& Self-Assembly Structural Diversity in Multinuclear Pd II Assemblies that Show Low-Humidity Proton Conduction Dipak Samanta and Partha Sarathi Mukherjee* [a] Abstract: Systematic investigation on synergetic effects of geometry, length, denticity, and asymmetry of donors was performed through the formation of a series of uncommon Pd II aggregates by employing the donor in a multicompo- nent self-assembly of a cis-blocked 908 Pd II acceptor and a tetratopic donor. Some of these assemblies represent the first examples of these types of structures, and their forma- tion is not anticipated by only taking the geometry of the donor and the acceptor building units into account. Analysis of the crystal packing of the X-ray structure revealed several H bonds between the counteranions (NO 3 À ) and water mole- cules (O ÀH···O =N). Moreover, H-bonded 3D-networks of water are present in the molecular pockets, which show water-adsorption properties with some variation in water af- finity. Interestingly, these complexes exhibit proton conduc- tivity (1.87  10 À5 –6.52  10 À4 Scm À1 ) at 296 K and low relative humidity (ca. 46%) with activation energies of 0.29–0.46 eV. Moreover, the conductivities further increase with the en- hancement of humidity. The ability of these assemblies to exhibit proton-conducting properties under low-humidity conditions makes these materials highly appealing as elec- trolytes in batteries and in fuel-cell applications. Introduction Biological systems utilize proteins as subunits to construct a di- verse array of self-assembled nanoscopic architectures for exe- cuting widespread functions that sustain the process of life. [1] Inspired by various natural systems, artificial model systems with such structural sophistication are drawing significant at- tention because of the tremendous impact on many aspects of chemistry such as gas separation, host–guest chemistry, cataly- sis, stabilization of reactive intermediates, and generation of unusual reaction products. [2] Self-sorting of multicomponent systems through the formation of metal–ligand bonds enables the creation of thermodynamically stable architectures by taking advantage of entropic and enthalpic driving forces. [3] These systems are likely to be highly symmetrical with structur- al resemblance to Platonic or Archimedean solids. [4] Recent in- vestigation on metal–organic assemblies with structural diver- sity gives us insight into the formation that opens the avenue for generating new systems. Controlling the geometrical princi- ples and stereo-electronic preferences of the building units allows us to construct intricate architectures. Serendipity can also permit the access to novel uncommon structures that are otherwise difficult to design. [5] Both approaches highlight how modification of geometry, length, and denticity of the ligands and introduction of asymmetry or the identity of the metal ions can influence the shape and size of the final architec- ture. [6, 7a] In this context, we have recently achieved a few un- common multinuclear Pd II assemblies by employing the di- verse coordination modes of polyimidazoles, decorated around an aromatic backbone. These assemblies have shown to be po- tential receptors for a variety of guest molecules or catalysts in organic transformations. [7] On the other hand, although the research in the field of proton-conductive materials has conceived much maturity, the demands for more advanced materials for alternate energy ap- plications are still urging. [8] Nafion has been widely used as a proton-conducting membrane in fuel cells under hydrous conditions. However, these membranes are very expensive and associated with hazardous manufacturing processes. They also suffer rapid dehydration at low-humidity conditions or at ele- vated temperatures, leading to the loss of conductivity and ir- reversible transition in the membrane microstructure. There- fore, discovery of better-conducting materials has gained im- mense interest in the research community. Scientists have ex- plored a few approaches to develop proton conductivity in various inorganic and organic materials. [9] In this context, metal–organic frameworks (MOFs) and coordination polymers (CPs) have been well recognized recently due to the easy con- struction and dynamic motion of protons in these structures. [10] Water chains and clusters, introduced in the intermolecular channels/pockets can develop proton conductivity in MOFs. [11] Chemical modification of organic ligands with sulfonic, carbox- ylic, phosphonic, or hydroxyl groups is also a route to give high conductivity. [12] Kim et al. have observed high anisotropic proton conductivity in cucurbituril-based porous organic mate- rials. [13] As observed in recent reports by Kitagawa and others, [a] D. Samanta, Prof. Dr. P. S. Mukherjee Department of Inorganic and Physical Chemistry Indian Institute of Science Bangalore 560 012 (India) Fax: (+ 91) 8023601552 E-mail : psm@ipc.iisc.ernet.in Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201305075. Chem. Eur. J. 2014, 20, 5649 – 5656  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5649 Full Paper DOI: 10.1002/chem.201305075