Phosphine-Ligated Cobalt(II) Acetylacetonate Complexes

Cobalt(II) acetylacetonate complexes bearing a phosphine ligand can be key intermediates or precursors to cobalt-based catalysts; however, they have been rarely studied, especially from a molecular structure point of view. This work is focused on the understanding of how different phosphines react with Co(acac)₂ (acac = acetylacetonate). To do so, a variety of analytical tools, including NMR and IR spectroscopy, X-ray crystallography, mass spectrometry, and elemental analysis, have been used to study the reactions and characterize the isolated products. These results have shown that the monodentate ligand, HPPh₂, binds to Co(acac)₂ weakly and reversibly to produce Co₂(acac)₄(HPPh₂), whereas the bidentate ligand, 1,2-bis(diphenylphosphino)ethane (dppe), interacts with Co(acac)₂ more strongly to yield a 1D coordination polymer of Co(acac)₂(dppe). 2-(Dicyclohexylphosphino)methyl-1 H-pyrrole (^CyPN^H), which is a pyrrole-tethered phosphine, forms an unusual 5-coordinate cobalt complex, Co(acac)₂(^CyPN^H), in which the pyrrole moiety participates in a bifurcated hydrogen–bonding interaction with the [acac]^– ligands. In contrast, another bidentate ligand, 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos), fails to react with Co(acac)₂, presumably due to its wide bite angle and difficulty in bridging two metals.