Multimodal Study of Secondary Interactions in Cp*Ir Complexes of Imidazolylphosphines Bearing an NH Group

Hydrogen bonding phenomena are explored using a combination of X-ray diffraction, NMR and IR spectroscopy, and DFT calculations. Three imidazolylphosphines R₂PImH (ImH = imidazol-2-yl, R = t-butyl, i-propyl, phenyl, 1a−1c) and control phosphine (i-Pr)₂PhP (1d) lacking an imidazole were used to make a series of complexes of the form Cp*Ir(L₁)(L₂)(phosphine). In addition, in order to suppress intermolecular interactions with either imidazole nitrogen, 1e, a di(isopropyl)imidazolyl analogue of 1b was made along with its doubly ¹⁵N-labeled isotopomer to explore bonding interactions at each imidazole nitrogen. A modest enhancement of transfer hydrogenation rate was seen when an imidazolylphosphine ligand 1b was used. Dichloro complexes (L₁ = L₂ = Cl, 2a−2c,2e) showed intramolecular hydrogen bonding as revealed by four X-ray structures and various NMR and IR data. Significantly, hydride chloride complexes [L₁ = H, L₂ = Cl, 3a−3c and 3e-(¹⁵N)₂] showed stronger hydrogen bonding to chloride than hydride, though the solid-state structure of 3b evinced intramolecular Ir−H···H−N bonding reinforced by intermolecular N···H−N bonding between unhindered imidazoles. These results are compared to literature examples, which show variations in preferred hydrogen bonding to hydride, halide, CO, and NO ligands. Surprising differences were seen between the dichloro complex 2b with isopropyl groups on phosphorus, which appeared to exist as a mixture of two conformers, and related complex 2a with tert-butyl groups on phosphorus, which exists in chlorinated solvents as a mixture of conformer 2a-endo and chelate 5a-Cl, the product of ionization of one chloride ligand. This difference became apparent only through a series of experiments, especially ¹⁵N chemical shift data from 2D ¹H−¹⁵N correlation. The results highlight the difficulty of characterizing hemilabile, bifunctional complexes and the importance of innocent ligand substituents in determining structure and dynamics.