Betaine Adducts of N-Heterocyclic Carbenes: Synthesis, Properties, and Reactivity

in European Journal Of Inorganic Chemistry (2009), (13), 1681-1699

N-Heterocyclic carbenes (NHCs) form stable zwitterionic adducts with a range of heteroallenes, ketenes, and allenes. Although the first representatives of this class of inner salts were first investigated ... [more ▼]

N-Heterocyclic carbenes (NHCs) form stable zwitterionic adducts with a range of heteroallenes, ketenes, and allenes. Although the first representatives of this class of inner salts were first investigated as far back as the 1960s, they have enjoyed a sustained interest from the chemical community over the years. Depending on the nature of their anionic moiety, NHC betaines display a very broad palette of reactivities and have found applications in various fields of organic synthesis and catalysis. In this Microreview, the synthesis, properties, and reactivity of NHC betaines are surveyed. The NHCs under consideration include ylidenes derived from imidazole, benzimidazole, imidazoline, thiazole, or triazole, and the heteroallenes investigated so far are carbon dioxide, carbon disulfide, isocyanates, isothiocyanates, and their selenium analogues. A historical background is provided for each type of adduct under consideration, but emphasis is placed mainly on developments that have appeared in the literature within the past few years. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [less ▲]

Synthetic, structural, magnetic, and Mossbauer spectral study of {Fe[HC(3,5-Me(2)pz)(3)](2)}I-2 and its spin-state crossover behavior

in European Journal Of Inorganic Chemistry (2002), (5), 1190-1197

The complex {Fe[HC(3,5-Me(2)pz)(3)](2)}I-2 (1) has been prepared from the reaction of FeI2 and HC(3,5-Me(2)pz)(3) (pz = pyrazolyl ring) in a 1:2 ratio. The complex is high-spin in both the solid state and ... [more ▼]

The complex {Fe[HC(3,5-Me(2)pz)(3)](2)}I-2 (1) has been prepared from the reaction of FeI2 and HC(3,5-Me(2)pz)(3) (pz = pyrazolyl ring) in a 1:2 ratio. The complex is high-spin in both the solid state and in solution at ambient temperature, In the solid state, magnetic susceptibility and Mossbauer spectral studies indicate that samples of the complex that have been crystallized and dried change completely from high-spin to low-spin upon cooling below 195 K, with a substantial two-step thermal hysteresis in the transition, The spin-crossover properties of I are sample dependent: powder samples do not change to low-spin at low temperatures. Two crystalline forms of I have been identified by X-ray crystallography, one form containing no solvent of crystallization and the other containing solvent, In both, there is only one iron site, with average Fe-N bond lengths of 2.18 and 2.17 Angstrom, values that confirm that the complex is high-spin. Single crystals of the nonsolvated form shatter at low temperatures whereas single crystals of the solvated form can be cooled to 110 K with no loss of crystallinity and remain fully high spin at this temperature, Crystallographic information: nonsolvated form is triclinic, P (1) over bar, a = 8,8062(2) Angstrom, b = 10.3549(2) Angstrom, c = 11.3549(2) Angstrom, a = 104.0768(10), beta = 110.2473(10)degrees, gamma = 92.6385(11)degrees, Z = 1; solvated form is monoclinic P2(1)/c, a 10.3214(5) Angstrom, b = 12.7753(7) Angstrom, c = 20.1995(11) Angstrom, alpha = 90degrees, beta = 97.5380(10)degrees, gamma = 90degrees, Z = 2. [less ▲]

Electrografting of acrylic and methacrylic monomers onto metals: influence of the relative polarity and donor-acceptor properties of the monomer and the solvent

in European Journal of Inorganic Chemistry (1998), 1998(11), 1711-1720

The possibility of grafting a series of alkyl polyacrylates and polymethacrylates onto a nickel cathode by electropoly-merization of the parent monomers has been investigated and has emphasized the ... [more ▼]

The possibility of grafting a series of alkyl polyacrylates and polymethacrylates onto a nickel cathode by electropoly-merization of the parent monomers has been investigated and has emphasized the critical importance of the solvent used. Indeed, the intensity of the inhibition peak, which is the electrochemical mark of the cathode passivation as result of the polymer grafting, clearly depends on both the polarity and the donor-acceptor properties of the solvent. The Gutmann concept is used to account for these experimental results. An increase in the donicity of the solvent used for the electrochemical medium has allowed, for the very first time, several polyacrylates and polymethacrylates [such as poly(ethyl acrylate), poly(methyl methacrylate), and poly(2-trimethylsilyloxyethyl methacrylate)] to be electrografted onto Ni. This observation is consistent with a competition process between the monomer and the solvent for being adsorbed on the cathode and amassing in its very close vicinity. The outcome of this competition is controlled by the relative polarity (in case of low donicity) and the relative donor-acceptor properties (when the difference is high enough) of the monomer/solvent pair, and by the monomer concentration (in case of weak competition). A semiquantitative relationship has also been observed between the monomer ability to be electrografted and the electron-accepting character of the vinyl β-carbon atom as measured by 13C NMR. [less ▲]