Aquation and Anation Kinetics of Rhenium(I) Dicarbonyl Complexes: Relation to Cell Toxicity and Bioavailability

The aquation reactions of four rhenium(I) dicarbonyl complexes, [Re(CO)₂(NN)(PR₃)(Cl)], where NN = 1,10-phenanthroline (Phen) and 2,9-dimethyl-1,10-phenanthroline (DMPhen) and PR₃ = 1,3,5-triaza-7-phosphaadamantane (PTA) and 1,4-diacetyl-1,3,7-triaza-5-phosphabicylco[3.3.1]nonane (DAPTA). Additionally, the anation reactions of the corresponding aqua complexes with Cl^- were investigated. Single crystals of [Re(CO)₂(DMPhen)(PTA)(Cl)]·DMF and [Re(CO)₂(DMPhen)(DAPTA)(Cl)] were obtained, and their structures were determined using X-ray diffraction. The Re-Cl interatomic distances are 2.4991(13) and 2.4922(6) Å, respectively, indicating a mild trans influence effect of the phosphine ligands. The rate constants, k_aq, for the aquation reactions of these complexes spanned a range of (3.7 ± 0.3) × 10^-4 to (15.7 ± 0.3) × 10^-4 s^-1 with the two Phen complexes having rate constants that are 2.5 times greater than those of the DMPhen complexes at 298 K. Similarly, the second-order anation rate constants (k_Cl) of the resulting aqua complexes, [Re(CO)₂(NN)(PR₃)(H₂O)]⁺, with Cl^- ions at 298 K varied between (2.99 ± 0.05) × 10^-3 and (6.79 ± 0.09) × 10^-3 M^-1 s^-1. Likewise, these rate constants for the Phen complexes were almost 2 times faster than those of the DMPhen complexes. The pK_a values of the four aqua complexes were determined to be greater than 9.0 for all of the complexes with [Re(CO)₂(Phen)(PTA)(H₂O)]⁺ having the highest pK_a value of 9.28 ± 0.03. From the pK_a values and the ratios of the aquation and anation rate contants, which give thermodynamic Cl^- binding constants, the speciation of the rhenium(I) complexes in blood plasma, the cytoplasm, and the cell nucleus were estimated. The data suggest that the aqua complexes would be the dominant species in all three environments. This result may have important implications on the potential biological activity of these complexes.