Synthesis and Characterization of Hydride−Alkynyl, Allenylidene, Carbyne, and Functionalized-Alkynyl Complexes Containing the [Os(<i>η</i>⁵-C₅H₅)(PⁱPr₃)₂]⁺ Fragment: The Complex [Os(<i>η</i>⁵-C₅H₅)(CCCPh₂)(PⁱPr₃)₂]PF₆, a New Type of Allenylidene Derivative from the Reactivity Point of View

Abstract

Complex Os(η5-C5H5)Cl(PiPr3)2 (1) reacts with equimolecular mixtures of TlPF6 and alkynes such as phenylacetylene and cyclohexylacetylene to give [OsH(η5-C5H5)(C⋮CR)(PiPr3)2]PF6 (R = Ph (2), Cy (3)). The structure of 2 in the solid state has been determined by X-ray diffraction analysis. The distribution of ligands around the metallic center can be described as a four-legged piano-stool geometry with the hydride and alkynyl ligands mutually transoid. The reaction of 1 with 2-phenyl-3-butyn-2-ol and TlPF6 leads to [OsH(η5-C5H5){C⋮CC(OH)MePh}(PiPr3)2]PF6 (4), which evolves into the hydride−enynyl complex [OsH(η5-C5H5){C⋮CC(Ph)CH2}(PiPr3)2]PF6 (5) in solution of chloroform. Treatment of 1 with 1,1-diphenyl-2-propyn-1-ol and TlPF6 affords [OsH(η5-C5H5){C⋮CC(OH)Ph2}(PiPr3)2]PF6 (6), which reacts with KOH in methanol to give the neutral compound Os(η5-C5H5){C⋮CC(OH)Ph2}(PiPr3)2 (7) by extraction of the hydride ligand. The addition of 1 equiv of HPF6 to the solutions of 7 leads to the allenylidene, [Os(η5-C5H5)(CCCPh2)(PiPr3)2]PF6 (8), which affords the dicationic carbyne derivative [Os(η5-C5H5)(CCHCPh2)(PiPr3)2](PF6)2 (9) by reaction with HPF6. The structure of 9 in the solid state has been also determined by X-ray diffraction analysis. In this case, the geometry around the osmium center is close to octahedral with the triisopropylphosphine ligands mutually cis disposed (P−Os−P = 105.12(8)°). Complex 8 also reacts with nucleophilic reagents; the reaction with CH3Li gives Os(η5-C5H5){C⋮CC(CH3)Ph2}(PiPr3)2 (10), whereas the reactions with acetone and methanol solutions of KOH afford Os(η5-C5H5){C⋮CC[CH2C(O)CH3]Ph2}(PiPr3)2 (11) and Os(η5-C5H5){C⋮CC(OCH3)Ph2}(PiPr3)2 (12), respectively. To understand the chemical behavior of 8, EHT−MO calculations on the model compounds Os(η5-C5H5)Cl(CCCH2)(PH3) (13) and [Os(η5-C5H5)(CCCH2)L(PH3)]+ (L = PH3 (14), CO (15)) have been also carried out. The results suggest that the behavior of 8 as nucleophile is a consequence of the high electron density of the allenylidene ligand, while the behavior as γ-electrophile is due to its cationic nature. In addition, we have determined by ab initio calculations the energies of stabilization by protonation of 13−15 with a naked proton. In the three cases the formation of the corresponding carbyne derivatives [Os(η5-C5H5)Cl(CCHCH2)(PH3)]+ (16; 267 kcal·mol-1), [Os(η5-C5H5)(CCHCH2)(PH3)2]2+ (17; 180 kcal·mol-1), and [Os(η5-C5H5)(CCHCH2)(CO)(PH3)]2+ (18; 157 kcal·mol-1) involves a stabilization of the system.