Crystalline phosphino(silyl)carbenes that readily form transition metal complexes

Phosphino(silyl)carbenes are known for their ineptitude to form transition metal complexes. We describe the synthesis of phosphino(silyl)carbenes bearing N-heterocyclic imine groups and show that these isolable, crystalline carbenes readily form stable copper(I) and gold(I) complexes. The solid-state structures of the free carbenes and their transition metal complexes are reported.


Experimental procedures Synthetic Details
General remarks: All manipulations, if not stated differently, were performed under an inert atmosphere of dry argon, using standard Schlenk and drybox techniques. Dry and oxygen-free solvents were employed. All glassware was oven-dried at 160 °C prior to use. 1 H, 13 C and 31 P NMR spectra were recorded at 300 K on Agilent DD2 600, Bruker AVANCE I 400, Bruker AVANCE III 400 or Bruker AVANCE II 200 spectrometers. Chemical shifts are given in parts per million (ppm) relative to SiMe4 ( 1 H, 13 C), 85% H3PO4 ( 31 P) and they were referenced to the residual solvent signals (CD3CN: 1 H δH = 1.94 ppm, 13 C δC = 118.26 ppm; C6D6: 1 H δH = 7.16 ppm, 13 C δC = 128.0 ppm; THF-d8: 1 H δH = 3.58 ppm, 13 C δC = 67.21 ppm) or internally by the instrument after locking and shimming to the deuterated solvent ( 31 P). Chemical shifts (δ) are reported in ppm. NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, p = pentet, sept = septet, m = multiplet, br = broad signal. Mass spectrometry was recorded using an Orbitrap LTQ XL (Thermo Scientific) spectrometer. HepatoChem EvoluChem TM LED 365PF (365 nm) was used for the irradiation experiments. Melting points were measured in glass capillaries sealed under argon gas by using a Stuart Melting Point Apparatus SMP3.

Preparation of 3a
Note: Until the irradiation step, this reaction was performed in the dark.
Trimethylsilyldiazomethane (0.826 mL, 2.0 M in hexanes, 1.65 mmol, 1.00 eq.) was dissolved in THF (20 mL) and filled into a Schlenk tube. The solution was cooled to -78 °C using a dry ice/acetone bath and n-butyllithium (1.03 mL, 1.6 M in hexanes, 1.65 mmol, 1.00 eq.) was added to the solution. The mixture was stirred at -78 °C for 30 min and subsequently transferred (via a PTFE cannula) into a separate Schlenk flask containing a stirred suspension of 1a (758 mg, 1.65 mmol, 1.00 eq.) in THF (30 mL), also cooled to -78 °C. The mixture was stirred for 80 min at -78 °C, then warmed to 0°C via an ice/water bath. While keeping the mixture cold, all volatiles were removed in vacuo and, subsequently, ice-cold n-hexane was added to the residue. The mixture was stirred for 5 minutes and then filtered into another Schlenk flask (via a PTFE cannula plucked with a glass filter). An orange solution was obtained ( Figure S     Single crystal X-ray diffraction analysis: Single crystals suitable for X-ray diffraction analysis were obtained during the synthesis (vide supra). The structure of 3a was confirmed.

Preparation of 3b
Note: Until the irradiation step, this reaction was performed in the dark.
A solution of (trimethylsilyl)diazomethane (2M in hexane, 0.9 mL, 1.80 mmol, 1.00 eq.) in THF (10 mL) was cooled to -78 °C using a dry ice/acetone bath and n-butyllithium (1.13 mL, 1.6 M in hexanes, 1.80 mmol, 1.00 eq.) was added to the solution. The mixture was stirred at -78 °C for 20 min and subsequently transferred (via a PTFE cannula) into a separate Schlenk flask containing a stirred suspension of 1b (819 mg, 1.80 mmol, 1.00 eq.) in THF (30 mL) at -78 °C. After gradually warming the reaction mixture to 0 °C, the now clear solution was evaporated in vacuo and the residue extracted with hexane (3 x 10 mL). After exposure to sunlight for three consecutive cloudy days, crystals of the carbene 3b formed. The supernatant hexane solution was decanted and the crystals washed with hexane (3 x 5 mL). After evaporation of the solvent, yellow, needle-like crystals of 3b (229 mg, 0.45 mmol, 25%) could be obtained.

Melting point: 170°C (decomposition).
Single crystal X-ray diffraction analysis: Single crystals suitable for X-ray diffraction analysis were obtained during the synthesis (vide supra). The structure of 3b was confirmed.

Preparation of 4
Note: The reaction, as well as storage of the product, were conducted in the dark.
3a (100 mg, 0.197 mmol, 1.00 eq.) and chloro(tetrahydrothiophene)gold(I) (63 mg, 0.197 mmol, 1.00 eq.) were added into a vial. While stirring the mixture, THF (3 mL) was added and the suspension stirred for 30 minutes at 21 °C. Afterwards, the slightly cloudy suspension was passed through a glass filter and all volatiles of the yellow solution were removed in vacuo. The product is obtained as a light yellow solid.  Remarks on the thermal stability of 4: Gradual heating of solid 4 in the melting point apparatus resulted in slow color change from light yellow to purple starting at 137 °C, indicating thermal decomposition. In a separate experiment, a solution of 4 in THF was gradually heated. Heating at 60 °C for 2 h resulted in slight precipitation of a purple solid, indicating slow thermal decomposition. Further heating at 80 °C showed greatly accelerated decomposition.
Single crystal X-ray diffraction analysis: Single crystals suitable for X-ray diffraction analysis were obtained by storing the filtrated reaction mixture at -35 °C for one week. The structure of 4 was confirmed.      Preparation of 6 3b (30.0 mg, 0.0590 mmol, 1.00 eq.) and CuOtBu (16.3 mg, 0.118 mmol, 2.00 eq.) were dissolved in THF-d8 (0.5 mL) and filled into an NMR-tube. The mixture immediately changed colour from yellow to light beige. A purification was not necessary.

NMR-Yield: Quantitative.
Optionally, 6 can be isolated as a white sold in quantitative yield after removal of all volatiles in vacuo.
Note: The NMR signals were assigned using 2D NMR experiments (vide infra). Single crystal X-ray diffraction analysis: Single crystals suitable for X-ray diffraction analysis were obtained by attempting the reaction in C6D6: After initial addition of the reactants, a suspension was obtained. The mixture was heated until dissolution of the precipitate and then let cool down. Colourless crystals were obtained. The structure of 6 was confirmed.

Crystallization of 7 and attempt of its targeted synthesis
Compound 5 was first prepared in THF. Due to its low solubility in this solvent, an attempt was made to recrystallize it in 1,2-difluorobenzene. Storage of the mixture for four months at 21 °C resulted in the formation of few crystals of 7, which were analyzed via X-ray diffraction analysis (vide infra).
To obtain 7 in a targeted synthesis, 5 (approx. 50 mg) was first prepared in THF (1 mL) by mixing equimolar amounts of 3b and CuOtBu. The supernatant THF solution was pipetted off and discarded. The remaining mixture was suspended in 1,2-difluorobenzene (0.5 mL) and monitored via 31 P NMR spectroscopy. After 2 h at 21 °C, no reaction was observed. However, heating at 60 °C for 16 h resulted in the formation of several species as indicated by 31 P NMR spectroscopy. Hence, a selective synthesis of 7 was not successful

Determination of %Vbur and steric maps
Computation of percent buried volume (%Vbur) values and steric maps was performed via the SambVca 2.1 web application. 5 The sphere are centered at the gold atom. Bond radii are scaled by 1.17, the sphere radius is set to 3.5 Å (if not stated differently), mesh spacing for numerical integration is set to 0.10 Å, H atoms are not included in the calculations. The investigated compounds are summarized in Figure S 37. The computed %Vbur values are summarized in Table S

X-ray Diffraction Studies
General: Single-crystal X-ray diffraction data of 1a, 1b, 3a, 3b, 4, 6 and 7: Crystals were selected under oil, mounted on glass capillaries, and then immediately placed in a cold stream of N2 on a diffractometer. Data were collected on a Bruker AXS detector using Mo-Kα radiation (λ = 0.71073 Å). Using Olex2, 8 the structures were solved with the ShelXT 9 structure solution program using intrinsic phasing and refined with the ShelXL 10 refinement package using Least Squares minimization.  Note: The highest residual electron density (Q1) is located about 1.8 Å adjacent to an imidazoline C atom, suggesting that it corresponds to a copper atom with an occupancy of 3%. One explanation would be that, similar to the formation of 7, CH activation has occurred and that the metallated species is present at about 3% in the crystal lattice. Because of the low occupancy, the lighter atoms cannot be located on the residual electron density map.

Computational studies General
The geometry optimizations and frequency calculations were performed with ORCA 5.0, 11 using the B3LYP 12 functional with a dispersion correction (D4) 13 . A triple zeta basis set (def2-TZVP) 14 with an RIJCOSX 15 approximation was used in all calculations. The absence of any imaginary frequencies confirmed that each optimized structure is at a local minimum.