Crystallographic characterization of the palladium(I) dimers, syn-Pd2Cl2(dppmMe)2 and Pd2Cl2(dpPM)2;: solution conformational behavior of syn- and anti-Pd2Cl2(dppmMe)2 and their (μ-Se) adducts [dppmMe = μ-1,1-bis(diphenylphosphino)ethane, and dppm = μ-bis(diphenylphosphino)methane]

An X-ray diffraction study on syn-Pd2Cl2(dppmMe)(2), syn-1, shows an unusual boat-like conformation of the eight-membered Pd2P4C2 ring. This conformation, containing equatorial Me groups, facilitates access of the Pd-Pd bond to small molecules such as CO, SO2, and elemental sulfur or selenium, and makes the syn isomer more reactive than anti-1. A comparison of bond angles around the Pd and P atoms in the syn- and anti-isomers reveals a more strained geometry of the former, which may also contribute to the stronger propensity of syn-1 to form A-frame adducts. Solution NMR/NOE studies on syn- and anti-1 and their (mu-Se) adducts reveal the structural rigidity of these complexes; the Me groups inhibit the interchange of axial and equatorial positions on the bridging methine C-atom, and solution structures correspond to those in the solid state. The X-ray structure of Pd2Cl2(dppm)(2) is, as expected, like that of the corresponding bromo, complex; both are analogous to that of anti-1 which adopts a chair conformation within both the fused, five-membered rings. (C) 2002 Elsevier Science B.V.. All rights reserved.