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.