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

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-eta-phenyl interaction is found in [Pd(eta(3)-allyl)(1,2-eta-Ph-1-kappa P)](+) (3), as indicated by NMR spectroscopy and by an X-ray study. The Rh(l) complexes [RhCl(NBD)(1-kappa P)] (4a) and [RhCl(COD)(1-kappa P)] (4b) were prepared, and their X-ray structures were determined. Chloride abstraction from 4a and 4b gave the corresponding cations [Rh(NBD)(1,2-eta-Ph-1-kappa P)](+) (5a) and [Rh(COD)(1,2-eta-Ph-1-kappa P)](+) (5b), which were fully characterized. Besides NMR spectroscopic data, the 1,2-eta-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-kappa P)(eta(6)-Ph-1-kappa P)](+) (6), whose eta(6)-arene coordination was studied by NMR spectroscopy and by X-ray diffraction. For the sake of comparison with the d8 complex 6, the crystal structure of the previously reported d(6) analogue [RuCl(2)(eta(6)-Ph-1-kappa P)](+) (7) was determined. This study shows that secondary :pi-arene-metal interactions turn phosphoramidite 1 to a four-, six-, or eight-electron donor, with increasing bond strength in this order.