Stable salts of the hexacarbonyl chromium(I) cation and its pentacarbonyl-nitrosyl chromium(I) analogue

Homoleptic carbonyl radical cations are a textbook family of complexes hitherto unknown in the condensed phase, leaving their properties and applications fundamentally unexplored. Here we report on two stable 17-electron [Cr(CO)6]•+ salts that were synthesized by oxidation of Cr(CO)6 with [NO]+[Al(ORF)4]− (RF = C(CF3)3)) in CH2Cl2 and with removal of NO gas. Longer reaction times led to NO/CO ligand exchange and formation of the thermodynamically more stable 18-electron species [Cr(CO)5(NO)]+, which belongs to the family of heteroleptic chromium carbonyl/nitrosyl cations. All salts were fully characterized (IR, Raman, EPR, NMR, scXRD, pXRD, magnetics) and are stable at room temperature under inert conditions over months. The facile synthesis of these species enables the thorough investigation of their properties and applications to a broad scientific community.


Table of Contents
Powder Diffraction. The powder diffractograms were recorded with the sample in a 0.5 mm thick capillary (Hilgenberg GmbH, wall thickness 0.01 mm) sealed with perfluoropolyalkylether oil (AB128330, ABCR GmbH & Co. KG), at about 100(10) K in the  range 3-42° with a STOE STADI P powder diffractometer with Mo-K radiation ( = 0.709300 Å) equipped with Ge-(111) monochromator and Mythen 1K detector. Data acquiring, processing and the calculation of powder diffractograms from single-crystal data were performed using STOE WinXPOW ® package. All powder diffractograms were background corrected.
UV/Vis spectroscopy. UV/Vis reflectance spectra were recorded at r.t. on a Thermoscientific Evolution 600 spectrometer equipped with an integration sphere. The baseline was measured against Spectralon ® . To protect the powdered samples from air and moisture during measurements, a special sample holder with a quartz window was used.
EPR spectroscopy. EPR spectra were recorded using a Bruker EMXplus continuous wave (cw) X-Band spectrometer with nitrogen cooling for 100 K measurements and liquid helium cooling for 4 K measurements, respectively. Samples were filled into fused silica glass tubes. Solvents were vacuum transferred to the samples and afterwards the sample tubes were sealed under vacuum. EPR spectra were analysed and simulated using the EasySpin MATLAB toolbox 20,21 .
Computational Details. Quantum chemical calculations were performed with the TURBOMOLE 22,23 program package (version 7.0). All investigated molecular structures were optimized at the density functional theory (DFT) and were run in redundant internal coordinates using the BP86 24,25 functional with the resolution-of-identity (RI) approximation 26,27 together with the basis set def2-TZVPP 28 and with dispersion correction (DFT-D3BJ) 29,30 . A fine integration grid (m4) and the default SCF convergence criteria (10 -6 a.u.) were used. All optimized structures were checked for minima (no imaginary frequencies) with the implemented module AOFORCE 31 and for proper spin occupancies using the implemented module EIGER. Entropic contributions to enthalpy and Gibbs free energy with inclusion of zero point energies (ZPE) were calculated at the BP86-D3BJ/def2-TZVPP level for standard conditions with the FREEH module. IR and Raman intensities were calculated with the Gaussian 32 software at BP86/def2-TZVP.
Correlation corrections were taken into account by Strongly-Contracted N-Electron Valence State Perturbation Theory (SC-NEVPT2) 44,45 . The CAS-SCF wave function served to calculate g-tensors for minimum structures of the [Cr(CO)6] •+ cation by the Effective Hamiltonian formalism 46 . The SA-CAS-SCF calculations were carried out in the basis of 13 doublet, 17 quartet and 1 sextet roots arising from the 6 S, 4 G, 4 P, 4 D and 2 I terms of the free Cr + cation. The active space consists of 9 electrons in 10 orbitals of the first and second chromium d-shell and two additional chromium-ligand binding orbitals, i.e. CAS (9,12).

Experimental glassware and pictures of complexes 1 and 2
Supplementary Figure 1. Double-Schlenk tube that was typically used for most reactions and crystallizations. Note that different varieties (sizes, Rettberg or J. Young valves) were used.

Synthesis of NO[F-{Al(OR F )3}2]
A double-Schlenk flask was equipped inside the glove box with Me3Si-F-Al(OR F )3 (1970 mg, 2.39 mmol, 2 eq.) and NO[PF6] (207 mg, 1.18 mmol, 1 eq.). The flask was equipped with a bubbler and SO2 (ca. 5 mL) was condensed onto the reaction mixture at -196 °C. The vessel was then carefully vented with Ar and the bubbler was opened towards the fume hood while the reaction mixture was stirred at -35 °C for 45 min (evolution of PF5!). Then, the volatiles were removed at 0 °C and a white solid was obtained, which was further dried in vacuo (Yield: 1700 mg, 1.12 mmol, 95%). Supplementary Figure 6. 14 N NMR spectrum (28.

Vibrational Analysis
The full vibrational analysis of compounds 1 to 4 is shown in Supplementary Table 2 and   Supplementary Table 3.
Supplementary   Bands of trace impurities are crossed-out: 2174/2173 cm -1 belong to a small contamination with residual [Cr(CO)6] + , 1821/1809 cm -1 might belong to the isotope-shifted A1 vibration, since it appears also in the spectra of single crystalshowever, since the shift is different for 3 and 4, we stated it as an unknown impurity.
The additional bands in the carbonyl region is due to minor interactions with the counter ion (which distorts the ideal C4v structure). Note that here the Cr-N and Cr-C vibrations at 657 cm -1 and 640 cm -1 ( § with overlapping Al   Table 4). Supplementary

Group Theory
In order to understand the obtained EPR-spectra and the underlying dynamic of the [CrCO6] •+ cation we optimized its structure applying different point groups. Removal of one electron from the neutral Cr(CO)6 d 6 complex breaks down its wave function symmetry resulting in an electronic B1g state and making an Oh symmetric structure impossible. Following the normal vibrational modes of the Oh structure leads to the point groups the cationic d 5 species may take.
The Eg symmetric vibrations according to an axial compression along the z-axis and stretching along the x-and y-axis (and vice versa) lead to D4h symmetric structures. We found (TPSSh-D3BJ/def2-TZVPP) a D4h symmetric structure, B1g electronic ground state, with compressed zaxis to be a local minimum whose electronic energy is +1.63 kJ mol -1 (DLPNO-CCSD(T)/def2-TZVPP) above the global minimum.
Following the T2g bending modes leads to a D2h symmetric distortion. However, this D2h * structure 2.04 kJ mol -1 above the global minimum is not a stationary point (cf. asterisk) and shows a gradient linked to stretchings along the principal axes, i.e. an admixture with the Eg symmetric stretching modes. The relaxed stationary D2h structure with B1g electronic ground state shows one imaginary frequency exposing it to be a transition state at +0.85 kJ mol -1 connecting two equivalent D3d symmetric structures with an A1g electronic ground state. The D3d structure arises from a linear combination of all three T2g bending modes of the parent Oh structure and was identified to be the global minimum structure.
We note, that the Ag electronic state of D2h point group is identical with the above mentioned

Hyperfine Coupling
The calculated hyperfine coupling constants of 53
In addition, we have done quite a few tests with our materials and using the equipment available to us in the chemistry department in Freiburg. Unfortunately, also with the new set up that was acquired only 3 years ago and supposedly able to treat CF3 groups, the test elemental analyses of electrochemically and spectroscopically extremely pure air-and water-stable The discrepancy between theoretical and experimental carbon content is small enough to be acceptable for an inorganic organometallic compound like this and is considered publishable, especially with the reasons given above. However, elemental analyses does not answer the question of the purity of the bulk materials any better than the sum of vibrational and NMR spectroscopy as well as pXRD which we thoroughly deployed here. This is the reason, why we trust the combination of these methods more than a doubtful combustion analysis with large deviations and tolerance thereof.

Powder XRD Data and Rietveld Refinement
In order to evaluate the phase purity of the bulk materials 1-4, powder XRD measurements were conducted at 100K. In the following section the experimental diffractogram and the from

Rietveld Refinement
Since Cr-N distances in the mixed complexes are expected to be shorter (see QM calculations), also their unit cell is slightly, but noticeably smaller than that of the all-carbonyl compounds.
To show this also for the microcrystalline bulk, we performed Rietveld-refinements of the powder data recorded at the same temperature like the single crystal data (100 K). Since the pXRD record higher angle data than the scXRD data, the resolution is better and the standard deviations are further reduced. Thus, also the bulk of the material show the smaller lattice parameters for the mixed NO-carbonyl complexes (see Supplementary Table 7  5145.08(7) (4). This difference of 11.05 Å 3 is also statistically relevant and supports the claim that the presence of the shorter Cr-N bond leads to slightly but significantly smaller unit cells even for the bulk samples. Supplementary Figure 63. Plot of the Rietveld refinement of 1.

Supplementary
Supplementary Figure 64. Plot of the Rietveld refinement of 2. [Cr(

Single-Crystal XRD Data
Supplementary Figure 67 shows the crystal structure of 1, Supplementary Data 2 gives full information on the crystallographic data.
Crystal structure of 1, the symmetry-generated atom sites are greyed out.
Supplementary Figure 67. Crystal structure of 1.  Crystal structure of 2, the symmetry-generated atom sites are greyed out.
Supplementary Figure 69 shows the crystal structure of 3, Supplementary Data 4 gives full information on the crystallographic data.

Modelling of the NO-disorder for [Cr(CO)5(NO)][Al(OR F )4]
A roughly equal distribution in the NO disorder (16%/4x17%/16%) led to the best model (Supplementary Figure 70). The R1 value changed from 2.52% to 2.46% when the disorder was included. A refinement of the N positions with a free variable, however, did not lead to a stable model. Supplementary Data 5 gives full information on crystallographic data.  Crystal structure of 4, the symmetry-generated atom sites are greyed out. Note that the nitrogen position cannot be crystallographically unambiguously assigned.

The resulting bond lengths are shown in Supplementary
Supplementary Figure 71 shows the crystal structure of 4, Supplementary Data 6 gives full information in crystallographic data.

Modelling of the NO-disorder for [Cr(CO)5(NO)][F-{Al(OR F )3}2]
The inclusion of 1/6 NO led to a change of the R1 value from 6.03% to 5.99% (Supplementary Figure 72). However, the NO ligands resulted in a slightly tilted octahedron. Supplementary Data 7 gives full information on crystallographic data.
Supplementary Figure 72. Disorder model for 4. The low bond precision for the Cr-N bond (Supplementary Table 9) as well as the tilted structure of the cation underline the question, if the refinement of a disordered NO actually yields a scientifically more accurate structure model. Therefore, we decided to leave the NO data out and just report the average bond lengths in the manuscript.

Hirshfeld Plots
The Hirshfeld plots show that in 1 slight interactions of O1 with the counterion disturb the local symmetry, leading to small distortions and therefore only a tetragonal symmetry and three crystallographically independent CO positions. For 2 however, the even less coordinating

Details on the gase phase energetics and DFT calculations
Details on the energetics and DFT calculations are given in Supplementary Table 10, details on the COSMO-RS calculations are given in  Supplementary Table 11. Supplementary

Additional information on the DFT calculations
Supplementary Figure   Supplementary Figure