Co–Ca Phosphonate Showing Humidity-Sensitive Single Crystal to Single Crystal Structural Transformation and Tunable Proton Conduction Properties

Abstract

A combination of humidity-dependent single crystal to single crystal (SC−SC) structural transformation and single crystal proton conductivity measurements is essential to elucidate the underlying proton transport mechanism in metal−organic framework materials. Herein, we report a new layered Co−Ca phosphonate [Co III Ca II (notpH 2 )(H 2 O) 2 ]ClO 4 •nH 2 O [abbreviated as CoCa•nH 2 O, where notpH 6 = 1,4,7-triazacyclononane-1,4,7-triyl-tris-(methylenephosphonic acid), C 9 H 18 N 3 (PO 3 H 2 ) 3 ]. CoCa•nH 2 O undergoes a reversible relative humidity (RH) dependent SC−SC structural transformation between CoCa•2H 2 O and CoCa• 4H 2 O at room temperature. Accordingly the continuous hydrogen bond network observed in CoCa, leading to a drastic decrease in proton conductivity by ∼5 orders of magnitude. The process is reversible; hence, the proton conductivity is tunable simply through humidity control. The AC impedance measurements using single crystals of CoCa•nH 2 O reveal that the [010] direction of H-bond extension is the preferred proton conduction pathway showing the greatest conductivity of 1.00 × 10 −3 S cm −1 at 25 °C and 95% RH. Although the [20−1] direction, which involves the phosphonate oxygen atoms in the H-bond network shows the lowest conductivity of 4.35 × 10 −8 S cm −1 at 25 °C and 95% RH, the ClO 4 − anions play a key role in not only connecting the lattice water molecules into a continuous hydrogen bond network but also assisting the proton diffusion between the lattice water molecules. This work provides a rare example of a proton conductive MOF with a well-illustrated proton conduction mechanism and is a promising humidity sensor for future applications.

Luminescent and Proton Conducting Lanthanide Coordination Networks Based On a Zwitterionic Tripodal Triphosphonate

“…10 -4 S•cm -1 (80 ºC) similar to those found for polycrystalline heterometallic Co-Ca phosphonate [Co III Ca II (notpH 2 )(H 2 O) 2 ]ClO 4 •nH 2 O [notpH 6 = 1,4,7triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid). 19 The proton conductivity for the chloride-free compounds oscillate between 2 . 10 -3 S•cm -1 and 8 .…”

From light to heavy alkali metal tetraphosphonates (M = Li, Na, K, Rb, Cs): cation size-induced structural diversity and water-facilitated proton conductivity

“…The field of metal phosphonate compounds has experienced constant growth during the last decades. 1 This is primarily due to the rich structural diversity of these hybrids and their potential applications, 2 including gas storage, [3][4][5] photoluminescence, 6,7 proton conductivity, [8][9][10][11][12][13][14] corrosion control [15][16][17][18][19][20] and metal ion absorption. 21,22 Phosphonic acids and their water-soluble salts find extensive use in several industrial and pharmaceutical applications, such as mineral scale inhibition, [23][24][25] dispersion, 26 scale dissolution, 27 osteoporosis treatment, [28][29][30] cancer treatment, 31 etc.…”

Nanoporous Metal Phosphonate Hybrid Materials as a Novel Platform for Emerging Applications: A Critical Review

“…Besides the Zr and Ti phosphonates, other metal phosphonates were also subjected for proton conduction studies. [169][170][171][172][173][174][175][176][177] Zheng and co-workers have systematically reviewed the research progress of proton conductive metal phosphonate frameworks including those involving main group metals, transition metals, lanthanide and actinide metals, and mixed metal phosphonates. [169] Phosphonate-based MOF materials such as Zn-MOF, [171,172] Co-MOF, [173] Ni-MOF, [173] Eu-MOF, [175] and La-MOF [177] have been proven to be efficient proton conductive materials for PEC.…”