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Substrate Uptake and Activation by Dimolybdenum and Ditungsten Hexaalkoxides–Factors Influencing the Cleavage of C–X Multiple Bonds in the Reactions Between [Mo 2 (OR) 6 ] (R t Bu or CH 2 t Bu) and Ar 2 CS, Et 2 NCN, and P( n Bu) 3
Author(s) -
Budzichowski T. A.,
Chisholm M. H.,
Folting K.
Publication year - 1996
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19960020119
Subject(s) - chemistry , adduct , aryl , toluene , bond cleavage , cleavage (geology) , stereochemistry , medicinal chemistry , crystallography , catalysis , alkyl , organic chemistry , materials science , fracture (geology) , composite material
The reaction between diarylthiones and [Mo 2 (OCH 2 t Bu) 6 ] in hydrocarbon solvents yielded [Mo 2 (OCH 2 t Bu) 6 ‐(μ‐S)(CAr 2 )] by cleavage of the CS double bond. The Lewis base adduct [Mo 2 (OCH 2 t Bu) 6 (μ‐S) (CPh 2 ) (PMe 3 )] ( 2 ) has been crystallographically characterized; it contains six‐ and five‐coordinate Mo atoms linked through μ‐S and μ‐OR groups. The rate of the cleavage of the CS bond has been studied by variable‐temperature 1 H NMR in [D 8 ]toluene. A Hammett plot shows that both electron‐donating and electron‐releasing substituents in the aryl groups enhance the rate relative to Ph 2 CS. The activation parameters for cleavage of the CS bonds in Ph 2 CS, ( p ‐MeOC 6 H 4 ) 2 CS, and ( m ‐CF 3 C 6 H 4 ) 2 CS exhibit essentially identical values for Δ S ‡ , while Δ H ‡ is somewhat smaller for the thiones bearing electron‐donating or ‐withdrawing substituents. We also studied the reversible binding of Et 2 NCN to [Mo 2 (O t Bu) 6 ] as a model system wherein the CN bond is reduced but not cleaved and the (MM) 6+ center oxidized. Finally, the reversible binding of P( n Bu) 3 to [Mo 2 (OCH 2 t Bu) 6 ], wherein the product exhibits no net oxidation of the metal centers, has been similarly studied. The activation parameters for these three classes of reactions are compared with previous studies of the reversible binding of CN − to [M 2 (OR) 6 ] compounds and the reaction 2[W 2 (O i Pr) 6 ] → [W 4 (O i Pr) 12 ]. We propose that the activation parameters associated with CS cleavage are associated with the formation of a μ‐η 1 ,η 2 ‐SCAr 2 reactive complex similar to others described elsewhere, [23] and that they result from a) the bimolecular nature of the reaction and b) reorganizational energy. The greater facility of CX multiple‐bond cleavage in reactions involving [W 2 (OR) 6 ] compounds relative to [Mo 2 (OR) 6 ] compounds, the substituent effects, and the dependence on R are discussed in terms of the orbital energy match between the MM and CX π and π * orbitals, and an analogy is made with neutral Diels‐Alder cycloaddition reactions.