Bifunctional Ligands: Evaluating the Role of Acidic Protons in the Secondary Coordination Sphere.

ABSTRACT

To evaluate bifunctional ligand reactivity involving NH acidic sites in the secondary coordination sphere of metal complexes, complexes where the proton has been substituted with a methyl group (NMe) are often investigated. An alternative strategy would involve substitution of the NH group for an O. This contribution considers and compares the merits of these approaches; the synthesis and characterization of cationic square planar Rh carbonyl complexes bearing diprotic bispyrazole pyridine ligand L1, and the bis-methylated pyrazole pyridine ligand L1Me are described. The syntheses and characterization of the novel monoprotic pyrazole isoxazole pyridine ligand L2 and aprotic bisisoxazole pyridine ligand L3, and their corresponding Rh carbonyl complexes are also described. The different CO stretching frequencies of all four Rh-complexes suggest that substitutions of NH with NMe, as well as O, lead to significant electronic differences, and these differences are further demonstrated to lead to different ligand addition/substitution reactivities of the four isoelectronic Rh-complexes. Overall, the results suggest that the electronic differences arising due to NH substitutions can be significant and must be accounted for prior to invoking the participation of the proton.