ROTATIONAL ISOMERS OF MO(VI) ALKYLIDENE COMPLEXES AND CIS-TRANS POLYMER STRUCTURE - INVESTIGATIONS IN RING-OPENING METATHESIS POLYMERIZATION

Low-temperature (-85-degrees-C) photolysis of complexes of the type syn-M(NR)(CHR')(OR'')2 (where M = Mo (usually) or W; R = 2,6-i-Pr2C6H3, 4-Br-2,6-i-Pr2C6H2, 2,6-Me2C6H3, 2-t-BuC6H4, 2-i-PrC6H4, 1-adamantyl; R' = CMe3, CMe2Ph; OR'' = OCMe3, OCMe2(CF3), OCMe(CF3)2, OC(CF3)3, OC(CF3)2(CF2CF2CF3), O-2,6-i-Pr2C6H3) in thf-d8, toluene-d8, or methylcyclohexane-d14 generated significant quantities of the anti rotamer. Anti to syn isomerization rate constants (k(a/s)) were determined by NMR methods and correlated with the nature of R, R', R'', and solvent. Activation parameters were calculated in toluene-d8 and thf-d8 for the series Mo(N-2,6-i-Pr2C6H3)(CHCMe2-Rh)(OR'')2 (where OR'' = OCMe2(CF3), OCMe(CF3)2, OC(CF3)3, or OC(CF3)2(CF2CF2CF3)). An estimate of k(a/s) for Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OCMe3)2 at 25-degrees-C was obtained using activation parameters. Values for k(a/s) were found to vary by up to 7 orders of magnitude (at 298 K), the smallest values for k./s being found in complexes that contain the most electron-withdrawing alkoxides in thf as a solvent. Equilibrium constants (K(eq) = k(a/s)/k(s/a)) at 25-degrees-C were obtained from 500 MHz H-1 NMR spectra and found to vary by up to approximately 2 orders of magnitude. Values for k(s/a) at 298 K were calculated and found to vary by up to 6 orders of magnitude in the same direction as k(a/s). 2,3-Bis(trifluoromethyl)norbornadiene (NBDF6) was added to photolyzed initiators of the type Mo(NR)(CHR')-(OR'')2 (where R = 2,6-i-Pr2C6H3, 2-t-BuC6H4, 1-adamantyl; R' = CMe3, CMe2Ph; OR'' = OCMe2(CF3), OCMe(CF3)2, OC(CF3)2(CF2CF2CF3)) in toluene-d8 and thf-d8 at temperatures where the rate of isomerization of anti rotamers to syn rotamers was negligible. The anti rotamer of Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)[OCMe(CF3)2]2 was found to be at least 2 orders of magnitude more reactive than the syn rotamer in thf-d8 and in toluene-d8. Results were similar for other complexes containing an arylimido ligand. The anti rotamer of every complex containing an arylimido ligand reacted with NBDF6 to yield a syn first insertion product that contained a trans C=C (anti --> syn,trans), and the syn rotamer produced a syn first insertion product that contained a cis C=C bond (syn --> syn,cis). For Mo(N-1-adamantyl)(CHCMe2Ph)[OCMe(CF3)2]2 both anti and syn isomers reacted at approximately the same rate with NBDF6; anti yielded a syn,trans first insertion product (anti --> syn,trans); but syn yielded an anti,trans first insertion product (syn --> anti,trans). The result of bulk polymerizations (40-100 equiv of NBDF6) at 25-degrees-C suggests a relationship between alkylidene rotamer isomerization rates and polymer cis/trans content when the initiator contains an arylimido ligand; high-cis polymers are formed when rotamer isomerization rates are negligible on the time scale of polymerization, while high-trans polymers are obtained when rotamer isomerization rates are fast on the time scale of polymerization. A mechanism is proposed for formation of cis polymers that consists of olefin attack (through the exo face) on the CNO face of the syn rotamer of the catalyst with C-7 of the monomer extending over the arylimido ring. Two plausible mechanisms for forming trans polymers are (i) a similar monomer approach to the CNO face of anti rotamers or (ii) monomer approach to the COO face of the catalyst in which the alkylidene ligand has rotated by up to 90-degrees. Bulk polymerizations with the 1-adamantylimido initiator containing OCMe(CF3)2 ligands do not yield trans or cis polymers selectively.