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

Abstract

. Recrystallization of a primary or secondary alkylammonium tetraphenylborate (TB) from approximately 1 : 1 mixtures of Me2C0 (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 de- 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) A 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 n 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 ir~itio study of the effect, on the geometry of the [H,c=NH,]+ ion, of a partial or complete replacement of the H atoms by one of the following substituents: F, C1, 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 EM and (in part) Lijwdin EL net charges on the atoms, and electron densities p, = p(r,) 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 [H'C=NH,]+ carries a significant negative net charge (both in terms of EM and e,), 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 a, + actzc(X) + aNtl,(X) of the numbers tz 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 ini...

New Energetic Complexes of Copper(II) and the Acetone Carbohydrazide Schiff Base as Potential Flame Colorants for Pyrotechnic Mixtures

“…Easy crystallization of the products can also be attributed to Schiff bases tending to crystallize quite readily from an acetone/water mixture. [14] Such Schiff-base coordination complexes of copper(II) have been drawing attention for some time because of their interesting coordination chemistry and biological applicability. [15,16] We were not able to obtain the chloride complex under similar conditions.…”

On the Electronic Structure of Ethidium

“…[50] The N=C bond length of N,N-dimethylisopropylideneammonium tetraphenylborate (1.30 ) is used as a standard equal to + 1 (a full double bond). [51] The NÀC bond length of 7-(a-(2-methoxycarbonylanilinobenzyl)quinoline-8-ol is taken as a single bond standard (at 1.47 ) and equal to zero charge. [52] Using these standards, a linear correlation is used to assign a charge of + 0.76 for the quaternary nitrogen (Figure 14a).…”

“…The N=C bond length of both N,N-dimethyl-isopropylideneammonium tetraphenylborate and 9-(10-methyl)acridinium iodide (both 1.30 ) are used as the standards for a double bond and are taken as a + 1 charge. [51,54] Using these standards, charges of + 0.28 and + 0.18 have been assigned to the 3-and 8-amines, respectively (Figure 14a). [55] An early computational study by Giacomoni and Le Bret also predicted positive p-charges for all of ethidiums nitrogen atoms.…”

“…[57,58] The charges on the nitrogen atoms are determined from bond-length/bond-order relationships observed in a high resolution X-ray structure of ethidium chloride as compared to simple standards. [50][51][52][53][54][55] A spacefilling model of ethidium (with hydrogen atoms omitted) illustrating the relative p-electron densities of individual atoms indicated by color and intensity is presented in b).…”

Spectroscopic and X-ray Crystallographic Evidence for Electrostatic Effects in 4-Substituted Cyclohexanone-Derived Hydrazones, Imines, and Corresponding Salts

“…Alkyl iminium salts are commercially available, 23 and simple protonated imines have been isolated and are amenable to X-ray crystallographic analysis. 24 Similar ions can be formed from hydrazones, which are typically protonated at the double-bonded nitrogen atom to form stable cations. 25 …”