Isomerization Mechanisms of Stereolabile tris- and bis-Bidentate Octahedral Cobalt(II) Complexes: X-ray Structure and Variable Temperature and Pressure NMR Kinetic Investigation
The isomerization dynamics of five labile octahedral Co(II) compounds have been investigated by variable temperature and pressure 1H and 19F NMR spectroscopy in dichloromethane solution. The X-ray crystal structure of the two tris-chelates, [Co(HFA)2bpic] (1) and [Co(TTFA)2bpy] (2), show a distorted octahedral arrangement of the 4 oxygen and 2 nitrogen donor atoms, with bidentate ligand bite angles smaller than 90. On the other hand, in the three bis-chelates, trans(N)-[Co(HFA)2(CH3py)2](3), cis(N)-cis(CF3)-trans(S)-[Co(TTFA)2(CH3py)2](4), and trans(N)-trans(CF3)-[Co(TTFA)2(CF3py)2](5), the replacement of the bidentate nitrogen donor ligands by two monodentate Rpy ligands leads to relaxed structures with almost regular octahedral arrangements of the donor atoms (HFA= 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato anion; TTFA= 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato anion; bpy=2,20-bipyridine; bpic=4,40-dimethyl-2,20-bipyridine). In solution the five complexes are stereolabile and all possible isomers are formed: from one for 1 up to five for 4 and 5. All cis-N isomers form pairs of enantiomers, whereas the trans-N isomers are achiral. A solid state structure/isomerization mechanism/rate correlation has been established for the isomerization dynamics of these Co(II) tris- and bis-chelates. The two tris-chelate complexes 1 and 2, with a distorted octahedral solid state structure, show one and three isomers in solution and isomerize/tautomerize very rapidly according to Bailar twist mechanisms. The three bis-chelate complexes 3, 4, and 5, with a close to octahedral symmetry in the solid state, show two, five, and five isomers, respectively. They isomerize/tautomerize 3 orders of magnitude slower as the tris-chelates, by an intramolecular dissociative mechanism involving a ring-opening of an arm of a bidentate ligand to form a TBP intermediate with a dangling bidentate ligand. The results of this first systematic investigation of the isomerization mechanisms of highly labile Co(II) complexes are supported by the NMR observed exchange paths (up to five for complexes for 4 and 5), the variable temperature (185 to 312 K) and pressure (up to 200 MPa) activation parameters, and a detailed analysis of the solid state structures.