Synthesis of High Oxidation State Bimetallic Alkylidene Complexes for Controlled ROMP Synthesis of Triblock Copolymers

Abstract

An X-ray study of [(THF)(RF6O)2(ArN)MoCH]2(1,4-C6H4) (ORF6 = OCMe(CF3)2; Ar = 2,6-diisopropylphenyl; THF = tetrahydrofuran; 1b), which is closely related to known [(DME)(RF6O)2(ArN)MoCH]2(1,4-C6H4) (DME = 1,2-dimethoxyethane; 1a), showed it to be the expected bimetallic species in which each end is approximately a trigonal bipyramidal monoadduct of a syn alkylidene with the THF coordinated to the NOO face of the metal trans to the MoC bond. Treatment of 1a with lithium tert-butoxide yielded [(t-BuO)2(ArN)MoCH]2(1,4-C6H4) (2). Addition of divinylferrocene to Mo(CHCMe2Ph)(NAr)(ORF6)2 produced the bimetallic species {(RF6O)2(ArN)Mo[CH(C5H4)]}2Fe (3), which upon treatment with lithium tert-butoxide produced a related tert-butoxide complex (4). X-ray studies of 3 and 4 showed them to be “syn/syn” bimetallic species related to 1b. In solution two resonances can be observed in proton NMR spectra in the alkylidene region for the “syn/anti” isomer of 1a, 2, 3, and 4; the total amount varies between 4 and 20% of the total. Bimetallic species 1a, 2, 3, and 4 have been shown to initiate at both ends and to produce homopolymers of 4,5-dicarbomethoxynorbornadiene (DCMNBD) and methyltetracyclododecene (MTD) in a living fashion. MALDI-TOF mass spectra of ferrocene-containing species have been obtained that are consistent with the polymerization process being living. Triblock copolymers (poly[(MTD)x/2(DCMNBD)y(MTD)x/2]) were prepared by adding y equivalents of DCMNBD to the bimetallic initiators followed (after consumption of DCMNBD) by x equivalents of MTD. These triblocks were shown to be of relatively high purity (free of homopolymer and diblock copolymer) and to have a relatively low PDI (≤1.2).