Synthesis and Alkyne Metathesis Activity of Pincer Rhenium Carbyne Complexes

While alkyne metathesis reactions involving d0 carbynes
(alkylidynes) are well documented, those involving well-defined non-d0 carbynes are still rare. This work reports the synthesis
and alkyne metathesis activity of d2 Re(V) carbyne complexes
supported by a [LXL]-type monoanionic [PNP]-pincer ligand, a [LLX]-type
monoanionic [PNO]-pincer ligand, and a [ONOH]-bidentate
ligand derived from the Schiff base 2-[(2-hydroxyphenyl)iminomethyl]phenol.
Treatment of Re(CCH2Ph)Cl2(PMePh2)3 (1) with bis(2-diphenylphosphino-4-tolyl)amine
(PNHP) and 2-{(2-diphenylphosphino-phenyl)iminomethyl}phenol (PNOH)
in the presence of NEt3 produced the pincer complexes Re(CCH2Ph)Cl(PMePh2)(PNP) (2) and Re(CCH2Ph)Cl(PMePh2)(PNO) (3), respectively.
The Schiff base 2-[(2-hydroxyphenyl)iminomethyl]phenol (HONOH) reacted with Re(CCH2Ph)Cl2(PMePh2)3 (1) to give Schiff base complex
Re(CCH2Ph)Cl(PMePh2)(ONOH) (4) (ONOH = o-O-C6H4–CHN-(o-C6H4OH)) bearing a [ONOH]-bidentate ligand. The [PNP]- and [PNO]-pincer complexes are catalytically
active for homometathesis of neat 1-methoxy-4-(1-propyn-1-yl)benzene
at 150 °C, while the complex bearing the [ONOH]-bidentate
Schiff base ligand is catalytically inactive under similar conditions.
The energy profiles for metathesis reactions of model pincer rhenium
alkyne-carbyne complexes have been calculated with DFT methods.