Solvothermal Synthesis of a Stable Coordination Polymer with Copper-I−Copper-II Dimer Units:  [Cu₄{1,4-C₆H₄(COO)₂}₃(4,4‘-bipy)₂]_n

Abstract

Metal coordination polymer chemistry is a growing field, its exploration driven in part by the search for novel magnetic materials 1,2 as well as for new microporous phases 3 and catalysts 4 which are complementary to zeolites. Recently, we reported the solvothermal synthesis of a highly porous yet stable coordination polymer from copper ions and trimesic acid [Cu 3 {1,3,5- 5 Central to the stability of 1 is the structural rigidity of its [Cu 2 (µ-O 2 CR) 4 L 2 ] dimer units (Figure 1). A recent report by Japanese workers of a related sorption polymer [Cu-{1,4-C 6 H 4 (COO) 2 }(H 2 O) x ] n , 6 prompted us to explore the solvothermal chemistry of the copper terephthalate system.Herein we report that under solvothermal or hydrothermal conditions, reaction of copper(II) nitrate, terephthalic acid, and 4,4′-bipyridine 7 results in the formation of an air-stable, openframework coordination polymer with mixed-valence [Cu 2 ] 3+ dimer subunits, [Cu 4 {1,4-C 6 H 4 (COO) 2 } 3 (4,4′-bipy) 2 ] n , 2. We believe that the polymer represents the trapping through "supramolecular precipitation" of the unstable Cu(I)-Cu(II) dimers which are produced under the reducing conditions prevalent in the reaction media at high temperatures.Compound 2 is formed phase-pure as small dark-blue blocks of ∼100 µm dimension in ∼65% yield from a solvothermal reaction (180 °C, 1 day) of [Cu(NO 3 ) 2 ]‚H 2 O, [1,4-C 6 H 4 (COOH) 2 ] and [4,4′-bipy] of 1:1:1 ratio in 50% aqueous ethanol. Larger crystal specimens up to 1 mm are formed, although in much reduced yield ( ∼20% of the isolated solids) from a parallel hydrothermal reaction with copper sulfate. The dark coloration suggested a rather novel structure for 2 which was confirmed by single-crystal diffraction. 8 Rather than the expected "paddle-wheel" arrangement of four bridging carboxylates, found in 1, an open-framework polymer containing dimeric [Cu 2 (µ-O 2 CR) 3 L 2 ] units with only three carboxylate bridges is found in compound 2. (Figure 2) Once the framework atoms had been refined, difference Fourier maps revealed no peaks of chemical significance, residual electron density maxima and minima were +0.92/-0.97 e Å -3 , both less than 1 Å from Cu. The necessity to charge balance the ligands in 2, indicate that the metals have a mixed Cu(I)-Cu(II) formulation. Other structural evidence for this comes from the unusually short Cu-Cu separation of 2.442(1) Å. This is consistent with the majority of [Cu 2 ] 3+ complexes determined previously, 9-11 which also have short "bonds". 12 Furthermore the bond valence sums indicate equivalence of the Cu centers and a delocalized 1.5+ "oxidation state" for each site.

Non‐Natural Eight‐Connected Solid‐State Materials: A New Coordination Chemistry

“…The most common nomenclature for such systems has been based [1][2][3] on the idea of two-, three-, four-, or six-connected materials in one, two and three dimensions. [4][5][6][7][8][9][10][11] Connectivities of five, seven, or higher are extremely rare, [12][13][14] and recent studies on inorganic/organic hybrid materials, especially in the area of metal-ligand coordination framework polymers, have enriched this area substantially. [15][16][17][18][19][20][21] However, highly connected materials remain scarce because the construction of such systems is severely hampered by the available number of coordination sites at the metal centers and the sterically demanding nature of organic ligands.…”

Synthesis, characterization, and luminescence of zinc(II) and cadmium(II) coordination complexes: [Zn(phen)$_{2}$(CH$_{3}$COO)](ClO$_{4})$, [Zn(bpy)$_{2}$(ClO$_{4})$](ClO$_{4})$, and [Cd(phen)$_{2}$(NO$_{3})_{2}$]

“…7 While the synthesis of fascinating self-assembly metal-organic coordination polymers occurs via bridging ligand and also through weak noncovalent interactions, such as π − π stacking or hydrogen bonding, 8−20 the construction of metal-organic compounds mainly depends on the nature of the organic ligands and metal ions. A considerable number of transition metal complexes using anionic O-donor ligands such as carboxylic acids 21,22 and neutral N-donor ligands such as phenanthroline, bipyridines, 23−26 pyrazine, 27,28 and triazines 29 have been reported during the last decade. In particular, 1,10-phenanthroline (phen) and 2,2'-bipyridine (bpy) ligands have been widely used to construct supramolecular architectures.…”

Remarkable solvent effects in the hydro‐ and solvothermal synthesis of copper‐1,10‐phenanthroline complexes

“…7 -12 Recently, a few mixedvalence Cu(II)/Cu(I) complexes, such as Cu 4 (ophen) 4 (tp) and Cu 4 (obpy) 4 (tp) (ophen = ophenanthroline; obpy = obipyridine and tp = terephthalate), 14,15 have been obtained through the use of the redox reaction of Cu(II) ions with mixed ligands under hydrothermal or solvothermal conditions. 3,13 In this paper, we obtained two very different complexes, [Cu(II)/Cu(I)] 2 (ophen) 6 (tpt) (tpt = trimesicate) (1) and Cu(I) 3 (CN) 3 (phen) 3 (2), through the use of different solvents. Under hydrothermal conditions, compound 1 was prepared, 3+ complexes determined previously.…”

“…Under hydrothermal conditions, compound 1 was prepared, 3+ complexes determined previously. 3,8,14,15,18,19 The bond valence sums of compound 1 molecule are the copper center as a delocalized 1.5+ oxidation state or a mixed Cu(I)-Cu(II) formulation. The XPS spectrum of 1 also supports its novel mixed-valence formulation.…”

“…According to the conditions for synthesis of 1, we find that the use of uniqueous anhydrous ethanol instead of water produced one novel compound [Cu 3 (CN) 3 Molecular packing as seen along the crystallo-graphic a axis for 1.…”