Hexameric Palladium(II) Terpyridyl Metallomacrocycles: Assembly with 4,4′‐Bipyridine and Characterization by TWIM Mass Spectrometry

Abstract

The construction of 2D or 3D materials using supramolecular chemistry principles has become an intriguing area of research. In particular, terpyridine-based building blocks have played a pivotal role in the construction of dimers, triangles, trigonal prisms, squares, 7–9] pentagons, and hexagons, based on their planar tridentate coordination mode and their facile potential for modification. Terpyridine (terpy) and its metal complexes are also of interest because of their well-known photophysical and electronic properties. Thus, the use of terpyridine for the construction of materials and molecular architectures with increasing complexity will continue to mature. However, the difficulty of obtaining single crystals suitable for X-ray structure determination means that other reliable techniques are essential for the characterization of macromolecular structures. Electrospray ionization (ESI) mass spectrometry has been applied in the identification and characterization under mild ionization conditions. In particular, the cold-spray (CSI) technique reported by Fujita and co-workers, and the Fourier transform mass spectrometry (FTMS) technique developed by Schalley and coworkers are the most prominent ESI-based methods. The work of Piguet and co-workers is also notable. Unfortunately, the signals that correspond to different charge states are superimposed and only a few isotope patterns of different charge states can be deconvoluted. Recently, traveling wave ion mobility mass spectrometry (TWIM-MS), a variant of ion mobility mass spectrometry (IM-MS), has been successfully applied to the detection and characterization of supramolecules. Ion-mobility-based separation enhances the resolving power of mass spectrometry by adding shapeand charge-dependent dispersion, which reduces isomer superposition and can deconvolute the isotope patterns of different charge states. Notably, isomeric linear and cyclic structures have been separated based on their different drift time in the ion mobility device. Herein, we report the 4,4’-bipyridine (bpy) assisted assembly of a hexagonal, dodeca Pd terpyridyl based macrocycle, and its characterization by NMR and TWIMMS. A recent example of the use of terpyridine and its Pd coordination for the formation of a metallocyclic rectangle was reported by Bosnich and co-workers, whereby two cofacially oriented, Pd terpyridine MeCN adducts were dimerized upon the addition of 4,4’-bipyridine. Our synthetic efforts began with an improved preparation of 1,3-bis(2,2’:6’,2’’-terpyridin-4’-yl)-5-tert-butylbenzene (1), which was isolated in 60% yield and exhibited identical H and C NMR spectra to that of the initially reported ligand. Ligand 1 was prepared by the reduction of commercially available 5-tert-butylbenzene-1,3-dicarboxylic acid with BH3·THF, followed by selective oxidation with pyridinium chlorochromate (PCC) and subsequent grinding with 2-acetylpyridine (4.05 equivalents) and NaOH to give an orange solid, which was then added to NH4OH and EtOH and heated at reflux for 24 hours. This ligand, which has coordination sites that are positioned 120 degrees apart, was treated with [Pd(MeCN)4](BF4)2 in dry MeCN to give [(1,3-bis(2,2’:6’,2’’-terpyridin-4’-yl)-5-tert-butylbenzene)Pd2(MeCN)2](BF4)4 2 in nearly quantitative yield (Scheme 1). Ligand 1 was initially insoluble in MeCN, but was readily solubilized when treated with [Pd(MeCN)4](BF4)2 in MeCN. The H NMR data confirmed the formation of adduct 2, with signals at d= 8.68 (s, 3’,5’-terpyH), 8.63-8.58 (m, 6,6’’-terpyH and 3,3’’-terpyH), 8.49 (t, 4,4’’-terpyH), and 7.89 ppm (t, 5,5’’terpyH); see Figure 1. A downfield shift of the tert-butyl singlet peak (from d= 1.51 to 1.57 ppm, Dd= 0.06 ppm) and the IR absorptions observed at 2334 and 2304 cm 1 assigned to the C N stretch also support the formation of a Pd adduct 2. [*] Dr. S. Perera, Dr. X. Li, Dr. M. Soler, A. Schultz, Prof. Dr. C. Wesdemiotis, Dr. C. N. Moorefield, Prof. Dr. G. R. Newkome Department of Polymer Science, Department of Chemistry, The University of Akron 302 Buchtel Common, Akron, OH 44325 (USA) Fax: (+1)330-972-2368 E-mail: wesdemiotis@uakron.edu newkome@uakron.edu Homepage: http://www.dendrimers.com [] These authors contributed equally to this work.