Chapter 10
Thermodynamic Studies of the Hydrogenation and Reductive Coupling of Carbon Monoxide by Rhodium(II) Porphyrins
Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: June 25, 1990 | doi: 10.1021/bk-1990-0428.ch010
Bradford B. Wayland, Virginia L. Coffin, Alan E. Sherry, and William R. Brennen Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323
Octaethylporphyrin rhodium II dimer, [(OEP)Rh]2, reacts with H and CO to produce an equilibrium distribution of hydride and formyl complexes (Equations 1-3). Thermodynamic and kinetic measurements for this system have 2
1,2
2 (OEP)Rh-H [(OEP)Rh] + H 2 (OEP)Rh-H + 2 CO 2 (OEP)Rh-CHO [(OEP)Rh] + 2 CO + H 2 (OEP)Rh-CHO 2
(1) (2) (3)
2
2
2
provided estimates for the effective Rh-Rh, Rh-H and Rh-C bond energies: (OEP)Rh-Rh(OEP) (~16.0 kcal mol ), (OEP)Rh-H (~62 kcal mol ), (OEP)Rh-CHO (~58 kcal mol ). Approximate bond energy-thermochemical relationships for organometallic reactions (Table I) have subsequently been used in anticipating new types of reactivity for the (OEP) Rh system and in guiding our efforts to modify r e a c t i v i t y through ligand steric and electronic effects. Reductive coupling of CO to form dimetal 1,2-ethanedione complexes and hydrocarbon reactivity are examples of anticipated [(OEP)Rh]2 reactivity. Ligand modifications suggested by thermodynamic and mechanistic considerations have been effectively used in achieving selectivity for CO reductive coupling and methane a c t i v a t i o n . This article summarizes the thermodynamic measurements and reasoning used in organizing and interpreting reactivity studies of rhodium macrocycles relevant to CO reduction. -1
-1
-1
3,4
5
6
7,8
9
0097-6156/90A)428-O148$06.00A) © 1990 American Chemical Society Marks; Bonding Energetics in Organometallic Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
10. WAYLAND ET AL.
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Hydrogénation and Reductive Coupling of CO
Table I: Thermochemical Estimates for Selected Reactions Relevant to the Hydrogénation and Reductive Coupling of CO (Continued on next page) (I)a,b ~ΔΗ° k c a l mol"
Reaction
C
1
(II) ~Δ£> (298K)112
Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: June 25, 1990 | doi: 10.1021/bk-1990-0428.ch010
Metalloformyls d. e. f. g.
M-H + CO H C - ( 0 ) C - C ( 0 ) C H ) . D i m e r i z a t i o n o f (TMP)Rh-CO must involve substantially larger electronic and s t r u c t u r a l r e a r r a n g e m e n t o f t h e Rh-CO u n i t compared t o that for CH3CO because the association energy 1
3
3
8
3
Marks; Bonding Energetics in Organometallic Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
10.
WAYLAND ET A L .
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Hydrogénation and Reductive Coupling of CO
Table I I I : Selected NMR and IR Parameters f o r the Formyl Group i n (por)Rh-CHO Complexes
2.82 194.4 29.1 200 1.8 1707 cm"
m
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