A 'magic bridge': effect of methylene chain length on the photochemistry of radical cations produced from bifunctional X-(CH2)n-Y molecules

Article abstract of DOI:10.1016/j.mencom.2009.09.012

The radical cations of aminoamides Me₂N(CH₂)_nC(O)NMe₂ and aminoethers Me₂N(CH₂)_nOMe reveal selective photoinduced intramolecular H transfer from methylene bridge at a specific ch

Full text of DOI:10.1016/j.mencom.2009.09.012

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Mendeleev
Communications
Mendeleev Commun., 2009, 19, 268–269
A ‘magic bridge’: effect of methylene chain
length on the photochemistry of radical cations
produced from bifunctional X–(CH2)n–Y molecules
Kirill B. Nuzhdin,a Aleksei V. Kobzarenko,a Igor I. Barabanovb and Vladimir I. Feldman*a,c
a Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow, Russian Federation.
Fax: +7 495 939 4870; e-mail: vladimir.feldman@rad.chem.msu.ru
b Institute of Chemical Kinetics and Combustion, Siberian Branch of the Russian Academy of Sciences,
630090 Novosibirsk, Russian Federation
c N. S. Enikolopov Institute of Synthetic Polymer Materials, Russian Academy of Sciences, 117393 Moscow,
Russian Federation
DOI: 10.1016/j.mencom.2009.09.012
The radical cations of aminoamides Me2N(CH2)nC(O)NMe2 and aminoethers Me2N(CH2)nOMe reveal selective photoinduced
intramolecular H transfer from methylene bridge at a specific chain length (n = 3).
The radical cations (RCs) of bridged X–(CH2)n–Y bifunc-
tional compounds (X and Y are functional groups) represent an
interesting class of model systems for molecular electronics,
radiation chemistry of macromolecules and radiobiology. It was
shown1,2 that symmetrical RCs of this kind (X = Y) usually
exhibited spin and charge delocalization, at least, at n < 3.
A delocalized structure was also found3 in methoxyacetone RC,
where X and Y are different, but the ionization energy of these
groups is close enough (ΔIPXY = 0.33 eV). On the other hand,
in the case of amidoesters (X = CONMe2, Y = MeOCO, n = 0–4),
spin density is mainly located at the amide nitrogen,4 which
results from substantially lower ionization energy of the amide
group (ΔIPXY ~ 1 eV). Nevertheless, the methylene bridge was
found to have a remarkable effect on the photochemical reac-
tions of amidoester RC, the most effective and specific intra-
molecular reaction occurring for n = 3. Here, we report a new
striking observation of crucial influence of the bridge length
on the photoinduced H transfer in bifunctional RCs containing
amino groups.
and methyl and methylene protons in the β-position to the
amine nitrogen atom (well defined and nearly isotropic). RCs
with a longer bridge (n = 3) give a large number of computed
conformers with close total energy. Meanwhile, in all the cases,
the most energetically favourable conformers are folded and
correspond to the shortest possible distances between functional
groups. The RC conformation manifests itself in variations
in methylene β-proton coupling constants and occurrence of
extra coupling with a δ-proton for certain folded conformers
(assuming free rotation of the methyl groups). The folded
conformers are characterized by large coupling with only one
of two methylene β-protons. Experimental EPR spectra for
all the studied species are in reasonable agreement with the
computational results (examples of AA-3 and AE-3 radical
cations are shown in Figure 1).
Warming the irradiated sample to 120–145 K results in
irreversible changes in the EPR spectra, which are converted to
triplets with the hyperfine splittings of 1.8–1.9 mT, which are
·
characteristic of >N–CH2 type radicals. This can be explained
A series of aminoamides (AA-n; X = Me2N, Y = CONMe2,
n = 1–3) were synthesized by different original methods. AA-1
was obtained from dimethylamine and methyl chloroacetate,
AA-2 was synthesized by the reaction of dimethylamine with
acrylic acid chloride, and AA-3 was obtained through a three-
stage procedure starting from γ-butyrolactone. Aminoethers
(AE-n, X = Me2N, Y = OMe, n = 2, 3) were obtained by known
methods. RCs were generated by the X-irradiation of frozen
solutions of the corresponding compounds (0.1 to 0.5 vol%) in
Freon 113 (CF2ClCFCl2) at 77 K. Experimental EPR studies were
complemented by theoretical DFT analysis (PBE functional,5
PRIRODA computational code6,7).
As revealed by quantum-chemical calculations, spin density
in all RCs under investigation is localized at the amino group
(at least, 60% of Hirshfeld spin population is located at the
amine nitrogen atom). This looks quite logical in view of
large ΔIPXY values (~1.9 eV for aminoamides and ~2.1 eV for
aminoethers, as estimated from the ionization energies of the
corresponding monofunctional molecules). Accordingly, the RCs
of aminoamides and aminoethers should exhibit major hyperfine
coupling for amine nitrogen nucleus (strongly anisotropic, only
weak and broad parallel features can be detected since a^ ~ 0)
by the ion–molecule reactions:
.
+
Y(CH2)nNMe2 + Y(CH2)nNMe2
·
Y(CH2)nNMeCH2 + Y(CH2)nN+(H)Me2
where Y = Me2NCO or MeO. Reactions of this kind typically
occur for various RCs in a Freon 113 matrix above 110 K.8
Note that we did not observe the formation of radicals of the
·RCHNR' type corresponding to proton abstraction from methylene
groups, although such species were calculated to be thermo-
·
dynamically more stable than the >N–CH2 type radicals (by
13.35 kcal mol–1 for AA-1). This result indicates that the reaction
is controlled by steric (kinetic) factors. A similar behaviour was
observed previously9,10 for the methylal RC.
Thus, the structure and thermal reactions of the studied
bifunctional RC are similar to those of amine RCs. Also, similar
to amine RC, the radical cations of bifunctional compounds
with short methylene bridge (n < 3, i.e., AA-1, AA-2 and AE-2)
are insensitive to visible light (l > 370 nm). However, RCs of
AA-3 and AE-3 reveal specific and selective photochemical
reactions. In particular, the photolysis of AA-3 RC with light
at l = 360–540 nm for ~1 h leads to transformation shown in
Figure 1 (spectrum 3). The resulting spectrum can be described
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Mendeleev Commun., 2009, 19, 268–269
as a four-line signal with an average splitting of ~2.25 mT
(weaker outer lines are probably due to residual primary RC).
Such a spectrum should belong to distonic RC, the product of
intramolecular H atom transfer from methylene bridge. The
presence of three protons with roughly similar coupling con-
stants (2.0–2.5 mT) is possible only for the interior-type radical
resulting from H transfer from the central methylene group
of the bridge. In this case, one should assume major coupling
with two β-protons and one α-proton (the latter one should be
essentially anisotropic, which is in agreement with lineshape of
the outer components of the quartet). According to the quantum-
chemical calculations, the most stable species is a distonic RC,
where the H atom is located between amine nitrogen atom and
carbonyl oxygen atom, thus forming a seven-membered pseudo-
cyclic structure (Figure 1). The formation of such a structure is
favoured by geometry reasons (the shortest distance between H
atom to transfer and carbonyl oxygen atom in the primary RC,
~0.277 nm).
The most efficient and specific phototransformation is char-
acteristic of AE-3 RC. Photolysis at l = 380–450 nm for only
7 min leads to the almost complete conversion of initial signal
to a three-line spectrum with a non-binomial intensity ratio
resulting from two slightly inequivalent protons. This signal
can be described reasonably as a doublet of doublets with
a(1H) = 1.74 and a(1H) = 2.33 mT (spectrum 6 in Figure 1).
Note that the transformation is nearly quantitative (the integrated
intensity is retained). The spectrum is clearly different from
·
that of >N–CH2 type radical, so it originates from the intra-
molecular H transfer from the methylene bridge. For obvious
reasons, it cannot be ascribed to an interior-type radical
·
(RCH2CHCH2R'), which should have, at least, three strongly
coupling protons. The most probable interpretation is concerned
·
with the formation of distonic RC of the RCH2CHOMe type
(major hyperfine coupling with one α and one β proton, the
second β proton giving a small coupling masked by linewidth):
hv
MeO(CH2)3NMe2+
MeO CH(CH2)2N+(H)(CH2)2.
·
·
ge
A
Quantum-chemical calculations show that two lowest electron
excited states of the AE-3 RC are characterized by high spin
population at p(π) or p(σ) orbitals of oxygen atom, which implies
high probability of H transfer from the adjacent methylene group.
An alternative explanation (H transfer from the methylene group
adjacent to nitrogen) is unfavourable from both thermodynamic
and structural viewpoints.
1
2
O
N
In summary, we have demonstrated that RC of Y(CH2)nNMe2
type compounds underwent efficient and selective intramole-
cular transformations induced by visible light at n = 3 (a ‘magic
bridge’). Such reactions do not occur for amine RCs or
analogues with a shorter methylene bridge. Most probably,
excitation of the primary RC results in electron transfer from
remote functional group (amide or ether) to amine nitrogen
followed by intramolecular H transfer from the methylene bridge.
The process is controlled by favourable folded conformation of
RC realized at n = 3. More detailed studies of the mechanism
of this unusual class of reactions are in progress.
O
Me2NCCH2CH2CH2NMe2+
·
O---H
Me2NC+
H2C
NMe2
CH2
3
·
B
5.0 mT
CH
O
N
N
ge
We are grateful to S. L. Levchenko and A. G. Mazhuga for the
synthesis of aminoethers. This work was supported by the Russian
Foundation for Basic Research (project no. 06-03-33104).
4
5
MeOCH2CH2CH2NMe2+
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·
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H
MeOCHCH2CHN+Me2
5.0 mT
·
Figure 1 (1) Experimental EPR spectrum obtained by X-irradiation of
AA-3 frozen solution in Freon 113; (2) isotropic simulation of the spectrum
of AA-3 RC: a(6H) = 2.45, a(1H) = 3.64 mT; (3) EPR spectrum obtained
after photolysis of AA-3 RC at l = 360–540 nm for 1 h; (4) experimental
EPR spectrum obtained by X-irradiation of AE-3 frozen solution in Freon 113;
(5) isotropic simulation of AE-3 RC: a(6H) = 2.61, a(1H) = 3.73, a(1H) =
= 0.45 mT; (6) EPR spectrum obtained after photolysis of AE-3 RC at
l = 360–540 nm for 7 min. Images A, B and C show calculated spin
density distribution in optimized conformations of the primary AA-3 RC,
distonic RC obtained by its intramolecular transformation, and primary
AE-3 RC, respectively.
10 V. I. Feldman, F. F. Sukhov, A. Yu. Orlov and N. A. Shmakova, J. Phys.
Chem. A, 2000, 104, 3792.
Received: 11th January 2009; Com. 09/3262
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