A Chiral Phosphoramidite beyond Monodentate Coordination: Secondary π-Interactions Turn a Dangling Aryl into a Two-, Four-, or Six-Electron Donor in d⁶ and d⁸ Complexes

Abstract

A study of the secondary interactions involving a pendant phenethyl group (NCH(CH 3 )Ph) of ( S a,R C,R C )-O,O′-[1,1′-binaphthyl-2,2′-diyl]- N, N ′-bis[1-phenylethyl]phosphoramidite ( 1 ) in d 8 and d 6 metal complexes is reported. A 1,2-η-phenyl interaction is found in [Pd(η 3 -allyl)(1,2 - η-Ph- 1 - κP )] + ( 3 ), as indicated by NMR spectroscopy and by an X-ray study. The Rh(I) complexes [RhCl(NBD)( 1 - κP )] ( 4a ) and [RhCl(COD)( 1 - κP )] ( 4b ) were prepared, and their X-ray structures were determined. Chloride abstraction from 4a and 4b gave the corresponding cations [Rh(NBD)(1,2 - η-Ph- 1 - κP )] + ( 5a ) and [Rh(COD)(1,2 - η-Ph- 1 - κP )] + ( 5b ), which were fully characterized. Besides NMR spectroscopic data, the 1,2-η-phenyl interaction is supported by an X-ray study of 5a . Upon standing in MeOH solution, 5b undergoes loss of the COD and forms [Rh( 1 - κP )(η 6 -Ph- 1 - κP )] + ( 6 ), whose η 6 -arene coordination was studied by NMR spectroscopy and by X-ray diffraction. For the sake of comparison with the d 8 complex 6, the crystal structure of the previously reported d 6 analogue [RuCl 2 (η 6 -Ph- 1 - κP )] + ( 7 ) was determined. This study shows that secondary π-arene−metal interactions turn phosphoramidite 1 to a four-, six-, or eight-electron donor, with increasing bond strength in this order.