"Triple-Decker Sandwich" Containing Planar {B2e2pd} Ring (E = S Or Se)

In an effort to generate triple-decker complexes comprising a {PdCl2}moiety in the middle deck, we have explored the reactivity of [(Cp*M)(2){mu-B2H2E2}], 1-4 (1: M = Co, E = S; 2: M = Co, E = Se; 3: M = Rh, E = Se; and 4: M = Ir, E = Se; Cp* =eta(5)-C5Me5), with [PdCl2(COD)] (COD = 1,5-cyclooctadiene). The reactions led to the formation of a series of trinuclear heterometallic triple-decker complexes, [(Cp*M)2{mu-B2H2E2Pd-(Cl)(2)}], 5-8 (5: M = Co, E = S; 6: M = Co, E = Se; 7: M = Rh, E = Se; and 8: M = Ir, E = Se). Formation of the complexes 5- 8 occurred almost instinctively as a single product with the elimination of the COD ligand. These complexes are examples of novel triple-decker species having a planar bridging palladacycle ligand, in which the Pd metal exists as Pd(II) in an uncommon pseudo-octahedral environment with an elongated M-Pd bonding interaction. The new species, 5-8, have been characterized spectroscopically, and the structures of 5-7 were confirmed by single-crystal X-ray diffraction studies. The structure and bonding of these molecules were further analyzed with the help of density functional theory studies that found a strong electron donation from the B2E2 (E = S or Se) fragment of the middle ring to the axial metals, while a weak bonding interaction between group 9 metals and Pd.