Conspectus Molecules and materials with easily tunable electronic structures and properties are at the forefront of contemporary research. π-Conjugation is fundamental in organic chemistry and plays ...a key role in the design of molecular materials. In this Account, we showcase the applicability of N-heterocyclic vinyl (NHV) substituents based on classical N-heterocyclic carbenes (NHCs) for tuning the structure, properties, and stability of main-group species (E) via π-conjugation and/or π-donation. NHVs such as (NHC)CR (R = H or aryl) are monoanionic ligands formally derived by the deprotonation of N-heterocyclic olefins (NHOs), (NHC)CHR. Further deprotonation of (NHC)CR (R = H) is viable, giving rise to N-heterocyclic vinylidene (NHVD) species such as (NHC)C. NHVs and NHVDs feature a highly polarizable exocyclic CNHCC bond because of the presence of adjacent π-donor nitrogen atoms. The nature of the NHC, in particular the π-acceptor property, has a direct consequence on the polarity of the CNHCC bond and hence on the magnitude of π-conjugation in the derived molecules. Thus, the electronic structure, especially the energy and shape of frontier molecular orbitals, HOMO and LUMO, of derived species can be fine-tuned by a judicious choice of the carbene unit. For instance, the HOMO of classical diphosphenes (RPPR) (R = alkyl or aryl) is invariably the phosphorus lone-pair orbital, while the PP π-bond is HOMO – 1 or HOMO – 2. In strong contrast, the HOMO of divinyldiphosphenes (R = NHV) is mainly the PP π-bond. This is owing to the π-conjugation, resulting in the lowering of the LUMO and raising of the HOMO energy. They have a remarkably small HOMO–LUMO energy gap (4.15–4.50 eV) and readily undergo 1e-oxidations, giving rise to stable radical cations and dications. By employing a similar approach, one can access divinyldiarsenes and the corresponding radical cations and dications as crystalline solids. The use of divinyldiphosphenes and divinyldiarsenes as promising ligands in the stabilization of metalloradicals has been shown. By a logical selection of singlet carbenes, stable 2-phosha-1,3-butadiene and 2-arsa-1,3-butadiene compounds, as well as related radical cations and dications, can be prepared as crystalline solids. The relevance of NHV ligands as potent π-donors has been demonstrated for the stabilization of elusive electrophilic phosphinidene and arsinidene complexes {(NHV)E}Fe(CO)4 (E = P or As). Moreover, stable singlet diradicaloid (NHC)CP2 and p-quinodimethane derivatives (NHC)CP22 based on an NHVD framework are accessible as stable solids. In this Account, a special emphasis is given to the contributions from this laboratory. The author hopes that this Account will serve as a useful reference guide for researchers interested in studying and applying NHV and NHVD scaffolds in modern molecular chemistry and materials sciences.
Classical N‐heterocyclic carbenes (NHCs) featuring the carbene center at the C2‐position of 1,3‐imidazole framework (i.e. C2‐carbenes) are well acknowledged as very versatile neutral ligands in ...molecular as well as in materials sciences. The efficiency and success of NHCs in diverse areas is essentially attributed to their persuasive stereoelectronics, in particular the potent σ‐donor property. The NHCs with the carbene center at the unusual C4 (or C5) position, the so‐called abnormal NHCs (aNHCs) or mesoionic carbenes (iMICs), are however superior σ‐donors than C2‐carbenes. Hence, iMICs have substantial potential in sustainable synthesis and catalysis. The main obstacle in this direction is rather demanding synthetic accessibility of iMICs. The aim of this review article is to highlight recent advances, particularly by the author's research group, in accessing stable iMICs, quantifying their properties, and exploring their applications in synthesis and catalysis. In addition, the synthetic viability and use of vicinal C4,C5‐anionic dicarbenes (ADCs), also based on an 1,3‐imidazole framework, are presented. As will be apparent on following pages, iMICs and ADCs hold potentials in pushing the limit of classical NHCs by enabling access to conceptually new main‐group heterocycles, radicals, molecular catalysts, ligands sets, and more.
Mesoionic carbenes (iMICs) and anionic dicarbenes (ADCs) are readily accessible by the deprotonation of C2‐arylated 1,3‐imidazolium salts, which are prepared by the direct C2‐arylation of classical N‐heterocyclic carbenes (NHCs). The implications of iMICs as potent σ‐donor ligands in organometallic catalysis and ADCs as unique building‐blocks in accessing conceptually new main‐group heterocycles with an annulated C4E2 ring are showcased.
Isolating stable compounds with low-valent main group elements have long been an attractive research topic, because several of these compounds can mimic transition metals in activating small ...molecules. In addition, compounds with heavier low-valent main group elements have fundamentally different electronic properties when compared with their lighter congeners. Among group 14 elements, the heavier analogues of carbenes (R2C:) such as silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:) are the most studied species with low-valent elements. The first stable carbene and silylene species were isolated as N-heterocycles. Among the dichlorides of group 14 elements, CCl2 and SiCl2 are highly reactive intermediates and play an important role in many chemical transformations. GeCl2 can be stabilized as a dioxane adduct, whereas SnCl2 and PbCl2 are available as stable compounds. In the Siemens process, which produces electronic grade silicon by thermal decomposition of HSiCl3 at 1150 °C, chemists proposed dichlorosilylene (SiCl2) as an intermediate, which further dissociates to Si and SiCl4. Similarly, base induced disproportionation of HSiCl3 or Si2Cl6 to SiCl2 is a known reaction. Trapping these products in situ with organic substrates suggested the mechanism for this reaction. In addition, West and co-workers reported a polymeric trans-chain like perchloropolysilane (SiCl2) n . However, the isolation of a stable free monomeric dichlorosilylene remained a challenge. The first successful attempt of taming SiCl2 was the isolation of monochlorosilylene PhC(NtBu)2SiCl supported by an amidinate ligand in 2006. In 2009, we succeeded in isolating N-heterocyclic carbene (NHC) stabilized dichlorosilylene (NHC)SiCl2 with a three coordinate silicon atom. (The NHC is 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes).) Notably, this method allows for the almost quantitative synthesis of (NHC)SiCl2 without using any hazardous reducing agents. Dehydrochlorination of HSiCl3 with NHC under mild reaction conditions produces (NHC)SiCl2. We can separate the insoluble side product (NHC)HCl readily and recycle it to form NHC. The high yield and facile access to dichlorosilylene allow us to explore its chemistry to a greater extent. In this Account, we describe the results using (NHC)SiCl2 primarily from our laboratory, including findings by other researchers. We emphasize the novel silicon compounds, which supposedly existed only as short-lived species. We also discuss silaoxirane, silaimine with tricoordinate silicon atom, silaisonitrile, and silaformyl chloride. In analogy with N-heterocyclic silylenes (NHSis), oxidative addition reactions of organic substrates with (NHC)SiCl2 produce Si(IV) compounds. The presence of the chloro-substituents both on (NHC)SiCl2 and its products allows metathesis reactions to produce novel silicon compounds with new functionality. These substituents also offer the possibility to synthesize interesting compounds with low-valent silicon by further reduction. Coordination of NHC to the silicon increases the acidity of the backbone protons on the imidazole ring, and therefore (NHC)SiCl2 can functionalize NHC at the C-4 or C-5 position.
Lewis Base Stabilized Dichlorosilylene Ghadwal, Rajendra S; Roesky, Herbert W; Merkel, Sebastian ...
Angewandte Chemie (International ed.),
July 20, 2009, Letnik:
48, Številka:
31
Journal Article
Recenzirano
Stable? You can bottle it! The base‐stabilized dichlorosilylene L1SiCl2 (see picture; L1=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) is stable at room temperature. L1SiCl2 can undergo a ...reaction with diphenylacetylene to form a trisilacyclopentene derivative. These compounds have been characterized by X‐ray crystallography and computational studies.
The first divinyldiarsenes {(NHC)C(Ph)}As2 (NHC=IPr 3 a, SIPr 3 b; IPr=C{(NAr)CH}2; SIPr=C{(NAr)CH2}2; Ar=2,6‐iPr2C6H3) are reported. Compounds 3 a and 3 b were prepared by the reduction of ...corresponding chlorides {(NHC)C(Ph)}AsCl2 (NHC=IPr 2 a, SIPr 2 b) with Mg. Calculations revealed a small HOMO–LUMO energy gap of 3.86 (3 a) and 4.24 eV (3 b). Treatment of 3 a with (Me2S)AuCl led to the cleavage of the As=As bond to restore 2 a, which is expected to proceed via the diarsane {(IPr)C(Ph)}AsCl2 (4). Remarkably, 4 as well as 2 a can be selectively accessed on treatment of 3 a with an appropriate amount of C2Cl6. Moreover, 3 a readily reacts with PhEEPh (E=Se or Te) at room temperature to give {(IPr)C(Ph)}As(EPh)2 (E=Se 5 a; Te 5 b), revealing the cleavage of As=As and E−E bonds and the formation of As−E bonds. Such highly selective stepwise oxidation (3 a→4→2 a) and bond metathesis (3 a→5 a,b) reactions are unprecedented in main‐group chemistry.
Making and breaking the As=As bond: The divinyldiarsene I with a strikingly small HOMO–LUMO energy gap (3.86 eV) was prepared by the reduction of the dichloroarsane II. The As=As bond of I can be readily cleaved with C2Cl6, restoring II. This transformation is also viable with AuCl. Complex I endured oxidative bond metatheses with PhEEPh (E=Se or Te), producing derivatives III–E.
The direct double carbenylation of 1,4-diiodobenzene and 4,4'-dibromobiphenyl with a classical N-heterocyclic carbene, SIPr (
) (SIPr = :C{
(2,6-iPr
C
H
)}
CH
CH
), by means of nickel catalysis gives ...rise to 1,3-imidazolinium salts (SIPr)(C
H
)(SIPr)(I)
(
) and (SIPr)(C
H
)
(SIPr)(Br)
(
) as off-white solids. Two-electron reduction of
and
with KC
cleanly yields Kekulé diradicaloid compounds (SIPr)(C
H
)(SIPr) (
) and (SIPr)(C
H
)
(SIPr) (
), respectively, as crystalline solids. Structural parameters and DFT as well as CASSCF calculations suggest the closed-shell singlet ground state for
and
. Calculations reveal a very low singlet-triplet energy gap Δ
for
(10.7 kcal mol
), while Δ
for
(29.1 kcal mol
) is rather large.
Blocking the C2 position of an imidazole‐derived classical N‐heterocyclic carbene (NHC) with an aryl group is an essential strategy to establish a route to mesoionic carbenes (MICs), which coordinate ...to the metal via the C4 (or C5) carbon atom. An efficient catalytic route to MIC precursors by direct arylation of an NHC is reported. Treatment of 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) with an aryl iodide (RC6H4I) in the presence of 0.5 mol % of Pd2(dba)3 (dba=dibenzylideneacetone) precatalyst affords the C2‐arylated imidazolium salts {IPr(C6H4R)}I (R=H, 4‐Me, 2‐Me, 4‐OMe, 4‐COOMe) in excellent (up to 92 %) yields. Treatment of {IPr(C6H5)}I with CuI and KN(SiMe3)2 exclusively affords the MIC–copper complex (IPrPh)CuI.
A highly atom‐economic route to C2‐arylated imidazolium iodides is reported. Palladium‐catalyzed direct CC cross coupling of an NHC with an aryl iodide readily affords the C2‐arylated 1,3‐imidazolium iodides in up to 92 % yield. The coupling products are insoluble and can be easily separated. This enables easy recycling of the catalyst. A mesoionic carbene copper complex (IPrPh)CuI is prepared in 91 % yield.
One‐electron reduction of C2‐arylated 1,3‐imidazoli(ni)um salts (IPrAr)Br (Ar=Ph, 3 a; 4‐DMP, 3 b; 4‐DMP=4‐Me2NC6H4) and (SIPrAr)I (Ar=Ph, 4 a; 4‐Tol, 4 b) derived from classical NHCs ...(IPr=:C{N(2,6‐iPr2C6H3)}2CHCH, 1; SIPr=:C{N(2,6‐iPr2C6H3)}2CH2CH2, 2) gave radicals (IPrAr). (Ar=Ph, 5 a; 4‐DMP, 5 b) and (SIPrAr). (Ar=Ph, 6 a; 4‐Tol, 6 b). Each of 5 a,b and 6 a,b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid‐state characterization of NHC‐derived carbon‐centered radicals 6 a,b by single‐crystal X‐ray diffraction is reported. DFT calculations indicate that the unpaired electron is mainly located at the original carbene carbon atom and stabilized by partial delocalization over the adjacent aryl group.
Crystalline radicals (IPrAr). and (SIPrAr). derived from classical N‐heterocyclic carbenes (NHCs; IPr=:C{N(2,6‐iPr2C6H3)}2CHCH, SIPr=:C{N(2,6‐iPr2C6H3)}2CH2CH2) were synthesized by one‐electron reduction of the corresponding C2‐arylated 1,3‐imidazoli(ni)um cations (see scheme). Cyclic voltammetry, EPR and X‐ray diffraction studies, and DFT calculations emphasized the key role of the C2 substituent in the stability of the NHC‐derived radicals.
Herein, the first stable anions KSIPrBp (4 a‐K) and KIPrBp (4 b‐K) (SIPrBp=BpC{N(Dipp)CH2}2, IPrBp=BpC{N(Dipp)CH}2; Bp=4‐PhC6H4; Dipp=2,6‐iPr2C6H3) 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.
One‐electron reduction of the C2‐biphenylated 1,3‐imidazolinium cation (2 a+) affords the radical 3 a, which undergoes further 1e‐reduction to yield the anion 4 a−. In the solid‐state, 3 a is monomeric while 4 a‐K has a hexameric tubular structure. In addition to 3 a and 4 a‐K, synthesis, structures, and reactivity of stable radicals and anions based on unsaturated N‐heterocyclic carbene frameworks have been presented.
Four‐membered biradicaloid compounds containing a N2E2 (E=main group element) framework have been thoroughly investigated; however, the synthesis of stable analogues with a C2P2 skeleton remains a ...challenge. Base‐mediated double C−H functionalization of IPr=CH2 (1) (IPr=CN(2,6‐iPr2‐C6H3)CH2) with PCl3 affords {(IPr)CP}2ClCl (2) as a royal blue solid. Treatment of 2 with KC8 yields the stable phosphorus biradicaloid (IPr)CP2 (3) featuring a four‐membered C2P2 ring. Compound 3 is diamagnetic and shows sharp and temperature‐independent NMR resonances, revealing its singlet biradicaloid nature. The stability of 3 is attributed to the σ‐ and π‐electron‐donating property of the N‐heterocyclic vinylidene (IPr)C group.
e‐Greening the blue: Two‐electron reduction of (IPr)CP)2ClCl (1) (IPr=CN(2,6‐iPr2C6H3)CH2) affords the N‐heterocyclic vinylidene‐stabilized singlet biradicaloid compound (IPr)CP)2 (2). Compound 2 is diamagnetic and exhibits temperature‐independent sharp NMR resonances.
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