Synthesis, Structures,
and Norbornene ROMP Behavior
of <i>o</i>-Aryloxide-N-Heterocyclic Carbene <i>p</i>-Cymene Ruthenium Complexes
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Abstract
Treatment of the <i>o</i>-hydroxyaryl imidazolium
proligands
(2-OH-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>C<sub>6</sub>H<sub>2</sub>)(R)(C<sub>3</sub>H<sub>3</sub>N<sub>2</sub>)<sup>+</sup>Br<sup>–</sup> (R = <sup><i>i</i></sup>Pr (<b>1a</b>), <sup><i>t</i></sup>Bu (<b>1b</b>), Ph
(<b>1c</b>), Mes (<b>1d</b>)) with 3 equiv of Ag<sub>2</sub>O afforded the corresponding silver complexes <b>2a</b>–<b>d</b>. The subsequent metal-exchange reactions with [(<i>p</i>-cymene)RuCl<sub>2</sub>]<sub>2</sub> at room temperature
yielded the desired <i>o</i>-aryloxide-N-heterocyclic carbene <i>p</i>-cymene ruthenium complexes <b>3a</b>–<b>d</b> in nearly quantitative yields. All the complexes were characterized
by <sup>1</sup>H and <sup>13</sup>C NMR, high-resolution mass spectrometry
(HRMS), and elemental analysis. The molecular structure of complex <b>3b</b> was determined by single-crystal X-ray diffraction analysis.
The ring-opening metathesis polymerization (ROMP) of norbornene (NBE)
with <b>3a</b>–<b>d</b> was studied. Among them,
complex <b>3d</b> showed high activity and efficiency toward
ROMP of NBE at 85 °C without the need for any cocatalyst, and
polymers with very high molecular weight (>10<sup>6</sup>) and
narrow
molecular weight distributions were obtained. This complex can also
catalyze the alternating copolymerization of NBE and cyclooctene (COE)