Vanadacyclobuta-(2,3)-diene rearrangement into a vanadium imido monoazabutadieneallenyl complex.

A cyclic alkyne polymerization catalyst undergoes, with heat or a Lewis base, a rearrangement to form a vanadium imido monoazabutadieneallenyl complex, which is no longer an active catalyst. [Display omitted] We show in this study how the catalyst for the cyclic polymerization of phenylacetylene, a high-spin vanadium(III) complex supported by the β-diketiminate (BDI) scaffold, [(BDI)V(κ2-C,C-Me 3 SiC 3 SiMe 3)] (1) (BDI = [ArNC(CH 3)] 2 , Ar = 2,6- i Pr 2 C 6 H 3), can convert thermally or with a Lewis base to a low-spin [VIII] complex [{ArNC(CH 3)CHC(CH 3)C(SiMe 3)CC(SiMe 3)}V=NAr] (2). Complex 2 was characterized spectroscopically by multinuclear NMR (1H, 13C, 29Si, 51V), IR, and UV–Vis in addition to single crystal X-ray diffraction analysis. Complex 2 was found to be inactive towards the polymerization of phenylacetylene, unlike its predecessor 1 , therefore suggesting that such rearrangement might be the modus operandi for catalyst deactivation. We propose the 1 → 2 transformation to involve a denticity change from κ2-C,C-Me 3 SiC 3 SiMe 3 to a κ1-alkylidene-alkylnyl, followed by an intra-molecular cross-metathesis pathway involving the V=C bond of the alkylidene-alkynyl ligand with the imine group of the BDI ligand to eventually form the vanadium imido ligand and monoazabutadieneallenyl (MADA−) moiety. [ABSTRACT FROM AUTHOR]