Dirhenium Compounds Containing the Diphosphine Ligands Ph2PCH2PPh2, Ph2PCH(CN)PPh2, or Ph2PCH(CH3)PPh2: A Comparative Study

When M2L10 complexes containing two rhenium(III) atoms in a quadruple bond react with an excess of the diphosphine ligand Ph2PCH2PPh2, the bond order decreases, and the rhenium is reduced to rhenium(II). When two diphosphine ligands are bonded to the dirhenium center, they can adopt a trans-trans, trans-cis, or cis-cis configuration. NMR spectra of compounds that adopt the cis-cis arrangement show unusually large downfield shifts for one of the bridgehead methylene hydrogens (P-CH2-P). To understand the unusual chemical shift, the hydrogens first have to be assigned. The strategy for making these assignments has been to try to make compounds where one of the hydrogens has been replaced with another group (e.g., CH3 or CN). All efforts to make cis-cis-type compounds containing Ph2PCH(CH3)PPh2 (dppmMe) or Ph2PCH(CN)PPh2 (dppmCN) have been unsuccessful. However, reactions of rhenium-containing starting materials with these derivatized ligands have produced compounds with unexpected chemical and physical properties. Furthermore, the work has allowed us to compare how the dirhenium chemistry of Ph2PCH(CH3)PPh2 (dppmMe) and Ph2PCH(CN)PPh2 (dppmCN) differs from the well-established chemistry of the parent diphosphine, dppm. Finally, one reaction mixture may have produced a sample of a compound with a trans-trans configuration of diphosphine ligands