Electron transfer by copper centers.

The molecular structure and dynamics of the photoexcited metal-to-ligand-charge-transfer (MLCT) state of [CuI(dmp)2]+, where dmp is 2,9-dimethyl-1,10-phenanthroline, in acetonitrile have been investigated by time-domain pump-probe X-ray absorption spectroscopy, femtosecond optical transient spectroscopy, and density functional theory (DFT). The time resolution for the excited state structural determination was 100 ps, provided by single X-ray pulses from a third generation synchrotron source. The copper ion in the thermally equilibrated MLCT state has the same oxidation state as the corresponding copper(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu−N distance 0.04 A shorter than that of the ground state [CuI(dmp)2]+. The results confirm the previously proposed “exciplex” structure of the MLCT state in Lewis basic solvents. The evolution from the photoexcited Franck-Condon MLCT state to the thermally equilibrated MLCT state was followed by femtosec...

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MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations.

Twelve related tripodal ligands have been synthesized in which the three legs linked to a bridgehead nitrogen are 2-methyl- or 2-ethylthioethyl and/or 2-pyridylethyl or -methyl. Utilization of both terminal methyl and ethyl groups on the thiaether legs was designed to determine whether slight differences in solvation or steric effects might cause detectable changes in properties. Inclusion of both methyl and ethyl linkages of the pyridines to the bridgehead nitrogen provides a comparison of the effect of five- versus six-membered chelate rings, respectively. For each of the tripodal ligands included in this work, the protonation constants and Cu(II) complex stability constants were carefully determined in aqueous solution at 25 °C, μ = 0.10 M (ClO4-). The CuII/IL redox potentials were also determined using slow-scan cyclic voltammetry, thereby permitting the stability of the Cu(I) complexes to be calculated. The stability constants for the twelve Cu(II) complexes range from 106 to 1017, increasing by 104−...

Influence of Coordination Geometry upon Copper(II/I) Redox Potentials. Physical Parameters for Twelve Copper Tripodal Ligand Complexes

Visible light excitation of [CuI(dmp)2](BArF), where dmp is 2,9-dimethyl-1,10-phenanthroline and BArF is tetrakis(3,5-bis(trifluoromethylphenyl))borate, in toluene produces a photoluminescent, metal-to-ligand charge-transfer (MLCT) excited state with a lifetime of 98 ± 5 ns. Probing this state within 14 ns after photoexcitation with pulsed X-rays establishes that a CuII center, borne in a CuI geometry, binds an additional ligand to form a five-coordinate complex with increased bond lengths and a coordination geometry of distorted trigonal bipyramid. The average Cu−N bond length increases in the excited state by 0.07 A. The transiently formed five-coordinate MLCT state is photoluminescent under the condition studied, indicating that the absorptive and emissive states have distinct geometries. The data represent the first X-ray characterization of a molecular excited state in fluid solution on a nanosecond time scale.

Rapid excited-state structural reorganization captured by pulsed X-rays.

Superoxide radical anion has been demonstrated to be a mediator of many disease states, including inflammatory, autoimmune, and cancerous diseases. As a consequence, we have developed a program to design, synthesize, and test synthetic low-molecular weight superoxide dismutase (SOD) mimics as potential pharmaceutical agents. A critical feature of the design of metal-based drugs is not only high activity as an enzyme mimic but also chemical stability. In this report we describe the synthesis and characterization of a series of C-substituted 1,4,7,10,13-pentaazacyclopentadecane, [15]aneN5, ligands, 1, and their corresponding Mn(II) complexes, 2 and 3, as their dichloro complexes, [Mn([15]aneN5)Cl2]. The purpose of the work is to probe the role that substituent (methyl and fused cycloalkyl) groups exert on the catalytic activity and stability of the complexes. All of the 18 new complexes described here are catalysts for the dismutation of superoxide, and substantial substituent effects are observed with kcat...

Synthesis, Characterization, and Stability of Manganese(II) C-Substituted 1,4,7,10,13-Pentaazacyclopentadecane Complexes Exhibiting Superoxide Dismutase Activity

The kinetics of electron-transfer reactions involving Cu II/I ([14]aneS 4 ) reacting with a series of selected counterreagents have been measured in aqueous solution at 25 o C, r=0.10 M (ClO 4 - ).

Gated electron-transfer behavior in copper(II/I) systems. Comparison of the kinetics for homogeneous cross reactions, NMR self-exchange relaxation, and electrochemical data for a copper macrocyclic tetrathioether complex in aqueous solution

The absolute magnitude of an “entatic” (constrained) state effect has never been quantitatively demonstrated. In the current study, we have examined the electron-transfer kinetics for five closely related copper(II/I) complexes formed with all possible diastereomers of [14]aneS4 (1,4,8,11-tetrathiacyclotetradecane) in which both ethylene bridges have been replaced by cis- or trans-1,2-cyclohexane. The crystal structures of all five Cu(II) complexes and a representative Cu(I) complex have been established by X-ray diffraction. For each complex, the cross-reaction rate constants have been determined with six different oxidants and reductants in aqueous solution at 25 °C, μ = 0.10 M. The value of the electron self-exchange rate constant (k11) has then been calculated from each cross reaction rate constant using the Marcus cross relation. All five Cu(II/I) systems show evidence of a dual-pathway square scheme mechanism for which the two individual k11 values have been evaluated. In combination with similar va...

Direct evidence for a geometrically constrained "entatic state" effect on copper(II/I) electron-transfer kinetics as manifested in metastable intermediates.

Effect of Conformational Constraints on Gated Electron-Transfer Kinetics. A Multifaceted Study on Copper(II/I) Complexes with cis- and trans-Cyclohexanediyl-[14]aneS4

Effect of Ligand Constraints upon the Stabilities and Potentials of Macrocyclic Polythiaether Complexes. Copper(II) and Copper(I) Complexes with Cyclohexyl and Phenyl Derivatives of [14]aneS4 in Water, 80% Methanol, and Acetonitrile

Studies have been conducted on the electron-transfer kinetics of the Cu(II/I) complexes formed with two dialcoholic derivatives of the macrocyclic tetrathiaether [14] aneS 4 , syn- and anti-3,6,10,13-tetrathiacyclotetradecane-1,8-diol. For Cu II / I (syn-[14]aneS 4 -diol), the self-exchange rate constant has been determined directly from NMR line broadening. For both Cu II / I L systems, extensive cross-reaction electron-transfer kinetics have also been measured using four different reductants and four different oxidants, spanning a wide range of reaction potentials

Electron-transfer kinetics of copper(II/I) complexes with dialcoholic derivatives of the macrocyclic tetrathiaether [14]aneS4. Effect of simple ring substituents upon gated behavior

Cynthia A. Salhi

Papers

Gated electron-transfer behavior in copper(II/I) systems. Comparison of the kinetics for homogeneous cross reactions, NMR self-exchange relaxation, and electrochemical data for a copper macrocyclic tetrathioether complex in aqueous solution

Direct evidence for a geometrically constrained "entatic state" effect on copper(II/I) electron-transfer kinetics as manifested in metastable intermediates.

Effect of Conformational Constraints on Gated Electron-Transfer Kinetics. A Multifaceted Study on Copper(II/I) Complexes with cis- and trans-Cyclohexanediyl-[14]aneS4

Effect of Ligand Constraints upon the Stabilities and Potentials of Macrocyclic Polythiaether Complexes. Copper(II) and Copper(I) Complexes with Cyclohexyl and Phenyl Derivatives of [14]aneS4 in Water, 80% Methanol, and Acetonitrile

Electron-transfer kinetics of copper(II/I) complexes with dialcoholic derivatives of the macrocyclic tetrathiaether [14]aneS4. Effect of simple ring substituents upon gated behavior