ISSN:
0947-6539
Keywords:
dihydrogen activation
;
heterolytic cleavage
;
hydrido complexes
;
rhodium
;
S ligands
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
[Rh(H)(L)(“buS4”)] complexes (L = CO (1), PCy3 (2); “buS4”2- = 1,2-bis[(2-mercapto-3,5-di-tert-butylphenylthio)ethane2-]) catalyze the D2/H+ exchange between D2 and EtOH protons in the presence of catalytic amounts of Brønsted acids. A mechanism and complete cycle for the heterolytic D2 cleavage are proposed that are based on characterization of key intermediates and monitoring of key reactions. The key intermediates are the thiol hydride complexes [Rh(H)(L)(“buS4”-H)]BF4, L = CO (3), PCy3 (4), the coordinatively unsaturated complexes [Rh(L)(“buS4”)]BF4, L = CO (5), PCy3 (6), which are the actual catalysts, and the deuterium-labeled derivatives of 1-4. Complexes 3 and 4 form from 1 and 2 by protonation with HBF4, and they release H2 to give 5 and 6. Complex 5 dimerizes in the solid state and was characterized by X-ray structure determination of 5·8CH2Cl2 (triclinic space group P\documentclass{article}\pagestyle{empty}\begin{document}$ \bar 1 $\end{document}, a = 1048.2(4) pm, b = 1430.0(5) pm, c = 1785.7(7) pm, α = 100.49(3)°, β = 102.92(3)°; γ = 103.68(3)°, Z = 1). Complex 6 is mononuclear and adds H2O or THF reversibly to give the highly labile [Rh(L)(PCy3)(“buS4”)]BF4, L = H2O (7), THF (8). CO is irreversibly added to give the stable [Rh(CO)-(PCy3)(“buS4”)]BF4 (9), whose high-frequency ν(CO) (2081 cm-1) indicates a relatively low electron density at the Rh center. Complex 6 also adds to H2 to give 4, which can be deprotonated by solid Na2CO3 or H2O to yield neutral 2. 1H NMR and 2H NMR spectroscopy revealed the scrambling of thiol protons and hydride ligands in 3 and 4 and its deuterium-labeled derivatives. This exchange of thiol protons for hydride ligands is explained by a transient [Rh(η2-H2)] species. Low-temperature 1H/2H NMR spectroscopy showed that protonation of 2 yields four diastereomers of 4 resulting from protonation of the four stereochemically nonequivalent lone pairs at the thiolate donors of 2. The relevance of these findings to H2 activation at transitionmetal sulfur sites in hydrogenases or hydrotreatment catalysts, and differences from the H2 cleavage achieved with other complexes not containing “built-in” Brønsted-basic centers, are discussed.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/chem.19970031224
