Heterobimetallic Pd–K carbene complexes via one-electron reductions of palladium radical carbenes

Unprecedented sequential substitution/reduction synthetic strategy on the Pd radical carbenes afforded heterobimetallic Pd–K carbene complexes, which features novel Pd–Ccarbene–K structural moieties.


Introduction
The inuence of Lewis acids in catalysis cannot be underestimated. While most efforts have focused on using them as the sole mediators of chemical transformations, [1][2][3][4][5] few examples have been reported that discuss the role of Lewis acids as coactivators in homogeneous catalysis. [6][7][8][9] These are related to heterogeneous processes, 10,11 with a singular example discussing the role of potassium ions on the hydrogenation of dinitrogen in the Haber-Bosch process, 12 and biological systems, with several examples reporting that Lewis acids are necessary cofactors that help modulate the redox properties of the oxygen-evolving complex's manganese cluster and likely its reactivity. [13][14][15][16][17] In organic synthesis, Shibasaki's rare-earth alkali-metal heterobimetallic complexes 18 are among the most enantioselective and broadly used catalysts to date, [19][20][21] but few studies discuss the role of Lewis acids in applications of late transition metals, in general, and of carbene complexes, in particular. [22][23][24] Transition metal carbenes are of vital importance to science and have witnessed a tremendous progress in their applications in the past few decades. [25][26][27][28][29][30][31][32] Late transition metal complexes with N-heterocyclic, 33,34 carbocyclic, 35 and heteroatom stabilized carbene ligands [36][37][38] have been extensively studied as a consequence; however, the corresponding non-heteroatom stabilized species, [M]CRR 0 ] (R, R 0 ¼ alkyl or H), are less explored. [39][40][41][42][43][44][45] This situation is even more pronounced for group 10 metals, likely because these metals are too electron rich to stabilize the M]C bond. Pioneering work by Hillhouse showed that a Ni(0) carbene containing a CPh 2 moiety could be isolated by the thermolysis or photolysis of its diphenyldiazoalkane precursor. 46,47 The isolation of the corresponding Pd and Pt carbenes is, however, more challenging than that of nickel complexes, [48][49][50][51][52][53][54] because of the highly reactive nature of the former. 55,56 On the other hand, these species are crucial intermediates in a variety of catalytic transformations, 57,58 such as palladium carbene mediated cyclopropanations, cross-coupling with diazo compounds, and migratory insertion reactions. 59 It is worth mentioning that only two examples of cationic Pd(II) carbene complexes are known (Chart 1, type A), synthesized via triate or hydride abstraction, and that their reactivity has not been studied. 48,49 A salt metathesis strategy was also reported for the synthesis of methanediide-based Pd(II) carbene complexes. 50,51 We have recently applied dehydrohalogenation reactions 60 [61][62][63] Interestingly, the Pd-C carbene bonds in these compounds are best described as ylide-like (Chart 1, type B), as demonstrated by their strong nucleophilic reactivity toward polar substrates (MeI, HCl, MeOH, para-toluidine), 61 strong Lewis acids, 62 C-H 64 and Si-H 65 bond activation reactions. Furthermore, our recent study on the redox-induced umpolung of palladium carbenes revealed that the radical carbene [{PCc(sp 2 )P} tBu PdI] (1, Chart 1, type C) bridges cationic and anionic carbenes via reversible oneelectron transfer processes. 66,67 We reasoned that the presence of iodide as a leaving group in 1 would facilitate its substitution with various anionic nucleophiles to afford new radical carbene species, which would generate Pd(II) carbenes with functional groups when subsequent one-electron reductions are applied. Herein, we report a sequential substitution/reduction reaction of the radical carbene 1 to form heterobimetallic Pd-K carbene complexes featuring amides or benzyl ligands that represent rare examples of late transition metal complexes containing Pd-C carbene -M units (Chart 1, type D). Such bonding motifs were recently proposed to be instrumental in the hydroarylation of dienes catalysed by rhodium carbodicarbene complexes, but structural characterization was not available. 22

Results and discussion
Synthesis and characterization of metal complexes Treatment of 1 with potassium amides, R 1 R 2 NK, in THF afforded new radical complexes, [{PCc(sp 2 )P} tBu PdNR 1 R 2 ] (2: , which were crystallized from n-pentane at À35 C as dark-green crystals in high yield. As observed for 1, 67 2 and 3 are silent by 1 H and 31 P NMR spectroscopy and are thermally robust. The effective magnetic moments m eff of 1.70 m B and 1.54 m B obtained using the Evans method 68 indicated an S ¼ 1/2 ground state for both compounds. EPR spectra of radicals 2 and 3 ( Fig. 1) exhibit similar hyperne patterns arising from an interaction of the unpaired electron with three pairs of magnetically equivalent protons, one nitrogen nucleus, and a 105 Pd nucleus. As the natural abundance of the magnetic 105 Pd isotope is 22.33%, each spectrum exhibits contributions from the non-magnetic palladium isotope (strong, well-resolved central signal) and the 105 Pd isotope (broad wings). Proton hyperne couplings were assigned to protons of the phenyl rings adjacent to the radical center.
The solid state molecular structures of 2 and 3 are consistent with their radical carbene nature (Fig. 2): both contain squareplanar palladium centers bound to sp 2 hybridized backbone carbons (S angles at C carbene are 359.9 for 2 and 360.0 for 3). The Pd-C carbene distances in 2 (2.019(2)Å) and 3 (2.024(2)Å) are comparable to each other and close to the value of 2.022(3)Å in 1. 67 Trigonal planar geometries were observed for both amide nitrogen atoms in 2 and 3, with the amide planes roughly perpendicular to the planes dened by C carbene , C(11), and C (21) or C(11) # (80.6 for 2 and 85.8 for 3). A longer Pd-N distance of 2.149(2)Å was observed in 3 compared to 2.0787(18)Å for 2, attributed to steric reasons.
Both radical complexes were subsequently treated with an equivalent of KC 8 in benzene, and the corresponding diamagnetic heterobimetallic complexes were obtained in high yield (Scheme 1). Both compounds are only soluble in ethereal solvents and were recrystallized by diethyl ether/n-pentane diffusion at ambient temperature. The solid state molecular structure of 5 was determined by X-ray diffraction studies (Fig. 3). Compound 5 exists as a notable polymer in which the anionic [{PC(sp 2 )P} tBu PdNPh 2 ] moieties are bridged by potassium ions through the carbene units and one phenyl ring of the amide group. The average K-C distances are slightly shorter when potassium binds the phenyl ring of the amide group rather than the carbene moiety (3.08Å vs. 3.17Å). The coordination sphere of potassium was further completed by an additional diethyl ether molecule. The C carbene carbon Scheme 1 Synthesis of heterobimetallic carbene complexes 4 and 5.  retains its trigonal planar geometry (S angles at C carbene are 359.9 ) with a Pd-C carbene distance of 2.043(2)Å, which is shorter than the corresponding value of 2.076(3)Å in [{PC(sp 2 )-P} tBu Pd(PMe 3 )]. 62 The trigonal planar geometry at the amide nitrogen was also retained, but the dihedral angle between the amide plane and the plane dened by C carbene , C(11) and C (21) was much smaller than that in 3 (59.4 vs. 85.8 ), probably due to the more congested environment found in the polymer.
Due to its poor solubility in non-coordinating solvents, the solution behaviour of 5 was studied in THF-d 8 . The 1 H NMR spectrum in THF-d 8 recorded at 298 K showed the peaks for the free Et 2 O at d ¼ 3.39 (q) and 1.12 (t) ppm, thus indicating its labile coordination that led to its displacement by the THF-d 8 molecules. Unlike the polymeric structure observed in the solid state, 5 exhibited a C 2 symmetry in the THF-d 8 solution, as evidenced by only a broad peak at d ¼ 48.64 ppm (Dn 1/2 z 870 Hz) in the 31 P{ 1 H} NMR spectrum, as well as three broad peaks for the i Pr (d ¼ 1.98 and 1.14 ppm) and t Bu (d ¼ 1.06 ppm) groups. Therefore, the polymeric structure of 5 is disrupted in the donor solvent and even in the presence of a small amount of THF-d 8 since the 1 H NMR spectrum of 5 in C 6 D 6 with 2 drops of THF-d 8 also showed C 2 symmetry.
Interestingly, the NPh 2 group on palladium exhibited three well-resolved peaks at d ¼ 7.35 (d), 6.80 (t), and 6.18 (t) ppm in the 1 H NMR spectrum of 5 in THF-d 8 , however, no resonances were observed from the phenyl rings of the supporting ligand in both 1 H and 13 C{ 1 H} NMR spectra. The variable temperature NMR spectra recorded in the temperature range of 208 to 323 K in THF-d 8 did not show any changes for these resonances. Only broadening of all the other peaks was observed in the 1 H NMR spectrum at 208 K, while a relatively sharp (Dn 1/2 z 320 Hz) peak was found in the 31 P{ 1 H} NMR spectrum (see Fig. S35 and S36 †). The phenyl rings of the supporting ligand are NMR silent in this whole temperature range. It is known that larger alkali metals such as potassium prefer anionic units that can delocalize the electron density, thus shiing the coordination of potassium away from the carbanion to the peripheral phenyl rings, leading to an h 6 -coordination mode. 69,70 Therefore, the observed solution behaviour might be explained by a fast motion of the potassium ions between the two phenyl rings of the supporting ligand that cannot be frozen out at 208 K, causing a C 2 symmetry on the NMR time scale. A comparable system has been reported for arene complexes of yttrium supported by a macrocyclic [P 2 N 2 ] ligand ([P 2 N 2 ] ¼ [PhP(CH 2 -SiMe 2 NSiMe 2 CH 2 ) 2 PPh]). 71 However, since all the signals from the phenyl rings of the supporting ligand could not be observed in the whole temperature range, a non-dynamic process caused by the formation of other polymers or oligomers containing bridging potassium ions cannot be ruled out. 72 The 1 H NMR spectrum of 4 in THF-d 8 also exhibited a similar C 2 symmetry as observed for 5. No coordination of diethyl ether was observed, even though 4 was crystallized from a diethyl ether/n-pentane mixture, indicating a slightly different bonding mode of potassium in 4, caused by the different amide substituent on palladium. Unlike those in 5, the phenyl groups from the supporting ligand in 4 exhibited three broad singlets at d ¼ 7.23, 6.61 and 6.54 ppm in the 1 H NMR spectrum. Only one sharp singlet at d ¼ 46.36 ppm was observed in the 31 P{ 1 H} NMR spectrum. Therefore, a similar polymeric chain can be proposed for complex 4 (vide infra). Notably, the proton of the NH group appears as a singlet at d ¼ 1.33 ppm, thus the formation of a possible [Pd]N p Tol] or [Pd-N p Tol-Pd] imido species is excluded.
Our study of the solution behavior of 4 and 5 shows that not only the polymeric structure can be disrupted by donor solvents such as THF, but that the mobility of the potassium cation increases as well. These ndings support the proposal by Meek and coworkers that the interaction of a rhodium catalyst for the hydroarylation of dienes with a Lewis acid has to be reversible in order to observe an increased activation of the substrate. 22 Heterobimetallic carbene complexes 73,74 containing K-C carbene -M units, in which the carbene is not stabilized by adjacent heteroatoms, are rare; 23,75 to the best of our knowledge, complexes 4 and 5 represent the only known characterized heterobimetallic carbene species possessing such binding motifs. The successful synthesis of amide substituted carbene complexes 4 and 5 from the radical precursor 1 prompted us to investigate other nucleophiles, especially those featuring an alkyl group. Interestingly, the reaction of 1 with two equivalents of PhCH 2 K in THF led to a dark-brown diamagnetic complex, K[{PC(sp 2 )P} tBu Pd(CH 2 Ph)] (6), in 64% yield, instead of the expected benzyl substituted Pd(II) radical carbene [{PCc(sp 2 )-P} tBu Pd(CH 2 Ph)] (7, Scheme 2). Using one equivalent of PhCH 2 K also led to 6, albeit with a lower conversion.
Similarly to 4 and 5, complex 6 is only soluble in ethereal solvents and its 1 H NMR spectrum recorded in THF-d 8 is consistent with a C 2 symmetric structure in solution. The 31 P { 1 H} NMR spectrum only shows a sharp singlet at d ¼ 48.52 ppm. A triplet at d ¼ 2.45 ppm ( 3 J PH ¼ 5.0 Hz) was observed for the benzyl CH 2 group in the 1 H NMR spectrum that correlates with a triplet at d ¼ 14.12 ppm ( 2 J PC ¼ 9.0 Hz) in the corresponding 13 C{ 1 H} NMR spectrum. The peak corresponding to the carbene carbon could not be assigned unambiguously in the 13 C{ 1 H} NMR spectrum, a situation previously observed for [{PC(sp 2 )P} H PdPMe 3 ], 61 as well as for other palladium carbene complexes. 50 As observed for 4 and 5, a discrepancy between the solid state and THF-d 8 solution structures of 6 exists that suggests a disruption of the polymeric chain by the donor solvent. In the 1 H NMR spectrum, three well-resolved peaks at d ¼ 7.20 (dt), 6.66 (td) and 6.52 (dd) ppm were assigned to the signals from the phenyl groups of the supporting ligand, which are very close to those of 4. Considering that benzyl and NH p Tol groups have similar steric proles, complexes 4 and 6 most likely possess similar solution structures. The 1 H NMR spectrum of 6 in C 6 D 6 with 2 drops of THF-d 8 also showed a C 2 symmetric structure. Notably, resonances of the phenyl groups of the supporting ligand are signicantly shied upeld in THF-d 8 compared to those in C 6 D 6 with only a small amount of THF-d 8 (from 7.61, 6.94, and 6.86 ppm to 7.20, 6.66, and 6.52 ppm, respectively), consistent with the generation in THF-d 8 of a monomer, in which the potassium ion interacts with only one carbene moiety and not with two, as observed in the solid state or in a nondonor solvent. 76 The formation of 6 is a combined substitution and reduction of radical complex 1: the benzyl group on palladium was introduced by substitution of iodide by a benzyl anion. Due to the reductive nature of the benzyl anion, the loss of an electron from it also reduced the radical species to an anionic carbene. This process is accompanied by the formation of the benzyl radical [PhCH 2 c], which dimerized to form PhCH 2 CH 2 Ph. Analysis of the crude reaction mixture by 1 H NMR spectroscopy indicated the formation of PhCH 2 CH 2 Ph, which was conrmed by comparison with an authentic sample. A minor palladium containing species was also observed, which showed two sets of peaks for the benzylic CH 2 groups at d ¼ 3.81 (s) and 2.94 ( 3 J PH ¼ 5.5 Hz) ppm in the 1 H NMR spectrum, and a sharp singlet at d ¼ 39.86 ppm in the 31 P{ 1 H} NMR spectrum. We tentatively assigned this species as complex [{PC(sp 3 )(CH 2 Ph)-P} tBu Pd(CH 2 Ph)], containing another benzyl group on the backbone carbon via radical coupling. Attempts to isolate this compound were hampered by its high solubility in aliphatic solvents.
Although the substitution reaction of 1 with PhCH 2 K led directly to the formation of the heterobimetallic carbene complex 6, the subsequent one-electron oxidation of 6 with [Cp 2 Fe][BAr F 4 ] afforded the radical complex 7 in quantitative yield (Scheme 2). The high solubility of 7 in aliphatic solvents prevented its separation from the byproduct, Cp 2 Fe, but its EPR spectrum (in the presence of Cp 2 Fe) in toluene at 298 K revealed a hyperne splitting from four pairs of magnetically equivalent protons and a 105 Pd nucleus ( Fig. S5 and S6 †). The smallest hyperne constant was attributed to the CH 2 group protons (0.08 mT), while the remaining proton hyperne constants (0.32, 0.19 and 0.12 mT) were assigned to the six phenyl ring protons of the supporting ligand, which are close to the corresponding values for 2 and 3. Accordingly, the one-electron oxidations of the heterobimetallic carbene complexes 4 and 5
Preliminary reactivity studies showed that the reaction of 6 with CH 3

CN (5 equivalents) in THF quantitatively afforded [{PC(sp 3 )HP} tBu Pd(CH 2 Ph)] (8, eqn (1)), which is the product of C-H activation, similarly to [{PC(sp 2 )P} H Pd(PMe 3 )]. 61,62
Given the presence of benzyl and amide ligands that could have been protonated by CH 3 CN, it is interesting to note that the former carbene carbon is more nucleophilic than these ligands, consistent with a localized negative charge on that carbon. (1) We also tested the direct nucleophilic substitutions of the parent carbene complex [{PC(sp 2 )P} tBu Pd(PMe 3 )] with p TolNHK, Ph 2 NK and PhCH 2 K. Although the formation of the heterobimetallic complexes 4 and 6 was conrmed ( p TolNHK, 65% and PhCH 2 K, 60% conversion) by 31 P{ 1 H} NMR spectra, some unknown species as well as unreacted starting material were also observed aer 24 h. Attempts to isolate pure 4 and 6 from these mixtures met with difficulty and were inefficient. The bulky nucleophile Ph 2 NK does not react with [{PC(sp 2 )-P} tBu Pd(PMe 3 )] under similar conditions, probably due to its steric hindrance. The sluggish substitution reactions observed for these nucleophiles could also be attributed to the relatively strong coordination of PMe 3 to palladium, therefore, the substitution/reduction strategy presented herein is preferable because it takes advantage of a facile redox process that provides a relatively easier access to heterobimetallic Pd-K carbene complexes.

DFT calculations
DFT calculations (B3LYP functional, LANL2DZ basis set) were performed on a model of the anion of 5, 5 0 , with the t Bu and i Pr groups replaced by H and methyl groups, respectively (Fig. 5). As was observed for [{PC(sp 2 )P} R Pd(PMe 3 )], 61,62 the HOMO of 5 0 shows a p type interaction between the carbene carbon p orbital and the appropriate symmetry d orbital of Pd, while the LUMO shows the corresponding s interaction, with both molecular orbitals having antibonding character. Thus the carbene moiety in these heterobimetallic carbene complexes is predicted to show some similar reactivity to that observed for [{PC(sp 2 )-P} R Pd(PMe 3 )], as was shown in eqn (1). 62,64,66 In order to study the inuence of the potassium cation on the Pd-C interaction, another simplied model, 5 00 , was considered, in which the interaction between potassium and a NPh 2 group from a neighboring molecule was replaced with a potassium benzene interaction. The HOMO of 5 00 (Fig. 5) resembles that calculated for 5 0 , in agreement with an electrostatic interaction between the potassium cation and the anionic carbon, while the LUMO is localized on the benzene ring coordinated to the potassium atom. The interaction between the cation and the carbene ligand observed in both the solid state structure and the computed model involves the delocalized p orbital of the two phenyl rings attached to the carbene atom. That carbon maintains its planar geometry in both 5 and 6, similarly to other delocalized p systems coordinated to potassium. [77][78][79] A similar situation was observed for 6. We studied the electronic structure of the free anion (6 0 ) and a model of the heterobimetallic carbene (6 00 ). In both, the iso-propyl groups and the tert-butyl groups were replaced by hydrogen atoms. For 6 00 , in order to simulate the coordination environments of both potassium atoms, we reduced the polymer to a dimer ( Fig. S24 and S25 †). Both HOMO and HOMOÀ1 for 6 00 are comprised of the lone pair on the carbenic carbon; LUMO and LUMO+1 are localized on the benzene rings coordinated to the terminal potassium cations. The planarity of the carbenic carbons can be attributed to the symmetric donation from the lone pair on these atoms to the two potassium atoms.

Conclusions
In conclusion, we showed that a substitution/reduction strategy employing the radical carbene [{PCc(sp 2 )P} tBu PdI] (1) can be utilized to synthesize a series of heterobimetallic Pd-K carbene complexes {[{PC(sp 2 )P} tBu PdX]K(OEt 2 ) n } (4: X ¼ NH p Tol, n ¼ 0; 5: X ¼ NPh 2 , n ¼ 1; 6: X ¼ CH 2 Ph, n ¼ 0) bearing functional groups on palladium. Polymeric structures were exhibited by these heterobimetallic Pd-K carbenes in the solid state, featuring unprecedented Pd-C carbene -K units that are not easily accessible by other synthetic strategies. The present carbenes complete the palladium carbene series containing a cationic (Chart 1, type A), an ylide-like (type B), a radical (type C), and a heterobimetallic (type D) carbene. The isolation of these novel species not only provides a fundamental understanding of their bonding and structural features, but also sheds some light on the role of alkalis in transition metal catalysis. Notably, the solution behaviour of these complexes showed that the interaction of the potassium ion with the carbene moiety is highly inuenced by the donating ability of the solvent, which led to a lower aggregation state of the complex; this may better represent species related to transition metal catalytic systems with alkali metals in polar solvents. More importantly, the exible interaction of potassium might also be crucial in terms of substrate activation, as proposed recently in the rhodium catalyzed hydroarylation of dienes, that a reversible interaction with Lewis acids enhance the reactivity. 22 Finally, the facile redox properties exhibited by these palladium carbenes also indicate their potential non-innocent ligand-based reactivity, and, therefore, future studies can lead to new and useful transformations in synthesis.

Experimental
All experiments are performed under an inert atmosphere of N 2 using standard glovebox techniques. Solvents hexanes, n-pentane, diethyl ether, and CH 2 Cl 2 were dried by passing through a column of activated alumina and stored in the glovebox. THF and THF-d 8 were dried over LiAlH 4 followed by vacuum transfer and stored in the glovebox, while C 6 D 6 was dried over CaH 2 followed by vacuum transfer, and stored in the glovebox. Complex 1, 67  were prepared according to literature procedures. p TolNHLi, p TolNHK and Ph 2 NK were prepared by deprotonation of amines with n BuLi and KH, respectively. 1 H, 13 C{ 1 H}, 31 P{ 1 H} NMR spectra were recorded on a Bruker DRX 500 spectrometer. All chemical shis are reported in d (ppm) with reference to the residual solvent resonance of deuterated solvents for proton and carbon chemical shis, and to external H 3 PO 4 , for 31 P chemical shis, respectively. Magnetic moments were determined by the Evans method 68,82,83 by using a capillary containing 1,3,5-trimethoxybenzene in C 6 D 6 as a reference. EPR spectra were recorded on a Bruker EMXplus EPR spectrometer with a standard X-band EMXplus resonator and an EMX premium microwave bridge, at microwave power of 2 mW, modulation frequency 100 kHz and amplitude 0.01 mT. Elemental analyses were performed on a CE-440 Elemental analyzer, or by Midwest Microlab. Gaussian 03 (revision D.02) was used for all reported calculations. The B3LYP (DFT) method was used to carry out the geometry optimizations on model compounds specied in text using the LANL2DZ basis set. The validity of the true minima was checked by the absence of negative frequencies in the energy Hessian.

Synthesis of [{PCc(sp 2 )P} tBu PdNH p Tol] (2)
p TolNHK (10.2 mg, 0.071 mmol) in 1 mL of THF was slowly added to a dark-green solution of 1 (50 mg, 0.067 mmol) in 1 mL of THF at À35 C. The resulted greenish slurry was allowed to stir at room temperature for 1 h. All volatiles were removed under reduced pressure and the residue was extracted with 6 mL of n-pentane, and ltered to give a dark-green solution. The volume of this n-pentane solution was reduced to about 0.5 mL under reduced pressure and stored at À35 C to give compound 2 as green crystals. Yield

Synthesis of [{PCc(sp 2 )P} tBu PdNPh 2 ] (3)
Ph 2 NK (19.5 mg, 0.094 mmol) and 1 (70 mg, 0.094 mmol) were mixed in 5 mL of THF and heated at 60 C for 6 days, during which time a greenish slurry was formed. All volatiles were removed under reduced pressure and the residue was extracted with n-pentane (3 Â 4 mL), and ltered to give a dark-green solution. The volume of the n-pentane solution was reduced to about 1 mL under reduced pressure and stored at À35 C to give compound 2 as green crystals. Yield 59 mg (80%).  KC 8 (8.6 mg, 0.064 mmol) and 3 (50 mg, 0.064 mmol) were mixed in 2 mL of C 6 H 6 at room temperature. The resulted greenish-brown reaction mixture was stirred at room temperature for 30 min. All volatiles were removed under reduced pressure and the residue was extracted with ether (3 Â 5 mL) and ltered to give a greenish-brown solution. The volume of this solution was reduced to about 3 mL under reduced pressure and layered with 9 mL of n-pentane at room temperature. Compound 5 crystallized from this solution as greenish brown crystals. Yield 41 mg (71%). PhCH 2 K (17.5 mg, 0134 mmol) in 1 mL of THF was slowly added to 1 (50 mg, 0.067 mmol) in 1 mL of THF at À35 C. The dark-red reaction mixture was stirred at room temperature for 2 hours. Volatiles were removed under reduced pressure and the residue was extracted with 8 mL of diethyl ether and ltered to give a dark-brown solution. The volume of this solution was reduced to about 1.5 mL under reduced pressure and layered with 7 mL of n-pentane. Compound 6 crystallized from this solution at À35 C as dark-brown solid. Yield 32 mg (64%). )P} tBu PdNH p Tol] À K + (4) (5 mg, 0.007 mmol) in 1 mL of diethyl ether at À35 C. The resulted brownish-green solution was allowed to stir at room temperature for 30 min. The volatiles were removed under reduced pressure and the residue was extracted with n-pentane. This solution was ltered and the volatiles were removed under reduced pressure to give a green solid. The EPR spectrum measured in toluene at 298 K is identical to that of a pure sample of 2 isolated by a different method (vide supra).
Oxidation of [{PC(sp 2 )P} tBu PdNPh 2 ] À [KOEt 2 ] + (5) [Cp 2 Fe][BAr F 4 ] (5.8 mg, 0.006 mmol) in 1 mL of diethyl ether was added to 5 (5 mg, 0.006 mmol) in 1 mL of diethyl ether at À35 C. The resulted green solution was allowed to stir at room temperature for 30 min. The volatiles were removed under reduced pressure and the residue was extracted with n-pentane and ltered. The volatiles were removed under reduced pressure to give a green solid. The EPR spectrum measured in toluene at 298 K is identical to that of a pure sample of 3 isolated by a different method (vide supra).

Synthesis of [{PCHP} tBu PdCH 2 Ph] (8)
CH 3 CN (4 mg, 0.1 mmol) in 0.5 mL of THF was added to a solution of 6 (15 mg, 0.02 mmol) in 1 mL of THF at room temperature. The dark-red solution immediately turned to orange, which was stirred for another 30 min. Volatiles were removed under vacuum to give an yellow oil, which was extracted into 3 mL of pentane, ltered and removal of all volatiles under reduced pressure afforded 8 as an yellow foam. Yield 14 mg (quantitative). For 8: 1 H NMR (500 MHz, 25 C, C 6 D 6 ): d ¼ 7.51 (d, 3 J HH ¼ 7.0 Hz, 2H, ArH); 7.46-7.44 (m, 4H, ArH); 7.26 (t, 3 J HH ¼ 7.8 Hz, 2H, ArH); 7.21 (dd, 3 J HH ¼ 8.5 Hz, 4 J HH ¼ 2.0 Hz, 2H, ArH); 6.96 (t, 3  with PhCH 2 K, p TolNHK and Ph 2 NK nucleophiles PhCH 2 K (2.8 mg, 0.022 mmol) in 1 mL of THF was added to carbene (15 mg, 0.022 mol) in 0.5 mL of THF at À35 C. The dark-red solution was then stirred at room temperature for 24 h. Volatiles were removed under reduced pressure and the residues were dissolved in C 6 D 6 (with 2 drops of THF-d 8 ) and monitored by 1 H and 31 P{ 1 H} NMR spectra. Same procedures were applied for p TolNHK and Ph 2 NK. The reactions with PhCH 2 K and p TolNHK showed conversion to the heterobimetallic carbene 6 and 4 in 60% and 65% conversion, respectively, based on the 31 P{ 1 H} NMR spectra. No conversion was observed for Ph 2 NK.