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 = (Et2PCH2CH2)(Ph)PCH2P(Ph)CH2CH2PEt2)
with 2 equiv of NiCl2·6H2O in EtOH produces soluble rac-Ni2Cl4(et,ph-P4) and precipitates meso-Ni2Cl4(et,ph-P4), allowing facile isolation of each bimetallic complex. Subsequent reaction with more than 250 equiv
of NaCN in H2O/MeOH releases the et,ph-P4 ligand and [Ni(CN)4]2-. The rac,trans- and meso,trans-Ni(CN)2(η2.5-et,ph-P4) form as intermediates in the cyanolysis of rac- and meso-Ni2Cl4(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(η2.5-et,ph-P4) intermediate complex is discussed.