New insights into the stoichiometric and catalytic reactivity of unsaturated Pd3(dppm)3COn+ clusters (n = 0, 1) towards halocarbons - First evidence for inorganic by-products
Résumé
The title clusters, Pd3(dppm)3(CO)+ and Pd3(dppm)3(CO)0 can be electrochemically generated from the 1- and 2-electron reductions, respectively, of the Pd 3(dppm)3(CO)2+ cluster [dppm = bis(diphenylphosphanyl)methane; Pd32+]. Pd 3+ reacts in a stoichiometric ratio with methyl iodide, MeI, and benzyl bromide, BzBr, in THF to provide the corresponding Pd 3(X)+ adducts (X = I, Br respectively) as inorganic products. Other products are Bz2 and PhMe for BzBr but, for MeI, no organic product was observed (since they are too volatile). In the presence of the same substrates, Pd30 also reacts in a stoichiometric ratio to form the same organics and the Pd3-(X)+ adducts (X = I and Br). However for MeI, the major inorganic product is the A-frame Pd2(dppm)2(Me)2I+ binuclear complex. For BzBr, the corresponding A-frame complex Pd2(dppm) 2(Bz)2Br+ could not be detected. The spin-trap agents, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and 5,5′-dimethyl-1- pyrroline N-oxide (DMPO), have been used to demonstrate the intermediacy of the radical Bz.. The catalytic generation of "Bz." was performed using two methods, i.e. 1) using a copper anode as the working electrode [Pd3-(Br)+ + Cu - e- → Pd 32+ + CuBr (s)] and 2) using a carbon cathode as the working electrode [Pd3(Br)+ + 2e- → Pd30 + Br-]. The chemical yields for Bz 2 vary between 50 and 56% and the Faradic yield is of the order of 90% for method 1 and between 52 and 59% for method 2 [taking into account the quantity of electricity necessary to reduce the catalyst Pd3-(Br) +]. The X-ray structure of Pd3(dppm)3(CO)(Br) + is presented and the following parameters were recorded: monoclinic space group P21/n, a = 10.6546(2), b = 37.1091(7), c = 212714(7) Å, β = 91.55(1)°, V = 8407,3(4) Å3, Z = 4, R1 = 0.0581 [I > 2σ(I)], wR2 = 0.1478 (all data). © Wiley-VCH Verlag GmbH & Co. KGaA, 2005.