Calculation of Ionization Energy, Electron Affinity, and Hydride Affinity Trends in Pincer-Ligated d(8)-Ir((PXCXP)-P-tBu4) Complexes: Implications for the Thermodynamics of Oxidative H-2 Addition

摘要

DFT methods are used to calculate the ionization energy (IE) and electron affinity (EA) trends in a series of pincer ligated d(8)-Ir((tBu(4))PXCXP) complexes (1-X), where C is a 2,6-disubstituted phenyl ring with X = O, NH, CH2, BH, S, PH, SiH2, and GeH2. Both C2v and C2 geometries are considered. Two distinct s-type ((2)A1 or (2)A) and p-type (2))1 or B-2) electronic states are calculated for each of the free radical cation and anion. The results exhibit complex trends, but can be satisfactorily accounted for by invoking a combination of electronegativity and specific p-orbital effects. The calculations are also used to study the effects of varying X on the thermodynamics of oxidative H2 addition to 1-X. Two closed shell singlet states differentiated in the C-2 point group by the d(6)-electon configuration are investigated for the five-coordinate Ir(III) dihydride product. One electronic state has a d(6)-(a)(2)(b)(2)(b)(2) configuration and a square pyramidal geometry, the other a d(6)-(a)(2)(b)(2)(a)(2) configuration with a distorted-Y trigonal bipyramidal geometry. No simple correlations are found between the computed reaction energies of H2 addition and either the IEs or EAs. To better understand the origin of the computed trends, the thermodynamics of H-2 addition are analyzed using a cycle of hydride and proton addition steps. The analysis highlights the importance of the electron and hydride affinities, which are not commonly used in rationalizing trends of oxidative addition reactions. Thus, different complexes such as 1-O and 1-CH2 can have very similar reaction energies for H2 addition arising from opposing hydride and proton affinity effects. Additional calculations on methane C-H bond addition to 1-X afford reaction and activation energy trends that correlate with the reaction energies of H-2 addition leading to the Y-product.