Complexes between Lithium Cation and Diphenylalkanes in the Gas Phase: The Pincer Effect

Abstract

The gas-phase lithium cation basicities (LCB values, Gibbs free energies of binding) of alpha,omega-diphenylalkanes Ph-(CH(2))(n)-Ph (n=2, 3, or 7) and 1,1-diphenylethane Ph-CH(Me)-Ph were investigated by means of Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry. Their structures, and those of the corresponding Li(+) complexes were optimized at the B3LYP/6-31G(d) level and their relative stabilities calculated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level. Whereas the most stable conformers of the free diphenylalkanes were found to adopt a completely stretched aliphatic chain connecting the two benzene rings, the most stable Li(+) complexes correspond to conformers in which the alkali metal cation interacts simultaneously with both benzene rings through the folding of the aliphatic chain ("pincer effect"). This chelation brings about a significant enhancement of the Li(+) binding enthalpies (LBE values), which were calculated to be approximately 75 kJ mol(-1) higher than those evaluated for conventional (singly coordinated) pi complexes in which the metal cation interacts with only one of the benzene rings. The increase of the corresponding lithium cation basicities, however, (Gibbs free energies of Li(+) binding, LCB values) was calculated to be smaller by approximately 15 kJ mol(-1) as the pincer effect is entropically disfavored. The good agreement between the calculated LCB values, assuming a statistical distribution of the different conformers present in the gas phase, and the experimental LCB values measured by means of FTICR mass spectrometry are considered indirect evidence of the existence of the pincer effect.

Consecutive losses of two benzyl radicals from the [M + Na]+ adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions

“…Prior experimental work using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and B3LYP density functional theory calculations revealed a so-called pincer effect when a Li + ion is solvated in the gas phase by 1,2-diphenylethane, higher a,v-diphenylalkanes and related molecules containing two flexibly oriented aromatic groups [30]. A similar behavior can be seen for the Na + adduct ions of the tribenzyl ether studied here.…”

“…Notably, and in contrast to protonated molecular species, understanding of the fragmentation of alkali metal-cationized molecules cannot be based on the fundamentals of "pure" organic chemistry. In turn, investigation of the gas-phase chemistry of such [M + metal] + ions of relatively simple organic compounds is of general interest because it cannot only help to extend the body of mass spectrometric fragmentation rules but also lead to more insights into the general interactions between alkali metal cations and organic species [25][26][27][28][29][30][31][32][33][34]. The present report is intended to add some interesting facets to these fields.…”

Computationals-Block Thermochemistry with the Correlation Consistent Composite Approach

“…Unlike the G3 model chemistry, the ccCA implementations used in this study have an almost zero MSE, suggesting less systematic error among the ccCA ∆H f values. The G3, G3B, and ccCA methods have similar enthalpies of formation for (LiCl) 3 , MgF, and NaCN, yet these values are significantly outside of the quoted experimental uncertainties. The ∆H f values of these three molecules may benefit from future experimental and theoretical validation to determine if the error lies with the model chemistries or the experimental value.…”

“…methods also each predict five molecules with an absolute deviation greater than (5.0 kcal mol -1 : BeO, (LiCl) 3 , MgF (with G3), Na 2 O (with G3B), and NaCN. Only four molecules have deviations greater than (5.0 kcal mol -1 with ccCA implementations: (BeF) 2 O, (LiCl) 3 , (NaF) 2 , and NaCN.…”

Gas-Phase Lithium Cation Basicity: Revisiting the High Basicity Range by Experiment and Theory

“…Nevertheless, the covalent contribution to the bonding of neutral ligands with Li + is significant [31,32]. Quite interestingly, the lithium cation binds relatively strongly to aromatic systems [33][34][35][36][37][38][39][40] that are weakly polar or nonpolar, providing evidence that bonding mechanisms other than pure ion/dipole are acting. For these reasons, it is useful to have access to an extended and consistent database on the interactions between Li + and various Lewis Electronic supplementary material The online version of this article (doi: 10.1007/s13361-014-0970-4) contains supplementary material, which is available to authorized users.…”