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.

Inspired by the proposed inner-sphere mechanism of the tungstoenzyme acetylene hydratase, we have designed tungsten acetylene complexes and investigated their reactivity. Here, we report the first intermolecular nucleophilic attack on a tungsten-bound acetylene (C 2 H 2 ) in bioinspired complexes employing 6-methylpyridine-2-thiolate ligands. By using PMe 3 as a nucleophile, we isolated cationic carbyne and alkenyl complexes.

The synthesis and characterization of a biomimetic system that can reversibly bind acetylene (ethyne) is reported. The system has been designed to mimic catalytic intermediates of the tungstoenzyme acetylene hydratase. The thiophenyloxazoline ligand S-Phoz (2-(4',4'-dimethyloxazolin-2'-yl)thiophenolate) is used to generate a bioinspired donor environment around the W center, facilitating the stabilization of W-acetylene adducts. The featured complexes [W(C2 H2 )(CO)(S-Phoz)2 ] (2) and [WO(C2 H2 )(S-Phoz)2 ] (3) are extremely rare from a synthetic and structural point of view as very little is known about W-C2 H2 adducts. Upon exposure to visible light, 3 can release C2 H2 from its coordination sphere to yield the 14-electron species [WO(S-Phoz)2 ] (4). Under light-exclusion 4 re-activates C2 H2 making this the first fully characterized system for the reversible activation of acetylene.

Bioinspired complexes employing the ligands 6‐tert‐butylpyridazine‐3‐thione (SPn) and pyridine‐2‐thione (SPy) were synthesized and fully characterized to mimic the tungstoenzyme acetylene hydratase (AH). The complexes [W(CO)(C2H2)(CHCH‐SPy)(SPy)] (4) and [W(CO)(C2H2)(CHCH‐SPn)(SPn)] (5) were formed by intramolecular nucleophilic attack of the nitrogen donors of the ligand on the coordinated C2H2 molecule. Labelling experiments using C2D2 with the SPy system revealed the insertion reaction proceeding via a bis‐acetylene intermediate. The starting complex [W(CO)(C2H2)(SPy)2] (6) for these studies was accessed by the new acetylene precursor mixture [W(CO)(C2H2)n(MeCN)3−nBr2] (n=1 and 2; 7). All complexes represent rare examples in the field of W−C2H2 chemistry with 4 and 5 being the first of their kind. In the ongoing debate on the enzymatic mechanism, the findings support activation of acetylene by the tungsten center.