Horst Elias

Copper(II) complexes with derivatives of salen and tetrahydrosalen: a spectroscopic, electrochemical and structural study

Abstract Salen type complexes, CuL, the corresponding tetrahydrosalen type complexes, Cu[H4]L, and N,N′-dimethylated tetrahydrosalen type complexes, Cu[H2Me2]L, were investigated using cyclic voltammetry, and electronic and ESR spectroscopy. In addition, the analogous copper(II) complexes with a derivative of the tetradentate ligand ‘salphen’ [salphen=H2salphen=N,N′-disalicylidene-1,2-diaminobenzene] were studied. Solutions of CuL, Cu[H4]L and Cu[H2Me2]L are air-stable at ambient temperature, except for the complex Cu(tBu, Me)[H4]salphen [H2(tBu, Me)[H4]salphen=N,N′-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-1,2-diaminobenzene]. Cu(tBu, Me)[H4]salphen interacts with dioxygen and the ligand is oxidatively dehydrogenated (–CH2–NH–→–Cue605N–) to form Cu(tBu, Me)[H2]salphen and finally, in the presence of base, Cu(tBu, Me)salphen. X-ray structure analysis of Cu(tBu, Me)[H2Me2]salen confirms a slightly tetrahedrally distorted planar geometry of the CuN2O2 coordination core. The complexes were subjected to spectrophotometric titration with pyridine, to determine the equilibrium constants for adduct formation. It was found that the metal center in the complexes studied is only of weak Lewis acidity. In dichlormethane, the oxidation Cu(II)/Cu(III) is quasireversible for the CuL type complexes, but irreversible for the Cu[H4]L and Cu[H2Me2]L type. A poorly defined wave was observed for the irreversible reduction Cu(II)/Cu(I) at potentials less than −1.0 V. The ESR spectra of CuL at both 77 K and room temperature reveal that very well resolved lines can be attributed to the interaction of an unpaired electron spin with the copper nuclear spin, 14N donor nuclei and to a distant interaction with two equivalent protons [∣ACu(iso)∣≈253 MHz, ∣AN(iso)∣≈43 MHz, ∣AN(iso)∣≈20 MHz]. These protons are attached to the carbon atoms adjacent to the 14N nuclei. In contrast to CuL, the number of lines in the spectra of the complexes Cu[H4]L and Cu[H2Me2]L is greatly reduced. At room temperature, only a quintet with a considerably smaller nitrogen shf splitting constant [∣AN(iso)∣≈27 MHz] is observed. Both factors, planarity and conjugation, are thus essential for the observation of distant hydrogen shf splitting in CuL. Due to the Cue605N bond hydrogenation, the coordination polyhedra of the complexes Cu[H4]L and Cu[H2Me2]L is more flexible and more sensitive to ligand modification than that of CuL. The electron-withdrawing effect of the phenyl ring of the phenylenediamine bridge is reflected in a reduction of the copper hyperfine coupling constants in Cu(tBu, Me)[H4]salphen and Cu(tBu, Me)[H2Me2]salphen complexes [∣ACu(iso)∣≈215 MHz].

Kinetics and mechanism of metal complex formation with N4-donor macrocycles of the cyclam type

Abstract The N 4 -donor macrocyclic ligand cyclam (1.4,8,11-tetraazacyclotetradecane) forms very stable complexes with nickel(II) and copper(II). With regard to kinetics and mechanism of complex formation, tetraaza cyclic ligands such as cyclam represent an interesting type of chelate ligands in that they are less flexible than aliphatic open-chain N-donor ligands and less rigid than cyclic N-donor ligands of the porphyrin type. The Eigen-Wilkins mechanism provides an adequate description of the kinetics of complex formation of nickel(II) and copper(II) with monodentate ligands and allows to correlate the rate of complex formation with the rate of solvent exchange on the solvated cations NiS 6 2+ and CuS 6 2+ (S = solvent). This review focuses on the factors controlling the rate and mechanism as well as the stereochemistry of complex formation of nickel(II) and copper(II) with N 4 -donor macrocyclic ligands of the cyclam type. The kinetic results obtained in aqueous solution are briefly reviewed, although their mechanistic interpretation is hampered by ligand protonation. The kinetic studies carried out in aprotic polar solvents such as N,N -dimethylformamide are reviewed and discussed in detail. In comparison to the Eigen-Wilkins mechanism and Eigen-Winkler mechanism, the aspect of metal-based rate control versus ligand-based rate control is emphasized.

Kinetics and Mechanism of Water Substitution at Half-Sandwich Iridium(III) Aqua Cations Cp*Ir(A−B)(H2O)2+/+ in Aqueous Solution (Cp* = η5-Pentamethylcyclopentadienyl Anion; A−B = Bidentate N,N or N,O Ligand)

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 6.4xa0×xa0104M−1s−1 (A−B = D-prol; L = TU) to 10.5 M−1s−1 (A−B = bpy; L = py) at 298xa0K. 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+.

The spectroscopic and structural properties of copper(II) complexes of the novel tridentate (ONO) pyridine N-oxide ligand Hpoxap

Abstract A tridentate ligand (Hpoxap=2-(o-hydroxyphenyliminomethyl)pyridine N-oxide) has been synthesised by the Schiff condensation of 2-pyridinecarboxaldehyde N-oxide with 2-aminophenol. A series of copper(II) complexes with this ONO-donor ligand has been prepared and their spectroscopic properties (electron, vibration, ESR) were studied. The structures for two complexes have been determined by X-ray analysis. The structure of [Cu(poxap)(OAc)] consists of isolated neutral molecules in which the copper(II) atom is situated in a slightly distorted square-planar surrounding. The structure of [Cu(poxap)(H2O)(NO3)] consists of neutral molecules which are interconnected by hydrogen bonds giving rise to a ladder structure of copper(II) ions with two different Cu⋯Cu distances of 5.27 and 7.41 A. The copper atoms are in a slightly distorted square-pyramidal environment with the tridentate ligand poxap− and the water molecule in the basal plane; the apical position being occupied by an oxygen atom of the nitrate anion. Magnetic susceptibility studies were undertaken and these confirm predominantly a dimeric character of the complex.

Kinetic investigation of sulfur(IV) oxidation by peroxo compounds R-OOH in aqueous solution

SummaryNormal and rapid-scan stopped-flow spectrophotometry in the range of 260–300 nm was used to study the kinetics of sulfur(IV) oxidation by peroxo compounds R-OOH (such as hydrogen peroxide, R=H; peroxonitrous acid, R=NO; peroxoacetic acid, R=Ac; peroxomonosulfuric acid, R=SO3−) in the pH range 2–6 in buffered aqueous solution at an ionic strength of 0.5 M (NaClO4) or 1.0 M (R=NO; Na2SO4). The kinetics follow a three-term rate law, rate=(kH[H]+kHX[HX]+kp)[HSO3−][ROOH] ([H] = proton activity; HX = buffer acid = chloroacetic acid, formic acid, acetic acid, H2PO4−). Ionic strength effects (I=0.05–0.5 M) and anion effects (Cl−, ClO4−, SO42−) were not observed. In addition to proton-catalysis (kH[H]) and general acid catalysis (kHX[HX]), the rate constant kp characterizes, most probably, a water induced reaction channel with kp=kHOH[H2O]. It is found that kH≠f(R) with kH(mean)=2.1·107 M−2 s−1 at 298 K. The rate constant kHX ranges from 0.85·106 M−2 s−1 (HX=ClCH2−COOH; R=NO; 293 K) to 0.47·104 M−2 s−1 (HX=H2PO4−; R=H; 298 K) and the rate constant kp covers the range 0.2·M−1 s−1 (R=H) to 4.0·104 M−1 s−1 (R=NO). LFE relationships can be established for both kHX, correlating with the pKa of HX, and kp, correlating with the pKa of the peroxo compounds R-OOH. These relationships imply interesting aspects concerning the mechanism of sulfur(IV) oxidation and the possible role of peroxonitrous acid in atmospheric chemistry. A UV-spectrum of the unstable peroxo acid ON-OOH is presented.

Kinetics of the oxidation of hydrogen sulfite by hydrogen peroxide in aqueous solution:: ionic strength effects and temperature dependence

Abstract Conductometry was used to study the kinetics of the oxidation of hydrogen sulfite, HSO−3, by hydrogen peroxide in aqueous non-buffered solution at the low concentration level of 10−5–10−6xa0M, typically found in cloud water. The kinetic data confirm that the rate law reported for the pH range 3–6 at higher concentration levels, rate=kH·[H+]·[HSO−3]·[H2O2], is valid at the low concentration level and at low ionic strength Ic. At 298xa0K and Ic=1.5×10−4xa0M, third-order rate constant kH was found to be kH=(9.1±0.5)×107xa0M−2xa0s−1. The temperature dependence of kH led to an activation energy of Ea=29.7±0.9xa0kJxa0mol−1. The effect of the ionic strength (adjusted with NaCl) on rate constant kH was studied in the range Ic=2×10−4–5.0xa0M at pH=4.5–5.2 by conductometry and stopped-flow spectrophotometry. The dependence of kH on Ic can be described with a semi-empirical relationship, which is useful for the purpose of comparison and extrapolation. The kinetic data obtained are critically compared with those reported earlier.

Effect of hydrogenation on electronic and distant magnetic properties in copper(II) complexes with derivatives of tetrahydrosalen and salen. X-ray crystal structure of [Cu{Bu,Me(saltmen)}] complex

Abstract The salen complex CuL and the corresponding tetrahydrosalen complex Cu[H4]L were investigated by ESR spectroscopy and molecular orbital calculations [H2L = N,N′-bis(3-tert-butyl-5-methylsalicylidene)-2,3-diamino-2,3-dimethylbutane; H2[H4]L = N,N′-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-2,3-diamino-2,3-dimethylbutane]. X-ray structure determination of CuL confirmed a slightly distorted planar geometry of the CuN2O2 coordination core. The ESR spectra of CuL at both 78 K and room temperature revealed that very well resolved lines cannot be attributed to the interaction with copper nuclear spin and 14N donor nuclei alone. Computer simulation showed that in addition to copper hyperfine (giso = 2.094, |ACu(iso)| = 270 MHz, room temperature) and nitrogen superhyperfine structure [|AN(iso)| = 46 MHz] a distant interaction with two equivalent protons is also present [|AH(iso)| = 23 MHz]. These protons are attached to the carbon atoms adjacent to 14N nuclei. In contrast the number of lines in the spectrum of the hydrogenated analogue Cu[H4]L is greatly reduced. At room temperature only a quintet with considerable smaller nitrogen shf constant [|AN(iso)|] = 25 MHz is observed. Thus, both factors planarity and conjugation, are essential for the observation of distant hydrogen shf splitting in CuL. The ESR findings are in good agreement with calculated spin densities by QR-INDO/1 method.

Solvatochromic, piezochromic and thermochromic behaviour of Mo(CO)4(N N) complexes

Abstract The solvent, pressure and temperature dependencies of the lowest energy metal to ligand charge transfer absorption bands were studied for a series of complexes of the type Mo(CO) 4 (N N ), where N N = 2,2′-bipyridine, 1,10-phenanthroline and biacetylbis(phenylimine). Throughout the series of complexes the absorption bands shift to shorter wavelength in more polar solvents or on increasing the pressure in a particular solvent, but to longer wavelengths on increasing temperature. These main tendencies can be accounted for in terms of solvent polarity and its dependence on pressure and temperature.

Fast Oxidation of Organic Sulfides by Hydrogen Peroxide by In Situ Generated Peroxynitrous Acid

A powerful oxidant and an unstable isomer of HNO3 , peroxynitrous acid ONOOH is generated by the fast reaction of H2 O2 with HNO2 in acidic medium [Eq. (1)]. If sulfides R2 S are present, ONOOH sulfoxidizes them in minutes. This reaction occurs faster than the decay of ONOOH to HNO3 and allows the fast preparation of sulfoxides with H2 O2 . (1).

Kinetics and mechanism of ligand substitution in β-diketone complexes of iron(III). Solvolysis controlling the substitution process in alcohol media

Abstract Conventional and stopped-flow spectrophotometry was used to study the kinetics of ligand substitution in a number of tris β-diketone iron(III) complexes, Fe(O ∩ O) 3 , by 8-hydroxyquinoline (=HO ∩ N) in alcohol media (O ∩ O − =anion of the β-diketones pentane-2,4-dione, 2,6-dimethylheptane-3,5-dione, 2,2,6,6-tetramethylheptane-3,5-dione, 1-phenylbutane-1,3-dione, 1,3-diphenylpropane-2,3-dione, and 1-(2-thienyl)-4,4,4-trifluorobutane-1,3-dione). As shown by spectrophotometry, the solutions of complexes Fe(O ∩ O) 3 in alcohols ROH are subject to solvolytic dissociation, leading to solvento species Fe(O ∩ O) 2 S 2 and to binuclear complexes [Fe(O ∩ O) 2 (RO)] 2 (S=ROH and RO − , respectively). The reaction of complexes Fe(O ∩ O) 3 with HO ∩ N in alcohol media, leading to Fe(O ∩ N) 3 , is triphasic. The corresponding first-order rate constants k 1 , k 2 , and k 3 are independent of the concentration of the entering ligand HO ∩ N and follow the order k 1 > k 2 > k 3 , with k 1 / k 2 ≈10 and k 1 / k 3 ≈10 2 . For a given system Fe(O ∩ O) 3 /HO ∩ N/ROH, the size of k 1 , k 2 , and k 3 correlates with the solvent polarity parameter E T (30). Rate constant k 1 describes the solvolytic dissociation of the complexes Fe(O ∩ O) 3 and rate constant k 3 the solvent-initiated splitting of the binuclear complexes [Fe(O ∩ O) 2 (RO)] 2 . Rate constant k 2 is assigned to the solvolytic dissociation of the intermediate complex Fe(O ∩ O) 2 (O ∩ N). Depending on the nature of the coordinated β-diketone and solvent ROH, k 1 ranges from 0.04 to 2 s −1 , k 2 from 0.007 to 0.2 s −1 , and k 3 from 0.002 to 0.01 s −1 at 298 K. The mechanism of the ligand substitution processes is discussed.