Coordination chemistry of phosphanyl amino acids: solid state and solution structures of neutral and cationic rhodium complexes

Abstract

Copper phosphide or arsenide complexes, [Cu(EPh₂)(neo)] (E = P, As, neo = 2,9-dimethyl-1,10-phenanthroline; trivial name: neocuprine) react selectively with the N-protected brominated serine derivatives, 2-(S)-(alkoxycarbonylamino)-3-bromomethylpropionates 1a–c (^ROCOSerBr, a: R = PhCH₂, b: tBu, c: Me) to give the corresponding phosphanylated or arsanylated amino acids, ^ROCOSerPhos (3a–c: Phos = PPh₂) and ^ZSerArs 7 (Ars = AsPh₂, Z = PhCH₂OCO). The dipeptide ^ZAlaSerPhos 3d was likewise prepared. The phosphanes 3a–d, and the arsane 7 reacted cleanly with [Rh₂(µ-Cl)₂(cod)₂] to give the rhodium(I) complexes [RhCl(cod)(^ZSerPhos)] 8, [RhCl(cod)(^BocSerPhos)] 9 (Boc = tBuOCO), [RhCl(cod)(^ZAlaSerPhos)] 10, and [RhCl(cod)(^ZSerArs)] 11 which were characterized by X-ray diffraction studies. A common structural feature is an intramolecular (N)H⋯Cl(Rh)-hydrogen bridge which according to NMR investigations remains intact in solution. The abstraction of chloride from the coordination sphere of Rh(I) in 8 or 10 has a profound structural impact. While in 8 and 10, the ligands bind in a monodentate fashion, via the phosphorus atom only, they serve as bidentate ligands via the phosphorus centre and the peptidic CO group in [Rh(cod)(κ²-^ZSerPhos)]PF₆12 and [Rh(cod)(κ²-^ZAlaSerPhos)]PF₆13. This causes also the amino acid residue structures to change from α-helix type in 8 and 10 to a β-sheet type in 12 and 13. Addition of chloride to 12 and 13 fully re-establishes the structures of 8 and 10. The complexes [RhCl(cod)(^ZSerPhos)] 8 and [RhCl(cod)(^BocSerPhos)] 9 show good activities in homogeneously catalyzed hydrogenations of olefins while the dipeptide complex 10 is less active. Phosphane addition to 8 greatly diminishes the catalytic activity. The cationic complex [Rh(cod)(κ²-^ZAlaSerPhos)]PF₆ shows low activity which, however, is greatly increased by addition of one equivalent of phosphane.