Comparison of the self-reactivity of [CpCo(PP)I]+ complexes, PP = Ph2P(CH2)nPPh2 (n = 1-4), leading to bridged and oxidized dangling PP. Reactions, spectral studies, and structures of [CpCo(dppm)I]I.CHCl3, CpCo[dppm(O)]I2, and Cp2Co2I4(.mu.-dpppent)

Insights into Formate Oxidation by a Series of Cobalt Piano-Stool Complexes Supported by Bis(phosphino)amine Ligands

A series of (cyclopentadienyl)cobalt(III) half-sandwich complexes (1-4) supported by bidentate bis(phosphino)amine ligands was synthesized and characterized by NMR spectroscopy, X-ray crystallography, and cyclic voltammetry. The Co^III-hydride complex 4-H bearing the bis(cyclohexylphosphine) ligand derivative was successfully isolated via protonation of the neutral reduced Co^I complex 5 with a weak acid. Experimental and computational methods were used to determine the thermodynamic hydride accepting ability of these Co^III centers and to evaluate their reactivity toward the oxidation of formate. We find that the hydride accepting ability of 1-4 ranges from 71 to 74 kcal/mol in acetonitrile, which should favor a highly exergonic reaction with formate through direct hydride transfer. Formate oxidation was demonstrated at elevated temperatures in the presence of stoichiometric quantities of 4, generating carbon dioxide and the Co^III-hydride complex 4-H in 72% yield.

Transition-metal complexes of group 12 with 1,1'-bis(phosphanyl)ferrocene ligands

The syntheses of four new cadmium and zinc complexes with 1,1'-bis(phosphanyl)ferrocene ligands and their phosphine chalcogenide derivatives are reported. The complexes were characterized by elemental analyses and IR, ¹H NMR, ³¹P NMR and electronic absorption spectroscopy. The crystal structures of dichlorido[1-diphenylphosphinoyl-1'-(di-tert-butylphosphanyl)ferrocene-κ²O,P]cadmium(II), [CdCl₂{(C₁₇H₁₄OP)(C₁₃H₂₂P)Fe}] or CdCl₂(κ²P,O-dppOdtbpf) (1), bis[μ-(tert-butyl)(1'-diphenylphosphinoylferrocen-1-yl)phosphinato-κ³O,O':O'']bis[chloridozinc(II)], [Zn₂{(C₉H₁₃O₂P)(C₁₇H₁₄OP)Fe}₂Cl₂] or [ZnOCl{κ²O,O'-Ph₂POFcPO₂(t-Bu)}]₂ (2), 1,1'-bis(di-tert-butylthiophosphinoyl)ferrocene, [Fe(C₁₃H₂₂PS)₂] or dtbpfS₂ (3), and [1,1'-bis(dicyclohexylphosphanyl)ferrocene-κ²P,P'][chlorido/cyanido(0.25/1.75)]zinc(II), [Zn(CN)_1.75Cl_0.25{(C₁₇H₂₆P)₂Fe}] or Zn(CN)₂(κ²-dcpf) (4), were determined crystallographically. Compound 1 has tetrahedral geometry in which the Cd^II centre is coordinated by one dppOdtbpf ligand in a κ²-manner and by two Cl atoms, while compound 2 displays a centrosymmetric dimeric unit in which two oxide atoms bridge the two Zn atoms to generate an eight-membered ring. Compound 3 revealed a sandwich structure with both phosphane groups sulfurized. In compound 4, the Zn^II atom adopts a tetrahedral geometry by coordinating to the 1,1'-bis(dicyclohexylphosphanyl)ferrocene ligand in a κ²-manner and to two cyanide ligands.

Fluorescence quenching and luminescence sensitization in complexes of Tb3+ and Eu3+ with humic substances

Intrinsic fluorescence quenching of humic substances (HS) and the sensitization of Ln3+ luminescence (Ln3+ = Tb3+, Eu3+) in HS complexes were investigated. Both measurements yielded complementary information on the complexation of metals by HS. Large differences between fulvic acids (FA) and humic acids (HA) were found. From time-resolved luminescence measurements it is concluded that a combination of energy transfer and energy back transfer between HS and Ln3+ is responsible for the observed luminescence decay characteristics. In the case of Eu3+, an additional participation of charge-transfer states is suggested. A new concept for the evaluation of the sensitized luminescence decays of Ln3+ was adapted.

Palladium(II) complexes of bis(phosphine)monooxide and bis(phosphine)monosulphide ligands bearing o-N,N-dimethylanilinyl substituents

Palladium(II) complexes of bis(phosphine)monooxide and bis(phosphine)monosulphide ligands with the general formula Ar2P(CH2)nP(E)Ar2 are described (n = 1 (dmapmE), E = O, S; n = 2 (dmapeE), E = O; Ar = o-N,N-dimethylanilinyl). The precursor, PdCl2(dmapm), which exists in solution as an equilibrium mixture of the P,P- and P,N-ligated isomers, reacts with peroxides to form PdCl2(P,N-dmapmO) and with sulphur to give [PdCl(P,N,S-dmapmS)]Cl, probably by direct oxidation of the "dangling" P atom of the P,N-isomer. PdCl2(dmape), which exists in solution as a mixture of PdCl2(P,P-dmape) and [PdCl(P,P,N-dmape)]Cl, reacts with hydroxide in H2O–CH2Cl2 under argon to give the face-to-face Pd(II)2 complex containing the cation [Pd2Cl2(µ-P,N:O-dmapeO)2]2+, which was isolated and crystallographically characterized as the PF6 salt. The remarkable structure, consisting of two face-to-face Pd(II) square planes in a head-to-tail orientation, reveals a 12-membered ring containing the two non-bonded metal centres (Pd—Pd = 4.873(2) Å). The dmapmS complex shows marginal activity as a catalyst for the hydration of maleic to malic acid.Key words: phosphine, phosphine oxide, phosphine sulphide, palladium.

Sequestering of Eu(III) by a GAAA RNA tetraloop

The site-specific binding of metal ions maintains an important role in the structure, thermal stability, and function of folded RNA structures. RNA tetraloops of the "GNRA" family (where N = any base and R = any purine), which owe their unusual stability to base stacking and an extensive hydrogen bonding network, have been observed to bind metal ions having different chemical and geometric properties. We have used laser-induced lanthanide luminescence and isothermal titration calorimetry (ITC) to examine the metal-binding properties of an RNA stem loop of the GNRA family. Previous research has shown that a single Eu(III) ion binds the stem loop fragment in a highly dehydrated site with a K(d) of approximately 12 microM. Curve-fitting analysis of the broad luminescence excitation spectrum of Eu(III) upon complexation with the tetraloop fragment indicates the possibility of two microenvironments that do not differ in hydration number. Binding of Eu(III) to the loop was accompanied by positive enthalpic changes, consistent with energetic cost of removal of water molecules and suggesting that the binding is entropically driven. By comparison, binding of Mg(II) or Mn(II) to the RNA loop, or Eu(III) to the DNA analogue of the loop, was associated with exothermic changes, consistent with predominantly outer-sphere coordination. These results suggest specific binding, most probably involving ligands on the 5' side of the loop.

Self-Assembled Dinuclear Lanthanide Helicates: Substantial Luminescence Enhancement upon Replacing Terminal Benzimidazole Groups by Carboxamide Binding Units

The segmental ligands bis{1-alkyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl}methane (alkyl = methyl (L(5)), ethyl (L(6))) react with lanthanide perchlorates (Ln = La, Eu, Gd, Tb) in acetonitrile to yield the f-f dinuclear homotopic triple-stranded helicates [Ln(2)(L(i)())(3)](6+) (i = 5, 6) under thermodynamic control. The crystal structure of [Tb(2)(L(6))(3)](ClO(4))(3)(MeCN)(2)(THF)(0.5)(EtOH)(0.5) (11a, C(124)H(145)N(26)O(31)Cl(6)Tb(2), triclinic, P&onemacr;, Z = 2) shows the wrapping of the ligands about a pseudo-C(3) axis passing through the metal ions. The Tb ions are 9-coordinate in facial pseudo-tricapped trigonal prismatic sites and are separated by 9.06 Å. (1)H-NMR and ES-MS data establish that the triple helical structure is maintained in solution. Spectrophotometric titrations (Ln = La, Eu) indicate log beta(23) = 24-25 and the formation of a 2:2 complex [Ln(2)(L(5))(2)](6+) (log beta(22) = 19-20). Quantum yield determination in acetonitrile shows that the terminal N,N-diethylcarboxamide groups in L(5) favor efficient intramolecular L(5) --> Eu(III) energy transfers leading to strong Eu-centered red luminescence, 50 times as intense as the luminescence observed when the carboxamide groups are replaced by substituted benzimidazole units in [Eu(2)(L(4))(3)](6+). Resistance toward hydrolysis also results from the use of carboxamide groups, and no quenching of luminescence is observed for [Eu(2)(L(5))(3)](6+) in moist acetonitrile up to 2.5 M water. The crucial role played by carboxamide groups for the control of structural, electronic, and photophysical properties is discussed. Replacing perchlorates by triflates allows the isolation of the dinuclear double-stranded helicate [Eu(2)(L(6))(2)(CF(3)SO(3))(4)(H(2)O(2))(2)](CF(3)SO(3))(2)(MeOH)(2)(H(2)O)(5)(.5), whose crystal structure (13a, C(85)H(106)Eu(2)F(18)N(16)O(30)S(6), monoclinic, C2/m, Z = 2) reveals a side-by-side arrangement of the two strands and 9-coordinate Eu ions linked through hydrogen-bonded water molecules.