Reactivity of Nickel Complexes Bearing P(C=X)P Ligands (X = O, N) Toward Diazoalkanes: Evidence for Phosphorus Ylide Intermediates

Nickel carbenes are attracting attention for the development of more sustainable catalysts, among others, for cyclopropanation. Intramolecular trapping of a nickel carbene intermediate with an olefin incorporated in a P(C=C)P Ni pincer complex had previously allowed the isolation of a nickelacyclobutane intermediate and a detailed characterization of its reactivity. Herein, we report the reactivity of related nickel pincer complexes bearing a ketone P(C=O)P or an imine P(C=N)P with diazoalkanes as the carbene precursor. The observed reactivity suggests, in both cases, the reaction of the transient nickel carbene with one of the phosphine arms to form phosphorus ylides that subsequently react with the unsaturated backbone. Density functional theory (DFT) calculations are used to shed light on the mechanisms of these reactions.


■ INTRODUCTION
Metal carbenes are key intermediates in several catalytic cycles, such as cyclopropanation and olefin metathesis.They are commonly synthesized through the reaction of a reduced metal complex with a precursor such as a diazoalkane (nitrogen extrusion).They can react with unsaturated compounds such as olefins to yield cycloaddition products such as cyclopropanes and can be inserted into X−H bonds.−4 In the growing body of research on base-metal catalysis, nickel has emerged as a good candidate for the development of environmentally friendlier catalysts. 5,6This has motivated previous studies on isolated nickel carbenes, which showed that they generally have a nucleophilic character and undergo transfer reactions with substrates as CO and ethylene yielding ketenes and cyclopropane products, respectively. 7−17 Previously, we had reported that an olefin tethered in the framework of a diphosphine pincer complex P(C�C)P could trap a nickel-carbene intermediate to yield a stable nickelacyclobutane (Figure 1), which allowed the study of its divergent reactivity relevant to both olefin metathesis and cyclopropanation processes. 18erein, we investigate the reactivity of nickel diphosphine pincer complexes bearing a ketone P(C�O)P and an imine P(C�N)P group toward diazo compounds.In the case of the ketone pincer, an unusual carbonyl olefination reaction is observed.For the imine ligand, on the other hand, the capture of the carbene fragment between one phosphine and the imine group is observed.DFT calculations suggest the formation of nickel carbenes from the reactivity of P(C�X)P Ni(0) complexes with diazoalkanes and the intermediate formation of phosphorus ylide for both reactions.

■ RESULTS AND DISCUSSION
To test the reactivity of (P(C�O)P)Ni complexes with diazoalkanes, we started with ( Ph dppb)Ni(BPI) complex 1 (Scheme 1), in which the ketone moiety is coordinated to the nickel center alongside an easily displaceable benzophenone imine coligand (BPI). 19Reaction with 1.6 equiv of diphenyldiazomethane at room temperature led to a single P-containing product (2) along with unidentified black solids after 1.5 h.In C 6 D 6 , compound 2 features two different 31 P{ 1 H} NMR spectral signals at 28.7 and −14.1 ppm.
An X-ray crystal structure determination of compound 2 revealed a metal-free structure resulting from the olefination of the ketone backbone with concomitant oxidation of one of the phosphine moieties (Figure 2). 20C37−C38 present a bond length of 1.350(3) Å, in good agreement with other reported tetraarylolefins. 21,22O1 and O2 are only partially occupied (52.4(5) %), which is consistent with the 31 P{ 1 H} NMR spectrum showing the presence of one phosphine and one phosphine oxide moiety per molecule.
Interestingly, instead of using 1, the reaction could also be carried out with only catalytic amounts of Ni(cod) 2 (15 mol %), resulting in full conversion of ligand 4 to 2 in 1.5 h (TON > 6).A control reaction without the nickel catalyst did not yield the olefination product (Supporting Information, SI Section S1), showing the critical role of nickel in this reaction.A similar catalytic reaction with bis(4-methylphenyl)diazomethane afforded the analogous compound 3 with 31 P{ 1 H} NMR spectral signals located at 28.6 and −14.1 ppm, respectively.A 1 H NMR spectrum in C 6 D 6 corroborated that structure 3 contains p-tolyl groups, with a singlet signal corresponding to two methyl groups at 1.93 ppm.
DFT calculations were used to gain insight into the mechanism of carbonyl olefination (Scheme 2).From complex 1, the exchange of benzophenone imine (BPI) for diphenyldiazomethane is slightly endergonic (+3.6 kcal/mol).Because similar ligand-exchange reactions of 1 have been shown experimentally to be rapid at room temperature, 19 the ligand-exchange mechanism was not investigated in detail.A change in the coordination mode of the diazo ligand from Nbound (5) to C-bound ( 6) is followed by facile N 2 extrusion  (ΔG ‡ = 16.6 kcal/mol) yielding nickel carbene 7 (−25.4kcal/ mol).Insertion of the carbene into the P−C bond to form a phosphorus ylide is readily feasible with a barrier of 20.6 kcal/ mol (ΔG ‡ = −4.8kcal/mol), yielding complex 8 (−18.6 kcal/ mol).The ylide complex is predicted to be energetically less stable than nickel carbene 7; nevertheless, complexation with solvent molecules to form an 18-electron complex could help in stabilization.Additionally, carbene insertion into the opposite (left) phosphine-nickel bond to form the other possible conformer had an isoenergetic transition state (ΔG ‡ = −4.8kcal/mol).The resulting conformer is a more energetic structure (−11.3 kcal/mol) and presents significant differences in geometry (SI Section S5.1.1).The ketone backbone is coordinated in all structures, and analogous structures without ketone coordination were not found.
From complex 8, no energetically accessible pathways for the formation of the organic product while the ligand is bound to nickel were identified.Two alternative routes involving the coupling of the phosphorus ylide with the ketone backbone were computed but were energetically prohibited under the experimental conditions (see SI Section S6.2.1).An alternative pathway starting with the formation of a nickelaoxetane was also considered, but the transition state from nickel carbene 7 was too high in energy (ΔG ‡ = 12.9 kcal/mol, overall barrier 38.3 kcal/mol, see SI Section S5.1.2).While nickel is clearly required for the reaction to take place, the final product does not bind Ni(0) and it is unclear from experiments at which moment nickel is released from the organic molecule.We hypothesize that Ni could be released from the ylide complex 9.This idea is further supported by the fact that reforming complex 5 from 8, ligand 4, and diphenyldiazomethane with the release of free ylide 9 is exergonic (−9.5 kcal/mol), even though a detailed elucidation of the ligand-exchange mechanism has not been attempted.
The pathway involving an intramolecular Wittig reaction from free phosphorus ylide 9 (after dissociation of nickel) was computed to be kinetically facile (Scheme 2, in blue).A first transition state to form strained oxaphosphetane 10 is readily accessible (ΔG ‡ = −13.7 kcal/mol).Opening of the oxaphosphetane is facile (ΔG ‡ = −17.0kcal/mol) yielding compound 2.These last results show that a metal-free process to obtain the product from the free ylide is plausible.
Next, we aimed to study the reactivity of a nickel complex featuring a P(C�N)P ligand.−52 To explore the trapping of a Ni-carbene intermediate with an imine, we started with (P Ph CNP Ph )Ni(PPh 3 ) complex 11, which contains an imine coordinated in η 2 (C,N) fashion. 53Reaction of 11 with three equivalents of Bis(4-methylphenyl)diazomethane yielded the new nickel complex 12 along with a small amount of phosphazine as a side product (Scheme 3). 35,54,55Only a small amount of phosphazine is observed even if the reaction is performed with 10 equiv of diazoalkane (see SI Section S3).In C 6 D 6 solution, complex 12 displays two 31 P{ 1 H} NMR spectral signals at 49.9 (d, J P−P = 259 Hz) and 30.8 (d, J P−P = 259 Hz) ppm, consistent with the phosphorus atoms occupying trans positions in a square-planar geometry.The methyl groups originating from the diazoalkane moiety appear Scheme 2. Proposed Mechanisms for the Formation of 2 from ( Ph dppb)Ni(BPI) a a Calculations were performed at B3LYP-GD3BJ/def2TZVP//B3LYP/6-31g(d,p) level of theory.Scheme 3. Reactivity of (P Ph CNP Ph )Ni(PPh 3 ) with Diazo Compounds Organometallics as inequivalent 1 H NMR singlets at 2.13 and 1.91 ppm.The hydrogen atom originally bonded to the C α atom of the imine moiety has shifted upfield to 5.80 ppm (J H−P = 28 Hz) and appears as a doublet due to coupling with one 31 P nucleus.Its associated carbon nucleus is found in the 13 C{ 1 H} NMR spectrum at 67.9 ppm, showing a loss of sp 2 character.Purification of the product proved to be challenging and was only achieved by crystallization from tetrahydrofuran (THF)/ hexamethyldisiloxane (HMDSO).Unfortunately, the obtained crystals were not of sufficient quality for X-ray diffraction.
Using a slightly different diazoalkane, bis(4methoxyphenyl)diazomethane, led to the analogous structure 13 that provided better quality crystals.The 31 P{ 1 H} NMR spectrum in C 6 D 6 shows the expected two phosphorus signals at 50.6 (d, J P−P = 258 Hz) and 30.9 (d, J P−P = 258 Hz) ppm.The methoxy groups appear at 3.31 and 3.14 ppm in the 1 H NMR spectrum of 13 in C 6 D 6 .The imine-derived CH proton resonates at 5.77 ppm (J H−P = 28 Hz).An X-ray crystal structure determination of complex 13 revealed that an intricate chemical transformation had taken place (Figure 3). 20The structure exhibits a square-planar nickel(II) center with the two phosphorus atoms of the chelating ligand in the trans positions.The coordination environment is completed with an amino group derived from the imine and a phenyl group transferred from one of the phosphines as X-type ligands.The carbene moiety is bound to C7, the carbon atom that belonged to the imine group, and to the phosphorus atom from which a phenyl group has migrated, forming a 5membered ring with nickel (P2−C32−C7−N1−Ni1).The bond lengths of C7−C32 1.573(2) and C7−N1 1.463(2) confirm the sp 3 hybridization of C7.
The reactions forming 12 and 13 from the imine complex 11 can be explained by a pathway involving a phosphorus ylide intermediate.In Scheme 4, a proposed pathway is shown starting from a nickel carbene (with the aromatic group on the nickel carbene truncated to phenyl groups).The nickel carbene complex 14 with end-on imine coordination was found to be slightly lower in energy than isomer 14a, where the imine is coordinated side-on (2.6 kcal/mol).Carbene insertion into the P−Ni bond is feasible (ΔG ‡ = 15.1 kcal/mol), yielding the phosphorus ylide complex 15 with the imine backbone coordinated side-on (−1.6 kcal/mol).A change of coordination mode yielding complex 16 (10.5kcal/mol) is followed by nucleophilic attack of the now uncoordinated ylide moiety on the imine carbon atom (ΔG ‡ = 16.4 kcal/mol), creating a new C−C bond.In the resulting complex 17, nickel is coordinated to one of the phenyl rings of phosphonium in an η 2 (C,C) fashion.A change of coordination mode (ΔG ‡ = 8.1 kcal/mol) yields an isoenergetic structure 18 with η 2 (C,P) coordination.From there, the transition state for oxidative addition is readily available (ΔG ‡ = 6.8 kcal/mol) yielding the final product (19).In contrast, a pathway involving the formation of an azanickelacyclobutane intermediate was computed with a higher overall barrier (32.0 kcal/mol; see SI 6.3.2).These calculations show that the observation of 19 as the final product is consistent with the initial formation of carbene intermediate 14, but it remains unclear how this intermediate forms under the reaction conditions.Generally, the synthesis of nickel carbenes from diazoalkanes starts with the formation of a nickel diazoalkane adduct in the η 1 (N) coordination mode.Subsequently, change of coordination mode to η 2 (C,N) is followed by nitrogen extrusion, yielding the desired nickel carbene. 7,12,18However, the formation of a (P Ph CNP Ph )Ni-[η 2 (C,N)-N 2 CPh 2 ] intermediate from 11 with the release of PPh 3 is strongly endergonic (28 kcal/mol, SI Section S5.2.3); the energy penalty for ligand exchange is already exceeding the expected barrier for a reaction at room temperature. 56dditional calculations showed that η 1 (N) coordination of the diazo compound without release of PPh 3 is facile owing to the hemilability of the C�N bond.However, subsequent η 2 (C,N) coordination is also prohibitively high in energy (30.8 kcal/mol, see SI Section S5.2.3) in energy.Alternative pathways involving the release of an organic carbene from an η 1 (N) diazo complex 57 were also found to be energetically inaccessible (≥34.7 kcal/mol, see SI Section S5.2.4).
In summary, DFT calculations identified a readily accessible mechanism for the formation of the final product 19 from putative carbene intermediate 14, but no energetically accessible pathway to form 14 from triphenylphosphine complex 11 was identified.It seems likely that the diazoadduct [(P Ph CNP Ph )NiPPh 3 ][η 1 (N)-N 2 CPh 2 ], whose formation is facilitated by the hemilabile behavior of the imine moiety, plays an important role.Possible pathways to generate a carbene intermediate from [(P Ph CNP Ph )NiPPh 3 ][η 1 (N)-N 2 CPh 2 ] may include radical (chain) processes or a single electron transfer step.In addition, an alternative pathway for the formation of complexes 12 and 13 that does not involve carbene intermediates cannot be formally excluded.

■ CONCLUSIONS
The reactivity of nickel diphosphine pincer complexes bearing a ketone (P(C�O)P) or an imine (P(C�N)P) group toward diazo compounds was investigated.Reaction of diaryldiazomethane with ( Ph dppb)Ni(BPI) resulted in intramolecular olefination of the backbone, yielding a tetrasubstituted olefin bearing a pendant phosphine oxide group.Interestingly, catalytic amounts of Ni(cod) 2 mediate the reaction of Ph dppb and a diazoalkane to form the same product.DFT calculations showed that the formation of phosphorus ylides by carbene  16) Å. 20 insertion in the Ni−P bond is feasible, likely followed by a metal-free intramolecular Wittig reaction.The reaction of (P Ph CNP Ph )Ni(PPh 3 ) with diaryl diazoalkanes illustrates a different reaction pathway likely involving a phosphorus ylide intermediate.It ultimately yields a bicyclic phosphine ligand by coupling the carbene fragment to both a phosphorus atom and the carbon atom of the imine group with a concomitant phenyl transfer from P to Ni.
These results illustrate the propensity of phosphinesupported Nickel carbene intermediates to form ylides by carbene transfer to a phosphine ligand.Pincer ligands like those used in this study provide competing sites for carbene migration: an unsaturated bond on one side and phosphine moieties on the other side.Previous work had found that reaction with a C�C bond to form a nickelacyclobutane is favored over ylide formation.The stark contrast with the results described here can be explained by the nucleophilic character of Ni-carbenes, resulting in a polarity mismatch with the electron-rich heteroelement of the C�O and C�N bonds.While ylide formation is often an undesired decomposition pathway, the catalytic olefination reaction described here shows that productive catalysis forming challenging C�C double bonds via ylide intermediates can be envisioned.

■ EXPERIMENTAL SECTION
Caution: Diazo compounds are high-energy compounds that present a potential risk of explosion.Their reactivity is highly dependent on their structure; the diaryl diazoalkanes used in this work present mild reactivity and are mainly sensitive to light.More information on the risks associated with diazoalkanes can be found in a review by Bull and co-workers. 58eneral Information.All reactants were purchased from commercial sources and used as received without further purification.Additionally, Ni(cod) 2 , OPPh 3 , and diazo compounds were stored in the glovebox.All of the reactions were performed under an N 2 (g) atmosphere using glovebox techniques.Deuterated solvents were purchased from Cambridge Isotope Laboratory Incorporation (Cambridge), degassed by freeze pump procedure, and stored over molecular sieves before use.Common solvents were dried using a MBRAUN MB SPS-80 purification system, except for THF, which was purified by distillation from a THF/Na/Benzophenone suspension.( Ph dppb)Ni(BPI), 19 (P Ph CNP Ph )Ni(PPh 3 ), 53 diazoalkanes (diphenyldiazomethane, bis(4-methylphenyl)diazomethane, b i s ( 4 -m e t h o x y p h e n y l ) d i a z o m e t h a n e ) , 5 9 a n d 2 , 2 ′ -b i s -(diphenylphosphino)benzophenone 60 were synthesized according to literature procedures. 1 H, 13 C, and 31 P NMR spectra (400, 101, and 161 MHz, respectively) were recorded on an Agilent MR400, Jeol JNM-ECZL G 400 MHz NMR with a Royalprobe HFX or a Varian AS400 spectrometer at 297 K. 1 H and 13 C NMR chemical shifts relative to tetramethylsilane are referenced to the residual solvent resonance. 31P NMR chemical shifts were referenced to 85% aqueous H 3 PO 4 solution, both externally.Infrared spectra were recorded using a Perking Elmer Spectrum One FT-IR spectrometer under a N 2 flow.Elemental analysis was conducted by Medac Ltd., Surret, United Kingdom.

-[ 2 -( D i p h e n y l p h o s p h i n o y l ) p h e n y l ] --[ 2 -(diphenylphosphino)phenyl]-2,2-diphenylethene (2).
Procedure from 1. Inside a drybox, ( Ph dppb)Ni(BPI) (100 mg, 0.126 mmol) was dissolved in benzene (10 mL).Subsequently, a solution of diphenyldiazomethane (40 mg, 0.20 mmol) in benzene (5 mL) was added dropwise.The solution was stirred for 1.5 h, and the formation of black solids was observed.The mixture was filtered, the volume reduced down to 7 mL under vacuum, and pentane (ca. 3 mL) was added.After 24 h, crystals were obtained, washed three times with cold pentane, and dried in vacuum to obtain the product as yellowish crystals (36 mg, 40%).

P r o c e d u r e f r o m 4 . I n s i d e t h e d r y b o
x , 2 , 2 ′ -b i s -(diphenylphosphino)benzophenone (200 mg, 0.36 mmol) and Ni(cod) 2 (15 mg, 0.054 mmol) were dissolved in 12 mL of toluene, and the mixture was stirred for 15 min.The solution was cooled down to −78 °C, and a solution of diphenyldiazomethane (210 mg, 1.08 mmol) dissolved in 3 mL of toluene was added dropwise.The solution was stirred for 15 min at −78 °C and for 1 h and 15 min at room temperature.The solvent was evaporated under vacuum until 5 mL of volume, and hexane was added until a white precipitate was observed.The solid was collected by filtration and washed with hexane until the solvent was colorless.The solid was taken out of the drybox, dissolved in 7 mL of toluene, and washed with brine.The organic fraction was dried over Na 2 SO 4 , and the solvent was evaporated.The solid was recrystallized in MeOH to yield 200 mg (77%) of the product.Crystals suitable for X-ray diffraction were obtained by layering a solution of 2 in benzene with pentane. 1
Blank Reaction.20 mg (0.036 mmol) of 2,2′-bis-(diphenylphosphino)benzophenone and 9.5 mg (0.036 mmol) of OPPh 3 as internal standard were dissolved in 2 mL of toluene, and a 31 P NMR spectrum was recorded.The solution was cooled down to −78 °C and a solution of Bis(4-methylphenyl)diazomethane (22.2 mg, 0.1 mmol) dissolved in 0.5 mL of toluene was added dropwise.The solution was stirred 15 min at −78 °C and 105 min at room temperature.A 31 P NMR spectrum was again recorded showing no detectable conversion of 2,2′-bis(diphenylphosphino)benzophenone.
Computational Methods.DFT calculations were performed using the Gaussian 16 software package version C.01. 61 Geometry optimizations were carried out in a vacuum at the B3LYP/6-31G(d,p) level of theory on all atoms.Frequency analyses on all stationary points were used to ensure that they are minima (no imaginary frequency) or transition states (one imaginary frequency).Transition states were optimized using the QST3 (synchronous transit-guided quasi-Newton number 3) method or using the opt = TS (Berny algorithm) keyword.The guess structures used as the starting point for TS optimizations were based on the results of relaxed potential energy surface scans (PES).ΔG°was calculated by single point calculation at the B3LYP-GDB3J/def2TZVP level of theory, adjusting the value with the thermal correction obtained at the B3LYP/6-31g(d,p) level of theory with temperature 298.15K and pressure 1 atm.