A Spirocyclic, Bimetallic, Carbon-Bridged Bis(iminophosphorano

of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada. Organometallics , 2010, 29 (1), pp 52–60. DOI: 10.1021/om900571k. Public...
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Organometallics 2010, 29, 52–60 DOI: 10.1021/om900571k

A Spirocyclic, Bimetallic, Carbon-Bridged Bis(iminophosphorano)methanediide Complex of Palladium(II): Synthesis, Crystal Structure, Solution Behavior, and DFT Bonding Analysis Guibin Ma, Robert McDonald, and Ronald G. Cavell* Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada Received July 2, 2009

The dimeric dilithium methanediide salt [Li2C(Ph2PdNSiMe3)2]2 ([Li2-1]2) reacted with either 1 or 2 equiv of [PdCl(allyl)]2 (allyl = CH2dCHCH2) in toluene or benzene to give exclusively the bimetallic Pd spirocyclic carbon-bridged complex [{η3-C3H5)Pd}2{μ,κ2:κ2-C(Ph2PdNSiMe3)2}] (2). Complex 2 was fully characterized by solution multinuclear NMR (1H, 31P, and 13C) spectroscopy in toluene-d8 and by X-ray crystallography. NMR reveals three coexisting isomers (labeled as: 2I1, 2I2, and 2I3) in the molar ratios 2/1.73/0.45 in solution. These isomers are attributed to different relative orientations of the allyl groups on Pd. The crystal and solid-state structure similarly shows the visible presence of isomers with differing allyl orientations. The isomer ratio in the solid is, however, not the same as in the solution. The minor isomer in the solid, 2I1, has an asymmetrical molecular configuration which would create inequivalent phosphorus atoms, whereas the symmetric isomeric forms (which comprise the major proportion of the complex in the crystal) display geometries with a pseudo 2-fold axis relating the two phosphorus centers, thereby creating NMR equivalence. This is consistent with the appearance in solution of one asymmetric isomer with inequivalent phosphorus atoms and two symmetric isomers with equivalent phosphorus centers. Variable-temperature NMR spectra showed that each of the three isomers present in solution suffers conformational exchange; however, full exchange was not observed in the 31P NMR-the different isomeric signals were clearly detectable up to 100 °C and were still separated at 115 °C. There is an apparent shift in isomeric distributions with temperature, but cooling the sample back to room temperature reestablishes the initially observed ratios of the three isomers. No thermal decomposition was observed in the VT NMR studies or in specific heating experiments to 115 °C. Under oxygen- and moisture-free conditions, 2 is remarkably inert; the allyl or the bridging carbon atom (which can be classified as a bridging carbene) are not displaced by CO. Interestingly, the 13C NMR chemical shift of the bridged Pd-C(carbene) appeared at a high field (-7.0 to -10.0 ppm), which indicates an electron-rich PCP carbene presence in 2. This appears to manifest its presence by relatively long Pd-C(carbene) bond lengths and shorter Pd-C(allyl) bond lengths. Gaussian DFT calculation and NBO analysis revealed a net back-donation from the Pd to the Ccarbene along with a delocalization throughout the backbone skeleton which stabilizes the molecular structure.

Introduction Many binuclear PdI-PdI complexes have received considerable attention over the past few years because of their ability to react with a wide range of substrates to provide *To whom correspondence should be addressed. E-mail: ron.cavell@ ualberta.ca. (1) Sui-Seng, C.; Enright, G. D.; Zargarian, D. J. Am. Chem. Soc. 2006, 128, 6508–6519. (2) Barder, T. E. J. Am. Chem. Soc. 2006, 128, 898–904. (3) Murahashi, T.; Kurosawa, H. Coord. Chem. Rev. 2002, 231, 207– 228. (4) Lin, W.; Wilson, S. R.; Girolami, G. S. Inorg. Chem. 1994, 33, 2265–2272. (5) Dupont, J.; Pfeffer, M.; Rotteveel, M. A.; De Cian, A.; Fischer, J. Organometallics 1989, 8, 1116–1118. pubs.acs.org/Organometallics

Published on Web 12/10/2009

important potential applications for organic and organometallic catalysis.1-6 Generally, phosphines are directly coordinated to palladium(I) and palladium(II) centers to modulate and stabilize the chemistry and inhibit reduction to palladium metal. To the best of our knowledge, there are few studies of the much smaller group of binuclear PdII-PdII complexes and other PdII-PdII combinations linked with different, nonphosphine, ligands.7 The past decade has seen a surge of interest in carbon-bound aminocarbene complexes, especially those of the N-heterocyclic carbene (NHC) ligands, and stable monomeric palladium complexes (6) Kannan, S.; James, A. J.; Sharp, P. R. J. Am. Chem. Soc. 1998, 120, 215–216. (7) Christmann, U.; Pantazis, D. A.; Benet-Buchholz, J.; McGrady, J. E.; Maseras, F.; Vilar, R. J. Am. Chem. Soc. 2006, 128, 6376–6390. r 2009 American Chemical Society

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of this ligand system have found well-developed applications in synthesis and catalysis.8 Only one carbon (and phosphorus) bound phosphinocarbene palladium complex is known.8d Several years ago, we9,10 and others11 reported the synthesis and characterization of the structurally unusual dimeric organolithium complex [Li2C(Ph2PdNSiMe3)2]2 ([Li2-1]2), which was easily obtained by double deprotonation of the methylene C atom of the parent ligand, CH2(Ph2PdNSiMe3)2 (H2-1),12,13 using alkyl- or aryllithium reagents. Subsequently we employed both [Li2-1]2 and H2-1 to prepare a number of novel “pincer” carbene complexes of group 4, group 9 and 10 transition metals,14-21 and lanthanides.22 These new mononuclear pincer or chelate20 complexes were prepared by direct deprotonation of the methylene carbon of the ligand either by reaction of H2-1 with special metal precursors (such as metal alkyls or metal silyl amides) or by metathetical reactions between [Li2-1]2, and metal chloride precursors. We also prepared homogeneous, bimetallic, bridging carbene complexes of Cr23 and Al.24 More recently we prepared a group of heterogeneous and homogeneous bridged bimetallic complexes containing Rh and/or Pd.21 Our results show that this versatile ligand can adopt pincer, bidentate chelate, or bridged structures to offer a variety of bonding modes with different metal centers (Scheme 1). The pincer complexes of 1 are formed by the binding of the methanediide carbon to the metal with a carbenelike M-C bond of order higher than 1. The pincer structure is then formed by chelation of the two N atoms. The related chelates20 form a similar higher order M-C carbene bond, (8) (a) Wang, C.-Y.; Liu, Y.-H.; Peng, S.-M.; Chen, J.-T.; Liu, S.-T. J. Organomet. Chem. 2007, 692, 3976–3983. (b) Loch, J. A.; Albrecht, M.; Peris, E.; Mata, J.; Faller, J. W.; Crabtree, R. H. Organometallics 2002, 21, 700–706. (c) Bertrand, G.; Diez-Barra, E.; Fernandez-Baeza, J.; Gornitzka, H.; Moreno, A.; Otero, A.; Rodriguez-Curiel, R. I.; Tejeda, J. Eur. J. Inorg. Chem. 1999, 1965–1971. (d) Teuma, E.; Lyon-Saunier, C.; Gornitzka, H.; Mignani, G.; Bacieredo, A.; Bertrand, G. J. Organomet. Chem. 2005, 690, 5541–5545. (9) Kasani, A.; Babu, R. P. K.; McDonald, R.; Cavell, R. G. Angew. Chem., Int. Ed. 1999, 38, 1483–1484. (10) Klobukowski, M.; Decker, S. A.; Lovallo, C. C.; Cavell, R. G. J. Mol. Struct. (THEOCHEM) 2001, 536, 189–194. (11) Ong, C. M.; Stephan, D. W. J. Am. Chem. Soc. 1999, 121, 2939– 2940. (12) Appel, V. R.; Ruppert, I. Z. Anorg. Allg. Chem. 1974, 406, 131– 144. (13) M€ uller, A.; Neum€ uller, B.; Dehnicke, K. Chem. Ber. 1996, 129, 253–257. (14) Babu, R. P. K.; McDonald, R.; Decker, S. A.; Klobukowski, M.; Cavell, R. G. Organometallics 1999, 18, 4226–4229. (15) Babu, R. P. K.; McDonald, R.; Cavell, R. G. Chem. Commun. 2000, 481–482. (16) Babu, R. P. K.; McDonald, R.; Cavell, R. G. Organometallics 2000, 19, 3462–3465. (17) Cavell, R. G.; Babu, R. P. K.; Kasani, A.; McDonald, R. J. Am. Chem. Soc. 1999, 121, 5805–5806. (18) Aparna, K.; Babu, R. P. K.; McDonald, R.; Cavell, R. G. Angew. Chem., Int. Edit. 2001, 40, 4400–4402. (19) Fang, M.; Jones, N. D.; Lukowski, R.; Tjathas, J.; Ferguson, M. D.; Cavell, R. G. Angew. Chem., Int. Ed. 2006, 45, 3097–3101. (20) Jones, N. D.; Lin, G.; Gossage, R. A.; McDonald, R.; Cavell, R. G. Organometallics 2003, 22, 2832-2841; Organometallics 2003, 22, 5378 (erratum). (21) Fang, M.; Jones, N. D.; Friesen, K.; Lin, G.-Y.; Ferguson, M. J.; McDonald, R.; Lukowski, R.; Cavell, R. G. Organometallics 2009, 28, 1652–1665. (22) Aparna, K.; Ferguson, M.; Cavell, R. G. J. Am. Chem. Soc. 2000, 122, 726–727. (23) Kasani, A.; McDonald, R.; Cavell, R. G. Chem. Commun. 1999, 1993–1994. (24) Aparna, K.; McDonald, R.; Ferguson, M.; Cavell, R. G. Organometallics 1999, 18, 4241–4243.

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Scheme 1. Pincer, Chelate, and Spirocyclic Complexes of the Bis(iminophosphorano)methanediide Ligand

but only one of the N atoms is bound to the metal. While the exact nature of the metal-carbon bond has been somewhat controversial, the higher bond order of the M-C link is supported by the fact that reagents (EH or heteroallenes) can be added across the metal-carbene bond to produce a methine center while maintaining a (slightly longer) M-C link. Our recent Rh/Pd21 spirocyclic bimetallic systems relate to work by others7,25 which showed unique catalytic properties of the system (as did our spirocyclic bimetallic dialuminum complex24,26), and so we were prompted to explore the synthesis of binuclear PdII-PdII complexes of this ligand system to investigate possible catalytic utility. Herein, we present the synthesis and solid-state structure of a bimetallic PdII-PdII complex formed from [Li2-1]2 and an organometallic PdII halide precursor and describe its behavior in solution.

Results and Discussion Synthesis and Normal Temperature Spectroscopy. When 1 equiv of [Pd(allyl)Cl]2 was mixed with 1/2 equiv of [Li2-1]2 for 10 min in benzene, the 31P{1H} NMR spectrum of the resultant solution showed a double doublet of resonances at 34.3 (intensity 1) and 43.3 ppm (intensity 1) with a 66.7 Hz 2 JP-P spin-spin coupling constant (verified by comparing two 31P NMR spectra at different fields), a single resonance at 37.4 ppm (intensity 1.73), and a second single resonance at 40.2 ppm (intensity 0.45). We attribute this set of signals (Figure 1a, intensity ratios of 2/1.73/0.45) to three chemically shifted isomers (2I1, 2I2, and 2I3) of the binuclear complex [Pd2(allyl)2-1] (Scheme 2) which arise from the different relative orientations of the allyl groups attached to the two Pd centers. The 1H NMR spectra of the trimethylsilyl and allyl groups (Figure 1b,c) also support this assignment. The synthetic reaction was very clean and essentially quantitative according to the (normal temperature, 30 °C) 31 P NMR spectrum, which also indicated a solution yield close to 100%. Previously, in analogous studies, a 1:1 reaction ratio of [Li2-1]2 with a Rh precursor19,21 gave the intermediate Li-Rh spirocyclic complex; therefore, we expected a restricted reaction ratio to proceed similarly to give the monopalladium-lithium [Pd(allyl)Li-1] complex. However, this mixed Pd-Li complex did not form-only the dimetalated Pd2 complex (2) was obtained (with the exact same solution 31P NMR spectrum as obtained above from the 2:1 Pd metal to ligand ([Li2-1]2) molar reaction ratio) along with a single 31P NMR signal at 14.26 ppm which corresponded to unreacted ligand [Li2-1]2 remaining in the solution (Scheme 2, bottom). For the 31P NMR spectrum, see the Supporting Information). Integrated intensities of all (25) Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. 2005, 44 366–374. (26) Cavell, R. G.; Aparna, K.; Babu, R. P. K.; Wang, Q. J. Mol. Catal. A 2002, 189, 137–143.

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Figure 1. Normal-temperature (30 °C) phosphorus-31 and partial 1H NMR spectra of the binuclear palladium complex 2: (a) 31 P spectrum; (b) 1H spectrum of methylsilyl groups; (c) 1H spectrum of central allyl group protons. The label numbers represent the signals assigned to each isomer: (1) 2I1; (2) 2I2; (3) 2I3. Scheme 2. Synthesis Reactions

the isomer complex signals were equal to the intensity of the remaining unreacted ligand signal, clearly demonstrating that only half of the ligand [Li2-1]2 had been consumed in the 1:1 reaction. This binuclear palladium complex [Pd2(allyl)2-1] (2) is comprised of a set of three isomers: one asymmetric isomer (2I1) which provides the doublet phosphorus NMR signal, and two symmetric forms, 2I2 and 2I3. As we show below (supported by X-ray crystallography results), these isomeric geometries most probably result from the differences in the relative mutual orientations of the coordinated allyl groups (Scheme 3) which make the two phosphorus (PCP) environments magnetically distinct in one isomer (2I1) where there is no symmetry relationship between the phosphorus centers and so develops the 2JP-P spin-spin coupling. The other two isomers, 2I2 and 2I3, possess a pseudo 2-fold axis relating the two phosphorus (PCP) centers, rendering them magnetically equivalent; thus, each of these two isomers shows only a single phosphorus resonance, each with a slightly different chemical shift. The crystal structure also shows that the central carbon has a distorted-tetrahedral environment; the allyl groups are oriented to the same side of the molecule and the Pd-N-P-C rings are not planar but rather are folded about the Pd-P axis. Similarly, three isomeric species appear in the roomtemperature 1H NMR in the same relative proportions. The methylsilyl group signals of 2 showed a pair of singlets at 0.11 and 0.06 ppm with equal unit intensity and two single peaks at 0.10 and 0.08 ppm with intensities 1.73 and 0.45,

Ma et al.

respectively. The four signals with 1:1:1.73:0.45 intensity ratios (Figure 1b) can be assigned to the three isomers 2I1, 2I2, and 2I3 described above. In addition, the -CH- proton signal of the palladium-coordinated allyl group (CH2CHCH2) showed a pair of seven peak groups at 5.02 and 4.59 ppm with equal unit intensity and another two sets of seven peaks at 4.87 and 4.63 ppm with intensities of 1.73 and 0.45, respectively (Figure 1c). All room-temperature 1H NMR signals thus correspond to the three isomers of 2 (2I1, 2I2, and 2I3). Isomers of 2 are observed in the crystal and molecular structures, in support of the assignment, and the interrelationship between the two systems is discussed below. Finally, the normal-temperature 13C{1H,31P} NMR spectrum showed the same pattern displayed by the 31P and 1H spectra; the four carbon signals due to Si(CH3)3, CH2CHCH2, CH2CHCH2, and CH2CHCH2 carbon atoms of the three isomers had intensity ratios close to 1:1:1.73:0.45 (Supporting Information), as observed in 31P and 1H spectra. The carbon signal of the (PCP) carbene appeared as a triplet with an intensity ratio of 1:2:1 in a high-field region (-6.5 to -10.5 ppm), showing a spin-spin coupling to the two phosphorus atoms (2JP-P = 90 Hz). In a 31P-decoupled spectrum, the central carbon resonance became a singlet. The correlation of the three-bond interaction between the C(carbene) and the allyl proton (CH2) was observed in a gHMBC 2D spectrum (Supporting Information). In comparison to our previously studied metal PCP complexes,20,27 the carbene 13C NMR chemical shift in 2 was unusual in that it appeared at a higher field, which indicates electron enrichment on the carbon center consistent with a dianionic carbon. Considering the fact that only the Pd(allyl) precursor gave a good pathway to the product, we suspect that the system represents a unique combination of Pd-C single bonding supported by stabilizing Pd-allyl back-donation. Variable-Temperature NMR Spectroscopy. Variable temperature 31P and 1H NMR studies (in toluene-d8) were undertaken to ascertain if isomer interconversion or conformational exchanges were occurring. The temperature was varied from -40 to þ115 °C. Proton signals visibly collapsed, but full spectral coalescence was not observed in the 31 P NMR spectrum. The 31P NMR signals of three isomers gradually approached each other, and the signal line widths became broad (Figure 2). At the highest temperature that could be reached (115 °C), the 31P NMR signals remained separated into three groups of broad signals and each group can still be associated with each isomer. It is, however, apparent that conformational exchange does occur between the three isomers on the NMR time scale. Within the variable-temperature range explored (from -40 to þ115 °C), the 31P NMR signal at 37.4 ppm shows an intensity increase of approximately 15.0% relative to the 31 P double doublet signals at 34.3 and 43.3 ppm. This suggests that the isomer species distribution changes over this temperature range. Returning the samples to 30 °C gave 31 P and 1H NMR spectra which were identical with those observed at the commencement of the VT experiment; thus, the isomer distribution equilibrium is reversible. The isomeric composition of the complex, consistent throughout all spectra, can be attributed to the differences in relative orientation of the two allyl groups which are fixed (27) Kubo, K.; Jones, N. D.; Ferguson, M. J.; McDonald, R.; Cavell, R. G. J. Am. Chem. Soc. 2005, 127, 5314–5315.

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Scheme 3. Representation of the Orientations of Three Isomers Observed in Solution As Viewed through the Spirocyclic Carbon Centera

a All hydrogen atoms have been removed, and only allyl, spiro, and phenyl ipso carbon atoms are shown. Relative allyl orientations are shown symbolically.

Scheme 4. Possible Variants in Bonding Modes of Allyl Groups to Metals28

Figure 2. Variable-temperature (in °C) 31P NMR spectra of the bimetallic complex 2 in toluene-d8. The identification labels of the three isomer species are defined in the caption to Figure 1.

in three arrangements, as illustrated in Scheme 3. The overall rigidity of the central core structure of the complex preserves these relative orientations. The possibility of isomers due to relative orientation of allyl groups was also supported by the crystal structure analysis (vide infra). The lesser spectral dispersion of the 1H NMR signals facilitates spectral coalescence of the allyl and silylmethyl proton signals in the accessible temperature range; thus, we did observe nearly final coalescence of the proton NMR spectra of the allyl group and virtually complete coalescence of the SiMe3 group signal (see Figures S2 and S3 in the Supporting Information). The 1H NMR of the central proton of the allyl groups demonstrated behavior similar to that shown by the 31P NMR spectrum between 30 and 80 °C. When the temperature was increased to 100 °C, however, the signals collapsed to two separated broad peaks which eventually overlapped at 115 °C to provide a large complex asymmetric peak (which is not, however, fully coalesced into a fast exchange single peak; Figure S4 in the Supporting Information). Thus, the allyl groups exhibit fluxional behavior which is typical of allyl complexes in general; however, the detailed mechanism of the process is not discernible without some extensive spin transfer experiments.

The methylsilyl protons, which span a very small chemical shift range, appeared to coalesce at a lower temperature. At -40 °C, the methylsilyl protons showed clearly four separated signals, as was the case for the room-temperature spectrum. Increasing the temperature resulted in a progressive signal collapse so that by 80 °C, the right and left pair of signals due to the major isomer had merged to become a single signal. At higher temperatures, these two signals arising from the two minor isomers also merged to become one signal by 80 °C and these two signals then merged to become one by 100 °C (see Figure S3 in the Supporting Information). These silyl groups are on the outer regions of the complex, and this, coupled with the very small spectral dispersion, means that no meaningful deductions about the allyl processes are likely to be extracted from the silyl 1H NMR variable-temperature spectra. It is interesting that, upon cooling to -40 °C, an additional group of three new allyl proton NMR signals appears between 4.95 and 5.07 ppm (Figure S3 in the Supporting Information) and, in addition, a pair of new signals from the CH2 portion of the allyl group appeared to high field (3.90 dd, 3.68 dd, 1.56 t, and 1.54 t ppm). We suggest that these additional signals arise from further reduced fluxionality within the allyl groups, possibly trapping a distinct allyl bonding structure such as the η3 (I), σ, π, (II), or η1 (III) bonding configurations (Scheme 4) discussed by others28 in each of the three major isomers. The room-temperature allyl proton spectra of each of the three isomers can be described (28) (a) Mann, B. E.; Pietropaolo, R.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1973, 2390–2393. (b) Becconsall, J. K.; Job, B. E.; O'Brien, S. J. Chem. Soc. A 1967, 423–430. (c) McClellan, W. R.; Hoehn, H. H.; Cripps, H. N.; Muetterties, E. L.; Howk, B. W. J. Am. Chem. Soc. 1961, 83, 1601– 1606. (d) Dehm, H. C.; Chien, J. C. W. J. Am. Chem. Soc. 1960, 82, 4429– 4430. (e) Trost, B. M.; Vranken, D. L. V. Chem. Rev. 1996, 96, 395–422.

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Ma et al. Table 1. Crystal Data and Structure Refinement Details for Complex 2 empirical formula formula wt cryst syst cryst dimens (mm) space group unit cell dimens a (A˚) b (A˚) c (A˚) V (A˚3) Z calcd density (g cm-3) temp, K μ(Mo KR) (mm-1) θ range for data collection (deg) index ranges

Figure 3. Perspective view of the major isomer of [{η3-C3H5)Pd}2{μ,κ2:κ2-C(PPh2NSiMe3)2}] (equivalent to 2I2) showing the atom-labeling scheme. Only the ipso carbons of the phenyl groups are shown. Isomers (labeled A and B) differing in the relative orientation of one allyl group were observed in crystals and are indicative of (but are not necessarily those of) the solution isomers 2I2 and 2I1, respectively. The inset shows the allyl arrangement in the alternate (nonsymmetric) isomer 2I1 (B). Non-hydrogen atoms are represented by Gaussian ellipsoids at the 20% probability level. Allyl group hydrogen atoms are shown with arbitrarily small thermal parameters; all other hydrogen atoms are omitted.

as AM2X2 spectra; however, one of the new species showed an A2M2X spectrum, with 14 lines for the center proton and 2 triplets for the two terminal protons, as described elsewhere in detail.28 Reactivity and Stability. As described, the reaction of [Li21]2 with the precursor [Pd(allyl)Cl]2 in either C6H6 or C7H8 worked very well; however, other Pd precursors, such as [Pd(cod)Cl2] (the cod analogue worked well in the case of Rh21), [Pd(CH3)(cod)Cl], [Pd(CH3CN)2Cl2], and [Pd(DMSO)2Cl2] in C6H6 or C7H8 did not produce successful results. In these latter cases the reaction solution quickly became black upon mixing the reagents; Pd(II) was reduced to palladium metal, and no Pd product containing the ligand system was observed. In contrast, the successful synthesis from [Pd(allyl)Cl]2 showed no evidence of Pd metal formation. In a separate test, heating the preparation reaction to 115 °C also showed no evidence of decomposition. The allyl substituent thus provides a critical stabilization of the Pd in this system. The reactivity of 2 with CO was examined (in view of the behavior of the related system21), and we have found that 2 is completely inert when exposed to 1 atm of CO for 24 h. No noticeable color changes occurred, and spectral 31P and 1H NMR analysis showed that no new species were formed. That the allyl substituents are not displaced by CO (vs the situation whereas a strongly bonded Pt-cod substituent was replaced by CO20) indicates that the Pd-allyl bonding is highly stabilized. Similarly, the allyl substituents are not replaced by CO in the bimetallic Pd(allyl)/Rh(cod) complex described elsewhere, whereas the cod on Rh is readily replaced by CO in this mixed-metal complex.21 Crystal and Molecular Structure. (a). General Features. The molecular structure of 2 (Figure 3) deduced from X-ray crystallography (details in Table 1) confirms that this

no. of rflns collected no. of indep rflns no.of data/restraints/params goodness of fit on F2 (all data) R1 (I > 2σ(I)) wR2 (all data) large diff peak and hole (e/A˚3)

C37H48N2P2Pd2Si2 851.69 orthorhombic 0.41  0.24  0.08 Pbca (No. 61) 20.0946(14) 18.6314(13) 20.8458(15) 7804.5(10) 8 1.450 193(2) 1.092 2.80-26.32 -25 e h e 25 -23 e k e 23 -26 e l e 26 58804 7990 7990/3/433 1.068 0.0302 0.0794 0.904 and -0.482

Table 2. Selected Bond Lengths (A˚) and Angles (deg) for Complex 2 Pd(1)-Pd(2) Pd(1)-N(1) Pd(1)-C(1) Pd(1)-C(2) Pd(1)-C(3) Pd(1)-C(10) Pd(2)-N(2) Pd(2)-C(4A) Pd(2)-C(5A) Pd(2)-C(6A) Pd(2)-C(4B) Pd(2)-C(5B) Pd(2)-C(6B) Pd(2)-C(10) P(1)-N(1) P(1)-C(10) P(2)-N(2) P(2)-C(10) Si(1)-N(1) Si(2)-N(2)

a

3.0525(4)a 2.137(2) 2.106(4) 2.094(4) 2.182(4) 2.134(3) 2.146(2) 2.104(18) 2.107(5) 2.182(17) 2.12(4) 2.110(10) 2.16(4) 2.141(3) 1.602(2) 1.732(3) 1.589(2) 1.725(3) 1.697(3) 1.707(2)

N(1)-Pd(1)-C(1) N(1)-Pd(1)-C(2) N(1)-Pd(1)-C(3) N(1)-Pd(1)-C(10) C(1)-Pd(1)-C(2) C(2)-Pd(1)-C(3) C(1)-Pd(1)-C(10) C(2)-Pd(1)-C(3) C(2)-Pd(1)-C(10) C(3)-Pd(1)-C(10) N(2)-Pd(2)-C(4A) N(2)-Pd(2)-C(5A) N(2)-Pd(2)-C(6A) N(2)-Pd(2)-C(4B) N(2)-Pd(2)-C(5B) N(2)-Pd(2)-C(6B) N(2)-Pd(2)-C(10) Pd(1)-C(10)-Pd(2) P(1)-C(10)-P(2) Pd(1)-N(1)-Si(1) Pd(2)-N(2)-Si(2) N(2)-P(2)-C(10) N(1)-P(1)-C(10)

173.84(14) 141.78(16) 108.89(13) 74.57(10) 38.34(16) 36.97(17) 107.19(13) 36.97(17) 141.82(17) 172.57(14) 175.2(4) 141.9(2) 110.7(3) 173.8(7) 144.6(4) 112.6(6) 73.70(10) 91.11(10) 133.04(16) 123.84(13) 126.94(13) 101.66(13) 101.78(13)

Nonbonded distance.

complex is indeed a binuclear palladium compound. Selected relevant bond lengths and angles are given in Table 2. The atomic bond angles surrounding each Pd metal, such as C(10)-Pd(1)-N(1) = 74.57°, N(1)-Pd(1)-C(3) = 108.88°, N(1)-Pd(1)-C(1)=173.85°, and C(10)-Pd(2)-N(2)=73.7°, C(10)-Pd(2)-C(4A)=106.78°, N(2)-Pd(2)-C(4A)=172.2°, show that the coordination environment around each Pd atom is a slightly distorted square plane. Two N atoms of the ligand 1 are involved in two four-membered chelate rings, and the ligand 1 adopts a κC,κN coordination mode to each of the two Pd atoms. The carbene C atom in 2 is a pseudo tetrahedral spirocyclic carbon center as expected; overall the structure is very similar to those presented in the Rh/Pd system.21 The two four-membered rings subtended from C(10) are not planar; both are folded about the P-Pd axis with one, C(10)-P(2)-N(2)-Pd(2), being more bent (23°) than the other (C(10)-P(1)-N(1)-Pd(1); 11°). Both Pd-allyl groups project to the same side of C(10).

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Complexes that contain chelating C, N ligands with a carbenic C are relatively rare; other examples are the Ruaminocarbene complexes [RuCp(L)(κ2N,N-py-NdC(H)Ph)][PF6] (L=CH3CN, PMe3, PPh3),29 the tris(pyrazolyl)borate ruthenium carbenes,30 and our recently reported bimetallic Rh/Pd complexes.21 (b). Allyl Substituents: Relationship between Solution and Solid Structures. As suggested above, the mutual spatial relationships between the two allyl groups is responsible for the appearance of isomers of 2 in both solid and solution states. The Pd(1)-bound allyl shows only one orientation in all molecules in the crystal structure, whereas the Pd(2)bound allyl group displays two conformational orientations in the crystal. The atoms labeled C(4A), C(5A), and C(6A) in the molecular structure (Figure 3) on Pd(2) is the dominant conformation representing about 65% of the molecules in the crystal. In this dominant conformation the phosphorus atoms are related by a pseudo C2 axis, making them magnetically equivalent. This conformation thus corresponds to the two solution isomers with singlet 31P NMR signals, 2I2 and 2I3 (Scheme 3). In the case of the alternate allyl orientation found in the crystal structure (Pd(1): C(1), C(2), and C(3); Pd(2): C(4B), C(5B), and C(6B)), the “B” isomer, which has both allyl groups oriented in the same direction (not “mirrored”) and on the same side of the molecule, has no symmetry operation relating the phosphorus atoms within the molecule. This asymmetric isomer, B (which represents about 35% of the molecules in the crystal), corresponds to the solution isomer 2I1 with inequivalent 31P NMR (doublet of doublets) signals. The solution and solid-state conformational pictures are in general accord with each other, but it appears that the isomeric distribution in each state differs in the relative proportions of isomers. Only the symmetric form of A represented as 2I2 (Scheme 3) appears to crystallize, possibly because the related form 2I3 (where we could posit that the allyl central CH units point “toward” each other instead of “away” (Scheme 3)) may develop some van der Waals interference between the H atoms which would inhibit crystallization. Although the solution and solid-state conformational pictures are in good general accord with each other, the difference in isomeric distributions between solution and solid suggests that a conformational change may accompany crystallization. Since all three forms appear to be in equilibrium with very small energy differences between the forms, this does not appear to be unreasonable. The observation of two allyl isomers in the solid structure does reinforce the solution behavior given herein. It is also notable that redissolution of the crystalline material gave solution NMR spectra with exactly the same isomeric distribution as observed in the original product; hence, the solution is in equilibrium. (c). Other Structural Features. The η3-allyl ligand is asymmetrically bound in 2, showing considerable differences in Pd-C(olefin) bond lengths. The bonds trans to the N-donor atom (C(1) and C(4A), Pd(1)-C(1) = 2.106(4), Pd(2)-C(4A) = 2.104(18) A˚) are shorter than those trans to the carbene C atom (C(3) and C(6A), Pd(1)-C(3) = 2.182(4), Pd(2)-C(6A)=2.182(17) A˚), suggesting a higher trans influence for the latter donor atom. This phenomenon was also (29) Standfest-Hauser, C. M.; Mereiter, K.; Schmid, R.; Kirchner, K. Organometallics 2002, 21, 4891–4893. (30) Ruba, E.; Hummel, A.; Mereiter, K.; Schmid, R.; Kirchner, K. Organometallics 2002, 21, 4955–4959.

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observed in our previously synthesized Rh/Pd heterobimetallic complex.21 Moreover, the average Pd-C(allyl) bond lengths in [{η3-C3H5)Pd}2{μ,κ2:κ2-C(Ph2PdNSiMe3)2}] (2; 2.129 A˚) are slightly shorter than those found in the Pd(II)-allyl-NCN (carbene) complex [{η3-C3H5)Pd}Cl(SIbiphen)] (2.149(5) A˚).31 The additional N-donor trans influence reinforces the allyl bonding interaction with palladium(II) in 2. This asymmetry may contribute to the orientational differences observed in the isomers. The Pd(1)-Pd(2) separation in 2 is 3.0525(4) A˚, which is consistent with no palladium metal-metal bonding interaction, whereas, in comparison, the metal separation distance in the recently discovered palladium(I) dimer Pd2Br2(P(t-Bu2)Ph)27 (Pd-Pd = 2.5706(4) A˚) is consistent with metal-metal bonding. The two Pd-C(carbene) distances in 2 are 2.134(3) A˚ (Pd(1)-C(10)) and 2.141(3) A˚ (Pd(2)-C(10)) (average 2.138(3) A˚), which are longer than those found in the palladium(II) N-heterocyclic carbene (NCN) complex (2.022(4) A˚),31 in the Pd(II) phosphinocarbene complex [(iPr2N)(P(t-Bu)2)PdCl2] (2.019(7) A˚), and in the palladium(II) complex trans-[PdCl2(C.cxa.fc.cxa.C)] (C.cxa.fc.cxa.C = 1,10 -di-tert-butyl-3,30 -(1,10 -dimethyleneferrocenyl)diimidazol-2ylidene) (2.058(3) A˚).32 This Pd-C(carbene) distance in 2 is also much longer than the monometallic Pt-C(carbene) distance (2.021(5) A˚20) observed in the chelated Pt complex, which we considered to be essentially a PtdC double bond. The longer distance in this case is consistent with the bridging nature of the carbene in Pd(II) complex 2. Both of the P-C(carbene) lengths in 2 are also appreciably shorter than that found for the parent P-C(carbene) in H2133 (P(1)-C(10) = 1.732(3), P(2)-C(10) = 1.725(3) A˚ vs 1.827(1) A˚), while the P-N distances are considerably longer (P(1)-N(1) = 1.602(3), P(2)-N(2) = 1.589(2) A˚ vs 1.539(2) A˚). In addition, the Pd-N bonds (Pd(1)-N(1) = 2.137(2) and Pd(2)-N(2) = 2.146(2) A˚) in 2 are longer than those of [PdCl2(Me3SiNdPEt3)2] (2.095 A˚)34 and [PdCl2PCH2oxMe2)] (2.058(2) A˚)35 but similar to those of related Pt complexes [PtCl(PEt3){CH(Ph2PdN-p-tolyl)(Ph2PNH-ptolyl)-κC,κN}][PtCl3(PEt3)] (2.124(8) A˚)36 and [PtCl(PPhMe2){CH(Ph2PdN-p-tolyl)2-κC,κN}] (2.132(4) A˚).37 The electronically delocalized effect inside the four-membered endocyclic ring introduces a structural bond length difference between H2-1 and bound 1. This observation is similar that in our previously studied complexes and is consistent with our suggestion of π-electron delocalization as discussed previously.14-21 Electronic Structure Calculations. Molecular geometry, NMR tensors, and bonding structure of the model compound [{η3-C 3 H5)Pd}2 {μ,κ 2,κ 2 -C(Me 2 PdNSiH3)2 }] (2a; Scheme 5), adapted from the conformation 2aI1 or 2aI2 observed in the solid state, were calculated at the DFT level with (31) Christophe, F.; Aline, M. F.; Stephane, B. L. Inorg. Chim. Acta 2007, 360, 143–148. (32) Coleman, K. S.; Turberville, S.; Pascu, S. I.; Green, M. L. H. J. Organomet. Chem. 2005, 690, 653–658. (33) M€ uller, A.; M€ ohlen, M.; Neum€ uller, B.; Faza, N.; Massa, W.; Dehnicke, K. Z. Anorg. Allg. Chem. 1999, 625, 1748–1751. (34) Miekisch, T.; Mai, H. J.; Meyer, R.; Kocker, R. M. Z.; Dehnicke, K.; Magull, J.; Goesmann, H. Z. Anorg. Allg. Chem 1996, 622, 583–588. (35) Braunstein, P.; Fryzuk, M. D.; LeDall, M.; Naud, F.; Rettig, S. J.; Speiser, F. J. Chem. Soc., Dalton Trans. 2000, 1067–1074. (36) Avis, M. W.; Vrieze, K.; Ernsting, J. M.; Elsevier, C. J.; Veldman, N.; Spek, A. L.; Katti, K. V.; Barnes, C. L. Inorg. Chem. 1995, 34, 4092–4105. (37) Avis, M. W.; Vrieze, K.; Ernsting, J. M.; Elsevier, C. J. Organometallics 1996, 15, 2376–2392.

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Scheme 5. Model Compound for DFT Calculationsa

a 2aI1 and 2aI2 correspond to the orientations in the crystal structure labeled as B (no symmetry) and A (pseudo C2 symmetry).

Gaussian 03.38a Geometry optimization showed that the two conformations labeled 2aI1 and 2aI2 possess nearly identical minimal energies, indicating they are both stable structures with the (symmetric) orientation “A” (e.g., 2aI2) lying 0.212 kcal/mol lower in energy than the asymmetric “B” orientation (2aI1); thus, 2aI2 is the marginally more stable isomer. This is in agreement with the observed population distribution in the solid state and is also consistent with the appearance of three isomers in an equilibrium in solution. The calculated NMR shielding tensors of 2a (Scheme 5) demonstrated that the isomer with allyl labeled as “A” (2aI2) has identical NMR shielding constants for the two P nuclei, but these constants were not equal in the isomer with the allyl in the “B” (2aI1) orientation, in agreement with the measured 31P NMR solution spectrum of the mixture of isomers. DFT bonding analysis revealed detailed multiple-bonding interactions between the two Pd metals, the carbene center, and the two allyl groups (calculation framework structure is shown in Figure 4). The molecular orbital coefficients indicate that all of the s, px, py, and pz atomic orbitals of the bridging carbon participate in bonding with the d orbitals of the two palladium metals. For example, MO-96 (E = -0.226 au) links carbon s with Pd(1) and Pd(2) dxz, dyz, and dx2-y2 orbitals. MO-100 (HOMO-1, E = -0.207 au), links carbon s and py orbitals with Pd(1) dz2, dxz, dyz, and dx2-y2 and Pd(2) dyz and dx2-y2 orbitals. The HOMO (MO-101, E = -0.184 au) is made up of carbon px and pz, Pd(1) and (38) (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery , J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Menucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Rega, N.; Salvador, P.; Dannenberg, J. J.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian03, Revision C02; Gaussian, Inc., Pittsburgh, PA, 2004. (b) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO version 3.1 included in the Gaussian Suite of Programs.

Figure 4. Calculation framework and selected molecular orbitals (MO) representing the bonding interactions between the carbene center, two palladium metals, and two allyls (from left to right): (top) framework showing the crystal and calculation atom labeling scheme, LUMO (MO-102), HOMO (MO-101); (middle) MO-100, MO-96, MO-94, and MO-88; (bottom) MO81, MO-76, and MO-75. The calculation results are consistent with the experimental NMR measurements and the crystal structure determination.

Pd(2) dxz, dyz, dx2-y2, and dxy, MO-94 (HOMO-7, E = -0.247 au) shows framework bonding of carbon pz with Pd(1) dz2 and dx2-y2 and Pd(2) dxz and dx2-y2, MO-88 (HOMO-13, E = -0.285 au) also connects the carbon py to Pd(1) dx2-y2 and Pd(2) dz2 and dx2-y2, MO-86 (E = -0.318 au) shows carbon py connecting with Pd(1) and Pd(2) dxz and dxy and MO-82 (E = -0.332 au) connecting carbon py with Pd(1) and Pd(2) dxz, dyz and MO-81 (E = -0.333 au) connecting carbon px with Pd(1) and Pd(2) dxz, dxy orbitals (Figure 4). Meanwhile conjugated orbital bonding chains were found within the molecular structure, for example, C(3)-Pd(1)-C(10)-Pd(2)-C(4) (MO-101, creating a py-dxz,yz,x2-y2,dxy-px,y -dxz,yz,x2-y2,dxy-pz orbital overlapping chain), C(3)-Pd(1)C(10)-Pd(2)-C(6) (MO-100, creating a pz-dz2,xz,x2-y2,yz -py-dyz,x2-y2-py,pz orbital chain), and N(1)-Pd(1)-C(10)-Pd(2)-N(2) (MO-81, creating a px-dxy,dxz-px-dxy,xz -px orbital chain) as well as the bonding interactions of the allyl groups and two palladium metal atoms presented in MO85 (E = -0.327 au), wherein the allyl atoms C(1) pz, C(2) py, C(3) pz connect with Pd(1) dxz, dyz, dxy and allyl carbons C(4) py, C(5) pz, and C(6) py connect with Pd(2) (i.e., Pd(2) dyz), MO-82 (E = -0.332 au) connecting allyl carbons C(1) pz, C(2) py, and C(3) pz with Pd(1) dxz, dyz and allyl carbons C(4) py, C(5) pz, C(6) py connect with Pd(2) dxz, dyz, MO-76 (E = -0.381 au), C(1) px, py, pz, C(2) px, pz, C(3) px, and Pd(1) dxy and MO-75 (E = -0.383 au), C(4) px, C(5) px, C(6) px, py, pz and Pd(2) dxz (Figure 4). All these bonding interactions together provide a strong core-bonding framework which accounts in part for the stability.

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Gaussian NBO analysis38b for the two isomer conformations of 2a show that the carbon bridges the two Pd metals (Pd-Ccarbene-Pd) with σ bonding and antibonding orbitals. On the basis of the NBO coefficients the bonding and antibonding orbitals can be described as Pd back-donation of two electrons to the Pd-Ccarbene bonding orbital (Pd(1), s0.11d0.89 f C s0.26p0.74; Pd(2), s0.15d0.85 f C s0.41p0.59) and carbene C(10) donating one electron to the Pd(1)-Ccarbene antibonding orbital (C s0.26p0.74 f Pd(1) s0.11d0.89 and C s0.41p0.59 f Pd(2) s0.15d0.85); thus, the overall net bond order is approximately 0.5 (Figure 5). Similarly, the Pd-C bonds to the allyl C3H5- group are formed by p orbitals of three carbons overlapping with Pd s and d hybrid orbitals to form Pd-Callyl bonds with a high electron occupancy in the bonding orbital compared to that in the antibonding orbital, thus accounting for the stability of the system. The d and s components participating in the Pd sd hybrid orbitals vary with the orientation directions of the allyl carbon atoms (for details see the Supporting Information). The NBO analysis at the second-order donor-acceptor interaction level confirmed the existence of electron delocalization interaction between the core-frame atoms,39 such as within the four four-membered rings, two rings subtended from the spiro carbon (C(10): C(10)-Pd(1)-N(1)-P(1), C(10)-Pd(2)-N(2)-P(2)) and two Pd-allyl rings (Pd(1)-C(1)-C(2)-C(3) and Pd(2)-C(4A)-C(5A)-C(6A)) (using the atom numbering scheme shown in the crystal structure) (Figure 3), which is in agreement with the crystal structure data discussed above. The electrons of Pd-allyl bonding orbitals were predicted to be delocalized over their antibonding orbitals and the surrounding Pd-N and Pd-carbon antibonding orbitals and a further Pd-carbon antibonding orbital delocalized on the bridging carbon Rydberg orbital. Thus, the orbital electron transfer from the allyl carbon atoms through Pd and finally to the bonding carbon atom has been demonstrated. This palladium metal back-donation of electrons to the carbene is similar to the behavior of some previously studied bis(phosphino)carbene-Pd(allyl) complexes,40 which however do not possess a direct carbon-palladium bond but are rather chelate diphosphorus complexes with a carbene unit connecting the two phosphorus arms. In that case a dianionic carbon is formed which exerts a π interaction with Pd. The C-Pd separations range from 2.9 to 3 A˚. In our present complex 2 an electron-rich bridging carbene is generated in the molecule which is consistent with the 13C NMR measurement of the bridging carbon chemical shifts.

59

Figure 5. NBO representation of the Pd(1)-C(10) (carbene) bonding (center) and antibonding orbitals (right). The crystal and calculation atom labeling schemes are identical at the left of the figure.

change at elevated temperatures with small changes in relative proportions of the isomers up to 115 °C. Returning the sample to room temperature produces the original distribution of isomers, suggestive of a chemical equilibrium which is consistent with the small energy differences calculated for two isomers. The 13C NMR chemical shift of the coordinated carbene C in 2 is found at a high field, indicating an electron-rich C(PCP) bridge. This is the first time we have seen such a highly shielded C atom in metal complexes of ligand 1. Gaussian 03 DFT B3LYP OPT calculations demonstrated that isomer 2aI1 (asymmetric) and 2aI2 (symmetric) are both stable structures very close in energy, with the latter being the most stable. Gaussian NBO analysis revealed that a Pd-Ccarbene bond is formed between Pd sd hybrid orbitals and the carbene C sp hybrid orbitals with a net bond order of approximately 0.5. The Pd-allyl antibonding orbital delocalization donates electrons to the Pd-N and Pd-carbene antibonding orbitals and finally transfers electrons to the carbene Rydberg orbital, thereby generating an electron-rich carbene center in the molecule.

Experimental Section

The dianionic ligand 1 formed the spirocyclic bimetallic dipalladium complex set 2, which contains a spirocyclic bridging carbene. The rigidity of the core structure and differences in allyl group relative orientations create three isomers in equilibrium in solution, and two forms are observed in the solid state. (The crystal structure could not differentiate between the two symmetric isomers but clearly revealed the asymmetric isomer). Variable-temperature NMR studies show that there is conformational ex-

General Considerations. All manipulations were performed either in an Ar-filled glovebox or under an Ar or N2 atmosphere using standard Schlenk techniques. Solvents were dried over appropriate drying agents and degassed by three freeze-pump-thaw cycles prior to use. The organolithium compound [Li2-1]2 was prepared according to our published procedures.9 The precursors used were made by literature methods41-43 or were purchased from either Strem or Aldrich. NMR spectra were normally recorded at ambient temperatures using C6D6 or toluene-d8 solutions of the complexes on a Varian i400 spectrometer (161.9 MHz for 31P, 100.6 MHz for 13C) and referenced to residual solvent proton (1H), solvent (13C), external 85% H3PO4 (31P). All J coupling values are given in Hz; abbreviations used are s = singlet, d = doublet, t = triplet, and m = multiplet. Elemental analyses were carried out at the Analytical and Instrumentation Laboratory, Department of Chemistry, University of Alberta. Synthesis of [{(η3-C3H5)Pd}2{μ,K2:K2-C(Ph2PdNSiMe3)2}] (2). To a benzene solution (10 mL) of [Li2-1]2 (0.172 g, 0.15 mmol) was added [Pd(allyl)Cl]2 (0.108 g, 0.30 mmol) as a solid in a single portion at room temperature. The resulting milky dark blue mixture was stirred at room temperature for a period of 2-3 h, over which time the mixture turned into a clear blue solution. The filtered benzene solution was evaporated on a

(39) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899–926. (40) Vignolle, J.; Gornitzka, H.; Maron, L.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. J. Am. Chem. Soc. 2007, 129, 978–985.

(41) Drew, D.; Doyle, J. R. Inorg. Synth. 1972, 13, 47–50. (42) Bailey, C. T.; Lisensky, G. C. J. Chem. Educ. 1985, 62, 896–897. (43) R€ ulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769–5778.

Summary and Conclusion

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dynamic vacuum line, and the remaining powder was dissolved in 5 mL of ether. The undissolved solid LiCl was removed by centrifugation. The blue solution was evaporated under vacuum to yield a fine yellow-green powder of 2 (0.241 g, yield 94%). This powder was dissolved in ether with addition of a small amount of benzene, and the mixed solution was transferred to a capped bottle and stored inside a -20 °C freezer overnight. Yellow-blue crystals of 2 formed. In the solid state, 2 is stable to air for short periods of time. Anal. Calcd for C37H48N2P2Pd2Si2: C, 52.1; H, 5.68; N, 3.29. Found: C, 51.8; H, 5.68; N, 3.27. 1H NMR: for 2I1 δ 0.056 (s, 9H, Si(CH3)3), 0.107 (s, 9H, Si(CH3)3), 2.625 (d, -CH2 (allyl), 3JH-H = 12.5), 2.66 (d, -CH2 (allyl), 3 JH-H = 12), 3.769 (dd, -CH2 (allyl), 3JH-H = 7, 2JH-H = 2.5), 3.847 (dd, -CH2 (allyl), 3JH-H = 7, 2JH-H = 2), 4.587 (7 peaks, 1H, -CH- (allyl), 3JH-H = 7), 5.018 (7 peaks, 1H, -CH- (allyl), 3JH-H = 7); for 2I2 δ 0.102 (s, 18H, Si(CH3)3), 2.551 (d, -CH2 (allyl), 3JH-H = 12.5), 3.147 (d, -CH2 (allyl), 3 JH-H = 7), 4.865 (7 peaks, 2H, -CH- (allyl), 3JH-H = 6.5); for 2I3 δ 0.075 (s, 18H, Si(CH3)3), 2.694 (d, -CH2 (allyl), 3 JH-H = 13), 3.692 (d, -CH2 (allyl), 3JH-H = 7), 4.625 (7 peaks, 2H, -CH- (allyl), 3JH-H = 7); 6.791-6.87 (m, Ph H), 7.134-7.351 (m, Ph H), 7.438-7.477 (m, Ph H), 7.616-7.657 (m, Ph H), 7.919-7.959 (m, Ph H). 13C{1H} NMR: for 2I1 δ -9.30 (t, PCP, 1JP-C = 90), 4.11 (s, Si(CH3)3), 4.31 (s, Si(CH3)3), 47.63 (s, -CH2 (allyl)), 49.48 (s, -CH2 (allyl)), 58.39 (s, -CH2 (allyl)), 59.41 (s, -CH2 (allyl)), 104. 73 (s, -CH- (allyl)), 107. 57 (s, -CH- (allyl)); for 2I2 δ -8.46 (t, PCP, 1JP-C = 90), 4.26 (s, Si(CH3)3), 49.39 (s, -CH2 (allyl)), 58.20 (s, -CH2 (allyl)), 104. 93 (s, -CH- (allyl)); for 2I3 δ -7.23 (t, PCP, 1JP-C = 90), 4.13 (s, Si(CH3)3), 47.69 (s, -CH2 (allyl)), 59.06 (s, -CH2 (allyl)), 107.05 (s, -CH- (allyl)), 127.09-127.31 (5 peaks, Ph C), 127.48-127.81 (5 peaks, Ph C), 129.83-130.19 (3 peaks plus 5 shoulders, Ph C), 131.60-132.56 (16 peaks, Ph C). 13C{1H,31P} NMR: 2I1, δ -9.28 (s, PCP), 4.12 (s, Si(CH3)3), 4.31 (s, Si(CH3)3), 47.63 (s, -CH2 (allyl)), 49.49 (s, -CH2 (allyl)), 58.40 (s, -CH2 (allyl)), 59.42 (s, -CH2 (allyl)), 104. 74 (s, -CH(allyl)), 107.58 (s, -CH- (allyl)); for 2I2 δ -8.43 (s, PCP), 4.27 (s, Si(CH3)3), 49.40 (s, -CH2 (allyl)), 58.21 (s, -CH2 (allyl)), 104. 94 (s, -CH- (allyl)); for 2I3 δ -7.25 (s, PCP), 4.25 (s, Si(CH3)3), 47.70 (s, -CH2 (allyl)), 59.06 (s, -CH2 (allyl)), 107.06 (s, -CH- (allyl)), 127.15-127.27 (3 peaks, Ph C), 127.53-127.77 (3 peaks, Ph C), 129.85-130.21 (7 peaks, Ph C), 131.65-132.53 (8 peaks, Ph C). 31 1 P{ H} NMR: for 2I1 δ 34.2, 43.3 (dd, 2JP-P = 67); for 2I2 37.4 (s); for 2I3 40.2 (s). (44) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.

Ma et al. Crystal Structure Determination. Suitable crystals of 2 were mounted on glass fibers by means of Paratone-N oil, and data were collected using graphite-monochromated Mo KR radiation (0.710 73 A˚) on a Bruker PLATFORM/SMART 1000 CCD diffractometer. The structure was solved by direct methods (SHELXS-97 44) and refined using full-matrix least squares on F2 (SHELXL-97 44). All the non-hydrogen atoms in the structure of the compound were refined with anisotropic displacement parameters. Selected crystal data and structure refinement details for 2 are given in Table 1. Computational Details. The geometric optimization and energy calculations were performed with the Gaussian03 program38 using the density functional theory (DFT) method. The Becke threeparameter hybrid functional with the Lee-Yang-Parr45 correlation functional (B3LYP)46 was employed for all calculations. For geometric optimization, the LANL2DZ basis sets for Pd and 3-21g for all other atoms were applied in the preliminary first step optimization; further optimization was carried out using the first step obtained data with the basis sets of LANL2DZ for Pd and 6-31G(d) for others. Once an optimized geometry was obtained, imaginary frequencies were checked at the same level by vibration analysis to verify the genuine minimum on the potential energy surface (PES) and to evaluate the zero-point energy (ZPE) correction. The DFT wave functions obtained at optimized structures were confirmed to be stable. Gaussian NBO analysis was carried out for the final optimized structures.

Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support of this work in terms of a Discovery Grant. We also thank the University of Alberta for the support which maintains the X-ray Crystallography and Analytical and Instrumentation Laboratories at the Department of Chemistry which were essential for this work. We thank a reviewer for detailed and helpful comments on the nature of the allyl exchange process. Supporting Information Available: A CIF file giving crystallographic data for complex 2, Tables S1-S8 and Figures S1-S9, and Gaussian 03 calculation results for complex 2. This material is available free of charge via the Internet at http:// pubs.acs.org. (45) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (46) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5642.