Polyacetylenes are interesting synthetic targets due to their application as optical limiters, membranes for gas separation, and as polymer template for the synthesis of functionalized polymers, along with their unique paramagnetic susceptibility and conductive properties. In particular, cyclic polymers of acetylenes are of utmost importance due to their distinctive physical properties compared to their linear counterpart. Nevertheless, little is known about their properties and applications due to their novelty and limitations of most synthetic approaches. To the best of our knowledge, there is only one reported tungsten-based active catalyst known thus far, for the one-pot direct synthesis of cyclic polyacetylenes using terminal alkynes. Herein, we represent cyclic polymerization of phenylacetylene using a vanadium alkylidyne complex supported by deprotonated -diketiminate, (L)V≡CtBu(OEt2) (L2−= ArNC(CH3)CHC(CH2)NAr, Ar = 2,6-iPr2C6H3), as the catalyst. This system provided us the opportunity to probe the mechanism of cyclic polymerization, the results of which will be discussed in this presentation. The reaction of the aforementioned vanadium alkylidyne with bulkier alkynes, tBu-and Mes- acetylene, allowed us to synthesize and characterize the pseudo-intermediates with vanadacyclobutadiene and deprotio-vanadacyclobutadiene scaffold, respectively, where in the latter case, the β-hydrogen is too acidic that protonates the deprotonated β-diketiminate ligand framework. We present additional evidence that a metal-ligand proton shuttling event is essential in the growth of the intermediate metallacycle. All the captured pseudo-intermediates (protio- and deprotio- vanadocyclobutadienes and deprotio-vanadocyclohexa-triene) are fully characterized structurally and spectroscopically. Based on mechanistic and computational studies, we propose that chain growth occurs via insertion of phenylacetylene monomers to the deprotio-vanadocyclobutadiene facilitated by proton-shuttling between the deprotio-vanadocyclobutadiene and -diketiminate ligand. There are a number of reports on the synthesis of metalocyclobutadienes with deprotonated -carbon, however, this is the first system showing insertion of further terminal alkynes to obtain the corresponding cyclic polymers. Size-exclusion chromatography and viscosity data have been collected to verify the cyclic topology of the synthesized polymers.

Cyclic polymerization of phenylacetylene with vanadium alkylidynes: A mechanistic study

ACS Fall 2020 Virtual Meeting

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