Convenient Synthesis of Aluminum and Gallium Phosphonate Cages

The reactions of AlCl3·6H2O and GaCl3 with 2-pyridylphosphonic acid (2PypoH2) and 4-pyridylphosphonic acid (4PypoH2) afford cyclic aluminum and gallium phosphonate structures of [(2PypoH)4Al4(OH2)12]Cl8·6H2O (1), [(4PypoH)4Al4(OH2)12]Cl8·11H2O (2), [(2PypoH)4Al4(OH2)12](NO3)8·7H2O (3), [(2PypoH)2(2Pypo)4Ga8Cl12(OH2)4(thf)2](GaCl4)2··8thf (4), and [(2PypoH)2(2Pypo)4Ga8Cl12(OH2)4(thf)2](NO3)2·9thf (5). Structures 1−3 feature four aluminum atoms bridged by oxygen atoms from the phosphonate moiety and show structural resemblance to the secondary building units found in zeolites and aluminum phosphates. The gallium complexes, 4 and 5, have eight gallium atoms bridged by phosphonate moieties with two GaCl4− counterions present in 4 and nitrate ions in 5. The cage structures 1−3 are interlinked by strong hydrogen bonds, forming polymeric chains that, for aluminum, are thermally robust. Exchange of the phosphonic acid for the more flexible 4PyCH2PO3H2 afforded a coordination polymer with a 1:1 Ga:P ratio, {[(4PyCH2PO3H)Ga(OH2)3](NO3)2·0.5H2O}x (6). Complexes 1−6 were characterized by single-crystal X-ray diffraction, NMR, and mass spectrometry and studied by TGA.