Reactions of oxygen containing ligands in the coordination sphere of permethylhafnocene and permethyltantalocene

<p>A series of terl-butylperoxide complexes of hafnium, Cp*₂Hf(R)(OOCMe₃) (Cp* = ((ηp>5p>-C₅Me₅); R = Cl, H, CH<sub>3, CH<sub>2CH<sub>3, CH<sub>2CH<sub>2CH<sub>3, CH<sub>2CH<sub>2CH<sub>2CH<sub>3, CH<sub>2CHMe₂, CH=CHCMe₃, C₆H<sub>5, meta-C₆H<sub>3(CH<sub>2)2) and Cp*(ηp>5p>-C₅(CH<sub>3)₄CH<sub>2CH<sub>2CH<sub>2)Hf(OOCMe₃), has been synthesized. One example has been structurally characterized, Cp*₂Hf(OOCMe₃)CH<sub>2CH<sub>3 crystallizes in space group P2₁/c, with a = 19.890(7)Ã…, b = 8.746(4)Ã…, c = 17.532(6)Ã…, β = 124.987(24)°, V = 2498(2)Ã…p>3p>, Z = 4 and R_F = 0.054 (2222 reflections, I > 0). Despite the coordinative unsaturation of the hafnium center, the terl-butylperoxide ligand is coordinated in a mono-dentate ligand. The mode of decomposition of these species is highly dependent on the substituent R. For R = H, CH<sub>2CH<sub>3, CH<sub>2CH<sub>2CH<sub>3, CH<sub>2CH<sub>2CH<sub>2CH<sub>3, CH<sub>2CHMe₂ a clean first order conversion to Cp*₂Hf(OCMe₃)(OR) is observed (for R CH<sub>2CH<sub>3, ΔHÇ‚ = 19.6 kcal•molp>-1p>, ΔSÇ‚ = -13 e.u.). These results are discussed in terms of a two step mechanism involving ηp>2p>-coordination of the terl-butylperoxide ligand. Homolytic O-O bond cleavage is observed upon heating of Cp*₂Hf(OOCMe₃) R (R = C₆H<sub>6, meta-C₆H<sub>3(CH<sub>3)₂). In the presence of excess 9,10-dihydroanthracene thermolysis of Cp*₂Hf(OOCMe₃)C₆H<sub>6 cleanly affords Cp*₂Hf(C₆H<sub>6)OH and HOCMe₃ (ΔHÇ‚ = 22.6 kcal•molp>-1p>, ΔSÇ‚ = -9 e.u.). The O-O bond strength in these complexes is thus estimated to be 22 kcal•molp>-1p>.p> <p>Cp*₂Ta(CH<sub>2)H, Cp*₂Ta(CHC₆H<sub>5)H, Cp*₂Ta(C₆H<sub>4)H, Cp*₂Ta(CH<sub>2=CH<sub>2)H and Cp*₂Ta(CH<sub>2=CHMe)H react, presumably through Cp*₂Ta-R intermediates, with H<sub>2O to give Cp*₂Ta(O)H and alkane. Cp*₂Ta(O)H was structurally characterized: space group P2₁/n, a= 13.073(3)Ã…, b = 19.337(4)Ã…, c = 16.002(3)Ã…, β = 108.66(2)°, V = 3832(1)Ã…p>3p>, Z = 8 and R_F = 0.0672 (6730 reflections). Reaction of terlbutylhydroperoxide with these same starting materials ultimately yields Cp*₂Ta(O)R and HOCMe₃. Cp*₂Ta(CH<sub>2=CHR)OH species are proposed as intermediates in the olefin hydride reactions. Cp*₂Ta(O₂)R species can be generated from the reaction of the same starting materials and O₂. Lewis acids have been shown to promote oxygen insertion in these complexes.p>

The Design and Synthesis of Transition Metal Complexes Supported by Non-innocent Ligand Scaffolds for Small Molecule Activation

<p>This dissertation focuses on the incorporation of non-innocent or multifunctional moieties into different ligand scaffolds to support one or multiple metal centers in close proximity. Chapter 2 focuses on the initial efforts to synthesize hetero- or homometallic tri- or dinuclear metal carbonyl complexes supported by para-terphenyl diphosphine ligands. A series of [M₂M’(CO)₄]-type clusters (M = Ni, Pd; M’ = Fe, Co) could be accessed and used to relate the metal composition to the properties of the complexes. During these studies it was also found that non-innocent behavior was observed in dinuclear Fe complexes that result from changes in oxidation state of the cluster. These studies led to efforts to rationally incorporate central arene moieties capable managing both protons and electrons during small molecule activation.p> <p>Chapter 3 discusses the synthesis of metal complexes supported by a novel para-terphenyl diphosphine ligand containing a non-innocent 1,4-hydroquinone moiety as the central arene. A Pdp>0p>-hydroquinone complex was found to mediate the activation of a variety of small molecules to form the corresponding Pdp>0p>-quinone complexes in a formal two proton ⁄ two electron transformation. Mechanistic investigations of dioxygen activation revealed a metal-first activation process followed by subsequent proton and electron transfer from the ligand. These studies revealed the capacity of the central arene substituent to serve as a reservoir for a formal equivalent of dihydrogen, although the stability of the M-quinone compounds prevented access to the PdII-quinone oxidation state, thus hindering of small molecule transformations requiring more than two electrons per equivalent of metal complex.p> <p>Chapter 4 discusses the synthesis of metal complexes supported by a ligand containing a 3,5-substituted pyridine moiety as the linker separating the phenylene phosphine donors. Nickel and palladium complexes supported by this ligand were found to tolerate a wide variety of pyridine nitrogen-coordinated electrophiles which were found to alter central pyridine electronics, and therefore metal-pyridine Ï€-system interactions, substantially. Furthermore, nickel complexes supported by this ligand were found to activate H-B and H-Si bonds and formally hydroborate and hydrosilylate the central pyridine ring. These systems highlight the potential use of pyridine Ï€-system-coordinated metal complexes to reversibly store reducing equivalents within the ligand framework in a manner akin to the previously discussed 1,4-hydroquinone diphosphine ligand scaffold.p> <p>Chapter 5 departs from the phosphine-based chemistry and instead focuses on the incorporation of hydrogen bonding networks into the secondary coordination sphere of [Fe₄(μ₄-O)]-type clusters supported by various pyrazolate ligands. The aim of this project is to stabilize reactive oxygenic species, such as oxos, to study their spectroscopy and reactivity in the context of complicated multimetallic clusters. Herein is reported this synthesis and electrochemical and Mössbauer characterization of a series of chloride clusters have been synthesized using parent pyrazolate and a 3-aminophenyl substituted pyrazolate ligand. Efforts to rationally access hydroxo and oxo clusters from these chloride precursors represents ongoing work that will continue in the group.p> <p>Appendix A discusses attempts to access [Fe₃Ni]-type clusters as models of the enzymatic active site of [NiFe] carbon monoxide dehydrogenase. Efforts to construct tetranuclear clusters with an interstitial sulfide proved unsuccessful, although a (μ₃-S) ligand could be installed through non-oxidative routes into triiron clusters. While [Fe₃Ni(μ₄-O)]-type clusters could be assembled, accessing an open heterobimetallic edge site proved challenging, thus prohibiting efforts to study chemical transformations, such as hydroxide attack onto carbon monoxide or carbon dioxide coordination, relevant to the native enzyme. Appendix B discusses the attempts to synthesize models of the full H-cluster of [FeFe]-hydrogenase using a bioinorganic approach. A synthetic peptide containing three cysteine donors was successfully synthesized and found to chelate a preformed synthetic [Fe₄S₄] cluster. However, efforts to incorporate the diiron subsite model complex proved challenging as the planned thioester exchange reaction was found to non-selectively acetylate the peptide backbone, thus preventing the construction of the full six-iron cluster.p>

Yang-Mills theory in six-dimensional superspace

<p>The superspace approach provides a manifestly supersymmetric formulation of supersymmetric theories. For N= 1 supersymmetry one can use either constrained or unconstrained superfields for such a formulation. Only the unconstrained formulation is suitable for quantum calculations. Until now, all interacting N>1 theories have been written using constrained superfields. No solutions of the nonlinear constraint equations were known.p> <p>In this work, we first review the superspace approach and its relation to conventional component methods. The difference between constrained and unconstrained formulations is explained, and the origin of the nonlinear constraints in supersymmetric gauge theories is discussed. It is then shown that these nonlinear constraint equations can be solved by transforming them into linear equations. The method is shown to work for N=1 Yang-Mills theory in four dimensions. p> <p>N=2 Yang-Mills theory is formulated in constrained form in six-dimensional superspace, which can be dimensionally reduced to four-dimensional N=2 extended superspace. We construct a superfield calculus for six-dimensional superspace, and show that known matter multiplets can be described very simply. Our method for solving constraints is then applied to the constrained N=2 Yang-Mills theory, and we obtain an explicit solution in terms of an unconstrained superfield. The solution of the constraints can easily be expanded in powers of the unconstrained superfield, and a similar expansion of the action is also given. A background-field expansion is provided for any gauge theory in which the constraints can be solved by our methods. Some implications of this for superspace gauge theories are briefly discussed. p>

Electron transfer in chemically and genetically modified myoglobins

<p>The temperature dependences of the reduction potentials (Ep>o'p>) of wildtype human myoglobin (Mb) and three site-directed mutants have been measured by using thin-layer spectroelectrochemistry. Residue Val68, which is in van der Waals contact with the heme in Mb, has been replaced by Glu, Asp, and Asn. At pH 7.0, reduction of the heme iron (III) in the former two proteins is accompanied by uptake of a proton by the protein. The changes in Ep>o'p>, and the standard entropy (ΔSp>o'p>) and enthalpy (ΔH<sup>o'p>) of reduction in the mutant proteins were determined relative to values for wild-type; the change in Ep>o'p> at 25°C was about -200 millivolts for the Glu and Asp mutants, and about -80 millivolts for the Asn mutant. Reduction of Fe(III) to Fe(II) in the Glu and Asp mutants is accompanied by uptake of a proton. These studies demonstrate that Mb can tolerate substitution of a buried hydrophobic group by potentially charged and polar residues, and that such amino acid replacements can lead to substantial changes in the redox thermodynamics of the protein.p> <p>Through analysis of the temperature dependence and shapes of NMR dispersion signals, it is determined that a water molecule is bound to the sixth coordination site of the ferric heme in the Val68Asp and in the Val68Asn recombinant proteins while the carboxyl group of the sidechain of Glu68 occupies this position in Val68Glu. The relative rhombic distortions in the ESR spectra of these mutant proteins combined with H<sub>2p>17p>O and spin interconversion experiments performed on them confirm the conclusions of the NMRD study.p> <p>The rates of intramolecular electron transfer (ET) of (NH<sub>3)₅Ru-His48 (Val68Asp, His81GIn, Cys110AIa)Mb and (NH<sub>3)₅Ru-His48 (Val68GIu,His81GIn,Cys110Ala)Mb were measured to be .85(3)sp>-1p> and .30(2)sp>-1p>, respectively. This data supports the hypothesis that entropy of 111 reduction and reorganization energy of ET are inversely related. The rates of forward and reverse ET for (NH<sub>3)₅ Ru-His48 (Val68GIu, His81 GIn, Cys110AIa)ZnMb -7.2(5)•10p>4p>sp>-1p>and 1.4(2)•10p>5p>sp>-1p>, respectively- demonstrate that the placement of a highly polar residue nearby does not significantly change the reorganization energy of the photoactive Zn porphyrin.p> <p>The distal histidine imidazoles of (NH<sub>3)₄isnRu-His48 SWMb and (NH<sub>3)₅Ru-His48 SWMb were cyanated with BrCN. The intramolecular ET rates of these BrCN-modified Mb derivatives are 5.5(6)sp>-1p> and 3.2(5)sp>-1p>, respectively. These respective rates are 20 and 10 times faster than those of their noncyanated counterparts after the differences in ET rate from driving force are scaled according to the Marcus equation. This increase in ET rate of the cyanated Mb derivatives is attributed to lower reorganization energy since the cyanated Mb heme is pentacoordinate in both oxidation states; whereas, the native Mb heme loses a water molecule upon reduction so that it changes from six to five coordinate. The reorganization energy from Fe-OH<sub>2 dissociation is estimated to be .2eV. This conclusion is used to reconcile data from previous experiments in our lab. ET in photoactive porphyrin-substituted myoglobins proceed faster than predicted by Marcus Theory when it is assumed that the only difference in ET parameters between photoactive porphyrins and native heme systems is driving force. However, the data can be consistently fit to Marcus Theory if one corrects for the smaller reorganization in the photoactive porphyrin systems since they do not undergo a coordination change upon ET.p> <p>Finally, the intramolecular ET rate of (NH<sub>3)₄isnRu-His48 SWMb was measured to be 3.0(4)sp>-1p>. This rate is within experimental error of that for (NH<sub>3)₄pyrRu-His48 SWMb even though the former has 80mV more driving force. One likely possibility for this observation is that the tetraamminepyridineruthenium group undergoes less reorganization upon ET than the tetraammineisonicotinamideruthenium group. Moreover, analysis of the (NH<sub>3)₄isnRu-His48 SWMb experimental system gives a likely explanation of why ET was not observed previously in (NH<sub>3)₄isnRu-Cytochrome C.p>