1-Methylimidazole adduct structure

X-Ray studies of crystals of pyridine complexes with trimethylsilyl bromide and iodide (264) and of N-methylimidazole adducts to trimethyl-chlorosilane (265) were performed. The tetracoordinate silicon structure of these complexes was proved. The distances between the halogen and silicon atoms in pyridine adducts are 4.359 and 4.559 A for bromine and iodine, respectively, which is approximately 2 A greater than the sum of the covalent radii and significantly longer (0.5 A) than the sum of the van der Waals radii. This result is consistent with the ionic structures of the complexes in the solid state. The distance from silicon to nitrogen (1.86 A) is evidently larger than the length of typical Si—N bonds (1.75 A), which... 

In (67 R1 = R2 = Me, R = H) with X = Br green or yellow complexes were obtained, according to the solvent used, with v(V=0) at ca. 980 and 900 cm-1, respectively.748 When X = C1, only the green-type complex may be obtained. The authors assume five-coordinate square pyramidal geometry for the green complexes and a chain structure -V=0 V=0 , similar to [VO(salnpn)], for the yellow complex. Solid adducts were isolated with 1-methylimidazole. 

Jameson, G. B., Molinaro, F. S., Ibers, etal., Models for the active-site of oxygen-binding hemoproteins - dioxygen binding-properties and the structures of (2-methylimidazole)-meso-tetra (a,a,a,a-ortho-pivalamidophenyl)porphyrinatoiron(II)-ethanol and its dioxygen adduct. J. Am. Chem. Soc. 1980, 102, 3224-3237. 

Me2SiCl2 forms with A-methylimidazole (NMI) a 1 2 adduct, but after sublimation crystals of a 1 4 adduct were obtained. The crystal structure analysis of the 1 4 adduct revealed a pentacoordinated silicon atom108 (equation 66). 

The cobalt(II) cyclidene product VI is reasonably stable toward autoxidation in the solid state, and a solid state IR spectrum can be obtained under aerobic conditions. The cobalt(II) complex is best identified by its ESR spectrum in an acetonitrile-1.5M M-methylimidazoIe solution, frozen to a glass at — 196 C a 2.3 and 2.00. Exposure of the thawed solution to O2 then causes replacement of this spectrum with that of the 1 1 Oj adduct % 2% 2.01 and 3% 2.08. At 20 °C in 2.5 M aqueous N-methylimidazole the complex binds dioxygen reversibly with an equilibrium constant Kq = 1.6 torr. An X-ray structure of the cobalt(II) complex confirms the presence of the commodious lacuna, which protects the coordinated dioxygen from autoxidation in polar sol vents.An X-ray structure determination of the dioxygen adduct, prepared by recrystallization of the complex from aqueous V-methylimidazole exposed to the atmosphere, confirms the existence and nature of the O, adduct. ... 

Figure 5 Structure of the O2 adduct of the Fe picket fence porphryin/1—methylimidazole complex ...

To inhibit binding of a second copper atom and to prevent possible hydrogen atom transfer reactions, the bridging oxime hydrogen was replaced with BF2 via treatment with Bp3-etherate in dioxane. The Cu(I) complex reacts with monodentrate (i.e., CO, 1-methylimidazole, acetonitrile) yielding five-coordinate adducts. Of course such a model is not an accurate mimic of the active sites of any copper protein but suggests that consideration of Cu(I) active site structures must include the possibility of five-coordination. 

The frequent presence of an imidazole-co-ordinated residue in an axial position, coupled with the suggestions that metal-imidazole bond rupture might play an important role in biological processes of coenzyme B12 and the cytochrome c system, indicates that a detailed characterization of the structure and dynamics of the imidazole-metal bond in such porphyrin complexes could have significant biological implications. La Mar and Walker present some preliminary data on the exchange of iV-methylimidazole (WMIm) on the bis-adduct of A MIm with [Fe(tpp)Cl] in CDCI3, and Kolski and Plane describe temperature-jump studies on the reaction between imidazole and the iron(m) derivative of ethylenediamine-substituted protoporphyrin IX in water. 

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