Neutral Formazan Ligands Bound to the fac-(CO)(3)Re(I) Fragment: Structural, Spectroscopic, and Computational Studies

[Image: see text] 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.