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2024

2024

Annulated 1,4-Disilabenzene-1,4-diide and Dihydrogen Splitting

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F. Ebeler, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, D. W. Szczepanik, R.  S. Ghadwal*

The isolation of silicon analogues of phenyl anions such as (C6H5)– and (C6H4)2– is challenging owing to their extremely high reactivity associated with their silylene character and weak C–Si π-interaction. Herein, we report the first annulated 1,4-disilabenzene-1,4-diide compound [(ADC)Si]2 (5) based on anionic dicarbene (ADC) scaffolds (ADC = PhC{N(Dipp)C}2; Dipp = 2,6-iPr2C6H3) as a green-yellow crystalline solid. Compound 5 is prepared by KC8 reduction of the Si(IV) chloride [(ADC)SiCl3]2 (3) or the cyclic bis-chlorosilylene [(ADC)SiCl]2 (4), which are also prepared for the first time. 5 is a neutral molecule and each of the two-coordinated Si(I) atoms has a lone pair and an unpaired electron. Experimental and theoretical data indicate delocalization of the silicon unpaired electrons, resulting in a 6π-electron C4Si2 ring in 5. The diradical character (y) for 5 amounts to 15%. At room temperature, 5 readily reacts with dihydrogen (H2) to form elusive bis-hydridosilylenes [(ADC)SiH]2 (Z)-6 and (E)-6. The [4+2]-cycloaddition of 5 and PhC≡CPh in yielding the barrelene-type bis-silylene [(ADC)SiCPh]2 (7) emphasizes the diradical reactivity of 5. With elemental sulfur, 5 results in the S2- and S3-bridged silathione derivatives [(ADC)Si(S)]2(μ-S2) (8a) and [(ADC)Si(S)]2(μ-S3) (8b). Moreover, treatment of 5 with Fe2CO9 affords the Fe(0) complex [(ADC)2Si2(µ2-CO)Fe2(CO)8] (9), in which each silicon atom serves as a two-electron σ-donor ligand and shares one electron with the bridging CO unit to form two Si–C bonds. The molecular structures of all compounds have been established by X-ray diffraction and representative compounds have been analyzed by quantum chemical calculations.

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N-Heterocyclic Carbene Analogues of Wittig Hydrocarbon

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H. Steffenfauseweh, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, B. de Bruin, R.  S. Ghadwal*

N-Heterocyclic carbene (NHC) analogues of Wittig hydrocarbon, i.e. [(NHC)2(Stil)] have been reported as crystalline solids. One-electron oxidations of[(NHC)2(Stil)] afford radical cations [(NHC)2(Stil)]●+, which can be further oxidized into the dications [(NHC)2(Stil)]2+. The CVs of [(NHC)2(Stil)]2+ are consistent with the synthetic viability of [(NHC)2(Stil)] and the radical cations [(NHC)2(Stil)]●+.

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Non-Kekulé meta-Quinodimethane Singlet Diradicals Based on Classical N-Heterocyclic Carbenes

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H. Steffenfauseweh, D. Rottschäfer, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, B. de Bruin, R.  S. Ghadwal*

Chem. Eur. J. 2024, 30, e202403029.

Diradicals based on a meta-quinodimethane (m-QDM) scaffold generally have a triplet ground state and are rather scarce. Herein, m-QDM-based non-Kekulé diradicals [3,3'-(NHC)2BP] (3-NHC) (NHC = SIPr = C{N(Dipp)CH2}2; IPr = C{N(Dipp)CH}2, Me-IPr = C{N(Dipp)CMe}2; Dipp = 2,6-iPr2C6H3; BP = 1,1'-C6H4C6H4) featuring N-heterocyclic carbene (NHC) pendants are reported as crystalline solids. The EPR spectra of 3-NHC show both allowed (Dms = 1) and forbidden (Dms = 2; 'half-field') transitions characteristic for triplet diradicals. Variable temperature EPR studies however reveal a singlet ground state for 3-SIPr. Consistent with the EPR spectra, calculations predict a remarkably small (open-shell) singlet-triplet energy gap (≤ 0.26 kcal/mol) for the 3-NHC compounds. The calculated singlet diradical character for the ground states of the 3-NHC compounds amounts to ~99%.

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Access to a peri-Annulated Aluminium Compound via C–H Bond Activation by a Cyclic Bis-Aluminylene

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A. Merschel, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, R. S. Ghadwal*

KC8 reduction of 2-I with a catalytic amount the NHC (IMe4) has been shown to afford the peri-annulated compound 4 as a colorless crystalline solid. The formation of 4 suggests intramolecular insertion of the putative bis-aluminylene 3 into the C–H bonds of HCMe2 groups. Calculations predict singlet ground state for 3 while the conversion of 3 into 4 is thermodynamically favored by 61 kcal/mol.

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Divergent Reactivity of a Cyclic Bis-Hydridostannylene: A Masked Sn(I) Diradicaloid

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F. Ebeler, B. Neumann, H.-G. Stammler, R. S. Ghadwal*

Reactivity (including catalytic activity) of the hydridostannylene (1-H2) with a variety of unsaturated (alkyne, carbonyl, azo) substrates has been reported. 1-H2 (e.g.) undergoes dehydrogenative [2+2]-cycloaddition with Ph2CO to form the 1,4-distannabarrelene 1-OCPh2, which readily liberates Ph2CO on exposure to H2 and regenerates 1-H2. Thus, 1-H2 behaves as a masked Sn-diradicaloid 1 that also affords 1-OCPh2on treatment with Ph2CO.

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Isolation of an Anionic Dicarbene Embedded Sn2P2 Cluster and Reversible CO2 Uptake

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F. Ebeler, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, R. S. Ghadwal*

Decarbonylation of a cyclic bis-phosphaethynolatostannylene [(ADC)Sn(PCO)]2 based on an anionic dicarbene framework (ADC = PhC{N(Dipp)C}2; Dipp = 2,6-iPr2C6H3) under UV light results in the formation of a Sn2P2 cluster compound [(ADC)SnP]2 as a green crystalline solid. The electronic structure of [(ADC)SnP]2 has been analyzed by quantum-chemical calculations. At room temperature, [(ADC)SnP]2 reversibly binds with CO2 and forms [(ADC)2{SnOC(O)P}SnP]. [(ADC)SnP]2 enables catalytic hydroboration of CO2 and reacts with elemental selenium and Fe2(CO)9 to afford [(ADC)2{Sn(Se)P}SnSe] and [(ADC)Sn{Fe(CO)4}P]2, respectively. All compounds have been characterized by multinuclear NMR spectroscopy and their solid-state molecular structures have been determined by single-crystal X-ray diffraction.

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Boosting the π-Acceptor Property of Mesoionic Carbenes by Carbonylation with Carbon Monoxide

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A. Merschel, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, R. S. Ghadwal*

Direct room temperature dimerization of carbon monoxide by anionic dicarbenes Li(ADC) has been reported to quantitatively yield (E)-ethene-1,2-bis(olate) bridged mesoionic carbene (iMIC) lithium compounds COen-[(iMIC)Li]2. They undergo 2e-oxidation to afford 1,2-dione bridged bis iMIC, COon-(iMIC)2 containing compounds while redox neutral salt metatheses yield COen-[(iMIC)E]2 compounds E = main group element).

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Ring-Opening of 1,3-Imidazole Based Mesoionic Carbenes (iMICs) and Ring-Closing Clicks: Facile Access to iMIC-Compounds

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A. Merschel, Y. V. Vishnevskiy, B. Neumann, H.-G. Stammler, R. S. Ghadwal*

Herein, ring-opening of mesoionic carbenes (iMICs) (iMIC = [ArC{N(Dipp)}2C(SiMe3)C:) (Dipp = 2,6-iPr2C6H3, Ar = Ph, 4-Me2NC6H4 or 4-PhC6H4) based on an 1,3-imidazole scaffold to yield N-ethynylformimidamide (eFIM) derivatives as crystalline solids (eFIM = {(Dipp)N=C(Ar)N(Dipp)}C≡CSiMe3) is reported. eFIMs are thermally stable under inert gas atmosphere and show moderate air stability (t1/2 = 3h for Ar = Ph). eFIMs are excellent surrogates of iMICs, which generally have a limited shelf-life, and readily undergo ring-closing click reactions with a variety of main-group as well as transition metal Lewis acids to form hitherto challenging iMIC-compounds in good to excellent yields. In addition to the relevance of eFIMs in the synthesis of iMIC-compounds, quantification of the stereoelectronic properties of a representative iMIC (Ar = Ph) by experimental and theoretical methods suggests remarkably σ-donor property and steric profile of these new ligand sets.

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