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Siegfried Schindler

Publications and source records attributed to Siegfried Schindler.

12 recordsLinked to original sources
PubMed2005

Chelate ring size variations and their effects on coordination chemistry and catechol dioxygenase reactivity of iron(III) complexes.

The catechol dioxygenase reactivity of iron(III) complexes using tripodal ligands was investigated. Increasing, as well as decreasing, chelate ring sizes in the highly active complex [Fe(tmpa)(dbc)]B(C6H5)4 (tmpa = tris[(2-pyridyl)methyl]amine; dbc = 3,5-di-tert-butylcatecholate dianion), using related ligands, only resulted in decreased reactivity of the investigated compounds. A detailed low-temperature stopped-flow investigation of the reaction of dioxygen with [Fe(tmpa)(dbc)]B(C6H5)4 was performed, and activation parameters of DeltaH++ = 23 +/- 1 kJ mol(-1) and DeltaS++ = -199 +/- 4 J mol(-1) K(-1) were obtained. Crystal structures of bromo-(tetrachlorocatecholato-O,O')(bis((2-pyridyl)methyl)-2-pyridylamine-N,N',N'')-iron(III), (mu-oxo)-bis(bromo)(bis((2-pyridyl)methyl)-2-pyridylamine-N,N',N' ',N''')-diiron(III), dichloro-((2-(2-pyridyl)ethyl)bis((2-pyridyl)methyl)amine-N,N',N' ',N''')-iron(III) and (tetrachlorocatecholato-O,O')((2-(2-pyridyl)ethyl)bis((2-pyridyl)methyl)amine-N,N',N' ',N''')-iron(III) are reported.

Journal Article↗
PubMed2004

Reaction behaviour of dinuclear copper(I) complexes with m-xylyl-based ligands towards dioxygen.

Intramolecular ligand hydroxylation was observed during the reactions of dioxygen with the dicopper(I) complexes of the ligands L(1)(L(1)=alpha,alpha'-bis[(2-pyridylethyl)amino]-m-xylene) and L(3)(L(3)=alpha, alpha'-bis[N-(2-pyridylethyl)-N-(2-pyridylmethyl)amino]-m-xylene). The dinuclear copper(I) complex [Cu(2)L(3)](ClO(4))(2) and the dicopper(II) complex [Cu(2)(L(1)-O)(OH)(ClO(4))]ClO(4) were characterized by single-crystal X-ray structure analysis. Furthermore, phenolate-bridged complexes were synthesized with the ligand L(2)-OH (structurally characterized [Cu(2)(L(2)-O)Cl(3)] with L(2)=alpha, alpha'-bis[N-methyl-N-(2-pyridylethyl)amino]-m-xylene; synthesized from the reaction between [Cu(2)(L(2)-O)(OH)](ClO(4))(2) and Cl(-)) and Me-L(3)-OH: [Cu(2)(Me-L(3)-O)(mu-X)](ClO(4))(2)xnH(2)O (Me-L(3)-OH = 2,6-bis[N-(2-pyridylethyl)-N-(2-pyridylmethyl)amino]-4-methylphenol and X = C(3)H(3)N(2)(-)(prz), MeCO(2)(-) and N(3)(-)). The magnetochemical characteristics of compounds were determined by temperature-dependent magnetic studies, revealing their antiferromagnetic behaviour [-2J(in cm(-1)) values: -92, -86 and -88; -374].

Journal Article↗
PubMed2003

Reversible binding of dioxygen by the copper(I) complex with tris(2-dimethylaminoethyl)amine (Me6tren) ligand.

At low temperatures, the mononuclear copper(I) complex of the tetradentate tripodal aliphatic amine Me(6)tren (Me(6)tren = tris(2-dimethylaminoethyl)amine) [Cu(I)(Me(6)tren)(RCN)](+) first reversibly binds dioxygen to form a 1:1 Cu-O(2) species which further reacts reversibly with a second [Cu(I)(Me(6)tren)(RCN)](+) ion to form the dinuclear 2:1 Cu(2)O(2) adduct. The reaction can be observed using low temperature stopped-flow techniques. The copper superoxo complex as well as the peroxo complex were characterized by resonance Raman spectroscopy. The spectral characteristics and full kinetic and thermodynamic results for the reaction of [Cu(I)(Me(6)tren)(RCN)](+) with dioxygen are reported.

Journal Article↗
PubMed2003

Syntheses and characterization of two dioxygen-reactive dinuclear macrocyclic schiff-base copper(I) complexes.

The dinuclear copper(I) complex [Cu(2)L(1)(CH(3)CN)(2)](ClO(4))(2) (1, L(1) = 3,6,9,17,20,23-hexaazatricyclo[23.3.1.1]triaconta-1(29),2,9,11(30),12(13),14,16,23,25,27-decaene) has been structurally characterized. As previously described, intramolecular ligand hydroxylation (at the aromatic ring) was observed when 1 was reacted with dioxygen. A stopped-flow analysis of the reaction of 1 with dioxygen under different conditions did not allow a "dioxygen intermediate" to be spectroscopically detected. Detailed NMR and electrochemical data on 1 are also presented and evaluated for the first time. No copper(II) complexes of L(1) could be characterized due to hydrolysis of the compounds. In contrast, complex 2-differing from 1 only in an increase in the size of the chelate rings-did not undergo intramolecular hydroxylation when it was oxidized. The crystal structure of 2 is also described.

Journal Article↗
PubMed2002

Copper-bispidine coordination chemistry: syntheses, structures, solution properties, and oxygenation reactivity.

Copper(I) and copper(II) complexes of two mononucleating and four dinucleating tetradentate ligands with a bispidine backbone (2,4-substituted (2-pyridyl or 4-methyl-2-pyridyl) 3,7-diazabicyclo[3.3.1]nonanone) have been prepared and analyzed structurally, spectroscopically, and electrochemically. The structures of the copper chromophores are square pyramidal, except for two copper(I) compounds which are four-coordinate with one noncoordinated pyridine. The other copper(I) structures have the two pyridine donors, the co-ligand (NCCH(3)), and one of the tertiary amines (N3) in-plane with the copper center and the other amine (N7) coordinated axially (Cu-N3 > Cu-N7, approximately 2.25 A vs 2.20 A). The copper(II) compounds with pyridine donors have a similar structure, but the axial amine has a weaker bond to the copper(II) center (Cu-N3 < Cu-N7, approximately 2.03 A vs 2.30 A). The structures with methylated pyridine donors are also square pyramidal with the co-ligands (Cl(-) or NCCH(3)) in-plane. With NCCH(3) the same structural type as for the other copper(II) complexes is observed, and with the bulkier Cl(-) the co-ligand is trans to N7, leading to a square pyramidal structure with the pyridine donors rotated out of the basal plane and only a small difference between axial and in-plane amines (2.15, 2.12 A). These structural differences, enforced by the rigid bispidine backbone, lead to large variations in spectroscopic and electrochemical properties and reactivities. Oxygenation of the copper(I) complexes with pyridine-substituted bispidine ligands leads to relatively stable mu-peroxo-dicopper(II) complexes; with a preorganization of the dicopper chromophores, by linking the two donor sets, these peroxo compounds are stable at room temperature for up to 1 h. The stabilization of the peroxo complexes is to a large extent attributed to the square pyramidal coordination geometry with the substrate bound in the basal plane, a structural motif enforced by the rigid bispidine backbone. The stabilities and structural properties are also seen to correlate with the spectroscopic (UV-vis and Raman) and electrochemical properties.

Chemical Phenomena↗
PubMed1999

Intramolecular Ligand Hydroxylation: Mechanistic High-Pressure Studies on the Reaction of a Dinuclear Copper(I) Complex with Dioxygen.

We provide a mechanistic study of a monooxygenase model system and detail low-temperature stopped-flow kinetics studies in acetone as solvent, employing both the use of rapid-scanning diode-array observation and variable high-pressure (20-100 MPa) techniques. The dicopper(I) complex employed is [Cu(2)(H-XYL-H)](2+) (1), with the H-XYL-H ligand wherein a m-xylyl group links two bis[2-(2-pyridyl)ethyl]amine units. This reacts with O(2) reversibly (k(1)/k(-)(1)) giving a peroxo-dicopper(II) intermediate [Cu(2)(H-XYL-H)(O(2))](2+) (2), which thereupon irreversibly (k(2)) reacts by oxygen atom insertion (i.e., hydroxylation) of the xylyl group, producing [Cu(2)(H-XYL-O(-))(OH)](2+) (3). Activation parameters are as follows: k(1), DeltaH() = 2.1 +/- 0.7 kJ/mol, DeltaS() = -174 +/- 3 J/(K mol); k(-)(1), DeltaH() = 80.3 +/- 0.8 kJ/mol, DeltaS() = 77 +/- 3 J/(K mol); k(2), DeltaH() = 58.2 +/- 0.2 kJ/mol, DeltaS() = -5.8 +/- 0.9 J/(K mol). These values are similar to values obtained in a previous study in dichloromethane. At low temperatures and higher concentrations, the situation in acetone is complicated by a pre-equilibrium of 1 to an isomer form. The present study provides the first determination of activation volumes for individual steps in copper monooxygenase reactions. The data and analysis provide that DeltaV()(k(1)) = -15 +/- 2.5 cm(3)/mol and DeltaV()(k(-)(1)) = +4.4 +/- 0.5 cm(3)/mol for formation and dissociation of 2, respectively, while DeltaV()(k(2)) = -4.1 +/- 0.7 cm(3)/mol; a volume profile for the overall reaction has been constructed. The significance of the findings in the present study is described, and the results are compared to those for other systems.

Journal Article↗
PubMed1998

Structural, Spectroscopic, Thermodynamic and Kinetic Properties of Copper(II) Complexes with Tripodal Tetraamines.

Spectroscopic, thermodynamic, and kinetic measurements have been made on aqueous solutions of copper(II) complexes of hexamethylated tren and trimethylated tren (one methylation per primary amine group of tren) with the objective of correlating the influence of geometry (trigonal bipyramidal, evident from UV/vis spectroscopy) and N-alkyl substitution in the ligand on these inherent properties. At 25.0 degrees C the protonation constants of Me(3)tren are not significantly different from those of tren and Me(6)tren, and the stability constant for the Cu(II) complex is of the same order of magnitude as that for the [Cu(tren)(H(2)O)](2+) complex ion. The pK(a) for deprotonation of the coordinated water molecule of [Cu(Me(3)tren)(H(2)O)](2+) is intermediate between the values for the complexes containing the unsubstituted and the fully substituted tren ligand. Substitution (pyridine for water) kinetics measurements employing stopped-flow and temperature-jump methods revealed different patterns of reactivity: pyridine replaces water in [Cu(Me(3)tren)(H(2)O)](2+) with a second-order rate constant of (4.4 +/- 0.8) x 10(2) M(-)(1) s(-)(1) at 25.0 degrees C, whereas the corresponding process for [Cu(Me(6)tren)(H(2)O)](2+) is relatively complex and is discussed in more detail. Substitution in the former complex ion is characterized in the forward and reverse directions, by DeltaH() = 60 +/- 8 and 51.9 +/- 0.9 kJ mol(-)(1), DeltaS() = 5 +/- 27 and -23 +/- 3 J mol(-)(1) K(-)(1), and DeltaV() = -8.7 +/- 4.6 and -6.2 +/- 1.1 cm(3) mol(-)(1), respectively. It is concluded that this reaction follows an I(a) mechanism, similar to that reported for the comparable reaction of [Cu(tren)(H(2)O)](2+). An X-ray structural determination on a crystal of [Cu(2)(Me(3)tren)(2)(CN)](ClO(4))(3).2CH(3)CN demonstrated trigonal bipyramidal geometry about each copper(II) center. As has been found in comparable complexes of tren and Me(6)tren, the axial nitrogen to copper bond is shorter than the equatorial nitrogen-copper bonds, and the angle made by N(axial)-Cu-N(equatorial) is less than 90 degrees (84.6-85.4 degrees ), signifying that each copper ion lies below the plane of the equatorial nitrogen atoms.

Journal Article↗
PubMed1998

Intramolecular Ligand Hydroxylation: Mechanistic Studies on the Reaction of a Copper(I) Schiff Base Complex with Dioxygen.

The dinuclear copper(I) complex of 1,3-bis[N-(2-pyridylethyl)formimidoyl]benzene, [Cu(2)(H-BPB-H)(CH(3)CN)(2)](BF(4))(2), as well as the 5-nitro derivative, [Cu(2)(NO(2)-BPB-H)(CH(3)CN)(2)](BF(4))(2), react with dioxygen to form phenolate-bridged complexes as products. In a detailed kinetic study activation parameters of DeltaH() = 47 +/- 9 kJ/mol, DeltaS() = -53 +/- 11 J/(mol K), and DeltaV() = -9.5 +/- 0.5 cm(3)/mol for the reaction of [Cu(2)(H-BPB-H)(CH(3)CN)(2)](BF(4))(2) with dioxygen were obtained which account, together with further kinetic findings, for the occurrence of an intermediate peroxo complex that cannot be observed spectroscopically. The crystal structures of the products of the reaction were determined. Crystal data: complex [Cu(2)(H-BPB-O)-OH(H(2)O)](BF(4))(2), formula C(22)H(24)B(2)Cu(2)F(8)N(4)O(3), monoclinic space group P2(1)/c, Z = 4, a = 10.122(2) Å, b = 28.711(6) Å, c = 9.283(2) Å, alpha = 90 degrees, beta = 100.78(3) degrees, and gamma = 90 degrees; complex [Cu(2)(NO(2)-BPB-O)-OH(H(2)O)(3)](BF(4))(2), formula C(22)H(27)B(2)Cu(2)F(8)N(5)O(7), triclinic space group, P&onemacr;, Z = 2, a = 10.144(2) Å, b = 10.7612(2) Å, and c = 16.000(4) Å.

Journal Article↗
PubMed1996

Mechanistic Studies of the Formation and Decay of Diiron(III) Peroxo Complexes in the Reaction of Diiron(II) Precursors with Dioxygen.

Mechanistic studies of the reactions of three analogous alkoxo-bridged diiron(II) complexes with O(2) have been carried out. The compounds, which differ primarily in the steric accessibility of dioxygen to the diiron(II) center, form metastable &mgr;-peroxo intermediates when studied at low temperature. At ambient temperatures, these intermediates decay to form (&mgr;-oxo)polyiron(III) products. The effect of ligand steric constraints on the O(2) reactivity was investigated. When access to the diiron center was unimpeded, the reaction was first-order with respect to both [Fe(II)(2)] and [O(2)] and the activation parameters for O(2) addition were similar to those for O(2) reacting with the dioxygen transport protein hemerythrin. When the binding site was occluded, however, reduced order with respect to [O(2)] was observed and a two-step mechanism was required to explain the kinetic results. Decay of all three peroxide intermediates involves a bimolecular event, implying the formation of tetranuclear species in the transition state.

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