Origin for Fluxional Structure of Tetracoordinate PdIIComplexes

Abstract

In the Pd-catalyzed cross-coupling reactions, the PdL2 complexes with the bulky and electron-rich L ligands (phosphines, NHCs, etc) are important compounds as a Pd(L)nprecusor. The catalytic activity of Pd(L)n-precusors was influenced by the electronic property of the strong σ-donor and the steric effect of the bulky L ligands. Due to the electron-transfer from the strong σ-donor L ligand to Pd, the oxidation state of Pd in the neutral [PdLnX] catalysts is zerovalent. In some X-ray crystallography results, the Pd([9]aneB2A)L2 complexes with mixed hard/soft tridentate ligand have been observed as a five-coordinate geometry with an apical (soft A...Pd) interaction. In Pd(η,η,ηC10H16N2Me2) complexes with an aza-macrocyclic ligand, the six coordination bonds of the three olefin units with Pd were symmetrically formed via three (olefin...Pd) interactions. Meanwhile, in hydration reactions on square planar [PdCl4] complexes, two vertical (H2O...Pd) and (HOH... Pd) interactions for the ligand exchange processes were investigated theoretically and experimentally. With increasing the strength of the fifth (solvent...Pd) interaction, the replacement of the equatorial (Pd-Cl) bond with (solvent...Pd) takes place via a five-coordinate transition states. Is there any relationship between the relative stability and fluxional configuration of the Pd isomers (exoand endotypes) and the apical (soft P...Pd) interaction in the Pd complexes? The unusual stereochemistry of square planar Pd complexes might be influenced by the fifth special (soft P...Pd) interaction, and the conformational rearrangement for the relative stability. The vacant bond on the zaxis allows the occupied dz2-orbital of Pd to interact with the Lewis acid ligand (σ-acceptor), but it is difficult for electron-rich substrates and Pd to interact apically (σ-donor ...filled dz2-orbital). Therefore, for Pd([9]aneB2A)L2, the apical (σ-donor...filled 4dz2-orbital) repulsion shown in Scheme 1 is suggested. In the present work, we have investigated the relative stabilities and extraordinary structures of the macrocyclic Pd([9]aneB2A)L2 complexes within the frameworks of the orbital interactions. The equilibrium geometric structures of tetracoordinate palladium [Pd([9]aneB2A)L2, Pd([9]aneBAB)L2 (A: P; B=N; L=Cl−, PH3)] complexes were fully optimized with the B3P86/6-311+G** (LanL2DZ for Pd) level using Gaussian 03. Optimized geometric structures of tetracoordinate Pd([9]aneN2P)L2 {Pd([9]aneN2P)L2} isomers including HOMO were represented in Figure 1 and their parameters were listed in Table 1. In the Pd complexes, Pd locates at the center of the tetracoordinate Pd complexes with a ([9]aneN2P) ligand and two L monodentates. In endo-Pd([9]aneN2P)Cl2