Helicity inversion and redox chemistry of chiral manganese(ii) cubanes

Abstract

The chiral ligand S-1,2-bis(1H-benzimidazol-2-yl)ethanol, 1, reacts with manganese(II) salts to form cubanes which readily undergo oxidation reactions leading either to a tetranuclear manganese(II,III) mixed valence complex 4 or to a tetranuclear complex of ligand 5 where the secondary alcohol has been oxidised to an enolate. N-methylation of ligand 1 slows the oxidation reaction and stable manganese(II) cubanes may be isolated. The fully methylated ligand 2 gives a cubane of opposite helicity to that found previously for 1 with cobalt. The inversion may be explained by conformational analysis. Cyclic voltammetry suggests that the manganese cubanes reported here are insufficiently robust to store oxidising equivalents as in the oxygen evolving system of photosystem II.

The Honeysuckle and the Bindweed Revisited: Synthesis and Stereochemistry of Extended Helical Structures from Coordination Complexes

“…The crystal structure of the complex [Co 4 (( R )‐ 1 ‐H) 4 (dpp) 2 ] 2+ (dpp=diphenyl phosphinate) [6] shows a similar structure to the ( S )‐enantiomer but of ( M )‐helicity, suggesting that the ligand configuration does determine the complex helicity. However, the crystal structure of the manganese complex prepared using the ligand ( S )‐ 2 which is methylated on the benzimidazole nitrogen ligands, [Mn 4 (( S )‐ 2 ‐H) 4 (dpp) 2 ] 2+ , shows ( M )‐helicity, Figure 3 and not the ( P )‐helicity seen with cobalt [5] …”

“…Although manganese complexes of ligand 2 were prepared by Deville, [5] we encountered problems with oxidation. An experimental method that avoided the problems with oxidation was developed.…”

“…However, the crystal structure of the manganese complex prepared using the ligand (S)-2 which is methylated on the benzimidazole nitrogen ligands, [Mn 4 ((S)-2-H) 4 (dpp) 2 ] 2 + , shows (M)-helicity, Figure 3 and not the (P)-helicity seen with cobalt. [5] This surprising change is due to a difference in the orientation of the ligand with respect to the helical axis. In (P)-[Co 4 ((S)-1-H) 4 (dpp) 2 ] 2 + (Figure 2) the plane of the 5-membered chelate ring is roughly perpendicular to the helical axis and the plane of the 6-membered chelate ring is roughly parallel to the helical axis: we will denote the perpendicular arrangement with an h for horizontal and the parallel arrangement with a v for vertical.…”

“…Ligands 1 and 2 are very similar and with cobalt(II) show no significant differences in their coordination chemistry. With manganese(II), ligand 1 often undergoes deprotonation at the imidazole nitrogen leading to undesired redox chemistry [5] …”

“…With manganese(II), ligand 1 often undergoes deprotona-tion at the imidazole nitrogen leading to undesired redox chemistry. [5] The asymmetric carbon of the ligand gives rise to two enantiomers distinguished by the Cahn-Ingold -Prelog symbols (R) and (S). The axial chirality generated by the twisting of the ligands around the cubane also gives rise to enantiomers distinguished by the symbols (P) and (M) corresponding respectively to right-handed and left-handed helices.…”