Variable-Temperature and Variable-Pressure Kinetic and Equilibrium studies of formation and dissociation of [V(H2O)5NCS]2+

The kinetics of formation and dissociation of [V(H₂O)₅NCS]²⁺ have been studied, as a function of excess metal-ion concentration, temperature, and pressure, by the stopped-flow technique. The thermodynamic stability of the complex was also determined spectrophotometrically. The kinetic and equilibrium data were submitted to a combined analysis. The rate constants and activation parameters for the formation (f) and dissociation (r) of the complex are: k/M ^?1 · S^?1 = 126.4, k/s^?1 = 0.82; ΔH /kJ · mol^?1 = 49.1, ΔH/kJ · mol^?1 = 60.6; ΔS/ J·K^?1·mol^?1= ?39.8, ΔSJ·K^?1·mol^?1 = ?43.4; ΔV/cm³·mol^?1 = ?9.4, and ΔV/cm³ · mol^?1 =?17.9. The equilibrium constant for the formation of the monoisothiocynato complex is K²⁹⁸/M ^?1 = 152.9, and the enthalpy and entropy of reaction are ΔH⁰/kJ · mol^?1 = ? 11.4 and ΔS⁰/J. K^?1mol^?1 = +3.6. The reaction volume is ΔV⁰/cm³· mol^?1 = +8.5. The activation parameters for the complex-formation step are similar to those for the water exchange on [V(H₂O)₆]³⁺ obtained previously by NMR techniques. The activation volumes for the two processes are consistent with an associative interchange, I_a, mechanism.     

Preparation and Properties of Bis[μ-S-(2-tiolatoethyl)-diphenylphosphine][(2-thiolatoethyl)diphenyl-phosphine]dinickel(+1) Ion,a Novel Unsymmetrical Thiolate-bridged complex

The title cation ( = Ni₂L) is formed in a variety of reactions (Schemes 1 and 2) in systems containing Ni²⁺ and (2-thiolatoethyl)-diphenylphosphine (= L^?) in the absence of coordinating anions at Ni²⁺/L^? ratios > 0.5 in apolar or moderately polar media. Solid [Ni₂L₃]CIO₄ and [Ni₂L₃]BPh₄ have been isolated. Job's plots confirm the Ni₂L- stoichiometry in solution. ³¹P-NMR data are consistent with ≥ 97% Ni₂L (vs. ? 3% of hypothetical Ni₃L) at equilibrium and support the suggested configuration (Fig. 2). The equilibrium between NiL₂ + NiL₂Br₂ and Ni₂L + Br^? varies with the solvent composition in CH₂₃Cl₂/EtOH mixtures. The rate of formation of Ni₂L₂Br₂ from Ni₂L and bromide (in high excess) in CH₂Cl₂ is first-order in [Ni₂L]_tot but depends on the ratio [Bu₄NBr]_tot/[Ni₂L₃ · ClO₄]tot, even at a high excess of bromide. This is interpreted by efficient competition in ion-aggregate formation between the small perchlorate concentration introduced as the counterion of Ni₂L, and the large excess of bromide.     

The Protonation of [Pt3(m̈-CO)3(PCy3)3]

The cluster [Pt₃(m?-CO)₃(PCy₃)₃] can be protonated with HBF₄ · OEt₂ to form the cluster [Pt₃(m?-CO)₃(PCy₃)₃](m?₃-H) ⁺BF ( 2 ). This unstable compound was isolated and characterised by NMR and IR spectroscopy.     

Réactions de substitution dans l'anion tétrachloropalladate(II) avec des diamines. I. Ethylènediamine

The replacement of Cl^? by ethylenediamine (en) in PdCl has been followed spectrophotometrically at 25°C and μ = 1 (NaClO₄); it proceeds in two steps leading to Pd(en)Cl₂ and Pd(en), respectively. The observed rate constants are discussed in terms of the mechanism proposed by Reinhardt [1] for the successive ammination reactions of PdCl.     

Oxygenation of Co(II) Complexes with Tripodal Ligands

The kinetics of O₂-uptake of five-coordinated Co²⁺/tren complexes (tren = 2,2′, 2″-tris(2-aminoethyl)amine) have been studied extensively. The kinetics of formation of (tren)Co(O₂, OH)Co(tren)³⁺ exhibits two steps. The rate law of O₂-addition, the first step, was of the form: rate = (k[H⁺] + kK_a)/([H⁺] + K_a) [Co(tren)²⁺][O₂]. Second-order rate constants k = 220 ± 19 M ^?1s^?1 and k = 1.8 ± .035 · 10³M ^?1s^?1 agreed well from O₂-uptake and (stopped-flow) spectrophotometric measurements. The protonation constant of the hydroxo complex obtained by equlibrium measurements (spectrophotometric and by pH-titration) in anaerobic conditions (pK_a = 10.03) agreed well with that derived from kinetic data (p K_a = 9.93); k and k are about a factor 100 smaller than those for the pseudooctahedral Co(trien) (H₂O). This and the fact that several other Co(II) complexes with five-coordinated geometry do not exhibit oxygen affinity led to the proposal that the oxygenation mechanism for Co²⁺/tren complexes involves fast preequilibria between Co(tren) (H₂O)²⁺ and Co(tren) (H₂O) and only the latter is assumed to be reactive. The enhanced rate at high pH is explained by rate determining H₂O-exchange in the O₂-addition step and the ability of coordinated OH^? to labilize the neighbouring H₂O. This mechanism is furthermore supported by the formation of one kinetically preferred isomer of the peroxo-bridged dicobalt(III) complex (O₂ cis to the tertiary N-atom) and the large negative activation entropy (?30 eu). The second step is the intramolecular bridging reaction: is independent of [Co(tren)²⁺] and [O₂] but exhibits a pH-dependence of the form k₃ = k′₃[H + ]/(K_a + [H⁺]); k_?3 ( = 5 · 10^?5 s^?1) was determined independently and from the two rate constants the equilibrium constant was calculated as ≈ 10⁵. The ligand combination as in Co(tren)²⁺ was shown to provide an excellent balance to form a reversible oxygen carrier; possible reasons for this are discussed.     

Complexes of 3,3′-Oxybis[(diphenylphosphino)methylbenzene] with Ni(II), Pd(II), Pt(II), Rh(I), and Ag(I). How Important is Backbone Rigidity in the Formation of trans-Spanning Bidenatate Chelates??

The bidentate diphosphine ligand, 3,3′-oxybis[(dipenylphosphino)methylbenzene] ( 1 ) forms monomeric, trans-square-planar complexes MX₂( 1 ) (M = Ni, Pd, Pt; X = Cl^?, Br^? I^?, and, in part, N, NCS^?, CN^?, NO) as well as Pt(H)Cl( 1 ), Pt(H)Br( 1 ), and RhCl(CO)( 1 ). Polymeric species have been observed with substitutionally inert metal centres: trans-[PtCl₂( 1 )]₂ and cis-[PtCl₂( 1 )]_n (mean value of n ≈ 4–5) ³¹P-NMR, and selected IR and UV/VIS parameters are reported. Ligand 1 shows a marked preference for trans-spanning and monomeric chelate formation, despite its various degrees of freedom of internal rotation in the lignad backbone. The readily available ligand 1 as well as analogues with other donor atoms, therefore, appear useful in most potential applications of trans-spanning chelate ligands. The crystal structure of AgCl( 1 )·0.5 (CH₃)₂C?O·0.39 C₆H₁₂ (space group C2/c,a = 21.02 Å, b = 14.57 Å, c = 24.79 Å, β = 99.77°, V = 7531.4 ų, Z = 8) confirms the presence of three-coordinate Ag( I ), with a coordination intermediate between a trigonal-planar and a T-shaped geometry (P-Ag-P = 145.61(8)°).     

L'action de l'ammoniac sur l'oxyde cuivrique. et les hydroxo-complexes de cuivre (II)

Using a new mathematical treatment, the nature and stability constants of the simple and mixed complex-species of copper(II) with hydroxyde and ammonia as ligands have been determined. The solubility curves of CuO in heterogeneous equilibrium have been identified in function of pH only and in function of pH and pNH_3tot at 25° and unit ionic strength (NaClO₄). The predominent species in the relatively dilute system limited by the ionic strength are [Cu²⁺], [Cu(OH)₂], [Cu(OH)], [Cu(OH)], [Cu(NH₃)], [Cu(NH₃)], [Cu(NH₃)], [Cu(NH₃) (OH)⁺], [Cu(NH₃)₃(OH)⁺] and [Cu(NH₃)₂(OH)₂].     

Cuprous complexes and dioxygen. VIII. Steric factors controlling one- or two-electron reduction of O2 by cuprous complexes with substituted imidazoles

In aqueous acetonitrile (AN), Cu (I) forms the complexes Cu(AN)L⁺ and CuL with a series of substituted imidazoles (L). Stability constants logK of Cu(AN)⁺ + L ? Cu(AN)L⁺ and logβ₂ were near 5 and 12, resp., log units for all ligands. The rate of autoxidation is described by ?d[O₂]/dt=[CuL]²[O₂](k_a/(1+k_b[CuL]) + (k_c[L]+k_d)/([CuL] + k_e[Cu])), implying competition between one- or two-electron reduction of O₂. The value of k_c decreases from 5500M ^?2S ^?1 for unsubstituted imidazole to about 40M ^?2S ^?1 for 2-methylimidazole or 1,2-dimethyl-imidazole and essentially zero for the corresponding 2-ethyl-derivatives. On the other hand, k_a and k_b are much less influenced by the nature of the ligands, all values being near 5 · 10⁴M ^?2S ^?1 and 10³M ^?1, respectively, for the complexes with the last four bases. Thus rather subtle sterical changes may strongly influence the relative importance of different pathways in the reduction of dioxygen by cuprous complexes.     

Singlet-Oxygen (1Δg) Production by Ruthenium(II) complexes containing polyazaheterocyclic ligands in methanol and in water

In the context of our studies on ruthenium(II) complexes containing polyazaheterocyclic ligands, we have determined the rate constants of quenching by molecular oxygen (k_q) of the metal-to-ligand charge-transfer-excited state of a series of homoleptic [RuL₃] complexes (where L stands for 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2′-bipyrazine (bpz), 4,7-diphenyl-1,10-phenanthroline (dip), diphenyl-1,10-phenanthroline-4,7-disulfonate (dpds), and 1, 10-phenanthroline-5-octadecanamide (poda)) in H₂O and in MeOH. These compounds are singlet-oxygen (O₂(¹Δ_g)) sensitizers, and quantum yields of singlet-oxygen production (Ψ_Δ) in both solvents are also reported. Values of k_q and Ψ_Δ depend on the nature of the ligand L and on the solvent, Ψ_Δ values showing a large range of variation (0.2 to 1.0). In MeOH, the only pathway for quenching of the excited [RuL₃] complexes by molecular oxygen is energy transfer: the fraction of quenched excited states yielding singlet oxygen (?) is unity for all compounds in the series investigated. Changing from MeOH to H₂O has several remarkable effects: higher k_q and lower Ψ_Δ values are observed; ? drops to ca. 0.5 except for [Ru(bpz)₃]²⁺. In fact, [Ru(bpz)₃]²⁺ is by far the weakest reductant in the series and behaves differently from the other complexes, with lowest k_q and Ψ_Δ values and a ? equal to 1 in both solvents. Results are interpreted on the basis of the role played by charge-transfer interactions between the sensitizer excited state and molecular oxygen in the quenching mechanism. Ru^II Complexes based on the 4,7-diphenyl-1, 10-phenanthroline (dip) ligand are very efficient and stable singlet-oxygen sensitizers with Ψ_Δ values close to unity in air-saturated MeOH.     

Synthesis and Crystal Structure of Tricaesium Heptaphosphide–Ammonia(1/3) Cs3P7·3 NH3

Recrystallization of Cs₃P₇ from liquid NH₃ yields the triammoniate Cs₃P₇·3 NH₃, which loses the weakly bound NH₃ of crystallization below 253 K. A low-temperature crystal-structure analysis shows that Cs₃P₇· NH₃ consists of a framework of heptaphosphanortricyclane anions P and Cs⁺ cations with NH₃ molecules completing the coordination of the cations. The framework is built from Cs₃P₇ layers connected by only few Cs…?P interactions, the interlayer gap being filled by a two-dimensional network of NH₃. The Cs₇P₇ part of the structure completes a family of alkali-metal-polyphosphide substructures which range from [RbP₇]^2? or [CsP₁₁]^2? chains over [Cs₂P_n]^? layers (n = 7, 11) to now [Cs₃P₇] frameworks.     

Complex formation equilibria of L-Amino-Acid amides with copper(II) in aqueous solution

Protonation and Cu(II) complexation equilibria of L -phenyhilaninamide, N²-methyl-L-phenylalaninamide, N², N²-dimethyl-L-phenylalaninamide, L -valinamide, and L -prolinamide have been studied by potentiometry in aqueous solution. The formation constants of the species observed, CuL²⁺, CuL, CuLH, CuL₂H and CuL₂H_?2, are discussed in relation to the structures of the ligands. Possible structures of bisamidato complexes are proposed on the ground of VIS and CD spectra. Since Cu(II) complexes of the present ligands (pH range 6–8) perform chiral resolution of dansyl- and unmodified amino acids in HPLC (reversed phase), it is relevant for the investigation of the resolution mechanism to know which are the species potentially involved in the recognition process.     

Mehrkernige Kobaltkomplexe. III. Über meso-[(en)2Co(OH)2Co(en)2] (NO3)4

Polynuclear Cobalt Complexes. III. On meso-[(en)₂Co(OH)₂Co(en)₂] (NO₃)₄ The structure of the title compound has been determined by X-ray analysis and refined to R = 0,060. The crystals are monoclinic, space group very probably C2/m, with cell dimensions a = 16,113(3), b = 10,946(2), c = 7,193(2) Å, β = 113,22(2)° and Z = 2. The configuration of the cation is ΔΛ IR. and UV./VIS. spectroscopic data are given.     

Spectroscopic,Electrochemical, and Kinetic Characterization of New Ruthenium(II) Tris-chelates Containing Five-Membered Heterocyclic Moieties

Synthesis, redox, photophysical, and photochemical properties of Ru(NN) complexes NN = 2-((2′-pyridyl)thiazole (pyth), 2-(2′-pyrazyl)thiazole (pzth), 2,2′-bithiazole (bth), 5-(2′-pyridyl)-1,2,4-thiadiazole (pytda), 2-(2′-pyridyl)imidazole (pyim), 1-methyl-2-(2′-pyridyl)imidazole (Mepyim), and 2-(2′-pyridyl)oxazole (pyox)) are described. Oxidation potentials for the Ru^3+/2+ couples in MeCN varied from about 0.80 V to 1.60 V vs. NHE. Three reduction waves were observed in all the cases except for Ru(pyim) and Ru(Mepyim) complexes and asigned to the one-electron reduction of each bidentate ligand. Absorption spectra contained bands in the UV (280–325 nm) and VIS (437–481 nm) regions which have been assigned to ligand-centered π-π* and metal-to-ligand charge-transfer dπ-π* transitions, respectively. Emission spectra at 77 K were determined for all the complexes presenting maxima in the 580–650-nm region, with vibrational progression in some of them. Only pyth, pzth, bth, and pytda tris-chelates showed luminescence at room temperature in aqueous solution, with quantum yields ranging from 0.0013 to 0.0095 and excited-state lifetimes from 55 to 390 ns, as determined from pulsed laser techniques. Their E_0–0 spetroscopic energies have been estimated from emission wavelength maxima at 77 K which, in turn, have allowed calculation of excited-state redox potentials. A plot of E_0–0 vs. ΔE_1/2, where ΔE_1/2 = E_1/2(3+/2+) ? E_1/2(2+/+), was linear with a slope of ca. 1.1 and a correlation coefficient of 0.999, demonstrating an identical nature of the orbital involved in spectroscopic and electrochemical processes. Photochemical properties of Ru(NN) complexes have been tested using methyl viologen (MV²⁺) in Ar-purged aqueous solution at pH 5. Stern-Volmer treatment has led to the determination of bimolecular quenching constants (0.5 to 2 × 10⁹m^?1·s^?1) which parallel electron-transfer free-energy changes. Homogeneous back-reaction of primarily produced MV^+· and Ru(NN) has been measured resulting to be slightly higher than diffusion control and independent of ligand nature. Rate constants for the scavenging of Ru(NN) by added edta have been also determined (1.7 to 8.2 × 10⁸M^?1 · S^?1). Under such conditions, net production of MV^+· is attained with quantum yields varying from 0.003 to 0.038 (single-shot laser results).     

Photophysical Properties and Photochemical Behaviour of Ruthenium(II) complexes containing the 2,2′-bipyridine and 4,4′-diphenyl-2,2′-bipyridine ligands

The temperature dependence of the emission lifetime of the series of complexes Ru(bpy)_n(4,4′-dpb) (bpy = 2,2′bipyridine, 4,4′-dpb = 4,4′-diphenyl-2,2′-bipyridine) has been studied in propionitrile/butyronitrile (4:5 v/v) solutions in the range 90–293 K. The obtained photophysical parameters show that the energy separation between the metal-to-ligand charge tranfer (³MLCT) emitting level and the photoreactive metal-centered (³MC) level changes across the series (ΔE = 3960, 4100, 4300, and 4700 cm^?1 for Ru(bpy)), Ru(bpy)2(4,4′-dpb)²⁺, Ru(bpy)(4,4′-dpb), and Ru(4,4′-dpb), respectively, where ΔE is the energy separation between the minimum of the ³MLCT potential curve and ³MLCT – ³MC crossing point. Comparison between spectral and electrochemical data indicated that the changes in ΔE are due to stabilization of the MLCT levels in complexes containing 4,4′-dpb with respect to Ru(bpy)²⁺₃. The photochemical data for the same complexes (as I^? salts) have been obtained in CH₂Cl₂ in the presence of 0.01M Cl^? upon irradiation at 462 nm. The complexes containing 4,4′-dpb are more photostable than Ru(bpy). Comparison between the data for thermal population of the ³MC photoreactive state and those for photochemistry indicated that the overall photochemical process is governed by (i) a thermal redistribution between the emitting and photoreactive excited states, and (ii) mechanistic factors, likely related to the size of the detaching ligand.     

Kristallstruktur und schwingungsspektrometrische Daten von cis-Na2[Pd(SO3)2en] · 4 H2O

Crystal Structure and Data from Vibrational Spectra of cis-Na₂[Pd(SO₃)₂en] · 4 H₂O The compound cis-Na₂[Pd(SO₃)₂en] · 4 H₂O (en = 1,2-diaminoethane) crystallizes in the orthorhombic space group Pnma with a = 623.7(2), = 1070.9(10), c = 1989.5(30) pm and Z = 4. In the [Pd(SO₃)₂en]^2? anions the trans-influence of the sulfite ligands manifests itself in long Pd? N bonds with short Pd? S distances. A set of Na⁺ ions is present in face-sharing octahedra Na(OH₂)₆⁺, forming rods [Na(OH₂)_6/2]⁺ parallel to [100]. A second set of Na⁺ ions is surrounded by two H₂O molecules and four O atoms from SO₃ ligands of two anions to form likewise octahedra with face-sharing, yielding rods [Na(OH₂)_2/2{(OSO₂)₂Pd en}_2/2]^? parallel to [100]. Comparatively low v(Pd? N)-frequencies reveal the trans-influence of the sulfite ligands also in the vibrational spectra.     

Cuprous Complexes and Dioxygen. Part 12.. Rate law and mechanism of the copper-catalyzed oxidation of ascorbic acid in aqueous acetonitrile

The copper-catalyzed oxidation of ascorbic acid (AscH₂) has been studied with a Clark electrode in aqueous MeCN. Cu^I or Cu^II may be equally used as the source of metal ion, without influence on the rate law. At sufficiently high [MeCN], the rate of the overall reaction is essentially given by the rate of Cu^I autoxidation: the reaction is of first order with respect to [Cu] and [O₂] and shows an inverse-square dependence on [MeCN] as observed for the autoxidation of Cu. The pH dependence is complicated by the combination of the intrinsic pH effect on autoxidation with an additional term in the rate law which is directly proportional to [AscH^?]. The latter term is explained by direct oxidation of the organic substrate by the primary dioxygen adduct of Cu^I, CuO. For [MeCN] < 0.7M , a gradual and pH-dependent transformation of this rate law and deviation from the first-order dependence on [O₂] is indicated.     

Preparation,characterization, crystal and molecular structure of Na2[N(CH2COO)3 99Tc(IV) (μ-O)2 99Tc(IV)N(CH2COO)3] · 6H2O

The title compound and its potassium analog have been prepared from corresponding aqueous solutions of ⁹⁹TcO at pH ≈? 2 with SO₂ as a reducing agent. An X-ray structure determination of the Na-salt showed Tc coordinated to the tetradentate N(CH₂COO) ligand (NTA). Two Tc-NTA moieties are joined via two bridging O-atoms into a four-membered Tc₂O₂ ring. The observed diamagnetism, a strong absorption band at 19 950 cm^?1, and a short Tc-Tc distance of 2.363 Å are typical for the Tc₂O₂-fragment with its strong metal-metal interaction. The structural trans-influence at Tc and the network of H-bonds are consistent with Tc in oxidation state IV.     

Structures and Electrical Properties of Some New Organic Conductors Derived from the Donor Molecule TMET (S,S,S,S,-Bis(dimethylethylenedithio) tetrathiafulvalene)

Crystal structures and electrical properties of radical-cation salts of the chiral organic donor TMET (S,S,S,S,-bis-(dimethylethylenedithio)tetrathiafulvalene) are described. Two structural types, 2:1 with octahedral anions Pf, AsF, SbF, I (incommensurate), and 3:2 with tetrahedral anions BF^?₄, CIO^?₄, ReO^?₄ are observed. Resistivity measurements between 2 and 298 K indicate that the 3:2 types are organic metals, while the other compounds are semiconductors. (TMET)₃(CIO₄)₂ is metallic down to about 120 K at ambient pressure and remains metallic down to 2 K at 8 kbar.     

A Unified Interpretation of Kinetic Data on the Acid-Induced Cleavage and of Product-Analysis Data on Spontaneous Cleavage of the Mono-ol Cation μ-Hydroxo-bis[pentaamminecobalt(III)] ([(NH3)5CoOHCo(NH3)5]5+)

Recent work on the spontaneous (= acid-independent) cleavage of the mono-ol cation, i.e. in Cl^?/ClO and NO/ClO mixed-electrolyte media has established (by analysis of anion-competition experiments) the existence of reactive ion pairs of the mono-ol cation with Cl^? and NO. Their existence must be allowed for in the analysis of the rate data for the acid-induced cleavage (pH 0–1) of the mono-ol cation in these mixed-electrolyte media. Thus, previous data for acidic Cl^?/ClO media have been re-interpreted in this work, and new data for NO/ClO media have been analyzed in the same sense. This analysis removes an apparent discrepancy in the orders of magnitude of ion aggregate stability constants between the mono-ol and similar binuclear cations.     

Stability,Formation and Dissociation Kinetics of the Pentacoordinate Ni2+ and CO2+ Complexes with 1,5-Diazacyclooctane-N,N′-diacetic Acid

The stability constants of the Ni²⁺ and Co²⁺ complexes with 1,5-diazacyclooctane-N,N′-diacetic acid (H₂DACODA) have been determined potentiometrically in 0.5M KNO₃ at 25°. Only M(DACODA) and M(DACODA)OH^? were observed. In addition the formation and dissociation kinetics of the pentacoordinate complexes M(DACODA) has been studied in aqueous solution using a stopped-flow technique. Formation follows the rate law v_f = k_f [M²⁺] [HDACODA^?]/[H⁺], which can be interpreted as a bimolecular process either between M²⁺ and DACODA^2? (k) or between MOH⁺ and HDACODA^? (k). The second order rate constants k are much higher than those expected from water exchange and can only be explained by a strong internal conjugate base effect. In the limiting case, however, this is equivalent to the second possible explanation, which assumes MOH⁺ and HDACODA^? as reactive species. The dissociation rate is given by v_d = (k^ML + k [H⁺]) · [M(DACODA)].