Unexpected Optically active Rh derivatives

The introduction of hydroxyl groups into ligand is able to confer high hydrophilic features to be transferred to the related metal systems.[Budapest] The atom-economy synthetic procedure adopted consists in the Cp- oxide ring opening and results in one-step, quantitative and stereoselective formation of multi-hydroxyl rac-1,2,4- C5H2[CH(CH2)3CHOH]3 CpOOO ligand1. Rh complexation of CpOOO gives rise to a novel class of water-soluble (Kpo/w 0.86) complexes (L,L)RhCpOOO (LL=NBD 1, COD 2, CH2CH2 3, CO 4,) characterized by their spectroscopic features (ESI-Ms, IR, 2D, n.O.e. NMR). The X-ray diffraction studies on 1a reveal in the crystal structure the occurrence of one couple of enantiomeric pairs, whilst the crystal packing shows an interesting self-organization in macrocycles network, promoted by strong intermolecular hydroxyl H-bonding. This effect has been exploited, by performing VT NMR experiments in different donor solvents (CDCl3, py, DMSO). Unpredictably, in the absence of chiral tag, 1 exhibits solvent-dependent chiroptical properties (CD, aD25), which are correlated with UV transitions and DFT calculations. The intra/inter molecular H-binding reveals to be crucial to drive the equilibrium of the observed atropisomers 1a and 1b, by varying the planar chirality on the two p-complexes (1R,2S,4R)CpOOO(RhNBD) 1a and (1S,2R,4S)CpOOO(RhNBD) 1a’, differing for the stereogenic centres sequence. The different helicity (SP or RP)2 of the propeller-like CpOOO ligand controls the conformation of the rotamers, driving the equilibrium between 1a (SP) ═ 1b (RP). Thermal treatment causes complete transformation of 1b in 1a. The thermodinamically favourite SP configuration alters the racemic mixture to become scalemic, giving rise to a detectable enantiomeric excess.