Electron-withdrawing substituents R in complexes [L n M(PR 2 )] influence the P-M bond length due to a decreased σ-donation and enhanced π-back-bonding, leading to an increased Lewis acidity of the metal ion and therefore strengthening the M-L bond to electron-rich ligands L. This influences the Lewis acidity and the redox behavior of corresponding transition-metal complexes, which is important for the design of optimized catalytic systems. To investigate this effect, the electronpoor phosphanes R 2 PH with R = C 2 F 5 , C 6 F 5 , 2,4-(CF 3 ) 2 C 6 H 3 were treated with Pd(F 6 acac) 2 (F 6 acac = hexafluoroacetylacetonato) [a] 3904 and Pd(acac) 2 (acac = acetylacetonato). While the reaction of the phosphanes with Pd(F 6 acac) 2 in all cases yielded the corresponding phosphido-bridged dinuclear palladium complexes [{(F 6 acac)Pd[μ-(PR 2 )]} 2 ], the compounds obtained in the reaction with Pd(acac) 2 were structurally more diverse. For R = C 2 F 5 , the dinuclear palladium complex [{(acac)Pd{μ-[P(C 2 F 5 ) 2 ]}} 2 ] was obtained, while the reaction with (C 6 F 5 ) 2 PH yielded a trinuclear palladium complex bridged by four phosphido units. All complexes were fully characterized, including X-ray crystallography.complexes with bridging dialkylphosphido units have been reported in the literature, [10] no X-ray structural data are available.The most conducive and feasible procedure to synthesize dinuclear palladium(II) complexes with bridging phosphido units was devised by Shmidt, Belykh and Goremyka. [11] They precisely describe the reaction of Ph 2 PH with Pd(acac) 2 (acac = acetylacetonato) which led to di-and trinuclear palladium complexes bridged by diphenylphosphido units. A related complex featuring chelating F 6 acac ligands (F 6 acac = hexafluoroacetylacetonato), albeit synthesized differently, was published by Röschenthaler et al. including an X-ray structural investigation. [12] Perfluoroalkyl and -aryl groups distinctly influence the electronic properties of phosphane ligands. The HOMO, which correlates with the negative ionization energy, as well as the LUMO, which correlates with the negative electron affinity, of the fluorinated phosphane derivatives are lowered (Figure 1, left), which is expressed in a decreased nucleophilicity of the Eur.
3905Scheme 3. Synthesis of palladium complex 4.Scheme 4. Synthesis of the trinuclear palladium complex 5.Scheme 5. Synthesis of tetrakis[2,4-bis(trifluoromethyl)phenyl]diphosphane, 6.
