Molybdenum (VI) Imido Complexes Derived from Chelating Phenols: Synthesis, Characterization and ε-Caprolactone ROP Capability

Reaction of the bulky bi-phenols 2,2'-RCH[4,6-(t-Bu)(2)C6H2OH](2) (R = Me (LH2)-H-1Me, Ph (LH2)-H-1Ph) with the bis (imido) molybdenum (VI) tert-butoxides [Mo (NR1) (NR2) (Ot-Bu)(2)] (R-1 = R-2 = 2,6-C6H3-i-Pr-2; R-1 = t-Bu, R-2 = C6F5) afforded, following the successive removal of tert-butanol, the complexes [Mo (NC(6)H(3)i-Pr-2-2,6)(2)L-1Me] (1), [Mo(NC(6)H(3)i-Pr-2-2,6)(2)L-1Ph] (2) and [Mo (Nt-Bu)(mu-NC6F5)(L-1Me)](2) (3). Similar use of the tri-phenol 2,6-bis (3,5-di-tert-butyl-2-hydroxybenzyl)-4-methylphenol ((LH3)-H-2) with [Mo(NC(6)H(3)i-Pr-2-2,6)(2) (Ot-Bu)(2)] afforded the oxo-bridged product [Mo(NC(6)H(3)i-Pr-2-2,6)(NCMe)(mu-O)(LH)-H-2](2) (4), whilst use of the tetra-phenols alpha,alpha,alpha',alpha'-tetrakis (3,5-di-tert-butyl-2-hydroxyphenyl)-p-or -m-xylene (LH4)-H-3p/(LH4)-H-3m led to {[Mo (NC(6)H(3)i-Pr-2-2,6)(2)](2)(mu-L-3p)} (5) or {[Mo(NC(6)H(3)i-Pr-2-2,6)(2)](2)(mu-L-3m)} (6), respectively. Similar use of [Mo(NC6F5)(2)(Ot-Bu)(2)] with (LH4)-H-3p afforded, after work-up, the complex {[Mo(NC6F5)(Ot-Bu)(2)](2)(mu-L-3p)}center dot 6MeCN (7.6MeCN). Molecular structures of 1, 2 center dot CH2Cl2, 3, 4 center dot 6MeCN, 6 center dot 2C(6)H(14), and 7 center dot 6MeCN are reported and these complexes have been screened for their ability to ring open polymerize (ROP) epsilon-caprolactone; for comparative studies the precursor complex [Mo(NC(6)H(3)i-Pr-2-2,6)(2)Cl-2(DME)] (DME = 1,2-dimethoxyethane) has also been screened. Results revealed that good activity is only achievable at temperatures of >= 100 degrees C over periods of 1 h or more. Polymer polydispersities were narrow, but observed molecular weights (M-n) were much lower than calculated values.