Tuning Cobalt(II) Phosphine Complexes to be Axially Ambivalent

We report the isolation and characterization of a series of three cobalt(II) bis(phosphine) complexes with varying numbers of coordinated solvent ligands in the axial position. X-ray quality crystals of [Co(dppv)(2)][BF4](2) (1), [Co(dppv)(2)(NCCH3)]-[BPh4](2) (2), and [Co(dppv)(2)(NCCH3)(2)][BF4](2) (3) (dppv = cis-1,2-bis(diphenylphosphino)ethylene) were grown under slightly different conditions, and their structures were compared. This analysis revealed multiple crystallization motifs for divalent cobalt(II) complexes with the same set of phosphine ligands. Notably, the 4-coordinate complex 1 is a rare example of a square -planar cobalt(II) complex, the first crystallographically characterized square-planar Co(II) complex containing only neutral, bidentate ligands. Characterization of the different axial geometries via EPR and UV-visible spectroscopies showed that there is a very shallow energy landscape for axial ligation. Ligand field angular overlap model calculations support this conclusion, and we provide a strategy for tuning other ligands to be axially labile on a phosphine scaffold. This methodology is proposed to be used for designing cobalt phosphine catalysts for a variety of oxidation and reduction reactions.

Automated summary

We report the isolation and characterization of a series of cobalt(II) bis(phosphine) complexes with varying numbers of coordinated solvent ligands. Comparison of their structures revealed multiple crystallization motifs for divalent cobalt(II) complexes with the same set of phosphine ligands. This study provides a strategy for tuning the axial variability of other ligands on a phosphine scaffold, and proposes a method for designing cobalt phosphine catalysts.