Two- and Four-Electron Alkyne Ligands in Osmium−Cyclopentadienyl Chemistry:  Consequences of the π_⊥→M Interaction

Abstract

The complexes Os(η 5-C5H5)Cl{η 2-HC⋮CC(OH)R2}(PiPr3) (R = Ph (1a), Me (1b)) react with TlPF6 to give [Os(η 5-C5H5){η 2-HC⋮CC(OH)R2}(PiPr3)]PF6 (R = Ph (2a), Me (2b)). The structures of 1a and 2a have been determined by X-ray diffraction. The comparative study of the data reveals a shortening of the Os−C(alkyne) distances on going from 1a to 2a, whereas the acetylenic bond lengths remain almost identical. Comparison of their 1H and 13C{1H} NMR spectra shows that the HC⋮ proton resonances and the chemical shifts of the acetylenic carbon atoms of 2a and 2b are substantially shifted toward lower field than are those of 1a and 1b. DFT calculations were carried out on Os(η 5-C5H5)Cl(η 2-HC⋮CR)(PH3) (R = H (A), R = CH3 (A CH3 )) and [Os(η 5-C5H5)(η 2-HC⋮CR)(PH3)]+ (R = H (B), R = CH3 (B CH3 )) model systems in order to study the differences in bonding nature of the two parent alkyne complexes, 1 and 2. Calculations give geometries very close to the X-ray-determined ones, and by using the GIAO method we succeed in qualitatively reproducing the experimental 1H and 13C chemical shifts. Both structural and spectroscopic changes can be explained by the participation of the acetylenic second π orbital (π⊥) in the metal−alkyne bonding. As we go from 1 to 2 or from A to B, the extraction of the chloride ligand transforms the 2-electron-donor alkyne ligand to a 4-electron-donor ligand, with both the π|| and the π⊥ orbitals donating to the metal and stabilizing the otherwise 16-electron unsaturated complex 2. Calculations also predict an increase of dissociation energies of the alkyne, and an enhancement in the energy of rotation of the alkyne, for complex B. Finally, Bader's atoms in molecules (AIM) analysis shows that differences in coordination nature are also reflected in the topological properties of electron density.

Mechanism of alkyne conversion to carbyne by 14- or 16-electron Os(H)2ClL2X (L=PiPr3; X=OTf or B(C6H3(CF3)2)4)

“…The 31 P{ 1 H} NMR spectrum showed only one chemical shift. Appearance of acetylene carbons (C(a) and C(b)) at 199.5 ppm and 185.7 ppm in 13 C NMR also confirms 4 electron donation from the acetylene [6][7][8][9][10][11]. Analogous carbon chemical shifts are seen for…”

“…An alkyne proton was observed at 14.1 ppm as a triplet (J = 2.7 Hz) of doublets (J = 3.3 Hz) which couples to 2 P and to a hydride, respectively. This unusual chemical shift indicates 4-electron donation from acetylene to the metal [6][7][8][9][10][11]. One of two hydrides in the resulting adduct 2a was observed at À0.8 ppm as a triplet of doublets of doublets, coupled to two phosphines, to the alkynyl proton and to the other hydride.…”

Reactions of Hydride Complexes of Ruthenium and Osmium with Propargylic Alcohols: Preparation of Chelate Vinyl ­Derivatives

“…This forms the equatorial plane of the coordination octahedron, and it is completed by the two remaining apical phosphite ligands. Coordination of 1,1-diphenyl-2-propyl-1-ol to transition metals has already been reported, [17,18] but this is the first-known instance where the alkyne carbon atom acts as a η 1 donor and C,O chelation is observed; all other cases involve η 2 coordination of the triple bond. The four-membered chelation ring is characterised by two very compressed bond angles on the C,O donor atoms: C26-C39-Ru 98.8(2)°and C26-O13-Ru 94.7(1)°.…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…A range of studies examining the fluxionality and isomerisation of hydrocarbyl ligands have been reported including ruthenium() dihydride complexes using parahydrogen, 178 alkyl isomerisation in very unusual three coordinate iron() complexes, 179 hydride-carbyne to carbene in a CpOs complex that does not proceed via single α-hydride migration, 180,181 osmium() alkyne ligands as 2 and 4-electron donors, 182 and metal migration in cycloheptatrienyl complexes. 183 Exchange processes in η 2 -silane complexes of ruthenium() have also been investigated 184 and include the structure and bonding of [CpRu(η 2 -HSiCl 3 )(PMe 3 ) 2 ] ϩ .…”