New Insights into the Stoichiometric and Catalytic Reactivity of Unsaturated Pd 3 (dppm) 3 CO n + Clusters ( n = 0, 1) Towards Halocarbons – First Evidence for Inorganic By‐Products

The title clusters, Pd 3 (dppm) 3 (CO) + and Pd 3 (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; Pd 3 2+ ]. 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 Bz 2 and PhMe for BzBr but, for MeI, no organic product was observed (since they are too volatile). In the presence of the same substrates, Pd 3 0 also reacts in a stoichiometric ratio to form the same organics and the Pd 3 ‐(X) + adducts (X = I and Br). However for MeI, the major inorganic product is the A‐frame Pd 2 (dppm) 2 (Me) 2 I + binuclear complex. For BzBr, the corresponding A‐frame complex Pd 2 (dppm) 2 (Bz) 2 Br + 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 [Pd 3 ‐(Br) + + Cu – e – → Pd 3 2+ + CuBr (s)] and 2) using a carbon cathode as the working electrode [Pd 3 (Br) + + 2e – → Pd 3 0 + 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 Pd 3 ‐(Br) + ]. The X‐ray structure of Pd 3 (dppm) 3 (CO)(Br) + is presented and the following parameters were recorded: monoclinic space group P 2 1 / n , a = 10.6546(2), b = 37.1091(7), c = 21.2714(7) Å, β = 91.55(1) o , V = 8407.3(4) Å 3 , Z = 4, R 1 = 0.0581 [ I > 2σ( I )], wR 2 = 0.1478 (all data). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)