Synthesis of Tungsten(VI) Imido Alkylidene Bispyrrolide Complexes via the Isocyanate Route

A general route to tungsten­(VI) imido alkylidene bispyrrolide complexes via the extrusion of CO2 from the reaction of one equivalent of an aryl isocyanate with WOCl4 to form the corresponding imido complexes is reported. The otherwise required 2-fold excess of an aniline derivative in this reaction sequence is circumvented in the reaction of W­(O)­Cl4 with Ar-NCO (Ar = 2,6-Me2-C6H3, 2,6-iPr2-C6H3, 2,6-Cl2-C6H3, 3,5-Me2-C6H3) to W­(NAr)­Cl4. In the subsequent steps, the scope of the synthesis of tungsten­(VI) (NAr) alkylidene complexes via the isocyanate route, which was so far limited to Ar = 2,6-Me2-C6H3 and 2,6-iPr2-C6H3, was extended to Ar = 2,6-Cl2-C6H3 and 3,5-Me2-C6H3. From those tungsten alkylidene complexes, the use of inexpensive and readily available reagents facilitates the synthesis of tungsten­(VI) bispyrrolide complexes of the type W­(NAr)­(CHCMe2Ph)­(Pyr)2(DME) and W­(NAr)­(CHCMe2Ph)­(Me2Pyr)2 (Ar = 2,6-Me2-C6H3, 2,6-iPr2-C6H3, 2,6-Cl2-C6H3, 3,5-Me2-C6H3; Pyr = pyrrolide, Me2Pyr = 2,5-dimethylpyrrolide), also avoiding the use of triflic acid. Thereby, both competitive yields with regard to previously published routes and high atom economy and thus cost-effectiveness are achieved. In addition, the peculiarities in the alkylation of complexes of the type W­(NAr)­(OtBu)2Cl2(THF) have been investigated and factors leading to undesired overalkylation are outlined.