Crowned Tetrameric Spirocyclic Water Chain:  An Unusual Building Block of a Supramolecular Metal−Organic Host

Abstract

A 3D supramolecular metal-organic host could be realized in solid in which the water assembly as a whole acted as a building block of the 3D framework with rectangular channels where guest molecules reside, adding a new dimension to water cluster research where the role of water has been shifted from its usual behavior of guest to that of a host. The crystal structure of the compound, {[Cu(mal) 2 ](picH) 2 ‚5H 2 O} n , 1 [mal ) malonate dianion, picH ) protonated 2-amino-4-picoline], synthesized from purely aqueous media, shows that the Cu(II)-malonate 2D coordination polymeric sheets are locked together by the water assembly that lines up perpendicular to the Cu(II)-malonate sheets constituting a 3D infinite supramolecular host where within the channels protonated 2-amino-4-picoline auxiliary ligands are accommodated. A spirocyclic tetrameric water chain having a dimeric crown on it describes the composition of the water assembly that runs down the c-axis.Water, an elixir of life on our planet, is ubiquitous and covers two-thirds of the globe. However, a complete understanding of water's behavior is still lacking, and the quest to understand water's anomalous behavior continues. 1 The realization that condensedphase behavior must be explained in terms of water clusters 2 led to the recent upsurge in water cluster research. Information on small water clusters in the gas phase was obtained through laser spectroscopic experiments, 3 supported by ab initio calculations. 4 Water's ability to form flexible hydrogen bonds with its neighboring molecules has led to the detection of various ordered water assemblies 5-10 in various deformed geometries. 11 The key to understanding the behavior of water is the precise structural data of various hydrogen-bonded networks. One can now even think of engineering the water assembly inside organic and metal-organic frameworks (MOFs) in a systematic way by gradually tuning its environment and studying its effect on the hydrogen-bonded networks. 8b,12 An understanding of this mutual interaction, especially when the components of the framework resemble molecular building blocks in biology, can possibly unravel the role of ordered water in bio-environments.The water tetramer, one of the main cluster components incorporated in models to simulate anomalous behavior 1 of water, has been predicted to be cyclic and planar 2b and has been structurally characterized within MOFs by a number of groups. 13 A polymeric chain structure having cyclic tetramers as its basic units has recently been discovered in organic crystalline hosts. 14,15 Herein we report a corner-sharing highly ordered tetrameric water chain with additional water forming a dimeric crown (Figure 1). The chains act as sidewalls, supporting 2D metal-carboxylate sheets, to generate a 3D supramolecular host having rectangular channels ( Figure 2). This unique host accommodates guest 2-amino-4-picoline ligands in their protonated forms within the channels. 16 The presence of a cyclic dimeric crown of the tetrameric water chain is u...

Chemical Design of Heterometallic Coordination Polymers Based on {Cu(Me2mal)2} Fragment

“…It should also be noted that only in the structures of [Cu(mal) 2 ] · Im · 2 H 2 O (Im = imidazole) and (NH 4 ) 2 [Cu(mal) 2 ] are the lateral contacts Cu–O less than 2.5 Å, giving robust layers and frameworks with coordination formula AT 11 2 and simple underlying nets of 4‐c sql and dia topologies, respectively (Figure S1b in the Supporting Information). In the compounds Kt[Cu(mal) 2 ] the weaker lateral Cu–O (2.5–3.2 Å) interactions caused by Jahn–Teller distortions usually lead to incorporation of the {Cu(mal) 2 } 2– complexes into chains [Kt + = 1,2‐bis(4‐pyridinium), adeninium cations] and layers (Kt + = H + , 2‐amino‐4‐picolinium, 2‐amino‐4‐methylpyridinium, methylammonium, propane‐1,3‐diammonium, diethanolammonium cations) , . Similarly in the complex 2 , the {Cu(Me 2 mal) 2 } groups are incorporated into simple chains (if Li–O bonds are ignored, Figure ) through long Cu–O contacts (2.944 Å).…”

Synthesis, crystal structure, and fluorescence properties of a novel coordination polymer [Zn(L)(4,4′-bipyridine)(H2O)] n · (H2O)4

“…It is well known that the vast H bond network formed is based on water clusters, which often appear in the complex crystals [18][19][20][21][22]. In this paper, four water molecules (O(1w), O(2w), O(3w), and O(4w)) formed a discrete water cluster with the oxygen atoms, which come from L (O(5)) and coordinated water molecules (O(1)), resulting in the discrete 1D chain of water cluster formed by a unit of (H 2 O) 4 .…”

Structure and stability of spiro-cyclic water clusters

“…Some of these clusters are not stable at higher level calculations. However, these spiro-cyclic structures (for example 6-6 and 4-4) are observed in certain crystals 48,49 suggesting that the stabilization of these structural motifs must be arising from additional interaction with the host molecules and crystal packing. In contrast, the 3-3, 3-4, 3-5, 3-6 and 4-4 spiro clusters seem to be stable even in the absence of any host lattice and associated packing effect.…”

“…48,49 Some of the spiro-cyclic and spiro-annulated water clusters optimized at the HF/6-311++G** level of theory are shown in figure 3. The possibility of forming such structures in the absence of a host or a crystal packing environment has been examined.…”