Abstract
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 hydride is presented. The monohydride (SiO)
3Zr–H is obtained by hydrogen treatment at ca. 150°C of the well-defined surface species Si–O–ZrNp
3 (Np=CH
2C(CH
3)
3). This surface complex is formally an 8 electron species and is consequently very electrophilic. Similarly, hafnium and titanium hydrides are obtained by treatment under hydrogen of Si–O–MNp
3 (M=Hf,Ti). In the case of titanium the reaction is not quantitative in the sense that a non-negligible amount of titanium(III) is formed. The tantalum hydride (SiO)
2Ta–H is obtained by hydrogen treatment at ca. 150°C of (Si–O)
x
Ta(CHC(CH
3)
3)(CH
2C(CH
3)
3)
3−
x
(
x=1,2), prepared by reaction of Ta(CHC(CH
3)
3)(CH
2C(CH
3)
3)
3 with the hydroxyl groups of silica. Examples of applications of these hydrides in the field of the activation of alkanes at moderate temperatures are then given. All these surface hydrides can achieve the hydrogenolysis of alkanes at low temperature. When the titanium hydride is used, a simultaneous reaction of skeletal isomerization occurs. In all cases, the mechanism of C–C bond cleavage passes through an elementary step of β-alkyl transfer. The mechanism of hydroisomerization observed with the titanium hydride 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. The zirconium hydride can also 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 the tantalum hydride. 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.