Reversible C-H bond activation at a triosmium centre: A comparative study of the reactivity of unsaturated triosmium clusters Os3(CO)8(μ-dppm)(μ-H)2 and Os3(CO)8(μ-dppf)(μ-H)2 with activated alkynes.

Heating a benzene solution of the unsaturated cluster Os 3 (CO) 8 (μ-dppm)(μ-H) 2 ( 1 ) [dppm = bis(diphenylphosphino)methane] with MeO 2 CC CCO 2 Me (DMAD) or EtO 2 CC CCO 2 Et (DEAD) at 80 °C furnished the dinuclear compounds Os 2 (CO) 4 (μ-dppm)(μ-η 2 ;η 1 ;к 1 -RO 2 CCCHCO 2 R)(μ-H) ( 3a , R = Me, 3b , R = Et) and the saturated trinuclear complexes Os 3 (CO) 7 (μ-dppm)(μ 3 -η 2 ;η 1 ;η 1 -RO 2 CCCCO 2 R)(μ-H) 2 ( 4a , R = Me, 4b , R = Et). In contrast, similar reactions using unsaturated Os 3 (CO) 8 (μ-dppf)(μ-H) 2 ( 2 ) [dppf = bis(diphenylphosphino)ferrocene] afforded only the trinuclear complexes Os 3 (CO) 8 (μ-dppf)(μ-η 2 ;η 1 -RO 2 CCHCCO 2 R)(μ-H) ( 5a , R = Me; 5b , R = Et) and Os 3 (CO) 7 (μ-dppf)(μ 3 -η 2 ;η 1 ;η 1 -RO 2 CCCCO 2 R)(μ-H) 2 ( 6a , R = Me; 6b , R = Et). Control experiments confirm that 5a and 5b decarbonylate at 80 °C to give 6a and 6b , respectively. Both 5a and 5b exist as a pair of isomers in solution, as demonstrated by 1 H NMR and 31 P{ 1 H} NMR spectroscopy. DFT calculations on cluster 5a (as the dppf-Me 4 derivative) indicate that the isomeric mixture derives from a torsional motion that promotes the conformational flipping of the cyclopentadienyl groups of the dppf-Me 4 ligand relative to the metallic plane. VT NMR measurements on clusters 6a and 6b indicate that while the hydride ligand associated with the dppf-bridged Os-Os bond is nonfluxional at room temperature, the second hydride rapidly oscillates between the two non-dppf-bridged Os-Os edges. DFT examination of this hydride fluxionality confirms a “windshield wiper” motion for the labile hydride that gives rise to a time-average coupling of this hydride to both phosphorus centers of the dppf ligand. Thermolysis of 6a and 6b in refluxing toluene yielded Os 3 (CO) 7 (μ-dppf)(μ-η 2 ;η 1 ;к 1 -CCHCO 2 R) ( 7a , R=Me; 7b , R=Et). The vinylidene moieties in 7a and 7b derive from the carbon-carbon bond cleavage of coordinated alkyne ligands, and these two products exhibit high thermal stability in refluxing toluene. [ABSTRACT FROM AUTHOR]