"Outlaw" Dipole-Bound Anions of Intra-Molecular Complexes

Using the example of silatranes XSi(OCH₂CH₂)₃N (X = Me, H, F, Cl), XS, it was found that the effect of the dipole-bound (DB) electron on the cage intramolecular complexes does not fit into the standard views. Upon the transition from XS to the DB anions XS, the unusual shortening of the internuclear Si···N distance is always observed. For X = Cl, it is equal to 0.15 Å, which is a record length for all DB anions known from the literature. The formation of DB anions with the cage structure has principal features, controlled not only by the "critical" value of the dipole moment (μ > 2.5 D), but also by a geometric factor, such as the degree of pyramidality of the N(CH₂)₃ moiety-the positive end of the molecular dipole of XS. It was a surprise that the effect of the substituent X on the extent of the structural rearrangement in the process XS → XS cannot be explained using the values of the electron detachment energy of XS or the initial strength of the coordination Si ← N bond in XS. The unique sensitivity of the silatrane geometry to the addition of an excess electron is governed by the rate of increase of their dipole moment with the shortening of the dative Si ← N contact. The conclusions drawn are supported by the high-accuracy CCSD and CCSD(T) calculations and the experimental (RET-PES) data. There is no real reason to doubt that the peculiarities of the formation of DB anions of XS can also be characteristic of many hundreds of their structural analogues XM(YCH₂CH₂)₃N (M = Si, Ge, Sn, Pb, Ti, Al, Cr, Fe, Ni...; Y = O, NR, CH₂, S), i.e., substituted 5-azabicyclo[3.3.3]undecans.