Rhenium tricarbonyl chloro of di-2-pyridylketone 4-aminobenzoyl hydrazone (dpk4abh), fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl]: Synthesis, spectroscopic and electrochemical properties

The reaction between di-2-pyridyl ketone 4-aminobenzoylhydrazone hydrate (dpk4abh.H2O) and [Re(CO)5Cl] in refluxing toluene gave fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl]. Hydrolysis of fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] to form fac-[Re(CO)3(κ3-N,O,N-dpkO,OH)] occurs in wet solvents. The identity of fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] was established from the results of its elemental analysis, spectroscopic measurements, and electrochemical properties. The infrared spectra of fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] show the facial coordination of the carbonyl (CO) groups, the coordination of dpkabh and none coordination of the carbonyl (CO) group. 1H-NMR measurements on fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] in d6-dmso and d7-dmf confirmed the coordination of dpkabh, and demonstrate sensitivity of fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] to its surroundings as manifested by the chemical shift variations observed in different solvents. Variable temperature 1H-NMR studies showed stronger interaction between d7-dmf and fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] compared to d6-dmso and the amine protons of coordinated dpk4abh undergo faster exchanges with solvated water protons compared to the amide proton. The electronic absorption spectra of fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] showed significant solvent dependence. In CH3CN, a single intra-ligand charge transfer (ILCT) transition appeared at 332 nm and assigned to π → π* of dpk followed by dpk → hydrazone charge transfer. In a protophilic solvent, such as dmf, two intra-ligand charge transfer (ILCT) transitions assigned to fac-[Re(CO)3(κ2-N,N-dpk4abh-H)Cl] and fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] appeared at 464 and 350 nm, respectively. Reversible interconversion between fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] and its conjugate base fac-[Re(CO)3(κ2-N,N-dpk4abh-H)Cl] was established in protophilic solvents by using a base to shift the equilibrium to the conjugate base and an acid to shift the equilibrium or conjugate base to the neutral form. Acid–base titrations of fac-[Re(CO)3(κ2-N,N-dpk4abh-H)Cl] or fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] show acids or bases in concentrations as low as 1x10−6 M can be detected and determined using protophilic solutions of fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl]. Electrochemical measurements on fac-[Re(CO)3(κ2-N,N-dpk4abh)Cl] in CH3CN and CH2Cl2 showed sequential irreversible redox processes consistent with the electrochemical reduction or oxidation of the hydrazone backbone.