CALORIMETRIC DETERMINATION OF THE HEATS OF PROTONATION OF THE METAL IN (METHYL-SUBSTITUTED CYCLOPENTADIENYL)IRIDIUM COMPLEXES, CP'IR(1,5-COD)

Titration calorimetry has been used to determine the enthalpies of protonation (DELTA-H(HM)) of the iridium in the Cp'Ir(1,5-COD) (Cp' = C5Me(x)H5-x, x = 0, 1, 3-5) complexes according to the following reactions: Cp'Ir(1,5-COD) + CF3SO3H (0.1 M) --> [Cp'Ir(H)(1,5-COD)]+CF3SO3-, at 25.0-degrees-C in 1,2-dichloroethane. The DELTA-H(HM) values become more exothermic from -22.8 +/- 0.2 kcal mol-1 for Cp' = C5H5 to -28.5 +/- 0.2 kcal mol-1 for Cp' = C5Me5. A plot of DELTA-H(HM) versus the number of Me groups on Cp' is linear; this result has been interpreted to indicate that the bulkiness of the Me group, even in the C5Me5 ligand, probably does not affect the DELTA-H(HM) values. Each Me group contributes -1.1 kcal mol-1 to DELTA-H(HM). Correlations between DELTA-H(HM) and the COD olefin H-1 NMR chemical shift of the Cp'Ir(1,5-COD) compounds and the Ir-H H-1 NMR chemical shift of the protonated species are also made. Equilibrium studies for the protonation of Cp'Ir(1,5-COD) show that the effect of each added Me group on DELTA-G-THETA is -0.89 kcal mol-1 and on DELTA-S-THETA is -0.7 eu. Thus, DELTA-S-THETA contributes little to the differences in equilibrium constants for protonation of the Cp'Ir(1,5-COD) complexes. A comparison of the common C5H5 and C5Me5 ligands shows that the replacement of C5H5 by C5Me5 increases the equilibrium constant for the protonation of Cp'Ir(1,5-COD) by 1900, makes DELTA-G-THETA more favorable by -4.5 kcal mol-1, causes DELTA-H(HM) to be more exothermic by -5.7 kcal mol-1, and reduces DELTA-S-THETA slightly by ca. -4 eu.