Crystal chemistry of tetraradial species. Part 3. Salts of aliphatic Schiff bases crystallizable from aqueous media: crystal structures of five iminium tetraphenylborates and a 6-31G* ab initio study of simple iminium ions
Authors: Knop Osvald , Cameron T. Stanley , Bakshi Pradip K. , Kwiatkowski Witold , Choi S.C. , Adhikesavalu D. 
Published: May 2006

Recrystallization of a primary or secondary alkylammonium tetraphenylborate (TB) from approximately 1:1 mixtures of Me2CO (or MeEtCO) and water yields, against expectation, the TB of the corresponding iminium cation and thus provides an easy route to a variety of aliphatic Schiff bases. The crystal structures of five such salts have been determined: A, [Me2C = NHMe]BPh4 (Cmcm, a = 10.747(2) Å, b = 15.641(4) Å, c = 13.453(4) Å, Z = 4); B, [Me2,C = NMe2,]BPh4 (Cmca, a = 13.196(4) Å, b = 14.469(3) Å, c = 25.122(4) Å. Z = 8); C, [Me,C = NHEt]BPh4 (P21/n, a = 12.146(3) Å, b = 14.099(2) Å, c = 14.177(3) Å, β = 96.23(2)°, Z = 4); D, [MeEtC=NHMe]BPh4 (Cmc21, a = 10.967(5) Å, b = 15.642(5) Å, c = 13.430(4) Å, Z = 4); E, [Me2C=NH(β-CH2CH2Ph)]BPh4 H2O (C2/c, a = 30.747 Å, b = 9.845(4) Å, c = 19.741(5) Å, β = 97.12(2)°, Z = 8). The building principles of these and related TBs are discussed in detail, as also is the tendency of the small iminium ions to orientational disorder. The range of the C=N bond length in these and other iminium salts is quite narrow, with 1.283(12) Å as the mean (uncorrected) value. Where a hydrogen atom(s) is attached to the iminium nitrogen, it will form a hydrogen bond (simple or bifurcated) with the aromatic π system of the nearest phenyl ring(s). The narrowness of the C=N bond-length range with respect to substitution on the cation has prompted a detailed 6-31G* ab initio study of the effect, on the geometry of the [H2C=NH2]+ ion, of a partial or complete replacement of the H atoms by one of the following substituents: F, Cl, CN, Li, and Na. The 46 structures optimized in planar geometry were supplemented by a number of optimizations of similarly substituted ethylenes as well as by a few optimizations in perpendicular geometry. Analysis of the numerical results involves the bond lengths and angles, Mulliken εM and (in part) Löwdin εL net charges on the atoms, and electron densities ρc = ρ(rc) at the bond critical points. Only a limited use has been made of comparisons involving the total electronic energies E (substitutional isomerism). Since the above substituents span the entire electronegativity scale, and CN provides for the possibility of conjugation, the effects they produce are expected to bracket the effects that would be produced by any other substituents. The calculations confirm the essential narrowness of the C=N bond-length range in iminium ions. They also show that, at least in the 6-31G* scheme, the nitrogen in [H2C=NH2]+ carries a significant negative net charge (both in terms of εM and εL), which is contrary to the conventional representation of the ion as H2C=N+H2. The trends observed in the parameters are smooth and highly self-consistent except for the tri- and tetrasubstituted ions containing Li or Na, where interligand repulsions may invalidate the assumption of planarity. Many of the parameters can be represented to a high degree of correlation as linear functions α0 + αCnC(X) + αNnN(X) of the numbers n of substituent atoms X on the C and N atoms. This representation makes it possible to estimate the contribution to the parameter value from the substitution at the distant atom of the C=N bond, i.e., the extent of localization of the effect produced by the substituent. The success of the linearization of the parameter variation raises the important question of whether the apparent additivity is genuine and intrinsic or merely a property of the 6-31G* optimization scheme. In other words, will additivity be retained, functionally, for sets of parameters obtained from higher level ab initio calculations, and will it be eventually verified by experiment?