Proton-Coupled Electron Transfer Studies on a Series of Osmium and Iridium Hydride Complexes
by Niyaz Alizadeh Kordestani
Date of Examination:2024-10-14
Date of issue:2024-11-15
Advisor:Prof. Dr. Sven Schneider
Referee:Prof. Dr. Sven Schneider
Referee:Prof. Dr. Dirk Schwarzer
Persistent Address:
http://dx.doi.org/10.53846/goediss-10871
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Abstract
English
Due to the important role of transition metal hydrides in organometallic catalysis, their properties and reactivity have been thoroughly investigated and reviewed. Considering their importance, substantial efforts were undertaken to categorize and parametrize thermodynamic trends in the periodic table, e.g., bond dissociation free energies (BDFE), hydricities and acidities of transition metal hydrides either as ground state reactivity or after photoexcitation in order to exploit them for catalytic transformations. The crucial influence of ligands on adjusting both the electronic and steric characteristics around the metal center is widely recognized, exerting a substantial impact on reactivity. Exploring the ligand-centered reactivity of transition metal complexes bearing "redox-active" ligands is currently a prominent focus in the fields of coordination chemistry[100,101,160]. These types of ligands demonstrate electron donor or acceptor behavior in which the redox events mainly occur on the ligands rather than the respective metal, facilitating the stabilization of a broad range of redox states and coordination modes. Tridentate, monoanionic P-N-P pincer type ligands are of this type and their properties and applications have been extensively studied in organometallic chemistry[102,103]. In addition to the PNP pincer type ligands, chelating ligands such as bipyridine can also tune the range of oxidation state a metal complex can posess. There have been several examples where 2,2’-bipyridine has been utilized as a building block for a PNN or NNN pincer ligands[111,112]. Over the years, the chemistry of Ru(II) and Os(II) polypyridyl complexes, especially those featuring 2,2’-bipyridine (bpy) have been extensively studied. A number of osmium (II) hydride complexes have been reported by Sullivan et al. bearing 2,2’-bipyridine ligand[161]. There have been several examples of osmium monohydride complexes over the past years, however, among them there has been only one example of an isolable osmium(III) hydride complex, [Os(en)2(py)H)]2+ reported by Taube et. al.[56]. Bianchini et. al.[77] have also reported the cyclic voltammogram of a osmium(II) hydride complex [OsIIHCl(PP3)] (PP3 = P(CH2CH2PPh2)3) which undergoes one electron oxidation, that is discussed below. However, no isolated [OsIIIHCl(PP3)]+ has been reported so far to the best of our knowledge. This encourages a systematic design of an osmium hydride complex with increased thermochemical stability and kinetic inertness toward oxidation. For this purpose two sets of ligands were utilized. An unsaturated PNP pincer ligand to stabilize the possible formed transition metal hydride radical cations and providing a robust framework to guarantee the access to various33 oxidation states. Also bipyridine ligand as a redox-active ligand providing access to lower reduction potentials and increased stability.
Keywords: Osmium, PCET, Hydrogen Atom Transfer
