Heteroleptic Trivalent Chromium In Coordination Chemistry: Novel Building Blocks For Addressing Old Challenges In Multimetallic Luminescent Complexes

Although less famous than low-spin trivalent cobalt found in [Co-III(NH3)(6-x)Cl-x]Cl3-x (x <= 3), which was exploited by Alfred Werner during the early part of the 20th century for establishing the basic rules of coordination chemistry, related trivalent chromium complexes exhibit comparable kinetic inertness, a rare property along the 3d-transition series. The associated slow isomerisation processes are compatible with the isolation of well-defined heteroleptic complexes. Some subtle energetic differences between Cr-III-X (X = halide, pseudo-halide, cyanide, solvent) and Cr-N bonds can be exploited for preparing stable and inert cis/trans-[Cr(N boolean AND N)(2)X-2](+) and fac/mer-[Cr(N boolean AND N boolean AND N)X-3] primary heteroleptic complexes incorporating multidentate nitrogen-containing chelate ligands. The use of these building blocks within the frame of the 'complex-as-ligand' strategy, or via the labilization of the remaining Cr-X bonds pave the way for the design of discrete polymetallic assemblies in which the photophysically appealing [(CrN6)-N-III] chromophores could find applications as functional light-converting devices. (C) 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).