Henric Steffenfauseweh

Silicon(IV) halides [{(Et ₃ B)anIPr}SiHCl ₂ ] ( 4 ) and [{(Et ₃ B)anIPr}SiBr ₃ ] ( 5 ) based on an anionic N ‐heterocyclic carbene (NHC) ligand (i.e., {(Et ₃ B)anIPr} = (Et ₃ B)C=C(H){N(Dipp)} ₂ C, Dip p = 2,6‐ i Pr ₂ C ₆ H ₃ ) are reported as crystalline solids. Compounds 4 and 5 featuring a four‐coordinated Si(IV) atom are prepared by reacting [Li{(Et ₃ B)anIPr}] ( 3 ) with HSiCl ₃ and SiBr ₄ , respectively. 4 and 5 are characterized by spectroscopic and X‐ray diffraction methods. Density functional theory (DFT) calculations have been performed to elucidate the electronic structures as well as the fluoride ion affinities (FIAs) of 4 and 5 . The calculated FIA, a measure of Lewis acidity, of 4 and 5 is about 46 or 55 kJ/mol larger than that of HSiCl ₃ and SiBr ₄ , respectively.

Zusammenfassung: Hier berichten wir über den Einfluss von C2‐Terphenyl‐Substitutionsmustern (d. h. p ‐Terphenyl vs. m ‐Terphenyl) auf das Redoxverhalten und die Stabilität der entsprechenden Radikale und Anionen, die von N ‐heterocyclischen Carbenen (NHCs) abgeleitet sind. Drei bekannte NHCs, SIPr ( 1a ), IPr ( 1b ) und Me‐IPr ( 1c ) (SIPr = C{N(Dipp)CH ₂ } ₂ , IPr = C{N(Dipp)CH} ₂ , Me‐IPr = C{N(Dipp)CCH ₃ } ₂ , Dipp = 2,6‐ i Pr ₂ C ₆ H ₃ ), wurden an der C2‐Position mittels 4‐Bromo‐ p ‐Terphenyl ( p ‐TerBr) und 5'‐Bromo‐ m ‐Terphenyl ( m ‐TerBr) unter Nickelkatalyse funktionalisiert, wodurch die entsprechenden Kationen [(NHC) p ‐Ter]Br ( 2a – c ) bzw. [(NHC) m ‐Ter]Br ( 3a – c ) erhalten wurden. Cyclovoltammetrie (CV)‐Messungen von 2a – c zeigen zwei deutlich reversible Redoxereignisse, während 3a – c eine reversible und eine irreversible bzw. quasi‐reversible Welle aufweisen. Die Reduktion von 2a – c und 3a – c mit KC ₈ führt zu stabilen Radikalen [(NHC) p ‐Ter] ^● ( 4a – c ) und [(NHC) m ‐Ter] ^● ( 5a – c ), die als kristalline Feststoffe isoliert werden konnten. Eine weitere Reduktion von 4a – c ergibt die diamagnetischen Anionen [(NHC) p ‐Ter]K ( 6a – c ‐ K ), was mit den elektrochemischen Daten übereinstimmt. Im Gegensatz dazu zeigen 5b und 5c unter ähnlichen Bedingungen keine Reaktivität gegenüber KC ₈ , während 5a (abgeleitet vom stärker elektrophilen NHC 1a ) zum entsprechenden Anion [(SIPr) m ‐Ter]K ( 7a ‐ K ) reduziert werden kann. Ausgewählte Verbindungen wurden mittels spektroskopischer Methoden und Einkristall‐Röntgenstrukturanalyse charakterisiert; computergestützte Studien untermauern die experimentellen Befunde. Die Ergebnisse verdeutlichen, wie sowohl das NHC als auch das C2‐Terphenyl‐Substituentenmuster die Eigenschaften und Stabilität der resultierenden Spezies beeinflussen.

Herein, we report the influence of C2‐terphenyl substitution patterns (i.e., p ‐terphenyl versus m ‐terphenyl) on the redox behavior and stability of the corresponding radicals and anions derived from N ‐heterocyclic carbenes (NHCs). Three well‐known NHCs; SIPr ( 1a ), IPr ( 1b ), and Me‐IPr ( 1c ) (SIPr = C{N(Dipp)CH ₂ } ₂ , IPr = C{N(Dipp)CH} ₂ ; Me‐IPr = C{N(Dipp)CCH ₃ } ₂ ; Dipp = 2,6‐ i Pr ₂ C ₆ H ₃ ); were functionalized at the C2 position using 4‐bromo‐ p ‐terphenyl ( p ‐TerBr) and 5′‐bromo‐ m ‐terphenyl ( m ‐TerBr) under nickel catalysis, yielding the corresponding cations [(NHC) p ‐Ter]Br ( 2a – c ) and [(NHC) m ‐Ter]Br ( 3a – c ), respectively. Cyclic voltammetry (CV) measurements of 2a – c reveal two distinct reversible redox events, while 3a – c exhibit one reversible and one irreversible or quasi‐reversible wave. Reduction of 2a – c and 3a – c with KC ₈ readily affords stable radicals [(NHC) p ‐Ter] ^● ( 4a – c ) and [(NHC‐ m ‐Ter] ^● ( 5a – c ), isolated as crystalline solids. Further reduction of 4a – c produces diamagnetic anions [(NHC) p ‐Ter]K ( 6a – c‐K ), consistent with the electrochemical data. In contrast, 5b and 5c are unreactive toward KC ₈ under similar conditions, while 5a (derived from the more electrophilic NHC 1a ) can be reduced to the corresponding anion [(SIPr) m ‐Ter]K ( 7a‐K ). Selected compounds have been characterized by spectroscopic techniques and single‐crystal X‐ray diffraction, with computational studies supporting the experimental findings. The results highlight how the NHC and the C2‐terphenyl substituent influence the properties and stability of the resulting species.

Die zweifache Deprotonierung der C2‐arylierten 1,3‐Imidazoliumsalze ((IPr‐Ar)X ( 1 ‐Ar)X (IPr‐Ar=ArC{N(Dipp)CH} ₂ ; Ar=Ph, 4‐Me ₂ NC ₆ H ₄ (DMP) oder 4‐PhC ₆ H ₄ (Bp); Dipp=2,6‐ i Pr ₂ C ₆ H ₃ ) mit n BuLi liefert die sogenannten anionischen Dicarbene Li(ADC) ( 2 ‐Ar) (ADC=ArC{N(Dipp)C} ₂ ). 2 ‐Ar können zur Herstellung einer Vielzahl von Hauptgruppenheterocyclen verwendet werden, ihre Strukturen im Festkörper sind jedoch bisher unbekannt. Die hier berichteten Einkristallröntgenbeugungsstudien zeigen eine acetylidartige [ArC{N(Dipp)}(Dipp)NC≡CLi)] _n ( 3 ‐Ar) dimere ( n =2) oder trimere ( n =3) Molekularstruktur für 2 ‐Ar. Die Umsetzung von 3 ‐Ph mit Et ₃ B führt zu dem monoanionischen Carben Li[(ADC)(BEt ₃ )] ( 4 ‐Ph) mit einem schwach koordinierenden Anion, welches in dieselbe Moleküleinheit eingebettet ist. 3 ‐Ar reagieren sofort mit CO ₂ und N ₂ O unter Bildung der Ringschlussprodukte Li[(ADC)(CO ₂ ) ₂ ] ( 5 ‐Ar) bzw. Li[(ADC)N ₂ O] ( 6 ‐Ar).

The two‐fold deprotonation of the C2‐arylated 1,3‐imidazolium salts (IPr‐Ar)X ( 1 ‐Ar)X (IPr‐Ar = ArC{N(Dipp)CH} ₂ ; Ar = Ph, 4‐Me ₂ NC ₆ H ₄ (DMP) or 4‐PhC ₆ H ₄ (Bp); Dipp = 2,6‐ i Pr ₂ C ₆ H ₃ ) with n BuLi affords the so‐called anionic dicarbenes Li(ADC) ( 2 ‐Ar) (ADC = ArC{N(Dipp)C} ₂ ). 2 ‐Ar can be used to prepare a variety of main group heterocycles, however their structures in the solid‐state remained hitherto unknown. Herein reported single‐crystal X‐ray diffraction studies reveal an acetylide type [ArC{N(Dipp)}(Dipp)NC≡CLi)] _n ( 3 ‐Ar) dimeric ( n = 2) or trimeric ( n = 3) molecular structure for 2 ‐Ar. Treatment of 3 ‐Ph with Et ₃ B cleanly yields the monoanionic carbene Li[(ADC)(BEt ₃ )] ( 4 ‐Ph) featuring a weakly coordinating anion embedded in the same molecular entity. 3 ‐Ar readily undergo reactions with CO ₂ and N ₂ O to form the ring‐closing products Li[(ADC)(CO ₂ ) ₂ ] ( 5 ‐Ar) and Li[(ADC)N ₂ O] ( 6 ‐Ar), respectively.

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 (Δm_s = 1) and forbidden (Δm_s = 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 singlet‐triplet energy gap (ΔE_ST ≤ 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 %.

N‐Heterocyclic carbene (NHC) analogues of Wittig hydrocarbon, [(NHC)(Stil)(NHC)] (3a‐c) (NHC = SIPr (1a) = C[N(Dipp)CH₂]₂, Dipp = 2,6‐iPr₂C₆H₃; IPr (1b) = C[N(Dipp)CH]₂; Me‐IPr (1c) = C[N(Dipp)CMe]₂ and Stil = C₆H₄CHCHC₆H₄) have been reported as crystalline solids. 3a‐c are prepared by two‐electron reductions of the corresponding bis‐1,3‐imidazoli(ni)um bromides [(NHC)(Stil)NHC)](Br)₂ (2a‐c) with KC₈ in >94 % yields. 2a‐c are accessible by the nickel catalyzed direct C−C coupling of NHCs (1a‐c) with (E)‐4,4'‐dibromostilbene. One‐electron oxidation of 3a,b yields the corresponding radical cations [(NHC)(Stil)NHC)]B(C₆F₅)₄ 4a,b. All compounds have been characterized by UV‐Vis/NMR/EPR spectroscopy as well as 2a, 3a, and 3b by single crystal X‐ray diffraction. The electronic structures of representative systems have been analyzed by quantum chemical calculations.

Die erste 1,4‐Distabenzol‐1,4‐diid‐Verbindung [(ADC)Sb]₂ (5) auf der Basis eines anionischen Dicarbens (ADC) (ADC=PhC{N(Dipp)C}₂, Dipp=2,6‐iPr₂C₆H₃) wird als bordeauxroter Feststoff berichtet. Verbindung 5, die einen zentralen sechsgliedrigen C₄Sb₂‐Ring mit formalen Sb^I‐Atomen aufweist, kann als ein Basen‐stabilisiertes cyclisches Bis‐Stibiniden betrachtet werden, in dem jedes der Sb‐Atome zwei freie Elektronenpaare trägt. 5 geht eine 2e‐Oxidation mit Ph₃C[B(C₆F₅)₄] ein und bildet [(ADC)Sb]₂[B(C₆F₅)₄]₂ (6) als ziegelroten Feststoff. Jedes der Sb‐Atome von 6 hat ein ungepaartes Elektron und ein freies Elektronenpaar. Die offenschalige Singulett‐Diradikal‐Lösung (broken‐symmetry) für (6)²⁺ ist um 2.13 kcal mol⁻¹ stabiler als die geschlossenschalige Singulett‐Lösung. Der diradikalische Charakter von (6)²⁺ beträgt nach SS‐CASSCF (state‐specific complete active space self‐consistent field) und UHF (unrestricted Hartree‐Fock) 39 % bzw. 36 %. Die Behandlung von 6 mit (PhE)₂ ergibt [(ADC)Sb(EPh)]₂[B(C₆F₅)₄]₂ (7‐E) (E=S oder Se). Die Reaktion von 5 mit (cod)Mo(CO)₄ führt zu [(ADC)Sb]₂Mo(CO)₄ (8).

The first 1,4‐distibabenzene‐1,4‐diide compound [(ADC)Sb]₂ (5) based on an anionic dicarbene (ADC) (ADC=PhC{N(Dipp)C}₂, Dipp=2,6‐iPr₂C₆H₃) is reported as a bordeaux‐red solid. Compound 5, featuring a central six‐membered C₄Sb₂ ring with formally Sb^I atoms may be regarded as a base‐stabilized cyclic bis‐stibinidene in which each of the Sb atoms bears two lone‐pairs of electrons. 5 undergoes 2 e‐oxidation with Ph₃C[B(C₆F₅)₄] to afford [(ADC)Sb]₂[B(C₆F₅)₄]₂ (6) as a brick‐red solid. Each of the Sb atoms of 6 has an unpaired electron and a lone‐pair. The broken‐symmetry open‐shell singlet diradical solution for (6)²⁺ is calculated to be 2.13 kcal mol⁻¹ more stable than the closed‐shell singlet. The diradical character of (6)²⁺ according to SS‐CASSCF (state‐specific complete active space self‐consistent field) and UHF (unrestricted Hartree‐Fock) methods amounts to 36 % and 39 %, respectively. Treatments of 6 with (PhE)₂ yield [(ADC)Sb(EPh)]₂[B(C₆F₅)₄]₂ (7‐E) (E=S or Se). Reaction of 5 with (cod)Mo(CO)₄ affords [(ADC)Sb]₂Mo(CO)₄ (8).

A series of different aryl‐linked (frustrated) Lewis pairs with perfluoroethylsilyl and ‐stannyl functions as Lewis acids has been prepared. The 1,2‐phenylene linked tin/phosphorus Lewis pair with a di‐tert‐butylphosphanyl unit as base shows a strong intramolecular donor‐acceptor interaction, but still readily adds hydrogen chloride. At the same time, the analogous silicon/phosphorus Lewis pair could not be obtained due to the decomposition of at least one perfluoroethyl group. With a sterically more demanding 2,2,6,6‐tetramethylpiperidine base, stable Lewis pairs could be obtained for both, silicon and tin. However, they do not show activity in small molecule activation with H₂, phenyl isocyanate, CS₂ or SO₂. Furthermore, 2‐benzyl‐linked tin/nitrogen and silicon/phosphorus Lewis pairs were synthesized. Studies on their reactions towards small molecules did not show any reactivity, probably caused by an unfavorable preorganization due to rotation of the benzyl position.