Interactions between silica materials and transition metal complexes: Reactivity and surface probes

• Main Author: Taylor, Colleen M
• Format: Dissertation
• Language: English
• Subjects: chemistry, inorganic

The focus of the work presented herein was to obtain chemical information regarding the influence of anchoring and grafting processes on the reactivity and speciation of coordination complexes in silica materials. This work was accomplished using two compounds to probe various surface coordination phenomena, ferrous tris-phenanthroline (ferroin) and cobalt 2,9-dimethyl-1,10-phenanthroline.
Ferrous tris-phenanthroline (ferroin) derivatives were used as reactivity probes to examine the effect of competing decomposition and stabilization reactions during the initial stages of a sol-gel reaction, affording rate constants for the immobilization process. These rate constants provide a new benchmark for defining ligand substitution rates at transition metal centers in terms of the processes that can have an impact on the survival of transition metal complexes during sol-gel immobilization or imprinting procedures.
Although the information regarding the rate of stabilization in sol-gel materials as a function of anchoring was informative in the ferroin study, the sol-gel approach can be inherently complex. The use of preformed silica to produce catalytic materials decreases the complexity of the synthesis procedure by reducing some of the variables associated with anchoring or grafting of TM complexes during the sol-gel reaction. We developed the transition metal probe [Co(2,9-dimethyl-1,10-phenanthroline)(solvent) 4]2+ ([Co(DMP)]2+) to examine the differences found by grafting to a preformed surface or in situ during the sol-gel reaction. The visible, X-ray absorption spectra and pH titration data of the surface adducts were interpreted in terms of a binding mode in which the Co(II) center has a highly distorted pseudo-C2v symmetry. The formation of the surface species in tetramethoxyorthosilicate (TMOS) sol-gels suggested a two proton-dependent binding event to form a species such as [Co(DMP)(SiO) 2] whereas, the adduct grafted on silica was more consistent with the formula [Co(DMP)(SiO)(H2O)]NO3. Using the sol-gel method, a systematic titration was performed to demonstrate the balance between desirable material properties of the sol-gel material and the maximum of an inner-sphere coordinated metal complex.
The unusual electronic spectroscopy of the surface complex was examined in detail to obtain useful information regarding the ligand field strength of the silanolate groups. This inorganic aspect of the surface coordination chemistry of [Co(DMP)(SiO)2] was accomplished through comparisons with four model complexes of the type [Co(DMP)X2], X = I −, Br−, Cl−, and N CS −. (Abstract shortened by UMI.)