Surface Organometallic Chemistry:  Synthesis and X-ray Characterization of Novel Silanolate Surface Models [Re₂(CO)₈(μ-H)(μ-OSiR₂R‘)] and of the First Models with Two Homo and Hetero Metal Carbonyl Fragments Linked to Vicinal or Geminal Silanols

Abstract

Reaction of [Re2(CO)8(THF)2] with R2R‘SiOH (R = Et, Ph; R‘ = Et, Ph, OH, OSiPh2OH) gives the new molecular models [Re2(CO)8(μ-H)(μ-OSiR2R‘)], which mimic a surface [Re2(CO)8(μ-H)(μ-OSi⋮)] species anchored to isolated, geminal, or vicinal silanols. The bridging structure has been confirmed by the X-ray structure of [Re2(CO)8(μ-H)(μ-OSiPh2OH)]. Thermal treatment (70 °C) of [Re2(CO)8(μ-H)(μ-OH)] with excess Et3SiOH is another way to form [Re2(CO)8(μ-H)(μ-OSiEt3)], which can be reconverted into the hydroxo complex by addition of excess water at the same temperature. Both [Re2(CO)8(μ-H)(μ-OSiEt3)] and [Re2(CO)8(μ-H)(μ-OH)] are readily transformed into [Re2(CO)10] under 1 atm of CO at 150 °C. These molecular models, which are the only neutral carbonyl rhenium complexes bearing silanolate ligands reported up to now, constitute a new tool to clarify the first step of the surface chemistry of the photochemical interaction of [Re2(CO)10] with a rather inert support such as silica. Although the free silanol OH of both [Re2(CO)8(μ-H)(μ-OSiPh2OSiPh2OH)] and [Re2(CO)8(μ-H)(μ-OSiPh2OH)] is much less reactive than the OH of the corresponding noncoordinated silanol, it reacts at 25 °C with excess [Re2(CO)8(THF)2] to give the first bimetallic models [Re2(CO)8(μ-H)(μ-OSiPh2OSiPh2O-μ)(μ-H)Re2(CO)8] and [Re2(CO)8(μ-H)(μ-OSiPh2O-μ)(μ-H)Re2(CO)8], respectively. The latter complex, less stable to hydrolysis, is of particular interest because it constitutes the first molecular model of two metal carbonyl fragments linked to geminal surface silanols, thus suggesting that this kind of bimetallic system may occur also on a surface. Remarkably, the reaction of the rather unreactive [Os3(CO)10(μ-H)(μ-OSiPh2OSiPh2OH)] cluster with the very reactive [Re2(CO)8(THF)2] affords [Re2(CO)8(μ-H)(μ-OSiPh2OSiPh2O-μ)(μ-H)Os3(CO)10], the first heterobimetallic molecular model whose structure has been determined by X-ray diffraction.

Synthesis, Spectroscopic, and X-ray Characterization of Rhenium Carbonyl Complexes with Different Silsesquioxanes, as Models That Mimic the Chemical Behavior and the Topology of the Silica Surface

“…Compound 6 mimics a surface [Re 2 (CO) 8 (µ-H)(µ-OSit)] species anchored to an isolated silanol of the silica surface, which could be the first step of the photochemical or thermal interaction of [Re 2 (CO) 10 ] with the silica surface. 9,10 Working in deuterated toluene, the reaction can be followed by 1 H NMR spectroscopy (by the appearance of a new signal at δ -10.64 ppm that falls in the region typical for hydrides bridging two Re atoms, as previously observed for [Re 2 (CO) 8 (µ-H)(µ-OSiR 2 R′)] complexes 14 ). The 13 C NMR spectrum shows three carbonyl signals in a 2:4:2 ratio (in agreement with a localized idealized C 2V symmetry), and 29 Si NMR spectroscopy shows three signals in a 3:4:1 ratio.…”

“…This is expected, since pentacarbonyl Re(I) species such as [Re(CO) 5 (OR)] (R ) alkyl, aryl) have so far eluded isolation, due to the strong cis-labilizing power of OR -groups, which favors the formation of dimeric complexes containing Re(CO) 3 units joined by bridging ORgroups. 13 Other models of rhenium carbonyl surface species were synthesized and structurally characterized, such as the species [Re 2 (CO) 8 (µ-H)(µ-OSiR 2 R′)] (R ) Et, Ph; R′ ) Et, Ph, OH, OSiPh 2 OH), 14 where the OSiR 2 R′ group may mimic the interaction with isolated, geminal, and vicinal surface silanols. By analogy with the known easy anchoring of [Os 3 (CO) 12 ] on the silica surface by oxidative addition to give the silicaanchored [Os 3 (CO) 10 (µ-H)(µ-OSit)] species, 1e as confirmed by surface models, 6 the latter Re complexes could be models of the silica-anchored [Re 2 (CO) 8 (µ-H)(µ-OSit)] species possibly formed by thermal or photochemical oxidative addition of [Re 2 (CO) 10 ] with the silica surface.…”

Synthesis, reactivity and X-ray crystal structure of tris(pentafluorophenyl)silanol (C6F5)3SiOH

“…NMR data (CDCl 3 , 298 K, 9.4 T). 19 3 . In a two necked flask, 1 (130 mg, 0.238 mmol) was dried 3 h at 80°C under vacuum, then dissolved at room temperature under nitrogen in freshly distilled diethyl ether (7 mL) and added with one equivalent of n-BuLi (0.15 mL, 0.24 mmol, 1.6 M in n-hexane).…”

“…The investigation on the structure and reactivity of silica-anchored organometallic species has been improved in the last two decades also through the complementary investigation on the synthesis, structure and reactivity of molecular models bearing silanolate ligands which mimic the silanol groups of the silica surface [1][2][3][4][5][6][7]. For instance, some of us reported evidence for the peculiar stabilization of the rhenium pentacarbonyl species [Re(CO) 5 OSi≡] on the silica surface, but the synthesis of its molecular model [Re(CO) 5 OSiR 3 ] was hampered by the fact that nucleophiles, such as Me 3 SiO -, are exceptionally good cis-labilizing ligands that facilitate the instantaneous loss of carbonyl ligands and further nucleophilic attacks to give dimeric units bridged by silanolate groups such as [Re 2 (CO) 6 (-OSiMe 3 ) 3 ] - [8].…”

Low-Temperature Nucleophilic Attack of Me3SiO−and MeO−on Rhenium(I) and Rhenium(0) Carbonyl Complexes

“…The understanding of the structure and reactivity of silica-anchored organometallic species has improved by the help of the complementary synthesis of molecular models bearing silanolato ligands which mimic the silanol groups of the silica surface. − To get a more detailed insight into the surface organometallic chemistry involving rhenium carbonyl species supported on silica, recently some of us investigated silanolato carbonyl rhenium complexes in order to confirm the evidence of the silica-anchored rhenium pentacarbonyl species [Re(CO) 5 OSi], generated under CO from silica-supported [Re(CO) 3 OH] 4 . Such a surface species was unexpected, since organometallic species of the type [Re(CO) 5 OR] (R = alkyl, aryl) have so far eluded isolation, due to the strong cis-labilizing power of OR − groups, which favors the formation of dimeric complexes containing Re(CO) 3 units joined by alkoxo bridging groups. , The attempted synthesis of a molecular model of the silica-anchored [Re(CO) 5 OSi] by reaction of [Re(CO) 5 Cl] with Me 3 SiONa did not give the expected [Re(CO) 5 OSiMe 3 ] derivative.…”