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

Abstract

Variable-temperature (13)C NMR spectra for a series of Fe(CO)(4)(PR(3)) 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 PCl(3) in THF and Et(3)N. 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 (31)P NMR spectrum of 5 indicate that 2 is a strong pi-acid and a weak sigma-base, commensurate with its lack of reactivity with CH(3)I. The trend in the decreasing basicity of 2 and related phosphines and phosphites was determined to be P(NMe(2))(3) > 3 > 4 > 1 > P(OPh)(3) > 2. IR data for a series of Rh(acac)(CO)(PR(3)) complexes indicate the trend in decreasing pi-acceptor ability to be 2 approximately 1 > 4 > P(OPh)(3) > 3 > PPh(3). 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 pi-acidity listed above. Variable-temperature (13)C NMR spectra for compounds 8-10 show a single doublet resonance for the carbonyls in the temperature range from -80 to 20 degrees C indicative of rapid intramolecular rearrangement of carbonyls between axial and equatorial sites. However, the (13)C NMR spectrum for 7 shows slowed axial-equatorial carbonyl exchange at 20 degrees C. The limiting slow-exchange spectrum is observed at -20 degrees 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 ((1)H, (13)C, and (31)P) spectroscopy, IR spectroscopy, mass spectrometry, and elemental analysis, compounds 2, 7, 9, and 10 were further characterized by X-ray crystallography.

Glacial acetic acid as an efficient catalyst for simple synthesis of dindolylmethanes

“…Their results also indicated that BIMs could potentially be applied as chemotherapeutic agents against tumors 1,3 . BIMs and tris(indolyl)methanes (TIMs), have been used as legands for the synthesis of many complex molecules and different properties of these complex molecules have been investigated [4][5][6][7][8][9] . The electron rich indole nucleus shows an enhanced reactivity towards carbon electrophiles that generally results in the formation of three substituted indole derivatives 10 .…”

Deprotonated diindolylmethanes as dianionic analogues of scorpionate bis(pyrazolyl)borate ligands: synthesis and structural characterization of representative titanocene and zirconocene complexes

“…Suitable dianionic and trianionic analogues of poly(pyrazolyl)borates can be envisioned via substitution of pyrrolyl [7] or bulkier indolyl moieties for pyrazolyl substituents in bis-and tris(pyrazolyl)methanes. In this regard, we recently demonstrated the utility of trianionic tri(pyrrol-2-yl)methane and tri(3-methylindol-2-yl)methane for the preparation of bulky, constrained p-acidic phosphines [8,9]. Here, we demonstrate that deprotonated di(3-methylindol-2-yl)phenylmethane (L 2 H 2 ) and 2-methoxyphenyldið3-methylindol-2-ylÞmethane ðL 0 2 H 2 Þ function as dianionic scorpionate chelates to transition metals.…”

Synthesis of N-Heterocycle Substituted Silyl Ligands within the Coordination Sphere of Iron

“…Additionally, the disappearance of the three distinct 13 C NMR resonances around 200 ppm for 3 (62) and appearance of a single resonance at 217.9 ppm for Na- 5 is consistent with the formation of a fluxional 5-coordinate structure with fast axial–equatorial exchange. 63−73 The independently synthesized ammonium salt NEt 4 - 2 74 readily undergoes substitution under the same conditions, showing that nucleophilic substitution is feasible on Cl 3 Si bound to Fe(0) and may take place after deprotonation of 3 . From a practical point of view, however, reactions involving NEt 4 - 2 are less well-behaved because the counterion is susceptible to Hoffman degradation, i.e.…”