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-R 2 -pyrrolyl) 2 C 6 H 3 (2,6-dipyrrolylphenoxide or ODPP R ) ligands in which R = i-Pr, Ph. W(NAr)(CH-t-Bu)(Pyr)(ODPP Ph ) ( 4a ; Ar = 2,6-disopropylphenyl, Pyr = pyrrolide) reacts readily with ethylene to yield a metallacyclobutane complex, W(NAr)(C 3 H 6 )(Pyr)(ODPP Ph ) ( 5 ). The structure of 5 in the solid state shows that it is approximately a square pyramid with the WC 4 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.