Structure and carbon dioxide reactivity of amido and imido titanium(IV) derivatives.

摘要

Approximately 10% of all fossil carbon mined on Earth is used in the manufacturing of organic chemicals. While there is disagreement concerning the longevity of global fossil petroleum reservoirs, it is clear that alternate sources of carbon will need to be developed as these supplies dwindle. The progressive increase in atmospheric carbon dioxide (CO2 ) from anthropogenic emissions makes it an attractive potential C1 synthon. Several methods have been employed to reduce CO2 to more useful molecules, but there has been no process developed to efficiently abstract carbon from CO2 by way of C-O bond cleavage. Titanium(IV) was explored as a possible catalyst to achieve such a process.; Isocyanate, via C-O bond cleavage, was observed from the reaction of titanium tetrachloride (TiCl4) with t-butylamine and CO2. Analogous reactions with propylamine also produced isocyanate, but in much lower yields. It is hypothesized that the N-alkyl substituents of primary amines play a significant role in determining the preliminary Ti-N bonding motif that dictates CO2 reactivity. Six new titanium(IV) compounds were prepared from primary and secondary amines, and crystallographically characterized. The structural diversity of these complexes has shed light on their respective CO2 reactivity. It is concluded that primary amines with sterically bulky N-alkyl groups result in terminal imido ligands while primary amines with smaller N-alkyl groups generate mu-N-imido bridges. Attempts were made to develop mu-oxo bridges in place of the mu-N-imido bridges, which appear to be inert to isocyanate extrusion with CO2. The single crystal X-ray structures demonstrate success in forming mu-oxo bridges by the addition of water to initial reaction mixtures, but this did not consistently prevent mu-N-imido bridges. More investigations are necessary in order to achieve a generalized synthetic preparation that yields terminal metal-imides with all primary amines. The results of this investigation are intended to direct future development of titanium(IV) species that catalytically activate CO2.