Oxide supported surface organometallic complexes as a new generation of catalysts for carbon-carbon bond activation

This paper reviews recent applications of well-defined silica-supported hydrides of the group 4 and 5 transition metals in the field of carbon-carbon and carbon-hydrogen bonds activation of alkanes. The synthesis and characterization of the zirconium and tantalum hydrides is presented. (≡SiO)₃M-H complexes (M=Ti 1, Zr 2, Hf 3) are obtained by hydrogen treatment at c.a. 150°C of ≡Si-O-MNp₃ (Np=CH₂C(CH₃)₃). These three surface complexes are formally 8 electrons species and are consequently very electrophilic. (≡SiO)₂Ta-H 4 is obtained by hydrogen treatment at c.a. 150°C of (≡Si-O)_xTa(=CHC(CH₃)₃)(CH ₂C(CH₃)₃)_3-x (x=1,2) and is also highly electrophilic. Examples of applications of these hydrides in the field of the activation of alkanes at moderate temperatures are then given. The complexes 1-3 can achieve the hydrogenolysis of alkanes at low temperature. With 1 a simultaneous reaction of skeletal isomerization occurs. With 2 and 3 the mechanism of C-C bond cleavage passes through an elementary step of β-alkyl transfer. The mechanism of hydroisomerization observed with 1 passes also by an elementary step of β-alkyl transfer but in this case the β-H elimination-olefin reinsertion occurs quite rapidly so that a skeletal isomerization also occurs. Complexes of type 2 or 3 were found to catalyze under olefin pressure the olefin polymerization, and under hydrogen pressure, the polyolefin hydrogenolysis. Here the equilibrium between the olefin insertion into a metal-alkyl and the β-alkyl transfer is shown to occur with the same catalyst in agreement with the concept of microreversibility. A new catalytic reaction, called "alkane metathesis" has been discovered with complex 4. By this reaction alkanes are catalytically transformed into higher and lower alkanes. The mechanism by which this reaction occurs is not fully understood. The products distribution, especially with labeled alkanes is explained by a concerted mechanism by which a Ta-C bond and a C-C bond of the alkane can be cleaved and reformed simultaneously via a kind of four centered σ-bond metathesis which has no precedent in classical organometallic chemistry.