Vanadium Alkylidyne Initiated Cyclic Polymer Synthesis: The importance of a Deprotiovanadacyclobutadiene Moiety.

Reported is the catalytic cyclic polymer synthesis by a 3d transition metal complex: a [VV] alkylidyne, [(dBDI)V≡CtBu(OEt2)] (1−OEt2), supported by the deprotonated β-diketiminate dBDI2− (dBDI2− = ArNC(CH3)CHC(CH2)NAr). Complex 1−OEt2 is a pre-catalyst for the polymerization of phenylacetylene to give cyclic pol-yphenylacetylene, whereas its precursor, complex [(BDI)V≡CtBu(OTf)] (2−OTf; BDI− = [ArNC(CH3)]2CH, Ar = 2,6-iPr2C6H3) and the zwitterion [((C6F5)3B−dBDI)V≡CtBu(OEt2)] (3−OEt2) exhibit low catalyst activity despite having the same alkyli-dyne ligand. Cyclic polymer topologies were verified by size-exclusion chromatography (SEC) and intrinsic viscosity studies. A component of the mechanism of the cyclic polymerization reaction was probed by isolation and full character-ization of 4- and 6-membered metallacycles as model intermediates. Metallacyclobutadiene (MCBD) and deprotio-metallacyclobutadiene (dMCBD) complexes, (dBDI)V[C(tBu)C(H)C(tBu)] (4−tBu) and (BDI)V[C(tBu)CC(Mes)] (5−Mes), respectively, were synthesized upon reaction with bulkier alkynes, tBu− and Mes−acetylene, with 1−OEt2. Furthermore, the reaction of the conjugate acid of 1−OEt2, (BDI)V≡CtBu(OTf) (2−OTf), with the conjugated base of phenylacetylene, LiC≡CPh, yields the doubly deprotio-metallabenzene complex, [Li(THF)4]{(BDI)V[C(Ph)CC(tBu)CC(Ph)]} (6). Protonation of the anionic 6-membered metallacycle 6, yields 6−H+, a catalytically active species towards the polymerization of phe-nylacetylene, for which the polymers were also confirmed to be cyclic by SEC studies. Computational mechanistic stud-ies complement the experimental observations and provide insight into the mechanism of cyclic polymer growth. The non-innocence of the supporting dBDI2− ligand and its role in proton shuttling to generate deprotio-metallacyclobutadiene (dMCBD) complexes that proposedly culminate in the formation of catalytically active [VIII] species is also discussed. This work demonstrates how a dMCBD moiety can react with terminal alkynes to form cyclic polymers.