Ligands rock & roll: Stepwise twisting of two cis-coordinated lopsided N-heterocycles in an octahedral bis(2-phenylazopyridine)-ruthenium(II) complex with seven atropisomers

摘要

H-1 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 -95degreesC: 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 C-2 axis in the alpha-[Ru(azpy)(2)] moiety, are two sets of identical pairs (BIB 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 65degreesC, 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 0degreesC (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 -95degreesC, 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)(2)Cl-2] which shows extraordinarly high cytotoxicity against tumor cell lines.