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From neutral iminophosphoranes to multianionic phosphazenates. The coordination chemistry of imino-aza-P(V) ligands
No full textThis review deals with the chemistry and coordination behaviour of imino-aza phosphorus(V) ligands focussing on s- and p-block as well as Group 11 and 12 metal complexes. Imino phosphorus(V) ligands contain one or more terminal R-N = P-units, which include iminophosphoranes R3PNR′, monoanionic diiminophosphinates [R2P(NR′)2]-, dianionic triiminophosphonates [RP(NR′)3]2- and trianionic tetraiminophosphates [P(NR′)4]3-. Aza-phosphorus(V) ligands feature bridging P-N = P units, which include cyclic and polymeric phosphazenes [R2PN]n. Imino-aza- phosphorus(V) ligands containing both imino and aza functions include linear diiminodiphosphazenates [N{R2P(NR′)2}2]- and multianionic poly(imino) cyclophosphazeantes such as [N4{RP(NR′)}4]4- and [N3{P(NR′)2}3]6-. Imino-aza phosphorus(V) ligands are assembled of three basic building blocks: the cationic tetravalent phosphonium centre (P), the anionic divalent amido function (N) and the terminally arranged R-group. The overall negative charge Z of the resulting ligand system is equal to the difference between the number of P and the number of N-centres: Z = n(P)-n(N). Imino-aza phosphorus(V) ligands are electron rich N-donor ligands which co-ordinate via both N(imino) and N(aza) functions and have been applied in numerous metal complexes in order to stabilise low coordination numbers, unusual oxidation states and bonding modes or serve as ligands in homogeneous catalysis. The R-group provides both steric bulk and solubility in non-polar solvents. Multianionic phosphazenates feature a polydentate ligand surface, which facilitates an extremely high metal load. P = N units of iminophosphoranes and phosphazenes have acceptor properties and enhance the acidity of α-alkyl and ortho-aryl protons. Deprotonation of P-alkyl and P-aryl iminophosphoranes give ligand systems featuring C,N chelating sites, which are also discussed. © 2002 Published by Elsevier Science B.V
Decarbonylation of phenylacetic acids by high valent transition metal halides
Full text linkTriphenylacetic acid underwent unusual decarbonylation when allowed to react with a series of halides of group 4-6 metals in their highest oxidation state, in dichloromethane at ambient temperature. Thus, the reaction of CPh 3 COOH with MoCl 5 , in 1:1 molar ratio, afforded the trityl salt [CPh 3 ][MoOCl 4 ], 1, in 79% yield, while the 1:2 reaction of CPh 3 COOH with NbF 5 afforded [CPh 3 ][NbF 6 ], 2, in 70% yield, NbOF 3 being the metal co-product. CPh 3 COOH reacted with NbCl 5 , TiF 4 and WOCl 4 to give mixtures of compounds, however the cation [CPh 3 ] + was NMR identified in each case. [CPh 3 ][NbCl 6 ], 3, was isolated from NbCl 5 and CPh 3 COCl, prior to being generated from CPh 3 COOH and PCl 5 . The reaction of CPh 3 COOH with TiCl 4 was non-selective, and the salt [CPh 3 ][Ti 2 Cl 8 (μ-κ 2 -O 2 CCPh 3 )], 4, was obtained in 18% yield. The decarbonylation reactions of CMePh 2 COCl and CMe 2 PhCOCl by means of NbCl 5 led to the indanes 5a-b, which were isolated in 79-97% yields after hydrolysis of the mixtures and subsequent alumina filtration of the organic phases. The reactions of CH(Ph) 2 COOH with NbCl 5 and WCl 6 afforded NbCl 4 (OOCCHPh 2 ), 6, and CHPh 2 COCl, respectively, as the prevalent species. CPh 2 (CH 2 CH 2 Br)COOH did not undergo CO release when allowed to interact with WCl 6 , instead selective intramolecular condensation to C(Ph) 2 C(O)OCH 2 CH 2 , 7, occurred. MeCCCOOH underwent hydrochlorination by WCl 6 to give MeC(Cl)CHCOOH, 8, in 72% yield. All the products were fully characterized by elemental analysis, IR and multinuclear NMR spectroscopy. In addition, the solid state structures of 1, 2, 4, 7, and 8 were elucidated by X-ray diffraction
Formation and structural characterization of a diiron aminoalkylidene complex with N-cyano substituent
Full text linkThe reaction of the isocyanide complex [Fe2Cp2(CO)3(CNMe)], 1Me, with BrCN (1.2 eq.), in acetonitrile at 60 °C, led to the low-yield isolation, after work-up, of the unprecedented N-methylcyanamido-(cyano)alkylidene complex [Fe2Cp2(CO)3{μ-C(CN)N(CN)Me}], 3. The synthesis of 3represents an uncommon example of C-C bond forming cyanylation reaction taking place at a metal complex despite competitive oxidative addition pathways; it proceeded with the presumable, intermediate formation of a N-methylcyanamido-alkylidyne precursor readily undergoing CN addition, and was accompanied by the production of minor amounts of [Fe2Cp2(CO)4] and monoiron(II) piano-stool compounds. Compound 3 was fully characterized by single crystal X-ray diffraction, elemental analysis, IR and NMR spectroscopy. The reaction of [Fe2Cp2(CO)4] with cyanogen bromide, under conditions resembling those employed for 1Me/BrCN, afforded a mixture of [FeCpBr(CO)2] and [FeCp(CN)(CO)2]
Non-precious metal carbamates as catalysts for the aziridine/CO2 coupling reaction under mild conditions
Full text linkThe catalytic potential of a large series of easily available metal carbamates (based on thirteen different non-precious metal elements) was explored for the first time in the coupling reaction between 2-aryl-aziridines and carbon dioxide, working under solventless and ambient conditions and using tetraalkylammonium halides as co-catalysts. The straightforward synthesis of novel [NbCl3(O2CNEt2)2],NbCl, and [NbBr3(O2CNEt2)2],NbBr, is reported. The niobium complexNbCl, in combination with NBu4I, emerged as the best catalyst of the overall series to convert aziridines with smallN-alkyl substituents into the corresponding 5-aryl-oxazolidin-2-ones
From 1,2-dialkoxyalkanes to 1,4-dioxanes: a room temperature transformation mediated by group 5 metal halides
No full textWe recently showed that niobium and tantalum pentahalides, MX5, suspended in chlorinated solvents, react readily with simple oxygenated organic molecules (ketones, aldehydes, amides, ureas, cyclic ethers) affording Lewis acid-base adducts. Subsequent C–H or C–O bond activation has been observed in a number of cases.1,2 Interestingly, when an excess of 1,2-dimethoxyethane (dme) is added to MX5 (M = Nb, Ta, X = Cl, Br, I), final formation of MOX3(dme), CH3X and 1,4-dioxane takes place, as result of unusual dme activation, see Figure 1.3 Analogously, the reaction of 1,2-dimethoxypropane with NbCl5 yields 2,5-dimethyl-1,4-dioxane, indicating a new route for the expeditious synthesis of methyl-substituted dioxanes (Figure 1). The role played by the halide and the influence of the stoichiometry employed will be discussed
Construction of Two-Faced (Hetero)hydrocarbyl Diiron Complexes Mediated by the Interplay of Ligands
Full text linkThe functionalized allylidene complex [Fe2Cp2(CO)(mu-CO){mu-eta 1:eta 3-C gamma(Fc)-C beta HC alpha(CN)NMe2}], 1 [Cp = eta 5-C5H5; Fc = CpFe(eta 5-C5H4)], reacted with isocyanides (CNR), in isopropanol solution at ca. 100 degrees C, to give the CO-substitution products [Fe2Cp2(CO)(mu- CNR){mu-eta 1:eta 3-C gamma(Fc)C beta HC alpha(CN)NMe2}] [R = CH2P(O)(OEt)2, 2a; R = 2-naphthyl, 2b; R = CH2C(O)(OEt), 2c], which were isolated in 61-84% yields. The bridging coordination of CNR in 2a-c is forced by a stabilizing electrostatic interaction between the nitrogen lone pair belonging to the {NMe2} group and the terminal CO ligand. Isocyanide methylation with methyl triflate proceeded with the inversion of stereochemistry at the C alpha carbon and led to [Fe2Cp2(CO){mu- CN(Me)R}{mu-eta 1:eta 3-C gamma(Fc)C beta HC alpha(CN)NMe2}]CF3SO3 (R = CH2P(O)(OEt)2), ([3a]CF3SO3; R = 2-naphthyl, [3b]CF3SO3), containing bridging allylidene and aminocarbyne ligands (68-74% yields). All products were fully characterized by IR and multinuclear NMR spectroscopy, and the structures of 2a and [3a]CF3SO3 were elucidated by X-ray diffraction studies. Density functional theory (DFT) calculations were extensively carried out to shed light on structural, mechanistic, and thermodynamic features
Reactions of diiron m-aminocarbyne complexes containing nitrile ligands
Full text linkThe acetonitrile ligand in the mu-aminocarbyne complexes [Fe2{mu-CN(Me)R}(mu-CO)(CO)(NCMe)(Cp)2][SO 3CF3] (R = Me, 2a, CH2Ph, 2b, Xyl, 2c) (Xyl = 2,6-Me2C6H3) is readily displaced by halides and cyanide anions affording the corresponding neutral species [Fe2{mu-CN(Me)R}(mu-CO)(CO)(X)(Cp)2 ] (X = Br, I, CN). Complexes 2 undergo deprotonation and rearrangement of the coordinated MeCN upon treatment with organolithium reagents. Trimethylacetonitrile, that does not contain acidic alpha hydrogens has been used in place of MeCN to form the complexes [Fe2{mu-CN(Me)R}(mu-CO)(CO)(NCCMe3 )(Cp)2][SO3CF3] (7a-c). Attempts to replace the nitrile ligand in 3 with carbon nucleophiles (by reaction with RLi) failed, resulting in decomposition products. However the reaction of 7c with LiCºCTol (Tol = C6H4Me), followed by treatment with HSO3CF3, yielded the imino complex [Fe2{mu-CN(Me)Xyl}(mu-CO)(CO) {N(H)C(CºCC6H4Me-4)CMe3}(Cp) 2][SO3CF3 ] (8), obtained via acetilyde addition at the coordinated NCCMe3
Trapping carbamates of α-Amino acids: One-Pot and catalyst-free synthesis of 5-Aryl-2-Oxazolidinonyl derivatives
Full text linkCarbonation of natural α-amino acids in water and subsequent cyclization with (2-bromo-1-arylethyl)dimethylsulfonium bromides led selectively to a family of 2-oxazolidinonyl derivatives, which were isolated without the needing of purification. Analogous conjugation of the 2-oxazolidinone skeleton with amino acids was previously realized by complicated and narrow-scope routes, whereby the amino acid core is built stepwise. Deprotonation of some of the products afforded the corresponding water-soluble carboxylates, while a straightforward, proof of concept esterification reaction of the carboxylic acid group yielded a menthol-alanine-oxazolidinone conjugate
Carbonyl-isocyanide mono-substitution in [Fe2Cp2(CO)4]: A re-visitation
Full text linkThe reactions of [Fe2Cp2(CO)4] with a series of isocyanides, CNR, were conducted in acetonitrile and afforded, after a thermal treatment, the mono-isocyanide derivatives [Fe2Cp2(CO)3(CNR)] [R = 1H-indol-5-yl, 1; CH2P(O)(OEt)2, 2; Cy = C6H11, 3; 4-C6H4OMe, 4; Xyl = 2,6-C6H3Me2, 5; Me, 6; 2-naphthyl, 7; Bn = CH2Ph, 8]. In order to avoid multiple substitution, the diiron reactant was used in a molar excess with respect to the isocyanide (1.6 equivalents; 1.1 for the synthesis of 8). The products were separated from unreacted [Fe2Cp2(CO)4] by chromatography or via reversible protonation, and finally isolated in 50–83% yields. IR and NMR spectroscopy indicate that the isocyanide ligand is bridging coordinated in 2 and 7, terminal in 3 and 5, while in the remaining cases a mixture of terminal- and bridging-CNR isomers is obtained. The molecular structure of 5 was ascertained by X-ray diffraction. In general, the coordination mode of the isocyanide is scarcely influenced by the environment (solvents with different polarities, solid state). Sluggish partial isocyanide migration from terminal to bridging position was recognized for 3 and 5 upon heating in refluxing toluene, a process which was reproduced by DFT calculations
Convenient synthesis of fluoride-alkoxides of Nb(V) and Ta(V): a spectroscopic, crystallographic and computational study
No full textThe synthesis and the spectroscopic characterization of fluoride-alkoxides of niobium and tantalum in the highest oxidation state are reported. Suspensions of MF5 (M = Nb, Ta) in a chlorinated solvent reacted with up to three equivalents of ROSiMe3 (R = Me, Et, Ph) to afford polynuclear derivatives and variable amounts of FSiMe3. Thus MF4(OR) (R = Et, Ph) and MF3(OR)(2) were obtained by selective 1 : 1 and 1 : 2 reactions almost exclusively as single isomeric products; otherwise mixtures of MF4(OMe) species were afforded from the equimolar reactions of MF5 with MeOSiMe3. The 1 : 3 reaction of TaF5 with MeOSiMe3 led to different forms of TaF2(OMe)(3). The synthesis of TaF(OPh)(4) was forced by high temperature conditions or the use of a large excess of PhOSiMe3. DFT studies were carried out in order to predict, in the distinct cases, the most stable structures of the metal products. The molecular structures of [NbF2(OPh)(2)(mu-F)](3) and [TaF(OPh)(3)(mu-OPh)](2) were ascertained by X-ray diffraction
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