Imine Metathesis Catalyzed by a Silica-Supported Hafnium Imido Complex

Abstract

The well-defined single-site silica-supported hafniaaziridine complex [(Si–O−)­Hf­(η2,π-MeNCH2)­(η1-NMe2)­(η1-HNMe2)] was prepared using surface organometallic chemistry. Upon thermal treatment under high vacuum, the grafted species was converted into the unprecedented hafnium imido bis-amido complex [(Si–O−)­Hf­(NMe)­(η1-NMe2)]. The surface complexes were characterized by elemental analysis and the following spectroscopic techniques: infrared, solid-state single and multiple quantum NMR, advanced DNP-SENS, and extended X-ray absorption fine structure. [(Si–O−)­Hf­(NMe)­(η1-NMe2)] catalyzed imine metathesis under mild conditions, and characterization of the reactivity showed that the imido exchange with N-(4-phenylbenzylidene)­benzylamine yielded [(Si–O−)­Hf (NCH2Ar)­(η1-NMe2)], demonstrating a kind of 2 + 2 mechanism involving the imine and the imido groups; the proposed reaction mechanism is also supported by DFT calculations.

Imine Metathesis by Silica-Supported Catalysts Using the Methodology of Surface Organometallic Chemistry

“…Single-site catalysts have been prepared by the reaction of an organometallic compound (or a coordination compound) with a very clean surface of highly dehydroxylated metal oxide (e.g., silica). Recently, we have identified surface organometallic fragments (SOMF) (e.g., M=C, M-C, M-H, M-NC, M=O, M=CR 2 , andM=NR ) that have a pivotal role toward targeted catalysis reactivity (e.g., alkane oxidation 1,2,3 , alkane metathesis 4,5 , alkene metathesis 6 , imine metathesis 7,8,9 ). [M]=NR fragments have received less attention; their characterization and reactivity studies remain limited, yet they can act as an intermediate in carbon-nitrogen transformation reactions 10 .…”

“…. The chemistry of metal-alkylidene-mediated olefin metathesis processes has broadened not only pure hydrocarbon organic synthesis possibilities 22 but also made it possible to deliver chemicals with new carbon-heteroatom double bonds 7,16 . Alkane metathesis was discovered later by Basset's group and requires multifunctional catalysts 23,24,25 .…”

“…Employing rigorous SOMC methodology, we have isolated two new well-defined hafnium nitrogen-containing fragments (metallaaziridine and metal-imido) that were fully characterized (FTIR, solid-state NMR, elemental microanalysis) 7 . Using highly dehydroxylated silica with isolated silanols, SiO 2-700 ( Figure 1) allows the isolation of well-defined single-site catalysts.…”

“…Using highly dehydroxylated silica with isolated silanols, SiO 2-700 ( Figure 1) allows the isolation of well-defined single-site catalysts. This work is considering the M-N, MNC, and M=N surface fragments (SFs) as key reaction intermediates that direct the reaction toward hydroaminoalkylation 26 or imine metathesis 7,8,32 . This study can potentially bring a better general understanding of transition metal imido functional groups bore by group IV metal grafted on silica.…”

“…This work is showing that 15 N DNP-SENS can provide a clear insight into the silica-supported nitrogen moieties (metallaaziridine and metal-imido). Herein, we isolate a silica-supported well-defined (imido) complex and demonstrate its capability as a highly efficient imine metathesis catalyst 7,33 .…”