A reversible structural interconversion involving [M(H₂pdc)₂(H₂O)₂]·2H ₂O (M=Mn, Fe, Co, Ni, Zn, H₃pdc=3,5-pyrazoledicarboxylic acid) and the role of a reactive intermediate [Co(H₂pdc)₂]

A new type of hydrogen bonded networks [M(H₂pdc)₂(H₂O)₂]· 2H₂O [M=Mn (1), Fe (2), Co (3), Ni (4), Zn (5); H₃pdc=3,5-pyrazoledicarboxylic acid] have been synthesized via hydrothermal reactions and their structures have been characterized. Upon a cooling-heating cycle, these compounds undergo a reversible structural interconversion process via hydration-dehydration: [M(H₂pdc)₂(H₂O)₂]· 2H₂O _+4H2O⇌^-4H2O [M(H₂pdc)₂]. The process is associated with distinct color changes. The dehydrated [M(H₂pdc)₂] (M=Mn, Fe, Co, Ni, and Zn) are amorphous and highly reactive. Further chemical reactions of these reactive intermediates show that they may act as effective precursors towards assembly of new supramolecular compounds that may otherwise be inaccessible by other synthetic routes. An interesting structure containing an "open-box" molecule [Co₄(Hpdc)₄(py)₁₂]·4 py·2 H₂O· 2CH₃OH (6) (py=pyridine) has been isolated by using dehydrated [Co(H₂pdc)₂] as the precursor, and its crystal structure has been analyzed. Crystal data for 1-6: monoclinic, space group P2₁/c and Z=2 with a= 10.186(2), b=12.473(2), c=6.831(1) Å, β=108.80(3)° (1); a=9.896(2), b= 12.402(2), c=6.810(1) Å, β=108.15(3)° (2); a=9.981(2), b=12.426(2), c= 6.807(1) Å, β=108.23(3)° (3); a= 9.896(2), b=12.402(2), c=6.810(1) Å, β=108.15(3)° (4); a=10.001(2), b= 12.430(2), c=6.834(1) Å, β=108.32(3)° (5); a=9.9617(1), b=18.5080(2), c= 28.4786(3) Å, β=93.076(1)° (6).