Harnessing the Flexibility of Peptidic Scaffolds to Control their Copper(II)‐Coordination Properties: A Potentiometric and Spectroscopic Study

Designing small peptides that are capable of binding Cu²⁺ ions mainly through the side‐chain functionalities is a hard task because the amide nitrogen atoms strongly compete for Cu²⁺ ion coordination. However, the design of such peptides is important for obtaining biomimetic small systems of metalloenyzmes as well as for the development of artificial systems. With this in mind, a cyclic decapeptide, C‐Asp, which contained three His residues and one Asp residue, and its linear derivative, O‐Asp, were synthesized. The C‐Asp peptide has two Pro? Gly β‐turn‐inducer units and, as a result of cyclization, and as shown by CD spectroscopy, its backbone is constrained into a more defined conformation than O‐Asp, which is linear and contains a single Pro? Gly unit. A detailed potentiometric, mass spectrometric, and spectroscopic study (UV/Vis, CD, and EPR spectroscopy) showed that at a 1:1 Cu²⁺/peptide ratio, both peptides formed a major [CuHL]²⁺ species in the pH range 5.0–7.5 (C‐Asp) and 5.5–7.0 (O‐Asp). The corrected stability constants of the protonated species (log K*_CuH(O?Asp)=9.28 and log K*_CuH(C?Asp)=10.79) indicate that the cyclic peptide binds Cu²⁺ ions with higher affinity. In addition, the calculated value of K_eff shows that this higher affinity for Cu²⁺ ions prevails at all pH values, not only for a 1:1 ratio but even for a 2:1 ratio. The spectroscopic data of both [CuHL]²⁺ species are consistent with the exclusive coordination of Cu²⁺ ions by the side‐chain functionalities of the three His residues and the Asp residue in a square‐planar or square‐pyramidal geometry. Nonetheless, although these data show that, upon metal coordination, both peptides adopt a similar fold, the larger conformational constraints that are present in the cyclic scaffold results in different behaviour for both [CuHL]²⁺ species. CD and NMR analysis revealed the formation of a more rigid structure and a slower Cu²⁺‐exchange rate for [CuH(C‐Asp)]²⁺ compared to [CuH(O‐Asp]²⁺. This detailed comparative study shows that cyclization has a remarkable effect on the Cu²⁺‐coordination properties of the C‐Asp peptide, which binds Cu²⁺ ions with higher affinity at all pH values, stabilizes the [CuHL]²⁺ species in a wider pH range, and has a slower Cu²⁺‐exchange rate compared to O‐Asp.     

Unraveling the Mechanism of Water Oxidation Catalyzed by Nonheme Iron Complexes

Density functional theory (DFT) is employed to: 1) propose a viable catalytic cycle consistent with our experimental results for the mechanism of chemically driven (Ce^IV) O₂ generation from water, mediated by nonheme iron complexes; and 2) to unravel the role of the ligand on the nonheme iron catalyst in the water oxidation reaction activity. To this end, the key features of the water oxidation catalytic cycle for the highly active complexes [Fe(OTf)₂(Pytacn)] (Pytacn: 1‐(2′‐pyridylmethyl)‐4,7‐dimethyl‐1,4,7‐triazacyclononane; OTf: CF₃SO₃^?) ( 1 ) and [Fe(OTf)₂(mep)] (mep: N,N′‐bis(2‐pyridylmethyl)‐N,N′‐dimethyl ethane‐1,2‐diamine) ( 2 ) as well as for the catalytically inactive [Fe(OTf)₂(tmc)] (tmc: N,N′,N′′,N′′′‐tetramethylcyclam) ( 3 ) and [Fe(NCCH₃)(^MePy₂CH‐tacn)](OTf)₂ (^MePy₂CH‐tacn: N‐(dipyridin‐2‐yl)methyl)‐N′,N′′‐dimethyl‐1,4,7‐triazacyclononane) ( 4 ) were analyzed. The DFT computed catalytic cycle establishes that the resting state under catalytic conditions is a [Fe^IV(O)(OH₂)(LN₄)]²⁺ species (in which LN₄=Pytacn or mep) and the rate‐determining step is the O?O bond‐formation event. This is nicely supported by the remarkable agreement between the experimental (ΔG^≠=17.6±1.6 kcal mol^?1) and theoretical (ΔG^≠=18.9 kcal mol^?1) activation parameters obtained for complex 1 . The O?O bond formation is performed by an iron(V) intermediate [Fe^V(O)(OH)(LN₄)]²⁺ containing a cis‐Fe^V(O)(OH) unit. Under catalytic conditions (Ce^IV, pH 0.8) the high oxidation state Fe^V is only thermodynamically accessible through a proton‐coupled electron‐transfer (PCET) process from the cis‐[Fe^IV(O)(OH₂)(LN₄)]²⁺ resting state. Formation of the [Fe^V(O)(LN₄)]³⁺ species is thermodynamically inaccessible for complexes 3 and 4 . Our results also show that the cis‐labile coordinative sites in iron complexes have a beneficial key role in the O?O bond‐formation process. This is due to the cis‐OH ligand in the cis‐Fe^V(O)(OH) intermediate that can act as internal base, accepting a proton concomitant to the O?O bond‐formation reaction. Interplay between redox potentials to achieve the high oxidation state (Fe^V?O) and the activation energy barrier for the following O?O bond formation appears to be feasible through manipulation of the coordination environment of the iron site. This control may have a crucial role in the future development of water oxidation catalysts based on iron.     

Physicochemical Characterization of Four Gadolinium(III) DTPA‐like Complexes

The analysis of ¹⁷O NMR transverse relaxation rates and EPR transverse electronic relaxation rates for aqueous solutions of the four DTPA‐like (DTPA = diethylenetriamine‐N,N,N′,N″,N″‐pentaacetic acid) complexes, [Gd(DTPA‐PY)(H₂O)]^? (DTPA‐PY = N′‐(2‐pyridylmethyl)), [Gd(DTPA‐HP)(H₂O)₂]^? (DTPA‐HP = N′‐(2‐hydroxypropyl)), [Gd(DTPA‐H1P)(H₂O)₂]^? (DTPA‐H1P = N′‐(2‐hydroxy‐1‐phenylethyl)) and [Gd(DTPA‐H2P)(H₂O)₂] (DTPA‐H2P = N′‐(2‐hydroxy‐2‐phenylethyl)), at various temperatures allows us to understand the water exchange dynamics of these four complexes. The water‐exchange lifetime (τM) parameters for [Gd(DTPA‐PY)(H₂O)]^?, [Gd(DTPA‐HP)(H₂O)₂]^?, [Gd(DTPA‐H1P)(H₂O)₂]^? and [Gd(DTPA‐H2P)(H₂O)₂] are of 585, 98, 163, and 69 ns, respectively. Compared with [Gd(DTPA)(H₂O)]^2? (τM = 303 ns), the τM value of [Gd(DTPA‐PY)(H₂O)]^? is slightly higher, but the other three complexes values are significantly lower than those of [Gd(DTPA)(H₂O)]^2?. This difference is explained by the fact that the gadolinium(III) complexes of DTPA‐HP, DTPA‐H1P, and DTPA‐H2P have two inner‐sphere waters. The ²H longitudinal relaxation rates of the labeled diamagnetic lanthanum complex allow the calculation of its rotational correlation time (τR). The τR values calculated for DTPA‐PY, DTPA‐HP, DTPA‐H1P, and DTPA‐H2P are of 127, 110, 142 and 147 ps, respectively. These four values are higher than the value of [La(DTPA)]^2? (τR = 103 ps), because the rotational correlation time is related to the magnitude of its molecular weight.     

Thermodynamic and Kinetic Study of Scandium(III) Complexes of DTPA and DOTA: A Step Toward Scandium Radiopharmaceuticals

Diethylenetriamine‐N,N,N′,N′′,N′′‐pentaacetic acid (DTPA) and 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA) scandium(III) complexes were investigated in the solution and solid state. Three ⁴⁵Sc NMR spectroscopic references suitable for aqueous solutions were suggested: 0.1 M Sc(ClO₄)₃ in 1 M aq. HClO₄ (δ_Sc=0.0 ppm), 0.1 M ScCl₃ in 1 M aq. HCl (δ_Sc=1.75 ppm) and 0.01 M [Sc(ox)₄]^5? (ox^2?=oxalato) in 1 M aq. K₂C₂O₄ (δ_Sc=8.31 ppm). In solution, [Sc(dtpa)]^2? complex (δ_Sc=83 ppm, ?ν=770 Hz) has a rather symmetric ligand field unlike highly unsymmetrical donor atom arrangement in [Sc(dota)]^? anion (δ_Sc=100 ppm, ?ν=4300 Hz). The solid‐state structure of K₈[Sc₂(ox)₇] ? 13 H₂O contains two [Sc(ox)₃]^3? units bridged by twice “side‐on” coordinated oxalate anion with Sc³⁺ ion in a dodecahedral O₈ arrangement. Structures of [Sc(dtpa)]^2? and [Sc(dota)]^? in [(Hguanidine)]₂[Sc(dtpa)] ? 3 H₂O and K[Sc(dota)][H₆dota]Cl₂ ? 4 H₂O, respectively, are analogous to those of trivalent lanthanide complexes with the same ligands. The [Sc(dota)]^? unit exhibits twisted square‐antiprismatic arrangement without an axial ligand (TSA′ isomer) and [Sc(dota)]^? and (H₆dota)²⁺ units are bridged by a K⁺ cation. A surprisingly high value of the last DOTA dissociation constant (pK_a=12.9) was determined by potentiometry and confirmed by using NMR spectroscopy. Stability constants of scandium(III) complexes (log K_ScL 27.43 and 30.79 for DTPA and DOTA, respectively) were determined from potentiometric and ⁴⁵Sc NMR spectroscopic data. Both complexes are fully formed even below pH 2. Complexation of DOTA with the Sc³⁺ ion is much faster than with trivalent lanthanides. Proton‐assisted decomplexation of the [Sc(dota)]^? complex (τ_1/2=45 h; 1 M aq. HCl, 25 °C) is much slower than that for [Ln(dota)]^? complexes. Therefore, DOTA and its derivatives seem to be very suitable ligands for scandium radioisotopes.     

Sterically‐Directed Consecutive and Size‐Selective Self‐Assembly of Palladium Diphosphane Complexes with an Ar‐BIAN Ligand: Unexpected Formation of Pentameric and Hexameric Aggregates

The coordination properties of N,N′‐bis[4‐(4‐pyridyl)phenyl]acenaphthenequinonediimine (L¹) and N,N′‐bis[4‐(2‐pyridyl)phenyl]acenaphthenequinonediimine (L²) were investigated in self‐assembly with palladium diphosphane complexes [Pd(P^P)(H₂O)₂](OTf)₂ (OTf=triflate) by using various analytical techniques, including multinuclear (¹H, ¹⁵N, and ³¹P) NMR spectroscopy and mass spectrometry (P^P=dppp, dppf, dppe; dppp=bis(diphenylphosphanyl)propane, dppf= bis(diphenylphosphanyl)ferrocene, and dppe=bis(diphenylphosphanyl)ethane). Beside the expected trimeric and tetrameric species, the interaction of an equimolar mixture of [Pd(dppp)]²⁺ ions and L¹ also generates pentameric aggregates. Due to the E/Z isomerism of L¹, a dimeric product was also observed. In all of these species, which correspond to the general formula [Pd(dppp)L¹]_n(OTf)_2n (n=2–5), the L¹ ligand is coordinated to the Pd center only through the terminal pyridyl groups. Introduction of a second equivalent of the [Pd(dppp)]²⁺ tecton results in coordination to the internal, sterically more encumbered chelating site and induces enhancement of the higher nuclearity components. The presence of higher‐order aggregates (n=5, 6), which were unexpected for the interaction of cis‐protected palladium corners with linear ditopic bridging ligands, has been demonstrated both by mass‐spectrometric and DOSY NMR spectroscopic analysis. The sequential coordination of the [Pd(dppp)]²⁺ ion is attributed to the dissimilar steric properties of the two coordination sites. In the self‐assembled species formed in a 1:1:1 mixture of [Pd(dppp)]²⁺/[Pd(dppe)]²⁺/L¹, the sterically more demanding [Pd(dppp)]²⁺ tectons are attached selectively to the pyridyl groups, whereas the more hindered imino nitrogen atoms coordinate the less bulky dppe complexes, thus resulting in a sterically directed, size‐selective sorting of the metal tectons. The propensity of the new ligands to incorporate hydrogen‐bonded solvent molecules at the chelating site was confirmed by X‐ray diffraction studies.     

Characterization of the FeV=O Complex in the Pathway of Water Oxidation

Hypervalent Fe^V=O species are implicated in a multitude of oxidative reactions of organic substrates, as well as in catalytic water oxidation, a reaction crucial for artificial photosynthesis. Spectroscopically characterized Fe^V species are exceedingly rare and, so far, were produced by the oxidation of Fe complexes with peroxy acids or H₂O₂: reactions that entail breaking of the O?O bond to form a Fe^V=O fragment. The key Fe^V=O species proposed to initiate the O?O bond formation in water oxidation reactions remained undetected, presumably due to their high reactivity. Here, we achieved freeze quench trapping of six coordinated [Fe^V=O,(OH)(Pytacn)]²⁺ (Pytacn=1‐(2′‐pyridylmethyl)‐4,7‐dimethyl‐1,4,7‐triazacyclononane) ( 2 ) generated during catalytic water oxidation. X‐ray absorption spectroscopy (XAS) confirmed the Fe^V oxidation state and the presence of a Fe^V=O bond at ≈1.60 Å. Combined EPR and DFT methods indicate that 2 contains a S=3/2 Fe^V center. 2 is the first spectroscopically characterized high spin oxo‐Fe^V complex and constitutes a paradigmatic example of the Fe^V=O(OH) species proposed to be responsible for catalytic water oxidation reactions.     

Complexation Properties of N,N′,N″,N′′′‐[1,4,7,10‐Tetraazacyclododecane‐1,4,7,10‐tetrayltetrakis(1‐oxoethane‐2,1‐diyl)]tetrakis[glycine] (H4dotagl). Equilibrium,Kinetic, and Relaxation Behavior of the Lanthanide(III) Complexes

Eu³⁺, Dy³⁺, and Yb³⁺ complexes of the dota‐derived tetramide N,N′,N″,N′′′‐[1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetrayltetrakis(1‐oxoethane‐2,1‐diyl)]tetrakis[glycine] (H₄dotagl) are potential CEST contrast agents in MRI. In the [Ln(dotagl)] complexes, the Ln³⁺ ion is in the cage formed by the four ring N‐atoms and the amide O‐atom donor atoms, and a H₂O molecule occupies the ninth coordination site. The stability constants of the [Ln(dotagl)] complexes are ca. 10 orders of magnitude lower than those of the [Ln(dota)] analogues (H₄dota=1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid). The free carboxylate groups in [Ln(dotagl)] are protonated in the pH range 1–5, resulting in mono‐, di‐, tri‐, and tetraprotonated species. Complexes with divalent metals (Mg²⁺, Ca²⁺, and Cu²⁺) are also of relatively low stability. At pH>8, Cu²⁺ forms a hydroxo complex; however, the amide H‐atom(s) does not dissociate due to the absence of anchor N‐atom(s), which is the result of the rigid structure of the ring. The relaxivities of [Gd(dotagl)] decrease from 10 to 25°, then increase between 30–50°. This unusual trend is interpreted with the low H₂O‐exchange rate. The [Ln(dotagl)] complexes form slowly, via the equilibrium formation of a monoprotonated intermediate, which deprotonates and rearranges to the product in a slow, OH^?‐catalyzed reaction. The formation rates are lower than those for the corresponding Ln(dota) complexes. The dissociation rate of [Eu(dotagl)] is directly proportional to [H⁺] (0.1–1.0M HClO₄); the proton‐assisted dissociation rate is lower for [Eu(H₄dotagl)] (k₁=8.1?10^?6 M ^?1 s^?1) than for [Eu(dota)] (k₁=1.4?10^?5 M ^?1 s^?1).     

Intramolecular Gas‐Phase Reactions of Synthetic Nonheme Oxoiron(IV) Ions: Proximity and Spin‐State Reactivity Rules

The intramolecular gas‐phase reactivity of four oxoiron(IV) complexes supported by tetradentate N₄ ligands ( L ) has been studied by means of tandem mass spectrometry measurements in which the gas‐phase ions [Fe^IV(O)( L )(OTf)]⁺ (OTf=trifluoromethanesulfonate) and [Fe^IV(O)( L )]²⁺ were isolated and then allowed to fragment by collision‐induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane (tmc) and N,N′‐bis(2‐pyridylmethyl)‐1,5‐diazacyclooctane ( L ⁸Py₂) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N′‐bis(2‐pyridylmethyl)‐1,2‐diaminoethane (bpmen) and N,N′‐bis(2‐pyridylmethyl)‐1,3‐diaminopropane (bpmpn) showed predominant oxidative N‐dealkylation for the hexacoordinate [Fe^IV(O)( L )(OTf)]⁺ cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe^IV(O)( L )]²⁺ cations. DFT calculations on [Fe^IV(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N‐dealkylation versus dehydrogenation of the diaminoethane linker for the hexa‐ and pentacoordinate ions, respectively, is dictated by the proximity of the target C? H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σ_CH–σ* overlap, all the reactions still proceed preferentially on the quintet (S=2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S=2 reactions for both hexa‐ and pentacoordinate complexes.     

Rational Electrostatic Design of Easy‐Axis Magnetic Anisotropy in a ZnII–DyIII–ZnII Single‐Molecule Magnet with a High Energy Barrier

Two novel trinuclear complexes [ZnCl(μ‐L)Ln(μ‐L)ClZn][ZnCl₃(CH₃OH)]?3 CH₃OH (Ln^III=Dy ( 1 ) and Er ( 2 )) have been prepared from the compartmental ligand N,N′‐dimethyl‐N,N′‐bis(2‐hydroxy‐3‐formyl‐5‐bromo‐benzyl)ethylenediamine (H₂L). X‐ray studies reveal that Ln^III ions are coordinated by two [ZnCl(L)]^? units through the phenoxo and aldehyde groups, giving rise to a LnO₈ coordination sphere with square‐antiprism geometry and strong easy‐axis anisotropy of the ground state. Ab initio CASSCF+RASSI calculations carried out on 1 confirm that the ground state is an almost pure M_J=±15/2 Kramers doublet with a marked axial anisotropy, the magnetic moment is roughly collinear with the shortest Dy?O distances. This orientation of the local magnetic moment of the Dy^III ion in 1 is adopted to reduce the electronic repulsion between the oblate electron shape of the M_J=±15/2 Kramers doublet and the phenoxo‐oxygen donor atoms involved in the shortest Dy?O bonds. CASSCF+RASSI calculations also show that the ground and first excited states of the Dy^III ion are separated by 129 cm^?1. As expected for this large energy gap, compound 1 exhibits, in a zero direct‐current field, thermally activated slow relaxation of the magnetization with a large U_eff=140 K. The isostructural Zn–Er–Zn species does not present significant SMM behavior as expected for the prolate electron‐density distribution of the Er^III ion leading to an easy‐plane anisotropy of the ground doublet state.     

Influence of Decreasing Solvent Polarity (1,4‐Dioxane/Water Mixtures) on the Acid–Base and Copper(II)‐Binding Properties of Guanosine 5′‐Diphosphate

The acidity constants of twofold protonated guanosine 5′‐diphosphate, H₂(GDP)^?, and the stability constants of the [Cu(H;GDP)] and [Cu(GDP)]^? complexes were determined in H₂O as well as in 30 or 50% (v/v) 1,4‐dioxane/H₂O by potentiometric pH titrations (25°; I=0.1M , NaNO₃). The results showed that in H₂O one of the two protons of H₂(GDP)^? is located mainly at the N(7) site and the other one at the terminal β‐phosphate group. In contrast, for 50% 1,4‐dioxane/H₂O solutions, a micro acidity‐constant evaluation evidenced that ca. 75% of the H₂(GDP)^? species have both protons phosphate‐bound, because the basicity of pyridine‐type N sites decreases with decreasing solvent polarity whereas the one of phosphate groups increases. In the [Cu(H;GDP)] complex, the proton and the metal ion are in all three solvents overwhelmingly phosphate‐bound, and the release of this proton is inhibited by decreasing polarity of the solvent. Based on previously determined straight‐line plots of log K vs. pK (where R represents a non‐interacting residue in simple diphosphate monoesters ROP(O^?)(?O)? O? P(?O)(O^?)₂, R? DP^3?), which were now extended to mixed solvents (based on analogies), it is concluded that, in all three solvents, the [Cu(GDP)]^? complex is more stable than expected based on the basicity of the diphosphate residue. This increased stability is attributed to macrochelate formation of the phosphate‐coordinated Cu²⁺ with N(7) of the guanine residue. The formation degree of this macrochelate amounts in aqueous solution to ca. 75% (being thus higher than that of the Cu²⁺ complex of adenosine 5′‐diphosphate) and in 50% (v/v) 1,4‐dioxane/H₂O to ca. 60%, i.e., the formation degree of the macrochelate is only relatively little affected by the change in solvent, though it needs to be emphasized that the overall stability of the [Cu(GDP)]^? complex increases with decreasing solvent polarity. By including previously studied systems in the considerations, the biological implications are shortly discussed, and it is concluded that Nature has here a tool to alter the structure of complexes by shifting them on a protein surface from a polar to an apolar region and vice versa.     

Kinetics and mechanism of oxidation of a ternary complex involving dipicolinatochromium(III) and DL‐aspartic acid by N‐bromosuccinimide

The kinetics of oxidation of [Cr^III(Dpc)(Asp)(H₂O)₂] (Dpc = dipicolinic acid and Asp = DL ‐aspartic acid) by N‐bromosuccinimide (NBS) in aqueous solution have been found to obey the equation: where k₂ is the rate constant for the electron transfer process, K₁ is the equilibrium constant for deprotonation of [Cr^III(Dpc)(Asp)(H₂O)₂], K₂ and K₃ are the pre‐equilibrium formation constants of precursor complexes [Cr^III(Dpc)(Asp)(H₂O)(NBS)] and [Cr^III(Dpc)(Asp)(H₂O)(OH)(NBS)]^?. Values of k₂ = 4.85 × 10^?2 s^?1, K₁ = 1.85 × 10^?7 mol dm^?3, and K₂ = 78.2 mol^?1 dm³ have been obtained at 30°C and I = 0.1 mol dm^?3. The experimental rate law is consistent with a mechanism in which the deprotonated [Cr^III(Dpc)(Asp)(H₂O)(OH)]^? is considered to be the most reactive species compared to its conjugate acid. It is assumed that electron transfer takes place via an inner‐sphere mechanism. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 394–400, 2004     

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure,Redox Properties,and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru^V?O complex [Ru^V(N₄O)(O)]²⁺, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol^?1, respectively. The overall free energy activation barrier (ΔG^≠=25.5 kcal mol^?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru^III(N₄O)(HOCl)]²⁺ (ΔG=15.0 kcal mol^?1) and the substitution of HOCl by a water molecule (ΔG^≠=10.5 kcal mol^?1). For water oxidation, nucleophilic attack on Ru^V?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol^?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [Ru^V(N₄O)(O)]²⁺ leads to the [(N₄O)Ru^IV? O₂? Ru^III(N₄O)]⁴⁺ complex with a calculated free energy barrier of 12.0 kcal mol^?1.     

Negative ion fragmentations of deprotonated peptides. The unusual case of isoAsp: a joint experimental and theoretical study. Comparison with positive ion cleavages

The following peptides have been examined in this study: GLDFG(OH), caeridin 1.1 [GLLDGLLGLGGL(NH₂)], 11 Ala citropin 1.1 [GLFDVIKKVAAVIGGL(NH₂)], Crinia angiotensin [APGDRIYVHPF(OH)] and their isoAsp isomers. It is not possible to differentiate between Asp‐ and isoAsp‐containing peptides (used in this study) using negative ion electrospray mass spectrometry. This is because the isoAsp residue cleaves to give the same fragment anions as those formed by δ and γ backbone cleavage of Asp. The isoAsp fragmentations are as follows: RNHCH(CO₂H)^?CHCONHR′ → [RNH^?(HO₂CCH?CHCONHR′)] → RNH^?+HO₂CCH?CHCONHR′ and RNHCH(CO₂H)^?CHCONHR′ → [RNH^?(HO₂CCH?CHCONHR′] → ^?O₂CCH?CHCONHR′+RNH₂. Calculations at the HF/6‐31+G(d)//AM1 level of theory indicate that the first of these isoAsp cleavage processes is endothermic (by +115 kJ mol^?1), while the second is exothermic (?85 kJ mol^?1). The barrier to the highest transition state is 42 kJ mol^?1. No diagnostic cleavage cations were observed in the electrospray mass spectra of the MH⁺ ion of the Asp‐ and isoAsp‐containing peptides (used in this study) to allow differentiation between these two amino acid residues. Copyright © 2009 John Wiley & Sons, Ltd.     

Strongly Coupled Cyclometalated Ruthenium Triarylamine Chromophores as Sensitizers for DSSCs

A series of anchor‐functionalized cyclometalated bis(tridentate) ruthenium(II) triarylamine hybrids [Ru(dbp‐X)(tctpy)]^2? [ 2 a ]^2?–[ 2 c ]^2? (H₃tctpy=2,2′;6′,2′′‐terpyridine‐4,4′,4′′‐tricarboxylic acid; dpbH=1,3‐dipyridylbenzene; X=N(4‐C₆H₄OMe)₂ ([ 2 a ]^2?), NPh₂ ([ 2 b ]^2?), N‐carbazolyl [ 2 c ]^2?) was synthesized and characterized. All complexes show broad absorption bands in the range 300–700 nm with a maximum at about 545 nm. Methyl esters [Ru(Me₃tctpy)(dpb‐X)]⁺ [ 1 a ]⁺–[ 1 c ]⁺ are oxidized to the strongly coupled mixed‐valent species [ 1 a ]²⁺–[ 1 c ]²⁺ and the Ru^III(aminium) complexes [ 1 a ]³⁺–[ 1 c ]³⁺ at comparably low oxidation potentials. Theoretical calculations suggest an increasing spin delocalization between the metal center and the triarylamine unit in the order [ 1 a ]²⁺<[ 1 b ]²⁺<[ 1 c ]²⁺. Solar cells were prepared with the saponified complexes [ 2 a ]^2?–[ 2 c ]^2? and the reference dye N719 as sensitizers using the I₃^?/I^? couple and [Co(bpy)₃]^3+/2+ and [Co(ddpd)₂]^3+/2+ couples as [B(C₆F₅)₄]^? salts as electrolytes (bpy=2,2′‐bipyridine; ddpd=N,N′‐dimethyl‐N,N′‐dipyridin‐2‐yl‐pyridine‐2,6‐diamine). Cells with [ 2 c ]^2? and I₃^?/I^? electrolyte perform similarly to cells with N719 . In the presence of cobalt electrolytes, all efficiencies are reduced, yet under these conditions [ 2 c ]^2? outperforms N719 .     

Study of the Ternary Complex Formation Between Vanadium(III)-Picolinic Acid and the Amino Acids: Cysteine,Histidine, Aspartic and Glutamic Acids

The complex species formed between vanadium(III)-picolinic acid (HPic) and the amino acids: cysteine (H₂Cys), histidine (HHis), aspartic acid (H₂Asp) and glutamic acid (H₂Glu) were studied in aqueous solution by means of electromotive forces measurements emf(H) at 25 °C and 3.0 mol⋅dm⁻³ KCl as ionic medium. Data analysis using the least-squares program LETAGROP indicates the formation of ternary complexes, whose stoichiometric coefficients and stability constant were determined. In the vanadium(III)-picolinic acid-cysteine system the model obtained was: [V(Pic)(H₂Cys)]²⁺, [V(Pic)(HCys)]⁺, V(Pic)(Cys) and [V₂O(Pic)(Cys)]⁺. The vanadium(III)-picolinic acid-histidine system contained the following complexes: [V(Pic)(HHis)]²⁺, [V(Pic)(His)]⁺, V(Pic)(His)(OH) and [V(Pic)₂(HHis)]⁺. In the vanadium(III)-picolinic acid-aspartic acid system the model obtained was: V(Pic)(Asp), [V(Pic)(Asp)(OH)]⁻ and [V₂O(Pic)(Asp)]⁺ and finally, in the vanadium(III)-picolinic acid-glutamic acid system the complexes: V₂O(Pic)₂(HGlu)₂, V(Pic)(HGlu)₂ and V(Pic)₂(HGlu) were observed.     

The Electron Transfer Series [MoIII(bpy)3]n (n=3+, 2+, 1+, 0, 1−), and the Dinuclear Species [{MoIIICl(Mebpy)2}2(μ2‐O)]Cl2 and [{MoIV(tpy.)2}2(μ2‐MoO4)](PF6)2⋅4 MeCN

The electronic structures of the five members of the electron transfer series [Mo(bpy)₃]ⁿ (n=3+, 2+, 1+, 0, 1?) are determined through a combination of techniques: electro‐ and magnetochemistry, UV/Vis and EPR spectroscopies, and X‐ray crystallography. The mono‐ and dication are prepared and isolated as PF₆ salts for the first time. It is shown that all species contain a central Mo^III ion (4d³). The successive one‐electron reductions/oxidations within the series are all ligand‐based, involving neutral (bpy⁰), the π‐radical anion (bpy^.)^1?, and the diamagnetic dianion (bpy^2?)^2?: [Mo^III(bpy⁰)₃]³⁺ (S=3/2), [Mo^III(bpy^.)(bpy⁰)₂]²⁺ (S=1), [Mo^III(bpy^.)₂(bpy⁰)]¹⁺ (S=1/2), [Mo^III(bpy^.)₃] (S=0), and [Mo^III(bpy^.)₂(bpy^2?)]^1? (S=1/2). The previously described diamagnetic dication “[Mo^II(bpy⁰)₃](BF₄)₂” is proposed to be a diamagnetic dinuclear species [{Mo(bpy)₃}₂(μ₂‐O)](BF₄)₄. Two new polynuclear complexes are prepared and structurally characterized: [{Mo^IIICl(^Mebpy⁰)₂}₂(μ₂‐O)]Cl₂ and [{Mo^IV(tpy^.)₂}₂(μ₂‐Mo^VIO₄)](PF₆)₂?4 MeCN.     

1,3,5-Trideoxy-1,3,5-tris(dimethylamino)-cis-inositol,an Efficient Ligand for Hard Trivalent Metal Ions. Determination of the stability constants of the FeIII,AlIII, and CuII complexes in aqueous solution

The complexes [Fe(tdci)₂]Cl₃ and [Al(tdci)₂]Cl₃ (tdci = 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol) were prepared and characterized by mass spectrometry, NMR spectroscopy, and magnetic-susceptibility measurements. The formation constants were determined in aqueous solution (25°, 0.1M KCl) by potentiometric titration. pK values of H₃(tdci)³⁺: 5.89, 7.62, 9.68; Fe^III complexes: log β_ML = 18.8, log β = 32.6; Al^III complexes: log β_ML = 14.3, logβ = 26.4. The protonated complex [FeH(tdci)₂]⁴⁺ has also been identified. In contrast to the high stability of the Fe^III and Al^III complexes, only weak interactions of tdci with Cu^II have been observed in aqueous solution (25°, 0.1M KNO₃).     

Electrochemical Investigation of Potassium Heptacyanorhenate(III) in Aqueous Solution

The electrochemistry of potassium heptacyanorhenate(III) in aqueous solution was studied by cyclic and by rotating disk voltammetry at planar microelectrodes. The results are consistent with a single, reversible electron transfer: Re(CN)^3?₇ + e?Re(CN)^4?₇ with E′₀ = 643 mV vs. NHE. A single protonation equilibrium is observed: Re(CN)^4?₇ + H⁺? Re(CN)₇H^3? with pK = 1.31 determined from combined voltammetric and pH data. The Re–CN bond appears to be kinetically inert, and none of the cyano complexes in other oxidation states of Re claimed in the literature was found in the potential range ? 2 V to + 1 V.     

Kinetics of the reaction of nitric oxide with polypyridylamine iron(II) complexes

The reactions of three polypyridylamine ferrous complexes, [Fe(TPEN)]²⁺, [Fe(TPPN)]²⁺, and [Fe(TPTN)]²⁺, with nitric oxide (NO) (where TPEN = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine, TPPN = N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,2-propylenediamine, and TPTN = N,N,N′,N′-tetrakis(2-pyridylmethyl)trimethylenediamine) were investigated. The first two complexes, which are spin-crossover systems, presented second-order rate constants for complex formation reactions (k_f) of 8.4 × 10³ and 9.3 × 10³ M^?1 s^?1, respectively (pH 5.0, 25 °C, I = 0.1 M). In contrast, the [Fe(TPTN)]²⁺ complex, which is in low-spin ground state, did not show any detectable reaction with NO. k_f values are lower than those of high-spin Fe(II) complexes, such as [Fe(EDTA)]^2? (EDTA = ethylenediaminetetraacetate) and [Fe(H₂O)]²⁺, but higher than low-spin Fe(II) complexes, such as [Fe(CN)₅(H₂O)]^3? and [Fe(bipyridine)₃]²⁺. The release of NO from the [Fe(TPEN)NO]²⁺ and [Fe(TPPN)NO]²⁺ complexes were also studied, showing the values 15.6 and 17.7 s^?1, respectively, comparable to the high-spin aminocarboxylate analogs. A mechanism is proposed based on the spin-crossover behavior and the geometry of these complexes and is discussed in the context of previous publications.     

Chromatographic Characterization of Molecularly Imprinted Microspheres Synthesized by Aqueous Microsuspension Polymerization：Influences of pH,Kinds and Concentration of Buffer on Capacity Factors

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