Group 9 complexes of an N-heterocycle carbene bearing a pentafluorobenzyl substituent: attempted dehydrofluorinative coupling of cyclopentadienyl and N-heterocycle carbene ligands

Treatment of N-methylimidazole with pentafluorobenzyl bromide produces 1-pentafluorobenzyl-3-methylimidazolium bromide (1), which reacts with silver(I) oxide to give the N-heterocycle carbene (NHC) complex 1-pentafluorobenzyl-3-methylimidazolin-2-ylidene silver(I) bromide (2). Complex 2 acts as a carbene transfer reagent giving the complexes [(η⁵-C₅Me₅)MCl₂(NHC)] (3a, M = Rh; 3b M = Ir) on reaction with [(η⁵-C₅Me₅)MCl(μ-Cl)]₂. An attempt to use intramolecular dehydrofluorinative coupling methodology to link the carbene and the pentamethylcyclopentadienyl ligands of [(η⁵-C₅Me₅)RhCl(CN^tBu)(NHC)]BF₄ was unsuccessful.     

Copper-catalyzed oxidative cleavage of carbon-carbon double bond of enol ethers with molecular oxygen

A novel CC bond cleavage reaction of aromatic enol ethers (1) to give ketones (2) using molecular oxygen as oxidant is described. Among the examined catalysts (Cu(II), Pd(II), Ru(II), and H⁺), CuCl₂ exhibited the highest activity. The reaction proceeded smoothly with several kinds of substrates.     

Phosphorus-supported multidentate coumarin-containing fluorescence sensors for Cu

Phosphorus hydrazides PhP(O)[N(Me)NH₂]₂, (S)P[N(Me)NH₂]₃, and N₃P₃[N(Me)NH₂]₆ were condensed with 7-diethylaminocoumarin-3-aldehyde (RCHO) to afford the corresponding hydrazones PhP(O)[N(Me)NCHR]₂ (1), (S)P[N(Me)NCHR]₃ (2), and N₃P₃[N(Me)NCHR]₆ (3). The structural characterization of 1-3 was carried out by their HRMS, ¹H and ³¹P{¹H} NMR spectra. The molecular structure of 2 was established by a single-crystal X-ray analysis. Interaction of 1 and 2 with various transition metal ions revealed substantial fluorescence enhancement upon interaction with Cu²⁺ enabling a selective detection mechanism for this metal ion. However, such a fluorescence enhancement was not observed in the case of 3. A 1:1 complex [2·Zn][ClO₄]₂·4CH₂Cl₂ was isolated in the reaction of 2 with Zn(ClO₄)₂·6H₂O. The molecular structure of this complex revealed that the Zn^II is encapsulated by the ligand utilizing a 3N, 3O coordination set.     

Organic reactions in water: an efficient one-pot synthesis of acyloxiranes from Baylis-Hillman adducts using hypervalent iodine

Treatment of Baylis-Hillman adducts with iodosobenzene (PhIO) in the presence of a catalytic amount of KBr in water at room temperature afforded the corresponding acyloxiranes in good yields.     

A turn-on and reversible fluorescence sensor with high affinity to Zn in aqueous solution

A new simple receptor 1 based on aminosalicylimine was prepared. It exhibited an ‘off–on fluorescence type’ mode with high sensitivity in the presence of Zn²⁺. In particular, this chemosensor could clearly distinguish Zn²⁺ from Cd²⁺. Also, it could be a reusable chemosensor because the addition of EDTA quenched the fluorescence of the Zn²⁺-2·1 complex. Furthermore, receptor 1 had a sufficiently low detection limit (68 nM) in aqueous solutions, which implies that 1 could sense the nanomolar concentration of Zn²⁺. Therefore, this sensor has the ability to be a practical system for the monitoring of Zn²⁺ concentrations in aqueous samples.     

Osmium assisted C-H activation and CN cleavage of N-(2′-hydroxyphenyl) benzaldimines. Synthesis, structure and electrochemical properties of some organoosmium complexes

Reaction of N-(2′-hydroxyphenyl)-4-R-benzaldimines (L-R, R = OCH₃, CH₃, H, Cl and NO₂) with [Os(PPh₃)₃Br₂] in refluxing 2-methoxyethanol in the presence of triethylamine affords two families of organoosmium complexes (1-R and 2-R). In both 1-R and 2-R complexes a benzaldimine ligand is coordinated to the metal center as tridentate C,N,O-donor. In the 1-R complexes, a bidentate N,O-donor imionsemiquinonate ligand, derived from the hydrolysis of another benzaldimine, and a PPh₃ ligand are also coordinated to osmium. In the 2-R complexes, a carbonyl, derived from decarbonylation of 4-R-benzaldehyde (derived from the same hydrolysis stated above), and two PPh₃ ligands take up the remaining coordination sites on osmium. Structures of the 1-Cl and 2-OCH₃ complexes have been determined by X-ray crystallography. All the 1-R and 2-R complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the 1-R complexes shows a reversible Os(III)-Os(IV) oxidation within 0.47-0.67 V (vs SCE), followed by an irreversible oxidation of the imionsemiquinonate ligand within 1.10-1.36 V. An irreversible Os(III)-Os(II) reduction is also displayed by the 1-R complexes within −1.02 to −1.14 V. Cyclic voltammetry on the 2-R complexes shows a reversible Os(II)-Os(III) oxidation within 0.29-0.51 V, followed by a quasi-reversible oxidation within 1.04-1.29 V, and an irreversible reduction of the coordinated benzaldimine ligand within −1.16 to −1.31 V.     

Theoretical studies on the imine germylenoid HNGeNaF and its insertion reaction with R-H (R = F, OH, NH2, CH3)

The geometries and isomerization of the imine germylenoid HNGeNaF as well as its insertion reactions with R-H (R = F, OH, NH₂, CH₃) have been systematically investigated at the B3LYP/6-311+G^∗ level of theory. The potential barriers of the four insertion reactions are 117.2, 172.6, 219.7, and 322.3 kJ/mol, respectively. Here, all the mechanisms of the four reactions are identical to each other, i.e., an intermediate has been formed first during the insertion reaction. Then, the intermediate could dissociate into the substituted germylene (HNGeHR) and NaF with a barrier corresponding to their respective dissociation energies. Correspondingly, the reaction energies for the four reactions are 185.0, 208.1, 224.4, and 266.9 kJ/mol, respectively, which are linearly correlated with the calculated barrier heights. Compared with the insertion reaction of HNGe: and R-H, the introduction of NaF makes the insertion reaction occur easily though it is more difficult to proceed than that of insertion reaction between H₂GeNaF and R-H. Furthermore, the effects of halogen (F, Cl, Br) substitution and inorganic salts employed on the reaction activity have also been discussed. As a result, the relative reactivity among the four insertion reactions should be as follows: H-F > H-OH > H-NH₂ > H-CH₃.     

Clay-anchored non-heme iron-salen complex catalyzed cleavage of CC bond in aqueous medium

Clay-anchored iron[N,N′-ethylenebis(salicylideneaminato)] complex, synthesized by direct exchange, oxidizes various olefins and chalcones in aqueous acetonitrile using hydrogen peroxide as terminal oxidant. Aldehyde and its derivatives are obtained as oxidation products by the cleavage of CC double bond. In comparison with the catalysis by iron-salen complex in solution, the clay catalyzed pathway not only increases the rate of reaction significantly, but also provides selective oxidation toward the aldehyde. Some chalcones also give very good yield in water, compared to the solution and clay catalyzed pathways.     

New bicyclic phosphorous ligands: synthesis, structure and catalytic applications in ionic liquids

New azadioxaphosphabicyclo[3.3.0]octane ligands showing a trans arrangement with regard to the two five-membered heterocycles, were obtained as a mixture of three conformers, in agreement with molecular modelling studies. The stability of oxaphosphane ligands was studied under basic catalytic conditions, monitored by NMR spectroscopy. Palladium catalytic systems containing these ligands were active in Suzuki C-C cross-coupling reactions between phenylboronic acid and aryl halides (bromide and chloride derivatives) bearing electron-donor or electron-withdrawing substituents, in both organic and ionic liquid solvents. The catalytic systems showed a high stability even under the most severe reaction conditions used in this work. The ionic liquid catalytic phase could be recycled up to ten times without significant activity loss.     

Synthesis and structures of the silyl bridged bis(indenyl) diruthenium complexes and a novel indenyl nonanuclear ruthenium cluster Ru9(μ6-C)(CO)14(μ3-η:η:η-C9H7)2

Thermal treatment of C₉H₇SiMe₂C₉H₇ and C₉H₇Me₂SiOSiMe₂C₉H₇ with Ru₃(CO)₁₂ in refluxing xylene gave the corresponding diruthenium complexes (E)[(η⁵-C₉H₆)Ru(CO)]₂(μ-CO)₂ [E = Me₂Si (1), Me₂SiOSiMe₂ (2)]. A desilylation product [(η⁵-C₉H₇)Ru(CO)]₂(μ-CO)₂ (3) was also obtained in the latter case. Similar treatment of C₉H₇Me₂SiSiMe₂C₉H₇ with Ru₃(CO)₁₂ gave a novel indenyl nonanuclear ruthenium cluster Ru₉(μ₆-C)(CO)₁₄(μ₃-η⁵:η²:η²-C₉H₇)₂ (5) with carbon-centered tricapped trigonal prism geometry, in addition to the diruthenium complex (Me₂SiSiMe₂)[(η⁵-C₉H₆)Ru(CO)]₂(μ-CO)₂ (4) and the desilylation product 3. Complex 4 can undergo a thermal rearrangement to form the product [(Me₂Si)(η⁵-C₉H₆)Ru(CO)₂]₂ (6). The molecular structures of 1, 2, 4, 5, and 6 were determined by X-ray diffraction.     

DFT study on the unsaturated germylenoid H2CGeNaF

The unsaturated germylenoid H₂CGeNaF was studied by using the DFT B3LYP method in conjunction with the 6-311+G(d, p) basis set. Geometry optimization calculations indicate that H₂CGeNaF has three equilibrium configurations, in which the p-complex is the lowest in energy and is the most stable structure. Two transition states for isomerization reactions of H₂CGeNaF are located and the energy barriers are calculated. For the most stable one, vibrational frequencies and infrared intensities have been predicted.     

Rhodium assisted CH activation of N-(2′-hydroxyphenyl)benzaldimines. Synthesis, structure and electrochemical properties of a group of organorhodium complexes

Reaction of a group of N-(2′-hydroxyphenyl)benzaldimines, derived from 2-aminophenol and five para-substituted benzaldehydes (the para substituents are OCH₃, CH₃, H, Cl and NO₂), with [Rh(PPh₃)₃Cl] in refluxing toluene in the presence of a base (NEt₃) afforded a family of organometallic complexes of rhodium(III). The crystal structure of one complex has been determined by X-ray crystallography. In these complexes the benzaldimine ligands are coordinated to the metal center, via dissociation of the phenolic proton and the phenyl proton at the ortho position of the phenyl ring in the imine fragment, as dianionic tridentate C,N,O-donors, and the two PPh₃ ligands are trans. The complexes are diamagnetic (low-spin d⁶, S = 0) and show intense MLCT transitions in the visible region. Cyclic voltammetry shows a Rh(III)Rh(IV) oxidation within 0.63-0.93 V vs SCE followed by an oxidation of the coordinated benzaldimine ligand. A reduction of the coordinated benzaldimine is also observed within −0.96 to −1.04 V vs SCE. Potential of the Rh(III)Rh(IV) oxidation is found to be sensitive to the nature of the para-substituent.     

Aliphatic carboncarbon bond activation of ketones by rhodium(II) porphyrin radical

Aliphatic carboncarbon bond activation of both enolizable and non-enolizable ketones occurred successfully with rhodium(II) porphyrin radical to give rhodium(III) porphyrin alkyls. Added Ph₃P promoted the yields of products.     

A rich chemistry of fluorinated imidoyl halides

A recent development of the chemistry of trifluoroacetimidoyl metals and trifluorolactimidoyl halides is described.     

Preparation of fluorine compounds of groups 13 and 14: a study case for the diagonal relationship of aluminum and germanium

The synthesis of carbaalanes of composition [(AlF)₆(AlNMe₃)₂(CR)₆] containing terminal fluorine atoms is described. The clusters have in common that the central core consists of eight aluminum and six carbon atoms. Six of the eight aluminum atoms are bearing six terminal fluorine atoms. The fluorination of (t-BuNCH₂AlH)₄ results in the formation of the aggregate (t-BuNCH₂AlF)₄. In group 14, the fluorine containing unsaturated compounds LGeF, LGeS(F), LGeSe(F), and LGeNSiMe₃(F) were prepared (LHC(CMeNAr)₂), Ar = 2,6-iPr₂C₆H₃ and Ar = 2,6-Me₂(C₆H₃)).     

Synthesis, structure, and magnetic properties of 2,2′-(buta-1,3-diyne-1,4-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide 1-oxyl)

An approach to the synthesis of nitronyl nitroxide 2,2′-(buta-1,3-diyne-1,4-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide 1-oxyl) (4) was developed. Compound 4 is the first diradical with nitronyl nitroxide groups directly linked through a diacetylene fragment. In solid phase the diradicals are arranged in stacks with parallel CC fragments, with the distances between the terminal carbon atoms of the neighboring diacetylene groups (T and d) being 6.170 and 4.466 Å, respectively, and the angle between the translation vector and the median line passing through the CCCC fragment of 45.9°. The values of T and d are outside the range of structural criteria allowing a topochemical reaction. Thus UV irradiation does not initiate solid phase polymerization of 4. After exposure at 373 K for 1 h the crystals of 4 turn dark-brown, become X-ray amorphous and lose the majority of their paramagnetic centers without significantly changing their mass. Upon further heating up to 400-420 K the product explodes, releasing about 360 kJ/mol of heat. A diluted solution of 4 in 1,4-dioxane produces an EPR spectrum typical of a strong exchange (a multiplet of nine broadened lines with A_4N = 0.35 mT), indicating the efficiency of the CCCC fragment as an exchange channel. The character of the experimental μ_eff(T) dependence for 4 indicates a strong intramolecular antiferromagnetic-type exchange interaction (J/k_B ∼ −104 K) and the dominating weak intermolecular ferromagnetic exchange.