Monoaryloxide Pyrrolide (MAP) Imido Alkylidene Complexes of Molybdenum and Tungsten That Contain 2,6-Bis(2,5‑R2‑pyrrolyl)phenoxide (R = i‑Pr, Ph) Ligands and an Unsubstituted Metallacyclobutane on Its Way to Losing Ethylene

We report the synthesis of Mo and W MAP complexes that contain O-2,6-(2,5-R2-pyrrolyl)2C6H3 (2,6-dipyrrolylphenoxide or ODPPR) ligands in which R = i-Pr, Ph. W­(NAr)­(CH-t-Bu)­(Pyr)­(ODPPPh) (4a; Ar = 2,6-disopropylphenyl, Pyr = pyrrolide) reacts readily with ethylene to yield a metallacyclobutane complex, W­(NAr)­(C3H6)­(Pyr)­(ODPPPh) (5). The structure of 5 in the solid state shows that it is approximately a square pyramid with the WC4 ring spanning apical and basal positions. This SP′ structure, which has never been observed as an actual intermediate, must now be regarded as an integral feature of the metathesis reaction.