Crystal structures of poly(aryl ether) dendrons with palladium scorpionate complexes at their focal point

Article abstract of DOI:10.1021/ic7005748

Novel palladium(II) complexes with bis(pyrazol-1-yl)methane ligands at the focal point of G0-G3 poly(aryl ether) Frechet-type dendrons are reported. The molecular structures of the metallodendrimer series G0, G1, and G2 [(dend)CH(3,5-Me₂pz)₂(PdCl₂)] have been determined by X-ray diffraction methods. The three structures show a similar three-dimensional organization of the metal complex, which is progressively engulfed by the branches with increasing dendrimer generation.

Full text of DOI:10.1021/ic7005748

Inorg. Chem. 2007, 46, 4793−4795
Crystal Structures of Poly(aryl ether) Dendrons with Palladium
Scorpionate Complexes at Their Focal Point†
Alberto Sa´nchez-Me´ndez, Ernesto de Jesu´s,* Juan C. Flores,* and Pilar Go´mez-Sal‡
Departamento de Qu´ımica Inorga´nica, UniVersidad de Alcala´, Campus UniVersitario,
28871 Alcala´ de Henares, Madrid, Spain
Received March 26, 2007
Novel palladium(II) complexes with bis(pyrazol-1-yl)methane ligands
at the focal point of G0 G3 poly(aryl ether) Fre´chet-type dendrons
Fre´chet-type dendrons (G1 and G2, Scheme 1) show dendron
arrangements that are extended and essentially planar (see
below).4 However, several studies have proposed different
degrees of back-folding both in the solid state and in
solution.3,9,10 We have recently suggested that bis(pyrazol-
1-yl)methanenickel(II) complexes are partially enclosed by
the dendritic arms of poly(aryl ether) dendrons to which they
were linked.11 These studies were carried out in a chloroform-
d1 solution, and our conclusions were based on the significant
−
are reported. The molecular structures of the metallodendrimer
series G0, G1, and G2 [(dend)CH(3,5-Me2pz)2(PdCl2)] have been
determined by X-ray diffraction methods. The three structures show
a similar three-dimensional organization of the metal complex,
which is progressively engulfed by the branches with increasing
dendrimer generation.
(4) Structures of G2 dendrimers based on 3,5-dihydroxybenzyl alcohol:
(a) Karakaya, B.; Claussen, W.; Gessler, K.; Saenger, W.; Schlu¨ter,
A.-D. J. Am. Chem. Soc. 1997, 119, 3296-3301. March, R. E. Acta
Crystallogr., Sect. B 1999, 55, 931-936. (b) Brevis, M.; Clarkson,
G. J.; Goddard, V.; Helliwell, M.; Holder, A. M.; McKeown, N. B.
Angew. Chem., Int. Ed. 1998, 37, 1092-1094. Brevis, M.; Clarkson,
G. J.; Helliwell, M.; Holder, A. M.; McKeown, N. B. Chem.sEur. J.
2000, 6, 4630-4636. (c) Vo¨gtle, F.; Plevoets, M.; Nieger, M.;
Azzellini, G. C.; Credi, A.; De Cola, L.; De Marchis, V.; Venturi,
M.; Balzani, V. J. Am. Chem. Soc. 1999, 121, 6290-6298.
(5) Other G2 structures: (a) Seyferth, D.; Son, D. Y.; Rheingold, A. L.;
Ostrander, R. L. Organometallics 1994, 13, 2682-2690. (b) de Groot,
D.; Eggeling, E. B.; de Wilde, J. C.; Kooijman, H.; van Haaren, R. J.;
van der Made, A. W.; Spek, A. L.; Vogt, D.; Reek, J. N. H.; Kamer,
P. C. J.; van Leeuwen, P. W. N. M. Chem. Commun. 1999, 1623-
1624. (c) Huang, B.; Paquette, J. R. Org. Lett. 2000, 2, 239-242. (d)
Wang, R.; Yang, J.; Zheng, Z.; Carducci, M. D.; Jiao, J.; Seraphin, S.
Angew. Chem., Int. Ed. 2001, 40, 549-552.
(6) Structures of transition-metal dendrimers (G1): (a) Seyferth, D.;
Kugita, T.; Rheingold, A. L.; Yap, G. P. A. Organometallics 1995,
14, 5362-5366. (b) Kriesel, J. W.; Ko¨nig, S.; Freitas, M. A.; Marshall,
A. G.; Leary, J. A.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120, 12207-
12215. (c) Leininger, S.; Stang, P. J.; Huang, S. Organometallics 1998,
17, 3981-3987. (d) Coupar, P. I.; Jaffre`s, P.-A.; Morris, R. E. J. Chem.
Soc., Dalton Trans. 1999, 2183-2187. (e) Kleij, A. W.; Klein Gebbink,
R. J. M.; van den Nieuwenhuijzen, P. A. J.; Kooijman, H.; Lutz, M.;
Spek, A. L.; van Koten, G. Organometallics 2001, 20, 634-647. (f)
Fujihara, T.; Obora, Y.; Tokunaga, M.; Sato, H.; Tsuji, Y. Chem.
Commun. 2005, 4526-4528. (g) Sato, H.; Fujihara, T.; Obora, Y.;
Tokunaga, M.; Kiyosu, J.; Tsuji, Y. Chem. Commun. 2007, 269-271.
(7) Hawker, C.; Fre´chet, J. M. J. J. Chem. Soc., Chem. Commun. 1990,
1010-1013. Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990,
112, 7638-7647.
Most dendrimers are flexible molecules, and therefore their
structures have been largely debated.1 de Gennes and Hervet,
for example, have postulated a model with the end groups
at the surface of the dendrimer,2a while Lescanec and
Muthukumar have predicted an inward folding of the chain
ends.2b Several computational and experimental techniques
have been used to gain structural information regarding
dendrimers in solution and the solid state,3 but only X-ray
diffraction techniques allow the acquisition of highly precise
data regarding the internal molecular conformations. Unfor-
tunately, the growth of single crystals of suitable quality has
been restricted mainly to small dendrimers because of a
variety of problems, in part due to the conformational
flexibility of these molecules.4-6
Poly(aryl ether) dendrons based on 3,5-dihydroxybenzyl
alcohol, developed by Fre´chet, are among the most popular
dendritic structures7 and have been widely used in organo-
metallic catalysis,8 usually after metal functionalization at
the core or focal point. The conformation adopted by the
dendritic arms of these catalysts in solution is important
because it determines the nanoenvironment and accessibility
of the metal centers. The few X-ray structures obtained for
† Dedicated to the late Prof. Trofimenko.
* To whom correspondence should be addressed. E-mail: ernesto.dejesus@
uah.es (E.deJ.), juanc.flores@uah.es (J.C.F.).
(8) For recent reviews about dendrimers, see: (a) Reek, J. N. H.; Are´valo,
S.; van Heerbeek, R.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.
AdV. Catal. 2006, 49, 71-151. (b) Me´ry, D.; Astruc, D. Coord. Chem.
ReV. 2006, 250, 1965-1979. (c) Gade, L. H., Ed. Dendrimer Catalysis;
Springer: Berlin, 2006. (d) Andre´s, R.; de Jesu´s, E.; Flores, J. C. New
J. Chem. 2007, doi:10.1039/b615761k.
(9) (a) Wooley, K. L.; Klug, C. A.; Tasaki, K.; Schaefer, J. J. Am. Chem.
Soc. 1997, 119, 53-58 and references cited therein. (b) Riley, J. M.;
Alkan, S.; Chen, A.; Shapiro, M.; Khan, W. A.; Murphy, W. R., Jr.;
Hanson, J. E. Macromolecules 2001, 34, 1797-1809 and references
cited therein.
‡ Crystal structure determinations: pilar.gomez@uah.es (P.G.-S.).
(1) (a) Dendrimers and Other Dendritic Polymers; Fre´chet, J. M. J.,
Tomalia, D. A., Eds.; Wiley-VCH: New York, 2002. (b) Newkome,
G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and Dendrons: Con-
cepts, Syntheses, Applications; Wiley-VCH: Weinheim, Germany, 2001.
(2) (a) de Gennes, P.-G.; Hervet, H. J. Phys., Lett. 1983, 44, L351-L352.
(b) Lescanec, R. L.; Muthukumar, M. Macromolecules 1990, 23,
2280-2288.
(3) (a) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV. 1999, 99,
1665-1688. (b) Bauer, B. J.; Amis, E. J. work in Chapter 11 of ref 1b.
(10) Naidoo, K. J.; Hughes, S. J.; Moss, J. R. Macromolecules 1999, 32,
331-341.
10.1021/ic7005748 CCC: $37.00
Published on Web 05/10/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 12, 2007 4793

COMMUNICATION
Scheme 1. Synthesis of the Palladium Complexesa
Figure 1. Molecular structure and numbering scheme for 1. Selected bond
lengths (Å): Pd-Cl(1) 2.2797(16), Pd-Cl(2) 2.2883(16), Pd-N(1) 2.031-
(5), Pd-N(3) 2.027(5), C(11)-N(2) 1.458(7), C(11)-N(4) 1.457(7). Selec-
ted bond angles (deg): Cl(1)-Pd-Cl(2) 89.10(6), N(1)-Pd-N(3) 85.69-
(19), Cl(1)-Pd-N(3) 91.87(15), Cl(2)-Pd(1)-N(1) 93.16(14), N(2)-C(11)-
N(4) 109.6(4). Dihedral angle (deg): Pd‚‚‚C(11)-C(12)-C(13) 112.0.
a (i) toluene, 80 °C, 3 h, 83-92% yield.
attenuation observed for the NMR longitudinal relaxation
times (T1) of protons situated in all of the dendritic layers
due to the effect of the paramagnetic nickel(II) center. Here
we report the preparation of diamagnetic palladium(II)
analogues and the elucidation of their single-crystal structures
up to the second generation.
The coordination chemistry of poly(pyrazoly-1-yl)alkane
ligands has made significant progress during the past few
years12,13 because of advances in their synthesis14 and because
of their wide range of attractive applications, particularly in
catalysis.15 We have combined scorpionate ligands with
metallodendrimers by taking advantage of the facile func-
tionalization of the bridging methine carbon atom in poly-
(pyrazol-1-yl)methane,16 first described by Reger et al.,17
which permits the easy preparation of the dendritic ligands
RCH(3,5-Me2pz)2 (I-IV, Scheme 1).11 The new complexes
1-4 were synthesized in warm toluene by displacement of
the diene ligand of [PdCl2(COD)] (COD ) η4-1,5-cyclooc-
tadiene) with I-IV (see the Supporting Information for
experimental details and characterization data). Coordination
Figure 2. Molecular structure and numbering scheme for 2 (molecule A).
Selected bond lengths (Å): Pd-Cl(1) 2.281(2), Pd-Cl(2) 2.292(2), Pd-
N(1) 2.046(7), Pd-N(3) 2.012(6), C(11)-N(2) 1.451(9), C(11)-N(4) 1.463-
(10). Selected bond angles (deg): Cl(1)-Pd-Cl(2) 90.29(8), N(1)-Pd-N(3)
86.3(3), Cl(1)-Pd-N(1) 92.2(2), Cl(2)-Pd-N(3) 91.05(19), N(2)-C(11)-
N(4) 110.3(6). Dihedral angle (deg): Pd‚‚‚C(11)-C(12)-C(13) 115.6.
of the ligands to the metal center causes a significant
downfield shift of the 1H NMR resonances due to the methyl
substituents at the 3 position of the pyrazolyl ring (see
Scheme 1) and to the methylene and ortho protons of the
benzyl group directly bonded to the palladacycle (∆δ ≈
+0.5, +1.7, and +0.5 ppm, respectively). The C, H, and N
elemental analyses and the IR and mass spectrometry (MS)
spectra are all consistent with the proposed structures.
The X-ray molecular structures of 1 (G0), 2 (G1), and 3
(G2) consist of discrete monometallic molecules (Figures
1-3, respectively). The structural parameters defining the
-CH(3,5-Me2pz)2PdCl2 moieties are almost identical for the
three complexes, with the palladium atoms exhibiting rather
square-planar geometries. The Pd(NN)2C palladacycles adopt
pronounced boat conformations in which the benzyl groups
attached to the methine carbons (C11) occupy axial positions,
thereby avoiding the steric hindrance that would arise in the
equatorial positions with the adjacent methyl groups at the
5 position of the pyrazolyl ring. This steric repulsion confers
rigidity on the metallacycle because no boat-to-boat confor-
mational exchange is observed in solution. The dendritic
substituents are oriented asymmetrically in relation to the
(11) Sa´nchez-Me´ndez, A.; Benito, J. M.; de Jesu´s, E.; de la Mata, F. J.;
Flores, J. C.; Go´mez, R.; Go´mez-Sal, P. Dalton Trans. 2006, 5379-
5389.
(12) Trofimenko, S. Scorpionates. The Coordination Chemistry of Poly-
pyrazolylborate Ligands; Imperial College Press: London, 1999.
(13) (a) Otero, A.; Ferna´ndez-Baeza, J.; Antin˜olo, A.; Tejada, J.; Lara-
Sa´nchez, A. Dalton Trans. 2004, 1499-1510. (b) Pettinari, C.;
Pettinari, R. Coord. Chem. ReV. 2005, 249, 525-543. (c) Pettinari,
C.; Pettinari, R. Coord. Chem. ReV. 2005, 249, 663-691. (d) Bigmore,
H. R.; Lawrence, S. C.; Mountford, P.; Tredget, C. S. Dalton Trans.
2005, 635-651.
(14) (a) Reger, D. L.; Grattan, T. C.; Brown, K. J.; Little, C. A.; Lamba,
J. J. S.; Rheingold, A. L.; Sommer, R. D. J. Organomet. Chem. 2000,
607, 120-128. (b) Julia´, S.; Sala, P.; del Mazo, J.; Sancho, M.; Ochoa,
C.; Elguero, J.; Fayet, J.-P.; Vertut, M.-C. J. Heterocycl. Chem. 1982,
19, 1141-1145. (c) Julia, S.; del Mazo, J. M.; Avila, L.; Elguero, J.
Org. Prep. Proced. Int. 1984, 16, 299-307.
(15) (a) Bigmore, H. R.; Dubberley, S. R.; Kranenburg, M.; Lawrence, S.
C.; Sealey, A. J.; Selby, J. D.; Zuideveld, M. A.; Cowley, A. R.;
Mountford, P. Chem. Commun. 2006, 436-438. (b) Lawrence, S. C.;
Ward, B. D.; Dubberley, S. R.; Kozak, C. M.; Mountford, P. Chem.
Commun. 2003, 2880-2881. (c) Milione, S.; Montefusco, C.; Cuenca,
T.; Grassi, A. Chem. Commun. 2003, 1176-1177.
(16) Sa´nchez-Me´ndez, A.; Silvestri, G. F.; de Jesu´s, E.; de la Mata, F. J.;
Flores, J. C.; Go´mez, R.; Go´mez-Sal, P. Eur. J. Inorg. Chem. 2004,
3287-3296.
(17) (a) Reger, D. L.; Semeniuc, R. F.; Smith, M. D. J. Organomet. Chem.
2003, 666, 87-101. (b) Reger, D. L.; Semeniuc, R. F.; Smith, M. D.
J. Chem. Soc., Dalton Trans. 2002, 476-477.
4794 Inorganic Chemistry, Vol. 46, No. 12, 2007

COMMUNICATION
(as is commonly observed, for example, in derivatives of
anisole).18 The singularity of the crystal structure of 3 arises
from the fact that this is the first case in which significant
back-folding is observed in a poly(aryl ether), with four of
their six branch units having Ψ between 71° and 86°.19
A
dependence of the structure and shape of dendrons on the
functional group at the focal point (or periphery) has been
observed previously and has been used to control the size
of supramolecular assemblies in bulk.20
Because crystallization is a kinetically controlled process,
crystal structures do not necessarily represent energy minima.
Moreover, both crystal-packing forces and intramolecular
interactions determine their energies. In spite of this, solid-
state studies are valuable as a guide to conformations that
are easily reachable in solution. In fact, the picture obtained
here in the solid state, according to which the dendritic wedge
does not spread but tends to enfold the palladium center,
essentially matches the conclusion of our previous report,
where related nickel complexes were studied in a CDCl3
solution by T1 measurements of their 1H NMR resonances.11
In other words, it is very likely that the accessibility of the
metal center in complexes 1-3 in a CDCl3 solution at any
given moment must be very close to those depicted in Figures
1-3. It should be noted, however, that the arrangement of
the poly(benzylphenyl ether) dendritic arms can be affected
by the nature of the solvent.3,9,10
Figure 3. Molecular structure and numbering scheme for 3. Selected bond
lengths (Å): Pd-Cl(1) 2.2852(15), Pd-Cl(2) 2.2826(12), Pd-N(1) 2.038-
(3), Pd-N(3) 2.018(3), C(11)-N(2) 1.451(5), C(11)-N(4) 1.454(5). Selec-
ted bond angles (deg): Cl(1)-Pd-Cl(2) 89.43(5), N(1)-Pd-N(3) 85.92-
(14), Cl(1)-Pd-N(3) 92.34(10), Cl(2)-Pd-N(1) 92.10(10), N(2)-C(11)-
N(4) 108.9(3). Dihedral angle (deg): Pd‚‚‚C(11)-C(12)-C(13) 114.5.
Table 1. Structural Parameters Determined by Single-Crystal X-ray
Diffraction for G2 Fre´chet-Type Poly(aryl ether) Dendrons
percentage of dihedral angles that
fall in the specified range (%)
mean Ψ
Φ (0 or 180 ( 15°) Ψ (180 ( 15°) Ω (0 ( 15°) (deg)
ref 4a
100
100
100
100
100
67
100
33
67
33
33
17
177
144
174
110
ref 4b
In summary, we have presented the single-crystal struc-
tures of a series of scorpionate dendrimers of palladium that
includes, to the best of our knowledge, the first higher
generation (G2) transition-metal dendrimer to be crystallo-
graphically characterized so far. The solid-state structure of
this G2 metallodendrimer shows that the dendritic wedges
tend to wrap up the palladium center rather than spreading.
Further synthetic, catalytic, and structural work with these
types of dendronized ligands is currently underway in the
context of palladium chemistry.
ref 4c
complex 3
palladacycle, with dihedral angles defined by the Pd‚‚‚C(11)
molecular axes and the C(12)-C(13) bonds in the range of
112.0-115.6°.
Several features can be highlighted regarding the confor-
mational arrangement of the dendritic substituents. First, it
looks as if there has been an onionlike growth on going from
1 to 3 in which the layer of a new generation is essentially
built on top of the preexisting structure of the preceding
generation. Second, the dendritic arms in 2 and 3 adopt a
disposition that is clearly nonplanar, with adjacent aromatic
rings mainly almost orthogonally. Finally, the dendritic
moiety in 3 tends to wrap up the palladium complex at the
core. The topology of the entire dendritic moiety is deter-
mined by the arrangements of the benzyl branches around
each ramification unit, which are defined by the three
dihedral angles involved in the ether linkage (Table 1).10 The
relative orientation of the aromatic rings is determined by
the values of Φ and Ω (which are 0 or 180° for a planar
structure), whereas the central dihedral angle Ψ is the most
important when considering the overall structure. The branch
is fully extended if this dihedral angle is 180° and completely
back-folded to the dendritic core when it is 0°. The
percentage of branches adopting a planar and extended
disposition ((15°) are summarized in Table 1 for all of the
structures of Fre´chet-type poly(aryl ether) G2 dendrimers
reported to date.4 The compounds referenced in Table 1 are
identical with 3 except in the function located at their focal
points. The dihedral angle Φ is almost 0 or 180° in all cases,
which means that the -CC6H3(OC-)2 core is virtually planar
Acknowledgment. We are grateful for financial support
from the Ministerio de Educacio´n y Ciencia (Project
CTQ2005-00795/BQU), the Comunidad de Madrid (Project
S-0505/PPQ/0328-03), and the Universidad de Alcala´ (FPI
grant to A.S.-M.). P. G.-S. is also indebted to “Factoria de
Cristalizacio´n” (Consolider-Ingenio 2010).
Supporting Information Available: Preparative details and
characterization data (NMR, IR, and MS) for the new compounds
and X-ray crystallographic information for complexes 1-3 in CIF
format. This material is available free of charge via the Internet at
http://pub.acs.org.
IC7005748
(18) Bellard, S.; Elliot, R.; McDonald, E. Acta Crystallogr., Sect. B 1982,
38, 669-671.
(19) Several crystal structures of dendrimers built of other motifs also
showed a tendency to back-folding. For example, see: Larre´, C.;
Donnadieu, B.; Caminade, A.-M.; Majoral, J.-P. Chem.sEur. J. 1998,
4, 2031-2036. Larre´, C.; Bressolles, D.; Turrin, C.; Donnadieu, B.;
Caminade, A.-M.; Majoral, J.-P. J. Am. Chem. Soc. 1998, 120, 13070-
13082.
(20) Percec, V.; Cho, W.-D.; Ungar, G. J. Am. Chem. Soc. 2000, 122,
10273-10281 and references cited therein.
Inorganic Chemistry, Vol. 46, No. 12, 2007 4795