Recent applications in catalysis of surface organometallic chemistry

This paper reviews recent applications of well-defined silica-supported hyd rides of the group 4 and 5 transition metals in the field of carbon-carbon and carbon-hydrogen bonds activation of alkanes. The synthesis and characte rization of the zirconium hydride is presented. The monohydride (=SiO)(3)Zr -H is obtained by hydrogen treatment at ca, 150 degrees C of the well-defin ed surface species =Si-O-ZrNp3 (Np = CH2C(CH3)(3)). This surface complex is formally an 8 electron species and is consequently very electrophilic. Sim ilarly, hafnium and titanium hydrides are obtained by treatment under hydro gen of =Si-O-MNp3 (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)(2)Ta-H is obtained by hydrogen treatmen t at ca. 150 degrees C of (=Si-O)(x)Ta(=CHC(CH3)(3))(CH2C(CH3)(3))(3-x) (x = 1,2), prepared by reaction of Ta(=CHC(CH3)(3))(CH2C(CH3)(3))(3) with the hydroxyl groups of silica, Examples of applications of these hydrides in th e field of the activation of alkanes at moderate temperatures are then give n. 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 beta-alkyl transfer. The mech anism of hydroisomerization observed with the titanium hydride passes also by an elementary step of beta-alkyl transfer but, in this case, the beta-H elimination-olefin reinsertion occurs quite rapidly so that a skeletal isom erization also occurs. The zirconium hydride can also catalyze under olefin pressure the olefin polymerization and under hydrogen pressure the polyole fin hydrogenolysis, Here the equilibrium between the olefin insertion into a metal alkyl and the beta-alkyl transfer is shown to occur with the same c atalyst in agreement with the concept of microreversibility. A new catalyti c reaction called 'alkane metathesis' has been discovered with the tantalum hydride. By this reaction, alkanes are catalytically transformed into high er and lower alkanes. The mechanism by which this reaction occurs is not fu lly 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 sigma-bond metathesis which has no precedent in classical organom etallic chemistry. (C) 1999 Elsevier Science B.V. All rights reserved.