Hydrogen-bond quenching of photodecarbonylation in the solid state and recovery of reactivity by co-crystallization

Article abstract of DOI:10.1039/b700073a

Intermolecular H-bonding between C=O (ketone) and HO (4′- hydroxyphenyl) quenches photodecarbonylation of 2,2,4,4-tetramethyl-1,3- di(4′-hydroxyphenyl)acetone 1 in the crystalline solid state, its reactivity is recovered by co-crystallization with the small organic molecule 4,4′-bicyclohexanone. The Royal Society of Chemistry.

Full text of DOI:10.1039/b700073a

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Hydrogen-bond quenching of photodecarbonylation in the solid state
and recovery of reactivity by co-crystallization{{
Jing Zhang, Milan Gembicky, Marc Messerschmidt and Philip Coppens*
Received (in Cambridge, UK) 4th January 2007, Accepted 22nd February 2007
First published as an Advance Article on the web 9th March 2007
DOI: 10.1039/b700073a
abilities. The analogue 2 (MonoOHTMK, Scheme 1), which has
Intermolecular H-bonding between CLO (ketone) and HO
4-hydroxy- and 49-methoxy-phenyl substituents, is prepared by
monomethylation of 1 with MeI in acetone. Crystalline solids of 1,
2 and 3 are obtained by recrystallization in chloroform, a mixture
of ether and hexane (1 : 1) and ethanol, respectively.§
(49-hydroxyphenyl) quenches photodecarbonylation of 2,2,4,4-
tetramethyl-1,3-di(49-hydroxyphenyl)acetone 1 in the crystalline
solid state, its reactivity is recovered by co-crystallization with
the small organic molecule 4,49-bicyclohexanone.
Even after 3 h no photoreactivity is observed when a powder
sample of 1 is irradiated with a focused 200 W medium-pressure
mercury lamp at RT, whereas under the same conditions 2 and 3
give the decarbonylated products in 30 and .95% yields,
respectively, although the substituents have similar electronic
It is well established that solid-state photochemical processes are
strongly affected by the details of the packing arrangement of the
reactants, which can be manipulated by crystal modification.1–5 In
the case of photodecarbonylation of b-substituted cyclohexanones,
for example, the absence or presence of the reaction has been
shown to depend on the molecular conformation imposed by
crystal packing requirements.6 Unlike in solution, the radicals or
radical pairs generated by photo-induced s-bond breaking
(Norrish type-I reaction) can not easily separate in the reaction
cavity in solids, so decarbonylation can not always compete with
triplet to singlet ISC which allows recombination of the unpaired
spins.7 A series of careful studies of the topochemical conditions
for these reactions have been reported.8,9
?
?
donating ability (the Hammett s p-OH and s p-MeO values are
20.40 and 20.27, respectively10) and are all in the para position.
The crystal structures of 1 (Fig. 1(a)) and 2 (Fig. S1, ESI{). show
…
that the difference in reactivity can be traced to an OH OLC
…
˚
hydrogen bond to the carbonyl oxygen of 1 (O O = 2.72 A),
which is absent in 2 (Fig. S1, ESI{), in which hydrogen bonded
chains are formed by the hydroxyl groups and the solvated water
molecules (Table S1, ESI{), and rare in benzylketones.11 In 1
hydrogen bonding between the hydroxyl groups extends the
H-bonded network into two dimensions (Fig. 1).
In the current work, we show that intermolecular H-bonding
between CLO (ketone) and HO (49-hydroxyphenyl) quenches
photodecarbonylation of 2,2,4,4-tetramethyl-1,3-di(49-hydroxy-
phenyl)acetone 1 (HOTMK, Scheme 1) in the crystalline solid
state inhibits reaction, and how the reactivity can be recovered by
co-crystallization with the small organic molecule 4,49 -bicyclohex-
anone in 1a (HOTMK?BCH), which eliminates the interaction
responsible for the quenching.
To the best of our knowledge, quenching of decarbonylation by
…
CLO H–O hydrogen bonding has never been examined in
crystalline solids, although it is important for solid-state photo-
chemistry. It is well known that in the liquid phase the lowest n, p*
state of aromatic ketones is capable of inter- and intramolecular
hydrogen abstraction from hydrocarbon, arene and alcohol
We chose 1 (HOTMK, Scheme 1) as subject for photodecarbo-
nylation, for (1) the similar electron donating ability of OH and
p-OMe in 3 (MeOTMK, Scheme 1), which was reported to exhibit
high reactivity in this reaction;9 (2) its synthetic versatility; (3) its
low pKa and oxidation potential, and strong hydrogen bonding
Scheme 1 Chemical structures of 1–3 (1: R1 = R2 = H, HOTMK; 2: R1 =
H, R2 = Me, MonoOHTMK; 3: R1 = R2 = Me, MeOTMK).
Department of Chemistry, State University of New York at Buffalo,
Buffalo, New York, 14260-3000, USA. E-mail: coppens@buffalo.edu;
Fax: (716)645-6948
{ Electronic supplementary information (ESI) available: Experimental
procedure; spectra and structure data for all complexes; Fig. S1–S5;
Table S1–S3. See DOI: 10.1039/b700073a
{ The HTML version of this article has been enhanced with colour
images.
Fig. 1 Hydrogen bonding in the structure of 1 which ‘‘turns off’’ the
˚
photodecarbonylation. Selected interatomic distances (A): O1(carbonyl
… …
O) O3A(phenyl O) 2.72, O3A(phenyl O) O2(phenyl O) 2.77.
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H-donors.12,13 In the case of substituted dibenzyl ketones hydrogen
abstraction and photodecarbonylation can both occur.12 It has
been argued that stabilization of the n-orbital by H-bond
formation to the carbonyl oxygen may lead to the non-reactive
triplet p, p* state being lower in energy than the n, p* excited
state.14 To examine the nature of the excited state, absorption and
emission spectra of 1–3 were recorded at 298 K (Fig. S3, ESI{) and
at 77 K (Fig. S4, emission only, ESI{), using 350 nm light for
excitation. The emission spectra of the three compounds are
essentially identical. Corresponding excited state lifetimes are less
than 200 ns for all three compounds. The similarity of the emission
spectra and lifetimes indicates that no inversion of the n, p* to p,
p* state energy levels occurs on hydrogen bonding. Evidence
presented by Turro and co-workers indicates the hydrogen
abstraction, which is very rapid, to occur from the excited singlet
state.12 In the confined space in the crystal the initial hydrogen
abstraction may be reversed rapidly when the excited state decays,
leading to the quenching of the a-cleavage reaction. Alternatively,
the hydrogen bond to the carbonyl oxygen may interfere with
escape of the CO group after initial a-cleavage leading to
recombination of the geminate radical pairs.
To obtain additional evidence for the effect of CLO hydrogen
bonding on the photo-reactivity we synthesized the related co-
crystals 1a and 2a with 4,49-biscyclohexanone (HOTMK?BCH
and 2MonoHOTMK?BCH, respectively). In neither of these, nor
in the previously reported 2,2,4,4-tetramethyl-1,3-diarylacetones9 is
the keto group involved in hydrogen bonding, and all are reactive.
Co-crystals 1a contain linear chains of alternating 1 and
4,49-biscyclohexanone molecules connected by hexanone
…
CLO H–O hydrogen bonds (Table S1, ESI{). No hydrogen
bonding to the keto group of the diaryl ketone occurs (Fig. 2). 2 : 1
…
cocrystals 2a of 2 with BCH showing similar CLO H–O
hydrogen bonding can also be grown (Fig. S2, ESI{). In the IR
spectra carbonyl stretching frequencies of compounds 2, 3, 1a and
2a occur at 1679–1698 cm21, whereas for 1 they are shifted to
1659 cm21 in accordance with the carbonyl hydrogen bonding
observed in the crystal structure (Table S2, ESI{).
Irradiation of powdered samples for 60 min under the
conditions described above yielded the decarbonylation products
of 1a and 2a in yields of 60 and 69%, respectively. 1H NMR
analysis of 1a dissolved in d6-DMSO after different irradiation
times exhibits a smooth progression upon increasing exposure
(Fig. 3(a) and (b)). Small amounts of an unidentified side product
Fig. 3 (a) 1H NMR of the phenyl group hydrogens in a d6-DMSO
(500 MHz) solution after room-temperature irradiation of crystalline
powders of 1a for different lengths of time. Arrows indicate photoinduced
signals. (b) Product formation of 1, 1a, 2 and 2a as a function of
irradiation time.
were also detected (,5% of total products). The initial product
formation shows a levelling-off attributed to degradation of the
crystal quality. The product yield for 2a is twice that of 2 indicating
the effect of dilution of the active species in the solid state.
Irradiation of single crystals of 2 and 3 at 90 K leads to reaction,
but in neither case is the crystal structure preserved on irradiation.
However, when 1a is irradiated for 30 min at 90 K, no
decarbonylation occurs and the crystal integrity is maintained,
although small (,+1% in a and c, +1u in b, Table S2, ESI{) but
significant changes in the unit cell dimensions occur. Fourier
difference maps subtracting the after- from the before-irradiation
densities clearly show a molecular shift of part of the molecules
(y30%) along the chain direction (Fig. 4), which is quantified by
˚
least squares as 0.6 A, and is accompanied by slight rotations of
Fig. 2 Hydrogen bonding in the structure 1a. Selected interatomic
…
…
˚
distances (A): O2 O5A 2.74, O3 O4 2.76.
the phenyl rings. The observations were reproduced in several
2400 | Chem. Commun., 2007, 2399–2401
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¯
˚
group P1, a = 7.74590(10), b = 11.0192(2), c = 16.6519(3) A, a =
3
109.2590(10), b = 92.6450(10), c = 90.7540(10), V = 1339.72(4) A , Z = 2,
˚
Dc = 1.221 Mg m23, R1 = 0.0376, wR2 = 0.0942, total reflections 5489,
observed reflections 4321. Compound 2a: C26H33O4, Mr = 409.52,
monoclinic, space group P21/c, a = 6.2893(2), b = 7.9701(2), c =
=
3
43.7044(11) A, b = 92.9470(10), V = 2187.84(10) A , Z = 4, Dc
˚
˚
1.243 Mg m23, R1 = 0.0409, wR2 = 0.1077, total reflections 5390, observed
reflections 4936. The data were collected on a Bruker APEX II CCD with
˚
Mo-Ka radiation (l = 0.71073 A) at 90(2) K. Reflections were reduced by
the SAINT program. The structures were solved by direct methods and
refined by a full-matrix least-squares technique based on F2 using the
SHELXL97 program. CCDC 1: 630993, 2: 630994, 1a: 630996, 2a: 630995.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b700073a
Fig. 4 (Left) Electron density change after 20 min exposure by 325 nm
light of a crystal of 1a (contour level 0.3 e A23). Plane through the central
1 (a) K. Kinbara, A. Kai, Y. Maekawa, Y. Hashimoto, S. Naruse,
M. Hasegawa and K. Saigo, J. Chem. Soc., Perkin Trans. 2, 1996,
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˚
atoms C1, C2, C3. The non-hydrogen atoms labelled ‘a’ belong to the
displaced molecule. (Right) As left, in the intermolecular HOTMK-BCH
H-bond region. Plane through the atoms C17, O3, O5.
ˇ ´
M. Nishio, CrystEngComm, 2004, 6, 130–158; (e) X. Gao, T. Friˇscic and
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successive experiments on different crystals. However when after
exposure at 90 K the crystals were warmed to room temperature,
the crystals did not remain transparent and turned into a white
solid. Subsequent NMR analysis of a solution in d6-DMSO
indicates a low-concentration presence of the same products as
obtained by room-temperature irradiation, i.e. that of combination
of the secondary geminate pair (2,3-bis(p-hydroxyphenyl)-2,3-
dimethylbutane), as well as a small amount of the unidentified side
product. It is possible that these products are formed on or close to
the surface of the crystal. The photoinduced structural changes in
the crystals indicate that a photon-triggered process does occur,
even though no changes were observed in atomic connectivity in
the bulk of the crystals. Low-temperature EPR experiments would
be most useful for further elucidation of these observations.
In summary, the evidence indicates that formation of an
intermolecular H-bond between the carbonyl (CLO) and phenol
(OH) groups of 2,2,4,4-tetramethyl-1,3-diarylacetone 1 in the solid
state quenches the photodecarbonylation reaction. Chemical
modification, such as substitution of one p-OH by p-OMe, leads
to alternative crystal packing and allows the reaction to proceed.
The reaction can also be turned on by co-crystallization with a
hydrogen-bond acceptor such as 4,49-biscyclohexanone, showing
how solid-state reactivity can be manipulated by applying the
methods of crystal engineering.
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We would like to thank Ivan Nemec for performing the
microanalyses, and Shao-Liang Zhang for careful reading of the
manuscript. We are grateful to Prof. M. A. Garcia-Garibay for
providing an early sample of 3. Support of this work by the
National Science Foundation (CHE0236317) is gratefully
acknowledged.
Notes and references
§ Crystal data: Compound 1: C19H22O3, Mr = 298.37, monoclinic, space
˚
group P21/c, a = 10.0719(2), b = 11.2639(2), c = 14.2688(3) A, b =
3
98.7710(10), V =1599.85(5) A , Z = 4, Dc = 1.239 Mg m23, R1 = 0.0354,
˚
wR2 = 0.0944, total reflections 3682, observed reflections 3214. Compound
2: C40H50O7, Mr = 642.80, monoclinic, space group P21, a = 12.517(3), b =
3
˚
˚
6.3855(13), c = 21.558(4) A, b = 90.98(3), V = 1722.7(6) A , Z = 2 Dc =
1.239 Mg m23, R1 = 0.0354, wR2 = 0.0991, total reflections 8471, observed
reflections 7987. Compound 1a: C31H40O5, Mr = 492.63, triclinic, space
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