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Ligand-Stabilized Bimetallic Colloids Identified by HRTEM and EDX.

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OAC
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the reaction of (R)-3c, which gives ( 9 - 7 c in good optical
yields. An explanation for this reaction behavior of the sr-sulfonyloxynitriles of aromatic cyanohydrins could possibly be
the additional stabilization of the benzyl cations by the nitrile group in the a-position.['ll
OSO,$'
I
I
Q- 5
Received: February 8, 1991 [Z 4433 IE]
German version: Angeu. Chem. 103 (1991) 866
CAS Registry numbers:
l a , 10021-63-3; Ib, 110905-95-8; lc , 10020-96-9; 3a, 133870-86-7; 3b, 13387087-8; 3c. 133870-88-9:4a, 129313-21-9; 5a, 133908-38-0;6b, 133870-89-0;7b.
23852-57-5; 7c, 25260-42-8.
NH
N3
(SF7
R'
L-CH&H,
L-CH&H'
CH,
+
Scheme 1. a) (R)-3a 1.5 mol CH,CO,K in dimethylformamide (DMF), 79 h.
20°C; 84% (S)-4a (ee = 96%). b) (R)-3a + 1.5 mol potassium phthalimide in
DMF. 6 d. 20°C; 70% (S)-5a (ee = 93%). Pht = phthaloyl. c) (R)-3b +
1.5 mol KN, in DMF, 3 d, 20°C; 78% (S)-6b ([a];' = - 36.1" (c = 0.77,
CH,CI,)). d) A solution of (R)-3b in Et,O was added dropwise to a solution
of LiAIH, in Et,O at - 80°C; after warming to room temperature within 2 h
the mixture was neutralized at - 7 0 T with K,HPO,/KH,PO, buffer ( p H 7);
64% (S)-7b ([&' = - 16.8" (c = 5.39. EtOH)). (R)-3c was allowed to react
under the same conditions; 5 6 % ( 9 - 7 c ([XI;' = + 31.7" (c = 2.59. CHCI,)).
purity of 96.3 YOee, it must be concluded that the conversion
of ( R ) - l a into (R)-3a proceeds almost free of racemization
and the nucleophilic substitution of (R)-3a leading to ( 9 - 4 a
solely involves an SN2-mechanism. Since aliphatic (8cyanohydrins, in contrast to the (R)-cyanohydrins, are not
accessible via the enzyme-catalyzed addition of hydrocyanic
acid to aldehydes,['"' this route provides a valuable extension
to the synthesis of aliphatic (9-cyanohydrins.
The reactions with potassium phthalimide leading to N phthaloyl-protected sr-aminonitriles (9-5['] and with potassium azide leading to the a-azidonitriles ( 8 - 6 also take place
at room temperature. a-Azidonitriles have thus far not yet
been described in the literature, neither as (R)- nor as (9enantiomers. That the substitutions of (R)-3b t o give ( 9 - 6 b
also proceed stereoselectively, was confirmed by hydrogenation of ( 8 - 6b in the known (253-1,2-diamino-4-methylpentane.r81The enantiomeric excess in ( 8 - 5 a was determined
with NMR-shift reagents to be 93 % re.
Aziridines['] are of equally great importance as intermediates in synthesis as oxiranes. The ready accessibility of the
compounds (R)-3opened up the possibility of preparing optically active aziridines (57-7 in a simple way by the method
of Ohta et aIJ4 b1 via hydrogenation of the cyano group with
LiAIH,, followed by intramolecular substitution. The optical purity of the compounds ( 8 - 7 b a n d ( 8 - 7 cobtained were
determined by comparison of their rotation values with data
quoted in the
though in the hydrogenation of
(R)-3c there arises the problem that difficultly separable 2phenylethylamine is formed as by-product.
a-Sulfonyloxynitriles (R)-3 show a pronounced dependence on structure in nucleophilic substitutions: the aliphatic compounds react with potassium acetate and other nucleophiles exclusively with inversion of configuration, the
aromatic compounds, on the other hand, generally with
racemization. Partial Walden inversion is found also upon
reaction of the aromatic compounds with cesium acetate but
only when trifluoromethanesulfonates are employed. A favoring of the SN2-reaction is also observed in the case of the
aromatic compounds by way of an intramolecular reaction
with formation of a three-membered ring, as follows from
874
6
VCH Verlugsge.~ell.~cha~t
mbH. W-6940 Weinheim. 1991
(11 a) W. Becker. H. Freund, E. Pfeil, Angew,. Cheni. 77 (1965) 1139; Angeir.
Chcm. Int. Ed. Engl. 4 (1965) 1079; b) F. Effenberger, T. Ziegler, S . Forster,
ibid. 99 (1987) 491 and 26 (1987) 458; c) F. Effenberger, B. Horsch. S.
Forster. T. Ziegler, Tetrahedron Leu. 31 (1990) 1249; d) U. Niedermeyer,
M:R. Kula, Angew. Chem. 102(1990) 423; Angen. Chem. Int. Ed. Engl. 29
(1990) 386; e) U. Niedermeyer, M.-R. Kula, DE-A 3823866 (July 14.
1988); E. Wehtje, P. Adlercreutz, Bo Mattiasson. Biorechnol. Bioeng. 36
(1990) 39.
[2] T. Ziegler, B. Horsch, F. Effenberger, Synthesis 1990, 575.
[3] a) F. Effenberger, U. Burkard. J. Willfahrt, Liebigs Ann. Chem. 1986,314,
and referencescited therein: b) U. Burkard, F. Effenberger, Chem. Ber. 119
(1986) 1594.
[4] a) I.A. Smith, Ber. Dtsch. Chem. Ges. 71 (1938'1634: b) K. Ichimura. M.
Ohta, Bull. Chem. Sor. Jpn. 43 (1970) 1443.
a) R. M. Dodson, H. W. Turner, 1. Am. Chem. Soc. 73(1951)4517: b) J.D.
London. I. Wellings, .lChem. Sot. iLondon) 1959, 1780.
F. Effenberger. B. Gutterer. T. Ziegler, E. Eckhardt, R. Aichholz. Liebigs
Ann. Chem. 1991, 47.
a) Yu M. Shafran, V.A. Bakulev, V.S. Mokrushin, Usp. Khim. 58 (1989)
250; b) J. Jenni. H. Kiihne. B. Prijs, Helv. Chim. Acta 45 (1962) 1163.
a ) S. Schnell, P. Karrer. Helv. Chim. Acra 38 (1955) 2036; b) H. Brunner.
M . Schmidt. G. Unger, Eur. 1. Med. Chem. 20 (1985) 509.
J. A. Deyrup in A. Hassner (Ed.): Heterocyclic Compounds, Vol. 42, Purr I ,
Wiley. New York 1983, p. 1 ff..
a) (S)-7b: H. Rubinstein. B. Feibush, E. Gil-Av, J. Chem. Soc. Perkin
Trnns. 2 1973, 2094; see also [4b]; b) (S)-7c: S. Fujita. K. Imamura. H.
Nozaki, Bull. Chem. Sac. Jpn. 44 (1971) 1975.
a) P. G. Gassman, T. T. Tidwell, Ace. Chem. Res. 16 (1983) 279; b) P. G.
Gassman. J. J. Talley, K. Saito, T. L. Guggenheim, M. M. Doherty, D. A.
Dixon. Prepr. Am. Chem. Soc. Div. Pet. Chem. 28 (1983) 334.
Ligand-Stabilized Bimetallic Colloids Identified
by HRTEM and EDX**
By Giinter Schmid,* Andreas Lehnerf, Jan-Olle Malm,
and Jan-Olov Bovin
Dedicated to Professor Hans-Georg von Schnering
on the occasion of his 60th birthday
Bimetallic clusters and colloids are of special interest for
two reasons. Firstly, they may serve as models for studying
the formation of different alloys. Secondly, it is possible to
save precious metal, like Pt, by optimizing the synthetic conditions so that only very thin surface layers occur.
Miner et al.['] described the synthesis of gold-platinum
and palladium-platinum alloys as monodisperse sols by
[*I Prof. Dr. G. Schmid, Dip1.-Chem. A. Lehnert
Institut fur Anorganische Chemie der Universitlt
Universitatsstrasse 5-7. W-4300 Essen 1 (FRG)
Dr. J. 0. Bovin. M. Sc. J.-0. Malm
National Center for HREM. Inorganic Chemistry 2
Chemical Center, P.O. Box 124. S-22100 Lund (Sweden)
[**I G . S. and A . L . are grateful t o the Fonds der Chemischen Industrie for
financial support. J.-0. B a n d J.-0. M . thank the Swedish Natural Science
Research Council and the Swedish National Energy Administration.
HRTEM = high-resolution transmission electron microscopy; EDX =
energy disperse X-ray microanalysis.
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Angeu. Chem. I n f . Ed. Engl. 30 ( 1 9 9 1 ) No. 7
simultaneous reduction of corresponding salt mixtures in
various molar ratios. They asserted that formation of goldplatinum alloys occurs at any atomic ratio, even if the two
metals show a broad miscibility gap between 2 and 85 wt.-Yo
A U . [ ~The
] homogeneous character of the various Au/Pt alloys was suggested to be proven by means of optical spectra,
sedimentation measurements, and electron microscopy investigations. The particle sizes used in the relevant investigations varied between 3-6 nm. The variations in the d,,,spacing could not be determined accurately enough by
transmission electron microscopy (TEM) to confirm the homogeneity of the particles being as claimed in the paper by
Miner et al.
This paper describes the synthesis of ligand-stabilized
bimetallic colloids and will identify the shell structure of such
particles by high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray (EDX) microanalysis.
Gold colloids with a diameter of 18 nmt3? can be covered
by platinum or palladium shells, if an aqueous solution of
the colloids is added to a solution of H,PtCI, or H,PdCl,
and H,NOHCl. The original red color of the gold
colloid then changes to brown-black. Addition of p H,NC,H,SO,Na
stabilizes the generated particles in the
same manner as P(m-C,H,SO,Na), the gold colloids,[31so
they can be isolated in the solid state. This kind of stabilization differs characteristically from former methods in the
literature when colloids were more or less embedded into
various polymers. The ligands used here behave like in complexes and clusters and impart the colloids “molecular”
character. The colloid shows a metallic luster. The HRTEM
studies of the gold-platinum and gold-palladium particles
show that there are considerable differences between their
structures even if the size is uniform (ca. 35 nm) in both
cases.
In the case of the Au/Pt particles we observe uniform
heterogeneous agglomerates, which have an average gold
content of around 15 atomic%. These agglomerates are built
up by a gold core surrounded by platinum crystals of about
5 nm. They are connected together in a rather “open” way,
as can be seen in Figure 1 (left). The EDX microanalysis
shows clearly that the outer layer of small crystals consists of
platinum (cf. Fig. 1, right). This is demonstrated by compar-
ing the spectrum obtained by focusing the electron beam
(diameter 30 nm) over the whole particle (lower left circle in
Fig. 1, left), which affords information about both Au and
Pt, with the one recorded when the beam only covers the
outer 5 nm of the particle (the circle to the right in Fig. 1 ,
left). The latter spectrum includes only signals from Pt, as
presented in the Au and Pt,, spectra compared in Figure 1
(right). The beam broadening (0.8 nm at 200 kV and 10 nm
crystal thickness) is negligible in small particles of this thickness.
The HRTEM images of the Au/Pt particles indicate that
the growth of the outer Pt layer is preceded by a pregrowth
of 5 nm small crystals of Pt. These small crystals are then
added to the surface of the 18 nm large Au particles.
Laser-optical diffractograms of images of the 35 nm large
particles show that the crystals constituting the whole particle can in principle be oriented in two different ways with
respect to each other (Fig. 2, left). The particle marked A has
almost all the small crystals oriented in the same crystallographic direction, as shown by the optical diffractogram
of A.
Fig. 2. Left: Two different types of Au/Pt particles imaged with HRTEM at
400 kV. The optical diffractogram of the whole particle A is shown below on the
left and the corresponding one of B below on the right. Right: a structurally
well-ordered Au/Pd particle imaged with HRTEM at 400 kV. The optical diffractogram of the whole particle is inserted.
92
9L
96
1O’x
98
R lkeVl
-
100 102
Fig. 1. Left: HRTEM image of gold/platinum particles supported on an amorphous carbon film. The selected EDX spectra (right) are recorded with the
electron beam positioned as the circles indicate in the picture on the left. The
counting time of the EDX spectrum recorded with the beam touching the edge
of the particle is ten times longer than in the case ofthe centered beam. ---edge
of particle. - center of particle. C = count rate, R = measuring range, L,
signifies the electron transition from the M shell into the L shell.
Angew Chem. Inr. Ed. Engl. 30 (1991) No. 7
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On the other hand, the optical diffractogram of B shows
a random orientation for the small Pt crystals making up the
outer layer of the particle. The reason for the two kinds of
intergranular structures is not fully understood but it may
well depend on the structure of the 18-nm Au core. Earlier
investigations[’] of the structure of gold colloids show that
the structure can either be single crystal without defects or a
multiply twinned structure. A single crystal structure of the
core may well give rise to the particle type A if the Pt crystals
add on epitaxial to the Au core, while the multiply twinned
structure gives several possibilities to different orientations
of the added-on Pt crystals.
In contrast to the gold platinum alloy, gold and palladium
form a continuous series of solid solutions.[’’ If an aqueous
solution of H,PdCl, is reduced by hydoxylamine in the presence of 18-nm gold colloids, followed by a complexation
Verlugsgesellschafi mbH, W-6940 Weinheim, 1991
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with p-H,NC,H,SO,Na, gold/palladium colloids can be isolated as a silver grey, metallic looking precipitate. Such colloids are soluble in water giving a brown color.
HRTEM investigations of these colloids show different
intergranular structural images than the Au/Pt combinations. Two different types of particles can be distinguished.
In Figure 2, right, one particle with the whole structure ordered is shown, as indicated by the optical diffractogram
inserted. A comparison of this particle with the Au/Pd particle A in Figure 2, left, shows that the Au/Pd particle has a
smoother surface-most likely resulting in a smaller surface
area. The dark nucleus of the Au/Pd particle can be interpreted as the gold core, which is surrounded by the lighter
shell of well-ordered palladium atoms. This will turn up
more clearly for the lighter Pd shell compared to Pt.
The second type of structure for the Au/Pd case is similar
to the type B of Au/Pt in Figure 2 in such a way that the
particle also gives an optical diffractogram of “polycrystalline” type (Fig. 3, left). The intergranular structure has
also in this case a smoother surface compared to the Au/Pt
case. The HRTEM image of the approximately spherical
specimen also shows a darker core.
4:
0
-
18 20 22 21 26 28 30 32 31 36
10-”x R lkeVl
Fig. 3. Left: HRTEM image of a “polycrystalline” Au/Pd particle recorded at
400 kV and with the corresponding optical diffractogram inserted. Right: A
comparison of EDX spectra recorded from a Au/Pd particle, with the electron
beam positioned as the circles indicate in Figure 1. The counting time of the
EDX spectrum recorded with the beam touching the edge of the particle is ten
particle edge, - particle center.
times longer than the centered case.
C = count rate, R = measuring range, Land M signify electron transitions into
the L and M shells, respectively, from higher states, not distinguishable if
a or 4.
---
The EDX spectrum of such a single particle shows a composition of about 17 atomic% of gold and 83 atomic% of
palladium. Analyses of peripheral parts of the particle (cf. in
Fig. 1) give a spectrum of almost pure palladium (cf. Fig. 3).
The composition of the colloids varies only a little. The colloid shown in Figure 3 has a diameter of about 36 nm. As the
nucleate gold colloids have an average diameter of 18.5 nm,
the Au:Pd ratio is calculated to be between 1:5 and 1:6. This
is in good agreement with analytical results, giving an average atomic ratio of about 1 :5 for Au/Pd particles.
During the observation, using a beam current intensity of
20-40 Acm-’, the particles were stable apart from some
surface rearrangements, which can always be seen on metallic surfaces. The X-ray microanalyses were performed with a
little less intense electron beam (- 5 Acm-’). In no case
could any structural changes in the particles be observed.
The sequence of the metals can also be reversed in this
kind of particles. For example, 15-nm palladium colloids can
be prepared by treatment of PdCl, in hydrochloric acid solution with trisodium citrate. Tetrachloroauric acid is added to
876
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Verlagsgesellschaft mbH. W-6940 Weinheim, 1991
the colloidal solution and is then reduced by hydroxylamine
to generate the outer shell of gold. The stabilization of such
Pd/Au colloids by P(m-C,H,SO,Na), yields isolable colloids. Their color is like that of pure ligand-stabilized gold
colloids.
From these investigations it can be assumed that bimetallic colloids can easily be prepared and stabilized. HRTEM
and EDX have proven to be useful methods for establishing
the structure of the layered colloids.
Experimental
Gold-platinum colloids: 1600 mL of completely demineralized water was heated
to boiling in a 2 L beaker. 16 mL of a 1 per cent solution of HAuCI, (5.00 gAu/
L) and 80 mL of a 1 per cent solution of trisodium citrate was added gradually
with vigorous stirring. The solution was then kept boiling for another hour.
Approximately 1500 mL of a ruby red gold sol containing particles of 1819 nm in diameter. was obtained, as observed in many electron microscopic
investigations. This gold colloid solution was stirred into a mixture of 116 mL
of a 1 per cent solution of H,PtCI, (5.00 gPt/L) and 100mL of 1 per cent
solution of H,NOHCI in 8 L of water and was then heated to 60°C. In the
course of 3 h the red color of the gold sol changed to the dark brown color of
platinum colloids. To stabilize the particles, 50 mg of H,NC,H,SO,Na was
added to the solution. Concentration in a water-jet pump vacuum followed
until coagulation began (decolorizing of the solution). The precipitate was
separated by centrifugation (5000 revolutions per minute) and dried under
vacuum.
Gold-palladium colloids: The gold sol, prepared by the procedure above, was
diluted to 8 L in a 10 L beaker. In the course of 8 h 83 mL of a 0.037 molar
solution of H,PdCI, (6.6 gPdC1, and 7.5 mL of a 1 n HCI in 1 L of H,O)
and 100mL of a 1 per cent solution of H,NOHCI were simultaneously
added dropwise. After 48 hours’ stirring at room temperature 2 g of
p-H,NC,H,SO,Na was added, and an isolation procedure carried out as described above. The product consisted of silver-grey gold-palladium colloids,
completely soluble in water.
Microanalysis and microscopy: The colloid was dispersed in methanol and
transferred onto a holey carbon support film. The sample was then loaded into
the microscope while still wet with methanol, and the high vacuum inside the
microscope column was used to evaporate the solvent. The ligands stabilizing
the colloids can be removed when the electron beam irradiates the area of
observation. High resolution electron microscopy was performed with a JEM4000EX. which was operated at an accelerating voltage of 400 kV and able to
achieve a structural resolution of 0.16 nm. The energy dispersive X-ray (EDX)
microanalysis was performed with a Link AN 10000 system coupled to a JEM2000FX STEM-microscope with a beam focusing ability of 30 nm. The EDX
spectra were evaluated with a software for thin foil analysis. Link RTS 2/FLS.
Received: Dezember 27, 1990 [Z 4356 IE]
German version: Angew. Chem. 103 (1991) 852
[l] R. S. Miner, Jr.. Namba, J. Turkevich, Proc. Vll. Intern. Congr. Catal..
Elsevier, New York 1981.
[2] M. Hansen, Consitutions of binary Alloys, McGraw Hill, New York 1958.
[3] G. Schmid, A. Lehnert, Angew. Chem. 101 (1989) 773; Angew. Chem. Int.
Ed. Engl. 28 (1989) 780.
[4] J. Turkevich, P. C . Stevenson, J. Hillier, Discuss. Furaday Soc. 11 (1951) 55.
[5] J.-0. Bovin et al., unpublished.
The Structure of IF;**
By Ali-Reza Mahjoub and Konrad Seppelt*
Dedicated to Professor Hans Georg von Schnering
on the occasion of his 60th birthday
The only main group element heptafluoride reported so
far, IF,, forms an adduct with NOF which has been formulated as NO@IFf.tllHowever, vibrational spectroscopic
studies permitted no unequivocal conclusions to be drawn
[*I Prof. Dr. K. Seppelt, Dip1.-Chem. A.-R. Mahjoub
lnstitut fur Anorganische und Analytische Chemie der Freien Universitat
Fabeckstrasse 34-36, W-1000 Berlin 33 (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie.
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Angen. Chem. Int. Ed. Engl. 30 (1991) No. 7
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