A good catch: N‐Heterocyclic carbenes (NHCs) form stable adducts with nitrous oxide (N2O) under mild reaction conditions. The adducts display unique reactivity as evidenced by an alkylation reaction which leads to a rupture of the NN bond.

N-Heterocyclic carbenes (NHCs) react at ambient conditions with nitrous oxide to give covalent adducts. In the crystal, all compounds show a bent N2O group connected via the N-atom to the former carbene carbon atom. Most adducts are stable at room temperature, but heating induces decomposition into the corresponding ureas. Kinetic experiments show that the thermal stability of the NHC-N2O adducts depends on steric as well as electronic effects. The coordination of N2O to NHCs weakens the N-N bond substantially, and facile N-N bond rupture was observed in reactions with acid or acetyl chloride. On the other hand, reaction with tritylium tetrafluoroborate resulted in a covalent modification of the terminal O-atom, and cleavage of the C-N2O bond was observed in a reaction with thionyl chloride. The coordination chemistry of IMes-N2O (IMes = 1,3-dimesitylimidazol-2-ylidene) was explored in reactions with the complexes CuOTf, Fe(OTf)2, PhSnCl3, CuCl2, and Zn(C6F5)2. Structural analyses show that IMes-N2O is able to act as a N-donor, as an O-donor, or as a chelating N,O-donor. The different coordination modes go along with pronounced electronic changes as evidenced by a bond length analysis.

Metal-mediated reaction between equimolar amounts of cis-[MCl2(CNR)2] [M = Pd, R = cyclohexyl (Cy) 1, tBu 2, 2,6-Me2C6H3 (Xyl) 3, 4-MeOC6H4 4; M = Pt, R = cyclohexyl (Cy) 5, 2,6-Me2C6H3 (Xyl) 7, 4-MeOC6H4 8] and benzophenone hydrazone, H2N−NCPh2, proceeds in CHCl3 under reflux for 8 h. The subsequent workup provides the carbene species cis-[MCl2{C(N(H)NCPh2)N(H)R}(CNR)] (M = Pd, 9−12; M = Pt, 13−15) in good (80−85%) isolated yields. Complexes 9−15 are air- and moisture-stable in the 20−80 °C temperature range and were characterized by elemental analyses (C, H, N), ESI+-MS, IR, and 1D (1H, 13C{1H}) and 2D (1H,1H-COSY, 1H,13C-HMQC/1H,13C-HSQC, 1H,13C-HMBC) NMR spectroscopies. In addition, the structures of two metallacarbenes, 11 and 14, were elucidated by single-crystal X-ray diffraction. The catalytic properties of 9−15 in the Suzuki−Miyaura cross-coupling of the aryl bromides 4-R2C6H4Br (R2 = H, Me, OMe, and NO2) with phenylboronic acid (in EtOH as a solvent, K2CO3 as a base, 80 °C), yielding biaryl species, were evaluated, and it was found that the palladium-aminocarbene species 9−12 exhibit a high catalytic activity (yields up to 97%, TONs up to 1.4 × 106).

Metal-mediated coupling between one or two isonitrile ligands in cis-[MCl 2 (CtNR) 2 ] [M = Pd, Pt; R = 2,6-Me 2 C 6 H 3 (Xyl), Bu t , cyclohexyl (Cy)] and N-phenylbenzamidine, HNdC(Ph)NHPh, proceeds with different regioselectivity upon varying R group. When the aromatic isonitrile is used (R = Xyl), N-phenylbenzamidine is coordinated to a metal by the HNdC moiety, and the nucleophilic attack proceeds via the NHPh center of the benzamidine giving [MCl{C(N(Ph)C(Ph)dNH)d NXyl}(CtNXyl)]. For R = Bu t , HNdC(Ph)NHPh is coordinated to a metal by the NHPh center, and the addition occurs via the HNdC center of the nucleophile to afford [MCl{C(NC(Ph)dNPh)d NBu t }(CtNBu t )]. With R = Cy, a mixture of two products that are derived from the addition of N-phenylbenzamidine by two nucleophilic centers was detected. The substituent R dependent reactivity was explored using theoretical (DFT) methods and interpreted as a result of the steric repulsions in one of the regioisomers of the addition products, when R = Cy and Bu t . All prepared species were fully characterized by elemental analyzes (C, H, N), high resolution ESI þ -MS, IR, 1D

The reactions between equimolar amounts of cis-[PdCl(2)(C[triple bond]NR)(2)] (R = cyclohexyl (Cy) 1, tBu 2, 2,6-Me(2)C(6)H(3) (Xyl) 3) and the acyclic nitrones O(+)N(R(2))=C(H)R(3) (R(2) = Me 5, R(2) = CH(2)Ph 6, R(3) = 4-MeC(6)H(4)) proceeded in C(6)H(6) at 5 degrees C for around 4 h and then the reaction systems were maintained at 20-25 degrees C for 20 h to provide the carbene complexes [PdCl(2){C(ONR(2)C(c)HR(3)) = N(d)R}(C[triple bond]NR)(C(c)-N(d))] (8-13) in good yields (54-70%). The latter species originated from the previously unreported metal-mediated [2+3] cycloaddition of nitrones to coordinated isonitriles. For the reactions of cis-[PdCl(2)(C[triple bond]NR)(2)] (1-3) and 5 or 6 performed in C(6)H(6) at 20-25 degrees C, the selectivity of the cycloaddition was lost, and the reactions gave mixtures of 8-13 and the complexes [PdCl(2){N(R(2))=CHR(3)}(C[triple bond]NR)] (14-19, ca. 75%) derived from intramolecular deoxygenation of the nitrones. The reactions of equimolar amounts of cis-[PdCl(2)(C[triple bond]NR)(2)] (1-3 and R = 4-MeOC(6)H(4) 4) and the nonaromatic cyclic nitrone (-)O(+)N(a)=CHCH(2)CH(2)C(b)Me(2)(N(a)-C(b)) (7) in CHCl(3) at 5 degrees C for around 2 h led to the carbene species [PdCl(2){C(ON(a)CMe(2)CH(2)CH(2)C(b)H) = N(e)R}(C[triple bond]NR)(N(a)-C(b))(C(b)-N(e))] (21-24), isolated in yields of 78-92%. When the reactions of cis-[PdCl(2)(C[triple bond]NR)(2)] (1-4) and 7 (1:1 molar ratio) were performed in CHCl(3) at 20-25 degrees C for around 1 h, the carbenes 21-24 and the complexes [PdCl(2){N(f)CMe(2)CH(2)CH(2)C(g)H}(C[triple bond]NR)(N(f)-C(g))] (25-28, ca. 35%), derived from the deoxygenation of 7, were identified by ESI(+)-MS and (1)H NMR analysis (after redissolution in CDCl(3)). The reactions of cis-[PdCl(2)(C[triple bond]NR)(2)] (1-4) and 7 (1:2 molar ratio) in CHCl(3) at 5 degrees C furnished mixtures of [PdCl(2){C(ON(a)CMe(2)CH(2)CH(2)C(b)H)=N(e)R}(N(f)CMe(2)CH(2)CH(2)C(g)H)(N(a)-C(b))(C(b)-N(e))(N(f)-C(g))] (29-32; ca. 25-30%), 25-28 (ca. 10-15%), and [PdCl(2)(ON(a)CMe(2)CH(2)CH(2)C(b)H)(2)(N(a)-C(b))]. The same reactions at room temperature afforded the complex [PdCl(2)(N(f)CMe(2)CH(2)CH(2)C(g)H)(2)(N(f)-C(g))] (33; ca. 20 %) in a mixture with 29-32 (ca. 5-10%) and 25-28.

A new method for the synthesis of industrially important azoimidazolium dyes is presented. The procedure is based on a reagent which is rarely used in the context of synthetic organic chemistry: nitrous oxide ("laughing gas"). N2O is first coupled to N-heterocyclic carbenes. Subsequent reaction with aromatic compounds through an AlCl3-induced C-H activation process provides azoimidazolium dyes in good yields.

Metal-mediated coupling between equimolar amounts of cis-[PdCl2(CNR1)2] [R1 = 2,6-Me2C6H3 (Xyl) 1, 2-Cl,6-Me-C6H3 2, cyclohexyl (Cy) 3] and H2NC5H3R2N [R2 = H, 2-aminopyridine 4; R2 = NH2, 2,6-diaminopyridine 5] proceeds smoothly for 12 h at 20–25 °C and leads to the diaminocarbene species [PdCl{C(NHC5H3R2 N)N(H)R1}(CNR1)]Cl (6–9). In the reaction of 2 with 5 (1:1 molar ratio), corresponding carbene 10 was detected only by high-resolution ESI+-MS in a mixture with other yet unidentified products. Addition of each of 6–8 to starting 1 or 2 (1:1 molar ratio) in the presence of excess solid K2CO3 in CHCl3 and heating of the reaction mixture for 12 h at 40 °C led to a novel type of dinuclear complexes, 11 (75% isolated yield) and 13 (65%). Similar dinuclear complexes 12 and 14, formed by addition of 7 or 10 to 2, were identified by high-resolution ESI+-MS in the mixture with other species (e.g., 7 and 10). Generation of 11–14 proceeds via a cascade reaction including addition of the amino group of a 2-aminopyridine to the metal-activated isonitrile, ring-closure, and coupling of the derived acyclic diaminocarbene complex with the yet unreacted starting material. Complexes 6–9, 11, and 13 were fully characterized by elemental analyses (C, H, N), high-resolution ESI+-MS, IR, and 1H and 13C{1H} NMR spectroscopies, while 10, 12, and 14 were identified by ESI+-MS. In addition, the structures of five complexes [6, 6a (the latter is a neutral species derived from the deprotonation of 6), 9, 11, and 13] were elucidated by single-crystal X-ray diffraction.

The synthesis and characterization of a new class of neutral aminyl radicals is reported. Monoradicals were obtained by reduction of azoimidazolium dyes with potassium. Structural, spectroscopic, and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation of a biradical with largely independent unpaired electrons. Both the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.