Structural organization of a {ruthenium[tris(bipyridyl)]}2+ complex by strong π–π stacking of a tethered 1,8-naphthalimide synthon: Impact on electrochemical and spectral properties
Graphical abstract
The {Ru(bipyridyl)2}2+ complex of the new ligand N-(5-methyl-2,2′-bipyridyl)-1,8-naphthalimide, containing both a bipyridyl and 1,8-naphthalimide group, has been prepared and shown to be organized into dimers in both the solid and solution phase by strong, directionally oriented π–π stacking. UV–VIS, fluorescence spectroscopy and electrochemical studies indicate that the 1,8-naphthalimide groups does not have a significant influence on the properties of the [Ru(bipy)3]2+ core.
Introduction
The advancement and subsequent successful implementation of crystal engineering is a goal for chemists working in the field of supramolecular chemistry. Investigations have been and are currently being performed to systematically study a variety of intermolecular interactions with the explicit intent of cataloging and controlling specific non-covalent interactions, thus facilitating the development of more advanced materials with predictable structures and properties [1]. These concepts of supramolecular chemistry and crystal engineering can be carried over to the quickly emerging field of artificial electron donor–acceptor dyads and triads. Systems of this type hold promise in realizing artificial electron transfer systems with the goal of duplicating, for example, nature’s most eloquent charge-transfer reaction, photosynthesis [2].
The reliability of the hydrogen bond has become ubiquitous within the realm of crystal engineering due to its directionality and associative strength; importantly these two characteristics of the hydrogen bond can be readily transferred to other systems. Arene–arene stacking interactions, in contrast, are much less predictable, because many orientations are possible that maximize the electrostatic attraction between the σ framework and the π electron density of the interacting groups [3]. Given that the order of stability for π–π stacking interactions is π-deficient–π-deficient > π-deficient–π-rich > π-rich–π-rich [3], we have been synthesizing ligands containing the electron withdrawing 1,8-naphthalimide synthon in order to maximize the strength of the interaction [4]. While the photophysical properties of 1,8-naphthalimide derivatives have been utilized [5], its π-deficiency has not been exploited extensively in crystal engineering. Our initial work has been the incorporation of this group into new poly(pyrazolyl)methane based ligands, with the intent of building strong π–π stacking interactions into metal complexes [4]. We have shown that not only do we observe strong π–π stacking interactions that are retained even in solution, but the interaction is generally directionally oriented with the naphthalimide groups organizing head to tail with their dipole vectors oriented at 180° antiparallel [4].
Another important reason to build the 1,8-naphthalimide and its N-substituted derivatives into transition metal complexes is that this group has been previously shown to undergo interesting electrochemistry and fluorescence, both of which can be tuned by substitution [5]. Taking into account these properties and the demonstrated utility of 1,8-naphthalimide to organize metal complexes into predictable structural arrangements and the interest in [Ru(bipy)3]2+ chemistry [6], we have coupled the two groups in a single compound. Herein is reported the design and synthesis of N-(5-methyl-2,2’-bipyridyl)-1,8-naphthalimide (L) and its ruthenium(II) complex [LRu(bipy)2]2+. This new complex combines the arene stacking interactions between two π-electron deficient, 1,8-naphthalimide moieties and the predictable and directional associative protocol of the naphthalimide group with the well-studied chemistry of the [Ru(bipy)3]2+ core [6], in order to determine the effect of the 1,8-naphthalimide moiety on the properties of the ruthenium complex.
Section snippets
General considerations
Unless otherwise specified, all operations were carried out under a nitrogen atmosphere by using either standard Schlenk techniques or a Vacuum Atmospheres HE-493 inert atmosphere dry box. Solvents for synthetic procedures and spectroscopic studies were dried by conventional methods and distilled under a N2 atmosphere immediately prior to use. All chemicals were purchased from Aldrich Chemicals. Mass spectrometric measurements recorded in ESI(±) mode were obtained on a Micromass Q-Tof
Synthesis
The new ligand, N-(5-methyl-2,2′-bipyridyl)-1,8-naphthalimide, was synthesized by the reaction of commercially available 1,8-naphthalimide, 5-(bromomethyl)-2,2′-bipyridine and potassium carbonate in refluxing acetone (Scheme 1). Reaction of this ligand and bis(bipyridyl)ruthenium(II) dichloride in refluxing ethanol and subsequent exposure to saturated, aqueous ammonium hexafluorophosphate resulted in high yield of {ruthenium[bis(bipyridyl)][N-(5-methyl-2,2′-bipyridyl)-1,8-naphthalimide]}(PF6)2 (
Conclusion
The synthesis of N-(5-methyl-2,2’-bipyridyl)-1,8-naphthalimide and {ruthenium[bis(bipyridyl)][N-(5-methyl-2,2’-bipyridyl)-1,8-naphthalimide]}(PF6)2 has been successfully carried out. In the solid state structure of 1, the 1,8-naphthalimide groups are π–π stacked in a 180° antiparallel arrangement. This π–π stacking interaction was also shown to be present in solution by PGSE-NMR analysis, further demonstrating the utility of the 1,8-naphthalimide synthon to organize the structures in both the
Supplementary data
CCDC 715283 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
Acknowledgment
The authors thank the National Science Foundation (CHE-0715559) for support of this research.
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