Can Nitriles Be Stronger Bases Than Proton Sponges in the Gas Phase? A Computational Analysis

Abstract

DFT calculations have been performed for a series of push-pull nitriles [(R2N)n(X═Y)iC≡N, where i = 0, 1, or 2, n = 1, 2, or 3, R2N = H2N, Me2N, or C4H8N, X = CH, N, or P, Y = CH or N]. The possible protonation N-sites (N-cyano, N-imino, and N-amino) have been examined and their proton affinities (PA) estimated. For all compounds in the series, even for those containing the guanidino, phosphazeno, and diphosphazeno pushing groups, the N-cyano atom is the favored site of protonation. The n-π conjugation strongly decreases the PA value of the pushing amino group in favor of the pulling cyano one. Nitriles with the phosphazeno groups [(R2N)3P═N-P(R2N)2═N and (R2N)3P═N] exhibit the strongest basicity in the series. Some of them (with PA > 1000 kJ mol(-1)) are stronger bases than DMAN, the so-called "proton sponge". Nitriles bearing the guanidino group [(R2N)2C═N] are less basic than those with the phosphazeno group [(R2N)3P═N] but more basic than those with the formamidino group (R2N-CH═N) containing the same substituent R. The N-imino atoms, present in the transmitter group (X═N, X = CH, N, or P), display PA values lower than those of the N-cyano site by more than 30 kJ mol(-1). When proceeding from the unsubstituted derivatives (R = H) to the methylated ones (R = Me), the Me groups at the N-amino atom increase the PA value of the N-cyano site for Me2N-X═Y-C≡N (X, Y = CH or N) by ca. 30-60 kJ mol(-1). For the guanidino and phosphazeno derivatives containing two and three amino groups, respectively, this effect is not additive. The four Me groups for (Me2N)2C═N-C≡N and the six Me groups for (Me2N)3P═N-C≡N increase the PA(N-cyano) values by only 30-50 kJ mol(-1). The C≡N bond lengths of the neutral forms are well correlated with the PA(N-cyano) values.