Water-Soluble Dendritic Iron Porphyrins: Synthetic Models of Globular Heme Proteins

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(71 a ) T Yamamoto. A. Yamamoto. S. Ikeda. J . A m C/i~tw. Sor. 1971. 93. 3350- 3359. h) R. Sustmmn. J. Lau, M . Zipp, Tritruhrdron L ~ t r . 1986.27.5207 - 5210; c) R Sustinann. J Lau. Chcm. Ber. 1986,119. 2531 -2541; d) R. Sustmann. J. Lau. M . Zipp. Rrt I . Pua Cliirn. Pa.vs-Bus 1986.105.356-359:e) R. Sustmann. P. Hopp, P. Holl. Trrvuhedron L P ~ / . 1989. 30. 689-692: F ) R. van Asselt. C. J. Elsebier. f i ~ / r i d i r ( / r o n IY94. 50. 323-334.

[8] The nickel-catalyzed bromine--zinc exchange reaction has been studied extensively in our Liboratories: a) A. Vaupel, P. Knochel. Zwuhcrhon Lrtt. 1994.35. 8349 8352: h) 1. Klement. P. Knochel. K. Chau. G. Cahiez. ihirl. 1994. 39. 1177 1180; c) S. Vettel. A. Vaupel. P. Knochel. ihid. 1995. 36. 1023-1026.

[9] M . J. Ro7em;i. S. AchyuthaRao. P. Knochel, J. Orx. C h m . 1992. 57. 1956- 1958.

[lo] F Langer. A. Devasagayaraj. P:Y. Chavant, P. Knochel, S.rnlr/r 1994. 410- 412.

[I I ] The oxidative addition of nickel or palladium(o) complexes to alkyl iodides may proceed hq. ii radical mechanism: a) A. V. Kramer. J. A. Labinger. J. S . Bradley. J A Oiborn. .l A m Clrrrn. Soc. 1974, 96. 7145-7147 b) A. V. Krainer. J. A. Osborn. rhirl. 1974. 96. 7832-7833; c) M . Chanon. Bull. Soc. C/iim. Fr. 1982. 11. 197-238.

1121 E. Negishi. . 4 < i . . Chew. Res. 1982. 15. 340-348.

Water-Soluble Dendritic Iron Porphyrins: Synthetic Models of Globular Heme Proteins** Peter J. Dandliker, Franqois Diederich," Jean-Paul Gisselbrecht, Alain Louati, and Maurice Gross*

Current interest in dendritic polymers stems from their unique structural properties and potential utility. Through judi- cious choice of branched building blocks and functional group chemistry, one can precisely control their shape, dimensions, density, polarity, flexibility, and solubility.['] Dendrimers with functional components located on the exterior surface,"] within the dendritic branches,r31 and at the interior core[41 are of particular interest since they exploit dendritic topology to modify the physical and chemical behavior of those components, giving rise to new materials with desirable properties.[51

Functional dendrimers could also serve as synthetic models of globular enzymes. Dendritic mimics of heme-containing proteins such as cytochrome c are particularly intriguing targets, since their study could help clarify the reasons for the large changes in redox potential of the Fe"'/Fe" couple observed in the biological systems.r6. 'I To systematically investigate the influence of peptide-like superstructure on iron porphyrin redox potentials in a purely synthetic system, we designed and synthesized the first and second generation dendrimers 1.FeCl and 2.FeC1, which contain an iron porphyrin core unit, and studied their redox behavior in both CH,Cl, and aqueous solution. We found that as the dendritic structure progresses from being relatively open in IaFeCI to densely packed in 2-FeC1, the redox potential for the Fe"'/Fe*' couple, measured in water, becomes 420 mV more positive. We attribute this substantial potential

['I Prof. F Diederich, Dr. P. J. Dandliker LdboratoriUm fur Organische Chemie, ETH Zentrum Universititstrasse 16. CH-8092 Zurich (Switzerland) Telefdx: Int. code + (1)632-1109 e-mail' Diederichkr org.chem.ethz.ch Prof. M. Gross, Dr. A. Louati. Dr. J.-P. Gisselbrecht Laboratoire d'Electrochimie et de Chimie Physique du Corps Solide U R A. au C . N. R. S. no 405 Faculte de Chimie. Universite Louis Pasteur 1 et 4. rue Blaise Pascal, F-67008 Strasbourg Cedex (France)

Zurich and the U. S. Office of Naval Research. [**I This work wis supported by a grant from the research council of the ETH

shift to the structured hydrophobic microenvironment created by the dendritic branches around the core iron poi phyrin of the higher generation.

The dendritic iron porphyrins 1-FeCI and 2.F'i:CI were synthesized from the zinc derivatives 3 and 4. respectively.[81 Acti- vation of the carboxylic acid end groups in 3 and 4 with N,N'-di- cyclohexylcarbodiimide (DCC) and I-hydroxybenzotriazole (HOBT) in triethyleneglycol monomethyl ether a j solvent, followed by preparative gel permeation chromatography (GPC) , yielded 1.Zn and 2.Zn in 92 and 89% yield. respectively (Schemes 1 and 2. Table 1 ) .

Table 1. Selected physical and spectroscopic data of' compounds I.FeCI, 2.Zn. 2.H,. and 2.FeCI [a].

1,FeCI: Viscous brown oil. FT-IR (CHCI,): 'v =l728,1615(C=O) cm-I ; ' H N M R (300MHr.CDCI,.298K):6 = - 0.8-5.9(br.signals,about280H).13.13(bs,4H. D,-H). 15.24 (bs, 4H, pI-H); porphyrin meso H (2H) and aryl H (6H) not visible; UV,'Vis (CHCI,): E.,,, ( E ) = 375 (sh. 57500), 409 (107000). 460 (sh. l IOOO) , 504 (13200), 572 ( 5 500). 644 (4400); FAB-MS: m/r (Oh): 3YS5.2 (100. [M-CI] ' , "C,,,13C,H,,,FeN,0,, requires 3955.9). 2.Zn: Viscous red oil. FT-IR (CHCI,): i. =1733, 1668 ( C = O ) c m - ' ; ' H N M R (300MHz. CDCI,. 298 K): 6 =1.12-1.42(br.m, 8H.0CH2CII,CH,). 1.71-1.83 (brm. 8H. OCH,CH,CH,), 2.45 (br.s, 24H. OCH,CH,. first generation), 2.44- 2.60 (br.m. 72H. OCH,CH,. second generation), 3.24-3.75 (in. 660H. dendritic ether and triethyleneglycol monomethyl ether protons). 330 (br.s. 8 H, OCH,CH,CH,). 4.14 (br.s, 72H, CO,CH,CH,). 6.14-6.26 (hr s. 16H. NH). 7.06 ( 6 . J = 7 5 Hz, 4H. rn-aryl H). 7.69 (1. J = 7.5 Hz, 2H.p-aryl H) , 8.94 (d. J = 3.9 Hz, 4 H . P2-H). 9.26 (d, J = 3.9 Hz, 4 H . Pl-H), 10.07 (s. 2H, f>r<,,\oH); l3C NMR (125MHz.CDCl3,298K):6 =23.8,31.8.34.6,36.8,58.7,59.6(2~).63.4,66.5(2 x).67.3, 68.8 ( Z x ) , 68.9, 70.2. 70.3(2x).71.6, 104.5, 105.1. I l l 6, 120.7. 129.8. 131.0, 131.5. 148.7. 150.2. 159.5. 170.9. 171.3. 172.0; UV Vis (CHCI,): E.,,, ( L ) = 394 (sh, 34700), 415 (468000). 545 (16200); MALDI-TOF-MS (positive ion. linear mode): rn/z: 11 330 ( [ M + Na]', 12C,,,i3CgH884N:,10:1~6bZn + Na requires 11 329). and fragments for loss of C[CH,0CH,CH,C0,(CH,CH,0),CH3],. 2.H,: Viscous dark purple oil. FT-IR (CHCI,): C =1733, 1667 (C=O)cm-'; ' H N M R (300MHz. CDCI,, 298K): 6 = - 3.13 (br.s. 2H. pyrrole NH). 1.23- 1.46 (br.m. 8 H . OCH,CH,CH,), 1.68-1.85 (br.m, 8H. OCH,CH,CH,), 2.35 (br.s, 24H. OCH,CH,, first generation), 2.49 (br.s, 72H. OCH,CH,, second generation), 3.20-3.70 (m. 660 H, dendritic ether and triethyleneglycol monomethyl ether protons). 3.79 (br.s, 8 H , OCH,CH,CH,), 4.14 (hr.s, 72H. CO,CH,CH,). 6.06-6.18 (br.s, 16H. amide NH), 7.05 (d. J = 8.0 Hz. 4H. wr-arylH), 7.68 (t. J = 8.0 Hz, 2H.p-aryl H). 8.92 (d, J = 4.5 Hr. 4H. p d - H ) . 9.25 (d. J = 4.5 Hz, 4H. B,-H), 10.12 (s, 2 H, meso-H); I3C NMR (75 MHz. CDCl,?, 298 K): 6 = 23.6, 31.8, 34.7.37.0,58.9,59.7(2~).63.5,66.6(2~).67.4,68.9(2~).69.0,70.4(3x),71.8, 104 1.105.3.111.S.118.9.130.5.131.1 ( 2 x ) , 144.7, 147.5, 159.5. 170.9, 171.4, 172.1: UVlVis (CHCI,): J.,,, ( E ) = 391 (sh. 67 600). 410 (331 000). 504 ( I 1500). 540 (5400), 577 (5000). 630 (500); MALDI-TOF-MS (positive ion. linear mode): mi:: 11 260 ( [ M + Na]'. L2C,,,i3C,H,,,N,,0,,, + Na requires 11 265). and fragments for successive ioss of C[CH,OCH,CH,CO,(CH,CH,O),CH,J, 2.FeCI: Viscous brown oil. FT-IR (CHCI,): C = 1738, 1667 (C = 0) cm- ' ; 'H NMR (300 MHz, CDCI,. 298 K): extremely broad spectrum with aliphatic signals between 6 = 1.7 and 4.5, 13.22 (br.s, 4H. D,-H). 15.25 (br.s, 4H, B,-H); porphyrinmesoH (2H)andarylH (6H)not visible;UV/Vis(CHCI,): i , , , a x ( c ) = 375 (sh. 69200). 415 (95500). 461 (sh, 11700). 505 (10700). 551 (7100), 645 (3100); MALDI-TOF-MS (positive ion. linear mode): r ~ i ; ~ : I 1 299 ( [ M - C l ] + , '*C,,2'3C,H,,,FeN,,0,,, requires 11 296), and fragments for successive loss of C[CH,OCH,CH,CO,(CH,CH,O),CH,I,.

[a] The matrices for FAB-and MALDI-TOF-MS were 3-nitrobenryl alcohol and 2,4,6-tr1hydroxyacetophenone, respectively

Demetalation of 1-Zn and 2.Zn with HCl in CHCI, gave the corresponding dendritic free-base porphyrins 1 *H, and 2-H, in 93 and 88 % yield, respectively. The Zn-H exchange was confirmed by the appearance of ' H N M R signals a t about 6 = - 3.13 (pyrrole NH), the appearance of four bands in the visible absorption spectrum characteristic of metal-free porphyrin, and correct mass spectral data (Table 1 ) . Heating 1 -H, or 2.H, at reflux in T H F with FeCI, followed by removal of the solvent, re-dissolution in CHCI,, and extensive washing with water provided 1.FeCI and 2.FeCl in 85 and 80 YO yield, respectively. The successful introduction of the metal ion was con-

Anxcm ('/wnr I n / . Ed. EngI. 1995, 34, N o . 23/24 (0 VCH Ver lu~ . s~rse l l s thu~/ mhH, 0-69451 Weinherm, 1995 0570-0833/95/3423-2725 6 10.00+ .25/0 2725

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in organic solvents.". In particular, Mo- menteau and co-workers synthesized bridged "basket-handle" iron porphyrins in which the bridging chains, containing ether or amide groups, were linked directly over the metihloporphyrin's rr-faces.["I The

the approach of solvent and counterions to the porphyrin and also exerted a marked through-space electrostatic effect, thus shifting the half-wave potential of the Fell'/ Fe" couple to more positive values. Given the steric bulk of the dendritic shell and the high local density of polar amide groups near the porphyrin ring in 1.FeCl and 2. FeCI, we expected similar effects to oper- ate in these systems.

amide-containing chains sterically hindered a

0 3 0 3 P3

Scheme 1. Synthesis ofdendritic porphyrins I.Zn. 1 .H2, and 1 .FeCI. a) DCC. HOBT. collidine, triethyleneglycol 1Qn: M = Zn monomethyl ether. 0°C to 2 0 ° C Zdays, 92%. b) 0 . 4 ~ aq.

,.HZ: M = H2 HCI, CHCI,, 20'C, 5 mm. 93%. c) FeCl,, THE reflux. 1 h, 8 5 % . l*FeCI: M = FeCl

HO

3

a -

4

Scheme 2. Synthesis of dendritic porphyrlns Z.Zn, 2 .H, . and 2.FeCI. a ) DCC. HOBT, collidine, triethyleneglycol monomethyl ether. 0 "C to 20"C, 2 days, 89%. b) 0.4 M aq. HCI, CHCI,. 20°C. 5 min. 88%. c) FeCI,. THF, reflux. 1 h. XO%.

firmed by the characteristic UV/Vis spectrum for iron(rI1) porphyrins, 'HNMR signals for the /l-pyrrole protons at about 6 = 13.2 and 15.2, and the expected mass spectral data (Table 1). All dendritic porphyrins 1 and 2 are viscous oils that dissolve remarkably well in solvents in the polarity range between water

(ET(30) = 63.1) and p-xylene (ET(30) = 33.1).L91 Control compound 6.FeCI without dendritic branches was synthesized from the free-base porphyrin 5 in a similar man- ner.

The effects of bulky super- 5: R=H, M = H 2 structure on the electrochemi-

have been studied extensively

RO OR

CFeCI: R = CH,(CH20CHZ)2CH20CH3, properties Of porphyrins M = FeCl

2-Zn: M = Zn

2*H2: M = H2 c 20FeCI: M = FeCl

Cyclic voltammetry in CH,Cl, (glassy carbon electrode, 0.1 M Bu,NPF, as supporting electrolyte) showed irreversible waves for the Fe'"/Fe" couple in 6.FeC1, typical of Fe"' porphyrin chlorides, and consistent with equilibrated dissocia- tion of the chloride counterion from the metal ion during electroreduction.["] In contrast, both 1 .FeCI and 2.FeCI showed reversible waves for the Fe"'/Fe" couple at -10 and +70 mV vs. the standard calomel electrode (SCE), respectively (Table 2).

Both 6-FeCI and I-FeCI showed waves for one-electron oxidation at + 1.06 V, and waves for one-electron reduction from Fe" to Fe' at - 1.29 and - 1.27 V, respectively, typical values for iron porphyrins (Table 2). The potential difference between the oxidation at + 1.06 V and reduction to Fe' was within the nor- mal value of 2.25f0.15 V reported for other metalloporphyrin complexes." 21 Compound 2-FeCI underwent reduction from

COMMUNICATIONS Tablc 7 . Hdf-wave potentials of the dendritic iron porphyrins in CHiCll [a].

6.1;eCI - 1.29 [b] - 0.59 [c.d] + I .06 [h]

I . FeCl - 1.27 [c] - 0.01 [f] + 1.06 [b] 2.keCl - I .46 [el + 0.07 11-1

+ 0.12 [c.e]

[a] Rcdox potciitials in CH,CI, determined by cyclic voltammetry (CV): El" (ap- proximated by (A: + 6 1 1 2 ) in V vs. SCE; supporting electrolyte 0.1 M Bu,NPF,, &I\ \> c;irboii %orking electrode, scan rate = 0.1 V s - ' . 7 = 293 K. [b] Quasi-re- vcrsible trniister [c] Irreversible transfer, peak potential from CV. [d] Cathodic peak polciitial. [el Anodic peak potential. [f] Reversible transfer.

Fe" to Fe' at - 1.46 V, that is, a t a 170 mV more negative potential than the reference compound 6.FeCI.

In order to assign the observed cyclic voltammetric signals to particular redox reactions of the core iron porphyrin, spec- troelectrochemical studies were performed at a platinum grid working electrode in CH,CI, with 0.1 M Bu,N PF, as supporting electrolyte. Compound 1 *FeCI exhibited well-defined isosbestic points on the first reduction step, and the absorption spectrum remained characteristic of an iron porphyrin with only small changes in the position of the Soret and Q bands. Following the second reduction step, the spectrum remained typical for the metalloporphyrin, displaying red shifts of 20 nm for the Soret band and 16 nm for the Q band. This behavior is characteristic of successive metal-centered reductions from Fe"' to Fe" and Fe' porphyrins. respectively. Compound 2.FeCI exhibited analogous behavior, which confirmed that the first reduction step corresponded to the Fe"'/Fe'' couple. The presumed generation of an Fe' species during the second reduction step at - 1.46 V vs. SCE could not be monitored, since it occurred too close to the accessible cathodic potential limit of the solvent.

To coinpare the redox behavior of the dendritic iron porphyrins to that of heme proteins more directly, cyclic voltammetric studies in water with 0.1 M Et,NC10, as the supporting electrolyte were carried out. For solubility reasons, only compounds 1-FeCI and 2.FeCI could be studied in aqueous solution. They both exhibited irreversible one-electron oxidation waves at the glassy carbon electrode at + 0.97 and + 1.09 V vs. SCE, respectively (Table 3). Compounds 1eFeCI and 2.FeCI

Tdhle 3. Half-nave potentials of the dendritic iron porphyrins in H,O [a]

Porphyrin E' '(Fe"/Fe') El '(Fe"'/Fe") EtX2

1. FeC'I - 1.26 [h] - 0.23 [c] + 0.97 [b] 2.FeCI - 1.26 (b] + 0.19 [c] + 1.09 [h]

[a] Rcdox potenmls in H,O determined by cyclic voltammetry (CV): E" (approx- imated hq (k; + Eh) 2 ) in V vs. SCE, supporting electrolyte 0.1 M Et,NCIO,; glassy carbon woi-king electrode. scan rate = 0.1 Vs ', T = 293 K. [h] Irreversible transfer, peah potential from CV. [c] Reversible transfer.

also exhibited two reductions, the second of which was irreversible and occurred at the same potential of - 1.26 V vs. SCE. In contrast. the increased density of the dendritic superstructure in 2.FeCl relative to I*FeCI caused the potential of the fully reversible Fe"'/Fe" couple of 2.FeCI to shift from -0.23 V to + 0.19 V and thus to become 420 mV more positive.

These results show that environmental polarity strongly influ- ences the redox potential of electrochemical reactions that occur at the heme metal center. In CH,CI,, the iron porphyrins in both the more open 1 .FeCl and the more densely packed 2.FeCI

experience similar microenvironments; hence similar potentials for the Fe"'/Fe" couple are measured. In water, the relatively open dendritic branches of IaFeCI d o not impede aqueous sol- vation of the iron porphyrin, whereas the densely packed dendritic superstructure of 2.FeCI significantly reduces contact between the heme and external solvent, destabilizing the oxidized. more charged state and shifting the redox potential to a more positive value.

The remarkable potential difference between 2.FeCI and 1.FeCI is similar to that (about 300mV) found between cytochrome c and a similarly ligated, more solvent-exposed cytochrome c heme ~c tapept ide . [ '~ ] Despite resemblance in globular shape and dimensions between 2.FeCI and cytochrome c,[l4] as suggested by computer modeling, further analogies between dendritic and biological system are currently limited by the difference in axial ligation to the heme iron. Whereas the protein uses a histidine-N atom and a methionine-S atom, the nature of the axial ligands of the dendrimer in water is different but remains to be determined. By covalently inserting the natural N- and S-ligands into dendritic iron porphyrins. their model char- acter for cytochrome c could be further extended in future work.

Received: July I?. 1995 IZ8198JEI German version: A n p i t . Cheiii. 1995, 1117. 2906- 2909

Keywords: dendrimers . electrochemistry . heme-protein mimics . iron compounds porphyrinoids

~~~~

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5147.

A New Strategy in Heterogeneous Catalysis: The Design of Cortex Catalysts Manfred T. Reetz," Stefan A. Quaker, Rolf Breinbauer, and Bernd Tesche

The majority of chemical transformations in industry are cata- lytic processes, more than 75% of which are carried out using heterogeneous catalysts.['] A large group of heterogeneous catalysts are the impregnation catalysts, prepared by impregnating solid supports with metal salts followed by calcination and reduction at high temperatures.['. '] More recently, solvated metal

as well as certain molecular metal complexes and metal clusters have been immobilized on commercial solid metal oxides.[41 Another current approach concerns surface organo- metallic chemistry according to which discrete catalytically ac- tive species are anchored on solid supports.r51 Although many of these catalysts have been characterized by a variety of techniques such as transmission electron microscopy (TEM) , energy dispersive X-ray analysis (EDX), and X-ray photoelectron spec- troscopy (XPS),['-6] the precise location of the metal particles or ligated metals on the support remains unclear. Generally, it is desirable to achieve metal dispersion preferably on the outer surface of the support, because this minimizes diffusion prob- lems. This is the reason why shell catalysts, especially those of the eggshell type, are often preferred. Accordingly, mm-sized pellets are treated in special impregnation procedures during which a metal-containing outer region several hundred pm thick is formed. However, it is likely that in this macroscopically visible region the metal particles are not exclusively fixed on the outer surface in a microscopic sense, but are also distributed deep in the pores of the solid support. In order to prevent this, we have now immobilized preformed and size-selectively prepared metal clusters on solid supports. By considering pore diameter and cluster size, it is possible to design catalysts in which metal particles of a specified size are found solely on the outer surface of the support.

Recently we described an electrochemical process for the pro- duction of nanostructured transition metal clusters (e.g. Ni, Pd, Pt, Co, Rh, Ru)['] and dimetallic clusters (e.g. Ni/Pd, Fe/Co, Fe/Ni, Pt/Sn)['. 91 stabilized by tetraalkylammonium salts. The diameter of the particles ranges between 1 and 10 nm and can be

[*] Prof. Dr. M. T. Reetz, Dr. S . A. Quaker, Dipl.-Ing. R. Breinbauer, Dr. B. Tesche Max-Planck-lnstitut fur Kohlenforschung Kaiser-Wilhelm-Platz 1, D-45470 Miilheim an der Ruhr (Germany) Telefax: Int. code +(208) 306 2985

controlled by adjusting the current density. The spherical clusters are surrounded by tetraalkylammonium salts that function as a stabilizing mantle; the visualization of this protective monomolecular layer is possible through a combination of transmission electron and scanning tunneling microscopy (TEM/STM) The solubility of these stabilized clusters is dependent on the ammonium salt; thus, those with (C,,H,,),N+Br- are soluble in nonpolar solvents such as pentane and those with betains such as C,,H,,- (CH,),N+CH,CH,CH,SO; are soluble in water.[81

Upon stirring solutions of these preformed clusters at room temperature in the presence of solid supports such as S O , or AI,O, in the form of powders or pellets, immobilization occurs without any change in cluster size.[', ' ' I This means that metal crystallites of a defined size and elemental composition can be specifically placed on such supports. In the case of pellets, shell catalysts[', are easily produced without the need for special preparation techniques necessary in conventional impregnation processes such as pretreatment of the solid support with certain salts, use of viscous solvents or additives, or special drying procedures. For example, impregnation of A1,0, pellets (Johnson Matthey 11838) with a 0 . 5 ~ THF solution of 3 nm sized (by TEM) (C,H, ,),N+Br--stabilized Pd clusters leads to rapid immobilization. Eggshell catalysts are obtained within 1-4 s, as shown by photographs of pellet cross sections (Fig. l a , b). The

Fig. 1 . Cross sections of A1,0, pellets (diameter 3.2 mm). a) After penetration with a 0 . 5 ~ tetrahydrofuran solution of (C,H,,),N+BrC-stabilized Pd clusters for 1 second. b) For 4 seconds. c) For 10 seconds. d) After penetration with an aqueous 0 . 5 ~ solution of betain-stahilized Pd clusters for 5 h.

extent of the uniform macroscopic penetration is time-dependent; the penetration is complete after 10 s (Fig. lc). In sharp contrast, interaction of the same pellets with a 0 . 5 ~ aqueous solution of betain-stabilized Pd clusters (3 nm by TEM) affords only eggshell-type catalysts (penetration < 100 pm), even after prolonged periods of interaction. For example, gentle shaking of such a black-colored solution of the cluster with the above A1,0, pellets for 5 h results in complete immobilization of the cluster (Fig. Id). Following decantation, the water is completely colorless and contains about 70% of the original betain stabilizer. If so desired, further washing can remove up to 95 YO of the stabilizer.

The dramatic difference in the rate and depth of penetration in going from (C,H,,),N+Br- to C,,H,,- (CH,),N+CH,CH,CH,SO; can be explained on the basis of different degrees of interaction of the stabilizer with the support. In the case of the (C,H,,),N+Br- stabilizer which has four lipophilic n-octyl chains at nitrogen, the interaction with the hydrophilic surface of AI,O, is relatively weak, and rapid macroscopic penetration occurs as the cluster-containing solvent moves into the pellet. In the case of the betain-stabilized clusters, strong interaction between the polar substituents of the stabilizer and the AI,O, surface lowers the rate and degree of

2728 0 VCH Verlagsgese1l.schqfi mbH, 0-69451 Weinheim, 1995 OS70-0833/9S/3423-2728 $l0.00+ .25/0 Angeu. Chem. In(. Ed Enxl. 1995, 34, No. 23/24

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