Luminescent Low-Valent Rhenium Complexes with 1,2-Bis(dialkylphosphino)ethane Ligands. Synthesis and X-ray Crystallographic, Electrochemical, and Spectroscopic Characterization

A series of low-valent rhenium phosphine complexes with the general formula [Re(dmpe)3−x(depe)x]2+/+ (x = 0−3), where dmpe is 1,2-bis(dimethylphosphino)ethane and depe is 1,2-bis(diethylphosphino)ethane, were synthesized and characterized. The reaction of [Re(benzil)(PPh3)Cl3] with the appropriate phosphine yielded the homoleptic tris complexes [Re(dmpe)3]+ and [Re(depe)3]2+, while the mixed-ligand complexes [Re(dmpe)2(depe)]+ and [Re(dmpe)(depe)2]2+ were prepared from [Re(dmpe)2Cl2]+ and [Re(depe)2Cl2]+, respectively. The oxidation state of the final product strongly depends on the donating properties of the ligand. Each complex, however, exhibits a diffusion-controlled, reversible one-electron transfer between ReI and ReII with formal reduction potentials, E°′, ranging from −0.09 to −0.28 V versus a ferrocene external standard. Subsequent oxidation to ReIII was found to be chemically irreversible. UV−vis and luminescence spectroelectrochemical techniques were used to study the spectral properties of the ReI and ReII forms. The ReII complexes are red in color and exhibit absorption features from 350 to 600 nm; the lowest-energy transition was assigned as a σ(P) to dπ(Re) ligand-to-metal charge-transfer (LMCT) transition. Excitation into the lowest-energy absorption band revealed rare examples of luminescent (Φ ≈ 0.07) LMCT excited states from d5 transition-metal complexes in a room temperature solution. Structural characterization of salts of both oxidation states of [Re(dmpe)2(depe)]2+/+ was also performed.