Entwicklung von neuartigen dinuklearen Molybdänkomplexen mit langlebigen angeregten Zuständen für die photochemische N2-Aktivierung
Zusammenfassung der Projektergebnisse
The initially proposed novel dinuclear molybdenum complexes could not be realized. The overall goal of the proposed project was the rational development of catalysts for the lightdriven activation of metal-bound N2. The primary goal toward this end was the establishment of long-lived MLCT excited states on Mo-N=N-Mo dimers, from which photochemical N2 activation could occur. The plan was to synthesis the most promising ligands and the new target complexes based on these ligands derived from the synthetic protocol of the earlier published complex [{(PhTpy)(PPh2Me)2Mo}2(μ2-N2)][BArF24]2 (PhTpy = 4’-Phenyl-2,2’,6’,2’’- terpyridine; ArF24 = (C6H3-3,5-(CF3)2)4). It was possible to synthesize most of the targeted ligands, metal precursors and counter anions but not the targeted complexes. The surprisingly difficult step was the reduction of the synthesized precursor complex [MoIIICl3(L)] (L = tridentate ligand) to the Mo(I) complex with Na(Hg)x or KC8 in the present of two equivalents of the monodentate isocyanide ligands. These synthetic steps were difficult in the view of the analytic (paramagentic complexes) and did not lead to the desired complex with the monodentate ligands. Therefore, the project was discontinued and the monodentate ligand was used to investigate possible luminescent transition metal complexes with Mn(I), Re(I) and Cr(III). These investigations led, amongst others, to a homoleptic manganese(I) complex with the aryliocyanide ligand. The monodentate ligand in this complex dissociates from the manganese(I) complex upon photo-excitation, due to the population of electronically excited states with antibonding character. Optical spectroscopic studies are complemented by structural, electrochemical and computational investigations, including detailed analysis of the vibrational properties of the manganese(I) complex. The key finding of this is that chelating isocyanide ligands seem essential to obtain a luminescent MLCT state, whereas monodentate arylisocyanides do not lead to photorobust coordination compounds with manganese(I). First investigations with Re(I) and monodentate isocyanide ligands exhibit also a certain photo instability as well as problems in the complexation. Furthermore, the synthesis and reduction with Cr(II) or Cr(III) precursors and e.g. Na(Hg)x did not result into a Cr(0) complex which could then be oxidized to Cr(III). These results show that the monodentate arylisocyanide ligands from the original proposal are not a good choice in terms of photostability and complexation with reduction reagents. The new focus for the former project could be based on the photochemical behavior of the well-known complex [(NH3)5OsII/III(μ-N2)OsII/III(NH3)5]5+ and a design based on a tetradentate tripodal ligand (e.g. tris(phosphino)silyl ligands, (SiPR3)- (R = Ph, iPr)) to avoid the need of a monodentate ligand.
Projektbezogene Publikationen (Auswahl)
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Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence. Inorganic Chemistry, 61(27), 10533-10547.
Ossinger, Sascha; Prescimone, Alessandro; Häussinger, Daniel & Wenger, Oliver S.
