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 V-(V) alkylidyne, [(dBDI)-V equivalent to C t Bu-(OEt2)] (1-OEt2), supported by the deprotonated beta-diketiminate dBDI(2-) (dBDI2- = ArNC-(CH3)-CHC-(CH2)-NAr, Ar = 2,6- (Pr2C6H3)-Pr-i). Complex 1-OEt 2 is a precatalyst for the polymerization of phenylacetylene (PhCCH) to give cyclic poly-(phenylacetylene) (c-PPA), whereas its precursor, complex [(BDI)-V equivalent to C t Bu-(OTf)] (2-OTf; BDI- = [ArNC-(CH3)]2CH, Ar = 2,6- i Pr2C6H3, OTf = OSO2CF3), and the zwitterion [((C6F5)3B-dBDI)-V equivalent to C t Bu-(OEt2)] (3-OEt2) exhibit low catalytic activity despite having a neopentylidyne 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 characterization of 4- and 6-membered metallacycles as model intermediates. Metallacyclobutadiene (MCBD) and deprotiometallacyclobutadiene (dMCBD) complexes (dBDI)-V-[C-( t Bu)-C-(H)-C-( t Bu)] (4- t Bu) and (BDI)-V-[C-( t Bu)-CC-(Mes)] (5-Mes), respectively, were synthesized upon reaction with bulkier alkynes, t Bu- ( t BuCCH) and Mes-acetylene (MesCCH), with 1-OEt2. Furthermore, the reaction of the conjugate acid of 1-OEt2, [(BDI)-V equivalent to C t Bu-(OTf)] (2-OTf), with the conjugated base of phenylacetylene, lithium phenylacetylide (LiCCPh), yields the doubly deprotio-metallacycle complex, [Li-(THF)4]-{(BDI)-V-[C-(Ph)-CC-(t Bu)-CC-(Ph)]} (6). Protonation of the doubly deprotio-metallacycle complex 6 yields 6-H+, a catalytically active species toward the polymerization of PhCCH, for which the polymers were also confirmed to be cyclic by SEC studies. Computational mechanistic studies complement the experimental observations and provide insight into the mechanism of cyclic polymer growth. The noninnocence of the supporting dBDI(2-) ligand and its role in proton shuttling to generate deprotiometallacyclobutadiene (dMCBD) complexes that proposedly culminate in the formation of catalytically active V-(III) species are also discussed. This work demonstrates how a dMCBD moiety can react with terminal alkynes to form cyclic polyalkynes.