Electron Transfer by Copper Centers

Assessment of the CCSD and CCSD(T) Coupled-Cluster Methods in Calculating Heats of Formation for Cu Complexes

Heats of formation for nine complexes of the form CuX(n) (X = Cu, H, O, OH, S, F, F(2), Cl, Cl(2)) were calculated using the CCSD and CCSD(T) coupled cluster methods with the 6-31G** and TZVP basis sets as well as the LANL2DZ basis set/pseudopotential on Cu with both the 6-31G** and TZVP basis sets applied to the nonmetal atoms. These values were compared with literature heat of formation values. A second order Douglas-Kroll-Hess relativistic correction was applied at the CCSD/TZVP and CCSD(T)/TZVP levels of theory. Overall, the CCSD(T)/TZVP level of theory with the relativistic correction was most suited for the heat of formation calculations possessing low absolute average error and RMSD and the ability to analyze each copper complex, except for the problematic case of copper(II) fluoride. Finally, experimental geometric parameters were compared with the calculated structures in such cases where these data were available. None of the investigated levels of theory predicted bond lengths consistently better than other methods, and it was determined that the most accurate bond length does not necessarily result in the most accurate calculated heat of formation value for a given complex.

Thiamacrocyclic lactones: new Ag(I)-ionophores

The syntheses of novel adamantane thialactones 5-12 are reported, and the results of the heavy- and transition-metal cation extraction experiments are described. The results are compared with those obtained with similar thiamacrocyclic ligands that have flexible chains of methylene groups incorporated into the macrocyclic framework as in 13-20. The results show that most of the hosts studied are very good in complexing the Ag(+) ion. The formation of complexes has also been demonstrated using NMR titration experiments for macrocycles 13 and 14 with AgTFA. Introduction of a single polycyclic molecule into the 15- to 18-membered rings increases the rigidity and preorganizes the ligand for complexation. However, two adamantane molecules embedded in the ring usually diminish the complexing ability of the ligand, primarily due to sterical effects of the bulky adamantane moiety that obstructs formation of an optimal geometry for binding the desired metal ion. The structures of macrocycles 5, 7, 9, 11, and 19 were determined by X-ray structure analysis, and their conformational properties are discussed. In the solid state, 7, 11, and 19 are organized into tubular fashion using C-H...O interactions. Also, two silver complexes with thialactone 13, Ag13 and Ag(13)(2), were prepared and characterized. The structure analysis of Ag13 and Ag(13)(2) reveals the formation of mononuclear and binuclear species with silver in ambivalent, tetrahedral coordination via sulfur and oxygen from trifluoroacetate anion.

A place for thioether chemistry in cellular copper ion recognition and trafficking

Over the last decade, cysteine thiolate ligands have been shown to be critical to the Cu(I) (cuprous) binding chemistry of many cytosolic metallochaperone and metalloregulatory proteins involved in copper physiology. More recently, the thioether group of methionine has begun to emerge as an important Cu(I) ligand for trafficking proteins in more oxidizing cellular environments.

The structure of the amyloid-beta peptide high-affinity copper II binding site in Alzheimer disease

Neurodegeneration observed in Alzheimer disease (AD) is believed to be related to the toxicity from reactive oxygen species (ROS) produced in the brain by the amyloid-beta (Abeta) protein bound primarily to copper ions. The evidence for an oxidative stress role of Abeta-Cu redox chemistry is still incomplete. Details of the copper binding site in Abeta may be critical to the etiology of AD. Here we present the structure determined by combining x-ray absorption spectroscopy (XAS) and density functional theory analysis of Abeta peptides complexed with Cu(2+) in solution under a range of buffer conditions. Phosphate-buffered saline buffer salt (NaCl) concentration does not affect the high-affinity copper binding mode but alters the second coordination sphere. The XAS spectra for truncated and full-length Abeta-Cu(2+) peptides are similar. The novel distorted six-coordinated (3N3O) geometry around copper in the Abeta-Cu(2+) complexes include three histidines: glutamic, or/and aspartic acid, and axial water. The structure of the high-affinity Cu(2+) binding site is consistent with the hypothesis that the redox activity of the metal ion bound to Abeta can lead to the formation of dityrosine-linked dimers found in AD.

A delicate electronic balance between metal and ligand in [Cu-P-Cu-P] diamondoids: oxidation state dependent plasticity and the formation of a singlet diradicaloid

Transition metal atoms often participate in redox reactions as catalytic sites, where ligand groups play an important role in orchestrating catalytic activity, especially in metalloenzymes. A major issue is to understand connections between oxidation state and geometry at the metal center, because geometric reorganization is directly related to reaction rate. In this article, we analyze an intriguing oxidation-induced geometrical change in [Cu-P-Cu-P] ring structures ( approximately 0.6 A change in metal-metal distance) using quantum chemical approaches. We find that the Cu-P interactions in the ring of the neutral species consist of four localized P --> Cu dative bonds. Successive oxidations extract electrons predominantly from P atoms on the ring rather than Cu sites. It emerges that as a result, the Cu-P interactions change and also exhibit partial Cu(3d) --> P donation, which causes the large distortion in geometry. We also find that the dication possesses a large degree of diradical character, forming a rare example of an observed species that is a singlet diradicaloid. This hypothesis is supported by our computational results as well as previously reported experimental features.

Periodic trends within a series of five-coordinate thiolate-ligated [MII(SMe2N4(tren))]+ (M = Mn, Fe, Co, Ni, Cu, Zn) complexes, including a rare example of a stable CuII-thiolate

A series of five-coordinate thiolate-ligated complexes [M(II)(tren)N4S(Me2)]+ (M = Mn, Fe, Co, Ni, Cu, Zn; tren = tris(2-aminoethyl)amine) are reported, and their structural, electronic, and magnetic properties are compared. Isolation of dimeric [Ni(II)(SN4(tren)-RS(dang))]2 ("dang"= dangling, uncoordinated thiolate supported by H bonds), using the less bulky [(tren)N4S](1-) ligand, pointed to the need for gem-dimethyls adjacent to the sulfur to sterically prevent dimerization. All of the gem-dimethyl derivatized complexes are monomeric and, with the exception of [Ni(II)(S(Me2)N4(tren)]+, are isostructural and adopt a tetragonally distorted trigonal bipyramidal geometry favored by ligand constraints. The nickel complex uniquely adopts an approximately ideal square pyramidal geometry and resembles the active site of Ni-superoxide dismutase (Ni-SOD). Even in coordinating solvents such as MeCN, only five-coordinate structures are observed. The MII-S thiolate bonds systematically decrease in length across the series (Mn-S > Fe-S > Co-S > Ni-S approximately Cu-S < Zn-S) with exceptions occurring upon the occupation of sigma* orbitals. The copper complex, [Cu(II)(S(Me2)N4(tren)]+, represents a rare example of a stable CuII-thiolate, and models the perturbed "green" copper site of nitrite reductase. In contrast to the intensely colored, low-spin Fe(III)-thiolates, the M(II)-thiolates described herein are colorless to moderately colored and high-spin (in cases where more than one spin-state is possible), reflecting the poorer energy match between the metal d- and sulfur orbitals upon reduction of the metal ion. As the d-orbitals drop in energy proceeding across the across the series M(2+) (M= Mn, Fe, Co, Ni, Cu), the sulfur-to-metal charge-transfer transition moves into the visible region, and the redox potentials cathodically shift. The reduced M(+1) oxidation state is only accessible with copper, and the more oxidized M(+4) oxidation state is only accessible for manganese.

S K-edge XAS and DFT calculations on square-planar NiII-thiolate complexes: effects of active and passive H-bonding

S K-edge XAS for a low-spin NiII-thiolate complex shows a 0.2 eV shift to higher pre-edge energy but no change in Ni-S bond covalency upon H-bonding. This is different from the H-bonding effect we observed in high-spin FeIII-thiolate complexes where there is a significant decrease in Fe-S bond covalency but no change in energy due to H-bonding (Dey, A.; Okamura, T.-A.; Ueyama, N.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 2005, 127, 12046-12053). These differences were analyzed using DFT calculations, and the results indicate that two different types of H-bonding interactions are possible in metal-thiolate systems. In the high-spin FeIII-thiolate case, the H-bonding involves a thiolate donor orbital which is also involved in bonding with the metal (active), while in the low-spin NiII-thiolate, the orbital involved in H-bonding is nonbonding with respect to the M-S bonding (passive). The contributions of active and passive H-bonds to the reduction potential and Lewis acid properties of a metal center are evaluated.

Copper-biomolecule complexes in the gas phase. The ternary way

This review deals with copper complexes of a variety of organic and bioorganic molecules that have been produced as gas-phase ions by electrospray and other ionization methods and studied experimentally by mass spectrometry and theoretically by ab initio and density functional theory computations. Ternary complexes of Cu((II)) allow one to modify the oxidation state and coordination sphere of the copper ion and thus induce novel fragmentations that involve redox and radical-based reactions. Structure elucidation, distinction, and quantitation of leucine and isoleucine isomers in peptides, distinction of enantiomers in chiral compounds, and sensitive detection of antibiotics are some of the highlights of mass spectrometry of ternary copper complexes. Binary copper complexes are mainly represented by Cu((I)) species in which the copper ion displays the properties of a weak Lewis acid.

Monocopper center embedded in a biomimetic cavity: from supramolecular control of copper coordination to redox regulation

The electrochemical behavior of diversely substituted Cu-N3-calix[6]arene, enzyme-like, "funnel" complexes is analyzed. The Cu(II)/Cu(I) redox process is regulated by the supramolecular organization of the Cu coordination. The presence of a "shoetree" alkyl nitrile guest molecule inside the host cavity is a prerequisite for a dynamic redox behavior. Combination of supramolecular CH-pi weak interactions with the calixarene cavity and electronic/steric effects from the N3 substituting groups (pyridine, imidazole, pyrrolidine) enforces the preferential geometrical pattern adopted by Cu. This dictates the pathway of the electron-transfer process and, thus, the thermodynamics and kinetics of the redox reaction in the framework of a square-scheme mechanism. The present observations recall strongly the redox control exerted by the protein matrix on copper proteins through biological concepts such as induced fit mechanism, protein foldings, and entatic and allosteric effects.

Copper(I) Complex O2-Reactivity with a N3S Thioether Ligand: a Copper−Dioxygen Adduct Including Sulfur Ligation, Ligand Oxygenation, and Comparisons with All Nitrogen Ligand Analogues

In order to contribute to an understanding of the effects of thioether sulfur ligation in copper-O(2) reactivity, the tetradentate ligands L(N3S) (2-ethylthio-N,N-bis(pyridin-2-yl)methylethanamine) and L(N3S')(2-ethylthio-N,N-bis(pyridin-2-yl)ethylethanamine) have been synthesized. Corresponding copper(I) complexes, [CuI(L(N3S))]ClO(4) (1-ClO(4)), [CuI(L(N3S))]B(C(6)F(5))(4) (1-B(C(6)F(5))(4)), and [CuI(L(N3S'))]ClO(4) (2), were generated, and their redox properties, CO binding, and O(2)-reactivity were compared to the situation with analogous compounds having all nitrogen donor ligands, [CuI(TMPA)(MeCN)](+) and [Cu(I)(PMAP)](+) (TMPA = tris(2-pyridylmethyl)amine; PMAP = bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine). X-ray structures of 1-B(C(6)F(5))(4), a dimer, and copper(II) complex [Cu(II)(L(N3S))(MeOH)](ClO(4))(2) (3) were obtained; the latter possesses axial thioether coordination. At low temperature in CH(2)Cl(2), acetone, or 2-methyltetrahydrofuran (MeTHF), 1 reacts with O(2) and generates an adduct formulated as an end-on peroxodicopper(II) complex [{Cu(II)(L(N3S))}(2)(mu-1,2-O(2)(2-))](2+) (4)){lambda(max) = 530 (epsilon approximately 9200 M(-1) cm(-1)) and 605 nm (epsilon approximately 11,800 M(-1) cm(-1))}; the number and relative intensity of LMCT UV-vis bands vary from those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {lambda(max) = 524 nm (epsilon = 11,300 M(-1) cm(-1)) and 615 nm (epsilon = 5800 M(-1) cm(-1))} and are ascribed to electronic structure variation due to coordination geometry changes with the L(N3S) ligand. Resonance Raman spectroscopy confirms the end-on peroxo-formulation {nu(O-O) = 817 cm(-1) (16-18O(2) Delta = 46 cm(-1)) and nu(Cu-O) = 545 cm(-1) (16-18O(2) Delta = 26 cm(-1)); these values are lower in energy than those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {nu(Cu-O) = 561 cm(-1) and nu(O-O) = 827 cm(-1)} and can be attributed to less electron density donation from the peroxide pi* orbitals to the Cu(II) ion. Complex 4 is the first copper-dioxygen adduct with thioether ligation; direct evidence comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 Angstrom}. Following a [Cu(I)(L(N3S))](+)/O(2) reaction and warming, the L(N3S) thioether ligand is oxidized to the sulfoxide in a reaction modeling copper monooxygenase activity. By contrast, 2 is unreactive toward dioxygen probably due to its significantly increased Cu(II)/Cu(I) redox potential, an effect of ligand chelate ring size (in comparison to 1). Discussion of the relevance of the chemistry to copper enzyme O(2)-activation, and situations of biological stress involving methionine oxidation, is provided.

The long and short of it: the influence of N-carboxyethyl versusN-carboxymethyl pendant arms on in vitro and in vivo behavior of copper complexes of cross-bridged tetraamine macrocycles

A cross-bridged cyclam ligand bearing two N-carboxymethyl pendant arms (1) has been found to form a copper(II) complex that exhibits significantly improved biological behavior in recent research towards (64)Cu-based radiopharmaceuticals. Both the kinetic inertness and resistance to reduction of Cu-1 are believed to be relevant to its enhanced performance. To explore the influence of pendant arm length on these properties, new cross-bridged cyclam and cyclen ligands with longer N-carboxyethyl pendant arms, 2 and 4, and their respective copper(II) complexes have been synthesized. Both mono- as well as di-O-protonated forms of Cu-2 have also been isolated and structurally characterized. The spectral and structural properties of Cu-2 and Cu-4, their kinetic inertness in 5 M HCl, and electrochemical behavior have been obtained and compared to those of their N-carboxymethyl-armed homologs, Cu-1 and Cu-3. Only the cyclam-based Cu-1 and Cu-2 showed unusually high kinetic inertness towards acid decomplexation. While both of these complexes also exhibited quasi-reversible Cu(II)/Cu(I) reductions, Cu-2 is easier to reduce by a substantial margin of +400 mV, bringing it within the realm of physiological reductants. Similarly, of the cyclen-based complexes, Cu-4 is also easier to reduce than Cu-3 though both reductions are irreversible. Biodistribution studies of (64)Cu-labeled 2 and 4 were performed in Sprague Dawley rats. Despite comparable acid inertness to their shorter-armed congeners, both longer-armed ligand complexes have poorer bio-clearance properties. This inferior in vivo behavior may be a consequence of their higher reduction potentials.

Real-time observation of the photoinduced structural change of bis(2,9-dimethyl-1,10-phenanthroline)copper(I) by femtosecond fluorescence spectroscopy: a realistic potential curve of the Jahn-Teller distortion

In copper(I) complex [Cu(dmphen)(2)]+ (dmphen = 2,9-dimethyl-1,10-phenanthroline), a "flattening" structural change is induced with 1MLCT excitation, which is a prototype of the structural change accompanied with Cu(I)/Cu(II) conversion in copper complexes. Femtosecond and picosecond emission dynamics of this complex were investigated in solution at room temperature with optically allowed S(2) <-- S(0) photoexcitation. Time-resolved emission was measured in the whole visible region, and the lifetimes, intrinsic emission spectra, and radiative lifetimes of the transients were obtained by quantitative analysis. It was concluded that the initially populated S(2) state is relaxed with a time constant of 45 fs to generate the S1 state retaining the perpendicular structure, and the D(2d) --> D(2) structural change (the change of the dihedral angle between the two ligand planes) occurs in the S(1) state with a time constant of 660 fs. The intersystem crossing from the S(1) state to the T(1) state takes place after this structural distortion with a time constant of 7.4 ps. Importantly, the temporal spectral evolution relevant to the structural change clearly exhibited an isoemissive point around 675 nm. This manifests that there exists a shallow potential minimum at the perpendicular geometry on the S1 surface, and the S1 state stays undistorted for a finite period as long as 660 fs before the structural distortion. This situation is not expected for the structural change induced by the ordinary (pseudo-)Jahn-Teller effect, because the distortion should be induced by the spontaneous structural instability at the perpendicular structure. This result sheds new light on the present understanding on the structural change occurring in the metal complexes.

A definitive example of a geometric "entatic state" effect: electron-transfer kinetics for a copper(II/I) complex involving A quinquedentate macrocyclic trithiaether-bipyridine ligand

The quinquedentate macrocyclic ligand cyclo-6,6'-[1,9-(2,5,8-trithianonane)]-2,2'-bipyridine ([15]aneS3bpy = L), containing two pyridyl nitrogens and three thiaether sulfurs as donor atoms, has been synthesized and complexed with copper. The CuII/IL redox potential, the stabilities of the oxidized and reduced complex, and the oxidation and reduction electron-transfer kinetics of the complex reacting with a series of six counter reagents have been studied in acetonitrile at 25 degrees C, mu = 0.10 M (NaClO4). The Marcus cross relationship has been applied to the rate constants obtained for the reactions with each of the six counter reagents to permit the evaluation of the electron self-exchange rate constant, k11. The latter value has also been determined independently from NMR line-broadening experiments. The cumulative data are consistent with a value of k11 = 1 x 10(5) M(-1) s(-1), ranking this among the fastest-reacting CuII/I systems, on a par with the blue copper proteins known as cupredoxins. The resolved crystal structures show that the geometry of the CuIIL and CuIL complexes are nearly identical, both exhibiting a five-coordinate square pyramidal geometry with the central sulfur donor atom occupying the apical site. The most notable geometric difference is a puckering of an ethylene bridge between two sulfur donor atoms in the CuIL complex. Theoretical calculations suggest that the reorganizational energy is relatively small, with the transition-state geometry more closely approximating the geometry of the CuIIL ground state. The combination of a nearly constant geometry and a large self-exchange rate constant implies that this CuII/I redox system represents a true geometric "entatic state."

63Cu NMR spectroscopy of copper(I) complexes with various tridentate ligands: CO as a useful 63Cu NMR probe for sharpening 63Cu NMR signals and analyzing the electronic donor effect of a ligand

63Cu NMR spectroscopic studies of copper(I) complexes with various N-donor tridentate ligands are reported. As has been previously reported for most copper(I) complexes, 63Cu NMR signals, when acetonitrile is coordinated to copper(I) complexes of these tridentate ligands, are broad or undetectable. However, when CO is bound to tridentate copper(I) complexes, the 63Cu NMR signals become much sharper and show a large downfield shift compared to those for the corresponding acetonitrile complexes. Temperature dependence of 63Cu NMR signals for these copper(I) complexes show that a quadrupole relaxation process is much more significant to their 63Cu NMR line widths than a ligand exchange process. Therefore, an electronic effect of the copper bound CO makes the 63Cu NMR signal sharp and easily detected. The large downfield shift for the copper(I) carbonyl complex can be explained by a paramagnetic shielding effect induced by the copper bound CO, which amplifies small structural and electronic changes that occur around the copper ion to be easily detected in their 63Cu NMR shifts. This is evidenced by the correlation between the 63Cu NMR shifts for the copper(I) carbonyl complexes and their nu(C[triple bond]O) values. Furthermore, the 63Cu NMR shifts for copper(I) carbonyl complexes with imino-type tridentate ligands show a different correlation line with those for amino-type tridentate ligands. On the other hand, 13C NMR shifts for the copper bound 13CO for these copper(I) carbonyl complexes do not correlate with the nu(C[triple bond]O) values. The X-ray crystal structures of these copper(I) carbonyl complexes do not show any evidence of a significant structural change around the Cu-CO moiety. The findings herein indicate that CO complexation makes 63Cu NMR spectroscopy much more useful for Cu(I) chemistry.

Highly active copper-based catalyst for atom transfer radical polymerization

Atom transfer radical polymerization (ATRP) generally requires a catalyst/initiator molar ratio of 0.1 to 1 and catalyst/monomer molar ratio of 0.001 to 0.01 (i.e., catalyst concentration: 1000-10,000 ppm versus monomer). Herein, we report a new copper-based complex CuBr/N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) as a versatile and highly active catalyst for acrylic, methacrylic, and styrenic monomers. The catalyst mediated ATRP at a catalyst/initiator molar ratio of 0.005 and produced polymers with well-controlled molecular weights and low polydispersities. ATRP occurred even at a catalyst/initiator molar ratio as low as 0.001 with copper concentration in the produced polymers as low as 6-8 ppm (catalyst/monomer molar ratio = 10(-5)). The catalyst structures were studied by X-ray diffraction and NMR spectroscopy. The activator CuIBr/TPEN existed in solution as binuclear and mononuclear complexes in equilibrium but as a binuclear complex in its single crystals. The deactivator CuIIBr2/TPEN complex was mononuclear. High stability and appropriate KATRP (ATRP equilibrium constant) were found crucial for the catalyst working under high dilution or in coordinating solvents/monomers. This provides guidance for further design of highly active ATRP catalysts.

First Investigation at Elevated Pressures To Confirm the Exact Nature of the Gated Electron-Transfer Systems: Volume Profiles of the Gated Reduction Reaction and Nongated Reverse Oxidation Reaction Involving a [Cu(dmp)2(solvent)]2+/[Cu(dmp)2]+ Couple (dmp = 2,9-Dimethyl-1,10-phenanthroline)

Redox reactions involving the [Cu(dmp)2]2+/+ couple (dmp = 2,9-dimethyl-1,10-phenanthroline) in acetonitrile were examined at elevated pressures up to 200 MPa. Activation volumes were determined as -8.8 and -6.3 cm3 mol-1 for the reduction cross-reaction by [Co(bipy)3]2+ (bipy = 2,2'-bipyridine) and for the oxidation cross-reaction by [Ni(tacn)2]3+ (tacn = 1,4,7-triazacyclononane), respectively. The activation volume for the hypothetical gated mode of the self-exchange reaction estimated from the reduction cross-reaction was -13.9 cm3 mol-1, indicating extensive electrostrictive rearrangement of solvent molecules around the CuII complex during the change in the coordination geometry before the electron-transfer step. On the other hand, the activation volume for the self-exchange reaction estimated from the oxidation cross-reaction was -2.7 +/- 1.5 cm3 mol-1. Although this value was within the range that can be interpreted by the concept of the ordinary concerted process, from theoretical considerations it was concluded that the reverse (oxidation) cross-reaction of the gated reduction reaction of the [Cu(dmp)2(CH3CN)]2+/[Cu(dmp)2]+ couple proceeds through the product excited state while the direct self-exchange reaction between [Cu(dmp)2(CH3CN)]2+ and [Cu(dmp)2]+ proceeds through an ordinary concerted process.

The Copper(II) Adduct of the Unstructured Region of the Amyloidogenic Fragment Derived from the Human Prion Protein is Redox-Active at Physiological pH

Prion diseases are caused by the misfolding and aggregation of the prion protein (PrP). Herein we provide evidence that the CuII adduct of the unstructured amyloidogenic fragment of the human PrP (PrP(91-126)) is redox active under physiological conditions. We have identified that the relevant high-affinity CuII binding region of PrP(91-126) is contained between residues 106 and 114. Both [CuII(PrP(91-126))] and [CuII(PrP(106-114))] have CuII Kd values of approximately 90 microM. Furthermore, the smaller PrP fragment PrP(106-114) coordinates CuII producing an electronic absorption spectrum nearly identical with [CuII(PrP(91-126))] (lambda max approximately 610 nm (epsilon approximately 125 M-1 cm-1)) suggesting a similar coordination environment for CuII. Cu K-edge X-ray absorption spectroscopy (XAS) reveals a nearly identical CuN(N/O)2S coordination environment for these two metallopeptides (2N/O at approximately 1.97 A; 1S at approximately 2.30 A; 1 imidazole N at approximately 1.95 A). Both display quasireversible CuII/CuI redox couples at approximately -350 mV vs Ag/AgCl. ESI-MS indicates that both peptides will coordinate CuI. However, XAS indicates differential coordination environments between [CuI(PrP(91-126))] and [CuI(PrP(106-114))]. These data indicate that [CuI(PrP(91-126))] contains Cu in a four coordinate (N/O)2S2 environment with similar (N/O)-Cu bond distances (Cu-(N/O) r = 2.048(4) A), while [CuI(PrP(106-114))] contains Cu in a four coordinate (N/O)2S2 environment with differential (N/O)-Cu bond distances (Cu-(N/O) r1 = 2.057(6) A; r2 = 2.159(3) A). Despite the differential coordination environments both Cu-metallopeptides will catalytically reduce O2 to O2*- at comparable rates.

Comparative study of donor atom effects on the thermodynamic and electron-transfer kinetic properties of copper(ii/i) complexes with sexadentate macrocyclic ligands. [CuII/I([18]aneS4N2)] and [CuII/I([18]aneS4O2)]

Studies have been conducted on the copper complexes formed with two sexadentate macrocyclic ligands containing four thioether sulfur donor atoms plus either two nitrogen or two oxygen donor atoms on opposing sides of the ring. The resulting two ligands, L, designated as [18]aneS(4)N(2) and [18]aneS(4)O(2), respectively, represent homologues of the previously studied Cu(ii/i) system with a macrocycle having six sulfur donor atoms, [18]aneS(6). Crystal structures of [Cu(II)([18]aneS(4)O(2))](ClO(4))(2) and [Cu(I)([18]aneS(4)O(2))]ClO(4) have been determined in this work. Comparison of the structures of all three systems reveals that the oxidized complexes are six coordinate with two coordinate bonds undergoing rupture upon reduction. However, the geometric changes accompanying electron transfer appear to differ for the three systems. The stability constants and electrochemical properties of both of the heteromacrocyclic complexes have been determined in acetonitrile and the Cu(II/I)L electron-transfer kinetics have been studied in the same solvent using six different counter reagents for each system. The electron self-exchange rate constants have then been calculated using the Marcus cross relationship. The results are compared to other Cu(II/I)L systems in terms of the effect of ligand geometric changes upon the overall kinetic behavior.

Dioxygen Reactivity of a Copper(I) Complex with a N3S Thioether Chelate; Peroxo−Dicopper(II) Formation Including Sulfur-Ligation

Employing a tetradentate N3S(thioether) ligand, LN3S, dioxygen reactivity of a copper(I) complex, [(LN3S)CuI]+ (1) was examined. In CH2Cl2, acetone (at -80 degrees C), or 2-methyltetrahydrofuran (at -128 degrees C), 1 reacts with O2 producing the end-on bound peroxodicopper(II) complex [{(LN3S)CuII}2(mu-1,2-O2(2-))]2+ (2), the first reported copper-dioxygen adduct with sulfur (thioether) ligation. Its absorption spectrum contains an additional low-energy feature (but not a Cu-S CT band) compared to the previously well-characterized N4 ligand complex, [{(TMPA)CuII}2(mu-1,2-O2(2-))]2+ (3) (TMPA = tris(2-pyridylmethyl)amine). Resonance Raman spectroscopy confirms the peroxo formulation {nu(O-O) = 817 cm-1 (16-18O2 Delta = 46 cm-1) and nu(Cu-O) = 545 cm-1 (16-18O2 Delta = 26 cm-1), in close analogy to that known for 3 {nu(O-O) = 827 cm-1 and nu(Cu-O) = 561 cm-1}. Direct evidence for thioether ligation comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 A}.

Thioether Sulfur Oxygenation from O2 or H2O2 Reactivity of Copper Complexes with Tridentate N2Sthioether Ligands

To model thioether-copper coordination chemistry including oxidative reactivity, such as occurs in the copper monooxygenases peptidylglycine -hydroxylating monooxygenase (PHM) and dopamine beta-hydroxylase (DbetaH), we have synthesized new tridentate N2S ligands LSEP and LSBz [LSEP = methyl(2-phenethylsulfanylpropyl)(2-pyridin-2-ylethyl)amine; LSBz = (2-benzylsulfanylpropyl)methyl(2-pyridin-2-ylethyl)amine)]. Both copper(I) and copper(II) complexes have been prepared, and their respective O2 and H2O2 chemistry has been studied. Under mild conditions, oxygenation of [(LSEP)CuI]+ (1a) and [(LSBz)CuI]+ (2a) leads to ligand sulfoxidation, thus exhibiting copper monooxygenase activity. A copper(II) complex of this sulfoxide ligand product, [(LSOEP)CuII(CH3OH)(OClO3)2], has been structurally characterized, demonstrating Cu-Osulfoxide ligation. The X-ray structure of [(LSEP)CuII(H2O)(OClO3)]+ (1b) and its solution UV-visible spectral properties [S-CuII LMCT band at 365 nm (MeCN solvent); epsilon = 4285 M-1 cm-1] indicate the thioether sulfur atom is bound to the cupric ion in both the solid (CuII-S distance: 2.31 A) and solution states. Reaction of 1b with H2O2 leads to sulfonation via the sulfoxide; excess hydrogen peroxide gives mostly sulfone product. These results may provide some insight into recent reports concerning protein methionine oxidation, showing the potential importance of copper-mediated oxidation processes in certain biological settings.

Synthesis and spectroscopic characterization of copper(II) tetraazaiminooxime macrocyclic complexes--a tetragonal distortion analysis

Herein we describe the synthesis and spectroscopic (infrared and UV-vis) analysis of [Cu(II)(dohpn)(L)](n+) (dohpn=imineoximic tetraazamacrocyclic ligand 2,3,9,10-tetramethyl-1,4,8,11-tetraazaundecane-1,3,8,10-tetraen-11-ol-1-olate) and L=SCN(-), I(-), Cl(-) (n=0) and 4-aminopyridine (ampy), 4,4'-bipyridine (bipy), imidazole (im), 2-aminopyrazine (ampz) and water (n=1+). The following order of the Jahn-Teller stabilization energy (cm(-1)) was observed: I(-)(6452)<Cl(-)(6477)<NCS(-)(6494)<ampy(6562)<im(6614)<bipy(6685)<ampz(6739)<H(2)O(7813) and the energy of the crystal field transition was correlated with the tetragonal distortion in the structures.

Redox-Triggered Interconversion between Piperidine Chair Conformations in a Cu(I/II) Complex

A redox-induced six-membered ring chair-chair conformational interconversion in a copper-coordinated trans-piperidine tripodal ligand is demonstrated. Each group of the 1,2,3-substituted ring can potentially ligate the metal; two equatorial groups ligate the metal in the Cu(I) state leaving a disassociated, axial group. However, all three groups (two axial and one equatorial) ligate the metal in the Cu(II) state. Exciton-coupled circular dichroism (ECCD) and 2D NMR were used to characterize the structures.

An electrochiroptical molecular switch: mechanistic and kinetic studies

Previously, we reported the synthesis and preliminary characterization of Cu(I/II) complexes of N,N-bis(2-quinilylmethyl)-(l)-methionine (Zahn, S.; Canary, J. W. Science 2000, 288, 1404-7). The chemically oxidized and reduced forms of the complexes gave nearly mirror image circular dichroism (CD) spectra as a result of reorganization of the inner coordination sphere of the copper atom. The reorganization involved exchange of oxygen for sulfur in proceeding from the Cu(II) to Cu(I) oxidation state and corresponding ligand conformational changes required to accommodate this exchange. In this paper, we demonstrate that the complex can be triggered by electrochemical means. The electrochemical and stereochemical details of the redox-induced ligand reorganization were probed by independent synthesis of alternative chemical intermediates, CD spectroelectrochemistry, curve fitting of cyclic voltammograms, CD titration, and scanning electrochemical microscopy (SECM). A square-type mechanism was most consistent with the data: the Cu(II) complex is reduced followed by a ligand reorganization step to give the stable reduced form of the complex. The Cu(I) complex is then oxidized in a fast step followed by another ligand reorganization. A millisecond time scale rate was found by SECM for one of the key conformational conversion steps.

Effect of constrained donor atom orientations on the stabilities, complexation kinetics, redox potentials, and structures of macrocyclic polythiaether Complexes. Copper(II) complexes with cyclopentanediyl derivatives of [14]aneS4 in 80% methanol

The two ethylene bridges in the macrocyclic tetrathiaether 1,4,8,11-tetrathiacyclotetradecane ([14]aneS(4)) have been systematically replaced by cis- or trans-1,2-cyclopentane to generate a series of new ligands that exhibit differing preferences for the orientation of the sulfur donor atoms while maintaining constant inductive effects. The resulting five dicyclopentanediyl derivatives, along with two previously synthesized monocyclopentanediyl analogues, have been complexed with Cu(II), and their stability constants, formation and dissociation rate constants, and redox potentials have been determined in 80% methanol/20% water (by weight). The crystal structures of the Cu(II) complexes with the five dicyclopentanediyl-[14]aneS(4) diastereomers as well as the structures for a representative Cu(I) complex and one of the free ligands have also been determined. The properties of these complexes are compared to previous data obtained for the corresponding cyclohexanediyl derivatives in an attempt to shed additional light on the influence of sterically constraining substituents upon the properties of macrocyclic ligand complexes.