Ligand Non-innocence and Single Molecular Spintronic Properties of AgII Dibenzocorrole Radical on Ag(111)

Article abstract of DOI:10.1002/anie.202016674

A facile method for the quantitative preparation of silver dibenzo-fused corrole Ag-1 is described. In contrast to the saddle conformation resolved by single-crystal X-ray analysis for Ag-1, it adopts an unprecedented domed geometry, with up and down orientations, when adsorbed on an Ag(111) surface. Sharp Kondo resonances near Fermi level, both at the corrole ligand and the silver center were observed by cryogenic STM, with relatively high Kondo temperature (172 K), providing evidence for a non-innocent Ag^II-corrole^.2? species. Further investigation validates that benzene ring fusion and molecule-substrate interactions play pivotal roles in enhancing Ag(4d(x²?y²))–corrole (π) orbital interactions, thereby stabilizing the open-shell singlet Ag^II-corrole^.2? on Ag(111) surface. Moreover, this strategy used for constructing metal-free benzene-ring fused corrole ligand gives rise to inspiration of designing novel metal–corrole compound for multichannel molecular spintronics devices.

Full text of DOI:10.1002/anie.202016674

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 11702–11706
International Edition:
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doi.org/10.1002/anie.202016674
doi.org/10.1002/ange.202016674
Corroles
Ligand Non-innocence and Single Molecular Spintronic Properties of
AgII Dibenzocorrole Radical on Ag(111)
Jialiang Xu+, Li Zhu+, Hu Gao, Chenhong Li, Meng-Jiao Zhu, Zhen-Yu Jia, Xin-Yang Zhu,
Yue Zhao, Shao-Chun Li,* Fan Wu,* and Zhen Shen*
Abstract: A facile method for the quantitative preparation of
silver dibenzo-fused corrole Ag-1 is described. In contrast to
the saddle conformation resolved by single-crystal X-ray
analysis for Ag-1, it adopts an unprecedented domed geometry,
with up and down orientations, when adsorbed on an Ag(111)
surface. Sharp Kondo resonances near Fermi level, both at the
corrole ligand and the silver center were observed by cryogenic
STM, with relatively high Kondo temperature (172 K), provid-
ing evidence for a non-innocent AgII-corroleC2ꢀ species. Further
investigation validates that benzene ring fusion and molecule-
substrate interactions play pivotal roles in enhancing Ag(4d-
(x2ꢀy2))–corrole (p) orbital interactions, thereby stabilizing
the open-shell singlet AgII-corroleC2ꢀ on Ag(111) surface.
Moreover, this strategy used for constructing metal-free
benzene-ring fused corrole ligand gives rise to inspiration of
designing novel metal–corrole compound for multichannel
molecular spintronics devices.
spin relaxation channels nowadays are highly desirable for
fabricating ultrafast miniaturized data storage devices and
logic gates.[2] Scanning tunnelling microscopy (STM) has been
demonstrated to be a powerful tool for studying spintronics
through investigating the interactions between the unpaired
molecular spins and the conduction band electrons of the
substrate.[3] Recent studies have shown that metal complexes
of porphyrins[4] and their analogues[5] efficiently couple their
delocalized unpaired p-electrons with the conduction band
electrons of the metal substrate thus been extensively studied
for molecular spintronics applications.[6] As a representative
branch of porphyrins, corrole can be viewed as a ring-
contracted porphyrinoid containing direct pyrrole-pyrrole
linkages and with the trianionic characteristics of the macro-
cyclic ligand. Corroles show remarkable coordination ability[7]
and chemical activities, which is widely used in the fields of
catalysts,[8] chemical sensors,[9] solar cells[10] and pharmaceut-
ical manufacturing.[11] Furthermore, by adjusting the HOMO
orbital energy of corrole molecule, the regulation of the
electron state could be easily achieved thus make it a promis-
ing candidate in spintronics.
Non-innocent ligands as special properties arousing from
d–p interaction[12] are widespread among metallocorroles,
especially among coinage metal corroles. Previous studies
have shown that copper corroles are non-innocent,[13] while
gold corroles are innocent,[14] depending on the metal
ðdx2ꢀy2 Þ–corrole(p) orbital interactions. However, describing
silver corroles[15] as innocent or non-innocent is very chal-
lenging. The crystal structures of silver triarylcorroles are
mildly saddled, suggesting the existence of some degree of
metalðdx2ꢀy2 Þ–corrole(p) orbital interactions in the silver case,
which is less than that of copper but more than that of gold.
Ghosh et al. reported a non-innocent silver b-octabromo-
meso-triarylcorrole[15d] based on its strongly saddled structure
and Soret maxima being sensitive to the nature of the para-
substituent, while a series of b-arylethynyl-substituted silver
corrole complexes are proved innocent.[15e] Therefore, study-
ing the intriguing subtle properties of silver corroles in single
molecular spintronics become attractive. Previously, we
reported a planar triplet ground-state CuII tetrabenzocorrole
with four fused benzene rings on the corrole ligand.[5c]
However, fused-ring expansions with other metal centers
have rarely been studied in detail in terms of the interactions
between metal centers and structurally modifying the corrole
ligands and their spintronic properties, since it is difficult to
remove the central copper ion or directly synthesize a free-
base tetrabenzocorrole ligand.
S
pintronics[1] is a novel technique that uses the electron spin
and magnetic moment to construct new electronic devices
rather than traditional electron charge, bringing great
improvements in heat dissipation, power consumption, and
speed. Traditional spintronics research mainly focuses on
transition metals and inorganic semiconductors, while the
advantage of organic molecules is that their electronic
structures and magnetic properties are relatively easy to
achieve effective spin control by changing the special external
conditions. Advances in molecular spintronics with multiple
[*] J. L. Xu,[+] H. Gao, C. H. Li, Dr. M. J. Zhu, Dr. Y. Zhao, Dr. F. Wu,
Prof. Dr. Z. Shen
State Key Laboratory of Coordination Chemistry, Collaborative
Innovation Center of Advanced Microstructures
Collaborative Innovation Center of Chemistry for Life Sciences
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing 210046 (P. R. China)
E-mail: wufan@nju.edu.cn
zshen@nju.edu.cn
L. Zhu,[+] Dr. Z. Y. Jia, X. Y. Zhu, Prof. Dr. S. C. Li
National Laboratory of Solid State Microstructures
School of Physics, Collaborative Innovation Center of
Advanced Microstructures, Nanjing University
Nanjing 210093 (P. R. China)
E-mail: scli@nju.edu.cn
Prof. Dr. S. C. Li
Jiangsu Provincial Key Laboratory for Nanotechnology
Nanjing University, Nanjing 210093 (China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016674.
Here, we described a novel dibenzo-fused silver corrole
Ag-1, prepared by the facile retro-Diels–Alder reaction of
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The molecular structures of
free-base corrole 1 and its silver
complex Ag-1 were unambigu-
ously determined by X-ray crys-
tallography analysis. Single crys-
tals of 1 and Ag-1 were obtained
by slow diffusion of n-hexane
into a toluene solution or recrys-
tallization from a mixture of
toluene and methanol, respec-
tively. The explicit crystal struc-
tures are shown in Figure 1, with
crystallographic data listed in the
Supporting
Information,
Table S1 and Figures S10, S11.
The average silver-nitrogen (Ag-
N) distances and the saddling
dihedral angles of various silver
corroles (Supporting Informa-
tion, Figure S1) are summarized
Scheme 1. Synthesis of dibenzo-fused silver corrole Ag-1.
a peripheral bicyclo[2.2.2]octadiene-fused (BCOD-fused)
corrole Ag-2. The latter compound Ag-2 was prepared
through a [2+2]-type condensation reaction, followed by
complexation with silver acetate (Scheme 1). Ag-1 adsorbed
on Ag(111) substrate was an open-shell singlet AgII-corroleC2ꢀ
state, which was found to adopt rare domed geometries that
are significantly different from the normal saddle configu-
ration resolved by single crystal X-ray analysis. In both domed
geometries, Kondo effects were observed at the center metal
as well as the ligand. Therefore, our research indicates that by
stabilizing the unpaired electrons of metal and the p radical in
the ligand provide a new pathway for developing multi-
channel molecular spintronics devices for data storage and
logic gates.
Other than copper corroles, research on fused-ring-
expanded metal corroles has not been reported due to
difficulties associated with their synthesis and purification.
Through a facile method, fused-ring-expanded free-base
corroles and the corresponding metallocorroles were success-
fully synthesized (Scheme 1). First, 2,17-dibicyclo-
[2.2.2]octadiene-fused corrole 2 was synthesized by the
[2+2] condensation reaction of the 1,9-bisphenyl-5-phenyl-
dipyrromethane 3 and 1,1’-bis(4,7-dihydro-4,7-ethano-2H-
isoindole) 4 (Supporting Information, Scheme S1). Complex-
ation of 2 with silver acetate in pyridine yielded the 2,17-
dibicyclo[2.2.2]octadiene-fused silver corrole Ag-2, which was
converted into the 2,17-dibenzo-fused silver corrole Ag-1 in
quantitative yield by heating at 2808C under vacuum.
Another approach to synthesizing Ag-1 involves first prepar-
ing the 2,17-dibenzo-fused corrole free base 1 by the retro-
Diels–Alder reaction of 2, followed by metal coordination;
however, 1 is relatively unstable in the air as well as the yield
in the metal complexation step is poor (Scheme 1). The
structures of the new macrocyclic products 1, 2 and their silver
complexes were characterized using various spectroscopic
means (Supporting Information). This method can also be
used to synthesize other fused-ring-expanded metallocor-
roles.
in the Supporting Information, Table S2. Ag-1 adopts mild
saddle configuration. The Ag-N distances in Ag-1 (1.958 ꢀ,
1.970 ꢀ) are slightly longer than those of the b-unsubstituted
silver triarylcorroles, such as Ag(TpFPC)[14] (1.943 ꢀ,
1.963 ꢀ) and Ag(TpMePC)[15a] (1.948 ꢀ, 1.960 ꢀ), but
smaller than those of the silver-octabromo-meso-triarylcor-
role, for example, Ag(Br8TpMePC) (1.983 ꢀ, 1.983 ꢀ).
However, the average value of torsion angles (c1ꢀc3, defined
in Table S2, 31.18) of Ag-1 are smaller than those of Ag-
(TpFPC) (32.58), Ag(TpMePC) (33.88) and Ag(Br8TpMePC)
(59.08). In addition, the mean plane deviation (m.p.d., defined
by 23 core atoms in Table S2) of Ag-1 (0.175 ꢀ) is the smallest
among the reported silver corroles. As well as the average
value of dihedral angles between the three meso-aryl rings
and the mean plane (Figure 1c, 57.58 on average) of Ag-1 are
larger than those of other silver corroles. These phenomena
declare the better planarity of Ag-1 than those reported silver
corroles.
To further illustrate the electronic states of Ag-1, an
isolated single Ag-1 molecule on Ag(111) has been charac-
terized by STM; experimental details are available in the
Figure 1. Top (up) and side (bottom) views of the X-ray crystal
structures of 1 (a, b) and Ag-1 (c, d) are shown with thermal ellipsoids
at 50% probability.[22]
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Figure 2. a) Molecular structure of Ag-1. b) STM image of a single Ag-1 molecule (size: 3ꢀ4 nm2, U= +200 mV, It =50 pA). c) dI/dV spectra of
the Ag-1 molecule (U= +100 mV, It =200 pA, Umod =2 mV). The red solid line is the fitted result using the Fano function. d) The half-width from
the dI/dV spectrum of Ag-1 on the Ag (111) film as a function of temperature and the fitted result. The error bars represent measurement
deviations from different molecules. (e, f) Differential conductance dI/dV spectra (U= +100 mV, It =200 pA, Umod =2 mV) of six different
characteristic sites of the two forms of Ag-1 whose centers are hollow (inset of (e)) and protruded (inset of (f)). g) Morphological image of an Ag-
1 molecule and it’s dI/dV map in the vicinity of the Fermi level (4ꢀ4 nm2, U= +100 mV, It =200 pA, Umod =10 mV).
Supporting Information. The Ag-2 precursor was thermally
sublimed onto an Ag(111) substrate in an ultra-high vacuum
(base pressure of 1 ꢁ 10ꢀ10 mbar).[16] During the sublimation
process, Ag-2 was converted into Ag-1 by a retro-Diels–Alder
reaction. The high-resolution STM topographical image
(Figure 2b) of a single molecule adsorbed on the Ag(111)
surface shows three bright bumps along with a ladder-shaped
tail, which matches the molecular structure of Ag-1 (Fig-
ure 2a).
The Kondo effect can be induced by the exchange
between the spin of a delocalized p-radical and the con-
duction electrons of the substrate.[17] Kondo resonance usually
features a peak, dip, or kink near the Fermi energy in the STS
spectrum,[18] and is useful for investigating the spin state of
a molecule. Figure 2c shows the differential conductance (dI/
dV) spectrum taken at the center of the Ag-1 molecule. A dip
near EF is evident in Figure 2c; this resonance feature was
fitted to the Fano resonance function:[19]
However, Ag-1 appears to adopt an unprecedented
“dome-like” configuration, rather than the usual “saddle-
like” configuration determined by X-ray crystallography. A
previous report indicated that substrate-molecule interactions
can change the configuration of a molecule adsorbed on
a substrate surface.[5c] In contrast to our previously reported
copper tetrabenzocorrole, we observed both a protruded spot
and a hollow area at the location of the central Ag atom in the
STM map of Ag-1. Most of the Ag-1 population adsorbed on
the Ag(111) surface adopts the “Ag-Down” configuration, in
which the central Ag is close to the substrate. As shown in the
inset of Figure 2e, a slight depression is observed at the center
of the “Ag-Down” configuration. On the other hand, the “Ag-
Up” configuration, in which the central Ag is positioned away
from the substrate, is only occasionally observed. The “Ag-
Up” configuration exhibits a protrusion at the molecular
center (inset of Figure 2 f). These inequivalent populations
may be due to the different formation energies of Ag-1 on the
Ag(111) substrate, and is easily understood in terms of the
coupling of the central Ag ion to the Ag substrate, which
lowers the formation energy; hence the central Ag ion in Ag-
1 tends to approach the substrate to increase molecule-
substrate coupling.
2
dIðVÞ ðe þ qÞ
ð1Þ
/
1 þ e2
dðVÞ
where e = (eVꢀe0)/G, e0 is the energy shift from EF, G is the full
width at half maximum (FWHM) of the Fano resonance, and
q is the Fano line-shape parameter. The good fit shown in
Figure 2c suggests that the observed resonance is due to the
Kondo effect of the Ag-1 molecule. The G values obtained at
various temperatures (T) are plotted in Figure 2d and are well
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
described by the formula: G ¼ 2 pkBT2 þ 2ðkBTkÞ accord-
ing to Fermi liquid theory.[17c] The Kondo temperature Tk was
determined to be about 172 K.
Notably, Kondo resonance is identified throughout the
entire ligand (including the three meso-aryl rings 1–3 and the
two fused benzene rings 5, 6) as well as the central Ag ion (4),
as shown in Figure 2e. This is in sharp contrast to the
previously studied Cu-tetrabenzocorrole,[5c] which showed
a Kondo effect only on the corrole macrocycle. As shown in
Figure 2g, the topographical image and the dI/dV map of the
Ag-1 molecule show that the Ag center has the least of
differential conductance, with larger ones appearing at the
surrounding ligands, implying that resonance at the silver
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center is most prominent, while that at the ligands is
“Ag-Down” configuration, the same conclusion could be
somewhat weaker. Kondo resonance was also observed on
the occasional “Ag-Up” configuration, as shown in Figure 2 f,
with similar results obtained.
drawn for the “Ag-Up” configuration. In addition, the ground
state energy of the “Ag-Down” configuration was 9.489 kcal
molꢀ1 lower than that of the “Ag-Up” configuration, which
quantitatively explains the dominance of the “Ag-Down”
configuration in terms of the distribution of adsorbed
molecules.
DFT calculations using the VASP code with a plane-wave
basis set were used to better understand the experimental
results of STM; details can be found in the Supporting
Information. Geometry optimization resulted in the open-
shell singlet Ag-1 on Ag(111) substrate adopting two differ-
ently oriented dome configurations (Figures 3a–d). These
calculation results are in good agreement with the exper-
imental structures that we refer to as the “Ag-Down” and
“Ag-Up” configurations. The distances between the molecule
and the silver substrate are 2.775 ꢀ for “Ag-Down” and
The spin-polarized partial density of states (PDOS)
spectrum of the Ag atoms in the differently oriented Ag-
1 molecules in the absorption system (Figure 3e) are predict-
ably asymmetric. The peaks near the Femi level that appear in
the spectra of both configurations provide a theoretical
possibility for observing the Kondo resonances in their
scanning tunneling spectra.[5a,b,21] The same conclusion can
be drawn from the two calculated spin density plots as well
(Figure 3 f for “Ag-Down” and Figure 3g for “Ag-Up”). The
lack of orthogonality between the three meso-aryl groups and
the mean plane (the averaged dihedral angles is 51.18 for “Ag-
Down” and 45.38 for “Ag-Up”) leads to the spin densities
distribution over almost the entire molecule, even onto the
meso-aryl groups, which is consistent with the Kondo
resonances results observed at the 1–3 sites. These properties
are obviously different from those of open-shell triplet
tetraphenylconjugated copper with plane configuration.[5c]
Combined with the Bader valences of the central Ag ion
results, with values of + 1.76 determined for the “Ag-Down”
and + 1.83 for “Ag-Up” configurations, we conclude that the
electronic state of Ag-1 adsorbed on the Ag(111) surface is an
open-shell singlet AgII-dibenzocorroleC2ꢀ.
ꢀ
3.721 ꢀ for “Ag-Up”. Meanwhile, the lengths of Ag N bond
are1.996 ꢀ and 2.010 ꢀ for “Ag-Down”, as for “Ag-Up” is
1.983 ꢀ and 1.985 ꢀ (Supporting Information, Table S3).
These bond lengths are much longer than those observed in
the crystal structure of Ag-1 and are closer to those of AgII
porphyrins.[20]
Three electronic states, namely the closed-shell singlet
AgIII-corrole3ꢀ, the open-shell singlet AgII-corroleC2ꢀ, and the
open-shell triplet AgII-corroleC2ꢀ radical were evaluated for
Ag-1 adsorbed on the Ag(111) substrate. The results listed in
the Supporting Information, Table S4 revealed that the open-
shell singlet state was the optimal ground state of Ag-1 in the
In summary, a novel free-base fused-ring-expansion
corrole and its silver complex (Ag-1) were successfully
synthesized for the first time. The electronic state of Ag-
1 on the Ag(111) substrate was studied by STM. Notably, two
unprecedented “dome-like” configurations (“Ag-Down” and
“Ag-Up” ) are formed on the Ag(111) substrate. Obvious
Kondo resonances near Fermi level, both at the corrole ligand
and the silver center, were observed by STM, with a relatively
high Kondo temperature (172 K). DFT calculations using the
VASP code with a plane-wave basis set show that Ag-
1 behaves as an open shell singlet AgII-corroleC2ꢀ when
adsorbed on the Ag(111) substrate. This is the first direct
spectroscopic observation of the existence of AgII corrole
radical. Meanwhile, this facile syntheses of the free-base and
silver dibenzocorrole complex provides a new approach to the
fine-tuning of interactions between the metal center and the
corrole ligand. Stabilization of the unpaired electrons of silver
and the p radical in the ligand provides a new pathway for the
design of multi-channel molecular spintronics devices for data
storage and logic gates. Efforts to further stabilize non-
innocent metallocorroles by fusion with naphthalene and
anthracene rings are currently underway.
Figure 3. Top and side views of optimized geometries of the a, b) “Ag-
Down” and c, d) “Ag-up” configurations in the adsorption system. The
white balls represent the unit cell, which contains 46 Ag atoms per
layer. e) Spin-polarized partial density of states (PDOS) spectra of the
Ag atom in the “Ag-Down” (red) and “Ag-Up” (blue) configurations.
The spin densities of the f) “Ag-Down” and g) “Ag-Up” configurations
on the Ag(111) substrate.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 21771102, 22071103, and
21911540069) to Z.S., (No. 22001119) to F.W. and (Nos.
11774149, 11790311) to S.L.
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Conflict of interest
The authors declare no conflict of interest.
[12] A. Ghosh, T. Wondimagegn, A. B. J. Parusel, J. Am. Chem. Soc.
2000, 122, 5100 – 5104.
Keywords: benzo-fused corroles · Kondo effect ·
non-innocence · silver corrole · spintronics
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contain the supplementary crystallographic data for this paper.
These data are provided free of charge by the joint Cambridge
Crystallographic Data Centre and Fachinformationszentrum
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structures.
Manuscript received: December 19, 2020
Revised manuscript received: March 9, 2021
Accepted manuscript online: March 10, 2021
Version of record online: April 16, 2021
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