Reversible Carbon–Carbon Bond Breaking and Spin Equilibria in Bis(pyrimidinenorcorrole)

Article abstract of DOI:10.1002/anie.201607237

Reversible homolytic dissociation of the bis(pyrimidinenorcorrole) σ-dimer was elucidated by means of variable temperature ESR and¹H NMR spectroscopy, mass spectrometry, and optical spectrocopy. Dehydrogenation of the σ-dimer yielded another dimer displaying a singlet–triplet equilibrium in solution, strong NIR absorption (1570 nm), and a narrow electrochemical HOMO–LUMO gap (0.74 V).

Full text of DOI:10.1002/anie.201607237

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201607237
German Edition: DOI: 10.1002/ange.201607237
Porphyrinoids
Reversible Carbon–Carbon Bond Breaking and Spin Equilibria in
Bis(pyrimidinenorcorrole)
´
Bin Liu, Takuya Yoshida, Xiaofang Li,* Marcin Ste˛pien, Hiroshi Shinokubo, and
Piotr J. Chmielewski*
Abstract: Reversible homolytic dissociation of the bis(pyrimi-
dinenorcorrole) s-dimer was elucidated by means of variable
1
temperature ESR and H NMR spectroscopy, mass spectrom-
etry, and optical spectrocopy. Dehydrogenation of the s-dimer
yielded another dimer displaying a singlet–triplet equilibrium
in solution, strong NIR absorption (1570 nm), and a narrow
electrochemical HOMO–LUMO gap (0.74 V).
R
eversible dissociation of carbon–carbon bonds has
attracted considerable attention because of the theoretical
and experimental significance of this phenomenon in various
areas of chemistry.[1–3] In some instances, the bonded and
dissociated state are so close in energy that dynamic bond
breaking occurs rapidly under ambient conditions;[1] other-
wise, an external stimulus (thermal or photochemical) is
required to drive the process.[3] Intramolecular dissociation
can be particularly facile, leading to various rearrangement
reactions.[1e,f,j,2,3a,c,e,g–k] However, the most intriguing processes
are those occurring in an intermolecular fashion as they
provide access to unique dynamic structures, such as self-
healing polymers.[1b] Here we report a bis(porphyrinoid),
linked through a C(sp3)–C(sp3) bond, which exists in solution
Scheme 1. Amination of norcorrolatonickel(II) and dissociation of 3
into monomer radical 3.
amination proceeded with the regioselectivity observed
previously for cyanation[5c] and nitration[6b] providing 3-
aminonorcorrolatonickel(II) 2 in 28% yield. To our surprise,
two additional products were formed due to ring expansion,
which were the dimeric species 3 (32%) and the previously
reported[8] 10-azacorrolatonickel(II)4 (ca. 3%). All com-
pounds were unambiguously identified and characterized
using NMR and HRMS spectra, and in the case of 2, also by
single-crystal X-ray diffraction analysis (Figure S33,
CCDC 1479588 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre).
In contrast to the parent antiaromatic norcorrole complex
1 and its amination product 2, the dimeric species 3 is non-
aromatic because the formation of the b–b linkage disrupts
the macrocyclic conjugation in both subunits. The most
distinctive feature of the 1H NMR spectrum of 3 (Figure S6a)
is an AA’XX’ system of H2(2’) and H3(3’) which gives rise to
characteristic multiplets at d 5.87 and 3.83 ppm (600 MHz,
300 K, CDCl3), in line with the presence of isochronous, yet
magnetically non-equivalent protons, and thus, with the
dimeric structure of 3. Further structural details, such as the
presence of sp3 carbon atoms at the bridging bond, the
presence of six-membered ring of pyrimidine, non-planarity
of the system, and the location of the bridge relative to the
pyrimidine ring, were established by means of 1D and 2D
homo- and heteronuclear NMR techniques. The NMR
spectra of 3 correspond to an effective C2 symmetry of the
dimer (with homochiral C3 and C3’ stereocenters) rather than
Ci (with heterochiral stereocenters), according to energy-
minimized DFT models of these two diastereomers and
variable-temperature experiments (173–300 K, CD2Cl2).
Despite the presence of two homochiral centers in 3, the
in an equilibrium with the corresponding monomeric radical.
2
=
This hemilabile dimer can be dehydrogenated to a C(sp )
C(sp2)-linked bis(porphyrinoid) species, which demonstrates
a singlet-triplet spin equilibrium associated with conforma-
tional decoupling of the two porphyrinoid p systems.
Nickel(II) norcorrole 1 is a recently discovered 16-p-
electron tetrapyrrolic porphyrinoid, which displays an anti-
aromatic character and a reactivity quite distinct from that of
aromatic oligopyrroles.[4–6] In an attempt to introduce an NH2
group into 1, we employed 4H-1,2,4-triazol-4-amine[7] in the
presence of solid sodium hydroxide in THF (Scheme 1). The
[*] B. Liu, Prof. X. Li
Key Laboratory of Theoretical Organic Chemistry and
Functional Molecules, Ministry of Education
School of Chemistry and Chemical Engineering
Hunan University of Science and Technology
Xiangtan, Hunan 411201 (China)
E-mail: lixiaofang@iccas.ac.cn
T. Yoshida, Prof. H. Shinokubo
Department of Applied Chemistry, Graduate School of Engineering
Nagoya University, Nagoya 464-8603 (Japan)
´
Prof. M. Ste˛pien, Prof. P. J. Chmielewski
Department of Chemistry, University of Wrocław
F. Joliot-Curie 14, 50383 Wrocław (Poland)
E-mail: piotr.chmielewski@chem.uni.wroc.pl
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607237.
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enantiomers could not be separated by chiral HPLC which
suggested that the stereocenters in 3 might be configuration-
ally unstable. The positive-mode ESI and MALDI mass
spectra of 3 revealed the molecular ion at m/z = 1181.37
([M+H]+) and an additional, singly charged peak at m/z =
590.19, corresponding to [M/2]+. A peak with an essentially
the same m/z ratio was also found in the negative-mode
spectra, providing further support for the dissociation of the
C3–C3’ linkage in 3 occurring in the ion source.
The weakness of the C3–C3’ bond in the dimer was further
demonstrated to result in reversible homolytic dissociation of
3 in solution. The optical spectrum of 3 was significantly
altered upon heating, and the toluene solution changed its
color from grass-green at room temperature to olive-brown at
353 K (Figure 1A). Variable-temperature ESR spectra of 3
molꢀ1 for dissociation and Ea = 5 ꢁ 1 kcalmolꢀ1 for recombi-
nation. The reactions in both directions were first-order,
indicating that for the recombination, the rate-determining
step might be associated with a structural change (likely, a p-
dimer-to-s-dimer rearrangement) that follows the actual
dimerization. Thermodynamic parameters for the dissocia-
tion were determined by variable-temperature spectropho-
tometry and values of DH0 = 20.2 ꢁ 1.5 kcalmolꢀ1, DS0 = 46 ꢁ
3 calmolꢀ1 Kꢀ1 and DG0298 = 6.5 ꢁ 2.4 kcalmolꢀ1 were deter-
mined from the vanꢁt Hoff equation.
The free-energy changes due to dissociation derived from
DFT calculations using the wB97XD functional were con-
sistent with the experimentally observed reversibility of the
process (counterpoise-corrected free energy DG0
=
298
3.92 kcalmolꢀ1). The DFT-optimized structure of the radical
3C, showed complete planarization of all b-pyrrole carbons,
reflecting the restored peripheral conjugation in the macro-
cycle. The calculated spin distribution in 3C had a negligible
intensity at the Ni center, and the unpaired electron was
mostly placed on the meso-carbons C5, C15 as well as on C1-
C4, C11, and C19 (Figure S21), confirming the p-radical
character of 3C. Appreciable Fermi contacts were calculated
by DFT for proton atoms H2, H3, H8, and H13 (aH = 2.82,
ꢀ10.60, ꢀ3.16, and ꢀ6.02 MHz, respectively) and nitrogen
atoms N21 as well as N23 (aN = 9.00, 4.38 MHz, respectively).
The calculated spin distribution slightly differs from that
observed experimentally, since the ESR spectrum was best
reproduced by simulation assuming interaction of the
unpaired electron with two equivalent nitrogen atoms (aN =
6.37 MHz) and four hydrogen atoms (aH = 6.23, 3.00, 2.44
(2H) MHz, Figure S21).
Figure 1. A) Temperature dependence of the UV/Vis-NIR spectra of 3
(2.85ꢀ10ꢀ4 m in toluene) in the range from 274 K (green trace) to
368 K (brown trace). The bold black line is the spectrum recorded at
298 K. B) ESR spectra of the toluene solution of 3 recorded at specified
temperatures. C) The inset shows 1H NMR signals of protons 5-p-Mes
of 3 at specified temperatures ([D8]toluene, normalized intensity
spectra).
In polar solvents, the green dimer 3 underwent a further
irreversible transformation to a brown product 5, containing
the dehydrogenated C3–C3’ linkage (Scheme 2). The reaction
recorded in a toluene solution, revealed the presence of
a radical species at room temperature, with a gradual, up to
six-fold increase of the signal intensity observed on going to
1
1008C (Figure 1B). The intensity of H NMR signals appre-
ciably decreased with respect to an internal standard as the
temperature increased, though chemical shifts of the signals
varied only slightly over the whole temperature range
([D8]toluene, 253–373 K), likely due to changes in solvation
(Figure 1C). These observations are indicative of a chemical
exchange, which is slow on the NMR time scale, between the
diamagnetic 3 and the radical dissociation product 3C, the
latter yielding NMR spectral lines that are too broad to be
detected. The radical content, estimated on the basis of
1H NMR signal integration, increased from about 12% at
300 K to almost 90% at 373 K. All spectral changes induced
by increasing the temperature were fully reversed upon
cooling and the cycle could be reproduced many times under
anaerobic conditions.
The response of the system to the temperature changes
was not instantaneous. Kinetics of the dissociation and
recombination processes were studied spectrophotometri-
cally in the temperature range of 30 to 458C, providing rate
constants kdiss = 1.36 ꢀ 10ꢀ2–3.00 ꢀ 10ꢀ2 sꢀ1 and krec = 1.62 ꢀ
10ꢀ2–2.60 ꢀ 10ꢀ2 sꢀ1 and activation energies Ea = 10 ꢁ 1 kcal
Scheme 2. Singlet–triplet equilibrium in 5.
was particularly effective when carried out in DMF giving
quantitative conversion over 30 minutes at 808C; in acetoni-
trile, the dehydrogenation took place at room temperature,
yielding crystalline product in moderate yield after four days.
The process is likely due to the presence of atmospheric
oxygen acting as a dehydrogenating agent, although the
reaction can be conducted also in less polar solvents under
catalytic and rigorously anaerobic conditions (608C, toluene,
glove box, Pd/C, 3 h, 50% yield). The loss of two hydrogen
atoms in 5 was confirmed by HRMS and its structure was fully
elucidated by a single-crystal X-ray diffraction analysis (Fig-
ure 2A, CCDC 1479589 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free
2
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2.0026, D = 0.0095 cmꢀ1), apart from a featureless radical
signal which may arise upon sample preparation through
spontaneous oxidation of 5 (Figure 2B). No Dms = 2 line was
observed. The rather small zero-field splitting parameter D is
consistent with the nonplanar structure of the dimer[14] and
indicates a weak dipolar interaction between the radical
centers.[15] That in turn, suggests separate delocalization of the
spin over each of the subunits.
1H NMR spectroscopy in solution indicates a fast chem-
ical exchange between the diamagnetic and paramagnetic
species, resulting in temperature-dependent variations of
chemical shifts, line widths, and longitudinal relaxation times
T1, observed for most resonances (Figures S24–S27). At low
temperatures (183 K, CD2Cl2), all signals are reasonably
sharp and can be assigned by 2D techniques, and their spin-
lattice relaxation times (T1 > 1 s) are typical for diamagnetic
species. At room temperature, however, some signals (e.g. H8
and H13) are extremely broadened and unobservable, while
others, such as H17 and H18, remain sharp enough to reveal
spin–spin splittings. All T1 relaxation times at 300 K are much
shorter than those observed at 183 K (0.04–0.5 s), in line with
the higher population of the paramagnetic form and/or faster
chemical exchange between the forms. Significantly, below
203 K, there are six distinct methyl signals of the meso-mesityl
substituents, proving that the rotation at the C3–C3’ bond is
frozen. The rotation becomes rapid at higher temperatures,
resulting in pairwise averaging of the o-Mes signals. The
frequency of rotation around the C3-C3’ bond can be
estimated as about 340–370 Hz at the coalescence temper-
atures of individual signals reflecting flexibility of the
molecular skeleton. That kind of structural flexibility prevents
the chromatographic separation of atropisomers observed in
the solid state by X-ray diffraction.[9b,10a,e] Unlike 3, dimer 5
revealed only weak temperature dependence of its optical
spectra (25–1008C, toluene) with only a slight increase of
absorption at 416 nm and a similar decrease at 950 nm
(Figure S28). These observations reflect a small contribution
of 5-T over the whole temperature range.
DFT calculations were carried out for 5 in open- and
closed-shell singlet (5-S) and triplet (5-T) states. Since the
open-shell singlet (modeled using the broken-symmetry
approach) gave no stable SCF solutions, we confined our
analysis to the restricted and unrestricted Hartree–Fock
approximation for 5-S and 5-T, respectively (Scheme 2). The
optimized geometry of 5-S (more stable by 0.39 kcalmolꢀ1
than 5-T) reproduces closely the features of the X-ray
structure (rC3-C3’ = 1.417 ꢂ; C2-C3-C3’-C2’ torsion qT =
142.88; dihedral angle qD = 42.38). The optimized structure
of 5-T is characterized by a significantly longer C3–C3’ bond
(1.468 ꢂ), lower qT torsion (120.58), and higher dihedral angle
between the mean planes (qD = 61.58). Relaxed potential-
energy scans performed along the qT coordinate for 5-S and 5-
T (qT = 20 to 1808) indicated that the triplet ground state is
preferred for the more perpendicular orientations of the
macrocyclic subunits (25 < qT < 1358 or 47 < qD < 908). The
E(5-S)–E(5-T) energy gap reaches 8–10 kcalmolꢀ1 at the
near-perpendicular orientation of the subunits (qT = 70–908,
Figure 2C), with the C3–C3’ bond length in 5-S sharply
increasing up to 1.48 ꢂ, a value typical for a single C(sp2)–
Figure 2. A) Molecular structure of 5 depicted with 50% thermal
ellipsoids. All hydrogen atoms are omitted. B) ESR spectrum of solid 5
at 77 K. Red trace: simulation of the S=1 system. C) Relaxed DFT
potential energy surface scans for 5-S (blue and green circles) and 5-T
(red circles) on C2-C3-C3’-C2’ torsion angle qT.
of charge from The Cambridge Crystallographic Data
Centre.). In solid state, 5 consists of two flat macrocyclic
subunits (with mean out-of-plane displacements of 0.052 and
0.069 ꢂ) forming a dihedral angle qD of 44.08 and a C2-C3-
C3’-C2’ torsion qT of 143.9(4)8. The unique C3–C3’ bond has
a length of 1.437(5) ꢂ, which is appreciably shorter than bond
lengths reported for b,b’-linked bis(porphyrins) (1.46–
1.47 ꢂ),[9] N-confused porphyrin dimers (1.463–1.490 ꢂ),[10]
bis(corrole) free base (1.490(8) ꢂ),[11a] porphyrin-chlorin
heterodimer (1.522(5)),[11b] bis(chlorin) (1.61 ꢂ),[12] or Cmeso
–
Cmeso bond in bis(porphyrin) complexes (1.47–1.51 ꢂ)[13] all
consisting of aromatic subunits linked by a single bond and
containing an even number of delocalized p-electrons in each
macrocyclic ring. Conversely, in 5, each macrocyclic p-system
contains 19 p-electrons, so that two electrons need to be
paired across the C3-C3’ bond to yield a closed-shell system.
The non-coplanarity of subunits in 5 was nevertheless thought
to destabilize the interannular p-conjugation, with the expect-
ation that the electrons might become unpaired and yield two
weakly interacting 19-electron systems. Interestingly, 5 was
found to be weakly paramagnetic in solid state, with an
effective magnetic moment meff of 0.58 mB at 1.8 K, which
increases up to 0.70 mB at 50 K without any further systematic
change up to 300 K. Thus, a small part of compound 5 is in
a paramagnetic state but exchange interaction between the
unpaired electrons appears to be weak. From a triplet-state
spin-only magnetic moment (2.83 mB), the estimated content
of the paramagnetic species at room temperature is about
6%. The present system differs from a diradical generated for
a doubly linked bis(corrole), wherein a strong exchange
interaction completely quenched the spin below 150 K in the
solid state.[14] The ESR spectrum of solid 5 at 77 K displayed
a signal typical of triplet-state biradicals[15] (gk = 2.0018, g? =
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C(sp2) bond (Figure S35). Thus, the observed energy increase
might in part be attributed to the strain introduced to the C3–
C3’ double bond in 5-S. The calculation of spin distribution in
the energy-minimized structure of 5-T indicates the highest
spin density at meso-carbon atoms of both subunits (C5, C15,
C5’, and C15’), bridging C3, C3’, and internal nitrogen atoms
N21, N21’ (pyrimidine) and N23, N23’. Altogether, the spin
distribution over each subunit closely resembles that calcu-
lated for 3C and roughly correlates with line widths, T1 values,
and the temperature dependence of 1H NMR chemical shifts
for the corresponding protons of 5 (Figures S24–S27).
Electrochemical measurements for 5 indicate an inter-
action between the subunits, which appears to be stronger
than in singly-bonded bis(porphyrinoids).[6a,9b,10a,e] Two rever-
sible oxidation couples (E1/2 = ꢀ0.10 and 0.11 V) and two
reduction couples (E1/2 = ꢀ0.84 and ꢀ1.05 V), accompanied
by the less accessible redox events at 1.02 Vand ꢀ2.22 V were
observed for 5 in CH2Cl2 (potentials vs. the internal ferrocene
standard). The narrow electrochemical HOMO–LUMO gap
(DE = 0.74 V) is in line with that predicted by DFT calcu-
lations (1.08 eV) and with the NIR absorption of 5 with an
onset of the lowest-energy band reaching 1700 nm (ca.
0.73 eV, Figure S30).
In conclusion, we have shown here an unprecedented ring
expansion of norcorrole which provides a straightforward
access to two spin-switchable bis(porphyrinoid) systems.
These two dimers, each consisting of two pyrimidine-contain-
ing norcorrole rings, undergo reversible homolytic cleavage of
either a s-bond or a p-bond, depending on the oxidation level,
thus leading to two distinct types of magnetic bistability. It is
likely that such equilibria are more common but seldom
identified and they may be responsible for the reactivity of
diverse p-aromatics. Considering the renewed interest in
functional organic radicals, usable as switches or dynamic
building blocks,[1a–d,2,3b,15d,16] the reactivity of 3C and the
coordination chemistry of the dimer 5, in conjunction with
its ability to adopt various oxidation and spin states, are
particularly attractive and will be further explored in our
laboratories.
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This work was supported by the National Natural Science
Foundation of China (Nos. 21371054 and 21671063), the
Scientific Research Fund of Hunan Provincial Education
Department (grant number 13A026), and by the Polish
National Science Center (grant number 2013/09/B/ST5/
00326). Quantum chemical calculations were performed in
´
the Wrocław and Poznan Centers for Networking and Super-
computing.
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Keywords: biradicals · free-radical dimerization · norcorrole ·
porphyrinoids · spin equilibria
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Received: July 26, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Porphyrinoids
´
B. Liu, T. Yoshida, X. Li,* M. Ste˛pien,
H. Shinokubo,
P. J. Chmielewski*
&&&& — &&&&
Reversible Carbon–Carbon Bond
Breaking and Spin Equilibria in
Bis(pyrimidinenorcorrole)
Porphyrinoid metal complexes: Amina-
tion of the nickel(II) complex of norcor-
role under mild conditions resulted in
C(sp3)–C(sp3)-linked bis(pyrimidinenor-
corrole) 1 which in solution exists in
equilibrium with a monomeric radical
(see picture). After dehydrogenation
1 shows a conformation-dependent sin-
glet–triplet equilibrium.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!