Probing the donor strength of yldiide ligands: synthesis, structure and reactivity of rhodium complexes with a PCylideN pincer ligand

Control of the metal ligand interaction by changes in the ligand protonation state is vital to many catalytic transformations based on metal–ligand cooperativity. Herein, we report on the coordination chemistry of a new PCy(H)N pincer ligand with a central ylide as donor site, which through deprotonation to the corresponding yldiide changes from a neutral L3-type ligand to an anionic L2X-type PCYN ligand. The isolation of a series of rhodium complexes showed that the strong donor ability of the neutral ylide PCY(H)N is further increased upon deprotonation, as evidenced by one of the lowest reported CO stretching frequencies in complex [(PCYN)Rh(CO)] (2) compared to other known rhodium carbonyl complexes. DFT calculations revealed that the high donor ability mostly results from the antibonding interaction of the pπ orbital at the ylide with the dxz orbital at rhodium, which enhances the backdonation into the π* orbital of the CO ligand. This unique interaction results in a rather long metal–carbon bond, but still a strong activation of the CO ligand in order to minimize repulsion between the filled orbitals at the rhodium and the ylide ligand. Accordingly, CO by phosphine replacement leads to a strong deviation from the square-planar geometry in the analogous phosphine complexes [(PCYN)Rh(PR3)] and an unusual reactivity with small alkyl halides, which upon oxidative addition add to the CO ligand, before inserting into the P–C bond in the pincer ligand. These results demonstrate the unique donor strength of yldiide ligands and their potential in the activation of strong bonds.


General information
All operations were performed using standard Schlenk or glovebox techniques under an Ar atmosphere unless indicated otherwise. Unless otherwise indicated, all solvents and reagents were used as received. Non-deuterated solvents were taken from a solvent purification system (MBRAUN SPS). Deuterated solvents were added to activated 3 Å molecular sieves. PCy2Cl 1 was prepared according to literature procedures. (acetylacetonato)dicarbonylrhodium(I) and chloro(1,5-cyclooctadiene)rhodium(I) dimer were purchased from Sigma and used as received.
NMR spectra were recorded on Avance-400 spectrometers at 25 °C if not stated otherwise. All values of the chemical shift are in ppm regarding the δ-scale. All spin-spin coupling constants (J) are printed in Hertz (Hz). To display multiplicities and signal forms correctly the following abbreviations were used: s = singlet, d = doublet, t = triplet m = multiplet, dd = doublet of doublet, br = broad signal. Signal assignment was supported by, HSQC ( 1 H / 13 C), HMBC ( 1 H / 13 C, 1 H / 31 P) correlation experiments.
IR-Spectra were recorded on a Thermo Nicolet iS5 FT-IR in transmission mode with an ATR module at 22°C. Abbreviations are as follows: w -weak, m -medium, s -strong.
Elemental analyses were performed on an Elementar vario MICRO cube elemental analyzer.
HRMS-ESI: An LTQ Orbitrap Velos (Thermo Fisher Scientific, Bremen, Germany) was used for direct infusion via a syringe pump. The heated desolvation capillary was set to 200°C and a spray voltage of 1.8 kV was supplied. In the tune file the LTQ Orbitrap was set to the following parameters (R = 30,000; IT = 500 ms; AGC Target = 1,000,000).
For details about the single-crystal X-ray diffraction analyses, see chapter 4.

Ligand Synthesis ( Ph BIPM Tol ) ligand L1.
Ligand L1 was prepared via a slightly modified procedure than previously reported. 2 In a 2-necked 500 mL Schlenk flask was added Bis(diphenylphosphino)methane (dppm) (1.00 eq, 10.0 g, 25.2 mmol) followed by 350 mL of anhydrous toluene. An addition funnel was attached to the RBF and filled with 50mL of anhydrous toluene. The reaction mixture was heated up to 60°C, a condenser was added and topped with a bubbler. Once the temperature reached 60°C, p-Tolyl azide (2.50 eq, 8.39 g, 63.0 mmol) was added to the 50 mL of anhydrous toluene in the addition funnel, and a medium speed addition was started over 15 minutes while heating. Once the addition was finished, the reaction was left to stir at 60°C for 4 hours total. The toluene was evaporated while keeping heating at 40°C until a beige powder was obtained. An orange impurity was removed by addition of 50 mL of diethyl ether and filtered off using a cannula. The solids are evaporated to obtain ligand L1 as an off-white powder (14.49g, 96.7%). In a 100 mL RBF Schlenk was added L1 (1 eq, 2 g, 3.36 mmol) in 40 mL of anhydrous THF. Then, the solution was cooled down at -78 degrees and a solution of n Butyl lithium (1.66 M in hexanes, 1.00 eq, 3.36 mmol, 2.02 mL) was added to the reaction mixture, without any significant color change. The reaction was left to stir at -78°C and slowly over 3h come back to room temperature. The solvent was then evaporated under reduced pressure, and 10 mL of anhydrous diethyl ether was added to facilitate the formation of a solid, then the solvent was evaporated. The lithiated ligand L1-Li was obtained as a beige powder (2.01 g, 89%). Colorless crystals suitable for X-ray diffraction analysis were grown through diffusion of pentane in a concentrated THF solution of L1-Li.

Method 1.
In a 100 mL Schlenk flask under an argon atmosphere was added ligand L1 (1.00 eq., 2.00 g, 3.36 mmol) and anhydrous THF (40 mL) and the flask as cooled to -78°C using an acetone/LN2 bath. Then, a n Butyl lithium solution (1.66 M in hexanes) (1.00 eq., 2.02 mL, 3.36 mmol) was added and left to stir at room temperature over 3 hours. The solvent was then evaporated under reduced pressure to give a beige powder. 10 mL of diethyl ether was then added, and the solid evaporated under reduced pressure to give the lithiated BIPM ligand L1-Li S4 (2.008 g, 2.991 mmol, 89%). L1-Li (1.05 g, 1.56 mmol, 1.00 eq.) was then added to a 100 mL Schlenk flask, and anhydrous THF was added (40 mL). The solution was cooled to -78°C using an ice bath, then dicyclohexylchlorophosphine (380 μL, 1.72 mmol, 1.1 eq.) was added and left to stir at room temperature for 1 hour before the solvent was evaporated. 15 mL of diethyl ether was added to solubilize the ligand but remove the LiCl formed, and the solution was evaporated under reduced pressure. A second portion of diethyl ether is added to remove some of the starting L1 ligand formed in the reaction as a side-product, and the solvent is removed under reduced pressure to obtain L2 as a yellow powder (1.02 g, 1.29 mmol, 82.7%).

Method 2.
In a 250 mL Schlenk RBF under an argon atmosphere was added the ligand L1 (1 eq, 4.00 g, 6.73 mmol) and 150 mL of anhydrous THF at 0° using an ice bath, then a n Butyl lithium solution (1,57 M in hexanes) (1.1 eq, 4.7 mL, 7.4 mmol) was added. In a second 250 mL Schlenk flask was added chlordicyclohexylphosphine (1.2 eq, 1.78 mL, 8.08 mmol) and 50 mL of anhydrous THF, and cooled down. The first solution was transferred into the second Schlenk and was stirred at 0°C for 2h. The solvent was evaporated under reduced pressure, and diethylether was added (100 mL), the LiCl filtered off, and the diethyl ether solution evaporated. Then, the resulting solid was washed 3 times with 50 mL pentanes and evaporated, giving the desired product as a yellow solid (4.11 g, 5.19 mmol, 77 % yield). Yellow crystals suitable for X-ray diffraction studies were grown from diffusion of pentane into a concentrated THF solution L2.   In a 100 mL schlenk was added ligand L2 (1 eq, 300 mg, 0.379 mmol) and chloro(1,5-cyclooctadiene)rhodium(I) dimer (0.6 eq, 112 mg, 0.227 mmol), followed by anhydrous THF (15 mL). The solution turned brown and was left to stir at room temperature for 24 hours before the solvent was removed under reduced pressure. The solid was washed 6 times with pentanes, and the remaining solid was dried under reduced pressure to afford a brown powder (304 mg, 0.327 mmol, 86%). Brown crystals suitable for X-ray diffraction studies were grown by diffusion of pentane on a concentrated THF solution of 1. Despite our best efforts and due to the reactive nature of complex 1, we were not able to obtain satisfactory elemental analyses.  In a Schlenk flask under an argon atmosphere was added ligand L2 (1.00 eq, 500 mg, 0.632 mmol) and acetylacetonato(dicarbonyl)rhodium(I) (1.00 eq., 163 mg, 0.632 mmol). The flask was taken out of the glovebox, and under an argon atmosphere was added anhydrous THF (25 mL), which produced effervescence through CO evolution and a color change to orange. The solution was left to stir at room temperature for 4 hours before removing the solvent until 2 mL remains, then 20 mL pentanes was added, resulting in a yellow solid crashing out of the solution. Half of the solvent was removed to promote additional solid crashing, and the solvent was filtered off via cannula. The remaining solid was washed 3 times with pentanes to afford 2 as a bright yellow powder (400 mg, 0.434 mmol, 69%). Orange crystals suitable for X-ray diffraction studies were grown by diffusion of pentanes on a concentrated THF solution of 2 at room temperature.   In a 100 mL Schlenk was added complex 1 (1.00 eq, 300 mg, 0.323 mmol), NaHMDS (1.10 eq, 65.2 mg, 0.355 mmol), and PPh3 (1.00 eq, 84.7 mg, 0.323 mmol), before adding 20 mL of anhydrous THF. The solution turned dark red, and was left to stir at room temperature for 1 hour. The solvent was evaporated under reduced pressure, then pentane was added, the solid was scratched from the walls, and further stirred in pentanes. The mixture was filtered using a cannula filter, and the solid was further washed three times with pentanes. Complex 3 was obtained as a brown solid that is sparsely soluble in organic solvents (256 mg, 0.222 mmol, 68.6%). Despite our best efforts, we were not able to obtain satisfactory elemental analysis due to the low solubility of 3 in all common organic solvents.

Note:
To probe whether all solid material of the reaction is indeed complex 3, we reacted the highly insoluble material with ethereal HCl. Indeed, the reaction mixture reacted with HCl to form a new species which we identified as the cationic, protonated version of complex 3 with Cl as counter anion. Accordingly, a new set of signals was observed by 31 P{ 1 H} NMR spectroscopy.

S8
The spectrum of the reaction mixture is shown in Figure S61. The high selectivity (> 95% according to 31P NMR) of this transformation clearly confirms the uniformity of the obtained solid. In a 100 mL Schlenk was added complex 1 (1.0 eq, 50 mg, 0.054 mmoles) and sodium hexamethyldisilazane (1.10 eq. 10.9 mg, 0.0590 mmoles). Then, 10 mL of anhydrous THF was added to the Schlenk, and quickly cooled down to -78°C. To the cold solution was added trimethylphosphine (10.0 eq. 46.9 μL, 0.538 mmoles). A color change from deep red to deep brown was observed immediately, and the solution was left to stir at low temperature for 30 minutes, before the solvent was removed under reduced pressure to afford a red residue. The residue was dissolved in diethyl ether and filtered through cannula. The solution was evaporated quickly as it decomposes under reduced pressure, giving a dark brown solid (34.0 mg, 65%, 0.0351 mmoles). Deep brown crystals suitable for X-ray diffraction analysis were grown from hexane diffusion into a concentrated deuterated benzene solution of 4 at room temperature under an atmosphere of argon.
Due to the highly sensitive nature of the complex under reduced pressure, and the decomposition of the complexes upon attempts to further purity through washings or even evaporation of crystals of the complex, we report the NMRs containing decomposition products and solvents, where the product is identified. In a 100 mL Schlenk was added complex 2 (1.00 eq., 192 mg, 0.208 mmoles) and anhydrous chloroform, followed by isopropyl iodide (10.0 eq., 357 mg, 210 L, 2.08 mmoles) and left to stir under argon for 72 hours at room temperature. A slight color change from light orange to darker orange was observed a few minutes after addition of isopropyl iodide. The solvent was then evaporated under reduced pressure and washed multiple times with pentanes to afford a brown powder of complex 5 (165 mg, 0.151 mmoles, 72.7%). In a J. Young NMR tube was added complex 2 (1.00 eq, 32.0 mg, 0.0347 mmoles), and 1 mL of anhydrous THF was added. The solution was cooled down to -30 °C. Then, methyl iodide (1.50 eq. 3.24 μL, 0.0521 mmoles) was added at -30°C. A color change from yellow to deep purple was observed. The solution was further stirred for 90 minutes. Black crystals suitable for X-ray diffraction studies were obtained by vapor diffusion of pentanes into the THF solution (27 mg, 73%, 0.025 mmol).  Note: Complex 6 was found to be unstable in THF solution. After 14 d, complete conversion to a decomposition product was observed, which could be identified as the C-H activation product, complex 9 by XRD analysis ( Figure S70). Figure

( Cy POC(Et)CP Ph N Tol )Rh III I(PPh2NTol), 7.
In a J. Young NMR tube was added complex 2 (1.00 eq, 30.0 mg, 0.0325 mmoles), and 1 mL of anhydrous toluene-d8 was added. The solution was cooled down to -30 °C. Then, ethyl iodide (1.50 eq. 3.9 μL, 0.0488 mmoles) was added at -30 °C. A color change from yellow to deep purple was observed. This reaction is slower than the MeI reaction. After 16 h the formation of complex 7 was noticed along with the little bit amount of starting material and the C-H bond activated product (see Figure S57). Black crystals of complex 7 suitable for Xray diffraction studies were obtained by vapor diffusion of pentane into the toluene solution. Complex 7 was not isolated due to formation of C-H bond activated product.

( Cy POC(CH2Ph)CP Ph N Tol )Rh III Br2(PPh2NHTol), 8.
In a J. Young NMR tube was added complex 2 (1.00 eq, 30.0 mg, 0.0325 mmoles), and 1 mL of anhydrous THF was added. The solution was cooled down to -30°C. Then, benzyl bromide (2 eq., 7.77 μL, 0.065 mmoles) was added at -30°C. A color change from yellow to deep purple was observed. The solution was stirred for 24 hours. Almost full conversion was observed, along with some impurities. Black crystals suitable for x-ray diffraction studies were obtained by vapor diffusion of pentanes into the THF solution (24.7 mg, 65%, 0.0325 mmol).                        (5)). Top spectrum is complex 1 (red), middle is complex 5 (burgundy) and bottom is complex 2 (black).

Crystallographic details
The X-ray diffraction data for the single crystals of L1-Li, L2, and 1-8 were collected on a Rigaku Synergy instrument (κ-goniometer) with a PILATUS3 R 200K hybrid pixel array detector using MoKα (0.71073 Å) or CuKα (1.54184 Å) radiation monochromated by means of multilayer optics. The performance mode of a PhotonJet microfocus sealed X-ray tube was 50 kV, 1.0 mA. Suitable crystals of appropriate dimensions were mounted on loops in random orientations. Preliminary unit cell parameters were determined with three sets of a total of 10 narrow frame scans in the case of a Mo-source and six sets of a total of 10 narrow frame scans at two different 2θ positions in the case of a Cu-source. The data were collected according to recommended strategies in an ω-scan mode. Final cell constants were determined by global refinement of reflections from the complete data sets using the Lattice wizard module. Images were indexed and integrated with "smart" background evaluation using the CrysAlisPro data reduction package (Rigaku Oxford Diffraction). Analysis of the integrated data did not show any decay. Data were corrected for systematic errors and absorption using the ABSPACK module: Numerical absorption correction based on Gaussian integration over a multifaceted crystal model and empirical absorption correction based on spherical harmonics according to the point group symmetry using equivalent reflections.
The structures were solved by intrinsic phasing using SHELXT-2018/2 and refined by the fullmatrix least-squares on F 2 using SHELXL-2018/3, [3][4] which uses a model of atomic scattering based on spherical atoms. Non-hydrogen atoms were refined anisotropically. The positions of the hydrogen atoms at the carbon bound to the rhodium atoms were determined by the difference Fourier maps, and these atoms were refined isotropically. The other hydrogen atoms were inserted at the calculated positions and refined as riding atoms.

4.2.
Crystal structure analysis of 1-Li H1 was found in the difference Fourier map and refined independently.

Crystal structure analysis of L2
H1 was found in the difference Fourier map and refined independently.

Crystal structure analysis of complex 1
The crystal structure contains a disordered cyclohexyl group and a disordered THF solvent molecule. The two parts of both disorders were refined using the PART keyword and free variables which refined to 0.795 (cyclohexyl) and 0.734 (THF), respectively. Furthermore, one carbon atom in the disordered THF molecule was refined using the ISOR restraint (0.01).
H1 was found in the difference Fourier map and refined independently. Figure S66. Structural representation of complex 1 with 50 % probability anisotropic displacement ellipsoids of non-hydrogen atoms. Hydrogen atoms, except for H1, as well as solvent molecules have been omitted for clarity. Only the major disordered moiety is shown.

Crystal structure analysis of complex 2
The crystal structure contains a disordered tolyl group, which was refined using the PART keyword and a free variable which refined to 0.451. SADI and FLAT restraints were used for the refinement of both parts.

Crystal structure analysis of complex 5
The asymmetric unit contains two molecules of the Rh complex as well as a toluene solvent molecule on an inversion center. An additional heavily disordered solvent molecule was present in the structure, but couldn't be refined. Therefore, the SQUEEZE option in platon was used to minimize the void connected with the solvent. The toluene solvent on the symmetry centre was refined using the PART -1 keyword and fitted to an ideal solvent using the FRAG option. In the refinement process ISOR and SIMU as well as EADP restraints were used.
H1 and H53 were found in the difference Fourier map and refined independently.

Crystal structure analysis of complex 8
The structure contains a disordered benzyl and cyclohexyl group. The two parts of both disorders were refined using the PART keyword and free variables which refined to 0.584 (benzyl) and 0.564 (cyclohexyl), respectively.

4.11.
Crystal structure analysis of complex 9.
The structure contains a disordered tolyl group that was refined using PART keyword and free variables which refined to 0.833.

General information and results
All calculations were performed without symmetry restrictions. Starting coordinates were obtained from the crystal structure analyses. Geometry optimizations and NBO analysis (Ref) were carried out with the Gaussian 16 (revision C.01) 6 program package, while the Bond order analysis was done with the Multiwfn program version 3.6. 7 The geometry optimization for the ligands and rhodium complexes were performed with the PW6B95D3 functional, 8 through the def-2TZV(f) basis set for C, H, N, P, O atoms and LANL2DZ for Rh, Cl, Br, and I. The metrical parameters of the energy-optimized geometries are in good agreement with those determined by X-ray diffraction. Harmonic vibrational frequency analyses were performed at the same level of theory as the optimizations to determine the nature of the structure. The vibrational frequency analysis showed no imaginary frequencies. Population analysis and NBO analysis were performed with the same functional but using the DEF2TZVP triplet-zeta basis set and the LANL2TZ(f) ECP for Rh, Cl, Br, and I.  The calculations were done at the PW6B95-D3/def2svp level of theory for C, H, N, P atoms

S72
The formation of complex 6 (and 7) is proposed to first proceed via the formation of acyl complex II. Due to the high electron shift to the carbonyl ligand in 2 this is an exergonic reaction and the acyl complex favoured over the classical oxidative addition complex I. Due to the higher energy of complex I relative to 2, it is reasonable to assume that the formation of II proceeds via a cooperative bond activation step, in which the methyl group directly is bound to the carbonyl carbon atom. In the next step, reductive elimination leads to the formation of ylide complex III, from which the carbonyl group might attack at the aminophosphine moiety to eliminate the amidophosphine and to form the final complex 6. For a detailed mechanistic understanding further calculations including conformational analyses, the inclusion of solvent effects and calculations of activation barriers as well as other mechanistic pathways are required. Figure S82. Optimized structures of the complexes shown in Figure S81.  Table S10. Caclulcated energies of the complexes shown in Figure S81.