Neutral Formazan Ligands Bound to the -(CO)Re(I) Fragment: Structural, Spectroscopic, and Computational Studies.

Metal complexes with ligands that coordinate via the nitrogen atom of azo (N═N) or imino (C═N) groups are of interest due to their π-acceptor properties and redox-active nature, which leads to interesting (opto)electronic properties and reactivity. Here, we describe the synthesis and characterization of rhenium(I) tricarbonyl complexes with neutral ,-bidentate formazans, which possess both N═N and C═N fragments within the ligand backbone (Ar-NH-N═C(R)-N═N-Ar). The compounds were synthesized by reacting equimolar amounts of [ReBr(CO)] and the corresponding neutral formazan. X-ray crystallographic and spectroscopic (IR, NMR) characterization confirmed the generation of formazan-type species with the structure -[ReBr(CO)(κ-(Ar-NH-N═C()-N═N-Ar))]. The formazan ligand coordinates the metal center in the '' form, generating a five-membered chelate ring with a pendant NH arm. The electronic absorption and emission properties of these complexes are governed by the presence of low-lying π*-orbitals on the ligand as shown by DFT calculations. The high orbital mixing between the metal and ligand results in photophysical properties that contrast to those observed in -[ReBr(CO)(L,L)] species with α-diimine ligands.