Force constants and vibration frequencies of the rhenium-oxygen bonds in the infrared region (200–4000 cm−1)

Remarkably stable dinuclear, diamagnetic rhenium(VI) salts with the [ReOCl4OReOCl4]2- anion

A reaction leading reproducibly to the stable salts containing mu-oxido-bis{cis-tetrachloridooxidorhenate(VI)} anion (systematic name: micro-oxido-bis{(OC-6-32)-tetrachloridooxidorhenate(VI)} anion) was described. A series of phosphonium salts was prepared in crystalline form and a detailed investigation of the complex anion structure was performed. The magnetic and spectroscopic properties are discussed on the example of the methyltriphenylphoshonium salt.

A vibrational spectroscopic study of the thermal behavior of Re2(CO)10 on zeolite Y

This work describes the thermal decomposition of Re2(CO)10 in zeolite Y, which generates the zeolite-supported complex Re0(CO)3(O-Z)3, where (O-Z) represents a framework oxygen. The adsorption and thermal decomposition of Re2(CO)10 was performed on dehydrated and hydrated zeolite NaY. The conversion to Re0(CO)3(O-Z)3 was not significantly affected by the presence of water within the supercages as the supported complex formed at around 200 °C no matter what the initial water content of the zeolite was. However, when the zeolite sample was treated at ≥200 °C for long periods, the adsorbed H2O became involved in the reactivity, playing a role as an oxidant. The subcarbonyl complex Re0(CO)3(O-Z)3 was oxidized, generating isolated Re2O7 or HReO4, or a mixture of both, which was dispersed inside the zeolite host. This result suggests that thermal decomposition of Re2(CO)10 on a hydrated zeolite under mild conditions can be a convenient route for preparation of zeolite-supported Re2O7 catalysts. Re0(CO)3(O-Z)3 was also formed in siliceous zeolite Y (SiY) upon heating. But it quickly decomposes into metal Re at around 200 °C, suggesting that the tricarbonyl species is destabilized in the almost natural zeolite framework. The present study also shows that FT-Raman spectroscopy is a useful technique for monitoring the behavior of the metal carbonyl species in zeolitic hosts.Key words: Raman spectroscopy, zeolite Y, rhenium carbonyl, rhenium oxide.

Preparation and electronic properties of rhenium(V) complexes with bis(diphenyl phosphino)ethane

Complexes of bis(diphenylphosphino)ethane (dppe) of the types ReOX3(dppe) and ReO(OR)X2(dppe) (with X= Cl or Br, and R = Me, Et, Pr, Ph, cyclohexyl (Cy), or -CH2CH2OH) were prepared to evaluate the influence of ligand changes on the low-energy d–d transitions in these Re(V) low-spin d2 systems. Arylimido compounds Re(NR)Cl3(dppe) (with R = Ph and p-ClC6H4) were also obtained. X-ray diffraction studies on ReO(OPr)Cl2(dppe), ReO(OPh)Br2(dppe), and ReO(OCy)Cl2(dppe) confirmed the presence of the trans O=Re-OR unit and showed that the structural characteristics of the Re-O-R segment are not affected by the presence of a bulky cyclohexyl substituent or an aromatic phenyl group. The structure of the arylimido complex Re(p-ClC6H4N)Cl3(dppe) was also determined. The electronic absorption spectra of the ReOX3(dppe) compounds include two low-energy components at ~11 500 and ~16 000 cm–1, assigned to the two spin-allowed d–d transitions expected for these low-symmetry systems. Substitution of the oxo ligand by an arylimido group has little effect on the lower-energy component, but moves the two components closer together. Replacing the halogen trans to the Re=O bond by an alkoxo group shifts the whole system to higher energies. These variations were found to correlate well with the energies of the frontier orbitals determined from DFT calculations. While attempting to prepare ReOCl3(dppe) from ReOCl3(PPh3)2, the Re(III) compound fac-ReCl3(PPh3){Ph2PC2H4PPh2(O)} was obtained, in which one end of dppe had become a phosphine oxide. Upon standing in DMSO, this compound gave the octahedral compound ReCl4{Ph2PC2H4PPh2(O)}, in which the formation of a chelate ring involving a phosphine and a phosphine oxide was ascertained by X-ray diffraction.Key words: rhenium, crystal structure, DFT calculations, d–d electron transitions.

Bidentate Lewis base adducts of methyltrioxorhenium(VII) and their application in catalytic epoxidation

Methyltrioxorhenium(VII) (MTO) forms octahedral adducts with bidentate Lewis bases. These complexes were isolated and fully characterized, including X-ray crystallography. The compounds display distorted octahedral geometry in the solid state with a tendency of disorder concerning the Re central atom. At elevated temperatures, they undergo rapid ligand-exchange reactions in solution. The ease of this ligand exchange depends mainly on the Lewis basicity of the ligand. The more Lewis basic the ligand is, the stronger the metal-ligand interaction is, as can be shown by NMR spectroscopy. All examined complexes are temperature stable but quite sensitive to light and moisture. In the presence of H(2)O(2), the complexes form very active and highly selective epoxidation catalysts. Peroxo complexes are generated, and at least one of the Re-N interactions is cleaved during this process. Total ligand dissociation only occurs in the case of very weakly coordinating bidentate ligands. The peroxo complexes of the MTO Lewis base adducts are, in general, more sensitive to water than MTO itself.

Re(V) complexes with an open-chain quadridentate ligand containing two amine and two amido donors. Synthesis, characterization, and solution equilibria of Re2O3(dioxo-tetH4)2 and [ReO(H2O)(dioxo-tetH4)]Cl (dioxo-tetH6 = 1,4,8,11-tetraazaundecane-5,7-dione)

We are interested in identifying mononuclear cationic [M(V)=O]3+ (M = Tc, Re) complexes for radiopharmaceutical applications. The open-chain ligand, 1,4,8,11-tetraazaundecane-5,7-dione-(dioxo-tetH6) with two amine and two amide donors, was selected for investigation since the literature led us to expect that a five-coordinate [Re(V)=O(dioxo-tetH4)]+ cation would dominate. Instead, the neutral mu-oxo bridged dinuclear complex, Re2O3(dioxo-tetH4)2 (1), and a salt of the six-coordinate mononuclear cation, [ReO(H2O)(dioxo-tetH4)]+ (2), were isolated; the structure of each was determined by X-ray crystallography. The cation (2) is unusual because it has a trans-oxo/aqua core. Such aqua compounds are rarely isolated, and the Re-OH2 distance is relatively short (2.185 A). The cation has two pKa values, 4.1 and 8.7, determined with visible spectroscopy. Since the Re-OH2 bond is short, the coordinated water is likely to be acidic. Thus the two pKa's are assigned to the stepwise deprotonation of the water ligand to give a trans-oxo/hydroxo neutral form and a trans-dioxo anion. Although 1 was the first product isolated following ligand exchange of ReOCl3(Me2S)(OPPh3) with dioxo-tetH6 under neutral conditions, it probably formed from the hydroxo mononuclear complex. Under concentrated conditions (approximately 300 mM) the dinuclear complex deposited from solution, but the 1H NMR spectra of 2 (approximately 20 mM) were consistent with the presence of only monomeric forms in D2O, pH 3-12. 1H NMR experiments demonstrated that in DMSO-d6 2 converts to 1 upon addition of base, consistent with the proposal that two units of the hydroxo monomer condense to give the dinuclear form. In addition, all spectra of pure 1 dissolved in DMSO-d6 included extra low intensity signals that were characteristic of the monomer. Thus, although 1 is favored over the neutral monomer in DMSO-d6, the two complexes exist as a mixture of equilibrating forms. Our results do not support the previous findings for the Re(V) complex with a macrocyclic diamine-diamide ligand related to dioxo-tetH6. The data indicate that the ability of an amido group to donate electron density to a Re(V) center is moderately greater than the donating ability of a neutral amine group.

Preparation and characterization of oxorhenium(V) complexes with 2,2'-biimidazole: the strong affinity of coordinated biimidazole for chloride ions via N-H...Cl- hydrogen bonding

N,N'-Dimethylbiimidazole and bipyridine (N-N) react with ReOCl3(OPPh3)(Me2S) to give mer-ReOCl3(N-N) compounds. Nonmethylated biimidazole forms a trans-O,O [ReOCl2(OPPh3)(biimH2)]+ cation, which is tightly associated with the Cl- counterion via N-H...Cl- hydrogen bonding. Hydrolysis of ReOCl3(biimMe2) in wet acetone (5% water) leads to the linear oxo-bridged dinuclear species [(OReCl2(biimMe2)2(mu-O)] containing chelated biimMe2. Acetone solutions containing only 1% water yield the bent oxo-bridged dinuclear species [(OReCl2)2(mu-O)(mu-biimMe2)2], where each Re center retains the ReO2Cl2N2 coordination but the biimMe2 ligands are bridging. The linear oxo-bridged [(OReCl2(biimH2)2(mu-O)] complex obtained with nonmethylated biimidazole includes two Cl- ions held via N-H...Cl- hydrogen bonds, leading to a dianionic [(OReCl2(biimH2...Cl)2(mu-O)]2- unit in the crystals of the PPh4+ salt. The compounds are characterized by IR and NMR spectroscopies, and the structures of [ReOCl2(OPPh3)(biimH2)]Cl, [(OReCl2(biimH2)2(mu-O)](PPh4Cl)(2).2H2O, and [(OReCl2)2(mu-O)(mu-biimMe2)2].acetone are determined by X-ray diffraction.