Bioinspired Tungsten Complexes Employing a Thioether Scorpionate Ligand

Neutral W(V) Complexes Featuring the W2O2(µ-O)2 Core and Amino Acids or EDTA Derivatives as Ligands: Synthesis and Structural Characterization

Multinuclear complexes of heavy metals, such as tungsten, have demonstrated considerable potential as candidates for advanced radiocontrast agents. Of particular interest is the development of stable non-ionic compounds with high metal content and reasonably low osmolality in solution. Accordingly, we have synthesized a series of neutral W(V) complexes that contain the W2O2(µ-O)2 core and amino acids or disubstituted EDTA derivatives as ligands. The compounds were prepared from the oxalatotungstate(V) complex via a convenient procedure utilizing microwave heating. Their detailed characterization was accomplished by electrospray ionization high-resolution mass spectrometry (ESI-HRMS), 1H and 13C NMR spectroscopy, elemental analysis, and X-ray crystallography. Further experiments to evaluate the utility of the complexes as radiocontrast media were precluded by their poor aqueous solubility.

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

A series of WIV alkyne complexes with the sulfur-rich ligand hydridotris(2-mercapto-1-methylimidazolyl) borate) (TmMe ) are presented as bio-inspired models to elucidate the mechanism of the tungstoenzyme acetylene hydratase (AH). The mono- and/or bis-alkyne precursors were reacted with NaTmMe and the resulting complexes [W(CO)(C2 R2 )(TmMe )Br] (R=H 1, Me 2) oxidized to the target [WE(C2 R2 )(TmMe )Br] (E=O, R=H 4, Me 5; E=S, R=H 6, Me 7) using pyridine-N-oxide and methylthiirane. Halide abstraction with TlOTf in MeCN gave the cationic complexes [WE(C2 R2 )(MeCN)(TmMe )](OTf) (E=CO, R=H 10, Me 11; E=O, R=H 12, Me 13; E=S, R=H 14, Me 15). Without MeCN, dinuclear complexes [W2 O(μ-O)(C2 Me2 )2 (TmMe )2 ](OTf)2 (8) and [W2 (μ-S)2 (C2 Me2 )(TmMe )2 ](OTf)2 (9) could be isolated showing distinct differences between the oxido and sulfido system with the latter exhibiting only one molecule of C2 Me2 . This provides evidence that a fine balance of the softness at W is important for acetylene coordination. Upon dissolving complex 8 in acetonitrile complex 13 is reconstituted in contrast to 9. All complexes exhibit the desired stability toward water and the observed effective coordination of the scorpionate ligand avoids decomposition to disulfide, an often-occurring reaction in sulfur ligand chemistry. Hence, the data presented here point toward a mechanism with a direct coordination of acetylene in the active site and provide the basis for further model chemistry for acetylene hydratase.