Complexes of lopsided N-donor heterocyclic bioligands: Has the electrostatic effect of the N2CH proton been overlooked in metallobiochemistry?

Abstract

Abstract

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

“…For complexes of the type cis-[Ru(bpy) 2 ], a-and b-[Ru(azpy) 2 ] being borderline complexes with respect to the stable cis bifunctional coordination to non-tethered purines, these studies prompted us to investigate this aspect using the model bases 1-methylimidazole, MeIm, and 1-methylbenzimidazole, MeBim. [31,32,44,48] 1-Methyl(benz)imidazole derivatives are useful DNA model bases in the investigation of the bifunctional binding of sterically encumbered octahedral complexes, [18,[24][25][26][27][28][29][30] and MeIm and MeBim form stable bisadducts with ruthenium bis(didentate) complexes. The imidazole H(ii) proton is structurally related to the H(8) atom of guanine derivatives, in addition, MeIm and MeBim possess a proton on the other side of the ligand, that is, H(iv), a most valuable (NMR) probe proton that is lacking in guanine derivatives (Scheme 1).…”

“…[48] Conformers: For two cis-coordinated lopsided ligands on a metal, the corresponding atoms can be on the same side (head-to-head, HH) or on opposite sides (head-to-tail, HT) of the ligand-metal-ligand plane. [18] As a lopsided ligand will preferably not orient in-plane with one of the coordination planes, [14] four staggered orientations are possible; in a "cis-bis" octahedral six-coordinate complex therefore, 16 conformers can be designed (Supporting Information, Figure S2). Rationalizing further, the monodentate ligands will orient with the head-to-tail axis of the heterocyclic ligands in a "parallel" fashion rather than an "orthogonal" orientation ("parallel" and "orthogonal" as in the projection used in Scheme 3 and Figure S2), as in the latter case there are severe clashes between the two cis-coordinated ligands and the didentate ligands.…”

“…The most abundant atropisomer of 2, D, has the MeBim ligands placed in such a way that the imidazole ring protons are oriented close to the aza bond of the respectively fac-coordinated azpy ligands. Besides the steric effects inhibiting easy rotation of the MeBim ligands (see Scheme 4), an electronic effect is possibly also stabilizing the orientations of the MeBim ligands in D. Marzilli et al, [18] have pointed out the orientation of imidazole ligands can be partly determined by the electrostatic interaction between the positive, d…”

“…A characteristic feature of HH atropisomers in cis-bisadducts of square-planar platinum, [16] as well as octahedral ruthenium complexes, [18,24] is the lopsided ligands causing one proton resonance (H8 in guanine, H(ii) in benzimidazoles) to shift up field, and the other one down field. This is indeed also observed for the imidazole protons H(ii) B and H(ii') B , for the methyl-group resonances, Me(i) B at 3.49 and Me(i') B at 4.04 ppm, and is paralleled, and even more pronounced, for the H(iv)/H(iv') protons.…”

“…[15][16][17] All factors influencing the orientation and dynamic behavior of lopsided ligands on cis-coordinated octahedral complexes are not as thoroughly understood, yet. However, it is of considerable interest to understand the coordinative binding of biologically available N-heterocycles to six-coordinate complexes, [18] like rutheni- 1 H NMR data of a-[Ru(azpy) 2 (MeBim) 2 ](PF 6 ) 2 (azpy = 2-phenylazopyridine, MeBim = 1-methylbenzimidazole), 2, revealed the presence of a total of seven atropisomers at À95 8C: 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 a-[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.…”