Triple-Bond Metathesis with Group 6 Nitrido Complexes: The Importance of Ligand Identity and the Development of XXX Pincer Ligands.

Abstract

Triple-bond metathesis is a useful methodology for the synthesis of new organic functionalities. Alkyne cross metathesis (ACM) has been employed in the synthesis of materials with unusual optical properties and in the construction of biologically relevant molecules. Installation of an alkyne functionality into a substrate is often more challenging than introduction of a nitrile group. Therefore nitrile-alkyne cross metathesis (NACM) is desirable to allow for greater flexibility in the construction of carbon-carbon triple bonds. Using NW(OC(CF3)2Me)3(DME) (1) as a catalyst, the utility of NACM is demonstrated in the synthesis of an arylene-ethynylene macrocycle via a shorter route than possible through ACM. Catalyst 1 was found to be sensitive to both polar and protic functional groups. Molybdenum NACM catalysts were designed in order to overcome the limited functional group tolerance of 1. The activation barrier for metalacycle formation is larger for Mo than W, therefore elevated temperatures are required to effect a complete NACM cycle with Mo complexes. Using the thermally robust triphenylsiloxide ligand, NMo(OSiPh3)3 (2) successfully catalyzed NACM at 185 °C. The triphenylsiloxide ligand was found to have a proper electron donor strength, as other Mo complexes did not catalyze NACM under similar conditions. Trianionic (XXX) pincer ligands were designed in order to lower the activation barrier required for Mo-catalyzed NACM. The compound 1,3-bis(4,5-(3,5-dimethylphenyl)-1H-imidazol-2-yl)benzene (3) was synthesized as an XXX pincer ligand precursor. While 3 proved challenging to install onto Mo complexes as desired, 3 was facilely activated by Zr(NMe2)4. Subsequent investigations of the Zr-XXX complexes revealed an unusual charge-switching feature in which the XXX ligand could be converted into a monoanionic (LXL) ligand. The XXX-LXL transformation could be effected through methylation, borylation, and silylation, with reversibility being possible through silyl group cleavage. Computational studies revealed differing electron donating strengths of the XXX and LXL ligands, suggesting that the pincer ligand charge could be used to alter the reactivity patterns of metal complexes. In order to realize the goal of using an XXX ligand for Mo catalysts, 1,8-bis(4,5-bis(3,5-dimethylphenyl)-1H-imidazol-2-yl)-3,6-di-tert-butyl-9H-carbazole was synthesized as a new XXX ligand precursor. Future studies will explore the application of the new ligand in catalysis of NACM.