A unique coordination environment for an ion: EXAFS studies and bond valence model approach of the encapsulated cation in the Preyssler anion

X-Ray absorption spectroscopy was used to probe the coordination of different encrypted cations in the Preyssler anions [M(n+)P5W(30)O(110)]((15-n)-)(M(n+)= Sr2+, Am3+, Eu3+, Sm3+, Y3+, Th4+, U4+ in decreasing order of ionic radius, IR), hereafter abbreviated [M(n+)PA](15-n)-. The increase of the M-W distance and the decrease of the M-P distance with increasing M ionic radius reveal that the M cation is displaced along the C5 axis within the Preyssler cavity. The slight change (0.07 A) of the M-O distance that does not correspond to the IR difference of 0.27 A confirms that the cavity retains its rigidity upon cation substitution. Geometric modeling of the encapsulated cation in the channel was performed for comparison to the EXAFS results. The position of the cation in the cavity was calculated as well as the M-O10, -W5 and -P5 distances. This modeling confirms the cation displacement toward the center of the Preyssler anion as the cation size increases, which is understood in terms of the non-homogenous electrostatic potential present within the cavity. The bond valence model approach was applied to obtain experimental bond valences. Only the bond valence sum (BVS) of Am3+ is close to its actual charge. Sums smaller than the actual valences of the +3 and +4 ions (2.39-2.63 for +3 cations, Y, Sm, Eu; 3.17 and 3.38 for +4 cations, U and Th, respectively) were obtained, and a larger sum (2.89) was obtained for Sr2+. The deviations from the formal M sums of the encapsulated ions are attributed to the rigidity of the Preyssler framework. The tendency toward coordinative unsaturation for electroactive cations, such as Eu3+, is thought to be the driving force for facile reduction. Unlike other inorganic chelating ligands, the Preyssler anion provides a unique redox system to stabilize an electroactive cation in a low oxidation state.     

On the origin of alternating bond distortions and the emergence of chirality in polyoxometalate anions

Alternating short and long bond length (ABL) distortions observed within the ring structures of molecular metal oxide anions or polyoxometalates (POMs) are reminiscent of the cooperative linear ABL distortions in perovskite d(0) metal oxides. We show herein that these distortions have a common origin: a pseudo Jahn-Teller (PJT) vibronic instability. Four POM structural types with different M(n)O(n) ring sizes are investigated herein using density functional theoretical methods: Lindqvist [M6O19](q-) (n = 4), Keggin alpha-[XM12O40](q-) (n = 6), Wells-Dawson alpha-[X2M18O62](q-) (n = 8), and Preyssler [(Na)P5W30O110](14-) (n = 10), where M = Mo(VI) and W(VI) and X = Si(IV), Ge(IV), P(V), As(V), S(VI), and Se(VI). Chirality is induced within the latter three structural types by the ABL ring distortions. The calculations confirm the PJT vibronic origin of the ABL distortions with good agreement between calculated geometries and published single-crystal X-ray diffraction data. Both theory and experiment show that the vibronic interaction and distortion magnitude increase for (1) molybdates relative to that of tungstates, (2) larger M(n)O(n) ring sizes, (3) increases in negative charge of the internalized fragments (O(2-) or XO4(q-)), and (4) d(0) versus d(n) metal oxidation states. The PJT vibronic coupling model explains these observations in terms of the energy gap between Kohn-Sham frontier molecular orbitals (MOs) concomitant with the propensity for metal-oxygen pi-bonding within the M(n)O(n) rings. The frontier MOs for the undistorted nuclear configurations are largely nonbonding pi-O(p) (occupied) and pi-M(d) (unoccupied) in character, where smaller HOMO-LUMO (H-L) gap energies lead to greater metal-oxygen pi-orbital mixing under the influence of the nuclear distortion. A reduction in pi-bond order decreases the distortion in mixed-valence POMs. Of the tungstates examined, only the Preyssler anion shows pronounced ABL ring distortions, which derive from its large ring size and concomitant small H-L gap.     

On the distributions of ion/neutral molecule clusters in electrospray and laser spray—A cluster division model for the electrospray process

The clustering of a medium-sized, involatile, neutral molecule, octyl beta-D-glucopyranoside (OG), with Na(+), Ca(2+), and Yb(3+) (M(z+)) ions in electrospray (ESI) was investigated using laser spray (LSI). Extensive distributions of [(M(z+))(i) (OG)(a)](n+)-clusters, extending beyond 50 kDa, were observed. The distributions were highly stable and reproducible and changed only marginally when concentrations of electrolyte or neutral compound were varied by orders of magnitude. Compared with ESI, laser spray yielded superior intensities, particularly of the larger clusters. The cluster distributions demonstrated a range of remarkable features. In particular, the Yb(3+)/OG cluster distribution was unusual. For example, no clusters with 35-52 or with 110-116 OG molecules were observed. The distribution pattern revealed that the clusters were formed as a result of cluster dissociations, such as [(Yb(3+))(3)(OG) ( approximately 110)W](9+) --> [(Yb(3+))(2)(OG)( approximately 90)W](6+) + [(Yb(3+))(1)(OG) ( approximately 20)W](3+), where W represents the water content at the time of dissociation. Based on this study, a cluster division model for electrospray of aqueous solutions of strongly solvated ions is proposed: the Rayleigh droplet disintegration process, which is well-established for the initial stages of electrospray, maintains its general character as it proceeds through a final regime of multiply charged cluster dissociations to the singly and multiply charged ions in mass spectrometry. In the dissociation of multiply charged clusters, the size of each daughter cluster is roughly proportional to the square of the cluster charge. Observed cluster distributions are consistent with a mixture of symmetric and asymmetric cluster dissociations.     

Encapsulation of cyanometalates by a tris-macrocyclic ligand tricopper(II) complex: syntheses, structural variation, and magnetic exchange coupling pathways

The reaction of [M(CN)(6)](3-) (M = Cr(3+), Mn(3+), Fe(3+), Co(3+)) and [M(CN)(8)](4-/3-) (M = Mo(4+/5+), W(4+/5+)) with the trinuclear copper(II) complex of 1,3,5-triazine-2,4,6-triyltris[3-(1,3,5,8,12-pentaazacyclotetradecane)] ([Cu(3)(L)](6+)) leads to partially encapsulated cyanometalates. With hexacyanometalate(III) complexes, [Cu(3)(L)](6+) forms the isostructural host-guest complexes [[[Cu(3)(L)(OH(2))(2)][M(CN)(6)](2)][M(CN)(6)]][M(CN)(6)]30 H(2)O with one bridging, two partially encapsulated, and one isolated [M(CN)(6)](3-) unit. The octacyanometalates of Mo(4+/5+) and W(4+/5+) are encapsulated by two tris-macrocyclic host units. Due to the stability of the +IV oxidation state of Mo and W, only assemblies with [M(CN)(8)](4-) were obtained. The Mo(4+) and W(4+) complexes were crystallized in two different structural forms: [[Cu(3)(L)(OH(2))](2)[Mo(CN)(8)]](NO(3))(8)15 H(2)O with a structural motif that involves isolated spherical [[Cu(3)(L)(OH(2))](2)[M(CN)(8)]](8+) ions and a "string-of-pearls" type of structure [[[Cu(3)(L)](2)[M(CN)(8)]][M(CN)(8)]](NO(3))(4) 20 H(2)O, with [M(CN)(8)](4-) ions that bridge the encapsulated octacyanometalates in a two-dimensional network. The magnetic exchange coupling between the various paramagnetic centers is characterized by temperature-dependent magnetic susceptibility and field-dependent magnetization data. Exchange between the CuCu pairs in the [Cu(3)(L)](6+) "ligand" is weakly antiferromagnetic. Ferromagnetic interactions are observed in the cyanometalate assemblies with Cr(3+), exchange coupling of Mn(3+) and Fe(3+) is very small, and the octacoordinate Mo(4+) and W(4+) systems have a closed-shell ground state.     

Oxalate-bridged dinuclear M2 units: dimers of dimers, cyclotetramers, and extended sheets (m = Mo, W, Tc, Ru, and Rh)

The electronic structures of D(4h)-M(2)(O(2)CH)(4) and the oxalate-bridged complexes D(2h)-[(HCO(2))(3)M(2)](2)(mu-O(2)CCO(2)) and D(4h)-[(HCO(2))(2)M(2)](4)(mu-O(2)CCO(2))(4) have been investigated by a symmetry analysis of their MM and oxalate-based frontier orbitals, as well as by electronic structure calculations on the model formate complexes (M = Mo and W {d(4)-d(4)}, Tc, Ru {d(6)-d(6)}, and Rh {d(7)-d(7)}). Significant changes in the ordering, interactions, and electronic occupation of the molecular orbitals (MOs) arise through both the progression from d(4) to d(7) metals and the change from second to third row transition metals. For M = Mo and W, the highest-occupied orbitals are delta based, while the lowest-unoccupied orbitals are oxalate pi based; for M = Tc, the highest-occupied orbitals are an energetically tight delta-based set of MOs, while the lowest-unoccupied orbitals are MM-based pi. For both Ru and Rh, the highest-occupied MOs are the MM pi* and delta*, respectively, while the lowest-unoccupied MOs, in both instances, are MM-based sigma. With the exception of M = Ru, all of the complexes are closed shell. From the progression M(2) --> [M(2)](2) --> [M(2)](4), we can envision the nature of bandlike structures for a 2-dimensional square grid of formula [M(2)(mu-O(2)CCO(2))](infinity). Only for Mo and W oxalates should good electronic communication between MM centers generate a band of significant width to lead to metallic conductivity upon oxidation.     

Electronic structure of 3d[M(H2O)6](3+) ions from Sc(III) to Fe(III): a quantum mechanical study based on DFT computations and natural bond orbital analyses

The metal-donor atom bonding along the series of 3d[M(H2O)6](3+) ions from Sc(3+) to Fe(3+) has been investigated by density-functional calculations combined with natural localized bond orbital analyses. The M-OH(2) bonds were considered as donor-acceptor bonds, and the contributions coming from the metal ion's 3d sigma-, 3d pi-, and 4s sigma-interactions were treated individually. In this way, the total amount of charge transferred from the water oxygen-donor atoms toward the appropriate metal orbitals could be analyzed in a straightforward manner. One result obtained along these lines is that the overall extent of ligand-to-metal charge transfer shows a strong correlation to the hydration enthalpies of the aqua metal ions. If the contributions to the total ligand-to-metal ion charge transfer are divided into sigma- and pi-contributions, it turns out that Cr(3+) is the best sigma-acceptor, but its pi-accepting abilities are the weakest along the series. Fe(3+) is found to be the best pi-acceptor among the 3d hexaaqua ions studied. Its aptitude to accept sigma-electron density is the second weakest along the series and only slightly higher than that of Sc(3+) (the least sigma-acceptor of all ions) because of the larger involvement of the Fe(3+) 4s orbital in sigma-bonding. The strengths of the three types of bonding interactions have been correlated with the electron affinities of the different metal orbitals. Deviations from the regular trends of electron affinities along the series were found for those [M(H2O)6](3+) ions that are subject to Jahn-Teller distortions. In these cases (d(1) = [Ti(H2O)6](3+), d(2) = [V(H2O)6](3+), and d(4) = [Mn(H2O)6](3+)), ligand-to-metal charge transfer is prevented to go into those metal orbitals that contain unpaired d electrons. A lowering of the complex symmetry is observed and coupled with the following variations: The Ti(3+)- and V(3+)-hexaaqua ions switch from T(h)() to C(i)() symmetry while the Mn(3+)-hexaaqua ion moves to D(2)(h)() symmetry. The loss of orbital overlap leading to a diminished ligand-to-metal charge transfer toward the single occupied metal orbitals is compensated by amplified bonding interactions of the ligand orbitals with the unoccupied metal orbitals to some extent.     

Correlated electrons in the Eu-exchanged Preyssler anion [EuP(5)W(30)O(110)](n-)

The Eu-encrypted Preyssler anion, [EuP5W30O110]n-, demonstrates unusual electronic properties as a function of applied potenial. The Preyssler anion itself, when exchanged with a nonredox active trivalent ion such as La3+, is electroactive in acidic solution, reversibly accepting up to 10 electrons under reducing conditions. Encrypted Eu, which is trivalent at rest potential, is reduced concomitantly with the framework. The details of this reduction are probed with Eu L3-XANES, coulometry, and magnetic susceptibility. The results are interpreted in terms of a bonding interaction between the Eu f-states and the W-O framework band, facilitated by the crossing of the framework Fermi level with the localized states. Such behavior has been previously observed in intermediate-valent and correlated-electron systems. The observation of such behavior in an isolated cluster opens the door for new avenues of research into correlated-electron behavior.     

New ionic crystals of oppositely charged cluster ions and their characterization

By reacting Keggin-type polyoxometalate cluster anions H(2)W(12)O(40)(6)(-) (metatungstate) or Co(II)W(12)O(40)(6)(-) (tungstocobaltate) with the large aluminum cluster polycation [Al(30)O(8)(OH)(56)(H(2)O)(26)](18+), Keggin ion based molecular ionic compounds [delta-Al(13)O(4)(OH)(24)(H(2)O)(12)][XW(12)O(40)](OH).nH(2)O (X = H(2) (1) and Co (2); n congruent with 20) and [W(2)Al(28)O(18)(OH)(48)(H(2)O)(24)][H(2)W(12)O(40)](2).55H(2)O (3) were obtained. The polygon-shaped cluster ions are packed alternately through intercluster hydrogen bonds as well as electrostatic interactions, leaving large pores, which result from the packing of large clusters. The clusters are arranged in square pyramidal geometries, showing face-to-face interactions between them. The isolation of metastable [delta-Al(13)O(4)(OH)(24)(H(2)O)(12)](7+) and the formation of a new transition metal substituted aluminum heteropolycation [W(2)Al(28)O(18)(OH)(48)(H(2)O)(24)](12+) in 1-3 result from the slow fragmentation and recombination of Al(30) in the presence of suitable counter cluster anions with similar shape and charge.     

六聚同多酸阴离子M_6O_(19)~(n-)(M=Mo,W,n=2;M=Nb,Ta,n=8)电子结构和化学性质的密度泛函理论研究

采用B3LYP方法在LanL2DZ水平上计算了六聚同多阴离子 (M6On-19,M =Mo和W ,n =2 ;M =Nb和Ta ,n =8)的电子结构 ,分析了它们的前线轨道、分子静电势 (MEP) .计算结果表明 ,Nb6O8-19和Ta6O8-19是电子给体 ,而Mo6O2 -19和W6O2 -19则是电子受体 ,这与它们在溶液中具有不同的化学性质是一致的     

Electronic properties of polyoxometalates: electron and proton affinity of mixed-addenda Keggin and Wells-Dawson anions

A series of systematic DFT calculations were conducted on Keggin [SiW(9)M(3)O(40)](n-), M = Mo, V, and Nb; and Wells-Dawson anions [P(2)M(18)O(62)],(6-) M = W and Mo; [P(2)M(15)M(3)'O(62)](m-), M = W and Mo, M' = W, Mo, and V to analyze the redox properties and the basicity of the external oxygen sites in polyoxometalates with nonequivalent addenda metals. The energy and composition of the lowest unoccupied orbitals, formally delocalized over the addenda atoms, determine the redox properties of a polyoxometalate. When a Mo(6+) substitutes one W(6+) in the 1:12 tungstate, the energy of the LUMO decreases and the cluster is more easily reduced. The tungstoniobates behave differently because the niobium orbitals insert into the tungsten band and the reduction of [SiW(9)Nb(3)O(40)](7-) yields the blue species SiW(9)Nb(3) 1e and not the cluster SiW(9)Nb(2)Nb(IV). In Wells-Dawson structures, the polar and equatorial sites have different electron affinities and the reduction preferentially occurs in the equatorial sites. Inserting ions with larger electron affinities into the polar sites can modify this traditional conduct. Hence, the trisubstituted [P(2)W(15)V(3)O(62)](9-) anion is reduced in the vanadium polar sites. By means of molecular electrostatic potential maps and the relative energy of the various protonated forms of [SiW(9)V(3)O(40)](7-) and [SiW(9)Mo(3)O(40)](4-), we established the basicity scale: OV(2) > OMo(2) > OW(2) > OV > OW > OMo. Finally, a continuum model for the solvent enabled us to compare anions with different total charges.     

Study of metal ion labeling of the conformational and charge states of lysozyme by ion mobility mass spectrometry

The efficiency of Zn(2+), Cu(2+), Ni(2+), Co(2+), Fe(2+) or Mn(2+) labeling of the conformational and charge states of lysozyme was studied in H(2)O solvent at pH 2.5-6.8. Labeling of lysozyme was conducted with 50 M, 100 M and 500 M excess of the metal ion, resulting in the number of metal ions attached to lysozyme increasing two-fold over this range. At pH 6.2-6.8, Zn(2+), Cu(2+), Ni(2+), Co(2+) and Mn(2+) labeled the highly folded 7+ conformer and the 8+ and 9+ partially unfolded conformers of lysozyme with the same number of metal ion tags, with only Fe(2+) exhibiting no labeling. Lysozyme conserved its charge after metal ion labeling which shows at each charge state the divalent metal ion is replacing two protons. As the pH is lowered to 4.7-5.0 and 2.5-2.9, the labeling of lysozyme by Zn(2+), Cu(2+), Ni(2+), Co(2+) or Mn(2+) decreased in efficiency due to increased competition from protons for the aspartate and glutamate binding sites. The metal ions preferentially labeled the highly folded 7+ and partially unfolded 8+ conformers, but labeling decreased as the charge of lysozyme increased. In contrast to the other metal ions, Fe(2+) exhibited labeling of lysozyme only at the lowest pH of 2.8. At higher pH, the oxidation of Fe(2+) and formation of hydroxy-bridged complexes probably make the Fe(2+) unreactive towards lysozyme.     

TD-DFT investigation of triple-stranded helicates with bis(benzene-o-dithiolato) ligands

The electronic spectra of triple-stranded complexes of Ti(IV) with bis(benzene-o-dithiolato) ligands (H(4)-L(1)=1,2-bis(2,3-dimercaptobenzamido)ethane; H(4)-L(2)=1,2-bis(2,3-dimercaptobenzamido)benzene) were investigated at the TD-DFT level of theory employing B3LYP/LANL2DZ. The influence of the nature (aliphatic or aromatic) of the spacer bridging both {Ti(S(2)C(6)H(3))(3)} units on the absorptive features of the dinuclear complexes was also studied. B3LYP/LANL2DZ leads to spectra accounting for four absorption bands. At the lowest-energy region, the most important transitions are due to ligand-to-metal charge transfer (LMCT), in which out-of-plane ligand-centered orbitals and titanium-based MOs are involved. In going to the blue-region, a third band was detected with excitations showing an important contribution from ligand-to-ligand charge transfer (LLCT) and indeed, a combined LMCT+LLCT character has been considered. This observation seems to arise from a decrease in the metallic character of the LUMO-derivatives involved in the excitations. The origin of the absorption band at the highest-energy part of the spectrum is assigned to a LLCT. The influence of the nature of the spacer on the molar absorption coefficients (?) for this band has been clearly observed. The complex bearing aliphatic spacer shows ? of about 5 E+4 M(-1) cm(-1), while the one containing an aromatic spacer present a value of ? of about 2 E+5 M(-1) cm(-1).     

Effects of cations on the hydrogen bond network of liquid water: new results from X-ray absorption spectroscopy of liquid microjets

Oxygen K-edge X-ray absorption spectra (XAS) of aqueous chloride solutions have been measured for Li(+), Na(+), K(+), NH(4)(+), C(NH(2))(3)(+), Mg(2+), and Ca(2+) at 2 and 4 M cation concentrations. Marked changes in the liquid water XAS are observed upon addition of the various monovalent cation chlorides that are nearly independent of the identity of the cation. This indicates that interactions with the dissolved monovalent cations do not significantly perturb the unoccupied molecular orbitals of water molecules in the vicinity of the cations and that water-chloride interactions are primarily responsible for the observed spectral changes. In contrast, the addition of the divalent cations engenders changes unique from the case of the monovalent cations, as well as from each other. Density functional theory calculations suggest that the ion-specific spectral variations arise primarily from direct electronic perturbation of the unoccupied orbitals due to the presence of the ions, probably as a result of differences in charge transfer from the water molecules onto the divalent cations.     

Electronic structure and chemical bonding in MO(n)- and MO(n) clusters (M = Mo, W; n = 3-5): a photoelectron spectroscopy and ab initio study

Photoelectron spectroscopy (PES) and ab initio calculations are combined to investigate the electronic structure of MO(n)- clusters (M = W, Mo; n = 3-5). Similar PES spectra were observed between the W and Mo species. A large energy gap between the first and second PES bands was observed for MO3- and correlated with a stable closed-shell MO3 neutral cluster. The electron binding energies of MO4- increase significantly relative to those of MO3-, and there is also an abrupt spectral pattern change between MO3- and MO4-. Both MO4- and MO5- give PES features with extremely high electron binding energies (>5.0 eV) due to oxygen-2p-based orbitals. The experimental results are compared with extensive density functional and ab initio [CCSD(T)] calculations, which were performed to elucidate the electronic and structural evolution for the tungsten oxide clusters. WO3 is found to be a closed-shell, nonplanar molecule with C3v symmetry. WO4 is shown to have a triplet ground state (3A2) with D2d symmetry, whereas WO5 is found to be an unusual charge-transfer complex, (O2-)WO3+. WO4 and WO5 are shown to possess W-O* and O2-* radical characters, respectively.     

簇离子Mo_3S_4M~（n＋）中M与Mo成键作用及电子光谱（M＝Fe、Ni，n＝4；M＝Cu，n＝5）

应用ＩＮＤＯ／Ｓ半经验量子化学方法，对簇合物离子Ｍｏ３Ｓ和Ｍｏ３Ｓ４Ｍｎ＋（Ｍ＝Ｆｅ、Ｎｉ，ｎ＝４；Ｍ＝Ｃｕ，ｎ＝５）分别进行分子轨道计算。根据计算得到的簇离子中的原子表观电荷和成键指标，说明Ｆｅ、Ｎｉ、Ｃｕ＋与Ｍｏ３Ｓ成键作用的相对强度依次是Ｆｅ－Ｍｏ＞Ｎｉ－Ｍｏ＞Ｃｕ＋－Ｍｏ。比较了用含组态作用的ＩＮＤＯ／Ｓ方法计算得到的电子跃迁能与实验得到的电子吸收光谱值，并讨论了吸收峰归属情况。对于Ｍ为Ｆｅ、Ｎｉ的簇离子Ｍｏ３Ｓ４Ｍ４＋，最低能量的电子跃迁吸收峰起源于异金属间电荷转移跃迁（ＭＭ’ＣＴ）；而Ｍｏ３Ｓ４Ｃｕ（５＋）簇离子观察到的吸收峰主要是Ｍｏ３Ｓ芯的局域内电荷转移跃迁。根据理论计算结果，由Ｃｕ＋离子到Ｍｏ３Ｓ的电荷转移跃迁谱线，大约在４６０００ｃｍ－１以上才能观察到吸收峰。从Ｍｏ３Ｓ４Ｆｅ４＋次低能量吸收峰的实验值１６６００ｃｍ－１和理论值１６５００ｃｍ－１与Ｍｏ３Ｓ的最低能量吸收峰的实验值１６６００ｃｍ－１和理论值１６９００ｃｍ－１比较，表明无论从理论上或实验上都能证实簇离子Ｍｏ３Ｓ４Ｆｅ４＋在能量为１６６００ｃｍ－１处的吸收峰是起因于Ｍｏ３Ｓ芯的局域内电荷转移跃迁。     

High-nuclearity 3d-4f clusters as enhanced magnetic coolers and molecular magnets

Four 52-metal-ion 3d-4f cluster complexes featuring a common core of Ln(42)M(10) (Ln = Gd(3+), Dy(3+); M = Co(2+/3+), Ni(2+)) were obtained through self-assembly of the metal ions templated by mixed anions (ClO(4)(-) and CO(3)(2-)). Magnetic studies revealed that the Gd(42)Co(10) and Gd(42)Ni(10) clusters exhibit the largest magnetocaloric effect (MCE) among any known 3d-4f complexes. Replacement of Gd(3+) ions with anisotropic Dy(3+) ions caused significant changes in the magnetic behavior of the clusters; both Dy(42)Co(10) and Dy(42)Ni(10) displayed slow relaxation of the magnetization.     

Binding modes in metal ion complexes of quinones and semiquinone radical anions: electron-transfer reactivity.

9,10-Phenanthrenequinone (PQ) and 1,10-phenanthroline-5,6-dione (PTQ) form 1:1 and 2:1 complexes with metal ions (M (n+)=Sc (3+), Y (3+), Mg (2+), and Ca (2+)) in acetonitrile (MeCN), respectively. The binding constants of PQ--M (n+) complexes vary depending on either the Lewis acidity or ion radius of metal ions. The one-electron reduced species (PTQ(-)) forms 1:1 complexes with M (n+), and PQ(-) also forms 1:1 complexes with Sc(3+), Mg(2+), and Ca(2+), whereas PQ(-) forms 1:2 complexes with Y(3+) and La(3+), as indicated by electron spin resonance (ESR) measurements. On the other hand, semiquinone radical anions (Q(-) and NQ(-)) derived from p-benzoquinone (Q) and 1,4-naphthoquinone (NQ) form Sc(3+)-bridged pi-dimer radical anion complexes, Q(-)--(Sc(3+))(n)--Q and NQ(-)--(Sc(3+))(n)-NQ (n=2 and 3), respectively. The one-electron reduction potentials of quinones (PQ, PTQ, and Q) are largely positively shifted in the presence of M (n+). The rate constant of electron transfer from CoTPP (TPP(2-)=dianion of tetraphenylporphyrin) to PQ increases with increasing the concentration of Sc(3+) to reach a constant value, when all PQ molecules form the 1:1 complex with Sc(3+). Rates of electron transfer from 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to PTQ are also accelerated significantly by the presence of Sc(3+), Y(3+), and Mg(2+), exhibiting a first-order dependence with respect to concentrations of metal ions. In contrast to the case of o-quinones, unusually high kinetic orders are observed for rates of Sc(3+)-promoted electron transfer from tris(2-phenylpyridine)iridium(III) [Ir(ppy)(3)] to p-quinones (Q): second-order dependence on concentration of Q, and second- and third-order dependence on concentration of Sc(3+) due to formation of highly ordered radical anion complexes, Q()--(Sc(3+))(n)--Q (n=2 and 3).     

Synthesis, structure, and electronic properties of RuN6 dinuclear Ru-Hbpp complexes

A series of RuN(6) dinuclear Ru-Hbpp complexes (Hbpp is the dinucleating tetraaza ligand 3,5-bis(pyridyl)pyrazole) of general formula {[Ru(II)(R(2)-trpy)(MeCN)](2)(μ-R(1)-bpp)}(3+), 10(3+)-14(3+), (R(1) = H, Me, or NO(2). and R(2) = H, Me, MeO; see Scheme 1) has been prepared from their Cl(-) or AcO(-) bridged precursors. The complexes have been characterized by UV-vis, NMR, CV, and some by X-ray. Complexes 10(3+)-14(3+), Ru(2)(II,II), were oxidized by 1 equiv in solution, leading to the mixed valence Ru(2)(II,III) complexes 10(4+)-14(4+) containing one unpaired electron and were characterized by EPR and UV-vis-near-IR, which showed metal-centered spin and the presence of low-energy IVCT bands. The H(ab) parameter indicates a relatively strong electronic coupling between the two ruthenium centers (class II). Further two electron oxidation in solution of the 10(3+)-14(3+) led to the formation of EPR silent Ru(2)(III,III) complexes 10(5+)-14(5+), that were further characterized by UV-vis-NIR. TD-DFT calculations are employed to assign the nature of the UV-vis transitions for the complexes in the various oxidation states, which are of metal to ligand charge transfer (MLCT) type for Ru(2)(II,II) and ligand to metal charge transfer (LMCT) type for Ru(2)(III,II) and Ru(2)(III,III).