115224-75-4Relevant academic research and scientific papers
(η2-trans-Cyclooctene)2Fe(CO)3 and related complexes: Structure and dynamic behavior
Angermund, Hildegard,Grevels, Friedrich-Wilhelm,Moser, Rainer,Benn, Reinhard,Krüger, Carl,Rom?o, Maria J.
, p. 1994 - 2004 (2008/10/08)
(η2-trans-Cyclooctene)(η2-olefin)Fe(CO) 3 complexes are synthesized by thermal olefin exchange [3, olefin = trans-cyclooctene, from (η2-cis-cyclooctene)2Fe(CO)3 (2) and trans-cyclooctene; 6, olefin = methyl acrylate, from (η2-methyl acrylate)2Fe(CO)3 and trans-cyclooctene] or CO photosubstitution [3, from Fe(CO)5 and excess trans-cyclooctene; 5, olefin = dimethyl fumarate, from (η2-trans-cyclooctene)Fe(CO)4 and dimethyl fumarate or from (η2-dimethyl fumarate)Fe(CO)4 and trans-cyclooctene]. The products are obtained as mixtures of diastereomers, some of which (3a, C2 isomer; 3b, Cs isomer; 5a) are separated by fractional crystallization. The reaction of 2 with (-)-trans-cyclooctene yields (-)-3a. Single-crystal X-ray diffraction analyses of racemic 3a and 5a establish a trigonal-bipyramidal geometry with the C=C units occupying two equatorial positions. Crystals of racemic 3a are monoclinic of space group P21/n with a = 12.069 (2) A?, b = 8.278 (1) A?, c = 18.815 (2) A?, β = 98.386 (9)°, and Z = 4. The unit cell contains two molecules of each 3a enantiomer. The structure was solved and refined to R = 0.034 and Rw = 0.037 by using 4218 independent reflections. Crystals of 5a are orthorhombic of space group P212121 with a = 7.811 (1) A?, b = 13.167 (1) A?, c = 20.432 (2) A?, and Z = 4. The structure was solved and refined to R = 0.037 and Rw = 0.045 by using 5506 independent reflections. All compounds are characterized by elemental analysis and IR (including CO force field parameters of 3a) and 13C NMR spectroscopy. The 13C NMR coordination shifts of the olefinic carbon atoms in 3a and 5a correlate (inversely) with the respective metal-carbon distances. Variable-temperature 13C NMR spectroscopy of 3b reveals three sequential dynamic processes: (a) olefin rotation (ΔH≠ = 8.0 ± 0.5 kcal mol-1, ΔS≠ = -5.1 ± 3 cal mol-1 K-1), (b) eq-CO/ax-CO two-site exchange (ΔH≠ = 13.8 ± 0.4 kcal mol-1, ΔS≠ = 2.8 ± 1.2 cal mol-1 K-1), (c) complete CO scrambling (ΔH≠ = 16.8 ± 0.8 kcal mol-1, ΔS≠ = 10.2 ± 3 cal mol-1 K-1). In the case of 3a the two CO site-exchange processes are indistinguishable. The dynamic behavior of the mixed bis(olefin) complexes 5a and 6 indicates that the two olefins rotate independently, and rotation of both of the olefins appears to be a prerequisite for CO scrambling. In conclusion, these observations are in accord with a sequence of three Berry pseudo-rotational steps, alternately using an olefin, a CO group, and the respective other olefin as the pivotal ligand.
