Activation and Photoinduced Release of Alkynes on a Biomimetic Tungsten Center: The Photochemical Behavior of the W-S-Phoz System.

Abstract

The synthesis and structural determination of four tungsten alkyne complexes coordinated by the bio‐inspired S,N‐donor ligand 2‐(4′,4′‐dimethyloxazoline‐2′‐yl)thiophenolate (S‐Phoz) is presented. A previously established protocol that involved the reaction of the respective alkyne with the bis‐carbonyl precursor [W(CO)2(S‐Phoz)2] was used for the complexes [W(CO)(C2R2)(S‐Phoz)2] (R=H, 1 a; Me, 1 b; Ph, 1 c). Oxidation with pyridine‐N‐oxide gave the corresponding W‐oxo species [WO(C2R2)(S‐Phoz)2] (R=H, 2 a; Me, 2 b; Ph, 2 c). All W‐oxo‐alkyne complexes (2 a, b, c) were found to be capable of alkyne release upon light irradiation to afford five‐coordinate [WO(S‐Phoz)2] (3). The photoinduced release of the alkyne ligand was studied in detail by in situ 1H NMR measurements, which revealed correlation of the photodissociation rate constant (2 b>2 a>2 c) with the elongation of the alkyne C≡C bond in the molecular structures. Oxidation of [WO(S‐Phoz)2] (3) with pyridine‐N‐oxide yielded [WO2(S‐Phoz)2] (4), which shows highly fluxional behavior in solution. Variable‐temperature 1H NMR spectroscopy revealed three isomeric forms with respect to the ligand arrangement versus each other. Furthermore, compound 4 rearranges to tetranuclear oxo compound [W4O4(μ‐O)6(S‐Phoz)4] (5) and dinuclear [{WO(μ‐O)(S‐Phoz)}2] (6) over time. The latter two were identified by single‐crystal X‐ray diffraction analyses.