Tuning Cobalt(II) Phosphine Complexes to be Axially Ambivalent

Abstract

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 ][BF 4 ] 2 (1) , [Co(dppv) 2 (NCCH 3 )][BPh 4 ] 2 (2) , and [Co(dppv) 2 (NCCH 3 ) 2 ][BF 4 ] 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.

Redox Behavior of Cobalt–Phosphine Complexes vs Their Catalytic Activity in Organozinc Compound Formation: Background for Mechanistic Investigations

“…Two pronounced reversible electrochemical processes were observed on the CV curve of Co 4 (dppe) 5 Cl 8 at E 1/2 = −0.44 V and E 1/2 = −1.00 V on the GC electrode (Figure ), which could be assigned to Co II /Co I and Co I /Co 0 processes, respectively. These potentials are close to the previously reported values for [Co­(dppv) 2 ]­[BF 4 ] 2 (dppv, cis-1,2-bis­(diphenylphosphino)­ethylene) . Good stabilization of Co I and Co 0 (according to high values of the redox potentials) can be explained by the presence of four soft phosphorus donor atoms in the [Co­(dppe) 2 Cl] + cation, which is a part of this complex.…”

“…These potentials are close to the previously reported values for [Co(dppv) 2 ][BF 4 ] 2 (dppv, cis-1,2-bis-(diphenylphosphino)ethylene). 38 Good stabilization of Co I and Co 0 (according to high values of the redox potentials) can be explained by the presence of four soft phosphorus donor atoms in the [Co(dppe) 2 Cl] + cation, which is a part of this complex. However, despite the presence of a reversible redox process, use of Co 4 (dppe) 5 Cl 8 as a catalyst in the reaction of aryl halides with zinc did not lead to the formation of an organozinc compound in high yield.…”