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Hindered Axial−Equatorial Carbonyl Exchange in an Fe(CO)4(PR3) Complex of a Rigid Bicyclic Phosphine

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posted on 16.08.2001, 00:00 by Thomas S. Barnard, Mark R. Mason
Variable-temperature 13C NMR spectra for a series of Fe(CO)4(PR3) complexes ligated by phosphatri(3-methylindolyl)methane (1), phosphatri(pyrrolyl)methane (2), P(N-3-methylindolyl)3 (3), and P(N-pyrrolyl)3 (4) are reported. Ligand 2 was prepared by reaction of tri(pyrrolyl)methane with PCl3 in THF and Et3N. Compound 2 is stable to methanolysis, hydrolysis, and aerial oxidation at room temperature. Reactions of 2 with selenium powder and Rh(acac)(CO)2 yield phosphatri(pyrrolyl)methane selenide (5) and Rh(acac)(CO)(2) (6), respectively. The carbonyl stretching frequency in the IR spectrum of 6 and the magnitude of 1JSe-P in the 31P NMR spectrum of 5 indicate that 2 is a strong π-acid and a weak σ-base, commensurate with its lack of reactivity with CH3I. The trend in the decreasing basicity of 2 and related phosphines and phosphites was determined to be P(NMe2)3 > 3 > 4 > 1 > P(OPh)3 > 2. IR data for a series of Rh(acac)(CO)(PR3) complexes indicate the trend in decreasing π-acceptor ability to be 21 > 4 > P(OPh)3 > 3 > PPh3. Phosphines 14 were reacted with Fe2(CO)9 to yield Fe(CO)4(1) (7), Fe(CO)4(2) (8), Fe(CO)4(3) (9), and Fe(CO)4(4) (10), respectively. IR data for 710 support the trend in π-acidity listed above. Variable-temperature 13C NMR spectra for compounds 810 show a single doublet resonance for the carbonyls in the temperature range from −80 to 20 °C indicative of rapid intramolecular rearrangement of carbonyls between axial and equatorial sites. However, the 13C NMR spectrum for 7 shows slowed axial−equatorial carbonyl exchange at 20 °C. The limiting slow-exchange spectrum is observed at −20 °C. Hindered carbonyl exchange in 7 is attributed to the rigid 3-fold symmetry and steric bulk of 1. In addition to characterization of the new compounds by NMR (1H, 13C, and 31P) spectroscopy, IR spectroscopy, mass spectrometry, and elemental analysis, compounds 2, 7, 9, and 10 were further characterized by X-ray crystallography.