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)(C 2 R 2 )(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(C 2 R 2 )(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 1 H 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 [WO 2 (S-Phoz) 2 ] (4), which shows highly fluxional behavior in solution. Variable-temperature 1 H NMR spectroscopy revealed three isomeric forms with respect to the ligand arrangement versus each other. Furthermore, compound 4 rearranges to tetranuclear oxo compound [W 4 O 4 (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.