Neutral Formazan Ligands Bound to the fac -(CO) 3 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 N , N -bidentate formazans, which possess both N═N and C═N fragments within the ligand backbone (Ar 1 -NH-N═C(R 3 )-N═N-Ar 5 ). The compounds were synthesized by reacting equimolar amounts of [ReBr(CO) 5 ] and the corresponding neutral formazan. X-ray crystallographic and spectroscopic (IR, NMR) characterization confirmed the generation of formazan-type species with the structure fac -[ReBr(CO) 3 (κ 2 - N 2 ,N 4 (Ar 1 -N 1 H-N 2 ═C( R 3 )-N 3 ═N 4 -Ar 5 ))]. The formazan ligand coordinates the metal center in the ' open ' 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 fac -[ReBr(CO) 3 (L,L)] species with α-diimine ligands.