Yikun Zhu’s research while affiliated with Albany State University and other places

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Publications (26)


(a) Crystal structure of K2‐1TR²⁻, ball‐and‐stick model. The THF molecules and hydrogen atoms are omitted for clarity. The external contacts are shown as green dashes. (b) Metal coordination pattern of 1TR²⁻ in different views, space‐filling model. (c) 2D layer of K2‐1TR²⁻, mixed model. Two independent K⁺ ions are shown in different shades of purple.
Crystal structures of (a) Rb2‐1TR²⁻ and (b) Cs2‐1TR²⁻, ball‐and‐stick models. All hydrogen atoms are omitted for clarity. (c) Metal coordination pattern of 1TR²⁻ in both compounds, along with the Rb−C and Cs−C distances, space‐filling model. 1D column packing in (d) Rb2‐1TR²⁻ and (e) Cs2‐1TR²⁻, space‐filling model. Independent Rb⁺/Cs⁺ ions are highlighted in different shades of orange/green.
(a) ¹H NMR investigation of the two‐fold reduction of TBCOT with K. (b) UV‐vis absorption spectra of all products.
Electrostatic potential surfaces of (a) Na‐1⁻, (b) Na2‐1TR²⁻, (c) Rb2‐1TR²⁻, and (d) Cs2‐1TR²⁻.
(a) Schematic 3D structure of the aromatic component of the Na2‐1TR²⁻, Rb2‐1TR²⁻, and Cs2‐1TR²⁻ products. The different sets of rings are color‐coded identified by the letters A and B. (b) Partial charges and aromaticity (NICS in ppm and HOMA) metrics for the respective rings in the different species.

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Stabilizing Contorted Doubly‐Reduced Tetraphenylene with Heavy Alkali Metal Complexation: Crystallographic and Theoretical Evidence
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January 2025

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6 Reads

Yikun Zhu

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Marina A. Petrukhina

The two‐fold reduction of tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT, or tetraphenylene, 1) with K, Rb, and Cs metals reveals a distinctive core transformation pathway: a newly formed C−C bond converts the central eight‐membered ring into a twisted core with two fused five‐membered rings. This C−C bond of 1.589(3)–1.606(6) Å falls into a single σ‐bond range and generates two perpendicular π‐surfaces with dihedral angles of 110.3(9)°–117.4(1)° in the 1TR²⁻ dianions. As a result, the highly contorted 1TR²⁻ ligand exhibits a “butterfly” shape and could provide different coordination sites for metal‐ion binding. The K‐induced reduction of 1 in THF affords a polymeric product with low solubility, namely [{K⁺(THF)}2(1TR²⁻)] (K2‐1TR²⁻). The use of a secondary ligand facilitates the isolation of discrete complexes with heavy alkali metals, [Rb⁺(18‐crown‐6)]2[1TR²⁻] (Rb2‐1TR²⁻) and [Cs⁺(18‐crown‐6)]2[1TR²⁻] (Cs2‐1TR²⁻). Both internal and external coordination are observed in K2‐1TR²⁻, while the bulky 18‐crown‐6 ligand only allows external metal binding in Rb2‐1TR²⁻ and Cs2‐1TR²⁻. The reversibility of the two‐fold reduction and bond rearrangement is demonstrated by NMR spectroscopy. Computational analysis shows that the heavier alkali metals enable effective charge transfer from the 1TR²⁻TBCOT dianion, however, the aromaticity of the polycyclic ligand remains largely unaffected.

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Stepwise reduction of an asymmetric π-expanded pyracylene towards the crystalline radical trianion

January 2025

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6 Reads

The chemical reduction of a pyracylene-hexa-peri-hexabenzocoronene-(HBC)-fused nanographene TPP was investigated with K and Rb metals to reveal its multi-electron acceptor abilities. The in situ reaction of TPP with the above alkali metals, monitored by UV-vis-NIR and ¹H NMR spectroscopy, evidenced the stepwise reduction process. The use of different solvents and secondary ligands enabled isolation of single crystals of three different reduced states of TPP with 1, 2, and 3 electrons added to its π-system. This provided a unique set of carbanions with gradually increasing negative charge for in-depth structural analysis of the outcomes of controlled electron addition to a non-planar and asymmetric nanographene, using X-ray crystallographic, spectroscopic, and theoretical tools. EPR spectroscopy measurements of the mono- and triply-reduced TPP products revealed distinct EPR splitting patterns. DFT calculations demonstrated a notable difference in the spin density distribution of these two open-shell products and provided insights into experimental EPR data. Moreover, the influence of the counterions on the “naked” TPP anions was illustrated computationally.



What a Difference an Electron Makes: Structural Response of Saddle‐Shaped Tetraphenylene to One and Two Electron Uptake

September 2024

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29 Reads

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2 Citations

Cyclooctatetraene (COT) and COT²⁻ dianion are well‐known as archetypical non‐aromatic and aromatic systems, respectively. However, despite a wealth of studies the effect of one electron addition to the eight‐membered ring remains equivocal. Herein, we report the first stepwise electron addition to tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT or tetraphenylene), accompanied by isolation and structural characterization of the mono‐ and doubly‐reduced anions. The X‐ray crystallographic study reveals only a small asymmetric distortion of the saddle‐shaped core upon one electron uptake. In contrast, the doubly‐reduced product exhibits a severely twisted conformation, with a new C−C bond separating the COT ring into two fused 5‐membered rings. The reversibility of the two‐fold reduction and bond rearrangement is demonstrated by NMR spectroscopy. In agreement with experimental results, computational analysis confirms that the reduction‐induced core rearrangement requires the addition of the second electron.


Unveiling the Multielectron Acceptor Properties of π-Expanded Pyracylene: Reversible Boat to Chair Conversion

May 2024

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8 Reads

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3 Citations

Journal of the American Chemical Society

In this work, the chemical reduction of a hybrid pyracylene-hexa-peri-hexabenzocoronene (HPH) nanographene was investigated with different alkali metals (Na, K, Rb) to reveal its remarkable multielectron acceptor abilities. The UV–vis and ¹H NMR spectroscopy monitoring of the stepwise reduction reactions supports the existence of all intermediate reduction states up to the hexaanion for HPH. Tuning the experimental conditions enabled the synthesis of the HPH anions with gradually increasing reduction states (up to −5) isolated with different alkali metal ions as crystalline materials. The single-crystal X-ray diffraction structure analysis demonstrates that the highly negatively charged HPH anions (−4 and −5) exhibit a drastic geometry change from boat-shaped (observed in the neutral parent, mono- and dianions) to a chair conformation, which was proved to be fully reversible by NMR spectroscopy. DFT calculations show that this geometry change is induced by an enhanced interaction between the coordinated metal ions and negatively charged HPH core in the chair conformation.


The anti-aromatic dianion and aromatic tetraanion of [18]annulene

March 2024

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108 Reads

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9 Citations

Nature Chemistry

Conjugated macrocycles behave differently from analogous linear chains because their electronic wavefunctions resemble a quantum particle on a ring, leading to aromaticity or anti-aromaticity. [18]Annulene, (CH)18, is the archetypal non-benzenoid aromatic hydrocarbon. Molecules with circuits of 4n + 2 π electrons, such as [18]annulene (n = 4), are aromatic, with enhanced stability and diatropic ring currents (magnetic shielding inside the ring), whereas those with 4n π electrons, such as the dianion of [18]annulene, are expected to be anti-aromatic and exhibit the opposite behaviour. Here we use ¹H NMR spectroscopy to re-evaluate the structure of the [18]annulene dianion. We also show that it can be reduced further to an aromatic tetraanion, which has the same shape as the dianion. The crystal structure of the tetraanion lithium salt confirms its geometry and reveals a metallocene-like sandwich, with five Li⁺ cations intercalated between two [18]annulene tetraanions. We also report a heteroleptic sandwich, with [18]annulene and corannulene tetraanion decks.


Homoleptic Rare-Earth-Metal Sandwiches with Dibenzo[a,e]cyclooctatetraene Dianions

February 2024

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32 Reads

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3 Citations

Inorganic Chemistry

A family of rare-earth complexes [RE(III) = Y, La, Gd, Tb, Dy, and Er] with doubly reduced dibenzo[a,e]cyclooctatetraene (DBCOT) has been synthesized and structurally characterized. X-ray diffraction analysis confirms that all products of the [RE(DBCOT)(THF)4][RE(DBCOT)2] composition consist of the anionic sandwich [RE(DBCOT)2]⁻ and the cationic counterpart [RE(DBCOT)(THF)4]⁺. Within the sandwich, two elongated π decks are slightly bent toward the metal center (avg. 7.3°) with a rotation angle of 35.9–37.6°. The RE(III) ion is entrapped between the central eight-membered rings of DBCOT2– in a η⁸ fashion. The trends in the RE–COT bond lengths are consistent with the variations of the ionic radii of RE(III) centers. The ¹H NMR spectra of the diamagnetic Y(III) and La(III) analogues illustrate the distinct solution behavior for the cationic and anionic parts in this series. Magnetic measurements for the Dy analogue reveal single-molecule magnetism, which was rationalized by considering the effect of crystal-field splitting for both building units analyzed by electronic structure calculations.


Stepwise deprotonation of truxene: Structures, metal complexation and charge-dependent optical properties

October 2023

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76 Reads

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3 Citations

As a planar subunit of C60-fullerene, truxene (C27H18) represents a highly symmetrical rigid hydrocarbon with strong blue emission. Herein, we used truxene as a model to investigate the chemical reactivity of a fullerene fragment with alkali metals. Monoanion, dianion, and trianion products with different alkali metal counterions were crystallized and fully characterized, revealing the core curvature dependence on charge and alkali metal coordination. Moreover, a ¹proton nuclear magnetic resonance study coupled with computational analysis demonstrated that deprotonation of the aliphatic CH2 segments introduces aromaticity in the five-membered rings. Importantly, the UV-vis absorption and photoluminescence of truxenyl anions with different charges reveal intriguing charge-dependent optical properties, implying variation of the electronic structure based on the deprotonation process. An increase in aromaticity and π-conjugation yielded a red shift in the absorption and photoluminescent spectra; in particular, large Stokes shifts were observed in the truxenyl monoanion and dianion with high emission quantum yield and time of decay. Overall, stepwise deprotonation of truxene provides the first crystallographically characterized examples of truxenyl anions with three different charges and charge-dependent optical properties, pointing to their potential applications in carbon-based functional materials.


The Dianion and Tetraanion of [18]Annulene

October 2023

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56 Reads

[18]Annulene, (CH)18, is one of the iconic molecules of organic chemistry. Sondheimer’s investigation of this compound in the 1960s provided a compelling endorsement for molecular orbital theory by showing that Hückel’s rule extends to molecules substantially larger than benzene. Molecules with circuits of 4n+2 π-electrons, such as [18]annulene (n = 4), are aromatic, with enhanced stability and diatropic ring currents (magnetic shielding inside the ring), whereas those with 4n π-electrons are antiaromatic and exhibit the opposite behaviour. In 1973, Oth, Woo and Sondheimer reported that [18]annulene can be reduced to an antiaromatic dianion (20 π-electrons). Here we show that their published structural assignment of this dianion was incorrect, and that [18]annulene can also be reduced to a stable aromatic tetraanion (24 π-electrons). The 1H NMR spectra of the dianion and tetraanion confirm that they are antiaromatic and aromatic, respectively, and indicate that [18]annulene adopts a C2v conformation in both reduced states that contrasts the virtual D6h symmetry of the neutral ring. The crystal structure of the tetraanion lithium salt confirms this geometry and reveals a metallocene-like sandwich, with five Li+ cations intercalated between two [18]annulene tetraanions. We also report a heteroleptic sandwich, with both [18]annulene and corannulene tetraanion decks.


Crystallographic evidence for global aromaticity in the di-anion and tetra-anion of a cyclophane hydrocarbon

September 2023

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56 Reads

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11 Citations

[24]Paracyclophanetetraene is a classic example of a macrocyclic hydrocarbon that becomes globally aromatic on reduction to the di-anion, and switches to globally anti-aromatic in the tetra-anion. This redox activity makes it promising as an electrode material for batteries. Here, we report the solid-state structures of the di- and tetra-anions of this cyclophane, in several coordination environments. The changes in bond length on reduction yield insights into the global aromaticity of the di-anion (26π electrons), and anti-aromaticity of the tetra-anion (28π electrons), that were previously deduced from NMR spectra of species generated in situ. The experimental geometries of the aromatic di-anion and anti-aromatic tetra-anion from X-ray crystallographic data match well with gas-phase calculated structures, and reproduce the low symmetry expected in the anti-aromatic ring. Comparison of coordinated and naked anions confirms that metal coordination has little effect on the bond lengths. The UV-vis-NIR absorption spectra show a sharp intense peak at 878 nm for the di-anion, whereas the tetra-anion gives a broad spectrum typical of an anti-aromatic system.


Citations (16)


... 26 The increased rigidity imparted by the transannular bond also prevents conformational rearrangements that are oen seen when switching between aromatic and antiaromatic states of larger molecules. [27][28][29][30][31][32][33][34] However, to date no crystallographic data of a pentalene and its corresponding pentalenide have been reported to compare the extent of any changes. ...

Reference:

Reversible formation of tetraphenylpentalene, a room temperature stable antiaromatic hydrocarbon
What a Difference an Electron Makes: Structural Response of Saddle‐Shaped Tetraphenylene to One and Two Electron Uptake

... Unlike Na metal, [23] the Rb + and Cs + ions tend to directly coordinate to the negatively charged π-surface, even in the presence of secondary ligands. [38][39][40][41] In both crystal structures of Rb 2 -1 TR 2À and Cs 2 -1 TR 2À (Figure 2), the two cations are asymmetrically η 5 -coordinated ( Figure 2cd, see Table S6 for more details) to the opposite external six-membered rings A and C of 1 TR 2À , in a similar manner as in K 2 -1 TR 2À . The coordination environment of each Rb + /Cs + ion is completed by 18-crown-6 ether (RbÀ O crown : 2.861 (2) (Table S6). ...

Unveiling the Multielectron Acceptor Properties of π-Expanded Pyracylene: Reversible Boat to Chair Conversion
  • Citing Article
  • May 2024

Journal of the American Chemical Society

... 26 The increased rigidity imparted by the transannular bond also prevents conformational rearrangements that are oen seen when switching between aromatic and antiaromatic states of larger molecules. [27][28][29][30][31][32][33][34] However, to date no crystallographic data of a pentalene and its corresponding pentalenide have been reported to compare the extent of any changes. ...

The anti-aromatic dianion and aromatic tetraanion of [18]annulene

Nature Chemistry

... 24 RE compounds comprising anionic and cationic dbCOT metallocenes are also accessible. 25 Taking into account the recent developments in RE chemistry, the dianionic dbCOT ligand serves as an excellent candidate for the generation of the first actinide complexes containing dbCOT in any oxidation state. The complexation of redoxactive actinide ions with the expanded π-surface of the dbCOT ligands holds great potential towards applications in switchable, 26 and conductive materials. ...

Homoleptic Rare-Earth-Metal Sandwiches with Dibenzo[a,e]cyclooctatetraene Dianions
  • Citing Article
  • February 2024

Inorganic Chemistry

... Unlike Na metal, [23] the Rb + and Cs + ions tend to directly coordinate to the negatively charged π-surface, even in the presence of secondary ligands. [38][39][40][41] In both crystal structures of Rb 2 -1 TR 2À and Cs 2 -1 TR 2À (Figure 2), the two cations are asymmetrically η 5 -coordinated ( Figure 2cd, see Table S6 for more details) to the opposite external six-membered rings A and C of 1 TR 2À , in a similar manner as in K 2 -1 TR 2À . The coordination environment of each Rb + /Cs + ion is completed by 18-crown-6 ether (RbÀ O crown : 2.861 (2) (Table S6). ...

Stepwise deprotonation of truxene: Structures, metal complexation and charge-dependent optical properties

... 21 In contrast, the corresponding macrocycle, paracyclophanetetraene (PCT; Scheme 1, top center), dissolves well in various organic solvents and the compound can be easily processed from solution. 17,22,23 Furthermore, conjugated macrocycles can also act as redox-active units, 17,[24][25][26][27][28][29] meaning that, unlike the attachment of aliphatic side chains, macrocyclization may even increase the specific capacity for use as a battery electrode material. In the case of PCT and its derivatives, the macrocyclic structure enables switching between a stable, locally aromatic neutral state (with concealed antiaromaticity) and a stable, globally aromatic doubly charged state (Scheme 1, top right), providing these macrocycles with excellent redox properties. ...

Crystallographic evidence for global aromaticity in the di-anion and tetra-anion of a cyclophane hydrocarbon

... [23,24] Compared to alkali metals with smaller ionic radii, Li + and Na + , which favor the formation of SSIPs with a variety of πligands, [25][26][27] the heavier alkali metals, K + , Rb + , and Cs + , tend to complex with the carbanions through direct metal ion coordination. [28][29][30] Particularly, the large size of Cs + ion and enhanced metal-π interactions favour the selective concave binding to various curved π-hosts, including several carbon bowls and nanographenes. [31][32][33][34][35][36][37] In this work, we investigate the chemical reduction behavior of TBCOT with heavy alkali metals and focus on metal coordination effects. ...

Chemical reduction of indenocorannulene: A two-electron addition stop
  • Citing Article
  • July 2023

Journal of Organometallic Chemistry

... In particular, the chemical robustness of Chichibabin's hydrocarbon can be enhanced by ring fusion and peripheral substitution, as illustrated by the recently reported derivatives of CH, such as 5,10dimesityldiindeno[1,2-a:2 0 ,1 0 -i]phenanthrene (DIPh), 4 its heterocyclic analogues DFFu, 5 DFPy and DFTh, 6 and some coronoid oligoradicaloid structures. [7][8][9] These systems exemplify the use of indeno fusion 10-12 as a strategy for elaborating stable di-and oligoradicaloids, which are of interest as organic semiconductors, 6,13,14 redox-active systems, 15 ion receptors, 8 and chiral materials. 16,17 Heterocycle fusion, showcased by DFFu, Chart 1 Difluorenoheteroles and the influence of ring fusion on spinspin interactions in indeno-fused oligoradicals. ...

Orbital Nature of Carboionic Monoradicals Made from Diradicals

... Despite these original binding abilities, to date, only solventseparated ion-products (SSIPs) have been isolated for TBCOT and its derivatives. [23,24] Compared to alkali metals with smaller ionic radii, Li + and Na + , which favor the formation of SSIPs with a variety of πligands, [25][26][27] the heavier alkali metals, K + , Rb + , and Cs + , tend to complex with the carbanions through direct metal ion coordination. [28][29][30] Particularly, the large size of Cs + ion and enhanced metal-π interactions favour the selective concave binding to various curved π-hosts, including several carbon bowls and nanographenes. ...

Stepwise reduction of a corannulene-based helical molecular nanographene with Na metal
  • Citing Article
  • March 2022

Chemical Communications

... Despite these original binding abilities, to date, only solventseparated ion-products (SSIPs) have been isolated for TBCOT and its derivatives. [23,24] Compared to alkali metals with smaller ionic radii, Li + and Na + , which favor the formation of SSIPs with a variety of πligands, [25][26][27] the heavier alkali metals, K + , Rb + , and Cs + , tend to complex with the carbanions through direct metal ion coordination. [28][29][30] Particularly, the large size of Cs + ion and enhanced metal-π interactions favour the selective concave binding to various curved π-hosts, including several carbon bowls and nanographenes. ...

Reversible structural rearrangement of π-expanded cyclooctatetraene upon two-fold reduction with alkali metals
  • Citing Article
  • February 2022

Chemical Communications