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

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, 1a; Me, 1b; Ph, 1c). Oxidation with pyridine-N-oxide gave the corresponding W-oxo species [WO(C2R2)(S-Phoz)(2)] (R=H, 2a; Me, 2b; Ph, 2c). All W-oxo-alkyne complexes (2a, 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 (HNMR)-H-1 measurements, which revealed correlation of the photodissociation rate constant (2b>2a>2c) with the elongation of the alkyne CC 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 (HNMR)-H-1 spectroscopy revealed three isomeric forms with respect to the ligand arrangement versus each other. Furthermore, compound 4 rearranges to tetranuclear oxo compound [W4O4(-O)(6)(S-Phoz)(4)] (5) and dinuclear [{WO(-O)(S-Phoz)}(2)] (6) over time. The latter two were identified by single-crystal X-ray diffraction analyses.