Exploiting Complementary Second-sphere Effects in Supramolecular Coordination Solids

Abstract

In the design of extended supramolecular solids, reliable synthons are a valuable commodity. This work concerns the complementary second-sphere coordination interactions between a highly preorganized hexasulphonated ligand, L, and aquated metal ions. Four second-sphere inclusion complexes [M(H²O)⁶]² L ·(S)³ (M/S: Mg/acetone, 1; Zn/acetone, 2, Mg/dioxane, 3; Zn/dioxane, 4) and three extended networks {[(M(H²O)³)²(L)]·(H²O)^14.5}^∞ (M=Cr, 5; Fe, 6; Al, 7) have been structurally characterized by X-ray crystallography. The second-sphere effects on the stabilization of the primary coordination sphere are illustrated by TGA experiments. In these assemblies, the potential of a new supramolecular synthon is illustrated, that being the complementary hydrogen-bonding interaction between cis-aquo ligands and sulphonate oxygen atoms.

Keywords:

• Second-sphere interactions
• Supramolecular chemistry
• Coordination networks
• Porous frameworks

Acknowledgements

We would like to thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for support. We also thank J. Ripmeester and G. Enright at the Steacie Institute of the National Research Council of Canada, for the use of their powder X-ray diffractometer, and J. Reid for running the samples.

Notes

The Future of Supramolecular ChemistryThe number and breadth of extended coordination structures being reported are growing at an exponential rate. There is still much untapped ground with respect to new ligands, varying cation–anion pairs, and altering synthetic conditions to yield new frameworks. Our feeling has been that, while new assembly motifs and structural topologies are of interest, they do represent only a means to an end, that being the development of new families of functional solids. Zeolite mimics with permanent porosity have been heralded as Holy Grails within this domain, and understandably so, given the structural variation incorporation of organic moieties brings. Numerous outstanding examples have been reported which illustrate this feature already. A feature which has been less explored are pliant coordination solids which exhibit reversible phase transitions with guest inclusion (the third-generation solids as defined by Kitagawa). While often not possessing measurable porosity, it can certainly be argued that reversible guest sorption constitutes functional porosity. It is almost a question of philosophy: “If a guest molecule exists in a pore and it can be removed and re-inserted, does it matter that the pore exists in its absence?” Our recent research has concerned a number of functionally porous solids. The appeal of this class of compounds is not so much that they will replace zeolitic solids, but more that they represent complements to rigid networks. Particularly with the incorporation of metal centres with some physico-chemical activity (magnetism, luminescence, and so on), the idea of solids switchable with guest inclusion is a prospect not available to a rigid network. There are likely many such networks which have been reported but have not been recognized as hosts with such potential as interest was focused on a permanently porous structure. It is important for researchers in the field to recognize and study the sorption properties of “soft” networks as these compounds represent the truly distinctive aspect of coordination solids vs. a metal oxide framework. With these compounds in mind, the domain of possible building units also expands to include “weaker” coordinate covalent and non-covalent interactions, as these would be expected to promote a greater structural flexibility. In this light, we have been examining second-sphere coordination as a means of assembling extended solids, and this is the topic of the following work.

George Shimizu (right) is originally from Winnipeg, Manitoba, Canada. He received a BSc in Chemistry from the University of Winnipeg and his PhD from the University of Windsor with Stephen Loeb. After NSERC postdoctoral stays in Birmingham (Fraser Stoddart) and the National Research Council of Canada (John Ripmeester), George joined the University of Calgary in 1998. He is currently an Associate Professor and Director of Graduate Studies in Chemistry.Sean Dalrymple was raised in Coxheath, Nova Scotia, Canada. He completed his BSc at St. Francis Xavier University and moved to Calgary to begin graduate studies in 1999. Sean is presently a PhD student and an NSERC Canada Graduate Scholarship holder.