Equatorial Coordination Plane Research Articles

1,4-Divinylphenylene-bridged diruthenium complexes with 2-hydroxypyridine- and 2- or 8-hydroxyquinoline-olate ligands

Abstract Four divinylphenylene-bridged diruthenium complexes [{Ru(CO)(P i Pr3)2(L-κ 2 [N,O] – )}2(μ-CH=CH-C6H4-CH=CH-1,4)] (2a–2d) with N,O-chelating 2-hydroxypyridine and 2-hydroxy- or 8-hydroxyquinoline ligands are presented. They were studied by NMR spectroscopy, electrochemical methods and, in their neutral and oxidized states, by IR, UV/Vis/NIR and, if applicable, by EPR spectroscopy. The experimental studies are complimented by (TD-)DFT calculations. Our results indicate that the pyridine-olate complexes 2a,b exist as three isomers with a ratio of about 78:20:2 that differ with respect to the orientation of the N and O donors relative to the CO and alkenyl ligands in the equatorial coordination plane. Only the isomer with both imine N donors trans to the alkenyl ligand is observed for complexes 2c,d with quinolinato ligands. All complexes undergo two consecutive, chemically and electrochemically reversible one-electron oxidations at low potentials. Our results indicate strong contributions of the divinylphenylene bridge to the redox processes and an even delocalization of the electron hole and the unpaired spin density over the entire π-conjugated divinylphenylene diruthenium backbone with only minor involvement of the peripherally attached κ 2 [N,O] – donor ligands.

Dual mode of voltammetric studies on Cu(II) complexes of His2 peptides: phosphate and peptide sequence recognition.

Copper(II) complexes of peptides with a histidine residue at the second position (His2 peptides) provide a unique profile of electrochemical behavior, offering signals of both Cu(II) reduction and Cu(II) oxidation. Furthermore, their structures with vacant positions in the equatorial coordination plane could facilitate interactions with other biomolecules. In this work, we designed a library of His2 peptides based on the sequence of Aβ5-9 (RHDSG), an amyloid beta peptide derivative. The changes in the Aβ5-9 sequence highly affect the Cu(II) oxidation signals, altered further by anionic species. As a result, Cu(II) complexes of Arg1 peptides without Asp residues were chosen as the most promising peptide-based molecular receptors for phosphates. The voltammetric data on Cu(II) oxidation for binary Cu(II)-His2 peptide complexes and ternary Cu(II)-His2 peptide/phosphate systems were also tested for His2 peptide recognition. We achieved a highly promising identification of subtle modifications in the peptide sequence. Thus, we introduce voltammetric measurement as a potential novel tool for peptide sequence recognition.

Probing the primary processes of a triazido-cobalt(III) complex with femtosecond vibrational and electronic spectroscopies. Photochemical selectivity and multi-state reactivity.

The elementary dynamics following 355 nm-excitation of the complex, mer-[Co(dien)(N3)3], were studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond-ultraviolet-pump/mid-infrared-to-near-ultraviolet probe spectroscopy in conjunction with electronic structure calculations based on density functional theory. Following the initial N3--to-Co charge transfer excitation, the parent complex undergoes an ultrafast metal-to-ligand back electron transfer (BET) within 2 ps thereby populating a metal-centered singlet excited state, 1MC, which can either repopulate the electronic ground state or cleave an azido ligand from the ligand sphere surrounding the metal center. From the asymptotic ground-state bleaching signal after 1 ns, a primary quantum yield for ligand loss of ca. 13% is estimated. The IR-spectrum of the product demonstrates that the photodissociation occurs selectively from the equatorial binding site thereby leading exclusively to the solvolysis product, mer-trans-[Co(dien)(N3)2(DMSO)]+, which features the solvent ligand in the equatorial coordination plane and the azides in the two axial positions. The remarkable photochemical selectivity is traced back to the initial BET and the nature of the intermediate state, 1MC, whose electronic structure entails occupancy of the σ-antibonding d(x2-y2)-orbital. A stereochemical scrambling at the stage of the primary penta-coordinated diazido product is kinetically inhibited on the singlet surface by an energy barrier of roughly 27 kJ mol-1. Primary penta-coordinated products that may be born on the triplet surface are funneled to their singlet ground-state preferentially from geometries with trans-oriented azido ligands thereby also preventing a stereochemical isomerization that could possibly arise from an intersystem crossing.

Photolabile Ru Model Complexes with Chelating Diimine Ligands for Light‐Triggered Drug Release

A series of water‐soluble photolabile model complexes of the general formula [Ru([9]aneS3)(chel)(py)]Cl2 ([9]aneS3 = 1,4,7‐trithiacyclononane, chel = chelating diimine) was prepared and fully characterized. The phototriggered release of pyridine with visible light as a function of the nature of the diimine {chel = 2,2′‐bipyridine (6) 1,10‐phenanthroline (7), 4,7‐diphenil‐1,10‐phenanthroline (8), dipyrido‐[3,2‐a:2′,3′‐c]phenazine (dppz, 9), 2,2′‐biquinoline (bq, 10)} was investigated. Our aim is to establish whether this type of complexes might be realistically used in the photo‐uncaging strategy of photoactivated chemotherapy (PACT) in the future. Compounds 6–9 present a MLCT absorption in the blue region of the visible spectrum. When irradiated with light at 470 nm, they rapidly and quantitatively release the coordinated pyridine. Complex 10 turned out to be quite different from the others in the series. Structure‐wise, in 10 the average plane of coordinated bq – owing to its steric demand – is remarkably tilted relative to the equatorial coordination plane [37.43(4)°, with the “front“ of the ligand pointing towards the axial py] and the orientation of py is nearly orthogonal compared to that found in 6 and 7 for minimizing steric clashes with bq. The low‐lying acceptor orbitals of bq induce a red‐shift of the MLCT absorption maximum to ca. 500 nm. Contrary to the expectations, complex 10 is more photostable compared to 6–9 and photo‐dissociation of both py and bq, in nearly equal amounts, occurs. A detailed theoretical investigation was performed on 10 (and on 6 for comparison), for explaining its peculiar spectral features and photochemical behavior.

Synthesis of Coordination Polymers of Tetravalent Actinides (Uranium and Neptunium) with a Phthalate or Mellitate Ligand in an Aqueous Medium.

Four metal-organic coordination polymers bearing uranium or neptunium have been hydrothermally synthesized from a tetravalent actinide chloride (AnCl4) and phthalic (1,2-H2bdc) or mellitic (H6mel) acid in aqueous media at 130 °C. With the phthalate ligand, two analogous assemblies ([AnO(H2O)(1,2-bdc)]2·H2O; An = U4+ (1) or Np4+ (2)) have been isolated, in which the square-antiprismatic polyhedra of AnO8 are linked to each other via μ3-oxo groups with an edge-sharing mode to materialize infinite zigzag ribbons. The phthalate molecules play a role in connecting the adjacent zigzag chains to build a two-dimensional (2D) network. Water molecules are bonded to the actinide center or found intercalated between the layers. With the mellitate ligand, two distinct structures have been identified. The uranium-based compound [U2(OH)2(H2O)2(mel)] (3) exhibits a three-dimensional (3D) structure composed of the dinuclear units of UO8 polyhedra (square antiprism), which are further linked via the μ2-hydroxo groups. The mellitate linkers use their carboxylate groups to connect the dinuclear units, eventually building a 3D framework. The compound obtained for the neptunium mellitate ([(NpO2)10(H2O)14(Hmel)2]·12H2O (4)) reveals oxidation of the initial NpIV to NpV under the applied hydrothermal synthetic conditions, yielding the neptunyl(V) (NpO2+) unit with a pentagonal-bipyramidal NpO7 environment. This further leads to the formation of a layered assembly of the square-frame NpO7 sheets via the bridging oxygen atoms from the neptunyl oxo groups, which further coordinate to the pentagonal equatorial coordination plane of the adjacent neptunium unit (i.e., cation-cation interactions). In compound 4, the mellitate molecules act as bridging linkers between the NpO7 sheets by using four of their carboxylage groups, eventually building up a 3D structure.

Synthesis, Characterization and Crystal Structure of trans-Aquahydroxobis(2,2-dimethyl-1,3-propanediamine)chromium(III) Diperchlorate

Chromium is the seventh most abundant element on earth. 1 It occurs in various oxidation states ranging from Cr(II) to Cr(VI) with trivalent and hexavalent states being the most stable and common in terrestrial environments. The coordination chemistry of chromium(III) complexes is relevant to third generation solar photoconversion scheme. 2 The geometrical isomerism and conformation in transition metal complexes with mixed ligands are very important in the medical application, and likely to be a major factor in determining the antiviral activity and its side-effect. 3,4 2,2Dimethyl-1,3-propanediamine (Me2tn, Fig. 1) is a bidentate ligand that can coordinate to the central metal ion by forming six-membered chelate ring with a chair conformation. The trans-[Cr(Me2tn)2(OH)(H2O)] 2+ can adopt either the trans or cis geometric isomer. In addition, two different kinds of conformation with respect to the chelate rings of Me2tn occur in the trans isomer, as shown in the bottom of Figure 1. The carbon atoms of the two chelate rings of the two conformers may be on the same side (syn conformer) or on the opposite side (anti conformer) of the coordination plane. In the case of trans-anti/syn-[Cr(Me2tn)2Cl2]Cl, trans-anti/ syn-[Cr(Me2tn)2Br2]2Br2·HClO4·H2O and trans-anti/syn[Cr(Me2tn)2Br2]Br, independent syn and anti conformational isomers were found within the same crystals. 5-7 However, structural analyses of trans-anti-[Cr(Me2tn)2(NCS)2]NCS·1/2H2O, trans-anti-[Cr(Me2tn)2Cl2]ClO4, and trans-anti-[Cr(Me2tn)2Cl2]2ZnCl4 indicated that two chelate rings of Me2tn ligands were only in the anti chair-chair conformation. 6-9 The different conformations of the two sixmembered chelate rings of Me2tn ligands may be dependent on the crystallographic disorder, packing forces, hydrated molecules and counter anions in the crystals. The factors which determine the stability of these geometric conformations are subtle and complicated. 5-9 X-ray crystallography is generally used to establish the presence of either conformation but the syn or anti conformer of the six-membered chelate rings cannot be readily discriminated by infrared and visible absorption spectroscopy. The elucidation of the factors that stabilize the either the syn- or anti-conformation in these complex cations continues to be of interest. Here we report the synthesis, crystal structure and spectroscopic properties of the trans-anti-[Cr(Me2tn)2(OH)(H2O)](ClO4)2 with the purpose of elucidating the influence of Odonor ligands occupying the axial positions and counter anions, on the geometry and the conformation referring to the position of the carbon atoms of the Me2tn chelate rings with respect to the equatorial coordination plane.

Structural, spectroscopic and redox properties of uranyl complexes with a maleonitrile containing ligand

The reaction of uranyl nitrate hexahydrate with the maleonitrile containing Schiff base 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile (salmnt((Et(2)N)(2))H(2)) in methanol produces [UO(2)(salmnt((Et2N)2))(H(2)O)] (1) where the uranyl equatorial coordination plane is completed by the N(2)O(2) tetradentate cavity of the (salmnt((Et(2)N)(2)))(2-) ligand and a water molecule. The coordinated water molecule readily undergoes exchange with pyridine (py), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF) and triphenylphosphine oxide (TPPO) to give a series of [UO(2)(salmnt((Et(2)N)(2)))(L)] complexes (L = py, DMSO, DMF, TPPO; 2-5, respectively). X-Ray crystallography of 1-5 show that the (salmnt((Et(2)N)(2)))(2-) ligand is distorted when coordinated to the uranyl moiety, in contrast to the planar structure observed for the free protonated ligand (salmnt((Et(2)N)(2))H(2)). The Raman spectra of 1-5 only display extremely weak bands (819-828 cm(-1)) that can be assigned to the typically symmetric O=U=O stretch. This stretching mode is also observed in the infrared spectra for all complexes 1-5 (818-826 cm(-1)) predominantly caused by the distortion of the tetradentate (salmnt((Et(2)N)(2)))(2-) ligand about the uranyl equatorial plane resulting in a change in dipole for this bond stretch. The solution behaviour of 2-5 was studied using NMR, electronic absorption and emission spectroscopy, and cyclic voltammetry. Complexes 2-5 exhibit intense absorptions in the visible region of the spectrum due to intramolecular charge transfer (ICT) transitions and the luminescence lifetimes (< 5 ns) indicate the emission arises from ligand-centred excited states. Reversible redox processes assigned to the {UO(2)}(2+)/{UO(2)}(+) couple are observed for complexes 2-5 (2: E(1/2) = -1.80 V; 3,5: E(1/2) = -1.78 V; 4: E(1/2) = -1.81 V : vs. ferrocenium/ferrocene {Fc(+)/Fc}, 0.1 M Bu(4)NPF(6)) in dichloromethane (DCM). These are some of the most negative half potentials for the {UO(2)}(2+)/{UO(2)}(+) couple observed to date and indicate the strong electron donating nature of the (salmnt((Et(2)N)(2)))(2-) ligand. Multiple uranyl redox processes are clearly seen for [UO(2)(salmnt((Et(2)N)(2)))(L)] in L (L = py, DMSO, DMF; 2-4: 0.1 M Bu(4)NPF(6)) indicating the relative instability of these complexes when competing ligands are present, but the reversible {UO(2)}(2+)/{UO(2)}(+) couple for the intact complexes can still be assigned and shows the position of this couple can be modulated by the solvation environment. Several redox processes were also observed between +0.2 and +1.2 V (vs. Fc(+)/Fc) that prove the redox active nature of the maleonitrile-containing ligand.

Structural effects of Cu(ii)-coordination in the octapeptide region of the human prion protein

The copper-binding ability of the prion protein is thought to be central to its function. The structural effects of copper coordination in the octapeptide region of the human prion protein have been investigated by molecular dynamics simulations. Simulations were performed with the apo state, in order to investigate the behavior of the region without copper ions, as well as with the octapeptide region in the presence of copper ions. While the structure of the apo state is greatly influenced by the interaction between the rings in the histidine, tryptophan and proline residues, the region shows evidence of highly ordered coordination sites in the presence of copper ions. The position of the tryptophan indole ring is stabilized by cation-pi interactions. Two stable orientations of the indole ring with respect to the equatorial coordination plane of copper were observed, which showed that the indole ring can reside on both sides of the coordination plane. The interaction with the indole ring was found to occur without a mediating axial water molecule.

High-Affinity Uranyl-Specific Antibodies Suitable for Cellular Imaging

Monoclonal antibodies (mAbs) have proved to be valuable models for the study of protein-metal interactions, and previous reports have described very specific antibodies to chelated metal ions, including uranyl. We raised specific mAbs against UO2(2+)-DCP-BSA (DCP, 1,10-phenanthroline-2,9-dicarboxylic acid) to generate new sets of antibodies that might cross-react with various complexed forms of uranyl in different environments for further application in the field of toxicology. Using counter-screening with UO2(2+)-DCP-casein, we selected two highly specific mAbs against uranyl-DCP ( K D 10-100 pM): U04S and U08S. Competitive assays in the presence of different metal ions (UO2(2+), Fe (3+), Zn2+, Cu2+, and Ca2+) showed that uranyl in solution can act as a good competitor, suggesting some antibody ability to cross-react with chelating groups other than DCP in the UO2(2+) equatorial coordination plane. Interestingly, one of the antibodies could be used for revealing uranyl cations in cell samples. Fluorescence activated cell sorting analyses after immunolabeling revealed the interaction of uranyl with human kidney cells HK2. The intracellular accumulation of uranyl could be directly visualized by metal-immunostaining using fluorescent-labeled mAb. Our results suggest that U04S mAb epitopes mostly include the uranyl fraction and its paratopes can accommodate a wide variety of chelating groups.

Molecular Dynamics Simulations of Cu(II) and the PHGGGWGQ Octapeptide

The interaction between Cu2+ and the copper-binding octapeptide region in the human prion protein has been investigated by molecular dynamics simulations. In total four different nonbonded and bonded models were used in the study. Charge sets containing atomic partial charges were developed for these models. Out of the considered models, the bonded model performed physically in the most correct way. The simulations with the bonded model showed that the water molecules in the axial position are very labile. The tryptophan indole ring can remain in a stable position on top of the equatorial coordination plane of copper without water mediation. Strong aromatic interaction was observed between the imidazole and indole rings. The nonbonded models showed a tendency for water-mediated interaction between the copper ion and different carbonyl oxygen atoms. In the case of the bonded model, a carbonyl group could also interact directly with the copper ion in one of the apical position.

Anhydrous Photochemical Uranyl(VI) Reduction: Unprecedented Retention of Equatorial Coordination Accompanying Reversible Axial Oxo/Alkoxide Exchange

In a dramatic reversal of the normal trend of observed reactivity in uranyl(VI) coordination chemistry, an unprecedented retention of the normally labile equatorial coordination plane accompanies facile and reversible axial oxo/alkoxide exchange during both the photochemical reduction of cationic uranyl(VI) phosphine-oxide complexes with organic substrates and subsequent hydrolysis of the uranium(IV) alkoxide complexes to regenerate the uranyl(VI) starting complex.

Synthesis and Crystal Structure of the U(VI) Complex with Triphenylphosphine Oxide {UO2[OP(C6H5)3]4}(ClO4)2

Data on synthesis and structure of the U(VI) complex with triphenylphosphine oxide (TPPO) are presented. Single crystals were grown and the structure of this compound was determined [monoclinic unit cell: a = 9.795(3), b = 25.246(6), c = 14.086(4) A, β = 99.86(2)°, space group P21/c, Z = 2, V = 3431.7(15) A3, dcalc = 1.534 g cm-3; Siemens P3/PC, MoKα radiation, graphite monochromator, ω/2thetas; scanning, 2947 unique reflections, direct method, R1 = 0.0401 for 2261 observed reflections, wR2 = 0.1194 for 2947 unique reflections]. The crystal structure consists of complex cations {UO2[OP(C6H5)3]4}2+ and ClO4- anions. The U(VI) atom has a tetragonal-bipyramidal oxygen surrounding with coordination number (CN) 6. The UO2 group is linear: the O = U = O bond angle is 180.0°, and the U = O bond length is 1.750(6) A. The square equatorial coordination plane of the bipyramid is formed by oxygen atoms of four TPPO molecules, the U-Oeq bond lengths vary from 2.289(6) to 2.307(6) A (average 2.298 A), and the Oeq-U-Oeq bond angles are close to 90°.

Synthesis, Crystal Structure, and Spectral Characteristics of the Np(V) Complex with Triphenylphosphine Oxide [NpO2(PO(C6H5)3)4]ClO4

Single crystals of [NpO2(PO(C6H5)3)4]ClO4 were grown and the structure of this compound was studied (CAD4 diffractometer, MoKα; triclinic unit cell: a = 9.140(2), b = 13.528(3), c = 13.918(3) A, α = 97.14(3)°, β = 108.17(3)°, γ = 91.75(3)°, space group P-1, Z = 1, V = 1618.0 A3, dcalc = 1.521 g cm-3; R1 = 0.0245 for 5342 observed reflections with F0 > 4σ(F0), wR2 = 0.0677 for 5601 unique reflections, 419 refined parameters). [NpO2(PO(C6H5)3)4]ClO4 consists of complex cations [NpO2(PO(C6H5)3)4]+ and disordered ClO4- anions. The Np(V) atom has tetragonal-bipyramidal oxygen surrounding with coordination number (CN) 6. The NpO2 group is linear, the O = Np = O bond angle is 180.0°, and the Np = O bond length is 1.797(2) A. The equatorial coordination plane of the bipyramid is formed by oxygen atoms of four triphenylphosphine oxide (TPPO) molecules. The Np-Oeq bond lengths vary from 2.434(2) to 2.442(2) A (average 2.438 A). The Oeq-Np-Oeq bond angle is close to the right angle (88.98°). The average bond lengths in TPPO ligands are P-O 1.498, P-C 1.798, and C-C 1.376 A; the average bond angles are O-P-O 111.12°, C-P-C 107.78°, P-C-C 120.2, and C-C-C 120.0°. The electronic absorption spectra (Shimadzu UV-3100, 900-1050 nm) and IR spectra (Specord-M80, 400-4000 cm-1) of crystalline [NpO2(PO(C6H5)3)4] were measured in an NaCl matrix. The absence of the main characteristic absorption band of the NpO2+ dioxocation (980 nm) in the electronic absorption spectrum suggests centrosymmetrical equatorial surrounding of the Np(V) atom. The narrow absorption band etaas(NpO2+) in the IR spectra in the region of 860 cm-1 suggests the absence of cation-cation interaction, and its position is consistent with a short Np-O bond length in neptunyl(V) groups.

Zigzag-Chain and Cyclic-Tetrameric Compounds Derived by Deprotonation of Mononuclear Copper(II) Complexes with N,N'-Bis(2-substituted-imidazol-4-ylmethyl- idene)-1,4-diaminobutane (2-Substituent = H, Me): Synthesis, Characterization, Structure, Substituent Effect, and Interconvertibility.

Two copper(II) complexes with N,N'-bis(2-substituted-imidazol-4-ylmethylidene)-1,4-diaminobutane (2-substituent = H, 1; Me, 2) were synthesized, and their deprotonation assembly behavior was studied. X-ray analyses of 1 and 2 revealed that the molecules assume a butterfly shape bent by a line of N(2)-Cu-N(4) where the angle of N(3)-Cu-N(5) representing the extent of the molecular bend is 150.9(2) degrees and 105.66(8) degrees for 1 and 2, respectively. Due to the molecular bend, two imidazole moieties (sites A and B) are recognized as two inequivalent groups for the deprotonation process in which the imidazole group of site A is in an equatorial CuN(3) plane, while that of site B is in a CuN(2) plane bent from the equatorial coordination plane. 1 dissociates a proton of site B first to give an infinite zigzag-chain compound 1', while 2 dissociates a proton of site A first to give a cyclic-tetranuclear compound 2'. 2' undergoes a further deprotonation to give an electrically neutral di-deprotonated complex 2". The interconversion between the protonated monomer and the deprotonated oligomer/polymer was investigated by pH-dependent potentiometric and absorption spectrometric titrations.

μ-Cyano-bis(2,2':6',2''-terpyridine-N,N',N'')dicopper(II) Perchlorate Acetonitrile, [Cu2(CN)(C15H11N3)2](ClO4)3.CH3CN

The [Cu2(terpy)2(CN)]3+ cation (terpy is 2,2′:6′,2′′-terpyridine) is non-centrosymmetric and contains two CuII centres bridged by a disordered cyano group. The overall geometry around both metallic centres can be described as a distorted octahedron. The equatorial coordination plane is occupied by the N atoms of the tridentate terpy ligand and by an atom (C* or N*) of the cyanide bridge. The axial positions for one Cu atom are occupied by O atoms of two perchlorate anions while, for the other Cu atom, a perchlorate ion and an acetonitrile molecule lie in trans positions.

An NMR and single-crystal X-ray diffraction structural study of Ru II [12]aneS 4 polypyridyl complexes

The synthesis of a series of Ru II complexes with the thioether ligand [12]aneS 4 (1,4,7,10-tetrathiocyclododecane) and various polypyridyl ligands was carried out using [Ru(DMSO) 4CL 2] (DMSO- = dimethylsulphoxide) as a starting material. The first synthetic step involved the introduction of the thioether ligand and the isolation of the compound [Ru([12]aneS 4)Cl 2]. The polypyridyl ligands were then exchanged to give a series of complexes [Ru([12]aneS 4)(X)] 2+ [X = 1,10 phenanthroline (phen), 2,2′-bipyridyl (bpy) and 4,7′-diphenyl-1, 10-phenanthroline (dip)]. Crystal structures of the metal complexes [Ru([12]aneS 4)(bpy)]Cl 2·4H 2O, [Ru([12]aneS 4)(phen)]Cl 2·4.25H 2O and [Ru([12]aneS 4)(dip)]Cl 2·2H 2O all show the Ru centre in a distorted cis-octahedral environment, in which the equatorial coordination plane is formed by two S-donor atoms of the macrocyclic ligand [12]aneS 4 and two N-donor atoms of a polypyridyl ligand : bpy, phen or dip. The hexa coordination is completed via the remaining sulfur atoms of the macrocycle. The distortion of the octahedral geometry is associated with the small cavity size of macrocyclic ligand and small bite angle of the bidentate polypyridyl ligands [77.9(3)° bpy, 79.6(3) phen and 77.4(4)° dip]. The small cavity size of [12]aneS 4 precludes the sulfur atoms of the macrocycle achieving axial positions of an ideal octahedron leading to angles, S axRuS ax, of 162.5(2), 161.9(1) and 162.0(1)° for the bpy, phen and dip complexes respectively. A 1H and 13C NMR study of the complexes [Ru([12]aneS 4)(phen)] 2+, [Ru([12]aneS 4)(bpy)] 2+ and [Ru([12]aneS 4)(dip)] 2+ was carried out. The 1H spectra exhibit broad lines for the H-2 and H-9 protons of the polypyridyl ligand and the CH 2 protons of the [12]aneS 4 ligand. A variable temperature study indicated that exchange processes were taking place in solution, most probably involving cleavage of the RuN bond as a result of the strain on the octahedral environment of the small cavity size of the [12]aneS 4 macrocycle.

Tetrakis[μ-(3-bromobenzoato)-μ-(2-dimethylaminoethanolato)-copper(II)

A cubane-type Cu 4 O 4 core has been found in the title tetrameric copper(II) complex {tetrakis(μ-3-bromobenzoato)-1κO: 2κO'; 2κO: 3 κO'; 3 κO: 4κO';-4κO:1κO'-tetrakis[μ-2-(dimethylamino)ethanolato] -1κN,1:2:3κ 3 O;2κN,2:3:4κ 3 O;3κN,3:4:1κ 3 O;4κN,4:1:2κ 3 O-tetracopper(II), [Cu 4 (C 7 H 4 BrO 2 ) 4 (C 4 H 10 NO) 4 ]}. The short Cu-O ethanolato bonds form an eight-membered ring folded in a boat-like conformation. The Cu...Cu distances vary between 3.121(2) and 3.796(3)A. Each Cu atom has a distorted octahedral environment. Two ethanolato O atoms, a carboxy O atom and an amino N atom form the equatorial coordination plane, with Cu-O bond lengths in the range 1.914(12)-1.954(14)A and Cu-N bond lengths in the range 2.011(12)-2.057 (13)A. The axial sites are occupied by an ethanolato O atom and an O atom of the carboxylate group, with Cu-O distances in the range 2.443 (8)-2.877 (9) A.

Double-Strand Cleavage of DNA by a Monofunctional Transition Metal Cleavage Agent

A copper-based transition metal complex has been designed which performs double-stranded cleavage of DNA in a nonrandom fashion. The complex, ((2S,8R)-5-amino-2,8-dibenzyl-5-methyl-3,7-diazanonanedioate)copper(II), presents an ammonium group on one side of the metal equatorial coordination plane to the DNA backbone phosphate groups, while the aromatic phenylalanine-derived side chains are constrained to the opposite side of the coordination plane toward the DNA groove. This structure was designed to bind at locations where phosphate groups are in proximity to accessible hydrophobic regions of the DNA. We have estimated single-strand break to double-strand break ratios for DNA strand scission by this complex under a variety of activation conditions, and they are substantially lower than that predicted by statistical models for a random DNA linearization process. This means that more double-strand breaks are produced per single strand break than can be accounted for by random coincident single-strand breaks. We have also investigated the formation of abasic sites, and found that at least as many abasic sites can be cleaved to linear DNA as are linearized in the initial cleavage reaction. We interpret this to mean that the complex binds both at the intact DNA surface for strand scission, and binds at nicked sites on the DNA (where the charged end groups of the nick are likely to be proximate to the accessible hydrophobic interior) for reactivation and complementary strand scission. Insofar as double-strand cleavage may be more potent biologically than single-strand cleavage as a source of lethal DNA lesions, the recognition characteristics of this complex may aid in the design of chemotherapeutic agents.

Phase Transition of Hexaamminechromium(III) Pentachlorocuprate(II)

The crystal of [Cr(NH 3 ) 6 ][CuCl 5 ] shows a phase transition at 259.2 K. An X-ray diffraction study of the cubic and tetragonal phases has been carried out at 295 and 120K, respectively. Although the arrangement of the metal complexes remained unchanged through the phase transition, the trigonal bipyramidal [CuCl 5 ] 3- ion, which has D 3h symmetry in the cubic phase, deformed into approximate C 2v symmetry in the tetragonal phase, as observed in the isomorphous [Co(NH 3 ) 6 ][CuCl 5 ] crystal. Drastic changes were observed in the equatorial coordination plane. One of three equatorial Cu-Cl bonds was elongated and the Cl-Cu-Cl bond angle formed by the two shortened Cu-Cl bonds expanded to 143.6(1)°.

Structure of catena-poly[{(2,2'-bipyridyl)(diperchlorato)copper(II)}-μ-4,4'-bipyridyl

Cu(C10H8N2)(CI0H8N2)(C104)2) , Mr = 574.5, orthorhombic, Pbcn, a=12.413(3), b = 14.645 (3), c = 12.287 (2) , V = 2233.6 (8) A 3, Z = 4, Dx = 1.708 g cm -3, A(Mo Kte) = 0.71069 ,~, /z = 13.12 cm- 1, F(000) = 1164, room temperature, R = 0.050 and wR = 0.049 for 1498 observed reflections. The coordination around Cu n is an elongated dis- torted octahedron. Two N atoms of 2,2'-bipyridyl and two N atoms from two 4,4'-bipyridyls form the equatorial coordination plane, and two perchlorate ions occupy the axial sites. The 4,4'-bipyridyl ligand bridges neighbouring Cu n atoms to form polymeric chains along the c axis in the crystal. The rings of 4,4'-bipyridyl are coplanar and make a dihedral angle of 114 ° with the equatorial coordination plane; the 4,4'-bipyridyl may provide a pathway for mag- netic superexchange interaction between the adjacent Cu ~I atoms. Introduction. In the last decade, a number of binu- clear transition-metal complexes bridged by hetero- cyclic aromatic diamines have been investigated as * To whom correspondence should be addressed.