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).