Synthesis and Characterization of Stable Iron Pentacarbonyl Radical Cation Salts

Abstract The open‐shell iron pentacarbonyl cation [Fe(CO)5].+ was isolated by deelectronation, i.e., the single‐electron oxidation of the parent neutral Fe(CO)5 using [phenazineF].+ as the [Al(ORF)4]− and [F‐{Al(ORF)3}2]− salt (RF=C(CF3)3; phenazineF=perfluoro‐5,10‐bis(perfluorophenyl)‐5,10‐dihydrophenazine). [Fe(CO)5].+[Al(ORF)4]− was fully characterized (scXRD analysis, IR, NMR, EPR, 57Fe spectroscopy, CV and SQUID magnetization study) and, apart from being a compound of fundamental interest, may serve as a precursor for low‐valent iron coordination chemistry.

unless stated otherwise. Values of the magnetic susceptibility were corrected for the core diamagnetism of the sample using tabulated Pascal's constants. [13] For simulation and analysis of the data, the program "JUlX2" written by Dr. Eckhard Bill (MPI CEC, Mülheim/Ruhr) was used.

Evans NMR Method
NMR samples were prepared under inert atmosphere in NMR tubes equipped with a gas-tight J.

Computational Details
The quantum chemical calculations were carried out with the TURBOMOLE software [14,15] using BP86 [16,17] or B3LYP [18][19][20] functionals with def2-TZVPP [21] basis sets, in combination with the D3BJ [22,23] dispersion correction. A fine integration grid (m4) and the default SCF convergence criteria (10 -7 a.u) were used. All frequency calculations (if applicable via the module AOFORCE [24] , otherwise via the module NUMFORCE) were simulated without scaling factor and a FWHM of 10 cm -1 . All calculated structures were checked for consistency in terms of geometric conversion, sensible electron occupations and the absence of imaginary vibrational frequencies.

Synthesis of Phenazine F
A 250 mL Schlenk tube was charged with LiN(C6F5)2 (7.1 g, 19.9 mmol) and under slow stirring and dynamic vacuum heated to 180 °C. The temperature was increased to 200 °C and the tap to the vacuum manifold was closed. After 10-60 min, the reaction proceeded instantaneous forming a white and black precipitate in the entire Schlenk tube. The residue was taken up in a mixture of Et2O (100 mL) and THF (50 mL). The organic layer was washed with HCl (10%, 30 mL) and the aqueous layer was extracted with Et2O (3 x 30 mL). The combined organic layers were washed with H2O (30 mL) and brine (30 mL), then dried over MgSO4. The solvent was removed under reduced pressure yielding a sticky dark brown residue. The residue was suspended in MeCN (2 pipettes) and washed with Et2O (2 pipettes) yielding off-white phenazine F (0.7 g, 9.99 mmol, 11%). 19

Synthesis of [Phenazine F ][Al(OR F )4] using Ag[Al(OR F )4]
Phenazine F (900 mg, 1.37 mmol) and Ag[Al(OR F )4] (1.47 g, 1.37 mmol, 1.00 eq.) were weighted into an H-cell equipped with a G4 frit. A mixture of SO2/Br2 was condensed onto the solids at -78 °C (ensuring an excess of Br2). The purple reaction mixture was stirred at ambient temperature for 18 h. The SO2/Br2 mixture was condensed into the empty arm of the H-cell to allow for complete precipitation of AgBr and poured back onto the crude mixture. The suspension was filtered by the internal frit and extracted three times by back condensing the SO2/Br2 mixture. The SO2/Br2 mixture was removed and the product dried under reduced pressure yielding dark purple [Phenazine F ][Al(OR F )4] (2.07 g, 1.27 mmol, 93%).

Synthesis of [Phenazine F ][Al(OR F )4] using NO[Al(OR F )4]
Phenazine F (500 mg, 0.76 mmol) and NO[Al(OR F )4] (758 mg, 0.76 mmol, 1.00 eq.) were weighted into a Schlenk tube. CH2Cl2 (20 mL) was added to the solids. The reaction mixture was stirred at ambient temperature for 18 h. Neutral NO gas was removed through three cycles by the freeze

Synthesis of [Phenazine F ][F-{Al(OR F )3}2] using NO[F-{Al(OR F )3}2]
Phenazine F (150 mg, 0.23 mmol) and NO[F-{Al(OR F ) 3 }2] (344 mg, 0.23 mmol, 1.00 eq.) were weighted into a Schlenk tube. CH2Cl2 (10 mL) was added to the solids. The reaction mixture was stirred at ambient temperature for 18 h. Neutral NO gas was removed through three cycles by the freeze pump thaw method. The solvent was removed under reduced pressure yielding dark purple
The reaction mixture was stirred at ambient temperature for 1.

Synthesis of [Fe(CO)5][Al(OR F )4] in 1,2,3,4-Tetrafluorobenzene using pristine [Phenazine F ][Al(OR F )4] from NO-Route
[Phenazine F ][Al(OR F )4] (400 mg, 0.25 mmol) was weighted into a Schlenk tube. A stock solution of Fe(CO)5 in 1,2,3,4-tetrafluorobenzene (2.46 mL, 0.25 mmol, 1.00 eq.) was added, after which the reaction mixture instantaneously turned dark green. The reaction mixture was stirred at ambient temperature for 1.5 h. The solution was layered with n-pentane and [Fe(CO)5][Al(OR F )4] (1) was obtained as dark green crystals suitable for scXRD (200 mg, 0.17 mmol, 70%). 13          (1). The structural model on which the refinement is based was taken from the single crystal study and was not further refined. A very good agreement with the measured powder diffraction pattern is observed (blue data points and green line). As can be seen from the difference plot (black and turquoise line), no further crystalline phases can be detected. Nevertheless, amorphous impurities cannot be excluded from the result.   (2).  Figure S23: Continuous wave EPR spectrum of solid 1 at the X-band (9.75 GHz) at 100 K. Figure S24: X-band EPR spectrum of 1 in a frozen solution of oDFB at 95 K. Figure S25: Simulated X-band EPR spectra. The 1 st species refers to solid 1 (red), 2 nd species refers to [Fe(CO)5oDFB] •+ (blue). Sim is the combination of the 1 st and 2 nd species (gray).

Magnetic Measurement of [Fe(CO) 5 ] •+ [Al(OR F ) ] -(1) in Solution and as Solid
Evans NMR method was used to determine the effective magnetic moment µeff in solution. [30,31] For this, four solutions with different concentrations of 1 in 4FB were prepared. TMS was added as reference molecule. The mass susceptibilities χg are given by Eq. (1): [32,33] = Where ∆ is the difference in frequency shift of the probe molecule, in reference to TMS in Hz, 0 is the NMR spectrometer frequency in Hz, is the mass of the probe per cm 3 of solution, 0 is the mass susceptibility of 4FB, 0 is the density of the pure solvent and is the density of 4FB.
For diluted solutions, Eq. (1) can be simplified to Eq. (2): [32,33] = 3∆ 4 0 (2) By multiplication of with the molecular mass of the solute the molar susceptibilities of a solute can be calculated (Eq. 3): Through linear regression according to Eq. (4) the molar susceptibilities of 1 in 4FB were calculated ( Figure S22). The paramagnetic molar susceptibilities can be determined according to Eq. (5): [13] = − (5) The diamagnetic contribution was estimated by determining the molar susceptibility of diamagnetic [Co(CO)5][Al(OR F )4] in 4FB in analogy to 1. Thus, the effective magnetic moment of 1 can be calculated by Eq. (7), where T is the temperature in Kelvin. [32] = 2.828√ (6) Figure S27: SQUID magnetization measurements of two independently synthesized, microcrystalline solid samples of [Fe(CO)5][Al(OR F )4] (1) in the temperature range 2-300 K with an applied magnetic field of 1 T. The solid lines represent the best global fit obtained for S = 1/2.
112 (2)            The position of the fluorine atoms and cation was obtained from the scXRD structure of [Fe(CO)5][Al(OR F )4] (1), representing the nearest contact point between the cation and the anion. A charge of -0.3e was chosen, due to calculations revealing this to be the partial negative charge typically residing in -CF3 groups of the aluminate anion (cf. related work [34] ).