Oxorhenium(V) Complexes with Phenolate–Oxazoline Ligands: Influence of the Isomeric Form on the O‑Atom-Transfer Reactivity

The bidentate phenolate–oxazoline
ligands 2-(2′-hydroxyphenyl)-2-oxazoline (1a,
Hoz) and 2-(4′,4′-dimethyl-3′,4′-dihydrooxazol-2′-yl)­phenol
(1b, Hdmoz) were used to synthesize two sets of oxorhenium­(V)
complexes, namely, [ReOCl2(L)­(PPh3)] [L = oz
(2a) and dmoz (2b)] and [ReOX­(L)2] [X = Cl, L = oz (3a or 3a′); X
= Cl, L = dmoz (3b); X = OMe, L = dmoz (4)]. Complex 3a′ is a coordination isomer (N,N-cis isomer) with respect to the orientation
of the phenolate–oxazoline ligands of the previously published
complex 3a (N,N-trans
isomer). The reaction of 3a′ with silver triflate
in acetonitrile led to the cationic compound [ReO­(oz)2(NCCH3)]­(OTf) ([3a′]­(OTf)). Compound 4 is a rarely observed isomer with a trans-ORe–OMe
unit. Complexes 3a, 3a′, [3a′]­(OTf), and 4 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 2a and 2b. A comparison of the two isomeric complexes 3a and 3a′ showed significant differences in activity: 87% 3a vs 16% 3a′ sulfoxide yield. When complex
[3a′]­(OTf) was used, the yield was 57%. Density
functional theory calculations circumstantiate all of the proposed
intermediates with N,N-trans configurations
to be lower in energy compared to the respective compounds with N,N-cis configurations. Also, no interconversions
between N,N-trans and N,N-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 3a, 3a′, and [3a′]­(OTf) proved the N,N-cis complexes 3a′ and [3a′]­(OTf) to be slower
by a factor of ∼4.