Surface Organometallic Chemistry: A New Synthesis of [Re2(CO)6(μ-OH)3]- from [Re(CO)3OH]4 Supported on a Silica Surface Added with K2CO3; Evidence of Its Thermal Binding to the Surface via Re−O−Si⋮ Bonds Confirmed by the Synthesis of the Molecular Models [Re2(CO)6(μ-OH)x(μ-OSiEt3)3-x]- (x = 0, 1, or 2) by Reaction of [Re2(CO)6(μ-OH)3]- with Et3SiOH

Chemistry of tantalum clusters in solution and on SiO2 supports: analogies and contrasts

Tantalum clusters have been synthesized from Ta(CH2Ph)5 on the surface of porous fumed SiO2. When these clusters are small, incorporating, on average, several Ta atoms, their chemistry is similar to that of molecular tantalum clusters (and other early transition-metal) clusters. For example, The Ta-Ta bonds in these small supported clusters have been characterized by extended X-ray absorption fine structure (EXAFS), IR, and UV-vis spectroscopy, being similar to those in molecular analogues. The redox reactions of the supported clusters, characterized by X-ray absorption near-edge structure, are analogous to those of early transition-metal clusters in solution. In contrast to the largest of these clusters in solution and in the solid state, those supported on SiO2 are raftlike, facilitating the substantial metal-support-oxygen bonding that is evident in the EXAFS spectra. Samples consisting of tantalum clusters on SiO2 catalyze alkane disproportionation and the conversion of methane with n-butane to give other alkanes, but catalytic properties of analogous clusters in solution have barely been explored.

A Re(CO)5 Fragment Bonded to a Polyhedral Oligomeric Hydroxysilsesquioxane as a Model which Gives Further Evidence to the Existence of an Unexpected Re(CO)5 Surface Species

Reaction of Re(CO)5O3SCF3 with (c-C6H11)7Si8O12O-Li+ at 273 K under a CO atmosphere affords the [Re(CO)5OR] (R = (c-C6H11)7Si8O12) derivative (1). 1 is the first example of a rhenium pentacarbonyl bearing an OR ligand (R = alkyl, aryl, or silyl) stable enough to be characterized, and it represents also the first molecular model of the surface [Re(CO)5OSi] species formed by reductive carbonylation of silica-supported [Re(CO)3OH]4. At room temperature, 1 loses one carbonyl ligand and dimerizes to afford {Re(CO)4[(mu-O)O12Si8(c-C6H11)7]}2 (2), which has been characterized by X-ray diffraction and is the first reported example of a rhenium tetracarbonyl mu-oxo-bridged dimer of the type [Re(CO)4(mu-OR)]2.

Surface organometallic chemistry — Carbonyl complexes of Re(I) with silanolates as models of silica anchored rhenium carbonyl species

Reaction of Me3SiONa with [Re(CO)5Cl] affords the new complex Na[Re2(CO)6(µ-OSiMe3)3], which could be generated via formation of [Re(CO)5OSiMe3] followed by immediate reaction with Me3SiO–. Substitution of some CO ligands by phosphines significantly decreases the electrophilicity of the Re(I) center, and therefore hinders further attack by Me3SiO–. Thus, fac-[Re(CO)3(Ph2PCH2CH2PPh2)OTf] (OTf is the triflate anion) reacts with Me3SiONa to give fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3], a molecular model of silica anchored [Re(CO)5OSi≡]. However, substitution of only one CO by triphenylphosphine is not enough to avoid the formation of [Re2(CO)6(µ-OSiMe3)3]–. While fac-[Re(CO)3(Ph2PCH2CH2PPh2)OSiMe3] is stable towards hydrolysis, [Re2(CO)6(µ-OSiMe3)3]– is readily hydrolyzed to [Re2(CO)6(µ-OH)(µ-OSiMe3)2]–, a molecular model of silica anchored [Re2(CO)6(µ-OH)(µ-OSi≡)2]–, whose structure has been determined by single crystal X-ray diffraction. Key words: surface organometallic chemistry, rhenium, silica, silanolate, molecular model.