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Chemical Applications of Electron Microscopy Topotactic Reactions.

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CONFERENCE REPORTS
The Scope of NMR Studies on Paramagnetic
Compounds
By K . E. Schwarzhans[*I
N M R techniques were originally assumed t o be suitable only
for the study of diamagnetic substances, in the same way that
ESR methods are only applicable t o paramagnetic compounds. About 10 years ago McConneNcll developed a model
showing that paramagnetic compounds also give N M R
spectra in almost all cases in which the electron relaxation
times are shorter than 10-6 seconds. The most striking feature
of the N M R spectrum of a paramagnetic compound is the
exceptionally large shift of the resonance signals compared
t o those in the spectra of comparable diamagnetic substances.
According to McConnell, these shifts arise from the delocalization of unpaired electrons over the whole molecule.
Contradictions between the McConnell model and experiment became apparent during N M R studies on the paramagnetic representatives of the bis(cyclopentadieny1)metal
complexes of the 3d elements. The IH-NMR spectra of this
class of substances becomes readily interpretable, however,
if direct transfer of unpaired electrons from partly occupied
d orbitals of the central metal atom to s orbitals of the ligand
hydrogen atoms is assumed t o occur in addition to electron
delocalization.
This postulate was tested by means of the IH- and l4N-NMR
spectra of a large number of substituted bis(cyc1opentadieny1)metal complexes and bis(salicyla1dehydato) and bis(salicylideneiminato) complexes of cobalt(r1) and nickel(1r)
containing pyridine and picolines as axial ligands. In every
case the sign and magnitude of the shift showed a strong
dependence on the position of the relevant nucleus in the
molecule. Shifts of up t o I % of the resonance frequency
were observed. In extreme terms, a strong direct electron
transfer, e.g. in the bis(cyclopentadieny1)metal complexes
studied, can be regarded as leading to a metal-hydrogencarbon bond. Results obtained by I . A . Ronova [2J from electron diffraction studies on bis(cyclopentadienyl)metal complexes, which show the cyclopentadienyl protons t o be deflected out of the plane of the ring in the direction of the
metal atoms, lend support to this concept.
Lecture at Berlin on February 17, 1969 IVB 198 IE]
German version: Angew. Chem. 81, 433 (1969)
[ * ] Dr. K . E. Schwarzhans
Anorganisch-chemisches Laboratorium
der Technischen Hochschule
8 Miinchen 2 , Arcisstrasse 21 (Germany)
[l] H . M. McConnelland D . B. Chesnut, J. chem. Physics 28,107
(1958).
[ 2 ] I. A. Ronova and N . V . Alekseev, 2. strukturnoj Chim. 7, 886
(1966); J. Struct. Chem. 821 (1967) (English translation).
The Structure of Molten Alloys in Relation to their
Properties
By S . Steeb [*I
Results of wide-angle X-ray studies on molten metals and
alloys were used for the plotting of electron distribution
curves from which four pieces of information could be obtained: Classification of the melt; partial and total coordination numbers; the Cowley short range order parameter; and
finally, as the most recent development, the quantitative
resolution of certain alloys into their structural components.
We employed our new technique for measurement of the
small-angle scattering of melts at temperatures up t o 800 “C.
For molten aluminum, the isothermal compressibility
460
calculated from small-angle scattering agrees well with that
obtained from measurements on the velocity of sound
(2.387 x 10-6 bar-1). Small-angle studies on molten AI-Sn
alloys evidence the presence of Sn clusters.
According t o recent theories, the compressibility and electrical resistance[z] of alloys can also be calculated from
X-ray diffraction data.
Lecture at Berlin on February 11. 1969 [VB 199 IE]
German version: Angew. Chem. 81, 4 3 3 (1969)
- .-
[ * ] Dr. S. Steeb
Max-Planck-Institut fur Metallforschung,
lnstitut fur Sondermetalle
7 Stuttgart 1, Seestrasse 92 (Germany)
[l] N . W . Ashcroft and D . C. Langreth, Physic. Rev. 159, 685
(1967).
[2] T. E. Faber and b. M . Ziman, Philos. Mag. I ] , 153 (1965).
Chemical Applications of Electron Microscopy:
Topotactic Reactions
By H . R . Oswald[*l
In 1919 V. Kohlschiitter introduced the term “Topochemical
Reaction” for chemical processes in which the properties of
the solid reaction products are determined by reactions which
take place on or in a solid substance, i.e. locally. “Topochemistry” ought to include all influences of structure and morphology of the starting materials and end products on the
course of reactions between solids and solids, solids and
liquids, and solids and gases.
Modern electron microscopy has proved itself an invaluable
tool for investigations in this field of chemistry. In addition
to the direct picture method - often carried out with the aid
of heating, cooling, and goniometer accessories - various
reproduction methods have been successfully introduced.
In the investigation of topochemical processes, electron
microscopical (i.e. morphological) findings must always be
combined with the results obtained by structural methods,
usually X-ray methods. Fine-range electron diffraction is
a n important method in this connection; also of importance are thermal analysis and specific surface area measurements.
In recent times, the term “topotaxy” or “topotactic reaction”
has developed within the broad field of topochemistry; it is
used to describe “reactions in which a product is formed that
has a crystalline orientation related t o that of the starting
material”. The mutual structure relations and course of the
topotactic rearrangement have been studied in detail on the
basis of the following three reactions:
1. Conversion of solid copper hydroxide salts (fluoride,
nitrate, nitrite, perchlorate, sulfate) into copper hydroxide in
sodium hydroxide solutions.
2. Oxidation of manganese(r1) hydroxide with molecular
oxygen (e.g. for a solid/gaseous system; influence of topotaxy o n the reaction kinetics).
3. Conversion in cobalt chloride solutions of zinc hydroxide
nitrate crystals, into zinc-cobalt hydroxide chlorides, which
can be prepared only by a topotactic route.
The definition of topotaxy as crystal structure nucleation and
growth relation of two or more solid phases has been discussed o n the basis of these studies. The three dimensional structure analogy of the topotactic partner claimed by several
authors can be dispensed with, for one- and two-dimensional
structure relations obviously also suffice. To delineate it
from epitaxy the topotactic reaction must embrace the whole
starting lattice.
Angew. Chem. internat. Edit.
Yol. 8 (1969) / No. 6
Topotactic reactions occur in all processes involving
participation of solids; they occur much more frequently
than has hitherto been assumed and must always be
taken into consideration in investigations on the interelation of structure, appearance and chemical reactivity. Thus,
topotaxy plays an important role in numerous branches of
applied chemistry such as sintering processes and corrosion
in aqueous solutions or in hot gases. The interesting biological
process of bone formation also involves topotactic reactions.
Lecture a t Bonn, February 4, 1969 [VB 201 IE]
German version: Angew. Chem. 81,470 (1969)
[*] Prof. Dr. H. R. Oswald
Anorganisch-chemisches Institut der Universitat
CH-8001 Zurich, Ramisstrasse 76 (Switzerland)
Stable Organic Lewis Acids
groups presumably lead t o coplanarity of only one COOR’
group and t o decoupling of the other. Accordingly, the K
band of (la)(v’ = 3 1 600 em-1) lies bathochromic to that of
(2a) (v’ = 36000 cm--I), which actually corresponds more to
the K band of methyl cinnamate (v‘ = 36500 cm-1).
The structure of the anion A 0 has been established by synthesis and by UV- and N M R spectroscopy.
The stability of the Lewis acids under discussion is limited by
hydrolytic cleavage of the C=-Cdouble bond (retro Knoevenagel reaction). This reaction rollows a complicated multistep
mechanism according to an overall first order rate law; the
rate constant of the overall reaction is dependent on pH and
has a maximum when p H FZ p K i . In the p K i determination
the retro Knoevenagel reaction is taken into consideration by
extrapolation t o time t =. 0.
The Lewis acid character of the olefins L is occasionally
adversely affected by acid H atoms or by acidification of H
atoms in the R1 and Rz groups.
By 0. E. Polansky[*l
Olefins of structure L (in which XI and X2 are strongly electron accepting groups that are fully conjugated with the C = C
double bond and R1 and Rz are groups that have little or n o
acceptor or donor properties) usually behave as electrically
neutral, reversibly titratable organic Lewis acids. They
combine with bases via the @-carbon atom with polarization
of the C = C double bond, the negative charge being distributed over the site of the XI and X2 groups so that only a relatively slight partial charge results on the @-carbonatom in
the anion AO.
(3)
The synthesis of the above Lewis acids can be achieved either
by direct condensation in a water extractor (in the case where
Y = 0) or by reaction of the corresponding N-alkylamine
(Y = NRI) in a glass autoclave.
R:,C=Y
R2
A0
L
The apparent pKL values of the Lewis acids L can be determined, inter alia, by potentiometric titration with the base
conjugated t o the solvent
PKL
=
PKL+ PKHB
(~KL
= true p K of Lewis acid, KHB = ionic product of the
solvent).
The substituents R affect the p K i of the Lewis acids to a
greater extent than they do the p K values of the corresponding
carboxylic- or ammonium acids; increasing acceptor strength
of the substituent XI and X2 lowers the p K i value.
The relatively low pKL values of ( 1 ) compared with those
of ( 2 ) are due to the positive entropy of reaction in the reaction of bases with (I). i n (1) the two C O O groups are coplanar; in (2) repulsive forces between the two COOR’
+
H,C
7’A
-HY
”( 7’
k2
The synthesis is limited: 1. Aldehydes of the constitution
RCH2CHO have hitherto a1ways afforded resinous products; 2. strong Lewis acids (such as (31) obviously immediately react with XlCH2X2 t o form Michael-addition
products ( 4 ) .
In accord with their Lewis acid character the olefins L catalyze the etherification of alcohols by diazo compounds as well
as the rearrangement of epoxides to carbonyl compounds.
They react with diazoalkanes at low temperature to give,
among other things, thermally labile pyrazol-1-ines. At
room temperature, on the other hand, the reaction is usually
characterized by spontaneous evolution of N z ; depending on
the substitution the reaction products that can be isolated are,
cyclopropanes, dihydrofurans, alkylation- or rearrangement
products.
Lecture at Basel on January 23, 1969 [VB 200 1El
German version: Angew. Chem. 81, 469 (1969)
[*I
Angew. Chem. internal. Edit. i Vol. 8 (1969) 1 No. 6
(4)
Prof. Dr. 0. E. Polansky
Institut fur Theoretische Chemie der Universitat
A-1090 Wien, Wahringer Strasse 38 (Austria) and
Max-Planck-Institut fur Kohlenforschung,
Abt. Strahlenchernie
433 MulheimiRuhr, Stiftstrasse 34 -36 (Germany)
46 1
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