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

The m-terphenyl isocyanide complex, HCo-(CNArMes2)(4) (Ar-Mes2)(4) (Ar-Mes2 = 2,6-(2,4,6-Me3C6H2)(2)C6H3), serves as a unique example of a well-defined isocyano analogue to HCo(CO)(4). Given the well documented Bronsted acidity of HCo(CO)(4) in both protic and nonprotic media, the Bronsted acidity of HCo(CNArMes2)(4) was assessed for a quantitative comparison. Acid bracketing experiments in THE solution revealed that HCo(CNArMes2)(4) has a Morris relative pK(alpha)(THF) value of 38.5-40.7, which is considerably higher than that of HCo(CO)(4) (pK(alpha)((calc))(THF) = 11.4) and thereby indicates insignificant Bronsted acidity. Furthermore, the relative acidity of HCo(CNArMes2)(4) rivals that of tetra-phosphine cobalt hydrides (i.e., HCo(PR3)(4); pK(alpha)((calc))(THF) >= 48), despite the good pi-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 THE solution revealed a regular progression of increasing p(alpha)(THF) values as isocyanides are added to the Co center. However, the monoisocyanide tricarbonyl hydride, HCo(CO)(3)(CNArMes2), possesses a pK(alpha)(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 sigma-donors than organophosphines is discussed within the context of both the Bronsted acidity and spectroscopic features of the HCo(CO)(n)(CNArMes2)(4-n)](-) monohydrides.