3D hydrogen bonded metal-organic frameworks constructed from [M(H 2 O) 6 ][M 0 (dipicolinate) 2 ] Æ mH 2 O (M/M 0 = Zn/Ni or Ni/Ni). Identification of intercalated acyclic (H 2 O) 6 /(H 2 O) 10 clusters Marina V. Kirillova, Alexander M. Kirillov, M. Fa ´tima C. Guedes da Silva, Maximilian N. Kopylovich, Joa ˜o J.R. Frau ´ sto da Silva, Armando J.L. Pombeiro * Centro de Quı ´mica Estrutural, Complexo I, Instituto Superior Te ´cnico, Av. Rovisco Pais, 1049–001, Lisbon, Portugal Received 31 October 2006; accepted 3 December 2006 Available online 16 December 2006 Dedicated to Prof. Piero Zanello on the occasion of his 65th anniversary and in recognition for his contributions to inorganic electrochemistry. Abstract New heterodinuclear Zn II /Ni II (1) and homodinuclear Ni II /Ni II (2) water-soluble and air stable compounds of general formula [M(H 2 O) 6 ][M 0 (dipic) 2 ] Æ mH 2 O have been easily prepared by self-assembly of the corresponding metal(II) nitrates with dipicolinic acid (H 2 dipic) in water solution at room temperature. The compounds have been characterized by IR, UV/Vis and atomic absorption spec- troscopies, elemental and X-ray single crystal diffraction (for 1 Æ 4H 2 O and 2 Æ 5H 2 O) 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 Æ 4H 2 O, the latter rep- resent a rectangular-grid 2D framework with multiple channels if viewed along the c crystallographic axis, while in 2 Æ 5H 2 O 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 lar- ger (H 2 O) 10 one with an unusual geometry (acyclic helical octamer with two pendent water molecules) by taking into account the hydro- gen bonds to water ligands in [Ni(H 2 O) 6 ] 2+ . The obtained Zn/Ni compound 1 relates to the recently reported family of heterodimetallic complexes [M(H 2 O) 5 M 0 (dipic) 2 ] Æ mH 2 O (M/M 0 = 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 (Co II > Ni II > Zn II > Cu II ) or aqua species (Co II < Ni II < Zn II < Cu II ). Ó 2006 Elsevier B.V. All rights reserved. Keywords: Crystal engineering; Dipicolinic acid; Heterometallic complexes; Hydrogen bonds; Metal-organic frameworks; Water clusters 1. Introduction Multidimensional metal-organic frameworks con- structed by means of hydrogen bonds constitute a recent class of compounds with high current interest in various areas, e.g. in supramolecular and host–guest chemistry, material science, crystal engineering, catalysis, molecular magnetism, etc. [1–11]. A considerable part of such metal- organic frameworks is based on functional components constructed from different aromatic mono- and polycarb- oxylic acids, which are able to generate robust and flexible H-bonding networks via extensive O–HO interactions. In particular, pyridine-2,6-dicarboxylic acid (dipicolinic acid, H 2 dipic) has been a recognized structural component in many metal-organic frameworks [12–19] not only because of the versatility of its coordination modes and affinity to form strong hydrogen bonds, but also due to the growing significance of dipicolinate containing com- pounds as various functional materials, in biochemistry 0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.12.016 * Corresponding author. Fax: +351 21 846 4455. E-mail address: pombeiro@ist.utl.pt (A.J.L. Pombeiro). www.elsevier.com/locate/ica Available online at www.sciencedirect.com Inorganica Chimica Acta 361 (2008) 1728–1737