55 cm (-1) shift in nu NO, suggesting a partially metal-centered process. The compounds [(Por)Ru(NO)(X)]BF 4, where X = pyridines, undergo a reversible one-electron reduction. The site of the reduction was determined by spectroelectrochemical studies to be NO-centered with a ca. -300 cm (-1) shift in nu NO. The EPR response of the NO (*) complexes was essentially unaffected by the variation in the substituted pyridines X. DFT calculations support the interpretation of the experimental results because the HOMO of [(TPP)Ru(NO)(X)] (+), where X = H 2O or pyridines, was calculated to be centered at the porphyrin pi system, whereas the LUMO of [(TPP)Ru(NO)(X)] (+) has about 50% pi*(NO) character. This confirms that the (first) oxidation of [(Por)Ru(NO)(H 2O)] (+) occurs on the porphyrin ring wheras the reduction of [(Por)Ru(NO)(X)] (+) is largely NO-centered with the metal remaining in the low-spin ruthenium(II) state throughout. The 4% pyridine contribution to the LUMO of [(TPP)Ru(NO)(py)] (+) is correlated with the stability of the reduced form as opposed to that of the aqua complex.

Establishing the NO oxidation state in complexes [Cl5(NO)M]n−, M = Ru or Ir, through experiments and DFT calculations

Predominantly NO-centered reduction was observed by EPR and IR spectroelectrochemistry to occur reversibly at low temperatures for [Cl(5)Ir(NO)](-). In contrast, the [Cl(5)Ru(NO)](2-) ion was found to undergo only irreversible reduction but reversible oxidation to a ruthenium(III) species at -40 degrees C. DFT calculations were used to establish the electronic structures and to rationalise the different stabilities. The calculations also reveal orientation-dependent energies and EPR properties between staggered and eclipsed conformations.

REDOX REACTIVITY OF COORDINATED LIGANDS IN PENTACYANO(L)FERRATE COMPLEXES

Publisher Summary This chapter discusses redox chemistry of small nitrogen-containing L ligands with bioinorganic relevance, particularly nitric oxide, NO, and its redox-interconverted forms NO + (nitrosyl) and NO - (nitroside, or its protonated form, HNO, nitroxyl). The coordination chemistry of transition metal cyanometallates has been of great and long-standing matter because of the fundamental issues related to their electronic structure and reactivity. The chemistry of redox-active ligands L in the pentacyano(L) ferrate(II) and -(III) complexes is of fundamental concern for disclosing the mechanistic features which are operative in stoichiometric and catalytic processes associated with oxidation, reduction, and disproportionation of L. This is particularly important for small molecules displaying a versatile coordination behavior, such as the N-binding species associated with the nitrogen redox cycle, which display complex, multi-electronic paths for oxidation or reduction in most cases, generally under kinetic rather than thermodynamic control. Another crucial factor influencing the redox reactivity of cyano complexes is the possibility of bridging one (or more) cyano ligands to an acceptor moiety.

The valence-localized decacyanodiruthenium(III,II) analogue of the Creutz–Taube ion. Completing the full d5/d6 triad [(NC)5M(μ-pz)M(CN)5]5−, M=Fe,Ru,Os; pz=pyrazine

Abstract The new ruthenium(II) complex ion [(NC) 5 Ru(μ-pz)Ru(CN) 5 ] 6− has been prepared and studied as the hexakis(tetraethylammonium) salt. Although spectroelectrochemical experiments in dichloromethane were affected by adsorption as they were in other solvents, the compound could be oxidized via two reversible one-electron steps with a mixed-valent pentaanionic intermediate. In comparison with the iron and osmium analogues, the system [(NC) 5 Ru(μ-pz)Ru(CN) 5 ] 5− is distinguished by less negative redox potentials, a smaller comproportionation constant K c of only 10 4.7 , a very broad (Δ ν 1/2 =4200 cm −1 ) symmetrical IVCT band at 1760 nm ( e =2600 M −1 cm −1 ) conforming with the Hush model for weakly coupled mixed-valent systems, the absence of an EPR signal even at 3.5 K and the appearance of infrared bands (CN, pyrazine ring vibrations) indicating localized valence on the time scale of 10 −12 s. Together with the high MLCT energies these results suggest a weaker metal–pyrazine interaction for the ruthenium system in comparison with the Fe and Os analogues reported previously. In relation to the Creutz–Taube ion, the substitution of ammine ligands by non-innocent cyanide ions attenuates the metal–metal interaction across the π accepting pyrazine bridge.

Disproportionation of hydroxylamine by water-soluble iron(III) porphyrinate compounds.

The reactions of hydroxylamine (HA) with several water-soluble iron(III) porphyrinate compounds, namely iron(III) meso-tetrakis-(N-ethylpyridinium-2yl)-porphyrinate ([Fe(III)(TEPyP)](5+)), iron(III) meso-tetrakis-(4-sulphonatophenyl)-porphyrinate ([Fe(III)(TPPS)](3-)), and microperoxidase 11 ([Fe(III)(MP11)]) were studied for different [Fe(III)(Porph)]/[HA] ratios, under anaerobic conditions at neutral pH. Efficient catalytic processes leading to the disproportionation of HA by these iron(III) porphyrinates were evidenced for the first time. As a common feature, only N(2) and N(2)O were found as gaseous, nitrogen-containing oxidation products, while NH(3) was the unique reduced species detected. Different N(2)/N(2)O ratios obtained with these three porphyrinates strongly suggest distinctive mechanistic scenarios: while [Fe(III)(TEPyP)](5+) and [Fe(III)(MP11)] formed unknown steady-state porphyrinic intermediates in the presence of HA, [Fe(III)(TPPS)](3-) led to the well characterized soluble intermediate, [Fe(II)(TPPS)NO](4-). Free-radical formation was only evidenced for [Fe(III)(TEPyP)](5+), as a consequence of a metal centered reduction. We discuss the catalytic pathways of HA disproportionation on the basis of the distribution of gaseous products, free radicals formation, the nature of porphyrinic intermediates, the Fe(II)/Fe(III) redox potential, the coordinating capabilities of each complex, and the kinetic analysis. The absence of NO(2)(-) revealed either that no HAO-like activity was operative under our reaction conditions, or that NO(2)(-), if formed, was consumed in the reaction milieu.

Properties of the mixed-valence binuclear complex ion, (NH3)5RuIII-NC-OsII(CN)5]−

Abstract The mixed-valence ion, [(NH 3 ) 5 Ru III -NC-Os II (CN) 5 ] − , was prepared in solution from [Ru(NH 3 ) 5 H 2 O] 3+ and [Os(CN) 6 ] 4− and was isolated as a potassium salt. A kinetic study of the formation reaction shows that a dissociative mechanism is operative, the process being rate-controlled by the loss of water from the Ru(III) moiety, as in related reactions with [Fe(CN) 6 ] 4− and [Ru(CN) 6 ] 4− . The binuclear ion shows a distinctive intervalence band, associated to charge transfer from Os(II) to Ru(III), at 830 nm (ϵ=3450 M −1 cm −1 ), with a noticeable asymmetry, probably associated to spin-orbit coupling. Shifts in the redox potentials at the metal centers on dimer formation are consistent with the Os(II)-Ru(III) formulation. By measuring solution Raman spectra in post-resonance conditions with respect to the IT band, activation of the bridging CN stretching, as well as of the terminal stretching modes, is observed; absolute distortion values for both of the modes can be calculated by applying a time-dependent analysis of the scattering problem. IR and Raman results of the solid samples are also presented. Theoretical models were applied to IT data, allowing an estimation of the delocalization parameter, α 2 , and the electronic coupling parameter, H AB . The results are compared with other valence-trapped binuclear systems.

The crystal and molecular structure of sodium hexacyanoosmate(II) decahydrate and related hexacyanometalate complexes

The crystal structures of sodium hexacyanoosmate, ruthenate and ferrate decahydrates, Na4M(CN)6·10H2O(M=Os, Ru, Fe), have been determined from X-ray diffraction data and refined by full matrix least-squares to final agreement values: R = 0.038, Rw = 0.039; R = 0.026, Rw = 0.041; R = 0.060, Rw = 0.043 for Os, Ru and Fe compounds, respectively. The compounds are isostructural and crystallize in the monoclinic space group P21/n, Z = 2, with a = 9.154, b = 11.506, c = 9.876 A, β = 97.95°; a = 9.146, b = 11.486, c = 9.867 A, β = 98.00°; a = 9.038, b = 11.450, c = 9.782 A, β = 97.57°, for Os, Ru and Fe compounds, respectively. The structure can be described as layers of hexacyanometallate anions, intercalated with layers of sodium polyhedra containing hydration water molecules and N atoms, perpendicular to the crystallographic ac plane. MetalC and CN distances for the hexacyanide anions are correlated with those from other structurally related moieties. The infrared spectra of the compounds are complementary with previous results for potassium salts.