The structures and energies of the lithium, sodium, and magnesium derivatives of the anions CH2CN- and CH2NC-. Solvation and aggregation of the lithium species

Abstract

The energetic and structural relationships among various C2H2NM isomers (M = Li, Na, MgH, and the anion) are explored by theoretical calculations. The cyanomethyl anion, CH2CN-, is predicted to have a pyramidal structure but with a very low inversion barrier of 0.3 kcal/mol (MP2/6-3l+G*//6-3l+G*). At the same level of ab initio theory, a significantly higher barrier of 2.8 kcal/mol is found for the isocyanomethyl anion, CH2NC-. r-Delocalization in CH2CN-results in a large stabilization relative to CH3-(calculated 45.3, experimental 44.4 kcal/mol). In contrast, the CH2NC-anion is stabilized (by 36.3 kcal/mol relative to CH3-) largely inductively by the sp-hybridized nitrogen. Extensive ab initio examination of the lithium, sodium, and magnesyl (MgH) derivatives of these anions (as well as those of isomeric structures) found lithium-bridged and, to a somewhat lesser extent, sodium-bridged geometries 7 and 12 to be favored. The magnesyl derivatives, 1 and 12, have more covalent character and nearly classical structures. The metallated ketenimine forms, M-N=C=CH2( 2 , M = Li, Na, MgH), are next lowest in energy. The metallated nitrile ylides, M-C=N=CH2, are substantially higher in energy than the bridged forms but also show the geometrical variations associated with increasing ionic character. Other C2H2NM isomers, including metallated aminoacetylenes and carbene complexes, are energetically unfavorable. The energy orderings of the acyclic C2HlNM isomers also reflect the relative stabilities of the parent systems (in kcal/mol), e.g., CH3CN (O.O), CH3NC (+20.8), H2C=C=NH (+35.3), HCC-NH2 (+51.4), HC=N==CH2 (+69.0), at the 6-31G*//6-31G* level. More revealing are the stabilization energies relative to CH3M or to NH2M, depending on the principal site of metal attachment of the metal derivatives based on the parent C2H3N structures. Thus, the amino (vs. NH2M) stabilization energies, 40.9 kcal/mol for Na-N=C=CH2 and 32.4 kcal/mol for Li-N=C=CH2 (3-21G//3-21G), are 70% and 56% those of the free anion; the value for M=MgH (23.3 kcal/mol) is decreased even further. Likewise, the methyl stabilization energies (vs. CH3M) of the CH,(M)CN forms (Li, 21.5; Na, 23.2; and MgH, 8.7 kcal/mol) are all substantially less than that of the free anion, 45.3 kcal/mol (CHICN-vs. CH3-). Similar attenuation is found for the CH2(M)NC species vs. the CHINC-anion. Aggregation and solvation effects were probed by MNDO calculations. The most stable unsolvated dimers of both LiCH2CN and LiCH2NC are indicated to prefer eight-membered-ring structures, but solvation favors the alternative four-membered-ring N-lithiated ketenimine dimer form of the former, which may be the structure in the solid state.The importance of cyano-stabilized carbanions as synthetic intermediates is underscored by Volume 31 of "Organic Reactions", the latest in a series of reviews devoted to this topic1 The sodium salt of propionitrile was first described almost a 100 years ago.2 Lithium derivatives of aliphatic nitriles were reported by Ziegler in 19323 and the corresponding...

The Structure of Lithiated Aminonitriles in Solution: Interpretation of their Experimental IR Spectra by Means of ab initio Calculations

“…Our MP2/6-31 G*//6-31 G* solvation energies for Id are more negative than those obtained with the MNDO method [5], which, on the other hand, are more similar to our HF/6-31 G*//6-31 G* results. While the use of water molecules as a solvent model is applicable in the case of 1 d. it fails for 2 b.…”

“…Con sequently, if there are, e.g., two molecules of the sol vent attached to the lithium atom in the keteniminelike monomer, there will be one in the corresponding nonclassical structure. Roughly estimating that the actual energy changes associated with the processes will be the same [5], we conclude from the numbers in Table 3 that the bridged compound will be less stabi lized in solution than 1 d. As to be expected, lc (which in contrast to the STO-3G results is also a minimum on the 6-31 G hypersurface of H3C2N 2Li) is almost isoenergetic with 1 a.…”

“…Inclusion of zero point vibrational energy calculated at the HF/6-31 G*//6-31 G* level slightly reduces this difference to 8.6 kcal/mol. This value is somewhat smaller than 9.7 kcal/mol obtained for the energy dif ference between 1 d and 2 b [5]. Thus, the effect of the amino group is to reduce the energy difference be tween the ketenimine-like and the nonclassical struc tures.…”

“…However, in the case of C6H5CHCNLi in THF at -108 °C the monomer/dimer ratio is clearly in favour of the monomer [5][6][7], while the same compound seems to be a dimer in dimethyl sulfoxide [8]. Fhe X-ray struc ture determination of lithiated phenyl acetonitrile complexed by tetramethylethylenediamine revealed that this molecule crystallizes as a dimer together with one molecule benzene in the asymmetric unit.…”

“…Although the tendency of lithium compounds to form oligomers not only in solution but also in the gasphase is notorious [3], the degree of association in solution is a sensitive function of the substituents X R, 2R, and 3R, of the nature of the solvent, and of the temperature [4], Thus, e.g., lithiated acetonitrile (D, X R2RN-and 3R replaced by hydrogen) dissolved in THF, is a dimer between the melting point of the solvent and room temperature [5]. However, in the case of C6H5CHCNLi in THF at -108 °C the monomer/dimer ratio is clearly in favour of the monomer [5][6][7], while the same compound seems to be a dimer in dimethyl sulfoxide [8].…”