The substitution of water in the half-sandwich complexes Cp*Rh(H2O)3 2+ and Cp*Ir(H2O)3 2+ (Cp* = η5-pentamethylcyclopentadienyl anion) by Cl-, Br-, I-, SCN-, py-CN (4-cyanopyridine), py-nia (nicotinamide), py (pyridine), TU (thiourea), and DMS (dimethylsulfide) was studied by stopped-flow spectroscopy at variable concentration, temperature, and pressure. The proton dissociation constants of the triaqua complexes, pK a = 6.47 (for rhodium) and pK a = 3.86 (for iridium), as well as the equilibrium constants for the formation of the dinuclear species (Cp*M)2(μ-OH)3 + were obtained by spectrophotometric titrations. The equilibrium constants K 1 for the formation of the monosubstituted complexes Cp*M(H2O)2L+/2+, as determined for anionic and neutral ligands L, lie in the range 102−105 M-1 and follow the sequences K(Cl-) < K(Br-) < K(I-) and K(py-CN) < K(py-nia) < K(py) < K(TU,DMS). Assuming the Eigen−Wilkins mechanism for the formation of the monosubstituted complexes, second-order rate constants k f,1 were corrected for outer sphere complex formation and for statistical factors to obtain rate constant k i‘ for the interchange step. The interchange rates k i‘ are nearly independent of the nature of L and very close to the rate of water exchange (k ex(Rh) = (1.6 ± 0.3) × 105 s-1 and k ex(Ir) = (2.5 ± 0.08) × 104 s-1). In all cases, i.e., for M = Rh and Ir and for L = anionic or neutral, the volume of the transition state is larger than that of the triaqua species. These findings support the operation of an I d mechanism without excluding a D mechanism. For a given ligand L, the substitution of another water molecule in the complexes Cp*M(H2O)2L+/2+ is by 1 order of magnitude slower than the substitution of the first water molecule in the triaqua species Cp*M(H2O)3 2+, as verified, for example, by k f,1 = 2.61 × 103 and k f,2 = 3.09 × 102 M-1 s-1 for M = Ir and L = py.
The perchlorate complexes of a series of half‐sandwich monoaqua cations Cp*Ir(A−B)(H2O)2+/+ with A−B = prol (D/L‐proline anion), picac (picolinic acid anion), R,R‐dach [(−)‐(1R,2R)‐1,2‐diaminocyclohexane], R,R‐dpen [(+)‐(1R,2R)‐1,2‐diphenylethylenediamine], phen (o‐phenanthroline), and bpy (2,2′‐bipyridine) (Cp* = η5‐pentamethylcyclopentadienyl anion) have been prepared and characterized. An X‐ray structure analysis of Cp*Ir(R,R‐dach)(H2O)(ClO4)2·H2O has revealed that the cation Cp*Ir(R,R‐dach)(H2O)2+ has a distorted pseudo‐octahedral coordination geometry. In the case of A−B = prol, crystallization from water led to the trinuclear complex [Cp*Ir(D‐prol)]3(ClO4)3, which has also been characterized by X‐ray structure analysis. The experimental data suggest that in aqueous solution the trinuclear proline complex dissociates to form the cation Cp*Ir(D‐prol)(H2O)+. The proton dissociation constants of the coordinated water in Cp*Ir(A−B)(H2O)2+/+ have been determined as pKa = 7.5 (A−B = bpy) and pKa = 7.1 (A−B = R,R‐dach and picac). Substitution of the water in Cp*Ir(A−B)(H2O)2+/+ by the monodentate ligands L = py (pyridine), DMS (dimethyl sulfide), TU (thiourea), and monodentate anions according to the Equation Cp*Ir(A−B)(H2O)2+/+ + L → Cp*Ir(A−B)L2+/+ + H2O has been studied by multi‐wavelength stopped‐flow spectrophotometry in aqueous solution at I = 0.2 M. This kinetic investigation, carried out at different concentrations, temperatures, and pressures, showed that the process obeys second‐order kinetics, where rate = kL[Cp*Ir(A−B)H2O2+/+][L]. The magnitude of the second‐order rate constant kL depends on the nature of both A−B and L. The data for kL have been found to range from 6.4 × 104 M−1s−1 (A−B = D‐prol; L = TU) to 10.5 M−1s−1 (A−B = bpy; L = py) at 298 K. The activation parameters for water substitution at Cp*Ir(A−B)(H2O)2+/+ (A−B = bpy, R,R‐dach, and picac) by L = TU have been evaluated. The activation volumes of ΔV≠ = +2.3, +7.4, and +7.3 cm3 mol−1, respectively, are supportive of an Id mechanism. The results regarding the kinetic lability of the coordinated water in the monoaqua cations Cp*Ir(A−B)(H2O)2+/+ are compared to those obtained for the triaqua cation Cp*Ir(H2O)32+.
Nanocomposites consisting of gelatin and hydroxyapatite as well as of gelatin and mixtures of hydroxyapatite and different amounts of octacalcium phosphate were prepared as bulk-materials. The composites were precipitated from aqueous solutions of CaCl 2 3 2H 2 O and (NH 4 ) 2 (HPO 4 ), respectively, with varying amounts of gelatin at 25 °C and pH 7. The influence of prestructuring effects of calcium and phosphate ions, respectively, on gelatin and by this on the precipitated materials was investigated in detail. X-ray-diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS), and high-resolution transmission electron microscopy (HR-TEM) revealed that the prestructuring components as well as the total amount of gelatin involved in the reactions have a substantial influence on the composition and shape of the nanocomposites formed. In the case of CaCl 2 3 2H 2 O used as the prestructuring agent for gelatin, hydroxyapatite is the inorganic phase obtained, independent of the initial amount of gelatin. By the prestructuring of gelatin with (NH 4 ) 2 (HPO 4 ), a strong dependency of the reaction products on the amount of gelatin was observed. Low gelatin quantities favor the formation of hydroxyapatite, whereas high gelatin concentrations lead to the formation of octacalcium phosphate. Moreover, the morphology of the composites changes gradually. Samples prepared by means of the Ca-prestructuring (CPS) reaction consist of small plate-like particles (∼50 nm  33 nm). When the PO 4 -prestructuring (PPS) reaction is used, the particle size is highly influenced by the amount of gelatin. Lower gelatin concentrations lead to small, plate-like particles (∼60 nm  35 nm), while higher gelatin concentrations cause the development of large foils (∼730 nm  410 nm). The thickness of the composite particles varies from 2 to 13 nm as determined by means of electron holography. The calcium phosphate-gelatin nanocomposites obtained by the precipitation reactions were investigated for use as dentine repair materials with a special focus on the closing of open tubuli of sensitive tooth necks.
Multi‐wavelength stopped‐flow spectrophotometry was used to study the kinetics of base hydrolysis of the octahedral cobalt(III) complex CoLCl2+ (2), in which the tetrapodal pentadentate ligand L has an NN4 donor set and forms a square‐pyramidal coordination cap [L = 2,6‐bis(1′,3′‐diamino‐2′‐methylprop‐2′‐yl)pyridine, 1]. The kinetic investigation, carried out at different temperatures, pressures and ionic strengths I, led to second‐order kinetics, rate = kOH [2][OH−], with kOH = 0.139 ± 0.001 M−1s−1 (I = 0.1 M) and kOH = 0.0570 ± 0.0004 M−1s−1 (I = 1.0 M) at 298 K. The temperature and pressure dependence of kOH resulted in ΔH‡ = 119 ± 3 kJmol−1 (I = 0.1 M) and ΔH‡ = 123 ± 3 kJmol−1 (I = 1.0 M), ΔS‡ = + 130 ± 13 Jmol−1K−1 (I = 0.1 M) and ΔS‡ = + 151 ± 15 Jmol−1K−1 (I = 1.0 M) and ΔV‡ = + 27.6 ± 0.6 cm3mol−1 (I = 1.0 M). The kinetic results support the operation of the conjugate base mechanism, Dcb, with intermediate deprotonation occuring cis to the leaving chloride ion. The observed inertness of complex 2 towards base hydrolysis is discussed on the basis of the special structural features of the ligand L. The pKa of the coordinated water in the aqua species CoL(H2O)3+(3) was determined to be pKa = 6.0 ± 0.1 at I = 0.1 M. The X‐ray crystal structure analysis of the hydroxo complex [CoL(OH)](ClO4)2 (4) shows a mononuclear terminal (hydroxo)CoIII complex, in which the ligand provides a regular square‐pyramidal NN4 donor cap for the octahedrally six‐coordinate CoIII ion [4, monoclinic, space group C2/m, a = 16.841(6) Å, b = 8.541(3) Å, c = 15.112(5) Å, β = 109.13(1)°, Z = 4]. Coordination of L is accompanied by the formation of 6 six‐membered chelate rings, all of which adopt a boat conformation. In the crystal lattice, pairs of cations are associated via four (hydroxo)O···H–N(primary amine) hydrogen bonds and related through an inversion center. Full spectroscopic data for 4 (1H‐, 13C‐NMR, IR, MS) are presented.
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