Unexpected Isolation of a Pentameric Metallomacrocycle from the FeII‐Mediated Complexation of 120° Juxtaposed 2,2′:6′,2′′‐Terpyridine Ligands

Self-assembly of specially designed building blocks has been widely employed in the construction of rigid supramolecular structures. Towards this goal, many successful strategies have been developed for the synthesis of metallomacrocycles, such as for trigonal, rectangular, hexagonal, heptagonal, and octagonal structures. In contrast, only a limited number of pentagonal shapes have been reported in the literature. Accordingly, building blocks require a suitable angle to generate five-sided motifs. For example, a carbazole unit that introduced a 1058 angle between two terpyridine ligands was readily self-assembled to give metallopentacycles. Tetragonal and trigonal metallomacrocycles formed by 608 juxtaposed bis(terpyridine) ligands possessing bendable alkyne spacers have been documented. However, the formation of unexpected products by using rigid linkers is rare. Herein, we report the isolation and characterization of a unique pentameric macrocycle, along with its anticipated hexameric homologue, generated from an Fe-mediated complexation of functionalized bis(terpyridine) ligands with a 1208 angle relative to the coordination sites. Reaction of 1 (Scheme 1) with 1.05 equivalents of FeCl2·4H2O in MeOH at 25 8C for 24 h gave the pentamer [Fe5(1)5 ACHTUNGTRENNUNG(NO3)10] (2a ; 4.3%) and the hexamer [Fe6(1)6ACHTUNGTRENNUNG(NO3)12] (2b ; 11.9%), which were isolated by gradient column chromatography eluting with H2O/MeCN/sat. KNO3(aq) (from 1:20:1 to 1:15:1; v/v/v). The H NMR spectrum of pentamer 2a revealed sharp singlets at 8.22 and 8.75 ppm for 2,6-ArHs and 4-ArH respectively, supporting the presence of the symmetric macrocycle, in contrast to linear oligomers that exhibit more complicated patterns. Other supportive data (H NMR spectroscopy) included an expected upfield shift for 6,6’’-tpyHs (d= 7.41 ppm, Dd= 1.34 ppm) and a downfield shift for 3’,5’tpyHs (d=9.65 ppm, Dd=+0.86 ppm) relative to the corresponding peaks in the uncomplexed ligand 1 (Figure 1). The H NMR spectrum of hexamer 2b exhibited a similar pattern, but the peaks of terpyridine part showed a slight downfield shift relative to the corresponding peaks in 2a presumably due to the additional metal centers in 2b resulting in added electron deficiency. In comparison with 2b, notably the 4-ArH peak of 2a has an upfield shift (Dd= 0.2 ppm) due to the enhanced crowded inner space. This proved to be a key discernable feature in recognizing, isolating and characterizing the otherwise remarkably similar fiveand sixmembered ring structures. The 2D COSY NMR spectra of the bis(terpyridine) ligand 1 and the self-assembled macrocycle 2a also ensured the proper assignments. Notably, at [a] Y.-T. Chan, Dr. M. Soler, Prof. G. R. Newkome Departments of Polymer Science and Chemistry The University of Akron, Akron, OH 44325-4717 (USA) Fax: (+1)330-972-2413 E-mail : newkome@uakron.edu [b] Dr. C. N. Moorefield, Prof. G. R. Newkome Maurice Morton Institute for Polymer Science The University of Akron, Akron, OH 44325 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200902988. Scheme 1. Unexpected and predetermined products 2a and 2b obtained by self-assembly of 1. a) FeCl2·4H2O, MeOH, 25 8C, 24 h (each Fe II macrocycle isolated as the polyNO3 salt; see Experimental Section).