A series of ferrocenyl substituted hydrazones (I–VII) derived from ferrocene carboxaldehyde and substituted hydrazides have been prepared and characterized by FTIR, 1H NMR spectroscopy, and crystallographic studies. The single-crystal X-ray analysis for III·0.5H2O·0.5CH3CN (CIF file CCDC no. 1968937) further authenticates the structural motif of the synthesized compounds. The C(11) of ferrocene carboxaldehyde is linked with N(1) of the hydrazide moiety with a bond length of 1.283(5) Å, confirming the binding of the two structural units present in the final product. They were preliminarily screened for their antimicrobial activity and demonstrate good results. The free radical scavenging activity for the compounds (III, IV) has been found to be more than 90% when compared with the ascorbic acid. The total antioxidant capacity and total reducing power assays for VI show significant activity whereas the data for the other compounds are also encouraging. Quantum chemical calculations at the DFT level predict that compound II is the softest while VII is the hardest within the series, resultantly II can be used as a synthon for further chemical reactions.

Compounds that can be labeled as "aromatic chameleons" are π-conjugated compounds that are able to adjust their π-electron distributions so as to comply with the different rules of aromaticity in different electronic states. We used quantum chemical calculations to explore how the fusion of benzene rings onto aromatic chameleonic units represented by biphenylene, dibenzocyclooctatetraene, and dibenzo[a,e]pentalene modifies the first triplet excited states (T1) of the compounds. Decreases in T1 energies are observed when going from isomers with linear connectivity of the fused benzene rings to those with cis- or trans-bent connectivities. The T1 energies decreased down to those of the parent (isolated) 4nπ-electron units. Simultaneously, we observe an increased influence of triplet state aromaticity of the central 4n ring as given by Baird's rule and evidenced by geometric, magnetic, and electron density based aromaticity indices (HOMA, NICS-XY, ACID, and FLU). Because of an influence of triplet state aromaticity in the central 4nπ-electron units, the most stabilized compounds retain the triplet excitation in Baird π-quartets or octets, enabling the outer benzene rings to adapt closed-shell singlet Clar π-sextet character. Interestingly, the T1 energies go down as the total number of aromatic cycles within a molecule in the T1 state increases. 

The aromaticity of cyclic 4nπ-electron molecules in their first ππ∗ triplet state (T1), labeled Baird aromaticity, has gained growing attention in the past decade. Here we explore computationally the limitations of T1 state Baird aromaticity in macrocyclic compounds, [n]CM's, which are cyclic oligomers of four different monocycles (M = p-phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 π-electrons in their main conjugation paths we find that for their T1 states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans ([n]CFU's) retain Baird aromaticity until larger n than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a 3[n]CFU with a specific n is more strongly Baird-aromatic than the analogous 3[n]CPP while the magnetic indices tell the opposite. To construct large T1 state Baird-aromatic [n]CM's, the design should be such that the T1 state Baird aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel aromaticity of one or a few monocycles and semilocalized triplet diradical character. Monomers with lower Hückel aromaticity in S0 than benzene (e.g., furan) that do not impose steric congestion are preferred. Structural confinement imposed by, e.g., methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T1 state aromaticity, we reveal the analogy to the Hückel aromaticity of the corresponding closed-shell dications yet observe stronger Hückel aromaticity in the macrocyclic dications than Baird aromaticity in the T1 states of the neutral macrocycles. © 2021 The Authors.

Baird's rule tells that the electron counts for aromaticity and antiaromaticity in the first ππ* triplet and singlet excited states (T1 and S1) are opposite to those in the ground state (S0). Our hypothesis is that a silacyclobutene (SCB) ring fused with a [4n]annulene will remain closed in the T1 state so as to retain T1 aromaticity of the annulene while it will ring-open when fused to a [4n + 2]annulene in order to alleviate T1 antiaromaticity. This feature should allow the SCB ring to function as an indicator for triplet state aromaticity. Quantum chemical calculations of energy and (anti)aromaticity changes along the reaction paths in the T1 state support our hypothesis. The SCB ring should indicate T1 aromaticity of [4n]annulenes by being photoinert except when fused to cyclobutadiene, where it ring-opens due to ring-strain relief. © 2017 by the authors.

The cyclopropyl (cPr) group, which is a well-known probe for detecting radical character at atoms to which it is connected, is tested as an indicator for aromaticity in the first ππ* triplet and singlet excited states (T1 and S1). Baird's rule says that the π-electron counts for aromaticity and antiaromaticity in the T1 and S1 states are opposite to Hückel's rule in the ground state (S0). Our hypothesis is that the cPr group, as a result of Baird's rule, will remain closed when attached to an excited-state aromatic ring, enabling it to be used as an indicator to distinguish excited-state aromatic rings from excited-state antiaromatic and nonaromatic rings. Quantum chemical calculations and photoreactivity experiments support our hypothesis; calculated aromaticity indices reveal that openings of cPr substituents on [4n]annulenes ruin the excited-state aromaticity in energetically unfavorable processes. Yet, polycyclic compounds influenced by excited-state aromaticity (e.g., biphenylene), as well as 4nπ-electron heterocycles with two or more heteroatoms represent limitations. 

Lignocellulosic biomass is renewable and one of the most abundant sources for the production of high-value chemicals, materials, and fuels. It is of immense importance to develop new efficient technologies for the industrial production of chemicals by utilizing renewable resources. Lignocellulosic biomass can potentially replace fossil-based chemistries. The production of fuel and chemicals from lignin powered by renewable electricity under ambient temperatures and pressures enables a more sustainable way to obtain high-value chemicals. More specifically, in a sustainable biorefinery, it is essential to valorize lignin to enhance biomass transformation technology and increase the overall economy of the process. Strategies regarding electrocatalytic approaches as a way to valorize or depolymerize lignin have attracted significant interest from growing scientific communities over the recent decades. This review presents a comprehensive overview of the electro-catalytic methods for depolymerization of lignocellulosic biomass with an emphasis on untargeted depolymerization as well as the selective and targeted mild synthesis of high-value chemicals. Elec-trocatalytic cleavage of model compounds and further electrochemical upgrading of bio-oils are discussed. Finally, some insights into current challenges and limitations associated with this approach are also summarized. © 2022 by the authors. 

The low-lying triplet state of a recently published compound (TMTQ) was analyzed quantum chemically in light of suggestions that it is influenced by Baird aromaticity. Two mesomeric structures describe this state: 1) a zwitterionic Baird aromatic structure with a triplet diradical 8π-electron methano[10]annulene (M10A) dicationic ring and 2) a Hückel aromatic with a neutral closed-shell 10π-electron ring. According to charge and spin density distributions, the Hückel aromatic structure dominates the triplet state (the Baird aromatic contributes at most 12 %), and separation of the aromatic fluctuation index (FLU) into α and β electron contributions emphasizes this finding. The small singlet-triplet energy gap is due to Hückel aromaticity of the M10A ring, clarified by comparison to the smaller analogues of TMTQ. Yet, TMTQ and its analogues are Hückel-Baird hybrids allowing for tuning between closed-shell 4n+2 Hückel aromaticity and open-shell 4n Baird aromaticity. 

The first hydrogenation step of benzene, which is endergonic in the electronic ground state (S 0), becomes exergonic in the first triplet state (T 1). This is in line with Baird's rule, which tells that benzene is antiaromatic and destabilized in its T 1 state and also in its first singlet excited state (S 1), opposite to S 0, where it is aromatic and remarkably unreactive. Here we utilized this feature to show that benzene and several polycyclic aromatic hydrocarbons (PAHs) to various extents undergo metal-free photochemical (hydro)silylations and transfer-hydrogenations at mild conditions, with the highest yield for naphthalene (photosilylation: 21%). Quantum chemical computations reveal that T 1-state benzene is excellent at H-atom abstraction, while cyclooctatetraene, aromatic in the T 1 and S 1 states according to Baird's rule, is unreactive. Remarkably, also CVD-graphene on SiO 2 is efficiently transfer-photohydrogenated using formic acid/water mixtures together with white light or solar irradiation under metal-free conditions. © The Author(s) 2016.

A series of new thiourea based carboxylic acids (Ia-Ie) were synthesized and characterized by elemental analysis, FTIR and NMR (1H and 13C) spectroscopy. They were preliminary bioassayed for their antibacterial, anifungal and urease inhibition activities. Molecular docking simulations were carried out to determine the probable binding mode of the synthesized compounds. The bioassay results showed that some of titled compounds exhibited encouraging results. 

Tributyltin(IV) derivative (1) was synthesized by reacting the ligand (A1) {2-[(2-methylpropanoyl)amino]propanoic acid} with tributyltin(IV)chloride in presence of potassium hydroxide. The compounds were characterized by elemental analysis, FTIR and multinuclear (1H, 13C and 119Sn) NMR spectroscopy. The 119Sn NMR data for (1) suggest that tin is five coordinated having trigonal bipyramidal molecular geometry. Molecular structures for (1) and (A1) were determined by single crystal X-ray analysis. Some new and novel structural features of the compounds, are reported first time. Tributyltin(IV) complex has distorted trigonal bipyramidal molecular geometry around tin in the solid state exhibiting single chain tetramer which may be ascribed as tetra-nuclear cage structure that is constructed from four tributyltin units and four ligand moieties. The tributyltin(IV) entities are bridged by one oxygen of the carboxylic group of one ligand and another oxygen of the amidic carbonyl of next ligand. The nature of Sn…O bond was analyzed with the help of natural bond orbital (NBO) analysis. The compounds were also screened in vitro for their anti-microbial and urease inhibition activities, and found some encouraging results. 

1,3,2,4-Diazadiboretidine, an isoelectronic heteroanalogue of cyclobutadiene, is an interesting chemical species in terms of comparison with the carbon system, whereas its properties have never been investigated experimentally. According to Baird's rule, Hückel antiaromatic cyclobutadiene acquires aromaticity in the lowest triplet state. Here we report experimental and theoretical studies on the ground- and excited-state antiaromaticity/aromaticity as well as the photophysical properties of an isolable 1,3,2,4-diazadiboretidine derivative. The crystal structure of the diazadiboretidine derivative revealed that the B2N2 ring adopts a planar rhombic geometry in the ground state. Yet, theoretical calculations showed that the B2N2 ring turns to a square geometry with a nonaromatic character in the lowest triplet state. Notably, the diazadiboretidine derivative has the lowest singlet and triplet states lying at close energy levels and displays blue phosphorescence.

Benzene exhibits a rich photochemistry which can provide access to complex molecular scaffolds that are difficult to access with reactions in the electronic ground state. While benzene is aromatic in its ground state, it is antiaromatic in its lowest ππ∗ excited states. Herein, we clarify to what extent relief of excited-state antiaromaticity (ESAA) triggers a fundamental benzene photoreaction: the photoinitiated nucleophilic addition of solvent to benzene in acidic media leading to substituted bicyclo[3.1.0]hex-2-enes. The reaction scope was probed experimentally, and it was found that silyl-substituted benzenes provide the most rapid access to bicyclo[3.1.0]hexene derivatives, formed as single isomers with three stereogenic centers in yields up to 75% in one step. Two major mechanism hypotheses, both involving ESAA relief, were explored through quantum chemical calculations and experiments. The first mechanism involves protonation of excited-state benzene and subsequent rearrangement to bicyclo[3.1.0]hexenium cation, trapped by a nucleophile, while the second involves photorearrangement of benzene to benzvalene followed by protonation and nucleophilic addition. Our studies reveal that the second mechanism is operative. We also clarify that similar ESAA relief leads to puckering of S1-state silabenzene and pyridinium ion, where the photorearrangement of the latter is of established synthetic utility. Finally, we identified causes for the limitations of the reaction, information that should be valuable in explorations of similar photoreactions. Taken together, we reveal how the ESAA in benzene and 6π-electron heterocycles trigger photochemical distortions that provide access to complex three-dimensional molecular scaffolds from simple reactants. 

Optically pure alcohols are abundant in nature and attractive as feedstock for organic synthesis but challenging for further transformation using atom efficient and sustainable methodologies, particularly when there is a desire to conserve the chirality. Usually, substitution of the OH group of stereogenic alcohols with conservation of chirality requires derivatization as part of a complex, stoichiometric procedure. We herein demonstrate that a simple, inexpensive, and environmentally benign iron(III) catalyst promotes the direct intramolecular substitution of enantiomerically enriched secondary and tertiary alcohols with O-, N-, and S-centered nucleophiles to generate valuable 5-membered, 6-membered and aryl-fused 6-membered heterocyclic compounds with chirality transfer and water as the only byproduct. The power of the methodology is demonstrated in the total synthesis of (+)-lentiginosine from D-glucose where iron-catalysis is used in a key step. Adoption of this methodology will contribute towards the transition to sustainable and bio-based processes in the pharmaceutical and agrochemical industries. © 2019, The Author(s).

New Zn(II) complexes (1–8) of general formula [Zn(R)2(L')2], where R = cinnamic acid (1–7), L' = 3-pyridine carboxamide (1), 2,2'-bipyridine (2), 3-pyridinecarbonitrile (3), 2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole (4), 2-methylimidazole (5), 2-phenylimidazole (6), 2-imidazolidinethione (7) and for (8), R = acetic acid; L' = 2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole)2, have been synthesized by a stoichiometric reaction between Zn(II) carboxylate(s) and the respective nitrogen-based ligands. The prepared compounds are analyzed by FTIR, NMR spectroscopy and single crystal X-ray diffraction techniques for their structural manifestations along with elemental analyses. The crystallographic data for 6 and 8 exhibit four-coordinated zinc having distorted tetrahedral molecular geometry. The thermogravimetric data demonstrate high stability of the compounds with gradual loss of acetate and thiadiazol fragments that ultimately lead to zinc oxide as the residual mass of 8. Quantum chemical calculations indicate that 7 is the softest in reactivity having the lowest band gap as determined from the energies of frontier molecular orbitals whereas 8 is highly polar with the highest dipole moment value of 12.61 D. The computational data are a complement to the success of the research.