Synthesis, X-ray crystal structure, and reactivity of Pd2(μ-dotpm)2 (dotpm = bis(di-ortho-tolylphosphino)methane)

Alkylation of PdCl2(dotpm) (dotpm = bis(di-ortho-tolylphosphino)methane) with n-butyllithium produces the binuclear Pd(0) complex Pd2(μ-dotpm)2 and the elimination byproducts 1-butene, cis-2-butene, trans-2-butene, butane, and octane. The dibutyl complex, Pd(dotpm)(n-Bu)2, is presumed to be the reaction intermediate. The crystal structure of Pd2(μ-dotpm)2 reveals that the methylene groups of the bridging dotpm ligands are located on opposite sides of the Pd2P4 unit, forming an 8-membered ring that is in an elongated chair conformation. The four phosphorus atoms are not coplanar, and the P1–P2–P3–P4 ring has a torsion angle of 13.8°, which minimizes the spatial interactions among the o-tolyl rings. The Pd–Pd bond distance is 2.8560(6) Å, which indicates that there is a weak “closed-shell” bonding interaction between the d10–d10 metal centers. Each palladium atom has a nearly linear geometry, and the eight methyl groups of the dotpm ligands shield the open coordination sites on the metal centers. Four methyl groups shield the metal atoms above and below the Pd2P4 ring cavity, and four methyl groups block the open metal sites outside of the Pd2P4 ring. The Pd2(μ-dotpm)2 complex readily undergoes oxidative addition of dichloromethane to form the rigid A-frame complex Pd2Cl2(μ-CH2)(μ-dotpm)2.