Light‐responsive and Protic Ruthenium Compounds Bearing Bathophenanthroline and Dihydroxybipyridine Ligands Achieve Nanomolar Toxicity towards Breast Cancer Cells^†

Abstract

We report new ruthenium complexes bearing the lipophilic bathophenanthroline (BPhen) ligand and dihydroxybipyridine (dhbp) ligands which differ in the placement of the OH groups ([(BPhen)2Ru(n,n′‐dhbp)]Cl2 with n = 6 and 4 in 1A and 2A, respectively). Full characterization data are reported for 1A and 2A and single crystal X‐ray diffraction for 1A. Both 1A and 2A are diprotic acids. We have studied 1A, 1B, 2A, and 2B (B = deprotonated forms) by UV‐vis spectroscopy and 1 photodissociates, but 2 is light stable. Luminescence studies reveal that the basic forms have lower energy 3MLCT states relative to the acidic forms. Complexes 1A and 2A produce singlet oxygen with quantum yields of 0.05 and 0.68, respectively, in acetonitrile. Complexes 1 and 2 are both photocytotoxic toward breast cancer cells, with complex 2 showing EC50 light values as low as 0.50 μM with PI values as high as >200 vs. MCF7. Computational studies were used to predict the energies of the 3MLCT and 3MC states. An inaccessible 3MC state for 2B suggests a rationale for why photodissociation does not occur with the 4,4′‐dhbp ligand. Low dark toxicity combined with an accessible 3MLCT state for 1O2 generation explains the excellent photocytotoxicity of 2.

Ruthenium Complexes with Protic Ligands: Influence of the Position of OH Groups and π Expansion on Luminescence and Photocytotoxicity

“…The protic 6,6′-dhbp ligand was combined with four spectator ligands (N,N = bpy, phen, dop, BPhen) to produce [(N,N) 2 Ru(6,6′-dhbp)]Cl 2 complexes ( 1 A , 2 A , 3 A , and 4 A , respectively) using previously reported synthesis procedures (see Figure 2 for structures) [ 25 , 26 , 28 ]. These complexes with OH groups near the metal center are contrasted herein with the 4,4′-dhbp complexes: [(N,N) 2 Ru(4,4′-dhbp)]Cl 2 complexes ( 5 A , 6 A , 7 A , and 8 A , using N,N = bpy, phen, dop, and BPhen, respectively, Figure 2 ).…”

“…Complexes 1 A – 4 A ( Figure 1 b and Figure 2 ) utilize the 6,6′-dhbp (dhbp = dihydroxybipyridine) ligand with hydroxy groups near the metal center, whereas complexes 5 A – 8 A ( Figure 1 c and Figure 2 ) utilize the 4,4′-dhbp ligand with distal hydroxy groups [ 11 , 25 , 26 , 27 , 28 , 29 ]. We chose the N,N spectator ligands as bpy, phen, dop, and Bphen (as shown in Figure 2 ) to gradually increase π expansion to improve singlet oxygen generation and cellular uptake (vide infra) [ 11 , 26 ]. Both dhbp ligands are diprotic, and the use of protic Ru anticancer compounds has been rare [ 30 ].…”

“…The terminology X A is used for the isolated dications ( 1 A – 8 A ), X B is used for the isolated neutral species ( 1 B – 8 B ), and a lack of subscript indicates an equilibrium mixture of A and B forms as dictated by the solution pH and the p K a values. The measured p K a values are given in the sixth column of Table 1 , and they are typically ~5–6 [ 25 , 26 , 28 ]. This indicates that complexes 1 – 8 are predominantly doubly deprotonated at physiological pH, but there can still be a significant amount of the OH bearing A form and the mono-deprotonated cationic species at physiological pH [ 29 ].…”

“…These complexes photodissociate (blue box in Figure 1 b) but the quantum yields (Φ PS ) are low ( Table 1 ) [ 28 ]. These complexes produce higher quantum yields of singlet oxygen (Φ Δ ) (purple circle in Figure 1 b and Table 1 ) vs. photodissociation (Φ PS ), and singlet oxygen generation appears to be responsible for the observed photocytotoxicity (only 3 A and 4 A show significant photocytotoxicity based on PI values, PI = phototoxicity index = EC 50_dark /EC 50_light , Table 1 and vide infra) [ 11 , 26 ]. This difference in toxicity across this series is proposed to be due primarily to uptake.…”

“…Complexes 5 A – 8 A lack steric strain near the metal center and, therefore, do not undergo photodissociation under the conditions used herein [ 25 , 26 , 28 , 35 ]. Thus, Figure 1 c shows only the singlet oxygen generation pathway for these compounds.…”