Catalytic Imido-Transfer Reactions of Well-Defined Silica-Supported Titanium Imido Complexes Prepared via Surface Organometallic Chemistry

Abstract

We expand the series of well-defined silica-supported titanium imido complexes of general formula (SiO)­Ti­(NtBu)­X­(py) n prepared via surface organometallic chemistry in order to analyze the effect of the X ligand on their catalytic properties. Two new surface complexes, (SiO)­Ti­(NtBu)­Cl­(py)2 (2s) and (SiO)­Ti­(NtBu)­Cp­(py) (3s), have been prepared and characterized with physicochemical techniques, and their performance in oxo/imido heterometathesis and other catalytic imido-transfer reactions has been studied and compared to that of the previously reported catalyst (SiO)­Ti­(NtBu)­(Me2Pyr)­(py)2 (1s; Me2Pyr = 2,5-dimethylpyrrolyl).

Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

“…While this review has concentrated on olen metathesis and demonstrated that SOMC can generate highly performant catalysts that can surpass in some cases their homogeneous analogues, one can see that this methodology can be readily transposed to related reactions, such as alkyne, 46,47,199,200 imine, 201,202 and oxo/imido 203,204 metathesis, which involve cleavage of p-bonded systems. These concepts can also be used to develop alkane metathesis reactions that involve similar elementary steps.…”

Design of 4‐Coordinate Ti Imido Aryloxide on the Surface of Silica for Catalytic Oxo/Imido Heterometathesis

“…85 % grafting (the typically observed value at quantitative consumption of the molecular precursor is 0.23 mmol Ti g À 1 ). [10,11] The Ti/N/C ratio 1 : 1.8 : 28.2 is higher in C than the expected value of 1 : 2 : 24. IR spectroscopy showed the disappearance of the band of isolated silanols at 3747 cm À 1 (Figure 2,b), although a broad feature observed in the region 3800-3500 cm À 1 indicated that certain amount of silanol groups remained on the surface.…”

“…The most important region of the spectrum is the area of the quaternary NCMe 3 resonance (40-80 ppm) as this signal is strongly sensitive to the state of N t Bu group. [1,11] It is well-known that for Ti complexes containing =N t Bu, À NH t Bu and NH 2 t Bu ligands, this resonance usually appears at ca. 70, 60 and 50 ppm respectively (see references given in Ref.…”

“…70, 60 and 50 ppm respectively (see references given in Ref. [11]). As shown in Figure 3,a, the spectrum of 2/SiO 2 contains three signals in this region, the major one at ca.…”

“…It has been welldocumented that the most reactive with respect to oxo/imido exchange M=NR moieties are formed by the transition metals in the left part of the d-block (in particular for group 4). [1][2][3][4][5][6][7][8][9] However the successful application of these highly reactive species as oxo/ imido heterometathesis catalysts only became possible with the development of well-defined silica-supported Ti imido complexes [10,11] prepared through Surface Organometallic Chemistry (SOMC), [12][13][14][15][16][17][18] such as Ti imido pyrrolyl complex 1/SiO 2 (Scheme 1,b) that is currently the most active catalyst for such reactions. [1,10,11,19,20] The efficiency of the supported systems is associated with the stabilization of Ti=O intermediates of the catalytic cycle (that readily dimerize in solution) through a covalent attachment to the surface of the support, thus preventing their bimolecular transformations.…”