A Lamellar Coordination Polymer with Remarkable Catalytic Activity

Abstract

A new positively charged lamellar coordination polymer based on a flexible triphosphonic acid linker is reported. [Gd(H 4 nmp)(H 2 O) 2 ]Cl·2H 2 O (1) [where H 6 nmp stands for nitrilotris(methylenephosphonic acid)] is obtained in a typical one-pot approach using water as a "green" solvent and by forcing the inclusion of additional acid sites by employing HCl in the synthesis. Compound 1performs as a heterogeneous, versatile acid catalyst, with outstanding activityin different organic reactions, namely, the alcoholysis of styrene oxide, acetalisation of benzaldehyde and ofcyclohexanaldehyde, and ketalisation of cyclohexanone. For all reaction systems, very high conversions were reached (92-97%) in only 15-30 min, under mild conditions (35 ºC, atmospheric pressure). Allied to these catalytic results, the material also exhibits interesting protonic conductivity values of 1.23x10-5 Scm -1 at 98% RH at 40 ºC.

Enhanced proton conductivity in a layered coordination polymer

“…–POH groups and coordinated water molecules of the 2∞[Gd(H 4 nmp)(H 2 O) 2 ] + cationic layer donate their hydrogen atoms to neighbouring water molecules of crystallization and to chloride anions. The conductivity of 1 at variable temperatures (40 and 60 °C) and relative humidity (RH, 20–98%) previously reported by us 4 reached 1.23 × 10 –5 S cm –1 at 40 °C and 98% RH, which compares favourably with those of other CPs/MOFs reported in the literature. 13 …”

“… 19 The “apparent” poor quality of these powder X-ray diffractograms is attributed to “poor” crystallinity of the product. However, following our previous work on 1 , 4 the observed in situ structural transformation at high temperatures and RH (above 80 °C and at 98% RH) of 1 into 2 , and the remarkable properties of the latter, prompted us to reinvestigate in more detail the structural features of this new compound.…”

“…In this report we explore how subtle structural changes between two related layered CPs can lead to a remarkable enhancement in proton conductivity: [Gd(H 4 nmp)(H 2 O) 2 ]Cl·2H 2 O ( 1 ) (previously reported by us) 4 converts in situ , at high humidity and temperatures, into [Gd 2 (H 3 nmp) 2 ]· x H 2 O ( 2 ) ( x = 1 to 4 water molecules; H 6 nmp stands for nitrilotris(methylenephosphonic)acid), which is shown to be a hydrophilic compound with variable interlayer space. The observed proton conductivities for the latter compound (which can also be directly isolated from typical and sustainable syntheses) are, to the best of our knowledge, some of the best reported conductivities to date, even surpassing those of commercial materials.…”

Pyrene Tetraphosphonate‐Based Metal‐Organic Framework: Structure and Photoluminescence

“…The high rigidity of the pyrene backbone gives rise to crystallisation problems. In previous reports we have used acid modulators, such as hydrochloric and sulfuric acid, to slow down the coordination process in order to obtain highly crystalline materials . These modulators both help to protonate the organic linkers and are part of the crystalline structure by coordinating to the metal centre or acting as charge‐balancing anions.…”

Coordination Polymers Based on a Biphenyl Tetraphosphonate Linker: Synthesis Control and Photoluminescence

“…In our group, the rich chemistry of phosphonic acid allowed the preparation of a variety of structures ranging from 1D to 3D networks by, in many cases, slight changes in experimental conditions. With this we were able to synthetize MOFs with a broad range of applications: from photoluminescence [28][29][30], to luminescent thermometers [31], heterogeneous catalyst [32,33] and/or proton conductors [34].…”