Fischer Carbene Complexes of Iridium(I) for Application in Catalytic Transfer Hydrogenation

New examples of the very rare class of iridium(I) Fischer carbene complexes (FCCs) are reported from the facile transmetalation from group 6 FCCs. Postcomplexation modification of either the carbene ligand or the ancillary coligands results in a tunable IrI metal center, for unprecedented application as a (pre)catalyst in a benchmark transfer hydrogenation reaction. The introduction of an aminocarbene ligand with a pendant N-donor moiety capable of hemilabile coordination yielded the best catalytic results with turnover frequencies reaching 445 h–1 and requiring 0.1 mol % catalyst and 0.5 mol % base loading, respectively.


S1.1 Measurement and Methods
General procedures. The preparation, purification and reactions of the complexes described were carried out under an atmosphere of dry, oxygen-free N2 or Ar gas using standard Schlenk techniques. All reactions were mechanically stirred and monitored by IR spectroscopy where relevant. The precursors [W(CO)5{C(OEt)Ar}] [Ar = p-DMA, Fc, Cp'Re(CO)3] and [Ir(cod)Cl]2 were prepared according to literature procedures. 1,2,3 Aluminum oxide 60 (particle size 0.05 -0.15 mm) was used as resin for all column chromatography separations. Anhydrous tetrahydrofuran (THF), diethyl ether (Et2O), and n-hexane were distilled over sodium metal and dichloromethane (DCM) was distilled over CaH2. All other reagents are commercially available and were used as received.
Nuclear magnetic resonance (NMR) spectra were recorded on Bruker Avance-III-300, Bruker Avance-III-400 and Bruker Avance-III-500 spectrometers using CDCl3, CD2Cl2, and C6D6 as solvents at 25 °C. The NMR spectra were recorded for 1 H at 300.13, 400.13 and 500.13 MHz, and for 13 C at 100.63 and 125.78 MHz. 2D NMR techniques (COSY, HSQC) were employed to assign signals that were otherwise ambiguous. Infrared spectroscopy was performed on a PerkinElmer Spectrum FT-IR spectrophotometer over the range 4000 -1400 cm -1 . Solution IR spectra were recorded in CH2Cl2 using a NaCl cell with a path length of ca. 1.0 mm. Melting points were measured with a Stuart SMP10 melting point apparatus. Mass spectral analyses were performed on a Bruker Compact Q-TOF mass spectrometer (Bruker Daltonics, Bremen, Germany) with a positive electron spray as the ionization technique by direct infusion at 0.3 mL min -1 . The m/z values were measured in the range of 50 -1000 in acetonitrile. Prior to analysis, the instrument was calibrated with sodium formate (5 mM) in resolution mode. Elemental analyses were carried out using an Elementar varioELcube CHNS-O analyser.

S1.2. Synthesis and characterization
Scheme S1.  [Ir(cod)Cl{C(NH n Pr)p-DMA}], 4. n PrNH2 was slowly added to a brown-orange solution of 1 (0.100 g, 0.20 mmol) in THF at room temperature and the colour immediately changed to yellow. The THF solvent was removed in vacuo. The resulting yellow paste was washed with n-hexane (2 x 5 mL) and cannula-S5 extracted with CH2Cl2 to give a yellow-lime powder after drying. Yellow crystals were obtained by slow diffusion of n-hexane (2 mL) into a concentrated CH2Cl2 solution of the powder. Yield = 0.099 g, 94%.

S3. TEP calculation
Complexes 1, 3, 4 and 6 were dissolved in solvent CH2Cl2 and CO(g) passed through the solution for approximately 10-20 minutes, for substitution of the cod-ligand with two carbonyl ligands (Scheme S2). The resultant dicarbonyl complexes were not purified and isolated, but the FT-IR spectra of the reaction mixtures in solution CH2Cl2 were recorded ( Figure S21), in order to obtain the two carbonyl stretching vibration frequencies required for TEP calculation.   S4. X-ray crystallography All crystals for single-crystal X-ray diffraction were grown by slow diffusion of n-hexane into a concentrated CH2Cl2 solution of the carbene complex at 4 °C. Single crystal X-ray diffraction data for complexes 1, 3 and 7 were collected at 173 K on a Bruker Apex II CCD diffractometer, while data for complexes 2, 4 and 5 were collected at 173 K using a Bruker Venture D8 Photon CMOS diffractometer, with a graphite-monochromated Mo-Kα (λ = 0.71073 Å) radiation using an Oxford Cryostream 600 cooler. All data reductions were carried out using the program SAINT+, version 6.02 6 and empirical absorption corrections were made using SADABS. 6 Space group assignments were made using XPREP. 6 The structures were solved in the WinGX 7 Suite of programs, using intrinsic phasing through SHELXT 8 and refined using full-matrix least-squares/difference Fourier techniques on F 2 using SHELXL-2017. 8 All C-bound H 1atoms were placed at idealized positions and refined as riding atoms with isotropic parameters 1.2 times those of their parent atoms. The amine-H atom positions in 4 and 5 were located and were refined. All diagrams and publication material were generated using OLEX2, ORTEP-3, 7 and PLATON. 9 Experimental details of the X-Ray analyses are provided in Table S2 below. a Y = midpoint of C(5)-C(6). b Y = midpoint of C(4)-C(5). c Y = midpoint of C(4)-C(5). d Y = midpoint of C(5)-C(6). e Y = midpoint of C(1)-C(2). f Y = midpoint of C(1)-C(2). g Y = midpoint of C(1)-C (8). h Y = midpoint of C(1)-C (8). i Y = midpoint of C(1)-C(2). j Y = midpoint of C(5)-C(8).  Figure S22. 1 H NMR spectrum in CDCl3 of the stability test reaction mixture for catalyst 6 Figure S23. 13 C{ 1 H} NMR spectrum of the stability test reaction mixture of 6 in CDCl3 S20

S6. Catalytic transfer hydrogenation
General procedure for transfer hydrogenation reactions.
The catalytic transfer hydrogenation reactions were done under an argon atmosphere in thick glass reaction tubes fitted with a greaseless high-vacuum stopcock. In a typical reaction, the schlenk tube was charged with a solution of acetophenone (2.0 mmol, 230 µL) in iso-propanol (4 mL), base (42 µL, 0.01 mmol of a 0.24 M KOH solution in iso-propanol), internal standard (n-decane, 100 µL), and the Iridium complex (0.002 mmol, 0.1 mol %). The mixture was stirred at 82 °C for the required number of hours. Conversions were determined by gas chromatography analysis under the following conditions: oven temperature 35 °C (2 min) to 220 °C at 20 °C/min with a flow rate of 1 mL/min using ultrapure He as carrier gas and Supelco Equity 1 capillary column, L x I.D. 30m x 0.32mm, df 0.25mm.