Screening of the Relaxivity of Gadolinium‐Loaded Periodic Mesoporous Silica Functionalized by Means of Soft Metalorganic Silylamide Grafting

Abstract

Soft metalorganic grafting of periodic mesoporous silica (PMS) materials is introduced as a method for obtaining nanostructured GdIII‐loaded silica particles with relatively high 1H relaxivity. The silylamide complex Gd[N(SiHMe2)2]3(THF)2 was grafted onto MCM‐41, SBA‐15, and SBA‐1, thereby providing PMSs of distinct symmetry as well as pore size and configuration. The materials were characterized by infrared spectroscopy, elemental analysis, powder X‐ray diffraction (PXRD), N2 physisorption, and scanning as well as transmission electron microscopy (SEM/TEM). Relaxivities up to 15.8 mM–1 s–1 were determined at high magnetic field strength (400 MHz). The nuclear magnetic resonance dispersion (NMRD) data of a calcined Gd[N(SiHMe2)2]3(THF)2@SBA‐1 material had a shape typical for slow‐tumbling systems, with a maximum at approximately 50 MHz. Temperature‐dependence studies of the relaxivity (r1) indicated that the grafted materials were characterized by an efficient water exchange. To maintain a high relaxivity, a low (1 wt.‐%) GdIII loading and a hydrophilic surface had to be provided. Varying the topology of the silica support only resulted in small variations in r1 of the hybrid materials.

High 1H Relaxivity Gadolinium‐Grafted Mesoporous Silica Materials

“…Considering the relatively low stability of alk(aryl)oxide complexes against hydrolysis, partially [42] or fully hydrolyzed surface species of type ϵSiO-Gd(OH) x (OH 2 ) y might be expected, similarly to the previously studied silylamide materials. [21] The fate of the ϵSiO-Gd bond is another issue, which was addressed by ICP-analysis of free Gd III ions in aqueous dispersions of the hybrid materials (Table S1). A leaching of only 1 % was observed for material 1b, corroborating the presence of strong ϵSiO-Gd bonds.…”

“…[28] This property might be turned into an advantage for the relaxivity, which profits from incomplete surface reactions, meaning low metal loading. [21] The alkoxide [Gd(OCH 2 CMe 3 ) 3 ] 4 , featuring a tetranuclear arrangement, [29] was considered a promising candidate based on a previous study of the reactivity of the cerium congener towards silica conducted in our group. [27] For comparison, we also used Gd(OC 6 H 3 tBu 2 -2,6) 3 as grafting precursor.…”

“…[21] Materials with only 1 wt.-% Gd were therefore synthesized, and the metal content precisely determined by ICP measurements. For precursor [Gd(OCH 2 CMe 3 ) 3 ] 4 , hybrid materials 1b (MCM-41), 2b (SBA-15), and 3b (SBA-1) with metal contents 1.30 wt.-%, 1.21 wt.-%, and 0.35 wt.-%, respectively, were obtained.…”

“…[23] However, when using silylamide Gd[N(SiHMe 2 ) 2 ] 3 (THF) 2 as a metal precursor, a side-reaction resulting in hydrophobic ϵSiO-SiHMe 2 moieties on the surface was found to have a negative effect on r 1 . [21] Further, the screening study did not show significant differences in r 1 correlated to using silica support materials with different topologies (MCM-41, SBA-15, SBA-1). These previous results indicated that for further improvements in r 1 of grafted mesoporous materials, one should consider modifying the type of surface species and ensure a hydrophilic surface and a maximum exchange of water from the bulk to the proximity of the metal centers.…”

“…The methods most frequently used to anchor Gd III to the silica particles are either incipient wetness impregnation with inorganic salts or tethering of macrocyclic chelates to the surface silanols via an amino linker. As an alternative synthesis route, we recently investigated Gd III -loaded Periodic Mesoporous Silica (PMS) materials obtained by surface metal-organic grafting, [21] by means of a direct reaction of the surface silanol groups with the moderately basic metal-organic precursor Gd[N(SiHMe 2 ) 2 ] 3 -(THF) 2 . [22] The grafting method allows for a unique control of the metal localization, metal coordination environment, and hybrid material design, opening a new avenue for understanding how the relaxometric properties of Gd III -loaded mesoporous silica can be improved in a controlled manner.…”