Metalloradical chemistry of rhodium(II) porphyrins : activation of carbon monoxide and methane / Alan Edward Sherry.

Format:

Book

Manuscript

Microformat

Thesis/Dissertation

Author/Creator:

Sherry, Alan Edward.

Contributor:

Wayland, Bradford B., advisor.

University of Pennsylvania.

Language:

English

Subjects (All):

Penn dissertations--Chemistry.

Chemistry--Penn dissertations.

Local Subjects:

Penn dissertations--Chemistry.

Chemistry--Penn dissertations.

Physical Description:

xix, 239 leaves : illustrations ; 29 cm

Production:

1990.

Summary:

This dissertation reports on the use of Rh(II) d$\sp7$ metalloradical complexes as vehicles for unprecedented chemical transformations including selective reductive coupling of carbon monoxide and selective activation of methane in benzene solvent. The broad scope of organometallic chemistry exhibited by the Rh(II) porphyrins complexes results from weak or non-existent M-M bonding and strong M-C bonds which produce favorable thermodynamics for these unusual reactions. The methane reaction with (TMP)Rh(II) is proposed to occur through a linear 4-centered transition state.

The rhodium-rhodium bonded complex of the bulky tetra (3,5-dimethylphenyl) porphyrin, ((TXP)Rh) $\sb2$, accomplishes selective reductive coupling of carbon monoxide at mild conditions (P$\sb{\rm CO}\sim$ 1 atm, T = 298 K). Reactivity studies using the stable metalloradical tetramesitylporphyrin rhodium(II), (TMP)Rh(II), have been used in elucidating the nature of carbon monoxide reductive coupling reaction. Addition of CO to (TMP)Rh(II) produces a unique 17 e$\sp-$ metallo-organic radical, (TMP)Rh$\cdot$CO, which dimerizes by C-C bond coupling. Analysis of the EPR parameters for (TMP)Rh$\cdot$CO reveals the presence of a bent M-CO fragment with substantial spin density at the carbonyl carbon which facilitates the coupling reaction.(UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\eqalign{\rm(TMP)Rh{\cdot} + CO&\rightleftharpoons \rm (TMP)Rh{\cdot} CO\cr\rm 2\ (TMP)Rh{\cdot} CO&\rightleftharpoons\rm (TMP)Rh{-}C(O){-}C(O){-}Rh(TMP)\cr}$$(TABLE/EQUATION ENDS)Rhodium porphyrin complexes are presently unique in achieving reductive coupling of CO to form $\alpha$-diketone complexes (M-C(O)-C(O)-M).

(TMP)Rh$\cdot$ is observed to react selectively with methane in benzene solvent and this result constitutes the first example of methane activation by a metalloradical species.$$\rm 2\ (TMP)Rh{\cdot} + CH\sb4 \rightleftharpoons (TMP)Rh{-}CH\sb3 + (TMP)Rh{-}H$$The reaction rate law and activation parameters (Rate = $\rm k\sb{f} \lbrack Rh\rbrack\sp2\ \lbrack CH\sb4\rbrack, \Delta H\sp\ddagger$ = 7.5 $\pm$ 1.0 kcal mol$\sp{-1}$, $\Delta$S$\sp\ddagger$ = $-$37 cal mol$\sp{-1}$ K$\sp{-1}$, k$\sb{\rm H}$/k$\sb{\rm D}$ (353 K) = 8 $\pm$ 1) are consistent with a linear four-centered transition state involving interaction of two metalloradicals with the methane substrate.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)

Metalloradical reaction mechanisms that involve multicentered interactions have also been invoked in reactions of (TMP)Rh(II) with H$\sb2$, CH$\sb3$NC, 2,6-xylyl isocyanide, P(Et)$\sb3$, and PPh$\sb3$: substrates which can achieve n-centered transition states with two metalloradicals react to produce thermodynamic products while those which cannot, exist as kinetically trapped intermediates.

Notes:

Supervisor: Bradford B. Wayland.

Thesis (Ph.D. in Chemistry) -- Graduate School of Arts and Sciences, University of Pennsylvania, 1990.

Includes bibliography.

Local Notes:

University Microfilms order no.: 91-01208.

OCLC:

187448243