Calculation
of Ionization Energy, Electron Affinity,
and Hydride Affinity Trends in Pincer-Ligated d<sup>8</sup>‑Ir(<sup>tBu4</sup>PXCXP) Complexes: Implications for the Thermodynamics of
Oxidative H<sub>2</sub> Addition
- Publication date
- Publisher
Abstract
DFT methods are used to calculate
the ionization energy (IE) and
electron affinity (EA) trends in a series of pincer ligated d<sup>8</sup>-Ir(<sup>tBu4</sup>PXCXP) complexes (<b>1</b>-X), where
C is a 2,6-disubstituted phenyl ring with X = O, NH, CH<sub>2</sub>, BH, S, PH, SiH<sub>2</sub>, and GeH<sub>2</sub>. Both <i>C</i><sub>2<i>v</i></sub> and <i>C</i><sub>2</sub> geometries are considered. Two distinct σ-type (<sup>2</sup>A<sub>1</sub> or <sup>2</sup>A) and π-type (<sup>2</sup>B<sub>1</sub> or <sup>2</sup>B) 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 π-orbital effects. The calculations
are also used to study the effects of varying X on the thermodynamics
of oxidative H<sub>2</sub> addition to <b>1</b>-X. Two closed
shell singlet states differentiated in the <i>C</i><sub>2</sub> point group by the d<sup>6</sup>-electon configuration are
investigated for the five-coordinate Ir(III) dihydride product. One
electronic state has a d<sup>6</sup>-(a)<sup>2</sup>(b)<sup>2</sup>(b)<sup>2</sup> configuration and a square pyramidal geometry, the
other a d<sup>6</sup>-(a)<sup>2</sup>(b)<sup>2</sup>(a)<sup>2</sup> configuration with a distorted-Y trigonal bipyramidal geometry.
No simple correlations are found between the computed reaction energies
of H<sub>2</sub> addition and either the IEs or EAs. To better understand
the origin of the computed trends, the thermodynamics of H<sub>2</sub> 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 <b>1</b>-O and <b>1</b>-CH<sub>2</sub> can have very similar
reaction energies for H<sub>2</sub> addition arising from opposing
hydride and proton affinity effects. Additional calculations on methane
C–H bond addition to <b>1</b>-X afford reaction and activation
energy trends that correlate with the reaction energies of H<sub>2</sub> addition leading to the Y-product
