Crystal Engineered Acid−Base Complexes with 2D and 3D Hydrogen Bonding Systems Using a Bisphosphonic Acid as the Building Block

The phosphonic acid iminobis(methylene phosphonic acid), NDP, [NH2(CH2PO3H)2][H3O], exists as
a zwitterion and can act as both a hydrogen bond donor and acceptor. NDP synthon was reacted with three organic
bases, 2,2‘-bipyridine, 4,4‘-trimethylene-dipyridine, and a hexaazamacrocyclic crown ether, to construct highly stable
hydrogen bonded structures. The 2,2‘-bipyridine complex 2 is layered, the layers being formed through hydrogen
bonding of the phosphonic acid groups. The layers are largely formed by strong PO−H···-O−P type hydrogen bonds
(2.49−2.62 Å) with the protonated bipyridyls held between the layers by strong NH+···O−P hydrogen bonds. In
complex 3, each amino-nitrogen atom is protonated leaving each phosphonic acid group negatively charged. The
NDP molecules align in hydrogen bonded rows in which the molecules are bonded together by quadruple hydrogen
bonds:  two P−O-···HO−P and two NH2+···O−P. The rows are then connected to each other by the NH2+···O−P
hydrogen bonds between the pyridyl and the phosphonic acid groups. In compound 4, the azacrown ether ring encloses
two NDP molecules with four such entities arranged in the unit cell according to the P21/n symmetry. There are 14
water molecules per ring filling the voids between the rings creating a three-dimensional hydrogen bonding network.