Towards larger polygonal architectures: synthesis and characterization of iron(II)- and ruthenium(II)-bis(terpyridine) metallomacrocycles.

Construction of supramolecular architectures employing the self-assembly of predesigned building blocks has drawn considerable attention in the past decades. Based on the combinatorial library proposed by Lehn, self-assembling constructs are often an equilibrium distribution of possible assemblies possessing comparable stabilities. Assemblies with similar components resulting in an equilibrating mixture of binary or multiple entities have been reported. Under appropriate conditions, the thermodynamically most stable species can be obtained in quantitative yield. Based on these tenants, many successful strategies have been developed for the synthesis of metallocycles with triangular, rectangular, pentagonal, and hexagonal shapes. In contrast, few examples of larger polygonal structures are documented. Towards larger finite architectures, subtle variation of building blocks could result in a dramatic change to the final products. In the self-assembly of bis(terpyridine) ligands possessing a 1208 angle between two ligating moieties, only hexameric metallomacrocycles were previously reported due to the difficulty of separation and characterization. More recently, Newkome et al. successfully isolated unexpected pentameric and heptameric macrocycles from the Fe-mediated complexation of rigid bis(terpyridine) ligands. Herein, we employ kinetically stable (in which tpy=2,2’:6’,2’’-terpyridine, M=Fe and Ru) connectivity to afford a series of homoand heteronuclear metallomacrocycles by means of stepwise assembly procedures. By using metals that form strong coordinative bonds (Fe and Ru), the self-assembly process becomes kinetically controlled and irreversible, thus allowing for the formation of macrocycles other than the thermodynamically most stable hexameric ring. Further, the use of elongated bis(terpyridine) dimers as precursors introduces steric constraints and increased rotational freedom, which obstruct the formation of small metallocycles, while facilitating the construction of novel, large metallocycles that would be difficult to realize otherwise. Traveling-wave ion-mobility mass spectrometry (TWIM-MS) and molecular modeling provide unique insight into the distinct ring sizes formed and their conformational flexibility. Reaction (Scheme 1a) of 3,5-bis(4’-terpyridinyl)anisole (1) with 1.05 equivalents of FeCl2·4H2O in a mixed solvent of MeOH/CHCl3 at reflux for 18 h gave the pentamer (2 ; 6%), hexamer (3 ; 15%), and heptamer (4 ; 3%), which were isolated by column chromatography (SiO2) eluting with H2O/ MeCN/sat.-KNO3(aq) (1:12.5:1; v/v/v). Subsequently, the counterions were exchanged from NO3 to PF6 by adding a slight excess of methanolic NH4PF6 (1m). The H NMR spectrum (Figure S1a in the Supporting Information) of pentamer 2 displayed four sharp singlets at d=9.41 (3’,5’-tpyH), 8.61 (4-ArH), 8.16 (2,6-ArH) and 4.34 ppm (OCH3), supporting the presence of a single homogeneous environment for complexed bis(terpyridine) ligands, in contrast to linear oligomers that have more complicated patterns. Other supportive data included an expected upfield shift in the H NMR spectrum for the 6,6’’-tpyH protons (d=7.27 ppm, Dd= 1.48 ppm) relative to the corresponding peaks of ligand 1 and one sharp peak in the C NMR spectrum at d=57.3 ppm (OCH3). Its structure was further confirmed by the intense ESI-MS peaks (Figure S2a in the Supporting Information) at m/z 1382.7, 1000.4, 771.3, 618.6, 509.5, and 427.7, corresponding to [M nPF6] ions (n=3–8), respectively. The H NMR spectra of macrocycles 3 and 4 revealed similar patterns, but all aromatic peaks exhibited a slight downfield shift relative to the corresponding peaks in 2, presumably due to conformational changes in the larger rings. The 4-ArH signal in pentamer 2 showed a significant upfield shift (Dd= 0.13 ppm) in comparison with hexamer 3, owing to the shielding by the adjacent pyridines in the more crowded inner space (Figure S1b in the Supporting Information). A similar phenomenon was observed in our previous work. Heptamer 4 has more conformational flexibility, minimizing this shielding effect; its 4-ArH signal was shifted further downfield (Figure S1c in the Supporting Information). Macrocycles 3 (MW=5498.5 Da) and 4 (MW= 6414.9 Da) were further verified by their ESI mass spectra (Figure S2b and S2c in the Supporting Information). Hexamer 3 gave rise to eight major peaks at m/z 1229.7, 954.7, [a] Dr. Y.-T. Chan, Dr. X. Li, Dr. C. N. Moorefield, Prof. Dr. C. Wesdemiotis, Prof. Dr. G. R. Newkome Department of Polymer Science, Department of Chemistry The University of Akron, 170 University Circle-RM501B Akron, OH 44325-4717 (USA) Fax: (+1)330-972-2368 E-mail : newkome@uakron.edu wesdemiotis@uakron.edu Homepage: http://www.dendrimers.com [] Drs. Chan and Li contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201100559.