The racemic and meso diastereomers of an electron-rich binucleating tetraphosphine ligand have been used to prepare homobimetallic rhodium norbornadiene complexes. The racemic bimetallic Rh complex is an excellent hydroformylation catalyst for 1-alkenes, giving both a high rate of reaction and high regioselectivity for linear aldehydes, whereas the meso complex is considerably slower and less selective. A mechanism involving bimetallic cooperativity between the two rhodium centers in the form of an intramolecular hydride transfer is proposed. Mono- and bimetallic model complexes in which the possibility for bimetallic cooperativity has been reduced or eliminated are very poor catalysts.

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The synthesis of the binucleating tetratertiary phosphine ligand system (R2PCH2CH2)(Ph)PCH2P(Ph)(CH2CH2PR2) (R = Et, Ph), LTTP, is described. The first synthetic step involves the reaction of PhPH2 with 1 /2 equiv of CH2C12 and a 56% aqueous solution of KOH in DMF to produce Ph(H)PCH2P(H)Ph in 45-55% isolated yield. The second step utilizes the 2,2'-azobis-(isobutyronitrile) (A1BN) free-radical-catalyzed addition of 2 equiv of R2PCH=CH2 (R = Et, Ph) with Ph(H)PCH2P(H)Ph in cyclohexane, which gives a nearly quantitative yield of LTTP. The reaction of 2 equiv of NiCl2-6H20 with the ethyl-substituted version of LTTP (eLTTP) in EtOH produces the bimetallic complex Ni2Cl4(eLTTP) (2). Both the meso-and racemic-diastereomeric forms of 2 readily crystallize from THF to produce THF-solvated crystals. The air-stable red-orange crystals are easily separated because the meso complex rapidly desolvates to give opaque crystals, while the racemic crystals retain the THF solvent and remain clear. Single-crystal X-ray structures of meso-and raoNi2Cl4(eLTTP) are presented. Crystal data for rac-Ni2Cl4(eLTTP): triclinic, Pi, a = 11.693 (6) k,b= 13.830 (3) A, c = 14.438 (3) k, a = 111.87 (2)°, ß = 110.95 (2)°, y = 94.37 (2)°, V = 1963 (1) A3, Z = 2. Crystal data for meso-Ni2Cl4(eLTTP): monoclinic, space group P2j/c, a = 16.919 (2) A, b = 13.656 (2) A, c = 15.966 (6) A, ß = 94.46 (2)°, V = 3678 (3) A3, Z = 4. Both structures have square-planar geometries with each half of the eLTTP ligand forming a five-membered bi in both structures are well separated, with Ni-Ni distances of respectively.