Ligands Rock & Roll: Stepwise Twisting of Two cis‐Coordinated Lopsided N‐Heterocycles in an Octahedral Bis(2‐phenylazopyridine)–Ruthenium(II) Complex with Seven Atropisomers

Abstract

1H NMR data of alpha-[Ru(azpy)2(MeBim)2](PF6)2 (azpy=2-phenylazopyridine, MeBim=1-methylbenzimidazole), 2, revealed the presence of a total of seven atropisomers at -95 degrees C: three head-to-tail, HT, isomers (A, C, and D), and four head-to-head, HH, isomers which, due to the presence of an intrinsic C2 axis in the alpha-[Ru(azpy)2] moiety, are two sets of identical pairs (B/B and E/E). The NMR data of 2 represent a unique example of a coordination compound that shows a variable temperature (VT) behavior with more, well-defined steps of slow-to-fast exchange of its atropisomers. At 65 degrees C, all atropisomers are in fast exchange; on lowering the temperature the sharp signals first broaden (at room temperature) and consecutively split up into two sets of relatively sharp signals, in slow exchange, at about 0 degrees C (D, 40 %, and the coalesced signals of ABBCEE, 60 %). Upon further cooling, the set of peaks belonging to D remain sharp until the lowest recording temperatures. The signals of the other set of resonances, on the other hand, first broaden again and then separate into two sets of broad peaks (C/E/E and A) and one set of sharp peaks (B and B in fast exchange); on lowering the temperature even more, these signals broaden once again and finally, at -95 degrees C, are split up into a total of four sets of signal (A, B/B, C, and E/E). At low temperatures, ROESY experiments revealed that atropisomerization occurs through the synchronous rotation of both MeBim ligands in the interconversion of the two "identical" HH atropisomers B and B, as well as in the interconversion between C and E/E. The HH rotamers B/B furthermore exhibit a slow-to-fast exchange atropisomerization behavior that is observed independently from the other dynamic processes in this compound. The versatile cis bifunctional binding of the DNA model bases (MeBim ligands) in 2 parallels the observation of alpha-[Ru(azpy)2Cl2] which shows extraordinarly high cytotoxicity against tumor cell lines.

Interaction between the DNA Model Base 9-Ethylguanine and a Group of Ruthenium Polypyridyl Complexes: Kinetics and Conformational Temperature Dependence

“…The observation of both H8-6A and H8-6Ta NOE crosscouplings supports this theory. The guanine derivatives, as well as other smaller model imidazole bases bound to ruthenium polypyridyl complexes, were found to be rotating fast on the NMR time scale, as observed in the cases of the smaller imidazole ligands, [35][36][37] not rotating at all in the cases in which the model base was stabilized by hydrogen bonds and electrostatic forces, 37,38 and slowly rotating in the intermediate cases. 33,34,36,37 The whole rotation process can be followed by variable-temperature 1D and 2D NMR, as described in this study.…”

Coordination of 9-Ethylguanine to the Mixed-Ligand Compound α-[Ru(azpy)(bpy)Cl2] (azpy = 2-Phenylazopyridine and bpy = 2,2‘-Bipyridine). An Unprecedented Ligand Positional Shift, Correlated to the Cytotoxicity of This Type of [RuL2Cl2] (with L = azpy or bpy) Complex

“…The model base 9-ethylguanine (9-EtGua) coordinates monofunctionally to both the α-[Ru(azpy) 2 ] and cis -[Ru(bpy) 2 ] moieties. , Interestingly, the 9-EtGua model base coordinated to the α-[Ru(azpy) 2 ] moiety can have two orientations, whereas 9-EtGua coordinated to the cis -[Ru(bpy) 2 ] moiety is fixed in one orientation . Combination of these 9-EtGua studies with detailed orientational studies using 1-methylbenzimidazole − have indicated that rather small differences in orientational freedom of the DNA model bases might explain the observed differences in cytotoxicity of α-[Ru(azpy) 2 Cl 2 ] and cis -[Ru(bpy) 2 Cl 2 ]. To confirm and further investigate this statement, DNA model base studies have also been performed with the hybrid species α-[Ru(azpy)(bpy)Cl 2 ], which will be presented in this Article.…”