Hindered axial-equatorial carbonyl exchange in an Fe(CO)4(PR3) complex of a rigid bicyclic phosphine

Variable-temperature C-13 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 (1)J(Se-P) in the P-31 NMR spectrum of 5 indicate that 2 is a strong pi -acid and a weak sigma -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 jr-acceptor ability to be 2 approximate to 1 > 4 > P(OPh)(3) > 3 > PPh3. Phosphines 1-4 were reacted with Fe-2(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 7-10 support the trend in or-acidity listed above. Variable-temperature C-13 NMR spectra for compounds 8-10 show a single doublet resonance for the carbonyls in the temperature range from -80 to 20 degreesC indicative of rapid intramolecular rearrangement of carbonyls between axial and equatorial sites. However, the C-13 NMR spectrum for 7 shows slowed axial-equatorial carbonyl exchange at 20 degreesC. The limiting slow-exchange spectrum is observed at -20 degreesC. 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 (H-1, C-13, and P-31) spectroscopy, IR spectroscopy, mass spectrometry, and elemental analysis, compounds 2, 7, 9, and 10 were further characterized by X-ray crystallography.