Synthesis and computational studies of palladium(I) dimers Pd 2X2(PtBu2,Ph)2 (X = Br, I): Phenyl versus halide bridging modes

Article abstract of DOI:10.1021/om060712j

The synthesis and characterization of the new palladium(I) dimer Pd ₂Br₂(μ-P^tBu₂Ph)₂ (8) is reported herein. The single-crystal X-ray analysis of this compound has shown that the phosphine acts as a bridging ligand between the two palladium centers, using the phosphorus atom as well as the ipso- and ortho-carbon atoms of the phenyl substituents. An analysis of the electronic structure indicates that the bond between the remote palladium center and the bridging ligand is stabilized by two distinct electronic components, one with the C=C bond of the arene, the other an agostic-type interaction with the P-C bond. The steric bulk at the phosphorus center causes distinct changes in the relative contributions of these two components and, hence, distorts the structure. In contrast, the iodide analogue (9) adopts a completely different structure, where the phosphines are terminally coordinated and the halide acts as a bridging ligand.

Full text of DOI:10.1021/om060712j

5990
Organometallics 2006, 25, 5990-5995
Synthesis and Computational Studies of Palladium(I) Dimers
Pd2X2(PtBu2Ph)2 (X ) Br, I): Phenyl versus Halide Bridging Modes
Ute Christmann,§,† Dimitrios A. Pantazis,‡ Jordi Benet-Buchholz,† John E. McGrady,*,‡
Feliu Maseras,†, and Ramo´n Vilar*,§,†
Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, U.K.,
Institute of Chemical Research of Catalonia (ICIQ), AVgda. Pa¨ısos Catalans 16, 43007,
Tarragona, Spain, WestCHEM, Department of Chemistry, Joseph Black Building, UniVersity of Glasgow,
G12 8QQ, U.K., and Unitat de Qu´ımica F´ısica, Edifici Cn, UniVersitat Auto`noma de Barcelona,
08193 Bellaterra, Catalonia, Spain
ReceiVed August 7, 2006
The synthesis and characterization of the new palladium(I) dimer Pd2Br2(µ-PtBu2Ph)2 (8) is reported
herein. The single-crystal X-ray analysis of this compound has shown that the phosphine acts as a bridging
ligand between the two palladium centers, using the phosphorus atom as well as the ipso- and ortho-
carbon atoms of the phenyl substituents. An analysis of the electronic structure indicates that the bond
between the remote palladium center and the bridging ligand is stabilized by two distinct electronic
components, one with the CdC bond of the arene, the other an agostic-type interaction with the P-C
bond. The steric bulk at the phosphorus center causes distinct changes in the relative contributions of
these two components and, hence, distorts the structure. In contrast, the iodide analogue (9) adopts a
completely different structure, where the phosphines are terminally coordinated and the halide acts as a
bridging ligand.
terization of the dinuclear halide-bridged species Pd2(µ-X)2-
(PtBu3)2 (X ) Br, 1; I, 2) shown in Scheme 1. These
compounds, which can be prepared by either a comproportion-
ation reaction or oxidative addition of halides to palladium(0)
species, have a strained Pd2(µ-X)2 core and show a rich
reactivity toward a wide range of small molecules (e.g., CO,
CNR, O2, and C2R2). A few years after these compounds were
first reported, Hartwig demonstrated that the bromide complex
Pd2(µ-Br)2(PtBu3)2 is an excellent precatalyst for the amination
of aryl halides.10 The high reactivity of this system has been
attributed to the in situ generation of monophosphine palladium
complexes that can readily undergo oxidative-addition of aryl
halides.
The unique catalytic properties of these palladium dimers with
low phosphine-to-palladium ratio11 prompted us to explore the
synthesis and properties of a new series of palladium dinuclear
complexes containing biphenyl phosphines (complexes 3-7 in
Scheme 1),9,12 where the biphenyl group adopts an unusual
η2:η2 bridging mode. Besides their unique structural properties,
these complexes have also proved to be excellent precatalysts
for the amination of aryl chlorides and bromides.
Introduction
Dinuclear PdI-PdI complexes have received considerable
attention over the past few years as a result of their ability to
react with a wide range of substrates, with important potential
applications in organometallic catalysis.1-6 The reactivity of the
dimers (whether the metal-metal bond is bridged or unsup-
ported) is highly dependent on the ligands coordinated to the
palladium centers, and so the development of synthetic routes
to this type of complex with ligands that have a wide range of
electronic and steric properties remains a subject of great
interest. Phosphines are particularly suitable in this regard since
their size and electronic properties can be readily modulated,
they coordinate easily to palladium, and the resulting compounds
can be monitored experimentally by means of 31P NMR
spectroscopy.
Over the past few years, we have investigated the synthesis,
reactivity, and catalytic properties of several palladium dimers
containing sterically demanding phosphines.7-9 More specifi-
cally, we have reported the preparation and structural charac-
* Corresponding authors. E-mail: r.vilar@imperial.ac.uk; j.mcgrady@
chem.gla.ac.uk.
Given the strongly contrasting structural and catalytic proper-
ties of the PtBu3 and PtBu2(Bph-R) dimers, we decided to
explore the properties of analogous dinuclear complexes con-
taining PtBu2Ph. This phosphine offers the possibility of
π-coordination via its phenyl ring, but the metal-phenyl
interactions are not expected to be as strong as those in the
PtBu2(Bph-R) analogues, where the phenyl ring can approach
the remote palladium center more closely. Herein we report the
§ Imperial College London.
† ICIQ.
‡ University of Glasgow.
Universitat Auto`noma de Barcelona.
(1) Sui-Seng, C.; Enright, G. D.; Zargarian, D. J. Am. Chem. Soc. 2006,
128, 6508.
(2) Barder, T. E. J. Am. Chem. Soc. 2006, 128, 898.
(3) Murahashi, T.; Kurosawa, H. Coord. Chem. ReV. 2002, 231, 207.
(4) Lin, W.; Wilson, S. R.; Girolami, G. S. Inorg. Chem. 1994, 33, 2265.
(5) Dupont, J.; Pfeffer, M.; Rotteveel, M. A.; De Cian, A.; Fischer, J.
Organometallics 1989, 8, 1116.
(6) Kannan, S.; James, A. J.; Sharp, P. R. J. Am. Chem. Soc. 1998, 120,
215.
(7) Dura-Vila, V.; Mingos, D. M. P.; Vilar, R.; White, A. J. P.; Williams,
D. J. J. Organomet. Chem. 2000, 600, 198.
(8) Vilar, R.; Mingos, D. M. P.; Cardin, C. J. J. Chem. Soc., Dalton
Trans. 1996, 4313.
(9) Christmann, U.; Vilar, R.; White, A. J. P.; Williams, D. J. Chem.
Commun. 2004, 1294.
(10) Stambuli, J. P.; Kuwano, R.; Hartwig, J. F. Angew. Chem., Int. Ed.
2002, 41, 4746.
(11) Christmann, U.; Vilar, R. Angew. Chem., Int. Ed. 2005, 44, 366.
(12) Christmann, U.; Pantazis, D. A.; Benet-Buchholz, J.; McGrady, J.
E.; Maseras, F.; Vilar, R. J. Am. Chem. Soc. 2006, 128, 6376.
10.1021/om060712j CCC: $33.50 © 2006 American Chemical Society
Publication on Web 11/15/2006

Palladium(I) Dimers Pd2X2(PtBu2Ph)2 (X ) Br, I)
Organometallics, Vol. 25, No. 26, 2006 5991
Scheme 1
Scheme 2
synthesis and characterization of the new palladium(I) dimers
Pd2Br2(µ-PtBu2Ph)2 (8) and Pd2(µ-I)2(PtBu2Ph)2 (9) including
the single-crystal X-ray crystallographic analysis of 8. An
analysis of the electronic structure of these dimers has also shed
some light on the factors that control the ligand coordination
modes.
filtration yielded a brown solid, which was washed with diethyl
ether and recrystallized from CH2Cl2/hexane.
The 31P{1H} NMR spectrum of this solid exhibits a singlet
at δ 76.9 ppm, while the FAB(+) mass spectrum shows the
expected fragmentation pattern for a compound with the
formulation Pd2Br2(PtBu2Ph)2. The highest intensity peak, which
is observed at 737 amu, can be assigned to [M - Br]+, while
a smaller peak at 818 amu corresponds to [M]+. Single crystals
of this compound (8) suitable for X-ray crystallographic analysis
were grown from a chloroform solution.
Results and Discussion
Synthesis and Structural Characterization of Pd2Br2(µ-
PtBu2Ph)2 (8). When 1 equiv of Pd2(dba)3 was mixed with 4
equiv of PtBu2Ph and stirred for 2 h, the 31P{1H} NMR spectrum
of the reaction solution showed a single resonance at 67.7 ppm
that corresponds to the chemical shift previously reported for
Pd(PtBu2Ph)2 (see Scheme 2).13 Addition of Pd(COD)Br2 to this
solution caused an immediate change in color from orange to
brown and a dark precipitate formed. After a further 3 h,
The molecular structure of 8 (Figure 1) confirms that this
complex is indeed a palladium(I) dimer, but the coordination
geometry is dramatically different from the PtBu3 species 1,
where the two bromide ligands occupy bridging positions. In
this case each phosphine is directly coordinated to one of the
palladium centers via the phosphorus atom but also to the second
palladium via the phenyl substituent on the phosphine. Bonds
to the ipso-carbon atoms of the phenyl rings are relatively short
(13) Mann, B. E.; Musco, A. J. Chem. Soc., Dalton Trans. 1975, 1673.

5992 Organometallics, Vol. 25, No. 26, 2006
Christmann et al.
Scheme 3. Previously Reported Dimers by Kurosawa15 (a)
and van Leeuwen14 (b)
distances are rather longer (2.703(4)/2.974(4) Å and 2.89/3.01
Å, respectively), indicating a slippage toward η1- rather than
η2-coordination of the arene ring. van Leeuwen has discussed
the nature of the bonding in the [(dppp)Pd]22+ cation (Scheme
3) using a highly simplified model where the dppp ligand was
replaced by two PH3 groups.14 Despite the complete absence
of a CdC double bond in their model, the Hartree-Fock-
optimized geometries still reproduced the tilting of the Pd-P
bonds toward the neighboring Pd atoms, with Pd‚‚‚P separations
of ca. 3.0 Å. As a result, the authors concluded that direct
Pd‚‚‚P interactions were the major factor in stabilizing the
bridging coordination mode of the phosphine. The Pd‚‚‚P
separations of 2.92 Å in 8 are not dissimilar to those in the
Figure 1. ORTEP plot (thermal ellipsoids shown at 50% prob-
ability level) of 8. Hydrogen atoms have been omitted for the sake
of clarity. Selected distances (Å) and angles (deg): Pd1-Pd2,
2.5706(4); Pd1-Br1, 2.5287(5); Pd2-Br2, 2.5018(5); Pd1-C1,
2.469(3), Pd1-C2, 2.403(3); Pd1-P2, 2.2440(8); Pd1-P1, 2.9217-
(8); Pd2-C15, 2.448(3); Pd2-C16, 2.421(3); Pd2-P1, 2.2416(8);
Pd2-P2, 2.8927(8); P1-C2, 1.834(3); P2-C16, 1.826(2);
C1-C2, 1.419(4); C15-C16, 1.411(4); Pd1-Pd2-Br2, 175.316-
(13); Pd2-Pd1-Br1, 172.843(11); Pd1-P2-C16, 108.00(9);
Pd2-P1-C2, 107.36(9); Pd1-C2-C1, 75.64(16); Pd2-C16-C15,
74.24(16).
2+
[(dppp)Pd]2 cation, suggesting that Pd‚‚‚P interactions may
also be important in this case, but the rather short and highly
symmetric Pd-Cipso and Pd-Cortho bonds noted above indicate
that interactions with the π-electron density of the arene ring
cannot be ignored.
In an attempt to resolve the nature of the bonding in this
system, we have conducted a series of calculations using density
functional theory. The optimized structure of 8, including the
phenyl and tBu groups on the phosphorus center, is summarized
in Figure 2a. The principal features of the X-ray structure are
well reproduced, in particular the Pd-Pd (2.61 Å), Pd-Br (2.56
Å), and Pd-P (2.28 Å) bond lengths. The geometry about the
palladium center is also in good agreement with experiment,
with optimized Pd-Cipso and Pd-Cortho bond lengths of 2.49
Å and a Pd‚‚‚P separation of 2.99 Å. A topological analysis of
the computed electron density using the atoms in molecules
(AIM)16 approach reveals the presence of bond critical points
along both the Pd-Cipso and Pd-Cortho vectors and the corre-
sponding bond paths exhibit a pronounced inward curvature in
the proximity of the carbon atoms, a situation that is typical of
an η2-coordination mode.17 Thus the molecular structure and
the electron density indicate that the bridge is stabilized in this
case by an interaction between the π-electron density on the
arene ring and the remote palladium.
In our previous study on complexes of the analogous biphenyl
phosphines,12 we highlighted the importance of the steric bulk
of the tBu groups in determining the strength of the interaction
between the metal center and the arene group. In order to explore
this question in the context of 8, we have reoptimized the
structure of two simplified model complexes, one where the
tBu groups on the phosphine have been replaced with methyl
groups (Figure 2b) and the other where they are replaced with
hydrogen atoms (Figure 2c). In the case of PMe2Ph, the tilt of
the arene ring is less pronounced, and as a result, the Pd-Cortho
distance (2.54 Å) is now considerably longer than Pd-Cipso (2.42
Å). These trends are even more exaggerated when the methyl
groups are replaced with hydrogen: the P-C bond is now
(2.403(3) Å [Pd1-C2] and 2.421(3) Å [Pd2-C16]), while those
to the ortho-carbons are slightly longer (2.469(3) Å [Pd1-C1]
and 2.448(3) Å [Pd2-C15]). All four distances are consistent
with a significant interaction between the arene ring and the
corresponding palladium atom. On the other hand, the
Pd(1)‚‚‚P(1) and Pd(2)‚‚‚P(2) separations of 2.92 Å suggest only
a weak interaction between the palladium centers and the
corresponding distal phosphine. The NMR spectroscopic data
of 8 in solution are fully consistent with the solid-state structure
shown in Figure 1: the resonance at δ 100.6 ppm in the
13C{1H} NMR spectrum can be assigned to the ipso-carbon,
and the shift to lower frequency relative to the free phosphine
(δ ) 138.6 ppm) suggests that the Pd-Cipso interaction is
retained in solution. At room temperature, however, the 13C
NMR spectrum shows only a single resonance (δ 127.1 ppm)
for the ortho-carbon atoms, suggesting either that the vibration
of the phenyl ring about the P-Ph bond is rapid on the NMR
time scale or that an η1-coordination mode is present in solution.
Bridging coordination of aryl phosphines is rare but not
unprecedented. In fact, two examples of palladium complexes
with this type of P-Ph bridging motif have been reported
previously, both of which are dicationic palladium(I) dimers:
the [(dppp)Pd]2(CF3SO3)2 species reported by van Leeuwen14
and [(PPh3)Pd(MeCN)]2(PF6)2, reported by Kurosawa15 (see
Scheme 3). In both of these compounds, the Pd-Cipso distances
(2.385(16) and 2.336(4) Å, respectively) are shorter than the
corresponding distances in complex 8, while the Pd-Cortho
(16) Bader, R. F. W. In Handbook of Molecular Physics and Quantum
Chemisty; Wilson, S., Vol. Ed.; John Wiley & Sons Ltd.: Chichester, U.K.,
2003; Vol. 2, p 770.
(17) Macchi, P.; Proserpio, D. M.; Sironi, A. J. Am. Chem. Soc. 1998,
120, 1447.
(14) Budzelaar, P. H. M.; Van Leeuwen, P. W. N. M.; Roobeek, C. F.;
Orpen, A. G. Organometallics 1992, 11, 23.
(15) Murahashi, T.; Otani, T.; Okuno, T.; Kurosawa, H. Angew. Chem.,
Int. Ed. 2000, 39, 537.

Palladium(I) Dimers Pd2X2(PtBu2Ph)2 (X ) Br, I)
Organometallics, Vol. 25, No. 26, 2006 5993
Figure 2. Optimized structures (bond lengths in Å) and partial molecular graphs of µ-PR2Ph isomers of Pd2X2(PR2Ph)2 (X ) Br, I) for R
t
) Bu, Me, H. Values in brackets are for the iodide species. In the molecular graphs red spheres indicate nuclei, yellow corresponds to
(3,-1) bond critical points, and green represents (3,+1) ring critical points.
almost parallel to the Pd-Pd axis and the arene ring is
perpendicular to the plane defined by the Pd-Pd-P bonds. As
a result, the distinction between the Pd-Cortho and Pd-Cipso
distances (2.64 and 2.38 Å, respectively) is even greater. The
structural changes associated with the removal of the tBu groups
are accompanied by substantial changes in the topology of the
electron density: the Pd-Cortho bond critical point present in 8
disappears in the PMe2Ph complex, while a new one emerges
between Pd and the remote phosphorus atom in the PH2Ph
analogue. Importantly, the charge density at the new Pd-P and
the Pd-Cipso critical points in the latter is identical (F ) 0.05),
suggesting that these two interactions are of similar magnitude.
Thus the bonding model that emerges from the analysis of the
electron density in the PH2Ph model system is rather different
from the complete molecule 8: in the former, the interaction
between the phosphine ligand and the remote palladium is
dominated by a P-C agostic bond, quite distinct from the
Pd-(CdC) interaction in the latter. The methylated complex
(PMe2Ph) lies between these two extremes, with only a single
critical point from the palladium to the ipso-carbon. While the
precise correspondence between AIM critical points and chemi-
cal bonds continues to be debated,18-20 our analysis of the
electron density distributions in these three closely related
systems clearly indicates that the interaction between the
phosphine group and the remote palladium in any given system
may contain contributions from a classic η2-CdC bond and also
from an agostic P-C interaction. Perturbations to the system
due, for example, to steric effects can therefore be interpreted
in terms of distortions along a continuum linking the η2-CdC
and P-C agostic limits. In the present case, the bulky tBu groups
influence the structure in a number of ways. First, and most
obviously, repulsions with the bromide ligands increase the
P-Pd-Br angle from 90° to 104°, imposing an almost collinear
Br-Pd-Pd-Br geometry, quite distinct from the very bent
structure in the model system 2c (Pd-Pd-Br ) 157°).21 More
subtly, the bulky groups increase the R-P-R angle from 101°
t
(R ) H, 2c) to 113° (R ) Bu, 2a), as a result of which the
(19) Maseras, F.; Lledos, A.; Costas, M.; Poblet, J. M. Organometallics
1996, 15, 2947.
(20) Bader, R. F. W. Chem. Eur. J. 2006, 12, 2896.
(21) Interestingly, a similar bending of the Pd-Pd-L angle is observed
2+
in both [(dppp)Pd]22+ (161°)14 and, to a lesser extent, [(PPh3)Pd(MeCN)]2
(172°),15 where the substituents on the phosphine are much less bulky. Our
AIM analysis of the full [(dppp)Pd]22+ system computed at the same level
of theory reveals that the electron density is very similar to that in the
simplified model system, with critical points between Pd and the semibridg-
ing P, as suggested by van Leeuwen, but also between the Pd center and
the ipso-carbon, indicating a P-C agostic structure.
(18) Poater, J.; Sola, M.; Bickelhaupt, F. M. Chem. Eur. J. 2006, 12,
2902.

5994 Organometallics, Vol. 25, No. 26, 2006
Christmann et al.
t
Figure 3. Optimized structures (bond lengths in Å) of µ-X isomers of Pd2X2(PR2Ph)2 (X ) Br, I) for R ) Bu, Me, H, and relative
energies (kcal mol-1) with respect to the µ-PR2Ph isomers of Figure 2. Values in brackets are for the iodide species.
Cipso-P-Pd angle contracts from 125° to 106° (the well-known
Thorpe-Ingold effect). Finally, the tBu groups prevent the arene
substituent from adopting its electronically preferred conforma-
tion where it lies perpendicular to the Pd-Pd-P bonds (as in
the model system 2c), instead forcing it to tilt toward a less
crowded conformation where the plane of the ring bisects the
tBu-P-tBu angle. All three of these factors contribute toward
a decrease in the separation between the remote palladium center
and one of the two ortho-carbon atoms and, hence, drive the
system toward the η2-(CdC) limit of the continuum.
complexes. This study reveals minima corresponding to the
halide-bridged isomers (Figure 3) in addition to the semibridged
phosphine species discussed previously (Figure 2). The geometry
of the Pd2(µ-I)2 core shown in Figure 3a is very similar to that
in the known structure of Pd2(µ-I)2(PtBu3)2, with optimized
Pd-Pd and Pd-I separations of 2.61 and 2.54 Å, respectively.
In the context of the current discussion, the critical feature is
that while the µ-halide structure is marginally less stable than
its semibridged phosphine counterpart for the bromide complex
(∆E ) 2.1 kcal mol-1), the opposite is true for the iodide
analogue (∆E ) -4.4 kcal mol-1), consistent with experiment.
We have noted previously that the precise nature of the bonding
within the semibridging architecture is highly sensitive to steric
factors, but the µ-halide structure seems relatively robust, and
the geometries are not strongly influenced by the removal of
Synthesis of Pd2(µ-I)2(PtBu2Ph)2 (9). Once the unexpected
semibridging coordination mode of the phosphine in 8 had been
established, we were interested to know whether the same
structural features would be present in the iodide analogue.
However, since the Pd(COD)I2 starting material is not readily
available and moreover is difficult to purify, a different synthetic
approach was required. We have previously established that
palladium(I)-halide dimers can be prepared by the reaction
between a palladium(0) compound and the corresponding halide
source (see Scheme 1).7 On the basis of this approach, 0.5 equiv
of CHI3 (previously shown to be a good source of iodide in the
synthesis of palladium dimers) was added to Pd(PtBu2Ph)2
(generated in situ from Pd2(dba)3 and 2 equiv of PtBu2Ph),
yielding a dark brown solution. The solvent was then removed
under reduced pressure and the residue washed with cold diethyl
ether and recrystallized from THF at low temperature to yield
Pd2(µ-I)2(PtBu2Ph)2 (9) as a purple solid. Repeated attempts to
obtain single crystals suitable for X-ray crystallography have
proved unsuccessful, but the formulation of 9 was confirmed
by elemental analysis and MALDI(+) mass spectrometry, which
revels a peak at 910 amu corresponding to the molecular ion
[M]+. Despite the lack of crystallographic data, we can infer
from the NMR characteristics of 9 that its structure is quite
different from that of 8. The 31P{1H} NMR spectrum of this
complex showed a singlet at 81.7 ppm, quite similar to the value
in 8, but the 13C{1H} NMR spectra of the two species are quite
distinct. In particular, the resonance for the ipso-carbon of
PtBu2Ph appears at δ 134.4 ppm in 9, a very similar value to
that of the free phosphine and very different from the δ 100.6
ppm resonance characteristic of the semibridging phosphine
architecture in 8. On the basis of this data, we therefore propose
that the iodide complex, 9, features the same µ-iodo architecture
seen in its PtBu3 analogue, Pd2(µ-I)2(PtBu3)2, rather than the
semibridging phosphine structure found in 8.
t
the Bu groups (compare Figures 3a, 3b, and 3c). Given the
very small energy differences between the two isomeric forms
in each case, it may be that subtle differences in the steric
interactions between the phosphine and the halide cause the
switch in structure.
Conclusions
The results presented here, together with our previous
investigations into the formation of palladium dimers with tBu
and biphenyl phosphines, demonstrate the important role of both
the steric and electronic properties of the phosphine in determin-
ing the final structure of the dinuclear compounds. While the
bulky and electron-rich phosphine PtBu3 yields the Pd2(µ-X)2-
(PtBu3)2 dimers (with terminal PtBu3 and bridging halides), the
biphenyl phosphines PtBu2(Bph-R) yield the [Pd2X(µ-X){µ-
PtBu2(Bph-R)}] species, where the metal centers are bridged
by both the terminal arene ring of the biphenyl moiety and one
halide. The molecular structure of Pd2Br2(µ-PtBu2Ph)2 (8) shows
a coordination geometry that is dramatically different from either
of these: each phosphine is directly coordinated to one of the
palladium centers via the phosphorus atom but also to the second
palladium via the phenyl substituent on the phosphine. Density
functional theory reveals that the interaction between the
bridging phosphine and the remote palladium center contains
contributions from both the CdC π and P-C σ bonds. Subtle
changes in the steric demand of the phosphine can alter the
relative contributions of these two components of the bond and,
hence, can cause significant changes in structure.
Experimental Details
To understand the adoption of different bonding modes of
the phosphine and halides in 8 and 9, we have conducted a
more detailed survey of the potential energy surface for both
General Comments. All experimental procedures were carried
out using standard high-vacuum and Schlenk line techniques under

Palladium(I) Dimers Pd2X2(PtBu2Ph)2 (X ) Br, I)
Organometallics, Vol. 25, No. 26, 2006 5995
an atmosphere of dry argon. Glassware was dried in an oven at
150 °C prior to use. CH2Cl2, hexane, THF, methanol, diethyl ether,
and toluene were dried using a solvent purification system (SPS,
Innovative-Technology) and degassed with argon prior to use. 1H,
13C, and 31P NMR spectra were recorded on a Bruker Avance 400
or Bruker Avance 500 spectrometer and referenced to residual 1H
and 13C signals of the solvents or 85% H3PO4 as an external
standard (31P). The compounds Pd(COD)Br2, [Pd2(dba)3]‚C6H6, and
PtBu2Ph were synthesized following literature procedures.22-24
Synthesis of Pd2Br2(PtBu2Ph)2 (8). Pd2(dba)3‚C6H6 (209 mg,
0.210 mmol) was dissolved in toluene (20 mL), and a solution of
PtBu2Ph (1.04 mL of a 0.773 M toluene solution, 0.840 mmol) in
toluene was added with stirring. After 2 h, Pd(COD)Br2 (105 mg,
0.281 mmol) was added and the reaction mixture stirred overnight
to yield a dark brown precipitate. After filtration the precipitate
was washed with diethyl ether (2 × 10 mL). Recrystallization from
CH2Cl2/hexane yielded a brown solid. Yield: 112 mg, (49%). Single
crystals suitable for X-ray crystallographic studies were grown from
(4)°, V ) 3506.6(11) Å3, Z ) 4, µ ) 3.643 mm-1, density ) 1.774
Mg/m3, R1 ) 0.0411(I > 2σ(I))/0.0850(all data) and wR2 )
0.0810(I > 2σ(I))/ 0.0921(all data), 10 601 reflections with I >
2σ(I), 16 064 independent reflections (Rint ) 0.0693) with a total
measured 54 952 reflections, goodness-of-fit on F2 ) 1.009, largest
diff peak (hole) ) 1.126 (-1.444) e Å-3
.
Computational Details. Full unconstrained geometry optimiza-
tions were carried out with the one-parameter hybrid mPW1PW91
density functional,25 which has been shown to handle reliably the
weak interactions in palladium-arene systems. All reported
structures were verified as true minima. Pd, Br, and I were described
with the Hay-Wadt effective core potentials along with the
corresponding valence basis sets (LANL2DZ).26-28 Extra sets of d
polarization functions were added to the halides (Rd ) 0.428 for
Br, Rd ) 0.289 for I).29 We note that the accuracy of relative
energies obtained with the LANL2DZ basis has been questioned
for correlated HF-based methods,30 but the inclusion of higher
angular momentum functions does not appear to be critical in DFT
calculations.31,32 The 6-31G(d) basis sets were employed for phos-
phorus and the atoms of the phenyl group, whereas the STO-3G
basis sets were used for the atoms of the tBu groups. All calculations
were performed with the Gaussian03 series of programs.33 Wave-
function files for the AIM analysis were obtained by single-point
calculations using the all-electron DGDZVP basis sets for Pd, Br,
and I.34 Subsequent analysis of the electron densities and construc-
tion of molecular graphs were carried out with the AIM2000
program.35
1
a chloroform solution. H NMR (CD2Cl2, 500 MHz): δ 8.30 (dd,
2
JHH ) 8.3 Hz, JHP ) 8.3 Hz, 4H, ï-Aryl-H), 7.87 (t, JHH ) 7.4
Hz, 2H, p-Aryl-H), 7.68 (dd, JHH ) 7.4 Hz, 4H, m-Aryl-H), 1.48
(d, 3JPH ) 14.8 Hz, 36H, C(CH3)3). 13C{1H} NMR (126 MHz, CD2-
Cl2): δ 136.3 (p-Aryl-C), 132.2 (d, JPC ) 8.6 Hz, m-Aryl-C), 127.1
(o-Aryl-C), 100.6 (ipso-Aryl-C), 38.8 (d, JPC ) 14.7 Hz, C(CH3)3),
31.2 (C(CH3)3). 31P{1H} NMR (202 MHz, CD2Cl2): δ 76.9 (s).
FAB-MS: m/z+ 818 [M]+, 737 [M - Br]+. Anal. Calcd for C28H46-
Br2P2Pd: C, 41.15; H, 5.67. Found: C, 41.05; H, 5.60.
Synthesis of Pd2I2(PtBu2Ph)2 (9). To a solution of Pd2(dba)3‚
C6H6 (102 mg, 0.102 mmol) in toluene (10 mL) was added
PtBu2Ph (265 µL of a 0.773 M toluene solution) in toluene. After
2 h CHI3 (20 mg, 0.051 mmol) was added as a solid. The reaction
solution was stirred for an additional hour, after which time the
solvent was removed under reduced pressure. The crude product
was recrystallized from THF at -4 °C to obtain a dark purple solid,
which was washed twice with cold diethyl ether and dried under
The supplementary crystallographic data for this paper (CCDC
613416) can be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223
336033 or e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgment. We thank EPSRC (U.K.) (GR/R74970/
01 and GR/R74963/01), the ICIQ Foundation (Spain), and
ICREA (Spain) for financial support and Johnson Matthey for
a loan of PdCl2. Dr. Gabriel Gonzalez and Mr. Kerman Go´mez
are thanked for their help with the NMR data. D.A.P.’s visit to
ICIQ and his calculations in CESCA/CEPBA were funded by
the HPC-EUROPA project (RII3-CT-2003-506079), with the
support of the European Community-Research Infrastructure
Action under the FP6 “Structuring the European Research Area”
Programme.
1
vacuum. Yield: 25 mg (21%). H NMR (400 MHz, THF-d8): δ
3
8.11-8.02 (m, 4H), 7.45-7.32 (m, 6H), 1.25 (t, JPH ) 7.00 Hz,
36H, C(CH3)3). 13C{1H} NMR (126 MHz, THF-d8): δ 137.8
(C-arom.), 134.4 (d, JPC ) 11.9 Hz, ipso-C-Aryl), 131.1 (d, JPC
)
10.1 Hz, C-arom.), 128.1 (d, JPC ) 4.69 Hz, C-arom.), 34.2 (d, JPC
) 5.55 Hz, C(CH3)3), 34.1 (d, JPC ) 5.55 Hz, C(CH3)3), 30.9
(d, JPC ) 4.69 Hz; C(CH3)3), 30.8 (d, JPC ) 4.16 Hz, C(CH3)3).
31P{1H} NMR (162 MHz, C6D6): δ 81.7 (s). MALDI-MS: m/z+
910 [M]+. Anal. Calcd for C28H46I2P2Pd: C, 36.90; H, 5.09.
Found: C, 36.91; H, 5.40.
Supporting Information Available: Cartesian coordinates and
absolute energies for all computed structures. Complete reference
for Gaussian03. Crystallographic data (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
X-ray Crystallography. Crystals of 8 (as a chloroform mono-
solvate) were obtained by crystallization in a saturated chloroform
solution at room temperature. The measured crystal was prepared
under inert conditions immersed in perfluoropolyether as protecting
oil for manipulation. Data collection: Measurements were made
on a Bruker-Nonius diffractometer equipped with a APPEX 2 4K
CCD area detector, a FR591 rotating anode with Mo KR radiation,
Montel mirrors as monochromator, and a Kryoflex low-temperature
device (T ) -173 °C). Full-sphere data collection was used
with ω and æ scans. Programs used: Data collection, Apex2 V.
1.0-22 (Bruker-Nonius 2004); data reduction, Saint+ Version
6.22 (Bruker-Nonius 2001); and absorption correction, SADABS
V. 2.10 (2003). Structure solution and refinement: SHELXTL Ver-
sion 6.10 (Sheldrick, 2000) was used. Crystal data for 8‚CHCl3:
C29H47Br2Cl3P2Pd2, M ) 936.58, monoclinic, space group P21/c,
a ) 11.368(2) Å, b ) 28.186(5) Å, c ) 11.873(2) Å, â ) 112.828-
OM060712J
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