SYNTHESIS AND CRYSTAL-STRUCTURE OF DINUCLEAR AND TETRANUCLEAR PALLADIUM DPPM COMPLEXES - [PD-2(MU-DPPM)(2)(VPY)CL] AND [PD-4(MU-DPPM)(4)(MU-X)(2)](CLO4)(2) (X = CL, BR AND I)

Reaction of the intermediate [Pd-2(mu-dppm)(2)](2+) which was prepared from [Pd-2(mu-dppm)(2)X-2] (X = Cl, Br and I) and AgClO4 in ethanol w ith 2-vinylpyridine (vpy) afforded dinuclear Pd(I) dppm complex [Pd-2( mu-dppm)(2)(vpy)Cl] (1). When the intermediate [Pd-2(mu-dppm)(2)](2+) was allowed to stand for a few days, tetranuclear Pd dppm complex [Pd- 4(mu-dppm)(4)(mu-X)(2)](ClO4)(2) (X = Cl (2); Br (3) and I (4)) was sy stematically formed. The structure for 1 and 2 were crystallographical ly determined. On complex 1 two Pd atoms are doubly bridged by two dpp m ligands to form an eight-membered Pd2P4C2 framework in a twist-chair conformation. Each Pd atoms is also coordinated by the Cl atom and th e N atom of vpy in unsymmetrical side-by-side mode. The Pd-Pd distance is 2.597(2) Angstrom and the torsion angle between two Pd2P4 plane is 43.2 degrees. On the other hand four Pd atoms on 2 are bridged by fou r dppm ligands to form a rectangular Pd-4 core, which is composed of t he shorter Pd-Pd separations (2.590(1) Angstrom) and the close Pd ... Pd separations (3.818(2) Angstrom). The later Pd ... Pd separation is also bridged by the Cl atom. The Pd-P distance (2.351(1) Angstrom) bri dging the close Pd ... Pd separation is rather longer than that (2.238 (2) Angstrom) bridging the shorter Pd-Pd separation. (C) 1998 Elsevier Science Ltd. All rights reserved.