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|Title:||Synthesis and characterization of osmium dihydrogen and ruthenium indenyl complexes|
Hong Kong Polytechnic University -- Dissertations
|Publisher:||The Hong Kong Polytechnic University|
|Abstract:||The pentacoordinated complex Os(PPh₃)₃Cl₂ reacts with KTp in isopropanol at room temperature to give TpOsCl(PPh₃)₂ (2), which reacts with dppe and dppp in dioxane to give TpOsCl(dppe) 3 and TpOsCl(dppp) 4, respectively. The X-ray structure of compound 2 has been determined and it shows that the Tp ligand is coordinated in tridentate form. The compound crystallized in the monoclince space group with a = 10.328(0), b = 24.957(5), c= 17.321(0)Å, β=92.81° and z= 4. The structure has been refined to R = 0.036, R' = 0.046. Protonation of TpOsH(L₂) (L₂ = (PPh₃)₂ 6, dppe 7, dppp 8) with HBF₄ produce the corresponding dihydrogen complexes [TpOs(n²-H₂)(L₂)]BF₄(L₂ = (PPh₃)₂ 9,dppe 10, dppp11). The pseudo-aqueous pKa values of 9 - 11 have been determined in dichloromethane, and they indicate that [TpOs(n²-H₂)(L₂)]BF₄ are more acidic than the Cp analogues trans-[CpOsH₂(L₂)]BF₄. Protonation of the n⁵-indenyl ruthenium complexes (n⁵-C₉H₇)RuH(L₂); [L₂ = dppm (16), dppe (17), dppp (18 PPh₃)(19)] in CD₂Cl₂ at - 60 ℃ with trifluoromethanesulfonic acid leads to the formation of the dihydrogen (n²-H₂) complexes or trans-dihydride monocations or mixture of both depending on the size of the chelating rings of the phosphine ligands. ¹H NMR study of the protonation of 16 - 19 with HBF⁴ revealed the initial formation of the dihydrogen complexes [(n⁵-C₉H₇)Ru(n²-H₂)(L₂)]BF₄ (L₂ = dppm 20, dppe 21, dppp 23) or dihydride complexes [n⁵-C₉H₇]RuH₂(L₂)]BF₄ ; (L₂ = dppe 22, dppp 24, (PPh₃)₂ 25)which slowly transformed irreversibly into [(n⁶-C₉H₈)RuH(L₂)]⁺ (26-29). Proton abstraction from (26-29) with triethylamine in dichloromethane regenerated (16-19).|
The reactivity of (n⁵-C₉H₇)RuH(dppm) 16 has been examined for its ability to activate C-H bonds. The complex activates aromatic C-H bonds of common solvent like benzene, toluene, chlorobenzene, nitrobenzene and pyridine under thermal condition, as probed by catalytic H/D exchange between H2 and the deuterated solvents; the C-H activation is further evidenced by the H/D exchange among H₂, D₂O, and the non-deuterated solvents. Mechanisms involving σ-metathesis steps are proposed to account for the H/D exchange. It is found that the electron-withdrawing substituents on the aromatic rings enhance the H/D exchange reactions between D₂O and the aromatic solvents in the D₂O/H₂/solvent systems. This is in accord with increased acidity of the aryl hydrogens with electron-withdrawing substituents. The fD values (fD = fraction of D in the organic solvents that undergo C-H activation) of mono-substituted benzenes increase in the order NH₂<CH₃<H~Cl<NO₂ for the 16 catalyzed H/D exchange reactions, this trend is in consonance with the electron withdrawing ability of the substituents on the aromatic rings. Selectivity in C-H activation is observed in benzene derivatives. Lack of H/D exchange in the ortho positions of toluene, chlorobenzene, nitrobenzene is attributable to steric hindrance of the substituents; and the high ortho selectivity in pyridine is due to the neighboring group participation effect. In addition, 16 is found to be an effective catalyst for the hydration of benzonitrile under thermal condition. A mechanism involving external and internal attack of the carbon atom of the benzonitrile is proposed.
|Description:||x, 180 leaves : ill. ; 30 cm.|
PolyU Library Call No.: [THS] LG51 .H577M ABCT 1999 Hung
|Rights:||All rights reserved.|
|Appears in Collections:||Thesis|
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