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N-Heterocyclic Carbene-Derived 1,3,5-Trimethylenebenzene: On-Surface Synthesis and Electronic Structure. J Am Chem Soc 2024; 146:13025-13033. [PMID: 38693826 DOI: 10.1021/jacs.3c14298] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/03/2024]
Abstract
1,3,5-Trimethylenebenzene (1,3,5-TMB), a 3-fold-symmetric triradical with a high-spin ground state, is an attractive platform for investigating the unique spin properties of π-conjugated triangular triradicals. Here, we report the on-surface synthesis of N-heterocyclic carbene (NHC)-derived 1,3,5-TMB (N-TMB) via surface-assisted C-C and C-N coupling reactions on Au(111). The chemical and electronic structures of N-TMB on the Au(111) surface are revealed with atomic precision using scanning tunneling microscopy and noncontact atomic force microscopy, combined with density functional theory (DFT) calculations. It is demonstrated that there is substantial charge transfer between N-TMB and the substrate, resulting in a positively charged N-TMB on Au(111). DFT calculations at the UB3LYP/def2-TZVP level of theory and multireference method, e.g., CASSCF/NEVPT2, indicate that N-TMB possesses a doublet ground state with reduced Cs symmetry in the gas phase, contrasting the quartet ground state of 1,3,5-TMB with D3h symmetry, and exhibits a doublet-quartet energy gap of -0.80 eV. The incorporation of NHC structures and the extended π-conjugation promote the spin-orbital overlaps in N-TMB, leading to Jahn-Teller distortion and the formation of a robust doublet state. Our results not only demonstrate the fabrication of polyradicals based on NHC but also shed light on the effect of NHC and π-conjugation on the electronic structure and spin coupling, which opens up new possibilities for precisely regulating the spin-spin exchange coupling of organic polyradicals.
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Enhancing Diradical Character of Chichibabin's Hydrocarbon through Fluoride Substitution. Chempluschem 2023; 88:e202300273. [PMID: 37409641 DOI: 10.1002/cplu.202300273] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2023] [Revised: 07/04/2023] [Accepted: 07/06/2023] [Indexed: 07/07/2023]
Abstract
In this work, 5-SIDipp [SIDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] (1) derived Chichibabin's hydrocarbon with an octafluorobiphenylene spacer (3) has been reported. The addition of two equivalents of 5-SIDipp with decafluorobiphenyl in presence of BF3 gives the double C-F bond activated imidazolium salt with two tetrafluoroborate anions, 2. Further reduction of 2 gives the fluorine substituted 5-SIDipp based Chichibabin's hydrocarbon, 3. Quantum chemical calculations suggested a singlet state of 3 with a singlet-triplet energy gap (ΔES-T ) of 3.7 kcal mol-1 , which is substantially lower with respect to the hydrogen substituted NHC-based Chichibabin's hydrocarbons (10.7 kcal mol-1 , B3LYP). As a result, the diradical character (y) of 3 (y=0.62) is also noticeably higher than the hydrogen substituted CHs (y=0.41-0.43). The ▵ES-T was found to be higher in CASSCF (22.24 kcal mol-1 ) and CASPT2 (11.17 kcal mol-1 ) for 3 and the diradical character (d) is 44.6 %.
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Revisiting the Reaction of IPr with Tritylium: An Alternative Mechanistic Pathway. Chemistry 2023; 29:e202203364. [PMID: 36445754 DOI: 10.1002/chem.202203364] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2022] [Revised: 11/29/2022] [Accepted: 11/29/2022] [Indexed: 11/30/2022]
Abstract
Despite a recent proposal on the mechanism of a single-electron transfer (SET) process between tritylium and 2,6-bis(diisopropylphenyl)imidazol-2-ylidene (IPr) based on evidence of transient IPr radical cation intermediate ([IPr]⋅+ ) formation, such oxidation is still contentious because of the high oxidation potential of N-heterocyclic carbenes. Our experimental analysis indicates that the appearance of deep purple color, previously considered to be from transient [IPr]⋅+ , originates from a zwitterionic intermediate (3 a), not a radical cation. Here, we propose an alternative mechanism for the reaction involving tritylium and IPr. This mechanism is noteworthy for explaining how [NHC-H]+ can be generated without the formation of transient [NHC]⋅+ , which has been frequently proposed as an intermediate for the reaction between NHC and oxidants. These results also show that a transient strong single-electron donor (3 a) could be generated by the alternative mechanism for oxidants using NHCs, which is a more feasible explanation for the reactivity of NHCs with oxidants.
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Luminescent Crystalline Carbon- and Nitrogen-Centered Organic Radicals Based on N-Heterocyclic Carbene-Triphenylamine Hybrids. Chemistry 2023; 29:e202203242. [PMID: 36331436 DOI: 10.1002/chem.202203242] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/17/2022] [Revised: 11/02/2022] [Accepted: 11/04/2022] [Indexed: 11/06/2022]
Abstract
Developing luminescent radicals with tunable emission is a challenging task due to the limitation of alternative skeletons. Herein, a series of carbene-triphenylamine hybrids were prepared by the direct C2-arylation of N-heterocyclic carbenes with 4-bromo-N,N-bis(4-methoxyphenyl)aniline. These hybrids showed multiple redox-active properties and could be converted to carbon-centered luminescent radicals with blue-to-cyan emissions (λmax : 436-486 nm) or nitrogen-centered luminescent radicals with orange emissions (λmax : 590-623 nm) through chemical reduction or oxidation, respectively. The radical species were characterized by electron paramagnetic resonance spectroscopy, ultraviolet-visible spectroscopy, and single-crystal X-ray diffractometry analysis. Notably, the corresponding nitrogen-centered radicals exhibited good stability in atmospheric air, and their thermal decomposition temperatures were determined to be above 200 °C. In addition, spectral and theoretical calculations indicate that all radicals exhibit anti-Kasha emissions.
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Crystalline Anions Based on Classical N-Heterocyclic Carbenes. Angew Chem Int Ed Engl 2023; 62:e202215244. [PMID: 36398890 PMCID: PMC10107637 DOI: 10.1002/anie.202215244] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2022] [Revised: 11/18/2022] [Accepted: 11/18/2022] [Indexed: 11/19/2022]
Abstract
Herein, the first stable anions K[SIPrBp ] (4 a-K) and K[IPrBp ] (4 b-K) (SIPrBp =BpC{N(Dipp)CH2 }2 , IPrBp =BpC{N(Dipp)CH}2 ; Bp=4-PhC6 H4 ; Dipp=2,6-iPr2 C6 H3 ) derived from classical N-heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a-K and 4 b-K are prepared by KC8 reduction of the neutral radicals [SIPrBp ] (3 a) and [IPrBp ] (3 b), respectively. The radicals 3 a and 3 b as well as [Me-IPrBp ] 3 c (Me-IPrBp =BpC{N(Dipp)CMe}2 ) are accessible as crystalline solids on treatment of the respective 1,3-imidazoli(ni)um bromides (SIPrBp )Br (2 a), (IPrBp )Br (2 b), and (Me-IPrBp )Br (2 c) with KC8 . The cyclic voltammograms of 2 a-2 c exhibit two one-electron reversible redox processes in -0.5 to -2.5 V region that correspond to the radicals 3 a-3 c and the anions (4 a-4 c)- . Computational calculations suggest a closed-shell singlet ground state for (4 a-4 c)- with the singlet-triplet energy gap of 17-24 kcal mol-1 .
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Gauging Radical Stabilization with Carbenes. Angew Chem Int Ed Engl 2022; 61:e202206390. [PMID: 35796423 PMCID: PMC9545232 DOI: 10.1002/anie.202206390] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2022] [Indexed: 11/29/2022]
Abstract
Carbenes, including N‐heterocyclic carbene (NHC) ligands, are used extensively to stabilize open‐shell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical‐stabilizing C‐donor ligands. With the large variety of C‐donor ligands experimentally investigated and their electronic properties established, we report herein their radical‐stabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π‐donation to‐ and π‐donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π‐donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group‐ and transition metal carbene complexes, and the quantification of redox non‐innocence.
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Gauging Radical Stabilization with Carbenes. Angew Chem Int Ed Engl 2022. [DOI: 10.1002/ange.202206390] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Nickel-Catalyzed Intramolecular 1,2-Aryl Migration of Mesoionic Carbenes (iMICs). Angew Chem Int Ed Engl 2021; 60:2969-2973. [PMID: 33155756 PMCID: PMC7898293 DOI: 10.1002/anie.202014328] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2020] [Indexed: 12/26/2022]
Abstract
Intramolecular 1,2-Dipp migration of seven mesoionic carbenes (iMICAr ) 2 a-g (iMICAr =ArC{N(Dipp)}2 CHC; Ar=aryl; Dipp=2,6-iPr2 C6 H3 ) under nickel catalysis to give 1,3-imidazoles (IMDAr ) 3 a-g (IMDAr =ArC{N(Dipp)CHC(Dipp)N}) has been reported. The formation of 3 indicates the cleavage of an N-CDipp bond and the subsequent formation of a C-CDipp bond in 2, which is unprecedented in NHC chemistry. The use of 3 in accessing super-iMICs (5) (S-iMIC=ArC{N(Dipp)N(Me)C(Dipp)}C) has been shown with selenium (6), gold (7), and palladium (8) compounds. The quantification of the stereoelectronic properties reveals the superior σ-donor strength of 5 compared to that of classical NHCs. Remarkably, the percentage buried volume of 5 (%Vbur =45) is the largest known amongst thus far reported iMICs. Catalytic studies show a remarkable activity of 5, which is consistent with their auspicious stereoelectronic features.
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The Reactivity of Isomeric Nitrenium Lewis Acids with Phosphines, Carbenes, and Phosphide. Chemistry 2021; 27:2861-2867. [PMID: 33137240 PMCID: PMC7898612 DOI: 10.1002/chem.202004798] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2020] [Indexed: 11/25/2022]
Abstract
Alkylation of spiro[fluorene‐9,3’‐indazole] at N(1) and N(2) with tBuCl affords the nitrenium cations [C6H4N2(tBu)C(C12H8)][BF4], 1 and 2, respectively. Compound 1 converts to 2 over the temperature range 303–323 K with a free energy barrier of 28±5 kcal mol−1. Reaction of 1 with PMe3 afforded the N‐bound phosphine adduct [C6H4N(tBu)N(PMe3)C(C12H8)]BF4] 3. However, phosphines attack 2 at the para‐carbon atom of the aryl group with concurrent cleavage of N(2)−C(1) bond and proton migration to C(1) affording [(R3P)C6H3NN(tBu)CH(C12H8)][BF4] (R=Me 4, nBu 5). Analogous reactions of 1 and 2 with the carbene SIMes prompt attack at the para‐carbon with concurrent loss of H. affording the radical cation salts [(SIMes)C6H3N(tBu)NC(C12H8).][BF4] 6 and [(SIMes)C6H3NN(tBu)C(C12H8).][BF4] 7, whereas reaction of 2 with BAC gives the Lewis acid‐base adduct, [C6H4N(BAC)N(tBu)C(C12H8)][BF4] 8. Finally, reactions of 1 and 2 with KPPh2 result in electron transfer affording (PPh2)2 and the persistent radicals C6H4N(tBu)NC(C12H8). and C6H4NN(tBu)C(C12H8).. The detailed reaction mechanisms are also explored by extensive DFT calculations.
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Nickel‐katalysierte intramolekulare 1,2‐Aryl‐Wanderung von mesoionischen Carbenen (iMICs). Angew Chem Int Ed Engl 2020. [DOI: 10.1002/ange.202014328] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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Air-Stable Oxyallyl Patterns and a Switchable N-Heterocyclic Carbene. Angew Chem Int Ed Engl 2020; 59:11516-11520. [PMID: 32277582 DOI: 10.1002/anie.202002669] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2020] [Revised: 04/10/2020] [Indexed: 01/22/2023]
Abstract
Oxyallyl derivatives are typically elusive compounds. Even recently reported "stabilized" 1,3-diaminooxyallyl species are still highly reactive and have short lifetimes at room temperature. Herein, we report the synthesis and preliminary study of mesoionic pyrimidine derivatives that feature 1,3-bis(dimethylamino)oxyallyl patterns with an unprecedented level of stabilization. The latter are not only insensitive towards air and moisture, but they are also compatible with the formation of an ancillary stable N-heterocyclic carbene moiety. As the oxyallyl pattern is proton-responsive, it allows the reversible switching of the electronic properties of the carbene, as a ligand.
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Saturated N-Heterocyclic Carbene Based Thiele's Hydrocarbon with a Tetrafluorophenylene Linker. Chemistry 2019; 25:16533-16537. [PMID: 31609519 DOI: 10.1002/chem.201904421] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2019] [Revised: 10/06/2019] [Indexed: 01/24/2023]
Abstract
The synthesis of a SIPr [1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] derived Kekulé diradicaloid with a tetrafluorophenylene spacer (3) has been described. Two synthetic routes have been reported to access 3. The cleavage of C-F bond of C6 F6 by SIPr in the presence of BF3 led to double C-F activated compound with two tetrafluoro borate counter anions (2), which upon reduction by lithium metal afforded 3. Alternatively, 3 can be directly accessed in one step by reacting SIPr with C6 F6 in presence of Mg metal. Compounds 2 and 3 were well characterized spectroscopically and by single-crystal X-ray diffraction studies. Experimental and computational studies support the cumulenic closed-shell singlet state of 3 with a singlet-triplet energy gap (ΔES-T ) of 23.7 kcal mol-1 .
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Tuneable Redox Chemistry and Electrochromism of Persistent Symmetric and Asymmetric Azine Radical Cations. Chemistry 2019; 25:12981-12990. [PMID: 31306523 DOI: 10.1002/chem.201902216] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2019] [Revised: 06/21/2019] [Indexed: 12/16/2022]
Abstract
Molecular organic radicals have been intensively studied in the last decades, due to their interesting optical, magnetic and redox properties. Here we report the synthesis and characterisation of persistent organic radicals from one-electron oxidation of redox-active azines (RAAs), composed of two guanidinyl or related groups. By connecting two different groups together, asymmetric compounds result. In this way a series of compounds with varying redox potential is obtained that could be oxidised reversibly to the mono- and the dicationic charge states. The accessible redox states were fully determined by chemical redox reactions. The standard Gibbs free energy change for disproportionation of the radical monocation into the dication and the neutral molecule in solution, estimated from cyclovoltammetric measurements, varies between 43 and 71 kJ mol-1 . While the neutral RAAs absorb predominately UV light, the radical monocations display strong absorptions covering almost the entire visible region and extending for some compounds into the NIR region. A detailed analysis of this highly reversible electrochromism is presented, and the fast switching characteristics are demonstrated in an electrochromic test device.
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Platforms for Stable Carbon‐Centered Radicals. Angew Chem Int Ed Engl 2019; 58:8978-8986. [DOI: 10.1002/anie.201900307] [Citation(s) in RCA: 84] [Impact Index Per Article: 16.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/09/2019] [Indexed: 11/12/2022]
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Azo‐MICs: Redox‐Active Mesoionic Carbene Ligands Derived from Azoimidazolium Dyes. Angew Chem Int Ed Engl 2019; 58:1764-1767. [DOI: 10.1002/anie.201813780] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2018] [Indexed: 11/06/2022]
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Azo‐MICs: Redox‐Active Mesoionic Carbene Ligands Derived from Azoimidazolium Dyes. Angew Chem Int Ed Engl 2019. [DOI: 10.1002/ange.201813780] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Stable Organic Radicals Derived from N‐Heterocyclic Carbenes. Chemistry 2018; 24:19110-19121. [PMID: 30058298 DOI: 10.1002/chem.201801560] [Citation(s) in RCA: 42] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2018] [Revised: 06/23/2018] [Indexed: 11/11/2022]
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Diradical Character Enhancement by Spacing: N-Heterocyclic Carbene Analogues of Müller's Hydrocarbon. Chemistry 2018; 24:16537-16542. [DOI: 10.1002/chem.201804524] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/04/2018] [Indexed: 11/11/2022]
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Silanylidene and Germanylidene Anions: Valence‐Isoelectronic Species to the Well‐Studied Phosphinidene. Angew Chem Int Ed Engl 2018; 57:11776-11780. [DOI: 10.1002/anie.201805936] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2018] [Revised: 07/03/2018] [Indexed: 11/06/2022]
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Silanylidene and Germanylidene Anions: Valence-Isoelectronic Species to the Well-Studied Phosphinidene. Angew Chem Int Ed Engl 2018. [DOI: 10.1002/ange.201805936] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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Carbene derived diradicaloids - building blocks for singlet fission? Chem Sci 2018; 9:6107-6117. [PMID: 30090299 PMCID: PMC6053972 DOI: 10.1039/c8sc01999a] [Citation(s) in RCA: 53] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2018] [Accepted: 06/25/2018] [Indexed: 12/13/2022] Open
Abstract
Organic singlet diradicaloids promise application in non-linear optics, electronic devices and singlet fission. The stabilization of carbon allotropes/cumulenes (C1, C2, C4) by carbenes has been equally an area of high activity. Combining these fields, we showed recently that carbene scaffolds allow as well for the design of diradicaloids. Herein, we report a comprehensive computational investigation (CASSCF/NEVPT2; fractional occupation DFT) on the electronic properties of carbene-bridge-carbene type diradicaloids. We delineate how to adjust the properties of these ensembles through the choice of carbene and bridge and show that already a short C2 bridge results in remarkable diradicaloid character. The choice of the carbene separately tunes the energies of the S1 and T1 excited states, whereas the bridge adjusts the overall energy level of the excited states. Accordingly, we develop guidelines on how to tailor the electronic properties of these molecules. Of particular note, fractional occupation DFT is an excellent tool to predict singlet-triplet gaps.
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