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Crystallographic Study of a Single Crystal to Single Crystal Photodimerization and Its Thermal Reverse Reaction.

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Crystallographic Study of a Single Crystal to
Single Crystal Photodimerization
and Its Thermal Reverse Reaction**
By Kathleeri Novak, Volker Enkefmann,* Gerhard Wegner,
and Kenneth B. Wagener
Topochemical reactions proceed in crystals under the
strict control of the crystal lattice. The best studied
topochemical reactions are [2 + 21 photocycloadditions[']
and the solid state polymerization of diacetylenes.12' In their
pioneering work on the [2 + 21 photodimerization of cinnamic acid derivatives, G. M. J. Schmidt et al. observed that
of many possible dimers, only the one whose symmetry is
already preordained in the crystal packing of the monomers
is formed.[31Photodimerizations of this type proceed heterogeneously, as d o most topochemical reactions; that is, at a
certain extent of conversion, phase separation occurs. This
destroys the monomer crystal and results in the formation of
microcrystalline aggregates. The driving force is the anisotropic changes in the lattice that occur when van der
Waals forces are transformed into chemical bonds in particular directions. In contrast, homogeneous topochemical reactions, for example the solid state polymerization of diacetylenes, are characterized by the persistence of the crystal
throughout the whole conversion range.141 The monomer
and polymer form a substituted mixed crystal in which
monomer and polymer are statistically distributed when
conversion is incomplete and occupy identical lattice positions. This is possible, because the accompanying chemical
reactions, which in part involve substantial atomic shifts, are
limited to only a few atoms in the center of the molecule,
while the substituents form a matrix that, as a first approximation, does not alter its position during the reaction.
The system shown in Scheme 1 and first described by
Hiinig and Hesse,15] does not only undergo the [2 + 21
photodimerization homogeneously, as described for individual cases in the literature,[41but also returns thermally to a
pure crystal of the monomer. It is thus possible in this case
to switch between monomer and dimer state without loss of
crystal quality.
sists for monochromatic irradiation with wavelengths up to
/z = 540 nm. However, if the wavelength corresponds to the
flank of the absorption, more than 150 nm longer than i
(1> 570 nm), the crystal is preserved over the whole reaction. Nevertheless, the choice of wavelength does not influence the crystal structure of the incipient dimer. The position
of the reflections and their intensities in the X-ray powder
diffractogram of the microcrystals obtained on UV irradiation correspond to those from single-crystal structure analyses. Figure 1 shows projections of views of the crystal structures of the monomer and the dimer, as well as of two
monomer-dimer mixed crystals that were isolated after different irradiation times.I6] In the mixed crystals, separate
Scheme 1. Reversible [2 + 21 photodmerization in the crystal
The key to this behavior lies in the choice of a suitable
wavelength for the photodimerization. If the wavelength of
the absorption maximum of the styrylpyrilium chromophore
= 420 nm) is used, the heterogeneous reaction typical
for cinnamic acid derivatives is observed: Initially yellow
microcrystalline aggregates of the dimer form on the surface
of the red crystals. On further irradiation the original
monomer crystals break up completely. This behavior per[*] Dr. V. Enkelmann, K . Novak, Prof. Dr. G Wegner
Max-Planck-Institut fur Polymerforschung
Ackermdnnweg 10, D-55021 Mainz (FRG)
Prof. Dr. K. B. Wagener
Department of Chemistry, University of Florida
Gainesville. FL 3261 1 (USA)
This work was supported by the 3 M Corporation of St. Paul, Minnesota.
We thank Dr. Jeremy Titman for recording the I3C N M R spectra.
1614 (0VCH VeriugsReseNsch~frmhH, 0-694SI Weinlterm, I993
Fig. 1. Sections of the crystal structures of a) the monomer, b) a monomerdimer mixture after 13% conversion, c) a monomer-dimer mixture after 67%
conversion, and d) the dimer for the reaction in Scheme 1.
atomic coordinates are found only for those C atoms directly
involved in the reaction. The occupation parameters of these
atoms were optimized in the refinement and agree with the
values obtained by IR spectro~copy.['~
The positions of other
atoms in the mixed crystal are identical within the error
limits of the structure analysis. The comparison of the crystal
structures depicted in Figure 1 reveals that in the course of
the reaction the positions of these atoms change substantially. For instance, the phenyl ring twists during the reaction by
18". Similar movements of the pyrilium ring and the counterion can also be detected. The change in conformation appears to be continuous, since the dihedral angle determined
from the mixed crystals lies between the two end states.
0570-0833/93/lIIf-I614 S 10.00+.25/0
Angew. Cliem. Int, E d Engl. 1993, 32, No. 11
The counterion and the tert-butyl groups are disordered in
the monomer at room temperature, and their two orientations are statistically occupied. This is probably caused by a
dynamic process. The solid state 13C MAS NMR spectrum
contains only one peak for the tert-butyl groups at room
temperature.fs1 Accordingly we assume that the dynamic
process is a jump rotation between the two sites. This is
frozen at low temperature. If the crystal structure is determined at 165 K, all disordered groups are present in only one
of the two orientations. Surprisingly, in the dimer the disorder no longer occurs at room temperature. Apparently the
cavities in the monomer in which these groups rotate are
changed to such an extent that the movement is no longer
possible after the reaction, although in the dimer no significantly smaller C-H contacts are observed.
The photochemically generated dimer is apparently in a
metastable state. At temperatures greater than 100 “C the
crystals of the monomer are reformed in a smooth reaction
during which the unexpectedly small heat of reaction of
1.8 kcal mol - is released.[’] Preliminary experiments show
that this is a reversible photochromic system. It is possible to
store holograms photochemically in several cycles with a
He-Ne laser (633 nm), to read them, and subsequently to
delete them
On recrystallization of the dimer from organic solvents
another modification of the crystal is obtained. A view of the
crystal structure of this form is illustrated in Figure 2.16’ In
this modification of the dimer, the disorder described for the
monomer is detected at room temperature. The dimer crystals formed on recrystallization can no longer be transformed back into the monomer, in contrast to the “as dimerized” form, but are stable at temperatures at which the latter
are cleaved within a minute.
It should be emphasized that topochemical control of the
reaction is not affected by the homogeneous or heterogeneous course of the reaction. Every solid-state reaction is
associated with anisotropic lattice changes. A steep concentration profile develops on irradiation into the absorption
maximum, and the photoreaction takes place in only a thin
layer on the crystal surface. As the lattices of monomer and
dimer are not identical, the crystallites of the dimer detach
from the surface of the crystal under these conditions. The
observation of the crystal surface of heterogeneous systems
therefore enables no conclusions about the mechanism of the
photoreaction. The directed material transport observed by
Kaupp[”] on the surface of cinnamic acid crystals under the
atomic force microscope during irradiation describes the
necessary structure of the crystallites under the chosen irradiation conditions, but does not support deductions on a
violation of the topochemical principle.
Received: May 10, 1993 [Z6072IE]
German version: Angew. Chem. 1993,105, 1678
Fig. 2. Section of the crystal structure of the recrystallized dimer of the reaction
in Scheme I
The ability to initiate a homogeneous topochemical photoreaction by irradiation into the outer absorption flank is
not restricted to the system described here. Crystals of the
oligomer of distyrylpyrazine have already been obtained in
the same
Some salts of styrylpyrilium monomers
with counterions like ClO,, BF,, ReO,, SbF,, AuCl;, or
SnC1; likewise form photoreactive crystals that can be
dimerized homogeneously in the same way. Preliminary experiments with a-trans-cinnamic acid indicate that here, too,
a dimerization can take place while the crystal is retained.“ ‘I
It seems possible that fu;ther photoreactions in crystals that
are heterogeneous under the usual irradiation conditions
may be single crystal to single crystal transformations on
irradiation into the outermost absorption flank.
AngLw. Chiwn. In!. G I . Engt. 1993,32, No. I 1
[1] For a comprehensive literature survey see V. Ramamurthy, K. Venkatesan.
Chem. Rev. 1987.87,433-481.
[2] V. Enkelmann, Adv. Puiym. Sci. 1984,63, 92-136.
[3] M. D. Cohen. G. M. J. Schmidt, F. I. Sonntag, J. Chem. Sue. 1964,20002013; G. M. J. Schmidt. ihirl. 1964, 2014-2021. For further work. see
G. M. J. Schmidt, et al. in Solid Slate Phofuchemis/rx(Ed.: D. Ginsburg).
Verfag Chemie, Weinheim, 1976.
[4] Homogeneous solid-state reactions known formerly: a) 2-benzylidenecyclopentanone and its p-bromo derivative: H. Nakanishi. W. Jones. J. M.
Thomas, M. B. Hursthouse. M. Motevalli, .l
Phw. Chem. 1981.85. 36361987,43.
3642; b) acridizinium salts: W. N. Wang, W. Jones, Te~ru/ze.clron
1273- 1279: c) photoreaction of the channel inclusion compound from
deoxycholic acid and acetophenone: H. C. Chang, R. Popovitz-Biro. M.
Lahav. L. Leiserowitz, J A m . Chem. Sue. 1982,104,614-616,d)oligomerization of distyrylpyrazine: H. G. Braun. G. Wegner. Makrumol. Chem.
1983, 184, 1103-1119; e) polymerization of butadiene in layered perovskites: B. Tieke, J. Pol-ym. Sci. Pulym. Chem. Ed. 1984,22. 2895-2921;
f) polymerization of diacetylenes: see [2] and references therein; g) solidstate reactions that form a new phase in the single crystal to single crystal
process: K. Cheng. B. Foxman, J Am. Chem. Soc. 1977.99.8102-8103:
h) racemization of a cobaloxime complex: Y Ohashi. K. Yanagi. T. Kuribara, Y. Sasada, Y Ohgo, ibrd. 1982,104, 6355-6359.
[5] K. Hesse. S. Hunig, Liehigs Ann. Chem. 1985,715-739.
[6] Crystallographic data: Enraf-Nonius CAD4 diffractometer. Cu,, radidtion, (2 = 1.5405 A). Annealing of the crystals in a stream of nitrogen. The
intensity of the reflections were measured with the O/$ probe. The scan
width dependent on the scattering angle (Aw = AW, + 0.14tanfJ) was selected according to the reflection width of the crystals: Am, varied only
marginally and lay between 0.4 and 0.6‘; structure resolved with direct
methods. The parameters of the H atoms were refined in “riding mode”.
Programs used: SIR88, Molen, CRYSTALS. Monomer at room temperature: a =10.3873(8). b =14.7855(8), c =16.4929(16)A, B =103.147(8).
V = 2466.5 A’. PZJc, Z = 4. pcalCd
=15.98 c m - ’ ; 3256
measured reflections of which 2071 observed (f>3u(1)), R = 0.090.
R, = 0.099. Monomer at 165 K : a = 10.3077(9). h =14.7763(7), c =
16.0772(12) A. fi =102.162(9), V = 2393.8 A’. P2,Ic. Z = 4. pcriid=
1.31 1 gcm-’, /i = 16.47 cm-’; 2859 measured reflections of which 1589
observed ( I >3u(1)), R = 0.063, R, = 0.074. Thermally regenerated
monomer at 165 K: a = 10.2845(10), b =14.7491(27). c =16.1026(27) A,
fi =102.210(9). V = 2387.3 A’, P2,/c, Z = 4, pInlcd= 1.320gcm-’.
1‘ =16.51 cm-’; 3294 measured reflections of which 2440 observed
(1>3u(1)), R = 0.031, R, = 0.034 Mixed crystal (13% conversion) at
165 K : a = 10.2941(14), b = 14.7400(12). c = 16.0874(16) A. fi =
102.328(8), V = 2385 A3. P2Jc. Z = 4, pcAIcd
= 1.322 g ~ m - ~p ,=
16.53 c m - ’ ;2820 measured reflections of which 2088 observed ( I > 3 u ( f ) ) ,
R = 0.068, R, = 0.080. Mixed crystal (67% conversion) at 165 K : a =
10.7497(12), h =14.2531(13). c =16.4773(15)k fi =105.267(7), V =
2435.5 A’, P2,/c, 2 = 4, prrled= 1.294 gem-’. p = 16.19 cm- : 321 5 measured reflections of which 1863 observed (1>30(1)), R = 0.090.
R, = 0.082. Dimer at room temperature: a = 10.8603(6),h = 14.1449(35),
c = 16.4046(8) A. p =106.324(8). V = 2418.5 A”. P2,lc. Z = 4. pcnicd=
1.298gcm-’. 11 =16.30cm-’; 3759 measured reflections of which 2744
observed ( I > 3u(1)), R = 0.056. R, = 0.053. Recrystallized dimer at room
temperature: u =14.3055(8). b = 16.2259(9), c = 21.5090(15) A, V =
4992.7 A’. Pcha, Z = 8, pLalcd= 1.257 gcm-’, p = 15.79 cm- ’ : 3203 measured reflections of which 1369 observed (1>3u(I)), R = 0.077,
R... = 0.071. Further details of the crvstal structure investigation may be
obtained from the Fachinformationszentrum Karlsruhe. Gesellschaft fur
m b H , 0-69451 Wemheim, I993
$10.00+ ,2510
wissenschaftlich-technische Information mbH, D-76344 Eggenstein-Leopoldshafen (FRG) on quoting the depository number CSD-57343. the
names of the authors, and the journal citation.
[7] The monomer was prepared as described in [S]. The photodimerization
WAS performed with a Coherent lnnova 90Kr laser. Eight to nine crystals
0.3 -0.5 mm long were oriented so that the (100) surface was irradiated and
slowly turned within a surface that was uniformly illuminated by means of
a beam expander. Two to three of these crystals were subjected to crystallographic analysis. while the rest were used for 1R spectroscopic studies.
The calibration was achieved by irradiation o f a mixture ofmonomer and
dimer with a Xe lamp followed by establishment of the degree of conversion by ' H N M R spectroscopy.
[ 8 ] The solid-state I3C NMR spectra were recorded with a Bruker MSL 300
spectrometer at 75.47 MHz The MAS (magic ungle spinning) frequency
was 3 kHz. and the C P (cross polarization) time 2 ms. A TOSS (total
suppression of spinning side bands) pulse sequence was used. The detection time was 35 ms for a 90 pulse length of 4.5 ps. For a spectral width
of 29.411 Hz. 2048 data points were collected.
191 Thermoanalytical experiments were performed on a Mettler DSC 30 instrument at a heating rate of 5 K m i n - ' .
[lo] A thick sinusoidal phase grating with a period of 17.3 pm was recorded on
the (100) Face of a crystal with two interfering beams from a 30mW H e Ne laser. Between writing periods, one of !he writing beams was blocked.
and the intensity of the diffracted reading beam measured with a photomultiplier tube. After the desired diffraction intensity had been reached,
the reading beam was attenuated by a factor of 2000 to avoid bleaching.
and the thermal decay of the grating was monitored continuously.
[I I ] V. Enkelniann. G. Wegner. K . Novak. K. B. Wagener, J A m . Chen?.Sor..,
in press. The dimerization of a-/runs-cinnamic acid was performed at a
wavelength of 360 nm and under these conditions is quantitative as a single
crystal to single crystal transformation.
(121 G. Kaupp. A n p w . Cheri?. 1992. 104, 606-609; Angew. C/?uni.h i t . Ed.
Engl. 1992. 31, 592-595.
The reason why 1 is stable and does not violate electron
counting rules is symmetry-based. Consider first the hypothetical molecule 2 with a D,,geometry. This is i ~ o l o b a l [ ~ . ~ ~
with 1 because of the relationship between PR and SIR,, as
shown in Scheme 1. An idealized orbital interaction diagram
Scheme 1
for 2 is present in Figure 1. The frontier orbitals of a carbenoid SiH, fragment consist of a radial spz hybrid orbital
pointing directly toward the Ni and a tangential p
Let us assume that intermixing between radial and tangential
combinations of the same symmetry is minimal. The twelve
resulting symmetry-adapted orbitals for the %,HI, ligand
are those shown on the right side of Figure 1. Three radial
[Ni(PtBu),] and [Ni(SiH,),] Are Isolobal,
Related to [In{Mn(C0),),]2-, and Have
16-Electron Counts""
By Huang Tang, David M . Hoffman,* Thomas A . Albright,*
Haibin Deng, and Roald Hoffmann*
Recently Ahlrichs, Fenske et al.['] reported the isolation,
structure determination, and calculations of the unique compound 1. The nickel and six phosphorus atoms in this com[Ni(PrBu),]
plex lie nearly in a plane. There is an interesting ambiguity in
the electron counting for 1.['.21In one extreme, the PtBu
groups can be counted as two-electron (2e) donors, leading
to a 22 electron count around the metal. Alternatively, the
normal P-P distances lead one to think of two-center twoelectron P-P bonds (i.e., (PtBu), is electronically saturated).
But then what holds the Ni(o) within the cavity of the (PtBu),
ring? There are also lone pairs on the phosphorus atoms of
the (PtBu), ring, sites for potential metal coordination. But
these d o not point to the ring center.
Prof. D. M. Hoffman. Prof. T. A. Albright, H. Tang
Department of Chemistry, University of Houston
Houston, T X 77204-5641 (USA)
Telefax. Int. code + (713)743-2787
Prof. R. Hoffmann. H . Deng
Department of Chemistry, Corneil University
Ithaca. NY 14853-1301 (USA)
Telefax. Int. code (607)255-5707
This work was supported by the Robert A. Welch Foundation, the
Petroleum Research Fund, administered by the American Chemical Society, the National Science Foundation, the University of Houston President's Research Enhancement Fund, and the State of Texas through the
Advanced Research Program. We also acknowledge the National Science
Foundation for an allocation of computer time at the Pittsburgh Supercomputing Center and the purchase of a Cray Y-MP/EL computer.
D. M. H. is a 1992-1994 Alfred P. Sloan Research Fellow.
#> VCH V e r l u ~ . ~ ~ ~ ~ . ~inhH.
e l l , sD-69451
~ ~ a f f Weinhein?.19Y3
Fig. 1. An idealized orbital interaction diagram for [Ni(SiH&] (2) (Dbhsymmetry).
( a t , + 1e1J and three tangential (blu + IeJ combinations
are Si-Si bonding and thus are at low energies. These molecular orbitals (MOs) are the delocalized equivalent of six localized Si-Si o bonds.
When the (SiH,), MOs are interacted with the valence
atomic orbitals (AOs) of a Ni atom, a simple picture
emerges. The Ni s and z2 orbital['] interact with the Si,H,,
a i gorbital to form a typical three orbital pattern,[41with the
3 10.00+ ,2510
Angrw. Chem. In!. Ed. Engl. 1993. 32. N o . 11
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crystals, reverse, thermal, crystallographic, reaction, stud, single, photodimerization
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