Synthesis of Para- and Meta-Phenylene Free Dicarbenes and Their Application as Proligands for Mo and Zr Complexes

A synthetic method was developed that provides access to free phenylene-bridged di-N-heterocyclic carbenes with sterically unencumbered substituents. Two free dicarbenes, p-phenylene-bridged 1a and m-phenylene-bridged 1b, were synthesized and isolated up to 86% yield. Free dicarbene 1a was synthesized and found to be highly crystalline, making it isolable in high purity and yield. Additionally, p-phenylene free dicarbene 1a was structurally characterized by SC-XRD. These free dicarbenes were used to synthesize neutral metal complexes in the absence of halogens, silver salts, or anionic counterions. A Mo bimetallic complex 3 was synthesized in 94% yield without oxidation of the metal center and was isolated and characterized spectroscopically. Using the m-phenylene free dicarbene 1b, the synthesis of (nBuCiCiCnBu) Zr trisamido complexes was improved from previous reports to obtain Zr pincer complex 4 in high purity. The free dicarbene method required only stoichiometric Zr(NMe2)4 and eliminated the halogens from the reaction mixture and the formation of complexes with mixed coordination spheres (monoamido and diamido).