Assembling Coordination Frameworks of Tetrakis[meso-(3,5

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Article

Assembling coordination frameworks of octacarboxymetalloporphyrins with poly-nuclear metallic nodes: mechanistic insights into the ameliorated synthesis Israel Goldberg, and Bharat Kumar Tripuramallu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00028 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 7, 2016

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Crystal Growth & Design

Assembling Coordination Frameworks of Tetrakis[meso– (3,5-biscarboxylphenyl)]-Metalloporphyrins with PolyNuclear Metallic Nodes: Mechanistic Insights into the Synthesis and Crystallization Process Bharat Kumar Tripuramallu and Israel Goldberg* School of Chemistry, Sackler Faculty of Exact Sciences, Tel-Aviv University, RamatAviv, 69978 Tel-Aviv, Israel. E-mail: [email protected], [email protected].

Abstract The article provides a deep insight into an improved synthetic methodology of crystalline

framework

solids

with

octa-topic

tetrakis-(3,5-dicarboxyphenyl)-

zinc/cobalt-porphyrin linkers and a variety of metallic nodes, using NaOH as a modulator to enhance crystal growth. In the given conditions of the supramolecular reaction poly-nuclear metallic clusters were formed in situ, leading to the construction of metalloporphyrin frameworks (MPFs) of diverse architectures perforated by wide intra-lattice voids. Synthetic procedures with Pr3+ ions yielded two different frameworks, labeled as PrMPF-1 (1) and PrMPF-2 (2). The former is sustained by heterometallic {NaPr (COO)4}n pillared synthons, revealing tetragonal interporphyrin channel voids. In the latter the porphyrin units are inter-coordinated by heterometallic tetranuclear {PrNa3(H2O)6(COO)9}3- clusters and the resulting structure exhibits hexagonal voids. Additional crystalline materials could be obtained with In3+ and Ga3+ cations as the inorganic component, yielding open 3D framework-structures InMPF (3) and GaMPF (4). These were found to be stabilized by the hetero-metallic In2Na(µ3-H2O)(COO)7 cluster-type and {NaGa(COO)4}n chain-type interaction synthons, respectively. When the transition metal ions Fe3+ or Mn3+ were used as the connecting reagents, homo-metallic tri-nuclear oxo-centered clusters (Fe/Mn)3(µ3O)(COO)7 were formed in the corresponding polymeric structures FeMPF (5) and MnMPF (6). In 6 adjacent tri-nuclear clusters are further interconnected by a formate bridge to form a covalently-linked [Mn3(µ3-O)]2 dimers. Similar reactions in the presence of the NaOH additive, but with Co2+ or Zr4+, yielded two isomorphous 3D supramolecular architectures (7; NaMPF-1 and 8; NaMPF-2) with tetragonally-shaped

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inter-porphyrin channel voids. Yet, in these structures the porphyrin-bridging {Na(COO)2}-n-chains were found to contain Na+ ions only. The solvent (DMF, water, and possibly dissociation products of hydrolysed DMF) accessible voids in the metalorganic frameworks 1-8 account for >50% of the crystal volume. Mechanistic aspects of the NaOH-modulated synthetic approach are discussed, characterizing the products at each step in the synthetic course and proposing a plausible reaction mechanism.

Introduction Metal organic frameworks deserve an increasing attention as an emerging class of porous functional materials in recent years.1-3 The most successful design strategy for constructing these crystalline frameworks is to take advantage of the versatile geometry of organic building units (linkers) and metal centers (uni- or multi-nuclear connecting nodes), which greatly influence the resulting architectures of the formed framework.4-6

Tetrapyrrole

macrocycles

or

porphyrin

building

blocks

are

quintessential for forming quadrangular open frameworks, owing to their bulkiness and D4h symmetry. Metalloporphyrin building blocks are structurally similar to natural photosynthetic pigments, hence the frameworks associated with these building blocks may exhibit light harvesting capability reminiscent of the natural photosynthetic systems.7-8 Moreover, metalloporphyrin frameworks often show excellent catalytic activity,

9

guest-molecule adsorption

10

and molecular separation

capacity .11 Reactions of poly-topic organic linkers with metallic centers, give rise to the formation of secondary building units (SBUs) or polyatomic cluster moieties which are formed in situ, initiating the nucleation and stabilizing the construction of the framework architectures.12 These clusters, along with the shape properties of the linkers, not only direct the size and geometry of the intra-lattice pores but also define the physical properties of the resulting framework in terms of magnetic,13 optical 14 and thermal behaviour.15 In most cases these SBUs are inter-connected via the molecular backbones of the linkers, but occasionally they are connected to each other to form secondary building chains (SBCs).16 The formation of clusters or chains featuring homo- or hetero-metallic, mixed- or uni-valent, neutral or ionic entities, solely depends upon the synthetic conditions and ligand geometries. These features affect also the dimensionality and the topological characteristics of the framework 2 ACS Paragon Plus Environment

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architectures.17-18 Several reports have addressed the construction of attractive framework solids involving metalloporphyrin linkers and multinuclear metallic cluster nodes.19-23 The octa-topic tetrakis-(3,5-dicarboxyphenyl)porphyrin scaffold is uniquely appealing to this end, having a large and rigid backbone and eight diverging functions with metal-coordination capacity. Choe et al. reported several pillared porphyrin homologues series featuring the paddlewheel clusters with tetracarboxy porphyrin linkers.24-25 Ma and coworkers reported highly porous metalloporphyrin frameworks through association of Co-trinuclear clusters and octatopic octa-carboxy porphyrin linkers.26 Devic et al. demonstrated recently a series of Fe-based bridging synthons, as isolated Fe3+ octahedra, diiron(II) paddle-wheel dimers, or extended [Fe3+(OH)O4]n chains, in the frameworks construction.27 Zirconium and hafnium were found to form higher nuclearity clusters, like Zr6, Zr8, Hf6 and Hf8, when reacted with tetrapodal porphyrin skeletons.28-30 In a similar context, our group reported earlier SBCs constructed from oxophilic lanthanoid ions with tetrapodal and octapodal metalloporphyrins entities.31-34 In a recent communication we reported an ameliorated synthetic methodology (using NaOH as a modulator) for obtaining crystalline framework materials, LnMPF1, from the fast-reacting octatopic tetrakis-(3,5-dicarboxyphenyl)-zinc-porphyrin linker (Zn-H8OCPP) with a series of lanthanide metal centers (Ln = Gd, Dy, Eu, Td, Dy).34 In view of the feasibility of this synthetic methodology in a wider context, we employed a similar approach in the present work to other metal ions from different groups of the periodic table. The mechanistic insights into the synthetic methodology were established by characterizing the products at different steps of the reaction, in relation to the composition of the final architectures. By using first-row transition dmetals (Mn, Fe), novel inorganic trinuclear metallic clusters were obtained in situ in the corresponding reactions with the octacarboxy porphyrin linker. In similar reactions with the trivalent Ga and In ions (column 13 of the periodic table) the SBUs involved either chain- or cluster-type heterometallic moieties that incorporate Na ions into the connecting synthon. Slight variation of the reaction conditions allowed the formation

of

two

topologically

different

frameworks

with

Pr-containing

heterometallic connectors. Somewhat surprisingly, in similar reactions with Co and Zr metal ions (in the presence of NaOH), the resulting porphyrin frameworks were found to be tessellated by Na ions only. The final framework structures have been unambiguously determined by single crystal X-ray diffraction. The crystal growth 3 ACS Paragon Plus Environment


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process was monitored by IR, TGA and PXRD, and the changes in the morphology/phase-type of the products during this process was monitored by scanning electron microscopy (SEM). It is evident from this study that the ability to obtain sizeable single crystals of the framework solids in question is largely enhanced in a reproducible manner by the NaOH-modulator approach.34 The hydroxyl ions supplied by the NaOH compete with the carboxylate anions for coordinating to the metal, slowing the reaction rate between the metal ions and the porphyrin and thus enhancing crystal growth of the solid polymeric product.

Experimental Section Materials and methods All the chemicals were received as grade reagents and used without further purification. The porphyrin building block tetrakis-(3,5-dicarboxyphenyl)-porphine (H8OCPP) and its metallated derivative, Zn-H8OCPP, were prepared according to literature procedures.35 Infrared spectra of solid samples were obtained on Bruker Tensor 27 system spectrophotometer in ATR mode. Powder X-ray diffraction patterns were

recorded

on

a

Bruker

D8-Advance

diffractometer

using

graphite

monochromated CuKα radiation.

Synthesis PrMPF-1 (1) was prepared as described in a previous report. 34 PrMPF-2 (2). To a solution of Zn-H8OCPP (2.5 mg, 2.5 µmol) in 300.0 µL DMF, a solution of Pr(NO3)3∙6H2O (8.7 mg, 20 µmol) in 100.0 µL of 1M HNO3 was added. Then the reaction mixture was sealed in a screw cap vial and heated at 120°C in a bath reactor. After two hours 300.0 µL of 1N NaOH was added to the reaction mixture and sonicated for few minutes to obtain a purple turbid solution which was then heated at same temperature for 7 days to obtain a block shaped crystals of PrMPF-2 along with white powder. The crystals were separated by filtration and washed with DMF, acetone and water several times, air dried for further characterizations For (Na/Pr)MPF-b, Yield : ~15 %. FTIR (cm-1): 3375, 2903, 2789, 2469, 1570, 1465, 1371, 1350, 1224, 1062, 1010, 769.


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InMPF (3) was prepared by the same procedure as that for 2 except that metal salt InNO3∙6H2O (6.0 mg, 20.0 µmol) was used instead of Pr(NO3)3∙6H2O. The resulting purple block crystals were collected by filtration in ~23.2% yield. FTIR (cm-1): 3400, 2929, 2354, 1648, 1491, 1439, 1371, 1339, 1256, 1099, 1046. GaMPF (4) was obtained according to the same synthetic procedure described above for 3, but Ga(NO3)3∙xH2O (5.1 mg 20.0 µmol) was used instead of the In-reagent. The reaction mixture was heated for two days before addition of NaOH and the block crystals were obtained by heating for 15 days in nearly 18.5 % yield. FTIR (cm -1): 3390, 2918, 2350, 1645, 1579, 1394, 1339, 1238, 1094, 1059, 771.

FeMPF (5). The preparation of 5 was similar to that of 4, but using the metal salt Fe(NO3)3∙9H2O (8.0 mg 20.0 µmol). Red block crystals were harvested by filtration in ~20.2 % yield. FTIR (cm-1): 3408, 2923, 2344, 1639, 1591, 1495, 1430, 1370, 1345, 1250, 1089. MnMPF (6). Red block crystals of 6 were obtained by following same synthetic procedure as described above, using Mn(NO3)2∙xH2O (4.4 mg, 25.0 µmol) as a source for Mn cations. Yield: ~17.5% FTIR (cm-1): 3411, 2934, 2352, 1621, 1582, 1529, 1437, 1360, 1325, 1237, 1065.

NaMPF-1 (7) was obtained by adding slowly the mixture of Co(NO3)3 (5.8 mg , 20.0 µmol) in 100.0 µL of 1M HNO3, to H8OCPP (2.4 mg, 2.5 µmol) dissolved in 300 µL of DMF. The resulting solution was then heated at 120°C for six hours in a bath reactor. Subsequently 300.0 µL of 1N NaOH was slowly added to it, sonicated for few minutes, and then followed by heating at 120°C for nearly 7-10 days to obtain needle shaped crystals of 7 in nearly ~30 % yield. FTIR (cm-1): 3423, 2978, 2348, 1655, 1494, 1446, 1328, 1256, 1002, 756.

NaMPF-2 (8). Anhydrous ZrCl4 (4.6 mg 20.0 µmol) was taken in 100 µL of 1M HCl and slowly added to Zn-H8OCPP (2.5 mg 20.0 µmol) in 300.0 µL of DMF solution and sonicated for 30 min at room temperature. The resulting reaction mixture was heated at 120°C for three hours to ensure complete precipitation. Then 300.0 µL of



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1N NaOH was added followed by sonication and heating at 120°C for 10 days, to get needle shaped crystals of 8 in ~25 % yield.

The different reactions, that yielded new crystalline products subjected to singlecrystal X-ray diffraction analysis, are summarized in Scheme 1.

-------------------------------------------------------------------------------------------------------

Scheme 1: Synthetic protocol of compounds presented in the study -------------------------------------------------------------------------------------------------------


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Single-Crystal Structure Determinations The X-ray measurements [Bruker-ApexDuo diffractometer, IμS microfocus MoKα radiation] were carried out at ca. 110(2) K on crystals coated with a thin layer of amorphous oil. These structures were solved by direct and Fourier methods and refined by full-matrix least-squares (using standard crystallographic software (SHELXTL-2014, SHELXL-2014)).36-37 They were found to contain severely disordered crystallization solvent (usually DMF and water, and occasionally also dissociation products of the DMF solvent, hydrolyzed in the solvothermal conditions of the supramolecular reactions) within the intra-lattice voids, which couldn't be modeled by discrete atoms. Correspondingly, the contribution of the disordered solvent moieties was subtracted from the diffraction pattern by the SQUEEZE procedure and PLATON software.38 The five-coordinate Zn(H2O)-OCPP entity was found to exhibit twofold disorder, with the Zn(H2O) fragment displaced in a given unit either above or below the porphyrin macrocycle. The solvent accessible voids in all the structures are in the range of 55-62% of the crystal volume. Due to the disordered solvent in such wide voids the analyzed crystals diffracted poorly, revealing additional minor disorder of the porphyrin component. Solvothermal reaction conditions in (often hydrolysed) DMF environment also provide ionic species (formate anions or dimethyl ammonium cations) to account for charge balance, when required. Usually, the formate entities are incorporated into the coordination framework, while the dimethyl ammonium species are entrapped and disordered within the wide interporphyrin lattice voids. The TOPOS software package was used to analyze the topological features of the available framework solids.39 The crystal data of the available structures are (excluding the disordered crystallization solvent and dissociation products of hydrolyzed DMF): (1) C52H30N4Na2O21Pr2Zn∙(xDMF, yH2O) Mr = 1439.97, orthorhombic, space group Imma (No. 74), a = 27.604(7), b = 29.333(6), c=13.110(3) Å, V = 10616(4) Å3, T = 110 K, Z = 4, µ(MoK) = 1.18 mm-1, (calcd) = 0.90 g·cm-3, 17085 reflections measured to  = 25.17, of which 4901 were unique (Rint = 0.074) and 2866 with I > 2(I).

Final R1 = 0.084 (wR2 = 0.230) for the 2866 data above the intensity

threshold, and R1 = 0.114 (wR2 = 0.246) for all unique data. Solvent accessible void



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space in the unit cell is 6194 Å3 (58% of the crystal volume), and the electron count therein is 2690e. CCDC 1445945. (2) C156H105N12Na6O69Pr2Zn3∙(xDMF, yH2O, 6Me2NH2+), Mr = 3867.38, trigonal, space group P321 (No. 150), a = b = 23.4820(8), c = 16.3180(5) Å, V = 7792.3(6) Å3, T = 110 K, Z = 1, µ(MoK) = 0.60 mm-1, (calcd) = 0.82 g·cm-3, 34299 reflections measured to  = 25.05, of which 9156 were unique (Rint = 0.039) and 8494 with I > 2(I). Final R1 = 0.028 (wR2 = 0.068) for the 8494 data above the intensity threshold, and R1 = 0.031 (wR2 = 0.069) for all unique data. The intralattice void space and electron-count accommodating the disordered solvent and dimethyl ammonium ions in the unit cell is 4658 Å3 (~60% of the crystal volume), and the electron count therein is 1839e. CCDC 1445946. (3): C172H114In8N16Na4O75Zn3∙(xDMF, yH2O) Mr = 4811.42, tetragonal, space group P4/mbm (No. 127), a = b = 30.142(2), c=16.470(1) Å, V = 14963(2) Å3, T = 110 K, Z = 2, µ(MoK) = 0.91 mm-1, (calcd) = 1.07 g·cm-3, 48256 reflections measured to  = 25.02, of which 7032 were unique (Rint = 0.084) and 4729 with I > 2(I). Final R1 = 0.050 (wR2 = 0.130) for the 4729 data above the intensity threshold, and R1 = 0.076 (wR2 = 0.138) for all unique data. The total solvent accessible void space in the unit cell is 8431 Å3 (approximately 56% of the crystal volume). After inclusion of part of the solvent (that could be modeled by discrete atoms), one DMF molecule and two water sites, in the finally refined structure, the remaining void space was assessed to be 7563 Å3 (51% of the crystal volume), and the electron count therein is 2449e. CCDC 1445947. (4): C52H30Ga2N4Na2O21Zn∙(xDMF, yH2O), Mr = 1297.59, orthorhombic, space group Imma (No. 74), a = 27.320(4), b = 29.924(5), c=12.665(3) Å, V = 10354(3) Å3, T = 110 K, Z = 4, µ(MoK) = 0.80 mm-1, (calcd) = 0.83 g·cm-3, 11387 reflections measured to  = 25.43, of which 4673 were unique (Rint = 0.12) and 1819 with I > 2(I). Preliminary R1 = 0.13 for the 1819 data above the intensity threshold. The poor quality of the crystals and of the diffraction pattern, as well as very low percentage of significant experimental data, this structure could not be fully refined with acceptable precision, and further calculations were interrupted at this preliminary stage. Solvent accessible void space in the unit cell is 6186 Å3 (nearly 60% of the crystal volume), and the electron count therein is 2501e.


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(5): C160H74Fe12N12O67Zn3∙(xDMF, yH2O) Mr = 4102.62, monoclinic, space group P2/m (No. 10) (pseudo Cmmm), a = 20.041(2), b = 16.259(2), c = 23.996(2) Å,  = 114.529(4), V = 7114(1) Å3, T = 110 K, Z = 1, µ(MoK) = 0.90 mm-1, (calcd) = 0.96 g·cm-3, 92350 reflections measured to  = 25.10, of which 13123 were unique (Rint = 0.091) and 7181 with I > 2(I). Final R1 = 0.059 (wR2 = 0.148) for the 7181 data above the intensity threshold, and R1 = 0.104 (wR2 = 0.158) for all unique data. The solvent accessible void space in the unit cell is 3942 Å3 (approximately 55% of the crystal volume), and the electron count therein is 1286e. CCDC 1445948. (6): C160H70Mn12N12O63Zn3∙(xDMF, yH2O) Mr = 4023.67, orthorhombic, space group Cmmm (No. 65), a = 19.959(3), b = 46.019(6), c = 16.271(2) Å, V = 14944(4) Å3, T = 110 K, Z = 2, µ(MoKa) = 0.78 mm-1, (calcd) = 0.89 g·cm-3, 22050 reflections measured to  = 25.0, of which 7025 were unique (Rint = 0.080) and 4177 with I > 2(I).

Final R1 = 0.055 (wR2 = 0.152) for the 4177 data above the intensity

threshold, and R1 = 0.084 (wR2 = 0.160) for all unique data. The solvent accessible void space in the unit cell is 8679 Å3 (about 58% of the crystal volume), and the electron count therein is 3167e. CCDC 1445949. (7): C52H22CoN4Na4O17∙(xDMF, yH2O, 4Me2NH2+) Mr = 1125.62, orthorhombic, space group Imma (No. 74) [pseudo tetragonal I41/amd], a = 28.691(4), b = 29.061(3), c = 12.492(1) Å, V = 10415(2) Å3, T = 110 K, Z = 4, µ(MoKa) = 0.22 mm-1, (calcd) = 0.72 g·cm-3, 35422 reflections measured to  = 25.08, of which 4822 were unique (Rint = 0.039) and 3856 with I > 2(I). Final R1 = 0.046 (wR2 = 0.129) for the 3856 data above the intensity threshold, and R1 = 0.056 (wR2 = 0.133) for all unique data. The total solvent accessible void space in the unit cell is 6485 Å3 (approximately 62% of the crystal volume), and the electron count therein is 2044e. CCDC 1445950. (8): C52H22N4Na4O17Zn∙(xDMF, yH2O, 4Me2NH2+) Mr = 1132.06, tetragonal, space group I41/a (No. 88) [pseudo I41/amd], a = b = 28.965(2), c=12.411(1) Å, V = 10413(2) Å3, T = 110 K, Z = 4, µ(MoKa) = 0.29 mm-1, (calcd) = 0.72 g·cm-3, 65072 reflections measured to  = 25.06, of which 4598 were unique (Rint = 0.035) and 3707 with I > 2(I). Final R1 = 0.045 (wR2 = 0.133) for the 3707 data above the intensity threshold, and R1 = 0.057 (wR2 = 0.137) for all unique data. The total solvent accessible void space in the unit cell is 6391 Å3 (approximately 61% of the crystal volume), and the electron count therein is 1916e. CCDC 1445951.



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Results and Discussion Rationalization of the NaOH-mediated synthetic methodology In our previous communication we reported a new synthetic methodology to obtain crystalline materials from fast reacting precursors like lanthanoid ions and multitopic porphyrin building blocks.34 NaOH has been used as a modulator to initiate the crystal growth and increase their size. In order to extend the applicability of this synthetic procedure and gain a deeper insight into the reaction methodology, several frameworks have been synthesized in this manner (their structures will be discussed in the 2nd part of this paper), and the synthesis-related observations are discussed in some detail. Initially we reacted Pr(NO3)3 with Zn-H8OCPP, followed by addition of NaOH to obtain PrMPF-1 (1). Framework 1 of [Zn(H2O)-OCPP)Pr2(H2O)4Na2]n composition is isomorphous with the Ln–porphyrin frameworks (Ln = Gd, Sm, Dy, Eu and Tb) we described in our earlier report.34 The reaction methodology related to all these examples can be divided into three steps: (a) formation of an initial precipitate by reaction of the porphyrin entity with the inorganic cations – this usually results in a non-crystalline precipitate that is insoluble in any common solvent, (b) decomposition of this precipitate by NaOH, and finally (c) rearrangement of the component species (the OH anions replace the carboxylates in the coordination shell of the transition metal), to give the crystalline framework material (Scheme 2). -------------------------------------------------------------------------------------------------------

Scheme 2. Stepwise description of the reaction/framework-formation progress with the Zn-H8OCPP linker. M' represent the potential exocyclic metal center.


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A slight deviation from this protocol by adding 1M HNO3 to the starting reaction mixture at the precipitation step, while preserving the other steps of the reaction pathway, yielded a new framework structure PrMPF-2 (2) of different topology. Although the initial purple precipitate in this case formed much faster than that obtained in the absence of the HNO3 (as in the synthesis of PrMPF-1), the IR spectra of both display the same characteristic features. Yet, the more acidic conditions in the synthesis of 2 resulted in an incomplete de-protonation of the metalloporphyrin moiety to Zn(H2OCPP)6- and a different connectivity pattern in 2 than in 1. It is plausible that presence of the acid in the reaction mixture affects the decomposition step through formation of large number of species in equilibrium, as shown below (M' = Pr in this case).

The phase purity of 2 was characterized also by PXRD and IR. (Figures S1 and S2 in Supplementary Information).

Based on the above aforementioned considerations we extended our investigations to other trivalent metallic reactants, following the modified synthetic protocol i.e. applying acidic conditions to increase the reactivity of the metal ions in the precipitation step, which is then characterized by faster kinetics. Moreover, addition of the acid ensures complete transfer of the porphyrin component from the solution to the precipitate. Elements in group 13 of the periodic table, e.g. In and Ga in their trivalent ionic form, provide excellent candidates to this end. The respective reaction with the indium nitrate reagent resulted indeed in quick formation of a purple precipitate. Subsequent addition of NaOH yielded eventually crystalline InMPF (3), revealing heterometallic [In2Na(COO)7] clusters as the interporphyrin bridging units. A strong IR band at 1690 cm-1 observed in the spectrum of the free Zn-H8OCPP material (attributed to C=O stretching) was absent in the IR spectrum of purple precipitate obtained in the precipitation step (Figure 1a), confirming that the porphyrin entity reacted with the In ions.40



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(a)

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(b)

(c)

(d)

Figure 1. (a) IR spectra of the different phases along the synthesis of 3. (b) TGA of initial precipitate and the final compound. (c) SEM micrographs of different phases during the crystal growth of 3. (d) PXRD of crystalline 3 along with simulated pattern from the crystal structure and initial precipitate. ------------------------------------------------------------------------------------------------------Thermogravimetric behavior of the initial precipitate and the final product is illustrated in Figure 1b. The initial precipitate starts decomposing above 200 °C, while the crystalline framework decomposes only at about 400 °C. This observation provides a clear indication that the initial precipitate does not contain the ordered framework structure. The bulk purity of 3 was confirmed by PXRD studies (Figure 1d). Powder diffraction spectrum of the initial purple precipitate lacks the characteristic pattern of crystalline solid. The crystal growth process was also studied by scanning electron microscopy (SEM) in order to assess the nature of the products at each reaction step (Figure 1c). Samples were taken directly from the reaction mixture at different intervals and drop casted on Si-wafer followed by evaporation of the solvent.

After three hours, the sample shows a large amount of shapeless 12 ACS Paragon Plus Environment

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precipitate which is consistent with the flat PXRD patterns. The second fraction was taken directly from the reaction mixture after 3 days. At this stage the reaction mixture is supposed to contain large amounts of indium hydroxide precipitate and a sodium salt of the Zn-OCPP linker. The SEM image at this point evidences a major portion of the solid hydroxide as well as needle-shape precipitate and a minor amount of block-shaped grains. Elemental analysis indicates that the material of needle morphology contains the sodium-porphyrin salt, whereas the composition of the block-type solid involves the porphyrin, sodium and indium moieties (Figure S3). After 6 days, mostly needle- and in larger quantity block-shaped (which represents the desired framework solid) species have been observed. The needle material at both stages was formed directly during the evaporation of the solvent, as most of the sodium-porphyrin ion-pair stays in solution during the reaction course. These observations support the proposed reaction pathway along which three major products are formed: M'x(OH)y (after addition of NaOH) and then Nax(ZnHtOCPP)y or M'xNay(ZnHtOCPP)z as the end product. If the (partial) exchange takes place between M'x(OH)y and Nax(ZnHtOCPP)y in the third stage of the proposed mechanism (Scheme 2), the heterometallic moiety M'xNay(ZnHtOCPP)z is formed. Otherwise, in the case of higher stability and reduced solubility of the metal hydroxides, formation of the Nax(ZnHtOCPP)y species would be preferred (Scheme 3). The outcome of similar reactions of the Zn-H8OCPP porphyrin linker with other metal ions will be rationalized accordingly. -------------------------------------------------------------------------------------------------------

Scheme 3. The possible reaction pathways at the rearrangement step. 13 ACS Paragon Plus Environment


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With the gallium (instead of indium) reactant a prolonged heating was required to obtain the initial precipitate in the precipitation step. The final product GaMPF (4) was obtained by heating the reaction mixture for about 15 days, indicating a weak affinity of the gallium ions towards porphyrin coordination. The resulting crystalline framework exhibiting {NaGa(H2O)2(COO-)4}n heterometallic synthons is structurally similar to that observed for the Pr analog (1) and the closely related multiporphyrin frameworks with lanthanoid ions described earlier.34 The successful formulation of crystalline framework solids with the octa-carboxy porphyrin ligand stabilized by the heterometallic inorganic clusters

(in PrMPF-2 and InMPF) or

heterometallic chains (in PrMPF-1 and GaMPF) involving Na+ at the inter-porphyrin coordination sites led us to extend our investigations towards other transition metals. Subsequent attempts involved reactions of the zinc-porphyrin linker with iron and manganese nitrates in the presence of HNO3, yielding at the end of the process frameworks FeMPF (5) and MnMPF (6). The hydroxides of the two metal ions formed in the decomposition step by addition of NaOH to the initial precipitates, Fe(OH)3 and Mn(OH)3, are largely soluble in the presence of excess amount of hydroxyl ions in the reaction mixture (as opposed to the rather insoluble hydroxides of Pr, In and Ga in the previous examples). Correspondingly, they may appear in the solution as different species, as it is exemplified (for the iron moiety) by the following equation: 41

As evidenced from the composition of obtained 3D frameworks 5 and 6, ion exchange occurred between the intermediate oxo/hydroxo iron species and the sodium salt of the metalloporphyrin linker. In these two solids the porphyrin entities are bridged

by

homo-metallic

oxo-centered

tri-nuclear

clusters

[M3(µ3-

O)(HCOO)(COO)7] (M= Fe3+, Mn3+), with a formate anion (from the hydrolyzed DMF) as part of the connecting synthon. Our intention to obtain heterometallic clusters was not successful in these examples. The presence of Na+ ions in the reaction mixture is not competent enough to push the reaction equilibrium towards the 14 ACS Paragon Plus Environment

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formation of heterometallic clusters. It is reasonable to assume that the smaller ionic radii of Fe3+ and Mn3+ (78.5 pm) than that of e.g. In3+ (94 pm) favors the formation of commonly observed homo-metallic Fe3O and Mn3O triangle oxo-clusters. The experimental and simulated PXRD of 5 confirm the phase purity of the final product (Figure S4). The present structure types are entirely different from framework ZJU-19 with Ni-OCPP linkers and Mn2+ connecting nodes reported recently by Wu et al.9 Coordination frameworks of the octa-carboxy porphyrin linkers with Fe3+ metal centers have not been reported before. Surprising results have been obtained when the porphyrin scaffolds H8OCPP or Zn-H8OCPP were reacted in similar experimental conditions with Co2+ and Zr4+ inorganic reactants and NaOH (Schemes 1 and 2). In both cases the end products [labeled as NaMPF-1 (7) and NaMPF-2 (8), respectively], represent supramolecular isomorphous frameworks sustained by Na+ ions only, without incorporation of the transition metals. The porphyrin units are held together in these structures by Na+carboxylate chains. In 7 the free-base porphyrin was used as the starting reagent, and was metallated by the Co2+ ion in an endocyclic, but not exocyclic, fashion during the reaction. Evidently, no ion exchange occurred between the Co(OH)2 hydroxide or its ionic components and the sodium-porphyrin salt, apart from incorporation of the metal ion in the macrocyclic core. Interestingly, in a related supramolecular reaction product between H8OCPP and Co ions reported by Ma et al. the porphyrin units were found to be bridged in the solid by Co3 trimeric cluster,26 as it has been also observed in a porous framework solid involving the tetra(carboxyphenyl)porphyrin scaffold and cobalt ions.42 Another attempt with ZrCl4 and Zn-H8OCPP as starting reactants yielded compound 8 (NaMPF-2), isomorphous to 7, namely with only Na-ions as the exocyclic connectors between the porphyrins. The high charge density of Zr4+ ions may prevent decomposition of the formed Zr(OH)4 base and lower the odds for exchange of the hydroxide anions by carboxylate anions in its coordination sphere. Further efforts to synthesize crystalline framework materials with other possible exocyclic connectors as Cr3+ and Al3+ by a similar synthetic methodology were not successful. After addition of NaOH a gelatinous precipitate is formed, which when subjected to prolonged heating transforms into e.g. Al2O3 and Cr2O3 colloidal material.43 Further synthetic details of reaction times and obtained phases are presented in the Table S1 and Figure S5 in Supporting Information.



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Description of crystal structures Structures of compounds 1 (PrMPF-1) and 4 (GaMPF) are isomorphous with the LnZnOCPP (Ln = Sm, Eu, Gd, Tb and Dy) open frameworks described in an earlier communication.34 They all adopt a (4,8)-connected topology and are perforated by 1D tubular zigzag channels of about 1.5 nm in diameter (Figure 2). These frameworks were found to possess permanent porosity. -------------------------------------------------------------------------------------------------------

Figure 2. Illustration of the LnMPF-1 single framework structure. The heterometallic [NaLn(H2O)2(COO)4]n chains aligned vertically are held together by the porphyrin linkers aligned horizontally.34

------------------------------------------------------------------------------------------------------As 1 and 4 represent formally new materials their crystal data (excluding unknown amount of crystallization solvent, a mixture of DMF and water, accommodated in the solvent-accessible voids) are presented in the Experimental section above. Compound 2 (PrMPF-2) crystallizes in trigonal space group P321 and represents a different framework architecture than in 1. In 2 the metalloporphyrin linker remains partly protonated, Zn-H2OCPP, as indicated by the geometry of the corresponding carboxylic acid functions (C=O 1.20 Å, C-OH 1.31 Å). Based on crystallographic analysis

the

framework

is

formulated

as

Pr2Na6(H2O)12[Zn(H2O)H2OCPP)]3∙6(CH3)2NH2. The lattice was found to contain also dimethylammonium cations to account for charge balance [the dimethyl formamide solvent readily hydrolyses under solvothermal conditions into formate anions (formic acid) and dimethylammonium cations (dimethylamine)]. The heterometallic cluster {PrNa3(H2O)6(COO)9}3- represents the SBU connector, crosslinking between the porphyrin units, that sustains the open 3D architecture (Figure 3a). The three Na ions in the cluster have a tetrahedral coordination environment with 16 ACS Paragon Plus Environment

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two oxygen atoms from water molecules and two oxygen atoms from carboxylate groups. These NaO4 tetrahedra are connected to the Pr3+ ions located at the centre of this triangle through carboxylate bridges. The central Pr is surrounded by nine Oatoms (PrO9) from six carboxylate bridges; three of them adopt the µ2-ηPrηNa coordination mode and the rest bind in a µ1-ηprηpr mode.

-------------------------------------------------------------------------------------------------------

(a)

(b)

(c) Figure 3. (a) Tetranuclear heterometallic building block {PrNa3(H2O)6(COO)9}3-. (b) The trigonally assembled framework and the intra-lattice hexagonal void, viewed down the c-axis of the crystal. (c) 3D framework architecture of 2 and its topological representation projected along c- axis (color code for topological representation: yellow 4-connected metalloporhyrin, green 6-connected metal cluster).



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The binding synthon in this structure acts as a six-connecting node. The Zn-H2OCPP metalloporphyrin building block acts as a four connecting node. The asymmetric nature of the partly protonated porphyrin moiety reduces the connectivity of the octacarboxy metalloporphyrin from the ideally 8-c node to a 4-c connecting node. Consistently, in the observed structure the basal plane of metalloporphyrin is slightly ruffled (Figure S6).44 The inter-linked six PrNa3 clusters and six metalloporphyrin linkers surround 1.5 nm wide intra-lattice hexagonal voids propagating parallel to the c-axis (Figure 3b). Three metalloporphyrins line the inner edge of the void channel, the other three line the outer edge, and the connecting clusters are located in between. The topology of the framework can be best explained by considering the metalloporphyrin as 4-c node and the PrNa3 cluster as 6-c node. The entire structure is thus a 4,6-connected binodal net with stp topology of point symbol {44.62}3{49.66}2 (Figure 3c). Red block tetragonal crystals of InMPF (3) were harvested by heating the reaction mixture of In(NO3)3 and Zn-H8OCPP in DMF at 120 °C, and adding NaOH in the following step. The molecular structure consists of two independent Zn(H2O)-OCPP porphyrins connected by an In2Na(H2O)(µ3-H2O)(HCOO)(COO)6, or shortly [In2Na(COO)7], cluster. Based on the crystallographic analysis, the 3D framework 3 is formulated as {In2Na(H2O)(µ3-H2O)(µ2-HCOO)}8{Zn(H2O)-OCPP}6. As shown in Figure 4a a unique heteronuclear In2Na cluster was formed in situ as part of the connecting node. The inter-coordinating synthon is composed of six carboxylate groups of six porphyrin molecules, one formate anion from hydrolyzed DMF and an Na(H2O) moiety. The two In and Na atoms in the cluster are connected through µ3H2O bridging molecule, constituting an equilateral triangle. The In ions are connected by the formate bridge in a µ2-η1η1 coordination mode, and then connected through the carboxylate bridge to the Na ion in a µ2-η1η1 fashion. The two indium ions are further bridged by two coordinating carboxylate groups in a µ1-η1η1 mode. The observed heterometallic In2Na(COO)7 synthons differs from other indium-carboxylate clusters [In3O(COO)6]+ and [In(COO)4]- reported earlier.17,45 The resulting supramolecular framework is perforated by three different sets of 1D channels running parallel to the c-axis of the crystal. As shown in Figure 4b, water molecules attached to Na protrude into the central square-shaped channel. The hydrophilic channel is surrounded by eight adjacent channels of two different types. Two co-facially aligned 18 ACS Paragon Plus Environment

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metalloporphyrin entities (at about 11 Å from each other) and two SBUs line the walls of the wider square channels. These are interspaced by narrower channel voids (Figure 4b). The SBU nodes represent typical six-connecting nodes with Cs symmetry, while the metalloporphyrin moiety can be considered as an eightconnecting node. Structure 3 can thus be best described as a (6, 8, 8) connected framework with msq network topology of vertex symbol (413·62)4(420·68)2(424·64)4 (Figure 4c). -------------------------------------------------------------------------------------------------------

(a)

(b)

(c)

Figure 4. (a) The connectivity scheme within the trinuclear indium cluster In2Na(COO)7. (b) Fraction of the framework, viewed down the c-axis of the crystal, showing the three different channel types (the water species axially bound to the twofold disordered Zn-centers are not shown). (c) Topological representation of framework 3 projected along c-axis (color code purple 6-connected cluster, yellow 8connected metalloporphyrin).



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The 3D framework of compound 5 (FeMPF) is characterized by the structural formula [{Fe3(H2O)(µ3-O)(µ2-HCOO)}8{Zn(H2O)-OCPP}6]n. It is composed of the metalloporphyrin linkers and the Fe2[Fe(H2O)](µ3-O) nodes formed in situ during the supramolecular reaction. In this structure the porphyrin-bridging synthons Fe2[Fe(H2O)](µ3-O)(HCOO)(COO)6 involve six carboxylate groups and one formate anion (Figure 5a).

-------------------------------------------------------------------------------------------------------

(a)

(b)

(c) Figure 5. (a) SBU representing the oxo-centered trinuclear homometallic Fe3O(COO)7 cluster in 5. (b) Projection of the framework structure down the b-axis, showing the channel voids. (c) Representation of 3D porous framework viewed along b-axis and its topological representation (color code: purple 6-connected cluster, yellow 8-connected metalloporphyrin).


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Unlike other cationic and mixed-valent Fe3O(COO)6 trigonal prismatic clusters in the literature,46

the aggregate observed in the present example is neutral and

univalent. In the trimetallic triangle all the Fe-ions are in the 3+ oxidation state and are interconnected by the µ3-O oxo-bridging unit. They have an octahedral or nearly octahedral coordination environment supplemented by the carboxylate and formate moieties. In the observed structure, the inorganic bridges can be considered as sixconnecting nodes that inter-coordinate the Zn(H2O)-OCPP linkers into a 3D open architecture. The latter is perforated by different types of 1D channels, propagating along the crystallographic axes (Figure 5b). Every metalloporphyrin entity in the framework is considered as a 8-connected node and the iron-containing cluster as a 6connected node, resulting in a (6,8,8) tri-nodal net with a new network topology (413·62)4(420·68)2(422·64)4 (Figure 4c). Red block crystals of MnMPF (6) were obtained directly from the reaction mixture, using NaOH as a modulator. The obtained product represent a 3D framework solid with stiochiometric formula {Mn3(µ3-O)(µ2-HCOO)}8{Zn(H2O)-OCPP}6. An oxocentered tri-nuclear Mn3 cluster was formed in situ, serving as the metallic node in the construction of the framework. All the Mn atoms in the cluster are in 3+ oxidation state [the original Mn(NO3)2 reagent was oxidized during the reaction] and form a neutral univalent homo-metallic oxo-centered trinuclear cluster Mn3(µ3-O)(µ2HCOO)(COO)6 or Mn3(µ3-O)(COO)7. Its composition is similar to that of the Fe3 cluster in the previous structure: the three manganese ions at the vertices of the triangle are interconnected by the µ3-oxo group and bridged by the carboxylate groups from Zn(H2O)-OCPP linker and formate anion (in a µ2-η1η1 coordination modes, Figure 6a). Trinuclear clusters composed by Mn3+ centers are rare in the literature; the majority of the known ones are not oxo-centered, but rather assembled in a linear fashion.9 In 6, two adjacent Mn3(µ3-O)(COO)7 clusters are further bridged by the formate anion in an anti µ2-η1η1 coordination mode, forming a covalently bonded [Mn3(µ3-O)]2[syn µ2-HCOO][anti µ2-HCOO](COO)12} aggregate (Figure 6b). Three different types of solvent-accessible channels perforate the crystal lattice parallel to the a-axis (Figure 6c). A central channel is surrounded by eight channels of two different types in similarity with previous observations in structures 3 and 5. Its walls constitute of four Mn3 clusters at vertices and four metalloporphyrin units bridging them, and its dimensions are approximately 9.5 x 9.5 Å2 (atom-to-atom distances). 21 ACS Paragon Plus Environment


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(a)

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(b)

(c)

(d)

Figure 6. (a) The trinuclear Mn3(µ3-O)(COO)7 SBU. (b) Two Mn3 clusters covalently connected by formate bridge into [Mn3]2 dimeric entity. (c) Fraction of the coordination framework viewed down the a-axis, displaying the three different channels. (d) Simplified topological representation along c-axis by considering the [Mn3]2 dimers as a single node (color code: purple 6-connected & 8-connected metalloporphyrins, yellow 10-connected cluster). ------------------------------------------------------------------------------------------------------In topological description of the observed framework the Mn3(µ3-O)(COO)7 cluster can be considered as a 7-connecting node (to six carboxylate groups and one formate moiety), and the metalloporphyrin linker as an 8-connecting node. In such case the coordination framework can be classified as a (7,8,8)-connected trinodal net with new network topology {32.413.54.62}4{32.420.56}2{420.68} (Figure S7). The topology can be simplified further, by considering the {[Mn3(µ3-O)]2[syn µ2-HCOO][anti µ2HCOO](COO)12} aggregate as a single 10-c node, to a (6,8,10) trinodal net with


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network topology {415}2{424.64}{435.610}2, involving then one 8-c and one 6-c metalloporphyrin connectors (Figure 6d). Numerous attempts to incorporate other divalent (Co2+), trivalent (Al3+, Cr3+) and tetravalent (Zr4+) metal centers into the frameworks by following the NaOHmodulated synthetic approach, yielded single-crystalline products only in reactions with Co- and Zr-reagents with free-base and Zn-metallated octacarboxy porphyrin, respectively. Crystallographic analysis of these products revealed that the resulting frameworks incorporate only Na-ions (but not the other metals, except for insertion of the Co2+ ion into the free-base porphyrin during the process) as exocyclic nodes. The two phases were labeled as NaMPF-1 (7) and NaMPF-2 (8), respectively. The two compounds are isostructural, with the [Na4(M(H2O)-OCPP)] (M = Co2+, Zn2+) repeating unit in the framework

The octapodal metalloporphyrin units are fully

deprotonated, being inter-linked by the sodium counter ions in the µ2-η2η2 coordination mode. The Na+ ions have a pseudo tetrahedral coordination environment of four carboxylate groups from four different metalloporphyrin scaffolds, and act as 4-connecting nodes. The Na-carboxylate interaction synthons form coordination chains/pillars that propagate along the c-axis of the crystal (Figure 7a). The metalloporphyrin entities (representing 8-connecting nodes) are aligned parallel to the ab plane of the crystal and inter-link between four neighboring pillars with their eight carboxylate residues. The resulting 3D framework exhibits 1D tubular channels along the c-axis. The distance between two central Co2+/Zn2+ ions in the co-facially arranged porphyrins is ~12.3 Å and the width the zig-zag tubular channels within the framework structure is ~13.4 Å (atom-to-atom distances). Applying the simplified "node-and-linker" topology,39 the observed structure can be described as a (4,8) connected framework with Schläfli symbol {4.53.62}4{44.54.616.84} (Figure 7b,c).



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(a)

(b)

(c)

Figure 7. (a) 1D chain constructed by coordination of the Na+ bridging ions with the surrounding carboxylate functions. (b) 3D framework of 7/8 viewed down the c-axis of the crystal, and (c) its node-and-linker-type topological representation (color code: green 4-connected cluster, red 8-connected porphyrin,).

Structure-related insights related to the improved synthetic methology Compounds 7 and 8 represent a relatively simple two component assembly in which only the metalloporphyrin and sodium entities construct the framework. The observed chain-topology of the connecting pillars in 7/8 shows similarity to the heterometallic binding nodes (zig-zag chains) of [NaPr(H2O)2(COO)4]n composition in structures 1 (as well as in 4, and all the previously described frameworks of this porphyrin with lanthanoid nodes)34. The similar topologies of the different synthons that inter-connect between the octa-dentate metalloporphyrin linkers in these structures is depicted in Figure 8a, all thus representing (4,8) nets. Not surprisingly, the unit-cells of structures 1, 4, 7 and 8 are nearly isometric. When the metalcoordination capacity of the metalloporphyrin is reduced from 8 to 4 by incomplete de-protonation as in 2, the geometry of the binding nodes (SBUs) changes from 1D chains (in 1) to tetra-nuclear clusters {PrNa3(H2O)6(COO)9}3- in 2. Correspondingly,


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the 4,8 connected framework with zig-zag channel voids in 1 is replaced by a 4,6 connected architecture with hexagonal voids in 2. -------------------------------------------------------------------------------------------------------

(a)

(b)

Figure 8. (a) The similar topology of the metal-carboxylate chain synthons in structures 7/8 (top) and in structure 1/4 (bottom). (b) Structure of the tetranuclear heterometallic binding clusters in 2. ------------------------------------------------------------------------------------------------------

Figure 9. Different types of Mn3 clusters observed in the literature. (a) Linear Mn3 cluster. (b) Trigonal-prismatic Mn3 cluster. (c) Distorted trigonal Mn3 cluster in 6. (d) Displacement of µ3-O2- from the mean plane of the Mn3+ ions in 6.



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In the Mn-derivative 6, an oxo-centered homometallic univalent trinuclear cluster was formed. Wu et al. reported structure ZJU-18 which consists of a linear Mn3 cluster frequently observed in coordination networks with carboxylate linkers (Figure 9a).9 Oxo-centered Mn3 clusters are also known. They are usually characterized by a D3h symmetry, with the µ3-O2- entities and the bound carboxylates lying in the plane of the Mn3 triangle (Figure 9b).46 In this context the {Mn3O} cluster in 6 is unique. The high-spin Mn3+-centers in the cluster have two different coordination environments and the µ3-O2- oxo-bridge is displaced from the mean plane of Mn3 by nearly 0.7 Å (Figures 9c,d). Moreover, two [Mn3O] units are covalently linked by a formate bridge to form [Mn3O]2 dimer. These features and the resulting magnetic anisotropy of the metallic clusters impart to this compound potentially interesting magnetic behavior,46-47 and provide an attractive implication of the present synthetic procedure. -------------------------------------------------------------------------------------------------------

(a)

(b)

Figure 10. Illustration of (a) D3h clusters In3 and Fe3,17,46,48 and (b) clusters In2Na and Fe3 of Cs symmetry in compounds 3 and 5. -------------------------------------------------------------------------------------------------------

In the iron compound 5 (FeMPF) a trinuclear univalent homo-metallic oxocentered cluster of [Fe3O] of Cs symmetry was formed (Figure 10b). The previously reported oxo-centered Fe3O clusters are in most cases of higher D3h symmetry (Figure 26 ACS Paragon Plus Environment

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10a), being composed of mixed valent {(Fe3+)2Fe2+O} or cationic {Fe3O(COO)6}+ species.46,48 -------------------------------------------------------------------------------------------------------

Table 1. List of the cluster-types formed in octacarboxy porphyrin-based frameworks.

Framework

Cluster

Composition

Ref.

NaMPF-1/2

1D chains

{Na(COO)2}-n

PrMPF-1

1D chains

{NaPr(COO)4}n

PrMPF-2

Tetranuclear

{PrNa3(H2O)6(COO)9}3-

MnMPF

Hexanuclear

{[Mn3(µ3-O)]2 [syn µ2-HCOO][anti

This

µ2-HCOO] (COO)12}

work

FeMPF

Trinuclear

Fe3(H2O)(µ3-O)(COO)6(HCOO)

InMPF

Trinuclear

In2Na(H2O)(µ3-H2O)(COO)6(HCOO)

GaMPF

1D chains

{NaxGay(COO)z}n

MMPF-2

Trinuclear

Co3(H2O)(µ3-OH)(COO)6

26

ZJU-18, ZJU19, ZJU-20 CZJ-4

Paddlewheel & Trinuclear Paddlewheel,

Mn2(COO)4Cl2 & Mn3(COO)4(μ-H2O)2(H2O)6 Zn2(COO)4(H2O)

9 19

ZJU-21

Paddlewheel

Cu2(COO)4(H2O)2

51

MMPF-4

Paddlewheel

Zn2(COO)3

20

------------------------------------------------------------------------------------------------------A previously unknown tri-nuclear hetero-metallic cluster In2Na was formed in structure 3 (InMPF). Zhang et al.45 and Ma et al.50 reported indium-based metalloporphyrin frameworks with octapodal and tetrapodal metalloporphyrin building blocks and monomeric [In(COO)4]− cluster SBUs. Homo-metallic In3 clusters of D3h symmetry are also well known to occur in coordination frameworks (Figure 10a).17,46 Formation of the hetero-metallic In2Na cluster nodes in 3 in this work is unique. It should be pointed out that, based on the present as well as the previous investigations, trivalent metal cations were found to be the most useful connectors (either in monomeric or trimeric form) in the effective construction of open singleframework architectures with the octa-dentate Zn-H8OCPP scaffold. The present 27 ACS Paragon Plus Environment


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synthetic strategy, employing NaOH in order to enhance crystal growth,34 yielded in structures 1-4 networks sustained by the heterometallic M'xNay (M = Pr, Ga, In) nodes. Frameworks 7 and 8 with sodium ions as inter-porphyrin connecting nodes provide another remarkable variant of these investigations. The nature (i.e., composition, geometry and nuclearity) of the various metallic nodes observed in the coordination frameworks with the octacarboxy porphyrin building blocks are shown in Table 1. Compounds 1-8 represent single-framework solids with crystalline architectures perforated by wide channel voids accessible to, and occupied by, the crystallization solvent. The arrangement of these channels in the different structures is shown in Figure S8).

Concluding remarks This article highlights the mechanistic insights into the ameliorated synthetic process of crystalline open-framework materials by reacting tetrakis-(3,5-dicarboxyphenyl)zinc/cobalt-porphyrin with various metal ions. The applied procedure employed NaOH as a modulator to enhance crystal growth and allow crystal structure analysis with home-laboratory diffraction equipment.34 A detailed investigation of the synthetic steps was carried out in the test case of compound 3. IR, TGA, PXRD and SEM analyses of the reaction intermediates were employed in order to clarify the mechanistic aspects of the overall process. These studies reveal that three major products M'x(OH)y, Nax(ZnHtOCPP)y and M'xNay(ZnHtOCPP)z exist in equilibrium after addition of NaOH. Two different frameworks with {NaPr(COO)4}n chains and hetero-metallic tetra-nuclear {PrNa3(H2O)6(COO)9}3- are formed in 1 and 2, respectively, by controlling the H+/OH- ratio in the reaction mixture. Structure 4 with Ga3+ ions is isomorphous with 1, showing 1D hetero-metallic chains of similar stoichiometry, {NaGa(COO)4}n, as interporphyrin connectors. Framework 3 is stabilized by the hetero-metallic tri-nuclear {In2Na(H2O)(µ3-H2O)(COO)6(HCOO)} interaction synthons. Similar reactions with Mn3+ and Fe3+ ions yielded frameworks 5 and 6 tessellated by homo-metallic oxo-centered tri-nuclear clusters of the cationic species. Application of other metal cations, Co2+, Zr4+, Al3+ and Cr3+, as potential inter-porphyrin connectors was not successful with the current synthetic methodology. Instead, crystalline coordination frameworks with sodium nodes (7 and 8) were obtained. After addition of NaOH in the decomposition step of the proposed reaction mechanism (Scheme 1), the reaction mixture contains M'x(OH)y hydroxides 28 ACS Paragon Plus Environment

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precipitates and a sodium salt of the porphyrin linker. The exchange in the rearrangement step of the OH anions in the coordination environment of the metal by carboxylate functions of the porphyrin depends on (among other factors) the relative strength of the M'-OH bonds. It may not be always feasible, leaving in such case the Na ions as the only available inter-porphyrin connectors. The use of excess NaOH as a modulator in the supramolecular syntheses referred to in this work has a major effect on the in-situ formation of the different homo-metallic and hetero-metallic nodes, and the resulting described single-framework solids with the octacarboxy porphyrin linkers in the form of sizeable single crystals. Prior to this study formulation and characterization of H8OCPP-based metal-organic-frameworks have enjoyed a rather slow progress in view of the synthetic difficulty in obtaining sizeable crystalline samples, as polycrystalline or amorphous solids are most frequently the outcome of common preparative procedures. On rare occasions, when small crystallites became available, their structural characterization by X-ray diffraction technique required access to synchrotron radiation sources due to the weak diffraction power of such crystals and high content of the disordered solvent trapped in the channel-perforated lattice. The NaOH-modulated preparative approach of the metalorganic frameworks in question put-forward in this work addresses this problem. Therefore, we envision that it will find use in the construction of open crystalline frameworks with a variety of other metal-organic frameworks based on multi-dentate building blocks.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.XXXXXXX. IR and PXRD spectra, further details of the synthetic process, topological representations of the crystal structure, X-ray crystallographic files for 1-3 and 5-8 (CIF).

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. * E-mail: [email protected] 29 ACS Paragon Plus Environment


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Notes\The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was supported by the Israel Science Foundation (grant no. 108/12)

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For the Table of Contents

Synopsis The applied procedure employs NaOH as a modulator providing a new pathway to the formulation of sizeable crystals of octa(carboxy)porphyrin-based open framework materials. This article highlights the different stages of the synthesis and characterizes the unique structural features of the MOF products.


Assembling Coordination Frameworks of Tetrakis[meso-(3,5

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