Mild activation of Ir[sbnd]Cl bond upon the interaction of pincer iridium hydride (tBuPCP)IrH(Cl) with acids and bases

Abstract

The coordination of nitrogen bases (CH3CN, PhCN and 2-hydroxymetylpyridine (HMPy)) to (tBuPCP)IrH(Cl) (1) was investigated by variable temperature NMR, IR, UV–vis spectroscopies combined with DFT/M06 calculations. Hydrogen bonding of 1 and its (tBuPCP)IrH(Cl)(CO) with alcohols was also studied. At room temperature the formation constants Kf of complex (tBuPCP)IrH(Cl)(PhCN) (2) is ca. 3.5 times higher than for complex (tBuPCP)IrH(Cl)(CH3CN) (3) (6.7 × 102 and 1.9 × 102 L mol−1, respectively). At that quantitative analysis of variable temperature UV–vis spectra gave ΔH° = −7.0 kcal mol−1 and ΔS° = −13.7 cal mol−1 K−1 for 3 and ΔH° = −5.8 kcal mol−1 and ΔS° = −7.1 cal mol−1 K−1 for 2. Relatively weak binding of 2-hydroxymethylpyridine to complex 1 yields two isomers, with HMPy ligand trans to hydride (4a) and trans to arene ring (4b) with formation constants Kf(4a) and Kf(4b) of only 28 and 18 L mol−1 at room temperature. Variable temperature measurements give ΔH°(4a) = −12.3 kcal mol−1, ΔS°(4a) = −35.5 cal mol−1 K−1 and ΔH°(4b) = −11.7 kcal mol−1, ΔS°(4b) = −34.5 cal mol−1 K−1. DFT calculations on (tBuPCP)IrH(Cl)(L) confirm preferential coordination of L = RCN, pyridines trans to the hydride ligand but show no alteration of IrH bond properties. Instead, the elongation of IrCl bond is revealed that correlates with the strength of the Ir-L binding. The second isomers with L coordinated trans to arene ring of the pincer ligand are slightly less stable (≤2.8 kcal mol−1 according to DFT calculations) and are observed only for pyridines. In 1 and its (tBuPCP)IrH(Cl)(L) derivatives, chloride ligand is able to serve as a proton accepting site forming Cl⋯HX hydrogen bonds in solution or in solid state. These interactions further activate the IrCl bond.