The Bis(ferrocenyl)phosphenium Ion Revisited

Abstract The bis(ferrocenyl)phosphenium ion, [Fc2P]+, reported by Cowley et al. (J. Am. Chem. Soc. 1981, 103, 714–715), was the only claimed donor‐free divalent phosphenium ion. Our examination of the molecular and electronic structure reveals that [Fc2P]+ possesses significant intramolecular Fe⋅⋅⋅P contacts, which are predominantly electrostatic and moderate the Lewis acidity. Nonetheless, [Fc2P]+ undergoes complex formation with the Lewis bases PPh3 and IPr to give the donor–acceptor complexes [Fc2P(PPh3)]+ and [Fc2P(IPr)]+ (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazole‐2‐ylidene).

Unless otherwise noted, NMR spectra were recorded at room temperature on a Bruker Avance Neo 600 MHz spectrometer. 1 H, 13 C{ 1 H}, 11 B{ 1 H}, 31 P{ 1 H} and 19 F NMR spectra are reported on the δ scale (ppm) and are referenced against SiMe4, BF3•Et2O (15% in CDCl3), H3PO4 (85% in water) and CFCl3, respectively. 1 H and 13 C{ 1 H} chemical shifts are reported relative to the residual peak of the solvent (CDHCl2: 5.32 ppm, for CD2Cl2) in the 1 H NMR spectra, and to the peak of the deuterated solvent (CD2Cl2: 53.84 ppm) in the 13 C{ 1 H} NMR spectra. 8 The assignment of the 1 H and 13 C{ 1 H} resonance signals was made in accordance with the COSY, HSQC and HMBC spectra.
The ESI HRMS spectra were measured on a Bruker Impact II spectrometer. Acetonitrile or dichloromethane/acetonitrile solutions (c =1•10 −5 mol•L −1 ) were injected directly into the spectrometer at a flow rate of 3 μL•min −1 . Nitrogen was used both as a drying gas and for nebulization with flow rates of approximately 5 L•min −1 and a pressure of 5 psi. Pressure in the mass analyzer region was usually about 1•10 −5 mbar. Spectra were collected for 1 min and averaged. The nozzle-skimmer voltage was adjusted individually for each measurement.

Synthesis and characterization of Fc2PCl (1)
To a pre-cooled (−80 °C) suspension of FcLi (10.77 g, 56 mmol) in THF (40 mL), i-Pr2NPCl2 (5.67 g, 28 mmol) was added. The reaction mixture was allowed to warm up to room temperature over the course of 12 h. A solution of HCl in Et2O (1.7 M, 20 mL, 34 mmol) was added slowly. The suspension was filtered over a pad of dry Celite to remove the insoluble i-Pr2NH2Cl. After the solvents were evaporated to dryness, CH2Cl2 (100 mL) was added and the suspension filtered over a pad of dry Celite to remove LiCl. All volatiles were evaporated to dryness and the remaining solid was washed with MeCN (3×25 mL) at rt and Et2O at 0°C (3×25mL). The remaining solid was dried under vacuum overnight. The title product was obtained as a yellow solid (7.72 g, 63%). Mp. 186-188 °C.

Synthesis and characterization of [Fc2P][BAr F 4] (2)
To a solid mixture of Fc2PCl (0.437 g, 1 mmol) and Na[BAr F 4] (0.886 g, 1 mmol) was added CH2Cl2 (10 mL) at room temperature. The reaction mixture was stirred for 15 minutes then filtered under argon through a syringe PTFE filter; a dark red-brown solution was obtained.
Hexane (50 mL) was added with vigorous stirring. The microcrystalline solid was allowed to settle, and the solution removed. The remaining solid was recrystallized once from 1,2difluorobenzene/hexane (4 mL/20 mL) and once from CH2Cl2/hexane (4 mL/20 mL). The title product was obtained as a black solid (1.

X-Ray diffraction studies
Intensity data of 1-4 was collected on a Bruker Venture D8 diffractometer at 100 K with graphite-monochromated Mo-K (0.7107 Å) radiation. All structures were solved by direct methods and refined based on F 2 by use of the SHELX program package as implemented in WinGX. 9,10 All non-hydrogen atoms were refined using anisotropic displacement parameters.
Hydrogen atoms attached to carbon atoms were located form the difference Fourier map and

Computational data
Starting from the solid-state molecular geometries of 1-4 structural optimizations were conducted by density functional theory (DFT) at the B3PW91/6-311+G(2df,p) 12,13 level of theory using Gaussian09. 14 For Fe a fully relativistic effective core potential (ECP10MDF) and a corresponding cc-pVTZ basis set was utilized. 15,16 Normal mode (or frequency) analysis proved that all stationary points were at least local minima. The wavefunction files were employed for a topological analysis of the electron density according to the Atoms-In-Molecules (AIM) space-partitioning scheme using AIM2000, 17  step size were computed. NCI grids were generated with NCIplot. 19 Molecular orbitals (MO) were extracted from the formatted checkpoint files with the cubegen subroutine of Gaussian09.
Natural bond orbitals (NBO) were calculated with NBO 5.9. 20 Bond paths are displayed with AIM2000, ELI-D and NCI figures are displayed with MolIso, 21 MO and NBO images are generated with GaussView 5. 22 The AIM topology parameters are collected in Table S3, whereas AIM charges (Q(AIM) are listed in Table S4.     92 For all bonds, ρ(r)bcp is the electron density at the bond critical point,  2 ρ(r)bcp is the corresponding Laplacian, ε is the bond ellipticity, G/ρ(r)bcp and H/ρ(r)bcp are the kinetic and total energy density over ρ(r)bcp ratios.