Roger Alberto

An efficient homogeneous intermolecular rhenium based photocatalytic system for the production of H2

We present an artificial photocatalytic model for photosystem I (PSI) using [ReBr(CO)(3)bipy)] (1) as a photosensitizer, [Co(dmgH)(2)] (2) as a hydrogen evolution reaction catalyst, and triethanolamine as an irreversible reductive quencher. Complex 1 is more robust in the long run, and turnover numbers were more than doubled in the present study as compared to the commonly used photosensitizer [Ru(bipy)(3)](2+). The quantum yield for hydrogen production with 1 was found to be 26 +/- 2% (H produced per absorbed photon). Forward electron transfer between 1(-) and 2 was found to occur at a rate close to diffusion control (k(1) = 2.5 +/- 0.1 x 10(8) M(-1) s(-1)). The rate of hydrogen production exhibited a linear dependence on the photon flux and a quadratic dependence on the total concentration of Co (k(obs) = 3.7 +/- 0.1 M(-1) s(-1)). Therefore, a second-order process in Co(III)-H is proposed. The process showed a complex dependence on [AcOH]. An excess of dimethylglyoxime was systematically added to the system to ensure the complete formation of 2 and reduce the portion of free [Co](solv)(2+), an efficient quencher of the excited state of 1.

Photocatalytic H2 production from water with rhenium and cobalt complexes.

Photocatalytic hydrogen production in pure water for three component systems using a series of rhenium-based photosensitizers (PS) and cobalt-based water reduction catalysts (WRC), with triethanolamine (TEOA) as an irreversible electron donor, is described. Besides the feasibility of this reaction in water, key findings are reductive quenching of the excited state of the PS by TEOA (k(q) = 5-8 × 10(7) M(-1) s(-1); Φ(cage) = 0.75) and subsequent transfer of an electron to the WRC (k(Co(III)) = 1.1 × 10(9) M(-1) s(-1)). Turnover numbers in rhenium (TON(Re), H/Re) above 500 were obtained, whereas TON(Co) (H(2)/Co) did not exceed 17. It is shown that the cobalt-based WRC limits long-term performance. Long-term performance critically depends on pH and the type of WRC used but is unaffected by the type of PS or the concentration of WRC. A quantum yield of 30% was obtained (H/photon).

Synthesis and characterization of open and sandwich-type polyoxometalates reveals visible-light-driven water oxidation via POM-photosensitizer complexes

The trivacant triruthenium-substituted polyoxometalate α-K6Na[{Ru3O3(H2O)Cl2}(SiW9O34)]·17H2O (1) and two sandwich-type Co- and Ni-containing silicotungstates K11Na1[M4(H2O)2(SiW9O34)2]·nH2O (M = Co2+, Ni2+) (2, 3) based on the same Keggin-type building block were obtained through variation of a solution-based protocol. All compounds were investigated with respect to their performance as visible-light-driven water oxidation catalysts (WOCs). Analytical characterizations and photocatalytic reactivity of (1) are correlated with results of B97D/Def2-TZVP computation. The WOC activity of (1) and (2) was found to arise from their immobilization into a recyclable POM–photosensitizer complex (POM–PS). These POM–PS complexes are formed in the initial phase of the reaction, and they prevent POM decomposition into colloidal oxide catalysts.

New Derivatives of Vitamin B12 Show Preferential Targeting of Tumors

Rapidly growing cells show an increased demand for nutrients and vitamins. The objective of our work is to exploit the supply route of vitamin B12 to deliver new derivatives of this vital vitamin to hyperproliferative cells. To date, radiolabeled ((57)Co and (111)In) vitamin B12 derivatives showed labeling of tumor tissue but also undesired high accumulation of radioactivity in normal tissue. By abolishing the interaction of a tailored vitamin B12 derivative to its transport protein transcobalamin II and therefore interrupting transcobalamin II receptor and megalin mediated uptake in normal tissue, preferential accumulation of a radiolabeled vitamin in cancer tissue could be accomplished. We identified transcobalamin I on tumors as a possible new receptor for this preferential accumulation of vitamin-mediated targeting. The low systemic distribution of radioactivity and the high tumor to blood ratio opens the possibility of a more successful clinical application of vitamin B12 for imaging or therapy.

[(Cp-R)M(CO)3] (M=Re or 99mTc) Arylsulfonamide, Arylsulfamide, and Arylsulfamate Conjugates for Selective Targeting of Human Carbonic Anhydrase IX†

Enhanced receptor selectivity: carbonic anhydrase inhibitors are relevant for both cancer diagnosis and therapy. Combining non-radioactive Re compounds with their radioactive (99m)Tc homologs enables the use of identical molecules for therapy and imaging (theragnostic). The syntheses and in vitro evaluation of [(Cp-R)M(CO)(3)] (Cp=cyclopentadienyl, M=Re, (99m)Tc) with R being a highly potent carbonic-anhydrase-targeting vector is reported.

3d element complexes of pentadentate bipyridine-pyridine-based ligand scaffolds: structures and photocatalytic activities.

The synthesis of the two penta-pyridyl type ligands pyridine-2,6-diylbis(dipyridin-2-ylmethanol) (PPy, 1) and bis-2,2-bipyridine-6-yl(pyridine-2-yl)methanol (aPPy, 2) is described. Both ligands coordinate rapidly to the 3d element cations Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II), thereby yielding complexes of the general composition [MBr(1)](+) and [MBr(2)](+), respectively. Further, the X-ray structures of selected complexes with ligands 1 and 2 are described. They show metal center dependent structural features and complexes with 2 exhibiting distinctly distorted octahedral geometries. Moreover, photocatalytic water reduction with [Co(II)Br(PPy)]Br (1c) and [Co(II)Br(aPPy)]Br (2c) as water reducing catalysts (WRC) was investigated. Both complexes showed catalytic activity in water when in presence of ascorbic acid as sacrificial electron donor and [Re(py)(bpy)(CO)3](+) (3) as photosensitizer (PS). Turnover numbers, TONs (H2/Co), up to 11,000 were achieved. Complex 2c was more active than 1c, whereas none of the other complexes showed any activity.

Syntheses, structural characterization and CO releasing properties of boranocarbonate H3BCO2H](-) derivatives

CO plays an important role in biological processes and molecules which release CO in a controllable way could therefore be used for medicinal purposes. Beside organometallic carbonyl complexes, boranocarbonate [H(3)BCO(2)H](-) is one of the most promising candidates but releases CO too rapidly. In order to delay the CO release, we have prepared boranocarbamates [H(3)BCONH-R](-) from [H(3)BCO(2)H](-) which comprise histamine, morpholine, aniline and ethylene-diamine bound via amides to the {H(3)BCO} moiety. The syntheses of the new derivatives is described together with their structural characterization. These compounds release CO at a much slower rate than the parent compound and are therefore potential CO releasing molecules for biological and medicinal application.

Metal-mediated retro Diels-Alder of dicyclopentadiene derivatives: a convenient synthesis of [(Cp-R)M(CO)3] (M = 99mTc, Re) complexes.

The synthesis of piano-stool type complexes [(Cp-R)99mTc(CO)3] from water and, if possible, directly from [99mTcO4]- is a persisting challenge. Such complexes are very convenient for labeling biomolecules since they are small in size and “innocent” with respect to noncovalent interactions in biological systems. We found that [(Cp-R)99mTc(CO)3] can be prepared directly from the dimerized Diels−Alder precursors and from [99mTcO4]-. Since the concentration of the respective monomers in the reaction solutions was undetectable, the formation of [(Cp-R)99mTc(CO)3] must be based on a metal-mediated retro Diels−Alder reaction with concerted Cp coordination. The authenticity of the formed 99mTc complexes was confirmed by comparing their HPLC retention times with those of the structurally characterized rhenium surrogates. Besides the principle access to piano-stool complexes for radiopharmaceutical studies, a metal-mediated retro Diels−Alder reaction under concomitant formation of the corresponding piano-stool comp...

99TcO4−: Selective Recognition and Trapping in Aqueous Solution

Too hot to handle: Unprecedented affinity and specificity for (99)TcO(4)(-) in aqueous solution was shown with the p-xylyl azacryptand in the hexaprotonated form. A crystal structure of the complex reveals how the anion fits within the cavity of the cage, and the formation of multiple H-bond interactions with protonated amino groups stabilize the adduct.

Metal complex mediated conjugation of peptides to nucleus targeting acridine orange: a modular concept for dual-modality imaging agents.

To target the nucleus of specific cells, trifunctional radiopharmaceuticals are required. We have synthesized acridine orange derivatives which comprise an imidazole-2-carbaldehyde function for coordination to the [Re(CO)₃](+) or [(99m)Tc(CO)₃](+) core. Upon coordination, this aldehyde is activated and rapidly forms imines with amines from biological molecules. This metal-mediated imine formation allows for the conjugation of a nuclear targeting portion with a specific cell receptor binding function directly on the metal. With this concept, we have conjugated the acridine orange part to a bombesin peptide directly on the (99m)Tc core and in one step. In addition, a linker containing an integrated disulfide has been coupled to bombesin. LC/MS study showed that the disulfide was reductively cleaved with a 60 min half-life time. This concept enables the combination of a nucleus targeting agent with a specific cell receptor molecule directly on the metal without the need of separate conjugation prior to labeling, thus, a modular approach. High uptake of the BBN conjugate into PC-3 cells was detected by fluorescence microscopy, whereas uptake into B16BL6 cells was negligible.