Monika Sieger

Establishing the NO oxidation state in complexes [Cl5(NO)M]n−, M = Ru or Ir, through experiments and DFT calculations

Predominantly NO-centered reduction was observed by EPR and IR spectroelectrochemistry to occur reversibly at low temperatures for [Cl(5)Ir(NO)](-). In contrast, the [Cl(5)Ru(NO)](2-) ion was found to undergo only irreversible reduction but reversible oxidation to a ruthenium(III) species at -40 degrees C. DFT calculations were used to establish the electronic structures and to rationalise the different stabilities. The calculations also reveal orientation-dependent energies and EPR properties between staggered and eclipsed conformations.

Rhenium(I) coordinated lumazine and pterin derivatives: structure and spectroelectrochemistry of reversibly reducible (6-ATML)Re(CO)3Cl (6-ATML=6-acetyl-1,3,7-trimethyllumazine)

Abstract A number of complexes between substituted lumazines (2,4-dioxo-(1H,3H)pteridines) or pterins (2-amino-4-oxo-(3H)pteridines) and Re(CO)3Cl have been synthesized and characterized electrochemically and spectroscopically. The structure of the (6-ATML)Re(CO)3Cl derivative (6-ATML=6-acetyl-1,3,7-trimethyllumazine) could be determined in [fac-(6-ATML)Re(CO)3Cl]·3C6H6. The rhenium(I) center coordinates in a rather symmetrical fashion through the O4–N5 α-carbonylimino chelate site of 6-ATML; the potentially available acetyl function is not involved in the metal coordination. The acetyl acceptor substituent facilitates the reversible one-electron reduction of this compound to a persistent anion radical complex [(6-ATML)Re(CO)3Cl] −, which is accompanied by variable low-frequency shifts of the six carbonyl stretching bands and by the appearance of a broad EPR signal.

Establishing the Chelating α‐Azocarbonyl Function in π‐Acceptor Ligands

Four-center two-step redox systems [see Eq. (1)] with coordinating heteroatoms in 1,4-positions have long played a prominent role in coordination chemistry as potentially noninnocent chelate ligands. Reducible a-diimines (E, E’= NR) including 1,4-diazabutadienes, o-quinonediimines or “polypyridines” of the 2,2’-bipyridine or 1,10phenanthroline type have thus been studied particularly under the aspect of light-induced charge transfer, while the more easily reduced a-diketones and especially o-quinones (E, E’= O) can exhibit the phenomenon of redox isomerism (“valence tautomerism”) in their transition-metal complexes. The related complexes of a-dithiolene ligands (E, E’= S), long known and recently reinvestigated, are often cited as prototypes of coordination compounds with partially covalent metal–donor bonds.