Chiral or not chiral? A case study of the hexanuclear metalloprisms [Cp*6M6(µ3-tpt-κN)2(µ-C2O4-κO)3]6+ (M = Rh, Ir, tpt = 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine)

Abstract

Cationic hexarhodium and hexairidium complexes with a trigonal prismatic architecture have been synthesised in good yield by self-assembly of the dinuclear oxalato-bridged complexes [Cp*₂M₂(µ-C₂O₄-κO)Cl₂] (M = Rh; 1: Ir; 2) with 2,4,6-tri(pyridine-4-yl)-1,3,5-triazine (tpt) in the presence of AgO₃SCF₃. The trigonal prismatic cations [Cp*₆Rh₆(µ₃-tpt-κN)₂(µ-C₂O₄-κO)₃]⁶⁺ (3) and [Cp*₆Ir₆(µ₃-tpt-κN)₂(µ-C₂O₄-κO)₃]⁶⁺ (4) have been isolated as their triflate salts. The single-crystal X-ray structure analysis of [3][O₃SCF₃]₆ shows two enantiomers in the racemic crystal (space group C2/c), the chirality being due to a twist of the two tpt units. By contrast, the single-crystal X-ray structure analysis of [4][O₃SCF₃]₆ shows a perfectly eclipsed conformation of the tpt units, so that 4 is not chiral in the crystal state (space group Fdc). However, in solution, enantiodifferentiation in the presence of the chiral anion Δ-BINPHAT is observed by ¹H NMR spectrometry not only in the case of 3, but also in the case of 4. This suggests that the iridium derivative 4, which is not chiral in the solid state, adopts chiral conformations in solution.