Dynamic dimer-monomer equilibrium in a cycloruthenated complex of [Re(η6-C6H6)2]. Chem Commun (Camb) 2020;56:10658-10661. [PMID: 32785303 DOI: 10.1039/d0cc04180g]

Unraveling the Mechanistic Details of Ru-Bis(pyridyl)borate Complex Catalyst for the Dehydrogenation of Ammonia Borane

Ru-Bis(pyridyl)borate complex (CAT) is an efficient catalyst for ammonia borane (AB) dehydrogenation. Although the mechanistic pathway of this catalyst has been theoretically investigated previously, the gap between the experimental findings and the computational results could not be bridged thus far. In our study, using density functional theory calculations, we elucidate the mechanism of AB dehydrogenation of CAT at a variable degree of ligand hydrogenation. Our results confirm that the acetonitrile ligands get reduced in the presence of AB and remain hydrogenated. Moreover, in line with experiments, we find that AB dehydrogenation on CAT proceeds via a concerted mechanism (with the free energy energetic span between 25.4 and 32.5 kcal/mol). We find that the ligand reduction alters the electronic structure and activity of CAT and the highest activity of the catalyst is expected at the fifth degree of hydrogenation of ligands with an energetic span of 25.4 kcal/mol. Additionally, the mechanism for the removal of molecular H₂ from the catalysts also alters with the degree of ligand hydrogenation. Furthermore, our results show that optimal H₂ binding free energy calculations can be used as a descriptor to identify the most active sites. Finally, this work demonstrates that ligand reduction improves the activity of the catalyst. These results highlight the importance of ligand hydrogenation in probing the activity and operating mechanism of the Ru-bis(pyridyl)borate complexes for AB dehydrogenation. Further, we identify a plausible dimer structure and rationalized experimental observation that the deactivation chemistry of this catalyst is different from the Shvo's catalyst.

Naphthalene Exchange in [Re(η6 -napht)2 ]+ with Pharmaceuticals Leads to Highly Functionalized Sandwich Complexes [M(η6 -pharm)2 ]+ (M=Re/99m Tc)

Bis‐arene sandwich complexes are generally prepared by the Fischer‐Hafner reaction, which conditions are incompatible with most O‐ and N‐ functional groups. We report a new way for the synthesis of sandwich type complexes [Re(η⁶‐arene)₂]⁺ and [Re(η⁶‐arene)(η⁶‐benzene)]⁺ from [Re(η⁶‐napht)₂]⁺ and [Re(η⁶‐napht)(η⁶‐benzene)]⁺, with functionalized arenes and pharmaceuticals. N‐methylpyrrolidine (NMP) facilitates the substitution of naphthalene with the incoming arene. A series of fully characterized rhenium sandwich complexes with simple arenes, such as aniline, as well as with active compounds like lidocaine and melatonin are presented. With these rhenium compounds in hand, the radioactive sandwich complexes [^99mTc(η⁶‐pharm)₂]⁺ (pharm=pharmaceutical) can be unambiguously confirmed. The direct labelling of pharmaceuticals with ^99mTc through η⁶‐coordination to phenyl rings and the confirmation of the structures with the rhenium homologues opens a path into molecular theranostics.