Crystal chemistry of tetraradial species. Part 8. Mix and match: cation geometry, ion packing, hydrogen bonding, and π–π interactions in cis-2,2′-bipyridinium(1+) and 1,10-phenanthrolinium(1+) tetraphenylborates — and what about proton sponges?

Abstract

The crystal structures at −20 °C of cis-2,2′-bipyridinium(1+) (BPTB, P21/n, a = 9.249(3), b = 14.093(7), c = 20.285(3) Å, β = 92.86(2)°, Z = 4) and 1,10-phenanthrolinium(1+) (PTB, P21/c, a = 11.194(2), b = 13.837(3), c = 18.303(3) Å, β = 107.82(1)°, Z = 4) tetraphenylborates have been determined. Inasmuch as 1,10-phenanthroline is an aromatically bridged cis-2,2′-bipyridine, monoprotonation results, in both systems, in the formation of an intra-cation N—H … N′ hydrogen bond, the geometric and spectroscopic properties of which we have investigated. The cation skeleton in PTB is planar to 0.03(2) Å; in BPTB the dihedral angle between the two cation ring planes is 5.2°. In the pale yellow PTB there are significant π–π stacking interactions that persist into solution. The effect of protonation on the geometry of the 2,2′-bipyridine and 1,10-phenanthroline systems is examined in considerable detail and compared with the corresponding effects in the paraquat(2+) and similar cations. On both geometric and spectroscopic (infrared spectra between 10 and 295 K) evidence, the N—H … N′ hydrogen-bonding interaction is stronger in BPTB; in PTB this interaction is among the weakest reported in crystals, the ν(NH) stretching frequency at 10 K being as high as 3279 cm−1. A detailed comparison of the geometries of the intra-cation N—H … N′ bonds in BPTB and PTB with those in classical and modified proton-sponge cations has led to the formulation of criteria useful in predicting the occurrence of proton-sponge-like properties. Key words: bipyridinium ions, hydrogen bonding, phenanthrolinium ions, proton sponges, tetraphenylborates.