Jean-Pierre Candy

Surface organometallic chemistry on metals: controlled hydrogenolysis of Me4Sn, Me3SnR, Me2SnR2, MeSnBu3 and SnBu4 (R = methyl, n-butyl, tert-butyl, neopentyl, cyclohexyl) onto metallic rhodium supported on silica

The controlled hydrogenolysis of MexSnR4u2006−u2006x (0 ≤ n x n ≤ 4; R = methyl, n-butyl, tert-butyl, neopentyl, cyclohexyl) onto Rh/SiO2 is followed by quantitative and qualitative analysis of evolved gases. Only MeH and RH are detected in the evolved gases. There is hydrogenolysis of the Sn–C bonds without any C–C bond hydrogenolysis, leading to formation of grafted organometallic fragments. Using various organotin compounds, MexSnR4u2006−u2006x, it has been possible to determine the regioselectivity of the hydrogenolysis of the Sn–C bonds. The initial selectivity is inversely proportional to the steric bulk of the alkyl group: tBu < Np < Bu. The formation of a five-coordinate tetraalkyl tin on the surface, Rh–SnMexR4u2006−u2006x (symmetry D3h), in which the bulkiest group, e.g., R, is away from the surface could explain these results. This surface five-coordinate tin species could eliminate an alkyl group, generally a methyl group, thus decreasing the steric bulk around the tin, into the equatorial plane of D3h, via a concerted hydrogen transfer-elimination mechanism to give Rh–SnMexu2006−u20061R4u2006−u2006x. Then, in the successive steps of the hydrogenolysis, the bulkiest group, R, would be eliminated.

Surface Organometallic Chemistry on Oxides, on Zeolites and on Metals

New concepts are slowly emerging from the overlap between organometallic chemistry and surface science. In particular the rules of molecular chemistry seem to apply quite well when organometallic complexes (mononuclear, polynuclear; transition metals, main group elements, lanthanides or actinides) react with a surface (oxide, zeolite, zerovalent metal). In the case of oxides it is thus possible to classify those surfaces by their chemical properties (e.g. acid-base, redox a.s.o.). Well defined surface organometallic fragments can also be prepared via organometallic complexes at the surface of oxides, zeolites and metals; it is thus possible to study on those fragments the real “elementary steps” of heterogeneous catalysis (oxidative addition, reductive elimination, insertion, C-C coupling, etc.). By means of supported heteropolynuclear complexes it is now possible to prepare bimetallic particles having the same composition as that of the starting complexes, a useful approach for the synthesis of tailor made catalysts. By means of organometallics it is possible to control the external pore size of zeolites and introduce a new approach of shape selectivation where the molecular steric control can be adjusted by the size of the organometallic fragment. Reaction of organometallics of main group elements with group VIII metals is a way to prepare well defined alloys of known composition which exhibit highly chemoselective properties in heterogeneous catalysis. It is also possible to graft organometallic fragments at the surface of metal particles and obtain a new generation of modified metallic catalysts extremely selective in a variety of catalytic reactions and which are already in practice in industry.

Silica grain labelling and EDX spectroscopy; evidencefor inter-grain diffusion of Pt(NH3)42+species during Pt/SiO2 catalyst preparation by ionicexchange

Labelling of silica grains and energy dispersive X-ray nspectroscopy (EDX) in a TEM-FEG (field emission gun) were used to ndemonstrate the migration of Pt(NH3)42+ nspecies from one grain to another during Pt/SiO2 catalyst npreparation by the ion-exchange procedure.

Unique selectivity for the dimerization of propene on RhSnSiO2 catalysts: implications on the mechanism of CC bond formation and cleavage on bimetallic catalysts

A series of bimetallic catalysts RhSnxSiO2 (x = 0.4, 0.7, 0.9, and 1.4) were synthesized by the reaction of the monometallic catalyst RhSiO2 with Sn(n-butyl)4 under hydrogen. Various chemical and spectroscopic methods indicated that the metals present were fully reduced, and that tin atoms rest on the surface, very slightly increasing particle size and producing isolated rhodium sites. The catalytic reactions of propylene/hydrogen mixtures in the presence of these bimetallic catalysts are compared with those of the monometallic RhSiO2 catalysts. The mechanistically interesting reactions observed are those of carbon-carbon bond formation and cleavage. For the monometallic catalyst, olefin homologation and hydrogenolysis were observed, reactions which invoked the transfer of C1 fragments from one olefin to another. For the bimetallic catalysts, a marked increase in the selectivity for C6 products was observed. The presence of hydrogen is necessary to this reaction but selectivity for C6 is enhanced when hydrogen is in deficit with respect to propylene. Selectivity for C6 increases with the surface rhodium to tin, RhsSn, ratio to a maximum at 0.9. Low temperature favors the formation of C6 and C2 products.