3D hydrogen bonded metal-organic frameworks constructed from [M(H2O)6][M′(dipicolinate)2] · mH2O (M/M′ = Zn/Ni or Ni/Ni). Identification of intercalated acyclic (H2O)6/(H2O)10 clusters

New heterodinuclear ZnII/NiII (1) and homodinuclear NiII/NiII (2) water-soluble and air stable compounds of general formula [M(H2O)6][M′(dipic)2] · mH2O have been easily prepared by self-assembly of the corresponding metal(II) nitrates with dipicolinic acid (H2dipic) in water solution at room temperature. The compounds have been characterized by IR, UV/Vis and atomic absorption spectroscopies, elemental and X-ray single crystal diffraction (for 1 · 4H2O and 2 · 5H2O) analyses. 3D infinite polymeric networks are formed via extensive hydrogen bonding interactions involving all coordinated and crystallization water molecules, and all dipicolinate oxygens, thus contributing to additional stabilization of dimeric units, metal-organic chains and 2D layers. In 1 · 4H2O, the latter represent a rectangular-grid 2D framework with multiple channels if viewed along the c crystallographic axis, while in 2 · 5H2O intercalated crystallization water molecules are associated to form acyclic nonplanar hexameric water clusters and water dimers which occupy voids in the host metal-organic matrix, with a structure stabilizing effect via host–guest interactions. The hexameric cluster extends to the larger (H2O)10 one with an unusual geometry (acyclic helical octamer with two pendent water molecules) by taking into account the hydrogen bonds to water ligands in [Ni(H2O)6]2+. The obtained Zn/Ni compound 1 relates to the recently reported family of heterodimetallic complexes [M(H2O)5M′(dipic)2] · mH2O (M/M′ = Cu/Co, Cu/Ni, Cu/Zn, Zn/Co, Ni/Co, m = 2, 3), what now allows to establish the orders of the metal affinity towards the formation of chelates with dipicolinic acid (CoII > NiII > ZnII > CuII) or aqua species (CoII < NiII < ZnII < CuII).