Aza[6]helicene Platinum Complexes: Chirality Control of cis–trans Isomerism

Abstract

It was serendipitously observed that cis-[PtCl2(NCEt)PPh3] reacted differently with either racemic or enantiopure 4-aza[6]helicene, giving respectively cis (racemic) and trans (enantiopure) [Pt(II)Cl2(4-aza[6]helicene)PPh3] complexes. This unexpected reactivity is explained through a dynamic process (crystallization-induced diastereoselective transformation) and enables a new aspect of reactivity in chiral transition-metal complexes to be addressed.

Synthesis and Structural Properties of Aza[n]helicene Platinum Complexes: Control of Cis and Trans Stereochemistry

“…This surprising result was identified as a dynamic process, namely a crystallization-induced diastereoselective transformation, due to the different solubilities of racemic and enantiopure 4-aza [6]helicene Pt complexes that displace the cis-trans equilibrium of Pt-precursors 5a-c in refluxing toluene (see Scheme 3). 4 As already described in ref. 4, complex 3a crystallizes in the triclinic P-1 space group with the presence of M and P 4-aza [6]helicenes.…”

“…4 As already described in ref. 4, complex 3a crystallizes in the triclinic P-1 space group with the presence of M and P 4-aza [6]helicenes. The geometry around the platinum atom is square planar (Figure 1).…”

“…As discussed for 3a, owing to the steric hindrance of the helicene moiety, the PPh 3 is stacked on one side of the pyridyl ring, thus generating planar chirality, with the pyPtCl 2 defining the chiral plane. 4,14 Indeed, C6-N-Pt-P and C4a-N-Pt-Cl1 torsion angles of -79.63 and -87.38° (pMchirality) £ are measured in the cis-4a molecule having the M-4-aza [6]helicene ligand, suggesting that the M-helicity induces a fixed pM-chiral planar sense. 4 The crystal packing is arranged to form parallel layers on the yz plane, as shown in Figure 4.…”

“…4,14 Indeed, C6-N-Pt-P and C4a-N-Pt-Cl1 torsion angles of -79.63 and -87.38° (pMchirality) £ are measured in the cis-4a molecule having the M-4-aza [6]helicene ligand, suggesting that the M-helicity induces a fixed pM-chiral planar sense. 4 The crystal packing is arranged to form parallel layers on the yz plane, as shown in Figure 4. The individual layers are connected by hydrogen bonds (distances 2.8-2.9 Å) of the type Cl2···H11 (helicene) and Cl1···H o (PPh 3 ) shown in Figure 4 in horizontal and vertical projection, respectively.…”

“…1b,3 Another important aspect is the new reactivity features that may originate from the different solubility properties of racemates and pure enantiomers, as recently demonstrated by some of us. 4,5 Therefore, there is considerable interest in the structural diversity of helicene-based coordination complexes and in the control of their supramolecular self-assembly properties via crystal engineering approaches.…”