Solvent-driven chiral redundancy of racemic polyol-Cp Rhodium complexes

We recently described the diastereoselective synthesis of racemic water-soluble polyol-Cp Rhodium species rac-(1,2,4)-CpOOORh(NBD) [CpOOO= C5H3[CH(CH2)3CH)OH]3 A [1]. We report on the unexpected chiral response in the absence of chiral auxiliaries or anisotropic medium, observed in polar-donor solvating media (thf, dmso, methanol) through a positive signal (CD), consistent with negative rotatory power values (25D = -12). The calculated DFT spectra describe the metal orbitals involved in the UV transitions (300-500 nm) and allow for the interpretation of CD results. Since the central and planar chirality effects are both cancelled out by the presence of racemate, the chiral behaviour in solution has to be ascribed exclusively to the preferred helicity of A conformers with minor energy. The dynamic VT NMR experiments in pyridine show a strong OH(1)•••OH(2) intramolecular hydrogen bond interaction. However, the marked bathochromic effect shown by the CD spectra of A suggests the presence of strong intermolecular H-bonding interactions (mean /T~12 ppb/K), which may be generated by supramolecular conglomerate networks with a preferred chirality. It should be pointed out that helicity inversion requires only slight geometrical alterations (~30o of the related dihedral angles) and low-energy barriers (experimentally and theoretically calculated as 7-8 Kcal mol-1). Therefore, we believe that chiroptical behavior (of the polyol species A is provoked by the donor effects of high polar solvents (dmso, MeOH). [1] S. Bordoni, P. Natanti, S. Cerini, R. Tarroni, M. Monari, F. Piccinelli, L. Busetto; Organometallics 2008, 27, 945–954 and references therein.