A neutral C4 cumulene 1 that includes a cyclic alkyl(amino) carbene (cAAC), its air-stable radical cation 1(·+) , and dication 1(2+) have been synthesized. The redox property of 1(·+) was studied by cyclic voltammetry. EPR and theoretical calculations show that the unpaired electron in 1(·+) is mainly delocalized over the central C4 backbone. The commercially available CBr4 is utilized as a source of dicarbon in the cumulene synthesis.

A series of new and known bis(imidazolium) chloride and bromide salts bridged by either a methylene group (1−8, 10a,b) or an ethylene group (9a,b) and bearing different N substituents (Me, Et, Bn, tBu, Mes) have been reacted with [Fe{N(SiMe 3 ) 2 } 2 ] 2 to yield the four-coordinate iron(II) complexes [LFeX 2 ] (11−20; X = Cl, Br; L = chelating bis(imidazolylidene) ligand). Molecular structures of six of these complexes have been characterized by X-ray crystallography, and selected examples have been characterized by 1 H NMR and UV−vis spectroscopy, cyclic voltammetry, Mossbauer spectroscopy, and SQUID magnetometry. In all cases the iron(II) is found in a distorted-tetrahedral environment; it is in the high-spin state and shows large quadrupole splittings in the range 3.67−4.03 mm·s −1 (δ = 0.73−0.81 mm·s −1 ). Subtleties of the metric parameters depend on the bridging unit between the two imidazolylidene groups, the peripheral N substituents, and the coligand (Cl or Br). In case of rather small (Me, Et) or flexible (Bn) N substituents the dicarbene species [LFeX 2 ] are formed together with ferrous tetracarbene complexes [L 2 FeX 2 ] (21−23), which are difficult to separate and could not be isolated in pure form. When the latter are dissolved in MeCN in the presence of residual [FeBr 2 (solv) x ], however, they transform into the ionic complexes [L 2 Fe(MeCN) 2 ][FeBr 4 ] (24−26), which have been characterized by single-crystal X-ray diffraction. They feature lowspin iron(II) (Mossbauer parameters δ ≈ 0.15 mm s −1 , ΔE Q ≈ 1.36 mm s −1 ) and distorted-octahedral structures with the two MeCN ligands in a cis configuration. Selected examples of the new dicarbene complexes [LFeX 2 ] have been tested as catalysts for the standard cross-coupling reaction between p-tolylmagnesium bromide and bromo-or chlorocyclohexane. They show moderate activity that appears to be generally lower than for related complexes with two monodentate NHC ligands, but the activities clearly depend on the peripheral N substituents and the linker between the two imidazolylidene groups; the best results are obtained for complex 19, which features a long ethylene bridge and bulky Mes substituents, and hence the most shielded metal center. ■ INTRODUCTIONN-heterocyclic carbenes (NHCs) have gained great popularity as ligands in organometallic chemistry and are finding widespread use in homogeneous catalysis. 1,2 They are usually viewed as strongly σ-donating ligands with little or negligible π back-bonding. Chemical and substitutional inertness is often mentioned among their particular advantages, and this is expected to be further amplified by incorporating the NHC moiety into a chelating scaffold. Mainly in the past decade a variety of iron complexes with monodentate NHC ligands 3−5 and several NHC ligated iron clusters have been published. 6 Some of the mononuclear iron NHC complexes 4,5 as well as species generated in situ from imidazolium salts and suitable iron precursors showed high activities in homogeneous catalysis. 7,8 However, only a f...

A neutral C 4 cumulene 1 that includes a cyclic alkyl(amino) carbene (cAAC), its air-stable radical cation 1C + , and dication 1 2+ have been synthesized. The redox property of 1C + was studied by cyclic voltammetry. EPR and theoretical calculations show that the unpaired electron in 1C + is mainly delocalized over the central C 4 backbone. The commercially available CBr 4 is utilized as a source of dicarbon in the cumulene synthesis.

The controversy over the absolute configuration of (+)-erythro-mefloquine, the less psychosis-causing enantiomer of the anti-malarial drug Lariam, has been resolved by Mosher ester crystallization. The configuration determined previously by physical methods is correct, whereas the configuration determined by three enantioselective syntheses is wrong.

For the structure of fluconazole [systematic name: 2-(2,4-difluorophenyl)-1,3-bis(1H-1,2,4-triazol-1-yl)propan-2-ol] monohydrate, C13H12F2N6O·H2O, a case study on different model refinements is reported, based on single-crystal X-ray diffraction data measured at 100 K with Cu Kα radiation to a resolution of sin θ/λ of 0.6 Å(-1). The structure, anisotropic displacement parameters (ADPs) and figures of merit from the independent atom model are compared to `invariom' and `Hirshfeld atom' refinements. Changing from a spherical to an aspherical atom model lowers the figures of merit and improves both the accuracy and the precision of the geometrical parameters. Differences between results from the two aspherical-atom refinements are small. However, a refinement of ADPs for H atoms is only possible with the Hirshfeld atom density model. It gives meaningful results even at a resolution of 0.6 Å(-1), but requires good low-order data.