THE DONOR PROPERTIES OF 1,8-BIS(DIPHENYLPHOSPHINO)-3,6-DIOXAOCTANE TOWARDS CATIONIC RHODIUM(I) SALTS REVISITED - X-RAY STRUCTURAL CHARACTERIZATION OF A QUADRIDENTATE MODE OF BONDING

The [(dppoo)Rh(COD)]+ cation (dppoo = (Ph2PCH2CH2OCH2)2) reacts with c hloroform at room temperature to give [RhCl2(dppoo)]+, whose structure has been established by X-ray structural determination on [RhCl2(dppo o)][PF6]. The dppoo ligand adopts a planar eta4 Mode of bonding, the t wo chlorine atoms being in the trans position. [(dppoo)Rh(COD)][ClO4] (2) reacts with dihydrogen at atmospheric pressure to give [(dppoo)Rh] [ClO4] (4) in which dppoo adopts the same mode of bonding, as deduced from spectroscopic data. The formation of 4 involves the transient for mation of unstable [(dppoo)Rh(H)2][ClO4] which has been detected by nu clear magnetic resonance (NMR) spectroscopy. Under carbon monoxide, [( dppoo)Rh(CO)2][ClO4] (6) is formed, and is readily decarbonylated to [ (dppoo)Rh(CO)][ClO4] (7) under vacuum. In 6, dppoo is trans bidentate. In 7, low temperature NMR experiments imply an equilibrium between et a3 and eta4 bonding for dppoo. 2 is a poor catalyst for the hydrogenat ion of alkynes and ketones and shows a low activity towards the hydrof ormylation of 1-hexene.