Homogeneous Model Complexes for Supported Rhenia Metathesis Catalysts

Abstract

A series of rhenium trioxo complexes (L− ReO 3 ) was synthesized, characterized, and demonstrated to be active in olefin metathesis. The relationship between perrhenate (ReO 4 − ), perrhenyl (ReO 3 + ), and metathesis-active rhenium complexes (L−ReO 3 ) was elucidated. Their chemical behavior can be tuned through the Lewis acid−base interaction. DFT calculations were performed for the metathesis reaction of L−ReO 3 with norbornene, which demonstrates that electron-withdrawing substituents or ligands are beneficial for olefin metathesis activity. Moreover, a rhenium− alkylidene complex was synthesized, which can be activated by B(C 6 F 5 ) 3 to afford an active metathesis catalyst. This system can be regarded as a homogeneous model of the hypothetical species present in the heterogeneous catalytic systems. The findings from the present study are consistent with our previous gas-phase studies and constitute the elucidation of the working principles for the metathesis-active rhenium species on the surface. ■ INTRODUCTIONThe system Re 2 O 7 /Al 2 O 3 and its related system, CH 3 ReO 3 (MTO)/Al 2 O 3 −SiO 2 , are effective heterogeneous catalysts for olefin metathesis at ambient temperature, even though the putative rhenium carbene active species is produced only in situ. Understanding of the mechanism on a molecular level for heterogeneous catalysis remains a challenge. Therefore, it is essential to synthesize homogeneous models of the hypothetical surface species present in the rhenium-based heterogeneous catalysts. These models can serve as a bridge between the realms of homogeneous and heterogeneous catalysis and help to elucidate the working principles for those heterogeneous catalysts. Our earlier experimental and computational studies on high-valent rhenium trioxo complexes in the gas phase 1 inspired the pseudo-Wittig mechanism 2 and have proven essential for elucidation of the structural requirements for a self-activating homogeneous metathesis catalyst. As is evident from Scheme 1, a reaction sequence from left to right would provide a route for a rhenium−oxo complex to react with an olefin to form a rhenium carbene by addition, rearrangement, and dissociation. As suggested by qualitative molecular orbital arguments, as well as high-level quantum chemical calculations, 1c this requires that (i) the rhenium center is four-coordinate, (ii) the supporting ligand is a σ-donor, and (iii) the σ-donor ligand is electron-deficient. In other words, the electronic property of L in L−ReO 3 plays an important role in metathesis activity.From a structural point of view, Re 2 O 7 can be divided into two fragments: a perrhenate anion (ReO 4 − ) and a perrhenyl cation (ReO 3 + ). Cleavage of Re 2 O 7 can be achieved in the presence of electrophiles or nucleophiles to give monomeric compounds of the general formula L−ReO 3 . 3 It was reported that Re 2 O 7 dissolves in polar solvents such as THF, CH 3 CN, and pyridine to form solvent adducts. 4 This unsymmetrical coordination enhances the reactivity of Re 2 O 7 toward alkyla...

Activation of Methyltrioxorhenium for Olefin Metathesis in a Zirconium-Based Metal–Organic Framework

“…The secondary building units (SBUs) are particularly relevant targets for functionalization because they can be thought of as nanoscale models of the metal oxides traditionally used as industrial catalyst supports. , Among supported OM catalysts, rhenium oxide-based systems stand out due to their activity at room temperature and their broad tolerance to a range of heterofunctionalized olefins when activated by main group alkyl species. ,, These characteristics contrast with molybdenum and tungsten systems, which are active only at significantly higher temperatures and are less tolerant of functionalized olefins. Important milestones in rhenium oxide chemistry were the discovery and efficient preparation , of methyltrioxorhenium (MTO), a molecule with diverse catalytic competency. − The most salient feature of this versatile model − for immobilized rhenium oxide species in the context of heterogeneous OM catalysis is its inability to catalyze this transformation until activated on an appropriate support. The surprisingly limited scope of supports capable of triggering OM activity from MTO include alumina, − silica–alumina, ,,− niobia, , and zeolite HY …”

Ligand Exchange-Mediated Activation and Stabilization of a Re-Based Olefin Metathesis Catalyst by Chlorinated Alumina

“…16−18 MTO can also be generated (or regenerated) in situ from inorganic perrhenates by treatment with a methylating agent, such as SnMe 4 or AlMe 3 . 19 In solution, MTO requires activation by a Lewis acid 20 such as AlCl 3 (in combination with SnMe 4 ) or AlCl x Me y . 21 However, deposition of MTO on an acidic oxide such as SiO 2 −Al 2 O 3 , 19,22−26 niobia, 23,27,28 zeolite Y, 29 γ-Al 2 O 3 , 23,30 or OMA 26,31,32 is even more effective.…”

“…Catalysts based on supported methyltrioxorhenium (MTO) have been proposed as more active alternatives to supported ReO x catalysts, facilitated by the development of efficient methods for MTO synthesis. − MTO can also be generated (or regenerated) in situ from inorganic perrhenates by treatment with a methylating agent, such as SnMe 4 or AlMe 3 . In solution, MTO requires activation by a Lewis acid such as AlCl 3 (in combination with SnMe 4 ) or AlCl x Me y . However, deposition of MTO on an acidic oxide such as SiO 2 –Al 2 O 3 , ,− niobia, ,, zeolite Y, γ-Al 2 O 3 , , or OMA ,, is even more effective.…”

Synthesis and structure of bis(m-terphenyl)zinc (2,6-Mes2 C6 H3)2 Zn

“…Differences are observed in the relative orientation of the phenyl rings. Like (C 6 F 5 ) 2 Zn (Sun et al, 1998), [6-(CF 3 )C 6 H 4 ] 2 Zn (Chisholm et al, 2005), the closely related (2,6-Naph 2 C 6 H 3 ) 2 Zn (Gridley et al, 2013), [2,6-(2,6-Xyl) 2 C 6 H 3 ] 2 Zn, [2,6-(3,5-Xyl) 2 C 6 H 3 ] 2 Zn, [2,6-Pmp 2 C 6 H 3 ] 2 Zn (Blundell et al, 2014) and the isoelectronic (2,6-Mes 2 C 6 H 3 ) 2 Hg (Niemeyer and Power, 1997), (2,6-Mes 2 C 6 H 3 ) 2 Zn adopts a nearly C s symmetry, while Mes 2 Zn (Cole et al, 2003;Krieck et al, 2009) and [3,5-(CF 3 ) 2 C 6 H 3 ] 2 Zn (Lai et al, 2012) possess a nearly C 2v symmetry. Presumably due to the large steric congestion, (2,4,6-t-Bu 3 C 6 H 2 ) 2 Zn (Westerhausen et al, 2005) and the related [2,4,6-(CF 3 ) 3 C 6 H 2 ] 2 Zn (Brooker et al, 1992) approach only C 2 symmetry.…”