Mild activation of IrCl bond upon the interaction of pincer iridium hydride (tBuPCP)IrH(Cl) with acids and bases

The coordination of nitrogen bases (CH 3 CN, PhCN and 2-hydroxymetylpyridine (HMPy)) to ( t Bu PCP)IrH(Cl) ( 1 ) was investigated by variable temperature NMR, IR, UV–vis spectroscopies combined with DFT/M06 calculations. Hydrogen bonding of 1 and its ( t Bu PCP)IrH(Cl)(CO) with alcohols was also studied. At room temperature the formation constants K f of complex ( t Bu PCP)IrH(Cl)(PhCN) ( 2 ) is ca. 3.5 times higher than for complex ( t Bu PCP)IrH(Cl)(CH 3 CN) ( 3 ) (6.7 × 10 2 and 1.9 × 10 2  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 K f ( 4a ) and K f ( 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 ( tBu PCP)IrH(Cl)(L) confirm preferential coordination of L = RCN, pyridines trans to the hydride ligand but show no alteration of Ir H bond properties. Instead, the elongation of Ir Cl 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 ( tBu PCP)IrH(Cl)(L) derivatives, chloride ligand is able to serve as a proton accepting site forming Cl⋯H X hydrogen bonds in solution or in solid state. These interactions further activate the Ir Cl bond.