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

✍ Scribed by Ghenadie Novitchi; Fabrice Riblet; Lothar Helm; Rosario Scopelliti; Aurélian Gulea; André E. Merbach

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 267 KB
• Volume: 42
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1438

✦ Synopsis

Abstract

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.