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Synthesis, characterization and reactivity of ruthenium porphyrin complexes containing metal-carbon bonds

University of British Columbia

The thesis describes developments in the organometallic chemistry of ruthenium porphyrin complexes, particularly their preparation, characterization and reactivity. Treatment of paramagnetic Ru(rV)(porp)Br₂ species, prepared via the interaction of [Ru(porp)]₂ with HBr in CH₂CI₂, with organolithiums or Grignard reagents yields the corresponding diamagnetic Ru(IV) complexes Ru(porp)R₂, where porp = the 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion(OEP): R = Ph, m-MeC₆H₄, p-MeC₆H₄, p-MeOC₆H₄, p-FC₆H₄, Me, Et; and porp = the 5,10,15,20-tetraphenylporphyrinato dianion(TPP): R = Ph. The spectral analyses (¹H NMR, UV/vis, and mass spectroscopies) of these complexes are fully consistent with the assigned structures, which are further verified by the X-ray crystallographical analyses of Ru(TPP)Br₂ and Ru(OEP)Ph₂. The dimethyl complex Ru(OEP)Me₂ is also formed along with Ru(OEP)(P[sup n]Bu₃)₂ by the reaction of Ru(III)(OEP)(P[sup n]Bu₃)Br and methyllithium, the process involving disproportionation of Ru(OEP)Me(P[sup n]Bu₃). The diamagnetic nature of the Ru(porp)R₂ complexes, as evidenced by their sharp and temperature-independent ¹H NMR chemical shifts, requires that the d⁴-electrons of the Ru(IV) are spin-paired in the lowest, doubly degenerate d[sub xz] and d[sub]yz orbitals; the porphyrin ring current results in upfield shifts for the axial ligand protons with respect to their 'normal' positions. An unusual feature in the Ru(OEP)Ph₂ structure is the considerable distortion of the (Ph)C-Ru-C(Ph) fragment from linearity. The Ru(porp)R₂ complexes are stable as solids and in solution under atmospheric conditions, but the metal-carbon bonds are readily cleaved either by reagents such as protonic acids, carbon monoxide and phosphines, or via thermolysis. A CO insertion product Ru(OEP)Ph(COPh) is observed in solution by ¹H NMR spectroscopy upon the reaction of Ru(OEP)Ph₂ with 1 atm CO in deuterated benzene at room temperature. The anaerobic thermolysis of the Ru(porp)R₂ complexes at 80 - 100°C yields the five-coordinate, low-spin, paramagnetic Ru(porp)R derivatives, and organic products that depend on the nature of the aryl or alkyl moiety R and the solvent The remarkable transformations shown below have been demonstrated, and niechanisms are proposed.[See Thesis for Diagram] The Ru(OEP)Ph complex has been characterized crystallographically and, together with Ru(OEP)Ph₂, these represent the first reported structures involving organoruthenium porphyrins. The temperature-dependent *H NMR shifts for the Ru(OEP)R species establish a single spin state (S = 1/2) over the temperature range studied (-60° to 70°C). Under appropriate conditions in benzene or toluene, the rate-(leternnning step for the thermal decomposition of the Ru(OEP)R₂ species (R = aryl) is the homolytic cleavage of the metal-carbon bond, and the temperature variation data for the rate constant of this step allow for an estimation of the Ru-C bond strength in solution. Such bond energies are critical for a better understanding of homogeneously catalyzed hydrocarbon reactions. Substitution effects on the Ru-aryl bond strengths were studied using four para- and meta-substituted phenyl complexes (p-Me, p-MeO, p-F, and m-Me); the bond energies are in the 29 - 33 kcal/mol range, and a Hammett ρ value of +1.7 describes the rate constant trend for the p-MeO, p-Me and unsubstituted phenyl systems. Because of their ccordinatively unsaturated nature, the Ru(porp)R complexes are very reactive toward reagents. The Ru(OEP)Ph species readily binds a second axial ligand L, such as pyridine or tri-n-butylphosphine, to form a six-coordinate derivative, and temperature variation data for the equilibrium constant for pyridine binding give a solution bond energy of 11.2 kcal/mol for the Ru-N(py) bond. The Ru(OEP)Me species on reaction with L (py or P[sup n]Bu₃) undergoes disproportionation to Ru(OEP)Me₂ and Ru(OEP)L₂. The Ru(OEP)Ph species reacts with carbon monoxide to generate Ru(OEP)(CO)[sub n] (n = 1 or 2). Bromination of Ru(OEP)Ph forms a paramagnetic Ru(IV)(OEP)Ph(Br) intermediate, characterized by ¹H NMR, en route to Ru(OEP)Br₂, while treatment of Ru(OEP)Ph with HX (X = Br, CI) yields the Ru(OEP)X₂ species. Reaction of the Ru(OEP)(X-C₆H₄) complexes (X = H, p-MeO) with in situ-generated phenyl radicals is close to diffusion-controlled (k ≈ (1.4 - 2.0) X 10⁹ M⁻¹h⁻¹ at 60° - 100°C), and leads to the formation of the mixed aryl species Ru(OEP)Ph(p-MeOC₆H₄). Of interest, the photosensitized O₂-oxidation of Ru(porp)R species (R = aryl) yields the μ-oxo dinuclear species [Ru(porp)R]₂O, the metal-carbon bond remaining intact, which is unusual for the interaction of organometallic metalloporphyrins with dioxygen under light.

Text

2010-10-13

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http://hdl.handle.net/2429/29126

Chemistry

University of British Columbia

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