Self-assembly of metallosupramolecular rhombi from chiral concave 9,9’-spirobifluorene-derived bis(pyridine) ligands

Abstract

SummaryTwo new 9,9’-spirobifluorene-based bis(4-pyridines) were synthesised in enantiopure and one also in racemic form. These ligands act as concave templates and form metallosupramolecular [(dppp)2M2L2] rhombi with cis-protected [(dppp)Pd]2+ and [(dppp)Pt]2+ ions. The self-assembly process of the racemic ligand preferably occurs in a narcissistic self-recognising manner. Hence, a mixture of all three possible stereoisomers [(dppp)2M2{(R)-L}2](OTf)4, [(dppp)2M2{(S)-L}2](OTf)4, and [(dppp)2M2{(R)-L}{(S)-L}](OTf)4 was obtained in an approximate 1.5:1.5:1 ratio which corresponds to an amplification of the homochiral assemblies by a factor of approximately three as evidenced by NMR spectroscopy and mass spectrometry. The racemic homochiral assemblies could also be characterised by single crystal X-ray diffraction.

Unexpected Self‐Assembly of a Homochiral Metallosupramolecular M4L4 Catenane

“…According to our previous results,4d–g we expected the straight‐forward formation of a homochiral [(dppp) 2 Pd 2 {( R )‐ 1 } 2 ](OTf) 4 complex when we mixed enantiomerically pure ligand ( R )‐ 1 with an equimolar amount of [(dppp)Pd(OTf) 2 ] in acetonitrile 8. Hence, we were surprised to obtain an ESI mass spectrum (Figure 1) and a 1 H NMR spectrum that clearly revealed the formation of a tetranuclear [(dppp) 4 Pd 4 {( R )‐ 1 } 4 ] complex of lower symmetry besides the expected dinuclear aggregate in an equilibrated mixture.…”

“…Hence, we were surprised to obtain an ESI mass spectrum (Figure 1) and a 1 H NMR spectrum that clearly revealed the formation of a tetranuclear [(dppp) 4 Pd 4 {( R )‐ 1 } 4 ] complex of lower symmetry besides the expected dinuclear aggregate in an equilibrated mixture. Such signals were not observed with a similar spirobifluorene‐based ligand that lacks the ethynyl groups and has the pyridyl groups directly attached to the spirobifluorene core 4g…”

“…As part of our ongoing program to elucidate general guidelines for the (diastereoselective) self‐assembly of metallosupramolecular aggregates,2–6 we have recently started a systematic study on the influence of rigid chiral concave building blocks on the outcome of the self‐assembly of bis(pyridine) ligands and palladium(II) and platinum(II) ions in the sense of chiral self‐sorting 4. 5 Cis ‐protected [(dppp)Pd(OTf) 2 ] or [(dppp)Pt(OTf) 2 ] (dppp=1,3‐bis(diphenylphosphino)propane) complexes, for example, self‐assembled into dinuclear metallosupramolecular rhombi upon coordination to bis(pyridine) ligands based on Tröger’s base,4d,e the 2,2′‐dihydroxy‐binaphthyl,4f and the 9,9′‐spirobifluorene scaffold 4g…”

Synthesis, Chiral Resolution, and Absolute Configuration of C2‐Symmetric, Chiral 9,9′‐Spirobifluorenes

“…In our group, 9,9′‐spirobifluorenes have proven their worth as versatile building blocks for artificial receptors10 as well as for metallosupramolecular aggregates 5e,5h. Being especially interested in the molecular recognition of chiral substrates and in the diastereoselective self‐assembly of metallosupramolecular aggregates, dissymmetric 9,9′‐spirobifluorenes attracted our interest and highlighted a need for enantiomerically pure derivatives.…”

“…In 1988, Toda published a procedure for the resolution of 2,2′‐dihydroxy‐9,9′‐spirobifluorene through clathrate formation with ( R , R )‐(+)‐2,3‐dimethoxy‐ N , N , N′ , N′ ‐tetracyclohexylsuccindiamide,11 which was refined and improved by Thiemann in 2005 12. Although this approach has been applied successfully by us5e,5h and others6e,6h,13 it is quite tedious. This is not only due to the fact that the tartaric acid derivative must be synthesised in three steps and tends to racemise during the second step.…”