Activation, Multiple-Bond Cleavage, and Fixation of Small Molecules Employing Group 6 Cyclopentadienyl, Amidinate (CpAm) Complexes
Yonke, Brendan Louis
Sita, Lawrence R
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Small molecules (e.g. N2, CO2, N2O) represent potential cheap and abundant chemical feedstocks. Despite the use of small molecules in various biosynthetic pathways, relatively few synthetic processes for the commercial utilization of small molecules exist. To investigate potential catalytic and stoichiometric methods for the activation, multiple-bond cleavage, and fixation of small molecules, Group 6 cyclopentadienyl, amidinate (CpAm) dinitrogen and multiple metal-ligand bonded complexes were investigated. Provided the weak activation of N2 in Group 6 dinitrogen complexes, these complexes were found to serve as M(II, d4) and M(IV, d2) synthons for the respective formation of Group 6 bis(carbonyl), bis(isocyanide), as well as mononuclear oxo and imido complexes. Moreover, a general route to Group 6 CpAm imido complexes of variable substitution was demonstrated upon the reaction of Group 6 CpAm dichloride complexes 48 and 49, respectively, with lithium amides, for steric amidinate deprotonation followed amido-imido tautomerization. Utilizing the synthesized mononuclear oxo and imido complexes various catalytic atom transfer reactions were demonstrated with their mechanistic details elucidated. These catalytic processes include the first ever oxygen atom transfer (OAT) reactions involving early transition metals including the synthesis of isocyanates from N2O and isocyanides, light-mediated degenerate OATs between CO2 and CO, and light-mediated non-degenerate OATs involving N2O and CO. Likewise, thermal-mediated nitrogen atom transfer (NAT) reactions were shown for the synthesis of isocyanates from organic azides (N3R) and CO. Key to this observed reactivity was the ability of multiply bonded metal ligands to undergo reaction with adjacent Lewis acidic ligands with this reactivity having been found to likewise facilitate interligand silyl group transfer in ð-loaded Group 6 CpAm oxo trimethylsilyl imido complexes. Despite direct utilization of the small molecules N2O and CO2 in OAT reactions, NAT reactions were found to require the use of organic azides. Therefore, in an effort to directly utilize N2 in NAT reactions, a novel photolytic N-N cleavage process was devised for the Group 6 dinitrogen complexes 52 and 53. This novel photolytic N-N cleavage represents the first time a ligand set has been shown to facility distinctly different N-N cleavage mechanisms for N2 bound to different metals.