A spirocyclic, bimetallic, carbon-bridged Bis(iminophosphorano)methanediide complex of palladium(II): Synthesis, crystal structure, solution behavior, and DFT bonding analysis

Article abstract of DOI:10.1021/om900571k

The dimeric dilithium methanediide salt [Li₂C(Ph ₂P=NSiMe₃)₂]₂ ([Li ₂-1]₂) reacted with either 1 or 2 equiv of [PdCl(allyl)]₂ (allyl = CH₂=CHCH₂) in toluene or benzene to give exclusively the bimetallic Pd spirocyclic carbon-bridged complex [{η³-C₃H₅)Pd}₂{u, κ²:κ²-C(Ph₂P=NSiMe ₃)₂}] (2). Complex 2 was fully characterized by solution multinuclear NMR (¹H, ³¹P, and 13C) spectroscopy in toluene-d_g 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 C_carbene along with a derealization throughout the backbone skeleton which stabilizes the molecular structure.

Full text of DOI:10.1021/om900571k

52 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
important potential applications for organic and organo-
metallic catalysis.1-6 Generally, phosphines are directly co-
ordinated 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 com-
plexes, especially those of the N-heterocyclic carbene
(NHC) ligands, and stable monomeric palladium complexes
Many binuclear PdI-PdI complexes have received con-
siderable 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.
(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,
Organometallics 1989, 8, 1116–1118.
J. E.; Maseras, F.; Vilar, R. J. Am. Chem. Soc. 2006, 128, 6376–6390.
r
pubs.acs.org/Organometallics
Published on Web 12/10/2009
2009 American Chemical Society

Article
Organometallics, Vol. 29, No. 1, 2010 53
of this ligand system have found well-developed applications
in synthesis and catalysis.8 Only one carbon (and phos-
phorus) bound phosphinocarbene palladium complex is
known.8d
Scheme 1. Pincer, Chelate, and Spirocyclic Complexes of the
Bis(iminophosphorano)methanediide Ligand
Several years ago, we9,10 and others11 reported the synthe-
sis 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(Ph2PdNSi-
Me3)2 (H2-1),12,13 using alkyl- or aryllithium reagents. Sub-
sequently 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 chlor-
ide 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,
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 proper-
ties of the system (as did our spirocyclic bimetallic dialumi-
num 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 organo-
metallic 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
2JP-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
(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.
1
Pd centers. The H 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)
31P NMR spectrum, which also indicated a solution yield
close to 100%. Previously, in analogous studies, a 1:1 reac-
tion ratio of [Li2-1]2 with a Rh precursor19,21 gave the
intermediate Li-Rh spirocyclic complex; therefore, we ex-
pected a restricted reaction ratio to proceed similarly to give
the monopalladium-lithium [Pd(allyl)Li-1] complex. How-
ever, 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
(12) Appel, V. R.; Ruppert, I. Z. Anorg. Allg. Chem. 1974, 406, 131–
144.
€
€
(13) Muller, A.; Neumuller, 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. Organo-
(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.
metallics 1999, 18, 4241–4243.
Catal. A 2002, 189, 137–143.

54 Organometallics, Vol. 29, No. 1, 2010
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 spec-
trum 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).
Figure 1. Normal-temperature (30 °C) phosphorus-31 and par-
tial 1H NMR spectra of the binuclear palladium complex 2: (a)
31P 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.
In comparison to our previously studied metal PCP com-
plexes,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-do-
nation.
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.
Variable-Temperature NMR Spectroscopy. Variable tem-
perature 31P and 1H NMR studies (in toluene-d8) were
undertaken to ascertain if isomer interconversion or con-
formational exchanges were occurring. The temperature was
varied from -40 to þ115 °C. Proton signals visibly col-
lapsed, but full spectral coalescence was not observed in the
31P 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, how-
ever, 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
31P 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
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) environ-
ments 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.
1
31P and H 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
Similarly, three isomeric species appear in the room-
temperature 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,
(27) Kubo, K.; Jones, N. D.; Ferguson, M. J.; McDonald, R.; Cavell,
R. G. J. Am. Chem. Soc. 2005, 127, 5314–5315.

Article
Organometallics, Vol. 29, No. 1, 2010 55
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
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 sepa-
rated signals, as was the case for the room-temperature
spectrum. Increasing the temperature resulted in a progre-
ssive signal collapse so that by 80 °C, the right and left pair of
signals due to the major isomer had merged to become a
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.
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
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).
1
allyl processes are likely to be extracted from the silyl H
NMR variable-temperature spectra.
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 pro-
ton 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 even-
tually 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 beha-
vior which is typical of allyl complexes in general; however,
the detailed mechanism of the process is not discernible
without some extensive spin transfer experiments.
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.

56 Organometallics, Vol. 29, No. 1, 2010
Ma et al.
Table 1. Crystal Data and Structure Refinement Details for
Complex 2
empirical formula
formula wt
cryst syst
C37H48N2P2Pd2Si2
851.69
orthorhombic
0.41 ꢀ 0.24 ꢀ 0.08
Pbca (No. 61)
cryst dimens (mm)
space group
unit cell dimens
˚
a (A)
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
˚
b (A)
˚
c (A)
3
V (A )
˚
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 ellip-
soids at the 20% probability level. Allyl group hydrogen atoms
are shown with arbitrarily small thermal parameters; all other
hydrogen atoms are omitted.
no. of rflns collected
no. of indep rflns
7990
no.of data/restraints/params
goodness of fit on F2 (all data)
R1 (I > 2σ(I))
7990/3/433
1.068
0.0302
0.0794
0.904 and -0.482
wR2 (all data)
large diff peak and hole (e/A )
3
˚
˚
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)
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)
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)
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 [Li2-
1]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 success-
ful 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 forma-
tion. 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
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)
P(1)-C(10)
P(2)-N(2)
P(2)-C(10)
Si(1)-N(1)
Si(2)-N(2)
a 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 1are
involved in two four-membered chelate rings, and the ligand 1
adopts a κC,κN coordination mode to each of the twoPd 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
morebent(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).
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

Article
Complexes that contain chelating C, N ligands with a
carbenic C are relatively rare; other examples are the Ru-
aminocarbene 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
Organometallics, Vol. 29, No. 1, 2010 57
observed in our previously synthesized Rh/Pd heterobime-
tallic 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). The additional N-donor trans
31
˚
(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 magne-
tically 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 orienta-
tion 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 conforma-
tional 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 con-
formational pictures are in good general accord with each
other, the difference in isomeric distributions between solu-
tion 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 ob-
servation 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.
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 interac-
tion, whereas, in comparison, the metal separation distance
in the recently discovered palladium(I) dimer Pd2Br2-
7
˚
(P(t-Bu2)Ph)2 (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 [(i-
˚
˚
Pr2N)(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-2-
ylidene) (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 ) observed in the chelated Pt complex, which we
20
˚
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 H2-
133 (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
34
˚
[PdCl2(Me3SiNdPEt3)2] (2.095 A)
and [PdCl2PCH2-
oxMe2)] (2.058(2) A) but similar to those of related Pt
35
˚
complexes [PtCl(PEt3){CH(Ph2PdN-p-tolyl)(Ph2PNH-p-
36
tolyl)-κC,κN}][PtCl3(PEt3)] (2.124(8) A) and [PtCl(PPh-
˚
Me2){CH(Ph2PdN-p-tolyl)2-κC,κN}] (2.132(4) A).37 The elec-
˚
tronically 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-C3H5)Pd}2{μ,κ2,κ2-C(Me2PdNSiH3)2}]
(2a; Scheme 5), adapted from the conformation 2aI1 or 2aI2
observed in the solid state, were calculated at the DFT level with
(c). Other Structural Features. The η3-allyl ligand is asym-
metrically 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-
(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.
˚
(4A) = 2.104(18) A) are shorter than those trans to the
€
€
€
(33) Muller, A.; Mohlen, M.; Neumuller, 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.
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 influ-
ence 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.
(37) Avis, M. W.; Vrieze, K.; Ernsting, J. M.; Elsevier, C. J. Organo-
Organometallics 2002, 21, 4955–4959.
metallics 1996, 15, 2376–2392.

58 Organometallics, Vol. 29, No. 1, 2010
Ma et al.
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 in-
dicate 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
Figure 4. Calculation framework and selected molecular orbi-
tals (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) MO-
81, 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 con-
jugated 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 MO-
85 (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.
(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.

Article
Gaussian NBO analysis38b for the two isomer conforma-
Organometallics, Vol. 29, No. 1, 2010 59
tions 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),
s
0.11d0.89 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
s
0.41p0.59 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
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.
-
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).
change at elevated temperatures with small changes in
relative proportions of the isomers up to 115 °C. Return-
ing the sample to room temperature produces the original
distribution of isomers, suggestive of a chemical equilibrium
which is consistent with the small energy differences calcu-
lated 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 de-
monstrated 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 anti-
bonding 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.
The NBO analysis at the second-order donor-acceptor
interaction level confirmed the existence of electron deloca-
lization 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 pos-
sess 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
Experimental Section
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 free-
ze-pump-thaw cycles prior to use. The organolithium com-
pound [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), ex-
ternal 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.
˚
separations range from 2.9 to 3 A. In our present complex 2
an electron-rich bridging carbene is generated in the mole-
cule which is consistent with the 13C NMR measurement of
the bridging carbon chemical shifts.
Summary and Conclusion
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-tempera-
ture NMR studies show that there is conformational ex-
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.;
(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) Rulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van
Bourissou, D.; Bertrand, G. J. Am. Chem. Soc. 2007, 129, 978–985.
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769–5778.

60 Organometallics, Vol. 29, No. 1, 2010
Ma et al.
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),
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 KRradiation (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 three-
parameter hybrid functional with the Lee-Yang-Parr45 correla-
tion 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) correc-
tion. The DFT wave functions obtained at optimized structures were
confirmed to be stable. Gaussian NBO analysis was carried out for
the final optimized structures.
3
2.625 (d, -CH2 (allyl), JH-H = 12.5), 2.66 (d, -CH2 (allyl),
=
3
2
3JH-H = 12), 3.769 (dd, -CH2 (allyl), JH-H = 7, JH-H
2.5), 3.847 (dd, -CH2 (allyl), 3JH-H = 7, 2JH-H = 2), 4.587 (7
3
peaks, 1H, -CH- (allyl), JH-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),
=
3JH-H = 7), 4.865 (7 peaks, 2H, -CH- (allyl), JH-H
3
6.5); for 2I3 δ 0.075 (s, 18H, Si(CH3)3), 2.694 (d, -CH2 (allyl),
3
3JH-H = 13), 3.692 (d, -CH2 (allyl), JH-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
1
-9.30 (t, PCP, JP-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
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.
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));for2I3 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).
31P{1H} NMR: for 2I1 δ34.2, 43.3 (dd, 2JP-P =67);for2I2 37.4 (s);
for 2I3 40.2 (s).
Supporting Information Available: A CIF file giving crystal-
lographic 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.
(44) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.