Synthesis and structural characterisation of the palladium N-heterocyclic carbene cluster complexes [Pd3(μ-CO)3(NHC)3] and [Pd3(μ-SO2)3(NHC)3]

Article abstract of DOI:10.1016/j.jorganchem.2009.09.018

The reduction of trans-[Pd(NHC)₂Cl₂] (NHC = IMes, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; IⁱPr₂ = 1,3-bis-isopropylimidazol-2-ylidene) with potassium graphite under an atmosphere of CO affords the pall

Full text of DOI:10.1016/j.jorganchem.2009.09.018

Journal of Organometallic Chemistry 695 (2010) 6–10
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and structural characterisation of the palladium N-heterocyclic
carbene cluster complexes [Pd3(
l-CO)3(NHC)3] and [Pd3(l-SO2)3(NHC)3]
*
Charles E. Ellul, Mary F. Mahon, Michael K. Whittlesey
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 August 2009
Received in revised form 2 September 2009
Accepted 10 September 2009
Available online 17 September 2009
The reduction of trans-[Pd(NHC)2Cl2] (NHC = IMes, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene;
IiPr2 = 1,3-bis-isopropylimidazol-2-ylidene) with potassium graphite under an atmosphere of CO affords
the palladium NHC carbonyl clusters [Pd3(
SO2 at room temperature yields the bridging SO2 complex [Pd3(
Complexes 1, 3 and 4 have been structurally characterised by X-ray crystallography.
Ó 2009 Elsevier B.V. All rights reserved.
l
-CO)3(NHC)3] (NHC = IMes, 1; IiPr2, 3). Treatment of 1 with
l
-SO2)3(IMes)3] (4) in quantitative yield.
Keywords:
N-heterocyclic carbenes
Palladium complexes
Cluster chemistry
1. Introduction
2. Results
Phosphine stabilised palladium clusters of the general type
[Pdn(CO)x(PR3)y] form the basis for a wide range of Pdn complexes,
varying from the high nuclearity species [Pd54(CO)40(PEt3)14] [1],
[Pd66(CO)45(PEt3)16] [2] and [Pd145(CO)x(PEt3)30] (x ꢀ 60) [3] all
Reduction of the mononuclear Pd(II) IMes complex [Pd
(IMes)2Cl2] [12] with potassium graphite in the presence of 1 atm
of CO at 70 °C for 16 h afforded the Pd(0) trimer [Pd3(
l-
CO)3(IMes)3] (1, Scheme 1), which was isolated as a deep red,
microcrystalline solid in 38% yield. The 1H NMR spectrum was con-
sistent with the molecule having high symmetry on the grounds of
there being only two methyl signals (in a 1:2 ratio) at d 2.34 and
2.04. The 13C{1H} spectrum showed two characteristic high fre-
quency resonances at d 249.2 and 198.8, which were assigned to
Pd–CO and Pd–CNHC on the basis of a 13C–1H HMBC correlation
from the lower frequency signal to the backbone IMes protons.
The IR spectrum of 1 (recorded in C6D6) showed a single band in
the bridging carbonyl region at 1796 cmÀ1, significantly lower in
the way down to the 3:3:3 and 3:3:4 clusters [Pd3(l-CO)3(PR3)3]
and [Pd3(l-CO)3(PR3)4]. These smaller clusters have been known
for close to 40 years, but there is only one example ([Pd3(
l
-
CO)3(PPh3)4]) that has been structurally characterised [4,5]. In
marked contrast, crystal structures have been reported for many
of the 3:3:3 and 3:3:4 Pt analogues for a range of phosphines,
including PCy3, PPh3, PPh2Bz and PtBu2Ph [6–8].
N-heterocyclic carbenes (NHCs) offer an alternative ligand set
to phosphines and have found applications in many areas of con-
temporary organometallic chemistry [9], although their use in me-
tal cluster chemistry is still relatively limited [10]. We now
describe the synthesis and characterisation of two novel NHC Pd
frequency than reported for [Pd3(l
-CO)3(PPh3)3] (1850 cmÀ1), [4]
consistent with the strong
r-donor power of NHCs.
A similar reaction of the N-alkyl Pd(II) precursor [Pd(IiPr2)2Cl2]
carbonyl species [Pd3(
methylphenyl)imidazol-2-ylidene) and [Pd3(
IiPr2 = 1,3-bis-isopropylimidazol-2-ylidene) [11], along with the
SO2 substitution product [Pd3( -SO2)3(IMes)3] (4). Structural eluci-
dation of all three species reveals that the bulk of the carbene N-
substituents has a significant impact on the planarity of the
l
-CO)3(IMes)3] (1, IMes = 1,3-bis(2,4,6-tri-
(2) with KC8/CO afforded the analogous N-isopropyl substituted
l
-CO)3(IiPr2)3] (3,
trimer [Pd3(l
-CO)3(IiPr2)3] (3, Scheme 1), which due to its low sta-
bility (the compound had to be maintained under a CO atmosphere
in solution, or rapidly turned black) could only be isolated in very
low (ca. 5%) yield. As for 1, the spectroscopic properties of 3 (a sin-
gle set of N-iPr proton resonances and a single low frequency
(1793 cmÀ1) carbonyl infra-red stretch) were indicative of a highly
symmetrical molecule.
l
Pd3(l-L)3 (L = CO, SO2) core.
Crystals of 1 and 3 (Fig. 1) suitable for X-ray crystallography
were obtained from THF/hexane solutions. Some asymmetry was
apparent in both the Pd–CO bond lengths and Pd–Pd distances
(Table 1). Thus, the Pd–CO ranged between 2.039(4)–2.076(3) Å
* Corresponding author. Tel.: +44 (0) 1225 383768; fax: +44 (0) 1225 382631.
E-mail address: m.k.whittlesey@bath.ac.uk (M.K. Whittlesey).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.09.018

C.E. Ellul et al. / Journal of Organometallic Chemistry 695 (2010) 6–10
7
2
8
2
Δ
2
2
i
Scheme 1.
Fig. 1. Molecular structures of 1 (left) and 3 (right). Thermal ellipsoids are shown at the 30% probability level. Both the solvent in 1 and hydrogen atoms throughout are
omitted for clarity. Primed labelled atoms in 3 are related to those in the asymmetric unit by the 1 À x, ½ À y, z symmetry transformation.
in 1 and 2.043(3)–2.077(3) Å in 3, while one of the Pd–Pd distances
in 1 showed a small but significant lengthening compared to the
remaining two (Pd2–Pd3, 2.6904(3) c.f. 2.6580(3) and
2.6520(3) Å). The two independent Pd–Pd distances were also mar-
ginally asymmetric in 3 (2.6624(3), 2.6773(3) Å).
bigger IMes ligand creates a greater deformation, with similarly
defined interplane angles of 0.9°, 4.5° and 7.8°, while the combina-
tion of IMes/SO2 in 4 results in comparative twists of 30.4°, 36.7°
and 34.7°. In both 1 and 4, it is notable that two of the bridging li-
gands lie to one side of Pd3 plane, while the remaining one is
accommodated on the opposite side. Simultaneously, the carbene
ligands also tilt relative to the mean plane through the palladium
centres, in a manner that minimises steric interaction with neigh-
bouring SO2 groups. This is evidenced by small deviations of the
carbene carbons from the mean-plane of the metals, but more
markedly, by the asymmetry induced in the Pd–CNHC–NNHC angles
within individual carbene fragments. In particular, these angle dif-
ferences span 1.3°, 1.7° and 9.5° in 1, 0°, 4.1° and 4.1° in 3 and 1.3°,
The application of a slow stream of SO2 through a THF solution
of 1 resulted in the red solution attaining a darker blood-red
appearance, from which the bridging tris-SO2 complex [Pd3(
SO2)3(IMes)3] (4, Scheme 1) was isolated in quantitative yield. As
with the -CO species, far greater success has been achieved in
structurally characterising Pt tris- -SO2 compounds than the palla-
dium analogues, with [Pd3( -SO2)3(PBz3)3] representing the lone
l-
l
l
l
exception [13]. The X-ray crystal structure of 4 was therefore
determined and is shown in Fig. 2.
7.8° and 25.5° in 4. By way of comparison to 4, [Pd3(
SO2)3(PBz3)3] [13] and [Pt3( -SO2)3(PR3)3] (R = Ph [14], Cy [15]) ex-
hibit planar M3( -SO2)3 arrangements, although non-planarity can
be induced sterically by increasing the cluster count from 42- to
44-electron, as in [Pt3( -SO2)3(PCy3)3(dppp)] [16]. It is also worth
noting that the positioning of two SO2 groups on one side of the
Pd3 plane in 4 dramatically reduces one of S–Pd–S angles (S1–
Pd3–S3, 122.17(2)° c.f. S1–Pd1–S2, 156.78(2)°; S2–Pd2–S3,
156.27(2)°) such that the structure cannot really be considered
l-
Scrutiny of the solid state structures of 1, 3 and 4 reveals some
interesting comparisons. Distortion of the palladium bound atoms
from the bridging ligands appears to reflect the steric demand of
these bridging fragments, or the bulk of the carbene substituents.
Thus, the most minimal distortions are observed with the smallest
NHC (IiPr2) in compound, 3, where the respective Pd–CCO–Pd mean
planes deviate from the plane of the Pd3 core by 0.0°, 4.5° and 4.5°
(the molecule has twofold rotation symmetry). In the case of 1, the
l
l
l

8
C.E. Ellul et al. / Journal of Organometallic Chemistry 695 (2010) 6–10
Fig. 2. Molecular structure of 4. Hydrogen atoms and solvent omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.
as a combination of three T-shaped ML3 fragments in the same way
as either the Pd or Pt [M3(l-SO2)3(PR3)3] species.
were performed by Elemental Microanalysis Ltd., Okehampton,
Devon.
4.2. Preparation of [Pd3(l-CO)3(IMes)3] (1)
3. Conclusions
[Pd(IMes)2Cl2] (200 mg, 0.25 mmol) and potassium graphite
(69 mg, 0.50 mmol) were added to an ampoule fitted with a J.
Youngs resealable valve. The vessel was purged with CO prior to
the addition of CO-saturated THF (10 mL). The resulting suspension
was then heated at 70 °C for 16 h. The red solution was cannula fil-
tered and the filtrate reduced to dryness to leave a mixture of red
and white solids. The red product was extracted in MeOH (2 mL),
Three new palladium N-heterocyclic carbene clusters have been
synthesised and, in contrast to their long established tertiary phos-
phine analogues, characterised by X-ray crystallography. The pres-
ence of bulky N-mesityl subsituents on the carbene helps to
stabilise Pd3(l-CO)3 against the degradation seen with the smaller,
more flexible IiPr2 ligand. However, the same steric bulk also forces
the bridging CO ligands (and SO2 ligands in 4) to adopt an unsym-
metrical arrangement around the Pd3 triangle. This may place lim-
itations on the usefulness of NHCs for the preparation of new, high
nuclearity Pd clusters.
filtered, reduced to dryness and the red solid of [Pd3(l-
CO)3(IMes)3] washed with hexane (10 mL) and dried under vac-
uum. Yield: 43 mg (38%). 1H NMR (benzene-d6, 400 MHz, 298 K):
d 6.81 (s, 2H, NCH), 6.14 (s, 4H, C6Me3H2), 2.34 (s, 6H, p-Me),
2.04 (s, 12H, o-Me). 13C{1H} NMR: d 249.2 (s, CO), 198.8 (s, NCN),
137.6 (s, N-i-C), 137.0 (s, C-p-Me), 136.3 (s, C-o-Me), 130.0 (s,
NCH), 119.1 (s, C6Me3H2), 21.7 (s, p-Me), 19.1 (s, o-Me). IR (ben-
4. Experimental
zene-d6, cmÀ1): 1796
(mCO) Anal. Calc. for Pd3C66N6O3H72
4.1. General comments
(1316.49): C, 60.21; H, 5.51; N, 6.38. Found: C, 57.63; H, 5.61; N,
6.45%. Repeated attempts to determine the CHN content gave a
consistently low % carbon.
All manipulations were carried out using standard Schlenk, high
vacuum and glovebox techniques. Solvents were purified on
MBraun SPS or Innovative Technologies solvent systems (toluene,
THF, hexane) or under a nitrogen atmosphere from purple solu-
tions of sodium benzophenone ketyl (benzene). Deuterated sol-
vents (Fluorochem) were vacuum transferred from potassium
(benzene-d6, toluene-d8, THF-d8) or CaH2 (CD2Cl2). CO (BOC,
99.9%) and SO2 (Aldrich, 99.9%) were used as received. [Pd(I-
Mes)2Cl2], [IMesH]Cl and KC8 were prepared according to the liter-
ature [12,17,18]. NMR spectra were recorded on Bruker Avance
300, 400 and 500 MHz NMR spectrometers and referenced (1H,
13C) as follows: benzene (d 7.15, 128.0), dichloromethane (d 5.32,
54.5), toluene (d 2.09, 20.4), THF (d 3.58, 25.4). IR spectra were re-
corded on a Nicolet Nexus FTIR spectrometer. Elemental analyses
4.3. Preparation of [Pd(IiPr2)2Cl2] (2)
Palladium dichloride (300 mg, 1.69 mmol), [IiPr2H]Cl (637 mg,
3.38 mmol) and caesium carbonate (4.9 g, 15.21 mmol) were sus-
pended in dioxane (10 mL) and heated at 80 °C for 16 h. After cool-
ing to room temperature an removal of the solvent under vacuum,
the residue was subjected to flash column chromatography
(CH2Cl2, silica) to give [Pd(IiPr2)2Cl2] as a white, microcrystalline
solid. Yield: 576 mg (70% yield). 1H NMR (CD2Cl2, 300 MHz,
298 K): d 6.96 (s, 2H, NCH), 5.65 (sept, 2H, JHH = 6.79 Hz, CHMe2),
1.59 (d, 12H, JHH = 6.79 Hz, CHMe2). 13C{1H} NMR: d 169.8 (s,

C.E. Ellul et al. / Journal of Organometallic Chemistry 695 (2010) 6–10
9
Table 1
Selected bond lengths (Å) and angles (°) for 1, 3 and 4.
1
3
4
Pd–Pd
Pd–CNHC
Pd–CO
2.6520(3), 2.6904(3) 2.6580(3)
2.6624(3), 2.6773(3)
2.088(4) 2.095(2)
2.043(3) 2.058(3) 2.077(3) 2.077(3)
2.7738(2), 2.7888(2) 2.8007(2)
2.080(2) 2.080(2) 2.095(2)
2.072(3) 2.076(3) 2.083(3)
2.039(4) 2.066(3) 2.061(3) 2.045(3)
2.054(4) 2.076(3)
Pd–SO2
2.2522(6), 2.2738(6), 2.2530(6),
2.2638(6), 2.2820(6), 2.2878(6)
OC–Pd–CO
Pd–CO–Pd
S–Pd–S
160.86(14), 156.98(14), 157.94(13)
80.48(13), 81.88(12), 80.12(13)
159.78(14), 158.61(9)
80.96(9), 80.27(13)
156.78(2), 156.27(2), 122.17(2)
Pd–S–Pd
75.869(18), 76.004(18), 75.571(19)
Table 2
Crystal data and structure refinement details for 1, 3 and 4.
Compound
1
3
4
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C69H79N6O3Pd3
1359.58
Triclinic
C30H48N6O3Pd3
859.94
Tetragonal
I41/a
12.8790(1)
12.8790(1)
43.1800(5)
90
C67H80N6O7Pd3S3
1496.75
Monoclinic
P21/n
22.1200(2)
12.5290(1)
24.2110(2)
90
93.008(1)
90
6700.63(10)
4
ꢀ
P1
12.3910(1)
12.9570(1)
22.5000(3)
96.435(1)
98.360(1)
115.831(1)
3153.85(5)
2
b (Å)
c (Å)
a
(°)
b (°)
90
90
c
(°)
U (Å3)
Z
7162.21(11)
8
1.595
Dc (gcmÀ3
l
)
1.432
0.897
1.484
0.946
(mmÀ1
)
1.527
F(0 0 0)
1394
3456
3064
Crystal size (mm)
Theta minimum, maximum for data
collection
0.30 Â 0.25 Â 0.13
0.20 Â 0.20 Â 0.15
0.25 Â 0.20 Â 0.15
3.74, 27.51
4.25, 27.51
3.56, 27.49
Index ranges
À16 6 h 6 16; À16 6 k 6 16;
À29 6 l 6 29
61 148
À16 6 h 6 16; À16 6 k 6 16;
À56 6 l 6 56
63 492
À28 6 h 6 28; À16 6 k 6 16;
À28 6 l 6 31
121 195
Reflections collected
Independent reflections, Rint
14 371, 0.0617
12 074
0.991
4122, 0.0405
3688
0.995
15 343, 0.0515
12 673
0.997
Reflections observed (>2
Data completeness
r)
Absorption correction
Multi-scan
Multi-scan
Multi-scan
Maximum, minimum transmission
Data/restraints/parameters
0.90, 0.83
14 371/0/731
1.026
0.738, 0.679
4122/0/198
1.199
0.831, 0.794
15 343/0/793
1.061
Goodness-of-fit (GOF) on F2
Final R1, wR2 [I > 2
r
(I)]
0.0492, 0.0974
0.0609, 0.1048
0.983, À0.816
0.0274, 0.0607
0.0327, 0.0629
0.612, À0.821
0.0316, 0.0724
0.0463, 0.0801
1.070, À1.189
Final R1, wR2 (all data)
Largest difference in peak, hole (eÅÀ3
)
NCN), 116.9 (s, NCH), 52.7 (s, NCHMe2), 23.6 (s, NCHMe2). Anal.
Calc. for PdCl2C18N4H32 (481.8036): C, 44.87; H, 6.69; N, 11.63.
Found: C, 44.52; H, 6.54; N, 11.58%.
to noise. IR (benzene-d6, cmÀ1): 1793 (
mCO). Elemental analysis
was precluded by the low stability of the complex in the absence
of CO.
4.4. Preparation of [Pd3(
l
-CO)3(IiPr2)3] (3)
4.5. Preparation of [Pd3(l-SO2)3(IMes)3] (4)
[Pd(IiPr2)2Cl2] (200 g, 0.415 mmol) and potassium graphite
(112 mg, 0.831 mmol) were loaded into an ampoule fitted with
a J. Young’s resealable valve. The vessel was purged with CO be-
fore CO-saturated THF (10 mL) was added. The suspension was
heated at 70 °C for 16 h to afford a dark red solution. This was fil-
tered and reduced in volume by slow bubbling of CO through the
[Pd3( -CO)3(IMes)3] (50 mg, 0.037 mmol) was dissolved in THF
l
(10 mL) in an ampoule fitted with a J. Young’s resealable valve. A
steady stream of SO2 gas was bubbled through the solution, which
resulted in the solution becoming warm and undergoing an imme-
diate colour change from red to blood-red. After the solution had
cooled (ca. 5 min), the stream of SO2 was stopped and the solvent
solution. [Pd3(
l
-CO)3(IiPr2)3] was isolated upon layering with CO-
removed in vacuo to give [Pd3(l-SO2)3(IMes)3] as a dark red solid.
saturated hexane. Yield: 6 mg (5% yield). NMR and IR spectra
were recorded under 1 atm CO to slow the rate of decomposition.
1H NMR (toluene-d8, 400 MHz, 298 K): d 6.53 (s, 2H, NCH), 5.21
(sept, 2H, JHH = 6.79 Hz, NCHMe2), 1.18 (s, 12H, JHH = 6.79 Hz,
Yield: 53 mg (98% yield). 1H NMR (THF-d8, 500 MHz, 298 K): d 7.12
(s, 2H, NCH), 6.79 (s, 4H, C6Me3H2), 2.33 (s, 6H, p-Me), 1.95 (12H, s,
o-Me). 13C{1H} NMR: d 188.7 (s, NCN), 138.4 (s, C-p-Me),136.9 (s, C-
o-Me), 136.8 (s, N-i-C), 129.6 (s, NCH), 124.6 (s, m-CH), 21.7 (s, p-
Me), 19.0 (s, o-Me). IR (benzene-d6, cmÀ1): 1261, 1099, 1017
13
1
*
NCHMe2). C{ H} NMR: d 196.1 (s, NCN), 115.4 (s, NCH), 52.4
*
(s, NCHMe2), 23.4 (s, NCHMe2). Due the small amount of product,
(m
SO). Repeated attempts to measure CHN analysis gave consis-
tently erroneous values for % carbon.
the 13C signal for Pd–CO could not be detected due to poor signal

10
C.E. Ellul et al. / Journal of Organometallic Chemistry 695 (2010) 6–10
[9] (a) For recent reviews of NHCs, see: S.P. Nolan (Ed.), N-Heterocyclic Carbenes
4.6. Crystallography
in Synthesis, Wiley-VCH, Weinheim, 2006;
(b) F.A. Glorius, Top. Organomet. Chem. 21 (2007);
Single crystals of compounds 1, 3 and 4 were analysed at 150 K,
using Mo(Ka) radiation on a Nonius Kappa CCD diffractometer. De-
(c) F.E. Hahn, M.C. Jahnke, Angew. Chem., Int. Ed. 47 (2008) 3122.
[10] (a) M.F. Lappert, P.L. Pye, J. Chem. Soc., Dalton Trans. (1977) 2172;
(b) A.J. Carty, N.J. Taylor, W.F. Smith, M.F. Lappert, P.L. Pye, J. Chem. Soc., Chem.
Commun. (1978) 1017;
tails of the data collections, solutions and refinements are given in
Table 2. The structures were solved using SHELXS-97 [19] and refined
using full-matrix least squares in SHELXL-97 [19]. Refinements were
generally straightforward, with the respective asymmetric units
comprising of one molecule, half of a molecule and one molecule
of the organometallic compound, plus a portion of solvent in two
of the structures.
In particular, 1 was seen to contain half of a hexane molecule
proximate to an inversion centre, while 4 crystallised with one
molecule of (ordered) THF. Crystallographic symmetry in 3 re-
sulted in Pd1, O2, C2 and C3 being located on a crystallographic
twofold rotation axis which serves to generate the remainder of
the molecule.
(c) J.A. Cabeza, I. del Río, S. García-Granda, V. Riera, M.G. Sánchez-Vega, Eur. J.
Inorg. Chem. (2002) 2561;
(d) J.A. Cabeza, I. da Silva, I. del Río, L. Martínez-Méndez, D. Miguel, V. Riera,
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(e) J.A. Cabeza, I. del Río, D. Miguel, M.G. Sánchez-Vega, Chem. Commun.
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(f) J.A. Cabeza, I. da Silva, I. del Río, M.G. Sánchez-Vega, Dalton Trans. (2006)
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(h) C.E. Cooke, T. Ramnial, M.C. Jennings, R.K. Pomeroy, J.A.C. Clyburne, Dalton
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(i) C.E. Cooke, M.C. Jennings, R.K. Pomeroy, J.A.C. Clyburne, Organometallics 26
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(j) C.E. Ellul, M.F. Mahon, O. Saker, M.K. Whittlesey, Angew. Chem., Int. Ed. 46
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Acknowledgements
(l) S. Milosevic, E. Brenner, V. Ritleng, M.J. Chetcuti, Dalton Trans. (2008) 1973;
(m) J.A. Cabeza, I. del Río, D. Miguel, E. Pérez-Carreño, M.G. Sánchez-Vega,
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Organometallics 27 (2008) 5777;
Financial support was provided by a Doctoral Training Award to
CEE. We thank Dr. A.D. Burrows (University of Bath) for invaluable
input at the start of this work.
(o) M.R. Crittall, C.E. Ellul, M.F. Mahon, O. Saker, M.K. Whittlesey, Dalton Trans.
(2008) 4209;
(p) L. Deng, R.H. Holm, J. Am. Chem. Soc. 130 (2008) 9878;
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García, Angew. Chem., Int. Ed. 48 (2009) 555;
Appendix A. Supplementary data
(r) J.A. Cabeza, I. del Río, J.M. Fernández-Colinas, E. Pérez-Carreño, M.G.
Sánchez-Vega, D. Vázquez-García, Organometallics 28 (2009) 1832;
(s) J.A. Cabeza, I. del Río, J.M. Fernández-Colinas, M.G. Sánchez-Vega,
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CCDC 743070, 743071 and 743072 contain the supplementary
crystallographic data for 1, 3 and 4. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2009.09.018.
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