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

Abstract

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.

Stereoisomers and functional groups in oxidorhenium(v) complexes: effects on catalytic activity

“…9,10 Over the last few years the chemistry of oxidorhenium(V) complexes and their application in homogeneous catalytic epoxidation of cyclooctene further developed. 6,[11][12][13][14][15][16][17][18] Compared to the impressive activities of MTO however, oxidorhenium(V) complexes had been inferior epoxidation catalysts, with the published complexes until 2014 only reaching turnovers of TON < 75. 17 Our initial research efforts concentrated on phenol-pyrazole (HpyzR) ligands, equipped with various substituents R (R = H, Me, OMe, Br, NO 2 ) on the phenol moiety ( Fig.…”

“…1), in perchlorate reduction chemistry. 12 Based on the ground laying work of the Abu-Omar group, a dissociative oxygen atom transfer (OAT) mechanism of redox-catalysis was postulated, with the Re atom cycling between Re(V) and Re(VII) (Scheme 1). [22][23][24][25] In perchlorate reduction catalysis, we could show that isomers play an important role for catalyst activity.…”

“…[22][23][24][25] In perchlorate reduction catalysis, we could show that isomers play an important role for catalyst activity. 11,12 For an octahedral complex with two ON-bidentate and two monodentate ligands like 3a-c, six stereoisomers could form in principle (Scheme 2). Isomers A and B have a trans arrangement of the oxido and X ligand, with the bidentate ON-ligand in the equatorial plane.…”

“…It could be shown that trans-4 is more active in perchlorate reduction compared to cis-4. 12 The initial step of the catalytic cycle is loss of the chlorido ligand, creating a vacant site on the rhenium atom. In cis-4, the neutral nitrogen atom of the oxazoline moiety is trans to the chlorido ligand, exerting a weaker trans-influence compared to trans-4, where the phenolate oxygen atom is trans to the chlorido ligand (Scheme 2).…”

“…This situation elongates the Re-Cl bond in trans-4 (2.4093(10) Å), compared to cis-4 (2.383(3) Å). 12 Hence the loss of chlorido ligand requires less energy for trans-4. The same is true for the subsequent steps in the catalytic cycle.…”