Reactivity of Electrochemically Generated Rhenium (II) Tricarbonyl α-Diimine Complexes: A Reinvestigation of the Oxidation of Luminescent Re(CO)3(α-Diimine)Cl and Related Compounds

The oxidative electrochemistry of luminescent rhenium (I) complexes of the type Re(CO)3(LL)Cl, 1, and Re(CO)3(LL)Br, 2, where LL is an α-diimine, was re-examined in acetonitrile. These compounds undergo metal-based one-electron oxidations, the products of which undergo rapid chemical reaction. Cyclic voltammetry results imply that the electrogenerated rhenium (II) species 1+ and 2+ disproportionate, yielding [Re(CO)3(LL)(CH3CN)]+, 7, and additional products. Double potential step chronocoulometry experiments confirm that 1+ and 2+ react via second-order processes and, furthermore, indicate that the rate of disproportionation is influenced by the basicity and steric requirements of the α-diimine ligands. The simultaneous generation of rhenium (I) and (III) carbonyl products was detected upon the bulk oxidation of 1 using infrared spectroelectrochemistry. The rhenium (III) products are assigned as [Re(CO)3(LL)Cl2]+, 5; an inner-sphere electron-transfer mechanism of the disproportionation is proposed on the basis of the apparent chloride transfer. Chemically irreversible two-electron reduction of 5 yields 1 and Cl−. No direct spectroscopic evidence was obtained for the generation of rhenium (III) tricarbonyl bromide disproportionation products, [Re(CO)3(LL)Br2]+, 6; this is attributed to their relatively rapid decomposition to 7 and dibromine. In addition, the 17-electron radical cations, 7+, were successfully characterized using infrared spectroelectrochemistry

Redox Properties of a Rhenium Tetrazolato Complex in Room Temperature Ionic Liquids: Assessing the Applicability of the Stokes-Einstein Equation for a Metal Complex in Ionic Liquids

The redox properties of a rhenium-tetrazolato complex, namely fac-[Re(CO)3(phen)L] (where L is 5-(4'-cyanophenyl)tetrazolate), have been studied by cyclic voltammetry in a range of common room temperature ionic liquids (RTILs) with different anions and cations. In all eight RTILs, one reduction and two oxidation peaks are observed. It is believed that the reduction peak corresponds to ligand reduction and the two oxidation peaks are two one-electron oxidations of the metal from Re(I) to Re(II) and Re(II) to Re(III). The redox potentials of the metal oxidations appear to be unchanged with the solvent; however, the potential for the reduction peak is more negative in RTILs containing the[P14,6,6,6]+ cation, suggesting a stabilization effect of the electrogenerated intermediate with the other RTIL cations studied (imidazolium and pyrrolidinium). Potential step chronoamperometric experiments were used to calculate diffusion coefficients of the complex in RTILs, and it was found that fac-[Re(CO)3(phen)L] diffuses very slowly through the RTIL medium. A plot of diffusion coefficient against the inverse of viscosity of the RTIL solvent showed a linear trend, suggesting that the Stokes-Einstein relationship generally applies for this complex in RTILs, but the coefficient on the denominator is likely to be closer to 4 (the “slip” limit) than 6 (the “stick” limit) when taking into account the hydrodynamic radius

Parametrization of the contribution of mono- and bidentate ligands on the symmetric C=O stretching frequency of fac-[Re(CO)3]+ complexes

A ligand parameter, IRP(L), is introduced in order to evaluate the effect that different monodentate and bidentate ligands have on the symmetric C≡O stretching frequency of octahedral d6 fac-[Re(CO)3L3] complexes (L = mono- or bidentate ligand). The parameter is empirically derived by assuming that the electronic effect, or contribution, that any given ligand L will add to the fac-[ReCO3]+ core, in terms of the total observed energy of symmetric C≡O stretching frequency (νCOobs), is additive. The IRP(CO) (i.e., the IRP of carbon monoxide) is first defined as one-sixth that of the observed C≡O frequency (νCOobs) of [Re(CO)6]+. All subsequent IRP(L) parameters of fac-[Re(CO)3L3] complexes are derived from IRP(L) = 1/3[νCOobs − 3IRP(CO)]. The symmetric C≡O stretching frequency was selected for analysis by assuming that it alone describes the “average electronic environment” in the IR spectra of the complexes. The IRP(L) values for over 150 ligands are listed, and the validity of the model is tested against other octahedral d6 fac-[M(CO)3L3] complexes (M = Mn, 99Tc, and Ru) and cis-[Re(CO)2L4]+ species and by calculations at the density functional level of theory. The predicted symmetric C≡O stretching frequency (νCOcal) is given by νCOcal = SR[∑IRP(L)] + IR, where SR and IR are constants that depend upon the metal, its oxidation state, and the number of CO ligands in its primary coordination sphere. A linear relationship between IRP values and the well-established ligand electrochemical parameter EL is found. From a purely thermodynamic point of view, it is suggested that ligands with high IRP(L) values should weaken the M−CO bond to a greater extent than ligands with low IRP(L) values. The significance of the results and the limitations of the model are discussed

Photochemistry in a 3D metal-organic framework (MOF): monitoring intermediates and reactivity of the fac-to-mer photoisomerization of re(diimine)(CO)3Cl incorporated in a MOF

The mechanism and intermediates in the UV-light-initiated ligand rearrangement of fac-Re(diimine)(CO)3Cl to form the mer isomer, when incorporated into a 3D metal–organic framework (MOF), have been investigated. The structure hosting the rhenium diimine complex is a 3D network with the formula {Mn(DMF)2[LRe(CO)3Cl]}∞ (ReMn; DMF = N,N-dimethylformamide), where the diimine ligand L, 2,2′-bipyridine-5,5′-dicarboxylate, acts as a strut of the MOF. The incorporation of ReMn into a KBr disk allows spatial distribution of the mer-isomer photoproduct in the disk to be mapped and spectroscopically characterized by both Fourier transform infrared and Raman microscopy. Photoisomerization has been monitored by IR spectroscopy and proceeds via dissociation of a CO to form more than one dicarbonyl intermediate. The dicarbonyl species are stable in the solid state at 200 K. The photodissociated CO ligand appears to be trapped within the crystal lattice and, upon warming above 200 K, readily recombines with the dicarbonyl intermediates to form both the fac-Re(diimine)(CO)3Cl starting material and the mer-Re(diimine)(CO)3Cl photoproduct. Experiments over a range of temperatures (265–285 K) allow estimates of the activation enthalpy of recombination for each process of ca. 16 (±6) kJ mol–1 (mer formation) and 23 (±4) kJ mol–1 (fac formation) within the MOF. We have compared the photochemistry of the ReMn MOF with a related alkane-soluble Re(dnb)(CO)3Cl complex (dnb = 4,4′-dinonyl-2,2′-bipyridine). Time-resolved IR measurements clearly show that, in an alkane solution, the photoinduced dicarbonyl species again recombines with CO to both re-form the fac-isomer starting material and form the mer-isomer photoproduct. Density functional theory calculations of the possible dicarbonyl species aids the assignment of the experimental data in that the ν(CO) IR bands of the CO loss intermediate are, as expected, shifted to lower energy when the metal is bound to DMF rather than to an alkane and both solution data and calculations suggest that the ν(CO) band positions in the photoproduced dicarbonyl intermediates of ReMn are consistent with DMF binding

Oncological Applications of Positron Emission Tomography with Fluorine-18 Fluorodeoxyglucose

Positron emission tomography (PET) is now primarily used in oncological indication owing to the successful application of fluorine-18 fluorodeoxyglucose (FDG) in an increasing number of clinical indications at different stages of diagnosis, and for staging and follow-up. This review first considers the biological characteristics of FDG and then discusses methodological considerations regarding its use. Clinical indications are considered, and the results achieved in respect of various organs and tumour types are reviewed in depth. The review concludes with a brief consideration of the ways in which clinical PET might be improved