Unprecedented Allenylidene Transfer from Chromium to Tungsten

Abstract

Pyrimidylallenylidene complexes 1 M = Cr (a), W (b)) were prepared in a one-pot procedure from readily available 2-ethynylpyrimidine, butyllithium, [(CO)5M(THF)], and triethyloxonium tetrafluoroborate. In addition to 1a,b, the homobinuclear allenylidene complexes 2a,b M = Cr, W) were formed. In 2a,b the second (CO)5M moiety is attached to the nonalkylated nitrogen atom of the pyrimidyl ring. Treatment of the chromium complex 1a with an excess of [(CO)5W(THF)] afforded the tungsten allenylidene complex 2b by transmetalation of the allenylidene ligand and addition of (CO)5W. The allenylidene ligands of other chromium allenylidene complexes [(CO)5CrCCC(R1)R2] could likewise be transferred to tungsten. In contrast, the reverse transmetalation from tungsten to chromium could not be achieved. DFT calculations indicate that the reaction proceeds by an associative rather than a dissociative pathway. The initiating reaction step is coordination of a (CO)5W fragment to the Cα−Cβ bond of the allenylidene ligand.