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)Cl-3] with the appropriate phosphine yielded the homoleptic Iris complexes I-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)(2)Cl-2](+) and Re(depe)(2)Cl-2](+), 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 Re-I and Re-II with formal reduction potentials, E-o', ranging from -0.09 to -0.28 V versus a ferrocene external standard. Subsequent oxidation to Re-III was found to be chemically irreversible. UV vis and luminescence spectro-electrochemical techniques were used to study the spectral properties of the Re-I and Re-II forms. The Re-II complexes are red in color and exhibit absorption features from 350 to 600 nm; the lowest-energy transition was assigned as a sigma(P) to d pi(Re) ligand-to-metal charge-transfer (LMCT) transition. Excitation into the lowest-energy absorption band revealed rare examples of luminescent (Phi approximate to 0.07) LMCT excited states from d(5) 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.