Manganese complexes with chelating and bridging di-triazolylidene ligands: synthesis and reactivity

Abstract

New manganese complexes bearing di-triazolylidene (di-trz) ligands are described. Depending on the wingtip substituents of the triazolylidene ligand and the synthetic procedure, two different ligand coordination modes were observed, i.e, bridging and chelating. A series of Mn(I) complexes of the general type fac-[Mn(di-trz^R)(CO)₃Br] (R = Me, Et, Mes) with a chelating di-trz ligand were prepared via Ag-transmetalation. In contrast, the in situ deprotonation of the triazolium salts with KOBu^t yielded the bimetallic Mn(0) complexes [Mn₂(CO)₈(μ-di-trz^R)] with a bridging di-trz ligand when short alkyl chains (Me, Et, i-Pr) are present as the N1 substituents of the triazolylidene ligand. The molecular structures of monometallic and bimetallic complexes were determined by X-ray diffraction studies. In addition, the cationic fac-[Mn(di-trz^Et)(CO)₂(PPh₃)₂]Br complex, a rare example of a dicarbonyl Mn(I) N-heterocyclic carbene, was obtained when fac-[Mn(di-trz^Et)(CO)₃Br] was irradiated with visible light in the presence of PPh₃. The crystal structure revealed a slightly distorted octahedral geometry around the Mn(I) centre, with the chelating di-triazolylidene ligand situated in trans position to the two CO ligands in the equatorial plane, and the two phosphine ligands occupying the axial positions. Cyclic voltammetry studies show reversible redox processes for the monometallic Mn(I) complexes, and a quasi-reversible EC mechanism for the oxidation of the bimetallic complexes. Infrared spectroelectrochemical studies along with DFT calculations for fac-[Mn(di-trz^Et)(CO)₃Br] suggest that the observed two consecutive reductions both occur at the metal centre.