Abstract

The direct reaction of 6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridazine-3-carboxylic acid (Me2LCOOH) with [(C4H8O)(CO)(3)Re(mu-Br)(2)Re(CO)(3)(OC4H8)] in dimethylformamide using a modified method produced a mixture of the known yellow [(N,N-Me2LCOOH)Re(CO)(3)Br] complex and a new red [(N,O-Me2LCOOH)Re(CO)(3)Br] isomer in a similar to 3:1 ratio. Structural analysis of the latter (Triclinic, P1, a = 7.418(3) & Aring;; b = 11.525(5) & Aring;; c = 12.241(5) & Aring;; alpha = 103.188(15)degrees, beta = 94.282(15)degrees, gamma = 97.647(15)degrees) indicates a twisted pyrazolyl group defining a 17.8(4)degrees dihedral angle with pyridazine. DFT modeling shows the N,O-coordination lowers the HOMO-LUMO gap from 2.65 to 2.28 eV compared to N,N-mode, consistent with the observed redshift of the MLCT absorption band. This effect arises from the pi-donating carboxylate oxygen destabilizing HOMO, while LUMO energies remain similar. MLCT emission bands appear at comparable energies, but N,O-coordination quenches radiative decay, reducing emission quantum yield by nearly two orders of magnitude versus the N,N-complex. Both complexes photosensitize singlet oxygen, confirming (MLCT)-M-3 emission character. Specific interactions with dimethylformamide likely enhance emission and singlet oxygen generation, especially for the N,N-complex.