Structural Mimics of Acetylene Hydratase: Tungsten Complexes Capable of Intramolecular Nucleophilic Attack on Acetylene

Abstract

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.

Bioinspired Nucleophilic Attack on a Tungsten-Bound Acetylene: Formation of Cationic Carbyne and Alkenyl Complexes

“…Longer reaction times led to the insertion of a second C 2 H 2 into the W–N bond forming [W(CO)(C 2 H 2 )(HCCH-6-MePyS)(6-MePyS)] ( 2 ), as previously observed in the unsubstituted analogue [W(CO)(C 2 H 2 )(HCCH-PyS)(PyS)]. 15 However, the additional methyl group significantly decreases intramolecular insertion because even after 24 h and repeated addition of C 2 H 2 only partial conversion is observed. Furthermore, in the absence of additional C 2 H 2 , 2 reacts reversibly to 1 under the elimination of acetylene or polyacetylene depending on the solvent.…”

“… 14 , 15 In a recent publication, the S,N-bidentate ligand pyridine-2-thiolate (PyS) was introduced to the tungsten(II) center to model the active site of AH as in Inter1 ( Scheme 1 ). 15 In contrast to previously reported structural model compounds, 14 the coordination of a second C 2 H 2 and subsequent insertion into the W–N bond occurred, showing that a second coordinated C 2 H 2 is sufficiently activated to undergo a nucleophilic attack. A similar behavior was observed in molybdenum and tungsten complexes, where a nucleophilic attack on one of two coordinated hexafluorobut-2-yne moieties took place, yielding a η 2 -vinyl complex.…”

“…Compared to the unsubstituted analogue [W(CO)(C 2 H 2 )(PyS) 2 ] with W–N distances of 2.161 and 2.212 Å, 1 exhibits slightly longer bonds [2.197(3) and 2.259(4) Å]. 15 As was already observed in [MoO 2 (6-MePyS) 2 ], the nitrogen atom in 6-MePyS is not able to bind to the metal center as tightly as it does in the unsubstituted version because of the methyl group in the ortho position. 25 …”

Synthesis and Reactivity of a Bioinspired Molybdenum(IV) Acetylene Complex

“…26 , 35 Recently, we observed the reversible insertion of a second C 2 H 2 into the W–N bond in [W(CO)(C 2 H 2 )(6-MePyS) 2 ] and especially in the unsubstituted analogue [W(CO)(C 2 H 2 )(PyS) 2 ]. 31 , 33 However, when 1 was stirred under a C 2 H 2 atmosphere for 24 h, insertion of a second C 2 H 2 did not occur.…”

Soft Scorpionate Hydridotris(2‐mercapto‐1‐methylimidazolyl) borate) Tungsten‐Oxido and ‐Sulfido Complexes as Acetylene Hydratase Models

“…While this might point toward a first‐shell mechanism in AH, future models capable of nucleophilic attack of water will have to provide support. The knowledge gained here as well as in our previous work on intramolecular nucleophilic attack on acetylene is nevertheless interesting as it represents the basis for the development of such future models.…”