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

The m-terphenyl isocyanide complex, HCo­(CNArMes2)4 (ArMes2 = 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(CNArMes2)4 was assessed for a quantitative comparison. Acid bracketing experiments in THF solution revealed that HCo­(CNArMes2)4 has a Morris relative pKαTHF value of 38.5–40.7, which is considerably higher than that of HCo­(CO)4 (pKαTHF(calc) = 11.4) and thereby indicates insignificant Brønsted acidity. Furthermore, the relative acidity of HCo­(CNArMes2)4 rivals that of tetra-phosphine cobalt hydrides (i.e., HCo­(PR3)4; pKαTHF(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(CNArMes2)4–n monohydrides and [Co­(CO)n(CNArMes2)4–n]− (n = 1–4) metalates were prepared and characterized. Acid bracketing studies on the [Co­(CO)n(CNArMes2)4–n]− metalates in THF solution revealed a regular progression of increasing pKαTHF values as isocyanides are added to the Co center. However, the monoisocyanide tricarbonyl hydride, HCo­(CO)3(CNArMes2), possesses a pKαTHF 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(CNArMes2)4–n monohydrides.