Influence of the Linker Length and Coordination Mode of (Di)Triazolylidene Ligands on the Structure and Catalytic Transfer Hydrogenation Activity of Iridium(III) Centers

A series of ditriazolylidene ligand precursors (2) containing alkyl or ether linkers of different lengths between the triazolium heterocycles (trzH–linker–trzH, with linkers (a) −, (b) CH2, (c) (CH2)2, (d) (CH2)3, and (e) CH2OCH2) have been prepared, and synthetic methods have been established to obtain selectively either Ctrz,Ctrz-bidentate chelating complexes (3) or Ctrz-monodentate coordinating triazolylidene iridium­(III) complexes with a pendant triazolium unit (4), as well as bimetallic complexes in which the ditriazolylidene ligand acts as a linker between the two metal centers (5). The linker has a distinct effect on the synthetic accessibility of these complexes, and on their reactivity. The structural properties of the complexes have been identified in solution and in the solid state and have been correlated with catalytic activity of the complexes in transfer hydrogenation catalysis. The best-performing catalysts derived from 3d and 4d achieved rates higher than 200 h–1. Catalytic activity profiles indicate distinctly different catalytic species for those complexes, while complexes 3a and 4a form the same active species.