A bench stable formal Cu(iii) N-heterocyclic carbene accessible from simple copper(ii) acetate

The first stable formal Cu(iii) NHC and its unusual reactivity with acetate are reported. Several products of this reaction are identified and fully characterised. It reactivity is extensively investigated and additionally explored by means of theoretical, electrochemical and isotope labelling experiments.


Single Crystal X-ray Diffraction General crystallographic details
Data were collected on an X-ray single crystal diffractometer equipped with a CCD detector (Bruker D8 Venture Duo IMS, APEX III, κ-CCD, λ = 0.71073 Å) equipped with a Helios optic monochromator (1,3) or a rotating anode (Bruker TXS) with MoK α radiation (λ = 0.71073 Å) and a Helios optic monochromator (2, 4) by using the APEX software package. 1 The measurements were performed on a single crystal coated with perfluorinated ether. The crystal was fixed on top of a microsampler and transferred to the diffractometer. The crystal was frozen under a stream of cold nitrogen. A matrix scan was used to determine the initial lattice parameters. Reflections were merged and corrected for Lorentz and polarization effects, scan speed, and background using SAINT. 2 Absorption corrections, including odd and even ordered spherical harmonics were performed using SADABS. 3 Space group assignments were based upon systematic absences, E statistics, and successful refinement of the structures. Structures were solved by direct methods with the aid of successive difference Fourier maps, and were refined against all data using SHELXLE 3 in conjunction with SHELXL-2014 4 . Hydrogen atoms were assigned to ideal positions and refined using a riding model with an isotropic thermal parameter 1.2 times that of the attached carbon atom (1.5 times for methyl hydrogen atoms). If not mentioned otherwise, non-hydrogen atoms were refined with anisotropic displacement parameters. Full-matrix least-squares refinements were carried out by minimizing Σw(Fo 2 -Fc 2 ) 2 with SHELXL-97 5 weighting scheme. Neutral atom scattering factors for all atoms and anomalous dispersion corrections for the nonhydrogen atoms were taken from International Tables for Crystallography. 6 Images of the crystal structures were generated by PLATON. 7

Special crystallographic details
Compound 1: The hexafluorophosphate counterions as well as the weakly coordinated diethyl ether are heavily disordered and partly located at special positions, leading to bigger ellipsoids in these cases and furthermore increased R-values.

Compound 3:
The copper atoms within the sandwich complex and the whole sandwich complex itself are rotationally disordered. Due to the high electron count of copper compared to the organic ligand frame the ligand related disorder cannot be resolved. Only two split layers with occupancy ratios of approximately 82:18 and 94:6 for copper atoms Cu1 and Cu3 have been included into the refinement. All remaining electron densities in the central complex plane were smaller and have thus been left "as is". One disordered, co-crystallized molecule of dichloromethane is located centrally in the bowl-shaped cavity, generated by the ligand at the ends of the complex. Due to disorder the atoms were refined isotropically. These circumstances, combined with the counter ion and related disorder also lead to the comparatively high R-values.

Preparation and characterisation of compound 2.
Although 2 is easily separated from a mixture with 1, due to its insolubility in MeCN, yields are very low (~10%) using synthetic protocols for 1 under anaerobic conditions. Thus, 2 being presumably formed during the reaction by Cu(I) acetate, (generated in situ; vide infra), a direct synthesis for 2 with Cu(I) acetate was developed, compensating the lack of internal base by the addition of external NBu 4 OAc. Eventually, 2 is best prepared by mixing H 4 LX 4 , Cu(I) acetate and NBu 4 OAc in a stoichiometric fashion (1:2:2) and heating for 30 min at 100°C in DMSO under an inert gas atmosphere (equation II).
It has to be noted, that presence of oxygen during the synthesis leads to significantly reduced yields of 2, since the formation of initially 1 and later the oxidized species 3 is favored under these conditions. After removal of the solvent, the crude product can be easily purified by washing with acetonitrile, yielding 2 as a white powder in good yields (~75 %). 2 is air stable in solid state, although it slowly reacts to 1 and 3, when dissolved in DMSO and exposed to air (see 5.3).  The single crystal structure of 2 is very similar to those of its higher homologues Ag and Au (Fig.  S1). The Cu-C bond lengths vary between 1.90 and 1.92 Å (Ag/Au: 2.09/2.03 Å) and the C-Cu-C angle lies between 159 and 163° (Ag/Au: 166/165°). The Cu-Cu distances range between 3.04 and 3.07 Å (Ag/Au: 3.20/3.15 Å), which is considerably longer than the sum of the respective van der Waals radii of 2.80 Å (Ag/Au: 3.44/3.32 Å). 8 Thus, the slight inward displacement from an ideal linear coordination of the Cu(I) ions, might be attributed to strain from the ligand moieties and not to any significant Cu(I) … Cu(I) interactions. The 1 H-NMR signals at 7.71 ppm and 6.99/6.40 ppm and the 13 C-NMR signals at 178.8, 122.1 and 64.0 ppm coincide very well with those of literature known Cu(I) bis(NHC) complexes. 9,10 Preparation and characterisation of compound 3.
The Cu 3 LO 2 compound 3 can also be obtained by a variety of different methods, though it is most favorably formed by mixing H 4 LX 4 , Cu(II) acetate and NBu 4 OAc in a stoichiometric fashion (1:2:2; equation III). In this way, 3 can be isolated in very good yields of >90% and in very high purity.
It is noteworthy that NBu 4 OAc is crucial for a clean formation of 2 and 3. Other acetate bases, such as NaOAc diminish the yield and purity of the respective compounds significantly, probably due to their worse solubility in DMSO/MeCN. Unlike 1 or 2, 3 is a slightly yellow powder. The crystal structure (Fig. S2) reveals a highly unusual structure for compound 3.  Complex 3, especially when compared to 2, displays very high stability towards air even in solution. It is completely stable towards oxygen and moisture up to 100°C in DMSO solution and in the solid state. Also, prolonged storage at 70°C results in no visual or spectroscopic changes. However, 3 is mildly sensitive to strong Brønsted acids such as hydrochloric-and triflic acid and treatment with those acids results in the smooth formation of the oxidized imidazolium salt 4 and the respective Cu(I)X salts. This process is easily reversible by addition of Cu(I) acetate and additional base.  2 (21.0 mg, 0.12 mmol) are dissolved in 1 mL of dry MeCN under inert atmosphere. The blue reaction mixture is stirred at r.t. for 5 min and then heated at 100 °C for 30 min. The colorless solution is cooled to r.t upon which a small amount of white precipitate forms. The suspension is transferred to a centrifuge tube and after centrifugation the clear colorless solution is decanted and precipitated with DCM (8 mL). The white precipitate is dissolved in MeCN (2 mL) and again precipitated with DCM (8 mL). This is repeated twice and the resulting white powder is washed with DCM, and dried at 70°C on air in a drying oven (32 mg, 0.042 mmol, 75 % yield). ESI MS analysis confirms an 18 O-content of 53 ± 0.7 %.

1 H NMR kinetic of the formation of 3 in DMSO-d 6 .
A NMR-tube was charged with H 4 L-PF 6 (10 mg, 0.011 mmol) and it was dissolved with degassed DMSO-d 6 (0.4 mL). A blank NMR (t=0) was taken and successively Cu(II)(OAc) 2* H 2 O (4.40 mg, 0.022 mmol) was added inside the Glovebox. The NMR-tube was well shaken and every 5 min. 1 H-spectra were collected at r.t. during the course of 60 min. During this time the color of the solution changed from blue to almost colorless. To complete the reaction from H 4 L-PF 6 to 1 the sample was heated at 40 °C for 10 min. and then stored at r.t. for 3 h. NBu 4 OAc (7.2 mg, 0.022 mmol) was added to the now colorless, clear solution and another spectra was collected. After heating to 100°C for 1 min. the yellow solution was cooled to r.t. and a final spectrum of 3 was collected.

1 H NMR kinetic of the formation of 3 in MeCN-d 3 .
A NMR-tube was charged with H 4 L-PF 6 (10 mg, 0.011 mmol) and it was dissolved with degassed MeCN-d 3 (0.4 mL). A blank NMR (t=0) was taken and successively Cu(II)(OAc) 2* H 2 O (4.40 mg, 0.022 mmol) was added inside the Glovebox. The NMR-tube was well shaken and every 5 min. 1 H-spectra were collected at r.t. during the course of 180 min. at 50°C. During this time the color of the solution changed from blue to almost colorless. To complete the reaction from H 4 L-PF 6 to 4 and 5 the sample was heated at 80 °C for 10 min. and then stored at r.t. over night. NBu 4 OAc (7.2 mg, 0.022 mmol) was added to the colorless solution at r.t., clear solution and after shaking another spectra was collected. After heating to 80°C for 5 min. the now yellow solution was cooled to r.t. and a final spectrum of 3 was collected.

1 H NMR of 2 at on air in DMSO-d 6 .
A NMR-tube was charged with 2 (10 mg, 0.012 mmol) and was dissolved with dry and degassed DMSO-d 6 (0.4 mL). A blank 1 H-NMR (t=0) was taken and during three weeks several 1 H NMR spectra were collected. Between the measurements, the solution was stored in an open NMR tube on air. After three weeks the NMR-tube was heated to 100°C for 30 min and the final NMR was collected, containing a mixture of 1 and 3. The former colourless solution turned green. The 13 C-NMR after 21 d confirms the presence of 1 in the solution.