Determining the Three-Dimensional Structures of Silica-Supported Metal Complexes from the Ground Up

Abstract

The immobilization of molecularly precise metal complexes to substrates, such as silica, provides an attractive platform for the design of active sites in heterogeneous catalysts. Specific steric and electronic variations of the ligand environment enable the development of structure–activity relationships and the knowledge-driven design of catalysts. At present, however, the three-dimensional environment of the precatalyst, much less the active site, is generally not known for heterogeneous single-site catalysts. We explored the degree to which NMR-based surface-to-complex interatomic distances could be used to solve the three-dimensional structures of three silica-supported metal complexes. The structure solution revealed unexpected features related to the environment around the metal that would be difficult to discern otherwise. This approach appears to be highly robust and, due to its simplicity, is readily applied to most single-site catalysts with little extra effort.

1H chemical shift anisotropy: a high sensitivity solid-state NMR dynamics probe for surface studies?

“…The second sample is a silica-tethered Zn phenanthroline complex (Section 4.2). Its structure was recently solved to a high level of precision using NMR-based distance measurements 39 and suggests secondary complex–support interactions that might hinder dynamics. The last sample is a monopodal grafted Zr metallocene complex (Section 4.3) for which two dynamic modes are expected, namely cyclopentadienyl (Cp) rotation, and a Si–O and/or O–Zr rotation.…”

“…The synthesis of the supported Zn phenanthroline complex was described elsewhere. 39 Cp 2 ZrOMe/SiO 2 . Using an Ar glovebox, Cp 2 Zr(CH 3 ) 2 (0.25 mmol, Strem) was dissolved in benzene (10 mL) and stirred with Davisil 635 silica (200 mg, Aldrich) that had been thermally treated under vacuum at 550 1C for 16 hours.…”

“…The obtained Cp 2 ZrMe/SiO 2 species is then converted to a Cp 2 ZrOMe/SiO 2 species by exposure to oxygen. [39][40][41]…”

The coordination chemistry of oxide and nanocarbon materials

“…In the Green formalism, 35 silanols on silica surfaces can act as either a X-type ligands to form uSi-O-M, or LX-type ligands that coordinate nearby siloxane bridges in addition to uSi-O-M. Silica surfaces are amorphous, prohibiting advanced X-ray diffraction studies of native or functionalized silica surfaces that are common in crystalline extended solids, 36 though recent advances in solid-state NMR and computational methods provide three-dimensional structures of sites present on oxide surfaces. [37][38][39] In the absence of extensive characterization to determine LX-type ligand coordination for silica-supported organometallics the X-type ligand behavior of silanols on silica is assumed. Using X-ray absorption methods κ Bu) 3 (uSi-O-Siu), 43 and κ 2 -uSiO-Lu[CH(SiMe 3 ) 2 ] 2 (uSi-O-Siu) 44 were shown to coordinate a nearby uSi-O-Siu…”

“…Silica surfaces are amorphous, prohibiting advanced X-ray diffraction studies of native or functionalized silica surfaces that are common in crystalline extended solids, 36 though recent advances in solid-state NMR and computational methods provide three-dimensional structures of sites present on oxide surfaces. 37–39 In the absence of extensive characterization to determine LX-type ligand coordination for silica-supported organometallics the X-type ligand behavior of silanols on silica is assumed. Using X-ray absorption methods κ 2 -SiO–Ta(CH t Bu)(CH 2 t Bu) 2 (Si–O–Si), 40 κ 2 -SiO–W(NAr)(CH t Bu)(CH 2 t Bu)(Si–O–Si), 41 κ 2 -SiO–Re(CH t Bu)(C t Bu)(CH 2 t Bu)(Si–O–Si), 42 κ 2 -SiO–W(O)(CH 2 t Bu) 3 (Si–O–Si), 43 and κ 2 -SiO–Lu[CH(SiMe 3 ) 2 ] 2 (Si–O–Si) 44 were shown to coordinate a nearby Si–O–Si bridge, Fig.…”