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Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
Supramolecular Chemistry of Some Metal Acetylacetonates with Auxiliary Pyridyl Sites Published as part of the Crystal Growth and Design Israel Goldberg Memorial virtual special issue. Chamara A. Gunawardana,† Abhijeet S. Sinha,† John Desper,† Marijana Đakovic,́ ‡ and Christer B. Aakeröy*,† †
Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10000 Zagreb, Croatia
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‡
S Supporting Information *
ABSTRACT: Hetero-bifunctional ligands can pave the way for elaborate metallo-supramolecular systems and are also useful for combining metal−ligand bonding with other types of noncovalent interactions. We synthesized two new pyridylacetylacetonate ligands, 3-(4-(pyridin-4-yl)phenyl)pentane2,4-dione (L1) and 3-(4-(pyridin-4-ylethynyl)phenyl)pentane-2,4-dione (L2), and explored their metal binding ability with selected di- and trivalent transition metal ions. As expected, the acetylacetonate ligation with metal dications remains consistent among four structures, [Cu(L1) 2(MeOH)2]n, [Co(L2)2]n, [Cu(L2)2(MeOH)2], and [Zn(L2)2(MeOH)2]; the metal is four-coordinate and resides in a square planar environment. Differences in the overall architectures arise basically from the role played by the terminal heterocycle (i.e., the pyridyl group). In [Cu(L1)2(MeOH)2]n and [Co(L2)2]n, the heterocyclic end directly binds to the metal (through vacant axial positions), thereby producing coordination networks. In [Cu(L2)2(MeOH)2] and [Zn(L2)2(MeOH)2], metal-methanol coordination and intermolecular O− H(methanol)···N(pyridine) hydrogen-bond interactions work in concert to weave those bis-acetylacetonate complexes into ribbon-like supramolecular polymeric arrays. Somewhat surprisingly, the only tris-chelated acetylacetonate complex characterized in this study, [Fe(L2)3], essentially exists as discrete dimeric aggregates.
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INTRODUCTION Inorganic−organic hybrid materials are structurally diverse and functionally versatile. The design and construction of such materials therefore represent an area of enormous relevance to fundamental crystal engineering and applied materials science alike.1−5 Even though the outcome of the attempted synthesis of such materials is influenced by many parameters, the nature of the organic linker or the ligand is one of the key factors that control both the structure and the subsequent properties of the resulting solids. So far, ligands containing carboxylates6−10 and nitrogen heterocycles11−18 (pyridine, imidazole, pyrazole, triazole, etc.) dominate the arena because of the intermediate strength of the coordinative bonds that simultaneously impart reliability and reversibility to the assembly process. This dynamic nature is of paramount practical synthetic importance as it allows for the self-correction of defects and for the reversal of undesirable binding events, thereby facilitating the formation of highly ordered as well as of thermodynamically stable structures. Carboxylate ligands possess several additional features that are responsible for their success. Being negatively charged, they can lead to electroneutral assemblies where no potentially disruptive counter-anions are present. Their cluster chemistry and node types with many metal ions, which serve as © XXXX American Chemical Society
secondary building units (SBUs) in the formation of network structures, are well-established, thereby making robust networks with predetermined topologies reasonably straightforward.6−10 Another family of compounds used for metal coordination is 1,3-diketonates or β-diketonates. When binding to metal ions, they act as bidentate ligands and form stable chelates comprising six-membered rings.19,20 The synergistic effect of resonance-stabilization and chelation drives the equilibrium toward complex formation. Acetylacetonate or 2,4-pentadionate (often abbreviated as acac) is the simplest and one of the most versatile members in this class of ligands. Its complexes, in most cases, are neutral and possess the formula M(acac)2 or M(acac)3, where Mn+ is typically a di- or trivalent metal ion. Numerous acetylacetone derivatives have been synthesized and studied in the context of coordination chemistry. Among those, centrally substituted (i.e., 3-substituted) acetylacetonates are much more common and have very similar binding vectors to the analogous carboxylates.21 For example, centrally Received: July 28, 2018 Revised: September 6, 2018
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DOI: 10.1021/acs.cgd.8b01140 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Scheme 1. : Acetylacetone Ligands Featuring Centrally-Attached Weaker Interaction Sites
substituted bis-acetylacetonates have been successfully employed in the assembly of numerous discrete, oligomeric structures (acyclic dimers, rhomboids, triangles, squares, etc.). The simplest is derived from tetraacetylethane, in which the two acac fragments are directly connected to each other at the C3 positions.22,23 In other cases, the relative orientation of acac moieties, and hence the coordination vectors, have been manipulated by carefully choosing the spacer and the substitution positions.24−34 Acetylacetonate ligands are, however, seldom used in the synthesis of metal−organic frameworks (MOFs) and other polymeric coordination compounds because they are generally stronger than carboxylate ligands35,36 and often produce amorphous or poorly crystalline solids.34 The structural characterization, which is an essential part of any systematic study, would unfortunately be impossible with such noncrystalline materials. Moreover, compared to carboxylic acids, acetylacetones are synthetically demanding and susceptible to hydrolysis via reverse Claisen condensation (retro-Claisen cleavage).37,38 One possible way of ameliorating β-diketonate ligands for the construction of crystalline extended solids is to combine them with relatively weaker metal-binding sites such as Ndonor substituents.39−44 Within this context, 3-cyanopentane2,4-dione and 3-(pyridin-4-yl)pentane-2,4-dione have gained the most attention during the past two decades (Scheme 1, A and B). Pioneering work by Maverick,45,46 Domasevitch,47−49 Nishikiori,50−52 Englert,53−59 and others60,61 shows that both molecules are effective for the construction of coordination polymers, especially heterobimetallic infinite assemblies. Though to a lesser extent, 3-(pyridin-4-ylmethyl)pentane-2,4dione,62 3-(2-(pyridin-4-yl)ethyl)pentane-2,4-dione,49 and 3(3,5-dimethyl-1H-pyrazol-4-yl)pentane-2,4-dione 63,64 have also been used (Scheme 1, C−E). In an attempt to introduce halogen-bonding secondary interactions into the metaldirected assemblies, Englert et al. reacted Fe(III) and Al(III) tris-acac complexes of 3-(pyridin-4-yl)pentane-2,4-dione with tetrafluorodiiodobenzene,65 and with a similar goal in mind, we studied a series of halophenyl- and haloethynylphenylsubstituted acac derivatives with divalent copper (Scheme 1, F and G).66,67 In the study presented herein, we have used elongated versions of 3-(pyridin-4-yl)pentane-2,4-dione, namely, 3-(4(pyridin-4-yl)phenyl)pentane-2,4-dione and 3-(4-(pyridin-4ylethynyl)phenyl)pentane-2,4-dione (i.e., L1 and L2, Scheme 2), for the explicit purpose of making polymeric metallosupramolecular assemblies. In these linear, ditopic, heterobifunctional ligands, the pyridyl and acac moieties are linked through either p-phenylene or p-phenylene-ethynylene spacers. The acac ligation could give rise to several different node types based on the coordination environment around the metal center, which, in turn, depends on the nature of the metal ion.
Scheme 2. : Structural Formulas of 3-(4-(Pyridin-4yl)phenyl)pentane-2,4-dione (L1) and 3-(4-(Pyridin-4ylethynyl)phenyl)pentane-2,4-dione (L2)
Divalent metal ions, for example, will likely form linear, rigid struts via tetra-coordination of two acac moieties, whereas octahedral tris-acac complexes derived from trivalent metal ions can serve as trigonal-planar nodes (Scheme 3a,b). Large, high-valent metal ions (e.g., Zr4+, lanthanoid ions) have the ability to accommodate even more acac units and produce higher connecting nodes. Moreover, as the metal acetylacetonates derived from L1 and L2 hold peripheral pyridyl units, they can undergo further metal binding. Square-planar bis-acac species, for instance, can assume other tetragonal geometries (octahedral and square-based pyramidal) when pyridyl moieties of neighboring bis-acac units bind to the open metal sites, thereby creating square-planar and T-shaped nodes, respectively (Scheme 3c,d).
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RESULTS AND DISCUSSION Our initial attempt at preparing L1 through direct crosscoupling of 4-hydroxy-3-(4-iodophenyl)pent-3-en-2-one (5) and 4-pyridinylboronic acid was not successful because the acac functionality did not withstand the reaction conditions and hydrolyzed into the corresponding phenylacetone. Hence, we had to follow an alternative, multistep pathway where 4(pyridin-4-yl)benzaldehyde was reacted with the biacetyltrimethyl phosphite (1:1) adduct to yield an oxyphosphorane intermediate, which, in turn, gave the desired product upon methanolysis (Scheme 4, see Experimental Section for details),68,69 but were still unable to isolate the pure form of the ligand. On the other hand, L2 was obtained relatively easily in moderate yields according to the synthetic route given in Scheme 5. Even though β-diketones can undergo keto−enol tautomerism and exist as a mixture of diketo and keto−enol forms, solution NMR suggests that 5, L1, and L2 remain in their keto−enolic form. This can be ascribed to the conjugation and intramolecular hydrogen bonding that shift this dynamic equilibrium in favor of the keto−enol tautomer. As evidenced from the single-crystal structural data of L2, the keto−enolic form is persistent in the solid-state as well, with an intramolecular O−H···O hydrogen bond. Indeed, the enolic proton is difficult to locate and is shared nearly equally by both oxygen atoms (Figure 1). The O···O separation of 2.436 Å mirrors the presence of this bridging proton. The B
DOI: 10.1021/acs.cgd.8b01140 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Scheme 3. (a−d) Node Types Anticipated for Metal Acetylacetonates of L1 and L2
Scheme 4. Synthetic Route to L1
Scheme 5. Synthetic Route to L2
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DOI: 10.1021/acs.cgd.8b01140 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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(Z-out distortion) and weakens the two Cu−N bonds significantly (Cu−N distance = 2.5214(13) Å). As shown in Figure 2a, the hexa-coordinated copper ions act as four-connecting square-planar nodes and build a twodimensional coordination polymer having 44 square lattice (sql) topology. Adjacent square grids are arranged in a staggered manner, but every other layer is superimposed, resulting in an ABAB-type stacking (Figure 2b,c). This layered arrangement creates isolated cavities (17.6% “free” volume when calculated with Mercury70 using contact surface, 1.2 Å probe radius, and 0.2 Å approximate grid spacing), which are taken up by methanol molecules (Figure 3a). In fact, each Cu(L1)2 unit has two closely associated methanol molecules linked by O−H(methanol)···O(acac) hydrogen bonds (Figure 3b), so the actual formula for this structure is [Cu(L1)2(MeOH)2]n. The reaction between L2 and copper(II) perchlorate offered a dark blue-green crystalline material. The structural examination revealed that the copper ion is tetracoordinated (chelated by two acac moieties) and sits in a distorted square planar geometry (Cu−O acac distance = 1.9397(16)− 1.9445(17) Å). The fifth and sixth coordination sites are occupied by methanol molecules (Cu−OMeOH distance = 2.445(2) Å). Any given [Cu(L2)2(MeOH)2] entity subsequently participates in four O−H(methanol)···N(pyridine) hydrogen bonds with two neighboring molecules, thereby creating a one-dimensional supramolecular chain (Figure 4a). The reaction of L2 with zinc(II) perchlorate produced colorless/light yellow crystals. With respect to the coordination environment around the metal center, the crystal structure shows features identical to those displayed by [Cu(L2)2(MeOH)2], with two acac moieties occupying the equatorial plane, leaving room for methanol molecules to coordinate in the axial positions (Zn−Oacac distance = 2.0285(13)− 2.0330(14) Å, Zn−OMeOH distance = 2.1977(16) Å). Once again, intermolecular hydrogen bonding between apically bound methanol molecules and pyridyl moieties organize these discrete [Zn(L2)2(MeOH)2] units into a ribbon-like infinite assembly (Figure 4b). Thermogravimetric studies show that both [Cu(L2)2(MeOH)2] and [Zn(L2)2(MeOH)2] start
Figure 1. Part of the crystal structure of L2.
acetylacetone segment is coplanar with the apical pyridyl ring and is perpendicular to the central phenyl ring, and the ligand is slightly bent along the plane defined by acetylacetone and pyridine. There are no obvious structure-directing intermolecular interactions in the overall packing. For metal binding studies, we used divalent Co(II), Cu(II), and Zn(II) and trivalent Fe(III) salts (associated with weakly coordinating anions). In most cases, the reactions led to instant precipitates or microcrystalline material which are insoluble in common organic solvents. To harvest single-crystals suitable for X-ray structural analysis, we therefore employed diluted solutions (
