Artificial metalloenzymes: The powerful alliance between protein scaffolds and organometallic catalysts

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Synthesis and characterisation of cis-dioxomolybdenum(VI) complexes of N-substituted 3-hydroxy-2-pyridinones

The butoxy- and methoxy-derivatives (HLc, and HLb, respectively) of 1-[(ethoxycarbonyl)methyl]-3-hydroxy-2-(1H)-pyridinone, HLa, were synthesized by transesterification and the mononuclear complexes [MoO2(L(a-c))2] were obtained by the reaction of the ligands with molybdenyl acetylacetonate. The three complexes were fully characterized by (1)H and (13)C NMR spectroscopy, mass spectrometry and elemental analyses. A variable temperature NMR study showed that the N-substituents do not have a significant influence on the racemisation process of the complexes. The comparison of the crystal structure of [MoO2(Lb)2] with [MoO2(La)2] confirmed that an additional methylene group does not affect the immediate coordination environment of the molybdenum centre. The methoxy-derivative HLb was also found to produce a dinuclear complex of composition [(MoO2(Lb)O)2], the structure of which was determined by X-ray diffraction.     

Synthesis of a cholesterol-appended Tb-DTPA complex by combined removal of tert-Butyl protecting groups and complexation of terbium(III)

The Tb(III)-complex of a cholesterol-DTPA conjugate was prepared in a one-step procedure from the tert-Butyl-protected form of the ligand through reaction with TbCl₃^.6H₂O and NaI in refluxing acetonitrile. The acid-labile carbamate linker that connects the chelating DTPA moiety with the cholesterol unit of the ligand stays intact under these conditions. The combination of deprotection of the ligand with terbium complexation reduces the overall number of synthetic steps required and thus increases the efficiency and overall yield of the reaction sequence.     

Spectroscopic and structural investigations reveal the signaling mechanism of a luminescent molybdate sensor

A heteroditopic ligand H(2)-L consisting of a dihydroxybenzene (catechol)-unit linked via an amide bond to a pyridyl-unit and its methyl-protected precursor Me(2)-L were synthesized, characterized, and their photophysical properties investigated. The three accessible protonation states of the ligand, H(3)-L(+), H(2)-L, and H-L(-), showed distinct (1)H NMR, absorption and emission spectroscopic characteristics that allow pH-sensing. The spectroscopic signatures obtained act as a guide to understand the signaling mechanism of the luminescent pH and molybdate sensor [Re(bpy)(CO)(3)(H(2)-L)](+). It was found that upon deprotonation of the 2-hydroxy group of H(2)-L, a ligand-based absorption band emerges that overlaps with the Re(dπ)→bpy metal-to-ligand charge transfer (MLCT) band of the sensor, reducing the quantum yield for emission on excitation in the 370 nm region. In addition, deprotonation of the catechol-unit leads to quenching of the emission from the Re(dπ)→bpy (3)MLCT state, consistent with photoinduced electron transfer from the electron-rich, deprotonated catecholate to the Re-based luminophore. Finally, reaction of 2 equiv of [Re(bpy)(CO)(3)(H(2)-L)](+) with molybdate was shown to give the zwitterionic Mo(VI) complex [MoO(2){Re(CO)(3)(bpy)(L)}(2)], as confirmed by electrospray ionization (ESI) mass spectrometry and X-ray crystallography. The crystal structure determination revealed that two fully deprotonated sensor molecules are bound via their oxygen-donors to a cis-dioxo-MoO(2) center.     

Synthesis, characterization, solid-state structures, and spectroscopic properties of two catechol-based luminescent chemosensors for biologically relevant oxometalates

The new heteroditopic ligand 2,3-dihydroxy-N-(1,10-phenanthroline-5-yl)benzamide (H2-L3) was synthesized and coordinated to [Ru(bpy)2(phen)]2+- and [ReBr(CO)3(phen)]-type luminophores (bpy = 2,2'-bipyridine and phen = 1,10-phenanthroline). The resulting chemosensors [Ru(bpy)2(H2-L3)]2+ and [ReBr(CO)3(H2-L3)] were fully characterized and their solid-state structures and spectroscopic properties were investigated to assess how the photophysical properties of the luminescent signaling units affect the performance of the sensors. [Ru(bpy)2(H2-L3)]2+ and [ReBr(CO)3(H2-L3)] both signal the presence and concentration of molybdate and vanadate in aqueous acetonitrile through a decrease in emission intensity. [ReBr(CO)3(H2-L3)] also detects tungstate. Due to the higher emission intensity of the Ru-based sensor, its detection limits for molybdate (43 microg L(-1)) and vanadate (24 microg L(-1)) are almost 1 order of magnitude lower than the ones achieved with the Re-based sensor. The optimum working pH of the chemosensors is determined by the pKa values of the 2-hydroxy-groups of the receptor units: pH 4 for [ReBr(CO)3(H2-L3)] and pH 3 for [Ru(bpy)2(H2-L3)]2+. Both sensors are selective: equimolar amounts of PO4(3-), SO4(2-), ReO4-, Mn(II), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) do not interfere with the detection of molybdate or vanadate.     

Eta(1)-2-pyrone metal carbonyl complexes as CO-releasing molecules (CO-RMs): a delicate balance between stability and CO liberation

An evaluation of the CO releasing ability of iron(II) and molybdenum(II) complexes has facilitated the discovery of the most rapid CO releaser, namely [Mo(CO)(3)(eta(5)-C(5)H(5))(eta(1)-{O}-C{=O}-O-CMe=CH-COMe=CBr)]BF(4) (CORM-F10), reported to date. The rate of CO release is related to the overall solution phase stability of the transition metal carbonyl complex. The cytotoxicity and vasodilatory properties of CORM-F10 have been determined.     

CO2 photoreduction with long-wavelength light: dyads and monomers of zinc porphyrin and rhenium bipyridine

Photocatalytic CO(2) reduction has been studied for two dyads with porphyrin covalently attached to rhenium tricarbonyl bipyridine moieties, and on separate components consisting of [Re(CO)(3)(Picoline)Bpy](+) and either zinc porphyrin or zinc chlorin. TONs decrease in the order: zinc porphyrin + Re > long spacer dyad > zinc chlorin + Re > short spacer dyad.