Thermochemical aspects of arene C-H activation by tantalum silyl complexes: Relative Ta-Si and Ta-C bond enthalpies

Article abstract of DOI:10.1021/om00021a048

Tantalum(III) silyl complexes, Cp₂Ta(L)(SiR₃) (L = PMe₃, CO), are reactive toward the C-H bonds of unhindered arenes such as benzene, toluene, and m-xylene. The sterically hindered silyl complex Cp₂Ta(PMe₃)(Si(t-Bu)₂H) (1) reacts in neat arenes to produce the corresponding aryl complexes, Cp₂Ta(PMe₃)(Ar), in high yields. However, complexes with smaller silyl ligands give equilibrium mixtures of silyl and aryl complexes in which the silyl is favored. Thermodynamic studies of this system have permitted the first direct comparison of Si-H and C-H bond activation by the same metal center and allow the estimation of relative Ta-C and Ta-Si bond dissociation enthalpies (BDE's). It is found that the Ta-Si bonds in 1 and in Cp₂Ta(PMe₃)(SiMe₃) (6) are respectively 5.4 and 7.9 kcal mol^-1 weaker than the Ta-phenyl bond in Cp₂Ta(PMe₃)(Ph) (3). However, metal-phenyl bonds are generally much stronger than metal-alkyl bonds and the Ta-Si BDE's are probably comparable to or greater than the strength of a tantalum-alkyl bond. As expected, the bulkier silyl exhibits the weaker Ta-Si BDE, but surprisingly the lower stability of 1 with respect to the phenyl complex 3 is primarily due to a large and favorable entropy change (34 ± 3 eu) resulting from the release of steric congestion upon converting the silyl into the phenyl complex. The high reactivity of tantalum silyls toward arene C-H bond activation can also be combined with the facile reaction of tantalum alkyls with silanes to yield the silane-catalyzed conversion of Cp₂Ta(L)(R) (R = alkyl; L = PMe₃, CH₂=CH₂) complexes into the corresponding aryl complexes. Complexes CP₂Ta(PMe₃)(Si(t-Bu)₂H) (1), Cp₂Ta(CO)(Si-(t-Bu)₂H) (8), and Cp₂Ta(CO)(SiMe₃) (9) have been structurally characterized by single crystal X-ray diffraction studies.