Metallacyclobuta-(2,3)-diene: A Bidentate Ligand for Stream-line Synthesis of First Row Transition Metal Catalysts for Cyclic Polymerization of Phenylacetylene

Described here is a direct entry to two examples of 3d transition metal catalysts that are active for the cyclic polymerization of phenylacetylene, namely, [(BDI)M{kappa 2-C,C-(Me3SiC3SiMe3)}] (2-M) (BDI=[ArNC(CH3)]2CH-, Ar=2,6-iPr2C6H3; M=Ti, V). Catalysts are prepared in one step by the treatment of [(BDI)MCl2] (1-M, M=Ti, V) with 1,3-dilithioallene [Li2(Me3SiC3SiMe3)]. Complexes 2-M have been spectroscopically and structurally characterized and the polymers that are catalytically formed from phenylacetylene were verified to have a cyclic topology based on a combination of size-exclusion chromatography (SEC) and intrinsic viscosity studies. Two-electron oxidation of 2-V with nitrous oxide (N2O) cleanly yields a [VV] alkylidene-alkynyl oxo complex [(BDI)V(=O){kappa 1-C-(=C(SiMe3)CC(SiMe3))}] (3), which lends support for how this scaffold in 2-M might be operating in the polymerization of the terminal alkyne. This work demonstrates how alkylidynes can be circumvented using 1,3-dianionic allene as a segue into M-C multiple bonds. We describe in this work how simple-to-prepare complexes composed of base metals such as Ti and V can be used as catalysts for the cyclic polymerization of phenylacetylene. These catalysts are prepared in one step by the treatment of the corresponding [(BDI)MCl2] with 1,3-dilithioallene [Li2(Me3SiC3SiMe3)] to form [(BDI)M{kappa 2-C,C-(Me3SiC3SiMe3)}] (BDI=[ArNC(CH3)]2CH-, Ar=2,6-iPr2C6H3; M=Ti, V), therefore circumventing the need to utilize transition metal alkylidynes as catalyst precursors.image