1,8-Bis(hexamethyltriaminophosphazenyl)naphthalene, HMPN: A Superbasic Bisphosphazene “Proton Sponge”

It is shown that a combination of Schwesinger's phosphazene base concept and the idea of the
disubstituted 1,8-naphthalene spacer, first introduced by Alder in paradigmatic 1,8-bis(dimethylamino)naphthalene (DMAN), yields a new superbase, HMPN, which represents the up to date most basic
representative of this class of “proton sponges”, as evidenced by the theoretically estimated proton affinity
PA = 274 kcal/mol and the measured pKBH+ (MeCN) 29.9 ± 0.2. HMPN is by nearly 12 orders of magnitude
more basic than Alder's classical 1,8-bis(dimethylamino)naphthalene (DMAN). The title compound, HMPN,
is prepared and fully characterized. The spatial structure of HMPN and its conjugate acid is determined by
X-ray technique and theoretical DFT calculations. It is found that monoprotonated HMPN has an
unsymmetrical intramolecular hydrogen bridge (IHB). This cooperative proton chelating effect renders the
bisphosphazene more basic than Schwesinger's set of “monodentate” P1 phosphazene bases. The density
functional calculations are in good accordance with the experimental results, providing some complementary
information. They conclusively show that the high basicity of HMPN is a consequence of the high energy
content of the base in its initial neutral state and the intramolecular hydrogen bonding in the resulting
conjugate acid with contributions to proton affinity of 14.1 and 9.5 kcal/mol, respectively.