Stepwise construction of silica-supported tantalum/iridium heteropolymetallic catalysts using surface organometallic chemistry

Abstract

A stepwise surface organometallic chemistry (SOMC) methodology consisting in using a silica-supported tantalum species, [(≡SiO)Ta(CH t Bu)(CH2 t Bu)2], as a reactive center to coordinate an iridium site, was developed to construct tantalum/iridium heterobimetallic edifices. The resulting material, MAT-2, exhibits enhanced catalytic performances-both in H/D isotopic exchange and alkane metathesis reactions-in comparison to MAT-1, which was prepared from the direct grafting on silica of a well-defined heterobimetallic Ta/Ir complex. We projected that the difference in catalytic activity was due to the presence of distinct active sites and we used a combination of advanced spectroscopic methods (IR, solid-state NMR and XAS spectroscopies) as well as modeling (computational studies and molecular models) to identify the structure of these surface species. These investigations point towards the presence of three types of active sites at the surface of MAT-2: the heterobimetallic surface species, [≡SiOTa(CH2 t Bu)2{IrH2(Cp*)}] 2-s, which is also found in MAT-1, as well as an unanticipated heterotrimetallic species, [≡SiOTa(CH2 t Bu)2{IrH2(Cp*)}], 3-s along with some unreacted monometallic Ta sites [(≡SiO)Ta(CH t Bu)(CH2 t Bu)2], 1-s. The well-defined trimetallic surface species 3-s was independently prepared and characterized to support this hypothesis. This study highlights the importance of the synthetic methodology used for the preparation of heterobimetallic species through SOMC, and the difficulty to obtain true single-sites surface species.

Strongly Polarized Iridiumδ−–Aluminumδ+ Pairs: Unconventional Reactivity Patterns Including CO2 Cooperative Reductive Cleavage

“…The hydride resonance is observed as a singlet at δ −16.80 ppm and integrates for 3H. This suggests rapid exchange among the three hydrides on the NMR time scale at room temperature, which is expected on the basis of literature precedents for metal–polyhydride systems. ,,, The IR spectrum of 1 exhibits two strong bands at σ 2144 cm –1 and 1973 cm –1 assigned to metal–hydride stretches. The most significant feature of the crystallographic structure of 1 , shown in Figure , is the Al–Ir distance of 2.406(2) Å, which is the second shortest reported distance for a Ir species bearing an Al atom in its coordination sphere. ,− The only species featuring a shorter Al–Ir bond of 2.382(1) Å is the pincer complex [PAlP]­Ir­(H) 4 (Scheme ) described by Yamashita and co-workers .…”

“…To our surprise, no followup studies emerged from this preliminary work to explore further the reactivity of this unconventional molecular object featuring a low-valent Ir 2 Al 2 core. Very recently, our group reported the preparation of heterobimetallic tantalum–iridium complexes using a similar alkane reductive elimination approach (Scheme , bottom) and demonstrated the ability of the resulting TaIr multiple-bond motif to promote H/D isotope exchange reactions with excellent catalytic performances. − …”

Mechanistic investigations via DFT support the cooperative heterobimetallic C–H and O–H bond activation across TaIr multiple bonds

“…Rapid exchange between the three hydrides is not unexpected on the basis of literature precedents for metal-polyhydride systems. 32,64,65 The IR spectrum of 4 ( Figure S3) shows a distinct absorption at 2103 cm -1 , which is assigned to a Ir-H stretching vibration. 31,32,[66][67][68] The identity of 4 was supported by single-crystal X-ray diffraction.…”

“…Furthermore this coordination environment and geometry in 4 is confirmed computationally (see below). Complex 4 is the result of the classical protonolysis of the three neopentyls from 1, 31,32,71,72 leading to the formation of phenoxides, as well as the HBOA of the fourth equivalent of phenol across the Ta=Ir bond. Note that when 1 is treated with less than 4 eq.…”

A family of rhodium(i) NHC chelates featuring O-containing tethers for catalytic tandem alkene isomerization–hydrosilylation

“…The latter reactivity indicates that the hydroxyl group in 3 might react with basic metal derivatives to yield heterobimetallic assemblies. Accordingly, in an attempt to synthesize a tantalum/rhodium heterobimetallic species, 25,26,36,37 complex 3 was treated with 1 equivalent of Ta(CH t Bu)(CH2 t Bu)3 at room temperature in THF. After reaction completion, the crude mixture was concentrated and stored at -40 °C to yield a new product, 4, as a yellow microcrystalline powder in 48% yield.…”