Copper β‐Diketonate Molecular Squares and Their Host–Guest Reactions

Abstract

In the field of porous supramolecular metal complexes, both molecular [1] and extended-solid [2] materials have been extensively studied in recent years. These materials are attractive due to their applications in gas storage [3] and host-guest chemistry.[4] Among the most often studied of these species are the metal-containing "molecular squares", that is, squareshaped porous tetrameric structures. These have been prepared by several approaches, the most common being the reaction of an organic bridging ligand with a metal complex that has available cis coordination sites (Figure 1 a). The bridging ligand is frequently a pyridine derivative (for example, 4,4'-bipyridine (4,4'-bpy)), [5] although bis-chelating ligands have also been used. [6] In these cases, the 908 "corners" in the molecular squares are provided by metal complexes. The resulting metal centers are usually coordinatively saturated, which makes it difficult for guest molecules to interact directly with the metal atoms. Herein we report molecular squares prepared by an alternative approach, in which bifunctional organic moieties serve as the "corners" and join metal atoms in the centers of the "sides" (Figure 1 b). [7] These molecular squares are formed in one step, by reaction of bis(b-diketone) ligands (1) (Scheme 1). The new squares (2) are capable of binding guests in both a s (for example, 4,4'-bpy) and p fashion (for example, C 60 ), and they are also surprisingly effective for hydrogen-gas storage, both at 77 K and at room temperature.Ligands 1 a and 1 b were synthesized by reaction of isophthalaldehyde with 2,2,2-trimethoxy-4,5-dimethyl-1,3,2-dioxaphospholene and 2,2,2-trimethoxy-4,5-diethyl-1,3,2-dioxaphospholene, respectively, at room temperature and heating the intermediate in methanol under nitrogen. [8] Mixing solutions of 1 a or 1 b in CH 2 Cl 2 with aqueous [Cu(NH 3 ) 4 ] 2+ produces 2 a or 2 b, respectively, which can be isolated as dark green solids in approximately 95 % yield.The angles between the two b-diketone moieties in the new ligands 1 are about 1208. Thus, we anticipated that their reaction with metal ions would yield hexamers, for example, [M 6 (m-pba) 6 ]. However, X-ray analysis of the crystalline products 2, which are formed in nearly quantitative yield, shows that they are actually molecular squares, with diameters of about 14 (Figure 2).[9] (We explored a variety of conditions for preparing 2, but found no evidence of other oligomers.) Thus, 2 a and 2 b are unusual examples of molecular squares in which the corners are organic bridging groups, and the metal atoms are in the centers of the sides.The angles between the b-diketone moieties in the structures of 2 a and 2 b are still approximately 1208. To accommodate this angle within an overall square shape, there must be some distortion elsewhere in the molecule. The two ligands are coplanar with the metal atoms in an undistorted [Cu(b-diketonate) 2 ] complexes, [10] while, in the present squares, the ligands are all bent away from coplanarity.The formation of...

Self‐Assembly of Nano‐Sized Neutral Metal–Organic Macrocycles from Bis(β‐diketone) Ligands

“…Accordingly, the target products of its self‐assembly with bivalent copper ions are supposed to be hexamers (Cu 6 L 2 6 ). Nevertheless, both previous studies and results herein display that they are actually molecular squares 16a. The dark‐green crystals of 2 (Cu 4 L 2 4 ) are obtained by using similar methods as that of square 1 .…”

“…Unfortunately, crystallinity of this type of square only exists inside the mother liquor (Figure S1 c in the Supporting Information), and the crystals lost solvent rapidly in air to give a green powder (Figure S1 d in the Supporting Information), making it impossible to do this experiment directly. Nevertheless, previous study has demonstrated that noncrystalline solids of this type of macrocycle still possessed cavities and could serve as a host for gas adsorption 16a. 17…”

Dinuclear Pyridine-4-thiolate-Bridged Rhodium and Iridium Complexes as Ditopic Building Blocks in Molecular Architecture

“…The search for new and complex structures has been a motivating factor for the intensive study of metalla-supramolecular chemistry. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] The sustained interest in this area is driven by the wide range of potential applications of these species in gas storage, 15,16 catalysis, [17][18][19][20] molecular magnetism, [21][22][23] optical materials, [24][25][26] sensoring [27][28][29] and many other areas.…”

Cyclic pyridyltriazole–Cu(ii) dimers as supramolecular hosts

“…8,9 Aida et al demonstrated cooperative binding of C 60 and diamine guests to a π-conjugated host, 10 and Yu et al have prepared supramolecular systems from dimetal units and multifunctional pyrazole ligands. 11,12 Our group has a long-term interest [13][14][15][16] in macrocyclic metal-organic compounds based on tetradentate β-diketonates and Cu 2+ . In an early study, we designed a metallo-supramolecular cage (Fig.…”

“…17 More recently, we prepared a β-diketonate molecular square, Cu 4 (m-pba) 4 , which accommodates 4,4′-bpy and C 60 as guests. 15 The molecular square described above, Cu 4 (m-pba) 4 , and its smaller congener Cu 2 (o-pba) 2 , are constructed from m-and o-phenylenebis(acetylacetonate) (m-pba 2− and o-pba 2− ) and Cu 2+ . 15,16 These reactions were expected to yield larger rings on the basis of ligand geometry (Cu 6 (m-pba) 6 and Cu 3 (o-pba) 3 , respectively).…”