Richness of isomerism in labile octahedral Werner‐type cobalt(II) complexes demonstrated by 19 F NMR spectroscopy: structure and stability

The cobalt(II) complexes [CoL 2 (R 2 ‐Py) 2 ] ( 1 – 4 ) where HL A = 1,1,1‐trifluoro‐5,5‐dimethyl‐2,4‐hexanedione, R 2 ‐Py = 4‐methylpyridine ( 1 ), HL B = 4,4,4‐trifluoro‐1‐(2‐thienyl)‐1,3‐butanedione, R 2 ‐Py = 4‐methylpyridine ( 2 ), 4‐phenylpyridine ( 3 ) and S ‐(−)‐1‐(4‐pyridyl)ethanol ( 4 ) were prepared by two‐step reactions. X‐ray structure analysis of [CoL A 2 (CH 3 ‐Py) 2 ] revealed the { trans (N)‐ trans (CF 3 )‐ trans } configuration for the complex obtained by crystallization from ethanol. A dynamic equilibrium between the five possible stereoisomers was observed for each complex 1 – 4 in solution by 19 F NMR spectroscopy. The criteria used for full NMR assignment (180–265 K) include comparison of integral ratios, cis (N) and trans (N) differentiation in presence of the chiral amine [ S ‐(−)‐1‐(4‐pyridyl)ethanol], effect of solvent polarity on the relative stabilities of the five isomers and observation of trans influences in a mixture of complexes. Thermodynamic parameters for the equilibria between the isomers of 2 in CD 2 Cl 2 (Δ H i,j , Δ S i,j and K i,j ) were obtained from signal integrals. The two trans (N) isomers are slightly more stable than the three cis (N) isomers at low temperature [Δ G ° i,j (max) = 2.8 kJ mol −1 at 179.8 K], but this stability difference almost vanishes with increasing temperature [Δ G ° i,j (max) = 1.0 kJ mol −1 at 265.0 K]. The values found for Δ H ° i,j are relatively small and largely entropy compensated. Copyright © 2004 John Wiley & Sons, Ltd.