Self-Assembly of 1D Double-Chain and 3D Diamondoid Networks Of

molecules

Article Self-Assembly of 1D Double-Chain and 3D Diamondoid Networks of Lanthanide Coordination Polymers through In Situ-Generated Ligands: High-Pressure CO2 Adsorption and Photoluminescence Properties

Chatphorn Theppitak 1,2, Suwadee Jiajaroen 1,2, Nucharee Chongboriboon 1, Songwuit Chanthee 1, Filip Kielar 3 , Winya Dungkaew 4, Mongkol Sukwattanasinitt 5 and Kittipong Chainok 1,*

1 Thammasat University Research Unit in Multifunctional Crystalline Materials and Applications (TU-McMa), Faculty of Science and Technology, Thammasat University, Pathum Thani 12121, Thailand; [email protected] (C.T.); [email protected] (S.J.); [email protected] (N.C.); [email protected] (S.C.) 2 Department of Chemistry, Faculty of Science and Technology, Thammasat University, Pathum Thani 12121, Thailand 3 Department of Chemistry, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand; ﬁ[email protected] 4 Department of Chemistry, Faculty of Science, Mahasarakham University, Maha Sarakham 44150, Thailand; [email protected] Citation: Theppitak, C.; Jiajaroen, S.; 5 Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand; Chongboriboon, N.; Chanthee, S.; [email protected] Kielar, F.; Dungkaew, W.; * Correspondence: [email protected]; Tel.: +66-86-339-5079 Sukwattanasinitt, M.; Chainok, K. Self-Assembly of 1D Double-Chain Abstract: Two new lanthanide-based coordination polymers, [Sm2(bzz)(ben)6(H2O)3]·0.5H2O(1) and 3D Diamondoid Networks of and [Eu(bbz)(ben)3](2), were synthesized and characterized. The described products were formed Lanthanide Coordination Polymers from in situ-generated benzoate (ben) and N’-benzoylbenzohydrazide (bbz) ligands, which were the through In Situ-Generated Ligands: products of transformation of originally added benzhydrazide (bzz) under hydrothermal conditions. High-Pressure CO2 Adsorption and Photoluminescence Properties. Compound 1 exhibits a one-dimensional (1D) double-chain structure built up from the connection 3+ Molecules 2021, 26, 4428. https:// of the central Sm ions with a mixture of bzz and ben ligands. On the other hand, 2 features a 3D 6 doi.org/10.3390/molecules26154428 network with a 4-connected (6 ) dia topology constructed from dinuclear [Eu2(ben)6] secondary building units and bbz linkers. High-pressure CO2 sorption studies of activated 1 show that maxi- Academic Editor: Charles L. mum uptake increases to exceptionally high values of 376.7 cm3 g−1 (42.5 wt%) under a pressure of B. Macdonald 50 bar at 298 K with good recyclability. Meanwhile, 2 shows a typical red emission in the solid state at room temperature with the decay lifetime of 1.2 ms. Received: 18 June 2021 Accepted: 19 July 2021 Keywords: coordination polymers; CO2 adsorption; hydrazide; in situ synthesis; lanthanide; Published: 22 July 2021 photoluminescence

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. Coordination polymers are a class of crystalline organic-inorganic materials built by connecting metal ions or clusters and organic bridges through coordination bonds [1]. These hybrid solid materials possess the intriguing architectures of variable dimensionality that allow potential applications in the fields of adsorption, luminescence, catalysis, and Copyright: © 2021 by the authors. magnetism [2–6]. Lanthanide-based coordination polymers (LnCPs) have drawn much Licensee MDPI, Basel, Switzerland. attention during the past few decades due to their unique optical properties, such as high This article is an open access article distributed under the terms and luminescence efficiency, long luminescence lifetimes, and narrow bandwidths, allowing conditions of the Creative Commons for potential applications in sensing, lighting, and integrated optics [7]. In comparison Attribution (CC BY) license (https:// with transition metal-based coordination polymers, the rational design and synthesis of 3+ creativecommons.org/licenses/by/ LnCPs is more difficult since Ln ions frequently show higher coordination numbers 4.0/). (typically from 6 to 12) and flexible coordination geometries. Moreover, various synthetic

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parameters, such as the ratio of reactants, solvent, reaction temperature, and pH can have an influence on the nucleation and crystal growth of the final products [8–11]. Therefore, experimental investigation of the impact of synthetic parameters on the crystallization process and self-assembly of LnCPs is still required. It is well known that Ln3+ ions have a high affinity for and prefer to bind to hard donor atoms. In this regard, oxygen- and/or nitrogen-containing organic ligands, in particular benzene- and pyridine carboxylates, have been widely used in the development of novel lanthanide-based luminescent materials. These types of ligands can adopt various coordination modes with respect to the central Ln3+ ions. They can also function as light-harvesting antennas for efficient sensitization of Ln3+ ions to enhance their emission intensity [12–14]. On the other hand, hydro (solvo) thermal reactions accompanied by in situ ligand synthesis have been developed as one of the most feasible strategies for the construction of novel coordination polymers [15]. For instance, a variety of intriguing structural topologies of tetrazole-based polymeric complexes have been successfully prepared through in situ [2 + 3] cycloaddition reaction of nitriles and an azide in the presence of metal salts under hydrothermal conditions [16,17]. Moreover, this synthetic approach can also be extended to a wide number of carboxylate-based coordination polymers derived from the hydrolysis of ester or cyano groups in water [18,19]. We are interested in the synthesis of novel crystalline coordination polymers with the aim of understanding their structure-property relationships [20–22]. In the present work, we intend to explore the possibility of synthesizing novel LnCPs using benzhydrazide (bzz) as a ligand. Herein, we report two new LnCPs, [Sm2(bzz)(ben)6(H2O)3]·0.5H2O(1) and 3D [Eu(bbz)(ben)3](2), obtained through in situ synthesis of N’-benzoylbenzohydrazide (bbz) and benzoate (ben) ligands from the reactions of bzz and lanthanide nitrate salts under hydrothermal conditions. The in situ ligand formation was clearly evidenced by single crystal X-ray diffraction analysis. Moreover, high-pressure CO2 sorption of the activated sample of 1 was investigated in the pressure range up to 50 bar at 298 K. Notably, the room temperature photoluminescence (PL) and lifetime decay behavior of 2 were also examined in the solid state.

2. Results and Discussion 2.1. Synthesis and Infrared Spectra As depicted in Scheme1, compounds 1 and 2 were obtained under the same conditions by the hydrothermal reactions of Ln(NO3)3·6H2O and bzz in a ratio of 1:2 and carried out at 130 ◦C for 96 h. The new ligands bbz and ben, generated in situ, were formed by the transformation of the bzz precursor. These new ligands can act as bridging and/or chelating ligands for Ln3+ ions through oxygen and nitrogen sites to promote the formation of the different polymeric structures viz. 1D chain and 3D network for 1 and 2, respectively. It should be noted that when the hydrothermal reactions were performed at temperatures above 140 ◦C, the starting material bzz was completely converted into the ben ligand through the cleavage of C-N bonds [23]. The FT-IR spectra (Figure S1 in the Supplementary Material) of 1 and 2 display characteristic absorption bands arising from the asymmetric and symmetric stretching vibrations of the carboxylate groups in the wavenumber ranges around 1600 and 1400 cm−1, sup- porting the presence of the ben ligand in their crystal structures. The absorption bands in the region 3126–3067 cm−1 can be ascribed to N-H stretching vibrations of the hydrazine or hydrazide groups of the ligands. For 1, the strong and broad absorption bands at 3600–3400 cm−1 are assigned as the characteristic absorption bands of O-H stretching of water molecules involved in hydrogen bonding [24]. All features of the FT-IR spectra of both compounds are in accordance with the results of single crystal X-ray diffraction analysis. Molecules 2021, 26, 4428 3 of 12 Molecules 2021, 26, x FOR PEER REVIEW 3 of 12

SchemeScheme 1. 1.Schematic Schematic representation representation of of the the in-situ in-situ formation formation of of ligands ligands ben ben and and bbz. bbz.

2.2.2.2. Structural Structural Descriptions Descriptions CompoundCompound1 1crystallizes crystallizes in in the the triclinic triclinic system system with with the the space space group groupP -1.P-1. As As shown shown inin Figure Figure1a, 1a, the the asymmetric asymmetric unitunit of 1 1containscontains two two crystallographically crystallographically independent independent Sm3+ 3+ Smions,ions, one onebzz ligand, bzz ligand, six ben six ligands, ben ligands, three three coordinated coordinated water water molecules, molecules, and one and disor- one 3+ disordereddered lattice lattice water water molecule molecule with with occupancy occupancy factors factors of 0.50. of 0.50. Both Both Sm3+Sm ions ionsin 1 adopt in 1 adoptsimilar similar eight-coordinate eight-coordinate geometries. geometries. Sm1 is Sm1coordinated is coordinated by five byoxygen ﬁve oxygenatoms from atoms dif- fromferent different ben ligands, ben ligands, one oxygen one oxygen atom and atom one and nitrogen one nitrogen atom from atom a frombzz ligand, a bzz ligand,and one andoxygen one oxygenatom from atom a coordinated from a coordinated water molecule water. molecule.Sm2 is coordinated Sm2 is coordinated by six oxygen by atoms six oxygenfrom different atoms from ben differentligands and ben two ligands oxygen and at twooms oxygen from coordinated atoms from water coordinated molecules. water The 3+ molecules.coordination The polyhedra coordination around polyhedra the Ln around3+ metal the centers Ln metal can be centers best candescribed be best as described distorted astrigonal distorted dodecahedral trigonal dodecahedral geometries geometries.. The Sm-O/N The Sm-O/Nbond lengths bond lengthsin 1 are in in1 arethe in range the range2.3088(16)–2.6290(19) 2.3088(16)–2.6290(19) Å (Table Å (Table S2 in S2 the in theSupplementary Supplementary Material), Material), which which is isnormal normal for for these types of compounds [25]. these types of compounds [25]. 2 The bzz ligand in these compounds exhibits only a bidentate-chelating η mode2 to bind The3+ bzz ligand in these compounds exhibits only a bidentate-chelating η mode to one Ln ion via3+ hydrazide oxygen and nitrogen sites, while the ben ligands adopt both bind one Ln 2 ion via hydrazide oxygen and nitrogen sites, while the ben ligands adopt monodentate η (O4–O12) and bidentate-bridging µ2 (O1, O2) modes. As can be seen from both monodentate η2(O4–O12) and bidentate-bridging µ2 (O1, O2) modes. As can be seen Figure1b, two neighboring Ln 3+ ions are bridged by carboxylate oxygen atoms of the ben from Figure 1b, two neighboring Ln3+ ions are bridged by carboxylate oxygen atoms of the ligands in a µ2 fashion to form a 1D single chain running parallel to the a axis. The Ln···Ln ben ligands in a µ2 fashion to form a 1D single chain running parallel to the a axis. The separations through the µ2-ben ligands in 1 are 5.0195(4) and 5.1423(4) Å. These chains are Ln···Ln separations through the µ2-ben ligands in 1 are 5.0195(4) and 5.1423(4) Å. These then cross-linked together in the c axis direction via the ben carboxylate oxygen (O1, O2) chains are then cross-linked together in the c axis direction via the ben carboxylate oxygen atoms, giving rise to the formation of a 1D double chain. The Sm···Sm separation between (O1, O2) atoms, giving rise to the formation of a 1D double chain. The Sm···Sm separation the single chains through the µ2-ben ligands is 5.9773(4) Å. Further stabilization of these 1D between the single chains through the µ2-ben ligands is 5.9773(4) Å. Further stabilization double chains is achieved by extensive O-H···O and O-H···N hydrogen bonds between the of these 1D double chains is achieved by extensive O-H···O and O-H···N hydrogen bonds coordinated water and amine NH2 donors and the carboxylate acceptors along with C-H···π arenebetween edge-to-face the coordinated interactions. water Furthermore, and amine theNH double2 donors chains and the are interconnectedcarboxylate acceptors with eachalong other with by C O-H-H······πO arene and O-H edge-to-face···N hydrogen interactions. bonds involving Furthermore, the lattice the waterdouble molecules, chains are terminalinterconnected carboxylate with oxygen each other atoms, by and O-H···O amine and NH O groups,-H···N alonghydrogen with additionalbonds involving N-H··· πthe interactionslattice water occurring molecules, between terminal bzz carboxylate molecules. oxygen As a result, atoms, a and 2D amine sheet isNH formed groups, which along propagateswith additional along N the-H···b axis.π interactions The overall occurring 3D supramolecular between bzz architecture molecules. results As a result, from thea 2D stackingsheet is offormed the 2D which sheets propagates via van der along Waals the interactions b axis. The along overall the 3Dc axis. supramolecular The geometrical archi- detailstecture of results hydrogen from bonds the stacking for 1 are of listed the 2D in sheets TableS3 via (in van the der Supplementary Waals interactions Material). along the c axis. The geometrical details of hydrogen bonds for 1 are listed in Table S3 (in the Sup- plementary Material).

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(a) (b)

(c)

Figure 1. ((aa)) Coordination Coordination environment of the Sm3+ ionion in in 11.. The The thermal thermal ellipsoids ellipsoids are are drawn drawn at at the the 30% 30% probability probability level level for non-hydrogen atoms. Hydrogen Hydrogen atoms atoms attached attached on on the the carbon carbon and and nitrogen nitrogen atoms atoms are are omitted for clarity. Symmetry codes: (i) (i) −−x, x1,− 1y−, 1y−,z 1; −(ii)z ;1 (ii)−x, 1 −yx, ,1 1−−z. y(b,) 1 One-dimensional−z.(b) One-dimensional double-chain double-chain structure structureof 1 viewed of along1 viewed the b along axis. Hydrogen the b axis. atoms attached on the carbon and nitrogen atoms are omitted for clarity. (c) Packing supramolecular structure of 1 viewed Hydrogen atoms attached on the carbon and nitrogen atoms are omitted for clarity. (c) Packing supramolecular structure approximately along the b axis; yellow colored space marks the contact surface of the void spaces. of 1 viewed approximately along the b axis; yellow colored space marks the contact surface of the void spaces.

InIn addition, addition, it it is is obvious obvious from from the the crystal crystal structure structure of 1 of described1 described above above that that the lat- the ticelattice and and the thecoordinated coordinated water water molecules molecules occupy occupy the intralayer the intralayer space spaceand the and free the space free betweenspace between the layers. the layers.According According to the void to theanalysis void analysisperformed performed with the PLATON with the PLATONprogram [26],program using [26 a ],probe using molecule a probe radius molecule of 1.4 radius Å and of 1.4a grid Å and interval a grid of interval0.2 Å, the of simulated 0.2 Å, the 3 solvent-accessiblesimulated solvent-accessible void space void of 1 spaceis estimated of 1 is estimated to be 5.9% to (146.0 be 5.9% Å (146.0) of the Å crystal3) of the volume crystal aftervolume removing after removing the water the molecules. water molecules. The potential The potentialvoid space void of 1 space calculated of 1 calculated in mercury in [27]mercury using [ 27a probe] using radius a probe of radius1.2 Å is of illustrated 1.2 Å is illustrated in Figure in1c. Figure 1c. ForFor 22,, the the hydrothermal hydrothermal in in situ situ synthesis synthesis of of the the new new bbz bbz ligand ligand involves involves the the formation formation ofof the the C-N bond. This ligand is flexible ﬂexible and able to connectconnect twotwo EuEu3+3+ centerscenters through the oxygenoxygen sites, sites, allowing allowing the the generation generation of of a a hi high-dimensionalgh-dimensional coordination coordination framework. This compoundcompound crystallizes crystallizes in in the the orthorhombic orthorhombic PnnaPnna spacespace group and the asymmetric unit consistsconsists of of one Eu 3+ ion,ion, three three ben ben ligands, ligands, and and a a half half of of two two bbz bbz ligands, ligands, which which lie lie on on an inversioninversion center. center. As As illustrated illustrated in Figure 22,, thethe EuEu3+ ionion in in 22 isis eight-coordinated eight-coordinated with with six six oxygenoxygen atoms fromfrom ﬁvefive different different ben ben ligands ligands and and two two oxygen oxygen atoms atoms from from two two bbz bbz ligands, ligands,adopting adopting a slightly a slightly distorted distorted square square antiprismatic antiprismatic geometry. geometry. The Eu-O The Eu bond-O lengthsbond lengths range rangefrom 2.277(2)from 2.277(2) to 2.559(2) to 2.559(2) Å (Table Å (Table S4 in the S4 Supplementaryin the Supplementary Material), Material), and are and comparable are com- parablewith other with reported other reported (EuO8) compounds(EuO8) compounds [28]. [28].

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3+ FigureFigureFigure 2.2. 2. Coordination Coordination environment environment of ofof the thethe Eu EuEu3+3+ ion ionion in in in 2 2.2 .The. The The thermal thermal thermal ellipsoids ellipsoids ellipsoids are are are drawn drawn drawn at at atthe the the 30%30%30% probabilityprobability probability levellevel level for for for non-hydrogen non-hydrogen non-hydrogen atoms. atoms. atoms. Hydrogen Hydr Hydrogenogen atoms atoms atoms attached attached attached on on on the the the carbon carbon carbon and and and nitrogen nitro- nitro- atomsgengen atoms atoms are omitted are are omitted omitted for clarity. for for clarity. clarity. Symmetry Symmetry Symmetry code: code: code: (i) 3/2 (i) (i)− 3/2 x3/2, 1−−x−x, y,1 ,1−z−y.y, ,z z. .

UnlikeUnlikeUnlike 11 ,1, the,the the ben ben ben ligand ligand ligand in in in2 2 adopts2 adopts adopts two two two coordination coordination coordination modes modes modesviz viz .viz a. bidentate-chelating .a a bidentate-chelat- bidentate-chelat- 2 2 ηingingmode η η2 mode mode (O1, (O1, O2)(O1, O2) andO2) and aand bidentate-bridging a a bidentate-bridging bidentate-bridgingµ2 mode µ µ2 2mode mode (O3–O6). (O3–O6). (O3–O6). As seen As As seen fromseen from from Figure Figure Figure3a, two 3a, 3a, 3+ 3+ symmetry-relatedtwotwo symmetry-related symmetry-related Eu Euions Eu3+ ions areions bridged are are bridged bridged by four by by µfour four2-bridging µ µ2-bridging2-bridging carboxylate carboxylate carboxylate groups groups groups from from fourfrom differentfourfour different different ben ligandsben ben ligands ligands to create to to create create a dinuclear a a dinuclear dinuclear [Eu2 [Eu(ben) [Eu2(ben)2(ben)6] secondary6]6 ]secondary secondary building building building unit unit unit (SBU) (SBU) (SBU) in whichinin which which the the midpointthe midpoint midpoint of the of ofSBU the the SBU liesSBU on lies lies a twofoldon on a a twofold twofold rotation rotation rotation axis. The axis. axis. Eu The ···TheEu Eu···Eu Eu···Eu separation separation separation across theacrossacross dinuclear the the dinuclear dinuclear SBU is 4.2842(4)SBU SBU is is 4.2842(4) 4.2842(4) Å. As depicted Å. Å. As As depicted depicted in Figure in 3inb, Figure Figure these 3b, SBUs3b, these these are SBUs linkedSBUs are are through linked linked bbzthroughthrough ligands bbz bbz (not ligands ligands shown) (not (not in shown) shown) a µ2-bridging in in a a µ µ2-bridging2 coordination-bridging coordination coordination mode, leading mode, mode, to leading theleading formation to to the the offor- for- a 3Dmationmation network. ofof aa The3D3D network. Eunetwork.···Eu separations TheThe Eu···EuEu···Eu via sepasepa therations bbzrations ligands viavia the arethe bbz 8.1379(4)bbz ligandsligands and areare 8.2993(4) 8.1379(4)8.1379(4) Å. The andand network8.2993(4)8.2993(4) is Å.Å. further TheThe networknetwork sustained isis by furtherfurther N-H··· sustainedsustainedO and C-H byby··· N-H···OON-H···O hydrogen andand bonds C-H···OC-H···O that hydrogenhydrogen occur between bondsbonds thethatthat carboxylate occur occur between between oxygen the the atomscarboxylate carboxylate and the oxygen oxygen hydrazine atoms atoms NH and and groups the the hydrazine hydrazine or the phenyl NH NH groups protongroups ofor or thethe the ligands.phenylphenyl proton proton In addition, of of the the thereligands. ligands. are In alsoIn addition, addition, weak C-H there there··· πare areinteractions also also weak weak C-H··· involvingC-H···ππ interactions interactions the phenyl involv- involv- rings ofinging the the the bzz phenyl phenyl and benrings rings ligands of of the the (Tablebzz bzz and and S5 ben ben in theligands ligands Supplementary (Table (Table S5 S5 in in Material).the the Supplementary Supplementary From a topological Material). Material). point of view, the dimer [Eu (ben) ] SBU and the2 bbz ligand6 can be regarded as 4- and FromFrom a a topological topological point point of of 2view, view, 6the the dimer dimer [Eu [Eu2(ben)(ben)6] ]SBU SBU and and the the bbz bbz ligand ligand can can be be 2-connectedregardedregarded asas nodes, 4-4- andand respectively, 2-connected2-connected as nodes,nodes, Figure respec3respecc shows.tively,tively, The asas network FigureFigure 3c structure3c shows.shows. of TheThe2 could networknetwork be 6 simpliﬁedstructurestructure of asof 2 2 a could could 4-connected be be simplified simplified (6 ) dia as asnet a a 4-connected 4-connected as depicted (6 in(66) Figure6 )dia dia net net3d. as as depicted depicted in in Figure Figure 3d. 3d.

(a(a) ) (b(b) )

Figure 3. Cont.

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Figure 3. Views of (a) the binuclear [Eu2(ben)6] SBU, (b) the packing arrangements of SBUs along a axis, (c) the simplified Figure 3. Views of (a) the binuclear [Eu2(ben)6] SBU, (b) the packing arrangements of SBUs along a axis, (c) the simplified 4-connected node of the binuclear SBUs and 2-connected node of the ligand, and (d) the a 4-connected (66) dia topology in 4-connected node of the binuclear SBUs and 2-connected node of the ligand, and (d) the a 4-connected (66) dia topology in 2. 2. 2.3. PXRD and TGA Analysis 2.3. PXRD and TGA Analysis The phase purities of 1 and 2 were confirmed by PXRD analysis. As shown in Figure4a, The phase purities of 1 and 2 were confirmed by PXRD analysis. As shown in Figure the experimental PXRD patterns of the bulk samples are in accordance with the simulated 4a, the experimental PXRD patterns of the bulk samples are in accordance with the simu- patterns derived from crystallographic data, indicating phase purity of the as-synthesized lated patterns derived from crystallographic data, indicating phase purity of the as-syn- samples. Thermal stabilities of 1 and 2 were also examined by TGA carried out under N thesized samples. Thermal stabilities of 1 and 2 were also examined by TGA carried out 2 atmosphere from room temperature to 650 ◦C. The results of this experiment are shown under N2 atmosphere from room temperature to 650 °C. The results of this experiment are in Figure4b. For 1, the first step of weight loss of 5.3% from 50 to 115 ◦C corresponds to shown in Figure 4b. For 1, the first step of weight loss of 5.3% from 50 to 115 °C corre- the removal of coordinated and lattice water molecules (calcd 5.1). Thereafter, a weight sponds to the removal of coordinated and lattice water molecules (calcd 5.1). Thereafter, loss of 11.6%, due to the decomposition of the bzz ligand, occurs in the second step a weight loss of 11.6%, due to the decomposition of the bzz ligand, occurs in the second from 150 to 350 ◦C (calcd 11.5%). The resulting structure is stable up to 425 ◦C, and stepthen from begins 150 toto collapse,350 °C (calcd corresponding 11.5%). The to resulting the decomposition structure is of stable the organic up to 425 components. °C, and thenMeanwhile, begins to thecollapse, TGA curvecorresponding of 2 displays to the two decomposition main steps of ofweight the organic loss. components. The first step Meanwhile,between 255 the and TGA365 curve◦C, with of 2 displays a weight two loss main of 30.9%, steps corresponds of weight loss. to theThe loss first of step the be- bbz tweenmolecule 255 (calcdand 365 31.8%). °C, with The a second weight weight loss of loss 30.9%, of 38.7% corresponds from 470 to to the 560 loss◦C isof attributed the bbz moleculeto the decomposition (calcd 31.8%). ofThe the second ben ligands weight (calcd loss of 48.1%) 38.7% withfrom the470 collapseto 560 °C of is the attributed skeleton tointo the decomposition unidentified products. of the ben Itligands is interesting (calcd 48 to.1%) note with that the the collapse heated of samplethe skeleton of 2 intoafter unidentifiedthe first weight products. loss stepIt is ininteresting the TGA to experiment note that the retained heated its sample structural of 2 after integrity. the first The weightresulting loss material step in the shows TGA significant experiment changes retained in its the structural PXRD patterns integrity. in The comparison resulting toma- the terialas-synthesized shows significant sample, changes as can be in seen the PXRD in Figure patterns4c. This in could comparison be attributed to the toas-synthesized the structural sample,changes as arising can be fromseen thein Figure removal 4c. of This the could bbz ligands be attributed upon heating, to the structural which leads changes to the arisingformation from of the a newremoval phase. of the It is bbz clearly ligands visible upon that heating, four different which leads phases to canthe beformation recognized of a innew the phase. variable It is temperature clearly visible PXRD that four patterns. different As can phases be seen, can be the recognized initial phase in (identifiedthe variable as temperaturethe as-synthesized PXRD patterns.2) can be As transformed can be seen, into the a secondinitial phase phase (identified when the sample as the as-synthe- is heated to sized300 ◦2C.) can Clearly, be transformed the PXRDpattern into a second of this secondphase when phase the shows sample well-defined is heated crystallinityto 300 °C. Clearly,and the the main PXRD peak pattern positions of this of the second experimental phase shows data well-defined match quite wellcrystallinity with those and of the the mainreported peak 1Dpositions polymeric of the structure experimental of europium-benzoate data match quite compound,well with those i.e., of [Eu the3(ben) reported9][28 ]. ◦ 1DThis polymeric phase II structure was stable of europi up toum-benzoate 400 C, and thencompound, underwent i.e., [Eu phase3(ben) transformation9] [28]. This phase into IIan was amorphous stable up to state 400 (phase°C, and III)then at underwent 500 ◦C. Notably, phase transformation the appearance into of newan amorphous peaks was stateobserved (phase upon III) at further 500 °C. heatingNotably, from the appearance 600 to 800 ◦ ofC, new suggesting peaks was a new observed phase upon (phase fur- IV) therbeing heating produced. from 600 This to possibly 800 °C, suggesting results from a new the pyrolysis phase (phase of the IV) 1D being chain produced. frameworks This in possiblyair. New results sharp from diffraction the pyrolysis peaks appearof the 1D at 2chainθ = 7.23, frameworks 19.88, 28.32, in air. 32.79, New and sharp 47.01 diffrac-◦ in the tionpatterns, peaks whichappear is at consistent 2θ = 7.23,with 19.88, the 28.32, pattern 32.79, of Euand2O 473 .01°(PDF: in 00-034-0392).the patterns, which Thus, itis can con- be sistentconcluded with fromthe pattern the above of Eu phenomena2O3 (PDF: that00-034-0392). the 3D diamond Thus, it network can be concluded of 2 can be from converted the aboveinto thephenomena 1D chain structurethat the 3D of [Eudiamond3(ben) 9network] through of crystalline-to-crystalline 2 can be converted into transformation the 1D chain upon heating the sample to about 300 ◦C.

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Molecules 2021, 26, 4428 7 of 12 structure of [Eu3(ben)9] through crystalline-to-crystalline transformation upon heating the sample to about 300 °C.

(a) (b)

(c)

Figure 4. (a) ComparisonComparison of the experimental PXRD patterns of 1 and 2 with the simulatedsimulated diffraction patterns from the single crystal X-ray diffraction determination.determination. (b)) TGATGA curvescurves ofof 11 andand 22.(. (c) Variable temperaturetemperature PXRDPXRD patternspatterns ofof 22..

2 2.4. CO CO2 AdsorptionAdsorption Isotherms Isotherms of of 11 As mentioned above,above, thethe crystallographic crystallographic analysis analysis revealed revealed that that compound compound1 has 1 has a 1D a 1Dpolymeric polymeric double double chain chain structure structure with with small sm voidall void space space to accommodate to accommodate water water molecules. molecules.On the On other the hand,other 2hand,exhibits 2 exhibits a dia topology a dia topology with dense with networkdense network structure. structure. Considering Consid- its eringporous its structure, porous structure, the CO2 adsorption the CO2 adsorption isotherms isotherms of 1, up to of 50 1, bar, up wereto 50 investigatedbar, were investi- using gateda high-pressure using a high-pressure volumetric gasvolumetric adsorption gas analyzeradsorption at 298analyzer K to explore at 298 K the to relationshipexplore the relationshipbetween structure between and structure adsorption and adsorption capacity. Prior capacity. to the Prior adsorption to the adsorption measurement, measure- the ment,crystalline the crystalline sample of sample1 (~50 of mg) 1 (~50 was mg) degassed was degassed at 110 at◦C 110 (1 °C◦C/min) (1 °C/min) under under vacuum vacuum(2.25 mTorr(2.25 mTorr)) for 24 for h. As24 h. shown As shown in Figure in Figure5a, the 5a, activated the activated sample sample of 1 displayed of 1 displayed a type a typeIII isotherm III isotherm according according to the to IUPAC the IUPAC classiﬁcation classification [29], which [29], indicateswhich indicates a weak a adsorbent– weak adsorbent–adsorbateadsorbate interaction. interaction. In the low-pressure In the low-pres regionsure (1–10 region bar), (1–10 the activatedbar), the activated sample adsorbed sample adsorbedvery little very CO2 ,little which CO is2, not which surprising is not surprising as its structure as its contains structure a smallcontains void a space.small void The space.amount The of adsorbedamount of CO adsorbed2 in 1 increased CO2 in 1 gradually increased with gradually increasing with gasincreasing pressure, gas which pressure, may whichbe related may to be crystal related structure to crys changestal structure including changes pore including expansion pore and expansion the speciﬁc and interactions the spe- cificbetween interactions the CO 2betweensorbate andthe CO the2 sorbentsorbate 1and. Apparently, the sorbent the 1. maximumApparently, CO the2 adsorption maximum 3 −1 COvalue2 adsorption of 1 reached value an of exceptionally 1 reached an high exceptionally volumetric high uptake volumetric capacity uptake of 376.7 capacity cm g of 376.7(42.5 wt%)cm3 g− at1 (42.5 49.8 wt%) bar without at 49.8 saturation.bar without Notably, saturation. 1 shows Notably, reversible 1 shows CO reversible2 desorption CO2 accompanied with a small hysteresis. This observation probably originates from the

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desorption accompanied with a small hysteresis. This observation probably originates fromadsorbent-adsorbate the adsorbent-adsorbate interaction, interaction, and this also andindicates this also thatindicates the adsorption that the adsorption does notreach does notequilibrium reach equilibrium within the within measurement the measurement pressure limit pressure of 50 limit bars. of 50 bars.

(a) (b)

Figure 5. (a) Adsorption–desorption isotherms of 1 for CO2 at 298 K. (b) PXRD patterns of 1 before and after CO2 adsorp- Figure 5. (a) Adsorption–desorption isotherms of 1 for CO2 at 298 K. (b) PXRD patterns of 1 before and after CO2 adsorption. tion.

Furthermore, the CO2 adsorption-desorption repeatability of 1 was tested as well. The resultFurthermore, demonstrates the CO that2 adsorption1 can be repeatedly-desorption used repeatability for at least of two 1 was consecutive tested as cycles well. withoutThe result losing demonstrates signiﬁcant that adsorption 1 can be repeatedly capacity. In used addition for at toleast this, two the consecutive PXRD results cycles in withoutFigure5b losing show thatsignificant the crystallinity adsorption of capacity.1 can be maintainedIn addition after to this, two the cycles PXRD of adsorption. results in FigureHowever, 5b show the main that diffraction the crystallinity peak at of 2 θ1 can≈ 5.93 be ◦maintainedwas signiﬁcantly after two shifted cycles to aof higher adsorption. angle However,2θ ≈ 6.69◦ theand main diffraction diffraction peaks peak at 2 atθ ≈ 2θ11.03 ≈5.93°◦, 11.88was significantly◦, and 17.73◦ shifteddisappeared, to a higher indicating angle 2structuralθ ≈6.69° and change diffraction on complete peaks removalat 2θ ≈11.03°, of solvent 11.88°, molecules. and 17.73° Regrettably, disappeared, we indicating failed to structuraldetermine change the crystal on complete structure removal of the desolvated of solvent1 molecules.due to poor Regrettabl diffraction.y, we failed to de- termine the crystal structure of the desolvated 1 due to poor diffraction. 2.5. Photoluminescence Properies of 2 2.5. PhotoluminescenceThe solid-state photoluminescence Properies of 2 properties and lifetime decay behavior of 2 were investigatedThe solid-state at room photoluminescence temperature and the proper resultsties are displayedand lifetime in Figuredecay 6behavior. Under excitation of 2 were investigatedat 297 nm (Figure at room S3 in temperature the Supplementary and the Material),results are2 displayedexhibits the in characteristic Figure 6. Under transitions excita- 3+ 5 7 5 7 5 7 tionof Eu at 297ion nmviz (Figure. D0 → S3 Fin1 the(592 Supplementary nm), D0 → F Material),2 (612, 620 2 nm),exhibitsD0 the→ characteristicF3 (652 nm), tran- and 5D → 7F (702 nm). Among these, the strongest emission peak centered at 620 nm is sitions0 of 4Eu3+ ion viz. 5D0 → 7F1 (592 nm), 5D0 → 7F2 (612, 620 nm), 5D0 → 7F3 (652 nm), and assigned to the electric dipole transition of 5D → 7F , which is hypersensitive to the 5D0 → 7F4 (702 nm). Among these, the strongest 0emission2 peak centered at 620 nm is as- environment in the vicinity of the Eu3+ ions. The strong intensity of this transition is signed to the electric dipole transition of 5D0 → 7F2, which is hypersensitive to the environ- responsible for the bright red emission with CIE coordinates of (0.650, 0.337), while the ment in the vicinity of the Eu3+ ions. The strong intensity of this transition is responsible peak at 590 nm, being second in terms of emission intensity, corresponds to the magnetic for the 5bright 7red emission with CIE coordinates of (0.650, 0.337), while the peak3+ at 590 dipole D0 → F1 transition, which is insensitive to site symmetry around the Eu ion. It nm, being second in terms of emission intensity, corresponds to the magnetic dipole 5D0 can be clearly seen, in the emission spectrum of 2, that the intensity of the electric dipole → 7F1 transition, which is insensitive to site symmetry around the Eu3+ ion. It can be clearly transition is stronger than that of the magnetic dipole transition (I(5D → 7F )/I(5D → 7F ) seen, in the emission spectrum of 2, that the intensity of the electric0 dipole2 transition0 is1 = 4), which implies that the Eu3+ ion resides in a low-symmetry site without an inversion stronger than that of the magnetic dipole transition (I(5D0 → 7F2)/I(5D0 → 7F1) = 4), which center [30]. Moreover, only one peak is found for the 5D → 7F transition, indicating implies that the Eu3+ ion resides in a low-symmetry site without0 an1 inversion center [30]. that there is only one Eu3+ ion in the asymmetric unit [31], which agrees with the crystal 5 → 7 Moreover, only one peak is found for the D0 F5 1 transition,7 indicating3+ that there is only structure analysis above. Notably, the lifetime for D0 → F2 of Eu ion in 2 was calculated one Eu3+ ion in the asymmetric unit [31], which agrees with the crystal structure analysis to be 1.2 ms. It should be noted that there are no emission peaks observed in 1 upon above. Notably, the lifetime for 5D0 → 7F2 of Eu3+ ion in 2 was calculated to be 1.2 ms. It excitation at various wavelengths from 254 to 420 nm with UV light. should be noted that there are no emission peaks observed in 1 upon excitation at various wavelengths from 254 to 420 nm with UV light.

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(a) (b)

(c)

FigureFigure 6. (a 6.) Emission (a) Emission spectra spectra and colorimetricand colorimetric coordinates coordinates (insert), (insert), (b) the (b 1931) the CIE 1931 chromaticity CIE chromaticity diagram, diagram, and (c) and emission (c) emis- ex intensitysion intensity decay for decay2 (λex for= 2972 (λ nm). = 297 nm).

3. Experimental3. Experimental Section Section 3.1.3.1. Materials Materials and and Methods Methods

AllAll starting starting materials, materials, i.e., i.e., Sm(NO Sm(NO3)3)·36H·6H22O,O, Eu(NOEu(NO3)3·6H·6H22O,O, and and benzhydrazide benzhydrazide (bzz) (bzz)were were of ofreagent-grade reagent-grade quality, quality, and and were were obtained obtained from from commercial commercial sources sources without without fur- furtherther purification.purification. Elemental (C, H, N) analysis was determined with with a a LECO LECO CHNS CHNS 932 932elemental elemental analyzer. analyzer. FT-IR FT-IR spectra spectra were were reco recordedrded on on a a PerkinElmer model model Spectrum Spectrum 100 −1 100spectrometer spectrometer using using ATR ATR mode, mode, in in the the range range of of 650–4000 650–4000 cm cm−1. P-XRD. P-XRD measurements measurements were werecarried carried out out on on a aBruker Bruker D8 D8 ADVANCE ADVANCE X-ray X-ray powder powder diffractometer diffractometer using using Cu-K Cu-Kαα (λ = (λ =1.5418 1.5418 Å). Å). TGA TGA thermograms thermograms were were acquired acquired in N in2 Natmosphere2 atmosphere on a on TGA a TGA 55 TA 55 instru- TA ◦ instrument.ment. The The measurement measurement was was performed performed from from am ambientbient temperaturetemperature upup to to 650 650 C°C with with a ◦ −1 a heatingheating rate rate of of 10 10C °C min min.−1 CO. CO2 adsorption2 adsorption isotherms isotherms in in the the pressure pressure range range of 0.1–50 of 0.1–50 bar bar at 298 K were measured on a Quantachrome iSorb HP1 analyzer. Ultrapure (99.995%) at 298 K were measured on a Quantachrome iSorb HP1 analyzer. Ultrapure (99.995%) CO2 CO2gasgas was was used used for for the the adsorption adsorption measurements measurements.. The The PL PLspectra spectra and and emission emission decay decay curves curveswere were measured measured at room at room temperature temperature using using a Horiba a Horiba Scientific Scientific model model FluoroMax-4 FluoroMax-4 spec- spectrofluorometer.trofluorometer.

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3.2. Synthesis of [Sm2(bzz)(ben)6(H2O)3]·0.5H2O (1)

A mixture of Sm(NO3)3·6H2O (44.7 mg, 0.1 mmol) and bzz (27.3 mg, 0.2 mmol) in distilled H2O (4 mL) was placed in a Teﬂon-lined reactor. The mixture was stirred at room temperature for 30 min, sealed in a 15 mL stainless steel autoclave, and then placed in an oven. The reaction mixture was kept at 130 ◦C under autogenous pressure for 96 h. After cooling to room temperature, the crystalline product was ﬁltered, washed with distilled H2O, and dried in air at room temperature. The product was obtained as light-yellow needle-like crystals with a yield of 84% (37.5 mg) based on Sm(NO3)3·6H2O. Anal. calc. for C49H46N2O17Sm2: C, 47.63%; H, 3.75%; N, 2.27%. Found: C, 47.54%; H, 3.49%; N, 2.56%. FT-IR (ATR, ν/cm-1, s for strong, m for medium, w for weak): 3430(w), 3058(w), 1593(m), 1532(s), 1492(m), 1403(s), 1307(w), 1180(s), 1070(w), 1025(w), 845(w), 712(s), 684(m).

3.3. Synthesis of [Eu(bbz)(ben)3] (2)

The procedure was the same as that for 1, except that Sm(NO3)3·6H2O was replaced by Eu(NO3)3·6H2O (44.9 mg, 0.1 mmol). The product was obtained as light-yellow block- shaped crystals with the yield of 47% (21.1 mg) based on Eu(NO3)3·6H2O. Anal. calc. for C35H27EuN2O8: C, 35.66%; H, 3.42%; N, 17.82%. Found: C, 35.43%; H, 3.41%; N, 17.35%. FT-IR (ATR, ν/cm-1): 3155 (w), 3055 (w), 3002 (w), 2896 (w), 1643 (m), 1587 (m), 1504 (s), 1477 (s), 1398 (s), 1305 (m), 1173 (w), 1066 (w), 1020 (w), 921 (w), 834 (w), 695 (m).

3.4. X-ray Crystallography Suitable crystals of compounds 1 and 2 were mounted on MiTeGen micromounts using paratone oil (Hampton Research). X-ray diffraction data were collected using a Bruker D8 QUEST CMOS PHOTON II operating at T = 296(2) K. Data were collected using ω and φ scans and using Mo-Kα radiation (λ = 0.71073 Å). The total number of runs and images was based on the strategy calculation from the program APEX3 and unit cell indexing was refined using SAINT [32]. Data reduction was performed using SAINT and SADABS were used for absorption correction. The integrity of the symmetry was checked using PLATON [26]. The structure was solved with the ShelXT structure solution program using combined Patterson and dual-space recycling methods [33]. The structure was refined by least squares using ShelXL [34] using the OLEX2 [35] interface. In the final refinement cycles, all non-H atoms were refined anisotropically. All C-bound H atoms were placed in calculated positions and refined using a riding-model approximation with C-H = 0.93 Å and Uiso(H) = 1.2 Ueq(C), while all N- and O-bound H atoms were located in a difference Fourier map and refined with N-H = 0.86–0.89 Å and O-H = 0.84 ± 0.01 Å, respectively. The lattice water molecule in 1 presents a positional disorder that was refined as two contributions with a 0.80:0.20 occupancy ratio. The benzene rings of the ligands in 1 and 2 were found to be disordered over two 50% occupancy sites. The details of crystallographic parameters, data collection, and refinements for all complexes are listed in Table S1 in the Supplementary Material.

4. Conclusions Two new mixed ligand LnCPs were synthesized based on in situ-generated ligands formed under similar hydrothermal conditions. The participation of the ben and bbz ligands formed in the transformation of bzz molecules plays an important role in the formation of the structural differences between the network structures of compounds 1 and 2. Compound 1 shows a 1D double chain, while compound 2 features a 3D network 6 with a 4-connected (6 ) dia topology. Activated compound 1 shows that maximum CO2 uptake increases to exceptionally high values of 376.7 cm3 g−1 (42.5 wt%) under a pressure of 49.8 bar at 298 K with good recyclability. Meanwhile, 2 exhibits a typical red emission in the solid state at ambient temperature with a decay lifetime of 1.2 ms. We believe that the hydrothermal in situ hydrolysis reactions of hydrazide ligand described here will provide an alternative strategy of obtaining novel crystalline LnCPs. Further systematic investigations are currently underway with the aim of exploring various aromatic hydrazide groups Molecules 2021, 26, 4428 11 of 12

containing ligands for hydrothermal in situ self-assembly reactions. We anticipate that this will result in many more novel LnCPs with fascinating structural properties, which will help us to understand the possible reaction mechanism of the in situ ligand synthesis and provide new insights toward the development of crystal engineering of LnCPs.

Supplementary Materials: The following are available online, Figure S1: FTIR spectra for 1 and 2, Figure S2: View of C···H-π interactions for 2, Figure S3: Solid-state excitation spectrum for 2, Table S1. Summary of crystal data and structure refinement for 1 and 2, Table S2: Selected bond lengths (Å) for 1, Table S3: Hydrogen-bond geometry (Å, ◦) for 1, Table S4: Selected bond lengths (Å) for 2, Table S5: C···H-π interactions (Å, ◦) for 2. Author Contributions: Funding acquisition, K.C. and M.S.; investigation, K.C. and M.S.; method- ology, C.T., S.J., N.C., S.C., F.K., W.D., and K.C.; validation, K.C.; visualization, C.T., S.J., and K.C.; writing—original draft, K.C.; writing—review and editing, C.T., N.C., S.J., S.C., F.K., W.D., M.S., and K.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by Thammasat University Research Fund, Contact No. TUFF02/2564, and was also partially funded by the Thailand Research Fund (RTA6180007). C.T. would like to acknowledge the Graduate Development Scholarship 2020, National Research Council of Thailand (Contact No. 15/2563) and the NSTDA STEM Workforce (scholarship No. SCA-CO-2561- 6014-TH). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in supplementary material. Acknowledgments: The authors thank the Faculty of Science and Technology, Thammasat University, for funds to purchase the X-ray diffractometer and thermogravimetric analysis. The Thammasat University Research Unit in Multifunctional Crystalline Materials and Applications (TU-McMa) is also gratefully acknowledged. Conflicts of Interest: The authors declare no conflict of interest. Sample Availability: Samples of the compounds 1 and 2 are available from the authors.

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