A Well-Defined Isocyano Analogue of HCo(CO)4. 2: Relative Brønsted Acidity as a Function of Isocyanide Ligation

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• 作者：Alex E. Carpenter ; Chinglin Chan ; Arnold L. Rheingold ; Joshua S. Figueroa
• 刊名：Organometallics
• 出版年：2016
• 出版时间：July 25, 2016
• 年：2016
• 卷：35
• 期：14
• 页码：2319-2326
• 全文大小：491K
• 年卷期：0
• ISSN：1520-6041

文摘

The m-terphenyl isocyanide complex, HCo(CNAr^Mes2)₄ (Ar^Mes2 = 2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃), serves as a unique example of a well-defined isocyano analogue to HCo(CO)₄. Given the well documented Brønsted acidity of HCo(CO)₄ in both protic and nonprotic media, the Brønsted acidity of HCo(CNAr^Mes2)₄ was assessed for a quantitative comparison. Acid bracketing experiments in THF solution revealed that HCo(CNAr^Mes2)₄ has a Morris relative pK_α^THF value of 38.5–40.7, which is considerably higher than that of HCo(CO)₄ (pK_α^THF_(calc) = 11.4) and thereby indicates insignificant Brønsted acidity. Furthermore, the relative acidity of HCo(CNAr^Mes2)₄ rivals that of tetra-phosphine cobalt hydrides (i.e., HCo(PR₃)₄; pK_α^THF_(calc) ≥ 48), despite the good π-acidity properties of the isocyano unit. To systematically determine the effect of substituting an isocyanide for a CO ligand on the acidity of the Co–H unit in HCoL₄ complexes, the full series of HCo(CO)_n(CNAr^Mes2)_4–n monohydrides and [Co(CO)_n(CNAr^Mes2)_4–n]⁻ (n = 1–4) metalates were prepared and characterized. Acid bracketing studies on the [Co(CO)_n(CNAr^Mes2)_4–n]⁻ metalates in THF solution revealed a regular progression of increasing pK_α^THF values as isocyanides are added to the Co center. However, the monoisocyanide tricarbonyl hydride, HCo(CO)₃(CNAr^Mes2), possesses a pK_α^THF value of 28.6–32.5, which is also significantly higher than that of HCo(CO)₄ and the monophosphine complex HCo(CO)₃(PPh₃). Accordingly, the unconventional ability of isocyanide ligands to function as stronger σ-donors than organophosphines is discussed within the context of both the Brønsted acidity and spectroscopic features of the HCo(CO)_n(CNAr^Mes2)_4–n monohydrides.