Ferdinand Belaj

Ion pairing in oxidorhenium( v ) complexes triggers striking solvatochromism via ion-pair charge transfer (IPCT), a phenomenon mechanistically elucidated through combined experiment and theory.

In this manuscript, we report on the formation and isolation of the di-nuclear rhenium(VI) µ-oxido bridged complex {[ReOCl₂(L1)]}₂(µ-O) (1), which we nick-named “deep purple” due to its intense purple color both in the solid state as well as in solution, and its surprising catalytic activity in oxyanion reduction. Complex 1 was obtained as the unintended product in an attempt to synthesize oxidorhenium(V) complex [ReOCl(L1)₂], using the O,N-bidentate phenol-dimethyloxazoline ligand HL1, equipped with two electron-donating tert-butyl groups on the phenol ring. Single crystal X-ray diffraction analysis finally revealed the surprising µ-oxido structure of 1, with two paramagnetic Re(VI) centers and a mixed cis/trans-chlorido arrangement on the two Re centers. Via a targeted synthesis in cyclohexane at 60 °C under ambient conditions, high yields of over 90% of 1 were obtained. To explain the source of the highly stereoselective formation of 1, its formation was examined by DFT calculations at the B3LYP-D3BJ/dhf-TZVPP@B3LYP-D3BJ/dhf-SVP level of theory, with solvent effects included via the COSMO model. In contrast to isoelectronic µ-oxido bridged Mo(V) complexes, 1 proved to be highly active in catalytic oxyanion reduction, namely perchlorate and nitrate reduction. With a catalyst loading of 5 mol% of 1, a conversion of 90% was observed for perchlorate. These findings highlight that in rhenium chemistry, µ-oxido bridged complexes like 1 still show promising catalyst activities for oxyanion reduction and potentially also other redox transformations.

One very unique feature of oxidorhenium(V) complexes is their dual catalytic activity in both reduction of stable oxyanions like perchlorate ClO₄ ⁻ and nitrate NO₃ ⁻ as well as epoxidation of olefins. In our ongoing research efforts, we were interested to study how an electron-withdrawing ligand would affect both these catalytic reactions. Hence, we synthesized the novel bidentate dimethyloxazoline-dichlorophenol ligand HL1 and synthesized oxidorhenium(V) complex [ReOCl(L1)₂] (1). Then, catalytic experiments were conducted showing that non-redox epoxidation activity is indeed enhanced, but redox catalysis via oxygen atom transfer (OAT) activity was reduced for ClO₄ ⁻ and NO₃ ⁻ reductions. From one nitrate reduction experiment, a small amount of the singly-oxidized dioxidorhenium(VI) complex [ReO₂(L1)] (2) could be isolated, confirming the successful reduction sequence of nitrate to nitrite NO₂ ⁻ (2e⁻ reduction) to NO (1e⁻ reduction). Furthermore, ligand L1 displayed a richer than usually observed coordination chemistry, allowing for the isolation of complexes [ReOCl₂(SMe₂)(L1)] (trans-3a), [ReOCl₂(OPPh₃)(L1)] (3b) and [ReCl₃(OPPh₃)(L1)] (3c). Complexes 1 and 3a-b were tested in cyclooctene epoxidation, 1 was additionally investigated as an oxyanion reduction catalyst of perchlorate and nitrate. All compounds HL1, 1, 2 and 3a–c could be characterized by single-crystal X-ray diffraction, besides other routine analyses.

Cu(II) complexes with meso-HMPAO and d,l-HMPAO were synthesized and characterized using single-crystal X-ray diffraction analysis, FTIR spectroscopy, cyclic voltammetry, and mass spectrometry. The electrochemical data indicated that the complexes bind to DNA molecules, specifically through guanine. These findings suggest that the new compounds have significant potential for further investigation, particularly for their anticancer activity and as models for the development of radiopharmaceuticals with 64Cu.

Various 4-aminotetrahydropyridinylidene salts were treated with aldehydes in an alkaline medium. Their conversion to 5-substituted β-hydroxyketones in a one-step reaction succeeded only with an aliphatic aldehyde. Instead, aromatic aldehydes gave 5-substituted β-aminoketones or a single δ-diketone. The new compounds were characterized using spectroscopic methods and a single crystal structure analysis. Some of them showed anticancer and antibacterial properties.

Various 4-aminotetrahydropyridinylidene salts were treated with aldehydes in alkaline medium. Their conversion to 5-substituted -hydroxyketones in a one-step reaction succeeded only with an aliphatic aldehyde. Instead, aromatic aldehydes gave 5-substituted -aminoketones or a single -diketone. The new compounds were characterized by spectroscopic methods and a single crystal structure analysis. Some of them showed anticancer and antibacterial properties.

Multinuclear complexes of heavy metals, such as tungsten, have demonstrated considerable potential as candidates for advanced radiocontrast agents. Of particular interest is the development of stable non-ionic compounds with high metal content and reasonably low osmolality in solution. Accordingly, we have synthesized a series of neutral W(V) complexes that contain the W₂O₂(µ-O)₂ core and amino acids or disubstituted EDTA derivatives as ligands. The compounds were prepared from the oxalatotungstate(V) complex via a convenient procedure utilizing microwave heating. Their detailed characterization was accomplished by electrospray ionization high-resolution mass spectrometry (ESI-HRMS), ¹H and ¹³C NMR spectroscopy, elemental analysis, and X-ray crystallography. Further experiments to evaluate the utility of the complexes as radiocontrast media were precluded by their poor aqueous solubility.