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Crystal Growth in Zeolite Y Revealed by Atomic Force Microscopy.

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Furthermore, the aggregates formed can act as receptors and
bind small molecules (water) or ions (Li+), stabilizing unusual
geometries in its cavity. This shows some relevance to the function of enzymes and antibodies that can stabilize the geometry
of reaction intermediates by a combination of stereoelectronic
effects and hydrogen bonding networks."
Received. January 11, 1996 [Z8711 IE]
German version: Angen. Chem. 1996, 108. 1299 - 1300
Keywords: catechol * complexes with oxygen ligands . selfassembly supramolecular chemistry * titanium compounds
-
[l] Reviews: D. S. Lawrence, T. Jiang, M. Levett. Ciiem. Rei'. 1995.95.2229; E. C.
Constable, Te~ruhedron1992. 48. 10013. and references therein.
[2] A. F. Williams. C. Piguet, G. Bernardinelli. Angeir. CIim?. 1991. 103. 1530,
Angew. Chrin. Inr. Ed. Engl. 1991. 30, 1490; G. Bernardinelli. C. Piguet. A. F.
Williams, ;hid. 1992, 104. 1626 and 1992. 31. 1622; R. Kriimer. J.-M. Lehn, A
De Cian, J. Fischer, ;bid. 1993, 105, 764 and 1993, 32, 703: K. T. Potts. C . P.
Horwitr. A. Fessak, M. Keshavarz-K. E. Nash, P. J. TOScdnO. J Ain C h ~ m .
Soc. 1993. 115, 10444; D. Zurita. P. Baret, JLL. Pierre, Nrii.. J. C/i~ii7.
1994. 18.
1143: C. Piguet. G. Hopfgartner, A. F. Williams. J.-C. G. Biinzli. J. Clietii. Soc.
Chiw Cornmiin. 1995. 491.
131 D L. White. K. N. Raymond, J Am. C/ic,tn.SOC.1985, 107.6540: E. J. Corey.
C. L Cywin. M. C. Noe, Tetmhedron L p t t . 1994, 35, 69: E. J. Enemark,
T. D. P. Stack. Angeii. Chem 1995, 107. 1082. AnReii. C h m . In/. E d Engl.
1995, 34, 996.
[4] M. Albrecht, S Kotila, Angeir. C/7en7 1995. 107. 2285. Angrw. Clieiir. I n / . Ed.
Engl. 1995, 34. 2134.
[5] M. Albrecht. Swthesis 1996, 230.
[6] Li4[(l)3Ti2] ' T NMR (100 MHz, BB/DEPT. [D,]methanol): d = 158.9 (C).
157.5(C), 128.0(C). 120.4(CH), 118.4(CH). 110.7 (CH). 32.6(CH2): UVWIS
(methanol): i = 209, 274, 363 nm: positive ion FAB-MS (glycerine): m/: =
839 [H,Li,[(l),Ti,]+].
845 [H2Li3[(1)3Ti2]+]. 851 [HLi,[(l),Ti,]'];
C,,H3,Li,0,,Ti,.6H,0.5MeOH(1118.6): calcd: C 50.47. H 5.59; found C
SO. 16. H 5.54.
[7] Crystals of Li,[(l),Ti2].2 H 2 0 . 6 D M F were obtained by slow diffusion ofether
in a solution of the compound in DMF/water ( 1 O : l ) . Crystal data for
Li,[(l),Ti2]-2H,0 6 D M F . Formula C,,H,,Li,N,O,,Ti,,
formula weight
1324.83, brownish yellow plates (0.4 x 0.3 x 0.2 mm')), monoclinic. space group
C 2 k ( N o . 15). a = 1208.7(2), h = 2280.0(3), c = 2337.2(2) pm. fi = 100.26(1)".
V = 6338 x 10" pm'.
Z = 4. F(OO0) = 2776, T = - 100°C. pca,ed=
1.388 g cm
= 3.3 cm- I , empirical absorption correction from Y scans
(CmjnmdX = 0 940/0.999), Enraf-Nonius MACH3 diffractometer. j. =
71.073 pm, w-20 scans, 5504 reflections measured (ih.- k ,
/,2Hm,, = 49')
of which 5363 were independent and 3066 observed [/>2u(I)], 449 refined
parameters, R = 0.050. w R 2 = 0.122, residual electron density max./min. 0.50/
- 0.36 e k '. The structure was solved by direct methods (SHELXS-86) and
refined against F 2 (SHELXL-93). hydrogen atoms were introduced to their
calculated positions and refined isotropically as riding atoms, the hydrogen
atoms of the crystalline water molecules were located from the difference
Fourier map. Asymmetric unit was one half of the monomer and the other half
was generated by symmetry operation - .Y, J. 0.5 - .: One D M F molecule was
disordered (59-41) and it was refined using geometrical and and thermal restraints. The figures were drawn with the schakal program. Further details of
the X-ray crystal structure analysis are available upon request from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen. on
quoting the depository number CSD-404667
[S] For a further example of diastereoselectivity in the self-assembly of achiral
components, see for example: R. W. Saalfrank, B. Homer, D. Stalke, J. Salbeck. A n p - . Cltein. 1993, 1Uj31223. A n p c Chein Int. Ed. Engl. 1993, 32.
1179.
191 J.-M. Lehn. Angiw Chein 1988. 100.92: Angeii . Chmm. In!. Ed. Engl. 1988,27,
90.
[lo] M. Albrecht, H. Rottele. P. Burger. unpublished results.
1111 J. Arnold. D. Y Dawson. C. G. Hoffman, J A m Cfrrm. Soc. 1993. 115.
2707.
[12] H . Sugimoto. M. Mori. M. Masuda. T Taga. J. Chem. Soc Chin. Commun.
1986,962.
[13] F. A Schroder, H. P. Weber. Actu Cryst. B 1975. 31. 1745; R. J. Morris, G. S.
Girolami. Po/i./iedi-un 1988. 7, 2001.
[14] F. E. Hahn. M. Keck, K. N. Raymond, Inorg. Clieni 1995, 34, 1402.
[lS] See. for example: P. G. Schulz. R. A. Lerner. Scirnce 1995. 269. 1835.
'.
+
Crystal Growth in Zeolite Y Revealed by
Atomic Force Microscopy**
Michael W. Anderson,* Jonathan R. Agger,
John T. Thornton, and Nicola Forsyth
Zeolites form one of the most important classes of heterogeneous catalyst. Their microporous nature and consequent high
surface area impart both high selectivity and high activity. Although well over one hundred zeolite-like compounds can now
be synthesized with a wide range of pore geometries their synthesis remains very much a "black art". Generally, zeolites are
synthesized from an alkaline aluminosilicate gel that sometimes
contains organic cations. At least two main routes to crystallization have been identified: first, gel globules crystallize without
transport of material resulting in globular, highly twinned crystals; second, single zeolite crystals grow by transport of nutrient
from the gel phase to a growing zeolite crystal. It is the second
route that is dealt with in this study.
Novel frameworks are often produced by incorporating large
organic cations into the synthesis mixture, but whether these
cations act as structure templates or as structure blockers is not
well understood. There are a number of examples of syntheses
in which the organic molecule fits very snugly into the resulting
tunnel structure, for example the tetrapropylammonium ion in
ZSM-5, and it is tempting in these cases to think of the molecules as templates. However, often the same structure can be
made with other organic molecules or with no organic molecules
at all-again ZSM-5 is a good example of a zeolite that can be
made with a wide variety of organic molecules o r simply in the
presence of N a + ions.
Zeolite Y (structure designation FAU) is probably the single
most important zeolite from a catalytic standpoint, since it is
used for the catalytic cracking of petroleum. An interesting recent example of structure-directing through the use of organic
molecules is the synthesis of the hexagonal polymorph of zeolite
Y (structure designation EMT) in the presence of [18]crown-6
and Na' ions."] Both FAU and EMT can be constructed by
linking sodalite cages (truncated octahedra) through double sixmembered rings. The relationship of the sodalite cages is as for
zinc blend in FAU and as for wurtzite in EMT. It has been
shown"] that a [Na([l8]crown-6)]+ macrocation is located within the oblate cages of E M T into which it sits very snugly. The
structure of EMT was originally proposed by BreckL3]and was
known as Breck structure six (BSS) but proved elusive to synthesize. The cubic polymorph of zeolite Y on the other hand is
extremely easy to synthesize and can be prepared from an aluminosilicate gel containing only Na' ions. As EMT and FAU are
constructed from the same building units they easily form random intergrowths, and this has been observed in many preparations containing a variety of organic molecules.[41Like E M T the
FAU structure can also be prepared by using a crown ether
I*]
[**I
1210
$> VCH Ver/ag.s~rse//schufi
inhH. 0.69451 Wenihe;tn. 1996
Dr. M. W. Anderson. Dr. J R. Agger
Department of Chemistry, UMIST
PO Box 88, Manchester. M60 1QD (UK)
Fax: Int. code +(161)236 7677
e-mail: M. Anderson@ umist.ac.uk
J. T. Thornton
Digital Instruments,
520 E. Montecito St.. Santa Barbara, CA (USA)
Dr. N. Forsyth
L. 0 .T. Oriel.
1 Mole Business Park. Leatherhead. KT22 7AU (UK)
We thank the European Commission through ESPRIT and EPSRC for funding J. R. A.. Osamu Terasaki for useful discussions and Ian Brough for the
scanning electron micrographs.
$ 15.00+ 2jjO
057/J-U833/96~3511-1210
Angen. Chem. Inl. Ed. Engl. 1996, 35, No. I 1
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([15]crown-5), and it has been shown that by mixing[18]crown-6
with [15]crown-5 controlled intergrowths of the two materials
can be
A mechanism has been postulated", in
which [18]crown-6 acts to stabilize the EMT structure during
crystallization. This mechanism views the crystallization in terms
of a layer growth (supported by high-resolution electron microscopy (HREM) of surface growth steps) in which [18]crown-6
decorates a growth layer. This decoration occurs at pocket sites
on the growing surface and forces the next growing surface to
grow with a mirror symmetry rather than an inversion symmetry
to the decorated layer. Such mirror related growth gives EMT
rather than FAU structure. The postulation of layer growth was
based primarily on electron microscopy which only provides a
projected image through the crystal, therefore, although the presence of growth steps was identified, the true morphology of the
related terraces circumscribed by these steps was unclear.
Atomic force microscopy (AFM) has recently been used to
monitor the surface features in zeolite minerals['01 and also the
details o f crystal growth of a number of organic[' and inorganic"21 materials. In this present study we have utilized AFM to
scan thc surhce of crystals of FAU and EMT synthesized by
using crown ether templates to reveal the growth face morphology. The crown ether preparations result in relatively large
( > 1 mm), defect-free crystals compared with conventional routes
for the synthesis of zeolite Y making study by AFM feasible.
Scanning electron micrographs of FAU and EMT samples
(Fig. 1) reveal characteristic octahedral and hexagonal crystal
4
Fig. 1 a ) AFM image (TappingMode) of FAU strucIurr shoving the ( I 1 1 ) c Face
The triangular terraces are growth steps each about 1 5 nm in height. This vnluc w;is
derived from section analysis (b), which shows height and separation ofcrystal steps
(il: lareral shift in the YJ plane). The colored arrows correspond 10 the folloc\.ing step
heights and (in parentheses) lateral step separations. respectively ired: 1.5 nm
(33 nm); green: 1.7 nm (39 nm): black: 1.4 nm ( 2 5 nm).
Fig. I . Scanning electron micrographs of a ) a FAU crystal showing octahedral
habit with a crystal edge length of about 1 pin and b) an E M T crystal showing
hexagonal prism habit with edge length about 1 pm.
morphology, respectively. In FAU the faces of the crystal are all
{ l l l } c , in EMT the top and bottom faces are {OOI}, and the
side faces are {loo),,, where c and h depict cubic and hexagonal
phases, respectively. Figure 2a shows the AFM image of one of
the i l l 1 i faces of FAU. The triangular nature of the face with
edge length about 1 pm is immediately apparent. On the crystal
surface a host of triangular terraces are observed with the triangles rotated by 60'' to the crystal edges. A section analysis
(Fig. 2b) reveals that the triangular terraces have a highly uniform thickness of approximately 1.5 nm. This thickness is in
very good agreement with the thickness of one faujasite layer of
1.43 nm, calculated crystallographically, (see [1 101 projection in
Scheme I ) and in very good agreement with the step heights
observed previously by HREM."]
A number of important features are observed in the AFM
image of FAU structure: 1) the triangular edges of the terraces are
rotated by 60 about [I 111, from the crystal edges; 2) the orientation of all triangular terraces is the same; 3) the spacing between
the terrace edges decreases towards the edges of the crystal.
The first feature is at first surprising. One might expect the
orientation to be the same; however, the rotated sense of the
terraces will result in layer growth which circumvents the whole
crystal as illustrated in Scheme 1 in which eight growing terraces
are shown with one highlighted in black. The growing terraces
nucleate on the {I 111 face and have a triangular appearance, as
the [11 I ] projection shows. In these projections vertices represent silicon or aluminum atoms linked by oxygen bridges. The
oxygen atoms are not shown for clarity but lie approximately
half way along each line. The trigonal symmetry of one faujasite
sheet can be seen in the [I 1 11 projection where U and D represent up and down, respectively, and indicate displacement of
sodalite cage building units. This trigonal symmetry is reflected
in the triangular appearance of the growing terraces. As the
triangular terraces enlarge they will merge with other growing
terraces on other {I 11 lCfaces to form the six-sided faces of a
truncated octahedron. Such a growth front ensures that the
crystal will grow in a uniform manner on all [ 11 1 j c faces. If the
triangular terraces were to grow in the same sense as the crystal
faces, each face would in effect grow independently ofneighboring faces which might result in more irregular crystals.
The second feature. that is the fact that on an) given crystal
face, all triangular terraces grow in the same sense is due both
to the threefold symmetry of the faujasite layer (Scheme 1) and
to the relationship between interconnected layers. A pure cubic
faujasite crystal is composed of an ABCABC stacking of faujasite layers. The different layers are shifted laterally to one another but are not rotated. A rotation of one layer by 60- would
result in an unfavorable planar fault and the creation of a small
amount of EMT structure. For example a layer stacking
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’1
I’
h2-
Fig. 3. Linedr plot of terrace number n versus area coefficient h2 (arbitrary units)
for growing terraces. The inset shows a schematic representation of kink growth.
the supersaturation of a synthesis medium drops, the first faces
to stop growing are the low index
such as
In
FAU, however, the only faces exposed are { 11 1 and therefore
these will all behave in an equivalent manner. The regular nature
of the surface terraces coupled with the excellent linear relationship between terrace area and growth time is compelling evidence that these terraces are indeed fundamental units in the
growth process and that zeolite Y grows by a layer mechanism.
The definitive, yet challenging, experiment will be an in situ
A F M study to monitor crystal growth.
Ic
Scheme I Schematic representation of terrace growth on a [ l l l : c face of a FAU
crystal.
ABCAB*ABCABC which is shown with the EMT section in
bold and the one rotated B layer highlighted with an asterisk. In
order to force this unfavorable rotation between interconnected
layers [I8]crown-6 must be used as a structure-directing agent.
The last feature, that is the decreasing spacing between terrace
edges has been observed previously in HREM images[’] and can
be explained in terms of the growth mechanism. If the terraces
grow by a terrace-ledge-kink (TLK) mechanism (see Fig. 3 inset), growth rate perpendicular to a growing face is governed by
the nucleation time of a new terrace. If it is assumed that this
terrace nucleation time is a constant, the terrace number provides a unit measurement of time (the smallest triangular terrace
is numbered as one). The size of each terrace will depend upon
the time for which it has been growing (that is those with a low
terrace number have been growing for the shortest period and
will therefore be smallest). If the terraces grow by deposition of
nutrient from solution at a kink site (a site with constant area),
the area of a terrace will increase at a constant rate. Consequently, a plot of growing time (or terrace number) versus terrace area
(or area coefficient, which is the square of any linear parameter
of the triangle-we have used the square of the triangle height,
17’) should be linear. Terrace steps will consequently get closer and
closer together as the step separation is related to the square root
of the terrace area. Figure 3 shows a linear relationship between
the terrace growth time (terrace number n) and terrace area coefficient hZ which is consistent with such a growth mechanism.
It should be remembered that analysis by AFM, as with analysis by HREM, is essentially a post-mortem" on a crystal
growth. It is not possible to determine whether the
surface structure has altered fOllOWlng removal Of the crystals
from the supernatant solution. Further, it is well known that as
1212
$
;
VCH Verlag,gesellsrhu~
mhH, D-69451 Weinhemi. 1996
Fig. 4 a) A F M image (TappingMode) of E M T structure showing a Partially obscured (OOl), face. These terraces are about 1.5 nm in height, as revealed by the
section analysis (b). corresponding t o one faujasite sheet ( d . lateral shlft in z1’
plane). The colored arrows correspond to the following step heights: red: 1.0 nm:
green: 1.5 nm; black: 1.5 nm
0570-0~33iY6i3511-1212~
15.00+ .25:0
Angar. Cliem. In?. E d EngI. 1996. 35, “ I .
1I
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Figure 4a shows the AFM image of one partially obscured
{OOl ) h face of EMT. The hexagonal nature of the face with edge
length about 0.6 pm is immediately apparent. Though the overall terrace topology on the crystal surface is not as well defined
as for FAU, both a hexagonal nature and a diminishing terrace
width towards the edge of the crystal are evident. This hexagonal terrace morphology is a consequence of the symmetry generated along the [OOI], direction by the ABAB stacking of Paujasite layers. Although individually the A and B layers have
threefold symmetry (see Scheme 1) they are rotated with respect
to one another by 60" thereby generating a pseudo sixfold axis.
Section analysis again reveals that the terraces are approximately 1.5 nm thick and this thickness is highly uniform. The growth
terraces of the EMT structure are less well defined than those of
the FAU structure which is a reflection of the larger number of
defects in EMT than FAU.[']
E.vpeuinimtttl Procedure
The zeolites weresynthesized in the system 10Si0,: l.OA1,0,:2.4Na,0~140H,O:
1.0
crown ether. The siwrces of material were 40wt% colloidal silica (HS-40 Ludox);
40wt'Y' sodium aliiminilte solution, [15]crown-5 and [18]crown-6 (Aldrich) Gels
were aged for two dabs al room temperature followed by crystallization in Teflon
bottlcs for eight d a \ s a t 95 C .
AFM images were recorded o n a Nanoscope Multimode Microscope from Digital
Instruments operating in TappingMode. Samples were secured on an adhesive surface to prevent lateral movement. A first order planefit wasconducted on the images
i n the \ and I' directions to level the crystal terraces. and simulated illumination is
used t o emphasize the crystal steps. Dark areas around crystal edges are due to the
surface topograph? possessing a greater slope than the side of the tip. Thus. these
areits contiiin information on the tip shape only and d o not contain topographical
information
Received: October 30, 1995 [28513IE]
German version: Angtw. Cheni. 1996. 108. 1301 -1304
Keywords: atomic force microscopy
*
crystallization
- zeolites
111 E. Delprato, 1.. Delmotte. J. L. Guth. L Huve, Zeolites 1990, 10. 546-552.
121 C. Baei-locher. L. B. McCusker. R. Chiapetta. Micruporoxs Moter. 1994, 2,
269-2x0
131 D. W Breck. AYJ/~IP
Mo/ec.u/ur S i n v s . Wiley. New York. 1974, p.56.
[4] J. M. Thomas. M. Audier. J. Klinowskl. J. Clieiii. Soc. Clieni. Conimun. 1981,
1221 1222. M. M. J. Treacy. J. M. Newsam. R. A. Beyerlein, M. E. Leonowicz. D E W. V;iughan. ihid 1986.1211-1213: J M. Newsam. M. M. J. Treacy,
D. E. W. Vaughan. K. G . Stohmaier. W. J. Mortier. ihrrl. 1989. 493-495: J. A.
Martens. P. A. Jacobs. S. Cartlidge, Zeolires 1989, 9, 423 -427; S. Ernst. G. T.
Kokotailo. J. Weitkamp, ihirl. 1987. 7. 180-182: M. M. J. Treacy. J. M.
Nc*som. D E. W. Vaughan, R. A Beyerlein. S. 3. Rice, C. 8 . deGruyter.
Miirw. Kc,\. S I I ( ..Ywip. /'roc. 1988. 111. 177.-190; D. E. W. Vaughan (Exxon),
Eur. Por A p p l 315461. 1988(C/wiii.Ahstr. I(J8. 115126m);D. E. W.Vaughan.
M. G. Barrel (Exxon). US-A4333859, 1982(C/irm.Ahsrr. 98. 7517011);J. Jiric
Ahsir. 86. 92954r). ihid. 4021 331,
(Mobil Oil). h i d 3971983. 1976 (C/~iwi.
1977 (C/rmi .4/uti-. 86. 19578313):V.Fulop. G Borbely, H. K. Beyer, S. Ernst.
J. Weitkamp. .I. < h i 7 i . Soc. Furarlut. ? ~ - N I I JI. 1989, 85. 2127-2139.
[ S ] M. W. Anderwn. K . S. Pachis. F. Prebin. S. W. Carr. 0 . Terasaki. T. Ohsuna,
V. Alfredsson. .I Chriii Soi.. Chwi Coi?iii?uri 1991, 1660- 1664.
[6] 0 Terasaki. T Ohsuna, V Alfredsson. 1-0.Bovin, D. Watanabe. S W. Carr,
M. W. Anderson. Clicwi M u r r r . 1993. 5. 452-458.
[7] T. Ohsunn. 0 Terasakl. V. Alfredsson, J:O. Bovin. D. Watanabe, S. W Carr.
M. W Andei-son. Pi-OC.R. Soc. Lonfk~irA . in press.
[8] J. P. Ai-hancet. M E. Davis. Cliw7. Mutw 1991. 3. 567 569.
[9] S. L. Burkett. M E Davis. Microporous Murer. 1993. 1. 265-282.
[ l o ] A. 1.Weisenhorn. J E. MacDougall. S A. C. Gould. S. D. Cox, W. S. Wise, J.
Masaie. P Mnibald. V. B. Elings, G D. Stucky. P. K. Hansma. Sc.irnre 1990,
247. I330 1313. J. E. MacDougdll. S. D. Cox. G. D. Stucky. A. L. Weisenhorn. P K . Hiin\m>i. W. S. Wise. Z~o/iIe.\1991. /I. 429-433. M. KOmlyama.
T. Y;ishima. . l p .I Appl. Phi..s. Purr 1 1994. 33. 3761 -3763.
[ l l ] S. Mannc. J. P Cleveland. G. D. Stucky. P. K. Hansma. J. Cryst. Groil-r/7 1993.
13/1. 333 -340: S D Durbin. W. E Carlson. rhid. 1992, 122. 71 79.
[12] P. E. Hillner. A J. Gratz, S. Manne. P. K. Hansma. Geology 1992, XJ. 359362. P. E. Hillner. S. Manne. A. J. Grdtz. P. K. Hansma, C'ltrmnicro.s<opj
1992.42. 13x7 1393: A. J. Gratz, S. Manne. P K. Hansma. Science 1991.751.
1343 I346
[ 131 W K . Burton. N. Cabrera. F. C. Frank. Plirl. 7 h i . s . R. Soi.. A 1951. 243.
299 3sx
Controlled Assembly of Nanosized
Metallodendrimers**
Wilhelm T. S. Huck, Frank C . J. M. van Veggel,* and
David N. Reinhoudt*
There is considerable interest in the synthesis of well-defined
structures of nanometer dimensions."' These structures can be
constructed by formation of covalent bonds, but this requires
multistep synthesis.[21Therefore, various other strategies have
been developed that rely on self-assembly through noncovalent
interactions, for example hydrogen bonds. Whitesides et aI.I3l
obtained stable rosettelike structures constructed of melamine
and barbituric acid units held together by strong hydrogen
bonds. The nanotubular assemblies prepared by Ghadiri et
are another beautiful example of hydrogen-bonded assemblies.
Dative bonds to transition metals can also be employed in selfassembly. Lehn et al. have applied the coordination of oligopyridines to transition metals to form triple helices, ladder polymers, and molecular grids.[51Previously, we have described the
self-assembly of small aggregates and ribbonlike polymers using
the uranyl cation.[61In this paper we describe the synthesis of
metallodendrimers by controlled assembly.
Dendrimers are attractive, nanosize compounds with very
specific architectures.[" *I] Dendrimers can be synthesized by
following either a convergent or a divergent route,['] in which an
increasing number of new covalent bonds are formed in each
generation. A few dendrimers containing transition metals have
been reported.["] Van Koten et al.["] have used metalated dendrimers as homogeneous catalysts. Balzani and co-workers reported the synthesis of metallodendrimers containing transition
metals in every generation, which relied on sequential reactions
on the metal centers and protection/deprotection of ligands." *I
In contrast, Achar and Puddephat built dendrimers by oxidative
additions to Pt" complexes." 31
We have recently described the synthesis of large organopalladium spheres by "genuine" ~ e l f - a s s e m b l y . ~Now
' ~ ~ we report
here that we can control this process and use this method for the
synthesis of first-, second-, and third-generation metallodendrimers. Our approach is based on controlled assembly of building blocks that contain all the necessary information. We make
use of the coordination chemistry of Pd" and have combined in
building block BB-Cl two kinetically inert tridentate "pincertype" ligands and one labile coordinating cyano group
(Scheme 1). The nucleus Go has in this case C , symmetry with
three Pd centers.
The temporary protection of the metal center by a strongly
coordinating CI - ion prevents coordination by cyano groups.
Further growth of Pd complex G o is achieved by replacing the
CI- ion for a noncoordinating BF, ion, by reaction with
AgBF,. Subsequent addition of three equivalents of the protected building block BB-Cl yields the next generation dendrimer G , . By repeating this sequence twice it is possible to
build the third-generation metallodendrimer G , .
The synthesis of BB-CI and G o is outlined in Scheme 2. The
S-C-S pincer ligand was prepared in seven steps from dihydroxy[*] Dr. Ir F C J. M. van Veggel. Prof. Dr. Ir. D. N. Reinhoudt.
.
Drs W.T S. Huck
Laboratory of Organic Chemistry and MESA Research Institute
University of Twente
Box 217. NL-7500 A € Enschede (Netherlands)
P. 0.
Fax: Int code +(53)4894645
e-mail: orgchemio ct.utwente.nl
[**I
We thank the Dutch Foundation for Chemical Research (SON) for financial
support. We are grateful to Prof. N. Nibberink and R. Fokkens (Institute of
Mass Spectrometry, University of Amsterdam) for ES-MS measurements.
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