Article pubs.acs.org/IC
Multiple-Step Humidity-Induced Single-Crystal to Single-Crystal Transformations of a Cobalt Phosphonate: Structural and Proton Conductivity Studies Zhong-Sheng Cai,† Song-Song Bao,† Xi-Zhang Wang,‡ Zheng Hu,‡ and Li-Min Zheng*,† †
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, People’s Republic of China ‡ Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, People’s Republic of China S Supporting Information *
ABSTRACT: Humidity-induced multiple-step single-crystal to single-crystal (SC-SC) transformations are observed in the cobalt phosphonate (NH4)3[Co2(bamdpH)2(HCOO)(H2O)2] (1), where bamdpH 4 is (benzylazanediyl)bis(methylene)diphosphonic acid, [C6H5CH2N(CH2PO3H2)2]. Under highhumidity conditions (95% RH), compound 1 experiences hydrolysis at 60 °C which is accompanied by the transformation from a double-chain structure of compound 1 into a single-chain structure of [Co(bamdpH2)(H2O)2]·2H2O (2). When the humidity is below 10% RH, part of the lattice water in compound 2 can be released, forming a third phase, [Co(bamdpH2)(H2O)2]·H2O (3). The structural transformation processes have been monitored by infrared and proton conductivity measurements.
■
INTRODUCTION Metal−organic frameworks (MOFs) or coordination polymers (CPs) have attracted intense current interest owing to their potential applications in gas storage and separation,1 catalysis,2 sensing,3 magnetism,4 proton conduction,5 etc. Of particular interest are those showing solid-state phase transitions triggered by external stimuli, which is often accompanied by a distinct change in functions.6 A single-crystal to single-crystal (SC-SC) transformation can provide a precise visualization about the structural changes during the phase transitions, and hence a structure−property relationship can be well illustrated.7 So far, a number of MOFs or CPs have been reported to experience SC-SC transformations induced by heat,8 light,9 redox,10 chemical reactions,11 and guest exchange.12 In contrast, humidity-induced SC-SC transformations have rarely been described.13 Metal phosphonates, as an important class of organic− inorganic hybrid materials, have received increasing attention recently due to their significance in the fields of catalysis,14 ion exchange,15 magnetism,16 and proton conductivity.17,18 Although solid-state phase transitions have been reported in metal phosphonate systems together with drastic changes in their physical and chemical properties,19 those showing SC-SC transformations are extremely rare.20−23 Albrecht-Schmitt and co-workers reported a few nanotubular uranyl phosphonates which can exchange ions in a SC-SC manner in solution.20 © XXXX American Chemical Society
Reversible SC-SC transformations were also observed in the cobalt phosphonates Co 2 (pbtcH)(2,2′-bpy) 2 (H 2 O) and Co2(pbtcH) (phen)2(H2O) (pbtcH5 = 5-phosphonatophenyl1,2,4-tricarboxylic acid) upon dehydration and rehydration, which involve the breakage and reconstruction of covalent bonds.21 More recently, we found that the number of lattice waters in a Co−Ca phosphonate can be controlled by humidity, resulting in a remarkable change in the proton conductivity of the material.22 In this paper, we report the new double-chain Co II phosphonate (NH4)3[Co2(bamdpH)2(HCOO)(H2O)2] (1), where bamdpH 4 is (benzylazanediyl)bis(methylene)diphosphonic acid (C6H5CH2N(CH2PO3H2)2). It experiences a SC-SC structural transformation at 60 °C and 95% relative humidity (RH), forming the single-chain compound [Co(bamdpH2)(H2O)2]·2H2O (2). The humidity-induced SC-SC transformation of compound 1 is unusual, because the process involves the hydrolysis of a bridging ligand of HCOO−, unlike the aforementioned Co−Ca phosphonate, which involves only the change of lattice water. The process can be monitored by infrared (IR) spectra and proton conductivity measurements. Furthermore, compound 2 can also undergo a SC-SC Received: February 25, 2016
A
DOI: 10.1021/acs.inorgchem.6b00413 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. Crystallographic Data for 1−3 empirical formula fw cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dc (g cm−3) F(000) goodness of fit R1, wR2 (I > 2σ(I))a R1, wR2 (all data)a Δρmax, Δρmin (e Å−3) CCDC no. a
1
2
3
C19H41Co2N5O16P4 837.31 monoclinic C2/c 34.454(11) 6.358(2) 15.408(5) 110.757(5) 3156.1(17) 4 1.762 1728 1.00 0.0312, 0.0811 0.0376, 0.0848 0.70, −0.40 1429479
C9H21CoNO10P2 424.14 monoclinic P21/c 16.847(4) 6.2741(16) 15.814(4) 106.321(4) 1604.1(7) 4 1.756 876 1.001 0.0804, 0.2027 0.1136, 0.2209 0.845, −0.969 1429480
C18H36Co2N2O17P4 794.23 monoclinic P21/c 16.261(5) 6.2566(19) 15.639(5) 114.182(4) 1451.5(7) 2 1.826 824 1.004 0.0952, 0.2486 0.1192, 0.2666 1.696, −1.903 1429481
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2. pattern (Figure S3 in the Supporting Information). Anal. Calcd for C18H36Co2N2O17P4: C, 27.22; H, 4.57; N, 3.53. Found: C, 27.10; H, 4.57; N, 4.27. Single-Crystal Structure Determination. Single crystals of the three compounds were selected for indexing and intensity data collection on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. The data were integrated using the Siemens SAINT program,25 with the intensities corrected for Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector face plate. Absorption corrections were applied. The structures were solved by direct methods and refined on F2 by full-matrix least squares using SHELXTL.26 All of the nonhydrogen atoms were located from the Fourier maps and were refined anisotropically. All H atoms were refined isotropically with the isotropic vibration parameters related to the non-hydrogen atoms to which they are bonded. The crystallographic data of compounds 1−3 are given in Table 1, and the selected bond lengths and angles are given in Tables 2−4, respectively.
transformation below 10% RH, leading to a third phase, [Co(bamdpH2)(H2O)2]·H2O (3).
■
EXPERIMENTAL SECTION
Materials and Measurements. The reagents and solvents employed were obtained from commercial sources and used without further purification. (Benzylazanediyl)bis(methylene)diphosphonic acid (bamdpH4) was prepared according to the literature.24 Elemental analyses for C, H, and N were performed on an Elementar Vario MICRO elemental analyzer. The infrared spectra were recorded on a Bruker TENSOR 27 IR spectrometer with pressed KBr pellets in the 400−4000 cm−1 region. Thermogravimetric (TG) analyses were performed on a Mettler Toledo TGA/DSC 1 STARe instrument in the range of 30−500 °C under a nitrogen flow at a heating rate of 10 °C min−1. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 ADVANCE XRD instrument using Cu Kα radiation. The UV−vis spectra were measured on a PerkinElmer Lambda 950 UV/ vis/NIR spectrometer. Mass spectra were recorded on a Shimadzu GC-MS QP2010 plus instrument. The humid environment was controlled by an Espec Corp. SH-221 humidity control oven. The impedance measurements were carried out under multiple different environmental conditions by the conventional quasi-four-probe method, using gold paste and gold wires (50 μm diameter) with a Solartron SI 1260 Impedance/Gain-Phase Analyzer and 1296 Dielectric Interface in the frequency range of 1 MHz to 0.1 Hz. Synthesis of (NH4)3[Co2(bamdpH)2(HCOO)(H2O)2] (1). Co(NO3)2·6H2O (596 mg, 2 mmol) and bamdpH4 (590 mg, 2 mmol) were added to 22 mL of distilled water and 28 mL of HCONH2. The pH value was adjusted to 3.55 with 0.5 M NaOH. Then the mixture was transferred to a 180 mL Teflon-lined autoclave and kept at 140 °C for 1.5 days. After the mixture was cooled to room temperature, red crystals were obtained. The crystals were washed with distilled water and dried in air. Yield: 311.7 mg (18.6% based on Co). The purity of the phase was confirmed by a PXRD pattern (Figure S1 in the Supporting Information). Anal. Calcd for C19H41Co2N5O16P4: C, 27.25; H, 4.94; N, 8.36. Found: C, 27.11; H, 5.17; N, 8.44. Synthesis of [Co(bamdpH2)(H2O)2]·2H2O (2). Compound 2 was obtained by keeping the crystals of compound 1 at 60 °C and 95% RH for 2 days. The purity of the phase was confirmed by a PXRD pattern (Figure S2 in the Supporting Information). Anal. Calcd for C9H21CoNO10P2: C, 25.48; H, 4.99; N, 3.30. Found: C, 25.47; H, 4.94; N, 3.89. Synthesis of [Co(bamdpH2)(H2O)2]·H2O (3). Compound 3 was obtained by keeping the crystals of compound 2 at 25 °C and
