HETEROLYTIC ACTIVATION OF DIHYDROGEN AT TRANSITION-METAL SULFUR SITESIN COORDINATIVELY UNSATURATED [RH(L)(S-BU(4))]BF4 COMPLEXES, INVOLVING NEUTRAL HYDRIDES, THIOL HYDRIDES, AND THIOL-HYDRIDE PROTON SCRAMBLING (L = CO, PCY3 - S-BU(4)(2-) = 2-MERCAPTO-3,5-DI-TERT-BUTYLPHENYLTHIO]ETHANE(2-))

[Rh(H)(L)(''S-bu(4)'')] complexes (L = CO (1), PCy3 (2); ''S-bu(4)''(2 -) = 1,2- -mercapto-3,5-di-tert-butylphenylthio)ethane(2-)]) catalyze the D-2/H+ exchange between D-2 and EtOH protons in the presence of ca talytic amounts of Bronsted acids. A mechanism and complete cycle for the heterolytic D-2 cleavage are proposed that are based on characteri zation of key intermediates and monitoring of key reactions. The key i ntermediates are the thiol hydride complexes [Rh(H)(L)(''S-bu(4)''-H)] BF4, L = CO (3), PCy, (4), the coordinatively unsaturated complexes [R h(L)(''S-bu(4)'')]BF4, L = CO (5), PCy3 (6), which are the actual cata lysts, and the deuterium-labeled derivatives of 1-4, Complexes 3 and 4 form from 1 and 2 by protonation with HBF4, and they release H-2 to g ive 5 and 6. Complex 5 dimerizes in the solid state and was characteri zed by X-ray structure determination of 5.8 CH2Cl2 (triclinic space gr oup <P(1)over bar> a = 1048.2(4) pm, b = 1430.0(5) pm, c = 1785.7(7) p m, alpha = 100.49 (3)degrees, beta = 102.92(3)degrees; gamma = 103.68( 3)degrees, Z = 1). Complex 6 is mononuclear and adds H2O or THF revers ibly to give the highly labile [Rh(L)(PCy3)(''S-bu(4)'')]BF4, L = H2O (7), THF (8), CO is irreversibly added to give the stable [Rh(CO)(PCy3 )(''S-bu(4)'')]BF4 (9), whose high-frequency v(CO) (2081 cm(-1)) indic ates a relatively low electron density at the Rh center, Complex 6 als o adds to H-2 to give 4, which can be deprotonated by solid Na2CO3 or H2O to yield neutral 2. H-1 NMR and H-2 NMR spectroscopy revealed the scrambling of thiol protons and hydride ligands in 3 and 3 and its deu terium-labeled derivatives. This exchange of thiol protons for hydride ligands is explained by a transient [Rh(eta(2)-H-2)] species. Low-tem perature H-1/H-2 NMR spectroscopy showed that protonation of 2 yields four diastereomers of 4 resulting from protonation of the four stereoc hemically nonequivalent lone pairs at the thiolate donors of 2. The re levance of these Findings to H-2 activation at transition-metal sulfur sites in hydrogenases or hydrotreatment catalysts, and differences fr om the H-2 cleavage achieved with other complexes not containing ''bui lt-in'' Bronsted-basic centers, are discussed.