Iridium(III) and Rhodium(III) Half-Sandwich Coordination Compounds with 11H-Indeno[1,2-b]quinoxalin-11-one Oxime: A Case of Spontaneous Resolution of Rh(III) Complex

Abstract

Two half-sandwich iridium(III) and rhodium(III) complexes with 11H-indeno[1,2-b]quinoxalin-11-one oxime (IQ-1) ligand were prepared by the reaction of the proligand with [M(Cp*)Cl2]2 (M = Ir, Rh) dimers. The reaction between IQ-1 and [Ir(Cp*)Cl2]2 in methanol gave the complex [Ir(Cp*)(IQ-1)Cl] (1), which crystallized in a centrosymmetric space group as a true racemate. Whereas complex [Rh(Cp*)(IQ-1)Cl] (2) in the form of a racemic conglomerate was obtained by the reaction of [Rh(Cp*)Cl2]2 and IQ-1 in methanol. The crystal structures of complexes 1 and 2 (R and S enantiomers) were determined by X-ray diffraction analysis, and the structural features were compared in order to understand the structural factors leading to the spontaneous enantiomer resolution of the rhodium(III) complex. In the crystal packing of 1, intermolecular C–H···C contacts between a pair of enantiomers link the molecules into centrosymmetric dimers and lead to the formation of heterochiral crystals of 1. In contrast, the intramolecular contacts CH···Cl and CH···C in complex 2 bind all three ligands around the chiral Rh(III) metal center. In addition, a combination of intermolecular CH···O and CH···C contacts leads to the formation of a homochiral supramolecular structure. These interactions altogether reinforce the spontaneous resolution in complex 2.