Noncovalent Interactions Electrostatic Site Attachment of Divalent Counterions to Rodlike Ruthenium( ii ) Coordination Polymers Characterized by EPR Spectroscopy** Dariush Hinderberger, Oliver Schmelz, Matthias Rehahn, and Gunnar Jeschke* Coordination polymers which are formed from connecting organic bischelating ligand molecules (“ligand monomers”) through transition metals have received much attention in the past decade. [1] The use of transition-metal ions such as ruthenium( ii ) or osmium( ii ) provides materials with kineti- cally inert coordinative bonds that are of interest as potential (electro-)optical or magnetic nanomaterials. [1e–h] Such systems are also very well suited as model systems for the study of polyelectrolyte behavior, since they have a well-defined rigid conformation in solution which reduces the complexity of the polyelectrolyte behavior by limiting the conformational degrees of freedom. [2a,b] Polyelectrolytes are macromolecular substances that are soluble in water or other ionizing solvents. [2] They dissociate into macromolecular ions that carry multiple charges (poly- ions) together with an equivalent number of ions of small charge and opposite sign. Despite their important role in many fields of scientific research, such as molecular biology (DNA, RNA) and nanotechnology, [3–5] the interplay of electrostatic interactions between macro-ions and counter- ions is not yet fully understood. [2a,b,6a–6c] Comparison between theoretical and experimental results has proved difficult in the past, as additional complexity is introduced in experi- ments by solvation effects, specific interaction between counterions and charged groups of the polyelectrolyte, and conformational changes of flexible polymer chains which cannot be separated properly from electrostatic effects. Herein we consider coordination polymers formed by divalent Ru II ions that are connected by bis(2,2’:6’,2’’-terpyr- idine)-based rigid spacer molecules of variable lengths (Scheme 1) to study the condensation of counterions [6d–g] to a stiff, relatively weakly charged polyion. [6] In the study we use a combination of continuous wave (CW) [7] and pulse [8] EPR spectroscopy on paramagnetic counterions that we have recently used to obtain a local, dynamic picture of counterion condensation to the flexible polyelectrolyte poly(diallyldime- thylammonium chloride). [9] For this strongly charged poly- electrolyte we were able to demonstrate that a significant portion of divalent counterions is transiently bound to the quaternary ammonium groups of the polyion with a lifetime that is lower than 1 ns. We call this process dynamic electro- static attachment. [9] Herein we raise the question whether such dynamic electrostatic attachment is also observable for a more weakly charged Ru II coordination polymer in which the carriers of the twofold positive charges are sterically shielded by bulky terpyridine ligands. In particular, we use double electron- electron resonance (DEER) spectroscopy [10] to characterize the spatial distribution of dianionic spin probe molecules (Fremy)s salt (FS), potassium nitrosodisulfonate, Scheme 1) on a length scale between 1.8 and 5 nm. As this length scale includes the separations of direct and next-neighbor ions of the coordination polymers, such experiments should provide a clear signature of site-attachment [6g] of the counterions along the backbone of the rodlike coordination polymer. By examining distance distributions of the spin-carrying counter- ions we should also be able to obtain a semiquantitative Scheme 1. Molecular structures of the spin probe FS (Fremy’s salt) and the rodlike coordination polymers containing divalent ruthenium ions as charged groups (1 and 2), with the number of repeat units n = 20–25. [*] Dr. D. Hinderberger, Priv.-Doz.Dr. G. Jeschke Max-Planck-Institut für Polymerforschung Postfach 3148, 55021 Mainz (Germany) Fax: (+ 49)6131-379-100 E-mail: jeschke@mpip-mainz.mpg.de O. Schmelz, Prof. Dr. M. Rehahn Deutsches Kunststoff-Institut (DKI) and Ernst-Berl-Institut für Technische und Makromolekulare Chemie Technische Universität Darmstadt Petersenstrasse 22, 64287 Darmstadt (Germany) [**] This work was supported by the Deutsche Forschungsgemeinschaft (Schwerpunktprogramm 1051 “High-Field EPR in Biology, Chemis- try, and Physics”). We thank Manfred Schmidt for helpful discus- sions and Christian Bauer for technical support. Communications 4616 # 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200460500 Angew. Chem. Int. Ed. 2004, 43, 4616 –4621