Synthesis, characterization, kinetic investigation and biological evaluation of Re(i) di- and tricarbonyl complexes with tertiary phosphine ligands

Abstract

Rhenium(I) di- and tri-carbonyl complexes of the form fac-[Re(CO)₃(L,L′-Bid)X] and [Re(CO)₂(L,L′-Bid)X₂], where X = aqua (H₂O), methanol (CH₃OH), triphenylphosphine (PPh₃), 1,3,5-triaza-7-phosphaadamantane (PTA), tricyclohexylphosphine (PCy₃) and L,L′-Bid = O,O′ bidentate ligands (tropolone = TropH and 3-hydroxyflavone = FlavH) and N,O bidentate ligands (8-hydroxyquinoline = QuinH, 5,7-chloro-8-hydroxyquinoline = diCl-QuinH and quinoline-2,4-dicarboxylic acid = QuinH₂), were synthesized and unambiguously characterized by ¹H-, ¹³C-and ³¹P-NMR, IR, UV/Vis and micro-analysis. The crystal structures of four complexes, namely fac-[Re(CO)₃(QuinH)(H₂O)]·H₂O (5), fac-[Re(CO)₃(Quin)(PPh₃)] (11), fac-[Re(CO)₃(diCl-Quin)(PPh₃)] (12) and [Re(CO)₂(Trop)(PPh₃)₂]·2C₆H₅CH₃ (20) were obtained. Re–P bonding distances for 11 and 12 are 2.4948(8) and 2.4908(8) Å, respectively, indicating the effect of the electron-withdrawing substituents of the diCl-Quin⁻ ligand. The second-order rate constants for the substitutions of methanol at 25.1 °C in fac-[Re(CO)₃(L,L′-Bid)(CH₃OH)] (L,L′-Bid = Trop, Flav and QuinH) type complexes by different entering phosphine ligands (PPh₃, PCy₃, and PTA) varied between 7.23(7) × 10⁻⁵ and 1.32(3) × 10⁻³ M⁻¹ s⁻¹ and were found to depend on the coordinated bidentate ligand (in general k₁ (QuinH) < k₁ (Trop) < k₁ (Flav)). The toxicity of fac-[Re(CO)₃(QuinH)(PTA)], fac-[Re(CO)₃(Trop)(PTA)], fac-[Re(CO)₃(Trop)(PPh₃)] and fac-[Re(CO)₃(Flav)(PPh₃)] on the cervical cancer HeLa and epithelial RPE-1 cell lines was then evaluated. Complex fac-[Re(CO)₃(Flav)(PPh₃)] (16) and fac-[Re(CO)₃(Trop)(PPh₃)] (13) displayed the highest cytotoxicity with IC₅₀ values of 12.21 ± 0.17 μM and 13.35 ± 0.94 μM, respectively in HeLa cells. Interestingly, a small selectivity towards cancer over non-cancerous cells was observed for these compounds (IC₅₀ = 18.41 ± 3.16 μM and >25 μM in RPE-1 cells).