Alexander G. Tskhovrebov

Covalent Capture of Nitrous Oxide by N‐Heterocyclic Carbenes

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 NN bond.

Adducts of Nitrous Oxide and N-Heterocyclic Carbenes: Syntheses, Structures, and Reactivity

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 [2+3] Cycloaddition of Nitrones to Palladium-Bound Isonitriles

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.

Sequential N–O and N–N Bond Cleavage of N-Heterocyclic Carbene-Activated Nitrous Oxide with a Vanadium Complex

Chemically induced bond cleavage of nitrous oxide typically proceeds by rupture of the N-O bond with concomitant O-atom transfer and liberation of dinitrogen. On a few occasions, N-N bond scission has been observed instead. We report a reaction sequence involving an N-heterocyclic carbene and a vanadium complex that results in cleavage of both the N-O bond and the N-N bond.

Different Routes for Amination of Platinum(II)-Bound Cyanoguanidine

The consecutive addition of AgSO(3)CF(3) (1 or 2 equiv) and cyanoguanidine (1 or 2 equiv, respectively) to the platinum(II) precursor [PtI(2)(tmeda)] leads to the cis-[PtI(tmeda){NCN=C(NH(2))(2)}](SO(3)CF(3)) (1.(SO(3)CF(3))) or cis-[Pt(tmeda){NCN=C(NH(2))(2)}(2)](SO(3)CF(3))(2) (2.(SO(3)CF(3))(2)) complexes. The reaction between 1.(SO(3)CF(3)) or 2.(SO(3)CF(3))(2) and the excess of R(2)NH (R = H, R(2) = C(5)H(10)) in EtOH gives the triazapentadiene compounds cis-[Pt(tmeda){NHC(NR(2))NC(NH(2))NH}](SO(3)CF(3)) (3.(SO(3)CF(3)) and 4.(SO(3)CF(3)), correspondingly). Protonation of these species results in cis-[Pt(tmeda){NHC(NR(2))NHC(NH(2))NH}](SO(3)CF(3))(2) ([3.H](SO(3)CF(3))(2) and [4.H](SO(3)CF(3))(2), respectively). The interaction of solid 2.(SO(3)CF(3))(2) and the gaseous RNH(2) (R = H, Me) leads to cis-[Pt(tmeda){NHC(NHR)NHC(NH(2))NH}(2)](SO(3)CF(3))(2) (5.(SO(3)CF(3))(2) and 6.(SO(3)CF(3))(2), respectively). Treatment of an acetone solution of 2.(SO(3)CF(3))(2) with an aqueous NH(3) or the reaction of 5.(SO(3)CF(3))(2) with Me(2)CO produces the triazine complex cis-{Pt(tmeda){NH=CNHC(Me)(2)}NHC(NH)(2)N}(2)(SO(3)CF(3))(2) (7.(SO(3)CF(3))(2)). The reaction of 5.(SO(3)CF(3))(2) with Me(2)CO also leads to 7.(SO(3)CF(3))(2). All new complexes were characterized by elemental analyses (C, H, N), electrospray ionization mass spectrometry, IR, and (1)H and (13)C NMR spectroscopies. The structures of 1.(SO(3)CF(3)), 2.(SO(3)CF(3))(2), 3.(SO(3)CF(3)), [4.H](SO(3)CF(3))(2), 5.(SO(3)CF(3))(2), and 7.(SO(3)CF(3))(2) were determined by single-crystal X-ray diffraction.

Insertion of zerovalent nickel into the N-N bond of N-heterocyclic-carbene-activated N2O.

Metal-mediated cleavage of the N-N bond is a rarely observed phenomenon in the chemistry of nitrous oxide (N2O). We demonstrate that, upon activation of N2O with N-heterocyclic carbenes, zerovalent nickel is able to insert into the N-N bond to give nitrosyl complexes.

cis-Dichloridobis(2-isocyano­phenyl 4-meth­oxy­benzoate)palladium(II) chloro­form monosolvate

In the title compound, [PdCl2(C15H11NO3)2]·CHCl3, the PdII atom adopts a slightly distorted square-planar coordination geometry composed of two Cl atoms in cis positions and two C atoms from isocyanophenyl ligands. The molecular conformation is stabilized by π–π stacking interactions [shortest centroid–centroid distance = 3.600 (1) Å] between substituted benzene rings of different ligands. The crystal packing is characterized by C—H⋯O and C—H⋯Cl interactions involving the chloroform solvent molecules.

Palladium(II)-Stabilized Pyridine-2-Diazotates: Synthesis, Structural Characterization, and Cytotoxicity Studies

Well-defined diazotates are scarce. Here we report the synthesis of unprecedented homoleptic palladium(II) diazotate complexes. The palladium(II)-mediated nitrosylation of 2-aminopyridines with NaNO2 results in the formation of metal-stabilized diazotates, which were found to be cytotoxic to human ovarian cancer cells.