Investigation of Monomeric versus Dimeric fac-Rhenium(I) Tricarbonyl Systems Containing the Noninnocent 8-Oxyquinolate Ligand

Synthesis and characterization of the dimeric [fac-Re(R-OQN)(CO)(3)](2) and monomeric fac-Re(R-OQN)(CO)(3)(CH3CN) complexes are reported where R = unsubstituted, 2-methyl, 5,7-dimethyl, or 5-fluoro and OQN = 8-oxyquinolate. Facile solvolysis of the dimeric systems is observed in coordinating media quantitatively yielding the monomer complexes in situ. Due to poor synthetic yields of the dimeric precursors, a direct synthetic strategy for isolation of the acetonitrile monomer complexes with an improved yield was developed. The fac-Re(CH3CN)(2)(CO)(3)Cl complex was easily generated in situ as a convenient intermediate to give the desired products in quantitative yield via reaction with the appropriately substituted 8-hydroxyquinoline and tetramethylammonium hydroxide base. Key to the success of this reaction is the precipitation of the product with triflic acid, whose conjugate triflate base is here noncoordinating. Furthermore, isolation of the solvated single crystal [fac-Re(FOQN)(CO)(3)](mu-Cl)[fac-Re(FHOQN)(CO)(3)]center dot CH3C6H5 has allowed a unique opportunity to access a possible reaction intermediate, giving insight into the formation of the [fac-Re(R-OQN)(CO)(3)](2) dimeric systems. Spectroscopic features (UV-vis, FTIR, and H-1 NMR) of both monomeric and dimeric structures are discussed in terms of the pi-electron-donating ability of the oxyquinolate ligand. Interpretation of these electronic effects and the associated steric properties is aided by single-crystal X-ray diffraction, electrochemical, and DFT/TD-DFT computational studies.