Interaction of Triosmium Complexes with Hydrogen Chloride: A Model for Fine-Tuning Regioselective Protonation in Metal Clusters

The reaction of Os₃(CO)₁₀(CNR)(NCMe) (1) with HCl was studied and this system was found to be a good model for observing the fine-tuning of site-selective protonation in metal complexes. Three products including the protonated species [(-H)Os₃(CO)₁₀(CNR)(NCMe)]⁺Cl^– (2), the bridging aminocarbyne complex (µ-Cl)Os₃(CO)₁₀(µ₂-C = NHR) (3), and the hydrido derivative (-H)Os₃Cl(CO)₁₀(CNR) (4) were obtained for the reaction. The site of protonation, either on the Os center or on the nitrogen atom of coordinated isocyanide, was tuned in a sensitive manner by the nature of the coordinated isocyanides, the polarity of the solvents, and the strengths of the acids, leading to different product distributions. The more electron withdrawing isocyanides (CNCH₂Ph, CNPh) favor the formation of the aminocarbyne complex 3. In a nonpolar solvent like cyclohexane the reaction afforded 3 as the main product. Furthermore, complex 2 was converted to (-H)Os₃(CO)₁₀(₂-CONHR) (5) upon hydrolysis, in which the coordinated isocyanide was transformed to a carboxamido group. This was verified by obtaining the deuterated species (-H)Os₃(CO)₁₀(₂-CONDPr) when [(-H)Os₃(CO)₁₀(CNPr)(NCMe)]⁺Cl^– (2a) was treated with D₂O in THF. The molecular structures of (µ₂-Cl)Os₃(CO)₁₀(µ₂-C = NHCH₂Ph) (3b) and (-H)Os₃Cl(CO)₁₀(CNPr) (4a) were determined by X-ray diffraction analyses. Complex 3b contains both bridging chloride and bridging aminocarbyne groups, whereas 4a contains a terminal chloride, a terminal isocyanide and a bridging hydride.