Facile H/D Exchange at (Hetero)Aromatic Hydrocarbons Catalyzed by a Stable Trans-Dihydride N-Heterocyclic Carbene (NHC) Iron Complex

Earth-abundant metal pincer complexes have played an important role in homogeneous catalysis during the last ten years. Yet, despite intense research efforts, the synthesis of iron PCcarbeneP pincer complexes has so far remained elusive. Here we report the synthesis of the first PCNHCP functionalized iron complex [(PCNHCP)FeCl2] (1) and the reactivity of the corresponding trans-dihydride iron(II) dinitrogen complex [(PCNHCP)Fe(H)2N2)] (2). Complex 2 is stable under an atmosphere of N2 and is highly active for hydrogen isotope exchange at (hetero)aromatic hydrocarbons under mild conditions (50 °C, N2). With benzene-d6 as the deuterium source, easily reducible functional groups such as esters and amides are well tolerated, contributing to the overall wide substrate scope (e.g., halides, ethers, and amines). DFT studies suggest a complex assisted σ-bond metathesis pathway for C(sp2)–H bond activation, which is further discussed in this study.


General Information
All reactions were performed at room temperature either by using standard Schlenk techniques or by using an N2-filled M. Braun Glovebox unless otherwise specified. Glassware was oven dried at 140 °C for at least 2h prior to use, and allowed to cool under vacuum. All reagents were used as received unless mentioned otherwise. Anhydrous iron chloride (FeCl2·1.5THF) [1] and 2,10-Di-tert-butyldipyrido [1,2-c;2',1'-e]imidazol-6-thione [2] were synthesized according to published procedures. [1][2] Tert-butyl lithium (1.7 M in pentane) and chlorodiisopropylphosphine were purchased from Sigma Aldrich and Alfa Aesar respectively. Anhydrous unstabilized tetrahydrofuran (THF) and diethyl ether (Et2O) were purchased from Sigma Aldrich and used as received. The 1 H, 13 C{ 1 H} spectra were recorded on Bruker AVANCE III 200, 300, 400, and 500 NMR spectrometers at room temperature unless mentioned otherwise. All chemical shifts (δ) are reported in ppm, and coupling constants (J) are in Hz. The 1 H and 13 C{ 1 H} NMR spectra were referenced using residual solvent peaks in the deuterated solvent. The 31 P chemical shifts are reported relative to the internal lock signal. Deuterated solvents (CDCl3, and C6D6) were purchased from Cambridge Isotope Laboratories, dried over calcium hydride, degassed by three freeze-pump-thaw cycles and vacuum-transferred prior to use. Atmospheric positive electrospray ionization time-of-flight mass spectrometry (TOF MS ES + ) and high-resolution mass spectrometry (HRMS) were performed on a Waters QTOFMS Xevo G2 spectrometer in the positive ion mode.

Physical Methods
Single-crystal X-ray diffraction. For compounds A1 and 1 low temperature (100K) diffraction data were collected using a Bruker SMART APEX II diffractometer coupled to an APEX II CCD detector with graphite monochromatic MoKα (λ = 0.71073 Å) radiation. All diffractometer manipulations, including data collection, integration, and scaling were carried out using the Bruker APEXII software. [3] Absorption corrections were applied using SADABS. [4] For compound 2, low temperature (100K) diffraction data was collected on a Rigaku XtaLAB AFC12 (RINC) with a Kappa single CCD detector and a micro-focus sealed X-ray tube with monochromatic CuKα (λ = 1.54184 Å) radiation. All diffractometer manipulations, including data collection and integration were carried out using CrysAlisPro (Rigaku Oxford Diffraction, 2020). Absorption corrections were applied using CrysAlisPro (Rigaku Oxford Diffraction, 2020), with numerical absorption correction based on gaussian integration overa multifaceted crystal model. Empirical absorption correction were applied using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. All structures were solved by direct methods using SHELXS [5] and refined against F 2 on all data by full-matrix least squares with SHELXL-2014 or SHELXL-2018 [6] using established refinement techniques. [7] All nonhydrogen atoms were refined anisotropically. All hydrogen atoms were included into the model at geometrically calculated positions and refined using a riding model. The isotropic displacement S6 parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms they are linked to (1.5 times for methyl groups). All air-and moisture-sensitive manipulations were carried out using standard Schlenk and cannula techniques or in an MBraun inert atmosphere drybox containing an atmosphere of purified nitrogen.

Synthetic Procedures
Synthesis of pro-ligand A1. In the glovebox, to a solution of 2,10-Di-tert-butyldipyrido [

Substrate scope for alkyne hydroboration
General procedure for the HIE using C6D6 as deuterium source: Inside a glovebox, an oven-dried 4 mL vial was charged with substrate (0.1 mmol) and a stock solution of catalyst 2 in C6D6 (0.5 mL, 0.005 mmol, 5 mol%) with tetraethylsilane (0.01 mmol, 50 µL from 0.2 mM stock solution) as internal standard. The reaction mixture was transferred to J-Young tube and heated at the specified temperature.
The amount of deuteration was determined by 1 H NMR spectroscopy, by integrating the product peaks with respect to internal standard. The same procedure was repeated without internal standard. After the after specified time, the solution was filtered through sort plug of alumina to remove the iron catalyst.          . Figure S24. 1 Figure S25. 13 Figure S31. 13 Figure S43. 13 Figure S49. 13                  S52 Tables   Table S1. Selected bond angles and distances for A1 and complexes 1 and 2.

Evaluation of energy barriers of C-H activation step at X-substituted benzenes (X = F, Me, Me2N).
In order to evaluate effects of substituents on Gibbs free energies of TS1_R we performed calculations for hydrogen substituted (dihydrides) transition states TS1_R. Specifically in these studies, we have opted to use the dihydride as the model system since, computationally, the only difference between hydrogen and deuterium atoms are in the vibrational modes. As a result, the respective potential energy surfaces (PESs) will be nearly identical except for their relative energies. Typically, PESs that include deuterium are slightly higher in energy (ca. 0.4 kcal/mol).

The search of lowest energy isomers of R-substituted TS1.
Possible isomers of TS1_R (R = F, Me, Me2N, MeO) were found ( Fig S77. and Fig S78) and their energies were compared (Table S3. and Table S4.) in order to choose the lowest Gibbs free energy isomers for further comparison (Table S5.).