Graduate / 0485

Graduate / 0485

See Abstract on Front Page 7 / Dissertation (MSc (Chemistry))--University of Pretoria, 2007. / Chemistry / unrestricted

Fischer carbene complexes of the Group VI transition metals (Cr, Mo and W) containing at least two or three different transition metal substituents, all in electronic contact with the carbene carbon atom, were synthesized and studied both in solution and in the solid state. For the complexes of the type [M(CO)5{C(OR)R’}], the substituents chosen included (hetero)aromatic (benzene or thiophene) rings π-bonded to a chromium tricarbonyl fragment or ferrocene as the R’-substituent, while the OR-substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1 (M = Cr, R = Et, R’ = Fc), 2 (M = W, R = Et, R’ = Fc), 5 (M = Cr, R = TiCp2Cl, R’ = Fc), 6 (M = W, R = TiCp2Cl, R’ = Fc), 7 (M = Mo, R = TiCp2Cl, R’ = Fc), 12 (M = Cr, R = TiCp2Cl, R’ = 2-thienyl) and 13 (M = Cr, R = TiCp2Cl, R’ = [Cr(CO) 3 (η 6-phenyl)]). Direct lithiation of the ferrocene with n-BuLi/TMEDA at elevated temperatures, followed by the Fischer method of carbene preparation, also resulted, in most cases, in the formation of the novel biscarbene complexes with bridging ferrocen- 1,1’-diyl carbene ligands [μ-Fe{C5H4C(OEt)M(CO) sub>5}2] (3: M = Cr, 4: M = W) or the unusual bimetallacyclic bridged biscarbene complexes [{μ-TiCp2O2-O,O’}{μ- Fe(C5H4)2-C,C’}{CM(CO) 5}2] (8: M = Cr, 9: M = W, 10: M = Mo). It was attempted to prepare the mixed heteronuclear biscarbene complex 11 [W(CO) 5C{μ-TiCp2O2- O,O’}{μ-Fe(C5H4)2-C,C’}CCr(CO) 5], however the complex could not be fully characterized. The investigation was expanded to include Group VII transition metals Mn and Re, and using the same methodology, the manganese complexes isolated included [MnCp(CO2{C(OR)Fc}] (22: R = Et, 24: R = TiCp2Cl), 23 [μ- Fe{C5H4C(OEt)MnCp(CO) 2}2] and 25 [{μ-TiCp2O2-O,O’}{μ- Fe(C5H4)2- C,C’}[CMnCp(CO) 2}2]. The different reactivity of the binary dirhenium decacarbonyl precursor complex, compared to that of the Group VI complexes, resulted in the formation of a range of complexes. The target compounds [Re2 (CO) 9{C(OR)Fc}] (26: R = Et, 31: R = TiCp2Cl), 27 [μ-Fe{C5H4C(OEt)Re2 (CO) 9}2] and 33 [{μ- TiCp2O2-O,O’}{μ-Fe(C5H4)2-C,C’}[CRe2 (CO) 9}2] were isolated displaying a variety of different geometric isomers. In addition, acyl (30) and aldehyde (32) decomposition products, as well as hydrido (29), and hydrido acyl hydroxycarbene (34) complexes and the unique dichloro-bridged biscarbene complex (28) were also characterized. Most of these complexes displayed Re-Re bond breaking, and two probable mechanisms, either radical or ionic, were proposed involving either hydrogen transfer or protonation followed by hydrolysis. Finally, the structural features and their relevance to bonding in the carbene cluster compounds of the Group VI transition metals were investigated as they represent indicators of possible reactivity sites in multimetal carbene assemblies. The possibility of using DFT calculations to quantify the effect of metal-containing substituents on the carbene ligands was tested and correlated with experimental parameters by employing methods such as vibrational spectroscopy, molecular orbital analysis, and cyclic voltammetry. The best results were obtained from the cyclic voltammetric studies, where the localized metal centre’s oxidation potential correlated to both the calculated HOMO energy, and the effect of both the heteroatom substituent and the (hetero)arene substituent, as well as different combinations of the above. / Thesis (PhD)--University of Pretoria, 2010. / Chemistry / unrestricted

This investigation is concerned with determining whether a carbon substituent or a silicon substituent on the carbon terminus of a silicon-carbon double bond has a more stabilizing effect. Two different 2-substituted silaethylenes were generated at the same time by pyrolyzing 1, 1-dimethyl-2-neopentyl-4- (dimethylalkoxysilyl) silacyclobutanes in a nitrogen flow system. The results of these pyrolyses, both neat and in the presence of a trapping reagent, show that the silaethylene with a silicon substituent on the carbon terminus was favored approximately two to one over the silaethylene with a carbon substituent. This datum, along with other observations and hypotheses discussed, leads to the suggestion that the silicon substituenton the carbon terminus of the silaethylene bond has a more stabilizing effect than the carbon substituent.

No description available.

The effect of substituents on the stabilities of cumulenes-ketenes, allenes, diazomethanes and isocyanates and related systems-alkynes, nitriles and nitrile oxides is studied using the density functional theory (B3LYP, SVWN and BP86) and ab initio (HF, MP2) calculations at the 6-31G* basis set level. Using isodesmic reactions, correlation between stabilization energies of cumulenes and substituent group electronegativities (c BE) is established and the results from DFT and MP2 methods are compared with the earlier HF calculations. Calculations revealed that the density functional methods can be used to study the effect of substituents on the stabilities of cumulenes. It is observed that the cumulenes are stabilized by electropositive substituent groups from s -electron donation and p -electron withdrawal and are destabilized by electronegative substituent groups from n-p donation. The calculated geometries of the cumulenes are compared with the available experimental data.High level ab initio and density functional theory calculations have been used to study the energetics of low-barrier hydrogen bond (LBHB) systems. Using substituted formic acid-formate anion complexes as model LBHB systems, hydrogen bond strength is correlated to the pKa mismatch between the hydrogen bond donor and the hydrogen bond acceptor. LBHB model systems are characterized by the 1H-NMR chemical shift calculations. A linear correlation between the calculated hydrogen bond strength and the predicted 1H-NMR chemical shift was established. It is concluded that the pKa matching within the enzyme active site of the two species involved in the LBHB is important to maximizing catalytic stabilization.

Title: Phosphinoferrocene ligands with polar amide substituents Author: Hana Charvátová Department: Department of Inorganic Chemistry Supervisor: prof. RNDr. Petr Štěpnička, Ph.D. Abstract: This thesis is focused on phosphino-urea ferrocene ligands that are still rather neglected in the literature. It describes the synthesis of novel polar amides and hydrazides of 1'-(diphenylphosphino)ferrocene-1-carboxylic acid (Hdpf) with or without ethylene linker bearing various urea and guanidine terminal functional groups. Urea and guanidine derivatives with ethylene bridge can be prepared from Hdpf and appropriate amine with amidation agents. Phosphine ureas without the linker are accessible from reactions of primary amide of Hdpf with suitable acylation agents while analogical guanidine is obtained from guanylation of amide-amine generated from Hdpf acylbenzotriazole and ethylenediamine. Reaction of the acylbenzotriazole with free guanidine leads to [1-(diphenylphosphino)ferrocenecarbonyl]guanidine hydrochloride. These ferrocene ligands were used to prepare four types of palladium(II) complexes, viz. where L denotes the newly synthesized ligands and LNC is 2- [(dimethylamino)methyl]phenyl-C,N auxiliary chelating ligand. Catalytic efficiency of complexes with ethylene bridge was tested on reactions of aromatic...

Yes / An experimental and computational investigation is conducted into the role of substituents in retro Diels-Alder extrusion of CO2 from 2-oxa-bicyclo[2.2.2]oct-5-en-3-ones. We provide the first experimental evidence that loss of CO2 from the cycloadducts significantly depends on the nature and position of the substituents. For example, we show that whilst 5-carboethoxy-2-pyrone undergoes a more facile cycloaddition that 3-carboethoxy-2-pyrone, the cycloadduct from the latter pyrone undergoes a more facile loss of CO2 than the cycloadduct from the former pyrone. / EPSRC, Yorkshire Cancer Research, Yorkshire Enterprise Fellowships

Os compostos N-metil-1,8-naftalimida (NI), 4-fenóxi-N-metil-1,8-naftalimida (PNI) e 4-naftóxi-N-metil-1,8-naftalimida (NNI) foram sintetizados e caracterizados por técnicas usuais de caracterização de compostos orgânicos. As propriedades fotofísicas desses compostos foram estudadas tanto no estado estacionário quanto resolvidas no tempo. Com base nessas medidas, verificamos que os espectros de absorção e de emissão apresentam deslocamento batocrômico quando os grupos fenóxi e naftóxi são introduzidos na posição C-4. Quando estes compostos são comparados com NI, os espectros de absorção apresentam deslocamentos de 27 e 28 nm e os de emissão de 50,2 e 65,4 nm para o PNI e NNI, respectivamente. Os compostos possuem altos rendimentos quânticos (= 0,50-0,92) em solventes apolares e baixos em solventes polares apróticos e próticos (= 0,12-0,014), que pode ser uma conseqüência da estabilização do estado singlete (S1). A intensidade de emissão dos compostos em dioxano decresce com adição de água, e a supressão ocorre pela combinação estática e dinâmica em conseqüência de uma interação específica soluto-solvente. O caráter doador-retirador dos substituintes foi avaliado por voltametria cíclica, onde os substituintes fenóxi e naftóxi garantem um caráter eletrodoador aos compostos correspondentes (PNI e NNI), uma vez que apresentam, respectivamente, seus potenciais de redução mais negativos - 1,187 e -0,985 V em relação a -0,829 V do NI. Numa tentativa de compreender melhor o comportamento desses substituintes, implementamos cálculos de DFT (Teoria do Funcional de Densidade). Estes sugerem que as propriedades fotofísicas dos compostos podem estar intrinsecamente relacionadas com as suas geometrias podendo assim explicar o deslocamento mais pronunciado na emissão em virtude de um maior grau de conjugação entre o grupo aromático substituinte e a naftalimida. / The compounds N-methyl-1,8-naphthalimide (NI), 4-phenoxy-N-methyl-1,8- naphthalimide (PNI) and 4-naphthoxy-N-methyl-1,8-naphthalimide (NNI) were synthesized and characterized by usually technique for organics compounds. The steady state and time-resolved photophysical properties of the compounds were studied. Both absorption and fluorescence spectra are red-shifted when the electron donor phenoxy and naphthoxy group is introduced at C-4 position. When compared to NI, the spectral shift in acetonitrile for PNI and NNI is 27 and 28 nm for the absorption, and the fluorescence emission is 50,2 and 65,4 nm, respectively. The compounds PNI and NNI show high quantum yields in non-polar aprotic solvents (f = 0,50- 0,92), and low ones in polar non-protic and protic solvents (f = 0,12-0,014), which can be assigned to stabilization of the singlet state (S1). The emission intensity of the PNI and NNI decrease by addition of water to dioxane solution, and the fluorescence quenching occurs by combination of dynamic and static contribution ascribed to specific solute?solvent interaction. The substituent donor-acceptor character has been evaluated by means of cyclical voltammetry showing that PNI in the ground state has a the higher donor character than NNI, since the reduction potentials are -1,187 and - 0,985 V, respectively. As an attempt to comprehend the behavior of these substituents, Density Functional Theory (DFT) calculations were performed. These calculations suggest that the photophysical properties oh the compounds may be intrinsically related to their geometries, thus explaining the more pronounced shift on the emission spectra, as a consequence of a higher degree of conjugation between the substituent aromatic-group and naphthalimide moiety.