Crystal-Engineered Three-Dimensional Hydrogen-Bonding Networks Built with 1,3,5-Benzenetri(phosphonic acid) and Bipyridine Synthons

The phosphonic acid 1,3,5-benzenetri(phosphonic acid) (BTP; 1,3,5-[(OH)2PO]3-C6H3) can act as both
a hydrogen bond donor and acceptor. BTP was reacted with two organic bases, 2,2‘-bipyridine and 4,4‘-bipyridine,
in a 1:1 molar ratio. The bis-deprotonation and tris-deprotonation trigger the formation of self-complementary 3D
hydrogen bonding architectures. Layers of the 2,2‘-bipyridine compound are formed through hydrogen bonding of
the phosphonic acid groups existing in the dianionic motif {1,3-[(OH)PO2]2-5-[PO(OH)2]-C6H3}2-. The ratio of BTP
to 2,2‘-bipyridine is 1:1.5, with two of the phosphonic acid protons being transferred to the nitrogen atoms, leaving
a negatively charged oxygen and a proton on each of the phosphonic acid groups. Discrete dimers are formed by
three very short PO−H···O−P type hydrogen bonds (2.45−2.48 Å) and one short PO−H···OP hydrogen bond at
2.56 Å interconnecting all dimers into extended layers in the bc plane. The protonated bipyridyls penetrated into
the layers by formation of two NH+···O−P hydrogen bonds (2.72, 3.14 Å) to complete the 3D network. The compound
with 4,4‘-bipyridine is triclinic P1̄, in which the ratio of BTP to bipyridine is 1:1. Each amino nitrogen atom is
protonated, leaving each phosphonic acid group negatively charged. One additional positive charge is provided by
a hydronium ion. The BTP molecules align in hydrogen-bonded staircase chains along the b axis, in which the
molecules are bonded together by double hydrogen bonds:  P−O-···HO−P. The staircases are then interwoven into
each other by the NH+···O−P hydrogen bonds between the pyridyl and the phosphonic acid groups.