Dispersion Force Effects on the Dissociation of "Jack-in-the-Box" Diphosphanes and Diarsanes

作者：Guo Jing Dong; Nagase Shigeru^*; Power Philip P

来源：Organometallics, 2015, 34(10): 2028-2033.

DOI：10.1021/acs.organomet.5b00254

The dissociation of the sterically encumbered diphosphanes and diarsanes [:E{CH(SiMe3)(2)}(2)](2) (E = P or As) and [:E{N(SiMe3)(2)}(2)](2) (E = P or As) into :E{CH(SiMe3)(2)}(2) or :E{N(SiMe3)(2)}(2) radical monomers was studied computationally using hybrid density functional theory (DFT) at the B3PW91 with the 6-311+G(d) basis set for P and As, and the 6-31G(d,p) basis set for other atoms. The structures were reoptirnized with the dispersion corrected B3PW91-D3 method to estimate dispersion force effects. The calculations reproduced the experimental structural data for the tetraalkyls with good accuracy. Without the dispersion correction, negative dissociation energies of -10.3 and -6.5 kcal mol(-1) were calculated for the phosphorus and arsenic tetraalkyls, indicating that the radical monomers are more stable. In contrast, the incorporation of dispersion force effects afforded high, positive dissociation energies of +37.6 and +37.1 kcal mol(-1) that favored dimeric structures. The dissociation energies (without dispersion) calculated for the tetraamido-substituted dimer are also negative, but changed to positive values of +29.3 and +32.5 kcal mori upon optimization with the D3 dispersion term. In contrast to earlier calculations, which indicated that the release of accumulated strain energy within the tetraalkyl dimers was the driving force for dissociation to monomers (i.e., the "Jack-in-the-Box" molecular model), the current calculations show that dispersion force attractive interactions exceed those of ligand relaxation and stabilize the dimeric structures. Single-point MP2 (second-order Moller-Plesset perturbation theory) calculations including dispersion effects afforded dissociation energies of 30.4 and 30.8 kcal mol(-1) for the tetraalkyl species, suggesting that the addition of the D3 dispersion term to the B3PW91 functional may overestimate such forces by 7-8 kcal mol(-1). It is concluded that the balance of dispersion forces and entropic effects are the major determinants of the dissociation equilibria.

• 出版日期2015-5-25