Magnetic Archimedean Tessellations in Metal-Organic Frameworks

Abstract

The properties of graphene nanoribbons are highly dependent on structural variables such as width, length, edge structure, and heteroatom doping. Therefore, atomic precision over all these variables is necessary for establishing their fundament l properties and exploring their potential applications. An iterative approach is presented that assembles a small and carefully designed molecular building block into monodisperse N-doped graphene nanoribbons with different lengths. To showcase this approach, the synthesis and characterisation of a series of nanoribbons constituted of 10, 20 and 30 conjugated linearly-fused rings (2.9, 5.3, and 7.7 nm in length, respectively) is presented. The discovery of fullerenes, nanotubes, and graphene has stimulated the exploration of synthetic low-dimensional carbon nanostructures. Among these, quasi-one-dimensional atomically precise substructures of graphene, known as graphene nanoribbons (NRs), combine the one-atom thickness of graphene with the structure-dependent metallicity of carbon nanotubes. NRs have unique electronic, optical and mechanical properties and are considered promising candidates to develop new technologies for electronics, photonics, and energy conversion, among others. The properties of NRs are highly dependent on several structural variables such as width, length, edge structure, and heteroatom doping. Therefore, atomic precision over these variables is necessary for establishing their fundamental properties and exploring their potential applications. The edge structure of NRs influences their metallicity and their photonic properties. The size of the energy gap of NRs is strongly influenced by the width. For example, energy gaps > 1.4 eV are expected for NRs with sub-nm widths. The length is also an important variable in NRs, as the size of the energy gap decreases with increasing length until saturation. Also, lengths of more than 5 nm constitute a structural prerequisite to explore the potential of NRs in single NR field-effect transistors. Even if there have been enormous advances in the synthesis of NRs, current approaches do not allow the attainment of atomic precision over width, length, and edge structure simultaneously on NRs of more than 5 nm in length. Top-down methods such as cutting graphene or unzipping carbon nanotubes by means of lithography or etching have been applied to prepare NRs, but they do not provide atomic precision over any structural vari ble. Bottom-up onsurface synthesis, 9] in-nanotube synthesis, and solution polymerisation methods provide atomically precise control over the edge and width of the NRs, but do not provide atomic precision over th length. A promising approach that can provide simultaneously atomic precision over edge, width, and length is multistep organic synthesis in solution. In fact, several families of monodisperse NRs with more than 2 nm in length have been reported 12] that evolve from acenes, naphthalene, pyrene, perylene, coronene, and rylene derivatives, among others. However, until now, only NRs with lengths approaching 5 nm have been obtained by this approach. This is up to 18 fused aromatic rings in a linear arrangement and up to 23 fused aromatic rings in an armchair arrangement. Approaching the synthesis of NRs more than 5 nm in length from an organic chemistry perspective is very challenging because of the large number of different synthetic and purification steps that have to be individually optimised and also because of the high tendency of large aromatic systems to aggregate in solution, which makes difficult, and in some cases even hamper, their synthesis, purification, characterisation, and processing. Herein we report an iterative approach that assembles a small molecular building block into NRs of different lengths, opening up a new route for the preparation of monodisperse NRs. To showcase this approach, we describe the synthesis of a series of NRs constituted of 10, 20, and 30 linearly-fused aromatic rings (with 2.9, 5.3, and 7.7 nm in length, respectively), which include the longest monodisperse NRs reported to date (Scheme 1; Supporting Information, Figure S1). Remarkably, the whole NR series is soluble in chlorinated [*] Dr. D. Cortizo-Lacalle, J. P. Mora-Fuentes, Prof. Dr. A. Mateo-Alonso POLYMAT, University of the Basque Country UPV/EHU Avenida de Tolosa 72, 20018 Donostia-San Sebastian (Spain) E-mail: [email protected] Dr. K. Strutyński, Prof. Dr. M. Melle-Franco CICECO—Aveiro Institute of Materials epartment of Chemistry, University of Aveiro 3810-193 Aveiro (Portugal) E-mail: [email protected] Prof. Dr. A. Saeki Department of Applied Chemistry, Graduate School of Engineering Osaka University, Suita, Osaka 565-0871 (Japan) Prof. Dr. A. Mateo-Alonso Ikerbasque, Basque Foundation for Science 48011 Bilbao (Spain) Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201710467. ⌫ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited, and is not used for commercial purposes. Angewandte Chemie Communications 703 Angew. Chem. Int. Ed. 2018, 57, 703 –708 ⌫ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim single crystalline compound is known to incorporate this chemical species, Na+(en)(bipy•–) (en = ethane-1,2-diamine).[14] The reaction of sodium with an excess of bipy in CH3CN affords an intensely dark-blue colored solution. The X-band electron paramagnetic resonance (EPR) spectrum of the frozen solution yields a single narrow resonance at g = 2.00 (Fig. S1, Supporting Information), compatible with the formation of an organic radical and similar to that of Na+(en)(bipy•–) (g = 2.00429).[14] The addition of solid GdI3 to the bipy•–/bipy solution results in an immediate formation of a dark-blue microcrystalline powder which exhibits a broad EPR signal (Fig. S2, Supporting Information). The elemental analyses of C, H, N, Gd, I, and Na indicate a formulation of GdI2(bipy)5/2×CH3CN and no presence of Na+ (cf. Supporting Information). Careful layering of the darkblue bipy•–/bipy solution onto CH3CN-covered solid GdI3 affords block-like, dark-blue crystals of Gd suitable for singlecrystal X-ray diffraction. Gd crystallizes in the tetragonal I4122 space group and features an ideal snub square tessellation of GdI2(bipy)5/2×xCH3CN (cf. Fig. 2a). Figure 2. (a) Single-crystal X-ray structure of Gd viewed perpendicular to one of the snub square tiling layers. (b) Single-crystal structure of the trace impurity phase Gd¢. Color codes: Gd, turquoise; I, purple; N, blue; C, grey. H atoms and co-crystallized CH3CN molecules have been omitted for clarity. The coordination geometries of the two crystallographically independent Gd centers are almost identical. The Gd3⁄4I bond lengths of 3.0437(7)–3.054(2) Å are slightly longer than those found in trans-[GdI2(thf)5] (thf = tetrahydrofuran) of 3.00 Å,[15] albeit significantly shorter than the Eu3⁄4I bonds found in trans-[EuI2(thf)5] of 3.22–3.24 Å,[16] corroborating the presence of Gd(III), and not the extremely rare Gd(II), in Gd.[17] The N3⁄4Gd3⁄4N angles are in the range of 68.1(2)° to 75.3(3)° and together with the I3⁄4Gd3⁄4I linearity (179.23(4)°, 177.90(5)°) reflect the close proximity of the local coordination environment to D5h symmetry. The presence of Gd(III) necessitates one bipy•– radical ligand per GdI2(bipy)5/2 formula unit. Goicoechea and coworkers have previously shown that the inter-pyridinic bond in bipy shortens by ~4% upon one-electron reduction.[14] According to that, in Gd, the two crystallographically independent Gd centres are each surrounded by three bipy0/•– ligands with longer C3⁄4C bond lengths of 1.49(3)–1.51(2) Å and two bipy0/•– ligands with short C3⁄4C bond lengths of 1.45(2) Å corresponding to a reduction of ~4%. Interestingly, the anticipated localized bipy•– ligands span the edges of the triangles in only one direction of the plane which leads to the formation of {Gd4(bipy)4} rhombi (Fig. S3). The presence of mixed valency in the bipy0/•– scaffold could be expected to lead to strong inter-valence charge transfer (IVCT) transitions in the midor near-infrared spectrum as e.g. observed in transition metal complexes of mixed-valent 2,2¢-bipyridine0/•–.[18] However, no such IVCT bands could be observed in Gd (Fig. S6, Supporting Information) which may, tentatively, be attributed to the weakly covalent nature of metal-ligand bonds and the localization of the unpaired electron, as inferred from crystallography. Notably, in each crystallization vial, a few needle-shaped dark-blue crystals of Gd¢ were systematically obtained. The structural analysis of Gd¢ revealed an identical chemical composition of GdI2(bipy)5/2×xCH3CN, but Gd¢ crystallizes in the triclinic P1 space group and resembles an elongated triangular tiling similar to the previously reported YbI2(bipy)5/2 structure (Fig. 2b). However, in contrary to YbI2(bipy)5/2, the tilting angle, a (cf. Fig. 1b), departs significantly from 90° and amounts to 101°. Thus, the Gd centres could, as well, be considered as approaching defect 6-fold nodes, corresponding to a = 120°. Similarly to Gd, two fifths of the bipy display short inter-pyridinic C3⁄4C bonds of an average length of 1.44(2) Å and three fifths exhibit normal, longer C3⁄4C bonds of 1.51(1)–1.53(2) Å, echoing the existence of Gd(III) and both bipy and bipy•– in Gd¢. Use of DyI3 reveals an identical behavior and yields DyI2(bipy)5/2×xCH3CN (Dy). Dy is isostructural to Gd with slightly contracted bond lengths. Notably, no traces of an elongated triangular tessellation phase could be observed for Dy. The room temperature value of the magnetic susceptibilitytemperature product,