Can an Amine Be a Stronger Acid than a Carboxylic Acid? The Surprisingly High Acidity of Amine–Borane Complexes

Abstract

The gas-phase acidity of a series of amine-borane complexes has been investigated through the use of electrospray mass spectrometry (ESI-MS), with the application of the extended Cooks kinetic method, and high-level G4 ab initio calculations. The most significant finding is that typical nitrogen bases, such as aniline, react with BH(3) to give amine-borane complexes, which, in the gas phase, have acidities as high as those of either phosphoric, oxalic, or salicylic acid; their acidity is higher than many carboxylic acids, such as formic, acetic, and propanoic acid. Indeed the complexation of different amines with BH(3) leads to a substantial increase (from 167 to 195 kJ mol(-1)) in the intrinsic acidity of the system; in terms of ionization constants, this increase implies an increase as large as fifteen orders of magnitude. Interestingly, this increase in acidity is almost twice as large as that observed for the corresponding phosphine-borane analogues. The agreement between the experimental and the G4-based calculated values is excellent. The analysis of the electron-density rearrangements of the amine and the borane moieties indicates that the dative bond is significantly stronger in the N-deprotonated anion than in the corresponding neutral amine-borane complex, because the deprotonated amine is a much better electron donor than the neutral amine. On the top of that, the newly created lone pair on the nitrogen atom in the deprotonated species, conjugates with the BN bonding pair. The dispersion of the extra electron density into the BH(3) group also contributes to the increased stability of the deprotonated species.

Some Interesting Features of Non-Covalent Interactions

“…42 Other Lewis acids different from beryllium, as boron or aluminum, can also induce significant acidity enhancements. [36][37][38][39] Similar closed-shell interactions as those described for beryllium compounds are found in boranes BX 3 and alanes AlX 3 , compounds that suffer a significant distortion when forming dative bonds with Lewis basis as it was previously shown on preceding sections. Therefore, the intrinsic acidity of phosphine is notably different from that of phosphine-boranes (seventeen orders of magnitude greater in terms of the corresponding equilibrium constant), but the most relevant finding is that these theoretical predictions are fully ratified by the experimental evidence.…”

“…37 This acidity enhancement is even larger when the phosphine is replaced by the corresponding amine, and the acidity gap between aniline-borane and the free aniline is 136 kJ mol -1 . 39 It is worth noting that the acidity enhancement is also larger for phosphine-alanes 38 than for the corresponding phosphine-borane analogues, an unexpected result since boranes should be, in principle, better Lewis acids than alanes. Once again, the reason behind this phenomenon is the greater stabilization of the deprotonated phosphine which is significantly larger in complexes with alanes than in complexes with boranes.…”

“…36 Several examples of this kind were studied in detail by our group in recent years. [37][38][39][40][41] As a general rule, it has been observed that when a given compound forms a beryllium bond, the conjugated base becomes a much better electron donor than the acid from which it comes as compared with the same acid-base pair in absence of beryllium bonds, resulting in an acidity enhancement triggered by the beryllium bond. This acidity enhancement is the consequence of the significant amount of charge transferred from the Lewis base towards the available empty orbitals of the beryllium compound, accompanied, as explained in previous sections, by the corresponding deformation mainly of the BeX 2 subunit.…”

Acidity enhancement of unsaturated bases of group 15 by association with borane and beryllium dihydride. Unexpected boron and beryllium Brønsted acids

“…This theoretical model has been proven to provide reliable results for similar complexes when compared with experimental values and high-level ab initio methods such as G3 and G4 theories. 30,32 Nevertheless we have assessed its reliability to reproduce the gas-phase acidities of the systems under investigation in this paper using as a reference G4 theory 40 calculations, for a suitable set of benchmark cases. Obviously, the B3LYP/6-311+G(3df,2p)// B3LYP/6-31+G(d,p) model is not applicable to Sb containing compounds, for which the aforementioned basis sets are not available, and where relativistic effects cannot be considered negligible.…”

“…30,32 These acidity enhancements are even larger when the Lewis acid is a beryllium derivative, 34 an electron deficient system, that as borane or alane behaves as a very strong Lewis acid. Some evidence seems to indicate that the effect depends on the nature of the Lewis base active site, since in general it has been found that the acidity enhancement is about twice as large in amine-boranes 32 than in phosphine-boranes. 30 There are four aims of this paper: (i) to investigate the acidity trends down group 15 of elements, more specifically when the heteroatom is directly bonded to an unsaturated moiety, in particular to a vinyl or to an ethynyl group, (ii) to analyze the effect that the association of these compounds either with BH 3 or BeH 2 has on their intrinsic acidities, (iii) to explore whether reactivity changes are directly related to the deformation of the base, the acid or both, and (iv) to explore the possibility that this association may lead to a change in the nature of the group losing the proton.…”