Separating the racemic and meso diastereomers of a binucleating tetraphosphine ligand system through the use of nickel chloride.

The reaction of a 1:1 mixture of rac- and meso-et,ph-P4 (et,ph-P4 = (Et(2)PCH(2)CH(2))(Ph)PCH(2)P(Ph)CH(2)CH(2)PEt(2)) with 2 equiv of NiCl(2).6H(2)O in EtOH produces soluble rac-Ni(2)Cl(4)(et,ph-P4) and precipitates meso-Ni(2)Cl(4)(et,ph-P4), allowing facile isolation of each bimetallic complex. Subsequent reaction with more than 250 equiv of NaCN in H(2)O/MeOH releases the et,ph-P4 ligand and [Ni(CN)(4)](2-). The rac,trans- and meso,trans-Ni(CN)(2)(eta(2.5)-et,ph-P4) form as intermediates in the cyanolysis of rac- and meso-Ni(2)Cl(4)(et,ph-P4). These have been characterized by X-ray crystallography. The unusual partial isomerization of the meso- to rac-et,ph-P4 ligand via the monometallic trans-Ni(CN)(2)(eta(2.5)-et,ph-P4) intermediate complex is discussed.

A bimetallic hydroformylation catalyst: high regioselectivity and reactivity through homobimetallic cooperativity.

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.