Oxorhenium(V)
Complexes with Phenolate–Oxazoline
Ligands: Influence of the Isomeric Form on the O‑Atom-Transfer
Reactivity
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Abstract
The bidentate phenolate–oxazoline
ligands 2-(2′-hydroxyphenyl)-2-oxazoline (<b>1a</b>,
Hoz) and 2-(4′,4′-dimethyl-3′,4′-dihydrooxazol-2′-yl)phenol
(<b>1b</b>, Hdmoz) were used to synthesize two sets of oxorhenium(V)
complexes, namely, [ReOCl<sub>2</sub>(L)(PPh<sub>3</sub>)] [L = oz
(<b>2a</b>) and dmoz (<b>2b</b>)] and [ReOX(L)<sub>2</sub>] [X = Cl, L = oz (<b>3a</b> or <b>3a′</b>); X
= Cl, L = dmoz (<b>3b</b>); X = OMe, L = dmoz (<b>4</b>)]. Complex <b>3a′</b> is a coordination isomer (<i>N</i>,<i>N</i>-cis isomer) with respect to the orientation
of the phenolate–oxazoline ligands of the previously published
complex <b>3a</b> (<i>N</i>,<i>N</i>-trans
isomer). The reaction of <b>3a′</b> with silver triflate
in acetonitrile led to the cationic compound [ReO(oz)<sub>2</sub>(NCCH<sub>3</sub>)](OTf) ([<b>3a′</b>](OTf)). Compound <b>4</b> is a rarely observed isomer with a <i>trans</i>-ORe–OMe
unit. Complexes <b>3a</b>, <b>3a′</b>, [<b>3a′</b>](OTf), and <b>4</b> were tested as catalysts in the reduction
of a perchlorate salt with an organic sulfide as the O acceptor and
found to be active, in contrast to <b>2a</b> and <b>2b</b>. A comparison of the two isomeric complexes <b>3a</b> and <b>3a′</b> showed significant differences in activity: 87% <b>3a</b> vs 16% <b>3a′</b> sulfoxide yield. When complex
[<b>3a</b>′](OTf) was used, the yield was 57%. Density
functional theory calculations circumstantiate all of the proposed
intermediates with <i>N</i>,<i>N</i>-trans configurations
to be lower in energy compared to the respective compounds with <i>N</i>,<i>N</i>-cis configurations. Also, no interconversions
between <i>N</i>,<i>N</i>-trans and <i>N</i>,<i>N</i>-cis configurations are predicted, which is in
accordance with experimental data. This is interesting because it
contradicts previous mechanistic views. Kinetic analyses determined
by UV–vis spectroscopy on the rate-determining oxidation steps
of <b>3a</b>, <b>3a′</b>, and [<b>3a′</b>](OTf) proved the <i>N</i>,<i>N</i>-cis complexes <b>3a′</b> and [<b>3a′</b>](OTf) to be slower
by a factor of ∼4
