Department of Chemistry
Permanent URI for this collectionhttps://hdl.handle.net/1807/16837
Established in 1859, the Department of Chemistry, with its advanced research facilities, is dedicated in conducting top notch research and providing excellent learning experiences for future great scientists. With 50 faculty members, 30 staffs, and over 200 graduate and post-doctoral students, the department has its focus on a wide range of research within chemistry (analytical, biological and organic, environmental, inorganic, physical, polymer and material, as well as chemical physics). However, advancement does not rely on single studies or research area, thus, the department also provides interdisciplinary research with the medical sciences and engineering faculties for an integrative approach.
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Item Open Access Luminescence of 1,3,6-trisubstituted pyrazolo[3,4-d][1,2,3]triazoles(Canadian Science Publishing, 2023-03-01) Liang, Qiuming; Hayashi, Kasumi; Philippi, Franka; Song, DatongThe photophysical properties of four 1,3,6-trisubstituted pyrazolo[3,4-d][1,2,3]triazoles were studied by a combination of UV–vis and fluorescence spectroscopy as well as density functional theory (DFT) and time-dependent DFT calculations. All four compounds are weakly fluorescent. Upon protonation with trifluoroacetic acid, all compounds display a drastic luminescence turn-on. The addition of excess trifluoroacetic acid further enhances the luminescence intensity, which is tentatively attributed to the formation of extended hydrogen bonding network rather than the formation of doubly protonated 1,3,6-trisubstituted pyrazolo[3,4-d][1,2,3]triazoles. All new compounds were characterized by nuclear magnetic resonance and X-ray crystallography.Item Open Access [4+1] cyclization of α-diazo esters and mesoionic N-heterocyclic olefins(Royal Society of Chemistry (RSC), 2023-03-27) Liang, Qiuming; Zeng, Yimin; Mendez Ocampo, Pedro A.; Zhu, Hui; Qu, Zheng-Wang; Grimme, Stefan; Song, DatongPrompted by the recent stepwise mechanistic proposal for the Huisgen [3+2] cycloaddition reaction between enamine and α-diazo ester, where the nucleophilic addition of the enamine carbon onto the terminal nitrogen of diazo ester is crucial, we examined the possible use of N-heterocyclic olefins (NHOs) as highly electron-rich dipolarophiles in these reactions. The mesoionic NHOs derived from 1,2,3-triazoles undergo fast [4+1] cycloaddition to give 3-(triazolium-4-yl)-(3H)-pyrazol-4-olates at room temperature. The reaction mechanism has been explored through experimental and DFT computational studies.Item Open Access Recent advances of mesoionic N-heterocyclic olefins(Royal Society of Chemistry (RSC), 2022-06-01) Liang, Qiuming; Song, DatongThis Perspective article highlights the recent development of mesoionic N-heterocyclic olefins (mNHOs), where the exo-cyclic olefinic carbon is not bonded to strongly electron-withdrawing groups. The unquenched basicity and nucleophilicity of the exo-cyclic olefinic carbon make mNHOs strong σ-donors and enable unique reactivity patterns.Item Open Access Dioxygenation of unprotected mesoionic N-heterocyclic olefins(Royal Society of Chemistry (RSC), 2021-09-22) Liang, Qiuming; Hayashi, Kasumi; Li, Longfei; Song, DatongWe report the dioxygenation of mesoionic N-heterocyclic olefins (mNHOs) using molecular dioxygen. For 1,2,3-triazole-derived mNHOs possessing a vinyl proton and at least one acidic C–H group, they are oxidized into the corresponding triazolium benzoate salts, whereas those without vinyl proton or an acidic C–H group are oxidized into triazolium oxide and ketones/aldehydes.Item Open Access Constructing fused N-heterocycles from unprotected mesoionic N-heterocyclic olefins and organic azides via diazo transfer(Royal Society of Chemistry (RSC), 2021-05-19) Liang, Qiuming; Hayashi, Kasumi; Zeng, Yimin; Jimenez-Santiago, Jose L.; Song, DatongMesoionic N-heterocyclic olefins (mNHOs) were first reported last year and their reactivity remains largely unexplored. Herein we report the reaction of unprotected mNHOs and organic azides as a novel synthetic route to a variety of pyrazolo[3,4-d][1,2,3]triazoles, an important structural motif in drug candidates and energetic materials. The only byproduct aniline can be easily recycled and converted back to the starting organic azide, in compliance with the green chemistry principle. The reaction mechanism has been explored through experimental and computational studies.Item Embargo Bifunctional Ruthenium Catalysts for endo-Selective Cycloisomerization of Nucleophile-Functionalized Terminal Alkynes(American Chemical Society (ACS), 2024-11-13) Garcia Mayerstein, Hector A.; Song, DatongThe catalytic cycloisomerization of nucleophile-functionalized alkynes is a useful method for the synthesis of heterocyclic compounds with 100% atom economy. Group 8 catalysts give high endo-selectivity in these transformations due to their ability to invoke metal-vinylidene intermediates. However, all known group 8 catalysts have relatively low activities and require high temperatures. Here, we report bifunctional ruthenium catalysts that enable the cycloisomerization of a large variety of substrates at low catalyst loadings and ambient temperature with turnover frequencies as high as 200 s–1.Item Open Access Reactivity of a Piano-Stool Iron Complex toward Boranes(American Chemical Society (ACS), 2023-04-21) Liang, Qiuming; Garcia Mayerstein, Hector A.; Song, DatongThe binding and activation of boranes by transition-metal compounds are relevant to catalytic hydroboration reactions. Herein, we report the stoichiometric reactivity of a piano-stool iron complex toward a series of boranes, where the joint action of the metal center and the ligand active site has been observed, as well as the catalytic activity of this iron complex toward the hydroboration of various N-heterocycles.Item Open Access [2Fe–2S] Cluster Supported by Redox-Active o-Phenylenediamide Ligands and Its Application toward Dinitrogen Reduction(American Chemical Society (ACS), 2021-05-27) Liang, Qiuming; DeMuth, Joshua C.; Radović, Aleksa; Wolford, Nikki J.; Neidig, Michael L.; Song, DatongAs prevalent cofactors in living organisms, iron–sulfur clusters participate in not only the electron-transfer processes but also the biosynthesis of other cofactors. Many synthetic iron–sulfur clusters have been used in model studies, aiming to mimic their biological functions and to gain mechanistic insight into the related biological systems. The smallest [2Fe–2S] clusters are typically used for one-electron processes because of their limited capacity. Our group is interested in functionalizing small iron–sulfur clusters with redox-active ligands to enhance their electron storage capacity, because such functionalized clusters can potentially mediate multielectron chemical transformations. Herein we report the synthesis, structural characterization, and catalytic activity of a diferric [2Fe–2S] cluster functionalized with two o-phenylenediamide ligands. The electrochemical and chemical reductions of such a cluster revealed rich redox chemistry. The functionalized diferric cluster can store up to four electrons reversibly, where the first two reduction events are ligand-based and the remainder metal-based. The diferric [2Fe–2S] cluster displays catalytic activity toward silylation of dinitrogen, affording up to 88 equiv of the amine product per iron center.Item Open Access Syntheses and Reactivity of Piano-Stool Iron Complexes of Picolyl-Functionalized N-Heterocyclic Carbene Ligands(American Chemical Society (ACS), 2021-11-15) Liang, Qiuming; Song, DatongThe complex [FeClCp*(HL)] (1; where HL = 3-methyl-1-(2-picolyl)-imidazol-2-ylidene) was synthesized from the reaction of the in situ generated HL ligand and [FeClCp*(TMEDA)] (where TMEDA is N,N,N′,N′-tetramethylethylenediamine). The deprotonation of 1 with KHMDS led to the removal of a pyridylic proton and the dearomatization of the pyridine ring of the HL ligand, forming [Cp*(L)Fe(μ-N2)FeCp*(L)] (2) under N2 or [(FeCp*)2(μ-H)(μ-L)] (3) under Ar. Complex 2 splits H2 across the L– ligands and the iron centers to give [FeCp*(H)(HL)] (4). Complex 4 readily converts to [Cp*(L″)Fe(μ-N2)FeCp*(L″)] (5) under N2, where the L″– ligand chelates to the metal center through the carbene carbon and a pyridyl carbon. The reactions of 2 with PhSiH3 and Ph2SiH2 give silyl complexes 6 and 7, respectively. The compounds 2, 4, and 5 are active (pre)catalysts for the dehydrogenative coupling of dimethylamine–borane.Item Open Access Reactivity of an Unprotected Mesoionic N-Heterocyclic Olefin(American Chemical Society (ACS), 2020-11-09) Liang, Qiuming; Hayashi, Kasumi; Song, DatongWe report a novel mesoionic N-heterocyclic olefin (mNHO) derived from a 1,2,3-triazolium salt. With the carbenic position unprotected, the mNHO can tautomerize into the mesoionic carbene (MIC) form at room temperature. The reactivity of this new mNHO with various group 13 Lewis acids and CO2 have been studied experimentally and computationally.Item Open Access Reactivity umpolung (reversal) of ligands in transition metal complexes(Royal Society of Chemistry, 2024-02-14) Morris, RobertThe success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts and the understanding of metalloenzyme mechanisms. Labelling a metal and ligand with Seebach donor and acceptor labels helps to identify whether a reaction involving the intermolecular attack on the ligand is displaying native reactivity or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxides, imides/nitrenes, alkylidynes, methylidynes, and nitrides. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the pKaLAC (LAC is ligand acidity constant) of the theoretical hydride complexes [H-[M]-L]+ formed by the protonation of pair of valence d electrons on the metal in the [M-L] complex. Preliminary findings indicate that a negative pKaLAC indicates that nucleophilic attack by a carbanion or amine on the ligand will likely occur while a positive pKaLAC indicates that electrophilic attack by strong acids on the ligand will usually occur when the ligand is nitrile, carbonyl, isonitrile, alkene and eta-6-arene.Item Embargo Degradable π-Conjugated Polymers(American Chemical Society, 2024-04) Uva, Azalea; Michailovich, Sofia; Hsu, Nathan Sung Yuan; Tran, HelenThe integration of next-generation electronics into society is rapidly reshaping our daily interactions and lifestyles, revolutionizing communication and engagement with the world. Future electronics promise stimuli-responsive features and enhanced biocompatibility, such as skin-like health monitors and sensors embedded in food packaging, transforming healthcare and reducing food waste. Imparting degradability may reduce the adverse environmental impact of next-generation electronics and lead to opportunities for environmental and health monitoring. While advancements have been made in producing degradable materials for encapsulants, substrates, and dielectrics, the availability of degradable conducting and semiconducting materials remains restricted. π-Conjugated polymers are promising candidates for the development of degradable conductors or semiconductors due to the ability to tune their stimuli-responsiveness, biocompatibility, and mechanical durability. This perspective highlights three design considerations: the selection of π-conjugated monomers, synthetic coupling strategies, and degradation of π-conjugated polymers, for generating π-conjugated materials for degradable electronics. We describe the current challenges with monomeric design and present options to circumvent these issues by highlighting biobased π-conjugated compounds with known degradation pathways and stable monomers that allow for chemically recyclable polymers. Next, we present coupling strategies that are compatible for the synthesis of degradable π-conjugated polymers, including direct arylation polymerization and enzymatic polymerization. Lastly, we discuss various modes of depolymerization and characterization techniques to enhance our comprehension of potential degradation byproducts formed during polymer cleavage. Our perspective considers these three design parameters in parallel rather than independently while having a targeted application in mind to accelerate the discovery of next-generation high-performance π-conjugated polymers for degradable organic electronics.Item Open Access Trans Ligand Determines the Stability of Paramagnetic Manganese(II) Hydrides of the Type trans-[MnH(L)(dmpe)2]+ Where L is PMe3, C2H4, or CO(ACS Publications, 2023-05) Rennie, Benjamin E; Price, Jeffrey S; Emslie, David J H; Morris, Robert HParamagnetic metal hydride (PMH) complexes play important roles in catalytic applications and bioinorganic chemistry. 3d PMH chemistry has largely focused on Ti, Mn, Fe, and Co. Various MnII PMHs have been proposed as intermediates in catalysis, but isolated MnII PMHs are limited to dimeric high-spin MnII structures with bridging hydrides. In this paper, a series of the first low-spin monomeric MnII PMH complexes are generated by chemical oxidation of their MnI analogues. This series is of the type trans-[MnH(L)(dmpe)2]+/0 where the trans ligand L is PMe3, C2H4, or CO [dmpe is 1,2-bis(dimethylphosphino)ethane], and the thermal stability of the MnII hydride complexes was found to be strongly dependent on the identity of the trans ligand. When L is PMe3, the complex is the first example of an isolated monomeric MnII hydride complex. In contrast, when L is C2H4 or CO, the complexes are only stable at low temperatures; upon warming to room temperature, the former decomposed to afford [Mn(dmpe)3]+, accompanied by ethane and ethylene, whereas the latter eliminated H2, generating [Mn(MeCN)(CO)(dmpe)2]+ or a mixture of products including [Mn(κ1-PF6)(CO)(dmpe)2], depending on the reaction conditions. All PMHs were characterized by low-temperature electron paramagnetic resonance (EPR) spectroscopy, and stable [MnH(PMe3)(dmpe)2]+ was further characterized by UV-vis and IR spectroscopy, Superconducting Quantum Interference Device magnetometry, and single-crystal X-ray diffraction. Noteworthy spectral properties are the significant EPR superhyperfine coupling to the hydride (∼85 MHz) and an increase (+33 cm-1) in the Mn-H IR stretch upon oxidation. Density functional theory calculations were also employed to gain insights into the acidity and bond strengths of the complexes. MnII-H bond dissociation free energies are estimated to decrease in the series of complexes from 60 (L = PMe3) to 47 kcal/mol (L = CO).Item Open Access 35+1 challenges in materials science being tackled by PIs under 35(ish) in 2023(Elsevier, 2023-08) Allen, Monica; Bediako, Kwabena; Bowman, William J.; Calabrese, Michelle; Caretta, Lucas; Cersonsky, Rose K.; Chen, Wen; Correa, Santiago; Davidson, Rachel; Dresselhaus-Marais, Leora; Eisler, Carissa N.; Furst, Ariel; Ge, Ting; Hook, Andrew; Hsu, Yi-Ting; Jia, Chunjing; Lu, Jianfeng; Lunghi, Alessandro; Messina, Marco S.; Moreno-Hernandez, Ivan A.; Nichols, Eva; Rao, Reshma; Seifrid, Martin; Shulenberger, Katherine Emily; Simonov, Alexandr N.; Su, Xiao; Swearer, Dayne F.; Tang, Evelyn; Taylor, Mercedes K.; Tran, Helen; Trindade, Gustavo F.; Truby, Ryan; Utzat, Hendrik; Yang, Ying; Yee, Daryl W.; Zhao, Shenlong; Cranford, SteveHere we highlight 35 (+1) global researchers approximately under the age of 35. The annual cohort was self-generated by initial seed invitations sent by the editorial team, with each contributor inviting the next in a self-selecting unrestricted (nominally supervised) manner. The final collection is an inspiring look at the challenges the current generation of materials researchers are tackling, demonstrating the interdisciplinarity of materials science.Item Open Access Biobased, Degradable, and Conjugated Poly(Azomethine)s(ACS Publications, 2023-02) Uva, Azalea; Lin, Angela; Tran, HelenCarotenoids are a class of biobased conjugated molecules that bear a resemblance to the substructure of polyacetylene, a well-known conductive but insoluble polymer. Solubility is an important physical attribute for processing materials using different techniques. To impart solubility in polymers, alkyl side chains are often included in the molecular design. While these design strategies are well explored in conjugated systems, they have not been implemented with carotenoids as a building block in polymers. Here, we show a series of carotenoid-based polymers with varying side chain lengths to tune solubility. Using carotenoid and p-phenylenediamine-based monomers, degradable and biobased poly(azomethine)s were synthesized via imine polycondensation. Maximum solubilities corresponding to the varying alkyl chain lengths were quantitatively determined by ultraviolet-visible (UV-vis) absorption spectroscopy. Since carotenoids are biobased with known degradation products, the effect of acidic and artificial sunlight-promoted degradation was systematically investigated using UV-vis spectroscopy, 1H nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, gel permeation chromatography (GPC), and high-resolution mass spectroscopy (HRMS). Our polymer system was found to have two modes of on-demand degradation, with acid hydrolysis accelerating the rate of polymer degradation and artificial sunlight generating additional degradation products. This work highlights carotenoid monomers as viable candidates in the design of biobased, degradable, and conjugated polymers.Item Open Access Conductive and elastic bottlebrush elastomers for ultrasoft electronics(Springer Nature, 2023-02) Xu, Pengfei; Wang, Shaojia; Lin, Angela; Min, Hyun-Kee; Zhou, Zhanfeng; Dou, Wenkun; Sun, Yu; Huang, Xi; Tran, Helen; Liu, XinyuUnderstanding biological systems and mimicking their functions require electronic tools that can interact with biological tissues with matched softness. These tools involve biointerfacing materials that should concurrently match the softness of biological tissue and exhibit suitable electrical conductivities for recording and reading bioelectronic signals. However, commonly employed intrinsically soft and stretchable materials usually contain solvents that limit stability for long-term use or possess low electronic conductivity. To date, an ultrasoft (i.e., Young's modulus <30 kPa), conductive, and solvent-free elastomer does not exist. Additionally, integrating such ultrasoft and conductive materials into electronic devices is poorly explored. This article reports a solvent-free, ultrasoft and conductive PDMS bottlebrush elastomer (BBE) composite with single-wall carbon nanotubes (SWCNTs) as conductive fillers. The conductive SWCNT/BBE with a filler concentration of 0.4 - 0.6 wt% reveals an ultralow Young's modulus (<11 kPa) and satisfactory conductivity (>2 S/m) as well as adhesion property. Furthermore, we fabricate ultrasoft electronics based on laser cutting and 3D printing of conductive and non-conductive BBEs and demonstrate their potential applications in wearable sensing, soft robotics, and electrophysiological recording.Item Open Access Thienoisoindigo-based recyclable conjugated polymers for organic electronics(Royal Society of Chemistry, 2023-12) Nozaki, Naoya; Uva, Azalea; Matsumoto, Hidetoshi; Tran, Helen; Ashizawa, MinoruImine-based semiconducting polymers based on thiophene-flanked diketopyrrolopyrrole (TDPP) are widely used to realize naturally disposable electronic devices. However, TDPP easily decomposes under mildly acidic conditions, limiting its potential for use in recyclable systems. Herein, we designed and synthesized two chemically recyclable thienoisoindigo (TII)-based polymers bearing an imine bond. These polymers were prepared from polycondensation reactions of the dialdehyde-functionalized monomer TII-(CHO)2 with p-phenylenediamine (PD) to produce p(TII-PD) and with 2,6-naphthalenediamine (2,6ND) to produce p(TII-2,6ND), respectively. Using ultraviolet-visible-near infrared spectroscopy, nuclear magnetic resonance, and mass spectroscopy, we examined the recyclability of both polymers. Under mildly acidic conditions, the imine-based polymers fully degrade into the original TII-(CHO)2 in as little as one day. Moreover, the recovered TII-(CHO)2 monomer is chemically stable for up to 6 months under acidic conditions, allowing us to isolate the monomer in high yield (>90%). Using the recovered TII-(CHO)2 monomer, we prepared recycled polymers, re-p(TII-PD) and re-p(TII-2,6ND). The recycled polymers displayed nearly the same electrical properties as the pristine polymers, with field-effect transistor mobilities in the order of 10−2–10−3 cm2 V−1 s−1. These results demonstrate the versatility of the TII-based monomer unit for developing fully recyclable semiconducting polymers.Item Open Access A Ruthenium Protic N-Heterocyclic Carbene Complex as a Precatalyst for the Efficient Transfer Hydrogenation of Aryl Ketone(American Chemical Society, 2022-07-22) Tsui, Brian T. H.; Sung, Molly M. H.; Kinas, Jenny; Hahn, F. Ekkehardt; Morris, Robert H.The neutral azole precursor to a protic NHC (pNHC) ligand, 6-((4,5-diphenyl-1H-imidazol-1-yl)methyl)-2,2’-bipyridine (3), was prepared from 6-(bromomethyl)-2,2’-bipyridine (2) and 4,5-diphenylimidazole. Complex [RuCl(pNHC-bpy)(PPh3)2](PF6) (4) bearing a protic, bipyridine-tethered NHC ligand was prepared by refluxing 3 with RuCl2(PPh3)3 and KPF6 in methanol and was characterized by NMR spectroscopy, mass spectrometry, elemental analysis and a single crystal X-ray diffraction study. The hydrido complex [RuH(pNHC-bpy)(PPh3)2](PF6) (5) was prepared by reaction of 4 with NaBH4 in ethanol and characterized by NMR and FTIR spectroscopy. Complex 5 was used as catalyst (0.1 mol% loading) in the transfer hydrogenation (TH) of a range of alkyl/aryl ketones in basic iso-propanol at 60 °C. Bulky alkyl groups or ortho-substituted aryl groups at the ketones slowed down or inhibited the catalytic transformation. The addition of an excess of triphenylphosphine also slowed the catalysis, providing an indication for a mechanism involving phosphine dissociation, while the addition of an excess of elemental mercury had only a small effect on the conversion. The importance of potassium cations in the mechanism is consistent with the observation of reduced catalytic conversion when [2,2,2]-cryptand was present or when 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was used as the base. A plausible homogenous catalysis mechanism involving the inner sphere addition of hydride to the substrate in the transition state TS1 is supported by DFT calculations where the potassium ion has replaced the hydrogen atom of the N–H group in the protic NHC.Item Open Access Designing materials acceleration platforms for heterogeneous CO2 photo(thermal)catalysis(Elsevier, 2023-05) Wang, Andrew; Bozal-Ginesta, Carlota; Hari Kumar, Sai Govind; Aspuru-Guzik, Alán; Ozin, Geoffrey A.Materials acceleration platforms (MAPs) combine automation and artificial intelligence to accelerate the discovery of molecules and materials. They have potential to play a role in addressing complex societal problems such as climate change. Solar chemicals and fuels generation via heterogeneous CO2 photo(thermal)catalysis is a relatively unexplored process that holds potential for contributing toward an environmentally and economically sustainable future and is therefore a very promising application for MAP science and engineering. Here, we present a brief overview of how design and innovation in heterogeneous CO2 photo(thermal)catalysis, from materials discovery to engineering and scaleup, could benefit from MAPs. We discuss relevant design and performance descriptors and the level of automation of state-of-the-art experimental techniques, and we review examples of artificial intelligence in data analysis. Based on these precedents, we finally propose a MAP outline for autonomous and accelerated discoveries in the emerging field of solar chemicals and fuels sourced from CO2 photo(thermal)catalysis.Item Open Access Electronic Insights into Aminoquinoline-Based PNHN Ligands: Protonation State Dictates Geometry While Coordination Environment Dictates N-H Acidity and Bond Strength(Royal Society of Chemistry, 2022-06-14) Gradiski, Matthew V.; Rennie, Benjamin E.; Lough, Alan J.; Morris, Robert H.A variety of transition metal complexes bearing aminoquinoline PNHH’-R ligands R = Ph (L1H), Cy (L2H) and their amido analogues are reported for rhodium(I) ([Rh(L1H)(PPh3)]+ 1 and Rh(L1)(PPh3) 2), cobalt(II) (Co(L1)(Cl) 4), and iron(II) ([Fe(L1H)2]2+ 5, Fe(L1)2 6, and [Fe(C5Me5)(L1H)]PF6 7). The acid-base and redox properties of the amido complexes 2, 6, and their protio parent complexes 1, and 5 permit the determination of the pKa and bond dissociation free energy (BDFE) of their N–H bonds while the ligand scaffold is coordinated to metal centres of square planar and octahedral geometry, respectively. From relative concentrations obtained by the use of 31P{1H} NMR spectroscopy, a pKaTHF value of 14 is calculated for rhodium complex 1, 6.4 for iron complex 5, and 24 for iron complex 7. These data, when combined with elecrochemical potentials obtained via cyclic voltammetry, allow the calculations of BDFE values for the N–H bond of 69 kcal/mol for 1, and of 55 kcal/mol for 5.