Neutral Formazan Ligands Bound to the fac-(CO)₃Re(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 ?-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 (Ar1-NH-N?C(R3)-N?N-Ar5). 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(κ2-N2,N4(Ar1-N1H-N2?C(R3)-N3?N4-Ar5))]. 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 ?*-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.