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.

亚氨基双(亚甲基膦酸)（iminobis(methylene phosphonic acid)，简称NDP，分子式为[NH₂(CH₂PO₃H)₂][H₃O]）以两性离子（zwitterion）形式存在，兼具氢键给体与氢键受体的双重角色。NDP合成子（synthon）与三种有机碱——2,2'-联吡啶（2,2‘-bipyridine）、4,4'-三亚甲基联吡啶（4,4‘-trimethylene-dipyridine）以及六氮杂大环冠醚（hexaazamacrocyclic crown ether）——反应，构建得到高稳定性的氢键组装结构。
2,2'-联吡啶配合物2呈层状结构，层间通过膦酸基团的氢键作用形成。该层状结构主要由强P-O-H···O-P型氢键（键长范围2.49~2.62 Å）构成，质子化联吡啶阳离子通过强NH⁺···O-P氢键固定于层间。
配合物3中，每个氨基氮原子均被质子化，使得每个膦酸基团带负电荷。NDP分子排列为氢键连接的链状结构，分子间通过四重氢键结合：2个P-O⁻···H-O-P氢键与2个NH₂⁺···O-P氢键。随后，各链之间通过吡啶基团与膦酸基团间的NH₂⁺···O-P氢键相互连接。
化合物4中，氮杂冠醚环包裹2个NDP分子，晶胞（unit cell）按照P2₁/n空间群对称性排布4个该结构单元。每个冠醚环周围存在14个水分子，填充环间空隙并形成三维氢键网络（three-dimensional hydrogen bonding network）。