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Reactions of small molecules with rhodium porphyrin complexes

University of British Columbia

The reaction of Rh(OEP)(H) (OEP - 2,3 J,8,12,13,17,18-octaemylporphyrin) with O₂ in organic solvents was examined by visible, proton and phosphorus nuclear magnetic resonance spectroscopy (nmr) as a candidate system for the net insertion of O₂ into the metal-hydride bond. The nmr data for a thermally unstable Rh[superscript]Ⅲ(OEP)(OOHXH₂0) mtennediate were obtained at -40 °C in CD₂C1₂- Addition of O₂ to CH₂CI₂ solutions of Rh(OEP)(H) with triphenylphosphine resulted in reaction with solvent forming Rh(OEP)(Cl)(PPh₃) and Rh(OEP)(CH₂Cl)(PPh₃) as the major species (in equimolar amounts), without concomitant formation of OPPh₃. The sensitivity of the reaction of Rh(OEP)(H) with O₂ to initial conditions, and the observation of products containing Rh (Ⅱ) and Rh-CH₂Cl, suggest a radical mechanism. Initiation/tenniriation: 2Rh(OEP)(H) ⇌ [Rh(OEP)]₂ + H₂ [Rh(OEP)]₂ ⇌ 2Rh(OEP)- and in the presence of PPh₃ [Rh(OEP)]₂+ PPh₃ ⇌ -Rh(OEP)(PPh₃) + Rh(OEP)- Propagation: Rh(OEP)- + O₂ ⇌ Rh(OEP)(O₂) Rh(OEP)( O₂) + Rh(OEP)(H) → Rh(OEP)(OOH) + Rh(OEP)- Teimination: Rh(OEP)(OOH) + Rh(OEP)(H) →[Rh(OEP)]₂ + H₂O +½O₂ and in the presence of PPh₃ •Rh(OEP)(PPh₃) + CH₂CI₂ ⇌ Rh(OEP)(Cl)(PPh₃) + ∙CH₂C1 Rh(OEP)∙ + ∙CH₂C1 ⇌Rh(OEP)(CH₂Cl) The reaction of Et₃N with Rh(por)(a)(L) (por = OEP or 5,10,15,20-tetramesitylporphyrin(TMP), L = H₂O, (CH₃)₂CHOH, CH₃CN) gave products, identified by visible (scanning and stopped-flow) and proton nmr spectroscopy to be Rh(por)(CH(Me)NEt₂), Et₂NH as Rh(por)(Cl)(Et₂NH), and Rh(por)(CH₂CHO). This indicated amine C-H and C-N cleavage and net one-electron reduction of Rh(Ⅲ) to Rh(Ⅱ) species ([Rh(OEP)]₂). Of other amines examined, aniline, N-methyl-diphenylamine and N-ethyl piperidine all formed complexes of the type Rh(OEP)(Cl)(amine). Proton Sponge and tributylamine with Rh(OEP)(Cl)(H₂O) give [Rh(OEP)]₂ as a major product under anaerobic conditions. Only the mechanism of the Et₃N system was examined in detail. Stopped-flow data indicated that Et₃N initially replaced L in the Rh[superscript]Ⅲ(por)(Cl)(L) species to form a Rh[superscript]Ⅲ(por)(Cl)(Et₃N) complex. An equilibrium involving reduction of Rh[superscript]Ⅲ to Rh[superscript]Ⅱ resulted and, in aromatic solvents under anaerobic conditions, slow reaction of Rh[superscript]Ⅱ(por) with Et₃N occurred to give Rh(por)(CH(Me)NEt₃). In CH₂CI₂, reaction of Et₃N with Rh(por)(Cl)(L) was fast, giving a mixture principally of Rh(por)(CH₂Cl)(amine) and the ɳ¹-ylidic enamine complex, Rh(por)(Cl)(CH₂CH=NEt₂) with minor mounts of Rh(por)(CH(Me)NEt₃). The Rh(por)(Cl)(CH₂CH=NEt₂) and Rh(por)(CH(Me)NEt₃) species were unstable, both ultimately giving Rh[superscript]Ⅲ(por)(Cl)(Et₂NH). The synthesis, along with characterization by visible, proton and phosphorus nmr spectroscopy, of the previously structurally characterised, mono-phosphine complex Rh(OEP)(Cl)(PPh₃) are presented. Additionally, spectroscopic evidence is presented for solution equilibria of Rh(OEP)(Cl)(PPh₃) involving formation of Rh(OEP)(L)₂+ (L = PPh₃, P[superscript]nBu₃) via thermal reactions, and Rh(OEP)(Cl)(L') (L'= CO, THF, CH₃CN) via photochemical processes. Thermodynamic data (ΔH⁰ = -33 ± 2 kJ mol⁻¹ and ΔS⁰ = -146 ±8 J K⁻¹mol⁻¹) have been estimated for formation of the bis(triphenylphospnine) species in CH₂CI₂: Rh(OEP)(Cl)(PPh₃) + PPh₃ [symbol omitted] [Rh(OEP)(PPh₃)₂]+ + Cl- Some complexes of Rh[superscript]Ⅲ(OEP) containing l,4-bis(diphenylphosphino)butane (dppb), l,2-bis((diphenylphospliino)ethane (dppe), bis(diphenylphosphino)methane (dppm) and 1,3-bis(diphenylphosphho)propane (dppp) have been characterized in situ using visible, proton and phosphorus nmr spectroscopy. The diphosphines (PP) are bound to Rh(Ⅲ) porphyrins in a monodentate dangling fashion (Rh[superscript]Ⅲ(OEP)(Cl)(PP) and [Rh[superscript]Ⅲ(OEP)(PP)₂]+), or a bridging mode between two metal centers ([Rh[superscript]Ⅲ(OEP)(Cl)]-μPP-[Rh[superscript]Ⅲ(OEP)(Cl)]).

Text

2011-02-10

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http://hdl.handle.net/2429/31158

Chemistry

University of British Columbia

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